title: Preparation and Properties of Electrospun Polyacrylonitrile Fiber Assemblies
Creators: name: Liao, Xiaojian
Publisher: name: University of Bayreuth
Year: 2020
Subjects: subject: Electrospun
subject: Polyacrylonitrile
subject: nanofiber Assemblies
subject: 540

description: 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 contribution of n-π* transitions resulting from the nitrogen atoms in the cyano groups, intersystem crossing occurs that leads to the formation of highly mobile triplet states with quantum energy of about 2.5 eV and a lifetime up to 1 s. Meanwhile, these designed 2D fibrous membranes are also mechanically robust and flexible, with a high specific tensile strength of 534 ± 28 MPa*cm3/g and toughness of 79 ± 7 J/g. The combination of efficient polarized deep blue luminescence, room temperature phosphorescence, triplet-triplet annihilation (non-coherent photon upconversion), mechanically robustness and flexibility of these fibers opens up new avenues for applications of AIE polymers. As shown in the Section 2.3, the pure PAN, highly aligned yarns, consisting of thousands of individual fibrils, were used to investigate the impact of the short fiber length distribution on the compressibility of the sponges. A new method for the preparation of short electrospun fiber dispersions with a controlled fiber length with low coefficient of variation (CV) was presented by cryo-microcutting of multifibrillar, highly oriented, pure PAN yarns. These short fibers exhibit tunable aspect ratios and narrow fiber length distribution, represented by the low CV values in the range of 15 to 20%. The 3D fibrous sponges obtained from these dispersions by freeze-drying showed similar pore size and pore size distribution but significantly higher compression strength and modulus for sponges with a low CV value of short fiber length. From this, we postulate that the mechanical properties of sponges could be tailored over a wide range by adjusting the fiber length, which provides an additional tool for the tuning of sponge properties in addition to chemical treatments. In conclusion, the underlying context of this thesis is creating fiber assemblies with tailored properties through the organization of fiber alignment, hierarchical structures and interactions of fibers in different dimensions. Three kinds of fiber assemblies with unique characteristics in different dimensions were prepared and investigated. The designed 1D fibrous yarns with highly alignment and connection of nanofibrils possess a combination of high strength and toughness. A mesoscopically ordered 2D fibrous membrane made from non-conjugated polymer exhibits polarized blue photoluminescence. The impact of the fiber length distribution on the compressibility of 3D porous sponges is analyzed quantitatively.