Synthesis and properties of polyurethanes from aromatic diol and diisocyanates

Abstract

Six kinds of polyurethane homopolymers and random copolymers were prepared with varying combinations of diols and diisocyanates by solution polymerization. For diol components, bishydroxyhexoxybiphenyl (BHHBP) was used as aromatic diol and hexanediol as aliphatic diol. Toluene 2,4-diisocyanate (TDI) and o-tolidine diisocyanate(TODI) were used for aromatic diisocyanate components. The thermal properties and mesomorphic behaviour were studied by using differential scanning calorimeter and polarizing optical microscope. The polyurethane prepared from BHHBP and TDI gave multiple endotherms in DSC thermograms but, contrary to the reported liquid crystallinity of the polymer, any mesomorphic texture was not observed. The temperature and frequency sweep were made to obtain information on rheological properties of the polymer using rheometrics dynamic spectrometer. The observed rheological behaviour confirmed the absence of liquid crystallinity. The measured viscoelastic parameters such as storage modulus(G``) and loss modulus(G") were interpreted in view of Han and Jhon``s theory. The polyurethane synthesized from hexanediol and TODI formed liquid crystalline structure, which was manifested by stir-opalescence observed through polarizing optical microscope. the typical liquid crystalline behaviour was also observed in the subsequent viscoelastic measurement: yield behaviour, relatively long relaxation time and shear thinning at low frequency range. The other polyurethane copolymers did not show clear mesomorphism. The crystal melting temperatures of the polyurethane random copolymers exhibited a eutectic point which is in relatively good agreement with the theoretical prediction of Flory. It was found that the copolymerization with monomers markedly different in chemical structure not only disrupts the structural regularity of the crystal but hinders the mesophase formation.