Use of associating polymers as multifunctional thickeners: studies of Their structure in aqueous solutions via nmr, qels, fluorescence, And rheology measurements

Abstract

The solution properties of an associating polymer were studied by NMR, quasi‐elastic light scattering (QELS), fluorescence, and rheology measurements. An associative thickening (AT) polymer was designed having a nonionic poly(ethylene oxide) backbone with long alkyl chains at both ends to achieve high viscosity even at relatively high salt concentrations and over a wide pH range. This study focuses on the associative state of the polymer in aqueous solutions at various polymer concentrations. In a fluorescence probe study using pyrene a spectral change in the I3/I1 ratio was observed for pyrene at a polymer concentration (Cp) of 3 x 10‐4%, indicating an apparent critical concentration (cmc) of the amphiphilic polymer. The viscosity, self‐diffusion coefficient (Dsel), and hydrodynamic size (Rh) distribution measurements at various Cp all suggest that there is a second transition at Cp≅ 0.4%. Although we observed the discontinuity in viscosity, Dsel, and Rh at Cp≅ 0.4%, no changes in the relaxation times (T1 and T2) were recognized for either the alkyl chain or the ethylene oxide moiety of the polymer at C p= 0.1–1%. These data suggest that there are no structural changes or phase transitions at Cp≅ 0.4%, but that intermicellar networks are presumably formed by bridging of the end alkyl groups of the polymer, which is driven by hydrophobic forces. Because the polymer forms networks by hydrophobic interaction and the polymer itself is nonionic, the viscosity of the polymer solution was influenced very little by either the addition of salt or a pH change, as would be expected. The dynamic viscoelastic study revealed that the polymer solution exhibits a single mode Maxwell type relaxation behavior with a terminal relaxation time of about 0.61 s, which imparts a unique flow appearance to the polymer solutions. The time course measurements of the dynamic elastic modulus of the stratum corneum revealed that the polymer has excellent potential for skin softening. It was concluded that the associative thickening polymer not only is a useful thickener with a salt and pH tolerance but also has beneficial skincare effects.