Study of the properties and structure of fluoropolymer films after space exposure on the Mir space station

Exposure in low Earth orbits (LEO) leads to alteration in properties of both outer and inner (screened) fluoropolymer films F4-MB and FEP-100A. Space environment factors have an effect primarily on the open surface of outer films. The volume and mass of the FEP-100A films accessible to the direct impact of space environment factors underwent nonequivalent changes, indicating an increase in the average density of the polymer. The surface of these films showed contact-angle anisotropy, thereby indicating the formation of spatially oriented structures on the surface. The study of the surface properties of the films revealed that the surface tension and the work of adhesion increased and both outer and (to a lesser extent) inner films became more hydrophilic after exposure. The hydrophilization process prevalently occurred during the first 28 months of exposure in LEO.     

Study of one-side aluminized polyimide films exposed on the Mir orbital space station

During exposure on the Mir space station, external polyimide films aluminized on one side substantially lost mass and mechanical strength and their outer surfaces became hydrophilic. The surface tension and polar plots of brightness of the exposed films acquired an anisotropic character. Spatially-oriented nano- and microsized entities, whose size and shape depend on the time of exposure, were detected by the scanning electron and atomic-force microscopy techniques on the outer surfaces of external films. The orientation axes of drops on the surface of the films, polar plots of brightness, and spatially oriented structures coincided with the direction of the station movement. It was assumed that one of the main factors of outer space responsible for mass loss and the formation of the anisotropic nano- and microsized structures is the impact of a flux of oxygen atoms onto the surface of the external film. The efficiency of the reaction of atomic oxygen with polyimides was estimated. A possible (photoelectron) mechanism of the influence of the metal on polyimide degradation is discussed.     

Characterization of polyimide films exposed at the mir orbital space station

Polyimide films remained unchanged upon an exposure at the Mir orbital space station under quartz plates, which protected them from bombardment by atomic oxygen with an energy of ～5 eV and transmitted solar radiation with λ ≥ 200 nm. The films underwent considerable degradation under the simultaneous action of atomic oxygen and solar radiation. The large weight loss and the mechanical destruction of one-sidedly aluminized polyimide films were caused by the participation of an aluminum coating in the polyimide destruction process. The accelerated degradation of the aluminum-polyimide composition can be explained by the photoeffect mechanism, the scavenging of photoelectrons emitted from aluminum by imide and benzene groups, and the formation and photochemical conversion of light-sensitive centers.     

Characterization of polymer materials exposed at the Mir orbital space station

The results of studies of polyimide and fluoropolymer films after a prolonged exposure at the Mir orbital space station are reported. The weight loss of external polyimide films was ～40–60%; the weight of FEP-100A fluoropolymer films remained unchanged, and the weight of F-4MB films increased by ～50%. The external and internal surfaces of polyimide films were hydrophilized. The external surface of a polyimide film acquired anisotropic properties, which manifested themselves as the shape anisotropy of liquid drops, the surface tension and the work of adhesion, the light scattering circular diagrams, and in the formation of spatially oriented fractal structures. The appearance of anisotropic properties is a consequence of the arrangement of a film on the station surface at an angle to the flow of atomic oxygen. The possible reactions of fast and scattered oxygen atoms, which are responsible for the chemical and structural transformations of polyimide, are discussed.     

