Measurements of Hydrodynamic Parameters for Nematic 5CB

Skarp, Kent; Lagerwall, S. T.; Stebler, B.

doi:10.1080/00268948008072401

Cited by 176 publications

(57 citation statements)

References 13 publications

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“…For example, the liquid crystal elastic constants have been measured previously for 5CB at room temperature [13,19] and have values of about 1 10 ÿ11 N. We measured the rubber modulus in a cone-and-plate rheometer and obtained a value of approximately 220 Pa for a 10 wt. % gel and 50 Pa for a 5 wt.…”

Section: Prl 96 147802 (2006) P H Y S I C a L R E V I E W L E T T E mentioning

confidence: 99%

Buckling Instability in Liquid Crystalline Physical Gels

et al. 2006

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In a nematic gel we observe a low-energy buckling deformation arising from soft and semisoft elastic modes. We prepare the self-assembled gel by dissolving a coil -side-group liquid-crystalline polymercoil copolymer in a nematic liquid crystal. The gel has long network strands and a precisely tailored structure, making it ideal for studying nematic rubber elasticity. Under polarized optical microscopy we observe a striped texture that forms when gels uniformly aligned at 35 C are cooled to room temperature. We model the instability using the molecular theory of nematic rubber elasticity, and the theory correctly captures the change in pitch length with sample thickness and polymer concentration. This buckling instability is a clear example of a low-energy deformation that arises in materials where polymer network strains are coupled to the director orientation. DOI: 10.1103/PhysRevLett.96.147802 PACS numbers: 61.30.ÿv, 61.41.+e, 82.70.Gg Liquid-crystalline elastomers and gels combine the orientational order of liquid crystals and the translational order of a polymer network. The coupling of these two effects in liquid-crystalline elastomers and gels gives rise to unique phenomena such as soft elasticity in which certain director rotations and network strains cost no free energy in the ideal case. Soft elastic modes are predicted and well understood theoretically by a molecular theory of nematic rubber elasticity [1][2][3]. However, a significant limitation in testing existing theories is the difficulty in preparing well-characterized networks. Many approaches have been undertaken to prepare liquid crystal elastomers, including free-radical polymerization of bulk liquid crystal acrylates and diacrylates [4], free-radical polymerization of diacrylates in the presence of liquid crystals [5,6], and side-coupling reactions with poly(methylsiloxane) [7]. Unfortunately, these approaches result in nematic elastomers and gels with heterogeneities in the polymer network and, in the case of many gels, phase separation of the polymer network from the liquid crystal host [8,9]. More recently, self-assembly of block copolymers has been used as an approach to create model liquid crystal networks [10 -12]. We have utilized self-assembly of a triblock copolymer that has a narrow molar mass distribution and a very long midblock to produce well-defined nematic gels [10,11]. This gel facilitates comparison to present theories which assume the material is lightly cross-linked and single phase.In this Letter, we present a study of the effects of nematic rubber elasticity in thin layers of uniformly aligned gels contained between rigid parallel plates. Using polarized optical microscopy, we observe a banded texture that arises in response to small temperature changes. We propose that this texture represents a buckling deformation made possible by the presence of soft and semisoft elastic modes. We formulate a description based on the molecular theory of nematic elastomers that incorporates soft and semisoft elasticity as well as l...

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Section: Prl 96 147802 (2006) P H Y S I C a L R E V I E W L E T T E mentioning

confidence: 99%

Buckling Instability in Liquid Crystalline Physical Gels

et al. 2006

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“…12 The LC gels are formed by dissolving an ABA triblock copolymer having polystyrene (PS) endblocks (''A'') and a very long side-group liquid crystalline polymer (SGLCP) midblock (''B'') in 4-pentyl-49-cyanobiphenyl (5CB), a wellcharacterized nematic LC. 13,14 The SGLCP midblock is designed to be soluble in both the isotropic and nematic phases of the solvent. Unfavorable interactions between the PS endblocks and nematic solvent cause segregation into PS-rich domains that function as the physical crosslinks of a spacefilling polymer network.…”

