Nondilute ternary systems

Stepanek, et al. report a careful study of the very-short-time part of 5(, r) of polymer solutions and simple solvents(64). Two rapidly-relaxing diffusive modes were clearly found. A mode whose amplitude is independent of polymer concentration was identified as corresponding to thermal diffusion in the solvent. A second mode whose amplitude was linear in polymer concentration appears to correspond to polymer self-diffusion, made visible through coupling to polymer motions. 

There is a limited number of studies of light scattering spectra of polymer polymer solvent systems in which neither polymer component is dilute. Theoretical models treating this problem can be traced back to work of Phillies(65,66) much of the experimental work tests the polymer-focused calculations of Benmouna, eta/. (67-69). 

Borsali, et al. studied dynamic light scattering spectra of 970 kDa polystyrene 950 kDa polymethylmethacrylate toluene. PMMA and toluene form an isorefrac-five pair(70). Semiquanfitafive experimental tests were made of the theoretical work of Benmouna, et a/. (67). When neither polymer was dilute, the observed spectrum was biexponenfial, even though only one macrocomponent scatters light. The mode 

Borsali, et al also studied mixtures of polystyrene and polydimethylsUoxane in tetrahydrofuran (which is isorefractive with polydimethylsiloxane) and in toluene, which is a zero average contrast solvent for these polymers and conditions(72). Measured spectral forms (one or two relaxations), relaxation times, and their concentration dependences were consistent with the Benmouna model(67). 

Giebel and co-workers studied dynamic light scattering spectra of polydimethylsiloxane and polymethylmethacrylate in several solvents as a function of the relative concentration of the two polymers(75,76). At the fixed total polymer concentration, the polymers were reasonably expected to be nondilute. Comparison was made with theoretical results of Benmouna, et al, with particular attention to the zero average contrast condition(67). A strong variation of some spectral parameters with composition was described weU by Benmouna-type models and a small number of free parameters(67). 

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Dielectric relaxation thus resembles self-diffusion. Both processes observe the motion of single macromolecules through a uniform albeit fluctuating background. In a two-component polymer-solvent system, dielectric spectroscopy reveals the effect of intermacromolecular interactions on single-molecule size and reorientation. Dielectric measurements on a three-component polymer-polymer-solvent mixture, in which a tracer polymer has a nonzero type-A dipole and a potentially nondilute matrix polymer has none, can be used for example to separate the effects of probe and matrix molecular weights on dielectric relaxation. This motif in the comparative study of binary and ternary solutions appears repeatedly below. Finally, dielectric measurements on block copolymers in which some copolymer subchains have been inverted end-to-end or have no dipole moment allow one to observe internal motions and dynamic cross-correlations of subchains. 

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