Methods and systems to form skinless foams

Abstract

Methods and systems to form skinless foams are described. The methods and systems utilize a gas-pressurized chamber to infuse a material with the gas and a heated fluid, heated above the glass-transition temperature of the material, to contact the material and generate a skinless foam from the material. The systems and methods generate foams that are skinless upon formation, eliminating the need for skin removal.

Description

METHODS AND SYSTEMS TO FORM SKINLESS FOAMSSTATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0001] This invention was made with government support under Grant No. 2301430, awarded by the National Science Foundation (NSF). The government has certain rights in the invention.CROSS-REFERENCE TO RELATED APPLICATION

[0002] This application claims priority to U.S. Provisional App. No. 63 / 733,815, filed on December 13, 2024 and incorporated by reference herein in its entirety.FIELD OF THE DISCLOSURE

[0003] The current disclosure provides methods and systems to form skinless foams The methods and systems utilize a gas-pressurized chamber to infuse a material with a gas The methods and systems also use a fluid, heated above the glass-transition temperature of the material, to contact the material. These systems and methods generate foams that are skinless upon formation, eliminating the need for skin removal.BACKGROUND OF THE DISCLOSURE

[0004] Solid-state foaming is an advanced technique for producing microcellular and nanocellular polymer foams, which are characterized by their internal structure with cell sizes ranging from 10 m to 1 nm. The solid- state foaming process includes three primary stages, gas saturation, depressurization, and nucleation of cells. Initially the untouched polymer is placed in a high pressure chamber and is saturated with an inert gas, typically CO2, until it reaches an equilibrium concentration within the polymer. The equilibrium concentration is the maximum solubility of the polymer and is dependent on the saturation time, pressure and temperature. Once the chamber is vented, the now saturated polymer is removed, causing a rapid release of gas from its surface. The polymer is then placed inside of a heating bath to raise the polymer above its glass transition temperature. The sudden change of pressure and temperature induces a thermodynamic instability, resulting in the nucleation and growth of bubbles within the polymer.

[0005] A major drawback to these described processes is that the formed polymers have a thick layer of nonfoamed skin surrounding the foamed center. This skin layer means that there is not a fully porous structure through the entire polymer, which inhibits potential applications of the microstructure, for example applications that require fluids to penetrate the entire foam thickness. Removing the non-porous skin layer has been attempted to achieve a skinless microcellular foam These attempts, however, have proved unsuccessful as they either require postfoaming processing, which can damage pore structure, or leave a thin layer of skin behind.SUMMARY OF THE DISCLOSURE

[0006] This Summary is provided to introduce a selection of concepts in a simplified form that is further described below in the Detailed Description. This Summary is not intended to identify all key features or essential features of the claimed subject matter, nor is it intended to be used alone as an aid in determining the scope of the claimed subject matter.

[0007] The present disclosure describes methods and systems for forming skinless foams, and the skinless foams formed thereby.

[0008] Methods of the present disclosure for generating a skinless foam can include using a gas to pressurize a chamber containing a material to a pressure and maintaining the pressure in the chamber until a saturation state of the gas is reached within the material. In implementations, the material in the chamber can then be heated with a fluid at a temperature that is greater than or equal to a glass transition temperature of the material, thereby generating the skinless polymer foam. In implementations, the method includes maintaining the pressure in the chamber as the material is contacted with the fluid and heated.

[0009] Systems of the present disclosure can include a chamber configured to contain a material and operate at elevated pressures and temperatures, a gas source fluidically connected to the chamber and configured to contain a gas, and a flow control device coupled between the chamber and the gas source. Systems described herein can also include a fluid reservoir fluidically connected to the chamber and configured to contain a fluid at elevated temperatures and pressures, and a flow control device coupled between the fluid reservoir and the chamber.

[0010] Skinless foams formed by the methods and systems described herein include thermoplastic materials having pores disposed therein with a width of 5 nanometers (nm) to 100 micrometers (pm). Specifically, a portion of the pores are exposed at a surface of the thermoplastic material at a time the foam is formed, for example 10% to 90% of the surface of the thermoplastic material is the exposed pores.

[0011] The systems and methods described herein are the first to allow for the formation of fully porous (skinless) foams These systems and methods thereby allow for improved efficiency in the production of skinless foams for example, for use as selectively permeable membranes for waterproofing, battery scaffolds, and durable water filters, among other uses.BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 illustrates a schematic of an example isobaric foaming system.

[0013] FIG. 2 illustrates an example isobaric foaming process.

[0014] FIG. 3 illustrates gas concentration over time for polyetherimide (PEI) samples normalized by thickness.

[0015] FIG. 4 illustrates a traditional solid-state foaming process (top) compared to an example isobaric foaming process of the present disclosure (bottom). Keeping the sample under a continuous pressure prevents the release of gas from the skin layer.

[0016] FIG. 5 illustrates a schematic of an example isobaric foaming system.

[0017] FIG. 6 illustrates recycled polyethylene (rPET) foam cross section at an angle, showing an un-foamed layer comparable to cell wall thickness.

[0018] FIG. 7 illustrates scanning electron microscope (SEM) images of example foamed polyethylene (PET) with cell wall sized skin. Panel a): 300 m scale; panel b): 100 pm scale; and panel c): 50 pm scale.

[0019] FIG. 8 illustrates SEM images of example nanocellular polycarbonate foam morphology: panel a) near- surface skin layer (30 pm scale); panel b) bulk cellular core (5 pm scale); panel c) primary cell with secondary nanopores (3 pm scale); and panel d) close-up of nanoscale pores (500 nm scale).

[0020] FIG. 9 shows an example CO2 Saturation vs Temperature graph of Polycarbonate, saturated at 25°C, room temperature and foamed at 100°C.DETAILED DESCRIPTION

[0021] Common processes of solid-state foaming have been to create microcel I u I ar and nanocellular foams from various amorphous and semi-crystalline polymers, such as polylactic acid (PLA), polyvinyl chloride (PVC), and polycarbonate (PC). “Foaming” is a process in which a gas is generated or introduced into a material, such as a polymer, to create a cellular or sponge-like structure containing many small bubbles or voids. As the gas forms and expands within a polymer matrix, it creates dispersed gas pockets that “blow up” the material into a foam. The extent and character of the foaming can be controlled by factors such as the rate of gas generation, viscosity of the polymer, temperature, and any applied pressure or confinement The resulting foam can have reduced density, improved insulation, cushioning, or flexibility.

[0022] The basic steps of solid-state foaming of thermoplastics are to saturate the thermoplastic material with a gas, and then to nucleate gas bubbles within the material. The polymer material is first placed in a pressure vessel under high pressure of an inert gas, such as carbon dioxide (CO2). The pressure differential between the inside of the polymer and the surrounding high pressure gas will cause the gas to diffuse into the polymer until it reaches an equilibrium state. Once the polymer is saturated, bubbles can be nucleated by creating thermodynamic instability. This can be achieved, for example, by rapidly dropping the pressure and increasing the temperature of the material, for example by submerging the material in a hot oil or water bath. These changes cause a sudden decrease in the gas solubility, which drives the gas out of the polymer and into nucleated bubbles

[0023] Microcellular foams can have a cell size in the range of 1 M to 10pim, while nanocellular foams can have a cell size of less than 1 piM. Cell size is attributed to the saturation of the polymer, for example, a sample with a greater CO2 content will have a decreased cell size, while a sample with a lower CO2 will have an increased cell size. It has also been shown that decreasing the saturation temperature, for example to -30°C, increases the solubility of the sample, and a cell size of 21 nm can be achieved. The development of polymer nanocellular foams is an expanding area of study because nanocellular foams have been found to provide unique and advantageous properties that are not available with larger celled foams. Some of the advantageous properties of nanocelluar foams include low thermal conductivity, low dielectric constant, and high specific strength.

[0024] A major drawback to previous foaming processes, however, is that solid-state foaming using polymers form a thick layer of non-foamed skin surrounding the foamed center Non-porous skin layer formation is caused by a desorption state near the surface of the polymer where the dissolved gas has exited the polymer, for example due to a pressure imbalance. A pressure imbalance can happen when the external pressure surrounding the polymer decreases, such as when the polymer is removed from a pressurized saturation chamber and moved into a heated bath. Therefore, following the release of high pressure from the saturation chamber, gas will actively escape from the surface of the polymer and when the polymer is then foamed in the hot bath, limited foaming will occur in the surface layer of the polymer and an impermeable skin with no microcellular structure will form.

[0025] This skin layer prevents a fully porous “open-cell” structure through the entire polymer. “Open-cell” refers to how the porous structure in a foam appears interconnected rather than containing discrete cells separated by cell walls, also known as “close-cells”. An open-cell structure allows fluids to penetrate the entire foam thickness. The layer of non-foamed skin that is generated in prior polymer foaming methods limits access to the foamed core structure, which inhibits potential applications of the microstructure. The production of fully nanoporousarchitectures opens the door to entirely new applications of foamed materials, such as use as filers and catalysis. As such, much focus has been directed towards the removal of this skin region.

[0026] While removing the non-porous skin layer from polymer foams has been attempted, all approaches have been performed post-foaming. For example, Jose (Jose, et al., American Society of Mechanical Engineers, Nov. 2016, Vol. 14) used a sacrificial layer to decrease the skin layer and then punch through the remaining skin later with a CC>2 laser, and Morisaki (Morisaki, et al., Polymer, Mar. 2008, 49(6):1611-1619) used super critical CO2 to foam polymethyl methacrylate (PM MA) with little desorption time. These attempts proved unsuccessful, as they either required post-foaming processing, which can damage pore structure, or they left a thin layer of skin.

[0027] Other methods to remove the non-porous skin layer from a porous foam include polishing the skin layer, but this approach can damage the polymer structure underneath, creating another even denser skin-like layer. Drilling through the skin layer to connect the porous structure to the outside of the skin also results in damage. Reactive ion etching and laser ablation were also unsuccessful because porosity is not well controlled. Furthermore, these methods are only reasonable with small samples due to inefficiency, limiting the scale up and industrial implementation of these methods.

[0028] Further attempts to create skinless nanofoams have focused on using diffusion barriers. One such study used a metallic mold as a gas diffusion barrier which achieved a homogeneous structure, however it restricted the expansion of the plastic during foaming. Restriction of the sample meant that it was not possible to significantly lower the density using this method, which is an essential feature of nanofoams to give them their unique properties. In addition, the saturation time was significantly higher due to the gas having to travel through the greatest dimension of the sample instead of being able to travel through the full surface area of the sample. As described above, existing solutions for generating skinless foams face many challenges and, despite considerable investigation, have not produced suitable industrial scale results.

[0029] Various implementations of the present disclosure relate to methods and systems for generating skinless microcellular and nanocellular polymer foams, that are skinless upon formation. No existing solution for removing or eliminating non-porous skin from polymer foams has attempted to eliminate desorption of the saturation gas from the surface of the polymer. As described herein, reducing the time of desorption and keeping the concentration gradient of the gas within the polymer consistent from saturation of the polymer to heating and nucleation of the gas achieves a skinless polymer foam at the time of foam formation. In some examples, an isobaric solid-state foaming process can be utilized to create micro- and nanofoamed polymers with a porous structure homogenously distributed throughout their thickness. Allowing microcellular (or nanocellular) foams to be created with no surrounding skin allows the internal structure to be accessed and controlled for multiple applications. Producing skinless microcellular (or nanocellular) foams at foam formation allows scaling up and improved efficiency of solid- state polymer foaming Combining this with the ability to control the size of cells, multiple thermoplastics can be fabricated using an inert, scalable, process. Additionally, the methods and systems described herein can improve efficiency by achieving a close to net zero use of gas. For example, by using CO2 to saturate a polymer, and supercritical CO2 to heat the polymer, the used or excess CO2 can be captured with minimal waste.

