Recent progress in electrospun nanofibers: Reinforcement effect and mechanical performance

Composite materials are becoming increasingly important as structural materials for aeronautical and space engineering, naval, automotive, and civil engineering, sporting goods, and other consumer products. Fiber‐based reinforcement represents one of the most effective manufacturing strategies for enhancing the mechanical strength and other properties of composite materials. Electrospinning has gained widespread interest in the last two decades because of its ability to fabricate continuous ultrafine nanofibers with unique characteristics. The impact of electrospinning on fiber synthesis and processing, characterization, and applications in drug delivery, nanofiltration, tissue scaffolding, and electronics has been extensively studied in the past. In this article, the authors have focused on a comprehensive review of the mechanical performance and properties of electrospun nanofibers as potential reinforcements as well as their advanced nanocomposites. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1171–1212     

A review on electrospun nanofibers for multiple biomedical applications

Electrospinning is a well-known technique since 1544 to fabricate nanofibers using different materials like polymers, metals oxides, proteins, and many more. In recent years, electrospinning has become the most popular technique for manufacturing nanofibers due to its ease of use and economic viability. Nanofibers have remarkable properties like high surface-to-volume ratio, variable pore size distribution (10–100 nm), high porosity, low density, and are suitable for surface functionalization. Therefore, electrospun nanofibers have been utilized for numerous applications in the pharmaceutical and biomedical field like tissue engineering, scaffolds, grafts, drug delivery, and so on. In this review article, we will be focusing on the versatility, current scenario, and future endeavors of electrospun nanofibers for various biomedical applications. This review discusses the properties of nanofibers, the background of the electrospinning technique, and its emergence in chronological order. It also covers the various types of electrospinning methods and their mechanism, further elaborating the factors affecting the properties of nanofibers, and applications in tissue engineering, drug delivery, nanofibers as biosensor, skin cancer treatment, and magnetic nanofibers.     

Tensile deformation of electrospun nylon‐6,6 nanofibers

Nylon‐6,6 nanofibers were electrospun at an elongation rate of the order of 1000 s^?1 and a cross‐sectional area reduction of the order of 0.33 × 10⁵. The influence of these process peculiarities on the intrinsic structure and mechanical properties of the electrospun nanofibers is studied in the present work. Individual electrospun nanofibers with an average diameter of 550 nm were collected at take‐up velocities of 5 and 20 m/s and subsequently tested to assess their overall stress–strain characteristics; the testing included an evaluation of Young's modulus and the nanofibers' mechanical strength. The results for the as‐spun nanofibers were compared to the stress–strain characteristics of the melt‐extruded microfibers, which underwent postprocessing. For the nanofibers that were collected at 5 m/s the average elongation‐at‐break was 66%, the mechanical strength was 110 MPa, and Young's modulus was 453 MPa, for take‐up velocity of 20 m/s—61%, 150 and 950 MPa, respectively. The nanofibers displayed α‐crystalline phase (with triclinic cell structure). © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1482–1489, 2006     

Phosphorylated polyacrylonitrile‐based electrospun nanofibers for removal of heavy metal ions from aqueous solution

The phosphorylated polyacrylonitrile‐based (P‐PAN) nanofibers were prepared by electrospinning technique and used for removal of Cu²⁺, Ni²⁺, Cd²⁺, and Ag⁺ from aqueous solution. The morphological and structural properties of P‐PAN nanofibers were characterized by scanning electron microscope and Fourie transform infrared spectra. The P‐PAN nanofibers were evaluated for the adsorption capacity at various pH, contact time, and reaction temperature in a batch system. The reusability of P‐PAN nanofibers for the removal of heavy metal ions was also determined. Adsorption isotherms and adsorption kinetics were also used to examine the fundamental adsorption properties. It is found that the P‐PAN nanofibers show high efficiency, and the maximal adsorption capacities of metal ions as calculated from the Langmuir model were 92.1, 68.3, 14.8, and 51.7 mg/g, respectively. The kinetics of the heavy metal ions adsorption were found to follow pseudo‐second‐order rate equation, suggesting chemical adsorption can be regarded as the major factor in the adsorption process. Sorption/desorption results reveal that the obtained P‐PAN nanofibers can remain high removal efficiency after four cycles.     

