Self-assembly Behaviors Of Azo-copolymers Containing Polypeptide Block And Their Photoisomerization

This article reported study of influences of two kinds of rigid structures??-helix secondary structure formed by intramolecular hydrogen bonding of polypeptide and?-?stacking of azobenzene?to self-assembly behaviors and photoisomerization.The rigid structures were constructed in one copolymer systems,forming block?A-b-?B-g-C??,star shaped??C??A??B??and graft?C-b-?B-g-A??molecular structures.Effects of internal and external factors to morphologies of aggregations were investigated in detail,and influences of block ratio and molecular aggregation degree to isomerization behaviors were well studied.?1?Self-assembly behaviors of block graft copolymer poly[6-?4'-methoxy-4-oxy-azobenzene?hexyl methacrylate]-b-poly[?-poly?ethylene glycol?methyl ether group-L-glutamate]?PAzoMA-b-?PELG-g-MPEG??in tetrahydrofuran?THF?solution were studied by fourier transform infrared spectroscopy?FT-IR?,scanning electron microscopy?SEM?,transmission electron microscopy?TEM?,optical microscope?OM?and dynamic light scattering?DLS?,results demonstrated that when THF:H₂O=5:3?v/v?,aggregations were bowl shaped vesicles with diameter of 493-554nm,the forming mechanism of the cave on surfaces of vesicles was heterogeneous shrinkage caused by high rigidity of aggregations.When THF:H₂O=1:1?v/v?,diameter of bowl shaped vesicles increased to 1.30?m due to increase of the average number of polymer chains in an aggregate(N_agg).This is the first report of giant vesicles self-assembled by block graft azobenzene-containing copolymer with polypeptide block,and copolymer packed in a head-to-head arrangement to form a bilayer structure in vesicle walls.Photoisomerization behaviors of copolymers in solvent and aggregations were studied by UV-Vis spectroscopy,results demonstrated that in aggregations,azobenzene of copolymer with lower and higher block ratio of PAzoMA block was mainly in non-association state and J-aggregation state respectively,this was because?-helix secondary structure formed by polypeptide block can loosen the package of copolymer chains in aggregations.?2?Self-assembly behaviors of 3-miktoarm star terpolymer poly?ethylene glycol?methyl ether-poly[6-?4'-methoxy-4-oxy-azobenzene?hexyl methacrylate]-poly??-benzyl-L-glutamate???MPEG??PAzoMA??PBLG??in THF solution were studied by FT-IR,SEM,TEM,atomic force microscope?AFM?and DLS,results demonstrated that when initial concentration was 0.1mg/mL,and THF:H₂O=5:3?v/v?,the aggregations was spindle shaped micelles,the length,width and height of the micelles were 213nm,81nm and 57nm respectively.PBLG and PAzoMA arms packed parallelly along the long axis of spindle shaped micelles.Because the distance between MPEG chains on aggregations was large and MPEG chains can not reduce the interfacial energy between hydrophobic micelle core and solvent efficiently,terpolymer chains were inclined to aggregate into spindle shaped aggregations with low interfacial area.Photoisomerization behaviors of terpolymers in solvent and aggregations were studied by UV-Vis spectroscopy,results demonstrated that azobenzene in aggregations was mainly in non-association state and the isomerization rate decreased indistinctively compared with azobenzene in solvent,this was due to homogeneous micelle cores formed by PAzoMA and PBLG and?-?stacking interaction between azobenzene side groups on PAzoMA and benzyl side groups on PBLG,azobenzene in the non-association state had large free volume and high isomerization rate.?3?Self-assembly behaviors of block graft copolymer poly?ethylene glycol?methyl ether-b-poly[?-three branched azobenzene group-L-glutamate]?MPEG-b-PELG-g-?MOAPB₆?₃?in THF solution were studied by FT-IR,SEM and TEM,results demonstrated that when initial concentration was 0.6mg/mL,and THF:H₂O=1.5:1?v/v?,the aggregations were bowl shaped vesicles with diameter of 329nm.When initial concentration was 0.9mg/mL,and THF:H₂O=2:1?v/v?,the aggregations were giant large compound vesicles?LCV?with diameter of 1.82?m,polypeptide blocks of copolymers packed vertically and formed a four-layered structure in LCV walls,the forming mechanism of LCV was as follows:copolymer chains first aggregated into vesicles,due to the weak repulsion force among solvophilic MPEG chains on the surface of vesicles and insufficient reduction of interfacial energy between hydrophobic vesicle wall and solvent,the vesicles aggregated into LCV to decrease interfacial area.Photoisomerization behaviors of copolymers in solvent and aggregations were studied by UV-Vis spectroscopy,results demonstrated that isomerization degree of azobenzene maintained high after copolymers aggregated,and azobenzene in aggregations was mainly in non-association state,this was because the adequate free volume provided by long azobenzene side groups of polypeptide block.