Effect of Polymer-Nanoparticle Interaction Strength on Viscoelastic Creep Attenuation in Polymer Nanocomposites

The use of polymer nanocomposites (PNCs) in long-term structural applications requires understanding the mechanisms by which nanoparticles (NPs) affect viscoelastic creep behavior in PNCs. Parameters of interest include NP size, loading, dispersion morphology, and polymer-NP interaction strength. In this study, we examine the creep behavior of silica/poly(2-vinylpyridine) (P2VP) nanocomposites consisting of 13 and 52 nm diameter silica NPs and 200 kg/mol P2VP. Polymer-NP interactions and dispersion quality are controlled by functionalizing the NPs with varying densities of octylsilane. Creep behavior is measured using an accelerated dynamic mechanical analysis (DMA) method and the results are correlated with polymer-NP interaction strength and dispersion. We find that smaller bare NPs delay creep failure by ~10x relative to the larger bare NPs. With octyl-capping, NP dispersion is reduced, shortening the time to creep failure in the small NP systems. For large NPs, the time to creep failure is unaffected by octyl-capping. These results suggest that the dominant reinforcement mechanism is the development of contiguous regions of slowed dynamics via percolated polymer-NP networks, which does not occur for the larger NPs, rather than individual polymer-NP interaction strength.