Liquid crystalline polymers history

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. 

Polymer mixtures have attracted some historical interest,320 as have liquid crystalline polymers.321 Several papers have examined the history of conducting polymers322 and of organic superconductors,323,324 the first having been discovered by Jerome in 1980. 

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. 

There has been a great deal of interest in thermotropic, liquid crystalline polymers in the past twenty years or so since the discovery of useful materials based on them. Many critical factors such as structure of mesogenic units, presence and structure of flexible spacers or rigid kinks, molecular weight and its distribution, and thermal history influence thermal, physical and thermotropic properties of liquid crystalline polymers(1-13). 

In tracing back the history of the liquid crystalline polymer itself, the German scientist D. Vorlander should be mentioned. It was he who first pointed out that the long shape of the polymer does not prevent the polymer from exhibiting liquid crystallinity. The polymeric liquid crystallinity was first found in the tobacco mosaic virus in solution around 1940, and later found in the poly-peptide solution. The initial theoretical basis for the rigid liquid crystalline polymer is attributed to the... 

The majority of polymer blends containing elastomeric, thermoplastic, and/or liquid crystalline polymers are processed by melt extrusion at some point in their history. After melt extrusion with intensive mixing, the morphology of an immiscible polymer blend on a microscopic scale will often consist of a dispersed phase of the more viscous polymer in a continuous matrix of the less viscous polymer (depending upon the relative amounts and viscosities of the two polymers in the blend). A good analogy from every-day experience is a dispersed mixture of viscous oil in an immiscible water matrix. 

In order to characterize polymeric fluids and to test rheological equations of state it is customary to use simple, well defined flows. The two main flows are simple shear and simple elongational. These are shown schematically in Figure 1. In shear flow, material planes (see Figure 1) move relative to each other without being stretched, whereas in extensional flow the material elements are stretched. These two different flow histories generate different responses in not only flexible chain polymers but in liquid crystalline polymers. When these flows are carried... 

D. Done and D. G. Baird, The effect of thermal history on the rheology and texture of thermotropic liquid crystalline polymers, Polymer Engineering Science, 27, 816-822 (1987). 

A history of the industrial development of thermotropic polymers would not be complete without a brief review of preceding technology, that is, the discoveries and developments made in lyotropic polymers. Thus, the timeline of milestones in liquid crystalline polymers proceeds from the initial observation of small molecule liquid crystallinity to the discovery of lyotropic and thermotropic high performance polymers and on through to the recent commercialization of thermotropic polyesters with the introduction of the Vectra (Celanese Corporation) and Xydar (Dartco Manufacturing) families of engineering resins. 

Weiss, R. A., Chung, N., and Kohli, A., The effect of deformation history on the morphology and properties of blends of polycarbonate and a thermotropic liquid crystalline polymer, Polym. Eng. Sci. 29 512 (1989). 

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... 

Marrucci G, Maffettone PL (1993) Liquid crystalline polymers. Pergamon, New York Mead DW (1994) Determination of molecular weight distributions of linear flexible polymers from linear viscoelastic material functions. J Rheol 38 1797-1827 Moldenaers P, Yanase H, Mewis J (1990) Effect of shear history on the theological behavior of lyotropic liquid crystals. Liq Cryst Polym 24(1990) 370-380 Muir MC, Porter RS (1989) Processing rheology of liquid taystal polymers a review. Mol Cryst Liq Cryst 169 83-95... 

Thermotropic liquid crystalline polymers (TLCPs) show its liquid crystallinity in melt phase (Brehmer and de Jeu 2012 Shibaev et al. 1984 Popa-Nita et al. 2009). Thermotropic phases are those that occur in a certain temperature range. If the temperature is raised too high, thermal motion will destroy the ordering of the LC phase, pushing the material into a cmiventional isotropic liquid phase. At too low a temperature, most LC phases will form a conventional anisotropic crystal. Many thermotropic LCs shows a variety of phases as temperature is altered (National Materials Advisory Board 1990 Popa-Nita et al. 2009 Davidson 1999). The melt temperature and the thermal history stored within the polymer system plays a vital role in determining the liquid crystallinity of TLCPs. 

The thermal behavior of liquid crystalline main chain polymers is more complicated. Depending on thermal history, different transitions may occur. The DSC curves of samples quenched from the melt may show the glass transition (positive ACp), cold crystallization (exotherm), melting (endotherm)and the mesophase-isotropic liquid transition (endotherm)[ Fig. I6]. ... 

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