Using Chemical Heterogeneity, Physical Clustering, and Cross-Linking to Control Penetrant Activated Motion in Dense Polymer Materials

The microscopic Self Consistent Cooperative Hopping Theory is employed to study the effect of polymer matrix chemical heterogeneity and selective matrix-penetrant attractions on the thermally activated dynamics of dilute penetrants. Three model polymer matrices are considered. (i) Spatially homogeneous copolymer melts where the penetrant is selectively attracted to a fraction of monomers. (ii) Cross-linked homopolymer networks where the penetrant is attracted to permanent cross-links. (iii) Associating copolymers where sticky monomers self-assemble into disordered microdomains and the penetrant is attracted to either the effectively immobilized sticky monomers or the mobile nonsticky monomer strands. An optimal value of selective monomer fraction is shown to produce a large enhancement of the selectivity of penetrant activated dynamics based on its size compared to athermal or fully attractive homopolymer matrices. The rich behavior involves an interplay of preferential solvation, attractive monomer density, and collective elastic effects on penetrant hopping. Increasing the effective penetrant-polymer attraction to the clustered monomers results in a sharp dynamic slowdown of the penetrant hopping rate. This behavior is in qualitative contrast to when the penetrant is attracted to the nonsticker mobile strand monomers, where at large attraction strengths penetrant hopping proceeds via vehicular-like mechanism nearly slaved to the matrix activated structural relaxation. Longer range microdomain scale correlations of sticky monomer clusters in the associating copolymer matrix have minimal direct effects on the penetrant activated hopping rate, in contrast to the degree of local clustering of sticky monomers. The theory is compared with recent experiments that probed how penetrant-cross-link attraction modifies the penetrant diffusivity. The effect of a range of other controllable polymer matrix and penetrant parameters on the penetrant activated dynamics is analyzed, suggesting novel design principles for achieving highly selective penetrant transport. Limitations of the theory, and other activated penetrant motion dynamical regimes not addressed, are discussed.