Structure And Dynamics Of Charged Colloidal Disks In Colloid-Polymer Mixtures

Book excerpt: Complex fluid mixtures of colloids and polymers are extensively used in several conventional and emerging technological applications. Particles self-assemble under different conditions to form colloidal glasses and gels and it often leads to the development of unusual viscoelastic features. In the case of aspherical particles, shape anisotropy and physical aging effects add to the existing complexities so the implementation of a strategic formulation method to improve performance and stability remains a critical challenge. This thesis presents a comprehensive analysis of particle interactions in mixtures of charged disk-shaped colloids and weakly-adsorbing polymers like poly(ethylene oxide) (PEO). Here, we discuss the behavior of suspensions containing laponite® and PEO of molecular weights (Mw) varying between 4.6 and 300 kg/mol and at concentrations within the dilute through concentrated regimes. Techniques such as rheology, light (DLS) and x-ray scattering (XPCS and USAXS) were used to understand and characterize the effect of chain number and length on the macroscopic behavior, microstructure and dynamics of particles in these colloid-polymer mixtures. Laponite® suspensions gradually transition from a homogeneous fluid to a structurally arrested phase. With the addition of polymers, rheological measurements show that in addition to the typical re-entrant behavior observed in the dilute polymer phase, there is another onset of stabilization close to the semi dilute polymer regime. While DLS and XPCS results show three different regimes in the microstructural dynamics along these transitions, USAXS measurements indicate the presence of only finite sized ellipsoidal structures that are directed by polymer-particle interactions. We believe that the arrested phase is a glassy system as no large scale structure is observed. On the other hand, adding high Mw PEO results in the formation of strong colloidal gels where laponite® particles act as junctions in the colloid-polymer network. Nanometer to micron-sized clusters form with the addition of PEO chains larger than the minimum required for polymer-clay bridging. Increasing the concentration of PEO changes the density of clusters and this directly affects the bulk elasticity of the material. The results thus serve as an excellent benchmark to understand how to effectively formulate an anisotropic colloid-polymer mixture for an application.