Effect of soft segment molecular weight on the microcellular foaming behavior of TPU using supercritical CO₂

This study investigates the foaming behavior of TPU samples with three different soft segment (SS) molecular weights of 1000, 2000, and 3500 g/mol. The variation of SS length did not only enhance the flexibility of the TPU samples but could also ease the hard segment (HS) crystallization with various size/closed packed structure with much wider HS crystal melting peak. Such broadened melting peak expanded the foam processing window in TPUs with higher SS length. Moreover, the results showed that the increase in SS length could not only enhance the foam expansion behavior but could also enhance the heterogeneous cell nucleation and more importantly minimize the shrinkage of the TPU foams as a result of broadened HS crystallization. The saturation temperature, pressure, and time could also differently influence the formation of HS crystals in each TPU with different structure which could eventually differently affect the TPUs’ foaming behavior.