Solvent diffusion in the presence of semipenetrable, rodlike polymers

Abstract

The translational self-diffusion coefficient of a solvent (D) in two rodlike polymer/solvent systems, poly(benzy1 glutamate) in dimethylformamide (PBLG/DMF) and poly(n-butyl isocyanate) in benzene (PBIC/benzene), was measured by pulsed gradient spin echo nuclear magnetic resonance. D decreases monotonically with polymer concentration at all temperatures and polymer rod lengths studied. DMF diffusion was independent of PBLG molecular weight and dropped more than a factor of 5 as 50 vol 7% polymer was approached. Using known properties of PBLG, including the mixing of its sidechains with solvent, the obstruction of DMF diffusion by the presence of PBLG could be accounted for using the solvent penetrable prolate ellipsoid model of Jonsson et al. (Colloid Polym. Sci. 1986, 264, 77). Benzene diffusion in the presence of PBIC can also be understood using the same model. The obstruction to solvent diffusion by 'fuzzy rods" was compared to that observed for random coil polymers. We show that in the dilute and semidilute regions this approach can also model experimental results with only a modest reduction of solvent diffusion coefficient within the domain of the polymer molecule. IntroductionThe translational diffusion of small molecules in simple liquids has been studied for many years. Self-diffusion measurements, those not involving a concentration gradient, are typically made by isotopic tracer1 or pulsed gradient spin echo nuclear magnetic r e s o n a n~e~,~ measurements. When a second component becomes of colloidal dimensions and is impenetrable, it is effectively stationary with respect to the motion of the solvent and presents simply an obstruction about which the solvent must diffuse. If the solvent diffusion is followed for distances that are long compared to the dimensions of the obstruction, the observed diffusion coefficient will be reduced, dependent on the volume fraction and geometry of the obstruction. In addition to obstruction, dispersed particles may have specific interactions with the solvent, e.g., hydration of ionic groups or hydrogen bonding, or the solvent may have a limited though finite solubility in the particle. Each of these will have an effect on the solvent diffusion and frequently is the reason for solvent self-diffusion ~tudies.l.~-~ A number of models have been presented which may be used to correlate the concentration and temperature dependence of the diffusion coefficient of the s o l~e n t .~-'~ One of the most clear-cut experimental systems is that of charge-stabilized spheroidal latex particles, which may be produced with diameters ranging from hundreds of angstroms to tens of microns. In aqueous dispersions of poly(methy1 methacrylate) latex, agreement with predictions is achieved up to 20 vol % latex, with solvent diffusion falling off more rapidly than predicted at higher concentrations.1° Deviations are considered to be the result of solvent interaction with the latex. With polystyrene latex, where solvent uptake should be minimal, both latexI3 and solvent14J5 diffusion have be...

Comparison of various measurements of microscopic friction in polymer solutions

“…Earlier work by Róssler et al at much lower temperatures in PS/toluene solutions indicated a very broad distribution of solvent rotation times. 26 Both of these results are qualitatively consistent with our observation in the PS/THF system.…”