Highly-functionalized ultrathin special polyester composite film and forming process thereof

Abstract

The invention provides a high-functionalization ultrathin special polyester composite film and a forming process thereof, and belongs to the technical field of functional polymer composite materials. According to the method, PET and cellulose acetate are blended and subjected to electrostatic spinning to prepare an ultrathin fiber felt, cellulose acetate is selectively removed through cellulase enzymolysis to form a through nano porous network, and then a surface-modified porous PET framework is placed in an acetobacter xylinum culture solution to grow bacterial cellulose nanofibers in situ. The preparation method comprises the following steps: preparing a three-dimensional gel network, interweaving with PET fibers to form the three-dimensional gel network, carrying out solvent gradient replacement to tert-butyl alcohol, freeze-drying, and finally depositing silicon dioxide on the surface by atomic layer deposition to obtain the chemically inert ultrathin composite film with a flat surface. The problems that in the prior art, due to the fact that interface defects and impurities are introduced through functional treatment, material uniformity and electrical reliability are reduced, and a conventional flexible polymer film is prone to creep shrinkage at high temperature are solved, and the flexible polymer film is suitable for being applied to substrate films of high-end electronic devices such as capacitors and semiconductors.

Description

Technical Field

[0001] This invention relates to the field of functional polymer composite materials technology, specifically to a highly functionalized ultrathin special polyester composite film and its molding process. Background Technology

[0002] Polyethylene terephthalate (PET) film has become an indispensable substrate material in flexible electronic devices, film capacitors, and specialty packaging due to its excellent mechanical properties, dimensional stability, insulation, and cost advantages. Especially in the fields of new energy vehicles, energy storage systems, and high-end consumer electronics, the performance of PET substrate film, as a carrier for aluminized films, metallized capacitor films, or functional coatings, directly determines the reliability, lifespan, and safety of the end products. Market requirements for substrate films are becoming increasingly stringent, demanding not only high purity, ultra-thinness, and excellent mechanical strength, but also extremely low dimensional fluctuations and long-term stability under extreme environments such as high temperature and high humidity to meet the extreme requirements of next-generation high-performance devices for substrate materials.

[0003] To meet the demands of specialized fields, PET films often require specific performance enhancements. For example, CN121064608A discloses a polyester film for blue adhesive tape and its preparation method. By compounding multiple components such as thermoplastic polyurethane, organically modified montmorillonite, and polyimide micropowder into a high-viscosity PET matrix, and utilizing high-shear melt blending and biaxial stretching processes, the dielectric strength, flexibility, and flame retardancy of the film are synergistically improved. However, this method is essentially a macroscopic composite of a multiphase system. The interfaces between a large number of heterogeneous fillers and the matrix may become electrical weaknesses, making it difficult to meet the extreme requirements of highly uniform, pure, and defect-free materials for applications such as capacitor films. Furthermore, the different thermal expansion behaviors of the components at high temperatures may exacerbate dimensional instability. In addition, applying surface functionalized coatings to PET films is another effective way to improve their performance. CN118652463A discloses a phosphorus-modified nitrile compound flame-retardant modified BOPET film and its preparation method. A mixed coating solution composed of diaminonitrile, dialdehyde, and phosphorus phenanthrene oxide is coated onto the surface of a pre-fabricated PET substrate. Subsequently, a precisely controlled multi-gradient heating process triggers the in-situ polymerization reaction of the components in the coating solution on the PET surface, generating a flame-retardant coating mainly composed of phosphorus phenanthrene nitrile macromolecular compounds. The advantage of this method is that it does not affect the bulk crystallization and properties of PET. However, the introduced coating and the PET substrate are mainly physically attached or have limited chemical bonding, posing challenges to interfacial adhesion and long-term stability. After thermal cycling or mechanical stress, there is a risk of coating peeling or performance degradation.

[0004] In summary, achieving intrinsic, three-dimensional nanoscale synergistic reinforcement of PET films without introducing heterogeneous interface contamination remains a significant challenge. Therefore, in pursuit of ultimate performance for high-end capacitor films and aluminized film substrates, there is an urgent need for an ultrathin functional composite film that can intrinsically, uniformly, and robustly reinforce materials at the three-dimensional nanoscale. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a highly functionalized ultrathin specialty polyester composite film and its molding process. This invention utilizes a sacrificial template method to prepare a porous PET film precursor via electrospinning, followed by plasma hydrophilization treatment and in-situ microbial synthesis, and then deposits silica on the surface. This constructs a highly functionalized PET film with an atomically flat, dense, and chemically inert surface on top of the robust porous structure of the composite film body. This solves the technical problems of PET film functionalization introducing interface defects and impurities that compromise material uniformity and electrical reliability; the difficulty in fabricating large-area, robust films from nanoporous materials; and the tendency of conventional flexible polymer films to creep, shrink, and exhibit poor dimensional stability at high temperatures. This material can be used as a base film for capacitor metallization films, flexible sensors, and wearable electronic devices.

