Polymers with Liquid-Crystalline Order

Because lyotropic liquid-crystalline polymers cannot be extruded, injection molded, or blown into films, other polymers that can be melt processed have been developed. These thermotropic liquid-crystalline polymers convert to a mesophase when the sohd polymer is heated to a temperature above the crystalline melting point. Thus, these polymers show three thermal transitions. In increasing order of temperature, these are glass transition temperature, erystalline melting 

Reprinted from Polymer, vol. 21, Aharoni, S. M. Rigid Backbone Polymers XVII. Solution Viscosity of Polydisperse Systems, pp. 1413-1422, Copyright 1980, with kind permission from Elsevier Science Ltd., The Boulevard, Langford Lane, Kidlington 0X5 1GB, UK. 

Crystallization in a thermolropic liquid-crystalline polymer is again a process of nucleation and growth [54]. It has been shown that the process can be followed easily using dynamic mechanical analysis (see Chap. 12) [55], in which we measure the stress response of the material to an imposed small-amplitude sinusoidal shear strain. Differential scanning calorimeter (DSC) data on the kinetics of crystallization show that the process is describable by an Avrami equation [56]. 

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Baird, D. G. The rheology of polymers with liquid-crystalline order, in Rheology, Vol. 3, Applications (ed.) Astarita, G., Mannicci, G., Nicolais, L., p. 647, New York, Plenum, 1980... 

D.G. Baird, "Rheology of Polymers with Liquid Crystalline Order," in Liquid Crystalline Order in Polymers, Academic, New York, 1978, pp. 237-259. 

Copolymers of mesogenic with non-mesogenic vinyl monomers provide the opportunity to diversify the properties of polymers with liquid crystalline order. 

Hydrophobically modified polybetaines combine the behavior of zwitterions and amphiphilic polymers. Due to the superposition of repulsive hydrophobic and attractive ionic interactions, they favor the formation of self-organized and (micro)phase-separated systems in solution, at interfaces as well as in the bulk phase. Thus, glasses with liquid-crystalline order, lyotropic mesophases, vesicles, monolayers, and micelles are formed. Particular efforts have been dedicated to hydrophobically modified polyphosphobetaines, as they can be considered as polymeric lipids [5,101,225-228]. One can emphasize that much of the research on polymeric phospholipids was not particularly focused on the betaine behavior, but rather on the understanding of the Upid membrane, and on biomimicking. So, often much was learnt about biology and the life sciences, but little on polybetaines as such. 

Orientational order plays an important role in solid polymers. It is often induced by industrial processing, for example in fibers and injection- or compression-modulated parts. In polymers with liquid-crystalline properties of the melt or solution, the anisotropies generated by the flow pattern are particularly pronounced. In order to improve the mechanical properties of polymer fibers or films, the degree of orientation is intentionally enhanced by drawing. At the same time, anisotropy of mechanical properties can result in low tolerance to unfavourably directed loads. In many liquid-crystalline polymers, in the mesophase near the transition to the isotropic phase, electric or magnetic fields can induce macroscopic orientational order [1]. Natural polymers such as silk protein fibers, which are biosynthesized and spun under biological condition, also have good mechanical properties because of their ordered structure [2]. 

Some polymers manifest liquid crystalline ordering, which does not have the full long-range three-dimensional periodicity of crystallinity but is far more ordered than amorphicity. Since many excellent books and articles have been published on such polymers and the author does not have much that is new to add to this background information, very little will be said about polymer liquid crystallinity in this book. Van Krevelen [3] has reviewed liquid crystallinity in polymers in a readable manner and discussed its effects on properties for which quantitative structure-property relationships are available. Adams et al [41] have published a valuable compendium of articles covering the theory, synthesis, physical chemistry, processing and properties of liquid crystalline polymers. Woodward [42] has discussed and illustrated liquid crystallinity in polymers with many beautiful micrographs. 

In a polymer, the rod-like structures can be attached as side groups—side-chain liquid crystal polymers or with the skeletal backbone—main chain liquid crystal polymers (Donald et al. 2006). The latter usually exhibit liquid crystal characteristics at elevated temperatures, while some side-chain liquid crystal polymers exhibit liquid crystalline order at room temperature. A number of more ordered smectic phases can be observed as well as chiral, nematic, and smectic phases (Donald et al. 2006). 

A general concept for describing all kinds of order in chain molecules, ranging from crystalline order to liquid crystalline order and then to order in oriented and isotropic amorphous polymers, is introduced in the third article written by Pieper and Kilian. After the presentation of the basic concept, experimental results obtained on different polymers including phases with rotatory segmental motion are discussed. 

Blumstein, A. and Hsu, E. C. Liquid crystalline order in polymers with mesogenic side groups in Blumstein, A. ed. Liquid Crystalline Order in Polymers . Academic Press, New York, NY 1978, p. 105... 

The second group involves polymers with three-dimensional ordering of side branches (e.g., those forming Mj-phaseXTable 5). On X-ray patterns of these polymers 3-4 narrow reflexes at wide angles are observed. As a rule, the authors define this type of structure as crystalline, or ascribe a smectic type of structure, characteristic for ordered smectics in SE or SH phases. The heats of transition from anisotropic state to isotropic melt are usually small and do not exceed the heats of transition smectic liquid crystal — isotropic melt . The similarity of structural parameters of three-dimensionally ordered smectics and that of crystalline polymers of the type here considered, make their correct identification quite a difficult task. 

A more recent trend in polymer materials research is the hybridization of cellulosic polysaccharides with inorganic compounds natural and synthetic layered clays, silica, zeolites, metal oxides, and apatites are employable as nanoscale components. In addition, if mesoscopic assemblies such as liquid-crystalline ordering are used in the construction of new compositional systems, the variety of functionalized cellulosic materials will be further expanded. 

This article deals with some topics of the statistical physics of liquid-crystalline phase in the solutions of stiff chain macromolecules. These topics include the problem of the phase diagram for the liquid-crystalline transition in die solutions of completely stiff macromolecules (rigid rods) conditions of formation of the liquid-crystalline phase in the solutions ofsemiflexible macromolecules possibility of the intramolecular liquid-crystalline ordering in semiflexible macromolecules structure of intramolecular liquid crystals and dependence of die properties of the liquid-crystalline phase on the microstructure of the polymer chain. 

Recent work focuses on non-classical mesogenes which are built up by self-assembly. One example is a family of polymers containing disk-like groups which form no liquid crystalline phase, but ean act as an electron acceptor or donor. Charge transfer complexation with a complementary low molecular mass compound induces nematic or columnar discotic liquid crystalline order [153,154]. Figure 13 demonstrates this with the example of a polyester, bearing electron-rich tetra(alkoxy)tri-phenylene-units in the main chain, mixed with the electron deficient aromatic 2,4,7-trinitro-9-fluorenone (TNF). While the pure polymer shows a non-ordered isotropic melt, a columnar phase appears on addition of TNF. 

More reliable AH values can be obtained from direct measurements of the heat evolved during polymerization [34—39], or from the difference between the heats of combustion of the amorphous polymer and the liquid monomer. With respect to the very low AHp values, as compared with the heats of combustion, the latter method is very inaccurate. The evaluation of calorimetric data is more difficult for partially crystalline polymers, for which the crystallinity as well as heat of crystallization must be known [38]. Equations (19)-(21) can be applied to such monomer polymer equilibria for which the equilibrium monomer concentrations at different temperatures are available with sufficient precision [28—30, 40]. The latter method is also limited to completely amorphous polymers because the crystalline ordered areas do not take part in the monomer-polymer equilibrium [21]. 

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