Polymer solution small-molecule rotation

Small-molecule rotational diffusion in polymer solutions... 

In a small molecule, rotational motion of a fluorophore in solution occurs on a ps timescale and the observed emission is thus averaged over all molecular orientations. In a synthetic polymer whole chain motion is in general very slow, but local motions may occur on the same timescale as fluorescence, and if these local motions, such as segmental motion, determine the rate of a process leading to emission, then the appropriate form of the rate expression for such motion must be employed, as suggested earlier [8]. 

The possibility of covalent interfullerene bond formation in solid and in some of its salts, results in a variety of dimers and polymers. Both the dimerization and the polymerization of Cgo are characteristic solid state reactions, up to now, neither of them were carried out in solution. In the fee lattice of C g the concentration of the reacting molecules is about 3 orders of magnitude higher than that in solution. The free rotation of the molecules allows for the geometrical conditions required by the transition state of the polymerization and the rigid polymer rods or sheets can form by a small rearrangement of the precursor structure. 

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 ... 

This chapter has examined the translational and rotational motion of small molecules in mixtures, highly viscous simple liquids, and polymer solutions. In low-viscosity liquids, probes are found to show Stokes-law behavior, so that D and A are both t]. In more viscous fluids, for small probes Stokes-law behavior is replaced by a dependence of D and A on as first described by Heber-Green(8). 

Chapter 5 considers translation and rotation by solvent molecules in small-molecule liquids and polymer solutions. Correlations between solution properties are already more complex than might have been expected. At small rj, the diffusion coefficient and equivalent conductance of small-molecule probes in simple liquids scale as At larger rj, D and A are instead The boundary between small and large t] seen in the literature is uniformly near 5 cP. It is unclear why this particular value of r should not be system-specific. In contrast to smaU-molecule probes, mesoscopic probes such as polystyrene latex spheres in potentially highly viscous mixed solvents such as water glycerol retain D T/ri behavior over three or more orders of magnitude in rj. 

The addition of a polymeric solute to a small-molecule solvent affects translational diffusion, viscosity, and rotational diffusion of solvent and other small molecules in solution. For polymer concentrations 4> < 0.4, the solvent selfdiffusion coefficient follows D exp(—a). The constant a is linear in the probe s molecular volume but is independent of polymer molecular weight. At larger concentrations (/) > 0.4, the simple-exponential dependence of D on concentration is replaced by a stretched-exponential concentration dependence, D and dD/dc both appearing continuous through the transition. The effects of added polymer on solvent self-diffusion and on the diffusion of small-molecule probes are clearly not the same. 

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