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Microscopic perspective of polymer glasses during physical aging and mechanical deformation : a computational study of dynamical heterogeneity, plasticity, and soft vibrational modes as the link to the molecular structure

University of British Columbia

Microscopic dynamics and mechanical response of polymer glasses are studied in four projects using molecular dynamics simulations of a simple bead-spring model. The first project studies the interplay between physical aging and mechanical perturbation. Structural, dynamical and energetic quantities are monitored in the recovery regime following aging and uniaxial tensile deformation periods. The total engineering strain is found to control a continuous transition from transient to permanent mechanical rejuvenation: After deformation in the pre-yield regime all quantities quickly reset to pre-deformation values, while deformation around the yield point results in the erasure of aging history. Deformation in the post-yield regime, however, drives the system into a distinct thermodynamic state. In the second project, I introduce an efficient algorithm that detects microscopic relaxation events, which are the basis of aging dynamics and plasticity. I use this technique to calculate the density-density correlations from the spatio-temporal distribution of so called hops in quiescent polymer glasses at different temperatures and ages. Correlation ranges are extracted and I analyze the size distributions of collaboratively rearranging groups of particles. Furthermore, I spatially resolve dynamical heterogeneity (DH) as hop-clusters, and I compare cluster growth, as well as volume distribution during aging with the four-point dynamical susceptibility Χ₄ as the established measure of DH. The third and fourth project use the hop detection technique to investigate the link between relaxation events and local structure. Quasi-localized low-energy vibrational modes, called soft modes, are found to correlate with the location and direction of hops. In the third project, I analyze the temperature- and age-dependence of this correlation in quiescent polymer glasses, and I show that the soft modes are long lived structural features. The fourth project extends the analysis to mechanically deformed polymer glasses. I find that the spatial correlation of hops and soft modes is reduced to pre-aging values after deformation in the strain softening regime. This reveals an additional perspective on mechanical rejuvenation and substantiates the findings from the first project. In the strain hardening regime the correlation increases, and this novel effect is linked to a growing localization of the soft modes.

Text

2015-04-30

Attribution-NonCommercial 2.5 Canada

http://hdl.handle.net/2429/53018

Physics

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc/2.5/ca/