(560g) Associations in Reversibly Bonded Networks

Polymer networks present an unrealized, important, and interdisciplinary opportunity to exert molecular-level, chemical control on material macroscopic properties. Reversible network formation resulting from homobonding (A–A) associative interactions such as hydrophobic, dipolar, or other homotypic interactions includes both intra- and inter-molecular cross-linking. Intrachain association causes loop formation which serves as defects in the network, but also as stored length which may toughen the gel. Hybrid Monte Carlo/molecular dynamics simulations of a minimal coarse-grained model of polymers with associative groups distributed evenly along the chain contour and a unique “fractal cactus” model explain the looping at different scales. Meanwhile, two-component solutions with heterotypic (A–B) associations, such hydrogen bonding, metal–ligand, electrostatic, or other pairwise associative interactions, form alternately cross-linked networks with exclusively interchain associations. Homogeneous A–B networks are most easily stabilized near stoichiometric conditions between A and B associative groups, resulting in a re-entrant sol–gel–sol transition as the overall composition is altered. Further, the chemical incompatibility between the A and B polymers drives a competition between attractively and repulsively-driven phase separation, leading to microphase formation and eutectic behavior. Accordingly, both homo- and hetero-binding associative interactions slow molecular relaxations leading to a more general description of the sticky rouse and reptation models.