[0006] This invention discloses a molding process for a high-functionality ultrathin special polyester composite film, the specific technical solution of which is as follows: Step 1: Polyethylene terephthalate and cellulose acetate are dissolved together in a mixed solvent to prepare a spinning solution for electrospinning. The prepared composite nanofiber felt is collected on a receiving device and then immersed in a cellulase solution for incubation under constant temperature conditions. After incubation, the fiber felt is removed and repeatedly washed with deionized water until the washing solution is clear. It is then dried to obtain a porous PET skeleton.

[0007] Step 2: Place the dried porous PET skeleton in an oxygen plasma treatment device, introduce oxygen and start the plasma treatment. Then transfer the skeleton to a liquid culture medium inoculated with Acetobacter xylinum and place it in a constant temperature incubator for incubation. After the incubation is completed, remove the skeleton that is impregnated with bacterial cellulose from the culture medium to obtain PET wet gel.

[0008] Step 3: The wet gel is sequentially immersed in ethanol aqueous solutions of increasing concentration for displacement. Each displacement is allowed to stand for a period of time. Then the gel is immersed in pure tert-butanol for displacement. Finally, the gel is rapidly frozen and transferred to a freeze dryer for drying.

[0009] Step 4: Place the dried composite porous membrane in the reaction chamber of the atomic layer deposition equipment, evacuate and heat to the set temperature, and sequentially pulse-introduce silicon source precursor and oxygen source precursor into the chamber. After each introduction, purge with inert gas and repeat the cycle multiple times. After the cycle is completed, cool to room temperature in the chamber and remove the film.

[0010] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. By combining sacrificial templates with biosynthesis, a fully interpenetrating three-dimensional nanonetwork of PET and bacterial nanocellulose was constructed, enabling the film to simultaneously possess high porosity, high specific surface area, and intrinsic high strength, high toughness, and high thermal dimensional stability derived from cellulose nanofiber reinforcement.

[0011] 2. Enzymatic hydrolysis, microbial synthesis, and tert-butanol freeze-drying are all relatively mild physical and biological processes, which minimize the damage to the fine structure caused by strong chemical or high-temperature treatments, and achieve a perfect transfer from wet gel to dry thin film nanostructure.

[0012] 3. Atomic layer deposition technology is used to impart a continuous, shape-preserving, and precisely controllable amorphous silicon dioxide layer to the film surface, providing an ideal surface that is atomically flat, chemically inert, insulating, and has good adhesion to metals, which can directly meet the stringent requirements of high-end electronic applications for substrates. Attached Figure Description

[0013] Appendix Figure 1 This is a process flow diagram for preparing the PET film of the present invention; Appendix Figure 2 The graph shows the breakdown field strength data of the example and comparative samples after different cycles. Appendix Figure 3 The graph shows the average breakdown field strength and Weibull parameter data for the examples and comparative samples; Appendix Figure 4 The figures show strain-temperature variation data for the examples and comparative samples. Appendix Figure 5 The graph shows the dielectric performance test data of the example and comparative samples at 1 kHz. Detailed Implementation

[0014] The following embodiments further explain and illustrate the technical solutions of the present invention. It should be specifically noted that each specific embodiment is a concretization and explanation of the technical solution and should not be considered as a limitation on the scope of protection of the present invention. Those skilled in the art still have the right to modify the technical solutions of these embodiments and make equivalent substitutions for some or all of the technical features, and these modifications or substitutions do not change the essence of the corresponding technical solutions, nor do they cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions described in the present invention.

