A Unified Picture of the Local Dynamics of Poly(dimethylsiloxane) across the Melting Point

Abstract

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 timetemperature 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 β 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.

Neutron scattering investigations on methyl group dynamics in polymers

“…Recent QENS investigations in PDMS [93], PVAc [103] and in the blends hh-PP/PEP [111], and PEO/PMMA [116] have tackled the problem of methyl group classical hopping in the very high temperature range where secondary relaxations or motions involved in the glass transition process enter the dynamic window of neutron scattering. In that situation the time scales of such processes and methyl group hopping superpose.…”

“…Though the RRDM was initially developed for polymer systems, it must be stressed that all its basic assumptions can, in principle, be also applied to non-polymeric disordered systems (see Section VII). In the framework of the RRDM [25,[31][32][33][34][35][36][88][89][90][91][92][93][94], it is assumed that the only effect of the structural disorder on methyl group dynamics is to introduce a distribution of rotational barriers g(V 3 ), originated from the different local environments felt by the individual methyl groups. A non-trivial assumption is that the distribution of distances between neighbouring methyl groups does not introduce significant coupling forces for the smallest distances, which, in particular, would modify the single-particle Hamiltonian (9) leading to a more complex energy level scheme [95][96][97][98][99][100].…”

Recent progress on polymer dynamics by neutron scattering: From simple polymers to complex materials

“…Refs. [111][112][113][114]. During the period considered in this review, the impact of confinement on the structural relaxation was investigated by Sanz et al 37 by applying NSE techniques to deuterated semicrystalline poly(ethylene terephthalate).…”

Inelastic neutron scattering for investigating the dynamics of confined glass-forming liquids

“…Another polymer with a relatively good separation of MG-dynamics and T g -processes is PDMS [30][31][32][33][34][35][36]. In Fig.…”