Preparation and properties of pore-filling membranes based on sulfonated copolyimides and porous polyimide matrix

The porous polyimide films were prepared by a wet phase inversion process. The influence of coagulating non-solvent on morphology, pore size and porosity of porous films was investigated. A series of pore-filling sulfonated polyimide (PFSPI) membranes, which derived from a homogenous spongy-like porous polyimide film as matrix filled with sulfonated copolyimides, were prepared and characterized. These PFSPI membranes exhibited excellent thermal stability with desulfonation temperature of 283–330 °C and good oxidative stability in Fenton's agent due to the protective effect of porous polyimide matrix on the sulfonic acid groups. The swelling of PFSPI membranes could be effectively suppressed by the porous matrix, which leads to the excellent dimensional stability and good water stability of membranes. The PFSPI membranes exhibited high proton conductivity at elevated temperature. All the PFSPI membranes displayed better permselectivity as compared with Nafion 115, which is attributed to their much lower methanol permeability.     

Porous polyimide films prepared by thermolysis of porogens with hyperbranched structure

Two hyperbranched molecules, benzenetricarboxylic acid dendritic benzyl ether ester (BTRC–BE) and benzenetricarboxylic acid polyethylene glycol ester (BTRC–PEG), were prepared and tested as pore‐generating agents (porogens) for the preparation of porous polyimide. The hyperbranched molecules were thermally stable during the imidization process and completely decomposed well below the degradation temperature of polyimides, indicating that they possessed desirable thermal decomposition characteristics as porogens for the porous polyimide. From the SEM analysis dispersed domains were observed in the poly(amic acid) films containing BTRC–BE, whereas no phase separation was observed in the poly(amic acid) films containing BTRC–PEG. This may be attributable to the different polarities of the porogens because the hydrophobic BTRC–BE was phase‐separated in the hydrophilic poly(amic acid) matrix but BTRC–PEG was well mixed with poly(amic acid). The morphology developed in the poly(amic acid) film was retained after imidization and decomposition of the porogens; thus only BTRC–BE produced the porous polyimide. As the content of BTRC–BE decreased from 20 to 5 wt %, the pore size decreased from 390 ± 100 to 90 ± 50 nm and the pore density also decreased. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1711–1718, 2004     

Synthesis and characterization of dense and porous cellulose films

Whereas cellulose‐derived polymers are routinely used as membrane materials, the cellulose polymer itself is not directly used to synthesize dense/porous films for membrane applications. Recently, N‐methylmorpholine N‐oxide (NMMO) and dimethylacetamide (DMAc)/lithium chloride (LiCl) have been successfully employed for dissolving unmodified cellulose. This provides a strong rationale for reexamining the possibility of cellulose membrane fabrication using these solvents. By judiciously selecting solvents, casting conditions, and solvent exchange steps, we successfully synthesized dense/asymmetric‐porous cellulose films. The pore size and porosity of the porous films decreased systematically with increasing cellulose concentration. SEM analysis of the cross sections revealed an asymmetric skinned structure with monotonically increasing pore size away from the skin. The measured pore diameters were in the range 1.8–4.8 μm. Mechanical testing indicated that the dense films possessed tensile properties comparable to those of cellulose acetate (CA) films. Though nitrogen permeability values were comparable for cellulose and CA dense films, cellulose film permeability depended upon the type of drying protocol employed. Overall, these results demonstrate that processability need not be a constraint in the use of cellulose polymer for membrane fabrication. In selected applications, cellulose membranes could become a cost‐effective, environmentally friendly alternative to other more commonly employed membrane polymers. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Porous biodegradable polyester blends of poly(L‐lactic acid) and poly(ε‐caprolactone): physical properties,morphology, and biodegradation

