(670b) Nonlinear Mechanics of Polymer Glasses: Mechanical Hole-Burning Spectroscopy

There is increasing interest in the non-linear viscoelastic response of glassy polymers from both practical and fundamental perspectives. Here we take the ideas of mechanical hole burning spectroscopy, developed in our laboratories at Texas Tech University for polymer melts [1-3], and apply them to an amorphous polymer deep in the glassy state. The fundamental premise of hole burning spectroscopy in the vicinity of the glass transition temperature has been that the observation of a "hole" is the result of dynamic heterogeneities in the material [4]. We have found that in polymer melts and solutions this seems not to be the case, but rather that the type of dynamics determines the nature and presence of the hole [3]. In the present work we examine the same sort of problem, but now in the glass state itself.

Mechanical hole burning spectroscopy (MSHB) is, essentially, an extension of large amplitude oscillatory shear (LAOS) methods, but is performed using a linear probe response after a large sinusoidal "pump" cycle or cycles [5]. The procedure requires two experiments at a minimum, so that in one test the pump is applied followed by a positive step and a second test is performed in which the pump is followed by a negative step in strain. The difference in responses divided by 2 gives the “modified response” and this is then compared with the linear response, in this case the relaxation modulus. If the modified response is the same as the linear response, the material is in the linear regime. Holes are burned in specific circumstances and can be either in vertical or horizontal directions. In entangled polystyrene solutions holes have been observed in the transition region from near the rubbery plateau to the terminal flow regime, i.e., above the glass transition temperature [3].

Similar testing is being performed on amorphous polymers in the glassy state. The ultimate goal is to relate the hole burning event with the dynamic heterogeneity of the polymer as evidenced by a strong beta-relaxation, such as observed in poly(methyl methacrylate) and poly(ethyl methacrylate) near to ambient conditions [6]. The behavior of the materials with strong beta-relaxations will be compared with that of materials, such as polycarbonate and polysulfone that have very weak beta-relaxations [6].

References

[1] X. Shi and G.B. McKenna, “Mechanical Hole Burning Spectroscopy: Evidence for Heterogeneous Dynamics in Polymer Systems,” Phys. Rev. Lett., 94, 157801-1 157801-4 (2005).

[2] X. Shi and G.B. McKenna, “Mechanical hole-burning spectroscopy: Demonstration of hole burning in the terminal relaxation regime,” Phys. Rev. B., 73, 014203-1 – 014203-11 (2006).

[3] Q. Qin, H. Doen and G.B. McKenna, “Mechanical Spectral Hole Burning in Polymer Solutions,” Journal of Polymer Science: Part B: Polymer Physics, 47, 2047–2062 (2009).

[4] B. Schiener, R. Bohmer, A.Loidl and R.V. Chamberlin, “Nonresonant spectral hole burning in the slow dielectric response of supercooled liquids,” Science, 274, 752-754 (1996).

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

[6] A. Flory and G.B. McKenna, "Chemical Structure-Normal Force Relationships in Polymer Glasses," Polymer 45, 5211-5217 (2005).