COMPARISON OF DIFFUSION-COEFFICIENTS OBTAINED FROM TERNARY POLYMER-SOLUTIONS USING DYNAMIC LIGHT-SCATTERING AND ULTRA CENTRIFUGATION

Dynamic light scattering has been used to measure the interpenetration diffusion coeffient of ternary polymer solutions formed with 233 000 dalton polystyrene and a trace amount of 330 000 dalton PMMA dissolved in thiophenol. These measurements have been compared with the diffusi on coefficients obtained from ultracentrifuge studies of the same solu tions by observing the change in shape of the sedimentation boundary c urve with time. This novel use of ultra centrifugation to measure diff usion coefficients in ternary polymer solutions is possible because; ( i) the solvent is isorefractive and isopycnic with polystyrene, and (i i) the PMMA was the minority polymer in the solutions studied (concent ration 0.001 g g(-1)). Hence only the PMMA was visible and it sediment ed through an effective solvent of thiophenol and polystyrene. All mea surements were made at 25 degrees C. The diffusion coefficients obtain ed from the ultracentrifuge observations are in good agreement with th e interpenetration diffusion coefficients obtained from dynamic light scattering studies on the same solutions. Solutions formed at polymer concentrations greater than 0.1 g g(-1) suffered phase separation sinc e PMMA and polystyrene are incompatible polymers. The solution with po lymer concentration 0.0755 g g(-1) was too unstable for ultra centrifu gation studies, but was susceptible to investigation by dynamic light scattering. The interpenetration diffusion coefficient of this solutio n was found to be much smaller than those of the more dilute solutions . This is consistent with the prediction that the interpenetration dif fusion coefficient approaches zero as phase separation is approached.