Nematic smectic polymers

Liquid Crystalline Polymers at their Nematic-Smectic Transition 206... 

The thermal behavior of la-lg observed by DSC (Fig. 1) confirms the presence of mesophases and is typical of low molecular weight thermotropic LC materials (M). The lower T , for lb and Id are consistent with the higher entropy of activation for crystallization of odd-n spacers, demonstrated in several main chain LC polymers (23). The apparent absence of nematic-smectic transitions in the DSC... 

Identify nematic, smectic, and cholesteric structures in liqnid crystalline 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. 

While for nematic polymers the statistical distribution of the centers of gravity of the mesogenic side chains is compatible with a more or less statistical main chain conformation, for smectic polymers a three dimensional coil conformation is no longer consistent with the layered structure of the mesogenic side chains. The backbone has to be restricted in its conformation, which will cause a more pronounced interaction between the main chain and the anisotropically ordered mesogenic side chains, compared to nematic and cholesteric polymers. 

As already mentioned in Chap. 2.2. one of the most obvious features of the l.c. side chain polymers is their ability to become glassy. The glass transition can be observed by cooling nematic, cholesteric and smectic polymers depending on the chemical constitution of the system and is indicated e.g. by a bend in the V(T) curves as schematically shown in Fig. 8. Two questions are of interest which will be discussed in this chapter ... 

The article covers synthesis, structure and properties of thermotropic liquid-crystalline (LC) polymers with mesogenic side groups. Approaches towards the synthesis of such systems and the conditions for their realization in the LC state are presented, as well as the data revealing the relationship between the molecular structure of an LC polymer and the type of mesophase formed. Specific features of thermotropic LC polymers and copolymers of nematic, smectic and cholesteric types are considered. 

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. 

Analysis of flow curves of these polymers has shown that for a nematic polymer XII in a LC state steady flow is observed in a broad temperature interval up to the glass transition temperature. A smectic polymer XI flows only in a very narrow temperature interval (118-121 °C) close to the Tcl. The difference in rheological behaviour of these polymers is most nearly disclosed when considering temperature dependences of their melt viscosities at various shear rates (Fig. 20). 

For a nematic polymer in a transition region from LC to isotropic state, maximal viscosity is observed at low shear rates j. For a smectic polymer in the same temperature range only a break in the curve is observed on a lgq — 1/T plot. This difference is apparently determined by the same reasons that control the difference in rheological behaviour of low-molecular nematics and smectics 126). A polymeric character of liquid crystals is revealed in higher values of the activation energy (Ef) of viscous flow in a mesophase, e.g., Ef for a smectic polymer is 103 kJ/mole, for a nematic polymer3 80-140kJ/mole. 

The fusion of LC phases above Tcl causes a sharp change in the character of flow and the values of Ef for nematic and smectic polymers become closer. In an isotropic phase Ef for a polymeric smectic ( 140 kJ/mole) is only twice as large as Ef for a polymeric nematic (70—80 kJ/mole). In other words, the transition from LC phase to isotropic melt, accompanied by the liberation of mesogenic groups from the mesophase levels the differences in the character of flow of smectic and nematic polymers. The differences in Ef for isotropic phase are determined only by the differences in chemical nature of the main chain of smectic and nematic polymers. The values of Ef, in this case, are close to the Ef values for poly(butylmethacrylate) and poly(butylacrylate), respectively, which are structurally similar to polymers XI and XII except that they do not contain mesogenic groups. 

We turn to the relaxation processes observed in smectic polymers with different attachment of mesogenic groups to the macromolecular backbone and compare dielectric behaviour of smectic and nematic polymers having identical mesogenic groups but different main chain structure. 

A detailed comparative study of dielectric behaviour of smectic and nematic polymers was carried out for polymers of acrylic and methacrylic series, containing identical cyanbiphenyl groups (polymers XI and XII) 137 138>. The difference in structural organization of these polymers consists in a more perfect layer packing of smectic polymer XI (see Chaps. 4.1 and 4.2) with antiparallel orientation of CN-dipoles. This shifts the relaxation process of CN-dipole reorientation to a low frequency region compared to nematic polymer XII. Identification of Arrhenius plots for dielectric relaxation frequencies fR shows that for a smectic polymer the value of fR is a couple of orders lower than for a nematic polymer (Fig. 21). Though the values... 

