Light Scattering by Shear-Induced Defects of Nematic Liquid Crystalline Polymers

Патлажан, С. А.; Riti, Jean-Bernard; Navard, Patrick

doi:10.1021/ma946435m

Cited by 15 publications

(14 citation statements)

References 35 publications

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“…This choice is based partly on electron microscopy observations of a thermotropic LCP (27), comparative analysis of the vertical streak in HH and VV polaroid positions, and numerical tests (24). The pattern reproduces fairly well oscillatory behaviour of the vertical streak observed in the experiment (18).…”

Section: Discussionmentioning

confidence: 77%

“…In the beginning we discuss briefly the main points considered in reference (24). In this work the concept of loop defects described in the case of thermotropic polymers (26,27) was extended to lyotropic systems.…”

Section: Theoreticalmentioning

confidence: 99%

“…The unusual behaviour of the intensity in the streak discussed in the experimental section (see Figure 1) suggests that 3D diffraction from an assembly of loops gives the major contribution. In reference (24), it was shown that the strong oscillations of the intensity in the scattering curves are caused by the retardation of waves scattered by the set of rods lying in various levels of the sample. Each pair of nearest rods in a different depth can belong either to the same disclination or to different loops.…”

Section: Theoreticalmentioning

confidence: 99%

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Light Scattering from Lyotropic Textured Liquid-Crystalline Polymers Under Shear Flow

Патлажан

Riti

Navard

1995

ACS Symposium Series

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Small-angle light scattering (SALS) experiments performed on lyotropic liquid crystalline polymer (LCP) solutions under steady-state shear flow are discussed. The structure of scattering objects is described in terms of a simplified model assuming linear defects located in different levels and oriented along the flow direction. A light scattering pattern appearing as a bright streak with strong intensity oscillations is theoretically calculated. From the comparison of the theoretical and experimental scattering curves for the streak the following conclusions were made: 1) defects created under steady state shear flow in cholesteric LCP solutions are grouped into sets of assemblies; 2) an increase in shear rate is accompanied by a decrease of the average domain size.It is well-documented now that the rheology of main-chain polymer nematics at low shear rates strongly depends on coupling between the director orientation and the structure of texture (1). This has been shown in a series of key experiments dealing with transient and steady-state flow. In these experiments, such effects were observed as oscillatory responses in stress and conservative dichroism after sudden reverse or step increase in shear rate (2-5), slow elastic recoil after removal of shear stress (6), and dependence of moduli on the mechanical history of the sample (3,7,8).Peculiarities in the rheological behaviour of the liquid crystalline polymers (LCP) are the result of an instability of the director at low Deborah number (tumbling regime) (1,(9)(10)(11)(12)(13) caused by hydrodynamic torque exerted by shear flow. This instability is thought to cause spatial variations in the director field and to give rise to the formation of defects (13,14). The area of the sample per one defect line is referred to as a "domain". Its average spatial scale, a", was estimated by Marrucci (15) from the balance between Frank elastic distortion and viscous energies. It was derived that this scale decreases with an increase in the shear rate, γ, as γ 1/2

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Section: Discussionmentioning

confidence: 77%

Section: Theoreticalmentioning

confidence: 99%

Section: Theoreticalmentioning

confidence: 99%

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Light Scattering from Lyotropic Textured Liquid-Crystalline Polymers Under Shear Flow

Патлажан

Riti

Navard

1995

ACS Symposium Series

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“…43 -46 For band formation, Patlazhan et al represented recently a model of defects at low shear rate, grown with increasing shear, on the basis of the results of depolarized light scattering observed at low shear rates for lyotropic and thermotropic polymer liquid-crystals. 47 The model consists of a twist loop, fluctuating around the shearing plane, with a director varying from the flow direction to vorticity axis. Therefore, the striped pattern is transient, unless an extremely low shear field is used.…”

Section: Introductionmentioning

confidence: 99%

Banded Texture Induced by an Electric Field in Polymeric Liquid Crystalline Solution

