Polymer film formation with capillary action

Abstract

A method of forming a polymer film includes providing a substrate and a superstrate, and providing a precursor polymer solution. The precursor polymer solution is flowed by capillary action between the substrate and the superstrate. The precursor polymer solution is cured to form the polymer film. The method can be referred to as capillary crawl film formation (CCFF), and can be implemented in the simple, low-cost and / or highly scalable manufacture of thin polymer films.

Description

TITLE: POLYMER FILM FORMATION WITH CAPILLARY ACTIONCROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to United States Patent Application No. 63 / 697,063 filed on September 20, 2024, the entire contents of which are hereby incorporated herein by reference.FIELD

[0002] The present disclosure relates generally to methods of forming thin polymer films involving capillary action.BACKGROUND

[0003] The following is not an admission that anything discussed therein is prior art or part of the knowledge of persons skilled in the art.

[0004] Polymeric films have garnered interest throughout the years, owing to their suitability in modem applications such as in electrical and electronic manufacturing, in the chemical industry, in aerospace engineering and in the construction industry. Their large- scale processing and thin film production have also been a major research focus in the last decade, due to our ever-increasing need for personal electronics and the emergence of wearable gadgets. In essence, polymer films are thin organic or inorganic-based materials with thicknesses going from a few nanometers and up into the micrometer range. Although processability might include manually intensive processing steps and expensive equipment, the benefits of working on a smaller scale allow for finer tuning of material properties, in addition to the ability to minimize by-products and waste.

[0005] Polymer films have been produced by techniques which can be classified as either physical or chemical deposition methods. The most common deposition processes include extrusion processes, spin coating, spray coating, sputtering processes, chemical vapor deposition, film casting, dip coating, doctor blading, and printing techniques. As for the choice of deposition process, it usually depends on theapplication and type of material targeted. For instance, thin films have been employed in coatings that promote certain functionalities, like absorption capability, surface protection and electrical properties enhancement, and in device manufacture where such films function as substrates or active layers in different disciplines, such as in photovoltaics, sensory applications, and battery technologies. Materials such as polyvinylidenefluoride), polydimethylsiloxane, polyimide and even composites such as polyamide and polyetherurea have been made into thin films and they vary from producing thin film polymers as substrates, to making photoactive layers and even conductive battery cathodes with the addition of additives.

[0006] Polydimethylsiloxane (PDMS) is a widely used material that has been researched extensively for thin film applications, as this biocompatible transparent elastomer has desirable physical and chemical properties. With excellent flexibility, superior tensile strength and its ease of processability, PDMS has seen widespread use in biomedical engineering, microfluidics, wearable generators, sensors development, medicine and cosmetics. The ability to add microstructures to the surface of PDMS thin films is an advantage, and a main reason behind its extensive use as active material for wearable generators and substrates in microfluidic applications. Such structures are added via manual intensive multiple processing steps either via pre-processing using molds or via integration into the film post-processing through energy intensive procedures like plasma etching, with the aim of adding more functionality to the film, such as enhancing its electrical properties, or introducing microfluidic channels within the layer.

[0007] However, widespread adoption of PDMS has been hindered by many challenges still faced to this day. For instance, PDMS production relies for the most part on manual intensive multiple steps processing, while large quantities of material is employed to produce the thin films, generating unnecessary waste, up to two thirds of the material used. In addition, the production of microstructured PDMS films nowadays can rely on expensive equipment that increases in cost with finer dimension requirements. As well, to the best of our knowledge, there are no feasible large scale PDMS film production techniques adopted in the industry today and it may not be possible to produce largemonolithic structures with current techniques. The generated chemical and energy wastes as well as production methods with large scalability of complex structures still form a barrier for the mass-production of PDMS and its widespread adoption.INTRODUCTION

[0008] The following is intended to introduce the reader to the detailed description that follows and not to define or limit the claimed subject matter.

[0009] Herein, we disclose a facile, highly reusable technique, which can be referred to as capillary crawl film formation (CCFF), to produce smooth or microstructured polymer thin films, all while generating no waste, and capable of being implemented in large scale mass produced unmanned manufacturing.

[0010] The technique is based on the capillary action of solution-processed PDMS between plates. We show the versatility of our approach by tuning PDMS’s physical properties such as the film’s surface structure through the addition of macro- and microstructures, in addition to its transparency and hydrophobicity through the use of substrates with different adhesion properties with PDMS. Small- and large-scale sample production is also shown with sample areas produced as large as 163.84 cm2from 1 ml of solution. As for the thickness of the PDMS films, it was controlled by the amount precursor solution volume in conjunction with the surface area of the substrate. While these two control measures led to films from the nanometers to micrometers, thicker films can be obtained with the addition of separators, or using a layer-by-layer approach. The lowest thickness achieved through CCFF was shown to be 0.02 mm by employing a single drop of PDMS onto a PET substrate.

[0011] The technique’s mass production capability is demonstrated and its viability for vertical and horizontal solution climb using multiple stacked plate from a single precursor source. The potential for large 3D monolithic structures is also included. Furthermore, we demonstrate the capability of CCFF to add a protective encapsulation coating to a solar cell, allowing them to retain their original performance throughout the full span of testing. We also show the ability of using CCFF to heal tom PDMS films, byre-fusing the separated parts together while maintaining their exact original thickness and forming a large new polymeric film. Finally, we integrate the films into a capacitive pressure sensor setup and test their response under mechanical stress.