Polymerization,structure and thermal properties of ODPA-DMB polyimide films

A new high molecular weight polyimide based on 4,4-oxidiphthalic anhydride (ODPA) dianhydride and 2,2-dimethyl-4,4-diaminobiphenyl (DMB) diamine has been synthesizedvia a one-step polymerization method. This polyimide is soluble in phenolic solvents. Films from 7 to 30 m thick were cast from the polymer solution and show in-plane orientation on a molecular scale detected by Fourier transform infrared spectroscopy experiments. This anisotropic structure leads to anisotropic optical properties arising from two different refractive indices along the inplane and out-of-plane directions. ODPA DMB possesses high thermal and thermo-oxidative stability. The glass transition temperature has been determined to be 298 °C. Dynamic mechanical analyses show two relaxation processes appearing above room temperature: the - and the -relaxation processes. The -relaxation corresponds to the glass transition while the -relaxation is a secondary relaxation process associated with the non-cooperative subsegmental motion.Dedicated to Professor Bernhard Wunderlich on the occasion of his 65th birthdayYHK acknowledges the support from the Yonam Foundation, Korea. This work was also supported by the Center of Molecular and Microstructure of Composites (CMMC) of NSF/EPIC/Industry, SZDC gratefully acknowledges the support from his PYI Award (DMR-9157738) from the National Science Foundation.     

Interfacial structure and properties in polystyrene/epoxy bilayer films

The interfacial structure and properties of immiscible deuterated polystyrene (dPS)/epoxy bilayer films were investigated with neutron reflectivity as functions of the composition of the epoxy layer, the thickness of the dPS layer, and the annealing time. We have found that the interfacial width and its growth rate depend strongly on the compositions of the epoxy layer but only weakly on the thickness of the dPS layer. The effect of the resin/crosslinker composition on the interfacial width and its growth rate is likely due to the different near‐surface structures that result for different epoxy stoichiometries. For an ultra‐thin dPS film (thickness = 2R_g), the data suggest a slight suppression of the growth of the interfacial width that could be due to confinement effects for the long‐chain molecules such as have been previously reported for a thickness of less than approximately 4R_g, where R_g is the radius of gyration of polymer molecules. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2653–2660, 2002     

Surface structure of semicrystalline polyimide films

The surface structure of thin films based on poly[4,4′-bis(4″-N-phenoxy)diphenyl]amic acid of 1,3-bis(3′,4-dicarboxyphenoxy)benzene and the product of its thermal imidization—a semicrystalline polyimide—poly[4,4′-bis(4″-N-phenoxy)diphenyl]imide of 1,3-bis(3′,4-dicarboxyphenoxy)benzene—at various stges of thermal imidization and after melting and subsequent annealing has been studied by methods of transmission, scanning electron, and atomic force microscopies. The topological structure of the film surface has been described in terms of the discrete cluster model. Under heating to 200 and 280°C, a continuous network of the infinite cluster appears; subsequent annealing leads to disintegration of the network to discrete fragments that practically correspond to clusters in the starting poly(amic acid) film. The polyimide film heated to 280°C crystallizes in the form of needle crystals stable to the argon plasma. The surface morphology of polyimide films recrystallized from melt is of the spherulite character.     

Study of the interfacial structure in epoxy bilayer films using neutron reflection

The interfacial structure in thin epoxy bilayer films was investigated with neutron reflection. For each experiment, a mixture of crosslinker and deuterated resin was spun onto a chemically similar, fully cured, protonated epoxy film. The reflectivity measurements were performed before and after curing the top epoxy film. We focused on the extent of penetration of the components of the top layer into the network of the bottom layer. The effect of the cure temperature of each layer was examined. In addition, the effect of the initial molecular weight of the oligomers in the top layer was probed by the partial curing of the mixture before spinning. As deposited, the components of the top layer penetrated the bottom layer to an extent that was largely independent of the aforementioned factors. The principal observation was that an additional penetration occurred with curing. This additional penetration was dependent upon both the molecular weight of the top layer and the cure temperature of the top layer relative to the glass‐transition temperature of the bottom layer. A decrease in the thickness of the top layer with curing was also observed, which likely indicates some evaporation of oligomers. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1709–1718, 2002     

