Hydrophobic-electrostatic balance driving the LCST offset aggregation-redissolution behavior of N-alkylacrylamide-based ionic terpolymers

摘要

A series of random terpolymers composed of N-isopropylacrylamide (NIPAAm), 2-acrylamido-2-methyl-1-propanesulfonicacid (AMPS), and N-tert-butylacrylamide (NTBAAm) monomers were synthesized by free radicalpolymerization. The molar fraction of the negatively charged monomer (AMPS) was maintained constant (0.05) forall studied terpolymer compositions. Turbidity measurements were used to evaluate the influence of the relative amountof NIPAAm and NTBAAm, polymer concentration, and solution ionic strength on the cloud point and redissolutiontemperatures (macroscopic phase separation). Dynamic light scattering (DLS) was employed to elucidate some aspectsregarding the molecular scale mechanism of the temperature-induced phase separation and to determine the low criticalsolution temperature (LCST). The aqueous solutions of terpolymers remained clear at all studied temperatures;turbidity was only observed in the presence of NaCl. The cloud point temperature (CPT) determined by turbidimetrywas found to be systematically much higher than the LCST determined by DLS; nanosized aggregates were observed attemperatures between the LCST and the CPT. Both CPT and LCST decreased when increasing the molar ratio ofNTBAAm (increased hydrophobicity). It was found that above a critical molar fraction of NTBAAm (0.25-0.30) theaggregation rate suddenly decreased. Polymers with NTBAAm content lower than 0.25 showed a fast macroscopicphase separation, but the formed large aggregates are disaggregating during the cooling ramp at temperatures stillhigher than the LCST. On the contrary, polymers withNTBAAmcontents above 0.30 showed a slow macroscopic phaseseparation, and the formed large aggregates only redissolved when LCST was reached. These differences were explainedon the basis of a delicate balance between the electrostatic repulsion and the hydrophobic attractive forces, whichcontribute cooperatively to the formation of metastable nanosized aggregates.