Transitions crystallization

To develop this model into a quantitative relationship between T j, and the thickness of the crystal, we begin by realizing that for the transition crystal liquid, AG is the sum of two contributions. One of these is AG , which applies to the case of a crystal of infinite (superscript °o) size the other AG arises specifically from surface (superscript s) effects which reflect the finite size of the crystal ... 

An example of the determination of activation enthalpies is shown in Figs. 11 and 12. A valuable indication for associating the correct minimum with the ionic conductivity is the migration effect of the minimum with the temperature (Fig. 11) and the linear dependence in the cr(T versus 1/T plot (Fig. 12). However, the linearity may be disturbed by phase transitions, crystallization processes, chemical reactions with the electrodes, or the influence of the electronic leads. 

The Reynolds number in mixing operations must be > 200 The ratio of the mold filling and gelation time must not exceed 0.5 No volatile products must appear shrinkage must be compensated chemical reactions and phase transitions (crystallization) must proceed quickly, regardless of the size of the article... 

If you can only answer these questions in a muddled, confused manner, read the review Otherwise, skip to the first section on thermal transitions—crystallization. 

A second rich collection of references on the polymorphic behaviour of organic materials is the compilation by Deffet (1942). This contains information and references to primary sources on 1188 substances that exhibit polymorphism at atmospheric pressure and another 32 that exhibit polymorphic behaviour at elevated pressures. A typical entry contains the number of reported polymorphic forms, their melting points, temperature(s) of transition, crystal system, some physical properties, and literature references, of which there are nearly 1000. Substances are organized by empirical formula with an index organized by compound name (in French). 

Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) are the most widely used thermal analysis techniques. Both techniques have the same objective to examine thermal events in a sample by heating or cooling without mass exchange with its surroundings. The thermal events examined by DTA and DSC include solid phase transformation, glass transition, crystallization and melting. Differential emphasizes that analysis is based on differences between sample material and a reference material in which the examined thermal events do not occur. 

The temperature of a polymer s melting, glass transition, crystallization, and its solid-state annealing are all known to have pressure dependencies.High-pressure instruments, such as the Gnomix Inc. (Colar-ado, U.S.A.), have been developed to study PVT relationships in polymers. In these experiments, the sample is placed in an incompressible fluid and then the desired pressure is applied. Full details of this technique, as well as a collection of P-V-T relationships for a wide range of polymers up to 200 MPa ( 30,000 psi) and 400°C, have recently been published. These data have mainly been collected isothermally to report the effects of pressure and temperature on the volume of... 

A well designed and properly replicated DSC profile would yield such physical properties as melting (endothermic), solid-state transitions (endothermic), glass transitions, crystallization (endothermic), decomposition (exothermic) and dehydration or desolvation (endothermic), purity (of high purity compounds though much less reliable than high-performance liquid chromatography, HPLC). 

Exothermic process of phase transition - crystallization, characterized by peak in the region B at the temperature 143 °C runs after PETP devitrification. 

Differential scanning calorimetry monitors the energy required to maintain the sample and a reference at the same temperature as they are heated. A plot of heat flow (W/g or J/g) versus temperature is obtained. A thermal transition which absorbs heat (melting, volatilization) is called endothermic. If heat is released during a thermal transition (crystallization, degradation), it is called exothermic. The area under a DSC peak is directly proportional to the heat absorbed or released and integration of the peak results in the heat of transition. 

A very small contact angle tends to be formed also if the solid is a crystal, phase a is air, and (3 is the liquid phase of the crystal material. This implies that the crystal acts as an effective catalytic impurity for the formation of its own melt. This explains why very little overheating generally suffices to induce the phase transition crystal —> melt, as was mentioned in Section 14.1. 

Many suppository bases are made up of various acylglycerols. Hardening of suppositories is considered to result from various phase transitions, crystallization, and transesterification reactions in these lipids. The DSC thermograms of a semisynthetic, hard base triglyceride shown in Fig. 177 indicate that a polymorphc phase transition occurred during storage.709... 

DSC is a thermal analysis technique that is used to measure the temperatures and energy flows related to transitions in materials as a function of time and temperature.These measurements provide qualitative and quantitative information about physical and chemical changes that involve endothermic or exothermic processes or changes in heat capacity. Any event, such as loss of solvent, phase transitions, crystallization temperature, melting point, and degradation temperature of the plastic sample, result in a change in the temperature of the sample. The systems available cover a wide temperature range, e g., -60°Cto>l,500°C. 

Johnson and Miller (67) showed that cholesteryl propionate gave a curve with three transitions crystal - smectic at 99°C smectic - cholesteric at U0°C and cholesteric - isotropic at 110°C. However, on changing the heating rate to 30°C/min, they no longer detected the 110°C peak, but detected a new, small endothermic peak at about 64°C. 

For the better understanding of blend morphologies, the fundamental mechanisms of morphology development are discussed, viz. the liquid-solid phase transition (crystallization), the liquid-liquid phase separation e.g., spinodal decomposition under non-isoquench depth), as well as the complex mechanism of the morphology generation that results from the competition between these two transitions. The effects of chemical reactions and flow fields on morphology development have also been discussed. Finally, several evidences of a local structure in single-phase polymer-polymer mixtures are presented. 

DSC has been used [33] in the characterisation of the polymer, providing information on thermal properties such as the glass transition, crystallization and melting of the polymer. The DSC profile shows the appearance of two glass transitions, one at -63 to -71 °C, while the other occurs at 90-103 °C. There is a slight increase in the latter glass transition on inclusion of the paclitaxel into the polymer attributed to interactions between the aromatic electrons. The release of paclitaxel from the polymer after an initial burst follows a longer sustained release pattern. 

Introduction of branched terminal chains, providing that they are long enough, resulted in the reduction, as expected, of the temperature of the transition crystal-to-mesophase, as well as the thermal stability. " The nickel complex with the racemic 2-methyloctyl chains showed two mesophases, Cr 98 SmC 103 N 118 I, whereas that with the chiral 5-5-methylheptyl chains presented an SmC between 119°C and 159°C. 

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