[0030] As shown in relation to FIG. 1, methods and systems of the present disclosure for generating a skinless foam can include a system 100 that is configured to utilize a gas, for example an inert gas such as CO2, to pressurize a chamber 102 containing a material 104. In implementations, the chamber 102 is a high pressure chamber configured operate at elevated temperature and pressures and positioned to receive the gas from a gas source 106, for example a gas tank. In some cases, the material 104 includes a thermoplastic polymer, a biologically derived material, or any combination thereof. For example, the thermoplastic polymer can include polyethylene terephthalate (PET), polylactic acid (PLA), polyvinyl chloride (PVC), polycarbonate (PC), polystyrene (PS), polymethyl methacrylate (PM MA), polyetherimide (PEI), polyether ether ketone (PEEK), polyethylene (PE), low density polyethylene (LDPE), medium density polyethylene (MDPE), high density polyethylene (HDPE), acrylonitrile butadiene styrene (ABS), polyethersulfone (PES), polyetherketoneketone (PEKK), polyaryletherketone (PAEK), polyimide (PI), polysulfone (PSU), polyphenylsulfone (PPSU), thermoplastic polystyrene (TPS), or cyclic olefin copolymer (COC). In implementations, the biologically derived material can include a food material, such as bread or pasta, or a protein-based biopolymer. In implementations, the material 104 can include a blend of at least two thermoplastic polymers. In some cases, the material 104 can include a composite material, for example a material including at leastone of a carbon fiber, a glass fiber, or a nanoparticle.

[0031] In implementations, the chamber 102 is fluidically connected to the gas source 106 and a flow control device 108 can be coupled between the chamber 102 and the gas source. Systems of the present disclosure can include flow control devices configured to operate at elevated temperatures and pressures, such as solenoid valves, ball valves, plug valves, gate valves, globe valves, and needle valves. Flow control devices described herein can also include pumps, compressors, throttling control devices, etc. The systems described herein can also include necessary safety measures, pressure seal valves, pressure relief valves, etc., such as pressure relief valves 110 and 111.

[0032] In implementations, the flow control device 108 can be actuated to supply the gas to the chamber 102 to pressurize the chamber 102 to greater than or equal to a threshold pressure, for example a pressure in a range of 0.01 megapascals to 30 megapascals. The pressure is then maintained within the chamber 102 for a time sufficient to allow the gas to diffuse into the material 104. The threshold pressure and the duration of time the material 104 is kept under pressure by the gas can be selected based on a desired saturation sate of the material 104, i.e. a desired amount of gas dissolved into the material 104. In implementation, the saturation state of the material 104 can include a partial saturation of the gas within the material 104 or an equilibrium concentration of gas within the material 104. For example, the threshold pressure can include a pressure sufficient to dissolve at least 10 wt% of the gas into the material 104.

[0033] In implementations, once a desired saturation state of the gas is reached within the material 104, the chamber 102 can then be heated with a fluid at a temperature that is greater than or equal to a glass transition temperature of the material 104. The fluid can be fed to the chamber 102 from a fluid reservoir 112 fluidically connected to the chamber 102 and configured to contain the fluid at elevated temperatures and pressures. The fluid can be fed to the fluid reservoir 112 from a fluid source 114 In implementations, the fluid can include water, steam, an oil, or a supercritical fluid such as supercritical CO2, or supercritical N2. In implementations, the heatedfluid is saturated with a gas. In some cases, the fluid is saturated with the same gas that is used to pressurize the chamber 102 and dissolve into the material 104. For example, the system 100 can include a gas feed line from the gas source 106 that is fluidically connected to the fluid reservoir 112 and / or to the fluid source 114. The gas feed line can include a flow control device configured to supply the gas to the fluid, for example to increase or maintain the dissolved gas concentration in the fluid. Saturating the heated fluid with the pressurizing gas can prevent desorption of the gas from the material when the heating fluid is contacted with the material In some cases, the fluid is a supercritical form of the gas that is used to pressurize the chamber 102 and dissolve into the material 104. In implementations, the fluid source 114 can include one or more sources of gas, such as one or more gas tanks, that can include one or more flow control devices configured to supply the gas to the fluid reservoir 112, for example to increase or maintain a supercritical pressure in the fluid reservoir 112. In implementations, the fluid source 114 can include the gas source 106, such that the gas source 106 is fluidically connected to the fluid reservoir 112 by a gas feed line The gas feed line can include a flow control device configured to supply the gas to the fluid reservoir 112, for example to increase or maintain a supercritical pressure in the fluid reservoir 112.

[0034] The fluid in the fluid reservoir 112 can be held at a temperature greater than or equal to a glass transition temperature of the material 104 and at a pressure greater than or equal to the threshold pressure within the chamber 102. The system can include a flow control device 116, such as a needle valve, coupled between the fluid reservoir 112 and the chamber 102 and configured to actuate to supply the chamber 102 with the fluid at a temperature greater than or equal to the glass transition temperature of the material 104 and a pressure greater than or equal to the threshold pressure within the chamber 102. For example, feeding the fluid to the chamber 102 can include maintaining an isobaric environment within the chamber 102.

[0035] In implementations, when the fluid at a temperature greater than or equal to the glass transition temperature of the material 104, enters the chamber 102 containing the material 104, the fluid contacts and heats the material 104. In implementations, the pressure in the chamber 102 is maintained at or above the threshold pressure as the material 104 is contacted with the fluid and heated, which eliminates desorption time of the saturation gas from the surface of the material 104. Heating the material 104 with the fluid creates thermodynamic instability within the material 104 and a sudden decrease in gas solubility which drives the gas out of the material 104 and into nucleated bubbles generating a skinless foam. Keeping the concentration gradient of the gas within the material 104 consistent from gas saturation of the material 104 to heating and nucleation of the gas from the material 104 allows a skinless foam to be generated at the time of foam formation as the fluid heats the material 104.

[0036] In certain examples, skinless foams formed by the methods and systems described herein include thermoplastic materials having pores disposed therein with a width of 5 nanometers (nm) to 100 micrometers (pm). Specifically, a portion of the pores are exposed at a surface of the thermoplastic material at a time the foam is formed, for example 10% to 90% of the surface of the thermoplastic material is the exposed pores. For example, the system parameters can be selected based on desired foam properties, i.e. for example pore size. Parameters that can influence foam properties include the pressure and the temperature used during foaming the material 104, as well as the duration of time the material 104 is allowed to foam under these conditions. For example, the material104 can be heated by the fluid for a threshold amount of time to form a skinless foam having desired foam properties. After the threshold amount of time, the material 104 can be cooled to below a glass transition temperature of the material 104. In implementations, the chamber 102 containing the fluid and the material can be cooled, for example by a cooling source coupled to the chamber. In some cases, the cooling source can include a refrigerator, a thermoelectric cooler, a cooling material, or a combination thereof. For example, the cooling source can be a container 118 positioned around the chamber 102, for example a refrigerator, a thermoelectric cooler, or a heat exchange jacket. In some cases, the container 118 is removably coupled to the chamber 102. The container 118 can include a space in which a cooling material can be disposed For example, the container 118 can be configured to receive dry ice and / or a cooling fluid In implementations, the cooling source can include a cooling coil disposed within the chamber 102, the cooling coil configured to receive a cooling fluid at a thermodynamic state to remove heat from the interior of the chamber 102 and thereby cool the material 104. In implementations, the fluid can be removed from the chamber 102 to facilitate cooling the material 104.

[0037] The system 100, in various examples, further includes sensors, such as temperature sensors 120 and 122 and pressure sensors 121 and 123, and at least one computing device 124. The computing device 124 includes memory 126 and one or more processors 128. In various implementations, the memory 126 is volatile (such as random access memory (RAM)), non-volatile (such as read only memory (ROM), flash memory, etc.) or some combination of the two. The memory 126 stores instructions that, when executed by the processor(s) 128, causes the computing device 124 to perform various operations. For example, the processor(s) 128 can be a control system, such as a proportional-integral-derivative (PID) controller, that receives input from sensors within the system 100, such as temperature sensors 120 and 122 and pressure sensors 121 and 123, and executes instruction to the computing device 124 to actuate flow control devices within the system, such as flow control devices 108 and 116.

[0038] In various examples, the memory 126 stores methods, threads, processes, applications, objects, modules, any other sort of executable instruction, or a combination thereof. In some cases, the memory 126 stores files, databases, or a combination thereof. In some examples, the memory 126 includes, but is not limited to, RAM, ROM, electrically erasable programmable read-only memory (EEPROM), flash memory, or any other memory technology In some examples, the memory 126 includes one or more of CD-ROMs, digital versatile discs (DVDs), content-addressable memory (CAM), or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by the processor(s) 128 and / or the computing device 124. The processor(s) 128 includes a central processing unit (CPU), a graphics processing unit (GPU), both CPU and GPU, or other processing unit or component known in the art.

[0039] The computing device 124 further includes input / output devices 130 The input / output devices 130 collectively function as an interface between a user and the computing device 124, for instance. Input devices may be configured to receive an input from a user and includes at least one of a keypad, a cursor control, a touch- sensitive display, a voice input device (e.g., a microphone), a haptic feedback device (e.g., a gyroscope), or any combination thereof. Output devices include at least one of a display, a speaker, a haptic output device, a printer,or any combination thereof. In some cases, the input / output devices 130 include at least one transceiver configured to receive communication signals from and / or transmit communication signals to at least one external device (not illustrated).

[0040] FIG. 2 presents an example process 200 for generating a skinless foam This depicted process includes pressurizing a chamber containing a material with a gas; maintaining the pressure in the chamber to reach a saturation state of the gas within the material; maintaining the pressure in the chamber and contacting the material in the chamber with a heated fluid; heating the material with the heated fluid for a threshold amount of time, thereby nucleating the gas in the material to generate the skinless foam.

[0041] The systems and methods described herein are the first to allow for the formation of fully porous (skinless) foams These systems and methods thereby allow for improved efficiency in the production of skinless foams for use as selective permeable membranes for waterproofing, battery scaffolds, and durable water filters, among other uses.

[0042] Aspects of the disclosure are now described with additional details and options as follows: (I) Chambers and System Considerations; (l-A) Material Chambers; (l-B) Fluid reservoirs; (l-C) Control System Considerations; (II) Materials; (III) Gasses; (IV) Saturation Conditions; (IV-A) Pressures; (IV-B) Temperatures; (V) Heated Fluids; (VI) Foaming Parameters; (Vl-A) Heating; (Vl-B) Contact Time; (Vl-C) Cooling; (VII) Features of Formed Foams; (VIII) Uses of Formed Foams; (IX) Experimental Examples; (X) Exemplary Clauses; an (XI) Closing Paragraphs. These headings are provided for organizational purposes only and do not limit the scope or interpretation of the disclosure.

