Aggregation Behavior of Poly(3-hexylthiophene) in Solvent Mixtures: Linear Solvation Energy Relationship (LSER) Modeling and COSMO-RS Calculations

The solubility and self-assembly behavior of poly(3-hexylthiophene) (P3HT) in solvent mixtures, and the dominant intermolecular forces related to these attributes, have only been partially scrutinized. In this work, the extent of aggregation of amorphous P3HT and the structural order of P3HT aggregates (M-n approximate to 20 kDa and M-n approximate to 75 kDa) are investigated in 54 solvent mixtures of chloroform with acetone, acetonitrile, dichloromethane, ethyl acetate, ethanol, and n-hexane. Correlations between the molecular mass, the extent of aggregation, the structural order of the aggregates, and the nature of the solvent-P3HT interactions are studied using a (1) linear solvation energy relationship (LSER) modeling and (2) solubility parameter-based Flory-Huggins interaction parameters, chi. In addition, COSMO-RS calculations are used to assess correlations between the extent of aggregation and the activity coefficients, gamma , of P3HT. The results reveal that the extent of aggregation and the structural order of the P3HT aggregates are governed by different solvent-P3HT interactions. LSER modeling shows that the nucleophilicity and polarity of the solvent blends are the principal determinant of the structural order of P3HT aggregates. This result, which is based on an independent experimental method, supports previous computational results using COSMO-RS sigma-profiles to assess the nucleophilicity and polarity of the solvent.