Scattering studies of the structure of colloid-polymer suspensions and gels

Depletion-driven changes in the structure of hard-sphere particles (radius R) mixed with a nonadsorbing polymer (radius of gyration R_g) dissolved in good (athermal) and ideal (theta) solvents are systematically studied. Colloidal structure factors, S(q), are determined using slit-smeared and pinhole-collimated ultra-small-angle X-ray scattering and small-angle neutron scattering. A comparison of the structure factors extracted from the three methods demonstrates the validity of the available desmearing algorithms. Polymer additives alter the colloidal structure more for larger particle volume fractions (φ_c) and smaller size asymmetry ratios R_g/R. At fixed φ_c ∼ 0.40 and R_g/R = 0.6, increasing the reduced polymer concentration (c_p/c_p*) results in a monotonic shift to higher wavevectors of the location of the first peak in the structure factor, q*, and a nonmonotonic variation of the cage order parameter, S(q*), in a nearly solvent quality independent manner. Local structural correlations arrest as the gel state is entered. The reduced polymer concentration required for gelation is smaller in athermal solvents compared to its theta analogue and, in both cases, is well below the fluid-fluid spinodal boundaries. Comparisons between the measured structure factors and no-adjustable-parameter predictions of the polymer reference interaction site model theory shows near quantitative agreement over all wavevectors. When the gel phase is entered, strong differences between the theory and the experiment emerge, indicating the nonequilibrium nature of structural correlations in the nonergodic gel. Relative to equilibrium expectations, enhanced (reduced) fluctuations occur at small (intermediate) wavevectors. The combined experimental and theoretical results suggest that neither long wavelength fluctuations nor the local cage structure are the primary origin of the gelation transition.