[0043] (I) Chambers and System Considerations

[0044] (l-A) Material Chambers. The isobaric solid-state foaming systems described herein can include one or more chambers (material chambers) configured to control saturation, heating, and foaming conditions. The chamber can be designed as an adiabatic and isobaric chamber and configured to operate at elevated temperatures and pressures. For example, the chamber can be configured to operate at temperatures of 600 °C and pressures of 6000 psi (413 bar). The one or more chambers in the isobaric solid-state foaming systems described herein are configured to hold one or more material samples, such that the one or more material samples can be exposed to a substantially constant pressure of an inert gas (e.g., CO2) at a selected temperature for a predetermined time. In other implementations, an adiabatic or quasi-adiabatic chamber may be used, for example, to allow pressure and temperature changes to assist in nucleation. In some implementations, in-situ heating under constant saturation pressure can be provided to the chamber, such that the material within the chamber remains in the pressurized atmosphere of the gas while being brought to a foaming temperature, thereby minimizing gas loss prior to nucleation and cell growth. For example one or more heaters can be coupled to the chamber, for example an electric heater, a combustion heater, a heat exchanger, a hot fluid, an infrared heater, or a combination thereof.

[0045] In implementations, the one or more chambers can be fluidically coupled to a gas source configured to supply the gas to the chamber. A flow control device can be coupled between the chamber and the gas source to control the flow of gas into the chamber. For example the flow control device can include a valve, a pump, acompressor, or a combination thereof to supply the gas to the chamber and pressurize the chamber to the desired pressure. Systems of the present disclosure can include flow control devices configured to operate at elevated temperatures and pressures, such as solenoid valves, ball valves, plug valves, gate valves, globe valves, and needle valves. Flow control devices described herein can also include pumps, compressors, throttling control devices, etc. The systems described herein can also include necessary safety measures, pressure seal valves, pressure relief valves, etc., such as pressure relief valves.

[0046] In implementations, one or more cooling source can be coupled to the one or more chambers and configured to rapidly cool the foamed materials. In some cases, the cooling source can include a refrigerator, a thermoelectric cooler, a cooling material, or a combination thereof. For example, the cooling source can include a container positioned around the chamber, such as a compartment designed to receive dry ice, chilled fluid, or another cold source in thermal contact with the material chamber. For example the cooling source can include a refrigerator, a thermoelectric cooler, or a heat exchange jacket. In some cases, the container is removably coupled to the chamber. The container can include a space in which a cooling material can be disposed. For example, the container can be configured to receive dry ice and / or a cooling fluid. In implementations, the cooling source can include a cooling coil disposed within the chamber, the cooling coil configured to receive a cooling fluid at a thermodynamic state to remove heat from the interior of the chamber and thereby cool the material.

[0047] (l-B) Fluid reservoirs. In some implementations, systems of the present disclosure include one or more fluid reservoirs fluidically connected to the material chamber and configured to contain a fluid at or above a second pressure and at or above a second temperature. In implementations, the fluid reservoir is configured to operate at elevated pressures and temperatures. For example, the fluid reservoir can be configured to operate at temperatures of 600 °C and pressures of 6000 psi (413 bar). The system can also include a flow control device coupled between the fluid reservoir and the material chamber. For example, the flow control device can include a valve, a pump, a compressor, or a combination thereof, configured to supply the fluid to the chamber. The flow control device is configured to operate at elevated temperatures and pressures, for example, such that the fluid reservoir , the flow control device, and the material chamber can operate isobarically and / or adiabatically. The chambers and transfer pathways may be thermally insulated to reduce heat loss and enable precise control of the heating and quenching profiles.

[0048] In implementations, the fluid reservoir can be configured to supply the fluid to the material chamber by pressure driven flow. For example, the fluid reservoir can be held at a pressure greater than the pressure of the material chamber, thereby providing energy to the fluid to flow from the fluid reservoir into the material chamber. In cases where the fluid within the fluid reservoir is a liquid, the pressure driven flow can be gravity driven flow. For example, the fluid reservoir can positioned at a location above the material chamber along a gravity force vector, thereby creating a hydrostatic pressure of the fluid between the fluid reservoir and the chamber. This hydrostatic pressure can then be used to feed the fluid into the material chamber.

[0049] In implementations, the fluid reservoir can include a heat source, such as an electric heater, a gas heater, a heat exchanger, a hot fluid, or any combination thereof, configured to heat the fluid to the desired temperature.Heat sources described herein can include one or more heat exchangers configured to capture thermal energy from other flow streams within the system to recover heat from the system and improve efficiency

[0050] In implementations, the systems described herein can include a gas feed line fluidically connected to the fluid reservoir and / or to a fluid source that supplies the fluid to the fluid reservoir . The gas feed line can include a flow control device configured to supply gas to the fluid, for example to increase or maintain the dissolved gas concentration in the fluid.

[0051] The combination of chamber design, pressure and temperature control, insulation, and integrated gas / fluid feed lines may be selected to achieve reproducible saturation, controlled in-situ heating, and rapid quenching, resulting in solid-state foamed articles with desired density, cell size, and mechanical properties.

[0052] (l-C) Control System Considerations Systems of the present disclosure can include includes sensors, such as temperature sensors and pressure sensors, and at least one computing device. The computing device can includes memory and one or more processors. In various implementations, the memory is volatile (such as RAM), non-volatile (such as ROM, flash memory, etc.) or some combination of the two. The memory stores instructions that, when executed by the processor(s), causes the computing device to perform various operations. For example, the processor(s) can be a control system, such as a PID controller, that receives input from sensors within the system, such as temperature sensors and pressure sensors, and executes instruction to the computing device to actuate equipment within the system such flow control devices, heaters, coolers, etc.

[0053] For example, system which includes a material chamber configured to contain a material, a gas source fluidically connected to the chamber and configured to contain a gas, a first flow control device coupled between the chamber and the gas source, a fluid reservoir fluidically connected to the chamber and configured to contain a fluid at or above a second pressure and at or above a second temperature, and a second flow control device coupled between the fluid reservoir and the chamber can include a processor communicatively coupled with the first flow control device and the second flow control device. The processor can cause the system to pressurize the material chamber to the first pressure with the gas by actuating the first flow control device. The processor can cause the system to actuate the second flow control device to cause the fluid to enter the chamber, while maintaining the first pressure in the chamber.

[0054] In various implementations, the system can include one or more temperature sensors communicatively coupled with the processor and configured to detect a temperature of the fluid and a pressure sensor communicatively coupled with the processor and configured to detect a pressure in the chamber. In implementations, the processor can also communicate with a heat source coupled to the material chamber, and can cause the system to actuate the heat source to heat the gas and the material contained in the chamber to the first temperature. In implementations, the processor is can also communicate with a pump / compressor coupled to the fluid reservoir , the processor further configured to cause the system to actuate the pump / compressor to pressurize the fluid reservoir to the second pressure. In some cases, the processor can communicate with a heat source coupled to the fluid reservoir , and can cause the system to actuate the heat source to heat the fluid contained in the fluid reservoir to the second temperature.

[0055] In various examples, the memory stores methods, threads, processes, applications, objects, modules, any other sort of executable instruction, or a combination thereof. In some cases, the memory stores files, databases, or a combination thereof In some examples, the memory includes, but is not limited to, RAM, ROM, EEPROM, flash memory, or any other memory technology. In some examples, the memory includes one or more of CD-ROMs, DVDs, CAM, or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by the processor(s) and / or the computing device. The processor(s) includes a CPU, a GPU, both CPU and GPU, or other processing unit or component known in the art.

[0056] The computing device can also include input / output devices. The input / output devices can collectively function as an interface between a user and the computing device, for instance. Input devices may be configured to receive an input from a user and includes at least one of a keypad, a cursor control, a touch-sensitive display, a voice input device (e.g. , a microphone), a haptic feedback device (e.g., a gyroscope), or any combination thereof. Output devices include at least one of a display, a speaker, a haptic output device, a printer, or any combination thereof. In some cases, the input / output devices include at least one transceiver configured to receive communication signals from and / or transmit communication signals to at least one external device (not illustrated).

[0057] (II) Foaming Materials. The isobaric solid-state foaming systems and methods described herein can be applied to a wide variety of foaming materials. Suitable materials for foaming according to methods described herein include thermoplastic homopolymers, copolymers, and polymer blends that exhibit sufficient melt strength and gas solubility under the selected saturation and foaming conditions. Examples of such polymers include polyethylene terephthalate (PET), polylactic acid (PLA), polyvinyl chloride (PVC), polycarbonate (PC), recycled polyethylene (rPET), polystyrene (PS), polyetherimide (PEI), polyether ether ketone (PEEK), acrylonitrile butadiene styrene (ABS), polyethersulfone (PES), polyetherketoneketone (PEKK), polyaryletherketone (PAEK), polyimide (PI), polysulfone (PSU), polyphenylsulfone (PPSU), thermoplastic polystyrene (TPS), and cyclic olefin copolymer (COC). Additional suitable polymers may include polyethylene (PE), low density polyethylene (LDPE), medium density polyethylene (MDPE), high density polyethylene (HDPE), polypropylene (PP), polyamides (PA), thermoplastic polyurethanes (TPU), polymethyl methacrylate (PMMA), and other engineering or commodity thermoplastics. These materials may be used individually or in combination, for example as polymer blends, multilayer structures, or recycled / virgin resin mixtures, to tailor properties such as stiffness, toughness, density, thermal resistance, and cell morphology. The materials described herein can include a blend of at least two thermoplastic polymers.

[0058] The material can include a composite material comprising at least one of a carbon fiber, a glass fiber, or a nanoparticle. The material may further include additives commonly used in foamed thermoplastic compositions. For example, the material may include nucleating agents to promote uniform cell formation, chain extenders or branching agents to enhance melt strength, impact modifiers to increase toughness, colorants or pigments, and stabilizers (e.g., thermal or UV stabilizers) to improve long-term durability.

[0059] (III) Gasses. A variety of gasses may be used in the isobaric solid-state foaming processes described herein In implementations, the gas can be an inert gas that does not react with the material. Suitable gassesinclude nitrogen (N2), carbon dioxide (CO2), nitrous oxide (N2O), helium (He), argon (Ar), butane, propane, and combinations thereof. In some cases, CO2is preferred due to its relatively high solubility in many thermoplastics, its ability to achieve supercritical conditions under moderate temperature and pressure, and its comparatively low environmental impact. In some implementations, supercritical CO2 (SCCO2) may be used to enhance gas uptake and promote fine, uniform cell structures. In other implementations, CO2 may be introduced in alternative forms, such as CO2 pre-saturated into water or other carrier fluids, which can then be brought into contact with the polymer to facilitate controlled gas diffusion and subsequent foaming

[0060] Additional gasses may be used to tailor foam morphology, density, and processing conditions. For example, mixtures of CO2 and N2 may be selected to adjust nucleation behavior and cell growth kinetics. In certain implementations, gasses may be used alone, while in other implementations, the gasses may be used in combination with chemical agents that decompose to release gases during heating thereby increasing the total gas content available for foaming. The choice of gas, and its state (e.g., gaseous, liquified, supercritical), saturation pressure, and temperature may be selected based on the specific polymer, desired cell size distribution, target density reduction, and mechanical and thermal performance requirements of the resulting foamed article

[0061] (IV) Saturation Conditions. In the production of m icrocel I ul ar and nanocellular nanofoams, one of the most important relationships is the amount of dissolved gas in the starting material. For example, cell size is attributed to the saturation of the material, for example, a sample with a greater dissolved gas content will have a smaller cell size, while a sample with a lower dissolved gas content will have an increased cell size. Generally at or above 10 wt% of dissolved gas, nanopore formation will be more prevalent compared to more micropore formation at lower than 10 wt%. Furthermore, each starting material has a different threshold pressure required to dissolve a sufficient amount of gas, and this threshold is temperature dependent. It has been shown that decreasing the saturation temperature of the material can increase the gas solubility limit of the material.