[0012] Other aspects and features of the teachings disclosed herein will become apparent, to those ordinarily skilled in the art, upon review of the following description of the specific examples of the present disclosure.BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The drawings included herewith are for illustrating various examples of apparatuses and methods of the present disclosure and are not intended to limit the scope of what is taught in any way. In the drawings:

[0014] Figure 1 shows different approaches to prepare PDMS films (a) first detailing the CCFF process as a drawn schematic depicting the successive steps of the process, (b) and then showing a classic model of preparation of PDMS thin films, followed by (c) a scheme showing the surface tension and Laplace pressure impact on a horizontal capillary flow, along with (d) another representing the different forces at play during traditional vertical capillary flow, while showing the adhesion and cohesion forces.

[0015] Figure 2 shows an ultra-thin PDMS film prepared via CCFF using PET sheet substrates.

[0016] Figure 3 shows Capillary Crawl Film Formation or CCFF versatility (a) in a horizontal single large film production setup using (i) flexible PET substrates, where (ii) 1 ml of PDMS solution was placed at the center, followed by (iii) the preliminary spread caused by the placement of the superstrate before capillary action takes over, then (iv) the film formed between the PET sheets after capillarity occurs, and afterwards (v) the removal of the superstrate and peeling off the film, and finally (vi) the large PDMS thin film obtained from CCFF, (b) (i) in a vertical multi-plate setup for the simultaneous production of PDMS thin films, with (ii) a close-up view of a produced film with the vertical CCFF, (c) through the capability of fabricating stacked PDMS films with unique structures,(i) showing a cross-sectional look at the produced stacked film using base film made with PET, a second layer using an UHMWPE superstrata and a third layer using monofilament fabric superstrates, (ii) a closer look at the cross-section of the stacked film under light microscopy, and (iii) another microscopic top-down image of the stacked film, and lastly (d) creating three different films at distinct spots using diverse superstrates at the same time on a single substrate, by (i) placing small PET, PMMA and UHMWPE plates on top of PDMS solution, (ii) waiting for the separated films to cure, then (iii) showing the formed films after removing the top plates.

[0017] Figure 4 shows joined acrylic, PET and UHMWPE superstrates for simultaneous curing of a single large film with different surface structures on a single glass substrate, from (a) the initial solution drop and crawl phase, to (b) the formed film seen through the back of the glass substrate, and (c) the formed large PDMS film on glass with the superstrates removed.

[0018] Figure 5 shows PDMS films produced through CCFF using different substrates are shown, namely (a) (i) a large-scale film prepared between smooth flexible transparent PET substrates, (ii) a small section of smooth film as seen through confocal imagery at x5 magnification and (iii) the PET substrate surface shown under the confocal microscope at x5 magnification, as well as (b) (i) a second large-scale film prepared between textured rigid UHMWPE plates, (ii) a small section of structured film as seen through confocal imagery at x5 magnification and (iii) the UHMWPE substrate surface shown under the confocal microscope at x5 magnification.

[0019] Figure 6 shows PDMS films produced through CCFF using different substrates, namely (a) (i) a large-scale film prepared between smooth rigid acrylic substrates, (ii) a small section of smooth film as seen through confocal imagery at x5 magnification and (iii) the acrylic substrate surface shown under the confocal microscope at x5 magnification, as well as (b) (i) a second large-scale film prepared between textured flexible PTFE sheets, (ii) a small section of structured film as seen through confocal imagery atx5 magnification and (iii) the PTFE substrate surface shown under the confocal microscope at x5 magnification.

[0020] Figure 7 shows confocal imagery at x5 magnification showing the structuring capability of CCFF technique using (a-d) different monofilament-based fabrics as superstrata pattern templates.

[0021] Figure 8 shows surface roughness values for structured PDMS films and their respective top substrate templates.

[0022] Figure 9 (a-f) shows macrostructured PDMS films produced with CCFF technique, along with the top substrate structured stubs used and confocal imagery at x5 magnification.

[0023] Figure 10 shows UV-Vis spectroscopy to check light transmittance.

[0024] Figure 11 shows PDMS film formation through solution casting on a single substrate using (a) UHMWPE plate, (b) PTFE film, (c) PET film and (d) PMMA plate.

[0025] Figure 12 shows hydrophobicity testing of a water droplet on the PDMS and its respective substrate surfaces, prepared through (a-b) SRP-PMA, (c-d) TRP-UPE, (e- f) SFF-PET, (g-h) TFF-PFE.

