Low molecular weight liquid-crystalline

The molecular design of these materials is based on a central focal point to which mesogenic units are linked, so that restricted moleciflar topologies and limited molecular flexibility are induced [57]. They represent a class of materials intermediate between low molecular weight liquid-crystalline compounds and dendritic systems, and can be described as supermolec-ular entities or giant molecular systems made up of covalently attached... 

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... 

Bonding low-molecular-weight liquid crystalline molecules to silica gel particles, or... 

Polymers with side chain structure similar to that of low molecular weight liquid crystalline compounds can achieve various levels of organization in the bulk. These polymers are sometimes formed by polymerization of vinyl monomers that themselves exhibit mesomorphic behavior. In other cases, they can be obtained from monomers that do not form liquid crystalline states. At one extreme the structure of the polymer is highly organized, approaching that of crystalline polymers and giving rise to a number of x-ray diffraction peaks. At the other extreme the polymer chains are disorganized, with x-ray diffraction patterns that resemble those from amorphous polymers. 

Evans et al. systematically studied various polyimides derived from PMDA [56]. None of those polyimides was thermotropic, despite the fact that PMDA was expected to be a mesogenic unit because of its flat, rigid, and linear strue-ture. The authors concluded, we are unaware of any low molecular weight liquid crystalline compounds which contain the imide functional group. This combined with the results of the studies reported here suggests that it may be... 

Although the following remarks do apply to ordinary, low molecular weight liquid-crystalline compounds, the definitions are as well suitable for the description of molecular arrangement of polymers. 

PTFE is a special case. It has been shown that rubbed PTFE layers induce the oriented crystallization of a number of materials crystalline and liquid-crystalline polymers as well as low molecular weight liquid-crystalline materials [55]. The underlying orientation process is not elucidated in every case. In many instances, it seems that some form of graphoepitaxy may be involved since no clear-cut structural matching can be dehned. Another indicator is the parallel orientation of the deposited chain with the PTFE substrate, which would be compatible with some form of milder interaction— but, of course, does not rule out hard epitaxy. One example of hard epitaxy, which also results in a tilted orientation of the chain axis relative to the PTFE substrate chain orientation, will be analyzed shortly. 

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. 

Low-molecular-weight liquid-crystalline compounds have been known for about 100 years. However, main chain liquid crystal polymers (MCLCPs) have attained prominence only in the last 15 years. In 1956 Flory predicted lyotropic behaviour and this theoretical prediction was well demonstrated in the synthetic polymer area with the discovery by Kwolek of the aramids, e.g. poly(/ -phenyleneterephthamide). This led to interest in thermotropic main chain LCPs and, although aromatic polyesters which are thermotropic were described during this period in patents issued to and Carborundum Co., their liquid-crystalline... 

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>. 

Because the textures of liquid crystalline polymers (LCPs) are qualitatively similar to those of low molecular weight liquid crystals, they are interpreted in the same way. However, the microscopy of LCPs is less straightforward ... 

T. Kajiyama, H. Kikuchi, and K. Nakamura, Photoresponsive electrooptical effect of liquid crystalline polymer/low molecular weight liquid crystal composite system, Proc. SPIE 1911, 111-121 (1993). 

A second way to obtain liquid crystalline textures consists in heating up polymer samples, which display liquid crystalline phases, between glass slides or under the influence of external forces such as electric or magnetic fields. Textures will then form in a way similar to the case of low molecular weight liquid crystals. This was observed for the case of the polymers 1. On cooling down the textures can be frozen in since a glass transition occurs. Thus it becomes possible to keep a specific texture with interesting optical properties permanently. 

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