Effects of Temperature and Microstructure on Microdeformation of Polymer Glasses PDF Download
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Author: Larry Lance Berger
Publisher:
ISBN:
Category : Deformations (Mechanics)
Languages : en
Pages : 322
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Book Description
Author: Larry Lance Berger
Publisher:
ISBN:
Category : Deformations (Mechanics)
Languages : en
Pages : 322
Get Book Here
Book Description
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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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.
Author: Jianning Liu
Publisher:
ISBN:
Category : Bisphenol A.
Languages : en
Pages : 61
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This work investigates the origin of mechanical stress based on stress relaxation experiments of ductile and brittle polymer glasses. To learn more about the relaxation behavior of polymer glasses, a series of stress relaxation experiments in both extension and compression were carried out in pre-yield and post-yield regimes respectively. Tensile tests were carried out using ductile glasses such as bisphenol A polycarbonate (PC), and compression tests were performed based on brittle poly(methyl methacrylate) (PMMA) as well as PC. The stress relaxation is shown to speed up in linear proportion to the deformation rate in the post-yield regime. Such scaling behavior complements the in situ measurements of mobility as a function of the applied extensional rate1. Rate effect, strain effect and temperature effect were investigated. We studied how the relaxation dynamics depend on the prior deformation history, and delineate the results in term of our recently-proposed molecular model2.
Author:
Publisher:
ISBN:
Category : Dissertations, Academic
Languages : en
Pages : 684
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Author:
Publisher:
ISBN:
Category : Polymer literature
Languages : en
Pages : 390
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Author: M. V. Volkenshtein
Publisher:
ISBN:
Category :
Languages : en
Pages : 14
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By the method of dynamic heat capacity the softening of non-crystallizing vitreous polymers, such as polyvinylacetate and polystyrene, was investigated. It was established that the prog ress of heat capacity temperature substantially depended upon (1) heating rate, (2) annealment at temperatures lower than the vitrification temperature, and (3) the rate of the preliminary cooling. (Author).
Author: David Scott Fryer
Publisher:
ISBN:
Category :
Languages : en
Pages : 160
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Author: Jianning Liu
Publisher:
ISBN:
Category : Glass
Languages : en
Pages : 209
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"It is of great fundamental important and practical interest to understand what controls the mechanical properties of polymeric glasses, such as shear yielding, necking, crazing, strain hardening and the brittle-ductile transition. Despite the tremendous efforts in the past decades to explorer the mechanical instabilities of polymer glasses, many topics about the nature of glass transition, nature of stress remain vague and under extensive debates. In this dissertation, we carried out a series of mechanical tests to study the mechanical response of polymeric glasses in both uniaxial extension and compression. Based on our recent phenomenological molecular model, this work investigates the origin of mechanical stress based on stress relaxation and brittle-ductile transition experiments of polymer glasses. Different from previous models emphasizing the inter-segmental contribution in stress, our stress relaxation experimental results revealed the important role of chain network by intra-chain connectivity and chain uncrossability. In Capture III, in room temperature stress relaxation experiments, we studied the stress relaxation behaviors of four different commercial polymer glasses under both extension and compression large ductile deformation over a wide range of rate. It was found that the initial stress relaxation rate after holding post-yield deformation is linearly proportional to the rate of prior deformation. While the pre-yield stress relaxation is logarithmically slow. This rate rescaling behaviors indicates the surviving segmental mobility in absence of ongoing deformation was due to the yield induced activation process. In Capture IV, to elucidate the nature of stress during deformation and stress relaxation, temperature for stress relaxation was increased to near Tg. All the pre-yield stress would vanish within fast segmental relaxation time independent of rate, while the initial post-yield stress relaxation can be either faster or far slower than the segmental dynamics dependents on prior deformation rate. Residual stress after large post-yield relaxation was observed to retain significant levels on the time scale much longer than the time scale for all the rate range investigated near Tg. Supporting results by MD simulation shown the chain network is essential in the mechanical response of uniaxial compression of glassy polymers through the chain network's lateral resistance to the lateral expansion and contribute to compressive stress. In Capture V and VI, to understand the role of chain network under uniaxial compression, we systematically studied how the structural change of chain network dictates whether the uniaxial compression of polymer glasses is ductile or suffers brittle fracture. Those structural characteristics of the perceived chain network can be changed by variation of molecular weight, molecular composition, and anisotropic reconstruction through melt stretching"--Website of ETD."
Author:
Publisher:
ISBN:
Category : Dissertations, Academic
Languages : en
Pages : 606
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Author: American Chemical Society, Committee on Professional Training Staff
Publisher:
ISBN: 9780841214187
Category : Science
Languages : en
Pages : 1390
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