Crystallization heat, enthalpy

A sample of the polymer to be studied and an inert reference material are heated and cooled in an inert environment (nitrogen) according to a defined schedule of temperatures (scanning or isothermal). The heat-flow measurements allow the determination of the temperature profile of the polymer, including melting, crystallization and glass transition temperatures, heat (enthalpy) of fusion and crystallization. DSC can also evaluate thermal stability, heat capacity, specific heat, crosslinking and reaction kinetics. 

It is very likely that during the polymerization under the strict control of the monomer crystal lattice, a strain energy caused by molecular movements accumulates on the reacting crystals resulting in the endothermal formation of polymer crystals. The enthalpy difference between the crystalline and amorphous polymers (3.1 kcal/mol) corresponds to the heat of crystallization of as-polymerized poly-DSP crystals. 

The heat capacity is assumed to remain constant at 21.75 cal K mol assumed glass transition point of 960 K. Below that temperature the crystal heat capacity (j ) is assumed to be valid. S Ct, 298.15 K) is calculated in a manner similar to that used for the enthalpy of formation. 

Microcalorimetry is an extremely sensitive technique that determines the heat emitted or adsorbed by a sample in a variety of processes. Microcalorimetry can be used to characterize pharmaceutical solids to obtain heats of solution, heats of crystallization, heats of reaction, heats of dilution, and heats of adsorption. Isothermal microcalorimetry has been used to investigate drug-excipient compatibility [82]. Pikal and co-workers have used isothermal microcalorimetry to investigate the enthalpy of relaxation in amorphous material [83]. Isothermal microcalorimetry is useful in determining even small amounts of amorphous content in a sample [84]. Solution calorimetry has also been used to quantitate the crystallinity of a sample [85]. Other aspects of isothermal microcalorimetry may be obtained from a review by Buckton [86]. 

Thermal properties, such as the glass transition temperature, melting temperature, crystallization temperature, enthalpy, heat expansion, and heat deformation temperature (HDT) ... 

The heat effects accompanying a crystallization operation may be determined by making heat balances over the system, although many calculations may be necessary, involving knowledge of specific heat capacities, heats of crystallization, heats of dilution, heats of vaporization, and so on. Much of the calculation burden can be eased, however, by the use of a graphical technique in which enthalpy data, solubilities and phase equilibria are represented on an enthalpy-composition H x) diagram, sometimes known as a Merkel chart. 

Molar volume of element M Heat of formation Heat of crystallization Heat of mixing Hole formation enthalpy Relative thermal expansion Viscosity Phase shift... 

Poly(L-lactic acid) (PLEA) is hydrolytically unstable and does not withstand humid heat. The more and more extended radiation sterilization on medical wear imposes investigation of induced effects. It undergoes random chain scission, when subjected to ionizing radiation consequences of this phenomenon on the crystalline state can reflect the induced modifications. The linear decrease of crystallinity (Table 40) [02K3] describes the constant deterioration of molecular structure. Physical properties like melting enthalpy and crystallization heat which are sensitive to the modification in molecular size and interactions are adequately mitigated (Fig. 58). 

For all isothermal measurements the heats to be determined, such as heats of crystallization, heats of dissolution, heats of combustion, and other chemical reactions, are obtained in a rather straightforward matmer at the temperature of measurement. Outside of the instrument corrections, no further data treatment is necessary. The thermochemical data handling was illustrated in Fig. 2.14, using the examples of heats of combustion and enthalpies of formation. An entry into the extensive literature on heats of chemical reactions can be found by studying the references 6 and 52 at the end of the chapter. 

If appropriate enthalpy data are unavailable, estimates can be obtained by first defining reference states for both solute and solvent. Often the most convenient reference states are crystalline solute and pure solvent at an arbitrarily chosen reference temperature. The reference temperature selected usually corresponds to that at which the heat of crystallization A/ of the solute is known. The heat of crystallization is approximately equal to the negative of the heat of solution. For example, if the heat of crystallization is known at then reasonable reference conditions would be the solute as a soUd and the solvent as a Uquid, both at The specific enthalpies then could be evaluated as... 

At a given ambient water vapor pressure (usually the level found in the open atmosphere), the temperature of the material is raised so that the equilibrium water vapor pressure over the hydrated material is higher than the ambient water vapour pressure. Generally, heating up to 400 °C is sufficient to remove all the water of crystallization from materials. This removal of water yields a material which may contain some more strongly bound water. To remove this water, the material requires to be heated to a higher temperature (400-600 °C) so that the equilibrium water vapour pressure exceeds the ambient water vapour pressure. For near-complete removal of the last traces of water, temperatures as high as 1000 °C may be required. In addition to the heat required to raise the temperature of the material, heat is also required for the evaporation of water, which is an endothermic process. The enthalpy of evaporation increases as the water content, and hence the equilibrium water vapor pressure, decreases. 

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