[0026] Figure 13 shows CCFF’s versatility in different applications, with first (a) testing produced PDMS films using the capillary-based process under the four substrate conditions in capacitive sensor devices while showing the setup used for measurements along with capacitive response values to different loads applied to the sensors, then (b) encapsulating solar cells through the use of CCFF and recording relative PCE measurements while comparing base solar cells (control) to encapsulated solar cells and tracking the changes in PCE when stored inside a glovebox and in high humidity conditions, and finally (c) showing CCFF technique being used as a healing mechanism for (i) torn and separated PDMS films and (ii) the consequent large film produced after the addition of a small amount of fresh solution.DETAILED DESCRIPTION

[0027] Various apparatuses or methods will be described below to provide an example of an embodiment of each claimed invention. No embodiment described belowlimits any claimed invention and any claimed invention may cover apparatuses and methods that differ from those described below. The claimed inventions are not limited to apparatuses and methods having all of the features of any one apparatus or method described below, or to features common to multiple or all of the apparatuses or methods described below. It is possible that an apparatus or method described below is not an embodiment of any claimed invention. Any invention disclosed in an apparatus or method described below that is not claimed in this document may be the subject matter of another protective instrument, for example, a continuing patent application, and the applicant(s), inventor(s) and / or owner(s) do not intend to abandon, disclaim or dedicate to the public any such invention by its disclosure in this document.Experimental:Materials:

[0028] Sylgard 184 kit was purchased from DOW Coming. The transparent acrylic plates (PMMA), the ultra-high molecular weight polyethylene (UHMWPE) plates, and the high-density polyethylene (HDPE) plates were acquired from McMaster-Carr. Patterned monofilament-based fabrics were provided by Andritz Group. The flexible polytetrafluoroethylene (PTFE) sheet was obtained from Cascade Sciences. Indium tin oxide coated polyethylene terephthalate (PET) sheets were purchased from MSE supplies. Regular commercial tape was used to add separation between the plates. Conductive nickel on copper-plated polyester fabric tape (NCFT) was purchased from Mouser Electronics. Conductive silver printing ink (5-6 pQ.cm) and Methylene violet solution were obtained from Sigma-Aldrich. The weights used to apply force to the capacitive sensors were bought from Neewer.PDMS film preparation:

[0029] PDMS was prepared using a technique recommended by the manufacturer. First, a nitrogen gun was used to clean the mixing flask and ensure no contaminants were present. A 10:1 ratio of prepolymer to curing agent solution was poured into the flask and mixed vigorously using a glass rod until air bubbles were visible. The solution was thenplaced in a vacuum chamber and was left to degas for about 1 hour. The substrates were then cleaned through two step bath sonication by first placing them in sonicator bath submerged in soapy water for 20 minutes, followed by the same process using DI water only. The plates were then dried using kimwipes. Isopropanol (IPA) was then rubbed across the surface of the plates, followed by a final DI water wash and wrapping up by blow drying the substrates using a nitrogen gun. Adhesive tape was used to add separation between the plates when needed. The tape served as another means of film thickness control, though primary control of sample thickness was introduced through varying the volume of solution used or the size of the substrate. 1 ml syringes were used to draw precursor solution from the degassed PDMS melt, which was subsequently deposited in the middle of the bottom substrate. A superstrate, often the same as the substrate, was then gently placed on top to cover the solution. After leaving the PDMS to cure overnight, both substrates were separated by hand and the film, having assumed its final dimensions, was collected from the substrate surface.Vertical capillary setup:

[0030] Six 3x3 in2acrylic sheets were stacked against each other, with a piece of commercial tape placed on the top of each substrate to maintain a singular separation throughout all the plates. The collective plates were then taped together to maintain their stability throughout the experiment. They were then placed in a glass petri dish, where a prepared precursor PDMS solution was poured. The setup was then left standing overnight to allow for the PDMS to climb, and the thin films to form.Solar cell encapsulation:

[0031] The solar cells were prepared using a known technique. In short, the glass and ITO substrates were cleaned using the ultrasonication bath, followed by a nitrogen gas drying step and UV / Ozone (UVO) treatment. Methylammonium Lead Iodide (MAPbls) was used as the perovskite semiconductor, along with a tin oxide (SnO2) electron transport layer, spiro-OMeTAD as hole transport layer, and a 100 nm gold layer as the back-contact electrode. After the solar cell preparation was completed, a single drop ofPDMS solution was poured in the middle of solar cell using a 1 ml syringe. Then, a 1x1 cm2flexible PET sheet was used to cover the droplet, then the sample was set aside overnight until the PDMS completed its crawl and cured. Four different conditions were prepared: two encapsulated solar cells and two control, where a control and an encapsulated solar cell were stored inside the N2-filled glovebox, while the other two devices were stored outside in air under >70% humidity conditions. The top substrate was then removed by lifting a corner using fine blade, then peeling off the substrate. The solar cells’ relative power conversion efficiency (PCE) was tested daily and recorded over a period of one week.Capacitive pressure sensor preparation:

[0032] PDMS films were prepared under the four different substrate conditions, albeit with similar thicknesses of around 200 pm. These films were chosen as the active layers for the capacitive devices. 2x1 cm2samples were prepared from the films, then sandwiched between two flexible adhesive conductive NCFT fabric electrodes. Copper wires were attached to both sides of the devices using conductive silver paste to complete the connections to an oscilloscope, where multiple weights (50 g, 100 g, 500 g, 1000 g) were placed on top to test the different signals produced under increasing pressure loads.Characterization:

