New model for physical bond breaking in associating copolymer liquids

We combine ideas from polymer and glass physics to construct a new model for the mean bond breaking time scale of attractive sticker groups in associating copolymer liquids that form transient networks. The activated bond breaking event is argued to be a two-step process. The first step defines the primary alpha relaxation and involves the release of non-sticker dynamic caging constraints which can be strongly modified by long-lived physical crosslinks, for which a microscopic theory has been recently developed (Macromolecules 53, 4366 (2020)). The slower bond breaking process involves attractive stickers surmounting an association free energy barrier subject to a local frictional resistance which can be strongly affected by relaxation-diffusion decoupling induced by locally heterogeneous dynamics. The latter is strongly polymer chemistry specific, and related to the fragility of the alpha process. The ideas embedded in the model produce a consistent and good description of the bond breaking timescale for diverse polymer chemistries and architectures as a function of both temperature and fraction of sticky groups. Chemically sensible values for association free energies are deduced. In strong contrast, the existing phenomenological models are shown to incur qualitative failures.