Design, synthesis and thermotropic self-organization of dendronized polystyrenes with different length alkyl tails

A series of dendronized polystyrenes with different length alkyl tails, poly{3,4,5-tris(alkoxy)styrene} (denoted as PTAVB-m, where m is the number of carbons in the alkoxy groups, m = 6, 8, 10, 12, 14, 16, 18), were synthesized via conventional free radical polymerization. The chemical structure of the monomers was confirmed by H-1 NMR, C-13 NMR and mass spectrometry. Analyses of the corresponding polymers were performed with H-1 NMR, gel permeation chromatography and thermogravimetric analysis. The phase behavior of the polymers was investigated by a combination of techniques including differential scanning calorimetry, polarizing optical microscopy, temperature-variable FT-IR spectroscopy, small-angle X-ray scattering, and 1D and 2D wide-angle X-ray diffraction. The results showed that the length of the alkyl tails has significant effects on the phase structure and phase transition for the simplest dendronized polymers. When m = 6, PTAVB-6 was an amorphous polymer. When the tail length increases to 8 and 10, at low-temperature, the phase of the polymers was amorphous. However, at high-temperature, the phase of the polymers was columnar nematic formed by the main chain and side groups. Following a subsequent cooling, the LC phase disappeared, which indicated that a reentrant isotropic phase formed during the heating and cooling processes. Moreover, when the tail length was increased to 12 (m >= 12), prior to entering the reentrant isotropic phase, the polymers were crystalline. In this work, a reentrant isotropic phase was observed for the first time in the dendritic liquid crystalline polymers characterized by high molecular weight. This observation is of great importance for the preparation and use of a new type of liquid crystal material in different fields.