Structural Comparison of Langmuir−Blodgett and Spin-Coated Films of Poly(tert-butyl methacrylate) by External Reflection FTIR Spectroscopy and Two-Dimensional Correlation Analysis

The structures of Langmuir−Blodgett (LB) and spin-coated films of poly(tert-butyl methacrylate) (PtBMA) were investigated by means of external reflection Fourier transform infrared (FTIR) spectroscopy and two-dimensional correlation analysis. In addition, the structural relaxation of the films during heating was studied. At room temperature, the intensity ratio of the band at 1728 cm^-1 (assigned to the CO stretching mode) and the band at 1147 cm^-1 (assigned to the C−O stretching modes of the tert-butoxy group) in the external reflection infrared spectrum of the LB film was greater than that in the spectrum of the spin-coated film. However, the values of this intensity ratio for the LB and spin-coated films converged with increasing temperature. The results suggest that the structural difference between the two types of film is primarily due to strain in the tert-butoxy group in the LB film. The same trend was observed in the intensity ratio of the bands at 1255 and 1274 cm^-1 assigned to the C−C−O stretching mode. Two-dimensional (2D) correlation analysis was applied to the external reflection FTIR spectral data of the LB and spin-coated films of PtBMA to analyze the relaxation of the films during heating. In the synchronous spectra of both film types, the strongest autopeak corresponded to the C−O stretching mode, appearing at 1151 and 1168 cm^-1 for the LB and spin-coated films, respectively. This suggests that the conformation of the tert-butoxy group changes significantly with increasing temperature. The intensity of the autopeak at 1151 cm^-1 in the synchronous 2D spectrum of the LB film is much higher than the intensities of the other autopeaks, indicating that the C−O group in the LB film undergoes mainly local reorientation, which in turn indicates that the tert-butoxy group is greatly strained in the room-temperature LB film. On the other hand, the tert-butoxy group of the spin-coated film is less strained than that of the LB film because the intensity of the autopeak at 1168 cm^-1 in the synchronous 2D spectrum of the spin-coated film is slightly higher than the intensities of the other autopeaks. Furthermore, the analysis of the signs of asynchronous cross-peaks revealed that the following sequence of spectral changes is induced as the temperature increases:  1131 (bending of ^bbC−C−O and ^bbC−CO) → [1393 (CH₃ bending of α-methyl), 1151 (C−O stretching)] cm^-1 and 1133 → 1393 → 1168 (C−O stretching) cm^-1 for the LB and spin-coated films, respectively. This suggests that in both film types the first event is associated with reorientation in both the chain backbone and the side chain. During heating, the α-methyl group relaxes along with the C−O bond in the LB film but prior to the C−O bond in the spin-coated film.