[0015] This invention proposes a molding process for a high-functionality ultrathin special polyester composite film, the specific technical solution of which is as follows: 1. Sacrificial template electrospinning and enzymatic hydrolysis for pore formation A blended spinning solution of PET and cellulose acetate was prepared, and composite nanofiber mats were fabricated by electrospinning. The mats were then placed in a cellulase solution with a specific pH buffer and incubated at a constant temperature. Because cellulose acetate and PET are blended and solidified into fibers at the molecular chain scale, the cellulose acetate phase is uniformly dispersed at the nanoscale within the continuous PET phase. During subsequent enzymatic hydrolysis, cellulase specifically recognizes and hydrolyzes the glucosinolate bonds in the cellulose acetate molecular chains, catalyzing the breakage of these bonds and degrading the water-insoluble cellulose acetate polymer into water-soluble small-molecule sugars and acetic acid. This hydrolysis reaction proceeds gradually from the fiber surface inwards. The degradation products diffuse into the liquid phase through the concentration gradient and are removed. Due to the continuity of the initial distribution of cellulose acetate, its complete removal leaves not isolated cavities, but rather a winding, interconnected network of nanoscale pore channels extending from the fiber interior to between fibers. Since the formation of pores is achieved through a biocatalytic reaction, the reaction conditions are mild and only target the sacrificial template. This has no negative impact on the chemical structure, molecular weight, and crystallinity of the PET molecular chain. Thus, while obtaining high specific surface area and excellent pore connectivity, it avoids impurities, interface defects, or bulk damage introduced by physical pore-forming agent residues or strong chemical etching, ensuring the chemical purity, structural uniformity, and inherent mechanical strength of the porous framework material.

[0016] 2. Plasma hydrophilization and enhanced in-situ microbial synthesis A porous PET framework was treated with oxygen plasma and then immersed as a scaffold in a liquid culture medium inoculated with *Acetobacter xylinum*. Cultured under shaking conditions, a composite wet gel containing PET / bacterial cellulose was obtained. Through the physical bombardment and chemical reaction of high-energy reactive oxygen species on the PET fiber surface, chain scission and oxidation were induced, introducing a high density of oxygen-containing polar functional groups such as hydroxyl, carboxyl, and aldehyde groups onto the surface. This transformed the chemical properties of the PET surface from a low-energy state dominated by benzene rings and ester groups to a high-energy state rich in hydrophilic groups, significantly improving its surface free energy and wettability. Immersing the modified porous framework in the culture medium, its polar surface forms a strong hydration layer with water molecules, allowing the culture medium to completely wet the entire complex porous structure, providing the necessary liquid environment for subsequent biological processes. Furthermore, the polar groups on the modified PET surface can form hydrogen bonds, ionic bonds, and van der Waals forces with components such as lipopolysaccharides and proteins in the *Acetobacter xylinum* cell wall, significantly enhancing the initial adhesion strength and colonization density of the bacteria. This three-dimensional, interconnected porous network provides bacteria with a vast habitat surface and a uniform microenvironment. Colonizing bacteria utilize carbon sources to synthesize β-1,4-glucan chains via cellulase in the cell membrane, which are then secreted extracellularly through porins. Subsequently, the glucan chains self-assemble through hydrogen bonds to form microfilaments, which further aggregate into a three-dimensional nanofiber network. This newly formed bacterial cellulose grows in situ within the pores and surface of the PET framework, forming physical entanglements and interfacial hydrogen bonds with PET fibers, constructing a fully interpenetrating network structure from the molecular to the macroscopic level.

[0017] 3. Tert-butanol replacement and sublimation drying and shaping A composite wet gel containing a large amount of water was sequentially placed in ethanol-water solutions and pure tert-butanol of increasing concentrations for multiple displacements. Finally, the gel filled with tert-butanol was rapidly and deeply frozen, and then freeze-dried under high vacuum. Since ethanol has good miscibility with both water and tert-butanol, and its surface tension is intermediate between the two, using ethanol as an intermediate solvent for gradient displacement can gently reduce the surface tension of the solvent system, avoiding rapid shrinkage or collapse of the network due to drastic changes in solvent polarity. Furthermore, because the surface tension of tert-butanol is much lower than that of water, when water is completely replaced by tert-butanol, the liquid in the gel network voids becomes tert-butanol, significantly weakening the capillary force generated by the liquid. Subsequently, during freeze-drying, tert-butanol forms a solid. During the sublimation stage, the system maintains high vacuum and conditions below the triple pressure point of tert-butanol, providing a certain amount of heat, allowing the solid tert-butanol to sublimate directly from the lattice state to gaseous molecules and be removed without passing through the liquid phase. This sublimation process eliminates the capillary forces generated by the presence of liquid during the drying process, and avoids the tearing and compression damage to the pore structure caused by the growth of crystals from the outside to the inside. As a result, the PET / bacterial cellulose three-dimensional interpenetrating nanonetwork formed by the gel in the wet state can be completely preserved, and finally a dry porous composite membrane is formed.