Different types of electrospun nanofibers and their effect on microfluidic‐based immunoassay

Protein capturing on polymeric substrate of microfluidic devices is a key factor for the fabrication of immunoassay with high sensitivity. In this work, simple and versatile technique of electrospinning was used to produce electrospun nanofibrous membranes (e.NFMs) with high surface area as a substrate for microfluidic‐based immunoassay to increase sensitivity. It was found that the simultaneous use of e.NFM and 1‐Ethylethyl‐3‐(3‐dimethylaminopropyl)‐carbodiimide/N‐Hydroxysuccinimide hydroxysuccinimide as coupling agent has synergic effect on antigen immobilization onto the microchannels. It was found that the oxygen plasma technique for the creation of oxygen containing functional group like carboxyl and hydroxyl causes extreme leakage of solution through the microchannels. Thus, due to capillary effect, it is impossible to use hydrophilic substrate to modify microchannels. In order to compensate this problem, it is propose to utilize other type of polymer for the fabrication of nanofiber to answer this important question that if it is possible to enhance the sensitivity of immunoassay just by changing the polymer type? For this purpose, four different polymers, namely, polycaprolactone, poly lactic‐co‐glycolic acid, poly L‐lactic acid, and polyethersolfone were used as the based material for e.NFM fabrication. Results showed that compared with plain poly (dimethylsiloxane) surface of microchannels, poly lactic‐co‐glycolic acidand poly L‐lactic acid, which inherently contain end‐group of carboxyl in their chemical structure, can improve the protein immobilization, which leads to immunoassay signal enhancement through 1‐ethyl‐3‐(3‐dimethylaminopropyl)‐carbodiimide/N‐hydroxysuccinimide coupling chemistry, significantly.     

Enhanced conductivity composites for aircraft applications: carbon nanotube inclusion both in epoxy matrix and in carbonized electrospun nanofibers

The potential of carbonized electrospun nanofiber mats to render epoxy resin composites for aircraft applications electrically and thermally more conductive was investigated. The effect of carbon nanotube inclusion both inside the carbon nanofiber and in the epoxy resin matrix material was studied, in order to reveal any synergistic effects of multilevel presence of nanosized reinforcements on the conductivity and mechanical properties. The carbon nanotube inclusion into the carbonized nanofibers increased the electrical conductivity of the samples by 20–50% and the thermal conductivity by approximately three times leading to a higher value than that of the conventional composites. The preparation of layered composites with a conductive upper layer containing nonwoven carbon nanofabric and a load bearing lower layer with conventional unidirectional carbon fiber reinforcement can offer a cost‐effective and weight‐saving solution for the replacement of metal meshes in structural aircraft composites. Copyright © 2014 John Wiley & Sons, Ltd.     

Bioconjugation of alkaline phosphatase to mechanically processed, aqueous suspendible electrospun polymer nanofibers for use in chemiluminescent detection assays

Aqueous suspendible polymer nanostructures were prepared by simple microtome processing of electrospun nylon 6 nanofibers and were used to immobilize calf intestinal alkaline phosphatase (ALP) by either covalent or noncovalent bioconjugation chemistries. It was found that noncovalent immobilization of ALP to the mechanically cut nanofibers (mean length approximately 4 microm; mean diameter approximately 80 nm) using a multi-stacked, layer-by-layer (LBL) approach with the cationic polymer Sapphire II resulted in the highest enzyme loading (48.1 +/- 0.4 microg . mg(-1) nanofiber) when compared to other covalent immobilization methods based on glutaraldehyde crosslinking. The biofunctionalized nanofibers were also characterized for their chemiluminescent activity with the dioxetane substrate, CSPD. The results indicate that the kinetic parameters, K(m) and V(max), for the catalytic activity of the nanostructure-bound ALP enzyme were influenced by the particular types of immobilization methods employed. In terms of the overall catalytic performance of the various immobilized ALP systems, a single-stacked LBL assembly approach resulted in the highest level of enzymatic activity per unit mass of nanofiber support. To the best of our knowledge, this study represents the first report examining the preparation of mechanically shortened, aqueous dispersed electrospun polymer nanofibers for potential application as enzyme scaffolds in chemiluminescent-based assay systems.     

