Abstract
Three sets of novel side-chain liquid crystalline polymers with monosubstituted azobenzene moieties in the side-chain have been studied. These are poly(p-(4′-methoxy-4-oxyhexyloxy azobenzene) benzyl methacrylate) (PPHABM), poly(m-(4′-methoxy-4-oxyhexyloxy azobenzene) benzyl methacrylate) (PMHABM) and poly(o-(4′-methoxy-4-oxyhexyloxy azobenzene) benzyl methacrylate) (POHABM). The chemical structure of the monomers was confirmed by 1H NMR, 13C NMR spectroscopy and elemental analysis. The structural characterisation of the polymers was performed by 1H NMR spectroscopy and gel permeation chromatography, and their phase behaviour and liquid crystalline properties were studied using differential scanning calorimetry, polarised optical microscopy and wide-angle X-ray diffraction. The results show that the transitional behaviour of side-chain liquid crystalline polymers containing monosubstituted azobenzene moieties depends strongly on the position of the substituent on the azobenzene moiety; for example, the ortho-monosubstituted polymers do not form liquid crystalline phases, but all the para- and meta-monosubstituted polymers exhibit a smectic A phase. Furthermore, the glass transition temperature (Tg ) of the polymers decreases in the order, para > meta > ortho. For the PPHABM and PMHABM polymers the isotropic temperature (Ti ) and liquid crystalline range (ΔT, from Tg to Ti ) are found to be in the order, para > meta, although it is surprising that the associated enthalpy changes in these polymers is the opposite order, meta > para.
Acknowledgements
This research was financially supported by the National Nature Science Foundation of China (20874082), the Postgraduate Innovation Fund of Hunan Province (S2008yjscx10), the Scientific Research Fund of Hunan Provincial Education Department (06A068), the Key Project of the Chinese Ministry of Education for Science and Technology (No. 207075) and the New Century Excellent Talents in University (NCET–05–0707).