Studies of cyclic and linear poly(dimethyl siloxanes):7. Diffusion behaviour in a poor solvent

Abstract

The diffusion coefficients ( D) of cyclic and linear poly(dimethylsiloxanes) (PDMS) have been measured in bromocyclohexane at 288 K and 301 K. Bromocyclohexane has previously been reported to be a θ-solvent for high molar mass linear PDMS at 301 K, but the hydrodynamic radii reported here apparently show the effects of molecular expansion at both temperatures. In addition, the hydrodynamic radii of both linear and cyclic PDMS are found to be insensitive to whether the solvent is toluene or bromocyclohexane. The ratio of friction coefficients f r f l for the ring (r) and linear (l) molecules of the same number of segments ( x) is in good agreement with the theoretical value of 8 3π in the impermeable limit and with the experimental value found previously in toluene solution. As x decreases the ratio f r f l tends to unity, illustrating the increasing importance of free-draining at low molar mass.