Alumina/poly(methyl methacrylate) (PMMA) nanocomposites were synthesized by an in situ free-radical polymerization process with 38 and 17 nm diameter ␥-alumina nanoparticles. At extremely low filler weight fractions (Ͻ1.0 wt % of 38 nm fillers or Ͻ0.5 wt % of 17 nm fillers) the glass-transition temperature (T g ) of the nanocomposites drops by 25°C when compared to the neat polymer. Further additions of filler (up to 10 wt %) do not lead to additional T g reductions. The thermal behavior is shown to vary with particle size, but this dependence can be normalized with respect to a specific surface area. The nanocomposite T g phenomenon is hypothesized to be because of nonadhering nanoparticles that serve as templates for a porous system with many internal interfaces that break up the percolating structure of dynamically heterogeneous domains recently suggested to be responsible for the T g reductions in polymer ultrathin films [Long, D.; Lequeux, F. Eur Phys J E 2001, 4, 371]. The results also point to a far field effect of the nanoparticle surface on the bulk matrix.
Alumina/poly(methyl methacrylate) (PMMA) nanocomposites were synthesized using 38
and 17 nm alumina nanoparticles. At an optimum weight fraction, the resulting nanocomposites display
a room-temperature brittle-to-ductile transition in uniaxial tension with an increase in the strain-to-failure that averages 40% strain and the appearance of a well-defined yield point in uniaxial tension.
Concurrently, the glass transition temperature (T
g) of the nanocomposites drops by more than 20 °C.
The brittle-to-ductile transition is found to depend on poor interfacial adhesion between polymer and
nanoparticle. This allows the nucleation of voids, typically by larger particles (∼100 nm), which
subsequently expand during loading. This void formation suppresses craze formation and promotes
delocalized shear yielding. In addition, the reduction in T
g shrinks the shear yield envelope, further
promoting this type of yield behavior. The brittle-to-ductile phenomenon is found to require both larger
particles for void growth and smaller particles that induce the lowering of yield stress.
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