Homogeneous nucleation rate, polymer crystal

On the basis of the concept described above, we propose a model for the homogeneous crystallization mechanism of one component polymers, which is schematically shown in Fig. 31. When the crystallization temperature is in the coexistence region above the binodal temperature Tb, crystal nucleation occurs directly from the melt, which is the well-known mechanism of polymer crystal nucleation. However, the rate of crystallization from the coexistence region is considered to be extremely slow, resulting in single crystals in the melt matrix. Crystallization at a greater rate always involves phase separation the quench below Tb causes phase separations. The most popular case... 

The rate of nucleation, I, has been estimated by Turnbull and Fisher [25] from the shape of AG and the influence of local viscosity, governed by a free enthalpy AG,. In Fig. 3.61 this equation is listed. The rate I applies to the case that nucleation is unhindered. The first exponent of the equation expresses retardation of nucleation due to viscosity effects with the given parameters (see Sect. 5.6). It stops nucleation as the glass transition temperature is approached. The nucleation described is homogeneous nucleation and creates a continuous stream of new crystals in the remaining melt or solution. For polymers it takes a supercooling of about 50 K to overcome the... 

The crystallization of a polymer in the presence of solidified domains of the second phase takes place through a heterogeneous nucleation process. Since the rate of heterogeneous nucleation is higher than that of homogeneous nucleation, and since primary nucleation is the rate-controlling step for polymer crystallization, the crystallization rate is expected to be higher in such blends when compared to homopolymers (Nadkami and Jog 1991). 

The overall nucleation and crystalhzation rates of PLA tmder heterogeneous conditions are relatively higher than in homogenous conditions. The nucleation and crystallization rates of propylene-ethylene copolymer are increased tmder isothermal conditions. Addition of nucleating agent accelerates crystallization. Avrami equation is in popular use in the analysis of isothermal crystallization kinetics of polymers ... 

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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