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 mes ogenic side groups was estimated by Si-29 NMR under high-power proton decou pling. The second-order orientation degree [P-2] related to the orientation distribution of chemical shift tensors was calculated from the spectra. Fr om 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: h owever, if this conformation involves sterical hindrance, a parallel arrang ement can arise. The apparent contradiction to SANS data concerning backbon e conformation of such polymers is discussed; thereby it is noted that in s ome cases the two methods take a somewhat different point of view. Thus, wi thout further structural information there is no unambiguous connection bet ween mutual orientation of main and side chains on the one hand and the ave rage shape of the whole molecule on the other.