Thermal and mechanical properties of electrospun PMMA,PVC, Nylon 6, and Nylon 6,6

Poly(methyl methacrylate) (PMMA), poly(vinyl chloride) (PVC), Nylon 6, and Nylon 6,6 have been electrospun successfully. The nanofibers have been characterized by scanning electron microscopy (SEM), confirming the presence of bead free and fiber‐bead free morphologies. Thermogravimetric analysis (TGA) indicated differences between the thermal stability of PMMA nanofibers and PMMA powder. However, no significant differences were observed between the starting physical form (powder or pellet) of PVC, Nylon 6 and Nylon 6,6, and their corresponding electrospun nanofibers. Differential scanning calorimetry (DSC) demonstrated a lower glass transition temperature (T_g) and water absorption for PMMA electrospun nanofibers. Furthermore, electrospun Nylon 6 and Nylon 6,6 had a slight decrease in crystallinity. Tensile testing was performed on the electrospun nanofibers to obtain the Young modulus, peak stress, strain at break, and energy to break, revealing that the non‐woven mats obtained had modest mechanical properties that need to be enhanced. Copyright © 2007 John Wiley & Sons, Ltd.     

ZIF-8/改性聚丙烯腈电纺纳米纤维的制备及其高效去除水中孔雀石绿

通过原位生长方法，将最常见的金属有机骨架（MOFs）——沸石咪唑酯骨架材料（ZIF‐8）固定到羧甲基化聚丙烯腈静电纺丝纳米纤维（PAN‐COOH NFs）表面，得到ZIF‐8/PAN‐COOH NFs。通过扫描电子显微镜（SEM）、能量色散光谱（EDS）、X射线粉末衍射（XRD）和傅里叶变换红外光谱（FTIR）对合成的ZIF‐8/PAN‐COOH NFs形貌和结构进行表征，并深入研究ZIF‐8/PAN‐COOHNFs从废水中去除孔雀石绿（MG）的性能。研究发现： ZIF‐8/PAN‐COOH NFs对MG的吸附符合拟二级动力学方程，吸附过程可采用Langmuir等温线模型拟合，其对MG的最大吸附容量可达3 604 mg·g^-1。此外，ZIF‐8/PAN‐COOH NFs在染料吸附实验中表现出良好的分离功能和重复利用性。     

ZIF-8/改性聚丙烯腈电纺纳米纤维的制备及其高效去除水中孔雀石绿

通过原位生长方法,将最常见的金属有机骨架(MOFs)——沸石咪唑酯骨架材料(ZIF-8)固定到羧甲基化聚丙烯腈静电纺丝纳米纤维(PAN-COOH NFs)表面,得到ZIF-8/PAN-COOH NFs。通过扫描电子显微镜(SEM)、能量色散光谱(EDS)、X射线粉末衍射(XRD)和傅里叶变换红外光谱(FTIR)对合成的ZIF-8/PAN-COOH NFs形貌和结构进行表征,并深入研究ZIF-8/PAN-COOHNFs从废水中去除孔雀石绿(MG)的性能。研究发现： ZIF-8/PAN-COOH NFs对MG的吸附符合拟二级动力学方程,吸附过程可采用Langmuir等温线模型拟合,其对MG的最大吸附容量可达3 604 mg·g^-1。此外,ZIF-8/PAN-COOH NFs在染料吸附实验中表现出良好的分离功能和重复利用性。     

