Polysiloxane liquid crystal polymers

Figure 7. Molecular order in calamitic side-group polymers (a) Spacer model, (b) 3D-MAS-NMR spectrum of of a polysiloxane liquid crystal polymer, (c) order parameters of different segments of the molecule. For details see [48].

The concept of a side chain liquid crystal polymer has been demonstrated in a number of laboratories and is well documented in the literature (J ). Most of the side chain liquid crystalline polymers reported to date contain polysiloxane, polyacrylate or polymethacrylate main chains. More recent studies on the effect of backbone flexibility now include the use of flexible poly(ethylene oxide) or more rigid poly( a-chloroacrylate) chains. 

The side chain polysiloxane liquid crystalline polymers PI, P2 and P3 composed of the above three ferroelectric liquid crystals as the side groups exhibit the ferroelectric liquid crystal phase, their chemical formulae being respectively. 

H J Coles and R Simon. Electro-Optic Effects in a Smectogenic Polysiloxane Side Chain Liquid Crystal Polymer, in "Recent Advances in Liquid Crystalline Polymers", L L Chapoy (ed),... 

Gray, G. W., Synthesis and properties of side chain liquid crystalline polysiloxanes, in Side Chain Liquid Crystal Polymers (C. B. McArdle, ed.), Blackie, Gla.sgow, 1989, pp. 106-129. 

The most common liquid crystal polymers are of the side chain variety (see Chapter 4). In the case of polysiloxanes (Scheme 47), monomer units are prepared with an alkenic moiety (192) that adds to a pre-prepared siloxane backbone (193) to generate liquid crystal polymers (194). 

Liquid crystal polymers consist of two kinds. One is a principal chain liquid crystal polymer such as poly(/ -phenyleneterephthalamide) and the other is a side chain liquid crystal polymer having a liquid crystal component (mesogen) as the side chain. The liquid crystal polysiloxanes belong to the latter type of polymer. 

Engel M, Hisgen B, Keller R, Kreuder W, Reck B, Ringsdorf H, Schmidt HW, Tschmner P (1985) Synthesis, structure and properties of liquid crystalline polymers. Pure Appl Chem 57 1009-1014 Finkelmann H (1982) Polymer Liquid Crystals. Academic ftess. New York Finkelmann H, Rehage G (1980) Investigations on liquid crystalline polysiloxanes, 1. Synthesis and characterization of linear polymers. Makromol Chem Rapid Commun 1 31-34 Finkelmann H, Rehage G (1984) Liquid crystal side chain polymers. Liquid crystal polymers II/III. Adv Polym Sci 60/61 99-172... 

Ferroelectric side-chain liquid crystal polymers (fLCPs) have been synthesized only recently. (see Chapter 8). Because of the dilution of chiral side chains in the backbone matrix, the spontaneous polarization is weaker than in the monomer chiral smectics, although Kapitza and others recently reported polysiloxane-based fLCPs with spontaneous polarization comparable to that observed for monomer liquid crystals. 

Fig. 2.8.17 Director orientation 0 obtained from 2H NMR spectra from the polysiloxane backbone polymer with liquid-crystal side-chain 4-methoxyphenyl-4 -butenyloxybenzoate, as a function of the apparent extension rate, i. The...

Carr et al. [252] studied LB films made from preformed polymers based on the polysiloxane structure. Such materials have attracted attention as liquid crystal side chain polymers as they remain in a mesophase at low temperatures but, just for this reason, they seem unlikely materials to form stable LB films. 

Jt OLYSILOXANES WITH PENDANT SIDE CHAINS are interesting materials from both the theoretical and practical points of view. A number of polysiloxanes with various side chains, such as liquid crystals (i, 2), carbazole groups (3), electron-donor and electron-acceptor groups (4), polystyrene (5), and functional groups (hydroxyl or carboxyl) (6), have been synthesized. Polysiloxanes are known for their useful properties, which include flexibility, heat resistance, water repellence, and biological inertness. These properties, combined with the ease with which a tailored polymer structure can be prepared,... 

The mesogenic structure of a benzoic acid dimer has been introduced as a noncovalent cross-linker for polysiloxanes [79]. Polymer 57 exhibits a smectic C phase due to the dynamics of H-bonding. In contrast, mesomorphic order is locked in the solid state of poly[(4-acryloyl)benzoic acid] by polymerization in its mesophase [128]. No liquid-crystalline state is observed for this material because of the lack of flexibility of the structures. Main-chain-type polymeric liquid-crystal associates are formed from carboxyl-bifunctionalized aromatic compounds [129]. 

With time and improved synthetic protocols, larger templates (fuUerenes, dendrimers, nanoparticles, colloids, micelles, lipid bilayers, self-assembled block copolymers, oligonucleotides, DNA and proteins) have been imprinted [14] and the choice of matrices has expanded to liquid crystal polysiloxanes, carbon networks, zeolites, layered aluminophosphates and colloidal crystals, though organic polymer networks remain the dominant imprint casting medium [14]. 

Moreover, organic functionalization of a polymer chain can lead to improvement in the physical properties, such as thermal stability and mechanical strength of the resulting siloxanes (Figure 3.1). Appropriate substitution on the polysiloxane backbone can lead to diverse materials such as liquid crystals," crosslinking agents, conductive and electroluminescent polymers, nonlinear optical materials,and bactericides. ... 

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