Self-Assembly of a Thermally Responsive Double-Hydrophilic Copolymer in Ethanol Water Mixtures: The Effect of Preferential Adsorption and Co-Nonsolvency

Lower alcohols can induce a combined collapse-swelling de mixing transition (lower critical solution temperature (LCST)-type cononsolvency) in aqueous solutions of poly(N-isopropylacrylamide) (PNIPAM) by interacting with the polymer's amide groups. This interaction results in an increase of the total surface area of hydrophobic sites and destabilizes the chains. Here, we make use of this phenomenon to drive the counterintuitive self-assembly of a PNIPAM-containing double-hydrophilic graft copolymer in water ethanol mixtures at T << LCST. Rheological frequency sweeps are used to quantify the distinct solvation states of PNIPAM at various temperatures and ethanol concentrations. The energy stored through elastic deformation at the de-mixing transition is simply related to the solvent binding. We find that the storage modulus decreases progressively, but nonlinearly with ethanol concentration, which evidences a preferential solvation pattern. Analogously, through a combination of dynamic light scattering and transmission electron microscope analyses, we demonstrate that a low-temperature structure variation takes place by adding ethanol following a similar solvent-content morphology dependent model.