SmA-I transition

In contrast to the extensive experimental investigations of nematic, cholesteric and chiral SmC phases, comparatively little work has been done on tbe characterization of the physical properties of SmA and SmC elastomers. The elongation, A, has been measured as a function of temperature for constant external load for a number of different loads in SmA sidechain elastomers [6]. It was found that A increases monotonically as a function of temperature for a material, which has a SmA-I transition. In addition it was shown that the elasticity modulus, E, decreases monotonically with temperature. X-ray investigations on SmA phases in side-chain LCE have been performed [70]. It was found that for the family of compounds studied, the orientation of the mesogenic groups was always perpendicular to the direction of stretching. 

Fluctuation effects are large in polymeric liquid crystals even far from phase transition temperatures. For example, in a novel liquid crystalline elastomer with a SmA-I transition, under an external mechanical stress, it was found in a mean field limit that, well in the isotropic phase, nematic fluctuations dominate with a cross-over temperature closer to the transition where SmA fluctuations become more important [14]. 

The G-SmA-N-I transition temperatures of syndiotactic poly(6-[4 -(4"- -bu-toxyphenoxycarbonyl)phenoxyl)phenoxy]-hexyl methacrylate prepared by aluminum porphyrin initiated polymerizations also level off at approximately 25 repeat units [91]. Similarly, the glass and nematic-isotropic transition temperatures of poly[6-(4 -methoxy-4"- Z-methylstilbeneoxy)hexyl methacrylate] prepared by group transfer polymerization become independent of molecular weight at approximately 20 repeat units [48]. Both polymethacrylates reach the same transition temperatures as the corresponding polymers prepared by radical polymerizations, which have nearly identical tacticities. 

Table 4.1 GNPs doping concentration dependent SmA-N and N-I transition temperatures...

Figure 5.19 Predictions of the McMillan theory for the dependence of orientational (P2) and translational (a) order parameters on temperature. At o = 1.1 (where a is defined by Eq. 5.17), there is a first-order transition from the SmA phase to the I phase (at Tai)- At a = 0.85, a first-order transition from SmA to nematic occurs (at Tan) below the N-I transition (at Tni), which is always first order. At a = 0.6, the SmA-N transition is second order...

Shashidhar et al. [75] studied the influence of pressure on the SmA- (re-entrant) nematic and N-I phase boundaries of mixtures of 4-n-hexyloxy- and 4- -octyloxy-4 -cyanobiphenyl. The maximum pressure where the SmA and re-entrant nematic phase, respectively, still exist, decreases with increasing mole fraction, x, of the hex-yloxy homolog till at x 0.30 the SmA phase disappears. Just in this mole fraction region the slope of the N-I transition... 

Garland [10] studied second-order SmA-N and SmC-SmA phase transitions by very precise heat capacity measurements up to 300 MPa. Similar measurements of the critical heat capacity near the SmA-N transition were performed by Kasting et al. [93, 94]. McKee et al. [95] carried out orientational order determinations near a possible SmA-N TCP (p = 289 MPa, 140 °C) by NMR. From McMillan s theory [96] in the case of a TCP of the SmA-N (N ) transition at atmospheric pressure a value of 0.866 (model parameter S = 0) for r(SmA-N)/ T(N-I) follows. Thus a rough test by the corresponding transition temperatures at higher tricritical pressures is possible. 

Since the SmAj-N and SmA -N transitions are expected to be second order (low Ja n/Jn-i) values [97]) and the SmA,-SmAd transition first-order (symmetry arguments), the meeting point should be a bi-critical point. Moreover the topology of the p-T phase diagram obtained resembles that of a diagram exhibiting known bicritical points [106]. 

The experimental results confirm the theoretical prediction that the N-I transition volume increases when the N-I transition gets closer to a SmA-N transition. This is due to short-range smectic order. This influence of the nematic range on the N-I transition volume can be very obviously seen for the ninth member of the series which exhibits a SmA-N-I triple point at 92 MPa and 103 °C. 

Alternatively, it is also possible to have polar molecular systems where the mole-eules do not overlap with eaeh other to form a bilayer structure, but instead the molecules form lamellae where they are arranged in a disordered head-to-tail way so that a mono-layer structure results (see Fig. 6). This phase has been given the symbol SmA i, and transitions ean be found from monolayer SmA, to bilayer SmA2 and SmAj phases. 

Here L gives the separation (measured in units of layers) between the nearest film/ vapor interface and the layer in question. The fittings yield Lq= 0.24 0.01, u=0.37 0.02 for 90.4 [47], Lq=0.31 0.02, u=0.32 0.02 for 3(10)OBC [45], Lq=0.34 0.05, u=0.30 0.05 for 54COOBC [48] andLo=0.32 0.01,u=0.36 0.02 for 40.8 [49]. The experimental data and fitting results near the SmA-SmI transition of 90.4 are shown in Fig. 9. The critical exponent i)= 1/3 indicates that simple van der Waals forces are responsible for the interlayer interaction. Although the layer-by-layer transition near a first order transition has been theoretically predicted [52], that near a second order transition is totally counterintuitive. Further experimental and theoretical work is necessary to address this unresolved puzzle. 

Today we know that the majority of liquid crystals undergoing an SmA - SmC transition do not behave according to the de Vries model. This would seem to require a very low degree of interaetion between the constituents from layer to layer, or even within layers, i.e., a high degree of randomness, yet connected with smectic order. 

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