Equilibrium flexibility of a rigid linear conjugated polymer

Abstract

The unusual electronic and optical properties of many electroluminescent and conducting polymers arise from extended conjugation along the polymer backbone, which can also lead to insolubility, aggregation, and gelation. Synthetic efforts to produce an optimal structure require a balance between the persistence length and the effective conjugation length for the successful implementation of these materials in photonic and electronic devices. In this study, we have investigated the solution properties of a group of poly(phenyleneethynylenes) using a variety of light scattering techniques, including polarized and depolarized intensity measurements, dynamic light scattering, and size exclusion chromatography with a multiangle light scattering detector (SEC/LS). Interpretation of light scattering in the presence of absorption, fluorescence, and optical anisotropy is discussed. The molecular weights determined by light scattering encompassed the range from 10 × 105 to 5 × 106, with the root-mean-square radius of gyration as high as 250 nm. The results may be interpreted with a wormlike chain model to yield a persistence length of about 15 nm, so that these high-M polymers are coil-like in solution, rather than "rigid rods". This persistence length is still expected to be several times larger than the effective conjugation length.