Segmental Dynamics in Poly(methylphenylsiloxane) Networks by Dielectric Relaxation Spectroscopy

Abstract

The segmental dynamics of PMPS chains are determined in the bulk liquid and cross-linked networks of varying cross-link density by broad-band dielectric relaxation spectroscopy. A large fragility index [τ(T g*/T) dependence], F 1/2 = 0.77, is found, independent of cross-link density, molecular weight, and T g. Also independent of the above variables is the relaxation shape (characterized by a KWW β parameter of 0.45). Since these parameters quantifying the α process in PMPS networks are insensitive to cross-linking, it is concluded that the length scale of cooperatively rearranging domains in PMPS networks is smaller than the distance between cross-links, i.e., less than 5 nm, in agreement with the current consensus. An argument based on the temperature dependence of the dielectric relaxation strength, dipole moment, and ensemble average chain configuration was advanced, suggesting that the high fragility of PMPS has an intramolecular origin.

Quasielastic neutron scattering of poly(methyl phenyl siloxane) in the bulk and under severe confinement

“…The results of the analysis are included in the figure as well with the solid line being the total fit and the dotted and dashed lines the elastic and quasielastic contributions, respectively. For all curves, the stretching parameter ␤ of the KWW was kept constant at ␤ = 0.4, which is the value that gave the best fit for all wavevectors and is in good agreement with literature values obtained for the segmental relaxation of different polymers with a variety of experimental techniques, 1,[41][42][43][55][56][57] where 0.38Ͻ ␤ Ͻ 0.52 was found. It is clear that the elastic part is very small at this temperature, which shows that there are almost no immobile species in the polymer sample.…”

“…Finally, the relaxation times for the pure segmental relaxation at 300 K are in a relatively good agreement with those of earlier studies on PMPS. 25,32,[40][41][42]59 B. Dynamics of PMPS under severe confinement Figure 2 shows the temperature dependence of the energy resolved elastic scattering intensity for the PMPS25 nanocomposite ͓Fig. 2͑c͔͒ in comparison to those for the PMPS1 homopolymer ͓Fig.…”

“…39 Poly͑methyl phenyl siloxane͒ ͑PMPS͒ is a polymer that possesses both a methyl group and a pendant phenyl ring, so it is believed that it will show rich dynamics both below and above the calorimetric glass transition temperature. Nevertheless, a few studies exist in the literature where experimental techniques, such as quasielastic neutron scattering, 32 dielectric spectroscopy, 25,[40][41][42] and light scattering, 40,41,43 have been utilized. Most of these studies were performed above the polymer glass transition temperature where the phenyl flip and/or the segmental process were/was identified.…”

Influence of cross-linking on the segmental dynamics in model polymer networks

“…The influence of network formation on the relaxation behavior of polymer networks is an issue of considerable experimental effort. [5][6][7] Comparison between the dynamic behavior of a covalently entangled and cross-linked polymer, with low volume fraction of cross-links, has shown similar features in both the normal mode and the segmental relaxation regime. 8 However, previous studies have shown a strong influence of cross-link density on the ␣ relaxation characterized by a slowing down of the segmental dynamics as cross-links density, inducing connectivity, increases.…”

“…9,10 In many studies, the true impact of chemical network density is obscured by several side effects such as morphological heterogeneity and strong physical interaction in segmented polyurethanes, 7 incomplete chemical conversion in acrylates, 6 or anomalous chain flexibility of network strands in polysiloxanes. 5 Some of these side effects can be minimized by controlled preparation of model polymer networks with well-defined molecular architecture. It is known that the appropriate trimerization of isocyanate in the presence of tin compounds produces a regular isocyanourate network.…”