Determination of the viscoelastic Poisson's ratio of polymers using a combined torsional-axial DMA

摘要

The lateral contraction of a material when stressing the material in axial direction is described by the Poisson's ratio [1]. In case of viscoelastic materials, like polymers, this parameter is a function of time (frequency) and temperature and important for e.g. structural mechanics simulations [1,2]. Several methods are described to determine the viscoelastic Poisson's ratio. The methods can be classified into direct methods which directly measure the change of the dimensions of the specimen and indirect methods from which the measurement of two moduli like shear modulus and Young's modulus[3]. A new measuring device concept is introduced which combines an Electronically commutated (EC) motor as rotational top drive and a moving magnet linear drive as bottom drive to enable torsional and axial dynamic mechanical analysis (DMA) measurements on one single device. The contribution highlights a testing procedure using the above described combined torsional-axial dynamic mechanical analyzer (MCR 702 MultiDrive) and clamping fixtures for cylindrical as well as rectangular specimens in order to determine the viscoelastic Poisson's ratio of different solid polymers. Using a linear and a rotational measuring drive in one instrument enables the determination of complex Young's modulus |E*| as well as the complex shear modulus |G*| with a single sample in a continuous measurement run. Consecutive frequency sweeps at different temperatures in both, torsion and tension deformation modes were performed to obtain the viscoelastic Poisson's ratio, following the protocol proposed by Tschoegl et al. [2].The suitability of the method is further examined and discussed by comparing the results with literature data.