Self-modeling, mixture spectra

Remarkably, the H NMR spectrum of the mixture is simply equal to the sum of the H NMR spectra of its components. This spectroscopic earmark indicates that this mixture of molecules undergoes self-sorting. Self-sorting refers to the ability of a molecule or entity to efficiently distinguish between self and non-self even within a complex mixture. Self-sorting is commonly observed in Nature (e.g. the immune system) but is still relatively rare in designed supramolecular systems. The molecular clips described here represent a well-defined model system for studies of self-sorting. 

Sun and Wang report a series of studies of polystyrene polymethylmethacrylate mixtures (in benzene, dioxane, and toluene, respectively) using light scattering spectroscopy as the major experimental technique(78-80). Both polymers were in general nondilute. Neither polymer is isorefiractive with any of the solvents. The objective was to study the bimodal spectra that arise under these conditions and to show that the two relaxation times and the mode ampUtude ratio can be used to infer diffusion and cross-diffusion coefiBcients of the two components. Experimental series varied both the total polymer concentration and the concentration ratio of the two components. The theoretical model predicts a biexponential spectrum. The experimental data were fitted by a bimodal distribution of relaxation rates or by a sum of two Williams-Watts functions. The inferred self-diffusion coefiBcients of both species fall with increasing polymer concentration. 

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