Abstract
The dynamic behavior of glass-forming polymers is fundamentally altered under geometric confinement, a phenomenon primarily driven by enhanced molecular mobility originating at the free surface. However, a critical phenomenological discrepancy remains: the apparent contradiction between the nanoscale mobility inferred from local probing techniques and the microscale dynamic perturbations captured by macroscopic characterizations. This thesis resolves this long-standing length-scale controversy by quantitatively partitioning the multi-scale spatial propagations of surface-enhanced dynamics in the films.
By distinguishing varying relaxations dynamics across divergent film thicknesses, a novel "mobile surface bilayer" structural framework is established. Experimental investigations reveal that free surface perturbations comprise not only an outermost, highly mobile nanoscale layer but also a deeply penetrating microscale mobile sublayer. Crucially, the maximal spatial propagation length of this dynamic enhancement is found to be directly proportional to the inherent bulk fragility (m) of the glassy polymer. While the relatively “strong” networks of less fragile glasses suppress the perturbation, highly fragile systems exhibit profound dynamic heterogeneity, enabling surface-induced excitations to propagate extensively across microscale length scales into the bulk interior.
To elucidate the physical origin of this microscale sublayer, this thesis proposes the excitation of shear modes at the free surface, a mechanism supported by molecular simulations conducted in collaboration with Prof. Rui Zhang’s group. By analyzing atomic displacements at varying depths, a distinct signature of shear excitation was identified. Notably, these simulated results reinforce the experimental findings, indicating a direct correlation between bulk fragility, the magnitude of enhanced surface mobility, and the implicated characteristic propagation distance of the shear mode.
The long-range influence of this surface-enhanced mobility is further demonstrated in substrate-supported ultrathin films. Under extreme spatial confinement, the physical proximity of the free surface dynamically agitates molecules buried within the adsorbed layers (ALs). Contrary to the classical consensus defining ALs as irreversibly frozen or dynamically "dead" interfaces, depth-profiled isotope tracking via time-of-flight Secondary Ion Mass Spectrometry (TOF-SIMS) confirms that surface-induced fluctuations traversing the whole film promote macroscopic chain desorption and activate unexpected dynamic inter-diffusion at the substrate interface.
Collectively, this thesis elucidates a unified, multiscale physical framework for confined glassy polymers. It identifies a quantitative link between localized interfacial mobility to intrinsic fragility—a bulk thermodynamic property—establishing the free surface as a potent thermodynamic fluctuation source. This source is capable of driving large-scale dynamic enhancement that dominates the relaxation behavior of polymer systems even under microscale confinement. Such a profound influence has significant implications for applications utilizing micrometer-sized polymers, such as lithographic nanopatterning, the shelf-life stability of amorphous pharmaceutical coating, and the mechanical integrity of polymer-based micro-electromechanical systems (MEMS).