Thermoplastic elastomers based on strong and well-defined hydrogen-bonding interactions

In order to investigate the effects of strong and well-defined hydrogen bonding on the properties of thermoplastic elastomers (TPEs), we have applied two distinct and strongly dimerizing 2-ureido-4-[1H]pyrimidinone (UPy) quadruple hydrogen-bonding units as physical cross-linker. While the UPy groups consequently serve as the "hard phase" or "hard block" in these TPEs, an amorphous polyester has been used as the "soft phase" or "soft block". The UPy unit has been flanked with either a sterically demanding isophorone spacer or a linear hexamethylene, where these spacers have been derived from isophorone diisocyanate (IPDI) or hexamethylene diisocyanate (HDI), respectively. This difference on a molecular level leads to a homogeneous amorphous material in the IPDI case (polymer 1) and to a nanophase-separated material in the HDI case (polymer 2) as revealed by AFM and DSC experiments. Apart from this distinctive difference in morphology and nanoscopic organization, the macroscopic properties of both materials are also fundamentally different. In linear stress-strain and DMTA experiments, the homogeneous IPDI material 1 shows rubberlike behavior at room temperature, while the mechanical properties are strongly temperature dependent. The nanophase-separated HDI material 2 shows a combination of rubber and plastic behavior and displays a clear rubber plateau between 0 and 50 degrees C with little temperature dependence. The apparent activation energy of flow for the latter material, determined by multifrequency DMTA, is 135 kJ mol(-1).