Origin of the Free Surface Effect of Polymers with Different Sizes of Side Groups

The free surface effect in nanoconfined polymer systems is important. Hence, understanding the influence of polymer structures on the free surface effect is essential. Herein, the dynamics of the free surface of polystyrene, poly(4-methylstyrene), and poly(4-tert-butylstyrene) with p-substituted methyl and tert-butyl groups in the benzene ring were investigated. An enhanced free surface effect was exhibited by polymers with large side groups and high fragility, which was attributed to the high free volume of the surface and the long distance for this unusual free volume to propagate into the bulk. This phenomenon was proven by deuterium oxide vapor penetration into the film surface via neutron reflectivity. High levels of packing frustration could be relieved on the free surface of fragile polymers during film formation; the chain packing efficiency on the surface was reduced, resulting in a large and broad free volume distribution on the free surface. This study provides a deep understanding of the relationship between the free surface effect and the chemical and physical properties of polymers.