Polyimide‐Polydimethylsiloxane Copolymers. Evaluation of the Thermal and Electrical Properties

Poly(dimethylsiloxane‐amic‐acid)s have been prepared starting from a fluorinated dianhydride, namely 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride, and a mixture of an aromatic diamine and bis(aminopropyl)‐oligodimethylsiloxane of controlled molecular weight, in different ratios. A solution imidization procedure was used to convert them quantitatively to the corresponding polyimides. The polymers, easily soluble both in polar (N‐methylpyrrolidone) and less polar (chloroform) solvents, were prepared for film‐forming by casting from solution. The thermal behavior in dependence on composition was evaluated by thermogravimetric analysis and differential scanning calorimetry. The kinetic processing of thermogravimetric data was carried out using the Flynn‐Wall‐Ozawa and Kissinger methods. Electrical insulating properties of some polymer films were evaluated on the basis of dielectric constant and dielectric loss tangent and their variation with frequency.     

Photopiezoelectric Composites of Azobenzene‐Functionalized Polyimides and Polyvinylidene Fluoride

Light is a readily available and sustainable energy source. Transduction of light into mechanical work or electricity in functional materials, composites, or systems has other potential advantages derived from the ability to remotely, spatially, and temporally control triggering by light. Toward this end, this work examines photoinduced piezoelectric (photopiezoelectric) effects in laminate composites prepared from photoresponsive polymeric materials and the piezoelectric polymer polyvinylidene fluoride (PVDF). In the geometry studied here, photopiezoelectric conversion is shown to strongly depend on the photomechanical properties inherent to the azobenzene‐functionalized polyimides. Based on prior examinations of photomechanical effects in azobenzene‐functionalized polyimides, this investigation focuses on amorphous materials and systematically varies the concentration of azobenzene in the copolymers. The baseline photomechanical response of the set of polyimides is characterized in cantilever deflection experiments. To improve the photomechanical response of the materials and enhance the electrical conversion, the polyimides are drawn to increase the magnitude of the deflection as well as photogenerated stress. In laminate composites, the photomechanical response of the materials in sequenced light exposure is shown to transduce light energy into electrical energy. The frequency of the photopiezoelectric response of the composite can match the frequency of the sequenced light exposing the films.

     

锂电隔膜研究及产业技术进展

隔膜是锂离子电池的关键材料,在极片间起绝缘和提供离子通道的作用.本文以如何构建耐高温的隔膜、如何提高隔膜的离子电导率为重点,介绍了锂电隔膜研究及产业化技术进展,内容主要包括隔膜性能的影响因素、凝胶聚合物电解质膜、拉伸法制备隔膜及其改性、隔膜制造的新方法和新材料.最后展望了隔膜的发展前景.     

Polyimide/hollow silica sphere hybrid films with low dielectric constant

Abstract

Polyimide (PI)/hollow silica (HS) sphere hybrid films with low dielectric constant values (low-k) were synthesized via thermal imidization process using pyromellitic dianhydride (PMDA)/4,4′-oxydianiline (ODA) as the polymer matrix and HS spheres as inorganic particles with the closed air voids. The monodispersed HS spheres were synthesized via a one-step process, which means that the formation of silica shells and dissolution of the core particles (polystyrene particles) occurs in the same medium. The HS particles have uniform size of ca. 1.5 μm in diameter and ca. 100 nm in shell thickness. PI/HS sphere hybrid films synthesized using mixture of polyamic acid (PAA) and HS spheres prepared via one-pot process, which means that the production of PAA and HS spheres mixture occurs with the polymerization of PMDA and ODA in the same bottle. HS spheres of two different kinds (pristine HS spheres (PHS spheres) and amine-modified HS spheres (AHS spheres)) were used for the preparation of the hybrid films. With the varying contents of AHS spheres in the range of 1–10 wt%, the dielectric constants of the PI/AHS sphere hybrid films were reduced from 3.1 of pure PI to 1.81 by incorporating 5 wt% AHS. The dielectric constants of the PI/PHS sphere hybrid films were reduced to 1.86 by incorporating 5 wt% PHS. Organic–inorganic hybrid porous polyimides may be expected as prime candidates for polymeric insulators due to their high thermal stability, good mechanical properties, solvent resistance, and low-k.     

