Microstructure of dense colloid–polymer suspensions and gels

Abstract

A systematic experimental study of polymer-induced changes of the collectivestructure of model hard-sphere nanocolloids in the fluid and gel states has beencarried out using ultra-small-angle x-ray scattering. The focus is on small,non-adsorbing polymer depletants where a direct transition from the homogeneousfluid phase to a nonequilibrium gel state occurs with increasing polymeradditions. As the polymer concentration is increased in the homogeneous fluidphase, the low angle concentration fluctuations monotonically increase, thecharacteristic interparticle separation decreases and tends to saturate, andthe intensity of the cage order peak varies in a non-monotonic manner.These equilibrium structural changes depend in a systematic fashion oncolloid volume fraction and polymer–colloid size asymmetry, and are innear quantitative agreement with the parameter-free polymer referenceinteraction site model theory calculations. By combining the accurateequilibrium theory with experimental observations, the loss of ergodicityand nonequilibrium structure formation in the gel state can be deduced.Abrupt departures between theory and experiment on the ∼2–3particle diameter and greater length scales are observed as the gel boundary istraversed. The liquid-like local cage structure is arrested. Intermediate scalefluctuations are suppressed suggesting the formation of small, compact clusters.Large amplitude, Porod-like fluctuations emerge on large length scales due toquenched heterogeneities which are analysed using a random two-phase compositemodel. By combining the results of all the scattering experiments and theoreticalcalculations a qualitative real space picture of the gel microstructure isconstructed, and its mechanical consequences are qualitatively discussed.