Flow behavior of liquid crystalline polymer

Marrucci, G. Greco, F. Flow behavior of liquid crystalline polymers. Adv. Chem. Phys. 1993, 86, 331-303. 

Viola, G. G., and Baird, D. G., Studies on the transient shear flow behavior of liquid crystalline polymers, J. Rheol., 30, 601-628 (1986). 

Marrucci Giuseppe, and Greco Francesco. Flow behavior of liquid crystalline polymers. In Advances in Chemical Physics, eds. Prigogine L, Rice Stuart A., 331—404. Hoboken, NJ Wiley, 1993. 

Finally, this article documents the need for a more thorough understanding of all of the molecular parameters which influence the flow behavior of liquid crystalline polymers. A universal theory which describes the flow behavior of all LCP will only be developed when all characteristics are defined and accounted for which impact the rheology of LCP solutions. Moreover, a model liquid crystalline polymer solution should be one in which these factors can be systematically studied. 

Our intent, as mentioned earlier, is not to review all the studies concerned with liquid crystalline fluids but to compare their properties with flexible chain polymers, interpret their properties in terms of the domain structure, and look for correlations between flow characteristics and processing conditions. We first examine the behavior of liquid crystalline copolyesters in steady shear flow and in small strain dynamic oscillatory flow. 

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. 

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. 

Fihpe, S., M. Gidade, and J. Maia. 2006. Uniaxial extensional flow behavior of immiscible and compatibUized polypropylene/liquid crystalline polymer blends. Rheologica Acta 45 281-289. 

From practical considerations, two properties are of prime interest The effect of liquid crystalline behavior on viscosity and the ability of the polymer to retain the ordered arrangement in the solid state. Liquid crystalline behavior during the melt results in lower viscosity, because the rigid polymeric mesophases align themselves in the direction of the flow. As a result, the polymer is easier to process. Also, retention of the arrangement upon cooling yields a material with greatly improved mechanical properties. Several thermotropic liquid crystalline copolyesters are now available commercially. 

The formation of banded textures in thin-film samples of solutions of hquid crystalline polymers (LCPs), subjected to shear, has been reported in the literature since 1979 [15]. Because of the symmetrical properties of the liquid crystal solutions, large domains of weU-oriented polymer chains are formed during shear flow, while defects are squeezed into small regions. The shear accounts for an additional energy stored in the solution. When the shear is stopped, the system will first relax with a characteristic time fb to a transient state. In this state the distortion energy is minimized, and the orientational order is kept, resulting in a banded stmcture. This behavior is observed only if two conditions are fulfilled [16] ... 

Fig. 2 Schematic five types of SMPs depicted as a function of their thermal behavior. Plotted is the heat flow vs temperature as measured in a differential scanning calorimetry (DSC) experiment (a) Cat. A-1, chemically crosslinked tunorphous polymer network (Tirans = 7g) (b) Cat. A-11, chemically ciossfinked semicrystaUine polymer networks (Taims = 7m)> ( ) Cat. B-1, physically crosslinked thermoplastic with Tirans = T (d) Cat. B-11, physically crosslinked thermoplastic (Tams = I m) and (e) liquid crystalline polymer (Tlrans = T -n)...

In most cases, the flow properties of polymers in solution or in a molten state are Newtonian, pseudoplastic, or a combination of both. In the case of liquid crystal polymer solutions, the flow behavior is more complex. The profound difference in the rheological behavior of ordinary and liquid crystalline polymers is due to the fact that, for the flrst ones, the molecular orientation is entirely determined by the flow process. The second ones are anisotropic materials already at equilibrium (Acierno and Brostow 1996). The spontaneous molecular orientation is already in existence before the flow and is switched on, varying in space, over distances of several microns or less (polydomain). If one ignores the latter, one can discuss the linear case (slow flow) as long as the rate of deformation due to flow (the magnitude of the symmetric part of the velocity gradient) is lower than the rate at which molecules rearrange their orientational spread by thermal motions. 

What Is a Model Liquid Crystalline Polymer Solution Solvent Effects on the Flow Behavior of LCP Solutions... 

A difference in the defect structure of PBLG in BA versus PBLG in m-cresol may prove to be the underlying cause of this alignment behavior. Regardless, the results exemplify the need for a broader choice of model liquid crystalline polymer solutions when examining the flow-induced structure in liquid crystalline polymer solutions. Moreover, a more complete understanding of the important parameters that affect the flow of LCP solutions is needed so that a more universal theory can be developed which can predict flow behavior of non-model LCP solutions. 

Doi and Edwards (1986) have used a tube model to describe flow of semidilute suspensions of rods. Predicted behavior is qualitatively similar to their theory for entangled, flexible polymer chains (see Chapter 11). This approach has also been extended to describe the rheology of nematic liquid crystalline polymers (Doi and Edwards, 1986 Larson, 1988). 

The study of the behavior of polymer systems capable of liquid-crystalline ordering in an external orienting field is both fundamental and of great practical interest. The role of the external field can be played by an electric or magnetic field and by the effective field which arises if the solution is placed in inhomogeneous hydrodynamic flow or a melt is exposed to uniaxial extension. 

Kim SS, Han CD (1994) Effect of shear history on the steady shear-flow behavior of a thermotropic liquid-crystalline polymer. J Polym Sci Part B Polym Phys 32 371-381 Kiss G (1986) Anomalous temperature dependence of viscosity of thermotropic polyesters. J Rheol 30 585... 

White JL, Dong L, Han P, Laun HM (2004) Rheological properties and associated structural characteristics of some aromatic polycondensates including liquid-crystalline polyesters and cellulose derivatives. Int Union Pure Appl Chem 76(ll) 2027-2049 Wiberg G, Hillborg H, Gedde UW (1998) Assessment of development and relaxation of orientation in a sheared thermotropic liquid crystalline copolyester. Polym Eng Sci 38 1278-1285 Wilson TS, Baird DG (1992) Transient elongational flow behavior of thermotropic liquid crystalline polymers. J Non-Newt Fluid 44 85-112... 

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