[0018] 4. Surface smoothing and functionalization of atomic layer deposition The dried porous composite membrane was placed in an atomic layer deposition (ALD) chamber, and silicon and oxygen source precursors were sequentially introduced for deposition cycles. During pulse deposition of the silicon source precursor onto the composite membrane, its molecules diffused in the gas phase and penetrated into all the pores of the porous membrane, undergoing chemisorption reactions with active groups such as hydroxyl groups on PET and bacterial cellulose to form a monolayer saturated layer. Subsequently, an inert gas was used to purge unreacted precursors and byproducts. Then, pulse deposition was performed with the oxygen source precursor, reacting with the adsorbed monolayer to generate amorphous SiO2 and regenerate surface active sites. This constituted one deposition cycle. Only a one-atom-layer-thick film was deposited in each cycle, and the reaction occurred only on accessible surfaces. Therefore, the precursor could undergo a stable and uniform reaction through gas-phase diffusion. Multiple cycles allowed for conformal growth of the amorphous SiO2 film on the surfaces of PET and bacterial cellulose nanofibers. As deposition progresses, these uniformly coated SiO2 layers on the fiber surface gradually thicken, not only completely covering the nanoscale roughness of the fibers themselves, but also filling the gaps at the fiber intersections. Ultimately, a continuous, dense, and pinhole-free encapsulation layer is formed on all the inner and outer surfaces of the entire porous composite membrane. This amorphous SiO2 layer is firmly bonded to the porous composite membrane body through chemical bonding, which brings excellent chemical inertness, thermal stability, and electrical insulation to the composite membrane, while avoiding problems such as interface delamination or incomplete coating caused by traditional coating methods.

[0019] The following are some specific embodiments of the present invention, and Table 1 shows the raw material information used in the embodiments.

[0020] Table 1 Raw Material Information Table

[0021] Example 1 S1: Weigh 0.75g of PET granules and 0.25g of cellulose acetate and add them to a dry and clean glass bottle. Then add 9.0g of hexafluoroisopropanol and stir magnetically for 8 hours in a 35℃ water bath to make a 10wt% uniform spinning solution. Then inject the spinning solution into a 21G stainless steel needle with a +15kV voltage applied. Adjust the distance between the needle tip and the grounded aluminum foil receiving roller to 15cm. Perform electrospinning at an ambient temperature of 25℃ and a relative humidity of 40% for 6 hours to obtain PET / cellulose acetate composite nanofiber felt. Immerse the fiber felt in an acetate-sodium acetate buffer solution with pH=5.0 and add 0.1g of cellulase. Shake and incubate in a constant temperature water bath shaker at 50℃ and 80rpm for 48 hours. Then wash the fiber felt with deionized water until neutral and dry it in a vacuum drying oven at 60℃ for 12 hours to obtain a porous PET skeleton.

[0022] S2: The dried porous PET skeleton prepared in S1 was placed in the reaction chamber of an oxygen plasma processor and treated for 120 seconds at a power of 100W and an oxygen flow rate of 50cssm to make its surface hydrophilic. At the same time, HS liquid culture medium containing 2% glucose, 0.5% peptone, 0.5% yeast extract and 0.27% sodium dihydrogen phosphate was prepared and its pH was adjusted to 6.0. The culture medium was autoclaved at 121℃ for 20 minutes, cooled and inoculated with 5% xylobacterium xylose by volume. Then the plasma-treated PET skeleton was transferred to a conical flask containing 150mL of bacterial culture medium and incubated at 28℃ for 7 days to obtain a composite wet gel in which PET fibers and bacterial cellulose fibers are interwoven.

[0023] S3: The composite wet gel obtained in S2 was sequentially immersed in ethanol solutions with concentrations of 30%, 50%, 70%, 90%, and 100%, for 2 hours at each concentration gradient. Then, the gel was transferred to anhydrous tert-butanol and immersed for 12 hours. The gel filled with tert-butanol was placed at -45°C and frozen for 4 hours to solidify it. Then, the gel was placed in a refrigerated dryer and sublimated at a temperature of -80°C and a vacuum of 6.5 Pa. During this process, the temperature of the separator holding the gel was maintained at -20°C for 24 hours, which is the main stage of sublimation drying. Then, the temperature was raised to 0°C and maintained for 12 hours. Finally, the temperature was raised to 25°C and maintained for 12 hours to obtain a dry porous composite film.

[0024] S4: Cut the dried composite membrane into 2cm×2cm sizes and fix it on the sample stage of the atomic layer deposition equipment. Set the reaction chamber temperature to 120℃, use TDMAS as the silicon source precursor and deionized water as the oxygen source precursor, set the TDMAS pulse to 0.1s, purge with high-purity nitrogen for 10s, and the deionized water pulse to 0.05s, purge with high-purity nitrogen for 10s. This is one standard deposition cycle. Repeat the cycle 500 times to finally obtain an ultrathin PET / bacterial cellulose nanocomposite substrate membrane with SiO2 modified on the surface.

