Water-soluble polymers. 80. Rheological and photophysical studies of pH-responsive terpolymers containing hydrophobic twin-tailed acrylamide monomers

To examine the effects of microblock hydrophobicity on interpolymer associa tion behavior, a series of terpolymers composed of acrylic acid, methacryla mide, and a DiC(12)AM, DiC(14)AM, or DiC(16)AM twin-tailed hydrophobic mono mer have been synthesized using micellar polymerization methods, The terpol ymer incorporating the DiC(12)AM hydrophobe possesses the highest initial v iscosity and lowest critical yield stress in aqueous solution, while the te rpolymer incorporating the DiC(14)AM hydrophobic monomer showed a slightly lower initial viscosity and critical yield stress. The pH response of these terpolymers was examined via rheology and fluorescence energy transfer (NR ET) measurements. Dynamic frequency sweeps measured as a function of soluti on pH reveal viscoelastic behavior consistent with a simple Maxwell model. It is proposed that the number density of network junctions and the residen cy of hydrophobes in the junctions increase with pH. NRET measurements perf ormed on mixed, separately labeled DiC(12)AM terpolymers indicate the onset of association (C*) at similar to0.04 g/dL. Both NRET and steady shear vis cosity studies indicate reorganization of the terpolymer network at pH valu es between 6.5 and 9.5, allowing for greater interaction of fluorescence la bels and hydrophobes on separate chains.