In situ FTIR spectroscopic study of the effect of CO 2 sorption on H-bonding in PEG–PVP mixtures

Abstract

A study of the H-bonding between poly(ethylene glycol) (PEG) and polyvinylpyrrolidone (PVP) in the presence of supercritical carbon dioxide at various temperatures, pressures, different M w of PEG and PVP and different PEG/PVP ratios is presented. In situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy was used to investigate H-bonding by examining changes in the relative intensities and positions of peak maxima of the 2nd derivative ν(C O) bands associated with ‘free’ and H-bonded C O groups. In general, relative intensities of bands associated with H-bonded C O groups decreased upon CO 2 sorption and was accompanied by an increase in intensity of bands associated with ‘free’ C O groups. At the same time, these bands were shifted to higher wavenumbers. These shifts were attributed to the shielding effect of CO 2 molecules on H-bonding interactions between PEG and PVP. The magnitude of the effects of CO 2 shielding generally increased with decreasing polymer M w and increasing CO 2 content. However, upon CO 2 venting the extent of the H-bonding between PEG and PVP reappeared. The extent of H-bonding recovery was greatest for blends with low M w PEG ( M w : 4 × 10 2) and PVP ( M w : 9 × 10 3) and PEG content ≥0.54 wt% under mild conditions of pressure (80 bar) and temperature (35 °C). For the same low M w blends, increasing pressure to 150 bar, or temperature to 50 °C, showed poor H-bond recovery upon CO 2 venting. Overall, it was shown that supercritical CO 2-induced shielding of H-bonding interactions in polymer blends is reversible upon CO 2 venting, and the magnitude of both was influenced by processing conditions and blend composition.