Effect of Molecular Weight on Mesomorphic Behavior of Side-Chain Liquid-Crystalline Azopolymers

摘要

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 'H NMR and FT-IR spectroscopies. Analysis by gel permeation chromatography (GPC) showed average molecular weights (Mw) of 1.0×10~5, 7.3×10~4, and 4.5×10~4 g/mol for polymers Pl%, 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 Pl%. The trans-cis photo-isomerization 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.