单根电纺纳米纤维中的结构与分子取向研究

Electrospun nanofibers often exhibit unusual properties as compared to bulk materials, one of which is an exponential increase in modulus when reducing the fiber diameter below an onset value. This is due to their complex internal structure and molecular orientation, which unfortunately are not well understood so far, largely because the limited spatial resolution capability of the traditional characterization techniques employed. AFM-IR is a new spectroscopy technique capable of infrared spectroscopic analysis with a spatial resolution of ~50 nm, and is becoming a powerful structure characterization tool for studying various micro objects. Here we propose to study individual electrospun nanofibers by AFM-IR in conjunction with other techniques, to explore the internal structures, including core-shell structure, molecular orientation in both crystalline and amorphous phases, crystallinity, as well as the distribution of different phases and orientations along the radial and axial directions of the nanofiber, of fibers spun under various conditions. With the detailed structure information obtained, the critical electrospinning parameters that are responsible for the formation of the microstructure and molecular orientation will be identified. Then the relationship between the structure and the modulus will be investigated systematically. The outcome of this research will advance our understanding of structure-property relationship for electrospun nanofibers and provide guidance for production of polymer nanofibers of better properties.

静电纺丝聚合物纤维存在复杂的内部结构和分子取向，因而具有与本体材料相比不寻常的物理性能，其模量随直径减小而指数性增长。然而传统的研究手段缺乏足够的空间分辨力，导致人们对纤维的内部结构所知有限，对于影响纤维性能的结构因素尚不清楚。新近出现的原子力红外光谱技术（AFM-IR）将红外光谱的空间分辨力提高到约50纳米，成为纳微尺度结构和组成分析的有效方法。本项目以AFM-IR为主要手段，探索不同纺丝条件下获得的纤维的内部结构，包括核-壳结构、晶区和非晶区的分子取向、结晶度、不同相区及取向区沿纤维径向及轴向的分布等，以揭示影响纤维中分子取向和多级结构形成的关键因素，进而系统研究纤维结构和分子取向与模量之间的关系，为调控静电纺丝纤维的结构和性能、获得更高强度的电纺纳米纤维提供指导。

电纺纤维中存在复杂的内部结构，通常认为分子链取向对纤维性能具有重要影响。我们以具有纳米尺度分辨力的原子力红外（AFM-IR）为主要手段，以聚偏氟乙烯（PVDF）和聚丙烯腈（PAN）作为结晶和非晶聚合物的代表，研究单根电纺纤维的内部结构，首次建立了偏振AFM-IR分析分子取向的定量方法，发现同一纺丝条件下获得的纤维粗细不同，纤维中分子取向随直径增大而呈指数式下降；以LTA（local thermal analysis，纳米区域热分析）技术研究了单根纤维的熔点，发现纤维熔点随着直径的也呈现类似的变化趋势。两个研究结果说明射流拉伸和松弛导致的分子取向是决定纤维其他特性的关键因素，纤维直径和分子取向都是拉伸的结果，而不是二者互为因果关系。