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 (Rh) 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 (Rhapp) 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.

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