Insights into the quasi-static mechanics of transversely isotropic polymer foams

Owing to their favorable balance of mechanical performance, lightness, and cost-effectiveness, polymer foams are the material of choice in numerous structural applications, where knowledge of the relationship between their microstructure and mechanical behavior is needed for design purposes. This research investigates how microstructural anisotropy affects the quasi-static mechanics of transversely isotropic polymer foams. Such foams were designed and produced by expanding polystyrene preforms with different polydispersity indices in cylindrical molds via a batch-foaming method. Microstructural anisotropy was defined to include both the aspect ratio and orientation of the cell, allowing us to apply the well-known homogenized rectangular cell model by Gibson and Ashby (1997) and isolating the effect of microstructural anisotropy. We observed that Young's modulus increased more than threefold and non-linear properties more than twofold within a microstructural anisotropy range from 0.69 to 1.43. Additionally, it is shown that the rheology of the base polystyrene influenced anisotropy development. In conclusion, design maps for transversely isotropic foams are proposed. These findings improve our understanding of the process-structure-property relationship in anisotropic polymer foams, fostering a more effective and efficient use of such ubiquitous materials.