The phase transition temperatures of a liquid crystal determined from FT-IR spectra explored by principal component analysis

The FT-IR spectra of a thin layer of pure 4-chloro-2′-hydroxy- 4′-pentyloxyazobenzene (CHPAB) were studied as a function of temperature. A detailed analysis of the intensity variations was performed by a method based on principal component analysis (PCA). It was shown that the phase transition temperatures obtained by means of PCA and those determined by differential scanning calorimetry (DSC), the most widely used technique in the field, were nearly identical. The PCA results revealed that the transition from solid to a liquid crystalline (LC) phase (smectic A) is more drastic phase transition in terms of infrared absorption changes. The nematic to isotropic phase transition is much less infrared sensitive. Very much smaller absorption changes are associated with the transition between the smectic and nematic mesophases. The pattern of the intensity changes strictly is correlated with the orientation of the CHPAB molecules towards the surface windows due to the surface-induced homeotropic alignment of LC molecules. The important role of hydrogen bonding interaction on the observed transition is disclosed.