Investigation into the thermal and mechanical behavior of alumina/polymethylmethacrylate (PMMA) nanocomposites

Polymer nanocomposites have great potential as a class of materials that show unique combinations of thermal and mechanical properties not exhibited by micron-size particle filled systems. The goal of the present work is to synthesize and characterize nanocomposites where the interface between particle and polymer is very weak and subsequently evaluate the thermal and mechanical properties. Alumina/polymethylmethacrylate (PMMA) nanocomposites were synthesized by an in situ free-radical polymerization process. At an optimum weight percent, the resulting nanocomposites display a room temperature brittle-to-ductile transition with an increase in the strain-to-failure of more than 1000% and the appearance of a well-defined yield point when tested in uniaxial tension. Concurrently, the glass transition temperature (T _g) of the nanocomposites decreased by as much as 25°C, while the ultimate strength and the Young's modulus decreased by 20% and 15%, respectively. For comparison, composites containing micron size alumina particles were synthesized and displayed neither phenomenon. Solid-state deuterium NMR results showed enhanced chain mobility at room temperature in the nanocomposites and corroborate the observed T_g depression indicating considerable main chain motion at temperatures well below those observed in the neat polymer. The brittle-to-ductile transition is found to depend upon the poor interfacial wetting between polymer and particle allowing the nucleation of voids which subsequently expand when exposed to the stress field created in uniaxial tensile testing. This expansion relieves the triaxial stress state suppressing any craze formation and promoting shear yielding. The reduction in glass transition is determined to be due to the break-up of the percolating structure of slow dynamical domains recently hypothesized to be responsible for the T_g reductions found in polymer ultrathin films.