MICROSTRUCTURE AND PHASE-DIAGRAMS OF POLYMER GELS

Recently we developed [Panyukov and Rabin, Macromolecules 29 (1996) 79 60] a phenomenological theory of randomly cross-linked polymer network s, based on the separation of solid-like and liquid-like degrees of fr eedom and taking into account the frozen inhomogeneity of network stru cture. We calculated the scattering spectra of weakly charged, randoml y cross-linked polymer gels in good, poor and in Theta solvents [Panyu kov and Rabin, Macromolecules 29 (1996) 8530; Rabin and Panyukov, Macr omolecules 30 (1996) 301]. For some values of the thermodynamic parame ters, the competition between poor solubility, electrostatics and netw ork elasticity leads to the divergence of the structure factor at a wa ve vector q, signaling the onset of microphase separation in the gel. Depending on the choice of thermodynamic parameters, the characterist ic wavelength 1/q varies from microscopic to macroscopic length scale s. We have shown that the presence of long range elastic interactions in the network affects the phase diagrams of polymer gels in poor solv ent. Weakly charged gels deswell continuously upon decreasing the qual ity of solvent. At intermediate degrees of ionization the gel undergoe s a first order volume transition between two homogeneous states, whic h takes place at the spinodal. Strong hysteresis is predicted for the swelling and the deswelling transitions in this regime. Further increa se of the charge on the gel leads to the formation of an anisotropical ly deformed phase on the surface of the isotropic built phase. (C) 199 8 Elsevier Science B.V. All rights reserved.