Synthesis and Characterization of Polypropyleneimine Dendrimers Having Peripheral Mesogenic Groups: Homeotropic Orientation and Mesogen Structure

In order to estimate the effectiveness of mesogenic structure on the spontaneous homeotropic orientation of side-chain type liquid crystalline dendrimers, five mesogens, 4′-methoxybiphenyl (PPO1), 4-methoxyphenyl benzoate (BPO1), trans-4-(4-pentylcyclohexyl)phenyl (PC5), 4′-fluorobiphenyl (PPF), and 3′,4′,5′-trifluorobiphenyl (PPF3), were introduced to the terminal amino groups of a second-generation polypropyleneimine dendrimer. Among them, the PC5 mesogen exhibited superior homeotropic orientation during the slow cooling process from the isotropic melt. Using the PC5 mesogen, the effect of the spacer length (n) was also examined. Homeotropic orientation was observed only for n = 3 and 6, which exhibited a stable smectic A phase in a wide temperature range.