Poly Ethylmethylsiloxane Research Articles

Chain expansion and chain localisation in the homogenous regime of blends of liquid low molar mass polysiloxanes as revealed by neutron scattering investigations

Abstract Using small angle neutron scattering and neutron spin echo spectroscopy, two isotopic blends of low molar mass polydimethylsiloxane (PDMS) and polyethylmethylsiloxane (PEMS), and the corresponding binary blends d-PDMS/ p-PEMS and p-PDMS/d-PEMS (d: deuterated, p: protonated) were studied at the critical composition in the homogeneous regime. From the scattering data it becomes evident that coil dimensions and collective dynamics of these both blend systems behave significantly different. Compared to the isotopic mixtures, which exhibit the expected unperturbed chain dimensions and the typical Rouse relaxation, in the binary blends, which differ by a large shift with respect to the critical temperature, considerable coil expansion and spatially restricted Rouse relaxation occur. Both these structural and dynamic effects are in agreement with the model of droplet formation and chain localization, resulting from the existence of microscopic heterogeneities within the spinodal boundaries of the phase diagram. In addition, the observation of Rouse relaxation, spatially restricted to microscopic length scales, provides a new access to the molecular understanding of the critical slowing down of the mutual diffusion process, observed by photon correlation spectroscopy.

Neutron Scattering Investigations on the Statics and Dynamics of Polydimethyl- and Polyethylmethylsiloxane Melts

Elastic and quasielastic small angle neutron scattering experiments on low molecular mass isotopic polydimethylsiloxane (PDMS) and polyethylmethylsiloxane (PEMS) blends were performed in order to study the static and dynamic single and many chain behavior in both systems. Within experimental error no differences could be detected with respect to the fundamental chain properties: conformation and type of relaxation. In particular, the latter follows the predictions of the Rouse model up to Q-values of 0.2 Å–1. However, it was also observed that the relaxation rates are not the same. In both siloxanes single chains relax slightly faster than many chains. The origin of this effect is not known. Despite of these deviations the overall experimental findings favor the hypothesis that the collective or many chain properties of low molecular mass polymer melts can be well modeled by the related single chain behavior.