Effects of polymorphism on the Raman-active longitudinal acoustical mode frequencies of n-paraffins

Abstract

Raman spectroscopicmeasurements on the orthorhombic and monoclinic crystalline forms of n‐C36H74 and n‐C94H190 show that the frequencies of the longitudinal acoustical modes depend on the orientation of the chain axis relative to the planes containing the terminal methyl groups as well as the chain length. The frequency differences between the orthorhombic and monoclinic forms of a given n‐paraffin depend on the magnitude and direction of the regular stagger between neighboring end groups in the monoclinic forms. It is concluded that these frequency differences are most likely associated with variations in end group packing associated with differences in crystalline form. The implications of these observations to the use of Raman spectroscopic data to characterize the morphology of chain‐folded polyethylene crystals are discussed.