Diffusing-wave-spectroscopy investigation of latex particle motion in polymer gels

Dynamic light scattering studies of particle motion in cross-linked chemica l gels are limited by the fact that when the cross-linking ratio is increas ed, the scattered intensity becomes quenched, and time-dependent concentrat ion fluctuations contribute less and less to the overall scattering signal. The concentration fluctuations then occur at very small length scales, whi ch can become inaccessible to classical dynamic light scattering techniques . Diffusing wave spectroscopy can help overcome these experimental difficul ties. We extend this technique to the case of nonergodic systems, and apply it to the detection of the motion of probe particles trapped inside chemic ally cross-linked gels. We measure the mean square displacement of the nano metric probes, leading to the measurement of local mechanical properties of the gel matrix.