Pretransitional effect

L. Sallen, P. Sotta, P. Oswald. Pretransitional effects near the hexagonal-micellar phase transition of the CnEOA/HoO lyotropic mixture. J Phys Chem 707 4875-4881, 1997. 

In addition to the above effects, the intermolecular interaction may affect polymer dynamics through the thermodynamic force. This force makes chains align parallel with each other, and retards the chain rotational diffusion. This slowing down in the isotropic solution is referred to as the pretransition effect. The thermodynamic force also governs the unique rheological behavior of liquid-crystalline solutions as will be explained in Sect. 9. For rodlike polymer solutions, Doi [100] treated the thermodynamic force effects by adding a self-consistent mean field or a molecular field Vscf (a) to the external field potential h in Eq. (40b). Using the second virial approximation (cf. Sect. 2), he formulated Vscf(a), as follows [4] ... 

Price, F. P. and Wendorff, J. H. Transitions in mesophase forming systems. I. Transformation kinetics and pretransition effects in cholesteryl myristate. J. Phys. Chem. 75, 2839 (1971)... 

In practice there are three main difficulties in the use of this technique. The 2kF and 4kF scattering is not observed in all materials or at all temperatures. Fortunately, pretransitional effects (one-dimensional fluctuations) are often observed in a wide temperature range above Tc [114]. Second, the corresponding scattering is often very weak and requires special measurement techniques. For example, in the case of TTF-TCNQ, the intensity of 2kF satellites is 10 4 that of main reflections and the onedimensional scattering at 60 K is a further 10 2 to 10 3 weaker [133]. Third, to get p, one must decide whether the observed instability occurs at the 2kF or at the 4kF electronic wave vector. 

Dimova, R., Pouligny, B., and Dietrich, C. (2000) Pretransitional effects in dimyristoylphosphatidylcholine vesicle membranes optical dynamometry study. Biophysical Journal, 79 (1), 340-355. 

Use of Flow Alignment and Pretransitional Effect Toward Applications... 

We have seen that it can be difficult to reach the critical concentration required to observe an isotropic-anisotropic transition because concentrated suspensions of colloids are not always stable. However, orientation of flexible polymers as well as of anisotropic particles in suspension can be induced by flow, a phenomenon that has long been observed, reported, and studied. This phenomenon is especially strong when a pretransitional effect exists, which can be easily observed by the naked eye on a sample that is shaken between crossed polarizers (see for example the section on clays). In these systems, birefringence is induced via mechanical forces, like the shear stresses in a laminar flow ( Maxwell-dy-namo-optic effect ). 

CJearly near-neighbour correlations have to be allowed for in the molecular statistical approach to give a better description of the pretransition effects. A step in this direction was taken in which... 

Since there are very few dynamic experimental investigations of pretransitional effects [8], not much modeling has been reported to date either. Based on work for the macroscopic dynamics of the nematic-isotropic transition in sidechain polymers [27 -29], it has been suggested [28] that the non-meanfield exponent observed in dynamic stress-optical experiments [8] can be accounted for at least qualitatively by the mode-coupling model [28, 29]. Intuitively this qualitatively new dynamic behavior can be traced back to static nonlinear coupling terms between the nematic order parameter and the strain tensor. 

Dynamic mechanical measurements of the storage modulus G and the loss modulus G" in polydomain samples of cholesteric LCE have been described [9]. It turns out that G >G" throughout the whole cholesteric range and that both G and G" increase when approaching the cholesteric-isotropic and cholesteric-chiral SmC transition, respectively, thus indicating the presence of substantial pretransitional effects at both phase transitions. 

Alben, R., Pretransition effects in nematic liquid crystals model calculations. Mol. Cryst. Uq. Cryst., 13,193-231 (1971). 

Due to a pronounced optical anisotropy of ordered liquid crystals, reflection elhpsometry is a powerful experimental tool for the study of surface-modified liquid crystalline order. The sensitivity of the technique enables quantitative measurements of the nematic order parameter profile at the interfaces. Prom observation of the nematic wetting of the solid-liquid crystal interface, we have determined the values of the coupling energies between the surface and the nematic order. Even the pretransitional effects at solid-smectic liquid crystals can be studied with elhpsometry due to the strong coupling between the nematic and the smectic order. The time evolution of the complex structure of the liquid crystal adsorbate on a solid substi ate has also been successfully monitored using BAE. 

A simple example of the pretransitional effect is seen in the problem discussed in the previous section. Suppose that the external field is weak, then S is small in the isotropic phase, so that eqn (10.55) can be approximated by... 

The pretransitional effect can be also observed by viscoelasticity. Firstly we derive the constitutive equation from the kinetic equation. Again, eqn (10.39) in the presence of the velocity gradient k is rewritten to give a kinetic equation for (see eqn (8.149)) ... 

Note that the instantaneous modulus G, becomes small near the transition point due to the pretransitional effect. The viscosity i/o and the steady-state compliance are calculated as... 

The steady-state compliance has a minimum at p = p f2 and then increases again by the pretransitional effect. This has indeed been observed by Berry and coworkers. On the other hand the viscosity is unaffected by the pretransitional effect. (This is a result of the cancellation of the effect on the relaxation time t and on the rigidity modulus Gg.) Experimental results on the viscodastidty of the isotropic solution have been reviewed by Baird. ... 

In om discussions so far, the transition from one phase to another occurs at a specific temperature and the order parameter increases in the lower temperature phase starting either from zero or a non-zero value. What we have not discussed is what occurs above the phase transition in the higher temperatirre phase. Since the order parameter decreases more and more rapidly as the phase transition is approached from below, it is clear that the proximity of the phase transition is evident. The question is whether there is any evidence that the phase transition is near when approaching the transition temperature in the higher temperature phase. As we will see, there are pretransitional effects in the higher temperature phase also. 

FIG. 16 Water-hexadecane system (see Table 2). Behavior of the measured enthalpies of (O) hexadecane (AHt,) and ( ) water (A// ) as a function of increasing water concentration (C) in the interval 0.105 < C < 0.420. The linear relationship holds, as shown in Fig. 14, within the interval 0.105 < C < 0.3. Pretransitional effects occur in the range 0.316 s C 0.372 phase separation develops in the interval 0.372 < C < 0.42. In the latter range, the linear relationship still holds for the isotropic upper phase of biphasic samples until the transition occurs toward a monophasic liquid crystalline mesophase. (Redrawn from Ref. 23.)... 

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