Polymer crystallization, nucleation theoretical model

Abstract. We review how the nucleation mechanism of polymer crystallization could be assigned to intramolecular processes and what are the preliminary benefits for understanding some fundamental crystallization behaviors. The speculative concept of molecular nucleation and the theoretical model of intramolecular nucleation have been elucidated in a broad context of classical nucleation theory. The focus is on explaining the phenomenon of molecular segregation caused by polymer crystal growth. 

Although secondary nucleation theory was, for a period, widely accepted, it is now coming under increasing pressure, from experimental data, from computer simulation, and from new approaches to the fundamental process of crystalhzation. It is not clear at this stage whether all that is required is a few adjustments to the theory, or whether the idea of a nucleation barrier is flawed, or even if the idea that the crystal thickness seen is the fastest growing is correct. With the development of new theoretical tools, and the increased integration of theory with computer simulation, it is hoped that a more complete model for polymer crystallization can be developed. 

A theoretically advanced model for polymer crystallization under shear was developed by Eder and Janeschitz-Kriegl [31]. The main hypothesis is that dynamic nucleation (i.e., when nuclei appear due to shear) is different from static... 

A key problem is to theoretically establish the appropriate primary and secondary nucleation processes that are operative. This includes defining the geometry of the rate controlling nucleus, i.e. whether it is two-dimensional or three-dimensional. It should be recalled that the experimental results can fit either model. Of crucial importance is the chain conformation within the nucleus. Conventional type data are not discriminatory. They can be fitted by either a bundle type nucleus, or one in which the chains are regularly folded with adjacent re-entry. At present, either type of nucleus is consistent with the experimental results. It is important that it be recognized that it is not necessary for there to be a one-to-one relation between the nucleus structure and the mature crystallite that evolves. It is not necessary to assume a regularly folded chain nucleus in order for a mature lamellar crystallite to evolve. It is important that the crucial role of nucleation in polymer crystallization be taken from the realm of assumption and placed on a first principled theoretical base. Similar concerns can be expressed with respect to the transport term. 

It was often found that, contrary to the theoretical prediction, the value of n is noninteger (Avrami 1939). The Avrami model is based on several assumptions, such as constancy in shape of the growing crystal, constant rate of radial growth, lack of induction time, uniqueness of the nucleation mode, complete crystallinity of the sample, random distribution of nuclei, constant value of radial density, primary nucleation process (no secondary nucleation), and absence of overlap between the growing crystallization fronts. These assumptions are often not met in polymer (blend) crystallization. Also, erroneous determination of the zero time and an overestimation of the enthalpy of fusion of the polymer at a given time can lead to noninteger values for n (Grenier and Prud homme 1980). 

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