Dynamics in poly(dimethylsiloxane) melts: fluorescence depolarization measurements of probe chromophore orientational relaxation

Dynamics in poly(dimethylsiloxane) (PDMS) melts from T{sub g} + 75 to T{sub g} + 175 K have been measured by the fluorescence anisotropy decay of a probe chromophore. The reorientational dynamics of the probe chromophore, 5-(dimethylamino)-1-naphthalenesulfonamide (dansylamide), attached to a trifunctional silane, are characterized in a small molecule solvent, cyclohexanol, and compared to its reorientation in the polymer system. In cyclohexanol, the orientational dynamics obey the Debye-Stokes-Einstein equation with a thermal activation energy equal to that of the cyclohexanol viscosity. In contrast, the rate of reorientation of the probe dispersed in PDMS polymer melts does not reflect the bulk properties of the samples. The local dynamics are exponentially activated, with activation energies that are higher than that of the viscosity of the bulk material. This result is different than conclusion of analogous studies made on carbon-based polymers. Two possible explanations are given based on the unique characteristics of the silicon-oxygen bonds in PDMS. 69 refs., 9 figs., 3 tabs.