Semi-rigid LCPs

Figure 1. A DPD model for a flexible main-chain LCP with three semi-rigid units...

Figure 5. Snapshot of a modd LCP system of 1000 semi-rigid units of 12 beads (only a fraction of the units of the sample is shown).

Liquid crystalline polymers (LCPs) exhibit ordered molecular arrangements in the melt state. The polymer molecules of LCPs are generally semi-rigid or contain rigid units. The rigid units are either along the backbone or in the side branches. LCPs are viscoelastic and anisotropic. The interested reader can learn details about LCPs from Feng and Leal (1997), Larson (1999), Fan et al. (2003), and Klein et al. (2008). 

A typical semi rigid liquid crystal polymer which includes more than one mesogenic units are (a) Xydar which consists a copolyesters of 4, 4 -dihydroxybiphenol (BP), Phthalic acid (TA), and Hydroxy benzoic acid (HB A), (b) Vectra A950, a copolyesters of HBA and 2-Hydroxy-6-napthoic acid (HNA). Vectra B-950 is the only commercially available thermotropic LCPs containing amide linkages on the polymer main-chains. 

Because of the structural similarity between semi-rigid thermotropic polymers and LCPs, i.e. the presence of rigid moieties separated by flexible spacers, all polymers of this kind are commonly referred to as the main chain liquid crystal polymers (mcLCP). 

In this section, we consider the case of solutions of rigid or semi-flexible polymers which display one or several liquid crystalline phases in a given range of concentration. The main control parameter is not flie temperature as is the case for thermotropic LCPs but rather the concentration of polymer in the solvent. There are many different kinds of lyotropic LCPs. Some are synthetic like Kevlar which has become a very important structural material with mechanical properties comparable to those of steel. Some are natural like the Tobacco Mosaic Virus and like DNA which shows a nematic and a hexagonal phase. Some are mineral like the vanadium pentoxide ribbons. In the next section, we shall first describe the lyotropic system which is probably best known, namely the tobacco mosaic virus. 

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