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Investigation of Segmental Dynamics in Polymer Glasses During Constant Strain Rate Deformation

2014
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Release : 2014
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Book Synopsis Investigation of Segmental Dynamics in Polymer Glasses During Constant Strain Rate Deformation by :

Download or read book Investigation of Segmental Dynamics in Polymer Glasses During Constant Strain Rate Deformation written by . This book was released on 2014. Available in PDF, EPUB and Kindle. Book excerpt: We have used optical measurements of dye reorientation to investigate the molecular dynamics of polymer glasses during deformation. In our work we have developed new instrumentation to perform constant strain rate deformations of polymer glasses while simultaneously measuring segmental mobility. By expanding our deformation protocols from creep and recovery to constant strain rate we have found that our molecular mobility measurement is more closely relevant to the theories, models, and simulations that the polymer glass deformation community is developing. We found that mobility was enhanced during deformation up to the yield point by a factor of 40 to 160 compared to the undeformed glass. In the postyield regime, this enhancement of mobility remained constant during the deformation. Finally, we found that high strain rates lead to higher postyield mobility, and that the mobility was correlated with the local strain rate as we have found previously with creep deformation experiments. The experimental features of the mobility enhancement, and its correlation with strain rate, are compared to computer simulations and theoretical models. This work significantly increases our understanding of molecular mobility during different types of deformations and shows the generalities of the measurement across many different systems. We have now found that the correlation of mobility and local strain rate is extremely robust across temperatures, types of deformations (postyield), and polymer glasses. We have also determined that a decrease in dynamic heterogeneity is a result of the deformation being applied to the polymer glass. By finding these generalities across different polymer glasses under different deformations, we can help provide the tools to create more accurate predictive models and simulations of polymer glass deformation.

Segmental Dynamics of Polymer Glasses in Multi-step Deformation and in the Strain-hardening Regime

2022
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Release : 2022
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Book Synopsis Segmental Dynamics of Polymer Glasses in Multi-step Deformation and in the Strain-hardening Regime by : Enran Xing

Download or read book Segmental Dynamics of Polymer Glasses in Multi-step Deformation and in the Strain-hardening Regime written by Enran Xing. This book was released on 2022. 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.

Segmental Dynamics of Polymer Glasses Undergoing Deformation

2016
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Release : 2016
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Book Synopsis Segmental Dynamics of Polymer Glasses Undergoing Deformation by :

Download or read book Segmental Dynamics of Polymer Glasses Undergoing Deformation written by . This book was released on 2016. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Segmental Dynamics of Polymer Glasses During Deformation

2020
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Release : 2020
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Book Synopsis Segmental Dynamics of Polymer Glasses During Deformation by : Trevor Bennin

Download or read book Segmental Dynamics of Polymer Glasses During Deformation written by Trevor Bennin. This book was released on 2020. Available in PDF, EPUB and Kindle. Book excerpt: A probe reorientation technique is used to measure changes in the segmental dynamics of polymer glasses during and after deformation. In this thesis, experiments are performed on poly(lactic acid) (PLA) and lightly crosslinked poly(methyl methacrylate) (PMMA) glasses in which fluorescent probe molecules, N,N'-dipentyl-3,4,9,10-perylenedicarboximide (DPPC), are dispersed. Glasses are subject to constant strain rate deformation and cyclic loading/unloading using a custom-built deformation apparatus that allows for concurrent fluorescence detection. The work described in this thesis provides quantitative dynamics and mechanical data that can test existing models and theories that describe the nonlinear deformation of polymer glasses. This is expected to improve predictions of the mechanical properties of polymer glasses and expand the utility of these materials in engineering applications. The segmental dynamics of PLA glasses between Tg - 15 K and Tg - 25 K are monitored during uniaxial extension at constant strain rates from 6x10^(-6) to 3x10^(-5) s-1. Segmental relaxation times are decreased by up to a factor of 30 in the plastic flow regime relative to the undeformed state. In the plastic flow regime, the segmental relaxation time is related to the local strain rate via a power law. Additionally, is it observed that the segmental dynamics become more homogeneous during deformation. Comparisons to previous probe reorientation experiments on lightly crosslinked PMMA and various models of polymer glass deformation are discussed. The effects of cyclic loading/unloading on the segmental dynamics and mechanical properties of lightly crosslinked PMMA glasses between Tg - 10 K and Tg - 25 K are investigated. Sets of 5000 tensile loading/unloading cycles are performed, with cycle extension strains ranging from 0.003 to 0.007. After cycling, segmental dynamics either remained unchanged or were faster relative to an undeformed sample. Surprisingly, the mechanical properties were unchanged after cycling under all investigated conditions. No evidence of overaging was observed in the optical or mechanical measurements as a result of these cyclic loading/unloading experiments; comparison of the results to various simulations and experiments are discussed.

Segmental Dynamics of Polymer Glasses Under Active Deformation

2009
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Release : 2009
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Book Synopsis Segmental Dynamics of Polymer Glasses Under Active Deformation by : Hau-Nan Lee

Download or read book Segmental Dynamics of Polymer Glasses Under Active Deformation written by Hau-Nan Lee. This book was released on 2009. Available in PDF, EPUB and Kindle. Book excerpt:

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