Microstructure and phase diagrams of polymer gels

Recently we developed [Panyukov and Rabin, Macromolecules 29 (1996) 7960] a phenomenological theory of randomly cross-linked polymer networks, based on the separation of solid-like and liquid-like degrees of freedom and taking into account the frozen inhomogeneity of network structure. We calculated the scattering spectra of weakly charged, randomly cross-linked polymer gels in good, poor and in Θ solvents [Panyukov and Rabin, Macromolecules 29 (1996) 8530; Rabin and Panyukov, Macromolecules 30 (1996) 301]. 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 microphase separation in the gel. Depending on the choice of thermodynamic parameters, the characteristic wavelength 1/q* varies from microscopic to macroscopic length scales. We have shown that the presence of long range elastic interactions in the network affects the phase diagrams of polymer gels in poor solvent. Weakly charged gels deswell continuously upon decreasing the quality of solvent. At intermediate degrees of ionization the gel undergoes a first order volume transition between two homogeneous states, which takes place at the spinodal. Strong hysteresis is predicted for the swelling and the deswelling transitions in this regime. Further increase of the charge on the gel leads to the formation of an anisotropically deformed phase on the surface of the isotropic bulk phase.