Advanced Silsesquioxanes Based Composites: Synthesis, Characterization and Properties

Nidhin Divakaran, Smitha C. Sukumaran, Pravin R, Prasanta Kumar Behera

Vellore Institute of Technology, Vellore

Polyhedral oligomeric silsesquioxanes (POSS) are a class of important nanosized cage-like molecules, derived from hydrolysis and condensation of trifunctional organosilanes. A typical POSS molecule possesses a structure of cube-octameric frame work represented by the formula (RSiO1.5)n with an inorganic silica-like core (Si8O12) (~0.53 nm in diameter) surrounded by eight organic corner groups, one (or more) of which is reactive. Incorporation of nano sized POSS macromers into polymers has produced significant improvement in thermal and oxidative resistance as well as reduction in flammability of several POSS-based polymers; these systems then being suitable candidates in high-temperature and fire-resistant applications.

The present work investigates the effect of addition of a monofunctional POSS into a silicone polymer which contains POSS structure in the main chain. Monoallyl heptaphenyl POSS was synthesized by the hydrolysis and alkali catalysed condensation of phenyl triethoxy silane followed by capping with allyltrichlorosilanes. The silicone polymer containing POSS structure in the main chain was synthesised by acid catalysed condensation of tri and di functional alkoxysilanes. Both the polymers were characterized using IR, 1HNMR and TGA.

Incorporation of different weight loadings of the synthesized monofunctional POSS into silicone polymer containing POSS structure along with other reinforcement fillers like hexamethyl disilazane treated silica and curing via platinum catalyzed addition cure mechanism using poly(dimethyl-co-methylhydrogen) crosslinker resulted in the polymer composite with enhanced thermal stability and mechanical properties than the existing silicone systems. A transmission electron micrograph taken from the POSS reinforced polymers illustrates the molecular level dispersion of mono functional POSS that can be achieved in silicon resin via compounding. The studies have shown that the chemically incorporated POSS in polymeric systems exhibit better reinforcing characteristics than any physically incorporated reinforcements. The developed polymers are potential candidate for high temperature resistant coatings as well as a high temperature resistant adhesive for aerospace applications.

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