Influence of Low-frequency Vibrational Modes on Polytypism of n-alkanes

Abstract

Crystal growth behaviors and thermodynamical properties were investigated for the two polytypic structures of n-alkanes. The solution crystallization of n-hexatriacontane (n-C₃₆H₇₄) were followed by oblique transmission micro-FTIR spectroscopy, which showed that a polytypic transformation took place during the crystallization. A single crystal appeared as single-layer polytype Mon and then a domain of double-layer polytype Orth II was generated heterogeneously. The crystal transformed gradually to a single crystal of Orth II through a solution-mediated phase transition. Inelastic neutron scattering measurements showed that the driving force of this transformation came from the difference in the vibrational density of state. From the frequency of methyl torsional mode, it is suggested that the cohesive force at the lamellar interface is stronger in Mon than in Orth II.