[0062] The saturation conditions of the material, i.e. the pressure and temperature of the material when gas is dissolved into the material, therefore, are important aspects of the methods described herein because they can affect the size of cells and nucleation density of the foam.

[0063] (IV-A) Pressures. Constraints such as the necessary concentration of gas, saturation level of the material to reach the desired properties of the foam, system temperature, and time constraints can influence the desired operating pressure. For example, the pressure can include a pressure sufficient to dissolve at least 10 wt% of the gas into the material. In implementations, a saturation level of gas within the material can include a partial saturation of the gas within the material or an equilibrium concentration of gas within the material. In implementations, the desired saturation pressure depends on the material that is being saturated. For example, the pressure can be in a range of 0.01 megapascals to 30 megapascals, or higher. In a PEI sample, for example, a pressure of at least 5 MPa may be required to reach nanoscale pores. Below 5 MPa this, it has been shown that larger pores form. Higher saturation pressures can also decrease the amount of time it will take to saturate the sample, see FIG. 3 which shows gas concentration over time for PEI samples normalized by thickness. The saturation concentration of CO2 in PEI follows Henry’s law with a constant of 18.55 mg CCWg PEI / MPa at 21°C. In an industrial setting, operating the system at a higher pressure may be preferrable to produce the foams at a faster rate. The systemsand methods described herein can operate at constant pressure to produce skinless foams, therefore the desired saturation pressure may also be influenced and / or determined by the desired foaming properties of the material.

[0064] (IV-B) Temperatures. As noted above, the saturation temperature of the material can increase the gas solubility limit of the material. Therefore the operating temperature can be adjusted based on the material, the desired gas solubility level, and the desired foam structure. In implementations, the saturation temperature of the material chamber will be less than the glass transition temperature of the material, for example room temperature (21 °C). Dissolving gas into a material also lowers the materials glass transition temperature.

[0065] (V) Heated Fluids. According to aspects of the present disclosure, once the a material is saturated to a desired saturation level with a gas, the material can then be heated to a foaming temperature by contact with a heated fluid to generate a skinless foam. The heated fluid provides efficient and uniform heat transfer to the material, enabling rapid and controlled transition from a saturated solid state to a softened or rubbery state suitable for gas nucleation and cell growth. The temperature of the heated fluid may be selected to be above the glass transition temperature (Tg) of the polymer and below its melting point (Tm) or degradation temperature. The choice of heated fluid, and its temperature and flow characteristics, may be selected based on the specific polymer and gas, desired heating rate, and target cell morphology and dimensional stability.

[0066] Suitable heated fluids include heated water, aqueous solutions, silicone oils, and other heat-transfer oils, and supercritical fluids such as supercritical CO2, supercritical N2. In implementations, the heated fluid is saturated with the pressurizing gas. Saturating the heating fluid can be achieved by supplying gas, for example from the pressurizing gas source within the system, to the heating fluid. In implementations, a gas feed line from the pressurizing gas source within the system can be fluid ically connected to a fluid reservoir containing the heating fluid and / or to a fluid source that supplies the fluid to the fluid reservoir . For example, if N2is used to pressurize a material, a heated fluid saturated in N2, for example supercritical N2, can be used to foam the material. The heated fluid saturated with the pressurizing gas prevents desorption of the gas from the material when the heating fluid is contacted with the material.

[0067] Common solid-state foaming methods use a hot silicone oil bath to perform the heating step of foaming. One reason oil can be used over water in current methods that involve depressurization and heating at ambient pressure is because water could not be in liquid form in many cases where foaming temperatures exceed 100°C at atmospheric pressure. The isobaric high pressure foaming methods presented herein allow water to be heated to elevated temperatures. Additionally, water is much less expensive than silicone oil, the physical properties of water are well understood, and allows for higher solubility of gases within the fluid. A higher concentration of gas in the fluid can reduce the diffusion rate out of a material sample, reducing skin formation. For example, the solubility of CO2in PEI is 100 mg CO2per gram of PEI. Assuming a density of 1.3 g / cm3, this is equivalent to a CO2concentration of 0.143 g / cm3in PEI. The solubility of CO2at 150°C and 5 MPa is 0.35 mol / kg of water. The density of water at these conditions is 0.919 kg / L, so the concentration of CO2can be 0.0142 g / cm3. Therefore, the concentration of CO2in PEI is significantly higher than in water. In this example scenario, there will still be a concentration gradient from the material sample to the water, however it will be less compared to heating with an unsaturated fluid.

[0068] In implementations, the heating fluid can be contained in a fluid reservoir . The temperature of the fluid reservoir can be at or above the set foaming temperature, for example at or above the glass-transition temperature of the material. The necessary foaming temperature depends on the glass transition temperature of the polymer because of the mechanism of nucleation and cell formation during foaming. The glass transition temperature varies depending on the concentration of gas dissolved in a material at saturation, which is directly proportional to the chosen saturation pressure. For example, a possible foaming temperature range of 80-170°C was found from literature for a PEI sample saturated at 5 MPa. Within this range, the density of the final product also depends on the foaming temperature, with density decreasing as temperature increases.

[0069] (VI) Foaming Parameters. The foaming temperature (Tf) and saturation pressure (Ps) are the strongest factors governing the morphology of solid-state polymer foams. Saturating at Ps= 1 MPa produces microfoams with cell sizes between 3piM to 5 M, while saturating at Ps= 5MPa produces nanofoams with much smaller cell sizes, ranging from 15nm to 40nm, among the smallest reported for solid-state nano-foaming (Costeux, J Appl. Polym Sci., 2014, 131(23); Pinto, et al„ Chem. Eng. J„ 2014, 243:428-435; Guo, et al„ Polymer, 2015, 70:231- 241). Going from low to high saturation pressures significantly increases the number of nucleation sites, from 1016to 1020cells / m3respectively. The foaming temperature serves to promote bubble growth by increasing expansion rates, thereby reducing prand transitioning the foam from a closed-cell to an open-cell structure. For example, nanofoams produced at Tf= 175 °C have larger cells (40nm) compared to those produced at 7 = 135 °C (15nm). Similarly, microfoams produced at Tt = 205 °C have larger cell sizes (5pim) than those produced at Tt = 175 °C (3pm)

[0070] As the relative density prdecreases from 80% to 49%, significant morphological changes become evident. In the lowest relative density microfoams, the structure is highly porous and some of the larger cells have coalesced, creating larger voids As the relative density decreases from 77% to 42%, there are more pronounced changes in cell structure compared to microfoams. In the highest density nanofoams, there are very fine cells 15nm to 20nm in size This morphology suggest limited foam expansion due to the lower foaming temperatures 7} and thicker cell walls. As density decreases, the cell walls become thinner and the morphology suggests a transition from a closed-cell to an open-cell structure.

[0071] The process begins by exposing a thermoplastic polymer to high pressures of gas, generally CO2, at a specific saturation pressure (Ps). The gas, which serves as the blowing agent, diffuses into the polymer until it reaches an equilibrium concentration. The material is then heated by a heating fluid set at a designated foaming temperature Tf that is above the glass transition temperature Tsof the polymer-gas system. The rapid temperature increase softens the polymer and forces the absorbed gas out of equilibrium, where it subsequently nucleates as nanopores (Miller, et al., Polymer, 2009, 50(23):5576-5584; Van Loock, et al., Proc. R. Soc. A: Math. Phys. Eng. Sci., 2019, 475(2226):20190339). As the nanopores grow, the polymer matrix deforms to create a foam structure.

[0072] The choice of saturation pressure (Ps) is critical in determining the foam’s morphology and properties as it directly influences cell nucleation density (Sridhar, et al., Polym. Eng. Sci., 2024). In implementations, gas absorption can be determined gravimetrically through hourly mass measurements until desired saturation conditions are met. For example, At 1 MPa, a polymer absorbs 35.8 mg / g, whereas at 5MPa, absorption in thesame polymer increases to around 110 mg / g. The absorption profile follows the diffusion model of Sridhar et al., 2024. This higher absorption at increased pressures not only affects the final foam properties but also influences how the concentration changes over time and through the thickness of the polymer sample. To ensure the polymer is uniformly infused with gas and to achieve the desired saturation conditions for the subsequent expansion phase, saturation times of, for example, 96 hours for 1 MPa and 48 hours for 5MPa can be used.

[0073] (Vl-A) Heating. Foaming temperatures can be tuned to achieve corresponding cell sizes, for example ranging from 3 pm to 5 pm and 15 nm to 40 nm, with relative densities ranging from 42%— 80% In implementations, a foaming temperature (Tt) that is above the glass transition temperature (Tg) of the polymer-gas system is important because this temperature softens the polymer and forces the absorbed gas out of equilibrium, where it nucleates as nanopores. As the nanopores grow, the polymer matrix of the material deforms to create a foam structure. To achieve uniform bubble expansion and prevent defects like curling or warping, it is crucial to carefully control the foaming temperature Tf and the ensure that the material sample is fully engulfed by the heating fluid. The foaming temperature serves to promote bubble growth by increasing expansion rates, thereby reducing prand transitioning the foam from a closed-cell to an open-cell structure For example, nanofoams produced at Tf = 175 °C can have larger cells (40 nm) compared to those produced at Tf = 135 °C (15 nm). Similarly, microfoams produced at Tf = 205 °C can have larger cell sizes (5 pm) than those produced at Tf= 175 °C (3 pm).

[0074] (Vl-B) Contact Time. The contact time between the a saturated material sample and the heated saturated fluid may be selected to provide sufficient cell nucleation and growth while avoiding excessive foaming, collapse, or degradation. In certain implementations, the material can maintain contact with the heated fluid for a predetermined time on the order of seconds to several minutes, for example from 10 seconds to 10 minutes, such as from 30 seconds to 5 minutes, or from 1 minute to 3 minutes. In some implementations, suitable contact times may include discrete values such as 2 minutes or 5 minutes, depending on the polymer type, sample thickness, gas concentration, and target foam morphology. Longer contact times may permit more extensive cell growth and lower final density, while shorter contact times may favor finer cell structures and improved dimensional stability. The appropriate contact time may be determined experimentally and may vary based on process conditions such as the saturation pressure, foaming temperature (e.g , relative to the glass transition temperature of the polymer), and the thermal properties of the polymer and heated fluid.

