Intermolecular order of poly-(2,5-dimethyl-para-phenylene vinylene) and poly-(para-phenylene vinylene) - A comparison

Poly-(para-phenylene vinylene) (2b, PPV) and poly-(2,5 -dimethyl-para-pheny lenevinylene) (Ib) were prepared via the formation of a double bond by ther mal elimination of an octylsulfinyl side group at 200 degreesC under vacuum . X-Ray diffraction experiments revealed a certain degree of long-range ord er within both polymers. However, considerable differences in the diffracti on pattern were observed, the packing of the polymer chains within the bulk structure being responsible for these differences. Within the crystal stru cture of 2b. neighbouring PPV chains pack in a zig-zag arrangement (herring bone pattern). Analysis of the diffraction pattern of Ib on the basis of th e crystal structure of a model compound (2,5,2 ' ,5 ' -tetramethylstilbene) revealed that the molecular planes of neighbouring polymer chains pack par allel to each other. A model for the crystal structure of Ib is given. The change of packing from a herringbone to a parallel arrangement of the molec ular planes is related to the introduction of methyl groups as side chains to PPV