Phase-Specific Raman Analysis of n-Alkane Melting by Moving-Window Two-Dimensional Correlation Spectroscopy

We use moving-window two-dimensional correlation spectroscopy (MW-2DCOS) for phase-specific Raman analysis of the n-alkane (C₂₁H₄₄) during melting from the crystalline solid phase to the intermediate rotator phase and to the amorphous molten phase. In MW-2DCOS, individual peak-to-peak correlation analysis within a small subset of spectra provides both temperature-resolved and spectrally disentangled Raman assignments conducive to understanding phase-specific molecular interactions and chain configurations. We demonstrate that autocorrelation MW-2DCOS can determine the phase transition temperatures with a higher resolving power than commonly-used analysis methods including individual peak intensity analysis or principal component analysis. Besides the enhanced temperature resolving power, we demonstrate that asynchronous 2DCOS near the orthorhombic-to-rotator transition temperature can spectrally resolve the two overlapping peaks embedded in the Raman CH₂ twisting band in the orthorhombic phase, which had been only predicted but not observed due to thermal broadening near the melting temperature.