From Shape Recovery to Super-Expansion: Dual-Stage Actuation in Polyurethane Hybrid Foams

Abstract

This study demonstrates that thermally expandable microspheres (TEMs) can serve as latent expansion fillers in shape-memory polyurethane foams (SMPFs), enabling a dual-stage, thermally triggered volumetric expansion beyond the conventional shape-memory response. The SMPFs were designed using a semicrystalline soft segment and a flexible cross-linker, with water acting as a foaming agent. The addition of TEMs (5–15 wt %) resulted in foams exhibiting a distinct two-step expansion behavior following thermo-mechanical compression. Upon heating above the transition temperature (Ttrans), the foams displayed an initial expansion governed by the shape-memory effect (SME) of the polyurethane matrix, enabling nearly a complete recovery (Rr ∼ 100%) of the original geometry. Further heating to approximately 140 °C induced a second, pronounced volumetric expansion driven by the thermal expansion of the encapsulated microspheres. The extent of volumetric expansion was strongly dependent on the TEMs content, with the highest expansion ratio (∼58%) observed for foams containing 15 wt % TEMs. In addition, the secondary expansion led to a noticeable increase in pore size and pore wall thickness and a significant reduction in density. The foams demonstrated thermally switchable jamming when inserted in a tubular glass, where secondary expansion increased radial pressure and load-bearing capacity to stabilize a metal rod. These findings demonstrate a promising strategy for designing smart foams with programmable and enhanced expansion and shape-morphing capabilities.