[0025] Example 2 The preparation method is the same as in Example 1, except that: S1: Weigh 0.9g of PET granules and 0.1g of cellulose acetate and add them to a dry and clean glass bottle. The enzymatic hydrolysis pH of cellulase is 4.8. The temperature of the constant temperature water bath shaker during enzymatic hydrolysis is 45℃, and the mixture is shaken and incubated for 36h. S2: The plasma processing power is 50W, the processing time is 60s, the glucose concentration in the culture medium is 1.5%, the temperature of the constant temperature incubator is set to 26℃, and the incubation time is 5 days; S3: The temperature for gel freeze-curing is -50℃, and the temperature of the separator where the sample is located during the main stage of sublimation drying is -25℃, and the time is 12h. S4: The reaction chamber temperature is set to 80℃, the TDMAS pulse time is 0.05s, the deionized water pulse time is 0.03s, the nitrogen purging time is 8s, the total number of cycles is 200, and the remaining steps are the same.

[0026] Example 3 The preparation method is the same as in Example 1, except that: S1: Weigh 0.6g of PET granules and 0.4g of cellulose acetate and add them to a dry and clean glass bottle. The enzymatic hydrolysis of cellulase is performed at pH 5.5. The temperature of the constant temperature water bath shaker is 55℃ during enzymatic hydrolysis, and the mixture is shaken and incubated for 72h. S2: The plasma processing power is 200W, the processing time is 300s, the glucose concentration in the culture medium is 3.0%, the temperature of the constant temperature incubator is set to 30℃, and the culture time is 10 days; S3: The temperature for gel freeze-curing is -20℃, the temperature of the separator where the sample is located during the main stage of sublimation drying is -5℃, and the maintenance time for the second and third stages is 24h. S4: The reaction chamber temperature is set to 150℃, the TDMAS pulse time is 0.2s, the deionized water pulse time is 0.15s, the nitrogen purging time is 20s, the total number of cycles is 1000, and the remaining steps are the same.

[0027] Example 4 The preparation method is the same as in Example 1, except that: S1: Weigh 0.8g of PET granules and 0.2g of cellulose acetate and add them to a dry and clean glass bottle. The enzymatic hydrolysis pH of cellulase is 5.2. The temperature of the constant temperature water bath shaker during enzymatic hydrolysis is 52℃, and the mixture is shaken and incubated for 60h. S2: The plasma processing power is 150W, the processing time is 200s, the glucose concentration in the culture medium is 2.5%, the temperature of the constant temperature incubator is set to 26℃, and the incubation time is 8 days; S3: The temperature for gel freeze-curing is -30℃, the temperature of the separator where the sample is located during the main stage of sublimation drying is -10℃, and the time is 20h. The second and third stages are both maintained for 20h. S4: The reaction chamber temperature is set to 140℃, the TDMAS pulse time is 0.15s, the deionized water pulse time is 0.1s, the nitrogen purging time is 16s, the total number of cycles is 800, and the remaining steps are the same.

[0028] Comparative Example 1 The preparation method is the same as in Example 1, except that: S1: Add 5wt% CaCO3 particles to the PET particle solution in hexafluoroisopropanol, mix and stir well, then electrospin to obtain PET / CaCO3 composite fiber felt, then immerse the fiber felt in 1mol / L dilute hydrochloric acid to remove CaCO3 particles, form a porous PET skeleton and dry it. S2: The bacterial nanocellulose hydrogel is broken up and added to water to disperse it evenly into a suspension. The dried porous PET skeleton is immersed in the suspension, stirred evenly, and then vacuum filtered to allow the bacterial nanofibers to attach to the surface and shallow pores of the PET skeleton. Then it is dried. The remaining steps are the same.

[0029] This comparative example prepares an ultrathin PET composite film by physical blending of pore-forming material and physical blending of added reinforcing fibers.

[0030] Comparative Example 2 The preparation method is the same as in Example 1, except that: S3: Place the composite wet gel in a -20℃ freezer for 12 hours to freeze solidify, then transfer it to a refrigerated dryer chamber with a vacuum of 10 Pa and a temperature of -20℃ for 36 hours of sublimation drying, then raise the temperature to 0℃ and maintain it for 12 hours. The remaining steps are the same.

[0031] This comparative example prepares a porous PET composite membrane without tert-butanol using a conventional freeze-drying method.

