Download or read book Segmental Dynamics of Polymer Glasses in Multi-step Deformation and in the Strain-hardening Regime written by Enran Xing and published by . This book was released on 2022 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: A probe reorientation technique is applied in this thesis to study the segmental dynamics of polymer glasses during deformation. Lightly-crosslinked poly(methyl methacrylate) (PMMA) glasses containing fluorescent molecules, N,N'-dipentyl-3,4,9,10-perylenedicarboximide (DPPC), are used as the model systems. Constant strain rate, creep, and cyclic deformations are performed using a custom-built apparatus, which allows for quenching samples with cold nitrogen gas and simultaneous optical measurements. The results of this thesis are compared with theoretical and simulation works. It's expected that a further understanding of the microscopic nature behind the nonlinear deformation of polymer glasses will promote the application of this class of materials. Multi-step deformations with simultaneous dynamics measurements are performed to test the capability of segmental dynamics in predicting complicated mechanical behaviors. During a multi-step (constant strain rate- creep- constant strain rate) deformation, while a more prominent strain-softening after yield is observed in the second constant strain rate step, the acceleration of segmental dynamics in this step is at least one order of magnitude smaller than the value during the initial constant strain rate deformation beyond yield. This result indicates that that the change of segmental dynamics cannot be the single mechanism that dictates the mechanical behavior of polymer glasses, which is a common hypothesis behind most current theories. The segmental dynamics in the strain-hardening regime are measured on the PMMA glasses with and without melt-stretching at 23K and 33K below the glass transition temperature, with engineering strain rates from 10^(-4.6)/s to 10^(-4)/s. The melt-stretched PMMA, which shows a stronger strain-hardening behavior, has segmental dynamics about 0.05 decade faster than PMMA without melt-stretching. And at a given true strain rate, the segmental dynamics of PMMA without melt-stretching are accelerated in the deep strain-hardening from the value just beyond yield point, by up to 0.15 decade at the 0.8 true strain. The results are also discussed in the context of simulation and theoretical works. The acceleration of segmental dynamics of PMMA glasses in the pre-yield regime is measured with a modified photobleaching protocol, in which a deformation occurs within a single optical measurement. Using cyclic deformation protocols, we show that deformations with the same absolute value of strain rate but different peak strains have different influences on the segmental dynamics. For different deformation protocols, the acceleration of the segmental dynamics falls onto a universal curve with respect to the peak strains. Comparison of the results with theoretical predictions is discussed.