Characterization of the thermal behavior of poly(hydroxybutylate) by principal component analysis-based two-dimensional correlation spectroscopy.

Principal component analysis-based two-dimensional (PCA2D) correlation spectroscopy, combined with the eigenvalue manipulating transformation (EMT) of a spectral data set, was applied to the temperature-dependent IR spectra of poly(hydroxybutylate) (PHB). In asynchronous PCA2D correlation spectrum, we clearly captured the existence of two components in the crystalline band of the C=O stretching mode, well-ordered primary crystals observed at lower wavenumber and less ordered secondary crystals observed at higher wavenumber, which is not readily detectable in the 1D spectra. By lowering the power of a set of eigenvalues associated with the original data, subtle differences in the thermal responses of PHB, which are difficult to observe even by conventional 2D correlation analysis, are revealed. When the contributions from minor factors are enhanced by eigenvalue manipulating transformation, intensities of bands assignable to the amorphous component of PHB are accentuated more in the C-O-C stretching region, while the intensities of bands assignable to the crystalline component become most prominent in the CH stretching region. However, the 2D correlation between CH and C=O stretching region reveals that the spectral intensity change of the CH(3) stretching bands at 2975 cm(-1) contains a component due to the amorphous contribution.