Specific interactions in polymer + CO₂ + cosolvent systems: experiment and modeling
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Systems in which there are strong specific interactions between the polymer and CO₂ are of interest in a number of applications including polymer foaming, coating and impregnation. Unfortunately, experimental data on the phase behavior of such systems are relatively scarce, as are models that explicitly consider specific interactions in such systems. The overall goal of this work was therefore to develop a method for the measurement of specific interactions in polymer + CO₂ systems and to apply such measurements to the development of a thermodynamic model for polymer solutions. This work demonstrates that in situ ATR-FTIR spectroscopy may be used to quantify specific interactions in CO₂ + polymer- systems that incorporate carbonyl, ether, siloxane and sulfone groups. However, carbonyl stretching frequencies cannot be used to quantify such interactions between CO₂ and carbonyl polymers, contrary to what has been suggested in the literature. This is because blue shifts in the carbonyl stretching frequencies were observed in the ATR-FTIR spectra of CO₂ + PVAc, CO₂ + PMMA, CO₂ + PLA, and CO₂ + PLGA85 systems. These CO₂ induced blue shifts can be attributed to dielectric effects, and therefore cannot be used to quantify specific interactions in these systems. We propose the use of the temperature dependence of the CO₂ bending mode to quantify specific interactions in CO₂ + carbonyl polymers. With this method, the enthalpies of association for C=O...CO₂ specific interactions were found to be between -7 and -10 kJ/mol in the order: CO₂ + PVAc > CO₂ + PCL ≈ CO₂ + PLA > CO₂ + PLGA85 > CO₂ + PMMA. The method was also extended to other CO₂ philic polymers, leading to enthalpies of association in the order CO₂ + PEG > CO₂ + PVAc > CO₂ + PSF > CO₂ + PMSSQ >> CO₂ + PVDF&PS. Specific interactions in polymer + CO₂systems were also investigated via NVT molecular dynamics simulations Such interactions were found to decrease in the order: CO₂...C-O-C > CO₂...O=C-O > CO₂...Si-O-Si. In addition, the association distance was identified to be 3.2 Å. Finally, CO₂ accessibility was found to decrease in the order PVAc > PVMK > PLA > PMA. It was also confirmed that 96 % of associated CO₂ molecules interact with one carbonyl group in these systems. A ternary extension of the Compressible Lattice Model (CLM) was developed and the enthalpy of specific interactions obtained from ATR_FTIR spectra was incorporated into the model to correlate and predict phase behavior in polymer + CO₂ + cosolvent systems. This work shows that model parameters obtained from binary data can be used to predict ternary system behavior with average absolute deviations between calculated and experimental values (AAD) less than 10%. The Sanchez-Lacombe lattice-fluid partition function was extended to associated systems by incorporating an association factor obtained from the Compressible Lattice Model. The resulting Associated Sanchez-Lacombe (ASL) EOS has the same form as the SL EOS, but includes the effects of specific interaction in the calculation of lattice energies, and chemical potentials. We demonstrate that ASL model parameters obtained from correlation of sorption equilibria can be used to predict swelling of polymers with AAD less than 10%. In summary, specific interactions between CO₂ and C=O and other CO₂-philic groups have been quantified using in situ ATR-FTIR spectroscopy. The results have been directly incorporated into a lattice model that is able to correlate cloud points, and sorption equilibria, using a single parameter. The model is therefore likely to be beneficial in many applications involving polymer + CO₂ or polymer + CO₂ + cosolvent systems including polymer impregnation, coating, foaming, and polymer membranes for CO₂ capture. An EOS formulation for the model has been derived for the calculation of swelling in these systems.