N phase transitions

Brodskii, A. N. Phase Transitions in the Double Layer at Electrodes 26... 

Finally, dispersions of MWCNT in chiral nematic liquid crystals were studied as well. These experiments suggested no change in the helical twisting characteristics of the chiral nematic phase. However, the MWCNTs were thought to disrupt the translational order in the SmA phase (decrease of the SmA-N phase transition) yet follow the twist of the nematic director in the chiral nematic phase [498]. 

As another approach to fast I-N phase transition, the optical switching behavior of LMW and polymer azobenzene LCs has been explored by means of reflection-mode analysis (Figure 10). 45 On laser-pulse irradiation, it is possible to switch the... 

Entry Polymer Initiator n Phase transition Reference... 

Table 13 (XX-n) Phase transitions (°C) and enthalpy changes (kj moL, in parentheses) for poly- ...

The thermodynamics of the I-N phase transition has been extensively investigated for resolving the issue concerning the order of the transition. Following the Ehrenfest scheme, a phase transition is classified into a first-order transition or a second-order one, depending upon the observation of finite discontinuities in the first or the second derivatives of the relevant thermodynamic potential at the transition point. An experimental assessment of the order of the I-N transition has turned out to be not a simple task because of the presence of only small discontinuities in enthalpy and specific volume. It follows from high-resolution measurements that I-N transition is weakly first order in nature [85]. 

There exist pre-transition effects in the isotropic phase heralding the I-N phase transition. Such pre-transition effects, which are consistent with the weakly first-order nature of the I-N transition, can be attributed to the development of short-range orientational order, which can be characterized by a position-dependent local orientational order parameter Q(r), where all component indices have been omitted [2]. In the Landau approximation, the spatial correlation function < G(0)G(r) > has the Omstein-Zemike form < G(0)G(r) exp(—r/ )/r, where is the coherence length or the second-rank orientational correlation length. The coherence length is temperature-dependent and the Landau-de Gennes theory predicts... 

Figure 96 Variation of the transition temperatures of complexes (213) with alkoxy and V-alkyl chain length. Tm melting, Ts-n SmC-to-N phase transition temperature, Tc clearing.

The handedness inversion behaviors upon UV irradiation were observed in both wedge cells and homeotropic cells with the cholesteric (N )—nematic (N)— cholesteric (N ) phase transition sequence as confirmed by the Cano s lines and fingerprint texmres (Fig. 5.12). The mechanism of the helix inversion is proposed... 

Pertsev, N.A., Tagantsev, A.K., Setter, N. Phase transitions and strain-induced ferroelectricity in SrTiOs epitaxial thin films. Phys. Rev. B 61(N 2), R825-R829 (2000)... 

This density wave is usually considered as a complex order parameter pi = exp (itt) of the smectic A phase in the Landau expansion or fi-ee energy at the SmA-N phase transition. Typically, when there is no distortion, one assumes cpi = 0 at z = 0 and operates only with the wave amplitude pi as the real part of the order parameter. 

Kaneko, K. Miwa, Y. Nakamura, N., Phase Transition Behavior and Electro-Rheological Effect of Liquid Crystalline Cyclic-Siloxanes with Fluorine Atoms. J.Appl. Polym. Sci. 2007,105,2474-2481. 

Fig. 9 H-NMR spectrum of a conventional side-chain LCE doped with an aD2-8CB liquid crystal recorded deep in the nematic phase (a) and in the vicinity of the PN-N phase transition (b). The spectrum (a) reflects the distribution of the domain-director alignment, and (b) reflects the distribution of the local order parameter...

In this section, the nature of the PN-N phase transition in LCEs will be presented, as revealed by H-NMR and supported by ac calorimetry. It will be demonstrated that the smooth phase transition in LCEs is a manifestation of both the field conjugate to the local order parameter (which in LCEs takes up close-to critical values) and the pronounced heterogeneity of LCEs, in the sense of distributed random fields. Particular attention will be paid to the description of a simple model based on the LdG approach, which considers these two features of LCEs. This model predicts the temperature profiles of the first and the second moment of the H-NMR spectral lines. The accordance of this model with the experimental results will be discussed. Finally, the idea of a smeared criticality in LCEs will be presented. 

From these observations, the calorimetric measurements (Fig. 5b) and the observations from other studies in the search for an appropriate description of the PN-N phase transition in LCEs, it seems that we start from two radically different, but not mutually exclusive, assumptions ... 

Fig. 13 Schematic representation of an effective, i.e. time averaged, molecular shape and the continuous temperature evolution of the order parameter S of mesogenic molecules in LCEs according to two possible scenarios (a) the supercritical scenario in which the order parametea-5 is hmnogeneous and S(T) is continuous because of the supercritical value of the internal field G, and (b) the heterogeneous sctmaiio in which the domain structure exists in the LCE. In each domain, the PN-N phase transition occurs at a different temperature 7pn n, distributed symmetrically around the mean value. o- Ihis results in a smeared transition and the continuous temperature dependence of the avtaage order parameter (S), even if the phase transition in individual domains is discontinuous...

In the previous sections, the basic principles of the PN-N transition in LCEs and the experimental techniques were introduced to the reader. The issue of a smeared criticality observed in LCEs was introduced in Sect. 4. In this section, the experimental results providing an insight for the understanding of the PN-N transition are presented. These data were obtained by deuteron NMR and ac calorimetry on side-chain and main-chain LCEs. The distinct role of each parameter that affects the critical behaviour of the PN-N phase transition of LCEs will be demonstrated in different subsections. These parameters influence the relative strength of the locked-in mechanical field G and, as demonstrated in the previous sections, they may alter the order of the PN-N transition. 

As shown in this review, the critical properties of the PN-N phase transition in LCEs can be controlled by adjusting various chemical and physical parameters during and after their synthesis, e.g. the density of crosslinkers, the externally applied stress during crossUnking, the temperature of the second crosslinking stage and the content of the dopant liquid-crystalline molecules. 

---