[0032] Comparative Example 3 S4: Cut the dried composite film into 2cm × 2cm pieces and fix them on the sample stage. Using a magnetron sputtering system with SiO2 as the target, evacuate the deposition chamber to 5.0 × 10⁻⁶. -4 Below Pa, argon gas was used as the sputtering gas, the working pressure was maintained at 1.0 Pa, 150 W of radio frequency power was applied, the deposition rate was set to 0.05 nm / s, and the deposition time was 600 s. In this way, a SiO2 layer was deposited on the surface of the porous membrane to obtain a PET composite film with SiO2 deposited on the surface. The remaining steps were the same.

[0033] This comparative example prepares an ultrathin PET composite film by depositing a SiO2 layer on the surface using PVD technology, which has poor shape retention.

[0034] Experimental Example 1 Thin film samples prepared in Examples 1-4 and Comparative Examples 1-3 were used. Five points were randomly selected on the surface of each sample using a thin film thickness gauge to measure the thickness and calculate the average value. A hemispherical stainless steel electrode with a diameter of 3 mm was selected. The sample was completely immersed in high-purity dimethyl silicone oil. During installation, the sample was placed flat between the two electrodes. A DC voltage was then applied from zero at a constant rate of 0.5 kV / s, while simultaneously monitoring the circuit current. When the current instantaneously exceeded the 1 mA threshold, the voltage value at that moment was recorded as the breakdown voltage. Subsequently, a new test point at least 5 mm away from the previous breakdown point or edge was selected on the same sample for the next test. This process was repeated eight times. Based on the recorded breakdown voltage and the average thickness of the sample, the breakdown field strength for each test was calculated using the formula:

[0035] Where Eb is the breakdown field strength of the sample, kV / mm; Ub is the breakdown voltage measured in the experiment, kV; and d is the thickness of the sample measured in mm. Subsequently, the corresponding Weibull scale parameter α and Weibull shape parameter β were simulated using software. The test results are shown in Table 2 and... Figure 2 , 3 As shown: Table 2. Breakdown strength test results of the examples and comparative samples.

[0036] From Table 2 and Figure 2 , 3As can be seen, the average breakdown field strength and Weibull shape parameter of the example sample are significantly higher than those of the comparative sample. This indicates that the example sample formed a complete and uniform PET / bacterial cellulose fully interpenetrating network through sufficient plasma treatment and biosynthesis, which greatly enhanced the mechanical integrity of the bulk. Moreover, the chemical and structural uniformity of the material is excellent, and the defect density is extremely low. In contrast, the comparative sample, due to the chemical etching to remove the pore-forming agent, attacked the PET molecular chains, leaving chemical damage and stress concentration points on the pore walls, which became the preferred channels for electrical breakdown. Furthermore, the added bacterial cellulose fibers and the PET skeleton were only physically adsorbed, with weak interfacial bonding and a large number of microscopic voids and defects, which led to a decrease in the insulation strength of the sample. At the same time, physical blending resulted in inconsistent pore size and shape, a loose skeleton, and therefore a higher average thickness. The distribution of the added fibers was also difficult to be uniform. These numerous randomly distributed interface defects and structural inhomogeneities cause breakdowns to occur at random weak points, resulting in extremely high data dispersion. In Comparative Example 2, the growth and sublimation of water ice crystals cause irreversible mechanical damage to the soft PET / bacterial cellulose nanonetwork, leading to pore collapse, network breakage, and the formation of a chaotic structure with both dense and fragile areas. This results in a reduction in average thickness. Furthermore, because this structural damage is random and uneven, the distribution of weak points in the material is completely irregular, leading to low breakdown strength and extremely dispersed data. In Comparative Example 3, the linear deposition characteristics of magnetron sputtering prevent it from covering the interior of the porous membrane and the back of the fiber. The resulting SiO2 coating is discontinuous and dense, with a large number of pores directly exposed. Moreover, the random and uncontrollable distribution of these defects makes random breakdowns highly likely, resulting in the greatest dispersion and significant damage to the insulation strength of the sample. The average thickness of the sample is also the greatest.

[0037] Experimental Example 2 The samples prepared in Examples 1-4 and Comparative Examples 1-3 were cut into strips of 10mm × 3mm. The samples were placed vertically on the quartz sample holder of the thermomechanical analyzer, with the bottom fixed and the top gently contacting the quartz probe of the instrument. The test temperature was set to 30℃~150℃, the heating rate was 5℃ / min, nitrogen was used as the protective gas, and a static force of 0.02N was applied to the sample. The test program was started, and continuous data on the change of sample length with temperature were recorded. Linear fitting was performed on this data curve, and the average linear coefficient of thermal expansion (CTE, ppm / ℃) of the sample was calculated. The test results are shown in Table 3 and... Figure 4 As shown.

