Main Chain Liquid Crystalline Polymers with Spacers

MAIN CHAIN LIQUID CRYSTALLINE POLYMERS WITH SPACERS... 

Most main chain liquid crystalline polymers are composed of mesogenic units linked by spacers with varying degrees of flexibility, such as repeated methylene, oxyethylene, or siloxane groups, etc., in order to reduce the... 

Huang W, Han CD (2006b) Dispersion characteristics and rheology of organoclay nanocomposites based on a segmented main-chain liquid-crystalline polymer having side-chain azopyiidine with flexible spacer. Polymer 47(12) 4400 410... 

LCPs combine the mechanical properties of polymers with the order of LCs. The low molar mass mesogenic units, which form a LC phase, consist of a rigid core. If this core is extended, so-called main chain liquid crystalline polymers (MCLCPs) are obtained. A second method to obtain LCPs is to connect low molecular weight LC (LMWLC) units via flexible spacers. This approach allows the preparation of side chain liquid crystalline polymers (SCLCPs) (Box 1). 

Thus, the remoteness of mesogenic groups from the backbone provided by a polymethylene spacer secures them sufficient autonomy from the main chain. On the other hand, the fact that mesogenic groups are chemically linked with the main chain of the macromolecule assists their cooperative interaction. This is why comblike polymers have come to be accepted as convenient matrices for constructing LC polymers. Already a few hundred liquid-crystalline polymers with various mesogenic side groups have been synthesized. 

Table 1. Liquid Crystalline Polymers with Main Chain Mesogenic Groups and Flexible Spacers...

In the case of very long rigid rods it is very difficult for the bend deformation to appear, shown in Figure 6.3. This is due to the steric overlap effect. However, a real liquid crystalline polymer chain is somewhat flexible, which makes bend deformation possible. This is the case for those liquid crystalline polymers with flexible spacers in their main chains. Even for those not having flexible spacers in their main chains, a bend configuration can also exist so long as their persistence length is smaller than the bend radius. 

Fig. 5.9. Differential thermal analysis curve of a liquid crystalline polymer with flexible spacers in the main chain IPoiy(oxy-2,2 -dimethylazoxybenzene-4,4 -diyloxydodecanedioyl)]. Heating at 50 K/min after cooling at 50 K/min. LC glass transition at 290 K. Crystallization of the mesophase at T,. = 345 K. Disordering transition at = 385 K. Transition to the isotropic phase Ti = 420 K, ASi = 9.8 J/(K mol). Drawn after Ref.

For a side chain liquid crystalline polymer [8] with a poly(acrylate) main chain and a 2- or 6-fold aliphatic spacer (Fig. 21.13) one finds three relaxation processes The -relaxation, the 8-relaxation, and the dynamic glass... 

Thus, the subject of the review are linear and cyclolinear etement-organic polymers forming a spedfic type of the mesophase state. The structure of the mesoj iases and the formation of liquid crystalline macromolecules, in wiiidi classical mesogens in the main or side chains are linked by flexible polysiloxane or poly-phosphazene spacers are similar to the liquid-crystalline polymers with organic spacers. Structure and properties of these polymers have been reviewed recently and, therefore, will not be considered in our article. 

Up to now, liquid crystalline polymers with side mesogenes have not been employed for significant applications. The presence of a spacer between the backbone and the mesogene causes decoupling between the main chain movement and the mesogene. 

In the procedures below, methodology is described to facilitate medium scale (ca. 100 g) synthesis of a main-chain thermotropic liquid crystalline polymer containing ethylene glycol units as a flexible spacer between the rigid aromatic units. Two methods are described melt polymerization, and polymerization in a heat-transfer solvent with an inorganic medium (Claytone). For melt polymerization, the material is obtained as an extremely rigid solid, while in the... 

The ease of forming the smectic mesophase by this class of side-group type liquid crystalline polymers has rendered a great possibility in synthesizing polymeric chiral smectic materials useful in non-linear optics, transducers, pyroelectric detectors and display devices (Chapter 6). The first polymer forming a chiral smectic-C phase was synthesized by Shibaev et al. (1984). It has a polymethacrylate main chain, a long polymethylene spacer, and a mesogenic unit attached at the end with a chiral moiety (polymer (3.60)). Since then, a lot of polymers with chiral mesophases have been synthesized and studied (Le Barny and Dubois, 1989). 

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