Polyimide ceramers : composition, morphology and properties

Polyimides have become important materials in the manufacture of a large number of technical products, e.g. varnishes, coatings etc, as they can fulfill a number of requirements. One such requirement is that they have to withstand high temperatures without deterioration of their properties. The work is based on the expectation that the temperature resistance and thermal stability of polyimides can be enhanced by introducing inorganic heat resistant materials, such as silicates, to form hybrid mixtures known as "ceramers". The systems considered in this work are based on linear aromatic polyimides and silica networks produced by the sol-gel route. Solutions of polyamic acid and partially polymerised tetraethoxysilane (TEaS) solutions are converted into ceramer films by solvent evaporation, followed by imidisation and condensation reactions through stepped temperature rises up to a maximum of 350°C. Phase separation is prevented by the use of small amounts of compatibilising agent glycidyloxypropyltrimethoxysilane (GOTMS) or by addition of triethylamine or tributylamine catalysts. By controlling the rate of condensation reactions in the two components different morphologies are obtained, varying from semi-interpenetrating networks of linear poyimides within highly crosslinked silica chains to finely dispersed heterogeneous systems exhibiting etiher a co-continuous or particulate microstructure. As each phase consists of interpenetrating networks of the two components at different concentrations, it is inferred that phase separation takes place through a spinodal decomposition. The crosslinking density of the silica network was reduced by diluting the tetraethoxysilane component with minor quantities of· dimethyldiethoxysilane (DMES). The properties of polyimide ceramers were found to be intermediate between those exhibited by the individual components, depending on the details of the morphological structure. The most interesting characteristics of these systems are: (a) high modulus, high glass transition temperature and dimensional stability at temperatures greater than 250·C, (b) suppression of anisotropy in polyimide films by addition of silica, and (c) negative coefficients of thermal expansion below 100·C. The causes of this anomalous behaviour are not clear at this stage but it appears to be related to large volumetric changes taking place within the silica network as a consequence of the absorption and desorption of small amounts of water.