[0033] Keyence Laser Confocal microscope (VK X-1000) was used for surface roughness and structural imaging at both the micro and macro levels. The PDMS film thicknesses were measured with the help of a Mitutoyo thickness gauge. A TBS1000B series oscilloscope was used to collect the voltage data generated during the capacitive sensor testing. The solar cell PCE was obtained using a Sciencetech SF300-A solar simulator, accompanied by a Keithley 2450 source meter for l-V measurements and powered by Sciencetech power supply 601 -300. Cary 5000 UV-Vis-NIR spectrophotometer was used to obtain the light transmittance data of the various thin films produced between 200 and 1000 nm wavelengths. Polymer film tensile strengthmeasurements were conducted using an in-house built tensile stress vs strain testing device.Results and discussion:

[0034] CCFF technique is based on fluid capillarity concepts, which describe the interaction between the polymer solutions and the solid surfaces of the substrates. In essence, the higher the adhesive force in comparison to the inherent material’s cohesive force is, the further the liquid solution will climb through capillarity. Figure 1 (a) depicts a representation of the complete CCFF process, as applied using rigid substrates to prepare thin PDMS films, though flexible substrates are also compatible with this technique. Once the PDMS solution was prepared, 1 ml volume was extracted using a syringe, and was consequently poured onto the ready bottom substrate as depicted in the first step. Step 2 is the introduction of the top superstate which is then gently placed to cover the resting melt solution. The solution then starts its climb in the third step until it reaches its final form seen in step 4, based on the surface tension force applied at the surface of the substrate, the contact angle determining the strength of adhesive forces as opposed to the cohesive forces of the material, and the overall pressure difference between the capillary pressure from the solution flow and the opposing surrounding Laplace pressure applied at the surface of the liquid. Once the film forms, the last step is the retrieval process which is now possible through separation of the plate substrates and removal of the PDMS film. In contrast, Figure 1 (b) shows a classic PDMS sample preparation technique, where step 1 represents the solution being poured in a petri dish, followed by the curing of the film as a second step, then the retrieval of the PDMS mold post-curing in step 3, the sample cutting forth step to finally separate and obtain the polymer film in step 5. While CCFF uses controlled amounts of initial precursors, exactly what is needed for the films produced, the classic technique results in waste, often more than 50% of the amount used.

[0035] Capillary action, as a concept, refers to the phenomena where surface tension forces at the fluid-fluid and fluid-solid interfaces lead to the climbing or crawl of the polymer solution along the surface of the solid substrates, in relation to the cohesiveand adhesive forces at play between the polymer molecules themselves and the polymer- substrate molecules respectively, as seen in both Figures 1 (c) and 1 (d). The capillary forces can act in both the vertical and horizontal directions. In CCFF, the capillary forces act in a horizontal direction, meaning that while the gravitational force is not at play, it is the pressure that surrounds the flowing liquid that acts as the opposing force as shown in Figure 1 (c). In this case, capillarity follows the Young-Laplace equation:2y cos(0)AP = — - — rWith AP representing the pressure difference between the flow of the liquid inside the capillary and the Laplace pressure applied by the opposing force restricting its motion, y the surface tension, 0 the contact angle between the liquid and the adjacent surfaces, and r the radius of the tube.

[0036] Similarly, for the flow of liquid between two vertical surfaces, as shown in Figure 1 (d), capillarity obeys the following equation:Where h represents the height of liquid rise, p the density of the solution and g the gravitational force.

[0037] Surface tension represents the ability of a liquid’s interfacial particles to resist the attraction forces of the other surface, due to the cohesiveness within its own molecules. The intermolecular forces such as Van Der Waals interactions, oppose the adhesive forces, between the fluid and the capillary surface. That counterbalance is what determines whether the liquid flow is driven forward or restricted. Surface tension can be obtained through the following equation:FY =~With F representing the cohesive forces between the molecules, L the length over which the force acts, and y the liquid’s surface tension.

