Small-angle neutron scattering (SANS) has been used to investigate the conformation of linear and cyclic poly(dimethylsiloxane)s (PDMS) in chemically identical, undiluted blends. SANS measurements have been carried out on (1) linear hydrogenous (H) mixed with linear deuterated (D) PDMS and (2) cyclic H mixed with cyclic D PDMS. The conformational behavior of the cyclic and linear polymers is studied over a wide range of molar mass and composition. Isotopic blends of linear PDMS are shown to adopt conformations that agree well with theoretical predictions for Gaussian random-coil polymers and confirm previous SANS studies. As expected for chains obeying Gaussian statistics, the mean radii of gyration, R g, scale with the weight-average molar mass as Rg ∝ Mw 0.5 . A detailed study of H/D cyclic PDMS mixtures is presented, and we demonstrate that, since Rg ∝ Mw 0.4 , highly flexible cyclic polymers in the melt adopt an even more compact conformation than that of unperturbed rings. This behavior confirms previous predictions based on computer simulations and theoretical studies. The results are in excellent agreement with computer simulations and theoretical predictions reported in the literature.

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.

It has been suggested that, due to topological constraints, rings in the melt may assume a more compact shape than Gaussian chains. In this paper, we exploit the availability of narrow fractions of perdeuterated linear and cyclic polydimethylsiloxane ͑PDMS͒ and, through the analysis of the small angle neutron scattering ͑SANS͒ profiles, demonstrate the difference in scattering properties of linear and cyclic PDMS molecules. As expected for Gaussian chains, for the H/D linear PDMS samples, log-log plots of the scattered intensity versus scattering vector Q display a Q ͑−2͒ dependence. However, for H/D cyclic blends, the scaling exponent is higher than 2, as predicted by computer simulations reported in the literature. We show that cyclic molecules in bulk display the characteristic maximum in plots of scattered intensity versus Q ͑−2͒ that is expected on the basis of Monte Carlo calculations and from the Casassa equation ͓E. F. Casassa, J. Polym. Sci. A 3, 605 ͑1965͔͒. It is also shown that, for rings, the Debye equation ͓P. Debye, J. Appl. Phys. 15, 338 ͑1944͔͒ is no longer appropriate to describe the SANS profiles of H/D cyclic blends, at least up to M w Ϸ 10 000. For these samples, the Casassa form factor gives a better representation of the SANS data and we show that this function which was developed for monodisperse cyclics is still adequate to describe our slightly polydisperse samples. Deviations from all above observations are noted for M w Ͼ 11 000 and are attributed to partial contamination of cyclic samples with linear chains. The failure of both the Debye and the Casassa form factors could be due to contamination of the cyclic fractions by linear polymers or to a real conformational change.

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