Nematic gel

Pii-8, like Pii-5, is designed to self-assemble at high pH. Comparable shifts in properties were found when studying Pn-8 solutions in 130 mM NaCl (Fig. 12). The p-sheet to random coil transition is lowered by 3 pD units and the pD range over which it forms nematic gels is extended down to pD 8.5 in aqueous NaCl... 

Fig. 12 Pn-4 (a, b) and Pn-8 (c, d) P-sheet variation measure by IR circles) and NMR triangles). 10 mg mL peptide solutions prepared in (a, c) D2O and (b, d) 130 mM NaCl in D2O. For Pii-4, I nematic gel, II flocculate, III nematic fluid, IV isotropic fluid. For Pn-8, I isotropic fluid, II biphasic solution. III nematic gel. Adapted from Carrick et al. [23]. Copyright 2007, with permission from Elsevier...

Figure 6 Schematic diagram showing arrangement of fibrils in nematic fluids (a) and the fiber-like junctions in nematic gel states (b) (Aggeli et al., 2003a, b).

We now start the case (2) to focus on the new effects of nematic gels, and then discuss the general case (3). 

At low temperatures the nematic gel coexists with excess solvent, i.e., the i-N biphasic coexistence. Above the triple point TJ, excess solvent coexists with an isotropic gel, i.e., the i-I coexistence. Also, above this temperature, the isotropic and the nematic phases of a gel can coexist, i.e., I-N phases. Beyond the phase gap, a single nematic phase exists. The I-N region terminates at the = 1 axis. This is at T the N-I transition temperature of the undiluted network which, in turn, is close to Tni, the transition temperature of the uncrosslinked polymer melt from which the network derives. The nematic order is not expected for any 4> for T above. This is the limit of stability for even the undiluted case. 

A qualitative understanding of why new phase equilibria are possible can be seen from a plot of the free energy of nematic gels with such a common tangent drawn in, see Figure 2.34. 

Figure 2.34. The free energy of nematic gels vs. volume fraction where = 0.

I) isotropic gel (II) and (III) nematic gel. Tt is the i-I-N triple-point temperature. (From Warner Wang, 1992a.)... 

Figure 2.36. The phase diagram of nematic gels where det (v) < 0 and vt = 1.1, vr = 1.2. (From Wang Warner, 1997.)...

Nematic gels are very interesting systems, thus deserving further study. Actually, these systems are being studied experimentally for applications. Examples are polymer dispersed liquid crystal displays are sometimes dispersed not in a polymer, but in a polymer network. Displays by means of the polymer stabilized cholesteric texture change, are also achieved in crosslinked systems. In addition, the chiral smectic phase has been obtained in such systems as well. Other types of liquid crystal gels have been applied or are expected to be applied in such devices. 

Fig. 12a,b. Phase diagram of clay suspensions vs clay and NaCl concentrations, (circles, F) Flocculated samples (squares, IL) isotropic liquid samples (lozenges, IG) isotropic gel samples (crosses, NG) nematic gel for a bentonite b laponite. (Reprinted from [4b], copyright (2000) from John Wiley and Sons)... 

Figure 22 shows the phase diagram of these suspensions. The suspensions are isotropic at low concentrations. The nematic phase is found at higher concentration and is separated from the isotropic liquid by a biphasic (N/I) region. Moreover, a sol-gel transition also takes place in the middle of the nematic phase. Nematic sols display typical Schlieren textures (Fig. 23) whereas samples of nematic gels sometimes show banded textures if they were submitted to flow on filling the optical capillary tubes. 

Light Scattering Electro-Optical Effects of Nematic Gels. 226... 

Fig. 14 Polarized optical photomicrographs of a mixture of Cy(AzCN)2 (3wt%) and 5CB and schematic illustrations of their structures (a) isotropic liquid state at 120 C (b) nematic gel state at room temperature before UV irradiation (c) chiral nematic LC phase (LC sol state) at room temperature after UV irradiation of the nematic gel (d) chiral nematic gel state at room temperature after maintaining a chiral nematic phase...

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