Study On The Structure And Self-assembly Behaviors Of The Amphiphilic Dendritic Copolymers

A series of amphiphilic dendritic copolymers(ADPs)with different degrees of amidation(DA)were synthesized by the amidation reaction of polyethyleneimine(HPEI)with dendritic palmitate tails.The hydrodynamic behaviors and the self-assembly process were explored.1.The hydrodynamic radii(R_h)of the polymers were investigated via dynamic light scattering(DLS).It was found that R_h increased with increasing DA,and a sharp increase was noted when DA was near 51 percent.The intrinsic viscosity was measured using an Ubbelohde capillary viscometer to understand the structure evolution of the ADPs.The results showed that the intrinsic viscosity decreased slowly with increasing DA.The Fox-Flory formula was employed to calculate the radius of gyration(R_g).The ratio of R_g and R_h was around 0.77,which indicated that the ADPs were compact.The solvent effects on R_h were investigated which showed that the R_h in chloroform was bigger than that in a poor solvent for HPEI.However,R_h remained unchanged with the increasing volume fraction of poor solvent for HPEI from twenty percent to eighty percent.The solvent quality has no obvious influence on the relationship between R_h and M_n before DA=51 percent,but a sharper slope is noted after DA=51 percent.This phenomenon can be explained by the compact structure of the ADPs.The R_h values remained constant for the synthesized DA within the measured temperature range in chloroform and glycol dimethyl ether.2.The self-assembly process of ADPs with a hydrophilic core and a hydrophobic shell was investigated via laser light scattering.The self-assembly occurs via a fast step and a slow step with different relaxation times.At the critical micelle concentration(CMC),the fusion of small micelles results in the rapid increase of the micelle size in the fast step.The slow step is associated with equilibrium through the fission and fusion of the micelles.The micelle size increases with the unimer concentration,which leads to a lower micelle concentration.The lower micelle concentration makes the relaxation time of the fast step increase with increasing unimer concentration.However,the fusion of larger micelles at higher concentration is more efficient to increase micelle size.The fusion of small micelles with large micelles at higher concentration accelerates the approaching equilibrium of the micelles except for the fission and fusion of micelles.3.The self-assembly process of ADPs from unimers to the final equilibrium micelle is monitored by the laser light scattering.Based on the change of the scattering intensity and hydrodynamic radius with time the detailed self-assembly kinetics is analyzed.The self-assembly contains two steps with different relaxation times.In the first step,the unimers associates quickly to form small micelles and then the small micelles fuse to form large micelles.In the second step,through the fission/fusion of the micelles the self-assembly process approaches equilibrium.The relaxation times of the two steps decreases with the concentration,while the concentration has little influence on the R_h.With the increasing DA,the driving force of the self-assembly increases and the relaxation times decrease.