Folded-chain nucleation

Sawada et al. [110] and the authors of this Chapter [104,111] have proposed another theory, the bundle-like nucleation theory, for the mechanism of ECC formation. Both groups of workers suggested that crystallization under high pressure starts from partially extended-chain nucleation rather than from the folded-chain nucleation as proposed by Hikosaka [103,104]. This theory was established on the basis of the following facts ... 

Finally we would like to draw attention to low molecular weight results and their analysis on the basis of surface nucleation theory. The theory was originally developed for infinitely long chains and cannot easily be applied to extended or once-folded chain crystallization. Therefore any discrepancies in this area would not be surprising and would not discredit the theory at higher molecular weights. 

Extended chain crystal (ECC) Folded chain crystal (FCC) Growth Growth rate Induction period Melt relaxation Molecular weight Nucleation Nucleation rate Nucleus Optical microscope (OM) Polyethylene Polymer Power law ... 

Figure 30 illustrates nine examples of the structures obtained in these simulations. It is clear that the chains group into crystallized kebabs on the shish surface. There are very few areas where the chains are partially or completely stretched under the influence of the shish template. The dominant mode of crystal nucleation on the shish is the growth of folded chains grouped into lamellar nuclei. Also, some of the chains do not join the central structure but drift away from it leaving a large gap on the shish between them. These simulations show clearly that the presence of the ordered template (the shish) influences the nucleation of lamellae and the formation of kebabs. 

It has been previously shown [76] that the rate of formation of homogeneously nucleated folded chain nuclei, I, is given by... 

Eagland et al. ( 3) propose a different scheme. The helix formation is compared to a first order reaction concerning only individual chains. In the first step the helices are nucleated and stabilized by the solvent. Next, the chains slowly fold back and the helical sequences associate by hydrogen bonds. Van der Waals interactions or entanglements between the folded chains are responsible for the gel gormation (see Figure 1-b). 

Once nucleated, crystallization proceeds with the growlh of folded chain ribbon-like crystallites called lamellae. The arrangement of polymer chains in... 

Ghosh, S.K. et al.. Power law of molecular weight of the nucleation rate of folded chain crystals of polyethylene, Macromolecules 35 (18), 6985-6991, 2002. 

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