[0075] (Vl-C) Cooling. The cooling or quenching step can be configured to rapidly reduce the temperature of the foamed or foaming polymer specimen below its glass transition temperature (Tg) in order to “freeze” the cellular morphology and prevent further cell growth, coalescence, or collapse. In certain implementations, the foamed material is brought into thermal contact with a cold medium such as dry ice, liquid nitrogen, chilled water brine, or refrigerated air. For example, the saturation chamber containing the polymer specimen may be packed or surrounded with dry ice for a predetermined period of time on the order or seconds to tens of minutes, such as from 1 minute to 20 minutes, from 3 minutes to 15 minutes, or from 5 minutes to 10 minutes Suitable discrete quench times may include, for instance, 5 minutes or 10 minutes in contact with dry ice, depending on the sample thickness, polymer type, and degree of foaming at the time of quench In other implementations, liquid nitrogen or another cryogenic fluid may be used to provide an even more rapid quench, for example by briefly immersing thespecimen or exposing it to a cold vapor phase to quickly arrest cell growth. The appropriate cooling time and cooling rate can be determined based on the desired final density, cell size distribution, and dimensional stability of the foam article. The quench time may be controlled, for example, by the duration of exposure to the cold medium, the amount of dry ice or cryogen added, or by monitoring the material sample or chamber temperature and terminating the cooling step once a target temperature or structural condition has been reached. In implementations, the heating fluid can be removed from the material chamber, to move the fluid out of contact with the material and facilitate cooling.

[0076] (VII) Features of Formed Foams. The mechanical performance of foam-like materials is closely tied to their cell size and structural characteristics. Naturally occurring foams generally feature smaller cell sizes and exhibit ductile behavior (Marsavina & Constantinescu, Springer Vienna, 2015, pp. 119-190; Marsavina & Linul, Fract. Eng. Mater. Struct., 2020, 43(11 ):2483-2514), whereas synthetic foams with larger cell sizes are prone to brittle failure (Barnhart, Prog. Mater Sci , 2001 , 559-U3; Rusch, J. Appl Polym. Sci., 1970, 14(5);1263-1276). Larger cell walls often show limited plasticity, resulting in rapid crack growth and limiting their effectiveness in applications requiring energy absorption and toughness (Bi, et al., Meeh. Mater, 2020, 145:103368). Creating structures at small length scales offers significant advantages. Metals (Li & Ebrahimi, Adv. Mater, 2005, 17(16):1969-1972), polymers (Brown, J. Mater. Sci., 1982, 17:469-476; Patel, et al., 2024, Addit Manuf. 84:104113; Quagliato, et al., Mater. Lett, 2022, 329:133121), and materials that are inherently brittle at larger scales such as silicon (Ostlund, et al., Adv. Funct. Mater., 2009, 19(15):2439-2444; Issa, et al., Mater. Today, 2021 , 48:29-37), glassy carbon (Albiez & Schwaiger, MRS Adv., 2019, 4(5): 133-138), and metallic glass (Chen, et al., Appl. Phys. Lett., 2015, 106(6):061903) begin to exhibit ductile behavior when scaled down to micro or nano length scales

[0077] In implementations, the systems and methods described herein can produce skinless polymer foams including a thermoplastic material, pores disposed in the thermoplastic material, a width of the pores being in a range of 5 nanometers to 100 micrometers, and a portion of the pores exposed at a surface of the thermoplastic material at a time the foam is formed. The skinless foams presented herein include pores on the surface of the foam that are “pristine” or “as fabricated”, meaning they form during the foaming process itself. The quality of the pores is not interrupted or damaged by cutting, piercing or otherwise machining a skin away. In implementations, the portion of the pores exposed at the surface of the thermoplastic material include 10% to 90% of the surface of the thermoplastic material. In implementations, the pores within the microcellular and nanocellular foams presented herein include open cells

[0078] (VIII) Uses of Formed Foams. As described herein, producing completely porous polymer foams in a single step eliminates an impermeable solid skin from the surface of the porous foams, enhancing functionality in key industries. Traditional dense foams have low permeability and rigidity, limiting their use in electrical and biomedical fields. Skinless microcellular or nanocellular morphology has advantages in filtration, energy storage, dielectric applications, and biomedical engineering. Skinless nanofoams are very promising in filtration technology owing to their improved permeability. While polymer foams are common in air and liquid filtration, they are compromised by a dense surface skin layer that retards fluid flow. The skinless nanofoams of the presentdisclosure overcome this limitation, enhancing the permeability of gases and liquids, making them suitable in microfiltration, ultrafiltration, as well as in gas-separation membranes. Water filtration, drug processing, and gasseparation systems depend on mechanically stable materials with controlled pore sizes. Skinless nanofoams are a light, tunable option that enhances selectivity and throughput Polymer foams have a key application in dielectric materials, which enhance electrical properties in capacitors, insulators, and shielding. Nanocellular voids lower the effective dielectric constant of a material significantly, and hence foams can find application in high-frequency electronics and EMI shielding. Conventional foams have nonuniform dielectric response due to a dense skin layer. Nanofoams without a skin, being totally porous, facilitate uniform electrical behavior, enabling better performance in flexible electronics, radar absorbers, and aerospace applications

[0079] Polymer foams are increasingly seen in the field of energy storage, including in batter separators and super-capacitors. Skinless nanofoams enhance ion transfer, enabling faster charge-discharge cycling in lithium- ion batteries. The interconnected nanoporosity of the foams boosts electrolyte uptake, lowers internal resistance, and enhances energy density. Such properties are essential for next-generation energy storage, where the requirement for power density and efficiency must be balanced without sacrificing material integrity. Skinless nanofoams will also enhance biomedical applications in drug delivery, medical devices, and tissue engineering. Open pores in scaffolds are needed for cell adhesion, growth, and nutrient supply. Dense surface layers in conventional foams impede cell infiltration, diminishing scaffold efficacy in regenerative medicine. Skinless nanofoams provide higher permeability, promoting vascularization and tissue integration. In drug delivery, controlled-porosity polymeric nanofoams release therapeutics at a controlled rate, improving implantable device efficacy.

[0080] Skinless polymer foams are also being research for acoustic insulation, lightweight composites, and microfluidic devices. Low-density nanofoams are utilized effectively in aerospace and architecture to attenuate sound across a broad frequency range. Their vibration damping and structural integrity make them suitable for automative interiors, aircraft components, and sound-proofing panels. In microfluidic and lap-on-chip devices, skinless nanofoams provide precise fluid control due to their open-cell structure, benefiting diagnostic and analytical applications.

[0081] (IX) Exemplary Clauses. The following clauses provide various examples of the present disclosure. However, the scope of the disclosure is not limited to any of the clauses listed herein.EXAMPLE CLAUSES

[0082] 1 . A method of generating a skinless polymerfoam, the method including: using a gas to pressurize a chamber to a pressure that is greater than or equal to a threshold pressure, the chamber containing a material; maintaining the pressure that is greater than or equal to the threshold pressure until a saturation state of the gas is reached within the material; and contacting the material, in the chamber at the pressure that is greater than or equal to the threshold pressure, with a fluid for a threshold time and at a temperature that is greater than or equal to a glass transition temperature of the material, thereby generating the skinless polymer foam.

[0083] 2. The method of clause 1 , wherein the material includes a thermoplastic material, a biologically derived material, or a combination thereof.

[0084] 3. The method of clause 2, wherein the material is the thermoplastic material and the thermoplastic material includes at least one thermoplastic polymer selected from the group consisting of polyethylene terephthalate (PET), polylactic acid (PLA), polyvinyl chloride (PVC), polycarbonate (PC), polystyrene (PS), polymethyl methacrylate (PMMA), polyetherimide (PEI), polyether ether ketone (PEEK), polyethylene (PE), low density polyethylene (LDPE), medium density polyethylene (MDPE), high density polyethylene (HDPE), acrylonitrile butadiene styrene (ABS), polyethersulfone (PES), polyetherketoneketone (PEKK), polyaryletherketone (PAEK), polyimide (PI), polysulfone (PSU), polyphenylsulfone (PPSU), thermoplastic polystyrene (TPS), and cyclic olefin copolymer (COC).

[0085] 4. The method of clause 3, wherein the thermoplastic material includes a blend of at least two thermoplastic polymers.

[0086] 5. The method of any of clauses 1-4, wherein the material is a composite material including at least one of a carbon fiber, a glass fiber, or a nanoparticle.

[0087] 6. The method of any of clauses 1 -5, wherein the gas includes nitrogen ( N2) , carbon dioxide (CO2) , nitrous oxide (N2O), helium (He), argon (Ar), butane, propane, or a combination thereof.

[0088] 7. The method of any of clauses 1 -6, wherein the threshold pressure includes a pressure sufficient to dissolve at least about 10 wt% of the gas into the material.

[0089] 8. The method of any of clauses 1-7, wherein the threshold pressure is in a range of about 0.01 megapascals to about 30 megapascals.

[0090] 9. The method of any of clauses 1-8, wherein the saturation state includes a partial saturation of the gas within the material or an equilibrium concentration of gas within the material.

[0091] 10. The method of any of clauses 1-9, wherein the fluid includes at least one of water, supercritical CO2, supercritical N2, steam, or an oil.

[0092] 11. The method of any of clauses 1-10, wherein the fluid is saturated with the gas.

[0093] 12. The method of any of clauses 1-11 , wherein using the gas to pressurize the chamber is performed at a temperature that is less than the glass transition temperature of the material.

[0094] 13. The method of any of clauses 1-12, wherein contacting the material, in the chamber at greater than or equal to the threshold pressure, with the fluid for the threshold time and at the temperature that is greater than or equal to the glass transition temperature of the material includes maintaining an isobaric environment within the chamber.

[0095] 14. The method of any of clauses 1-13, wherein contacting the material, in the chamber at greater than or equal to the threshold pressure, with the fluid for the threshold time and at the temperature that is greater than or equal to the glass transition temperature of the material includes moving the fluid into the chamber containing the material and holding the fluid in the chamber for the threshold time.

[0096] 15. The method of any of clauses 1-14, wherein the threshold time includes a time sufficient to form a desired pore size in the material.

[0097] 16. The method of any of clauses 1-15, wherein contacting the material, in the chamber at the pressure that is greater than or equal to the threshold pressure, with the fluid for the threshold time and at thetemperature that is greater than or equal to the glass transition temperature of the material generates pores in the material, a width of the pores being in a range of about 5 nanometers to about 100 micrometers.

[0098] 17. The method of clause 16, wherein a portion of the pores are exposed at a surface of the material.

[0099] 18. The method of clause 16 or 17, wherein the pores include open cells.

[0100] 19. The method of any of clauses 1-18, the chamber being a material chamber, wherein contacting the material, in the material chamber at the pressure that is greater than or equal to the threshold pressure, with the fluid for the threshold time and at the temperature that is greater than or equal to the glass transition temperature of the material includes: opening a flow control device between the material chamber and a fluid reservoir containing the fluid, thereby allowing the fluid to flow from the fluid reservoir into the material chamber containing the material.

[0101] 20. The method of clause 19, wherein the fluid in the fluid reservoir chamber is at a pressure greater than the pressure of the material chamber, thereby providing energy to the fluid to flow from the fluid reservoir chamber into the material chamber.

[0102] 21. The skinless polymer foam generated by the method of any of clauses 1-20.

