The role of molecular volume and the shape of the hole transport molecule in the morphology of model charge transport composites

Abstract

We present a study of the morphology and molecular interactions in a model charge transport composite with 1,1-bis(di-4-tolylaminophenyl) cyclohexane (TAPC) as the hole transport molecule in bisphenol-A polycarbonate (BPAPC) and cyclohexyl polycarbonate, also known as bisphenol-Z polycarbonate (PCZ). Solution NMR shows that while there is aromatic interaction between the phenyl groups of the polycarbonate and TAPC, the broadening of the peaks corresponding to the latter indicates a decrease in the rotational motion. FTIR spectroscopy also exhibits frequency shifts of the aromatic C–H absorption peaks, which parallels the extent of the depression of the glass transition temperature (Tg) of the polycarbonate. These are compared with the previous results for N,N-diphenyl-N,N-bis(3-methylphenyl)-[1,1-biphenyl]-4,4-diamine (TPD) and tri-p-tolylamine (TTA), and the differences are rationalized on the basis of the molecular shape and van der Waals volume of the small molecules. It is proposed that when the polycarbonate is in a random coil conformation, spherical small molecules (e.g., TAPC and TTA) reduce the glass transition temperature much more than a rodlike small molecule (e.g., TPD). Annealing at a temperature just below the Tg of the polycarbonate was used as a means of simulating accelerated ageing. Upon annealing, phase separation and crystallization of TAPC occurs and leads to a recovery of the Tg of the polymer significantly. The Tg recovery in this case is much more significant than in the case of TPD. The average crystal sizes are about ten times smaller than the crystals obtained in the case of TPD for the same temperature of annealing. To enhance the charge mobility, it might actually be advantageous to induce submicron crystals of the small molecule, while keeping the film transparent.