Sul-containing fluorinated polyimides for optical waveguide device

Novel sul-containing fluorinated polyimides have been synthesized by the reaction of 2,2′-bis-(trifluoromethyl)-4,4′-diaminodiphenyl sulfide (TFDAS) with 1,4-bis-(3,4-dicarboxyphenoxy)benzene dianhydride (HQDPA), 2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA), 4,4′-oxydiphthalicanhydride (ODPA) or 3,4,3′,4′-biphenyl-tetracarboxylic acid dianhydride (s-BPDA). The fluorinated polyimides, prepared by a one-step polycondensation procedure, have good solubility in many solvents, such as N-methyl-2-pyrrolidinone (NMP), dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), cyclohexanone, tetrahydrofuran (THF) and m-cresol. The molecular weights (M_n's) and polydispersities (M_n/M_w's) of polyimides were in the range of 1.24 × 10⁵ to 3.21 × 10⁵ and 1.59–2.20, respectively. The polymers exhibit excellent thermal stabilities, with glass-transition temperatures (T_g) at 221–275 °C and the 5% weight-loss temperature are above 531 °C. After crosslinking, these polymers show higher thermal stability. The films of polymers have high optical transparency. The novel sul-containing fluorinated polyimides also have low absorption at both 1310 and 1550 nm wavelength windows. Rib-type optical waveguide device was fabricated using the fluorinated polyimides and the near-field mode pattern of the waveguide was demonstrated.     

BPDA/PPD/OTOL聚酰亚胺纤维的力学性能、形貌和结构

在3,3’,4,4’-联苯四酸二酐(BPDA)和对苯二胺(PPD)体系中引入3,3’-二甲基联苯胺(OTOL),显著改善了纤维的力学性能。当n(PPD)∶n(OTOL)=70∶30时,纤维的拉伸强度可达到改性前的2倍,其拉伸强度和拉伸模量分别为1.50和80 GPa。SEM照片显示了纤维的断面为圆形且没有孔穴,也没有明显的"皮芯"结构和原纤结构。WAXD和SAXS分析表明,纤维的轴向堆积和分子链取向在热牵伸过程中得到改善。     

Enhancement of properties of polyimide/silica hybrid nanocomposites by benzimidazole formed hydrogen bond

Polyimide/silica hybrid nanocomposites were prepared by sol–gel method without coupling agent. A novel diamine with a benzimidazole group, 2‐(4‐aminophenyl)‐5‐aminobenzimidazole (PABZ), was introduced to copolymerize with 4,4'‐oxydianiline (ODA) and pyromellitic dianhydride (PMDA) to synthesize polyimide (PI) matrix. The compatibility between PI and silica was improved by hydrogen bonds formed between silica phase and the –NH– group on benzimidazole of the new diamine. Highly transparent hybrid films were obtained when silica content reached as high as 30 wt%. SEM results show that silica particles with sizes much smaller than that in PMDA‐ODA/silica system disperse homogeneously in the PI matrix. Differing from most hybrid systems without coupling agent, the tensile strength of PABZ system increases from 152 MPa to 165 MPa with silica content increasing from 0 to 20 wt%, while, it decreases linearly in PMDA‐ODA system. DMA analysis shows that the introduction of PABZ largely increases the glass transition temperature (Tg) for all silica contents, which is suggested to be due to the more rigid structures and stronger interaction between the two phases. Meanwhile, the decomposition temperature and char yields at 800 °C are both higher than that of pure PIs. The structures of the hybrid films were identified by FTIR spectra, which indicate that different silica morphologies are developed, resulted from the hydrogen bonds between benzimidazole and silica phase. Copyright © 2011 John Wiley & Sons, Ltd.     

表面改性纳米BaTiO3杂化聚酰亚胺薄膜的合成及其热性能

以3,3’,4,4’-联苯四酸二酐和4,4’-二氨基二苯醚为原料,通过原位聚合和流延成膜法制备了不同纳米BaTiO3含量的聚酰亚胺(PI)/BaTiO3杂化膜。采用不同的硅烷偶联剂对纳米BaTiO3进行表面改性,发现γ-巯丙基三乙氧基硅烷(KH590)的改性效果较好。大部分杂化膜在质量损失5%的温度都较纯膜有不同程度的升高,最多升高了18℃,说明PI/BaTiO3杂化膜较纯膜具有更好的耐热性能,且热分解温度均高于520℃。     

Gas transport properties of asymmetric polyimide membranes prepared by plasma‐based ion implantation

In this study, we report the gas permeance and selectivity of the asymmetric polyimide membrane prepared by plasma‐based ion implantation (PBII). The asymmetric polyimide membranes were prepared using a dry–wet phase inversion process, and the surface skin layer on the membrane was implantated by He ions at 2.5 keV. The asymmetric membranes treated by PBII were measured using a high vacuum apparatus with a Baratron absolute pressure gauge at 76 cmHg and 35°C. The (O₂/N₂) and (CO₂/CH₄) selectivities in the He⁺‐implanted asymmetric membrane at 60 sec resulted in 1.5 and 1.8 time increases, respectively, when compared to those of the asymmetric membrane before PBII. On the other hand, the O₂ and CO₂ permeances in the asymmetric membrane after PBII decreased with an increase in the He⁺ treatment time. In this paper, we addressed, for the first time, the gas permeation behavior of the asymmetric polyimide membranes prepared by PBII. Copyright © 2009 John Wiley & Sons, Ltd.