Dissociation and thermal characteristics of poly(acrylic acid) modified pluronic block copolymers in aqueous solution

The effect of salt on the dissociation behavior of a Pluronic-polyacrylic acid penta-block copolymer (P85PAA65) was examined. The average Gibbs energy corresponding to the energy for extracting a proton from P85PAA65 copolymer chains decreased with increasing NaCl concentration, suggesting that the addition of salt favors the dissociation process. A fluorescence study showed a two-step dissociation in which the formation of polymer complexes composed of a hydrophobic core surrounded by a hydrophilic shell was observed at pH 3.7, where the hydrodynamic radius (R_h) of the micellar structure increased from 145.6 to 302 nm due to coulombic repulsion of slightly charged PAA segments. As a consequence, the pyrene's first and the third vibronic peaks (I₁/I₃) ratio increased from 1.408 to 1.580, indicating a less hydrophobic or loose-coiled conformation compared to at a higher pH. At a pH of 7.22, the dissociation of the P85PAA65 copolymer was complete, yielding a dissociated polymer chain with apparent hydrodynamic radius (R_h^app) of about 3.2 nm. A reduction of the critical micelle temperature (CMT) attributed to the hydrogen bonding between Pluronic and PAA segments was observed.