Thermal and mechanical properties of chemically bonded aramid-silica nano-composites

Thermally stable aramid-silica nano-composites have been prepared via the sol-gel process. Two types of aramid matrices were used. Polyamide chains having no pendant hydroxyl groups were prepared by reacting a mixture of 1,3- and 1,4-phenylenediamine in 65:35 mole ratio with equivalent amount of terephthaloyl chloride (TPC) in dimethylacetamide (DMAc) as solvent. The silica network was developed in the matrix by hydrolysis and condensation of various proportions of tetraethoxysilane (TEOS) and this system was used as reference. The matrix having pendant hydroxyl groups was synthesized by the co-polymerization of a mixture of phenylenediamines and 2,4-diaminophenol with equivalent amount of TPC in DMAc. The hydroxyl groups on the chain were reacted with isocyanatopropyltriethoxysilane (ICTOS), which together with TEOS produced silica network structure bonded with the polymer chain. The morphology, thermal and mechanical properties of both types of composites, with and without chemical bonding with silica network have been studied. The glass transition temperature as determined from the maxima of tan δ data using dynamic mechanical thermal analysis showed a large increase in case of bonded system in comparison to the unbonded system. The value of the storage modulus was also found to be considerably higher in case of bonded system. The values of thermal expansion coefficient decreased with inclusion of silica in the matrix. The thermal decomposition temperature of these hybrids in air was in the range of 480-500 °C.