Dynamics and Structure of Unentangled Associative Polymers

Synthetic and biological molecules capable of forming networks via reversible bonds have highly desirable properties, including reconfigurability and stability. These networks can be viscoelastic, which is a characteristic of polymers in the molten state. Vitrimers are a class of chemically associative polymer networks that undergo reversible bond-exchange reactions (BERs) at a constant cross-linking density under external stimuli. By combining molecular dynamics and Monte Carlo simulations, we studied the structure, dynamics, and viscoelastic behavior of vitrimers. Vitrimers with short chain lengths but high bond-exchange energy barrier U_b and cross-linker concentration r_BA are found to follow reptation-like dynamics, which is a model used to describe highly entangled (long chain) polymeric systems. In the reptation model, a highly entangled chain is considered to move back and forth (reptate) along the contour of a fictitious tube created by the topological constraints imposed by surrounding chains. Here, the reptation-like motion is observed in unentangled vitrimers by demonstrating subdiffusive fractional scaling exponents of the mean-square displacements of inner monomers and center of mass with time characteristic of the reptation model before diffusive behavior. The variation of U_b is found to have little influence on the static structures of vitrimers, yet it strongly influences the transition from a viscous liquid to a reversible elastomer. Our results provide guidelines for the development of theoretical models of vitrimers and biopolymers with associative groups.