Pre-processing history dependent foaming behavior and morphing of thermoplastic polyurethane

Abstract

Thermoplastic polyurethanes (TPUs) foams are valued for their low density, energy absorption, low thermal conductivity, and, in general, tunable properties. These features make them ideal for applications such as sportswear, flexible electronics, shape memory sensors, and soft robotics. Foaming of thermoplastics is highly affected by the state of the polymer in terms of molecular chain arrangement, crystallinity, and residual stresses, which, in turn, are strongly influenced by the pre-processing history of the material. These complexities, together with multiphase morphology, make TPU foaming particularly challenging towards high expansion ratios. Preforms with different thermal and deformation histories are here selected to serve as models for exploring the relationship between pre-processing and foaming, with and without the addition of fillers. We analyze the expansion ratio, foam morphology, microstructural features, and post-foaming shrinkage of neat TPU, 3D-printed TPU structures, and TPU composites with multi-walled carbon nanotubes and aluminum nanoparticles using the batch foaming technique under varying processing conditions. Results show the critical role of pre-processing (in terms of printing parameters) and addition of fillers in influencing the foamability, and highlight microstructural control through pre-processing as a key strategy to tailor TPU foams for advanced structural and functional applications. • Foamability of 3D printed, neat and loaded, TPU is compared to as-received granules. • Pre-processing history critically controls foam density, morphology and shrinkage. • Microstructural tailoring controls gas transport and foaming. • Exploitation of local differences in foamability leads to foam-induced morphing.