Micro-phase-separation behavior of amphiphilic polyurethanes involving poly(ethylene oxide) and poly(tetramethylene oxide)

Abstract

The temperature dependence of thermal, morphological, and rheological properties of amphiphilic polyurethanes was examined with differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), rheological measurements, and Fourier transform infrared spectroscopy. Multiblock (MPU) and triblock (TPU) polyurethanes were synthesized with two crystallizable segments - poly(ethylene oxide) (PEO) as a hydrophilic block and poly(tetramethylene oxide) (PTMO) as a hydrophobic block. DSC and WAXS measurements demonstrated that the microphase of MPUs in the solid state is dominantly affected by the PEO crystalline phase. However, high-order peaks were not observed in the SAXS measurements because the crystallization of the PEO segments in MPUs was retarded by poor sequence regularity. The microphase in the melt state was induced by the hydrogen bonding between the NH group of hexamethylene diisocyanate linkers and the ether oxygen of PEO or PTMO blocks. As the temperature increased, the smaller micro-phase-separated domains were merged into the larger domains, and the liquidlike ordering was eventually disrupted because of the weakening hydrogen bonding. However, the fully homogeneous state of an MPU with a molar ratio of 5/5 PEO/PTMO (MPU55) was not confirmed even at much higher temperatures with both SAXS and rheological measurements. However, the SAXS patterns of TPU showed weak but broad second-order peaks below the melting temperature of the PEO block. Compared with MPU55, the ordering of the TPU crystalline lamellar stacks was enhanced because of the high sequence regularity and the low hydrogen-bonding density.