The effect of mixed soft segment on the microstructure of thermoplastic polyurethane

In this work, we fixed the hard segment content (Ch%) and synthesized thermoplastic polyurethane (TPU) elastomers with one-soft segment (HTPB-PU, PTMG-PU, and PCL-PU) and bi-soft segment (PTMG-HPU and PCL-HPU), using 4,4′-diphenylmethane diisocyanate, 1,4-butanediol, and different oligomer diols as raw materials. This work was used to explore the impact of two polar diols (polytetrahydrofuran diol [PTMG]; polycaprolactone diol [PCL]) of the hydroxy-terminated polybutadiene (HTPB)-based TPU on the microstructure and macroscopic properties. Using Fourier transform infrared spectroscopy, X-ray diffraction, and differential scanning calorimetry to characterize TPU, the results showed that the introduction of PCL was better than PTMG in promoting microphase mixing and crystallization, and reducing microphase separation. This result was closely related because the carbonyl group in PCL was stronger than the ether bond in PTMG in forming hydrogen bonds with ─NH bond. Through mechanical test and isopropyl alcohol (IPA) resistance test, the results showed that HTPB-based TPU with PCL (PCL-HPU) significantly improved tensile strength and elongation at break, and only a small reduction of Young's modulus was observed. PCL-HPU had the best retention of the IPA resistance of HTPB-based TPU.