Preparation and Properties of Electrospun Polyacrylonitrile Fiber Assemblies

Abstract

The aim of this thesis was to prepare and investigate the electrospun fiber assemblies in different dimensions from one-dimensional (1D) fibrous yarns, two-dimensional (2D) fibrous membranes to three-dimensional (3D) fibrous sponges. The underlying concept was to organize the fiber alignment, hierarchical structures and interactions including the macroscopic fibril orientation in the yarns, the interconnection between the fibrils and the macromolecular orientation in the fibrils. Some designed strategies were carried out to pursue different performance in these electrospun fiber assemblies. There is an intrinsic conflict in materials science between high strength and toughness, which is achieved in man-made single polymer nanofibers with ultrasmall diameter (less than 100 nm) by electrospinning. However, these single nanofibers are not robust enough to handle real-world applications. Natural fibers, such as dragline and recombinant spider silk, achieve the combination of high strength and toughness as well but their applicability is limited by either low availability or high prices for various applications. A straightforward concept to combine high strength and toughness was presented through the preparation of 1D polyacrylonitrile (PAN) fibers by yarn electrospinning, which creates fibers consisting of thousands of aligned fibrils in combination with a specified amount of a interconnecting linker – poly(ethylene glycol) bisazide (PEG-BA) (Section 2.1). Stretching at 160 °C with a stretch ratio of eight times could induce a high alignment of fibrils, and the adjacent annealing at 130 °C for 4 h under tension could lead to an azide cycloaddition reaction between the nitrile groups of PAN and bisazide groups of PEG-BA. The microstructures and crystal orientation of yarns with different stretch ratios at 160 °C were characterized by scanning electron microscopy (SEM), polarized Raman spectroscopy, wide/small-angle X-ray scattering and in situ X-ray diffraction measurements. The yarns obtained have a high tensile strength of 1236 ± 40 MPa, a modulus of 13.5 ± 1.1 GPa and a high toughness of 137 ± 21 J/g, which are similar properties to those of dragline spider silk. Furthermore, designed 2D polarized photoluminescent fibrous membranes from non-conjugated polymer were prepared and investigated, as shown in the Section 2.2. We designed three main rules for developing polarized photoluminescent materials from low-cost and easy-processable non-conjugated polymers or aggregation-induced emission (AIE) polymers: i) Electrospinning the non-conjugated or AIE polymer (PAN) into 2D fibrous membrane, ii) aligning the crystal arrangement by sufficient heat-stretching at 160 °C with a stretch ratio at 6 times, and iii) using functional cyano groups that show light-emitting when interactions between adjacent cyano groups lead to the formation of a more extended π-system. The obtained fibrous membrane exhibited a photoluminescence quantum yield of 30–32 %, which is remarkable for a simple non-conjugated polymer. Due to the…