Molecular Orientation and Instability of Liquid Crystalline Polymers in Shear Flows and Flows with Mixed Shear and Extensional Kinematics PDF Download
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Author: Bruce Darrell Bedford
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Category :
Languages : en
Pages :
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Author: Bruce Darrell Bedford
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Category :
Languages : en
Pages :
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Author: Domenico Acierno
Publisher: Springer Science & Business Media
ISBN: 940091511X
Category : Technology & Engineering
Languages : en
Pages : 331
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Liquid crystal polymers (LCPs) have many strange properties that may be utilized to advantage in the processing of products made from them and their blends with isotropic polymers. This volume (volume 2 in the series Polymer Liquid Crystals) deals with their strange flow behaviour and the models put forward to explain the phenomena that occur in such polymers and their blends. It has been known for some time that small ad ditions of a thermotropic LCP to isotropic polymers not only gives an improvement in the strength and stiffness of the blend but improves the processability of the blend over that of the isotropic polymer. In the case of lyotropic LCPs, it is possible to create a molecular composite in which the reinforcement of an isotropic polymer is achieved at a molecular level by the addition of the LCP in a common solvent. If the phenomena can be fully understood both the reinforcement and an increase in the proces sability of isotropic polymers could be optimized. This book is intended to illustrate the current theories associated with the flow of LCPs and their blends in the hope that such an optimization will be achieved by future research. Chapter 1 introduces the subject of LCPs and describes the ter minology used; Chapter 2 then discusses the more complex phenomena associated with these materials. In Chapter 3, the way in which these phe nomena may be modelled using hamiltonians is fully covered.
Author: Kwan Hongladarom
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Languages : en
Pages :
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Author: Victor Manuel Ugaz
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Category :
Languages : en
Pages :
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Although liquid crystalline polymers (LCPs) offer considerable commercial potential for the production of lightweight materials possessing exceptional mechanical strength, the influence of flow on molecular orientation remains poorly understood in melt processable thermotropic systems. In this thesis, a combination of materials and experimental techniques are employed which allow structural measurements to be obtained under a variety of flow conditions in the melt. In situ x-ray scattering techniques are used as a structural probe to directly measure the influence of flow on molecular orientation, and to identify the presence of other structural arrangements which would otherwise be unobservable. First, we use this structural data to identify a link between the development of a hexagonal microstructure and the onset of region I shear thinning in lyotropic poly(gamma-benzyl-L-glutamate) in m-cresol. Next, two different model thermotropic systems are studied, a polyester and polyether respectively, which incorporate molecular architectures specifically designed to provide a stable nematic phase over a wide temperature range. We present measurements of molecular orientation in steady shear flow, during relaxation after cessation of shear flow, and in the transient periods during shear flow inception and reversal. Under certain combinations of temperature and shear rate, the polyether is found to display a dramatic transition in molecular alignment to a condition where orientation is favored parallel to the vorticity direction, rather than parallel to the flow direction as is typically observed. Both materials exhibit dynamic behavior in the nematic phase consistent with flow alignment, as opposed to director tumbling behavior which is often observed in lyotropic LCPs. This hypothesis is supported by comparison with an adaptation of the Ericksen transversely isotropic fluid model and the Larson-Doi model, both of which account for the polydomain texture characteristic of LCP materials. The Larson-Doi model yields better qualitative predictions of the evolution of orientation and stress during flow reversal, suggesting that distortional elasticity plays a significant role in this type of flow. However, some discrepancies remain between the model predictions and the experimentally observed behavior, suggesting that distortional elasticity may play a different role in the rheology of thermotropic LCPs than in lyotropes.
Author:
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Category : Rheology
Languages : en
Pages : 782
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Author: Wesley R. Burghardt
Publisher:
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Category :
Languages : en
Pages : 364
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Author: Chinh T. Nguyen
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Category : Microstructure
Languages : en
Pages : 220
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Author: Mark Naoshi Kawaguchi
Publisher:
ISBN:
Category :
Languages : en
Pages : 512
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Author: A.I. Leonov
Publisher: Springer Science & Business Media
ISBN: 9401112584
Category : Technology & Engineering
Languages : en
Pages : 492
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This monograph presents theoretical and experimental studies of flows of elastic liquids. Falling into this category are particularly the melts and concentrated solutions of such flexible-chain polymers as polyethylene, polyisobutylene and polypropylene, all of which are widely used in polymer processing. These polydisperse polymers vary greatly, from batch to batch, in their mechanical properties and 20% variation in a property is believed to be good enough. l 7 All recent books - devoted to the rheology of polymers do not answer the question of which constitutive equations should be used for solving the fluid mechanic problems of polymer processing in the usual case of an appreciable nonlinear region of deformation where nonlinear effects of shear and extensional elasticity are very important. Viscoelastic constitut ive equations cited commonly (see, e.g. Refs 5 and 6) do not describe simultaneously even the simplest cases of deformations, viz. simple shear and uniaxial extension. Moreover, some of them are internally inconsist ent and sometimes display highly unstable behaviour in simple flows without any fundamental reasons. Even more respected molecular ap free from these defects.
Author: Witold Brostow
Publisher: Springer Science & Business Media
ISBN: 1461557992
Category : Technology & Engineering
Languages : en
Pages : 534
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may never overcome the effects of hysteresis and stress (see Chapters 6 and 12). The first sentence of the reference work, Handbook of Liquid Crystals, reads: The terms liquid crystals, crystalline liquid, mesophase, and mesomorphous state are used synonymously to describe a state of aggregation that exhibits a molecular order in a size range similar to that of a crystal but acts more or less as a viscous liquid: [2] In other words, molecules within a liquid crystalline phase possess some orientational order and lack positional order; furthermore, the shape of a liquid crystalline sample is determined by the vessel in which it is contained rather than by the orientational order of its aggregated molecules. The authors recognized the limitations and imprecision of this definition but, like others preceding them, could not devise a simple and generally applicable one that is better. Regardless, the terms 'liquid crystal' and 'mesophase' should not be used interchangeably. As mentioned above, all liquid crystals are mesophases, but all mesophases are not liquid crystals. Recent studies, employing elaborate and sophisticated analytical techniques, have permitted finer distinctions between classical crystals and mesophases. At the same time, they have made definitions like that from the Handbook of Liquid Crystals somewhat obsolete for reasons other than terminology. One part of the problem arises from the use of a combination of bulk properties (like flow) and microscopic properties (like molecular ordering) within the same definition.
