Critical opalescence of polymer solutions

Abstract

The light scattering of polymer solutions is calculated with a simple model based on a local description of the polymer segment density. The following eqn (3.10) is found for finite concentrations of a single polymer in a solvent Kc(1 + cos²θ)/R_θ= dΠ/RT dc+(16π²〈r²〉/3λ²)M^–1 ₂ sin²θ/2. It is consistent with experiments of Benoit and Picot at the theta point. The same equation also describes the critical opalescence. The Debye-l-parameter is much smaller than predicted by earlier theories and is in much better agreement with experiment. It is also suggested that the critical volume fraction found from light scattering is in fact the volume fraction at the maximum of the spinodal. This explains the discrepancy which is found between critical volume fractions of polydisperse polymers as determined by light scattering and phase-separation studies. The light scattering of a symmetrical system containing two polymers in a common solvent is also formulated. Two types of fluctuations are found: an asymmetrical one in which both polymers fluctuate as a whole and a symmetrical one in which only the composition fluctuates. The last type shows critical opalescence near the point of incompatibility. The predicted Debye l parameter for the range of molecular interaction is much larger than for a single polymer in a solvent.