Quantitative comparison of stress relaxation and chain exchange in triblock copolymer networks

Triblock polymers can self-assemble into spherical micelles in the bulk and in a selective solvent for one of the blocks. Micelles formed from BAB triblock polymers, where the end blocks form the micelle core, have numerous practical applications. For example, these materials are often utilized as thermoplastic elastomers, where the B blocks assemble into glassy cores that are connected by inter-micelle bridges formed from elastic A blocks. These bridges serve as dynamic physical crosslinks, resulting in materials with temperature-dependent relaxation behavior. It has long been assumed that macroscopic stress relaxation of these networks is intimately related to molecular-level chain pullout and motion; however, direct measurements of the latter are experimentally difficult. Here, we present the first measurements of molecular-level chain exchange in concentrated triblock networks using time-resolved small-angle neutron scattering (TR-SANS). We show that stress relaxation occurs many orders-of-magnitude more rapidly than chain exchange, which is a very counter-intuitive result. However, we are able to quantitatively explain this difference by accounting for dispersity within core block lengths and thereby establish a direct relationship between molecular-level chain movement and stress relaxation in triblock networks.