[0103] 22. A skinless polymer foam including: a thermoplastic material; pores disposed in the thermoplastic material, a width of the pores being in a range of about 5 nanometers to about 100 micrometers; and a portion of the pores exposed at a surface of the thermoplastic material at a time the foam is formed.

[0104] 23. The skinless polymer foam of clause 22, wherein the thermoplastic material includes at least one of PET, PLA, PVC, PC, PS, PMMA, PEI, PEEK, PE, LDPE, MDPE, HDPE, ABS, PES, PEKK, PAEK, PI, PSU, PPSU, TPS, or COC.

[0105] 24. The skinless polymer foam of clause 22 or 23, wherein the portion of the pores exposed at the surface of the thermoplastic material expose about 10% to about 90% of the surface of the thermoplastic material.

[0106] 25. The skinless polymer foam of any of clauses 22-24, wherein the pores include open cells.

[0107] 26. A filter including the skinless polymer foam of any of clauses 22-25.

[0108] 27. A system including: a chamber configured to operate at or above a first pressure and at or above a first temperature and to contain a material; a gas source flu idical ly connected to the chamber and configured to contain a gas; a first flow control device coupled between the chamber and the gas source; a fluid reservoir fluidically connected to the chamber and configured to contain a fluid at or above a second pressure and at or above a second temperature; and a second flow control device coupled between the fluid reservoir and the chamber.

[0109] 28. The system of clause 27, wherein the chamber is configured to fluidically seal and operate at a high pressure and a high temperature.

[0110] 29. The system of clause 27 or 28, wherein the first temperature is a temperature less than a glass transition temperature of the material.

[0111] 30. The system of any of clauses 27-29, wherein the first pressure is in a range of about 0.01 megapascals to about 30 megapascals.

[0112] 31. The system of any of clauses 27-30, wherein the material includes at least one of PET, PLA, PVC, PC, PEI, PEEK, PE, LDPE, MDPE, HDPE, ABS, PES, PEKK, PAEK, PI, PSU, PPSU, TPS, or COC.

[0113] 32. The system of any of clauses 27-31, wherein the gas includes N2, CO2, or a combination thereof.

[0114] 33. The system of any of clauses 27-32, wherein the first flow control device includes a valve, a pump, a compressor, or a combination thereof.

[0115] 34. The system of any of clauses 27-33, wherein the fluid reservoir is configured to fluidically seal and operate at a high pressure and a high temperature.

[0116] 35. The system of any of clauses 27-34, wherein the fluid reservoir is positioned at a location above the chamber along a gravity force vector creating a hydrostatic pressure of the fluid between the fluid reservoir and the chamber.

[0117] 36. The system of any of clauses 27-35, wherein the fluid includes water, super critical CO2, steam, or an oil.

[0118] 37. The system of any of clauses 27-36, wherein the second temperature is at or above a glass transition temperature of the material.

[0119] 38. The system of any of clauses 27-37, wherein the second pressure is in a range of about 0.01 megapascals to about 30 megapascals.

[0120] 39. The system of any of clauses 27-38, wherein the second flow control device includes a valve, a pump, a compressor, or a combination thereof.

[0121] 40. The system of any of clauses 27-39, further including a cooling source coupled to the chamber.

[0122] 41. The system of clause 40, wherein the cooling source includes a refrigerator, a thermoelectric cooler, a cooling material, or a combination thereof.

[0123] 42. The system of any of clauses 27-41 , further including a heat source coupled to the chamber.

[0124] 43. The system of clause 42, wherein the heat source includes an electric heater, a combustion heater, a hot fluid, an infrared heater, or a combination thereof

[0125] 44. The system of any of clauses 27-43, further including a pump, a compressor, or a combination thereof, coupled to the fluid reservoir .

[0126] 45. The system of any of clauses 27-44, further including a heat source coupled to the fluid reservoir

[0127] 46. The system of clause 45, wherein the heat source includes an electric heater, a gas heater, a hot fluid, or a combination thereof.

[0128] 47. The system of any of clauses 27-46, further including: a processor communicatively coupled with the first flow control device and the second flow control device, the processor being configured to: pressurize the chamber to the first pressure with the gas by actuating the first flow control device; and actuate the second flow control device to cause the fluid to enter the chamber, while maintaining the first pressure in the chamber.

[0129] 48. The system of clause 47, wherein the processor is further communicatively coupled with a heat source coupled to the chamber, the processor further configured to: actuate the heat source to heat the gas and the material contained in the chamber to the first temperature.

[0130] 49. The system of clauses 47 or 48, wherein the processor is further communicatively coupled with a pump / compressor coupled to the fluid reservoir , the processor further configured to: actuate the pump / compressor to pressurize the fluid reservoir to the second pressure.

[0131] 50. The system of any of clauses 47-49, wherein the processor is further communicatively coupled with a heat source coupled to the fluid reservoir , the processor further configured to: actuate the heat source to heat the fluid contained in the fluid reservoir to the second temperature.

[0132] 51. The system of any of clauses 47-50, further including: a temperature sensor communicatively coupled with the processor and configured to detect a temperature of the fluid; and a pressure sensor communicatively coupled with the processor and configured to detect a pressure in the chamber.

[0133] (X) Experimental Examples. The following Experimental Examples provides some implementations of the present disclosure. However, implementations are not limited to the specific Experimental Example described below.EXPERIMENTAL EXAMPLE 1

[0134] Abstract: Solid-state foaming is a process that takes a thermoplastic polymer supersaturated with inert gas that is heated to nucleate small pores, producing micro- or nanoporous foams. Saturation is done in a high pressure chamber, often with CO2 as the working gas, but traditional fabrication processes require samples to be taken out of the chamber prior to heating, resulting in formation of a solid ’’skin” at the surface where gas desorbs. In the present experimental example, it is demonstrated that solid-state foaming can be done in an isobaric process wherein samples are heated while still under pressure, resulting in the formation of micro- and nanoporous foams without a skin layer. A custom in-situ foaming chamber was developed that utilizes superheated water that is flowed into a material chamber while under pressure to initiate foaming. Two different thermoplastic polymers were investigated: recycled polyethylene terephthalate (rPET) and polycarbonate (PC). The RPET was saturated at room temperature and 4 MPa CO2pressure and foamed at 76°C, while the PC was saturated at-10°C at 195 MPa CO2pressure and foamed at 100°C. The heating process produced a microcellular closed-cell rPET with average pore sizes of 30pim, and a hierarchical microcellular PC with 5pm cells and 100nm pores in the cell walls. Both of these samples had a continuous porosity through their thickness (e.g., with the only ’’skin” layer being a single cell wall thick). This novel demonstration of an isobaric foaming process to produce ’’skinless” micro- and nanocellular foams opens the door to more advanced applications where fully nanoporous foams could be produced in a single step, i.e. without an additional step to remove a skin after foam formation, for use as selective permeable membranes for waterproofing, battery scaffolds, and durable water filters.Methods

[0135] The isobaric foaming process involves heating up samples while remaining at an equilibrium pressure to prevent the release of gas from the surface. A schematic of this process as compared to traditional solid-state foaming practices is illustrated in FIG. 4. The materials used in this process and a more detailed schematic of the system are discussed in the sections below.Material

[0136] Foaming experiments were performed on two different thermoplastic polymers: rPET and PC. The rPET was supplied by Phoenix Technologies International LLC. These were selected due to their low glass transition temperature (Tg) (e.g., 75°C). The sample was 0.5 mm thick and was cut into 20 mm by 20 mm sections. It was saturated with CO2 at 4 MPa in room temperature (25°C), for intervals of 24, 48 and 72 hours. This pressure was selected to prevent crystallization of the PET at higher pressures, eliminating an extraneous process variable.Apparatus Overview

[0137] This system used a 100 ml custom pressure chamber, controlled by an Omega iSeries PID controller. The water reservoir was built using a 12-inch pressure rated pipe, with a 1 -inch inner diameter and a maximum volume of 150 ml. The heating element includes an electric heat jacket, wrapped around the water reservoir chamber, controlled by an independent temperature controller in conjunction with a thermocouple within the vessel. Components were connected with 1 / 4 inch stainless steel tubing, with safety relief and depressurization valves placed at multiple key points. Both chambers were fed by the same commercial gas cylinder, supplying medical grade CO2. While the system is rated for pressures up to 10 MPa and temperatures of 176°C, safety release valves were chosen at 6 MPa to allow a comfortable factor of safety.

[0138] This setup utilized gravity driven flow to force the heated water into the material chamber A schematic of this system is shown in FIG. 5. As the water chamber is above the material chamber and the system is at equal pressure, the material chamber can be flooded with saturated water by opening one valve, bypassing the need for a pump. A similar effect could be achieved if the heated fluid were kept at a higher pressure to drive the flow, but the gravity driven flow here was done for simplicity.Procedure

[0139] The square PET samples were first sealed inside the saturation chamber. The reservoir was then filled with 100 ml of distilled water, and sealed using a ball valve. The system was pressurized with CO2, controlled with a PID controller, and set to 580 psi or 4 MPa. Once the saturation time was complete, the electric heat jacket was activated to raise the temperature of the saturated water When the desired temperature was reached, after 45 minutes, the connecting valve was opened, and the material chamber was flooded. To ensure the chamber was fully flooded, a 5 minute wait time was prescribed. Following this, the decompression valve was used to gradually return the system to ambient pressure, after which the chamber was opened and samples removed.Results

[0140] Through this process, a specimen was successfully foamed under high pressure. The specimen was saturated at 4 MPa and 25°C for 24 hours, after which it was foamed at 76°C. The specimen was freeze-fractured in liquid nitrogen, sputter coated with platinum, and imaged with a ThermoFisher Apreo 2s Scanning Electron Microscope.

[0141] From these micrographs of the cross section of the specimen, it was seen that no skin was present exceeding the thickness of the cell walls themselves. Using Imaged to analyze the micrograph, the average cell diameter was determined to be 20 25 pm with a skin thickness of 1.37 pm.

[0142] The cross section of the PET is shown in FIG. 6. The cell size is largest at the center of the foam and is smallest at the edge. The surface of the sample can also be seen at the bottom of the image, and small pores,which might connect to the internal structure, can be seen. The sample was placed at a slight angle to see the surface and see the entire cross section. FIG. 7 shows progressively closer views (panel a): 300 m scale; panel b): 100 pm scale; and panel c): 50 pm scale) of the surface layer of the foam in FIG. 6.

[0143] The specimen was analyzed using a Scanning Electron Microscope (SEM) (ThermoFisher Apero 2S). First the samples were cooled using liquid nitrogen and then fractured to achieve a brittle cross-sectional surface area. To image the cross-section, the sample was coated with platinum using sputter coater (Laica EM ACE600). Measurements were performed on the micrographs using imageJ / Fiji. The cell size was captured by taking the average of the diameter of 50 cells.

[0144] Once foamed the sample lost its flat structure and was curled due to the thermal shock during the foaming process. The surface of the sample was smooth which is similar to other micro cellular foams.DiscussionIsobaric Foaming Process

[0145] By eliminating the desorption step that causes gas to form a skin layer around the sample, a material that is uniform throughout its cross-section was created. During testing, PET was successfully foamed under pressure without the need for a heated bath. In the present example, the described system couples saturation and nucleation by performing each step sequentially in the same chamber and substantially the same pressure. By eliminating the need to depressurize the sample, the desorption time can be substantially eliminated, and substantially skinless foam can be created. The smallest thickness recorded is less than 2 micrometers, on the order of the cell wall thickness. This is a unique result not seen previously through the solid-state process.

