Crystal thickness/ thickening

Thickening occurring at 78 °C from twice-folded (fold length 12 nm) to once-folded (fold length 18 nm) crystals of C294H590 and was accompanied by a complete loss of the initial crystal shape to a fine dendritic structure with 20-30 nm wide stripes of thickened material in a matrix of material with the original crystal thickness [51]. 

The use of different alkanes with different crystal thicknesses and numbers of folds within the crystal enables us to assert with confidence that the contraction in the lattice is due to the thickening process. The fact that the alkanes, due to their strict monodispersity, form crystals with only a few closely defined thicknesses enables this contraction to be seen much more clearly than it is in polyethylene. In polyethylene a range of different crystal thicknesses may be present, and the thickening process generally occurs over a wider range of temperatures, smearing out any step-like effect that may exist. 

This shows that by means of synchrotron radiation, for the first time the primary lamellar thickening on melt crystallization in polyethylene could be observed. This enabled also the determination of the true crystal thickness as a function of supercooling AT. Most satisfyingly it was found to be closely identical to that obtained in solution crystallization were no isothermal thickening takes place. In conclusion, the hitherto existing gap between melt and solution crystallization has been removed and the corresponding f versus AT curves brought in coincidence. 

The Thompson-Gibbs equation predicts a linear relationship between the melting point and the reciprocal of the crystal thickness (Figure 6.2). In practice, it is difficult to obtain the melting point and crystal thickness data, as the experiments involved in determining the melting point allow crystal thickening to occur. 

The logarithmic-time dependence of crystal thickness can be deduced easily. By assuming a frictional barrier (AfJ for chain-sliding diffusion proportional to the lamella thickness (/), thus the thickening rate of monolayer lamellar crystal under a certain temperature is... 

The crystal thickness can also be estimated by converting melting-point data through application of the Thompson—Gibbs equation (eq. (7.21)). This method is indirect and has to be calibrated. The non-equilibrium nature of polymer crystal melting is another major obstacle. Both crystal thickening and superheating have to be inhibited or at least controlled. 

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