Dilatational phase transitions

A Dilatational phase transition is a special displacive transformation in which there is a coordination change. The symmetry is not necessarily increased at the higher-temperature side of the transition point. The transition of the CsCl structure to the NaCl structure is one example. These transitions are rapid and may have high latent heats. 

Over a long period of time experimental results on amphiphilic monolayers were limited to surface pressure-area ( r-A) isotherms only. As described in sections 3.3 and 4, from tc[A) Isotherms, measured under various conditions, it is possible to obtain 2D-compressibilities, dilation moduli, thermal expansivities, and several thermodynamic characteristics, like the Gibbs and Helmholtz energy, the energy cmd entropy per unit area. In addition, from breaks in the r(A) curves phase transitions can in principle be localized. All this information has a phenomenological nature. For Instance, notions as common as liquid-expanded or liquid-condensed cannot be given a molecular Interpretation. To penetrate further into understanding monolayers at the molecular level a variety of additional experimental techniques is now available. We will discuss these in this section. 

As a simple depiction of a hpid bilayer, one can consider it as a film or a slab, which may be curved, compressed or dilated, and sheared. At physiological temperatures most natural hpid membranes are fluid. Therefore, within this slab, the hpid molecules are free to move. Below the hpid phase transition temperature, single-component membranes crystalhze. In this so called gel phase, the relative mohon of hpids and membrane inclusions is principally hindered. The fluidity of the membrane and resistance to shear in the plane of the film are characterized by the shear viscosity, % (or the diffusion coefficient of the hpids). Typical values... 

The lattice parameters show gradual changes, with lowering temperature, indicating that there is no distinct structural phase transition down to 104 K (Fig. 4). Tlie temperature dilation of the a-axis is not linear with temperature. The a-axis remains constant down to 260-270 K, where it starts to expand gradually, with upper curvature down to 104 K, though the real separation of two anion layers (a sinP) is almost unchanged. 

Watkins, J. J. (1998)Phase transitions in polymer blends and block copolymers induced by selective dilation with supercritical CO2, Presented at the NATO Advanced Study Institute on Supercritical Fluids, Kemer, Turkey, July 12-24, 1998 [Manuscript is included in this volume]. 

PHASE TRANSITIONS IN POLYMER BLENDS AND BLOCK COPOLYMERS INDUCED BY SELECTIVE DILATION WITH SUPERCRITICAL COi... 

This method allows study of first- and second-order phase transitions of pure compounds. The mesogenic sample is enclosed in a rigid but dilatable metallic cell. The sample pressure and temperature are simultaneously recorded. The thermobarograms obtained exhibit a clear change of slope at the phase transition. The measuring apparatus is called a metabolmeter and requires only a very small amount of mesogen. 

Since the Cu(NCS)2 and Cu[N(CN)2]Br salts exhibit abnormal temperature-depen-dent resistivities down to around 100 K, the associated lattice parameters have been carefully investigated. The Cu(NCS)2 and Cu[N(CN)2]Br salts show no distinct structural phase transition down to 20 K except for ordering of the ethylene groups and a slight anomaly in the thermal dilation of the a-axis down to 80 K. - These results suggest that the Cu[N(CN)2]Br salt has weaker donor-anion atomic contacts and... 

An impact modifier is a rubber phase dispersed in particulate form throughout the matrix of a polymer solid. Unlike plasticizers, the rubber particles retain their intrinsic properties as a separate phase. The glass transition temperature of the parent matrix is not lowered by the addition of an impact modifier. The rubber particles do two things to the parent matrix phase (2,3,4) they act as stress concentrators (i.e., a large strain will start in the matrix near the interface) and they enhance the multi-axiality in stress. As multiaxial tensile strength near the interface further enhances dilatation, which shortens the mechanical relaxation time, the otherwise brittle polymer solid of the matrix will undergo plastic deformation in the vicinities of the rubber particles. 

Several transitions have been proposed to occur within ICs. One of them is condensation into an anisotropic liquid phase, due to interactions between molecules in different channels [148]. This has been claimed to occur at remarkably high temperature when the phenomenon of dilation of the lattice of tubes is taken into account [142, 143]. That is a collective phenomenon, analogous to the phonon-mediated superconductivity, but with a higher energy scale associated with the enhanced binding within the channel. 

Semi-empirical rules, which correlate the static glass transition temperature Ty from differential thermal analysis or dilatometry with the dynamic T(J taken from the tan 8 or E" peak, may be used with caution in analyzing two-phase systems with a dispersed rubbery phase. The dynamic Tg depends on the rubber phase volume, and it may be shifted further toward lower temperature for effectively crosslinked and grafted rubber particles because of dilatation. 

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