A unified picture of the local dynamics of poly(dimethylsiloxane) across the melting point

The local dynamics of poly(dimethylsiloxane) (PDMS) has been investigated by quasi-elastic neutron scattering (QENS). Methyl group reorientations dominate the QENS spectra up to 215 K (i.e., below the melting temperature, T_m ˜ 235 K). The dynamics of the CH₃ groups is interpreted in terms of a model function consisting of elastic and quasi-elastic components, the latter given by a Gaussian distribution of Lorentzian lines. Above T_m, the QENS spectra are analyzed considering two processes: (a) the methyl group rotation and (b) the segmental motion. The activation energy for the latter is 14.6 kJ/mol, in excellent agreement with rheological data. Moreover, in agreement with the latter, the intermediate scattering function, I(Q,t), computed via the inverse Fourier transform, follows time-temperature superposition according to the rheological shift factor. The contribution of the segmental motion to the scattering function I(Q,t) was fitted with a stretched exponential function (or its Fourier transform in the frequency domain). The fitted stretching exponent ß for segmental motion is 0.61 in both frequency and time domain, much higher than 0.5 (Rouse model), but in agreement with theoretical results realistically accounting for the chain stiffness. QENS studies of segmental motion in PDMS had indicated that the experimental data followed the Rouse model up to a very large Q, well beyond the validity range of the model. We suggest that the rotational motion of the methyl groups is responsible for this observation.