Poly(L ‐lactic acid) (PLLA), poly(ε‐caprolactone) (PCL), and their films without or blended with 50 wt% poly(ethylene glycol) (PEG) were prepared by solution casting. Porous films were obtained by water‐extraction of PEG from solution‐cast phase‐separated PLLA‐blend‐PCL‐blend‐PEG films. The effects of PLLA/PCL ratio on the morphology of the porous films and the effects of PLLA/PCL ratio and pores on the physical properties and biodegradability of the films were investigated. The pore size of the blend films decreased with increasing PLLA/PCL ratio. Polymer blending and pore formation gave biodegradable PLLA‐blend‐PCL materials with a wide variety of tensile properties with Young's modulus in the range of 0.07–1.4 GPa and elongation at break in the range 3–380%. Pore formation markedly increased the PLLA crystallinity of porous films, except for low PLLA/PCL ratio. Polymer blending as well as pore formation enhanced the enzymatic degradation of biodegradable polyester blends. Copyright © 2006 Society of Chemical Industry     

Macro and meso porous polymeric materials from miscible polysulfone/polyimide blends by chemical decomposition of polyimides

Novel macro and meso porous polysulfone materials were prepared from miscible blends of polysulfones with a phenylindane containing polyimide by selective chemical decomposition of the polyimide phase using a dilute hydrazine or tetraammonium hydroxide solution in methanol. It was found that the pore size of the material is affected by the backbone structure of the polysulfone selected. The compatibility between the polysulfones and the polyimide is influenced by the polysulfone structure. This in turn affects the pore size and the pore size distribution of the final porous material. Polyether sulfone was found to form most compatible blends that in turn leads to a porous material with the smallest pore size, a meso porous material. The meso porous polyether sulfones are transparent films, with uniform pore sizes in the range of 30 nm, while bisphenol A polysulfone based porous materials are opaque with pore sizes in the range of 200 nm.     

Thermal conductive performance of organosoluble polyimide/BN and polyimide/(BN + ALN) composite films fabricated by a solution‐cast method

Organosoluble polyimide (PI)/ceramic composite films with different BN or (BN + AlN) contents were under investigation for their thermal conductive performances. The chosen polyimide constituted by 4,4′‐oxydiphthalic dianhydride/2,2‐bis(3‐amino‐4‐hydroxyphenyl)hexafluor opropane could be dissolved and cast into thin films at room temperature. The commercially available BN and AlN fillers up to a volume ratio of 0.6 were added to the polyimide and their thermal conductive performances were measured. BN powders needed a surface precoating treatment to avoid sedimentation. The dense and flexible PI/BN composite films, after a drying treatment at 200°C, showed high thermal conductivity of 2.3 W/m·K⁻¹ at a BN volume ratio of 0.6, as compared with 0.13 W/m·K⁻¹ for pristine polyimide. However, in the case of PI/(BN + AlN) composite films, thermal conductive performance degraded because the films became highly porous at the higher AlN content. POLYM. COMPOS., 2013. © 2013 Society of Plastics Engineers     

Single Solvent‐Based Film Casting Method for the Production of Porous Polymer Films

A single solvent‐based film casting process for fabricating porous polymer films is developed in this study. The porous film is produced by mixing concentrated polylactic acid (PLA)/chloroform solution (20 wt%) and fresh chloroform solvent is followed by film casting. The average pore sizes of the films produced are seen to increase from 2.1 (±0.1) µm to 6.4 (±0.2) µm with increasing ratio of concentrated PLA solution and fresh solvent from 1:2 to 1:4. Functional groups of PLA after casting into porous film are confirmed via Fourier transform infrared spectroscopy analysis. Cytocompatibility studies (via Alamar Blue assessment) utilizing MG‐63 cells on the porous PLA films reveal an increase in cell metabolic activity up to 8 d postseeding. In addition, these direct cell culture studies show that the porous membranes support cell adhesion and growth not only on the surface but also through the porous structures of the membrane, highlighting the suitability of these porous films in tissue engineering applications.     

