Low-mass liquid crystals

As is for low mass liquid crystals, incorporation of kinked moieties will result in destructive effects on the liquid crystallinity of polymers (Figure 3.4). 2,2-diphenylpropane, diphenylmethane, diphenyl ether, diphenyl ketone, 1,2-phenylene, 1,3-phenylene, and 1,2-naphthalene are examples of kinked moieties used in the modification of liquid crystalline polymers. They are very effective in destroying the linearity of rigid rods. Polymers with kinked units have less crystallinity and lower phase transition temperatures. Appropriate use of kinked units is thus of help from case to case. However, the type and amount of kinked units should be carefully determined so as to maintain desirable liquid crystallinity. 

Nevertheless in polymeric liquid crystals the same types of orientational defects and thus the same types of textures as present in the low mass counterparts have been observed. The textures often formed by polymers are the threaded texture, the schlieren texture and the focal conic texture of smectics. As is for low mass liquid crystals, the texture is a consequence of defects (disclinations and dislocations, refer to Chapter 1) present in the liquid crystal and is characteristic of a specific type of the phase. The texture examination has become a very useful tool in the determination of the type and nature of the polymeric liquid crystals. 

Unlike low molar mass liquid crystals, these materials do not undergo a nematic-isotropic transition. Instead, they adopt liquid crystal behaviour throughout the region of the phase diagram for which they are in the melt. Above a particular temperature, rather than adopting an isotropic liquid structure, they decompose. 

There are now three major shape classifications of low molar mass liquid crystals - rod-like (calamitic), disc-like (discotic) and bent-core. The last of these is the most recent, and while examples of bent mesogens have been known for some years, it is only since the mid-1990s that the area has attracted widespread attention [2],... 

A review of the literature demonstrates some trends concerning the effect of the polymer backbone on the thermotropic behavior of side-chain liquid crystalline polymers. In comparison to low molar mass liquid crystals, the thermal stability of the mesophase increases upon polymerization (3,5,18). However, due to increasing viscosity as the degree of polymerization increases, structural rearrangements are slowed down. Perhaps this is why the isotropization temperature increases up to a critical value as the degree of polymerization increases (18). 

Note 1 The rotational viscosity coefficients are of the order of lO -lO" Pa s for low-molar-mass liquid crystals for polymeric liquid-crystals their values depend strongly on the molar mass of the polymer. 

Attaching non amphiphilic or amphiphilic liquid crystalline molecules as side chains to linear, branched or crosslinkedpolymers yields liquid crystal (l.c.) side chain polymers, which can exhibit the liquid crystalline state analogously to the conventional low molar mass liquid crystals. The l.c.-side chain polymers combine the specific, anisotropic properties of the liquid crystalline state with the specific properties of polymers. 

The systematic synthesis of non amphiphilic l.c.-side chain polymers and detailed physico-chemical investigations are discussed. The phase behavior and structure ofnematic, cholesteric and smectic polymers are described. Their optical properties and the state of order of cholesteric and nematic polymers are analysed in comparison to conventional low molar mass liquid crystals. The phase transition into the glassy state and optical characterization of the anisotropic glasses having liquid crystalline structures are examined. 

As can be seen in H, Kelkers l) excellent review on the history of liquid crystals, investigations on liquid crystalline polymers already exist before F. Reinitzer in 1888 gave the very first description of a low molar mass liquid crystal (1-l.c.). While, however, 1-l.c. s have become an extensive field of research and application during the past decades, these activities on l.c. polymers have come rather late. The research on l.c. polymers during the last years is mainly joined with activities in material science and tries to realize polymers with exceptional properties. These exceptional properties are expected because of the combination of the physical anisotropic behavior of l.c. and the specific properties of macromolecular material. 

From thermodynamic investigations invaluable qualitative and quantitative information is provided with regard to the phase transitions and vicinity of transitions of polymers and conventional low molar mass liquid crystals. Furthermore they give information about the kind of transition and phase stability relations, which are necessary to test theories or to evaluate new theoretical considerations. Owing to... 