[0146] In the present example, the described process is an isobaric process which relies on the change of CO2concentration that happens within the pocket of CO2.Microcellular and Nanocellular Foams

[0147] In this exploration, PET was foamed to have microcellular structures. Nanocellular foams using PC have also been reached, due to polycarbonates’s high CO2absorption. This allows for high concentrations of CO2inside of the polymer when nucleation occurs. To create an open-cell foam using this system, saturation was performed at higher pressures to increase the gas concentration within the polymer. In addition, thermal management was increased to facilitate higher temperatures and heating rate. Different polymers have different equilibrium gas concentrations. The CO2absorption of a polymer is based on the saturation temperature, pressure, and polymer structure. Other polymers, such as PC, have shown higher equilibrium concentrations indicating a higher probability of creating an open-cell foam.Saturation Chamber Design Considerations

[0148] The current saturation chamber has a diameter of 3 cm and a depth of 10 cm, limiting the sample size. T o allow larger samples, volume of the chamber was increased, and flow rate of saturated water into the saturation chamber was increased. Future designs should allow the saturation chamber to be completely filled, ideally directing flow in from the top and out from the bottom

[0149] In order to increase gas concentration, saturation at low temperatures is desired, ideally ranging from 0 to -10°C A low temperature controlled saturation chamber could be utilized to accomplish this. The foamingtemperature range of the system can be expanded by increasing the heat capacity of the foaming agent, thereby increasing thermal mass, and reducing the heat loss in transfer into the sample. This could entail using a liquid with a higher heat capacity, such as oil, or increasing the volume of the thermal mass.Summary and Perspectives

[0150] This novel foaming method has successfully produced microcellular PET foams, characterized by the uniform pore distribution, and the absence of the outer skin layer. This validates the feasibility of scaling up the process with the goal of manufacturing substantial quantities of a skinless polymer foam, which can be tailored to various applications.

[0151] By modifying the solid-state process, and foaming under isobaric conditions, gas desorption is effectively prevented, allowing for the microcellular structure throughout the foam. These findings indicate the potential to achieve skinless, open-celled nanofoams. This innovation holds significant promise towards developing novel materials, with applications towards membrane technologies such as water filters, catalysts and coatings.EXPERIMENTAL EXAMPLE 2

[0152] Abstract: Polymer foams are ubiquitous, found throughout society from disposable packaging to electrical superstructures. The inherent cellular structure is accompanied by a reduced material density and facilitates access to material performance bespoke to intended applications. The solid-state foaming process, while less prevalent than continuous melt-foaming, is unique in that it can yield a cellular structure in polymer sheets at the micro- and nanoscale, without the introduction of additional chemicals, at processing temperatures far below the corresponding melting points of their constitutive polymer blends. The fundamental driving mechanism of this process is the introduction of a thermodynamic instability to a polymer sheet saturated with an inert gas, in the form of a rapid drop in external pressure and increase in temperature. However, a limitation of this process is the presence of an un-foamed surface, or “skin”, sandwiching the cellular core Various efforts have been made in fabricating these foams without said skin, allowing access to the cellular core of these materials and enabling new applications in filtration, energy storage, and medical implants, to name a few. Towards the goal of fabricating skinless foams, the feasibility of a modified solid-state process was conceived and investigated, in which the thermodynamic instability is driven solely by an increase in temperature under constant pressure To accomplish this, specimens were saturated with medical-grade CO2 at pressures on the order of 5 MPa in a sealed vessel. Once sorption equilibrium was reached, the vessel was flooded with superheated water to rapidly heat the polymer. Through this approach, species of foamed PC and rPET were generated with a cellular structure across the sheet thickness, and an un-foamed surface thinner that the walls of individual foam cells.Thermodynamic Motivation

[0153] Gas Diffusion: When the supersaturated polymer is returned to lower pressure, gas immediately begins to diffuse from the surface. The change in the gas concentration profile across the sheet thickness is governed by Fick’s law, represented in one dimension as:where C is the gas concentration and D is the diffusivity constant dependent upon the specific gas-polymer system. The primary parameter determining the thickness of the surface “skin” is the concentration gradient. It was hypothesized that inducing cell nucleation while maintaining a uniform concentration profile across the thickness will eliminate the skin region near the polymer surface.

[0154] Dual-Mode Sorption: Prior work shows that the equilibrium concentration within a glassy polymer is well described by the dual-mode sorption model:where: / cD(Tsat) is Henry’s constant, where: CH' (Tsat) is the Langmuir capacity constant, where: b Tsat') is the Langmuir affinity parameter.This model combines a linear Henry’s law term with a Langmuir term representing gas trapped in microvoids.

[0155] Decoupling the Dual-Mode Model: To decouple temperature and pressure effects in foaming, data from Miller, et al., Polymer, 2009, 50(23):5576-5584, was used to reconstruct isobaric concentration profiles. FIG. 9 shows equilibrium concentration versus temperature at 5 MPa, 3MPa, and 0.1 MPa (atmospheric). The schematics illustrate how a constant-pressure temperature ramp (isobaric foaming) differs from the classical depressurization approach.Experimental

[0156] Parameters for Characterization: There are several key parameters to characterize cellular materials, and polymer foams specifically. The first is the relative density of the foam, defined as the ratio of the foam density to that of the unmodified precursor material:Pfoam Pr = ~ -PsolidFrom the relative density, the void fraction (porosity) can be calculated as:Vf = 1 - Pr

[0157] A key metric of any cellular material is the average cell size, , measured as the mean diameter of n cells from SEM micrographs:The cell nucleation density, No, relates cell size and relative density by:

[0158] Materials: PC and rPET were selected from their high CO2 affinity and established foaming behavior. Sheets (1 .0mm thick) were cut into 50 x 50 mm specimens and dried at 80 °C for 24h under vacuum to remove moisture.

[0159] Apparatus: A custom pressure vessel was constructed to enable in-situ heating under constant pressure. A secondary heating chamber above the saturation zone was filled with superheated water, which flooded into thespecimen chamber upon valve actuation. CO2 was pre-saturated into the water to minimize desorption from the polymer during foaming. An insulated enclosure (Cryo Container) around the vessel reduced heat loss and allowed rapid quench by dry ice. A schematic of this setup is shown in FIG. 5. A custom pressure vessel was configured to enable in-situ heating under constant saturation pressure. A secondary heating chamber above the saturation vessel contained water heated slightly above the target foaming temperature. Upon valve actuation, this superheated water flooded the specimen chamber by gravity - eliminating the need for an external pump. Subsequent design iterations added a CO2 feed into the water line, pre-saturating the heated water and preventing polymer desorption during foaming. An insulated enclosure around the vessel minimized heat loss during flooding and allowed rapid quenching with dry ice. A schematic of this system is shown in FIG. 5.Experimental Procedure

[0160] Specimen Saturation: Following a 24hr drying cycle at 80°C under vacuum, 50 x 50 mm specimens were sealed in the saturation vessel. Deionized water filled the heating chamber, and medical-grade CO2 was flowed for 60 seconds to purge air. Both chambers were pressurized to 5 0 MPa and held for 48hrs to reach sorption equilibrium.

[0161] Specimen Foaming: The water chamber was heated to 100°C using a flexible resistance heater. Once the set-point was reached, the isolation valve was opened, allowing CO2-saturated hot water to flood the specimen chamber and trigger cell nucleation.

[0162] Specimen Quench: Two minutes after flooding, the insulated enclosure was packed with dry ice to rapidly cool the vessel and freeze the evolving foam structure. After 10 minutes, the dry ice was removed, the system vented and drained, and specimens were retrieved.

[0163] Characterization: Cross section of the frozen specimens were prepared by scoring with a razor blade and fracturing in liquid nitrogen (LN2) for 2 minutes. Samples were then sputter-coated with platinum and imaged on a Thermo-Fisher Apreo 2S SEM. Cell diameters were measured over at least n > 50 pores using ImageJ / Fiji.Results and Discussion

[0164] Specimen Imaging: Successful foaming was achieved for both polymer types. The polycarbonate specimens exhibited a homogenous cellular morphology at the core, with an average skin thickness of 3pim - much smaller than previously reported for similar conditions. Primary cell diameters ranged from 10 to 14 m, with secondary nanopores between 100 and 200nm, confirming nanoscale features via this isobaric process. SEM micrographs are shown in FIG. 8. Images of rPET cross sections (FIG. 6) revealed larger average pores and a residual skin no thicker than the cell walls, demonstrating effective skin reduction via isobaric foaming.Conclusion

[0165] This experiment demonstrated an isobaric foaming technique for fabricating micro- and nanocellular polymer foams, substantially eliminating the un-foamed surface layer typical of solid-state foaming. The required thermodynamic instability was induced by a rapid temperature increase - via superheated, CC>2-saturated water - while maintaining constant pressure. This approach mitigated CO2 desorption at the polymer surface and, in turn, effectively decreased skin thickness. In rPET, any residual skin was of comparable thickness to the intra-cellular walls. Polycarbonate foams showed hierarchical structures with primary cells of 3pim and secondary nanopores of100-200nm. Maintaining elevated pressure during heating suppresses near-surface depletion zones that otherwise form a dense, un-foamed skin. In certain PC samples, partially spinodal-like cell formation produced secondary nanocellular features, expanding design possibilities and indicating a pathway toward fully nanoporous architectures. Overall, the results confirm that isobaric solid-state foaming can produce foams without a dense outer surface, addressing a key limitation in manufacturing high-performance cellular polymers. By controlling saturation parameters and heat input under constant pressure, this method paves the way for skinless micro- and nanocellular foams with broad functional capability in filtration, energy storage, dielectric, and biomedical applications.

[0166] (XI) Closing Paragraphs. All references cited are incorporated by reference herein in their entirety.

[0167] The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be used for realizing implementations of the disclosure in diverse forms thereof.

[0168] As will be understood by one of ordinary skill in the art, each implementation disclosed herein can comprise, consist essentially of or consist of its particular stated element, step, or component. Thus, the terms “include” or “including” should be interpreted to recite: “comprise, consist of, or consist essentially of.” The transition term “comprise” or “comprises” means has, but is not limited to, and allows for the inclusion of unspecified elements, steps, ingredients, or components, even in major amounts. The transitional phrase “consisting of” excludes any element, step, ingredient or component not specified. The transition phrase “consisting essentially of” limits the scope of the implementation to the specified elements, steps, ingredients or components and to those that do not materially affect the implementation. As used herein, the term “based on” is equivalent to “based at least partly on,” unless otherwise specified.

[0169] Unless otherwise indicated, all numbers expressing quantities, properties, conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present disclosure. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. When further clarity is required, the term “about” has the meaning reasonably ascribed to it by a person skilled in the art when used in conjunction with a stated numerical value or range, i.e. denoting somewhat more or somewhat less than the stated value or range, to within a range of ±20% of the stated value; ±19% of the stated value; ±18% of the stated value; ±17% of the stated value; ±16% of the stated value; ±15% of the stated value; ±14% of the stated value; ±13% of the stated value; ± 12% of the stated value; ±11 % of the stated value; ± 10% of the stated value; ±9% of the stated value; ±8% of the stated value; ±7% of the stated value; ±6% of the stated value; ±5% of the stated value; ±4% of the stated value; ±3% of the stated value; ±2% of the stated value; or ±1% of the stated value.

