Epitaxy Involving Fold Surfaces of Polymer Crystals

43 Epitaxy Involving Fold Surfaces of Polymer Crystals 

AU epitaxial crystallization examples considered so far deal with contact planes that are parallel to the chain axis. They are the most frequent and indeed also the most relevant ones, both scientifically and from a technological viewpoint. However, for the sake of completeness, it is necessary to mention epitaxies that involve planes of type hkl, and most prominently the folds planes. 

As indicated, epitaxial crystallization implies contact planes of the substrate and deposit that are crystallographic. In polymers, only hkO planes should be considered since they are parallel to the chain axis. By contrast, planes of type hkl cut the chain axis, which does not generate an exposed, free contact face, since the chains continue on both sides of the plane, possibly with a change in orientation, as in kink-bands. However, exposed planes of type hkl exist in polymer crystals, as they are associated with chain folding. This lamellar folds surface is by necessity disordered or, at least, not crystallographically ordered. A priori, the folds surface would not be thought of as a contact plane in any epitaxial crystallization. However, a few examples of epitaxies that involve the end, fold surface of polymer crystals have been reported. 

A somewhat similar conclusion can be reached in a different experiment. As indicated earlier, it is possible to vaporize, for example, PE. The vapors condensed and deposited on a cold substrate crystallize under very harsh conditions, well below the temperature of homogeneous nucleation ( 80°C). When the PE vapors are deposited on the fold surface of PE single crystals (or 

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