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

Abstract

We perform Raman spectroscopic measurements on normal alkanes (CnH2n+2) to quantify the n dependence of 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 the alkanes studied that melt via a rotator phase (21 ≤ n ≤ 37), we find that conformational disorder can be quantified by the loss of NCCT mass fraction, which systematically decreases with increasing chain length. For those that melt directly via the crystal phase (n ≥ 40), we observe NCCT conformational mass fractions that are independent of chain length but whose disordered mass fraction increases with length. These complement prior IR measurements which measure disorder via gauche conformations, but have not been able to measure the mass fraction of this disorder as a function of n. An interesting feature of the three-state analysis when applied to alkanes is that the measured fraction of disordered chain conformations in the rotator phase of (10 to 30) % greatly exceeds the mass fraction of gauche bonds (1 to 7) % as measured from IR; we reconcile this difference through DFT calculations.

The rheo-Raman microscope: Simultaneous chemical, conformational, mechanical, and microstructural measures of soft materials

“…27,28 Our analysis of the spectra in the CH 2 twist region follows a similar protocol used to quantify consecutive trans and amorphous conformers in alkanes and polyethylenes. 17,29 The Raman spectra are fit using two Lorentzian peaks: a narrow peak of approximately 4 cm −1 FWHM centered at 1296 cm −1 and a broader peak of approximately 25 cm −1 FWHM at 1303 cm −1 . These fits are used to calculate the integrated area I of each peak.…”

Determining conformational order and crystallinity in polycaprolactone via Raman spectroscopy

“…Much of this work has been done on the characterization of alkanes and polyethylenes to understand the possible ordered chain sequences that occur prior to crystallization. [7, 8] Raman measurements of PCL have primarily focused on identifying peaks that occur in the semicrystalline state versus the melt to give qualitative indicators of crystallinity. [9] However, more quantitative measures of crystallinity are required for developing structure-property relationships.…”

“…Purely experimental identification of single-chain order occurs when chains undergo non-crystalline structural transitions, as with the crystal-rotator transition in alkanes. This transition can be observed and quantified via standard peak fitting techniques,[8] but multivariate analysis techniques such as principal component analysis (PCA) have also been identified as a way to identify peaks sensitive to conformational changes in melting alkanes. [19] PCA has also been used to identify potential hydrogen-bonded chain conformers in poly(acrylic acid) based on subtle changes in the carbonyl stretch region of the Raman spectrum.…”

Intermediate phases during solid to liquid transitions in long-chain n-alkanes

“…Recently, changes in the fingerprint region of the Raman spectra of the n -alkane CH 21 H 44 and of some n -alkanes with 21 < n < 60 during melting were studied. 22 , 23 …”