Title of Thesis

Polyimide/silica Hybrids Using Imide-modified Silica Network Structures

Author(s)

Muhammad Khalil

Abstract
Polyimide/silica (PI/silica) hybrids incorporating imide-modified silica (IM-silica) network structures have been prepared through the sol-gel process from solution mixtures of poly(amic acid) (PAA) and tetraethoxysilane (TEOS) containing alkoxysilane-terminated amic acids with various degrees of polymerization. Two types of PAAs were employed. The first was obtained from the reaction of oxydianiline (ODA) with pyromellitic dianhydride (PMDA) in dimethylacetamide as a solvent. The second was prepared from the reaction of ODA with a mixture of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride and PMDA. The hybrid films obtained by solvent elution technique were thermally cured to carry out imidization process and silica network formation in the PI matrix. These hybrid films were transparent and flexible with silica contents as high as 30 wt.%. The effect of introducing IM-silica network on structure and properties of these hybrids has been studied and compared with the systems where reinforcement of the matrix was achieved using neat silica generated from TEOS.
The introduction of long and flexible imide spacer group in the silica network has promoted the formation of extremely small silica domains of uniform shape and size evenly dispersed in the matrix. The interfacial interaction between the phases was reflected by relatively large increase in the Tg of PIs on inclusion of IM-silica as compared to the pure one. The high surface-area of the interconnected silica domains shifted the decomposition profiles to much higher temperatures by providing barriers to heat and mass transport in the hybrid material. Moreover, the increased interfacial interaction between chemically bonded and physically intertwined components of these hybrids restricts the segmental motion of polymer and thus slowing down the diffusion of oxygen in the matrix, thereby reducing the oxidative decomposition of the polymer. The thermal decomposition temperature of the hybrid with 30 wt.% hexaimide-modified silica was nearly 260 °C higher than that of the pure PI matrix. The hybrids with IM-silica offered much better tensile properties than the hybrids with pure or unmodified silica. Reinforcement of existing and new PIs by this method offers an opportunity for
improving their thermo-oxidative stability without degrading their mechanical strength.The dielectric properties of the hybrids with IM-silica are less dependent on the applied frequency; thus enabling them to be used over a wide frequency range.

1.1 Materials
1.2 Sol-Gel Chemistry of Metal Alkoxides
1.3 Sol-Gel Derived Organic/Inorganic Hybrid Materials

2.1 Polyimides
2.2 Polyimide/Silica Hybrids
2.3 Previous Works on Polyimide/Silica Hybrids
2.4 Present Work

3.1 Chemicals
3.2 Reaction Conditions
3.3 Synthesis of Polyimide/Silica Hybrids
3.4 Characterization of Polyimide/Silica Hybrids

4.1 Results and Discussion

5.1 Results and Discussion