Porous biodegradable polyesters. II. Physical properties,morphology, and enzymatic and alkaline hydrolysis of porous poly(ϵ‐caprolactone) films

Porous poly(?‐caprolactone) (PCL) films were prepared by water extraction of poly(ethylene oxide) (PEO) from their solution‐cast phase‐separated blend films and the dependence of their blend ratio [X_PCL = PCL/(PEO + PCL)] and molecular weight of PEO on the porosity, pore size, crystallinity, crystalline thickness, mechanical properties, morphology, and enzymatic and alkaline hydrolysis of the porous PCL films were investigated. The film porosity or extracted weight ratio was in good agreement with the expected values, irrespective of X_PCL and molecular weight of PEO. The maximum pore size was larger for the porous films prepared using PEO having a lower molecular weight, compared with films prepared using PEO having a higher molecular weight at the same X_PCL. Differential scanning calorimetry of the porous PCL films revealed that their crystallinity and crystalline thickness were almost constant, regardless of X_PCL and molecular weight of PEO. The Young's modulus and tensile strength of the porous films decreased, whereas the elongation‐at‐break increased with decreasing X_PCL. The enzymatic and alkaline hydrolysis rates of the porous films increased with a decrease in X_PCL and an increase in the molecular weight of PEO. The porous PCL films having Young's modulus in the range of 2–24 kg/mm² and enzymatic hydrolysis rate in the range of one‐ to 20‐fold that of the nonporous PCL film could be prepared by altering X_PCL and the molecular weight of PEO. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 2281–2291, 2001     

Preparation of porous thin films of a partially aliphatic polyimide

A thermally labile polymer, poly(propylene glycol), was modified to obtain PPG having an amino end group. PPG was incorporated into a partially aliphatic polyimide based on an alicyclic dianhydride, and this afforded triblock copolymers containing various amounts of PPG blocks. The thermal properties of the copolymers were investigated by thermogravimetric analysis and differential scanning calorimetry. The thermal decomposition of the PPG block in the copolymers was carried out at 240°C under various pressures to obtain porous polyimide films. The pores remained during the thermolysis under a reduced pressure of 710 mmHg, whereas they collapsed under (near) atmospheric pressure. The pore size increased as the amount of the PPG block in the copolymers increased. The dielectric constants of the porous polyimides varied from 2.60 to 2.42 with the original copolymer composition. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 532–538, 2006     

Fabrication of porous polylactic acid films assisted by dip‐coating and template leaching techniques

A new simple method was developed to fabricate porous polylactic acid (PLA) films coated directly on substrates. The PLA films with a controllable thickness were realized using a dip‐coating technique. The pore structure of the resulting porous PLA films was tailored by utilizing phase inversion process and template leaching techniques. The experimental results show that the resulting PLA films became thicker when using the coating solution with a higher viscosity or applying a faster withdrawal speed. The porous structures (pore size and void density) of the resulting PLA films are significantly influenced by the polymer concentration, the nonsolvents, and the addition of poly(ethylene glycol) templates. The analyses of solubility parameters was utilized to explain the porous structures of the resulting PLA films in details. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Preparation of Nanoporous Polyimide Films from Poly(urethane‐imide) by Thermal Treatment

Nanoporous polyimide films were prepared in two steps. The first step is the preparation of poly(urethane‐imide) films by casting blend solutions containing various weight percentages of poly(amic acid) and phenol blocked polyurethane prepolymer (from 1,6‐hexamethylene diisocyanate and poly(ethylene glycol)). Three poly(amic acid)s were obtained from biphenyltetracarboxylic dianhydride (or) 2,2‐bis(3,4‐dicarboxyphenyl)hexafluoropropane dianhydride with 1,4‐phenylenediamine (or) 2,5‐dimethyl‐1,4‐phenylenediamine. Poly(urethane‐imide) films were characterized by density and surface energy measurements, AFM, DSC, TMA, mechanical properties and TGA. In the second step, these films were thermally treated above 300 °C to give nanoporous polyimide films. During thermal treatment, less thermally stable urethane domains decomposed, leaving porous polyimide films. The presence of pores was confirmed by scanning electron microscopy (SEM). The dielectric constant of the polyimide film was found to decrease with increasing amounts of urethane content.

