Double-Network Formation and Mechanical Enhancement of Reducing End-Modified Cellulose Nanocrystals to the Thermoplastic Elastomer Based on Click Reaction and Bulk Cross-Linking

In addition to being a renewable nanomaterial, cellulose nanocrystals (CNCs) exhibit a high specific modulus and are widely used as a reinforcing phase (filler) to improve the mechanical performance of polymeric materials. In these composite systems, the filler–matrix, filler–filler, and matrix–matrix interactions are critical factors that govern the mechanical properties of the composites. Inspired by the idea of combining these three interactions, we design a novel composite system of reducing an end-modified CNC-enhanced thermoplastic elastomer [styrene–butadiene–styrene copolymer (SBS)] with click reaction and bulk cross-linking. The strong linkage between the nanocrystals and SBS (filler–matrix) is first achieved by the thiol–ene click reaction induced by UV irradiation in the liquid compounding process, accompanied by the preservation of surface hydroxyl groups on nanocrystals and therefore the formation of a stable percolation network (filler–filler). The matrix–matrix network is further constructed in the composite by chemical self-cross-linking of bulk SBS with a post-irradiation treatment during molding process. Benefiting from these three strong interactions, a remarkable improvement in mechanical performance is accomplished for the fabricated composite, exhibiting simultaneous increases in strength (239%), modulus (411%), work of fracture (330%), and elongation at break (7%) in comparison with those for the pure SBS material. Finally, the percolation, Halpin–Kardos, and double-network models with three interactions are applied to compare the theoretical and experimental data for mechanical properties and further discuss the enhancing mechanism for the composites.