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 ΔT. The dependence of the slope on the molecular length l could be approximated by dG/d(ΔT) ∝ l^-8.5 for the three alkanes studied here. The morphological features, as well as the differences between growth and nucleation kinetics, are discussed.