Fluids, structured liquid crystalline polymers

Kupferman, R. Kawaguchi, M.N. Denn, M.M. Emergence of structure in a model of liquid crystalline polymers with elastic coupling. J. Non-Newton. Fluid Mech. 2000, 91, 255-271. 

For example, if the time for the process to occur is faster than the longest relaxation time, then the fluid behaves more like an elastic solid. For the liquid crystalline systems there seems to be two relaxation times which are important. One is the time for relaxation of orientation and the other is the time for relaxation of stress. Whereas these phenomena are connected for flexible chain polymers, they seem to separate for liquid crystalline polymers. In other words, there are several stress free states for LCP. Some of the behavior observed may be partly due to the copolymer nature of thermotropic systems. However, the lyotropic systems based on the polyamide structure also exhibit similar behavior. 

What we have attempted to do here is to present rheological tests for identifying the development and relaxation of orientation and structure in liquid crystalline polymers. Because these fluids are typically quite turbid, it is difficult to use rheo-optical techniques. The interpretation of the rheological tests must then come partly from studies on quenched solid specimens. In summary, it is believed that a detailed set of rheological tests based on the transient response of LCP can be used to evaluate various liquid crystalline polymers and identify processing conditions which will lead to the optimum physical properties. 

The terms liquid crystal and mesophase are interchangeable. Meso, in Greek, means between , so a fluid can be called a mesophase if it has some properties that are characteristic of crystals. The ability of the fluid to form a liquid crystal is due to the molecules ability to align with each other and create local ordering. So, liquid crystalline polymers are those polymers that form liquid crystalline phases either in solution or in the melt. Molecules that form a mesophase are usually rod-like or disc-like. In the case of rodlike polymers, such as poly(p-phenyleneterephthalamide) (PPTA), the rigidity of the backbone is primarily responsible for the formation of a mesophase. The rigidity is, of course, dependent on a variety of factors such as the nature of the solvent used, the temperature of the solution, and the chemical structure of the molecule. 

The principle of the conservation of angular momentum is often used to argue for the symmetry of the extra stress that is, Xxy = Tyx, and so forth. In that case there are six independent components, not nine. The angular momentum argument requires an explicit but often unstated assumption that there is no structure in the fluid that is capable of generating local torques, which seems generally to be the case for polymers (except perhaps for liquid crystalline polymers), and, except for a brief introduction to liquid crystals in Chapter 13, we will assume stress symmetry throughout. 

New mathematical techniques [22] revealed the structure of the theory and were helpful in several derivations to present the theory in a simple form. The assumption of small transient (elastic) strains and transient relative rotations, employed in the theory, seems to be appropriate for most LCPs, which usually display a small macromolecular flexibility. This assumption has been used in Ref [23] to simplify the theory to symmetric type of anisotropic fluid mechanical constitutive equations for describing the molecular elasticity effects in flows of LCPs. Along with viscoelastic and nematic kinematics, the theory nontrivially combines the de Gennes general form of weakly elastic thermodynamic potential and LEP dissipative type of constitutive equations for viscous nematic liquids, while ignoring inertia effects and the Frank elasticity in liquid crystalline polymers. It should be mentioned that this theory is suitable only for monodomain molecular nematics. Nevertheless, effects of Frank (orientation) elasticity could also be included in the viscoelastic nematody-namic theory to describe the multidomain effects in flows of LCPs near equilibrium. 

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... 

Many engineering thermoplastics (e.g., polysulfone, polycarbonate, etc.) have limited utility in applications that require exposure to chemical environments. Environmental stress cracking [13] occurs when a stressed polymer is exposed to solvents. Poly(aryl ether phenylquin-oxalines) [27] and poly(aryl ether benzoxazoles) [60] show poor resistance to environmental stress cracking in the presence of acetone, chloroform, etc. This is expected because these structures are amorphous, and there is no crystallinity or liquid crystalline type structure to give solvent resistance. Thus, these materials may have limited utility in processes or applications that require multiple solvent coatings or exposures, whereas acetylene terminated polyaryl ethers [13] exhibit excellent processability, high adhesive properties, and good resistance to hydraulic fluid. 

Polymers that exhibit liquid crystallinity, either in the melt or in their solutions, typically consist of comparatively rigid structures that confer high extension on the backbone of the macromolecule. This molecular feature is obviously conducive to the axial order that is the mark of a nematic fluid. ... 

A similar approach has been used to produce materials with a chiral (cholesteric) structure by performing the experiments described above in the presence of a low molecular weight chiral liquid crystalline material (Figure 9.6). The chiral material is not covalently attached to the network and can be removed subsequently to produce an imprinted chiral structure. As before, the polymer displays a nematic mesophase between the glass transition (Tg 33°C) and the transition to an isotropic fluid (rN, 128°C). 

Liquid crystal polymers have created a great deal of interest in recent years finding a number of commercial applications ranging from high-strength engineering plastics to optical display devices. A liquid crystal molecule possesses anisotropy and, as a mobile fluid, can spontaneously order. It therefore exhibits some of the properties of a liquid (mobility, flow) as well as a degree of order usually associated with a crystalline structure. 

Liquid crystals (LCs) are described as a fluid phase that flows like a liquid and is oriented in a crystalline manner. LCs are divided into two types thermotropic LCs, where the LC phase transition is dependent on temperature or lyotropic LCs, where the LC phase transition occurs as a function of solvent concentration. To introduce liquid crystallinity to conjugated polymers, LC moieties can be introduced to the polymer side chains for side chain-type liquid crystallinity. On the other hand, designing conjugated polymers with rigid main chain structures with flexible alkyl side chains for solubility enables main chain-type liquid crystallinity. 

C. Noel and J. Virlet, "DSC, Miscibility and X-ray Studies of the Thermotropic Liquid Crystalline Polyesters with Aromatic Moieties and Flexible Spacers in the Main Chain", in "Liquid Crystals and Ordered Fluid, A. Griffin and A. Johnson eds., Plenvim Press, New-York, vol. 1+, 1+01 (198I+) Finkelmann, "Synthesis, Structure, and Properties of Liquid Crystalline Side Chain Polymers", in "Polymeric Liquid Crystals", A. Ciferri, W.R. Krigbaum and R.B. Meyer eds. Academic Press, New-York (I982)... 

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. 

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