Packing, Phase Separation, and Interface Compatibility of Polymers with Reversible Cross-Links

Reversible cross-links introduce structural reorganization in a polymer matrix. Two key features that are particularly relevant when reversible cross-links are used to compatibilize polymer interfaces are how they affect the (i) bulk polymer density and (ii) interfacial activity of the cross-link groups. To probe these issues, we model both a bulk polymer melt and a thin film of a polymer melt, both with explicit small molecular cross-linkers, in the associative limit, i.e., when the number of cross-links is fixed over time. We show that the bulk density and the distribution of stickers within a polymer matrix are strongly influenced by their size and interactions with the base polymer. Specifically, when the cross-linkers are chemically compatible with the base polymer, the overall packing fraction increases, regardless of cross-linker size, while it decreases when cross-linkers are incompatible with the polymers. Similarly, the cross-linkers segregate preferentially to the polymer–air interface when they are incompatible with the polymer chains, leading to a reduction in interfacial tension. These results demonstrate the key role of cross-linker-polymer interactions and cross-linker size on the structural and interfacial properties of polymer melts with reversible cross-links. We draw parallels between nanoparticle-filled polymer systems and polymers cross-linked by molecular-scale cross-linkers, as both introduce localized constraints that modify chain packing, dynamics, and interfacial properties.