End-on and side-on nematic liquid crystal dendrimers

Pastor, Laura, Barberá, Joaquín, McKenna, Mark, Marcos, Mercedes, Martín-Rapún, Rafael, Serrano, José Luis, Luckhurst, Geoffrey R. and Mainal, Azizah (2004) End-on and side-on nematic liquid crystal dendrimers. Macromolecules, 37 (25), 9386-9394. (doi:10.1021/ma048450p).

Abstract

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 H-1, C-13 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 H-2 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.

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