Molecular weights and molecular weight distributions, together with the thermal transitions of poly-XXIII-m-n are summarized in Table 15. From these data it can be concluded that inducing smectic layering requires a minimum of eight methylene units and three difluoromethylene units or three methylene units and four difluoromethylene units. All other polymers with m=3,4 exhibited nematic mesophases. Polymers with m=2 displayed only nematic mesophases, or in the case of n=3 no mesophase at all. Because of the thermal transition data, the authors concluded that the polymers were not microphase-separated. 

Blinov, L. M., Barberi, R., Kozlovsky, M. V., Lazarev, V. V., and de Santo, M. P. Optical anisotropy and four possible orientations of a nematic liquid crystal on the same film of a photochromic chiral smectic polymer. / Nonlinear Opt. Phys. Mat. 9, 1 (2000). 

The re-entrant or Tammann loop-shape phase diagram as observed in proteins is also found in other systems and has been connected to exothermic disordering [88]. In this particular case, nematic - smectic A transitions in liquid crystals and the phase behaviour of a crystalline polymer, poly(4-methyl-pentene-l), the phase behaviour can be understood by... 

In the Ni phase, Sa > 0, but SB < 0. The conformation of the backbone chain is discus-like, i.e., is oblate shape, in which the mean square end-to-end distance along the director, (R%) is less than the perpendicular component (R%). In the smectic phase the anisotropy of two components is greater than that in the nematic phase. At the extreme case the backbone is surpassed in a plane, which was predicted by Renz Warner (1986) to exist in smectic polymers where backbones become confined between smectic layers formed by side chains, and has been seen by Moussa et al. (1987). In the extreme case, the backbone becomes a two-dimensional random walk when SB = — and Sa = 1. 

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). 

Utracki and Lyngaae-Jprgensen [2002] observed several common aspects of exfoliated CPNCs and liquid-crystal polymers (LCPs). Similar six-phase structures are predicted for CPNCs and observed in LCPs isotropic, nematic, smectic-A, columnar, house of cards, and crystal [Porter and Johnson, 1967 Balazs et al., 1999 Ginzburg et al., 2000]. These phases in CPNCs originate in a balance between the thermodynamic interactions, clay concentration, and platelets orientation, while in LCPs they depend mainly on temperature. Since it is more difficult on the one hand to prepare disk-shaped than rigid-rod molecules, and on the other to develop flow theory for LCPs with disk moieties, the number of publications on the latter systems is small [Ciferri, 1991]. 

Perhaps one of the most important applications of chiral induction is in the area of liquid crystals. Upon addition of a wide range of appropriate chiral compounds, the achiral nematic, smectic C, and discotic phases are converted into the chiral cholesteric (or twisted nematic), the ferroelectric smectic C and the chiral discotic phases. As a first example, we take the induction of chirality in the columns of aromatic chromophores present in some liquid-crystalline polymers. " The polymers, achiral polyesters incorporating triphenylene moieties, display discotic mesophases, which upon doping with chiral electron acceptors based on tetranitro-9-fluorene, form chiral discotic phases in which the chirality is determined by the dopant. These conclusions were reached on the basis of CD spectra in which strong Cotton effects were observed. Interestingly, the chiral dopants were unable to dramatically influence the chiral winding of triphenylene polymers that already incorporated ste-reogenic centers. 

Observed structures of a lyotropic material are classified into three categories nematic, smectic, and cholesteric. Nematic and cholesteric mesophases can be readily identified by microscopic examination. The existence of a smectic mesophase is not well defined and is only suggested in some cases. Solvent, solution concentration, polymer molecular weight, and temperature all affect the phase behavior of lyotropic polymer solutions. In general, the phase transition temperature of a lyotropic solution increases with increasing polymer molecular weight and concentration. It is often difficult to determine the critical concentration or transition temperature of a lyotropic polymer solution precisely. Some polymers even degrade below the nematic isotropic transition temperature so that it is impossible to determine the transition temperatures. Phase behavior is also affected by the polymer molecular conformation and intermolecular interactions. 

A phase separation during polymerization occurred when a nematic monomer was polymerized to a smectic polymer, because the polymer was insoluble in the monomer. 

No phase separation was observed when an isotropic monomer was polymerized to a nematic or smectic polymer. The nematic or smectic polymers are soluble in the isotropic monomer. When a certain conversion is obtained, a phase transition from isotropic to nematic or smectic phase takes place. This phase transition, caused by variation of the mole fraction of the monomer and the polymer, is comparable with a phase transition caused by varying the temperature at constant composition. 

There have been several recent reports of such smectic polymers although the majority of type i pol3mieric materials are nematic. 

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