Unexpected Effect of Small Nanoparticles: A Paradigm Shift for Polymer Nanocomposites*

The addition of nanoscale fillers with strong attraction to polymers causes significant changes to the polymers’ dynamics and mechanical properties. The challenge of polymer nanocomposite (PNC) research is to understand the critical microscopic parameters (e.g. chain rigidity, molecular weight, nanoparticle geometry) that control the emergent macroscopic properties of PNCs. In this work, we investigated the effects of adding very small (~1.8nm) POSS nanoparticles to a polymer (P2VP) melt. We found unexpectedly large increases in Tg (~35K) and fragility for the POSS-PNCs compared to PNCs with conventional 10-50nm silica particles. Further, the POSS-PNCs at high temperatures show essentially no viscosity increase in comparison to the neat polymer, making them highly attractive for practical applications. We ascribe this unusual behavior to two unique properties of PNCs with small nanoparticles: (i) fast mobility of small nanoparticles, and (ii) relatively short chain – nanoparticle desorption time. These results reveal a new approach for the design of PNCs by exploiting the unique properties of small nanoparticles, i.e. by tuning their mobility and chain desorption time.