Twist phase transitions

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]. 

The binary indium-rich transition metal compounds show a variety of different indium networks. In the structures of T2lns (T = Ti, Hf), the indium atoms form infinite two-dimensional (2D) planar networks, which can be described as a tessalation of triangles, squares, and pentagons. The 2D network is also found in the structures of the Lasins compounds, which contain well-defined indium square pyramids and some intercluster indium-indium distances that are only 11-15% greater than the average intracluster bond lengths. The apparent Ins " cluster in Lasins can be described as a closed shell nido-deltahedron, and the compound structurally as a Zintl phase. The /S-Ysins compounds with a similar structure to Lasins present a slow first-order phase transition to a-YsIns at high temperature. Clusters in the low-temperature a-YsIns phase are twisted and Joined by short bonds at trans-basal positions into chains that are more weakly interconnected into a three-dimensional structure. 

Now that we have discussed all of the structures of the phases that play a part in the formation of twist grain boundary phases, let us now consider the events that can occur at a nematic to smectic phase transition. At a normal chiral... 

All physical parameters mentioned above are material specific and temperature dependent (for a detailed discussion of the material properties of nematics, see for instance [4]). Nevertheless, some general trends are characteristic for most nematics. With the increase of temperature the absolute values of the anisotropies usually decrease, until they drop to zero at the nematic-isotropic phase transition. The viscosity coefficients decrease with increasing temperature as well, while the electrical conductivities increase. If the substance has a smectic phase at lower temperatures, some pre-transitional effects may be expected already in the nematic phase. One example has already been mentioned when discussing the sign of Ua- Another example is the divergence of the elastic modulus K2 close to the nematic-smecticA transition since the incipient smectic structure with an orientation of the layers perpendicular to n impedes twist deformations. 

Biphenyl The molecule (Fig. 5.16) is not planar in the gas phase the angle between the two planes is about 45°. In the biphenyl crystal, the molecules are planar. At 40 K and at 17 K, there are structural phase transitions within the biphenyl crystal. The molecules thereby lose their centres of inversion. Below 40 K, the molecule is twisted by about 10° around the central C-C axis. This means that the inter-molecular coupling is larger than the intramolecular torsion potential and that the coupling between torsional and lattice oscUlations cannot be neglected in every case, but instead can even induce phase transitions. A similar conclusion holds for the coupling of low-frequency bending vibrations (Fig. 5.16, lower part) to external lattice vibrations (see e.g. [19, 20]). 

Figure 16.4 Salt-dependent buckling of circular DNA molecules. (A) Schematic representation of the preparation of the over twisted 90-base-pair circular DNA. (B) A pronounced phase transition, from circular to buckled DNA, is observed at physiological conditions (concentrations of 50-200 mM]. Q is an order parameter that quantifies the DNA circle s supercoiling. Figure reproduced with permission from Savelyev et al., 2011b.

A vital property of these model proteins is that they are more ordered above the transition temperature defined by the binodal or coexistence line in Figure 5.3. The polymer component of this water-polypeptide system becomes more ordered or structured on increased temperature from below to above the transition. This behavior is the inverse of that observed for most systems, as discussed above. In particular, we developed the term inverse temperature transition when the precursor protein and chemical fragmentation products of the mammalian elastic fiber changed from a dissolved state, and therefore when molecules were randomly dispersed in solution, to a state of parallel-aligned twisted filaments as the temperature was raised from below to above the phase transition. - ... 

As considered in Chapter 5, there are many reasons for referring to the essentially unique phase transition utilized by biology as an inverse temperature transition. Furthermore, here we note that biology s inverse temperature transition bears equivalence to the phase transition of the heat engine that gave birth to thermodynamics, but with an inverse twist. 

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