Molecular sieving by polymer solutions: dependence on particle and polymer size, independence of polymer entanglement.
The previous postulate of collision and displacement mechanisms of molecular sieving based on a biphasic plot of retardation coefficient vs particle radius ("R-plot") was extended and modified in four ways: i) A wider size range of particles and polymers confirmed the biphasic nature of the R-plot and, in addition, revealed a third mechanistic phase in the largest size range of particles and polymers which exhibits a positive slope in plots of retardation coefficient vs log (particle or polymer size), presumably denoting a collision mechanism. ii) Peaks of retardation in polyethyleneglycol (PEG) solutions were found with a particle M(r) of 10(7) independently of the M(r) of PEG, and with a PEG M(r) of 4 x 10(5) independently of particle M(r), showing that the retardation mechanism is not qualitatively a function of the particle/polymer size ratio as postulated previously, although quantitatively retardation is directly related to the size of particle and polymer. iii) Items i) and ii) were confirmed using band width in lieu of mobility measurement. iv) The entanglement threshold, c*, was found to decrease monotonically across the entire polymer size range in which the triphasic retardation takes place. Thus c* cannot be the sole cause for a non-monotonic change of retardation or normalized relative bandwidth with polymer size and particle size. Moreover, Ferguson plots across c* do not reflect it in any way.