Factors Influencing the
Structure and Properties of Nanocomposites Derived by Sol-Gel Technique
Abhijit Bandyopadhyay[1]
and Anil K. Bhowmick[2]
Indian Institute of Technology
Kharagpur- 721302
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 (< 100 nm) have been
formed, whereas these aggregate with less polar (and more hydrophobic) polymer
matrices and also at higher TEOS concentrations (mostly > 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 ³ pH ³ 1.0) and 1:2 TEOS:H2O
mole ratio at room temperature (25 0C) 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 mm), 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.