A new quantitative theory for polyelectrolytes in salt free dilute solutions is developed. Depending on the electrostatic interaction strength, polyelectrolytes in solutions can undergo strong stretching (with polyelectrolyte dimension R_g\sim l_B^{ 1/3}N, where l_B is the Bjerrum length and N is the number of the chain segments) or strong compression (with R_g\sim l_B^{ -1/2}N^{ 1/3}). A strong polymer collapse occurs as a first-order phase transition due to accompanying counterion condensation.
Categories: cond-mat.soft cond-mat.stat-mech