Recyclable Polymer Network Nanocomposites and Composites via Dynamic Covalent Bonds: Achieving Full Cross-link Density and Other Property Recovery after Reprocessing

The development of covalent adaptable networks, sometimes called vitrimers, holds promise for overcoming long-standing recycling and polymer circularity issues associated with conventional, permanently cross-linked thermosets which cannot be recycled for high-value use. It is also important to demonstrate that properties of reprocessable networks can be optimized to meet ongoing demands for high-performance materials. We have designed several network nanocomposites and composites that demonstrate that such performance demands can be met while maintaining polymer circularity and contributing to sustainability. In one example, we used a strategy based on nitroxide-mediated polymerization to synthesize reprocessable networks and network composites containing alkoxyamine dynamic bonds. The networks, including those synthesized from lab-grade polybutadiene and industrial-grade natural rubber/carbon black composites, exhibit full cross-link density recovery and essentially no creep at 80 °C, where alkoxyamine cross-links are nearly static, after multiple molding cycles at 140/160 °C, where alkoxyamine cross-links are dynamic. The ability to “turn on” and “arrest” dynamic chemistry over a relatively narrow temperature (T) window is due to the relatively high activation energy (∼120 kJ/mol) and thus strong T-dependence of the alkoxyamine dissociation reaction. In a second example, we fabricated reprocessable polyhydroxyurethane (PHU) network nanocomposites reinforced with reactive polyhedral oligomeric silsesquioxanes (POSS). With functionalized POSS as a fraction of the cross-linkers, the PHU–POSS network nanocomposites exhibit enhanced storage modulus at the rubbery plateau region relative to neat PHU network. With up to 10 wt% POSS, the network composites undergo melt-state reprocessing at 140 °C with 100% cross-link density recovery. The hydroxyurethane dynamic chemistry leads to excellent creep resistance at T up to 90 °C and is unaffected by reactive incorporation of POSS. This study demonstrates the effectiveness of POSS as nanofillers for designing high-performance, organic−inorganic dynamic PHU networks with excellent reprocessability. We will also describe situations where the presence of nanofillers can lead to incomplete property recovery after reprocessing.

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