1 research outputs found
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掳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路 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路C, (b) suppression of anisotropy in polyimide films
by addition of silica, and (c) negative coefficients of thermal expansion below
100路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