A series of oligo(m-phenylene ethylene)s having triethylene glycol ester-linked side chains (i.e., 2-[2-(2-methoxyethoxy)ethoxy]ethyl esters) were prepared, and their supramolecular organization in the solid-state was studied. These oligomers are amphiphilic, having a nonpolar aromatic hydrocarbon backbone that is appended with flexible polar side chains. Oligomers containing eight or more monomers are viscoelastic waxy solids at room temperature, exhibiting birefringent textures that reversibly melt into isotropic fluids upon warming. As the chain length increases, the materials stiffen and the melting point rises asymptotically, suggesting homologous packing for all members of the series. These observations are in accord with the behavior of conventional short-chain polymers, whereby melting point depression results from the volume fraction of chain ends. Small- and wide- angle X-ray diffraction investigations were carried out to elucidate the details of chain packing. Samples slowly cooled from the isotropic melt develop lamellar organizations with longitudinal spacings that depend linearly on the chain length. The relationship between the calculated repeat unit length and the experimentally observed increment in lamellar spacing per repeat unit suggests that the chains are oriented 30°from the layer normal. Macroscopically aligned samples showed two sets of mutually orthogonal lateral reflections whose d-spacings are nearly independent of chain length. A model for the solid-state organization of these oligomers is proposed. Solution- and solid-state UV spectra are presented and interpreted by considering the relative abundance of cisoid vs transoid conformers under these various conditions.
ASJC Scopus subject areas
- Colloid and Surface Chemistry