Liquid crystalline state formation requirements

Two approaches to the attainment of the oriented states of polymer solutions and melts can be distinguished. The first one consists in the orientational crystallization of flexible-chain polymers based on the fixation by subsequent crystallization of the chains obtained as a result of melt extension. This procedure ensures the formation of a highly oriented supramolecular structure in the crystallized material. The second approach is based on the use of solutions of rigid-chain polymers in which the transition to the liquid crystalline state occurs, due to a high anisometry of the macromolecules. This state is characterized by high one-dimensional chain orientation and, as a result, by the anisotropy of the main physical properties of the material. Only slight extensions are required to obtain highly oriented films and fibers from such solutions. 

An approach based on the virial expansion suffers from the difficulty of evaluating higher coefficients for highly asymmetric particles and from the non-convergence of the virial series at the concentrations required for formation of a stable nematic phase Lattice methods therefore take precedence over the virial expansion as a basis for quantitative treatment of the liquid crystalline state. 

It is not excluded that this mechanism is observed during the formation of fibres from X-500 The authors of this work pointed out that when the fibre was heated to 250-300 °C, its spontaneous elongation took place. Note that to attain higher orientation of a polymer in a fibre, it is necessary not only to transfer it to the liquid crystalline state but also to orient the liquid crystalline domains formed along the axis of the fibre. This orientation of the domain in which the macromolecules have been already mutually ordered requires not too high a draw ratio (the theoretical value must be <2). Indeed, experiments have shown that at the draw ratio of 1.53 to 1,70 the modulus (E) and the tensile strength (a) of the fibre at thermal treatment increase, which can be seen from the table compiled according to the results of this work. 

In contrast to these lyotropic mesogenic materials, which require the presence of a solvent in order to produce a mesophase, thermotropic mesogens are those compounds which exhibit a mesophase in the melt state at temperatures above the crystalline solid state and before the formation of an isotropic melt Mesophase-forming polymers may possess either one of the two basic structures shown in Fig. 1 the polymer may either contain the mesogenic group (the part of the polymer molecule which is responsible for liquid crystallinity) directly in the main chain, or the mesogen may be present as a pendant group in the side chain. 

The greater stability of the crystalline state of networks formed from unoriented but crystalline chains compared with networks formed from amorphous polymers, can be explained in the same way as for networks formed from axially oriented natural rubber. Although prior to network formation the crystallites are randomly arranged relative to one another, portions of chains are still constrained to lie in parallel array. The cross-linking of the predominantly crystalline polymer cannot, therefore, involve the random selection of pairs of units. The units that can be paired are limited by the local chain orientation imposed by the crystalline structure. An increase in the isotropic melting temperature of such networks would therefore be expected. It can be concluded that orientation on a macroscopic scale is not required for partial order in the liquid state to develop. Concomitantly a decrease in the entropy of fusion will result, which reflects the increase in molecular order in the melt. This is an important concept that must be kept in mind when studying the properties of networks formed in this manner. This conclusion has important implications in studying the properties of networks formed from unoriented crystalline polymers. 

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