含有刚性聚肽亲溶剂链段的共聚物自组装行为

Self-Assembly Behavior of Copolymers Containing Solvophilic Segments of Rigid Polypeptides

首先采用阴离子开环聚合的方法合成了聚苯乙烯-b-聚(γ-苄基-L-谷氨酸酯)(PS-b-PBLG)聚肽嵌段共聚物,然后通过与甲氧基聚乙二醇的酯交换反应部分取代PBLG链段的侧链苄基,得到聚苯乙烯-b-聚(γ-苄基-L-谷氨酸酯-co-聚乙二醇-L-谷氨酸酯)(PS-b-P(BLG/PEGLG))接枝改性嵌段共聚物。使用溶液自组装的方法,以四氢呋喃-N,N-二甲基甲酰胺(THF-DMF)混合溶剂(体积比为1∶1)为起始共溶剂,水为选择性溶剂,制备了PS-b-PBLG和PS-b-P(BLG/PEGLG)共聚物的自组装胶束。使用扫描电子显微镜(SEM)、透射电子显微镜(TEM)、原子力显微镜(AFM)、紫外-可见光光谱仪(UV-Vis)和激光光散射仪(LLS)等测试表征了共聚物自组装体的形貌。研究表明,PS-b-PBLG共聚物自组装形成了球形粒子,而PS-b-P(BLG/PEGLG)接枝改性嵌段共聚物自组装形成了新颖的碗状核壳型粒子。这些组装体中,较为亲水的PBLG或P(BLG/PEGLG)刚性链段形成了外壳,较为疏水的PS链段形成了内核。通过改变共聚物结构(PEG接枝率和接枝长度等)和自组装条件(温度和加水速率等)调控自组装结构。当PEG接枝率较高或接枝链长度较长时,纳米碗的开口较大,并可转变为纳米环。根据研究结果提出了纳米碗形成的机理及其形貌的转变规律。PS-b-P(BLG/PEGLG)接枝改性嵌段共聚物在THF-DMF溶剂中溶解良好,加水诱导共聚物自组装可形成以PS为核、P(BLG/PEGLG)为壳的自组装初级胶束;随着加水量的增加,初级自组装胶束内部的有机溶剂向外扩散,而水不能进入胶束内核,因此胶束发生塌缩形成纳米碗。当未接枝PEG或PEG含量较低时,聚肽链段堆积较为紧密导致外壳较为坚硬,当有机溶剂向外扩散时,胶束形貌不易变形,因此保持了球形结构;而当PEG含量更高时,壳层P(BLG/PEGLG)聚肽链段的运动能力更强,当有机溶剂向外扩散时,胶束开口程度较大,甚至形成纳米环结构。

Polypeptides are amino acid polymers.Due to their unique rigid chain structure and excellent biocompatibility,their self-assembled structures have the advantages of easy regulation,simple preparation,and rich morphology,which have attracted much attention in fields such as biomedicine and nanomaterials.In this paper,the polystyrene-block-poly(γ-benzyl-Lglutamate)(PS-b-PBLG)polypeptide-based block copolymers were obtained via anionic ring opening polymerization.Through transesterification reaction between the benzyl and polyethylene glycol,polystyrene-block-(poly(γ-benzyl-Lglutamate)-co-poly(ethylene glycol)-L-glutamate)(PS-b-P(BLG/PEGLG))block graft copolymers were obtained.The self-assemblies were prepared by a selective precipitation method.PS-b-PBLG or PS-b-P(BLG/PEGLG)copolymers were first dissolved in THF-DMF mixture solvent.By adding water to the solution,core-shell particles were obtained,in which the hydrophobic PS segments formed the spherical core and the hydrophilic P(BLG/PEGLG)segments formed shell.The morphologies and structures of the formed aggregates were characterized by scanning electron microscopy(SEM),transmission electron microscopy(TEM),atom force microscopy(AFM),ultraviolet-visible spectrophotometer(UV-Vis),and laser light scattering measurements(LLS).PS-b-PBLG copolymers form spherical micelles,and PS-b-P(BLG/PEGLG)copolymers form novel bowl-like nanostructures.The effects of length and degree of grafting,assembling temperature and rate of the water addition on the morphologies of the aggregates self-assembled from PS-b-P(BLG/PEGLG)were investigated.With increasing the degree of grafting and graft length,the hollows of bowl-like micelles increase,and eventually toroids are formed.The formation mechanism of nanobowls is proposed.With the induction of water into PS-b-P(BLG/PEGLG)copolymers solution in THF-DMF mixture solvent,primary micelles with a PS core and a P(BLG/PEGLG)shell are formed.As water content increases,the organic solvent within the primary self-assembled micelle core diffuses outward,while the core of the micelles remains impermeable to water.Consequently,the micelles collapse into nanobowls.For the PS-b-PBLG copolymers or the P(BLG/PEGLG)copolymers with low PEG graft ratio,the polypeptide segments in the shell are densely packed,resulting in a relatively rigid shell.During the outward diffusion of the organic solvent,the micellar morphology is less prone to deformation,and a spherical structure is maintained.When the PEG content is higher,the polypeptide segments in the shell exhibit greater mobility.As the organic solvent diffuses outward,the micelles display a larger hollow,and in some cases,even form nanotoroid structures.