Main-Chain Stiffness and Helical Conformation of a Poly(quinoxaline-2,3-diyl) in Solution

Abstract

Light and small-angle X-ray scattering and viscosity measurements in tetrahydrofuran at 25 °C were made for nine helical poly[5,8-dimethyl-6,7-bis(propoxymethyl)quinoxaline-2,3-diyl] samples ranging in the weight-average molar mass Mw from 8 × 103 g mol–1 to 6 × 105 g mol–1 to determine the particle scattering function, the radius of gyration, and the intrinsic viscosity as a function of Mw. The dimensional and hydrodynamic properties were consistently explained in terms of the Kratky–Porod wormlike chain. The helix pitch per residue h (or the contour length per residue) and the chain stiffness parameter λ–1 (the Kuhn segment length or twice of the persistence length) were estimated to be h = 0.19 nm and λ–1 = 43 ± 3 nm. The former parameter corresponds to an internal rotation angle of about 120° which is substantially the same as the most stable helical structure estimated from the internal rotation potential. The latter one (λ–1) indicates that the poly(quinoxaline-2,3-diyl) has a rigid helical main cha...