Stretched-Exponential Relaxation of Electric Birefringence in a Polydisperse Colloidal Solution

The relaxation of electronic birefringence has been measured in a polydisperse solution of rodlike ionic micelles of cetylpyridinium chloride. The asymptotic behaviour of the relaxation is found to be characterized by a stretched-exponential: R ( t ) ≈ exp [−( t /τ) α ]. The value of the exponent α is 0.25 in the dilute regime. As the concentration of rods exceeds the critical concentration for entanglement c *, α grows progressively reaching the value 0.5. A simple theoretical model is presented to explain the dilute-regime results.