Influence of solvents properties on morphology of electrospun polyurethane nanofiber mats

Segmented polyurethane (SPU) nanofiber mats were prepared by electrospinning technique using the combination of four different solvents viz. tetrahydrofuran, N,N′‐dimethyl formamide, N,N′‐dimethyl acetamide, and dimethyl sulfoxide. Morphology of the electrospun nanofibers was examined by field emission scanning electron microscope. Experimental results revealed that the morphologies of polyurethane nanofiber mats have been changed significantly with the solvent selection for the electrospinning. It was observed that the diameters and morphology of the SPU nanofibers were influenced greatly by the use of combination of solvents. The uniform polyurethane nanofibers without beads or curls could be prepared by electrospinning through the selection of combination of good conductive and good volatile solvent viz. 7.5 wt/v% of SPU in N,N′‐dimethyl formamide/tetrahydrofuran (30 : 70 v/v) solutions at 20 kV applied voltages and volume flow rate of 1 ml/min. On the basis of the results obtained from this investigation, it has been established that solvent selection is one of the driving factors for controlling the morphology of the polyurethane electrospun nanofiber mats. The well‐controlled morphology of electrospun polyurethane nanofiber mats could be useful for many potential industrial applications such as in biomedical, smart textiles, nanofiltration, and sensors. Copyright © 2013 John Wiley & Sons, Ltd.     

Electrospun polymer nanofibers: Mechanical and thermodynamic perspectives

This article reviews and discusses some open problems concerning polymer materials of reduced sizes and dimensions. Such objects exhibit exceptional physical properties when compared with their macroscopic counterparts. More specifically, abrupt increases in polymer nanofiber elastic modulus have been observed when diameters drop below a certain value. In addition, temperature dependence of elastic modulus is highly influenced by fiber diameter. Mechanical (macroscopic) analyses have failed to provide satisfactory explanations for the mechanisms ruling such features, calling for detailed microscopic examination of the systems in question. A hypothesis bridging the current knowledge gaps is presented. The key element of this hypothesis is based on confinement of the supermolecular microstructure of polymer nanofibers and its dominant role in the deformation process. This suggestion challenges the commonly held view suggesting that surface effects are the most significant parameters impacting mechanical and thermodynamic nanofiber behaviors. The review will focus on the mechanical and thermodynamic properties of electrospun polymer nanofibers, selected as representatives of nanoscale polymer objects. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Synthesis and mechanical properties of para‐aramid nanofibers

Scalable, bottom‐up chemical synthesis and electrospinning of novel Cl‐substituted poly(para‐phenylene terephthalamide) (PPTA) nanofibers are herein reported. To achieve Cl‐PPTA nanofibers, the chemical reaction between the monomers was precisely controlled, and dissolution of the polymer into solvent was tailored to enable anisotropic solution formation and sufficient entanglement molecular weight. Electrospinning processing parameters were studied to understand their effects on fiber formation and mat morphology and then optimized to yield consistently high quality fibers. Importantly, the control of relative humidity during the fiber formation process was found to be critical, likely because water promotes hydrogen bond formation between the PPTA chains. The fiber and mat morphologies resulting from different combinations of chemistry and spinning conditions were observed using scanning electron microscopy, and observations were used as inputs to the optimization process. Tensile properties of single Cl‐PPTA nanofibers were characterized for the first time using a nanomanipulator mounted inside a scanning electron microscope (SEM), and fiber moduli measuring up to 70 GPa, and strengths exceeding 1 GPa were achieved. Given the excellent mechanical properties measured for the nanofibers, this chemical synthesis procedure and electrospinning protocol appear to be a promising route for producing a new class of nanofibers with ultrahigh strength and stiffness. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 563–573     

The potential use of electrospun polylactic acid nanofibers as alternative reinforcements in an epoxy composite system