[0038] As described above, two substrates are used to initiate the CCFF process; the substrates can be rigid or flexible, and may contain structures on either side. As the superstrate is placed on top of the solution, omnidirectional flow of the solution occurs, driven at first by the applied weight force applied of the top plate. As the top plate rests, the applied weight force dissipates and solution flow becomes solely controlled by the higher adhesive forces of the melt with the respective substrate, compared to its own cohesive forces. This pushes the solution spread farther beyond the original film formed until the point where the melt’s spread length gets high enough that the surface tension force becomes exceedingly elevated, ergo limiting the capillary climb. The final film shape takes place, and the solution then sets. The area and thickness of the produced films are controlled by two main factors: (1 ) the substrate dimension which affects the omnidirectional length over which the solution will travel; and (2) the solution initial volume which helps mitigate the initial spread induced by superstrate weight before omnidirectional crawl and therefore the final film thickness. To test these parameters and the limitations of the CCFF technique, film thickness control was studied by first varying the working surface area of the substrates. Using acrylic plates, 1 ml of solution was deposited between 3x3, 6x6 and 12x12 in2plates. Post curing, the samples were then retrieved, and the measured thicknesses were 110, 70 and 50 pm respectively. This showed that the thin film thickness can be controlled by changing the length over which the capillary force acts, ergo the surface area of the substrates. A second experiment was ran by varying the volume of solution deposited between 6x6 in2acrylic plates. At increasing volumes of 1 , 3 and 5 ml of PDMS, the obtained thicknesses were recorded to be 50, 160, and 230 pm respectively. Using the same principles tested prior, we attempted to develop an ultra-thin PDMS film using CCFF. A single drop of solution was deposited between 3x3 in2sheets, meaning that the same steps seen in Figure 1 (a) were followed, albeit with different substrate dimensions and much less volume to promote low thickness film formation. And after curing was completed, the film thickness was measured and found to be 20 pm, as depicted in Figure 2. The final film did not cover the plate completely as the used solution volume was not enough. At that stage, we believethat the atmospheric pressure surrounding the solution became larger than the applied capillary pressure, which halted the solution’s crawl, promoting molecular cohesion forces over the adhesion forces between PDMS molecules and the surfaces of the PET substrates forming the capillary, in turn holding the solution in place while it cures over time. Having said that, the ultra-low thickness achieved with CCFF is still within the lower- end range of what has been previously reported, and what can be found in the industry at the moment, in addition to producing these ultra-thin films at larger scale than other alternatives, with the added benefit of much control of the film’s other dimensions as well. For example, adding a small weight pressure would remedy the faced issue and create a complete 3x3 in2and 20 pm thin PDMS film, while an increase in pressure would even push the thickness lower and produce larger films. The introduction of separators and weights can be one approach to further manipulate the produced film thickness and exceed the CCFF technique’s limitations. For instance, separators if introduced can increase the range of targeted film thicknesses by manipulating the overall surface tension faced by the solution throughout its climb. The addition of a separator is mainly used to produce films thicker than 300 pm. Increasing the volume and reducing the plate dimensions present limitations in terms of the capillary crawl of the solution, as there is a finite distance we can decrease in terms of the crawl length available, and the solution volume has a threshold after which the solution will cross the border of the plates and continue its spread beyond the substrates. Specifically, the solution will climb below the substrate and proceed to form a secondary film below the aforementioned substrate, and between it and the surface it rests on. We hypothesize that the reason for this is that the capillary pressure remains strong enough at the edge of the substrate, that capillary action continues beyond it, forcing the liquid to crawl below the substrate and continue its climb until either the pressure becomes lower than that of the surrounding atmospheric pressure, or the curing and solidification processes commence. We believe that the introduction of separators in this case increases the volume of air surrounding the PDMS melt, inducing more atmospheric pressure, and lowering the impact of the capillary pressure, which in turn lowers the capillary climb to the point where the solution will againstop within the confines of the substrate dimensions, ensuring no spillage or double film form, all the while gaining access to thicker PDMS film formation through CCFF. As for added weights, the added pressure increases the initial applied weight force, forcing a larger initial solution spread, prior to the dissipation of this force’s impact and the beginning of the capillary action. In essence, the crawl will start from a later stage, essentially going on to produce thinner films. This approach is effective for the production of ultra-thin films in the very low micron and nanometer ranges. The choice of substrate has primarily shown to have an impact on the contact angle at the solution-substrate interface through the adhesive and cohesive force relation between the melt and the chosen substrate, ergo affecting the overall final film spread and thickness as well, though to a lesser degree than the aforementioned parameters in the substrate materials studied. Once the curing is complete, the plates are then separated for sample retrieval. The produced thin film, having controlled dimensions, thickness and surface structure, is ready for use in a subsequent application immediately. In other film fabrication approaches, such as the traditional PDMS film formation and curing technique shown in Figure 1 (b), in addition to possible necessary additional equipment and / or energy requirements, films may have to be post-processed to obtain the final shape, dimensions and even thickness, adding more steps and complexity to the approach. Further steps may be required for structuring the PDMS surface too. All these processes can generate a lot of waste material and can be energy intensive. They are also inappropriate processes for efficient mass production of PDMS thin films. CCFF on the other hand does not require as many steps, is compatible with all dimensions and a wide range of thicknesses, allows for inherent structuring of the film during the formation phase, and is even capable of mass production of PDMS films via a vertical solution climb approach.

