Molecular motions in crystalline and mesomorphic forms of poly(diethylsiloxane)

Abstract

Molecular motions in low molar mass (46200 g mol-') poly(diethylsi1oxane) (PDES) are studied by 2H NMR. From DSC measurements the existence of the crystalline phases a,, B, and a2, /I2, as well as a pronounced mesogenic phase a, , is clearly established. Based upon calculations of averaged field gradient tensors based on cone models a detailed motional analysis in the different phases has been conducted. At low temperatures the only motion present besides methyl rotation are librations of the ethyl groups with amplitudes below 25". The motion in the conformational disordered phase is characterized by conformational changes within the side chains, accompanied by occasional rotations through angles larger than 70" within the backbone. The flexibility of the ethyl groups, however, is limited by steric hindrance, prohibiting a full rotation. This situation is observed also in the meso phase where there is an additional rotational degree of freedom around the main chain axis.

The NMR-WEBLAB: An internet approach to NMR lineshape analysis

“…25, best agreement is obtained if the methyl group is jumping between site "1" and site "2" as indicated in Fig. 8a, so that Z is perpendicular to z2 .…”

“…As a last example, we want to demonstrate that WEBLAB can also be used to analyze more complicated situations as, for example, those encountered in Z H NMR spectra of poly(diethylsiloxane) (PDES) in its various phases [25]. Although the PDES chains themselves are highly flexible, their packing in the bulk leads to a number of solid as well as anisotropic molten mesophases, in which different internal motions as well as restricted rotations of the chains around their mean axis are activated and manifest themselves in the 2H NMR spectra.…”

“…8. Model for the calculation of a complex motion observed in PDES [25]. It is assumed that there are fast jumps of the ethylene side chains (only one is shown) around the Si-C axis z,.…”

“…(25) must be computed.Integrals over the surface of the other octants of the "northern" hemisphere are obtained by replacing Tj in Eq. (24) successively by (-i.…”

Molecular dynamics in poly[(R)-3-hydroxybutyric acid] biopolymer as studied by NMR

“…Comparison of the experimental values of d and i with those calculated for various models of motion makes possible an unambiguous identification of the motion nature [10][11][12][13].…”