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 (Mn ≈ 20 kDa and Mn ≈ 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, χ. In addition, COSMO‐RS calculations are used to assess correlations between the extent of aggregation and the activity coefficients, γ, 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 σ‐profiles to assess the nucleophilicity and polarity of the solvent.
The aggregation behavior of P3HT is investigated at the interface of orthogonal solvents for P3HT. The changeable characteristics of P3HT aggregate dispersions, for example, extent of aggregation and intrachain order, are studied by varying (1) the interfacial area, (2) the poor solvent used to induce aggregationdichloromethane (DCM), hexane (HEX), and acetonitrile (AcN)and (3) the relative composition of the good solvent, chloroform (CF), and poor solvents. The results are compared to those observed using rapid injection of the solvent. Miscibility gap values (Dd) provide a reasonable justification of the assembly behavior of P3HT in the solvent mixtures in terms of the kinetics of polymer aggregation and the kinetics of solvent mixing at the interface. Atomic force microscopy (AFM) is used to analyze the morphology of films processed from dispersions with disparate characteristics, but having the same solvent composition, for example, 70:30 CF:HEX or 60:40 CF:DCM. Based on the disparity of the kinetics and miscibility gap values, the prevalence of specific structural motifs in the films, for example, spheroids (globules) and fibers, is effectively rationalized in terms of the structural attributes of the aggregates in the liquid phase rather than the evaporation rate (boiling point) differences of the solvents in the mixture.
One method to improve the conductivity of conjugated polymers, like poly(3‐hexylthiophene) (P3HT), is to “chemically dope” them analogous to inorganic materials. One electron acceptor that has been used in tandem to p‐doped P3HT is 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ), and recently there has been much interest in the nature of the interactions between F4TCNQ and P3HT in the solution phase. To date, however, there are few reports that investigate the behavior of F4TCNQ‐doped P3HT in binary solvent mixtures. The study reported herein is an investigation of F4TCNQ‐doped P3HT in mixtures of chloroform (CF) with dichloromethane (DCM) or acetonitrile (AcN), wherein variations in the doping efficiency in these mixtures are observed using UV–vis absorption, Raman, and electron paramagnetic resonance spectroscopic techniques. The contrasting solubility and charge transfer behavior of F4TCNQ‐doped P3HT in CF:DCM and CF:AcN show that judicious selection of solvent mixtures may be exploited to improve the doping efficiency and solution processability of p‐doped P3HT dispersions.
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