Reversible thermal phase transition

From studies of the interactions between lipopolysaccharide and phosphatidyl ethanolamine in molecular monolayers at air-water interfaces, it was concluded that each lipopolysaccharide molecule is surrounded by approximately sixteen phosphatidyl ethanolamine molecules. The hydrocarbon chains of lipopolysaccharides can undergo a reversible thermal order-disorder transition, as shown by thermal phase-transition studies.The ordered conformation of lipopolysaccharides interpreted from wide angle JT-ray studies is considered less developed than that in normal phospholipid bilayers. 

Multiblock copolymers comprising NIPAAm and N,N-dimethylacrylamide (DMAAm) with variable length of blocks of both components prepared by reversible addition-fragmentation chain transfer polymerization were also investigated and their phase behavior was found to exhibit protein-like behavior exhibiting intramolecular collapse upon heating, forming unimolecular flower-like micelles above the thermal phase transition temperature." ... 

In conclusion, the number and kinds of thermal phase transitions of singlechain surfactant molecules vary from a simple phase transition, the solid crystalline to isotropic liquid, to a complex polymorphism and mesomorphism. More than one thermotropic state may exist in the same system, each being the stable phase within a particular range of temperature (and pressure). Phase transitions are usually reversible through all the intermediate forms to a structure that is thermodynamically stable at room temperature, or they are partially reversible through one or more, but not all, of these transitions, and the room temperature product is an undercooled form of a phase that is stable at some intermediate temperature [26]. 

The appearance of spontaneous polarization in the case of LuTaO is related to volumetric irregularities and ordering of the Li+ - Ta5+ dipoles, as is in the case of the similar niobium-containing compound Li4Nb04F. It can be assumed that the main difference between the two compounds is that the irregularities and the Li+ - Ta5+ dipoles are thermally more stable compared to the niobium-containing system. This increased stability of the dipoles leads to the reversible phase transition at 660°C. 

Lane 5 Reverse orientation of pLD-OC-40Pris Lane 6 Purified extract of E. coli strain XL-1 Blue containing no plasmid. Two rounds of thermally reversible phase transition purification were performed. 

Around 400 K there is a phase transition to a disordered but still cubic structure associated with the terminal oxygen atom in a WO4 tetrahedron which can migrate to another tetrahedron, thereby reversing the direction in which a pair of tetrahedra point. Nevertheless the same general atomic mechanisms are responsible for the negative thermal expansion up to the decomposition temperature. 

Structural polymorphism has been already reported as a peculiar solid-solid phase transition with a large spectral shift in the cast film of CgAzoCioN+ Br (chapter 4). The type 1 spectrum was thermally transformed to the type VI spectrum and then backed to the type I by the isothermal moisture treatment. The reversible spectral change between the type I and VI is a good experimental evidence of Okuyama s prediction on the molecular packing. Since the type VI state is assumed to be a metastable state, the isothermal phase transition to the type I state is expected to be induced by some external stimuli. Water molecules adsorbed to cast bilayer films might act as an accelerator of the phase transition. 

Photochemical switching of the phase transition is also found in the polyion complex film. Figure 29 shows reversible cycles of the absorption at 370nm by the coupling of the thermal and photoinduced phase transition of the complex film with carboxymethylcellulose 8. In conclusion, we indicate that the immobilized bilayer membranes containing the azobenzene chromophore are available to the erasable memory materials based on the phase transition triggered by thermal and photochemical processes. The polyion complex technique is clearly shown to be a very useful method for materialization of the immobilized bilayer membranes. 

MDSC is particularly useful for the study of reversible (related to the heat capacity) thermal reactions, and is less useful for non-reversing (kinetically controlled) reactions. Examples of reversible thermal events include glass transitions, heat capacity, melting, and enantiotropic phase transitions. Examples of non-reversible events include vaporization,... 

Switching also implies molecular and supramolecular bistability since it resides in the reversible interconversion of a molecular species or supramolecular system between two thermally stable states by sweeping a given external stimulus or field. Bistability in isolated molecules or supermolecules is, for instance, found in optical systems such as photochromic [8.229] or thermochromic substances or devices, in electron transfer or magnetic processes [8.239], in the internal transfer of a bound substrate between the two binding sites of a ditopic receptor (see Section 4.1 see also Fig. 33) [6.77]. Bistability of polymolecular systems is of a supramolecular nature as in a phase transition or a spin transition, both of which involve an assembly of interacting species. 

---