Mutual orientation of main chains and side chains in nematic liquid-crystalline side-chain polysiloxanes

The orientation of main-chain segments of some polysiloxanes containing mesogenic side groups was estimated by Si-29 NMR under high-power proton decoupling. The second-order orientation degree [P-2] related to the orientation distribution of chemical shift tensors was calculated from the spectra. From this conclusions were drawn about the degree of segment orientation for which a discussion of the transverse orientation distribution was necessary. It turns out that most of the samples investigated here have a structure in which the main-chain segments are perpendicular to the director field: however, if this conformation involves sterical hindrance, a parallel arrangement can arise. The apparent contradiction to SANS data concerning backbone conformation of such polymers is discussed; thereby it is noted that in some cases the two methods take a somewhat different point of view. Thus, without further structural information there is no unambiguous connection between mutual orientation of main and side chains on the one hand and the average shape of the whole molecule on the other.