Structure transition of aliphatic m,(6-)Polyurethane during heating investigated using in-situ WAXS, SAXS, and FTIR

Thermoplastic polyurethanes (TPU) produced in the reaction of hexamethylene diisocyanates (HDI) and diols are widely applied in industry. The phase transition behavior of the HDI-diol hard domain was investigated as a structural analog to conventional polyamides (PA), wherein an oxygen atom substituted a methylene group. This substitution brought about a distinct phase transition behavior; the apparent merging of two diffraction peaks into the intermediate peak was not observed; upon heating, the methylene groups orderly packed in the crystalline domain in a trans conformation, gradually transforming into a disorderly gauche conformation was also absent. Such a transition observed in this work is not in accordance with the Brill transition no matter that it is defined by apparent experimental criterion or essential molecular fundamental. The melting sequences of the phase transition, i) mesophase; ii) crystallites; iii) blocks mixing, were jointly revealed by multiple in-situ techniques, WAXS, FTIR and SAXS.