45 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°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
Development of Thermal Films for Greenhouse Applications Using Long Infrared Radiation Absorbers
Preparation and properties of elastic polyimide-silica composites using silanol sol from water glass
Microstructure and protonic conductivity of H3PO4-doped polyethylenimine–siloxane chemically covalently organic–inorganic hybrids
In Situ Sol-Gel Process of Polystyrene/Silica Hybrid Materials: Effect of Silane-Coupling Agents
The polystyrene–silica hybrid materials have been successfully prepared
from styrene and tetraethoxysilane in the presence of silane-coupling agents by an in
situ sol-gel process. Triethoxysilyl group can be incorporated into polystyrene as side
chains by the free-radical copolymerization of polystyrene with silane-coupling agents,
and simultaneously polystyrene–silica hybrid materials with covalent bonds between
two phases were formed via the sol-gel reaction. The 3-(trimethoxysilyl)-propyl-methacrylate
(MPS) systems were found to be more homogeneous than the corresponding
allytrimethoxysilane hybrid system of equal molar content. In the MPS-introduced
system, the thermal properties of the materials were greatly affected by the presence of
MPS. FTIR results indicate successful formation of the silica networks and covalent
bonding formation of coupling agents with styrene. The homogeneity of polystyrene–
silica systems was examined by scanning electron microscope and atomic force
microscope