Controlling the solvent, surfactant and ion-regulated swelling and elastic response of anionically-modified poly(N-isopropylacrylamide)-based hybrids through simultaneous macromolecular crowding

Abstract

An investigation is presented on how the ion specificities of anionically-modified thermosensitive hybrid systems formed by covalently cross-linked chains in presence of macromolecular crowding agent are affected by ion-pairing interactions. The effects of divalent and monovalent cations on poly(N-isopropylacrylamide-co-methacrylic acid)/polyacrylamide gels having controlled physical/mechanical properties were quantitatively investigated. A systematic investigation on solvent-composition induced transition of thermosensitive hybrids was performed in water–organic solvent (acetone, methanol, ethanol, and 2-propanol) mixtures. The volume fraction of solvent at which swelling of hybrids is minimum in co-nonsolvency zone varied and based on hydrophobicity and carbon chain length of alcohol, the swelling followed decreasing order; methanol > ethanol >2-propanol. The increase in both salt concentration and PAAm content in the precursor solution resulted in “salting out” behavior. The swelling was higher in presence of Cl⁻ anions compared to SO₄²⁻ anions and followed decreasing order for cations: Na⁺ > K⁺ > Mg²⁺. Salt-treatment caused significant changes in elasticity and swelling, while the elasticity of hybrids was correlated with alteration of salt concentration upon swelling in presence of kosmotropic ions. Surfactants interacted with hybrid system strongly enough to change the macroscopic properties. The swelling was lowest in cationic cetyl trimethylammonium bromide and cetyltrimethylammonium chloride solutions, while a greater swelling was observed in anionic sodium dodecylsulfonate solutions. A perspective for the molecular-specific nature of anionically modified poly(N-alkylacrylamide)-based hybrids is presented for potential applications to evaluate a wide range of surfactant, solvent and salt interactions.