Segmental dynamics of polymer glasses undergoing deformation: Effect of temperature and reversing deformation protocols

A probe reorientation technique is used to monitor changes in the segmental dynamics of polymer glasses as they undergo physical aging and deformation. This thesis focuses on lightly cross-linked poly(methyl methacrylate) (PMMA) glasses in which the optical probe N,N'-Dipentyl-3,4,9,10-perylenedicarboximide (DPPC) is dilutely dispersed. Deformations are performed within a home-built deformation apparatus which allows optical access to the samples. The work of this thesis provides a test of existing models and theories in the literature which describe polymer glass deformation. A full understanding of the deformation behavior of polymer glasses may allow these versatile materials to be used in a wider variety of applications.;The effect of temperature on segmental dynamics during flow-state deformation is studied using PMMA glasses between Tg-11 K and Tg-27 K deformed in tension at a series of constant engineering strain rates. These studies demonstrate that thermally-activated transitions are significant during flow, with calculated free energy barriers of ∼39 kTg. Furthermore, these free energy barriers during flow are reduced by only ∼10-15% as compared to the pre-deformation values, indicating that although deformation reduces thermal effects on dynamics, thermally-activated transitions remain a significant feature of flow-state dynamics. The reported effect of temperature is significantly larger than anticipated in the literature; a comparison of the results to existing models and simulations is discussed.;A series of reversing constant strain rate deformations is performed on a PMMA glass at Tg-7 K to separate contributions of proposed mechanisms which enhance segmental dynamics during deformation. We quantify the activity of the proposed rejuvenation mechanism using both probe reorientation and a mechanical experiment and find that for both techniques, rejuvenation gradually increases with strain, saturating at strains several times the yield strain. Our results describing the rejuvenation mechanism broadly agree with a theory of Chen and Schweizer. However, at low strains, the probe reorientation results show higher activity of the rejuvenation mechanism; these optical results agree with a recent simulation study. The difference between the optical and mechanical measurements, as well as a comparison to theoretical work in the literature is discussed.