Temperature dependence, polymer crystal crystallization kinetics

Some literature data concerning isothermal crystallization experiments of linear PE at 128 C [Doremus et al., 1958] have been fitted using different values for the parameters in Eqs 3.21 and 3.22 (Figure 3.15). The most accurate fit was obtained using the following parameters n = 4, k=3.7 X 10-1°, n = 2,k = 4x lO and C= 0.68. In the case of miscible polymer blends, the temperature dependence of the overall kinetic rate constant, k, can be calculated from [Boon and Azcue, 1968 Wunderlich, 1973 Hoffman, 1982] ... 

The crystalline phase typically grows as spherical aggregates called spherulites. However, other geometries such as disks or rods may be found with, as shown below, a consequent modification of the rate equation. M. Avrami [26] first derived these rate equations in the form used for polymer kinetics for the solidification of metals. The weight of the crystalline phase is calculated as a function of time at constant temperature. As will be described below, the temperature dependence of crystallization can be derived from classical nucleation theory. 

The behavior in the liquid state is essentially the same as amorphous polymers. In the transition region, an abrupt change in slope occurs as crystallization begins to take place. This is the crystallization temperature T, . If the material is cooled very rapidly, the crystallization rate can be depressed, depending on the crystallization kinetics. In fact, in some materials with sufficientiy slow crystallization kinetics, the crystallization can be almost completely suppressed by rapid cooling. A well-known example is polyethylene terephthalate (PET). If PET is rapidly quenched, it is almost completely amorphous with a density of about 1.33 g/-cm. If it is cooled slowly, it will crystallize with a resulting higher density of about 1.40 gZ-cm. ... 

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