A study of the morphologies and growth kinetics of three monodisperse n-alkanes: C122H246, C162H326 and C246H494

Abstract

An optical and electron microscopic study has been made of three monodisperse n-alkanes crystallising as extended-chain lamellae. All showed similar morphologies at the same supercooling and the same progressive changes of texture with crystallization temperature. This is, with increasing supercooling, from a morphology composed essentially of individual lamellae arranged in parallel stacks and radiating from common nuclei through coarse, somewhat branched microstructures whose parallel stacks diverge, to finer, more-branched pseudo-spherulitic textures. Quenched, once-folded, C246 shows a random spherulitic texture with a dominant/subsidiary microstructure as for polymeric systems. The kinetics of extended-chain growth increase linearly with supercooling, as predicted by Hoffman, except for a local dip at the onset of lamellar branching. This new phenomenon occurs within one quantized state of lamellar thickness in contrast to ‘self-poisoning’ when once-folded forms give way to extended-chain crystallization. When dominant lamellae are no longer parallel but diverge, their mutual splaying angles increase with supercooling for all the three n-alkanes studied. The values are less, for the same supercooling, for the longer homologues with their thicker lamellae. This behaviour is as expected of transient ciliation due to the excess of molecular length over that of the secondary nucleus and thereby reinforces previous evidence demonstrating the responsibility of ciliation for spherulitic development in polymers.