Chemically bonded silica-polymer composites from linear and branched polyamides in a sol-gel process

A new type of polymer-silica composite based on aramids of either linear or non-linear structure has been prepared successfully via the sol-gel process. The linear polyamide chains were prepared by the reaction of a mixture of m-phenylene and p-phenylene diamines and terephthaloyl chloride in dimethylacetamide. The non-linear (branched) polyamide chains (having an increased number of reactive ends) were synthesized using 1,3,5-benzenetricarbonyl chloride and terephthaloyl chloride to increase the mass average functionality of the monomer. A slight excess of acid chloride was added in both cases to produce amide chains with carbonyl chloride end groups, which permitted the chains to be end-capped with aminophenyltrimethoxysilane. Addition of tetramethoxysilane to a solution of the polymer, and its subsequent hydrolysis and condensation produced a silica network phase chemically bonded to the aramid chains. Films thus produced were yellow, and transparent for condentrations,of silica of up to 25 mass%. Tensile strengths increased gradually with increasing silica content up to this same concentration, but then decreased significantly. The overall values of the strength were found to be smaller for the non-linear aramid chains, relative to those of the linear, possibly because of the branches interfering with interchain interactions. Nonetheless, the increases in the tensile strength with increasing inorganic network phase were larger for the non-linear polymer, consistent with the goal of increasing the bonding between the organic and inorganic phases by increasing the number of amidophenyltrimethoxysilane chain ends. Increasing these strengths to values greater than those for the linear chains can probably be achieved by having the branched regions only near the ends of the chains. Since these transparent ceramers were found to withstand tensile stresses of the order of 175 MPa and had thermal decomposition temperatures around 460-475 degrees C, they may be very useful as matrices for fibre-reinforced composites.