Polymer/clay nanocomposites (PCNs) of poly(methyl methacrylate) and an organically modified clay, Cloisite 15a, were synthesized in situ with a suspension polymerization technique. The amount Of Clay present in the PCNs was varied to provide a better understanding of the effect of the clay on the properties of the polymer matrix. However, unexpectedly, we found that the concentration of clay had a dramatic impact on the molecular weight of the polymer matrix, and a relationship between the clay concentration and polymer molecular weight was determined. The PCNs were characterized with size exclusion chromatography (SEC), X-ray diffraction, transmission electron microscopy, and oscillatory shear rheology. From oscillatory shear rheology, the full master curves for the PCNs were obtained by application of the time-temperature superposition principle. To enable the effect of the clay on the rheology to be quantified, the experimental data was compared to the time-dependent diffusion model of des Cloizeaux for polydisperse polymer melts, which enabled the polydispersity to be incorporated through the use of the molecular weight distribution obtained via SEC. (C) 2009 Wiley periodicals, Inc. J Appl Polym Sci 113: 1307-1315, 2009