Bulk and hydration solvent-cosolvent contribution to cononsolvency effect

Cononsolvency is a phenomenon that decreases the solubility of a polymer when two individually good solvents are mixed together at specific ratios, causing insolubility of macromolecules. It also finds applications in polymer science in developing new functional materials.
Poly (N-isopropyl acrylamide) (PNIPAM) is a well-known polymer that displays cononsolvency when mixed with numerous solvent-cosolvent mixtures, including water-methanol, water-ethanol, water-isopropanol, and urea mixtures. It is known that poly(N,N-isopropyl acrylamide) (PDEA) in water-alcohol mixtures does not display cononsolvency behavior, although PNIPAM and PDEA have similar molecular structures. Namely, the solubility of PNIPAM decreases as the cosolvent mole fraction increases up to 0.35 mole fraction of methanol; PDEA continuously increases the phase transition temperature as the alcohol amount increases in the mixture resulting in a salting-in behavior. Although such discrepancy has been investigated before, the molecular mechanism that drives the aggregation still needs to be discovered and supported with spectroscopic data.
In this study, Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectra of the collapsed forms of PNIPAM and PDEA were collected by heating the samples on ATR crystal above its phase transition, which gives the opportunity to see the interactions between polymer aggregates and solvent-cosolvent molecules. Additionally, Self-Modelling Curve Resolution (SMCR) algorithm was employed to extract the hydration shell spectra of the macromolecule in their collapsed forms. The results show that the hydration properties of PNIPAM and PDEA with these water-alcohol mixtures are significantly different, leading to the presence or absence of cononsolvency behavior. It was also revealed that there are different hydration behaviors below and above the solvent mixtures around 0.1 mole fraction of alcohol, which correlates with the phase transition trends. Additionally, it is known that above a certain mole fraction, the phase transition of PNIPAM-water-methanol mixtures starts to increase again; we demonstrated this also for ethanol and isopropanol cosolvents with UV-Vis measurements.