Origin of the\nFree Surface Effect of Polymers with\nDifferent Sizes of Side Groups

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