A nanoporous polyimide film.     

Preparation of macroporous carbons from phase-inversion membranes

Preparation of porous carbons from phase-inversion membranes was investigated as a control method of pore structure in carbon materials. The structure in carbon films was estimated by means of electron microscopy, mercury porosimetry, and gas-adsorption methods. When phase-inversion membranes of Kapton-type polyimide were carbonized, they maintained the film form and gave macroporous carbon films having high porosity. However, micro- and mesopore structures in the carbon films were not influenced by phase inversion in the polymer stage, and, thus, the macroporous carbons had a molecular sieve property similar to that of carbons prepared from nonporous polyimide films. A macroporous structure in cellulose membranes was similarly maintained through the carbonization step, but some of these were fractured or deformed owing to the large shrinkage. Polymer membranes have a capability as porous carbon precursors if they satisfy two requirements: solid-state carbonization and relatively high carbon yield. A composite membrane of a macroporous carbon with a dense carbon having an impervious ability was readily produced by shaping at the precursor stage. © 1995 John Wiley & Sons, Inc.     

Synthesis and properties of low‐color polyimide/silica hybrid films

A series of polyimide precursors, poly(amic acid)s, containing propyltrimethoxysliane at two chain ends were prepared from 4,4′‐bis(4‐amino‐2‐trifluoromethylphenoxy)biphenyl ( I ) with six commercially available dianhydrides, followed by end‐capping with 3‐aminopropyltrimethoxysilane (APrTMOS). A new class of fluorine‐containing polyimide/silica composite films ( III ) with chemical bonds between the fluorinated polyimide backbone and the silica network has been synthesized from the APrTMOS‐terminated precursors with tetramethoxysilane via the sol‐gel process and thermal cyclodehydration. The resultant hybrid films were light‐colored, flexible, and tough. They had high levels of thermal stability associated with high glass‐transition temperatures (>251°C), 10% weight‐loss temperatures in excess of 527°C, and char yields at 800°C in nitrogen higher than 60%. For a comparative study, the analogous nonfluorinated polyimide/silica hybrid films ( III′ ), based on 4,4′‐bis(4‐aminophenoxy)biphenyl ( I′ ), and the neat fluorinated polyimide films ( IV ), based on diamine I , were also synthesized and characterized. The hybrid films of the fluorinated series III showed a higher transparency and less color intensity when compared with the nonfluorinated III′ analogs. They also revealed a lower refraction index and birefringence than pure polyimides ( IV ). © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 4046–4052, 2007     

Preparation of polyimide/siloxane‐functionalized graphene oxide composite films with high mechanical properties and thermal stability via in situ polymerization

An effective approach to prepare polyimide/siloxane‐functionalized graphene oxide composite films is reported. The siloxane‐functionalized graphene oxide was obtained by treating graphene oxide (GO) with 1,3‐bis(3‐aminopropyl)‐1,1,3,3‐tetra‐methyldisiloxane (DSX) to obtain DSX‐GO nanosheets, which provided a starting platform for in situ fabrication of the composites by grafting polyimide (PI) chains at the reactive sites of functional DSX‐GO nanosheets. DSX‐GO bonded with the PI matrix through amide linkage to form PI‐DSX‐GO films, in which DSX‐GO exhibited excellent dispersibility and compatibility. It is demonstrated that the obvious reinforcing effect of GO to PI in mechanical properties and thermal stability for PI‐DSX‐GO is obtained. The tensile strength of a composite film containing 1.0 wt% DSX‐GO was 2.8 times greater than that of neat PI films, and Young's modulus was 6.3 times than that of neat PI films. Furthermore, the decomposition temperature of the composite for 5% weight loss was approximately 30 °C higher than that of neat PI films. © 2015 Society of Chemical Industry     

Preparation of multi‐level honeycomb‐structured porous films by control of spraying atomized water droplets

