SCATTERING PROFILES OF CHARGED GELS - FROZEN INHOMOGENEITIES, THERMALFLUCTUATIONS, AND MICROPHASE SEPARATION

We calculate the scattering spectra of weakly charged, randomly cross- linked polymer gels in poor and in Theta solvents. For some values of the thermodynamic parameters, the competition between poor solubility, electrostatics, and network elasticity leads to the divergence of the structure factor at a wave vector q, signaling the onset of micropha se separation in the gel. At the transition, the random inhomogeneous equilibrium monomer density profile of the gel is replaced by a period ically modulated density distribution. Unlike all other known cases of microphase separation, partial reorganization of the equilibrium stat e takes place even before the onset of the transition and leads to the appearance of a peak in the structure factor. Depending on the choice of thermodynamic parameters, the characteristic wavelength of the mod ulation varies from microscopic to macroscopic length scales. The rami fications of our results for neutron and light scattering experiments and for studies of phase transitions in polymer gels are discussed.