[0170] Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

[0171] The terms “a,” “an,” “the” and similar referents used in the context of describing implementations (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. Recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context The use of any and all examples, or exemplary language (e.g. , “such as”) provided herein is intended merely to better illuminate implementations of the disclosure and does not pose a limitation on the scope of the disclosure. No language in the specification should be construed as indicating any non-claimed element essential to the practice of implementations of the disclosure

[0172] Groupings of alternative elements or implementations disclosed herein are not to be construed as limitations. Each group member may be referred to and claimed individually or in any combination with other members of the group or other elements found herein. It is anticipated that one or more members of a group may be included in, or deleted from, a group for reasons of convenience and / or patentability. When any such inclusion or deletion occurs, the specification is deemed to contain the group as modified thus fulfilling the written description of all Markush groups used in the appended claims.

[0173] Unless otherwise indicated, the practice of the present disclosure can employ conventional techniques of chemistry, thermodynamics, polymer science, mechanical engineering, and process engineering. These methods are described in the following publications: see, e.g., McIntosh, Organic Chemistry: Fundamentals and Concepts, De Gruyter, 2018; Cotton and Wilkinson, Advanced Inorganic Chemistry, Sixth Edition, John Wiley & Sons, 1999; Atkins, P., De Paula, J., & Keeler, J. Atkins’ Physical Chemistry, 11th ed., Oxford University Press, 2017; Green, D W., & Southard, M. Z. (2019). Perry's chemical engineers' handbook (9th ed.). McGraw-Hill Education; Reisel, J. R. (2022). Principles of engineering thermodynamics (2nd ed.). Cengage; and Sava, R., 2019, Mechanical Engineering: Designing and Applications, NY Research Press.

[0174] Certain implementations are described herein, including the best mode known to the inventors for carrying out implementations of the disclosure. Of course, variations on these described implementations will become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventor expects skilled artisans to employ such variations as appropriate, and the inventors intend for implementations to be practiced otherwise than specifically described herein. Accordingly, the scope of this disclosure includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by implementations of the disclosure unless otherwise indicated herein or otherwise clearly contradicted by context.

Claims

CLAIMSWhat is claimed is:1 . A method of generating a skinless polymer foam, the method comprising: using a gas to pressurize a chamber to a pressure that is greater than or equal to a threshold pressure, the chamber containing a material; maintaining the pressure that is greater than or equal to the threshold pressure until a saturation state of the gas is reached within the material; and contacting the material, in the chamber at the pressure that is greater than or equal to the threshold pressure, with a fluid for a threshold time and at a temperature that is greater than or equal to a glass transition temperature of the material, thereby generating the skinless polymer foam.

2. The method of claim 1, wherein the material comprises a thermoplastic material, a biologically derived material, or a combination thereof.

3. The method of claim 2, wherein the material is the thermoplastic material and the thermoplastic material comprises at least one thermoplastic polymer selected from the group consisting of polyethylene terephthalate (PET), polylactic acid (PLA), polyvinyl chloride (PVC), polycarbonate (PC), polystyrene (PS), polymethyl methacrylate (PMMA), polyetherimide (PEI), polyether ether ketone (PEEK), polyethylene (PE), low density polyethylene (LDPE), medium density polyethylene (MDPE), high density polyethylene (HDPE), acrylonitrile butadiene styrene (ABS), polyethersulfone (PES), polyetherketoneketone (PEKK), polyaryletherketone (PAEK), polyimide (PI), polysulfone (PSU), polyphenylsulfone (PPSU), thermoplastic polystyrene (TPS), and cyclic olefin copolymer (COC).

4. The method of claim 3, wherein the thermoplastic material comprises a blend of at least two thermoplastic polymers.

5. The method of claim 1 , wherein the material is a composite material comprising at least one of a carbon fiber, a glass fiber, or a nanoparticle.

6. The method of claim 1, wherein the gas comprises nitrogen (N2), carbon dioxide (CO2), nitrous oxide (N2O), helium (He), argon (Ar), butane, propane, or a combination thereof.

7. The method of claim 1 , wherein the threshold pressure comprises a pressure sufficient to dissolve at least about 10 wt% of the gas into the material.

8. The method of claim 1, wherein the threshold pressure is in a range of about 0.01 megapascals to about 30 megapascals.

9. The method of claim 1, wherein the saturation state comprises a partial saturation of the gas within the material or an equilibrium concentration of gas within the material.

10. The method of claim 1, wherein the fluid comprises at least one of water, supercritical CO2, supercritical N2, steam, or an oil.

11. The method of claim 1 , wherein the fluid is saturated with the gas.

12. The method of claim 1 , wherein using the gas to pressurize the chamber is performed at a temperature that is less than the glass transition temperature of the material.

13. The method of claim 1, wherein contacting the material, in the chamber at greater than or equal to the threshold pressure, with the fluid for the threshold time and at the temperature that is greater than or equal to the glass transition temperature of the material comprises maintaining an isobaric environment within the chamber.

14. The method of claim 1, wherein contacting the material, in the chamber at greater than or equal to the threshold pressure, with the fluid for the threshold time and at the temperature that is greater than or equal to the glass transition temperature of the material comprises moving the fluid into the chamber containing the material and holding the fluid in the chamber for the threshold time.

15. The method of claim 1 , wherein the threshold time comprises a time sufficient to form a desired pore size in the material.

16. The method of claim 1, wherein contacting the material, in the chamber at the pressure that is greater than or equal to the threshold pressure, with the fluid for the threshold time and at the temperature that is greater than or equal to the glass transition temperature of the material generates pores in the material, a width of the pores being in a range of about 5 nanometers to about 100 micrometers.

17. The method of claim 16, wherein a portion of the pores are exposed at a surface of the material.

18. The method of claim 16, wherein the pores comprise open cells.

19. The method of claim 1, the chamber being a material chamber, wherein contacting the material, in the material chamber at the pressure that is greater than or equal to the threshold pressure, with the fluid for the threshold time and at the temperature that is greater than or equal to the glass transition temperature of the material comprises: opening a flow control device between the material chamber and a fluid reservoir chamber containing the fluid, thereby allowing the fluid to flow from the fluid reservoir chamber into the material chamber containing the material.

20. The method of claim 19, wherein the fluid in the fluid reservoir chamber is at a pressure greater than the pressure of the material chamber, thereby providing energy to the fluid to flow from the fluid reservoir chamber into the material chamber.

21. The skinless polymer foam generated by the method of claim 1.

22. A skinless polymer foam comprising: a thermoplastic material; pores disposed in the thermoplastic material, a width of the pores being in a range of about 5 nanometers to about 100 micrometers; and a portion of the pores exposed at a surface of the thermoplastic material at a time the foam is formed.

23. The skinless polymer foam of claim 22, wherein the thermoplastic material comprises at least one of PET, PLA, PVC, PC, PS, PMMA, PEI, PEEK, PE, LDPE, MDPE, HDPE, ABS, PES, PEKK, PAEK, PI, PSU, PPSU, TPS, or COC.

24. The skinless polymer foam of claim 22, wherein the portion of the pores exposed at the surface of the thermoplastic material expose about 10% to about 90% of the surface of the thermoplastic material.

25. The skinless polymer foam of claim 22, wherein the pores comprise open cells.

26. A filter comprising the skinless polymer foam of claim 22.

27. A system comprising: a chamber configured to operate at or above a first pressure and at or above a first temperature and to contain a material; a gas source fluidically connected to the chamber and configured to contain a gas; a first flow control device coupled between the chamber and the gas source; a fluid reservoir fluidically connected to the chamber and configured to contain a fluid at or above a second pressure and at or above a second temperature; and a second flow control device coupled between the fluid reservoir and the chamber.

28. The system of claim 27, wherein the chamber is configured to fluidically seal and operate at a high pressure and a high temperature.

29. The system of claim 27, wherein the first temperature is a temperature less than a glass transition temperature of the material30. The system of claim 27, wherein the first pressure is in a range of about 0.01 megapascals to about 30 megapascals31. The system of claim 27, wherein the material comprises at least one of PET, PLA, PVC, PC, PEI, PEEK, PE, LDPE, MDPE, HDPE, ABS, PES, PEKK, PAEK, PI, PSU, PPSU, TPS, or COC.

32. The system of claim 27, wherein the gas comprises N2, CO2, or a combination thereof.

33. The system of claim 27, wherein the first flow control device comprises a valve, a pump, a compressor, or a combination thereof.

34. The system of claim 27, wherein the fluid reservoir is configured to fluidically seal and operate at a high pressure and a high temperature.

35. The system of claim 27, wherein the fluid reservoir is positioned at a location above the chamber along a gravity force vector creating a hydrostatic pressure of the fluid between the fluid reservoir and the chamber36. The system of claim 27, wherein the fluid comprises water, super critical CO2, steam, or an oil.

37. The system of claim 27, wherein the second temperature is at or above a glass transition temperature of the material.

38. The system of claim 27, wherein the second pressure is in a range of about 0.01 megapascals to about 30 megapascals.

39. The system of claim 27, wherein the second flow control device comprises a valve, a pump, a compressor, or a combination thereof.

40. The system of claim 27, further comprising a cooling source coupled to the chamber.

41. The system of claim 40, wherein the cooling source comprises a refrigerator, a thermoelectric cooler, a cooling material, or a combination thereof.

42. The system of claim 27, further comprising a heat source coupled to the chamber43. The system of claim 42, wherein the heat source comprises an electric heater, a combustion heater, a hot fluid, an infrared heater, or a combination thereof.

44. The system of claim 27, further comprising a pump, a compressor, or a combination thereof, coupled to the fluid reservoir45. The system of claim 27, further comprising a heat source coupled to the fluid reservoir .

46. The system of claim 45, wherein the heat source comprises an electric heater, a gas heater, a hot fluid, or a combination thereof.

47. The system of claim 27, further comprising: a processor communicatively coupled with the first flow control device and the second flow control device, the processor being configured to: pressurize the chamber to the first pressure with the gas by actuating the first flow control device; and actuate the second flow control device to cause the fluid to enter the chamber, while maintaining the first pressure in the chamber.

48. The system of claim 47, wherein the processor is further communicatively coupled with a heat source coupled to the chamber, the processor further configured to: actuate the heat source to heat the gas and the material contained in the chamber to the first temperature.

49. The system of claim 47, wherein the processor is further communicatively coupled with a pump / compressor coupled to the fluid reservoir , the processor further configured to: actuate the pump / compressor to pressurize the fluid reservoir to the second pressure.

50. The system of claim 47, wherein the processor is further communicatively coupled with a heat source coupled to the fluid reservoir , the processor further configured to: actuate the heat source to heat the fluid contained in the fluid reservoir to the second temperature.

51. The system of claim 47, further comprising: a temperature sensor communicatively coupled with the processor and configured to detect a temperature of the fluid; and a pressure sensor communicatively coupled with the processor and configured to detect a pressure in the chamber.

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