Temperature dependence crystallization from dilute solution

3 Temperature dependence crystallization from dilute solution 

The application of nucleation theory to the data, irrespective of the type selected, requires an accurate value of the equilibrium dissolution temperature, T°, because of the crucial role played by the undercooling in the analysis. The problem is that major differences in values have been reported for the same polymer-solvent system. For example, in the same investigation, depending on the extrapolation method used, T° for p-xylene is given as 118.6 2°C, for -octane it ranges from 127 1 °C to 124 2 °C, while in decalin it varies from 113.0 1.5 °C to 116.5 2 °C.(68) Among different investigations T values for p-xylene have been reported as 108.2 °C,(65), 110.5 1 °C (69) and 118.6 2°C.(68) The reported 

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In a similar way, a lower temperature is required to crystallize ice from salt water or from an alcohol-water solution than from pure water. Antifreeze substances added to an automobile radiator act on this principle. They dilute the water in the radiator and lower the temperature at which ice can crystallize from the solution. Again, the amount the freezing temperature is lowered depends upon the relative amounts of water and antifreeze compound. 

When homopolymers are crystallized from very dilute solutions, either lozenge-shaped platelets or crystals that possess a dendritic habit are formed. Some typical electron micrographs of the crystals precipitated from dilute solution are shown in Figs. 1.12 and 1.13. The crystal habit that is observed depends on the molecular weight of the polymer and the crystallization conditions, such as the temperature and the nature of the solvent. A very striking feature is that the platelets are only about 100 to 200 A thick. In conjunction with selected-area electron diffraction studies, it is shown that the chains are again preferentially oriented normal, or nearly so, to the basal plane of the platelet. Considering the... 

The effects of molecular weight on separation has been shown to be negligible over a wide molecular weight range of linear polyethylenes [119]. The reason the separation is not molecular weight dependent has been ascribed to a very controlled crystallization process from dilute solution. By using a very slow and controlled decrease in temperature (in some cases over the course of days), the crystallization process is more thermodynamically controlled and the kinetic effect of chain length on crystallization is diminished. 

The etch rate of silicon in KOH may vary from as low as 0.1 nm s in dilute solution at room temperature to as high as 200 nm s in 40% KOH near 90 °C. The etch rate of silicon single crystals always depends on the surface plane orientation in all dissolving solutions, but the effect is maximum for alkaline solutions. The (111) surface is the slowest etching plane in all reported investigations. This result can be convincingly correlated by the back bond theory, that is, the surface atoms on the (111) plane have 3 bonds connected to... 

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