No abstract

The synthesis, chemical characterization, and liquid crystalline behavior of two new series of poly(propyleneimine) (DAB) codendrimers are described. Both series were obtained by grafting onto the fourth-generation dendrimer (DAB−(NH2)32) various proportions of two types of promesogenic units:  one of them laterally attached and the other terminally attached. Both series differ in the terminally attached promesogenic unit, being chiral in series 1 and achiral in series 2. X-ray diffraction studies show that these compounds exhibit nematic or lamellar mesophases. For intermediate proportions, the smectic C mesophase, which was not exhibited by the homodendrimers, appears. Thus, the evolution between a nematic and an orthogonal lamellar mesophase occurs through a tilted mesophase. The chiral centers in series 1 permit the appearance of chiral nematic (N*) and chiral smectic C (SmC*) mesophases, which are studied. Deuterium NMR experiments reveal that the nematic formed by one codendrimer is uniaxial and not the anticipated biaxial nematic.

A group of nematic materials based on poly(propyleneimine) dendrimers (PPI) containing two types of mesogenic subunits were synthesized. The presence of different types of mesogenic group, the topology of the attachment to the core (end-on and side-on), and the chemical nature of the linking connection to the PPI scaffold (imine or amide) were used to tailor the mesomorphic properties. The materials were characterized by a variety of techniques including 1 H, 13 C NMR spectroscopy, GPC, MALDI-TOF MS, DSC, POM, and X-ray diffraction. The crystalline state, when it exists, was found to be suppressed for most of the materials once they had undergone the first transition to the isotropic phase. The majority of the compounds exhibited glass transitions near to or lower than room temperature and an accompanying wide temperature range mesophase. All of the side-on dendrimers synthesized exhibit a nematic mesophase whereas end-on dendrimers exhibit different mesophases depending on the length of the terminal chain of the mesogenic unit (nematic for n ) 2, 4, 5). The 2 H NMR technique was used to investigate the symmetry of the nematic phase formed by the side-on dendrimers (G ) 3). These studies revealed that the symmetry of the nematic phase is uniaxial. They also allow the investigation of the orientational order of the phase and the rate of field-induced director alignment.A new class of dendrimers which exhibit uniaxial nematic phases has been synthesized. These results indicate that the synthesis of these nanophase-structured dendrimers leads to macromolecules with LC properties similar to those of comparable low molar mass compounds.