Synthesis and phase structures of combined main-chain/side-chain liquid crystalline polymers with different-number azobenzene moiety in the side-chain based on mesogen-jacketed liquid crystalline polymers

Abstract

Three novel combined main-chain/side-chain liquid crystalline polymers with different number of azobenzene mesogens in the side chain per repeating unit have been synthesised. These are poly(2,5-bis{[3-(4′-methoxy-4-oxyhexyloxy azobenzene)benzyl]oxycarbonyl}styrene) (denoted as PABCS), poly(2,5-bis{[3,5-di(4′-methoxy-4-oxyhexyloxy azobenzene)benzyl]oxycarbonyl}styrene) (denoted as PDABCS) and poly (2,5-bis{[3,4,5-tri(4′-methoxy-4-oxyhexyloxy azobenzene)benzyl]oxycarbonyl}styrene) (denoted as PTABCS). The chemical structure of the monomers was confirmed by ¹H NMR and ¹³C NMR. The structural characterisation of the polymers was performed by ¹H NMR and gel permeation chromatography (GPC), and the phase structures and transitions of the polymers were studied using differential scanning calorimetry (DSC), polarised light microscopy (PLM) and one-dimensional (1D) wide-angle X-ray diffraction (WAXD). The results show that the number of azobenzene mesogens in the side chain per repeating unit in the combined main-chain/side-chain liquid crystalline polymers plays an important role in dominating the phase type preferences, and the polymers exhibit more versatile and intriguing liquid crystalline properties with increasing number of azobenzene mesogens. First, the main chains of PABCS construct a 2D-centred rectangular (Φ_R) scaffold and the packing of azobenzene-containing side chains inside the main-chain scaffold develops smecitc A (SmA)-like structure below isotropic temperature (T_i). Second, with increasing the number of azobenzene mesogens, the main chains of PDABCS develop a 2D-centred rectangular (Φ_R) scaffold at lower temperature and a 2D hexagonal columnar (Φ_H) phase at higher temperature, and the packing of side chains inside the main-chain scaffold undergoes the transition of SmA-like ↔ isotropic. Third, for PTABCS, the sequence of phase transitions followed is: 2D centred rectangular columnar phase (Φ_R) scaffold of main chain and SmB-like structure of side chain ↔ 2D centred rectangular columnar phase (Φ_R) scaffold of main chain and SmA-like structure of side chain ↔ 2D centred rectangular (Φ_R) scaffold of main chain and isotropic side chain ↔ 2D long-range-ordered hexagonal columnar phase (Φ_H) of main chain and isotropic side chain. The 2D long-range-ordered Φ_H phases of the polymers PDABCS and PTABCS still remain before the samples are completely decomposed.

Keywords:

• main-chain/side-chain liquid crystalline polymer
• mesogen-jacketed liquid crystalline polymer
• azobenzene moiety
• rectangular Φ_R
• hexagonal columnar Φ_H

Acknowledgements

This work is supported by the National Natural Science Foundation of China (NNSFC Grants Nos. 21106037, 21104062), the Scientific Research Fund of the Hunan Provincial Education Department (No. 11A040), Hunan Provincial Natural Science Foundation of China (No. 12JJ4015), the Scientific Research Project of Hunan Institute of Science and Technology (No. 2011Y04) and the Doctoral Foundation of Ministry of Education of China (No. 20114301120005).