Association Phenomena of Poly(acrylic acid)-b-poly(2-vinylpyridine)- b-poly(acrylic acid) Triblock Polyampholyte in Aqueous Solutions:  From Transient Network to Compact Micelles

An asymmetric model triblock copolymer of poly(acrylic acid)-b-poly(vinylpyridine)-b-poly(acrylic acid) (PAA-b-P2VP-b-PAA) was synthesized through anionic polymerization and explored in aqueous media. This block polyampholyte exhibits a rich phase behavior responding to pH changes. At low pH and low concentrations the polymer is molecularly dissolved. At pH close to 3.4 and above a critical concentration, a three-dimensional transient network is formed. The physical cross-links arise from electrostatic interactions among protonated positively charged P2VP units and negatively charged PAA units. In the intermediate pH region (4 < pH < 7) and around the isoelectric point, the polyampholyte is precipitated. The polymer is redissolved at higher pH, forming compact micelles with P2VP hydrophobic cores and extended PAA chains in the corona. The phase behavior of the system is reversible.