The role of casting solvents on the phase separation and morphological behavior of model TPD/polymer charge transport composites

Whether a polymer in solution exhibits an extended rod-like conformation or a coiled globular form would depend on its interaction with the solvent. Model charge transport composites of polycarbonate or polystyrene with the hole transport molecule TPD were prepared with various solvents. While the Tg’s of the polymers by themselves are unaffected by the choice of the solvent, a significant effect is found when films are cast with the polymer and TPD. A larger depression of the Tg of the polymer and a smaller extent of phase separation of TPD upon annealing were found with the composites prepared with 1,1,2,2-tetrachloroethane than with any other solvent used in this work. Infrared spectroscopy and viscosity measurements show that the above polymers adopt extended conformation in solution, which enhances the intercalation of TPD, when films are cast from this solvent. Whereas the previous studies were concerned with the effect of solvent on the charge transport behaviour of polymers such as polythiophenes, we show that the morphological stability is enhanced when the polymer adopts an extended conformation. This is further confirmed by the observation that the stability against phase separation is improved with the cyclohexyl polycarbonate compared to the bisphenol polycarbonate due to the restricted conformational flexibility of the former.