Glass Transitions in Polymer Nanocomposites

For polymers are under geometric confinement, it is generally believed that the glass transition temperature (T$_{\mathrm{g}})$ increases with favorable interfacial interactions. Experiments [1] and simulations [2] have reported that T$_{\mathrm{g}}$ increases almost proportionally to the attractive polymer-surface interactions. However, recent studies [3,4] have reported the contradictory finding that the T$_{\mathrm{g}}$ shift is rather modest and insensitive to the strength of interfacial attractions. In this study, we investigate the glass transition in polymer nanocomposites using molecular dynamics simulations. With attractive polymer-nanoparticle (NP) interactions, we find that T$_{\mathrm{g}}$ is increased by $\sim$ 3{\%} at moderate loadings and that the shift stays almost unchanged when the polymer-NP attractions are further increased by one order of magnitude. Both are in agreement with the recent experiments at comparable NP loadings [4]. We show that this is because the strongly adsorbed polymer segments do not participate in the glass transition. In other words, strong polymer-NP attractions create immobile polymer ``coatings'' around NPs that shield them from direct contact with the mobile polymers. \\[4pt] [1] Tate, R. S.; de Pablo, J. J.; Nealey, P. F. Journal of Chem. Phys. 2001, 115 (21), 9982-9990. \\[0pt] [2] Torres, J. A.; Nealey, P. F.; de Pablo, J. J. Phys. Rev. Lett. 2000, 85 (15), 3221$-$3224. \\[0pt] [3] Lu, H. Y.; Chen, W.; Russell, T. P. Macromolecules 2009, 42 (22), 9111$-$9117. \\[0pt] [4] Moll, J.; Kumar, S.K. Macromolecules 2012, 45 (32), 1131$-$1135.