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 ( $h_t$ ) remains elusive. Using dynamic mechanical analysis (DMA), we establish a strong correspondence between nanolayer mobility enhancement and $h_t$ , 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.