Two-dimensional FT-IR correlation analysis of the phase transitions in a liquid crystal, 4′- n-octyl-4-cyanobiphenyl (8CB)

Abstract

Generalized two-dimensional (2D)-IR correlation analysis has been applied to the temperature resolved IR spectra of 4′- n-octyl-4-cyanobiphenyl (8CB). The analysis revealed IR bands from different functional groups, i.e., CN, biphenyl ring and aliphatic tail that varied when the liquid crystal changed phase from isotropic to nematic or from nematic to smectic-A. When the entire data set is used, information from both phase transitions are encoded in the correlation analysis making unambiguous assignments impossible. A windowed autocorrelation analysis is applied and is able to locate transition points in the data. The data are then partitioned around a singular transition point and a generalized 2D-IR correlation analysis is performed. The correlation analysis from the partitioned data yields unambiguous correlation intensities from the isotropic to nematic phase transition and the nematic to smectic-A phase transition of 8CB.