[0039] The CCFF technique’s mechanical versatility was first explored. In Figure 3(a), the process detailed in Figure 1 (a) was displayed in all its steps, from the PET sheets and the placement of 1 ml of PDMS in Figures 3(a)(i) and 3(a)(ii) respectively, through to the initial spread in Figure 3(a)(iii), followed by crawl of the solution driven through capillary action in Figure 3(a)(iv), the peeling of the cured film from within the flexible PETsheets in Figure 3(a)(v), and finally the retrieved PDMS layer shown in Figure 3(a)(vi). Similarly, the CCFF approach was then applied though to a vertical setup. In this case, as shown in Figure 3(b)(i), multiple acrylic sheets were grouped and held together as one large unit. They were placed on top of uncured methylene violet infused PDMS solution and were set aside overnight to allow enough time for the solution to crawl upwards and cure between the plates. The PDMS melt climbed through the micrometer openings left between the plates, forming several individual PDMS sheets with an average thickness of 0.1 ± 0.01 mm, as shown in Figure 3(b)(ii). This approach opens the door for further large-scale mass production of the polymer. Another use of CCFF can be to produce diverse stacked PDMS layers with specific structures at each level. This is shown in Figure 3(c)(i), where a PDMS film, built by stacking three layers on top of each other, each with their own structure and properties, were cut in half to look at their cross-section and show the different stacked layers and how they connect with each other. A nonstructured thin PDMS film was produced initially through CCFF by sandwiching 0.3 ml of solution between PET substrates. The produced film was used as bottom layer to build on top of, or in other terms, a bottom substrate to be paired with an UHMWPE-based superstrata to build a second structured layer. Then, a third layer was built using the previously constructed film with a monofilament fabric-based superstrate, creating a further structured stack of PDMS layers, interconnected through the CCFF process. The layers were also apparent under microscopic magnification at the cross-section, seeing distinct structures at three different levels as depicted in Figure 3(c)(ii), with the surface of the film inheriting the last superstrate structure as seen by the top-down view in Figure 3(c)(iii). This approach can open up the potential for many applications. Specifically, in the field of microfluidics for example, where in plane two dimensional parallelization has been achieved, employing the stacking capability of CCFF, in addition to its compatibility with both horizontal and vertical crawl can open the door for out of plan three dimensional parallelization where the construction of various microstructures and channels on top of each other with defined thicknesses can allow for low cost easier build of 3D microfluidic channels and 3D PDMS monolithic large structures, without the use of soft lithography orany other complicated methods and expensive equipment. Lastly, a glass substrate, combined with structured UHMWPE and smooth PMMA and PET superstates were placed together to display the ability of forming multiple thin films separately or jointly on a single substrate. As seen in Figure 3(d)(i), three drops of PDMS solution were placed separately on the glass surface and left to cure and form discrete films, as depicted in Figure 3(d)(ii). After removing the superstates from the films’ surfaces, the PDMS layers formed on the glass substrate could be accessed, one of which showed structures as expected when employing the UHMWPE sheet, while the other two had smooth surfaces due to the use of flat PMMA and smooth PET superstates. The same approach was tested again for the formation of joined films, which were also shown to form a large single PDMS film, attached together from separate PDMS drops, and demonstrating unique structures based on the location of the used superstates. This is shown in Figure 4.

[0040] The CCFF technique’s capabilities were further studied. Specifically, two main parameters were explored when choosing substrate material: the flexibility of the substrate; and its surface structure. Hence, PMMA and PET were chosen as the smooth rigid (SRP-PMA) and flexible (SFF-PET) substrates respectively, whereas UHMWPE and PTFE were chosen as the textured rigid (TRP-UPE) and flexible (TFF-PFE) substrates respectively.

[0041] Confocal microscopy was used to study the morphology of the films produced as compared to the substrate’s surface as seen in Figure 5. Large-scale films were obtained through CCFF under the SFF-PET and TRP-UPE conditions, shown in Figures 5(a)(i) and 5(b)(i) respectively. The films produced under SRP-PMA and TFF- PFE conditions, along with close-up microscopic views of their surface structures against the substrate templates, are shown in Figure 6. Under SFF-PET condition, the thin films produced were smooth, displaying no surface structures whatsoever as seen in Figure 5(a)(ii), similarly to their template shown in Figure 5(a)(iii). However, using TRP-UPE conditions demonstrated the surface structure transfer capability of CCFF. The PDMS films, seen in Figure 5(b)(ii) in this case showed the exact structures as their template, seen in Figure 5(b)(iii), showing that the capillary action also favored vertical climb intothe microstructures of the substrates rather than simply the previously noted omnidirectional horizontal spread seen under non-structured conditions. CCFF makes it possible to grow a microstructured film directly in a single step. Different top substrates were employed to further test the texturing capability of the technique. The unique structures were added through the use of monofilament-based fabrics, 3D printed stamps, and metal electrodes as top substrates, opposite to non-structured bottom substrates. Confocal imagery of structured PDMS films and their respective top substrate templates were collected and shown in Figure 7. As seen from the images, the textures translated well onto the surface of the PDMS, ensuring the structure of the templates is inherited post-curing. In order to confirm the accuracy of the structuring process, surface roughness values (Sa) were collected for both the structured PDMS films and their respective templates. Figure 8 shows the Sa results obtained through roughness analysis from the confocal microscope. The Sa values of both films and substrates were similar in all cases, indicating that CCFF technique is able to produce thin structured films while accurately transferring the texture of the template substrates used. Figure 9 shows the used electrodes along with the resulting macrostructured PDMS films and a closer look at the macrostructures under the confocal microscope. On both micro- and macro- scales, the structures produced were replicas of the master templates, well defined and distinct. The addition of the structures, especially larger ones, came at the expense of PDMS’s inherent transparency, an important property for the material. To quantify this change, light transmittance through the samples was explored and the results were displayed in Figure 10. Non-structured PDMS showed near perfect light transmittance from the infrared and all through the visible range. However, with added structuring to the polymer’s surface, light transmittance starts declining. %T goes from nearly 100% to about 90% when the sample is prepared through the TFF-PFE condition, while it reaches about 30% with PDMS prepared under the TRP-UPE condition. These results also emphasize how the transparency of PDMS can also be tuned based on the choice of substrate material.