In the following we will contrast the phase behavior of a conventional non crystallizing polymer with that of a conventional, low molar mass liquid crystal. Thereafter we will discuss the experimental results on l.c. side chain polymers. 

In view of the effect of molecular mass on orientational phenomena the results of151) seem to be more explicable. In this work surprisingly low values for threshold voltage (U 8-40 V) and rise and decay times (x a 200 msec) were observed for an array of nematic polymers and copolymers. They are close to the corresponding values for low-molecular liquid crystals, which implies presumably that the polymers investigated were of low degrees of polymerization or had a very wide molecular mass distribution. 

The association and the generation of supermolecular liquid crystalline organizations of polymers in solution strongly depend on the molecular architecture of the macromolecules. From low molar mass liquid crystals it is well known that only particular molecular architectures cause the liquid crystalline state the... 

Before these results were published, polymer physicists and chemists mainly investigated only two phase-states, amorphous and crystalline. At the present time, along with these two states, the third phase-state of condensed systems, i.e. the liquid crystalline state, became very important. Here the situation turned out to be the same as in the case of low molar mass liquid crystals. In spite of the fact that historically the low molar mass substances in liquid crystalline state had been known for about a century, the intensive study of their properties began only after they had found an important practical application owing to a sharp change in optical properties of liquid crystals in electromagnetic fields (for visual displays) and as sensitive temperature indicators (in medicine). 

We shall mention here another property of liquid crystalline polymeric systems. As in the case of low-molar mass liquid crystals, when electric and magnetic fields are applied, liquid crstalline domains get oriented along the direction of the field. Rearrangement of a polymer structure under the effect of a magnetic field was demonstrated in for a PBA-dimethylacetamide system. However, the processes of... 

PHOTOINDUCED ALIGNMENT OF LOW MOLAR MASS LIQUID CRYSTALS... 

Although the technical applications of low molar mass liquid crystals (LC) and liquid crystalline polymers (LCP) are relatively recent developments, liquid crystalline behavior has been known since 1888 when Reinitzer (1) observed that cholesteryl benzoate melted to form a turbid melt that eventually cleared at a higher temperature. The term liquid crystal was coined by Lehmann (2) to describe these materials. The first reference to a polymeric mesophase was in 1937 when Bawden and Pirie (2) observed that above a critical concentration, a solution of tobacco mosaic virus formed two phases, one of which was bireffingent. A liquid crystalline phase for a solution of a synthetic polymer, poly(7-benzyl-L-glutamate), was reported by Elliot and Ambrose (4) in 1950. 

The uncrosslinked and the crosslinked polymers described in Table I still have some drawbacks. To begin with, the synthesis of polymers with strong lateral dipole moments (see polymer 3a, b in Table I (5)) is rather complicated, because the chiral groups have to be introduced prior to the polycondensation reaction (9), which they must survive unchanged. This limits the number of useful chiral groups and excludes e.g. chiral esters, which are well known from low molar mass liquid crystals (12). In addition the crosslinking has to... 

Starting from these polymers it is possible to introduce the chiral acids known from low molar mass liquid crystals (12) and to obtain the chiral homopolymers presented in Scheme III and Table III. These polymers show a high spontaneous polarization in the chiral smectic C phase (14) (see polymer 7, Table III) and selective reflection of visible light in the cholesteric phase (see polymer 9, Table III) (13). 

A phase and the smectic C phase. At a temperature of 105 °c, just in the smectic C phase, ferroelectric switching was achieved with a tilt angle of 26°. The mesogenic units were based upon the famous MHPOBC ferroelectric material which had a switch angle of approximately 25°. Thus this result demonstrated that the dendrimer is really acting as a collection of low-molar-mass liquid crystals. 

The basic structure of low molecular mass liquid crystals or monomers of liquid crystalline polymers is schematically shown below... 

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