Polymer crystallization crystal growth

It has been the aim of this review to present the theories of polymer crystal growth and to compare their different assumptions. We have stressed their strengths and failings and tried to show how these are related to the underlying models. However, it is left to the reader to judge for him/herself the correctness or otherwise in each case. To conclude we first make some cautionary remarks, then summarize the models and finally suggest avenues for future research. 

In the classical Lauritzen-Hoffman theory for the mechanism of polymer crystal growth [106], it is assumed that the observed lamellar thickness corresponds to those crystallites that happen to have the largest growth velocity. However, this picture is hard to reconcile with the experimental observation that the thickness of polyethylene single crystals can be modulated by varying the temperature at which they are grown [117,118]. In fact, simulations by Doye et al. [119,120] suggest that the observed lamellar thickness does... 

In order to record the dynamic polymer crystal growth process in-situ, two factors are significantly important. One is the use of a very high resolution technique. Such a technique can repeatedly record the same area without damage to or significant interactions with the sample. AFM has been proved to be a successful tool to fulfill this task. The other key factor is that the polymers must have an appropriate crystallization rate. It generally takes an AFM several minutes to produce an image. This requires that the polymer has a very slow crystallization rate. The crystallization rates of most semicrystalline polymers at room temperature are too fast. 

Polymer crystal growth is predominantly in the lateral direction, because folds and surface entanglements inhibit crystalliza- 4 don in the thickness direction. Neverthe-1 less, there is a considerable increase in the fold period behind the lamellar front during crystallization from the melt and, as we have j seen, polymers annealed above their crys-tallization temperature but below Tm also irreversibly thicken. Nevertheless, in most theories of secondary nucleation, the most i widely used being the theory of Lauritzen and Hoffman,28 it is assumed that once a part of a chain is added to the growing crystal, its. fold period remains unchanged. 

These approaches to crystal growth rely upon slow introduction of the component building-blocks, however, another approach is possible. The key issue in coordination polymer crystal growth is to slow the crystal formation process. This can be achieved by providing greater solubility of the building-blocks, or possibly small oligomers of... 

For many polymers, crystal growth can take place either from the melt or from dilute solution to yield single crystals. Crystal formations in polymers were studied intensively almost from the time of recognition of their existence in macromolecules. As a result, certain basic principles were established (1) The melt crystallization process is a first-order phase transition (see Section 1.4.5). (2) Crystallization from a molten polymer follows the general mathematical formulation for the kinetics of a phase change.Equilibrium conditions, however, are seldom if ever attained and complete crystallinity is not reached. 

C. J. Lee, Transport polymerization of gaseous intermediates and polymer crystals growth, J. 

Hu WB (2005) Molecular segregation in polymer melt crystallization simulation evidence and unified-scheme interpretation. Macromolecules 38 8712-8718 Hu WB, Cai T (2008) Regime transitions of polymer crystal growth rates molecular simulations and interpretation beyond Lauritzen-Hoffman model. Macromolecules 41 2049-2061 Jeziomy A (1971) Parameters characterizing the kinetics of the non-isothermal crystallization of poly(ethylene terephthalate) determined by DSC. Polymer 12 150-158 Johnson WA, Mehl RT (1939) Reaction kinetics in processes of nucleation and growth. Trans Am Inst Min Pet Eng 135 416-441... 

I.C. Sanchez and E.A. DiMarzio, Dilute solution theory of polymer crystal growth a kinetic theory of chain folding, J. Ghem. Phys. 55, 893-908 (1971). 

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. 

The regime-transition phenomena of polymer crystal growth have been well explained on the basis of the Lauritzen-Hoffman model [14,15]. Nevertheless, the assumptions about the details of secondary crystal nucleation can be replaced by the intramolecular crystal nucleation, without a substantial loss of semi-quantitative predictions about regime transitions. 

In a previous article [1] I considered from a personal perspective how in-situ real-space observation of the process of polymer crystal growth and melting can provide insights into the fundamentals of polymer crystallization at the molecular scale. There the aim was to concentrate on universal themes, and on how atomic force microscopy (AFM) could inform our understanding of free, unperturbed, crystal growth. At the same time, the impact of AFM... 

The recent development of video rate AFM has also been discussed and its first application to crystal melting has been shown. The possibility of obtaining images 1000 times faster than conventional AFM, with time resolution of milliseconds, should provide exciting new information in many areas of polymer crystal growth in the future. 

Polymer Crystal Growth derived from the expression... 

The changes in the rate are a consequence of the change in balance between free energy of the melt and that of the solid and its influence on the nucleation processes. The LH theory has been used extensively to analyse polymer crystal growth data and is able qualitatively to describe processes that occur in a number of polymer systems. Its success lies in its ability to describe the temperature dependence of both the initial crystal thickness (L ) and the linear growth rate (coj.). A large volume of data has been shown to fit the relationship ... 

---