Depolarized light scattering from critical polymer blends

Depolarized light scattering of binary polymer blends in disordered state n ear the demixing critical point is considered both theoretically and experi mentally. It is shown that the depolarized scattering in such systems is pr edominantly due to double scattering processes induced by composition fluct uations. For long enough polymer chains, this scattering is stronger than t he contribution from intrinsic anisotropy fluctuations. The general equatio n for the static and dynamic double scattering function is obtained in term s of the system structure factor. The scattering functions are calculated b oth analytically and numerically (dynamic part) for polymer blends. We foun d that the depolarized intensity depends on the polymerization degree N and the relative distance from the critical point tau = 1 - chi*/chi (where ch i is the Flory-Huggins interaction parameter and chi* its critical value) a s I-vh similar to N-2/tau(2), which is in good agreement with the experimen tal data. It is also shown that the dynamic scattering function is decaying non-exponentially. We calculate the relaxation rate and the non-exponentia lity parameter as functions of the scattering angle and tau. These theoreti cal predictions are compared with experimental data for three chemically di fferent blends.