Effect of Molecular Weight on Mesomorphic Behavior of Side-Chain Liquid-Crystalline Azopolymers
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Effect of Molecular Weight on Mesomorphic Behavior of Side-Chain Liquid-Crystalline Azopolymers Flores V. Daniela and Larios L. Leticia Departamento de Materiales Avanzados, Centro de Investigación en Química Aplicada, Saltillo, Coahuila, México. ABSTRACT Three methacrylic polymers bearing (phenylene)azobenzene moieties in the side-chain were synthesized via free-radical polymerization of monomer (E)-6-(4-((3’-cyano-4’-(hexyloxy) [1,1’- biphenyl]-4-yl) diazenyl) phenoxy) hexyl methacrylate using 1, 5 and 10 mol% of 1,1’azobis(cyclohexanecarbonitrile) (ABCN) as initiator. The chemical structures of monomer and polymers were confirmed by 1H NMR and FT-IR spectroscopies. Analysis by gel permeation chromatography (GPC) showed average molecular weights (Mw) of 1.0x105, 7.3x104, and 4.5x104 g/mol for polymers P1%, P5%, and P10%, respectively. These results indicate a clear dependence of the Mw on the amount of initiator used; the higher the amount of ABCN, the lowest the molecular mass. Thermotropic liquid-crystalline properties were analyzed by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). All polymers showed a liquid-crystalline behavior over a wide range of temperatures (>100°C) displaying smectic type mesophases. A small shift (around 8°C) upwards in the clearing temperature was observed on increasing the molecular masses from P10% to P1%. The trans-cis photoisomerization of polymers was studied in solution and in thin films by UV-Vis spectroscopy. High cis-isomer contents in solution (>90%) were reached in relative short irradiation times. INTRODUCTION In recent years, liquid-crystalline polymers having azobenzene moieties in the side chain (SCLCPs) have attracted considerable interest because they combine the self-organization properties of liquid crystals1 with the photo-sensitive response of azocompounds2. Among the most interesting SCLCPs are those in which the azobenzene moieties are attached to a methacrylic backbone, because they can be easily prepared by free-radical polymerization of mesogenic methacrylate monomers and show a relatively high thermal stability. In these polymers, the liquid-crystalline (LC) behavior is influenced by a number of factors, including the polymer backbone, spacer, mesogenic group, molecular weight3-5, etc. For instance, a large molecular anisotropy of the mesogen induces a rich polymorphism and high mesophase stabilities6,7. In such polymers, an improvement in the optical photo-induced response has also been observed compared to shorter mesogens. In the case of the molecular weight, an increase of the mesomorphic-mesomorphic and mesomorphic-isotropic phase transition temperatures is observed as the polymer molecular weight increase. Such effects exist up to certain degree of polymerization, and above which the thermal transition temperatures are independent on the molecular weight3-5. Nevertheless, as far as we know, the effect of molecular weight on the LC properties of side-chain polymers having highly anisotropic azobenzene mesogen has been lit
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