[0038] Table 3. Test results of thermal expansion coefficients for the examples and comparative samples.

[0039] From Table 3 and Figure 4It can be seen that the samples in the examples all have low CTE values, and the thermal strain-temperature curves are all smooth, highly linear lines, indicating that the internal structure of the samples is highly uniform. No phase change, structural relaxation, or destructive stress release occurred during the heating process. This ensures that when the samples are used as capacitor base films, no destructive stress will occur between them and the metal electrodes due to thermal expansion mismatch, thus ensuring long-term interface stability and device lifespan. The thermal strain-temperature curve of Comparative Example 1 has a higher slope, indicating that physical blending and the addition of reinforcing fibers cannot effectively construct a reinforcing network. HCl removal of the pore-forming agent damages the PET matrix. The added bacterial cellulose fibers are only bonded to the PET matrix by weak forces such as van der Waals forces, resulting in low interface strength. This cannot effectively constrain the thermal movement of the PET chain segments, leading to significant dimensional changes at high temperatures and poor thermal matching with the metal electrodes, failing to meet the dimensional stability requirements of capacitor base films. In Comparative Example 2, the conventional freeze-drying process destroyed the PET / bacterial cellulose three-micro-nano interpenetrating network in the wet gel. Due to the destructive force generated by the growth and sublimation of water ice crystals, the bacterial cellulose reinforcing phase loses its structural continuity, thus losing the nano-reinforcing effect. As a result, the thermal expansion properties of the sample are close to those of pure PET material. This material with high thermal expansion properties cannot be used as a substrate film for electronic devices. The CTE value of the sample in Comparative Example 3 is relatively high, indicating that the discontinuous and non-conformal SiO2 coating formed by magnetron sputtering cannot cover the inside of the pores and the back of the fiber. Therefore, it cannot provide an all-round surface tension effect like a continuous and uniform ALD film. The thermal expansion of the sample is still dominated by the unconstrained PET / bacterial cellulose composite substrate.

[0040] Experimental Example 3 The samples from Examples 1-4 and Comparative Examples 1-3 were placed flat at the center of the lower electrode of the dielectric material testing fixture. The upper electrode was adjusted to ensure complete contact with the sample surface without bubbles or wrinkles. The upper electrode was a circle with a diameter of 6 mm. A precision LCR meter was connected, and the parameters were adjusted to a frequency of 1 kHz and a test signal voltage of 1 Vrms. The test mode was parallel capacitance-loss (Cp-D). After starting the measurement, the capacitance value and loss factor D were recorded, where the loss factor D is equal to the loss tangent tanδ. The relative permittivity was calculated using the formula:

[0041] Where, ε r C is the relative permittivity of the material; p The measured capacitance is F; d is the average thickness of the sample, m; A is the area of ​​the upper electrode, 2.827 × 10⁻⁶. -5 m; ε0 is the vacuum permittivity, 8.854 × 10⁻⁶. -12 F / m.

[0042] Table 4. Dielectric property test results of the examples and comparative samples

[0043] From Table 4 and Figure 5 As can be seen, the relative permittivity and dielectric loss of the sample in the examples are both low, indicating that no impurity ions were introduced during the enzymatic hydrolysis and biosynthesis process. The dense SiO2 deposition layer also effectively suppressed charge injection and leakage current, meeting the basic requirements for manufacturing low-loss, high-stability capacitor substrates. In contrast, the sample in Comparative Example 1, due to the use of HCl etching to create pores, introduced Cl... - Impurities such as those found in the sample damage the PET molecular chain, increasing leakage current. Furthermore, the weak bonding interface between the fiber and the substrate becomes an interfacial polarization center, generating additional polarization losses under an alternating electric field. This leads to reduced capacitor efficiency and increased temperature rise, affecting device performance and safety. In Comparative Example 2, ice crystals damage the structure, resulting in the coexistence of defect and dense regions. This uneven structure induces strong interfacial and space charge polarization, and the bound water not completely removed from the pores also increases electrical losses. In Comparative Example 3, the uneven SiO2 coating on the surface forms a micro-interface with the exposed substrate, generating significant interfacial polarization under an electric field, leading to increased losses. This demonstrates that magnetron sputtering surface treatment cannot be used as a method for treating capacitor substrate films to achieve true surface passivation and improved insulation performance.

