Liquid crystal polymers history

Cogswell (1985) expressed it in the following words "To make the connection from the basic material properties to the performance in the final product, industrial technologists had to learn a new science". It is more or less so, that - for liquid crystal polymers -properties like stress history, optical and mechanical anisotropy, and texture seem to be independent variables this in contradistinction to the situation with conventional polymers. 

The book covers a wide range of topics within the field of polymer physics, beginning with a brief history of the development of synthetic polymers and an overview of the methods of polymerisation and processing. In the following chapter, David Bower describes important experimental techniques used in the study of polymers. The main part of the book, however, is devoted to the structure and properties of solid polymers, including blends, copolymers and liquid-crystal polymers. 

If this section of this chapter hardly reads as a chronological history, this is because the last 15 years have seen so many developments in different directions that a simple pattern of evolution does not exist. Instead, developments have occurred in an explosive way, emanating outward from the core of fundamental knowledge acquired up to the end of the 1970s. We have already looked briefly at display applications, liquid crystals from disc-shaped molecules, liquid crystal polymers, and metallomeso-gens, but to follow all the developments of recent years radiating out from the central core is hardly possible in a short chapter like this. 

In the field of conventional engineering thermoplastics we have a detailed understanding of the isotropic state, we appreciate the stress history deployed in a moulding process, we can measure relaxation phenomena and so predict residual orientation, and so we can deduce the property spectrum of a final product. If the time-scale between recognition of liquid crystalline phenomena in melts and its commercial exploitation appears protracted we need only note the observation of Professor J. L. White summing up at a recent conference in Kyoto, that, for liquid crystal polymers, stress history, optical anisotropy and texture are independent variables. In fact, to make the connection from basic material property to performance in the final product, industrial technologists have had to learn a new science. 

Class a, longitudinal liquid crystal polymers, earlier called main-chain polymers. A new name is necessary to distinguish them from the following classes jS and y and the subclasses (S, (R and Al. There are numerous examples of class a. We do not discuss them here in any detail since the entire Chapter 8 by MacDonald is devoted to thermotropic ones, while Chapter 6 by Northolt and Sikkema deals with lyotropic PLCs of the same class. Orientational and conformational order in longitudinal PLCs is discussed by Frangoise Lauprete in Section 3.1.3.1 would only like to note an account of the history of first polyesters in this class and of future perspectives by Jackson. ... 

When polymerization is effected at the air-water interface, monolayer polymer films may result. Such films have attracted a great deal of attention in recent years by virtue of their similarity to biological membranes. Polymeric films do not really fall within the strict definition of liquid crystal polymers. However, the molecular arrangement of the side chains resembles that found in many liquid crystal phases, most notably the lamellar phase, and could therefore realistically be described as one half of a lamellar sheet. Bilayer films, on the other hand, may be thought of as a single lamellar layer, and therefore the comparison becomes even more appropriate. The technical applications of polymer membranes are extremely diverse and for these reasons a short section will be devoted to their history, their uses and their current status. 

A crystalline or semicrystalline state in polymers can be induced by thermal changes from a melt or from a glass, by strain, by oiganic vapors, or by liquid solvents (40). Polymer crystallization can also be induced by compressed (or supercritical) gases, such as C02 (41). The plasticization of a polymer by C02 can increase the polymer segmental motions so that crystallization is kinetically possible. Because the amount of gas (or fluid) sorbed into the polymer is a direct function of the pressure, the rate and extent of crystallization may be controlled by controlling the supercritical fluid pressure. As a result of this ability to induce crystallization, a history effect may be introduced into polymers. This can be an important consideration for polymer processing and gas permeation membranes. 

As can be seen in H, Kelkers l) excellent review on the history of liquid crystals, investigations on liquid crystalline polymers already exist before F. Reinitzer in 1888 gave the very first description of a low molar mass liquid crystal (1-l.c.). While, however, 1-l.c. s have become an extensive field of research and application during the past decades, these activities on l.c. polymers have come rather late. The research on l.c. polymers during the last years is mainly joined with activities in material science and tries to realize polymers with exceptional properties. These exceptional properties are expected because of the combination of the physical anisotropic behavior of l.c. and the specific properties of macromolecular material. 

The big difference between normal isotropic liquids and nematic liquids is the effect of anisotropy on the viscous and elastic properties of the material. Liquid crystals of low molecular weight can be Newtonian anisotropic fluids, whereas liquid crystalline polymers can be rate and strain dependent anisotropic non-Newtonian fluids. The anisotropy gives rise to 5 viscosities and 3 elastic constants. In addition, the effective flow properties are determined by the flow dependent and history dependent texture. This all makes the rheology of LCPs extremely complicated. 

Figure 24. Biomesogenic structures a) (Bio)meso-gens displaying order-disorder distributions in CPK-presentation (left to right and top to bottom) hexa-n-alkanoyl-oxybenzene discoid - Chandrasekar s first non-rodlike liquid crystal [28 a, 51c] enantiomeric cholesteric estradiol- and estrone-derivatives [ 17 a, c, d, 26 f, 51 a, s, u] Reinitzer s cholesterolbenzoate [21, 22] - together with the acetate the foundation stones of liquid crystal history [21, 22] Kelker s MBBA -first liquid crystal fluid at ambient temperature [ 13 f, g] Gray s cyanobiphenyl nematics for electrooptic displays [25 a, 51 e] lyotropic lecithin membrane component [7 a, 14, 27 d, 52 a] and valinomycin-K -membrane carrier [7 a, 35] thermotropic cholesteryl-side-chain-modifiedpolysiloxanes with the combination of flexible main-chain and side-chain spacers [51 a, h] thermotropic azoxybenzene polymers with flexible main-chain spacers [51a] thermotropic cya-...

Polymer liquid crystals generally form unusual textures, accompanied by complex and unstable superstructures. Such textures depend on a sample history, especially mechanical and thermal histories, and also on the surface nature of the vessels. We have described the structural characteristics of polymer liquid crystals under a shear force by means of rheo-optical methods. ... 

Memory effects in polymer liquid crystals influence of thermal history of phase behavior... 

Phase behavior and morphology in conventional polymers are heavily dependent on the thermal history of the sample, as is obvious to anyone even remotely familiar with macromolecules. Polymer liquid crystals (PLCs) are clearly subject to similar constraints by virtue of their macromolecular identity. In addition, a number of thermal properties are specific to PLCs as a result of the interaction between macromolecular behavior and the molecular ordering characteristic of LC mesophases. This chapter focuses on just such features of thermal history, as revealed by the interplay of kinetic and thermodynamic factors observed in thermotropic polymers. 

The evidence and references cited support considering LCP s as liquid crystals it is also necessary to consider how they are similar or different from isotropic polymers. Solutions and melts of polymers are non-linear viscoelastic [18] - the stress depends not only on the instantaneous rate of deformation, but upon the entire history of the deformation. The material has a fading memory for previous configurations, the rate of fading memory for previous configurations, the rate of fading determined by a relaxation time or, in practice, by an entire spectrum of relaxation times. 

Drzaic Paul S. Putting liquid crystal droplets to work A short history of polymer dispersed liquid crystals. Liq. Cryst. 33 no. 11-12 (2006) 1281-1296. 

---