Controlled molecular conformation and morphology in poly(amidoamine) (PAMAM) and poly(propyleneimine) (DAB) dendrimers

The mesomorphic behavior of several new end-functionalized liquid-crystal-containing dendrimers is reported. These liquid-crystalline dendrimers are obtained by attaching promesogenic units to the termini of the preexisting first five generations of poly(amidoamine) (PAMAM) and poly(propyleneimine) (DAB) polymers by a condensation reaction; the promesogenic units are derived from salilcylaldimine bearing one, two, or three terminal aliphatic chains. All of the compounds were found to exhibit liquid-crystalline proper-ties, as deduced by polarized-light optical microscopy (POM) observations, differential scanning calorimetry (DSC) measurements, and X-ray diffraction (XRD) studies. The nature of the liquid-crystalline behavior of the dendrimers is directly correlated with the number of terminal chains grafted on the peripheral anisotropic groups. Indeed, the presence of one chain per mesogenic unit favors their parallel arrangement; hence, smectic mesomorphism. (SmA and SmC) is induced. Increasing the number of aliphatic chains (two and three) favors their radial disposition, leading to the formation of columnar structures (COl(H)), The differences in the thermal behaviors observed for these systems are discussed in terms of generation number, dendritic core, and number of terminal alkoxy chains. Considering the amphipathic character of the dendrimers (in this case, the incompatibility between the different constitutive molecular segments), which leads to microphase segregation, and the conformation of the dendritic core, a general model for the description of the molecular organization within the different types of supramolecular arrangements (lamellar and columnar mesophases) is proposed.