ELECTROSTATIC LAYER-BY-LAYER SELF-ASSEMBLY ON COLLOIDAL SPHERES AND CAPSULES COMPOSED OF AZO POLYELECTROLYTES

Abstract

In recent years,core-shell micro-particles and hollow capsules have aroused great research interests as new useful microstructures for many potential applications.In this work,core-shell spheres and hollow capsules containing azo polyelectrolytes were prepared by electrostatic layer-by-layer self-assembly and core-dissolution after the multiplayer deposition.In the electrostatic layer-by-layer self-assembling process,a side-chain azo polyelectrolytes (PEAPE) as polyanion was alternately adsorbed onto surfaces of polystyrene (PS) colloidal spheres together with a polycation of poly(diallyldimethylammonium chloride) (PDAC).PEAPE is a random copolymer composed of acrylic and azobenzene-containing acrylate units.PEAPE samples with degree of functionalization (DF) of 9.3% and 41.4% were used.Solutions of the polyelectrolytes (PEAPE and PDAC) in water and an aqueous medium of 0.1 mol/L NaCl were alternately added into the diluted suspension of PS latex for electrostatic layer-by-layer adsorption.After 15 min,PS spheres were separated and washed by high-speed centrifuge (ultracentrifuge,Hitachi CR 22G).Then the sediment was dispersed in Milli-Q water again for further experiment.The experiment steps were repeated several times until the required layer number was reached.The resulted core-shell particles were characterized by transmission electron microscopy (TEM,JEM-1200EX) and dynamic light scattering (DLS,Malvern Zetasizer 3000 HS).The layer-by-layer adsorption of PEAPE and PDAC was evidenced by the diameter increase from the TEM images and DLS measurements.The oscillation of Zete-potenctial after each adsorption step,which indicates the over charge compensation,also confirmed the successive self-assembly of azo polyelectrolytes.Changing the ionic strength of PDAC solution (in water or in aqueous medium of 0.1 mol/L NaCl) could obviously shift the Zete-potenctial of the colloids,but had little influence on the diameter increasing of the colloidal spheres.The photoisomerization of the azobenzenes was studied by UV-Vis spectroscopy.These azobenzene-containing core-shell particles show typical photochromic effect upon UV light irradiation.After adsorbed to colloidal surfaces,the H-aggregation of azobenzene groups decreased in certain degree and resulted in red shifts in UV-Vis spectrum.As a consequence,the azobenzene chromophores exhibited quickly photo-response behavior upon UV irradiation.To obtain hollow capsules with stable shells,a photosensitive cinnamate-containing polyelectrolyte (PAAEH,DF=27%),synthesized in authors' lab,was applied to induce covalent crosslinking in the shells of colloidal spheres.In the process,PAAEH was adsorbed onto the core-shell particles of PS/(PDAC/PEAPE)_3 through electrostatic self-assembly together with PDAC.In the process,PAAEH was dissolved in Milli-Q water with a concentration of 1 mg/mL and used as polyanions.After two or three layers of PDAC/PAAEH adsorbed,the core-shell colloidal spheres were exposed to UV light (18 mW/cm~2) for 30 min.The photosensitive shells were cross-linked upon UV-light radiation.Then PS core of colloidal spheres was removed by dissolving with tetrahydrofuran/H_2O (V∶V=1∶1).Finally,hollow capsules containing azo chromophores were obtained.Due to their photoresponsive composition, the core-shell particles and hollow capsules may have potential applications in areas such as bio-medicine and photoelectronics.