Linking Long-range Surface-Induced Mobility Enhancement and Intrinsic Fragility of Polymer Glasses

Surface-induced mobility enhancement propagates into polymer glasses over distances from nanometers to micrometers. The discovery of a mobile surface “bilayer” structure—comprising a nanoscale outer layer and a microscale sublayer—provides insight into this significant variability in propagation length. However, the origin of the extensive microscale sublayer (\(\:{h}_{\text{t}}\)) remains elusive. Using dynamic mechanical analysis (DMA), we establish a strong correspondence between nanolayer mobility enhancement and \(\:{h}_{\text{t}}\), both of which increase with the intrinsic fragility of polymer glass. Coarse-grained simulations confirm our experimental findings, revealing that enhanced molecular velocity at the surface initiates long-range shear-like excitation modes that penetrate deeper into the bulk as polymer fragility increases. This combination of experiment and simulation provides a unified framework that reconciles historical discrepancies regarding the free surface effect, establishing fragility as a critical parameter for predicting the relative sizes of surface-enhanced mobility and its propagation length across different polymers.