Reprocessable Polyhydroxyurethane Network Composites: Effect of Filler Surface Functionality on Cross-link Density Recovery and Stress Relaxation.

Conventional polymer composite networks cannot be recycled for high-value applications because of the presence of permanent covalent cross-links. We have developed reprocessable polyhydroxyurethane network nanocomposites using silica nanoparticles with different surface functionality as reinforcing fillers. The property recovery after reprocessing is a function of the interaction between the filler surface and the network matrix during the network rearrangement process. When non-reactive silica nanoparticles lacking significant levels of surface functional groups are used at 4 wt% (2 vol%) loading, the resulting network composite exhibits substantial enhancement in mechanical properties relative to the neat network and, based on values of rubbery plateau modulus, is able to fully recover its cross-link density after a reprocessing step. When nanoparticles have surface functional groups that can participate in dynamic chemistries with the reprocessable network matrix, reprocessing leads to losses in mechanical properties associated with cross-link density at potential use temperatures, along with faster rates and lower apparent activation energies of stress relaxation at elevated temperature. This work reveals the importance of appropriate filler selection when polymer network composites are designed with dynamic covalent bonds to achieve both mechanical reinforcement and excellent reprocessability, which are needed for the development of recyclable polymer network composites for advanced applications.