Determination of Solvent–Polymer and Polymer–Polymer Flory–Huggins Interaction Parameters for Poly(3-hexylthiophene) via Solvent Vapor Swelling

We report the use of solvent vapor swelling of ultrathin polymer films to determine Flory–Huggins solvent–polymer and polymer–polymer interaction parameters (χ_<i>i–j</i>) for poly­(3-hexylthiophene) (P3HT) and polystyrene (PS) over a wide solvent composition range. From the calculated interaction parameters, we constructed a polymer/polymer/solvent phase diagram that was validated experimentally. χ_{tetrahydrofuran–P3HT} (1.04 ± 0.04) and χ_{CHCl₃–P3HT} (0.99 ± 0.01) were determined through swelling of ultrathin P3HT films. Similar experiments using PS films gave χ_{tetrahydrofuran–PS} = 0.41 ± 0.02 and χ_CHCl₃–PS = 0.39 ± 0.01, consistent with literature values. As expected, these χ_<i>i–j</i> parameters indicated that P3HT is less compatible than PS with either solvent. From δ_PS (17.9 ± 0.2 MPa^1/2) and δ_P3HT (14.8 ± 0.2 MPa^1/2), determined through regular solution theory, we calculated χ_PS–P3HT = 0.48 ± 0.06 at 23 °C. The resulting phase diagram was validated by solution-based transmission measurements of PS/P3HT blends in <i>o</i>-xylene. Although we focused on PS/P3HT blends in this work, this approach is easily adaptable to other polymer/polymer combinations of interest