[0042] The effective crawl area of CCFF was then explored. The total available substrate area for the solutions to crawl on in all conditions was 195 cm2Under the different conditions, we first explored the impact capillarity has on each by comparing the areas of the films obtained by first using one substrate only and letting the solution rest and cure over time, as compared to using CCFF with both substrates and a 0.25 mm thick separation. The single substrate test results, as shown in Figure 11 , produced smaller films from 1 ml of PDMS solution compared to the ones produced using CCFF, seen in Figure 5. Specifically, under SRP-PMA, SFF-PET, TRP-UPE and TFF-PFE conditions, single substrate tests showed areas of 42.98 cm2, 26.775 cm2, 36.76 cm2and 26.625 cm2respectively, whereas using CCFF showed a major increase in polymer film area for all the conditions, with 104.13 cm2, 163.84 cm2, 58.065 cm2and 96.545 cm2for SRP-PMA, SFF-PET, TRP-UPE and TFF-PFE respectively.

[0043] The CCFF process also exhibited the ability to transfer some of the substrate’s properties as the solution climbs before the final curing stage. The hydrophobicity of the polymeric films was tested against their respective substrates, shown in Figure 12. The contact angles of a water droplet onto the substrates’ surface were 70.07°, 106.17°, 113.86° and 85.2° for the SRP-PMA, SFF-PET, TRP-UPE and TFF- PFE conditions respectively, whereas the produced PDMS films through these conditions produced contact angles of 70.83°, 106.39°, 115.24° and 87.61 ° respectively. The contact angles are practically equal which shows that the PDMS hydrophobicity can be tuned by changing the substrate material.

[0044] The influence of the top substrate’s weight on CCFF was then explored. Additional plates were placed on top of the thin lightweight PET substrates to distribute a total mass of 230 g and equal the substrate weight of the SRP-PMA condition. Under equal weight conditions, the produced film areas were very similar with SFF-PET condition 116.685 cm2film compared to SRP-PMA’s 104.13 cm2area. However, the average thickness of the films varied greatly, with 0.364 ± 0.018 mm and 0.086 ± 0.024 mm obtained with SFF-PET and SRP-PMA respectively. That indicates that not only did the added weight push the sample to better replicate the thickness, but it also showed adecrease in thickness variation throughout the sample, as it was approximately 5% deviation in this case. This was an improvement compared to 16% deviation from mean thickness obtained using SFF-PET with its original weight. As for the SRP-PMA condition, adding separation on all four sides of the bottom substrate lowered the percent deviation from 32% to 15%, but it was scaling down the substrate size, ergo changing the substrate dimensions, that showed the most promise with employing a 48.75 cm2area substrate and 0.3 ml volume solution used showing a film with an average thickness of 0.384 ± 0.014 mm or an about 3.7% deviation from the mean thickness result.

[0045] To further examine the versatility of the CCFF technique, pressure sensors were produced using PDMS films obtained from the various CCFF substrate conditions. The PDMS sensors were exposed to varying loads to check their response to pressure, as translated from the capacitance change within the material as depicted in Figure 13(a). In terms of results, the films reacted similarly to loads, showing a proportional relationship between the force applied and the voltage response generated. SRP-PMA based sensor showed increasing peak-to-peak sensitivity values ranging from 0.05 V / cm2at 0.5 N to 0.2 V / cm2at 10 N, with similar sensitivities recorded with TRP-UPE, while the devices from films produced with flexible substrates registered sensitivities in the range of 0.03 to 0.17 V / cm2produced when 0.5 N and 10 N of force were applied respectively.

[0046] The viability of CCFF as a means to add a protective coating on top of a deposited film of different material was tested. Solar cells have been some of the most promising and well researched sustainable technologies of the last decade, and perovskite solar cells, to be specific, have shown the ability to come close to silicon cells in terms of efficiency, at a much lower cost of production due to their solution processability. However, the effect moisture has on the overall long-term stability of perovskite materials has proven to be a key issue hindering progress in this field and limiting their viability as the light absorber layer in photovoltaic applications. A cost- effective sustainable encapsulation method could complement the low production costs of making these cells, along with increasing their lifetime usage. Therefore, we employed CCFF to coat perovskite solar cells, adding a layer of environmental protection to thematerial. Afterwards, J-V measurements were obtained to shed light on the power conversion efficiencies of these cells on a daily basis while monitoring the change in PCE over a period of two weeks. As seen in Figure 13(b), the solar cell efficiencies of the control samples fell short in terms of stability, as one control failed after 4 days, while the other device showed a continuous downward trajectory throughout the 7 days of testing. On the other hand, encapsulated solar cells have demonstrated that they retain their efficiency quite well, nearly maintaining their initial value throughout the whole week, without showing any signs of gradual loss in their PCE integrity. This experiment proves that the coating introduced helped increase the stability of the devices by up to 4 times their initial stability. More importantly, it shows that a simple cost-effective technique can function as an excellent coating technique while enhancing the stability of solar devices.

