Self-ordering in a 2D two-component system modelling the behavior of a mixture of two oppositely charged polyelectrolytes

A possible self-ordering mechanism in a mixture solution of two oppositely charged polyelectrolytes, is discussed. In the case of strongly charged polyelectrolytes, the electrostatic interaction of oppositely charged macromolecules results in formation of neutral complexes (ladders or scrambled-eggs). At low concentrations, these complexes are not connected each other, forming a suspension-like system. However, if the ionization level of the one of these polyelectrolytes is enough low (whereas the other one remains strongly charged), only charged monomers of the weakly charged polymer will form the coupled pairs with the oppositely charged monomers of strong polyelectrolyte. In such a case, the bridging phenomenon can be observed when the macromolecules of the weakly charged polymer can form reversible bonds with different macromolecules of strongly charged polyelectrolyte, playing a role of tie molecules. As a result, a percolated macromolecular network arises. Note that such a network can be formed even at low polymer concentrations corresponding to dilute solutions of neutral polymers. The above macromolecular network was analyzed in 2D case with the help of a relatively simple lattice model consisting of identical quantity of two types springs having different elasticities and equilibrium lengths. It turned out that such a system can demonstrate an orientational self-ordering when most springs of the one type will be orientated along one direction (for example, along x-axis), whereas most springs of other type will be orientated along other direction (along y-axis). The reorganization of the disordered system into the ordered one (and back) occurs, for example, under variation of the ionization level of weakly charged polymer by variation in the system pH. The surprised peculiarity of the model in question is that the system cross-over to the ordered state can occur as a phase transition both of the first and second kind depending on the system parameter values.