| Dynamic mechanical properties of fumed silica filled polymers are systematically studied at and above the glass transition temperatures, with highest filler concentration around 12 vol. percent.; Dramatic change in viscoelastic behavior is observed in filled melts, which includes high reinforcement at low strains and high nonlinearity in storage and loss moduli with strain. The reduction of dynamic modulus (nonlinearity) is recoverable with time if stress-free after large strain perturbations. The reinforcement, nonlinearity and the recovery behavior all show strong dependence on filler surface treatment, filler size and filler concentration, as well as the molecular weight of matrix polymers. The results suggest that filler-trapped entanglements and their release under applied strains, instead of filler structures and structure breakdown, are primarily responsible for the dynamic mechanical properties observed for the filled melts. Some substantial contributions to the reinforcement may also come from Langevin chain behavior, which becomes more important for nanocomposites because of the enhanced reference state bias of polymer chains. SEM pictures as well as small angle scattering data provide additional evidence that is also in favor of the entanglement-based mechanism.; Glass transition results show that all filled samples are thermo-rheologically simple just like most neat amorphous polymers, such as PVAc used in this study. The shift factors used for time-temperature-superposition of all samples form nearly identical curves versus temperature, regardless of filler concentration, with only one filled system (ST100L fumed silica) showing a little deviation at high filler concentrations. This filled system, which involves tethered silicone chains on the filler surface, is also the only exception to the universal dispersion of glass transition observed for other filled systems, i.e., identical normalized loss modulus peaks at Tg. The reinforcement in storage modulus agrees with conventional hydrodynamic models at several degrees below Tg, but increases substantially with temperature. Those results suggest that the filler-polymer energetic interactions are of a length scale of typical force fields, but far-field filler effects are also present through the long-range coupling of polymer chains, which agrees with the conclusions regarding the melt behavior. |
