NEUTRON AND X-RAY-SCATTERING STUDIES OF MODEL HYDROPHOBICALLY END-CAPPED POLY(ETHYLENE OXIDE) - AQUEOUS-SOLUTIONS AT REST AND UNDER SHEAR

A series of model hydrophobically end-capped poly(ethylene oxide) deri vatives was prepared in which the molecular weight of the main chain r anged between 2000 and 35000 and with paraffinic extremities of 12 or 18 carbon atoms. The structure of the aqueous solutions of these polym ers was investigated by neutron and X-ray scattering techniques in the whole range of composition. Phase diagrams were deduced from these st udies and complementary differential scanning calorimetry measurements . They exhibit (i) several monophasic domains corresponding to a homog eneous dispersed solution, a micellar solution and a micellar cubic ph ase, and (ii) several biphasic domains corresponding to a demixing bet ween a lamellar and a cubic phase at higher polymer concentration and a demixing between a dispersed solution and a micellar solution, at lo wer polymer concentration. The micellar solutions are locally organize d in a cubic array and correspond to a classical associative behavior. Transitions from the micellar solutions to the cubic phase are observ ed when temperature and molecular weight decrease. Both phases are com patible with the formation of micelle-like hydrophobic micro-domains o f spherical shape whose radius increases with concentration. The avera ge distance d between the micelles decreases when concentration increa ses. d is larger than the end to end distance of the PEO chain at low concentration and for high molecular weight and tends, at higher conce ntration, toward the same value for all the polymers, when plotted vs. the hydrophobic content. The concentration dependence of the viscosit y was studied in the micellar solution domain for two different polyme rs. Important differences are observed between polymers of the same mo lecular weight bearing end groups of 12 or 18 carbon atoms. Several co ncentration regimes can be distinguished. These solutions are mainly s hear-thinning and the shear rate threshold is much higher for polymers bearing C12 end groups. In a narrow range of concentration, they also exhibit a shear-thickening behavior, prior to the shear-thinning. Pre liminary neutron scattering experiments were performed on solutions wh ich exhibit Newtonian or shear-thickening and shear-thinning behaviors as a function of the shear rate. No modification of the scattering pa tterns was observed in the Newtonian regime. Under specific experiment al conditions, a phase separation is observed during the shear-thinnin g step.