Monzen

Hiraoka

Uematsu

et al. 1998

Polym J

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ABSTRACT:Using polarizing microscopy, we investigated the textural transformation and development of banded texture for cholesteric (nematic) liquid-crystalline solutions of poly(y-benzyl glutamate) (PBG)s in an in-plane electric field. The threshold value for producing the uniform director orientation depends on cell thickness, d, and is inversely proportional to d 112 • In the solution aligned uniaxially by a de field, the transient banded texture appears perpendicular to the field soon after inversion of the field direction. By using video-recording technique, detailed analysis of the texture can be performed and the following results are obtained: the appearance of banded texture is related to rotation of the director during realignment and two peaks of the optical intensity transmitted through the sample between crossed polarizers appear until the reorientation is completed; the band spacing remains constant during realignment and depends on the applied field strength; being a decreasing function of field strength. The effect of cholesteric twist on the kinetics of director rotation is studied for solutions of poly(y-benzyl-L-glutamate) (PBLG), poly(y-benzyl-o-glutamate) (PBDG) and racemic mixtures. A cholesteric twist was noted to affect kinetics of director rotation.KEY WORDS Poly(y-benzyl-L-glutamate) / Polymeric Liquid Crystalline / Electric Field / Banded Texture/ Threshold Value/ One distinctive phenomenon in the flow of lyotropic liquid crystalline polymers in the nematic or the cholesteric state is negative steady-state first normal stressdifference in shearing flow over a range of shear rates. For an isotropic fluid, the first normal stress difference, N 1 , is zero or positive over all rates of shear. However, Kiss and Porter found that in poly(y-benzyl-Lglutamate) (PBLG) solutions, positive values of N 1 were seen at low and high shear rates and negative values at intermediate shear rates. 1 They also found that the banded texture appeared at intermediate shear rates. The appearance of banded texture, as a striped pattern, perpendicular to the flow direction is particularly interesting. The characteristic banded texture has been observed during or after shear, in various main chain liquid-crystalline polymers, 2 -34 thermotropic systems as well as lyotropics. 10 -35 -40 Many models have been proposed to explain the mechanism of band formation. 23·41 ·42 However, the mechanism is not yet completely understood. Most theories consider that such behavior in lyotropic polymer liquid-crystals results from coupling between molecular tumbling motion and local molecular rotation during shear. 43 -46 For band formation, Patlazhan et al. represented recently a model of defects at low shear rate, grown with increasing shear, on the basis of the results of depolarized light scattering observed at low shear rates for lyotropic and thermotropic polymer liquid-crystals. 47 The model consists of a twist loop, fluctuating around the shearing plane, with a director varying from the flow direction to vorticity axi...

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“…The observed scattering pattern is certainly not unique to the proposed flow-induced structure. Other mechanisms could account for the observed structure factor; e.g., these patterns are characteristic of phase transitions in block copolymers, blends and binary polymeric solutions [15], and also shear-induced defects of nematic liquid crystalline polymers [16]. A direct visualization of the structure by means of shear microscopy will provide a deeper understanding of the physics underlying these patterns.…”

mentioning

confidence: 96%

“Tuliplike” Scattering Patterns in Wormlike Micelles under Shear Flow

Kadoma¹,

Egmond²

1996

Phys. Rev. Lett.

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Using small angle light scattering we report on the formation of a string phase in a semidilute solution of wormlike micelles (0.03M CTAB͞0.24M NaSal) under simple shear flow. The string phase appears as a bright narrow streak in the scattering pattern normal to the flow direction. We also study the relaxation behavior after cessation of flow, and using the Brochard-de Gennes model we are able to estimate the stress relaxation time t s and diffusion coefficient D c of the dominant relaxation mode.[S0031-9007(96)00227-X]

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Light Scattering by Shear-Induced Defects of Nematic Liquid Crystalline Polymers

Cited by 15 publications

References 35 publications

Light Scattering from Lyotropic Textured Liquid-Crystalline Polymers Under Shear Flow

Light Scattering from Lyotropic Textured Liquid-Crystalline Polymers Under Shear Flow

Banded Texture Induced by an Electric Field in Polymeric Liquid Crystalline Solution

“Tuliplike” Scattering Patterns in Wormlike Micelles under Shear Flow

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