Direct Comparison of Probe Reorientation and Linear Mechanical Measurements of Segmental Dynamics in Glassy Poly(methyl methacrylate)

Optical probe reorientation experiments and mechanical stress relaxation measurements in the linear response regime were performed on a polymer glass. Segmental dynamics of lightly cross-linked poly(methyl methacrylate) (PMMA) were monitored via both methods at temperatures between 9 and 24 K below the glass transition temperature (T-g) and over the course of 8 h of aging. Relaxation times from the two methods cover a range of similar to 1.7 decades. Similar results were obtained from the two experiments other than a slight difference in the aging rate. A difference of similar to 0.3 decades (a factor of similar to 2) is observed between the optically and mechanically obtained relaxation times, related primarily to the size of the probe. Both experiments give a value near 0.3 for the Kohlrausch-Williams-Watts (KWW) beta parameter. These results provide an important baseline for the interpretation of optical probe reorientation experiments during nonlinear deformation.