(648g) Reaction Coordinate for the Movement of Solvent Molecules in Glassy Amorphous Polymer

Diffusion in polymer is a fundamental physical process relevant to a wide range of applications. In glassy polymer, small molecules move around in a so-called "hopping" manner: they are constrained in microscopic cavities for extended periods of time, before jumping suddenly to different ones. A widely-adopted framework is therefore to coarse-grain the polymer matrix into a three-dimensional network of isolated sites (corresponding to penetrant-holding cavities) connected by hopping pathways. Predicting the transition rate along each pathway relies on the accurate calculation of the transition free energy barrier, which in turn requires the determination of the reaction coordinate. A method of identifying reaction coordinate from an ensemble of transition paths based on likelihood maximization has been developed in recent years. We apply this method to the diffusion of solvent molecules in glassy amorphous polymer. Order parameters capturing degrees of freedom in both solvent and polymer molecules are screened, and different models of reaction coordinate are optimized and quantitatively evaluated with likelihood maximization. A physical insight into the transition mechanism is also attained during this process. This method can be generalized to a broader range of problems in polymer materials and soft matter.