Abstract

Novel polyimide‐silica nanocomposites with interphase chemical bonding have been prepared using the sol‐gel process. The morphology, thermal and mechanical properties were studied as a function of silica content and compared with the similar composites having no interphase interaction. The polyimide precursors, polyamic acids (PAAs) with or without pendant hydroxyl groups were prepared from the reaction of pyromellitic dianhydride with a mixture of oxydianiline and 1,3 phenylenediamine or 2,4‐diminophenol in dimethylacetamide. The PAA with pendant hydroxyl groups was reacted with isocyanatopropyltriethoxysilane to produce alkoxy groups on the chain. The reinforcement of PAA matrices with or without alkoxy groups on the chain was carried out by mixing appropriate amount of tetraethoxysilane (TEOS) and carrying out its hydrolysis and condensation in a sol‐gel process. Thin hybrid films were imidized by successive heating up to 300C∘. The presence of alkoxy groups on the polymer chain and their cocondensation with TEOS developed the silica network which was interconnected chemically with the polyimide matrix. SEM studies show a drastic decrease in the silica particle size in the chemically bonded system. Higher thermal stability and mechanical strength, improved transparency, and low values of thermal coefficient of expansion were observed in case of chemically bonded composites.

Highlights

  • Composites where nanoscale inclusions are imbedded within matrix material have attracted increasing research attention in recent years

  • The inorganic networks are usually prepared by the sol-gel process [7, 8] to synergistically combine the properties of organic polymers with that of ceramic materials [9, 10]

  • Since the morphology of two-phase systems affects the mechanical properties of polymer hybrids, attempts have been made to improve the compatibility between PIs and silica by increasing interfacial interactions

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Summary

INTRODUCTION

Composites where nanoscale inclusions are imbedded within matrix material have attracted increasing research attention in recent years. The compatibility between PIs and silica generated in the sol-gel process using different tetraalkoxy silanes in the hybrids, is not as good as expected because of weak interactions between the pure organic and inorganic phases [12]. The morphology of silica particle produced in such matrices is totally different and particle size is much smaller for the same silica contents as compared to simple matrix without hydroxyl groups and this has a positive influence on the thermal and mechanical properties of the resulting hybrids. The reaction of hydroxyl groups on the polymer chain, is very slow with the alkoxy groups of TEOS and in the sol-gel process usually a physical bonding between the matrix and the silica network can be achieved. The morphology, thermal stability, visco-elastic and thermal mechanical properties of the PI-silica hybrid films were studied and related with the amount of silica and the extent of chemical bonding between the organic and inorganic phases

Materials
Preparation of PAA solution
Preparation of the PAA solution with pendant alkoxy groups
Sol-gel processing and imidization reaction
Measurements
RESULTS AND DISCUSSION
FTIR spectroscopy
Morphological studies
Visco-elastic properties
Thermal mechanical analysis
Thermal stability
CONCLUSIONS
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