Characterization of transition temperatures of a Langmuir-Blodgett film of poly(tert-butyl methacrylate) by two-dimensional correlation spectroscopy and principal component analysis

Abstract

External reflection FT-IR spectra of a Langmuir-Blodgett (LB) film of poly(tert-butyl methacrylate) (PtBMA) were measured at temperatures ranging from 26 to 136°C. The glass transition temperature (Tg) was determined from a two-dimensional (2D) mapping of the first derivative spectra of absorbance values against temperature over the wavenumber range 1100-1300 cm-1, which contains spectral features that are very sensitive to conformational changes. This mapping provides a surprisingly simple and direct method for detecting the value of Tg. The glass transition temperature determined from the 2D map was approximately 84°C. Another transition at 103°C, corresponding to the glass transition temperature of bulk PtBMA, was also detected from the 2D map. Principal component analysis (PCA) was employed to analyze the temperature-dependent FT-IR spectra. The glass transition temperatures (80°C; 100°C) of the PtBMA LB film determined by the score plot of PCA are consistent with those determined by the 2D map. Additionally, the loading vectors of PCA were found to give valuable insight into the molecular-level phenomena associated with the glass transition process. To gain more details about the polymer chain mobility, two-dimensional (2D) correlation analysis was performed on two sets of FT-IR spectra collected above and below Tg. In the synchronous 2D correlation spectrum obtained below the glass transition temperature (26-66°C), the observation that the strongest intensity change occurs at 1137 cm-1 indicates that the reorganization of the bending mode of the bbC-C-O and bbC-C=O bonds connected to the backbone (bb) chain and coupled to the C-O stretching mode of the tert-butoxy group is potentially the mechanism underlying the β-transition. This result is in good agreement with the presence of a transition (β-transition) at approximately 43°C obtained from the band around at 1137 cm-1 in the 2D mapping data.