Hard and soft confinement effects on the glass transition of polymers confined to nanopores

Abstract

We present results of molecular dynamics simulations for coarse-grained polymers confined in nanopores in a wide temperature range to investigate the factors that affect the glass transition. We focus on the influences of interaction strength, confinement size and the mobility of boundary on the static and dynamic properties of confined polymers, and further study their influences on the glass transition temperature Tg and the fragility Df, which quantifies how rapidly relaxation times vary with temperature T. For the immobile nanopore boundary (i.e. the hard wall confinement model), strong attractive interaction between wall and polymer induces slow polymer dynamics near the wall, while the weak interaction gives rise to a relatively enhanced monomer mobility. The mobile nanopore boundary (i.e. the soft wall confinement model) has a great influence on the shift of Tg and Df: it accelerates the structural relaxation of nearby monomers and leads to a lower Tg and a larger Df. The soft confinement effect is more obvious for nanopores with strong interaction than those with weak interaction. In addition, smaller confinement size leads to lower Tg of confined polymers, except for those confined in hard nanopores with strong attractive interaction. Our analysis demonstrates the change of Tg of confined polymers is mainly controlled by the surface effects originated from the polymer–wall interaction and the mobility of nanopore boundary.