Phase behavior, self-assembly, and interfacial tension of a dynamically linked polymer blend

Compatibilizing immiscible polymer blends is a challenge and a significant barrier to improving the recycling of polymers. Recent computational and experimental studies have demonstrated that the addition of dynamic cross-links (bonds between chains with a finite lifetime) provides a potentially viable approach to compatibilize blends. We implement a Metropolis-based algorithm for creating reversible dynamic bonds within molecular dynamics simulations, which allows us to systematically investigate the impact of dynamic cross-linking on phase behavior, self-assembly, and surface tension. We find that the presence of dynamic cross-linkers between different polymer types decreases both the critical temperature for phase separation and surface tension, i.e., improved compatibilization. We also show that the assembly of cross-linked chains can be mapped to a coarse-grained model of patchy particle self-assembly, where the effective number of sticky sites increases with chain length. The competing effects of inter-species cross-links vs self-cross-links (between the homopolymer chains of the same chemistry) are also systematically examined. Curiously, we find that when the propensity for cross-links between different polymer types is the same as the propensity for cross-links with the same type, the effect of dynamic bonds on phase behavior and surface tension is essentially neutralized, and thus, the blend behaves nearly the same as a cross-link-free blend. Moreover, when the propensity for self-cross-linking is higher, the blend becomes less miscible, so the type of cross-linking is critically important. In addition, we demonstrate that the surface tension for phase-separated systems with varying propensities to form dynamic bonds collapses to a master curve that is dictated by the interfacial density of distinct dynamic cross-links.