Dynamic of polymers, colloids and mixtures close to hard wall

Abstract

The main aim of this work is to study the interfacial dynamics of different soft matter systems such as polymer brushes, colloidal particles and mixtures. The main technique employed is the Evanescent Wave Dynamic Light Scattering, used to probe dynamics near a surface. A complete study on the collective dynamics of end-grafted polystyrene brushes in pure solvents, including good solvents and a theta solvent using the powerful technique of evanescent wave dynamic light scattering will be presented. Synthesis of a wide range of polymer brushes was carried out using the “grafting from” technique. Using different monomers such as polystyrene and polymethylmethacrylate and varying some parameters such as polymerization time and temperature, well-defined polymer brushes were achieved. All brushes had high molecular weight (106 g/mol) and different dry thickness, which were measured by ellipsometry as well as atomic force microscopy (AFM). Initially, we examine the dynamics of polymer brushes in good solvent, where the polymer brush is fully swollen and we had identified the theoretically predicted cooperative diffusion. The effect of grafting density in the dynamics was explored as well as the associated intensities. When the solvent quality changes from marginal to poor, the relaxation function was found to exhibit strong effect as compared with the smooth variation of the brush density profile. From a single exponential above 50°C, the correlation function became a two-step decay function. The fast decay was still assigned to the cooperative diffusion albeit slower than in the good solvent regime whereas the slow non-exponential and non-diffusive process might relate to microsegregated and/or chain dynamics in the present polydisperse brush. The relaxation function of the present three brushes with different grafting density revealed similarities and disparities between wet brushes and semidilute polymer solutions. Extending these studies on a polymer brush/particle system, we have shown how particle penetration in the brush can be measured and offer an estimate of the brush height and how different degree of penetration affects the measured surface diffusivities. It was shown that the presence of the brush affects the distribution of the particles, especially large hard sphere particles that were not able to penetrate the brush while smaller sized softer particles could partially penetrate the soft layer formed by the polymer brushes. The distribution of the particles was deduced from the penetration depth dependence of the scattering intensities. In addition, we further investigate the diffusivities of colloidal particles within swollen brushes, focusing on the effect of the brushes grafting density on both particle penetration and diffusivity. The diffusivities themselves of the colloidal particles were also an issue of investigation. Larger non-penetrating particles were seen to be slightly lubricated, the polymer brush providing a screening of the wall-particle hydrodynamic interactions. The smaller/softer penetrating particles lead to a more complex phenomenology. The slowing down appears to depend very strongly on the size of the particles. Finally, we use evanescent wave dynamic light scattering to investigate the Brownian motion of colloidal particles near a solid, planar surface. In the dilute regime, the diffusion coefficient near the interface was found to be much smaller than that for free diffusion in bulk solution. This was attributed due to the hydrodynamic interactions between the sphere and the solid wall, which slows down the motion. The wall-induced reduction of the self-diffusion was negligible at higher volume fractions due to interplay between the particle-wall and particle-particle hydrodynamic interactions. A simple model was proposed that captured the basic physical mechanism responsible for such behaviour, while a quantitative prediction of the weaker decay of the near-wall self-diffusion coefficient with volume fraction was offered by Stokesian Dynamics simulations. At high volume fractions, it seems that the particles exhibit similar behaviour either when in close proximity to another particle or to the wall. The same system was used to study the anisotropic diffusion of colloidal suspensions, varying the volume fraction of the colloidal suspensions, from the dilute to a concentrated regime. The near wall dynamics were compared to the associated bulk dynamics of the hard spheres and the main finding was that the parallel diffusivities in the concentrated suspensions behave similarly to the bulk dynamics in contrast with the perpendicular ones that are affected stronger by the presence of the wall.