Equilibrium melting temperature, polymer crystal nucleation

Temperature has a complex effect on crystallization rate. Initially, as the temperature falls below the equilibrium melting temperature, the crystallization rate increases because nucleation is favored. However, as the temperature continues to fall, the polymer s viscosity increases, which hampers crystallization. As a rule of thumb, a polymer crystallizes fastest at a temperature approximately mid-way between its glass transition temperature and its equilibrium melting temperature. 

To analyze the crystallization behavior of crystalliz-able polymer blends, it is suitable to distinguish between systems containing only one crystallizable component (crystalline/amorphous blends) and those containing both crystallizable components (crystalline/crystalline blends). In all cases, the crystallization of a polymer blend will take place in a defined temperature range, between the values of the glass transition temperature, Tg, and the equilibrium melting temperature of the crys-taUisable polymer, below Tg the chain mobility is inhibited, while at temperatures near T° the crystal nucleation does not occur. 

It is quite evident that nucleation plays an important role in polymer crystallization. It is dominant in the vicinity of the equilibrium melting temperature, as is manifested by the marked negative temperature coefficient. It is important, therefore, to define the particulars of the nucleation process that are operative in a given situation. In order to attain this objective, the analysis at this point is limited to isothermal... 

The crystallization of a polymer can only proceed in a temperamre range limited on the low temperamre side by the glass-transition temperature T and on the high temperature side by the equilibrium melting point T ). Below the mobility of the polymer chains is hindered, while in the proximity of crystal nucleation is inhibited. 

Growing numbers of commercial materials are blends of two or more polymers in which at least one of the components is a crystalline polymer. The crystallization in miscible blends is restricted to temperatures between the blend glass transition temperature and the equilibrium melting point, T ,e, i.e., to the crystallization temperature, Tc < Tm e- The difference, Dc = T ,e - Tc, depends on the cooling rate and the nucleation process. There are three mechanisms of the crystallization nucleation (Utracki 1989) ... 

Diluents/solvents affect the crystallization of polymers in several ways. One is that they lower the concentration of the polymer, limiting the rate of nucleation and growth. The other is that they lower the equilibrium dissolution temperature. Diluents/solvents can, therefore, be used to get controlled conditions for PSC with very regular crystal structures. Just as with composites cooled from a melt, CNTs can be used to nucleate PSCs in dilute and semi-dilute solutions as well. The precise control offered by dilution allows CNTs to crystallize polymer at temperatures above the homogeneous nucleation temperature. In some cases, this results in the transcrystallinity discussed in the last section, and in other... 

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