Crystalline transition

The dynamic mechanical properties of PTFE have been measured at frequencies from 0.033 to 90 Uz. Abmpt changes in the distribution of relaxation times are associated with the crystalline transitions at 19 and 30°C (75). The activation energies are 102.5 kj/mol (24.5 kcal/mol) below 19°C, 510.4 kJ/mol (122 kcal/mol) between the transitions, and 31.4 kJ/mol (7.5 kcal/mol) above 30°C. 

The high-temperature glassy or crystalline transitions of the linear aromatic and heterocyclic polymers were an important drawback for die structural aerospace applications, which need a good flow for the adhesive or composite formulations. 

Crystalline polyimide powders, 304 Crystalline transition temperature. See Melting temperature (Tm) Crystallization rate, for processing semicrystalline polymers, 44 CTE. See Coefficient of thermal expansion (CTE)... 

The phase transition of bilayer lipids is related to the highly ordered arrangement of the lipids inside the vesicle. In the ordered gel state below a characteristic temperature, the lipid hydrocarbon chains are in an all-trans configuration. When the temperature is increased, an endothermic phase transition occurs, during which there is a trans-gauche rotational isomerization along the chains which results in a lateral expansion and decrease in thickness of the bilayer. This so-called gel to liquid-crystalline transition has been demonstrated in many different lipid systems and the relationship of the transition to molecular structure and environmental conditions has been studied extensively. 

Metathesis reaction to prepare crystalline transition and B metal oxides from halides and sodium peroxide is initiated by a hot wire and can be highly exothermic and explosive in some instances. 

Partially Crystalline Transition Metal Sulphide Catalysts. Chiannelli and coworkers (6, 7, 8) have shown how, by precipitation of metal thio-molybdates from solution and subsequent mild heat-treatment many selective and active hydrodesulphurization catalysts may be produced. We have shown (18) recently that molybdenum sulphide formed in this way is both structurally and compositionally heterogeneous. XRES, which yields directly the variation in Mo/S ratio shows up the compositional nonuniformity of typical preparations and HREM images coupled to SAED (see Figure 2) exhibit considerable spatial variation, there being amorphous regions at one extreme and highly crystalline (18, 19) MoS at the other. 

It is pertinent to mention here that in the course of TGA many vital processes, for instance crystallization, crystalline transitions, pure fusion reactions, glass transitions, and solid-state reactions devoid of volatile components might not be indicated as they happen to cause little change in weight of the sample. TGA invariably describes with ample precision the stoichiometry related to chemical changes that are indicated during DTA by an endothermal or exothermal duration from the base-line. 

The chloroform solution of lipids (Solution A) is placed in a 50-mL round-bottomed spherical Quick-fit flask. Following evaporation of the solvent in a rotary evaporator at about 37°C, a thin lipid film is formed on the walls of the flask. The film is flushed for about 60 seconds with oxygen-free nitrogen (N2) to ensure complete solvent removal and to replace air. Two milliliters of distilled water and a few glass beads are added into the flask, the stopper is replaced, and the flask shaken vigorously by hand or mechanically until the lipid film has been transformed into a milky suspension. This process is carried out above the liquid-crystalline transition temperature (7/) of the phospholipid component of liposomes (> 7/) by prewarming the water... 

Black powder 95 200 Endotherm - overlapping peaks from crystalline transition of KNO3, transition and fusion of sulfur and vaporisation of volatile matter from charcoal. 

Investigators of works listed here as Refs 1, 2 3 have been concerned mostly wirh rates of propagative burning in gaseous mixtures and also for some solid-state systems in which neither fusion nor crystalline transitions were considered. Jackson (Ref 4) investigated the burning of quaternary compositions... 

DTA will also provide information on the melting, boiling, crystalline transitions, dehydration, decomposition, oxidation and reduction reactions. 

Laser pyrolysis has been shown to produce a wide variety of crystalline transition metal nitride and carbide nanoparticles with diameters as small as 2 nm. The nanopowders are in many cases monophasic and single crystalline. By varying the reaction conditions it is possible to control the particle diameter, and in some cases, the crystalline phase produced. Improvements in the synthesis are needed to control surface oxidation. 

Fig. 3. Models of the polytetrafluoroethylene chain above and below the crystalline transition at 19° C...

These changes are generally considered to be first-order crystalline transitions. The molecular conformation below and a-bove the 19° C transition are shown in drawings of molecular models in Fig.3 and the changes in the j I diffraction patterns of crystalline fibers at these transitions are illustrated in Fig. 4. Below 19° C the... 

DNA arranges into rectangular superlattice in the low-temperature gel phase of saturated cationic lipids [83, 84]. This is evidenced by two or three diffuse reflections in addition to the set of lamellar reflections these are attributed to DNA ordering both within the layer and across the lipid bilayers, from one DNA layer to another. These reflections index on a centered rectangular lattice. Noteworthy, DNA does not affect the gel-liquid crystalline transition temperatures of the lipoplexes [16, 19, 84]. This transition is associated with loss of the DNA inter-lamellar correlation. 

---