To examine the effects of microblock hydrophobicity on interpolymer association behavior, a series of terpolymers composed of acrylic acid, methacrylamide, and a DiC12AM, DiC14AM, or DiC16AM twin-tailed hydrophobic monomer have been synthesized using micellar polymerization methods. The terpolymer incorporating the DiC12AM hydrophobe possesses the highest initial viscosity and lowest critical yield stress in aqueous solution, while the terpolymer incorporating the DiC14AM 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 DiC12AM 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.

n-Octylamide-substituted copoly(sodium maleate-alt-ethyl vinyl ethers) [poly(SM-EVEs)] have been synthesized and fluorescently labeled with naphthyl and/or dansyl chromophores attached to the polymer backbone with either a long (C8) or short (C2) carbon spacer. The chromophores serve as monitors of the local environmental character of these amphiphilic macromolecules as they undergo the "polysoap-to-polyelectrolyte" transition. Intramolecular (closed) associations tend to predominate over a wide range of environmental conditions in aqueous media. The extent and reversibility of these interactions have been investigated by conducting viscometric measurements, light scattering, nonradiative energy transfer (NRET) quantum efficiency, and fluorescence quenching studies as a function of the solution pH. The results obtained for the 30% n-octylamide-modified poly(SM-EVE) in aqueous media have been contrasted with results obtained for 30% n-octylamide-modified poly(SM-EVE) in 6 M urea solution and unmodified poly(SM-EVE) in aqueous media to provide meaningful information concerning the effect of hydrophobic modification and pH responsiveness. The octylamide-modified polymer in aqueous media at low pH collapses into a compact aggregate due to the loss of polyelectrolyte character and an increase in hydrogen bonding. At moderate pH values, the modified polymer coil becomes slightly more expanded, as indicated by fluorescence and reduced viscosity measurements. At pH values between 7.0 and 8.0, the most expanded conformation is observed as determined by hydrodynamic size and nonradiative energy transfer (NRET) quantum efficiency measurements. NRET studies of both singlyand doubly-labeled, hydrophobically modified polymer solutions and fluorescence quenching in aqueous media are consistent with a globule-to-extended chain transition with increasing pH. At low pH, however, multichain complexes appear to form, even in dilute solution. At higher concentrations, above C*, multichain interactions are observed, consistent with associative thickening observed by viscometry. The onset values of C* in these systems as sensed by NRET are nearly one order of magnitude lower than those observed by viscometry. Also, the labels with long spacers are more sensitive than those with short spacers in detecting organized microdomains.

A series of terpolymers containing acrylic acid (AA), methacrylamide (MAM), and a twintailed hydrophobic monomer (DiC6AM, DiC8AM, or DiC10AM) were synthesized using micellar polymerization methods. These polymer systems were characterized using light scattering, viscometry, and fluorescence methods. Viscosity studies indicate that increasing the nonpolar character of the hydrophobic monomer (longer chain length or twin tailed vs single tailed) results in enhanced viscosity in aqueous solutions. The terpolymer with the highest hydrophobicity (MAM/AA/DiC 10AM terpolymer) exhibited the most pronounced viscosity increase with polymer concentration and thus the best associative thickening. Fluorescence nonradiative energy transfer measurements performed on this terpolymer indicate the onset of association (c*) at a concentration of ∼0.1 g/dL. Also, changes in the energy transfer efficiency and viscosity behavior as a function of pH are consistent with a concentration-dependent transition from closed to open associations.

Terpolymers composed of acrylic acid, methacrylamide, and DiC6, DiC8, or DiC10 twin-tailed hydrophobic monomers have been synthesized using micellar polymerization methods, and the interactions of these polymers with surfactants were investigated. These surfactants include sodium dodecyl sulfate (SDS), cetyl trimethylammonium bromide (CTAB), and Triton X-100. Viscosity measurements of DiC6AM and DiC8AM mixtures indicate little interaction with SDS, gelation with CTAB, and hemimicelle formation followed by polymer hydrophobe solubilization with Triton X-100. The DiC10AM terpolymer shows similar interaction behavior with CTAB and Triton X-100. However, the enhanced hydrophobic nature of the DiC10 polymer allows complex formation with SDS as confirmed by surface tensiometry. Fluorescence measurements performed on a dansyl-labeled DiC10AM terpolymer in the presence of increasing amounts of each of the surfactants indicate relative interaction strengths to be CTAB > Triton X-100 > SDS.

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