Inversion of the temperature dependence of crystallization rates due to onset of chain folding

Crystallization rate experiments performed on the uniform alkanes C₂₄₆H₄₉₄ and C₁₉₈H₃₉₈, by both differential scanning calorimetry and in situ X-ray diffraction (using a synchrotron source), have revealed that these rates, including both primary nucleation and growth, pass through a minimum with increasing supercooling. The first (expected) increase and subsequent (unsuspected) decrease correspond to extended-chain (E) crystallization, the renewed increase beyond the minimum corresponding to chain-folded crystallization with the fold period l being smaller than L but larger than L 2, where L is the extended chain length. The anomalously retarded crystallization with increasing supercooling, new even qualitatively, appears to arise through competition between extended- and folded-chain deposition. The attachment of folded chains evidently involves a much lower free energy barrier than does the attachment of extended chains. Even if the former process cannot lead to growth of stable chain-folded crystals above their melting temperature, it seriously hampers the only productive process, chain-extended crystallization. The observed effects, which have come to light owing to the availability of ultra-long and uniform n-alkanes, help to provide new insight into the primary stages of chain-folded crystallization, with many potential consequences, some of which are discussed.