Structure of carbosilane amphiphilic liquid-crystalline codendrimers in bulk and in thin (Langmuir) films

Abstract

Amphiphilic carbosilane liquid crystalline codendrimers with terminal mesogenic butoxy benzoate and phenolic groups have been synthesized for the first time. The structures and compositions of the synthesized codendrimers were characterized by NMR spectroscopy. The data from polarization optical microscopy, differential scanning calorimetry, and small angle X ray scattering demonstrated that all codendrimers possessed mesomorphic properties. They form thin (Langmuir) films at the water-air interface. The surface pressure-surface area isotherms were plotted. The Langmuir-Blodgett films of different thicknesses were obtained by the vertical dipping method on the solid substrate. The film structures were studied by grazing incidence X ray diffraction analysis.The studies of liquid crystalline (LC) dendrimers had long ago attracted attention of researchers working in the areas of both high molecular weight compounds and liquid crystals. 1-5 Unusual molecular topology of these systems that are prone to microsegregation and self assembling with the formation of a complicated hierarchy of supramo lecular structures evoked increased interest in studying the structure formation processes of these compounds in dilute solutions 6 and in the solid phase. 7 Undoubtedly, substantial information on the structure of LC dendrimers could be obtained by the study of their thin films prepared by the Langmuir-Blodgett method (LB films).However, to use this procedure, one needs, as a rule, amphiphilic molecules capable of specific orientation on the water surface with the polar groups arranged in the aqueous phase and nonpolar groups, in the organic phase. Carbosilane dendrimers, which have been studied in de tail in our previous works, 1,2,7 do not meet, unfortunately, these requirements. Therefore, we concentrated our attention on the synthesis of amphiphilic carbosilane LC codendrimers of the first, third, and fifth generations containing different ratios of hydrophilic (phenolic YOH) and hydrophobic (butoxybenzoate, Y´OBu) mesogenic groups (Fig. 1).As can be seen from Fig. 1, a dendrite molecule with random distribution of mesogenic and phenolic terminal fragments, in essence, is a biphilic codendrimer in which heterogeneous terminal groups are chemically bound to the carbosilane matrix through the aliphatic spacers con sisting of ten methylene units.It should be mentioned that the number of previously synthesized and studied codendrimers is rather small, although many LC dendrimers were described. 8,9The following tasks were set in the present work:(1) to develop a method for the synthesis of the LC codendrimers based on the carbosilane dendritic matrix;(2) to study the influence of the composition and gen eration number of the synthesized codendrimers on the formation of the LC structures;(3) to reveal the possibility of formation of the thin LB films from the synthesized LC codendrimers and to study their structures. Results and DiscussionSynthesis of codendrimers. We have developed a gen eral approach to the synthesis of amphi...