Dynamics of colloids in polymer solutions

• Main Author: Golz, Paul Michael
• Published: University of Edinburgh 1999
• Subjects: 541.33
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.651578

The dynamics of a dilute suspension of colloids diffusing in a simple liquid are well understood and are characterised by the Stokes-Einstein equation. However the validity of this equation for describing the diffusion of a colloid through a polymer solution has been questioned. In this work the motion of dilute poly-methylmethacrylate spheres diffusing through a solution of flexible polystyrene polymers has been studied. Dynamic light scattering was used to measure the self-diffusion of the spheres and the diffusion coefficient for this motion was found to exhibit time dependence. At short times the colloid moves faster than expected from a simple 'polymer solution as a continuum' assumption whereas at longer times this assumption appears valid. Solutions with polymer concentrations up to half the overlap concentration were investigated: the ratio of short to long time diffusion D^s/D^L was found to increase with concentration. The effect of changing the quality of the solvent from theta to good was also investigated and no qualitative difference was found. In these experiments it was found that the effect of scattering from the polymer was more significant that initially expected. In particular, the presence of cross scattering, in which the electric field scattered from the colloid is correlated with that from the polymer, can be as high as 30%, despite the polymer scattering being only a few percent. A method for both calculating and experimentally measuring the magnitude of the cross scattering has been described. A new experimental methodology is proposed to minimise the cross and polymer scattering, allowing for measurement of the colloid scattering alone.