TIME-RESOLVED RHEOMETRY

Applicability and limits of time-resolved rheometry have been analyzed for polymers which undergo change during a rheological measurement. P rocesses such as gelation, phase transition, polymerization or decompo sition affect the molecular mobility in these polymers and therefore t he rheological experiment. We propose to choose the well known effect of heating (or cooling) during the relaxation and analyze it as a para digm for rheometry on samples with changing molecular mobility. The te mperature change does not cause permanent changes in sample structure, but it affects the molecular mobility and it significantly interferes with the measurement if the temperature changes occur too fast. In th is study, time-resolved mechanical spectroscopy (TRMS) was used to exp erimentally investigate the effect of heating on the relaxation behavi or of a typical polycarbonate sample. Each data point in a cyclic freq uency sweep (CFS) was taken at a different state of the material; the data were interpolated using an interactive computer program. In this fashion, a single TRMS experiment yielded a master curve over eight de cades. A model for relaxation under non-isothermal conditions showed t he limitations of TRMS. It could be demonstrated that TRMS worked well for sufficiently small mutation numbers, i.e., for sufficiently small changes during the measurement. A critical mutation number of 0.9 was determined for the non-isothermal case beyond which the material resp onse became non-linear. This corresponds to a calculated relative chan ge of the shear stress amplitude of about 90%.