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Raman analysis of bond conformations in the rotator and premelting states of normal alkanes

Published

Author(s)

Anthony P. Kotula, Angela R. Hight Walker, Kalman D. Migler

Abstract

We perform Raman spectroscopic measurements on normal alkanes (CnH2n+2) with 21 {less than or equal to} n {less than or equal to} 60 with a specific focus on the conformational disorder that occurs below the melt temperature. We employ a three-state spectral analysis method originally developed for semi-crystalline polyethylene that posits crystalline, amorphous and non-crystalline consecutive trans (NCCT) conformations to extract their respective mass fractions. For 21 {less than or equal to} n {less than or equal to} 37, we observe that the NCCT mass fraction systematically decreases in the rotator phase with increasing temperature, as expected, but interestingly also with n, which provides a new insight into the disappearance of the stable rotator phase at high n. For n ≥ 40 we observe that NCCT conformations in the crystalline phase directly precede melting into the liquid phase. However, the Raman determined amorphous mass fraction of (10 to 30)% in the rotator phase greatly exceeds the IR determined mass fraction of gauche bonds (1 to 7)%. We reconcile this difference through ab-initio calculations of Raman spectra on prescribed chain conformations, which quantitatively explain the difference between measures of gauche bond mass fraction (from IR) and amorphous mass fraction (from Raman) and provides a theoretical justification for our analysis method. The Raman and IR analysis can then be combined to provide improved measures of the T and n dependence of gauche defects, which is critical for the development of theoretical and computational models.
Citation
Soft Matter
Volume
12
Issue
22

Keywords

Raman, alkane, rotator, gauche

Citation

Kotula, A. , Hight, A. and Migler, K. (2016), Raman analysis of bond conformations in the rotator and premelting states of normal alkanes, Soft Matter, [online], https://doi.org/10.1039/C6SM00182C (Accessed October 26, 2021)
Created May 9, 2016, Updated November 10, 2018