聚乙二醇-接枝-聚(γ-苄基L-谷氨酸酯)刚-柔接枝共聚物的合成及其自组装行为研究

Synthesis and Self-assembly of Poly(ethylene glyco1)-g-Poly(γ-benzyl-L-glutamate) Rod-Coil Graft Copolymers

通过负离子开环聚合,合成了以柔性亲水的聚乙二醇(PEG)为主链,刚性疏水的聚(γ-苄基L-谷氨酸酯)(PBLG)为侧链的PEG-g-PBLG刚-柔接枝共聚物.运用核磁共振氢谱(1H-NMR)、傅里叶变换红外光谱(FTIR)和凝胶渗透色谱(GPC)等表征了共聚物的结构、分子量及其分布.以共溶剂溶解、选择性溶剂透析的方法制备了自组装聚集体.利用扫描电子显微镜(SEM)、透射电子显微镜(TEM)、原子力显微镜(AFM)和激光光散射(LLS)等表征了共聚物自组装体的形貌和结构.研究发现,基于其特殊的拓扑结构,接枝共聚物的自组装行为表现出与一般规律不同的变化趋势.具有较短疏水PBLG侧链的PEG-g-PBLG可以自组装形成球形复合胶束,随着疏水PBLG侧链的增长,聚集体逐渐由复合胶束转变为囊泡,这种现象在已有的研究中鲜有报道.此外,降低初始聚合物溶液的浓度,共聚物自组装得到的聚集体的尺寸变小而分布变宽;反之,聚集体的尺寸增大而分布变窄.还利用耗散粒子动力学方法,验证了实验中聚集体的形貌变化,并给出了聚集体中的链段分布等在实验中较难得到的信息.

Poly（ethylene glyco1）-g-poly（γ-benzyl-L-glutamate）（PEG-g-PBLG） graft copolymers, bearing hydrophilic PEG backbone and hydrophobic PBLG side chain, were synthesized via ring opening polymerization of γ-benzyl-L-glutamate-N-carboxyanhydride（BLG-NCA） using PEG-g-NH2 as the initiator. The molecular weight and polydispersity index of the graft copolymers as well as conformation of PBLG chains were characterized by proton nuclear magnetic resonance（1H-NMR）, gel permeation chromatography（GPC）, and Fourier transform infrared spectrum（FTIR）, respectively. Self-assemblies were prepared by a selective solvent method with tetrahydrofuran（THF） as the initial common solvent and water as the selective solvent. The critical water content（CWC） for the graft copolymer aggregation was determined by turbidity measurement（OD）. The CWC value decreases from 17.3 wt% to 12.8 wt% gradually with the increase in PBLG length（the initial concentration is 0.5 g L,1）. The morphologies and structures of the formed aggregates were investigated by scanning electron microscopy（SEM）, transmission electron microscopy（TEM）, atom force microscopy（AFM）, and laser light scattering measurements（LLS）. The results show that the graft copolymers are able to self-assemble into spherical compound micelles when the PBLG side chain is relatively short. As the PBLG length increases, the aggregates gradually transform to rod-like aggregates and vesicles. In addition to the experiments, the self-assembly of the graft copolymers was investigated using dissipative particle dynamics（DPD） simulation based on a coarse-grained coil-g-rod graft copolymer model. The simulation results were in agreement with the experimental findings, and additional information such as chain distribution of copolymers in aggregates was obtained. Besides, it was found that the initial polymer concentration affected the aggregate size but no significant influence was observed on the aggregate morphology. When prepared at higher initial polymer concentration（1.0 g L,1）, the aggregates obtained were much larger in size and narrower in size distribution. Finally, based on both the experimental and the simulation results, a mechanism for the self-assembly of PEG-g-PBLG graft copolymers was proposed.