Disclination Structures in the Carbonaceous Mesophase

Zimmer, J.E.; White, J.L.

doi:10.1016/b978-0-12-025005-9.50011-4

Cited by 96 publications

(41 citation statements)

References 42 publications

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“…Understanding the disclination line, defect nucleation and stabilization mechanism in helicoidal plywoods can be used as a model for gaining insight into similar phenomenon in other lamellar systems, such as diblock co-polymers and SmA LCs [41]. A computational modeling study of texture formation in a nematic carbonaceous mesophase matrix with embedded circular carbon fibers, has concluded that the number and type of disclinations that arise invariably obey the experimentally observed Zimmer's rule; the disclination line strength is equal to -(N-2)/2, where N is the number of inclusions [42,43]. A Landau-de Gennes type model for defect coarsening process of the isotropic/smectic-A phase transition, has been used to simulate elementary a b dislocations, total dislocation content and defect annihilation laws for smectic disclinations has been established [44].…”

Section: Introductionmentioning

confidence: 99%

Modeling Textural Processes during Self-Assembly of Plant-Based Chiral-Nematic Liquid Crystals

Murugesan

Rey

2010

Polymers

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Biological liquid crystalline polymers are found in cellulosic, chitin, and DNA based natural materials. Chiral nematic liquid crystalline orientational order is observed frozen-in in the solid state in plant cell walls and is known as a liquid crystal analogue characterized by a helicoidal plywood architecture. The emergence of the plywood architecture by directed chiral nematic liquid crystalline self assembly has been postulated as the mechanism that leads to optimal cellulose fibril organization. In natural systems, tissue growth and development takes place in the presence of inclusions and secondary phases leaving behind characteristic defects and textures, which provide a unique testing ground for the validity of the liquid crystal self-assembly postulate. In this work, a mathematical model, based on the Landau-de Gennes theory of liquid crystals, is used to simulate defect textures arising in the domain of self assembly, due to presence of secondary phases representing plant cells, lumens and pit canals. It is shown that the obtained defect patterns observed in some plant cell walls are those expected from a truly liquid crystalline phase. The analysis reveals the nature and magnitude of the viscoelastic material parameters that lead to observed patterns in plant-based helicoids through directed self-assembly. In addition, the results provide new guidance to develop biomimetic plywoods for structural and functional applications.

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Section: Introductionmentioning

confidence: 99%

Modeling Textural Processes during Self-Assembly of Plant-Based Chiral-Nematic Liquid Crystals

Murugesan

Rey

2010

Polymers

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“…In addition to confirming that tha mesophase transformation proceeds differently in a fiber bundle than in bulk pyrolysis and that the matrix morphology is dominated by the alignment of mesophase layers parallel to the substrate, 2 the present results direct attention to the role of wetting and bloating phenomena in determining the extent and nature of the matrix porosity. Mesophase bloating, a familiar problem at the large scale of petroleum coking, 3 also operates at the micrometer scale of fiber interstices to produce pores that can agglomerate to varying degrees depending on the freedom of movement of individual filaments.…”

Section: Imentioning

confidence: 82%

Mesophase Behavior in Carbon Fiber Bundles.

White¹,

Ng²,

Sheaffer³

et al. 1982

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“…The solid like behavior is due to the domain texture, whose elastic solid-like nature is due to the presence of orientational defects. 44 …”

Section: Linear Viscoelasticity Of Monodomain and Polydomain Carboancmentioning

confidence: 99%

“…The selected K values are equal to 10 -7 Pa (T=T 3 ), 10 -6 Pa (T=T 2 ), and 10 -5 Pa (T=T 1 ), respectively, which imply that domain size is in the micron range, as observed experimentally. 44 Figure 7 shows the computed storage modulus ( ' mix G ) as a function of frequency (ω (rad/sec)) of a textured carbonaceous mesophase in the small frequency regime at three temperatures (T 1 <T 2 <T 3 ), computed using equation 14.…”

Section: Linear Viscoelasticity Of Monodomain and Polydomain Carboancmentioning

confidence: 99%

“…5,6,38,[42][43][44] The amount of elastic storage depends on the domain size (H), the defect charge (S), and the temperature, as follows: 5,6,38 (15) where c is a constant, K(T) is the characteristic Frank elastic constant or average of splay, bend, and twist. In the present study we take into account the defect elastic storage for three temperatures, T 1 <T 2 <T 3 .…”

Section: Linear Viscoelasticity Of Monodomain and Polydomain Carboancmentioning

confidence: 99%

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Linear viscoelasticity of textured carbonaceous mesophases

Lima¹,

Rey²

2006

J. Braz. Chem. Soc.

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Fibras de carbono de alto desempenho podem ser produzidas a partir de piche de petróleo anisotrópico usando o processo de extrusão-fiação. O piche mesomórfico é um cristal líquido discótico nemático termotrópico, que possui ordem na orientação das moléculas. Cristais líquidos são materiais que possuem textura, anisotropia e visco-elasticidade. Um fenômeno importante que é encontrado em cristais líquidos é a interação entre a textura devido à orientação das moléculas e o comportamento reológico, no qual a textura ocorre devido à presença de defeitos na orientação das moléculas. O processo de extrusão-fiação usado na fabricação de fibras de carbono a partir de precursores nemáticos usa uma cascata de escoamentos que incluem fluxo capilar convergente e extensional, os quais promovem a formação de uma variedade de estruturas, incluindo radial, concêntrica, randômica e bipolar, que darão propriedades especificas para as fibras de carbono resultantes. Portanto, a compreensão fenomenológica da interação entre a textura e a reologia do precursor nemático é necessária para o controle do processo e para a otimização do produto. Neste artigo, a interação entre a reologia e a textura de cristais líquidos discóticos nemáticos será enfocada e um modelo anisotrópico será desenvolvido para descrever o regime linear visco-elástico de piches mesofásico com textura. O modelo é capaz de prever todas as características observadas experimentalmente, incluindo a dependência dos módulos de armazenamento e de perda com a freqüência de oscilação e a sua dependência com a temperatura, assim como promover uma melhor compreensão do papel dos defeitos e da anisotropia no comportamento visco-elástico do piche anisotrópico.High performance carbon fibers can be produced from anisotropic petroleum pitch using the fiber spinning process. The mesophase pitch is a thermotropic discotic nematic liquid crystal possessing orientational order. Liquid crystals are textured, anisotropic, viscoleastic materials. An important phenomenon of liquid crystals is the interaction between orientational textures and rheology, where the texture is due to the presence of orientational defects. The fiber spinning process used in the carbon fiber manufacture from nematic precursors uses a cascade of processing flows that involves capillary, convergent, and extensional flows, which promotes the formation of a variety of textures that will give specific properties to the resultant carbon fiber. Hence a fundamental understanding of the impact of textures on rheology is needed for eventual process control and product optimization. In this paper we study the interaction of rheology and texture and develop a model of anisotropic linear viscoelasticity of textured mesophases. The model is able to predict all the features observed experimentally, including the frequency dependency of the loss and storage modulus and their temperature sensitivity, and provides a better understanding of the role of defects and anisotropy on pitch viscoelasticity.

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Disclination Structures in the Carbonaceous Mesophase

Cited by 96 publications

References 42 publications

Modeling Textural Processes during Self-Assembly of Plant-Based Chiral-Nematic Liquid Crystals

Modeling Textural Processes during Self-Assembly of Plant-Based Chiral-Nematic Liquid Crystals

Mesophase Behavior in Carbon Fiber Bundles.

Linear viscoelasticity of textured carbonaceous mesophases

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