三嵌段聚乳酸在溶剂中结晶驱动自组装

Crystallization-Driven Self-assembly of Poly(L-lactide)-b-Polydimethylsiloxane-b-Poly(L-lactide) Triblock Copolymers from Solvent

以端羟基聚二甲基硅氧烷(HO-PDMS-OH)为引发剂,引发L-丙交酯(L-LA)开环聚合,合成了4种三嵌段聚合物聚(L-丙交酯)-b-聚二甲基硅氧烷-b-聚(L-丙交酯)(PLLA-b-PDMS-bPLLA)。其中,聚二甲基硅氧烷(PDMS)含有74个结构单元,聚(L-丙交酯)(PLLA)的结构单元数分别是82、150、224和280。用凝胶渗透色谱仪(GPC)表征了聚合物分子量及分子量分布。将这4种三嵌段聚合物分别溶解在N,N-二甲基甲酰胺(DMF)中,在恒温水槽中静置48h,可分别组装成螺旋线聚集体以及"eye-like"聚集体。PLLA-b-PDMS-b-PLLA的组装主要由PLLA嵌段的结晶驱动。最后采用广角X射线衍射(WAXD)、透射电镜(TEM)、原子力显微镜(AFM)对聚集体进行了相关表征。

Triblock copolymers poly（L-lactide）-b-polydimethylsiloxane-b-poly（L-lactide）（PLLA-b-PDMS-bPLLA） were synthesized by ring-opening polymerization of L-lactide（L-LA） initiated with hydroxymethylpolysiloxane（HO-PDMS-OH）.The resulted block copolymers contained 74 dimethylsiloxane（DMS）residue units,and variable L-LA block which contains 82,150,224 and 280 PLLA units.The self-assemblies of these PLLA-b-PDMS-b-PLLA triblock copolymers were investigated in N,Ndimethylformamide（DMF）in order to avoid the adverse effect of water on the morphologies of the aggregates.The molecular weight of PLLA blocks greatly influences the morphologies of aggregates.Helical nanowires with several tens of micrometer in length were obtained in 10 mg/mL PLLA（140）-bPDMS（74）-b-PLLA（140） solution.While the self-assembly of PLLA（112）-b-PDMS（74）-b-PLLA（112） led to hierarchal＂eye-like＂crystals with about 2.6μm in long axis and about 1.5μm in short axis.PLLA（75）-b-PDMS（74）-bPLLA（75） could also assemble to ＂eye-like＂crystals but with smaller dimension.No assembly was found with copolymer PLLA（41）-b-PDMS（74）-b-PLLA（41） under the same condition due to the fact that the PLLA blocks were too short to crystalize in DMF.In addition,the morphology of aggregates was influenced by temperature as well.At-15 ℃ and 0 ℃,PLLA（112）-b-PDMS（74）-b-PLLA（112） self-assembled to nanowires accompanied with small diamond-shaped crystals and ribbons,respectively.Hierarchal＂eye-like＂crystals with different dimensions were obtained in the temperature range of 25—40℃.When the temperature was raised to 50 ℃,only small micelles could be observed in solution.Under higher polymer concentration（20 mg/mL PLLA（112）-b-PDMS（74）-b-PLLA（112））,＂eye-like＂crystals could be observed in the temperature range of 30—50 ℃,and the dimension decreased with the increasing temperature.In conclusion,the aggregate of PLLA-b-PDMS-b-PLLA was mainly controlled by the crystallization of PLLA block.If the PLLA blocks underwent rapid precipitation without sufficient folding for crystallization,the copolymer tended to assemble into helical nanowires（low temperature or high molecular weight）.In contrast,mild temperature and suitable PLLA length for folding and crystallization resulted in hierarchal ＂eye-like＂crystals.High temperature and too short PLLA block were not favorable for crystallization,therefore only micelles or nothing could be obtained.This study is a necessary complement to the crystallization-driven self-assembly of amphiphilic triblock copolymers.