Dynamic mechanical properties of linear and cross-linked polyurethane

Linear polyurethane melts were prepared by a polycondensation reaction of poly(oxypropylene) (POP) diol with a diisocyanate. Covalently cross-linked gels were obtained using three-armed star POP triol. The glass transition temperature and the viscoelastic properties were investigated as a function of the molar mass of the POP precursors. The variation of Tg is dominated by the density of urethane links. The loss peak of the shear modulus at high frequencies or low temperatures broadens with increasing density of urethane links. The gel modulus of end-linked POP triol decreases linearly with increasing molar mass of the precursors. The loss shear modulus of end-linked POP triol has a power law frequency dependence at low frequencies. The exponent of the power law dependence decreases with increasing molar mass of the precursors. Gels formed with POP triol with molar mass larger than 6 kg/mol show the effect of entanglements at intermediate frequencies.