Low-pressure carbon dioxide enhanced polymer chain mobility below the bulk glass transition temperature

The effects of low-pressure carbon dioxide (CO₂) on chain mobility of polystyrene (PS) and poly-(methyl methacrylate) (PMMA) thin films coated on a solid surface were studied using PS/deuterated PS/Si and PMMA/deuterated PMMA/SiO₂ configurations. The time evolution of chain diffusion below the bulk glass transition temperature (T_g) with and without CO₂ was measured by neutron reflectivity. It was found that polymer chains at the interface can self-diffuse at a temperature below their bulk T_g because of higher surface chain mobility and that adding CO₂, even at low pressures, can greatly enhance the chain mobility. When the interactions between the polymer chains and the substrate are weak as in the PS/Si configuration, the confining effects of the substrate are not significant, while in the case of the PMMA/SiO₂ configuration the interactions are strong enough to confine polymer chains onto the substrate at the nanoscale. Introducing CO₂ tends to alleviate this confining effect.