[0047] CCFF was lastly tested as a way to reconnect the separated parts of a large PDMS film. Three pieces of PDMS were cut from the previously formed polymer film under SFF-PET conditions, shown in Figure 13(c)(i). 0.3 ml of fresh PDMS was poured in the middle between the separated films on a smooth PET sheet, then the top substrate was added. The original films acted as substrate separators in this case, dictating the range of expected thickness the newly produced film will target. Post curing, the three cut pieces were rejoined as one large film, as seen in Figure 13(c)(ii), with the thickness being well controlled as well, as the new film demonstrated an average thickness of 0.35 ± 0.024 mm, showing an overall 6% deviation in thickness, and achieving a very similar thickness to the original film’s 0.364 ± 0.018 mm.Conclusion:

[0048] The CCFF technique disclosed herein is a promising approach towards simple, low-cost and highly scalable manufacture of thin polymer films. This capillary driven film production technique has shown the ability to structure the film’s surface during the setting stage of the process. The material’s physical properties were also tuned based on the choice of substrate, with areas produced through CCFF more than doubling in size compared to traditional solution casting and producing films as large 163.84 cm2from 1 ml volume, while also allowing for properties such as light transmittance andhydrophobicity to be controlled. Through choice of certain parameters such as type of substrate and its weight, the film’s overall thickness and its distribution were controllable and the variability of the thickness throughout a large film was below 5% from the mean thickness value. The films produced through CCFF proved useful as dielectric material in capacitive sensors, with sensitivities reaching 0.2 V / cm2The techniques were also employed as a coating method to add a protective encapsulation layer to the surface of perovskite solar cells. This helped mitigate their degradation by maintaining their initial performance much longer than unencapsulated devices in both a controlled nitrogen environment and in humid air conditions. The range of applicability of this method has the opportunity to be beneficial in many fields, especially for producing thin biocompatible microstructured polymer films for different technologies.

[0049] While the above description provides examples of one or more apparatuses or methods, it will be appreciated that other apparatuses or methods may be within the scope of the accompanying claims.

Claims

CLAIMSWe claim:1 . A method of forming a polymer film, comprising: providing a substrate and a superstrata; providing a precursor polymer solution; flowing the precursor polymer solution by capillary action between the substrate and the superstrate; and curing the precursor polymer solution to form the polymer film.

2. The method of claim 1 , comprising disposing the precursor polymer solution onto the substrate, and positioning the superstrate so that the precursor polymer solution is intermediate of the substrate and the superstrate and omnidirectional flow occurs by the capillary action.

3. The method of claim 1 or 2, comprising flowing the precursor polymer solution horizontally between the substrate and the superstrate.

4. The method of any one of claims 1 to 3, comprising flowing the precursor polymer solution vertically between the substrate and the superstrate.

5. The method of any one of claims 1 to 4, comprising controlling a thickness of the polymer film by at least one of: selecting working surface areas of the substrate and the superstrate; and selecting a volume of the precursor polymer solution.

6. The method of any one of claims 1 to 5, comprising controlling a thickness of the polymer film by selecting a dimension between the substrate and the superstrate.

7. The method of any one of claims 1 to 6, comprising controlling a thickness of the polymer film by applying force onto the precursor polymer solution by the substrate and the superstrate.

8. The method of any one of claims 1 to 8, wherein the polymer film is formed of polydimethylsiloxane (PDMS).

9. The method of any one of claims 1 to 8, wherein at least one of the substrate and the superstrate is flexible and formed of polyethylene terephthalate (PET).

10. The method of any one of claims 1 to 9, wherein at least one of the substrate and the superstrate is rigid and formed of poly(methyl methacrylate) (PMMA).11 . The method of any one of claims 1 to 10, wherein at least one of the substrate and the superstrate is textured, rigid, and formed of ultra-high molecular weight polyethylene (UHMWPE).

12. The method of any one of claims 1 to 11 , wherein at least one of the substrate and the superstrate is textured, flexible, and formed of polytetrafluoroethylene (PTFE).

13. The method of any one of claims 1 to 12, wherein at least one of the substrate and the superstrate is formed of glass.

14. The method of any one of claims 1 to 13, wherein the superstrate comprises at least one of a monofilament-based fabric, a 3D printed stamp, and a metal electrode for forming a macrostructure in the polymer film.

15. The method of any one of claims 1 to 14, comprising flowing a second precursor polymer solution by capillary action between the polymer film and the superstrate, and curing the second precursor polymer solution to form a second polymer film layered onto the first polymer film.

16. The method of claim 15, comprising flowing a third precursor polymer solution by capillary action between the second polymer film and the superstrate, and curing the third precursor polymer solution to form a third polymer film layered onto the second polymer film.

17. The method of any one of claims 1 to 14, wherein the substrate comprises an electrical component, and the polymer film encapsulates at least a portion of the electrical component.

18. The method of claim 17, wherein the electrical component comprises a solar cell.

19. The method of any one of claims 1 to 14, comprising, prior to the step of flowing, arranging an existing polymer film onto the substrate, and the step of flowing comprises engaging the precursor polymer solution with the existing polymer film so that the polymer film is joined to the existing polymer film.

20. The method of any one of claims 1 to 19, comprising retrieving the polymer film from between the substrate and the superstrate.

21. The method of claim 20, wherein the step of retrieving comprises separating the substrate and the superstrate, and optionally flexing at least one of the substrate and the superstrate.

22. A polymer film formed according to the method of any one of claims 1 to 21 .

23. An apparatus and / or a method comprising any combination of one or more of the features described above and / or illustrated in the drawings.

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