Phase transitions of a neutral polyampholyte

The behavior of a neutral polyampholyte, PA, chain with N monomers is analyzed in the framework of Flory's affine network theory of rubber elasticity. The PA chain, in addition to the neutral monomers, also contains an equal number of positively and negatively charged monomers. In order to account for the electrostatic interaction, the Debye-Huckel theory augmented by the Bjerrum's ideas of dipolar formation is used. We find that at high temperatures the chain exists in a dilute collapsed state with a radius of gyration scaling as N-v with v = 1/3 (microelectrolyte). For low temperatures the PA. resembles an affine network (microgel) with v = 2/5. In the theta solvent, v = 3/8. If the quality of the solvent is decreased, the microgel undergoes a collapse transition into a compact globular state. The high and low temperature regimes are separated by a first-order phase transition. The full phase-diagram of the PA is calculated as a function of temperature and quality of the solvent.