Hopping of Water in a Glassy Polymer Studied via Transition Path Sampling and Likelihood Maximization

Abstract

Diffusion of small molecules in amorphous polymers is known to follow a form of so-called hopping motion: penetrant molecules are trapped in microscopic cavities for extended time periods; diffusion is made possible by rare but fast jumps between neighboring cavities. Existing understanding of the hopping mechanism is based on the inspection of molecular images during individual molecular-dynamics trajectories. We focus on the diffusion of water molecules in a hydrophilic polymer below its glass transition temperature. The transition path ensemble of one hopping event is sampled with aimless shooting, a type of transition path sampling technique. In these trajectories, configurations of both the penetrant and the polymer change during the transition. Statistical analysis of the ensemble using likelihood maximization leads to a reaction coordinate of the transition, whose key components include the penetrant configuration and distances between penetrant-host atom pairs that have strong electrostatic interactions. Polymer motions do not contribute directly to the reaction coordinate. This result points toward a transition mechanism dominated by the penetrant movement. Molecular insights from this study can benefit the development of computational tools that better predict material transport properties, facilitating the design of new materials, including polymers with engineered drying properties.

Molecular Modeling Investigations of Sorption and Diffusion of Small Molecules in Glassy Polymers

“…It has been implemented for the study of a wide range of infrequent events, such as nucleation [99,100,101,102], protein folding and conformational changes [103,104,105,106], glassy dynamics [107], reactions [108,109,110] and penetrant diffusion in non-amorphous materials [111,112,113]. The use of TPS methods for the study of penetrant diffusion in polymeric systems is scarce and involves the study of diffusion of water in a glassy hydrophilic polymer [35]. TPS implementation in this case directly depends on the limited initial transition pathways extracted by MD simulations for the systems under study.…”

Correlated matrix-fluctuation-mediated activated transport of dilute penetrants in glass-forming liquids and suspensions

“…Multiple time and length scales are now important, and the existence of high barriers means that standard molecular dynamics or Monte Carlo simulations are difficult or impossible. 20,21 The limited existing models for activated penetrant diffusion are typically highly phenomenological, often built on ill-defined "free volume" ideas replete with adjustable fit parameters usually of unclear physical meaning. [22][23][24] Recently, we heuristically formulated a microscopic, force level, self-consistent nonlinear Langevin equation (SCNLE) theory 25 for activated barrier hopping and nonhydrodynamic diffusion of a hard sphere penetrant in dense hard sphere fluids.…”

Transition Path Sampling as Markov Chain Monte Carlo of Trajectories: Recent Algorithms, Software, Applications, and Future Outlook

“…[146][147][148] Nucleation processes tend to be very diffusive, [31,69,117,[149][150][151][152][153][154] and can require quite long paths (>10 ns), especially for molecular systems such as water where path lengths can even run into the 100 s of ns. [117] Transitions in small molecular systems have also been studied, for example, water cluster isomerization [155] and water evaporation, [156] diffusion of water, [157] water transport through pores, [158] polymer collapse collapse, [159,160] cavitation, [161] and coarse grained micelle fusion. [162] TPS also enables investigating of dynamical phase transitions in glassy systems, [56,[163][164][165][166][167][168] using the framework of large deviation theory.…”

“…Transitions in small molecular systems have also been studied, for example, water cluster isomerization [ 155 ] and water evaporation, [ 156 ] diffusion of water, [ 157 ] water transport through pores, [ 158 ] polymer collapse collapse, [ 159,160 ] cavitation, [ 161 ] and coarse grained micelle fusion. [ 162 ]…”