Achieving an Optimal T-g Change by Elucidating the Polymer-Nanoparticle Interface: A Molecular Dynamics Simulation Study of the Poly(vinyl alcohol)-Silica Nanocomposite System

摘要

The glass-transition temperature (T-g) of a polymer-nanoparticle composite system is strongly dependent on the local segmental dynamics of the polymers. These dynamics, in turn, are dependent on the types of interfacial interactions that exist between the nanoparticle and the polymers. In this work, we investigate the magnitude of T-g change with respect to the polymer-nanoparticle interactions by performing molecular dynamics simulations for a full-atomistic model of poly(vinyl alcohol) (PVA)-silica nanocomposites. Our segmental dynamics analysis and potential of mean force calculations indicate that the strong binding interaction between the hydroxylated-silica surface and the poly(vinyl alcohol) (PVA) is able to induce an increase in the T-g of the composite system with respect to the bulk PVA polymer system. While we expect that the T-g of the system will increase with an increasing amount of hydrogen bonds arising from the increasing surface hydroxylation, the trend of increasing T-g reaches a maximum when the surface is about 75% hydroxylated. Beyond 75% hydroxylation, we see a drop in the T-g. The detailed analysis of the interfacial hydrogen-bonding counts, strengths, and radial distributions sheds light on the underlying factors that induce the drop in the T-g beyond 75% hydroxylation. We found that the competition between inter-PVA-silanol and intra-silanol-silanol interactions is the key factor that contributes to the drop in T-g. Our results allude to the fact that the count and strengths of the different kinds of hydrogen bonds in a polymer-nanocomposite system can be modulated to enable the optimization of the T-g change desired for specific applications.