Claims

1. A molding process for a high-functionality ultrathin special polyester composite film, comprising composite film preparation and surface leveling layer preparation, characterized in that: The composite membrane preparation consists of porous PET framework preparation, composite wet gel preparation, and drying membrane preparation. The porous PET framework preparation involves adding cellulose acetate as a sacrificial template to PET electrospinning raw materials for composite formation, followed by enzymatic hydrolysis to remove the cellulose acetate and form a porous PET framework. The composite wet gel preparation involves surface modification of the porous PET framework, followed by in-situ growth and deposition of bacteria on the fiber surface and inside the pores to form an intertwined three-dimensional network structure. The drying membrane preparation involves solvent replacement of the composite wet gel with polar solvents of gradient concentrations, followed by freeze sublimation to remove the solvent. The surface smoothing layer is prepared by atomic layer deposition on the inner and outer surfaces of the composite membrane.

2. The molding process of a high-functionality ultrathin special polyester composite film according to claim 1, characterized in that, It is formed through the following steps: S1: Dissolve PET and cellulose acetate together in a mixed solvent to prepare a spinning solution for electrospinning. Collect the prepared composite nanofiber felt on a receiving device, then immerse it in a cellulase solution and incubate it under constant temperature conditions. After incubation, remove the fiber felt, wash it repeatedly with deionized water until the washing solution is clear, and then dry it to obtain a porous PET skeleton. S2: The dried porous PET skeleton is surface modified, and then the skeleton is transferred to a liquid culture medium inoculated with bacteria and placed in a constant temperature incubator for incubation. After the incubation is completed, the skeleton wetted by bacterial cellulose is taken out from the culture medium to obtain PET wet gel. S3: The wet gel is sequentially immersed in ethanol aqueous solution of increasing concentration for displacement, and allowed to stand for a period of time after each displacement. Then the gel is immersed in pure tert-butanol for displacement. Finally, the gel is frozen solidified and transferred to a freeze dryer for sublimation drying. S4: Place the dried composite porous membrane in the reaction chamber of the atomic layer deposition equipment, evacuate and heat to the set temperature, and sequentially pulse the silicon source precursor and oxygen source precursor into the chamber. After each pulse, purge with inert gas and repeat the cycle multiple times. After the cycle is completed, cool to room temperature in the chamber and remove the membrane.

3. The molding process of a high-functionality ultrathin special polyester composite film according to claim 2, characterized in that: The mass ratio of PET to cellulose acetate in S1 is 6:4 to 9:1, the pH of the cellulase solution is 4.8 to 5.5, the incubation temperature is 45 to 55°C, and the incubation time is 36 to 72 hours.

4. The molding process of a high-functionality ultrathin special polyester composite film according to claim 2, characterized in that: The surface modification treatment described in S2 involves impacting the porous PET framework with low-temperature oxygen plasma to increase hydrophilicity and surface-active functional groups; the bacteria described are Acetobacter xylinum.

5. The molding process of a high-functionality ultrathin special polyester composite film according to claim 2, characterized in that: The oxygen plasma treatment in S2 has a power of 50~200W and a treatment time of 60~300s; the glucose content in the liquid culture medium is 1.5~3%; the temperature for cultivation in the constant temperature incubator is 26~30℃ and the cultivation time is 5~10 days.

6. The molding process of a high-functionality ultrathin special polyester composite film according to claim 2, characterized in that: The freeze-curing temperature in S3 is -20~-50℃, the temperature of the sample on the partition during the main sublimation drying stage is -25~-5℃, and the total sublimation drying time is 36~72h.

7. The molding process of a high-functionality ultrathin special polyester composite film according to claim 2, characterized in that: The silicon source precursor in S4 is tetra(dimethylamino)silane, the oxygen source precursor is deionized water, and the inert gas is nitrogen.

8. The molding process of a high-functionality ultrathin special polyester composite film according to claim 2, characterized in that: The atomic layer deposition temperature in S4 is 80~150℃, the pulse time of the silicon source precursor is 0.05~0.2s, the pulse time of the oxygen source precursor is 0.03~0.15s, the inert gas purging time is 8~20s, and the number of cycles is 200~1000.

9. The high-functionality ultra-thin special polyester composite film prepared by the molding process of the high-functionality ultra-thin special polyester composite film according to any one of claims 1 to 8, characterized in that: The average thickness of the composite membrane is <20 μm.

10. The high-functionality ultrathin special polyester composite film prepared by the molding process of the high-functionality ultrathin special polyester composite film according to any one of claims 1 to 8, characterized in that: The composite film has an average breakdown field strength >200kV / mm, a coefficient of thermal expansion <30ppm / ℃, a relative permittivity ≤3.5, and a loss tangent <0.01.

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