Microstructure of colloid-polymer mixtures containing charged colloidal disks and weakly-adsorbing polymers

Structural arrest in an aging colloid-polymer mixture of charged colloidal disks and weakly interacting polymers is primarily governed by the spatial arrangement of microstructural domains in the system. Here we present a detailed ultra-small angle x-ray scattering (USAXS) study of the polymer induced microstructural changes occurring in systems containing laponite^® and poly(ethylene oxide) (PEO). Laponite^® suspensions are known to transition from a homogeneous fluid to an arrested phase as a function of time. USAXS results show that when polymer chains are added to this colloidal system, the microstructural changes are found to depend on (i) polymer-colloid size ratio, (ii) particle and polymer concentration. Large length scale structures (300–1000 nm) of non-uniformly varying size appear with increasing concentration of high molecular weight PEO (R_g/R ∼ 2.4). However with shorter PEO chains (R_g/R ∼ 0.5), we observe the formation of intermediary structures with a characteristic length scale of 50–60 nm. With increasing particle concentration, we observe a more compact structure for the high molecular weight PEO and no signature of a percolating network in the case of the low molecular PEO. We believe that these results add to our understanding of the complex aging behavior of clay-polymer systems.