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 association behavior, a series of terpolymers composed of acrylic acid, methacrylamide, and a DiC 12 AM, DiC 14 AM, or DiC 16 AM twin-tailed hydrophobic monomer have been synthesized using micellar polymerization methods. The terpolymer incorporating the DiC 12 AM hydrophobe possesses the highest initial viscosity and lowest critical yield stress in aqueous solution, while the terpolymer 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 (NRET) measurements. Dynamic frequency sweeps measured as a function of solution pH reveal viscoelastic behavior consistent with a simple Maxwell model. It is proposed that the number density of network junctions and the residency of hydrophobes in the junctions increase with pH. NRET measurements performed on mixed, separately labeled DiC 12 AM terpolymers indicate the onset of association ( C *) at ∼0.04 g/dL. Both NRET and steady shear viscosity studies indicate reorganization of the terpolymer network at pH values between 6.5 and 9.5, allowing for greater interaction of fluorescence labels and hydrophobes on separate chains.