The glass transition of polymers with different side-chain stiffness confined in free-standing thin films

The effect of confinement on the glass transition temperature Tg of polymeric glass formers with different side chain stiffness is investigated by coarse-grained molecular dynamics simulations. We find that polymer with stiffer side groups exhibits much more pronounced Tg variation in confinement compared to that with relatively flexible side groups, in good agreement with experiments. Our string analysis demonstrates that the polymer species dependence of dynamics can be described by an Adam-Gibbs like relation between the size of cooperatively rearranging regions and relaxation time. However, the primary effect of changing side-group stiffness is to alter the activation barrier for rearrangement, rather than string size. We clarify that free-surface perturbation is the primary factor in determining the magnitude of Tg variation for polymers in confinement: It is more significant for polymers having higher Tg and results in much more pronounced reduction of surface Tg and then the overall Tg of the polymers.