Lyotropic PLCs

First I want to draw attention to polymers wherein specific solute-solvent interactions do promote mesophase formation. There are two general classes of polymer-solvent systems wherein instances of mesomorphism result from intermolecular interactions that are closely related to those that stabilize the amphiphilic MLC lyotropics DSidechain polymers with amphiphilic sidechains appended to the polymer backbone may form lyotropic PLCs in water, 2) Block copolymers exhibit organized, gel-like phases when one block is preferentially solvated. The former PLCs are intimately related to lyotropic MLCs, however, compared to the monomer amphiphiles, new phases and often phase stability is influenced by the attachment of the amphiphilic core to the chain. Finkelmann and co-workers have recently considered such amphiphilic sidechain PLCs.(17)... 

The investigations of thermotropic PLCs commenced almost two decades after studies of mesophases employing rodlike polymers(21) and virus particles(22) in solution were initiated. Like their MLC analogues (aqueous solutions of amphiphilic molecules) the polymeric lyotropic liquid crystals which form in solutions of rigid, rodlike polymers (helical biopolymers, and more recently, semiflexible aromatic amides) constitute a distinct class of liquid crystals. Unlike the MLC analogues, however, the lyotropic PLCs are not necessarily stabilized by specific interactions between the polymer chain... 

Although instances of lyotropic PLCs predate studies of thermotropic PLCs, as they involved solutions of comparatively esoteric species — virus particles and helical polypeptides — studies of these liquid crystals were isolated to a few laboratories. Nevertheless, observations on these lyotropic PLCs did stimulate the first convincing theoretical rationalizations of spontaneously ordered fluid phases (see below). Much of the early experimental work was devoted to characterizing the texture of polypeptide solutions. (23) The chiral polypeptides (helical rods) generate a cholesteric structure in the solution the cholesteric pitch is strongly dependent on polymer concentration, dielectric properties of the solvent, and polymer molecular weight. Variable pitch (<1 - 100 pm) may be stabilized and locked into the solid state by (for example) evaporating the solvent in the presence of a nonvolatile plasticizer.(24)... 

More recently, lyotropic PLCs were catapulted into the forefront of polymer research when ultra-high strength polymer fibers were commercialized by spinning mesomorphic solutions of poly(p-aromatic amides), e.g., Du Pont s Kevlar. I have recently reviewed these aspects of PLCs.(25)... 

Figure 4.1 Structural model of fibers for thermotropic and lyotropic PLCs. (Reprinted with permission from [10], copyright 1993 Chapman Hall.)...

This section deals with thermotropic PLCs. The story starts, however, with lyotropic PLCs for which the long structural relaxation times of PLCs, compared to those of conventional flexible chain polymers, were first realized from rheo-optical studies [118]. Figure 10.12 presents the fundamentals of the important early discovery a specimen first oriented by shear flow which is suddenly switched off exhibits a rapid stress relaxation within a period of seconds. The decay of birefringence is however slower and it takes a very significant period of time. 

We first published this negative feature in 1978 for an m-cresol solution of the most definitively studied lyotropic PLC, i.e. PBLG [16]. 

This chapter has been both a retrospective of our work and an overview of some studies which have been conducted since about 1980 on the negative effect during shear of lyotropic PLCs and the formation of band structures transverse to the shearing direction upon cessation of shear. The two phenomena are a manifestation of (1) liquid crystalline order, how this order is affected by forces imposed on molecules and domains during shear, and (2) how the stored energy relaxes after... 

We hope that time will reveal that the first 20 years after our confirmation of the negative effect and band structures in lyotropic PLCs was simply the explanation phase, interesting as it may be to a small niche of specialists. We hope that the second 20 years will prove to be the exploitation phase. 

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. ... 

Kiss, G., Flow-Induced Phenomena of Lyotropic PLCs in Mechanical and thermophysical properties of polymer liquid crystals, W. Brostow Editor, Chapman and Hall, (London) (1998). 

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