The zero average contrast condition: Theoretical predictions and experimental examples

This paper deals with mixtures of homopolymers and copolymers in solvents under the zero average contrast condition. This condition is chosen in order to decouple the correlations due to polymer composition fluctuations from those due to polymer concentration fluctuations. This makes the measurement of these correlations much easier using techniques based on static light scattering, photoncorrelation spectroscopy, small angle neutron scattering, neutron spin echo or X-ray scattering. In the first part of the paper, a simple theoretical framework is developed to present the guidelines for the interpretation of the scattering data. In the second part, some experimental examples are discussed using the scattering techniques mentioned above. Data obtained from various systems involving alike (deuterated and ordinary) homopolymers and diblock copolymers as well as unalike species are considered. An example of mixtures of polyelectrolytes in aqueous solutions is also discussed within the framework of the zero average contrast condition. (C) 1997 Elsevier Science Ltd.