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Conformational changes in polymer films exposed to high-pressure CO2have been investigated with Fourier transform infrared (FT-IR) spectroscopy. The experimental setup, based on a custom-made stainless steel optical cell with CaF2windows, allows measurements in a CO2environment for pressures up to 6 MPa, in a temperature range from 293 to 353 K and in the mid-infrared (1000–4000 cm–1). Poly(methyl methacrylate) (PMMA), a polymer with a side group (C-type), was studied to monitor the spectral changes as a function of CO2pressure and was compared to poly(d,l-lactic-co-glycolic acid) (PLGA), a polymer without a side group (B-type). By monitoring the characteristic carbonyl bands, conformational changes that occur due to molecular interactions between the high-pressure CO2and the polymers were explored at a constant pressurization rate (0.02 MPa/min) and temperature. Spectral changes are observed only for PMMA, where the vibrational band at 1680 cm–1disappears with increasing pressure. The spectra of PLGA do not show any significant change in the presence of high pressure CO2in the investigated range. The behavior of the absorbance peak as a function of pressure and temperature highlights the presence of dynamic cross-links (DCs) between the side groups of PMMA films obtained by solvent casting below the glass transition temperature of the polymer. The spectral features are correlated using a model that accounts for CO2diffusion and the relaxation kinetics of the polymer chains in the thin film. The disappearance of the vibrational band attributed to the DCs for PMMA is related to the glass transition temperature, and a retrograde vitrification phenomenon is observed. This approach can be considered a useful alternative to magnetic suspended balance for the study of polymer–gas systems. [ABSTRACT FROM AUTHOR]