Flow Birefringence Studies Research Articles

Contribution to the discussion on the shape of the polystyrene molecule in dilute solutions by means of flow birefringence measurements

It is usually assumed that a chain molecule like polystyrene in dilute solutions is bent, and the question as to whether it is more or less rolled together has been put forward some time ago. Flow birefringence studies should be a fruitful approach to this question, but the theory of the Maxwell effect for chain molecules is very unsatisfactory. The author has recently developed the theory of the Maxwell effect for a solution of elastic impenetrable spheres. This model represents a limiting case like the model of a rigid sphere used successfully by Sadron and by Flory in the interpretation of viscosity measurements. The model could be a reasonable first approximation for flexible chain molecules, inasmuch as Debye and Bueche, as well as Kirkwood and Riseman find the impenetrable sphere as a limiting case in their more general theory of viscosity. The theory of the elastic sphere leads to some conclusions which are in agreement with experiments on the temperature dependence of the extinction angle at low gradients carried out with two samples of polystyrene. These results constitute a strong argument in favor of the model of a rather compact sphere for the polystyrene molecule. Also, the observed effect is completely different from the orientational effect characteristic of rigid molecules like tobacco mosaic virus; for polystyrene the origin of the phenomenon seems to be much more a deformation of an initially isotropic particle rather than an orientation of an elongated molecule. The results are on the other hand in disagreement with the theory of W. Kuhn and H. Kuhn based on a dumbbell model. In this short paper the author will summarize the results and put the emphasis on the possibilities of the flow-birefringence technique in the field of flexible chain molecules for the determination of the elasticity and the internal viscosity of these particles. Finally, a general method is suggested in order to check whether a chain molecule in solution is more or less rigid or flexible (extinction angle measurements at low gradients for several solvents with different viscosities).