Dynamic mechanical properties of linear and cross-linked polyurethane

Linear polyurethane melts were prepared by a polycondensation reaction of p oly(oxypropylene) (POP) diol with a diisocyanate. Covalently cross-linked g els were obtained using three-armed star POP triol. The glass transition te mperature and the viscoelastic properties were investigated as a function o f the molar mass of the POP precursors. The variation of T-g is dominated b y the density of urethane links. The loss peak of the shear modulus at high frequencies or low temperatures broadens with increasing density of uretha ne links. The gel modulus of end-linked POP triol decreases linearly with i ncreasing molar mass of the precursors. The loss shear modulus of end-linke d POP triol has a power law frequency dependence at low frequencies. The ex ponent 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. (C) 1999 The Society of Rheology. [S0148-6055(99)00906-2].