The Non-Linear Viscoelastic Response of Polycarbonate in Torsion: An Investigation of Time-Temperature and Time-Strain Superposition

Data are presented from torsional stress relaxationexperiments on a commercial polycarbonate. Tests were performedon samples over a range of torsional strains from 0.0025 to 0.08and at temperatures from 30 to 110°C at a fixed aging time of64,800 s (18 h). Following the scaling approach of Penn andKearsley [Trans. Soc. Rheol. 20 (1976)] we were able todetermine the stress relaxation response at shear strains to0.07. Then the individual data sets at each strain andtemperature could be described using a stretched exponential formrelaxation function. Over the range of temperatures studied thedata at each strain were superimposed using conventional time-temperature superposition. For strains up to the yield strainthe data at each temperature could also be superimposed to form amaster curve following the principle of time-strainsuperposition. Interestingly, the master curves found from time-strain superposition at each temperature did not have the sameform. Similarly, the master curves found from time-temperaturesuperposition at each strain did not have the same form.