Mechanical spectral hole burning in glassy polymers-Investigation of polycarbonate, a material with weak β-relaxation

Mechanical spectral hole burning (MSHB) has been used to investigate the nonlinear dynamics in polymers, ranging from melts, solutions, block co-polymers, and glasses. MSHB was developed as an analog to the dielectric spectral hole burning method, which is not readily applicable in polymers due to weak dielectric response. While similar holes were observed in both mechanical and dielectric hole burning, the interpretations were different. In the latter case, it has been argued that the holes are related to dynamic heterogeneity as related to an increase in the local temperature of molecular sub-ensembles (spatial heterogeneity), while in the former case, the holes have been related to the type of dynamics (rubbery, Rouse, etc.). Recent work from our laboratories used MSHB to investigate glassy poly(methyl methacrylate) and showed evidence of hole burning and supported the hypothesis that the origin of holes was related to dynamic heterogeneity as evidenced by the holes being developed near the strong beta -relaxation in PMMA. In this work, MSHB is used to study polycarbonate, which has a weak beta -relaxation, and the results are compared with those observed in PMMA. We observe that the polycarbonate exhibits weak holes and the nature of the holes with a change in pump amplitude and frequency is different than observed in PMMA. These results support the hypothesis that the hole burning observed in amorphous polymers below the glass transition temperature is related to the strength of the beta -transition, which, in turn, is related to molecular level heterogeneity in the material dynamics.

Automated summary

Mechanical spectral hole burning (MSHB) has been used to investigate the nonlinear dynamics in polymers, providing insights into dynamic heterogeneity and molecular level heterogeneity. Results from studying polycarbonate using MSHB show differences in hole characteristics compared to poly(methyl methacrylate), supporting the hypothesis that hole burning in amorphous polymers is related to the strength of beta-transition.