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Crystals of the long, uniform alkane, C294H590 can be grown from dilute solutions in a range of different integer folded forms. At suitable temperatures, folded crystals will thicken from one folded form to another. The morphology of the crystals as they grow depends both on the growth temperature and on the number of folds in the crystal. In this paper we report, for the first time, observations on the morphologies of crystals that have been grown, and allowed to thicken to varying extents, in DSC pans. From the DSC we have precise knowledge of the overall degree of crystallinity and extent of thickening that has occurred over the whole sample. On thickening from twice-folded to once-folded crystals, the initial crystal shape is lost and a fine, dendritic structure results, in which the thickened regions exist in a surrounding matrix of unthickened material; the thickened regions have a characteristic width of 20–30 nm. It is suggested that this characteristic width is caused by the interplay of solid state diffusion and the lattice strain caused by the reduction in top surface area. At lower temperatures, when thickening occurs from three times folded to twice-folded forms, the initial lozenge shape of the three-times folded crystals is maintained after thickening, forming “picture-frame” crystals, but with a fine web of thickened material within the frame. |
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