Dynamic Covalent Nanoparticle Network Enabling Shrinkage-Resistant and Closed-Loop Recyclable Microcellular Ethylene Copolymer Foams

Thermoplastic elastomer foams have attracted considerable attention owing to their lightweight nature, flexibility, and processability. However, their widespread application is limited by unstable cellular structures, severe post-foaming shrinkage, and poor recyclability. In this work, epoxy-functionalized ethylene copolymer (EGMA) was employed as the matrix, and a dynamic covalent network containing B-O bonds was constructed through the ring-opening reaction between 3-aminophenylboronic acid (APBA) and epoxy groups. Hydroxyl-rich SiO2 nanoparticles were further introduced to establish a dynamic nanoparticle network, yielding a thermoplastic elastomer foam with enhanced foamability, dimensional stability, and recyclability. The synergistic effect of the dynamic covalent network and SiO2 nanoparticles significantly improved the modulus, creep resistance, and melt stability of the elastomer. Under the optimized foaming condition of 80 °C and 20 MPa, a microcellular foam with an average cell size of 8.94 μm and a cell density of 3.73 × 1010 cell/cm3 was obtained. Compared with E-A and E-Si foams, the E-A-Si foam exhibited outstanding dimensional stability, retaining 94.09% of its original volume after 24 h. The dynamic B-O bonds enabled moderate topological rearrangement during foaming, promoting uniform nucleation and suppressing cell rupture. After foaming, SiO2 nanoparticles participated in a dynamic nanoparticle network through B-O-Si linkages, which effectively restricted stress relaxation and chain retraction, thereby stabilizing the cellular structure and reducing post-foaming shrinkage. Moreover, the E-A-Si foam maintained an intact cellular morphology and excellent elastic recovery after two closed-loop refoaming cycles. This work provides a new strategy for developing thermoplastic elastomer foams with high dimensional stability and closed-loop recyclability.