Shear flow of liquid-crystalline polymer solutions as investigated by small-angle light-scattering techniques

Ernst, B.; Navard, Patrick; Hashimoto, Takeji; Takebe, Toshihiko

doi:10.1021/ma00207a022

Cited by 35 publications

(20 citation statements)

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“…Flow light scattering patterns had a bright vertical streak parallel to the vorticity axis with four lobes and was similar to the patterns developed in references (20)(21)(22). The intensity in the vertical streak was processed using numerical image analysis.…”

Section: Methodssupporting

confidence: 55%

“…For example, the scattering curve cannot be solely described in terms of a single rod theory. The main contradictions with experiments of this simple model are the appearance of oscillations in the vertical streak and a drastic growth of the intensity at small values of scattering angles with increased shear rate as observed in (20,21).…”

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confidence: 57%

“…Experiments performed at low shear rate with lyotropic (18, [20][21][22] and thermotropic (23) LCP show that SALS scattering patterns have a bright streak along the vorticity axis (the vertical streak) and four lobes under crossed polarizers. These results confirm that the scattering objects have a strong alignment in the flow direction.…”

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confidence: 99%

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

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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: Methodssupporting

confidence: 55%

contrasting

confidence: 57%

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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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“…All the reported experiments have been performed by probing the x 2 -x 3 plane. The results at low shear rates obtained from lyotropic and thermotropic 16,18,23,26 polymers show that the light scattering patterns have common characteristics. Under crossed polarization, called HV (polarization direction H being along the flow direction (x 3 )), the scattering pattern consists of a bright streak along the vorticity axis (x 2 ) and four lobes.…”

Section: Introductionmentioning

confidence: 90%

“…6,7 On the experimental side, many results have been obtained from rheological techniques [8][9][10][11][12] and from various rheo-optical methods. 6,7,[13][14][15][16][17][18][19] Most of the results show that there are two basic flow behaviors in shear, depending on the shear rates. At low shear rates the characteristic features of the tumbling regime are observed, with very long transients, a low order parameter and the presence of many defects.…”

Section: Introductionmentioning

confidence: 99%

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

1996

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The depolarized light scattering patterns observed at low shear rates for lyotropic and thermotropic nematic main-chain liquid crystalline polymers show very similar aspects. They are composed of two main parts, a long streak perpendicular to the flow direction, with a strong intensity modulation, and four asymmetric lobes. These general features suggest a common origin for this pattern. A simplified model of the defects created in the low shear rate region of a thermotropic polymer (De'Nève, T.; Navard, P.; Kléman, M. Macromolecules 1995, 28, 1541, was used as the basic scattering unit. It consists of a twist loop, fluctuating around the shearing plane, with a director varying from the flow direction to the vorticity axis. The light scattering by a collection of such objects is theoretically calculated and compared to the experimental behavior of the vertical streak.

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Thermotropic mesomorphism of selected (2-hydroxypropyl)cellulose derivatives

Wojciechowski¹

2000

J. Appl. Polym. Sci.

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ABSTRACT:The thermotropic mesomorphism of some cellulose derivatives (CD) based on the (2-hydroxypropyl)cellulose (HPC) was investigated. Three types of derivatives: two esters (PPC, HxPC) and cyanoethyl derivative (CEPC) were prepared. The X-ray diffraction patterns of CDs were compared with the differential scanning calorimetry, thermooptical, and mechanical measurements within a broad range of the temperature. Two relaxation processes ␣ a and ␣ m , observed in the solid state of HPC, are also exhibited by all CDs, however, at lower temperatures. The ␣ m relaxations, which indicate the transition from frozen anisotropic phase to mobile liquid crystalline (LC)-phase, are shifted towards the lower temperatures with a corresponding increase in the d-spacing of the poly(saccharide) main chains of CDs (as seen in the X-ray measurements). The transition temperature to isotropic phase T ni as well as glass transition temperature T g (␣ a -relaxation) of the investigated CDs depend on the interactions between the lipophilic side chains and the hydrophilic poly(saccharide) main chains of CDs. These interactions are determined by the length and polarity of the lipophilic side chains. The observed changes in the transition temperature to isotropic phase T ni for CDs is consistent with the assumption that LC-organization of the poly(saccharide) main chains is stabilized by the lipophilic side-chains system. A significant increase in the length of the lipophilic side chains leads to nonlinear conformation, thus reducing the influence of van der Vaals forces, and consequently lowering T ni . The polymer with high polarity lipophilic side chains (CEPC) exhibits higher T ni in comparison to the ester derivative PPC with the same length of the side chains but having lower polarity. The stabilization effect of the lipophilic side-chains system on the LC-organization of the poly(saccharide) main chains is determined by the dynamic balance between length and polarity of the lipophilic side-chains system.

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Shear flow of liquid-crystalline polymer solutions as investigated by small-angle light-scattering techniques

Cited by 35 publications

References 1 publication

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

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

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

Thermotropic mesomorphism of selected (2-hydroxypropyl)cellulose derivatives

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