Honeycomb‐structured porous films have been widely applied in various industrial areas such as chemical sensors, tissue engineering, and micro reactors. In this article, one novel self‐assembly approach is proposed to fabricate well‐ordered polyphenylene oxide honeycomb films by a facile control of spraying ultrasonic humidifier atomized water droplets. Proper spraying retention time is necessary for porous films formation with highly uniform pore size. The effect of atomized water droplets flux on the pore size and the regularity of the hexagonal arrays were experimentally investigated. The pore size became larger with increasing the solution concentration. Especially, honeycomb films with two‐level pores were fabricated by spraying atomized water droplets two times and the influence of interval time on the two‐level honeycomb films formation was investigated. Apart from analysis of structural characteristics, self‐assembly mechanism was also discussed. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41163.     

Residual stress and mechanical properties of polyimide thin films

Four different structure polyimide thin films based on 1,4‐phenylene diamine (PDA) and 4,4′‐oxydianiline (ODA) were synthesized by using two different dianhydrides, pyromellitic dianhydride (PMDA) and 3,3′,4,4′‐biphenyltetracarboxylic dianhydride (BPDA), and their residual stress behavior and mechanical properties were investigated by using a thin film stress analyzer and nanoindentation method. The residual stress behavior and mechanical properties were correlated to the morphological structure in polyimide films. The morphological structure of polyimide thin films was characterized by X‐ray diffraction patterns and refractive indices. The residual stress was in the range of ?5 to 38 MPa and increased in the following order: PMDA‐PDA < BPDA‐PDA < PMDA‐ODA < BPDA‐ODA. The hardness of the polyimide films increased in the following order: PMDA‐ODA < BPDA‐ODA < PMDA‐PDA < BPDA‐PDA. The PDA‐based polyimide films showed relatively lower residual stress and higher hardness than the corresponding ODA‐based polyimide films. The in‐plane orientation and molecularly ordered phase were enhanced with the increasing order as follows: PMDA‐ODA < BPDA‐ODA < BPDA‐PDA ～ PMDA‐PDA. The PDA‐based polyimides, having a rigid structure, showed relatively better‐developed morphological structure than the corresponding ODA‐based polyimides. The residual stress behavior and mechanical properties were correlated to the morphological structure in polyimide films. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Synthesis and properties of ultralow dielectric constant porous polyimide films containing trifluoromethyl groups

In this research, a series of porous copolyimide (co‐PI) films containing trifluoromethyl group (CF₃) were facilely prepared via a phase separation process. The co‐PI were synthesized by the reaction of benzophenone‐3,3′,4,4′‐tetracarboxylic dianhydride (BTDA) with two diamines of 4,4′‐diaminodiphenyl ether (ODA) and 3‐trifluoromethyl‐4,4'‐diaminodiphenyl ether (FODA) with various molar ratios. The flexible and tough porous co‐PI films with about 300 μm thickness and 8～10 μm average diameter could be obtained by solution casting conveniently. The thermal properties of the obtained porous co‐PI films were excellent with a glass transition temperature at 270 °C ～ 280 °C and only 5% weight loss in temperature from 530 °C to 560 °C under nitrogen atmosphere. In addition, the dielectric and hydrophobic properties of porous co‐PI films were remarkably improved owing to the presence of trifluoromethyl groups (CF₃) in the polymer chains. Moreover, our synthesized porous co‐PI films also showed good mechanical properties. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44494.     

