Water-soluble copolymers .65. Environmentally responsive associations probed by nonradiative energy transfer studies of naphthalene and pyrene-labeled poly(acrylamide-co-sodium 11-(acrylamido)undecanoate)

The synthesis and photophysical characterization of terpolymers of acrylamide (AM) with the surface-active monomer sodium 11-(acrylamido)undecanoate (SA) and either [(1-naphthyl)methyl]acrylamide or [2-(1-pyrenesulfonamido)ethyl]acrylamide are reported. The pronounced spectral overlap between the excited state of the naphthylmethyl group and the ground state of the pyrenesulfonamido group allows analysis by nonradiative energy transfer (NRET) studies. Pyrene-labeled AM/SA copolymers exhibit strong associative behavior in aqueous solution, as evidenced by the viscosity profile as a function of polymer concentration. The concurrent increase in excimer emission with polymer concentration is reflective of interpolymer association between pyrene groups to form the dimeric excited state. NRET analysis of mixed solutions of naphthalene (NRET donor)- and pyrene (NRET acceptor)-labeled copolymers indicates sensitization of pyrene emission by naphthalene labels. The enhancement in acceptor emission at low polymer concentrations implies that intermolecular associations occur at polymer concentrations where association-driven viscosification is not observed. Open, intermolecular associations and interpolymer NRET are controllable by pH and electrolyte concentration. As ionic strength increases or pH decreases, open associations are broken up. This response had previously been reported as a macroscopic phenomenon by viscosity changes at higher polymer concentrations, but NRET studies confirm that the pH and salt responsiveness inherent to AMSA-based materials is also a molecular-level event.