Critical Neutron Scattering in a Polymer Blend above and below the Critical Point of Demixing: Critical Exponents and Amplitude Ratios

The critical behavior of a polymer blend was measured by small-angle neutron scattering above and below the critical temperature $T_c$, i.e., outside and inside the unstable region. The critical exponents and the amplitudes of the susceptibility and the correlation length were determined. For $T>\mathrm{}{T}_c$ the critical exponents $\gamma$ and $\nu$ are consistent with the renormalization-group predictions. Below $T_c$, however, deviations from the expected values were observed. Also experimental critical amplitude ratios (20-30)% smaller than predicted were found.