Preparation, structure and properties of porous polyimide films via PAA/PU alloy

A new route to porous polyimide (PI) films with pore sizes in the nanometer regime was developed. A polyamic acid (PAA)/polyurethane (PU) blend with PU as the disperse phase was first prepared via in situ polymerization of pyromellitic dianhydride and 4,4-oxydianiline in PU solutions. Porous PI films were obtained from PAA/PU films by thermolysis of PU at 360°C and imidization of PAA at 300°C, respectively. Fourier transform infrared spectroscopy and thermal gravimetric analysis were used to detect the imidization and thermolysis processes of PAA/PU blends under thermal treatment. The microporous structure of the PI films was observed by transmission electron microscopy. It was found that the size and content of pores increased with an increase in the PU mass fraction in the PAA/PU blend up to 20%. Because of the existence of nanopores, the dielectric constant of PI films decreased by a wide margin and was less than 2.0 at a PU mass fraction of 20%. It implies that this is an effective means to reduce the dielectric constant of PI, but it also causes the decrease of tensile strength and the rise of water absorption. Translated from Chemistry Journal of Chinese Universities 2006, 27(1): (in Chinese)     

Gas transport properties of annealed polyimide films

Sorption and permeation of CO₂ in various annealed polyimide (PI) films were investigated. Dual-mode sorption and partial immobilization models were used to analyze the data. Sorption of CO₂ in PI film quenched from above the glass transition temperature (T_g) is greater than in film as received. In fact, sorption is decreased over the entire pressure range by cooling the film slowly. These changes in sorption of CO₂ can be attributed to a change in the Langmuir sorption capacity C′_H by annealing, since the other dual-mode sorption parameters, k_D and b, are almost independent of annealing. The value of C′_H is increased by quenching, and decreased by slow cooling from above T_g. The two diffusion coefficients D_D and D_H according to the Henry and Langmuir modes, respectively, for CO₂ also depend markedly on annealing. Diffusion coefficients of quenched PI films are increased and those of film cooled slowly are decreased compared with values for PI film as received. The change in D_H is larger than that in D_D. The permeability coefficient of quenched PI films at 100 cmH_g is about 1.7 times that of PI film as received. The film structure formed by quenching can enhance permselectivity.     

Effects of monomer structure and imidization degree on mechanical properties and viscoelastic behavior of thermoplastic polyimide films

Two thermoplastic polyimides based on a common diamine (3,4′-ODA) were synthesized using different dianhydrides, namely ODPA and BPDA by a two step method. Molecular weight was controlled by using PA as an end capping agent. Effects of imidization degree on the mechanical properties and viscoelastic behavior of thermoplastic polyimide films were investigated. Film samples with varying degrees of imidization were characterized using FTIR, DMTA and tensile properties testing. It was found that two polyimides have different rates of imidization because of difference in monomer reactivity and molecular structure. It was observed that with an increase in imidization degree there was a decrease in thermoplastic response and a change in viscoelastic behavior from liquid-like to solid-like. With increase in imidization degree the tensile modulus and tensile strength of the films were increased, whereas elongation at break and tensile breaking energy were found to decrease after a certain imidization temperature.     

Effect of single-walled carbon nanotubes and carbon nanofibers on the structure and mechanical properties of thermoplastic polyimide matrix films

The effects of additives of single-walled carbon nanotubes prepared via electric-arc synthesis and carbon nanofibers produced via gas-phase synthesis on the crystallization capacities and mechanical and electric properties of composite films of a thermoplastic polyimide (PI) matrix based on 1,3-bis-(3,3′, 4,4′-dicarboxyphenoxy)benzene and 4,4′-bis-(4-aminophenoxy)biphenyl after their uniaxial drawing and additional annealing are studied. The use of these fillers induces the heterogeneous nucleation of a crystalline phase of PI on the nanoparticle surface. A higher specific interface area in the case of addition of carbon nanotubes relative to that of carbon nanofibers leads to the formation of the crystalline structure of PI with a small crystallite size and high imperfection. Uniaxial drawing leads to the formation of a supermolecular structure that is optimum for crystallization during additional annealing and removes the kinetic hindrances to crystal growth. The properties of these fillers have a significant effect on the orientation of the nanoparticles and the matrix macromolecules during the uniaxial drawing of the films, which is accompanied by an increase in the elastic modulus with an increase in the draw ratio and the ability of the composite films to undergo orientational crystallization during additional annealing.     

