Effect of Particle Size, Temperature, and Deformation Rate on the Plastic Flow and Strain Hardening Response of PMMA Composites

We present extensive studies of the nonlinear mechanics of neat, microparticle (MP) and/or nanoparticle (NP) filled PMMA. Rigid particles are used as probes that alter local chain packing, the degree of correlated molecular motion, and structural relaxation times. We show that the extent of these alterations are proportional to the specific particle-PMMA interface area and the spatial distribution of interparticle distances. Nanoparticles are used to maximize these probe effects because both their size (D) and the distance between them (D-NP) are of order the radius of gyration of the polymer chains at low NP volume fractions. We analyze the effects of MPs and NPs on the elastic moduli and postyield response for samples prepared with different thermal histories and deformed at a range of temperatures and strain rates. In particular, we exploit qualitative differences between the response of PMMA composites deformed at T-lo = T-g - 80 K and T-hi = T-g - 20 K to illustrate key features of the relationship between the mechanically rejuvenated yield stress, sigma(yr), and strain hardening modulus, G(H). The observed trends are supported by kinetic analyses and exploited to test recently proposed models of plastic flow and strain hardening. Our results should lead to an improved understanding of the factors controlling the ultimate performance of polymer composites.