Phase diagram, isotropic-nematic

Fig. 6.13 Phase diagram isotropic phase (Iso)-nematic (N)-smectic A (SmA)-reentrant nematic (Re-N) of poctyloxy-cyanobiphenyl in the piessure-temperatme coordinates [14]...

Let us enter the world of liquid crystals built by the purely entropic forces present in hard body systems. The phase diagram of hard spherocylinders (HSC) shows a rich variety of liquid crystalline phases [71,72]. It includes the isotropic, nematic, smectic A, plastic, and solid phases [73]. In a plastic crystal the particle centers lie on lattice sites, but the orientations of the... 

Unlike low molar mass liquid crystals, these materials do not undergo a nematic-isotropic transition. Instead, they adopt liquid crystal behaviour throughout the region of the phase diagram for which they are in the melt. Above a particular temperature, rather than adopting an isotropic liquid structure, they decompose. 

Figure 7.1 Illustration of different aggregation states obtained (from left to right) by increasing temperature crystal (K), smectic C (SmC), nematic (N) and isotropic (I). Row a shows macroscopic appearance of samples in row b, short-range microscopic ordering is represented (each bar represents a molecule) thermotropic phase diagram of row c illustrates relevant transition temperatures (Tm melting temperature Tsmc-N transition temperature between SmC and N Tc clearing temperature) row d shows different texture of different states as seen through polarizing microscope (with crossed polars, isotropic phase appears black).

The coagulation process can now be considered in perspective of a ternary polymer-solvent-nonsolvent system, A schematic ternary phase diagram, at constant temperature, is shown in Figure 8. The boundaries of the isotropic and narrow biphasic (isotropic-nematic) regions are based on an extension of Flory s theory ( ) to a polymer-solvent-nonsolvent system, due to Russo and Miller (7). These boundaries are calculated for a polymer having an axial ratio of 100, and the following... 

Note 3 Two TGBA structures are possible in one, the number of blocks corresponding to a rotation of the layer normal by 2ti is an integer, while in the other, it is a non-integer. Note 4 A TGBA is found in a phase diagram between smectic A and chiral nematic mesophases or between a smectic A mesophase and an isotropic phase. 

CB) [9]. The enhancement occurs in both chiral domains, although the dopant is nonchiral. The phase diagram with chemical structures of the two components are shown in Fig. 17. The B2 and B3 phases readily disappear by the introduction of a small amount of 5CB, and the B4 phase is stabilized. In the mixtures with more than 50 wt% 5CB, we observe a new phase transition at an almost constant temperature of 32 °C, corresponding to the Iso-N transition of 5CB. This transition point can be detectable by DSC [58] and texture observation, but not by X-ray analysis [59]. Based on the experimental fact of no layer structure changes, we can infer that P8-O-PIMB and 5CB molecules are nanosegregated. Namely helical nanofilaments are in the isotropic and nematic sea of 5CB in the B4 and lower phases, respectively. Hereafter we call these phases B4/Iso and B4/N phases, respectively. 

Fig. 17 Phase diagram of a binary mixture of bent- (P8-0-PIMB) and rod-shaped (5CB) molecules. Cartoon showing the phase structures are also shown. In the B4/Iso and B4/N phases, 5CB molecules are in the isotropic and nematic phases [9]...

Hgurc 4.14 (a) A typical phase diagram for a poly (p-phenylene terephthala-mide)-sulfuric acid system, a solution that is used to make Kevlar fiber (after Kikuchi, 1982). (b) Viscosity versus polymer concentration in PPTA/HjSO solution. Note the sharp drop in viscosity at about 20% which corresponds to a transition between isotropic and nematic liquid crystal phase. 

Fig. 9-6. Phase diagram of ferrocenes 11. Melting point nematic/isotropic liquid transition smectic A/isotropic liquid transition A isotropic liquid/smectic A transition O isotropic liquid/nematic transition.

Figure 5.14 (a) Temperature-volume fraction phase diagram for PBLG Ma, = 310,000) in DMF, where I denotes an isotropic phase, LC denotes a chiral nematic liquid-crystalline phase, and I + LC is a gel that is presumed to be two coexisting phases that are unable to separate macroscopically. (b) The x-volume fraction phase diagram predicted by the Flory lattice theory for rigid rods of axial ratio (length/diameter) = 150. (From Miller et al. 1974, with permission.)... 

Figure 14 shows the phase diagram of a flexible-semiflexible polymer blend at a constant pressure. Theoretical calculations and experimental results show that such mixtures can exhibit an isotropic-isotropic and isotropic-nematic phase separation. Our calculations are able to capture the isotropic-isotropic phase separation and serve to show that the origin of such a transition can be purely entropic. 

Figure 2.35(a) shows the phase diagram for vc = 1.1 and thus Vb = 1.21, that is where the network is a stronger nematic than the solvent. The strand length L = 10 and the isotropic Flory-Huggins parameter x = 0-4, correspond to a good solvent. These values will be used through out this section. 

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)... 

Fig. 2. Partial phase diagram of the binary mixture liquid crystal E7/silicone oil (Aldrich). is the E7 weight fraction. The Fig. shows an isotropic phase (1) above 60°C, a coexistence domain between an isotropic and a nematic phase (1+N) below 60°C, and a nematic phase (N) in the extreme right part of the diagram. The systems are quenched from the nematic domain to 20°C as indicated by the arrow...

Since the liquid crystal forms the continuous phase of the binary mixture, we are only interested in a small part of the total phase diagram. Weight fractions of the liquid crystal in the range 0.9 to 1 were used to determine the partial phase diagram of the mixture which is shown in Fig. 2. The system forms an isotropic (I) phase at high temperature, and a diphasic equilibrium between an isotropic and a nematic phase (N-i-I) at low temperature. A nematic domain (N) is found at intermediate temperatures and low silicone oil concentrations. As pointed out in the experimental section, the existence of this nematic domain has some importance prior to quenching the system to the diphasic region. The present mixture exhibits classical features usually observed in other mixtures of nematic liquid crystals and polymers or isotropic fluids [29,30]. 

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