Connecting local structure and transport of small molecules through glassy polymers

Understanding the relationship between structure and dynamics in disordered glassy materials is a key question in current scientific research. Several methods developed in the last decade have greatly improved understanding in this area, including the use of machine-learned softness fields to relate local structure with mobility and the propensity of particles to rearrange. For several model glass-formers it has been shown that rearrangement is an Arrhenius process in softness below the onset temperature of glassy dynamics with characteristic energetic and entropic scales. In this work we examine the diffusion of small particles through simple homopolymer melts below the onset of glassy dynamics and explore the relationship between softness and diffusion as the relative size and interaction potential of the diffusing particles changes. Additionally, we investigate how aging impacts transport inside the polymer glass by examining the time evolution of softness and the average energy required for particle rearrangement in the system. Characterizing this behavior provides an opportunity to better understand how softness may be used to predict or design for specific diffusion behavior, for example as applied to separations processes.