Variable-Temperature Fourier Transform Infrared Spectroscopic Investigations of Poly(3-Hydroxyalkanoates) and Perturbation-Correlation Moving-Window Two-Dimensional Correlation Analysis. Part II: Study of Poly(ε-Caprolactone) Homopolymer and a Poly(3-Hydroxybutyrate)—Poly(ε-Caprolactone) Blend

In the present study, variable-temperature Fourier transform infrared (FT-IR) spectra of a poly(ε-caprolactone) (PCL) homopolymer and a poly(3-hydroxybutyrate) (PHB)-poly(ε-caprolactone) (PCL) blend were analyzed by generalized two-dimensional correlation spectroscopy (2DCOS) and perturbation-correlation moving-window two-dimensional (PCMW2D) correlation spectroscopy. The C=O stretching vibration bands of PCL and PHB were employed to explore the structural changes in the PCL homopolymer and the PHB–PCL blend during the heating process. For the melting of PCL homopolymer in the temperature range of 50 to 70 °C, we observed in the synchronous and asynchronous 2D correlation spectra one crystalline (1724 cm−1) and one amorphous (1737 cm−1) C=O stretching vibration band, which are also detectable in the one-dimensional FT-IR spectra. This result was also confirmed by PCMW2D correlation spectroscopy. During the heating process of the PHB–PCL blend in the temperature range 30–200 °C, the PCMW2D correlation analysis provided detailed information. Thus, in the synchronous PCMW2D correlation spectrum the melting of PCL was observed in the temperature region between 30 and 70 °C. The recrystallization of PHB in the blend in the temperature range 70–120 °C was accompanied by a shift of the C=O stretching band from higher wavenumber (1724 cm−1) corresponding to an imperfect crystalline state to the lower wavenumber (1721 cm−1) characteristic of a well-ordered crystalline state. In the temperature range 120–200 °C the melting process of PHB in the blend is captured by a sharp transition from the crystalline (1722 cm−1) to the amorphous (1747 cm−1) C=O stretching band.