Small-molecule rotational diffusion in polymer solutions

4 Small-molecule rotational diffusion in polymer solutions 

In addition to the substantial literature on solvent and small-molecule translational diffusion, there is also a significant literature on small-molecule rotational diffusion. Experimental methods that report rotational diffusion behavior include VH tight scattering, as examined in different time domains with Fabry-Perot interferometry and photon correlation methods, nuclear magnetic resonance, oscillatory electrical birefringence, and time-resolved optical spectroscopy. 

(38) and Rizos, et a/.(39) studied VH scattering of Aroclor 1248 in which polystyrene or polybutadiene had been dissolved. The fast relaxation observed with a Fabry-Perot interferometer has a weak concentration dependence. In very cold samples, the fast relaxation disappears, and the time constant of the remaining relaxation (observed with photon correlation spectroscopy) has an exponential dependence on c. The sign of the dependence depends on the polymer adding polystyrene increases the solvent relaxation time, while adding polybutadiene reduces the solvent relaxation time. 

Gisser and Ediger(41) studied solvent and solute rotation with C and nuclear magnetic resonance. The selectivity of NMR allows separate measurement of reorientation times for multiple components of a mixture. Dilute polystyrene, polyisoprene, and polybutadiene were found to retard the rotational diffusion of the toluene solvent, polystyrene being modestly more effective as a retardant. Solvent Tr depends exponentially on polymer c, at least up to 90 g/1 of polymer. Gisser and Ediger also examined the small-molecule mixture chloronaphthalene  

An alternative physical probe of solvent motion is oscillatory electrical birefringence (OEB), as applied to Aroclor 1248 solutions of polystyrene and polybutadiene by Morris, et a/. (43). In these systems, OEB gives a solvent relaxation time that depends on temperature and polymer concentration. Measurements of r at the 

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