Liquid crystalline properties of poly(propylene imine) dendrimers functionalized with cyanobiphenyl mesogens at the periphery

Three generations poly(propylene imine) dendrimers with 4, 16, and 64 terminal amine groups have been functionalized with pentyloxycyanobiphenyl and decyloxycyanobiphenyl mesogens. The liq.-cryst. properties of these dendrimers have been studied in detail by differential scanning calorimetry, optical polarization microscopy, and X-ray diffraction. All the mesogenic dendrimers orient into a smectic A mesophase. Thermal properties are influenced to a large extent by the spacer length, showing g -> SA -> I transitions for the dendritic mesogens with the pentyloxy spacers and K -> SA -> I transitions for the ones with a decyloxy spacer. In the latter, the temp. range of the mesophase increases with dendrimer generation. Mesophase formation in the case of the pentyloxy series is more difficult compared with the corresponding decyloxy analogs, when the transition enthalpies and the kinetics of obtaining microscopic textures are considered. The effect of generation on mesophase formation cannot be clearly distinguished, although in the case of the fifth-generation dendrimer with a decyloxy spacer, microscopic textures could be obtained more easily, compared with the lower generations. X-ray diffraction measurements of oriented samples indicate that the cyanobiphenyl endgroups of both series orient into an antiparallel-overlapping interdigitated structure. The obsd. SA-layer spacings are independent of the dendrimer generation for both spacer lengths, indicating that the dendritic backbone has to adopt a completely distorted conformation, even for the higher generation