A forced torsional oscillator for dynamic mechanical measurements

A forced torsional oscillator (FTO) for measuring dynamic properties from 0.01 to 30 Hz and −190 to +250°C is described. It uses a digital transfer function analyzer to determine G′ and G″ directly. Errors such as clamping corrections, phase angle resolution, and instrument phase shift are evaluated. FTO measurements vs temperature on polymethylmethacrylate (PMMA) and polycarbonate compare well with free torsion pendulum data. Measurements on a silicone liquid vs frequency compare well to eccentric rotating disks data. The frequency sensitivity of the dynamic mechanical response of PMMA and polycarbonate is evaluated at 0.1, 1 and 10 Hz. The frequency dependence of the transitions is fit to an Arrhenius relation and activation energies calculated. A dimensionless transition temperature shift, ΔT*, giving the shift for a one decade frequency change divided by the temperature, is found to be approximately 0.01 for the glass transition and 0.07 for several secondary transitions in glassy polymers.