Thermodynamic and Structural Changes in Ion-Containing Symmetric Diblock Copolymers: A Small-Angle X-ray Scattering Study

Abstract

We present temperature-dependent SAXS measurements on symmetric poly(styrene-block-2-vinylpyridine) and poly(styrene-block-ethylene oxide) with added lithium triflate. Salt doping led to a strong increase of the order–disorder transition temperatures and increased domain spacings. Based on a detailed analysis of the scattering data close to the order–disorder transition, three contributions to the structural changes can be distinguished: an increased incompatibility between the different monomers, the additional volume of the added salt, and chain stretching due to coordination between polymer and salt. At the phase transition, i.e. at constant interaction parameter χ, for low concentrations the increase in domain size is quantitatively explained by the volume of the added salt, and at higher concentrations chain stretching sets in. Structural and thermodynamic effects are considerably stronger in PEO than in P2VP.