Factors Influencing the Structure and Properties of Nanocomposites Derived by Sol-Gel Technique

Abstract

Sol-gel technique has been widely practiced to synthesize in-situ polymer nanocomposites due to its mild processing conditions and also high optical transparency of the resultant composites. In this paper, we have discussed different factors that influence the formation of polymer-silica hybrid nanocomposites from solutions. Using tetraethoxysilane (TEOS) as silica precursor, it has been found that the silica domain sizes in the resultant hybrid composites largely depend on the polymer microstructure, TEOS concentration, solvent, TEOS-to-water mole ratio (for the hydrolysis of TEOS), pH and the temperature of gelation. With higher hydrophilicity of the matrix polymers, smaller silica particles ( 50 wt%). Interestingly, the silica domains still remain < 100 nm in size, if the polymerization and silica formation are simultaneous. Phase separation in these nanocomposites has been prohibited by strong polar-polar interfacial interactions between the polymers studied here and silica, as in the case of acrylic polymers or by hydrogen bonded interactions as in the cases of epoxidised natural rubber (ENR) and poly (vinyl alcohol) (PVA). Solvents with proper balance in hydrophilicity and polarity, high acidic pH (2.0 ≥ pH ≥ 1.0) and 1:2 TEDS:H 2 O mole ratio at room temperature (25 °C) generate nanocomposites, which are mechanically reinforced and also transparent. The improvement in tensile strength and modulus has been more than 200%. If these conditions are changed, even highly interactive polymers do not produce nanocomposites, contain big silica aggregates (2-3 μm), are not transparent and have low mechanical properties. In the later section of the paper, the effects of polymer-silica interactions on the solvent swelling, solution viscosity and dynamic mechanical properties of the hybrid nanocomposites have been discussed and mechanism of reinforcement of polymers by nanofillers has been revealed.