Low-molar-mass 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. 

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. 

Photoinduced Alignment of Low Molar Mass Liquid Crystals 166 Photoaffected Phase Behavior and the LCPT Photorecording 168 Conclusions 172... 

A liquid crystal dimer is composed of molecules containing two conventional mesogenic groups linked via a flexible spacer. These materials show quite different behaviour to conventional low molar mass liquid crystals and in particular their transitional behaviour exhibits a dramatic dependence on the length and parity of the flexible spacer. In this review a comprehensive overview of the relationships between molecular structure and liquid crystallinity in dimers is provided. This includes a description of the novel modulated and intercalated smectic phases exhibited by dimers. 

The initial interest in liquid crystal dimers was triggered, therefore, by the similarity of their behaviour to that of the semi-flexible main chain liquid crystal polymers. It soon became apparent, however, that the dimers are of significant fundamental interest in their own right and exhibit quite different behaviour to conventional low molar mass liquid crystals. These studies, for example, have resulted in the discovery of a new family of intercalated smectic phases. This review focuses upon the novel behaviour of dimers and how it may be understood at a molecular level. 

X-ray measurements [7, 11, 13-15, 17, 19, 20] performed on combined LC polymers show that the LC phases are analogous to low molar mass liquid crystals, which also show nematic, smectic A, smectic C and higher ordered smectic phases at different temperatures (see Figure 8). 

The extensive literature on low molar mass liquid crystals demonstrates that specific mesogens (specific chemical structures) tend to form specific mesophases, which vary somewhat with the length of the flexible substituent. We therefore expect that the type of mesogen, including the terminal substituent(s) and the length of the spacer should be the primary factors determining the specific mesophase(s) exhibited by a given SCLCP. The nature of the polymer... 

In order to determine whether or not the alkyl substituent is necessary for low molar mass liquid crystals to mimic the thermotropic behavior of laterally attached SCLCPs, both l,4-bis[(3 -fluoro-4 -n-alk-oxyphenyl)ethynyl]benzenes [193, 196]... 

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