Bonding structure and mechanical properties of carbon nitride bilayer films with Ti and TiN interlayer

Carbon nitride (CN_x) bilayer films with Ti and TiN interlayer were synthesized by cathode arc technique at various nitrogen pressures (P_N2). The dependences of microstructure and bonding composition of the films on the P_N2 and interlayer were analyzed by Raman spectroscopy and X‐ray photoelectron spectroscopy. Microstructure evolution consisting of the ordering and size of Csp² clusters, the faction of N–sp³/N–sp² bonds and graphite‐like/pyridine‐like configurations was dominated by P_N2, interlayer and annealing. The results showed that Ti and TiN interlayer decrease the atomic ratio of N/C and increase clustering Csp². High P_N2 induces the formation of C ≡ N and C ? N bonds, the increase of sp²‐bonding content and the growth of Csp² clusters. A large part of nitrogen atoms are coordinated with sp²‐hybridized carbon (minimum 71% for annealed CN_x monolayer). TiN/CN_x bilayer had a higher content of pyridine‐like configuration. Morphological characteristics of CN_x monolayer and bilayer mainly depend on the surface character (roughness and surface energy) of the sublayer. The internal stress in the as‐deposited Ti/CN_x bilayer is smaller, but it after annealing is higher than that of CN_x monolayer and TiN/CN_x bilayer. These results may be of interest for studying the CN_x films with controlled bonding composition and expected engineering properties. Copyright © 2014 John Wiley & Sons, Ltd.     

Microstructure and properties of organosoluble polyimide/silica hybrid films

Organosoluble polyimide/silica hybrid materials were prepared via the sol-gel process and their pervaporation properties were studied. The organosoluble polyimide (PI) was based on 4,4′-oxydiphthlic dianhydride (ODPA) and 4,4′-diamino-3,3′-dimethyldiphenylmethane (DMMDA). The surface chemical structure of polyimide/silica films was analyzed by Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS) and the results show that the completely hydrolysis of alkoxy groups of precursors and formation of the three-dimensional Si-O-Si network in the hybrid films. The morphology and the silica domain thus obtained were studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively. The silica particle size in the hybrid is in the range of 40-100 nm for the hybrid films when the amount of silica is less than 20 wt%. The strength and the modulus of the hybrid films are improved and the mechanical properties were found to be strongly dependent on the density of the crosslink. The glass transition temperature (T_g) of the hybrid films was determined by dynamic mechanical analysis (DMA) and the value increased 15-20 °C as the silica content increased. Furthermore, the pervaporation performances of the prepared hybrid films were also investigated for the ethanol/water mixtures at different temperature.     

Influence of curing temperature on the optical properties of fluorinated polyimide thin films

The influence of curing temperature (CT) on the optical properties of 6FDA/ODA poly(amic acid)-polyimide (PAA-PI) films was characterized by measuring ATR-FT-IR spectra, refractive index (RI) and birefringence of the films. The results showed that the infrared absorption intensity of characteristic peaks (IAICP) corresponding to the imide ring and the RI of PAA-PI films reached their maxima when the films had been cured at 270 °C, while the magnitude of birefringence (|Δn|) of the films reached its minimum as CT rose up to 330 °C. However, the RI decreased as CT was between 270 °C and 330 °C. Both the RI and |Δn| of the film increased obviously when CT increased after 330 °C. We think this is due to the interchain crosslink reaction (ICCR) above 330 °C and can be an evident proof of ICCR. And the evidences supporting ICCR was also discussed via IR differential spectra.     

