Rheological and photophysical studies of environmentally-responsive associations in aqueous solutions of water-soluble polyelectrolytes
Reported in this study was the synthesis of amphiphilic water-soluble polymers capable of forming organized hydrophobic microdomains in aqueous media. These materials were obtained by free radical copolymerization of acrylamide with the surface-active monomer sodium 11-(acrylamido)undecanoate (SA). Incorporation of fluorescent, hydrophobic monomers into the polymer structure was achieved by the free radical synthesis of terpolymers of acrylamide, sodium 11-(acrylamido)undecanoate, and ((1-naphthyl)methyl) acrylamide or (2-(1-pyrenylsulfonamido)ethyl) acrylamide. Incorporation of the fluorescent monomer (2-(1-pyrenylsulfonamido)ethyl) acrylamide yields a terpolymer with intermolecularly associative solution properties. Steady-state fluorescence emission studies reveal significant constriction of the polymer chain as pH decreases or electrolyte concentration increases. As a result, the nature of pyrene-pyrene association can be varied from inter- to intramolecular as electrolyte or acid concentration increases. Fluorescence quenching studies indicate that the salt-induced chain contraction may enhance the organization of mixed polymeric micelles formed by SA and pyrene repeat units. The presence of a pH and salt inter-/intramolecular "trigger" has also been verified by nonradiative energy transfer studies of mixed solutions of naphthalene- and pyrene-labeled AMSA copolymers. Hydrophobically modified, water-soluble terpolymers based on maleic anhydride and ethyl vinyl ether were evaluated for their ability to act as a host for hydrophobic molecules in water. The degree of naphthalene sequestration in solutions of hydrophobically modified poly (sodium maleate-alt-ethyl vinyl ether) was found to depend on hydrophobic modification, electrolyte concentration, and pH. As octyl substitution increased from 10 to 50 mole %, micellar domains formed by intrapolymer hydrophobic associations became more compact, and mobility constraints within micelles hindered phase transfer. Increasing sodium chloride concentration and decreasing pH also enhanced micellar organization. The presence of hydrophobic groups on the micellar surface in some cases was postulated to account for enhanced recruitment of naphthalene into hydrophobic domains. The proposed model of association involves the interaction of naphthalene with unmicellized polymer hydrophobic groups. Data from these studies are discussed in relation to previous viscosity and fluorescence studies that confirm the conformational changes driving the transition from an extended polyelectrolyte to a collapsed, globular structure.