Shear liquid crystalline polymers

Sgalari, G. Leal, L.G. Meiberg, E. Texture evolution of sheared liquid crystalline polymers numerical predictions of roll-cells instability, director turbulence, and striped texture with a molecular model. J. Rheol. 2003, 47 (6), 1417-1444. 

S. Yuanichi, M. Grosso, R. Keunings, S. Crescitelli, and P. Maettone, Prediction of chaotic dynamics in sheared liquid crystalline polymers, Rev. Lett, 86, 31-84, 2001. 

Viney C, Pumam W (1995) The banded microstructure of sheared liquid-crystalline polymers. Polymer 36 1731-1741... 

Shear modulus, polyamide, 138 Sheet molding compounds (SMCs), 30 Shoe sole products, 205 Shore hardness gauge, 243 Side-chain liquid crystalline polymers, 49 Side reactions, in transition metal coupling, 477... 

Optical and electro-optical behavior of side-chain liquid crystalline polymers are described 350-351>. The effect of flexible siloxane spacers on the phase properties and electric field effects were determined. Rheological properties of siloxane containing liquid crystalline side-chain polymers were studied as a function of shear rate and temperature 352). The effect of cooling rate on the alignment of a siloxane based side-chain liquid crystalline copolymer was investigated 353). It was shown that the dielectric relaxation behavior of the polymers varied in a systematic manner with the rate at which the material was cooled from its isotropic phase. 

Director Reorientation of Liquid Crystalline Polymers Under Shear and... 

Alternating dark and bright bands observed, following shear, in a wide range of main-chain nematic and chiral nematic liquid-crystalline polymers. 

In the second half of this article, we discuss dynamic properties of stiff-chain liquid-crystalline polymers in solution. If the position and orientation of a stiff or semiflexible chain in a solution is specified by its center of mass and end-to-end vector, respectively, the translational and rotational motions of the whole chain can be described in terms of the time-dependent single-particle distribution function f(r, a t), where r and a are the position vector of the center of mass and the unit vector parallel to the end-to-end vector of the chain, respectively, and t is time, (a should be distinguished from the unit tangent vector to the chain contour appearing in the previous sections, except for rodlike polymers.) Since this distribution function cannot describe internal motions of the chain, our discussion below is restricted to such global chain dynamics as translational and rotational diffusion and zero-shear viscosity. 

The zero-shear viscosity r 0 has been measured for isotropic solutions of various liquid-crystalline polymers over wide ranges of polymer concentration and molecular weight [70,128,132-139]. This quantity is convenient for studying the stiff-chain dynamics in concentrated solution, because its measurement is relatively easy and it is less sensitive to the molecular weight distribution (see below). Here we deal with four stiff-chain polymers well characterized molecu-larly schizophyllan (a triple-helical polysaccharide), xanthan (double-helical ionic polysaccharide), PBLG, and poly (p-phenylene terephthalamide) (PPTA Kevlar). The wormlike chain parameters of these polymers are listed in Tables... 

Experiments by Muller et al. [17] on the lamellar phase of a lyotropic system (an LMW surfactant) under shear suggest that multilamellar vesicles develop via an intermediate state for which one finds a distribution of director orientations in the plane perpendicular to the flow direction. These results are compatible with an undulation instability of the type proposed here, since undulations lead to such a distribution of director orientations. Furthermore, Noirez [25] found in shear experiment on a smectic A liquid crystalline polymer in a cone-plate geometry that the layer thickness reduces slightly with increasing shear. This result is compatible with the model presented here as well. 

B. Ernst and P. Navard, Shear flow of liquid-crystalline polymer solutions as investigated by small-angle light scattering techniques, Macromolecules, 23,... 

The rheological and flow properties of ordered block copolymers are extraordinarily complex these materials are well-deserving of the apellation complex fluids. Like the liquid-crystalline polymers described in Chapter 11, block copolymers combine the complexities of small-molecule liquid crystals with those of polymeric liquids. Hence, at low frequencies or shear rates, the rheology and flow-alignment characteristics of block copolymers are in some respects similar to those of small-molecule liquid crystals, while at high shear rates or frequencies, polymeric modes of behavior are more important. 

Burghardt, W.R. Molecular orientation and rheology in sheared lyotropic liquid crystalline polymers. Macromol. Chem. Phys. 1998, 199 (4), 471-488. 

Ugaz, V.M. Cinader, D.K. Burghardt, W.R. Origins of region I shear thinning in model lyotropic liquid crystalline polymers. Macromolecules 1997, 30 (5), 1527-1530. 

Han, W.H. Rey, A.D. Dynamic simulations of shear-flow-induced chirality and twisted-texture transitions of a liquid-crystalline polymer. Phys. Rev. E. 1994, 49, 597-613. 

Larson, R.G. Mead, D.W. Development of orientation and texture during shearing of liquid-crystalline polymers. Liq. Cryst. 1992,12, 751-768. 

Klein, D.H. Leal, L.G. Computational studies of the shear flow behavior of models for liquid crystalline polymers. AIChE Annual Meeting, San Francisco, November 16-21, 2003. 

Theimotropic liquid crystalline polymers can be formulated with high concentration of glass fiber to withstand working temperatures in excess of 300°C. In processing LCP, one problem arises. LCP orients itself in the direction of shear or flow - the process which benefits many materials but makes products from pure LCP excessively anisotropic. To balance mechanical properties it has been suggested that some quantities of short glass or mineral fibers be added to LCP. 

Small molecular mass liquid crystals do not respond to extension and shear stress. Liquid crystalline polymers may exhibit a high elastic state at some temperature due to the entanglements. However, the liquid crystalline network itself is an elastomer, showing rubber elasticity. In the presence of external stress, liquid crystalline networks deform remarkably and then relax back after the release of stress. The elasticity of liquid crystalline networks is more complicated than the conventional network, such as the stress induced phase transition, the discontinuous stress-strain relationship and the non-linear stress optical effect, etc. 

Figure 4.19. Banded texture of a mesogen-jacketed liquid crystalline polymer. The shearing direction is perpendicular to the direction of the parallel bands.

Lee (1988) further connected the viscosities of liquid crystalline polymers (in the unit of f]) with concentration and molecular length as Equation (6.37), where A is a constant less than unity which is associated with the stability of simple shear flow, is a dimensionless parameter associated with the interaction in the fluid, R = (/A(a n) is the second order parameter where a is the molecular long axis and P4 is the fourth rank Legendre polynomial. 

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