(419i) Dynamic Asymmetry within Interfacial Polymer Layers on Nanoparticles and Their Consequences on Macroscopic Mechanical Properties of Polymer Nanocomposites

Miscible polymer blends with different glass transition temperatures (T_g) are known to form confined interphases between glassy and mobile chains. We recently showed that nanoparticles adsorbed with a high-T_g polymer, poly(methyl methacrylate) (PMMA), and dispersed in a low-T_g matrix polymer, poly(ethylene oxide) (PEO), exhibited a liquid-to-solid transition at temperatures above T_gâ??s of both polymers. We reported that miscible polymers with asymmetric dynamics and composition reversibly stiffen upon heating with the inclusion of nanoparticles. In this talk, I will present the mechanical responses of other polymer combinations with varying T_g differences to reveal how the strength of particle-high T_g polymer attractions and crystallinity of the matrix polymer lead to their stiffening. In addition, I will discuss the rheological results on poly(vinyl acetate) (PVAc) and poly(methyl acrylate) (PMA) adsorbed nanoparticles to explore the strength of interphase interactions between particles and different polymers. Further, we studied the role of interfacial polymer on segmental and collective dynamics of the PEO matrix chains between sub-ns and 100 ns. Our spin-echo experiment results showed that the Rouse relaxation remains unchanged in weakly attractive composite systems compared to pure PEO. I will discuss tube dilation and disentanglements within weakly attractive composites (PMMA-adsorbed silica dispersed in PEO) and will present the decrease and recovery of the entanglements by vitrification and the mobility of the bound polymer, which is found to be in-line with the unusual softening-stiffening transition of our bulk polymer nanocomposites.