Growth and nucleation rate minima in long n-alkanes

The isothermal melt crystallization of the long pure n-alkanes, C150H302, C198H398, and C246H494, was studied by phase contrast and polarized optical microscopy. A custom-built T-jump cell was used. The crystal growth rates of C150H302, C198H398, and C246H494, and the rate of primary nucleation of C246H494 were measured directly. Both the growth and nucleation rates of crystals of C246H494 pass through a pronounced maximum and minimum with increasing supercooling. This unusual finding is in line with our previous observations of qualitatively the same behaviour of the overall crystallization rate of the alkanes C246H494 and C198H398 from both melt and solution. The rate minimum coincides with the transition between extended and once-folded chain crystallization. The slow down in nucleation and growth of extended-chain crystals is thought to be due to interference by frequent but unstable chain-folded depositions (surface 'self-poisoning'). Due to the high crystallization rates and experimental limitations, measurements could not be made at supercoolings which were sufficiently large to observe the minimum in C150H302 and C198H398, although an indication of the maximum is noticed in C198H398. In the vicinity of the extended-chain melting point the crystal growth rate G increases linearly with the increasing supercooling Delta T. The dependence of the slope on the molecular length I could be approximated by dG/d(Delta T) proportional to l(-8.5) for the three alkanes studied here. The morphological features, as well as the differences between growth and nucleation kinetics, are discussed. (C) 1996 Elsevier Science Ltd.