Some physical properties associated with the urethane group—II: Dielectric relaxation in thermoplastic polyurethane elastomers

Abstract

Dielectric permittivity and loss measurements have been made between 10 2 and 10 10 Hz on samples of polyethylene glycol 200 polyurethane, polyethylene glycol 600 polyurethane and polypropylene glycol 450 polyurethane elastomers. A high dielectric loss in the ultra high frequency region, like that observed (3) in N-ethyl urethane liquid, was not found. The polyethylene glycol polyurethanes exhibit low dielectric permittivity and loss factor at temperatures below a transition temperature measured by differential scanning calorimetry. Above this temperature, large dielectric permittivities and loss factors are observed at low frequencies, a phenomenon ascribed to the inclusion of a second phase of high interfacial polarizability within the elastomer matrix. Such observations support the two-phase structure of the elastomers proposed (1, 2) to explain their mechanical properties. The polypropylene glycol polyurethane also exhibits a high dielectric permittivity and loss factor at low frequencies, which phenomenon is best explained by the existence of Maxwell—Wagner two-phase interfacial polarization. However in this case the high losses and polarizabilities (and also elastomeric behaviour) are exhibited at temperatures below the transition temperature measured by differential scanning calorimetry. It is suggested that the true glass temperature of the polymer lies below −20°, but the dielectric results alone do not permit more than speculation as to the nature of the calorimetric transition.