Phase transition crystals

Frenkel D 1992 Computer simulations of phase transitions in liquid crystals Phase Transitions in Liquid Crystals ed S Martellucci and A N Chester (New York Plenum)... 

A single-to-single crystal phase transition was found to take place at 333 K in a new polymorph of ort/zo-ethoxy-trans-cinnamic acid [77]. In this study, the structures of the title compound obtained at two temperatures above the transition point were determined in addition to the structures of the stabilized forms existing at lower temperatures. It was found that the phase transition involved a cooperative conformational transformation coupled with a shift in layers of the constituent molecules. 

Brezesinski, T. Groenewolt, M. Antonietti, M. Smarsly, B. 2006. Crystal-to-crystal phase transition in self-assembled mesoporous iron oxide films. Angew. Chem. Int. Ed. 45 781-784. 

The results of differential scanning calorimetry(DSC) indicate the change in aggregation state. The trans micelle showed a main endothermic peak at 14 2°C(A H =1.0 kcal/mol), corresponding to a gel-liquid crystal phase transition, whereas the transition temperature for the cis micelle appeared at 11.9°C( AH = 0.8 kcal/mol). This is unequivocal evidence that the trans-cis photoisomerization is a sufficient perturbation to alter the state of molecular aggregation. 

By contrast, DTA curves for black powder and its ingredients have been obtained as shown in Figure 2.8. The DTA curves are more complex than those obtained by TG and include crystal phase transitions as described in Table 2.2. 

Crystal phase transitions are a possible target with present day computational means, when the transition is a smooth one and does not involve melting of the mother phase and subsequent recrystallization into the daughter phase. For crystalline OL-norleucine, an MD simulation has provided a detailed picture of the mechanism of a solid-solid second-order transition between two polymorphic crystal forms, showing concerted molecular displacements involving entire bilayers [61]. 

While cooling or warming crystals, phase transitions are not uncommon, but they are often overlooked. Many experiments can be useful to characterize a phase transition, as we will see in Sects 3 and 4. Diffraction, microscopy, spectroscopy, and calorimetry can all provide information on the structure, the energy, or the properties of a crystal. Their sudden change is the most obvious effect of a phase transformation and a matter of interesting study for scientists. 

Ammonium nitrate (AN) was considered early as an environment-friendly alternative to AP but its multiple crystal phase-transitions at low temperatures and its poor performance precluded its use. The nitramine-based propellants are also likely to emerge as potential eco-friendly propellants as the combustion products are non-toxic and non-smoky, although the present day nitramine propellants do not match the high performance and high burn rates of AP-based composite propellants [62, 63]. At the same time, high pressure exponent and unstable combustion prevent their application in large rockets due to safety considerations [20]. The inclusion of Al powder and other additives increases the burn rate and also eliminates the combustion instability. 

Cocrystallization in copolymers of a-olefins. II — Butene-1 copolymers and polybutene type II—I crystal phase transition. Polymer 7, 23 (1965). 

Yamada et al. [9,10] demonstrated that the copolymers were ferroelectric over a wide range of molar composition and that, at room temperature, they could be poled with an electric field much more readily than the PVF2 homopolymer. The main points highlighting the ferroelectric character of these materials can be summarized as follows (a) At a certain temperature, that depends on the copolymer composition, they present a solid-solid crystal phase transition. The crystalline lattice spacings change steeply near the transition point, (b) The relationship between the electric susceptibility e and temperature fits well the Curie-Weiss equation, (c) The remanent polarization of the poled samples reduces to zero at the transition temperature (Curie temperature, Tc). (d) The volume fraction of ferroelectric crystals is directly proportional to the remanent polarization, (e) The critical behavior for the dielectric relaxation is observed at Tc. 

The parameters of the JT distortions were calculated by the X -method for a series of crystals in good agreement with experimental melting temperatures [14]. The details of the theory and specific calculations seemingly require additional refinements, but the main idea of the JT origin of the liquid-crystal phase transition seems to be quite reasonable. This work thus makes an important next step toward a better understanding of the relation between the macroscopic property of SB and the microscopic electronic structure, the JT effect parameters. 

The gel to liquid crystal phase transition temperature of DHPC bilayers containing 5-oxo KSA or 12-oxo KSA, at the concentrations used here, is around 46 C (data not shown). 

DHPC l -dihexadecyl-sn-glycero-3-phosphocholine KSA potassium stearate 5-oxo KSA 5-oxo potassium stearate 12-oxo KSA 12-oxo potassium stearate 9HP 9-heptadecanone Tc temperature of the gel-to-liquid crystal phase transition of DHPC bilayers 43.41C... 

The rate and character of the molecular motions of both the molecules embedded in the lipid bilayer and lipid molecules themselves are strongly dependent on the temperature [19, 203], At a certain temperature tm, the gel-liquid crystal phase transition is known to occur for the membrane made of a synthetic lipid. For example, tm = 41.5 °C for the membranes from DPL. In the vesicles formed by a mixture of lipids, e.g. egg lecithin, the phase transition occurs smoothly rather than jumpwise and starts below 0 °C. Note that the permeability of lipid membranes increases notably upon transition from the liquid crystal state to the gel state [204]. 

FIGURE 9.6 Low-angle neutron scattering intensity as a function of scattering vector Q (A-1) for propylammonium vermiculite immersed in a 0.25 M propylammonium chloride solution at four different temperatures, T = 309 K (continuous line), 311 K (dashed line), 313 K (crosses) and 315 K (solid circles). The low-angle peak at 0.1 A-1 is due to a gel phase with a clay layer spacing of 60 A. The data therefore show the temperature-induced gel-crystal phase transition occurring between 311 and 313 K. Note that body temperature is 311 K and that cell fluids are typically 0.2 M. 

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