Mechanical spectral hole burning and nonlinear rheology of glassy polymers

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2021-08

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Abstract

The relaxation dynamics of glassy materials have attracted great attention because of its wide range of applications, due to which a fundamental understanding of the origins of the broadened relaxation response is necessary. Mechanical spectral hole burning (MSHB) methodology has been recently developed to study the heterogeneous dynamics in polymeric materials at temperatures around and above Tg. In the first part of the dissertation, MSHB has been applied to engineering polymers such as poly (methyl methacrylate) and polycarbonate, deep in the glassy state. These two materials exhibit different nature of secondary relaxation (β-relaxation) and the hypothesis of “hole burning in glassy polymers as related to the strength of β-relaxation” is tested. We find evidence of holes in both PMMA and polycarbonate which suggests inhomogeneous relaxation dynamics in the glassy state. The nature of these holes in both materials varies differently with strain and frequency. We also show evidence that the local heat dissipation, which is thought as the origin of dynamic heterogeneity in the dielectric hole burning works is not sufficient to explain the heterogeneity in MSHB in glassy polymers. Understanding the viscoelastic nature (linear and nonlinear) of polymers is essential in processing and applications. Small amplitude oscillatory shear (SAOS) tests provide a complete framework to characterize polymeric materials in linear regime. To go beyond linear regime, large amplitude oscillatory shear (LAOS) have been used to characterize nonlinear viscoelastic properties. LAOS tests have been used to characterize many soft materials such as polymer solutions, melts etc., yet it is seldom been applied on glassy materials. In the second part of the dissertation, we perform LAOS tests to characterize nonlinear viscoelastic nature of glassy PMMA. We first describe nonlinearity using the classic representation of Lissajous-Bowditch plots which interprets non linearity graphically through deviations from a elliptical curve on a stress-strain plot. Fourier transform analysis and Chebyshev polynomial method have also been used, which interprets nonlinearity in terms of parameters such as normalized third harmonic intensity (I_(3/1)) and normalized elastic and viscous Chebyshev coefficients (e_(3/1),v_(3/1)). We present the absolute harmonic intensity variation with strain and conclude that the (I_(3/1)) do not follow the expected quadratic scaling with applied strain amplitude. Normalized Chebyshev coefficients, e_(3/1),v_(3/1) suggests that the PMMA shows strain softening and shear thinning behavior with increase in strain amplitude. The work also shows a comparative description of the stress response on LAOS deformation with the predictions from the nonlinear viscoelastic model of the BKZ. BKZ predicted harmonics are also evaluated and compared with experimentally obtained results. Finally, harmonic analysis and Chebyshev method analysis is carried on a pure elastic material to understand whether the nonlinearity output in these methods is contributed by elastic and viscous part separately, or a combination of both.

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Keywords

Glassy Polymers, Heterogeneous Dynamics, PMMA, Polycarbonate, Nonlinear Viscoelasticity, Hole Burning

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