Supercooling mechanisms, polymer crystal

Wagner, J. Phillips, P. J., The Mechanism of Crystallization of Linear Polyethylene, and Its Copolymers with Octene, Over a Wide Range of Supercoolings. Polymer 2001,42, 8999-9013. 

Annealing of semicrystalline polymers is a difficult process to understand. Both microscopic and macroscopic defects are reduced upon sample exposme to temperatures somewhat below the crystalline temperature. Polymer crystals are metastable, ie the lamellae thickness and lateral size are generally determined by the degree of supercooling balanced by the side and end free energies. Therefore, as a function of time, chain-folded polymer crystals thicken on annealing to minimize the number of folds. Although the exact mechanism has yet to be determined... 

Solid polymer and gel polymer electrolytes could be viewed as the special variation of the solution-type electrolyte. In the former, the solvents are polar macromolecules that dissolve salts, while, in the latter, only a small portion of high polymer is employed as the mechanical matrix, which is either soaked with or swollen by essentially the same liquid electrolytes. One exception exists molten salt (ionic liquid) electrolytes where no solvent is present and the dissociation of opposite ions is solely achieved by the thermal disintegration of the salt lattice (melting). Polymer electrolyte will be reviewed in section 8 ( Novel Electrolyte Systems ), although lithium ion technology based on gel polymer electrolytes has in fact entered the market and accounted for 4% of lithium ion cells manufactured in 2000. On the other hand, ionic liquid electrolytes will be omitted, due to both the limited literature concerning this topic and the fact that the application of ionic liquid electrolytes in lithium ion devices remains dubious. Since most of the ionic liquid systems are still in a supercooled state at ambient temperature, it is unlikely that the metastable liquid state could be maintained in an actual electrochemical device, wherein electrode materials would serve as effective nucleation sites for crystallization. 

The first exponential term represents the transport of molecules to the growing nucleus with U the activation energy for this process and (T - To) the temperature difference between the crystallization temperature T and the temperature at which backbone motions substantially cease. For most polymers To is about 50°C below the glass transition temperature. The second exponential term represents the work required to form a critical nucleus where TS, is the melting point of an infinitely thick lamellar crystal, AT is the supercooling and AH the heat of fusion. The term C contains various interfacial energy terms and depends upon the precise mechanism of the nucleation process. For homogeneous nucleation... 

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