A study was carried out on formulations of organic-inorganic hybrids and their
subsequent use as matrices for unidirectional carbon fibre-reinforced composites. The
hybrids consist of low molecular weight polyimide precursors and silica which is
generated in-situ via the sol-gel route. A special feature of these systems is the use of
organofunctional trialkoxysilanes as coupling agents for the two phases and for controlling
the resultant morphology.
Enhancements are obtained in physicochemical, thermal and mechanical properties of
hybrids through morphological modifications achieved in the parent polyimide and silicate
materials. Small variations to the composition of the precursors display a substantial
effect both on the kinetics of the associated reactions and the final properties of hybrids,
often as a result of a change in miscibility of the organic and the inorganic components of
The processability of the matrix was evaluated with respect to the fabrication of
composites, which in this case is strongly influenced by the gelation behaviour of both the
organic pre-polymer and also the inorganic sol-gel component. The kinetics of gelation
reactions were examined by dynamic viscometry and by practical tests based on visual
observation of the cessation of flow. Differential scanning calorimetry, infrared
spectroscopy, thermogravimetric analysis and electron microscopy formed part of the
evaluation of the matrix materials. Composites were produced by application of the matrix
solution from a variety of formulations on pre-tensioned fibres, followed by vacuum drying
and curing under pressure at high temperatures. The properties of these composites
were determined by such methods as dynamic mechanical thermal analysis, flexural
testing and thermomechanical analysis.
From the results obtained in this study, it is concluded that the inclusion of silicate
phase in a polyimide matrix in the form of fine co-continuous networks improves the
thermal and mechanical properties of the base material, although these are dependent on
the overall silicate content and the amount of the coupling agent. High loadings of the
coupling agent can cause degradation by chain scission and a reduction of the
crosslinking density of the organic pre-polymer.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.