Synthsis And Properties Of Azobenzene Polymer Containing 1,3,4-oxadiazole Or 1,3,4-thiadiazole Unit

In this paper, five polymers containing azobenzene and 1,3,4-oxadiazole unit (p1, p2, p3, p4, p5) were synthesized by a two step polycondensation reactions with 5-(4-alkoxy phenylazo) isophthaloyl dihydrazide (6a,6b,6c) and two dichlorides. Polyimides containing azobenzene and 1,3,4-thiadiazole were synthesized with 5-(4-hexadecyloxy phenylazo) isophthaloyl dichloride and novel bis-1,3,4-thiadiazole amine compounds (la,lb). The structures of monomers and polymers were confirmed by IR, UV-vis and 1H NMR. The introduction of long chain of alkoxy azobenzene significantly improves polymers'(pl, p3, p4, p16-10) solubility in common organic solvents such as chloroform and tetrahydrofuran etc. p2, p5, p16-6 can dissolve in NMP easily. The molecular weights and molecular weight distributions of the polymers were determined by gel permeation exclusion chromatography (GPC). The Mw are 9774 g/mol (p1),19801 g/mol (p3),15612 g/mol (p4),22814 g/mol (p5), 28804 g/mol (p16-10),24832 g/mol (p16-6); PDI are1.625 (p1),1.611 (p3),1.709 (p4),1.723 (p5),1.708 (pl6-10),1.738 (p16-6), respectively. The intrinsic viscosities are 0.0210 dL·g-1 (p1),0.0394 dL·g-1 (p2),0.0791 dL·g-1 (p3),0.0562 dL·g-1 (p4),0.0819 dL·g-1 (p5),0.1161 dL·g-1 (p16-10),0.0932 dL·g-1 (p16-6), respectively. The rigidity of the conjugated polymers (p1-p5) in the main chain is better, and the intrinsic viscosity are small, the intrinsic viscosity of the nonconjugated polymers (p16-10, p16-6) in the main chain are relatively great. In addition, the intrinsic viscosity of pl-p5 and p16-10, p16-6 are creasing with the molecular weights creasing, respectively. The thermal stability of polymers are characterized by TG-DTA, the 5% weight loss of the pl-p5, p16-10 and p16-6 are 332℃,335℃,345℃, 347℃,359℃,320℃,322℃, respectively. The trans-cis photoisomerization of monomers and polymers take place under the radiation of 365 nm UV light. The efficiency of trans-cis photoisomerization (R), the content of cis-isomer (Y) and the rate constant of photoisomerization (Kp) of monomer (6a,6b,6c) and polymers (p1, p2, p3, p4, p5, p16-10, p16-6) are calculated, R are 83.14%(6a),83.54%(6b),88.65%(6c),59.32%(pi),64.15% (p2),76.79%(p3),71.78%(p4),71.26%(p5),82.66%(p16-10),73.68%(p16-6), respectively; Y are 87.34%(6a),87.76%(6b),93.04%(6c),62.50%(p1),67.35%(p2), 80.63%(p3),75.37%(p4),74.82%(p5),86.80%(p16-10),77.36%(p16-6), respectively; kp are 0.6130 min-1 (6a),0.7500 min-1 (6b),0.4610 min-1 (6c),0.6466 min-1 (p1),0.2078 min-1 (p2),0.3845 min-1 (p3),0.2501 min-1 (p4),0.2155 min-1 (p5),0.9003 min-1 (p16-10),0.9817 min-1 (p16-6), respectively. The R and Y of polymers are smaller than that of the monomers, which is attributed to the increased steric hindrance in polymers and the trans-cis photoisomerization of azobenzene is limited. The molar absorption coefficient (ε) and energy gap (Eg) were calculated,εare 2.8x10-4L·mol-1·cm-1(p1), 3.4×10-4L·mol-1·cm-1(p2),5.1×10-4 L·mol-1·cm·1(p3),7.9×10-4L·mol-1·cm-1(p4),6.5×10-4L·mol-1·cm-1(P5),3.6×10-4L·mol-1·cm-1 (p16-10),3.0x10-4L·mol-1·cm-1 (p16-6), respectively; Eg are 2.081 eV (p1),2.119 eV (p2), 2.275 eV (p3),2.226 eV (p4),2.153 eV (p5),2.348 eV (pl6-10),2.322 eV (p16-6), respectively. The dates indicate Eg of polymers are small between 2-3 eV, which shows the polymers synthesized have the property of electron transmission. The conjugated degree of conjugated polymers (pl-p5) in the main chain is near, and the Eg of polymer (pl-p5) are greater than p16-10 and p16-6, which indicates that the conjugated capability of the polymers containing oxadiazole is better than the polymers containing thiadiazole. Therefore, the conductive property of polymers containing oxadiazole is better than that of polymers containing thiadiazole. The emission peak of polymers have one wide peak when the concentration is 1x10-6g·mL-1, the peak of THF solution of polymer (p1, p3, p4, p16-10, p16-6) is located at about 375nm,360nm,410nm,375nm,395nm, and the peak of NMP solution of polymer (p2, p5) is located at about 472 nm,442 nm, respectively. All the polymers emitted indigo blue fluorescence. The results indicated that azobenzene polymers with and oxadiazole or thiadiazole containing long chain of alkoxy unit show good solubility, good thermal, trans-cis photoisomerzation property and fluorescence property, which can be used in the filed of optical and electrical functional polymer materials.