Too Much of a Good Thing? Thermodynamic Limits of Supramolecular Polymer Compatibilizers.

Recycling mixed plastic waste remains an unsolved challenge in materials science, largely because the constituent polymers are chemically incompatible and resist blending into useful materials. Compatibilizers address this by localizing at polymer-polymer interfaces to lower interfacial tension, but conventional designs based on block or graft copolymers require the tailored synthesis for each polymer pair. Here, we develop a theoretical description of supramolecular compatibilizers, which are homopolymers bearing complementary associative groups that reversibly bond at polymer-polymer interfaces. We show that the dilute addition of these compatibilizers lowers the interfacial tension, often outperforming conventional block copolymers; however, this benefit has a limiting behavior, terminating once the compatibilizer macrophase separates into a bond-rich third phase that wets the interface. The number of associative sites per chain, M, governs the existence and character of this wetting transition and, in part, determines the effectiveness of supramolecular compatibilizers. Specifically, the tendency for network formation at the interface in M > 2 compatibilizers strongly limits the thermodynamic reduction in interfacial tension. These results frame supramolecular compatibilization as a competition between interfacial association and bulk phase separation, establishing a thermodynamic limit on additive efficacy.