Descrição
A series of random terpolymers composed of N-isopropylacrylamide (NIPAAm), 2-acrylamido-2-methyl-1-propanesulfonic
acid (AMPS), and N-tert-butylacrylamide (NTBAAm) monomers were synthesized by free radical
polymerization. The molar fraction of the negatively charged monomer (AMPS) was maintained constant (0.05) for
all studied terpolymer compositions. Turbidity measurements were used to evaluate the influence of the relative amount
of NIPAAm and NTBAAm, polymer concentration, and solution ionic strength on the cloud point and redissolution
temperatures (macroscopic phase separation). Dynamic light scattering (DLS) was employed to elucidate some aspects
regarding the molecular scale mechanism of the temperature-induced phase separation and to determine the low critical
solution 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 turbidimetry
was found to be systematically much higher than the LCST determined by DLS; nanosized aggregates were observed at
temperatures between the LCST and the CPT. Both CPT and LCST decreased when increasing the molar ratio of
NTBAAm (increased hydrophobicity). It was found that above a critical molar fraction of NTBAAm (0.25-0.30) the
aggregation rate suddenly decreased. Polymers with NTBAAm content lower than 0.25 showed a fast macroscopic
phase separation, but the formed large aggregates are disaggregating during the cooling ramp at temperatures still
higher than the LCST. On the contrary, polymers withNTBAAmcontents above 0.30 showed a slow macroscopic phase
separation, and the formed large aggregates only redissolved when LCST was reached. These differences were explained
on the basis of a delicate balance between the electrostatic repulsion and the hydrophobic attractive forces, which
contribute cooperatively to the formation of metastable nanosized aggregates.
; Silva, Ricardo M. P. da 
