COMPATIBILITY ENHANCEMENT AND MICRODOMAIN STRUCTURING IN WEAKLY CHARGED POLYELECTROLYTE MIXTURES

The phase structure of a mixture of weakly charged polyelectrolytes in the presence of solvent, counterions, and low molecular salt is studied theoretically. The most important contribution to the free energy of this system is connected with the presence of charges and comes from the translational entropy of counterions and ions of low molecular salt. It is shown that the presence of even a small fraction of charges on the polymers leads in many cases to an essential increase of mixture compatibility. This theoretical prediction is confirmed experimentally. Compatibility enhancement is also achieved for the mixture of stiff and flexible macromolecules if one or both components carry some fraction of charged links. On the basis of a more detailed study of this system we conclude that although the region of macrophase separation in the presence of charged links diminishes, the remaining region of the phase diagram is divided into two parts: In one part we have a true molecular compatibility, while in the other part a microdomain structure is formed which is reminiscent of microdomain structures in melts of block copolymer. The physical reason for microdomain formation originates from the translational entropy of counterions and the impossibility of the violation of the condition of total electroneutrality of the system.