Section: Introductionmentioning

confidence: 99%

Rheological study of structural transitions in triblock copolymers in a liquid crystal solvent

et al. 2006

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Rheological properties of triblock copolymers dissolved in a nematic liquid crystal (LC) solvent demonstrate that their microphase separated structure is heavily influenced by changes in LC order. Nematic gels were created by swelling a well-defined, high molecular weight ABA block copolymer with the small-molecule nematic LC solvent 4-pentyl-49-cyanobiphenyl (5CB). The ''B'' midblock is a side-group liquid crystal polymer (SGLCP) designed to be soluble in 5CB and the ''A'' endblocks are polystyrene, which is LC-phobic and microphase separates to produce a physically cross-linked, thermoreversible, macroscopic polymer network. At sufficiently low polymer concentration a plateau modulus in the nematic phase, characteristic of a gel, abruptly transitions to terminal behavior when the gel is heated into its isotropic phase. In more concentrated gels, endblock aggregates persist into the isotopic phase. Dramatic changes in network structure are observed over small temperature windows (as little as 1 uC) due to tccche rapidly changing LC order near the isotropization point. The discontinuous change in solvent quality produces an abrupt change in viscoelastic properties for three polymers having different pendant mesogenic groups and matched block lengths.

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“…Poly ( hydrogenmass liquid crystalline (LC) materials by taking advantage of their ability to align in an electric field, but it has been found that their maximum viscosity change is only 50 Pa in shear stress at 300 s 01 of shear rate, or five times the zero-field viscosity, which is not sufficient for practical applications. [16][17][18][19][20] Recently, such liquid crystals have also been employed as the continuous phase in particle-dispersed ER fluids to improve their ER performance. 21,22 Our own attempts to obtain a larger ER effect, defined as the increase in shear stress upon application of an electric field, in LC materials began with a consideration of the physical structure and, particularly, their low molecular weight and the conjecture that their small ER effect might be attributable to the weak interaction between their LC domains and consequent mutual movement of these domains under applied shear even when they were aligned in an electric field.…”

Section: Introductionmentioning

confidence: 99%

Influence of dilution by polydimethylsiloxane on electrorheological effect of side-chain liquid crystalline polysiloxane

Inoue¹,

Ide²,

Oda³

1997

J. Appl. Polym. Sci.

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A liquid crystalline polysiloxane (LCP), having an ether bond in the spacer between its siloxane main chain and its mesogenic-group side chains, exhibited a very small electrorheological (ER) effect or increase in shear stress upon application of an electric field, but mixtures of the LCP and polydimethylsiloxane (PDMS) exhibited a sharply increasing ER effect with increasing PDMS content throughout the tested range of up to 0.5 of PDMS weight fraction. When phenyl-substituted PDMS (Ph-PDMS) at a weight fraction of 0.3 was used instead of PDMS, however, the ER effect decreased with increasing phenyl content and became nearly undetectable with Ph-PDMS having a phenyl content (ratio of substituted phenyl groups to initial methyl groups) of approximately 15%. DSC analyses showed that the ER effect of the LCP/ PDMS mixtures occurred undiminished throughout a temperature range in which LCP itself is an isotropic liquid in the absence of an applied electric field and suggested that the LC structure of the LCP was maintained even when it was diluted with PDMS in weight fractions of 0.5 or higher, but disrupted when diluted by a 0.3 weight fraction of Ph-PDMS having a 15% phenyl content. Optical microscopic observation of the mixtures of the LCP with a 0.3 weight fraction of PDMS or Ph-PDMS (15% phenyl content) showed that both consisted of uniformly dispersed micron-sized droplets which became elongated in the direction of the applied electric field when it was applied alone but became smaller when both the electric field and shear were applied. These results suggest that the phase separation between the LCP and the dilution oil, as well as the existence and orientation of LC domains, is essential for the generation of a large ER effect.

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Measurements of Hydrodynamic Parameters for Nematic 5CB

Cited by 176 publications

References 13 publications

Buckling Instability in Liquid Crystalline Physical Gels

Buckling Instability in Liquid Crystalline Physical Gels

Rheological study of structural transitions in triblock copolymers in a liquid crystal solvent

Influence of dilution by polydimethylsiloxane on electrorheological effect of side-chain liquid crystalline polysiloxane

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