Immiscible polydimethylsiloxane/polybutadiene vitrimer blends

Vitrimers are polymer networks cross-linked by dynamic covalent bonds that undergo associative exchange, where new bonds form before existing bonds break. Converting a polymer mixture into a vitrimer offers a promising strategy for compatibilizing otherwise immiscible polymers. In this work, we investigate immiscible vitrimer blends of polydimethylsiloxane (PDMS) and polybutadiene (PB) containing dioxaborolane cross-links that undergo associative exchange. Using photo-initiated thiol–ene click chemistry, we cross-link PDMS and PB in solution to prepare blends with varying PB content and cross-link density. The solution state morphology prior to cross-linking dictates the morphology of the resulting dry blends. Differential scanning calorimetry suggests that vitrimer blends exhibit enhanced mixing between PDMS and PB, whereas permanently cross-linked blends do not. Small-angle X-ray scattering shows that some vitrimer blends undergo microphase separation, forming cross-link aggregates. Rheological measurements indicate that associative cross-links influence the fast and slow relaxation regimes in distinct ways. Fast relaxations, which are sensitive to PB content, display thermorheological complexity due to the different temperature dependences of PDMS and PB segmental motions. In contrast, slow relaxations are independent of the PB content, obeying both time-temperature and time-composition superposition. Potential mechanisms governing the phase behavior and slow relaxation regime are discussed.