海藻酸钠-硫酸软骨素共混膜的结构及性能研究

利用溶液共混法成功制备了新型生物膜材料-硫酸软骨素共混膜,通过红外光谱、X-射线衍射、原子吸收光谱和扫描电镜对共混膜的结构进行了表征,并测定了不同配比共混膜的抗张强度、断裂伸长率,吸水率,同时考察了介质pH值和离子强度对共混膜吸水率的影响。结果表明:共混膜中海藻酸钠、软骨素之间具有较强的相互作用和良好的相容性,共混膜具有良好的力学性能。作为一种潜在的生物材料可望在生物医学领域得到应用。     

Synthesis and properties of ultralow dielectric porous polyimide films containing adamantane

A series of novel ultralow dielectric porous polyimide (PI) films containing adamantane groups was prepared via the thermolysis of polyethylene glycol (PEG) oligomers mixed into PI matrix. Scanning electron microscopy results indicated that the porous PI films showed closed pores with an average diameter of 120 ± 10 nm. Good thermal properties with 5% weight loss temperature of 499 °C in air atmosphere and glass transition temperature in excess of 310 °C were shown for porous PI films. Notably, the ultralow dielectric constant of porous PI films with 1.85 at 1 MHz was obtained and revealed via broadband dielectric spectroscopy. The effects of the chemical structure of the PI matrix and PEG content on the decomposition behavior of PEG and the performance of porous films were investigated. Wide‐angle X‐ray diffraction results indicated that the PI matrix with large d‐spacing generated weaker interactions between the PEG and PI backbone than those of PI matrix with small d‐spacing. As a result, the PEG for the PI matrix with large d‐spacing was completely decomposed. As indicated by the broadband dielectric spectroscopy results, lower dielectric porous PI films were prepared when the PEG contents in the PI matrix increased from 0 to 20 wt %. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 549–559     

Stress behaviors and thermal properties of polyimide thin films depending on the different curing process

The effect of high boiling point solvent on the residual stress behaviors of semiflexible structure poly(4,4′‐oxydiphenylene pyromellitimide) (PMDA‐ODA) and pseudo‐rodlike poly(p‐phenylene biphenyltetracarboximide) (BPDA‐PDA) polyimide was investigated. As a solvent, a mixed solution of 20 wt % cyclohexyl‐2‐pyrrolidone (CHP; bp = 307 °C) and 80 wt % n‐methyl‐2‐pyrrolidone (NMP; bp = 202 °C) was used. The effects of solvent system and imidizing history on the morphological structure, as well as residual stress, were significantly high in the BPDA‐PDA having high chain rigidity, but relatively low in the semiflexible PMDA‐ODA with low chain rigidity. In addition, rapidly cured films prepared from PAA (NMP/CHP) showed higher residual stress and a lower degree of molecular anisotropy than slowly cured film imidized from PAA (NMP). This was induced by high chain mobility in polyimide thin films prepared from PAA (NMP/CHP) during the thermal cure process. Therefore, molecular anisotropy, depending on the solvent system and imidizing history, might be one of the important factors leading to low residual stress in polyimide thin films. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2879–2890, 2000     

Effects of alicyclic moiety incorporation on the properties of polyimide/silica hybrid films

Polyimide (PI)/silica hybrid films were prepared from tetraethyl orthosilicate (TEOS) using a sol‐gel process as well as pyromellitic dianhydride and 4,4‐oxydianiline. 1,4‐Cyclohexanedicarboxylic acid (1,4‐CHDA) was added as a coupling agent. The PI/silica hybrid films were characterized by Fourier transform infrared spectroscopy, field emission scanning electron microscopy, differential scanning calorimetry and wide‐angle X‐ray diffraction. The thermal, tensile and dielectric properties of the hybrid films were measured. The results showed that the tensile and dielectric properties of the hybrid films improved with increasing silica concentration and 1,4‐CHDA content in the PI matrix. Covalent ester bonds were formed between SiOH groups of silica and carboxyl groups of 1,4‐CHDA. As a result, the silica particle size was reduced and dispersed homogeneously in the PI matrix, leading to increased tensile strength and tensile modulus of the typical hybrid film with 1,4‐CHDA (PI‐2), when compared with the PI/silica hybrid film without 1,4‐CHDA at the same silica contents. The presence of an alicyclic moiety containing silica in PI reduced the dielectric constant considerably to 2.83, which was lower than that of pristine PI. Copyright © 2016 John Wiley & Sons, Ltd.