The influence of poly(acrylic acid) on micellization and gelation characteristics of aqueous Pluronic F127 copolymer system

The influence of anionic homopolymer poly(acrylic acid) (PAA) of low (1800 g/mol) and high (450,000 g/mol) molecular weights on the micellization and gelation characteristics of aqueous Pluronic F127 (EO₉₇PO₆₉EO₉₇) triblock copolymer (with EO = ethylene oxide and PO = propylene oxide) was studied using differential scanning calorimetry, dynamic light scattering, spectrophotometry, tube inversion technique, and rheometry. It is found that the addition of PAA decreases the critical micellization temperature (CMT) of F127 due to their association via hydrogen bonding. This association becomes stronger at temperatures ≥CMT, arising from the increased affinity between the PO units and PAA. F127-PAA complexation results in lesser PO dehydration and smaller micellization enthalpy (∆H). With increasing PAA concentration, this effect becomes stronger and more F127 bound on PAA is present in the sample, eventually leading to vanishing ΔH. In the presence of PAA, gel region of F127 decreases with increasing PAA concentration due to the order disrupting effect of PAA on the arrangement of packed micelles. Both the micellization and gelation processes of F127 in the presence of PAA are thermoreversible. The PAA effects on both processes become considerably weak at high pH.