SYNTHESIS, CHARACTERIZATION AND SELF-ASSEMBLY OF AN AZOBENZENE-CONTAINING AMPHIPHILIC ABA TRIBLOCK COPOLYMER

Atom transfer radical polymerization (ATRP) was applied to prepare an amphiphilic azobenzene-containing ABA triblock copolymer PMAA-b-PMAZOCN-b-PMAA. In the presence of 4,4'-di(n-nonyl)-2,2'-bipyridine (bpy9) and CuBr (as the catalyst) the dibromo-initiator butylene di(2-bromoisobutyrate) was reacted with the azobenzene-containing monomer, 6-[4-(4'-cyanophenylazo)phenoxy]hexyl methacrylate (MAZOCN) which resulted in the macro-initiator. The absolute molecular weight of the macro-initiator was obtained by GPC-SLS (gel permeation chromatography-static light scattering) method. The dn/dc of the macro-initiator was predetermined as 0.257 mL/g by the refractometer. Then CuCl was used as the catalyst,and the macro-initiator further reacted with the tert-butyl methacrylate to obtain the triblock copolymer PtBMA-b-PMAZOCN-PtBMA as a precursor. From H-1-NMR, the polymerization of tBMA monomers initiated by the macro-initiator was proved by the presence of a rather strong resonance at delta = 1.43 which is corresponding to the tert-butyl group. A single Gaussian peak in the GPC curves further proved the narrow polydispersity of the resultant precursor triblock copolymer. The absolute molecular weight of the triblock copolymer was obtained by combination of the segement molar ratio determined from the characteristic H-1-NMR peaks and the absolute molecular weight of the macroinitiator. In the Final step, the tert-butyl groups were removed by the selective hydrolysis reaction with trifluoroacetic acid at room temperature. H-1-NMR analyses proved that most of the tert-butyl groups were hydrolyzed (conversion > 95 %) while the ester groups in the azo blocks were retained. These monomers and polymers were then studied with PLM (polarized light microscopy) and DSC (differential scanning calorimetry). The monomer, macro-initiator and the precursor triblock copolymer are all own smectic phase in the liquid crystalline region,while no characteristic liquid crystalline texture could be observed for the amphiphilic triblock copolymers. It is possibly because that, the presence of H-bonds between carboxyl groups hindered the development of the liquid crystalline phase. The amphiphilic ABA triblock copolymer possessed hydrophilic blocks in the outer part and the hydrophobic block in the central part. The vesicular aggregates of the polymer were obtained by gradually adding water, which is the good solvent of the hydrophilic block and the poor solvent of the azobenzene-containing liquid crystalline block, into the stirring THF solution of the amphiphilic copolymer. When the water content was more than 50 % volume ratio, an excessive amount of water was added to freeze the assembly of the copolymer. THF was removed from the mixed solvent by dialyzed against ultrapure water for days. The TEM (transmission electron microscopy) observation showed that the amphiphilic block copolymer self-assembled into vesicular aggregates with average diameters from 300 to 500 nm. By using a high-pressure mercury lamp as the light source, the vesicular aggregates transform to the colloidal spheres when irradiated with the ultraviolet light in the solid state.