Side-chain liquid crystalline polymers phase, nematic

In the latter two phases backbones have the spindle-like conformation, i.e., the prolate shape with (R%) > R p), the characteristic of main chain liquid crystalline polymers. Important means of investigating the conformations of side chain liquid crystalline polymers include small angle neutron scattering from deuterium-labeled chains (Kirst Ohm, 1985), or small angle X-ray scattering on side chain liquid crystalline polymers in a small molecular mass liquid crystal solvent (Mattossi et al., 1986), deuterium nuclear resonance (Boeffel et al., 1986), the stress- or electro-optical measurements on crosslinked side chain liquid crystalline polymers (Mitchell et al., 1992), etc. Actually, the nematic (or smectic modifications) phases of the side chain liquid crystalline polymers have been substantially observed by experiments. 

If the side chain liquid crystalline polymers goes into the smectic A phase from the nematic phase, the backbone chain is confined between two successive smectic layers, occasionally jumping into the neighboring layer gap. See Figure 2.30 where the cylinders denote side groups and thick lines represent backbones. The mean square end-to-end distances parallel and perpendicular to the director differs more than that in the nematic phase. 

We have only dealt with the main chain nematic networks so far. Actually many liquid crystalline networks are formed by crosslinking the backbones of side chain liquid crystalline polymers. The side chain nematic polymers have three nematic phases and their backbones have either prolate or oblate conformations, depending on their phase. It is expected that the rubber elasticity of a side chain nematic polymer network is more complex. For instance, the stress-induced Ni-Nm phase transition is predicted as the network shape transforms from oblate to prolate. Liquid crystalline networks have a bright potential in industry. 

Still in the isotropic phase, but closer to the phase transition temperature, a shear induced transition to the nematic phase occurs, see Fig. 6. Based on the equations presented here, such a behavior has been predicted theoretically quite some time ago [20, 21]. This phenomenon has been observed in lyotropic liquid crystals, in particular with wormlike micelles [5] and in side-chain liquid-crystalline polymers [35]. In Fig. 6, results are presented for = 1.3. For comparison, the highest temperature for which a metastable nematic phase exists, -d = 9/8 = 1.125, is included. For imposed shear rates, shear stress and consequently the viscosity jump smaller values at the induced phase transition. For imposed shear stress there is a jump to higher shear rates. 

Akihiko M. Novel biaxial nematic phases of side-chain liquid crystalline polymers. J Chem Phys 2012 137 224906. 

The study by Percec, Tomazos and Willingham (15) looked at the influence of polymer backbone flexibility on the phase transition temperatures of side chain liquid crystalline polymethacrylate, polyacrylate, polymethylsiloxane and polyphosphazene containing a stilbene side chain. Upon cooling from the isotropic state, golymer IV displays a monotropic nematic mesophase between 106 and 64 C. In this study, the polymers with the more rigid backbones displayed enantiotropic liquid crystalline behavior, whereas the polymers with the flexible backbones, including the siloxane and the polyphosphazene, displayed monotropic nematic mesophases. The examples in this study demonstrated how kinetically controlled side chain crystallization influences the thermodynamically controlled mesomorphic phase through the flexibility of the polymer backbone. 

The mesogenic behavior of 43 emphasizes the role played by the oc-tasilsesquioxane core when comparing with the related side-chain liquid-crystalline polysiloxanes [95,96]. It is remarkable that the thermal stability of the chiral nematic phase is similar in both the polymer and the dendrimer, suggesting that the cubic core does not perturb significantly the associations between the mesogens necessary to support the chiral nematic phase. 

FIGURE 5.3 Schematic representation of (a) nematic phase and (b) smectic phase for main-chain liquid crystalline polymers, showing the director as the arrow. The relative ordering is the same for side-chain-polymer liquid crystals. 

The redox properties of the ferrocene unit were also used to control the liquid-crystalline organization of side-chain liquid-crystalline poly(methacrylates) the reduced polymer gave rise to smectic C and smectic A phases whereas the oxidized polymer showed a nematic phase. ... 

An extensive study of possible side chain liquid crystalline materials has been undertaken.The majority of materials that have been studied have structures in which a mesogenic low molar mass entity is flexibly attached to a polymer backbone. The usual backbone is a flexible polymer and interestingly small-angle neutron scattering (SANS) experiments and X-ray scattering experiments have shown that the statistical random coil conformation of the polymer backbone is slightly distorted in the nematic phase and highly distorted in the smectic phase. 

In a polymer, the rod-like structures can be attached as side groups—side-chain liquid crystal polymers or with the skeletal backbone—main chain liquid crystal polymers (Donald et al. 2006). The latter usually exhibit liquid crystal characteristics at elevated temperatures, while some side-chain liquid crystal polymers exhibit liquid crystalline order at room temperature. A number of more ordered smectic phases can be observed as well as chiral, nematic, and smectic phases (Donald et al. 2006). 

This paper presents summaries of unique new static and dynamic theories for backbone liquid crystalline polymers (LCPs), side-chain LCPs, and combined LCPs [including the first super-strong (SS) LCPs] in multiple smectic-A (SA) LC phases, the nematic (N) phase, and the isotropic (I) liquid phase. These theories are used to predict and explain new results ... 

Extensive studies on photochromic liquid-crystalline polymers have been made by Krongauz et al,2 Liquid-crystalline phases caused marked colour changes of poly(acrylates)98 and poly(siloxanes) substituted with spiropyran side chains upon UV irradiation owing to the aggregation of the photomerocyanines." In contrast, spirooxazines attached to liquid-crystalline polymer backbones displayed no aggregation and hence exhibited normal photochromism similar to that in solution. Fulgimides bound covalently to the side chains of nematic liquid-crystalline polymers also showed normal photochromism. 

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