Correlation of residual stress and adhesion on copper by the effect of chemical structure of polyimides for copper‐clad laminates

BACKROUND: Polyimide films coated on copper are a potential new substrate for fabricating printed circuit boards; however, adhesion between the copper and polyimide films is often poor. The relations between residual stress and adhesion strength according to the development of molecular orientation of polyimide films with different chemical backbone structure coated on copper were studied. RESULTS: The effect of chemical structures on properties including the residual stress and the adhesion strength were widely investigated for four different polyimides. Diamine 4,4′‐oxydianiline (ODA) and dianhydrides 1,2,4,5‐benzenetetracarboxylic dianhydride (PMDA), 4′‐(hexafluoroisopropylidene)diphthalic anhydride (6FDA), 4,4′‐oxydiphthalic anhydride (ODPA) and 3,3′,4,4′‐benzophenone tetracarboxylic dianhydride (BTDA) were used to synthesize polyimide. In an attempt to quantify the interaction of thermal mismatch with the polyimide films depending on various structures, residual stress experiments between polyimide film and Cu? Si wafer were carried out over a range of 25–400 °C using in situ thin film stress analysis. A universal test machine was used to conduct 180° peel test (ASTM D903‐98) of polyimide film from cooper foil. The residual stress on Cu? Si (100) wafer decreased in the order 6FDA‐ODA > BTDA‐ODA > ODPA‐ODA > PMDA‐ODA, and the interfacial adhesion strength decreased in the order BTDA‐ODA (5 N mm^?2) > ODPA‐ODA > PMDA‐ODA > 6FDA‐ODA. The results may suggest that the morphological structure, degree of crystallinity of chain orientation and packing significantly relate to the residual stress and adhesion strength in polyimide films. Wide‐angle X‐ray diffraction was used for characterizing the molecular order and orientation and X‐ray photoelectron spectroscopy was used for the analysis of components on copper after polyimide films were detached to confirm the existence of copper oxide chemical bonding and to measure the binding energy of elements on the copper surface. CONCLUSION: In this research, it is demonstrated that BTDA‐ODA polyimide has a low residual stress to copper, good adhesion property, good thermal property and low dielectric constant. Therefore, BTDA‐ODA would be expected to be a promising candidate for a two‐layer copper‐clad laminate. Copyright © 2007 Society of Chemical Industry     

Fabrication of three‐dimensionally ordered microporous membrane by wet phase separation

We report a novel porous fluorinated polyimide membrane with a cylinder structure fabricated by a wet phase inversion process, which is formed by a ternary system, polyimide/solvent/water. The porous polyimide membranes consisted of a thin top porous layer and three‐dimensionally ordered cylinder micropores. The porous membrane‐forming solvents were N‐methylpyrrolidone containing nonsolvent additives such as alcohol, and the height and width of the cylinder structure were controlled by the solvents. Water fluxes through the porous polyimide membranes were measured using a stirred dead‐end filtration cell, and the fluxes of the porous membrane with the cylinder‐type structure were approximately three times greater than those of the membrane with the finger‐type structure. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3016–3021, 2004     

Water sorption behaviors of the BPDA‐based polyimide films depending upon the structural isomers of diamine

For the biphenyltetracarboxylic dianhydride (BPDA)‐based polyimide thin films, the water sorption behaviors were gravimetrically investigated by using a thin film diffusion analyzer. The water sorption behaviors of the polyimide thin films are quite different and strongly dependent upon the sort of polyimide. The diffusion coefficients of the polyimide thin films vary in the range of 1.6 × 10⁻¹⁰ to 12.4 × 10⁻¹⁰cm²/s and the water uptakes vary from 1.52 to 5.25 wt %. Both the diffusion coefficient and water uptake of the polyimide thin films are in the increasing order: BPDA‐pPDA < BPDA‐p,p′ODA < BPDA‐p,m′ODA < BPDA‐mPDA ∼ BPDA‐p,p′DDS < BPDA‐m,m′DDS. Specifically, the polyimide films with para‐oriented linkages in backbone structure showed relatively lower diffusion coefficient and water uptake than the corresponding polyimide films with meta‐oriented linkages because of the well‐developed crystalline structure and good intermolecular chain ordering. In addition, the polyimide thin films having higher chain order showed relatively lower diffusion coefficient and water uptake. The crystallinity and intermolecular chain ordering in the morphological structure are critical parameters in controlling the water sorption behaviors of the polyimide thin films. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2121–2127, 2001