Side-chain thermotropic polymers

NMR Studies of Orientational and Conformational Order in Side Chain Thermotropic Polymers... 

Side-Chain Thermotropic Liquid Crystal Polymers... 

Liu X H, Bruce DW, Manners I. 1997. Novel calamitic side chain metallomesogenic polymers with ferrocene in the backbone S5mthesis and properties of thermotropic liquid crystalline poly(ferrocenylsilanes). Chem Commun (3) 289 290. 

Prior to discussing LC elastomers, we will consider in Sect. 2 in some detail the conformations and chain anisotropy of their polymer counterparts because the polymer backbone generates the shape anisotropy and elastic response of the elastomer network. In this context, note that two different classes of thermotropic LC polymers exist main-chain and side-chain (comb-like), as depicted in Fig. 1). In side-chain LC polymers, the pendant mesogenic groups are linked to a linear polymer backbone... 

A unique but widely studied polymeric LB system are the polyglutamates or hairy rod polymers. These polymers have a hydrophilic rod of helical polyglutamate with hydrophobic alkyl side chains. Their rigidity and amphiphilic-ity imparts order (lyotropic and thermotropic) in LB films and they take on a F-type stmcture such as that illustrated in Fig. XV-16 [182]. These LB films are useful for waveguides, photoresists, and chemical sensors. LB films of these polymers are very thermally stable, as was indicated by the lack of interdiffusion up to 414 K shown by neutron reflectivity of alternating hydrogenated and deuterated layers [183]. AFM measurements have shown that these films take on different stmctures if directly deposited onto silicon or onto LB films of cadmium arachidate [184]. 

The separation of Hquid crystals as the concentration of ceUulose increases above a critical value (30%) is mosdy because of the higher combinatorial entropy of mixing of the conformationaHy extended ceUulosic chains in the ordered phase. The critical concentration depends on solvent and temperature, and has been estimated from the polymer chain conformation using lattice and virial theories of nematic ordering (102—107). The side-chain substituents govern solubiHty, and if sufficiently bulky and flexible can yield a thermotropic mesophase in an accessible temperature range. AcetoxypropylceUulose [96420-45-8], prepared by acetylating HPC, was the first reported thermotropic ceUulosic (108), and numerous other heavily substituted esters and ethers of hydroxyalkyl ceUuloses also form equUibrium chiral nematic phases, even at ambient temperatures. 

Effect of the Polymer Backbone on the Thermotropic Behavior of Side-Chain Liquid Crystalline Polymers... 

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). 

Figure 1.69 General structure of main-chain (M.C.) and side-chain (S.C.) LCPs. Adapted from T. S. Chung, The recent developments of thermotropic liquid crystalline polymers. Polymer Engineering and Science, 26(13), p. 903. Copyright 1986, Society of Plastics Engineers.

Shibaev, V. P., Moiseenko, V. M., Plate, N. A. Thermotropic liquid crystalline polymers, 3, Comb-like polymers with side chains simulating the smectic type of liquid crystals. Makromol. Chem. 181, 1381 (1980)... 

It was also shown in 1975 that incorporation of rigid-rod and mesogenic units into the side-chain of acrylate or methacrylate polymers could lead to thermotropic products21). The properties of these polymers, which are not further considered in this review, differ in a number of respects from those of polymers whose thermotropic behaviour depends upon the constitution of their main chain. 

This review deals with LC polymers containing mesogenic groups in the side chains of macromolecules. Having no pretence to cover the abundant literature related to thermotropic LC polymers, it seemed reasonable to deal with the most important topics associated with synthesis of nematic, smectic and cholesteric liquid crystals, the peculiarities of their structure and properties, and to discuss structural-optical transformations induced in these systems by electric and magnetic fields. Some aspects of this topic are also discussed in the reviews by Rehage and Finkelmann 27), and Hardy 28). Here we shall pay relatively more attention to the results of Soviet researchers working in the field. 

The general approach towards the synthesis of thermotropic LC polymers is confined to chemical binding of polymer chains with the molecules of low-molecular liquid crystals (with mesogenic groups, to be more precise), which may be incorporated either within the main chains or within the side chains of macromolecules2 16 11 28). 

Stiff rod-like helical polymers are expected to spontaneously form a thermotropic cholesteric liquid crystalline (TChLC) phase under specific conditions as well as a lyotropic liquid crystal phase. A certain rod-like poly(f-glutamate) with long alkyl side chains was recently reported to form a TChLC phase in addition to hexagonal columnar and/or smectic phases [97,98]. These properties have already been observed in other organic polymers such as cellulose and aromatic polymers. 

Thermotropic side chain liquid crystalline polymers, SCLCPs... 

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