The effects of para-, meta- and ortho-monosubstituted azobenzene moiety in the side chain on phase behavior of mesogen-jacketed liquid crystalline polymers

Three different mesogen-jacketed liquid crystalline polymers with monosubstituted azobenzene moiety in the side-chain have been studied. These are poly(2,5-bis{[para-(4′-methoxy-4-oxyhexyloxy azobenzene) benzyl] oxycarbonyl} styrene) (denoted as PPABCS), poly(2,5-bis{[meta-(4′-methoxy-4-oxyhexyloxy azobenzene) benzyl] oxycarbonyl} styrene) (denoted as PMABCS) and poly(2, 5-bis{[ortho-(4′-methoxy-4-oxyhexyloxy azobenzene) benzyl] oxycarbonyl} styrene) (denoted as POABCS). The chemical structures of the monomers were confirmed by 1H NMR, 13C NMR spectroscopy and elemental analysis. The structure characterization of the polymers was performed by 1H NMR spectroscopy and gel permeation chromatography (GPC), and the phase structures and transitions of the polymers were studied using differential scanning calorimetry (DSC), polarized light microscopy (PLM), and one- and two-dimensional (1D, 2D) wide-angle X-ray diffraction. The effects of monosubstituted azobenzene moiety in different positions on the liquid crystalline behaviors of the polymers were also investigated. The results show that the phase transitional behaviors of mesogen-jacketed liquid crystalline polymers containing monosubstituted azobenzene moiety depend strongly on the position of the substituent on the azobenzene moiety. We identify that PPABCS can form a hierarchically ordered structure with double orderings on both the nanometer and subnanometer length scales. Most likely, the thick main-chains of PPABCS obtained by “jacketing” the central rigid portion of side-chain to the polyethylene backbone construct a 2D centered rectangular scaffold. The azobenzene-containing side-chains pack inside the main-chain scaffold form smecitc A (SmA)-like structure and are perpendicular to the main-chains. We compared PPABCS with PMABCS and POABCS, and found that the hierarchically ordered structure of PMABCS was similar to that of PPABCS. It is surprising that the main-chains of POABCS also construct a 2D centered rectangular scaffold, however, the packing of azobenzene-containing side-chains inside the main-chain scaffold develops smecitc C (SmC)-like structure. Furthermore, the glass transition temperature (Tg), isotropic temperature (Ti) and liquid crystalline range (ΔT, from Tg to Ti) of the polymers decrease in the order, para > meta > ortho. It is very interesting phenomenon that the associated enthalpy changes of these polymers are also the same order, para > meta > ortho, which is different from those of MCLCPs and SCLCPs.

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