Morphological aspects and thermal behaviour of tri-p-tolylamine (TTA)-based model charge transport composites: A comparison with TPD

The morphology and thermal behaviour of model charge transport composites based on the hole transport molecule tri-p-tolylamine (TTA) in bisphenol-A polycarbonate (BPAPC), cyclohexyl polycarbonate (PCZ) and polystyrene (PS) were investigated. A very large decrease in the glass transition temperatures (Tg) of the host polymers is seen with the dispersion of TTA in the polymer matrix. The Tg reduces almost close to ambient temperature. This is much more pronounced than in the case of TPD (a tetraphenyl diamine) based composites [F. Khan, A.-M. Hor, P.R. Sundararajan, J. Phys. Chem. B 108 (2004) 117-126]. It is found that the extent of Tg depression is higher when the polymer is conformationally less flexible. FTIR study indicates a molecular level of interaction between TTA and the polymers used here. Upon annealing, phase separation and crystallization of TTA occurs at temperatures as low as 80 °C, and leads to a recovery of the Tg of the polymer. The Tg recovery is again much more significant than in the case of TPD. Sublimation of TTA occurs when the polycarbonate based composites are annealed at ≥120 °C. Re-plasticization upon annealing as suggested in the case of TPD/BPAPC is not observed with TTA/BPAPC. Due to such sublimation and the accompanying depletion of TTA, a photoreceptor, annealed at 140 °C failed to discharge. With polystyrene as the binder, sub-micron crystals of TTA are seen even with a low concentration of 30%. Chaining of such crystals is seen with a concentration of 50%, which could be a contributing factor for the enhanced charge mobility noted for polystyrene based charge transport composites.