Molecular weight, liquid crystalline order

This value coincides in the order with that proposed for low molecular-weight liquid crystalline substances in the swarm theory (6), 10 . In PBLG-dioxane preparation, orientation of the solution is not observed at 1125 V/cm, and is still weak at 2500 V/cm (29) a similar result is obtained from the X-ray diffraction photc raph of the sunilar liquid crystalline solution (52). There is every possibility that the orientation is caused by induced dipoles of the molecular cluster and/or by an electric current through the solution. This may be in accordance with the su estion that molecular association is of the antiparallel type in a low dielectric solvent... 

Binary mixtures of a flexible polymer and a low molecular weight liquid crystalline molecule, or a rigid rod-like molecule, are of interest because of their important technological applications in high modulus fibers, nonlinear optics, and electro-optical devices. These blends are basic materials for recent new technologies of liquid crystal displays [1,2], The performances of these systems are closely related to phase separations and conformations of polymer chains dissolved in a liquid crystalline phase. One of the main problems is to examine the location of various phases such as isotropic, nematic, and smectic phases, depending on temperature and concentration. To understand the thermodynamics and thermal instability of these blends, it is important to consider the co-occurrences between liquid crystalline ordering and phase separations. 

Sixou et al. (101) showed the circular dichroism of cholesteric CTA solutions in TFA depends on the CTA molecular weight. The intensity of the circular dichroic peak increases with molecular weight. Meeten and Navard (97) studied gel formation and liquid crystallinity in TFA-H2O solutions of CTA. When water was added to a liquid crystalline solution of CTA in TFA a gel phase formed presumably by the formation of crosslinks due to hydrogen bonding. They interpreted their results that liquid crystalline ordering involves both inter- and intramolecular forces. 

Information on the crystal to liquid crystal transitions is scarce and is to be treated with caution since partial crystallization is prominent and polymorphism of the smectic phase is frequent. Only the data on poly(acryloyloxybenzoic acid) (entry 2 of Table 5) have been extrapolated to 100% crystallinity. As with the low molecular weight liquid crystals, the total heat of transition is lower than expected for fully ordered crystals. Various combinations of two phase structures as suggested by Fig. 3 could be produced for the poly(acryloyloxybenzoic acid)21>. 

SINCE the discovery of liquid crystalline phenomenon for low molecular weight liquid crystals (LMWLCs) more than 100 years ago, anisotropic ordering behaviors of liquid crystals (LCs) have been of considerable interest to academe [1-8], In the 1950s, Hory postulated the lattice model for various problems in LC systems and theoretically predicted the liquid crystallinity for certain polymers [1-3], As predicted by the Hory theory, DuPont scientists synthesized lyotropic LCPs made of rigid wholly aromatic polyamide. Later, Amoco, Eastman-Kodak, and Celanese commercialized a series of thermotropic main-chain LCPs [2]. Thermotropic LCPs have a unique combination of properties from both liquid crystalline and conventional thermoplastic states, such as melt processibility, high mechanical properties, low moisture take-up, and excellent thermal and chemical resistance. Aromatic main-chain LCPs are the most important class of thermotropic LCPs developed for structural applications [2,4-7]. Because they have wide applications in high value-added electronics and composites, both academia and industry have carried out comprehensive research and development. 

Sometimes referred to as the fourth state of matter, the liquid crystalline state possesses the properties of both a liquid and a solid. The liquid crystalline state is usually associated with small molecules, but many polymeric systems exhibit similar types of order to those found in low molecular weight liquid erystals. It is appropriate to consider the factors that influence the formation of liquid crystalline phases in small molecules before considering polymer systems. 

As with solutions of extended-chain polyamides (12), the critical concentration is affected by polymer molecular weight, being higher as molecular weight decreases (Fig. 7). At low inherent viscosities solubility may be insufficient for the attainment of a liquid crystalline state. On the other hand, solvent-polymer interaction may be so strong as to prevent the development of liquid crystalline order. 

A common feature of different types of liquid crystal polymers (LCPs), e.g., thermotropic side-chain or main-chain (either stiff or with flexible spacers) polymers, is their slowed-down dynamics compared to low molecular weight liquid crystals (LCs). Often polymers can be quenched to a glassy state in which the liquid-crystalline order is preserved but motions are completely frozen out. Such liquid-crystalline glasses provide a unique opportunity to determine, in principle, the full orientational distribution function, whereas only its second moment is available from motionally averaged NMR spectra. Thus LCP studies have made fundamental contributions to LC science. 

The second group of systems exhibits the liquid-crystalline state only in the presence of a low-molecular-weight liquid (water, for example). These are really lyotropic liquid-crystalline systems in which self-ordering is caused by their amphiphilic character, i.e., the presence of lyophobic and lyophilic groups at the ends of the long molecules. Soaps, lipids, and some types of proteins are members of this group. The specific features of the phase transitions in them will not be examined here. 

The dependence of the liquid-crystalline ordering in films of polyorgano-silsesquisiloxanes on the molecular weight could also indicate that the formation of the mesomorphic structure begins on the molecular level [29]. 

The dependence of the equilibrium Kerr constants on the molecular weight (in the Gaussian region of Af) is an important feature of the EBF of mesomorphic comb-shaped molecules [19,79, 81] (Figs. 3.12 and 3.13) which is due to the presence of intramolecular liquid-crystalline order. 

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