(553c) Mechanical Spectral Hole Burning in Glassy Polymers

Glasses find extensive application in different fields such as optics, pharmaceuticals, aerospace, and electronics [1]. Polymeric glasses, in the deep glassy state exhibit excellent mechanical properties and are commonly used in engineering applications. Linear viscoelastic properties of polymers are well characterized by mechanical and di-electric spectroscopies, and to go beyond linear response, methods such as large amplitude oscillatory shear (LAOS) provide information related to the material response under large deformations (non-linear deformation). These LAOS tests have been applied extensively to soft materials such as polymer solutions and polymer melts [2], but very few works [3] have reported the non-linear dynamics of polymers in deep glassy state. Hence understanding of non-linear response of glassy polymers is important from both fundamental and practical perspective.

Hole burning spectroscopy has been used to investigate the non-linear dynamics of glass forming materials. Non-resonant spectral hole burning (NSHB) was originally developed by Schiener et al. [4] and that group attributed the presence of holes to dynamic heterogeneity. On the contrary, the majority of polymers exhibit low di-electric responses and, therefore, NSHB is not readily applied to these materials. In the present work we present results from mechanical spectral hole burning (MSHB) [5] experiments, a method derived from the initial ideas of NSHB [4], to investigate the non-linear dynamics of glassy polymers. MSHB has been used in our labs and has been successfully applied to polymer melts and solutions [6,7]. These prior works concluded that the nature and presence of holes are related to the type of dynamics rather than to dynamic heterogeneity as related to the glass transition event.

In the current work, MSHB has been applied to amorphous glassy poly(methyl methacrylate) (PMMA) to investigate the non-linear dynamics close to β transition [8]. These results are compared with those of polycarbonate which exhibits only a weak β transition, with the final aim to test the hypothesis that the hole burning event in the deep glassy state is related to the dynamic heterogeneity as reflected in the occurrence of a strong β transition. Our initial results on polycarbonate show a weak hole intensity compared to PMMA in the same experimental conditions. These initial results are consistent with the hypothesis that the β transition event in glassy polymers influences the hole burning event. Future tests will be carried out at different temperatures (i.e. distance from the β transition) and the impact on the hole burning event will be investigated.

References

[1] Kearns, K.L., et al., Hiking down the energy landscape: Progress toward the Kauzmann temperature via vapor deposition. The Journal of Physical Chemistry B, 2008. 112(16): p. 4934-4942

[2] Hyun, K., Wilhelm, M., Klein, C. O., Cho, K. S., Nam, J. G., Ahn, K. H., ... & McKinley, G. H. (2011). A review of nonlinear oscillatory shear tests: Analysis and application of large amplitude oscillatory shear (LAOS). Progress in Polymer Science, 36(12), 1697-1753.

[3] W. Davis and C. Macosko, “Nonlinear dynamic mechanical moduli for polycarbonate and PMMA,” J. Rheol. 22(1), 53–71 (1978)

[4] Schiener, B., et al., Nonresonant spectral hole burning in the slow dielectric response of supercooled liquids. Science, 1996. 274(5288): p. 752-754

[5] Shi, X. and G.B. McKenna, Mechanical hole burning spectroscopy: Evidence for heterogeneous dynamics in polymer systems. Physical review letters, 2005. 94(15): p. 157801

[6] Shi, X. and G.B. McKenna, Mechanical hole-burning spectroscopy: Demonstration of hole burning in the terminal relaxation regime. Physical Review B, 2006. 73(1): p. 014203

[7] Shamim, N. and G.B. McKenna, Mechanical spectral hole burning in polymer solutions: Comparison with large amplitude oscillatory shear fingerprinting. Journal of Rheology, 2014. 58(1): p. 43-62

[8] Mangalara, S. C. H., & McKenna, G. B. (2020). Mechanical hole-burning spectroscopy of PMMA deep in the glassy state. The Journal of Chemical Physics, 152(7), 074508.