This pilot study elaborates the development of novel epoxy/electrospun polylactic acid (PLA) nanofiber composites at the fiber contents of 3, 5, and 10 wt % to evaluate their mechanical and thermal properties using flexural tests and differential scanning calorimetry (DSC). The flexural moduli of composites increase remarkably by 50.8 and 24.0% for 5 and 10 wt % fiber contents, respectively, relative to that of neat epoxy. Furthermore, a similar trend is also shown for corresponding flexural strengths being enhanced by 31.6 and 4.8%. Fractured surface morphology with scanning electron microscopy (SEM) confirms a full permeation of cured epoxy matrix into nanofiber structures and existence of nondestructive fibrous networks inside large void cavities. The glass transition temperature (T_g) of composites increases up to 54–60 °C due to embedded electrospun nanofibers compared to 50 °C for that of epoxy, indicating that fibrous networks may further restrict the intermolecular mobility of matrix in thermal effects. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 618–623     

Electrospinning preparation and mechanical properties of PVA/HNTs composite nanofibers

In this paper, the PVA/HNTs composite nanofibers with well‐enhanced mechanical properties were successfully prepared by electrospinning technique. The structure and properties of the composite nanofibers were characterized by TEM, XRD, FT‐IR, and DSC. The results indicated that the highly oriented and dispersed HNTs wrapped in polymer matrix were achieved by inducing function during electrospinning processing. The mechanical properties of the PVA/HNTs composite nanofibers depended on HNTs content were investigated, which showed 72.4% increase in tensile strength at optimal filling content. Copyright © 2016 John Wiley & Sons, Ltd.     

The preparation and adsorption properties of electrospun aramid nanofibers

The focus of this work is the preparation of aramid nanofibers via electrospinning technology and the study of their adsorption properties. In this article, aramid nanofibers were prepared by electrospinning aramid fibers solution with the addition of lithium chloride (LiCl). It showed a good adsorption capacity when methylene blue (MB) was used as the model target. There were much larger adsorption amounts and faster kinetics of uptaking target species of electrospun aramid nanofibers to MB than that of electrospun polyethersulfone (PES) nanofibers. Compared with activated carbon, aramid nanofibers also have a much faster adsorption rate to MB. Aramid nanofibers were subsequently used to effectively remove endocrine disruptors such as bisphenol A (BPA), phenol (Phe), and p‐hydroquinone (BPhe) from their aqueous solutions. Additionally, molecule imprinted technology enhances aramid nanofibers with much higher adsorption amounts and special adsorption property for endocrine disruptors. These results showed that aramid nanofibers have the potential to be used in environmental applications. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Thermal properties and structure of electrospun blends of PVDF with a fluorinated copolymer

We report the structure and thermal properties of blends comprising poly(vinylidene fluoride) (PVDF) and a random fluorinated copolymer (FCP) of poly(methyl methacrylate)‐random‐1H,1H,2H,2H‐perfluorodecyl methacrylate, promising membrane materials for oil–water separation. The roles of processing method and copolymer content on structure and properties were studied for fibrous membranes and films with varying compositions. Bead‐free, nonwoven fibrous membranes were obtained by electrospinning. Fiber diameters ranged from 0.4 to 1.9 μm, and thinner fibers were obtained for PVDF content >80%. As copolymer content increased, degree of crystallinity and onset of degradation for each blend decreased. Processing conditions have a greater impact on the crystallographic phase of PVDF than copolymer content. Fibers have polar beta phase; solution‐cast films contain gamma and beta phase; and melt crystallized films form alpha phase. Kwei's model was used to model the glass transition temperatures of the blends. Addition of FCP increases hydrophobicity of the electrospun membranes. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 312–322     

Spectroscopic investigation of the composition of electrospun titania nanofibers

We investigate the structure and chemical nature of titania nanofibers synthesized by electrospinning techniques. Fourier transform infrared (FTIR) spectroscopy is used to identify CO₂ clathrates trapped within the nanofiber structures. These molecular species are formed during pyrolysis of the guide polymer. In addition, X‐ray photoelectron spectroscopy (XPS) identifies silicon within the nonwoven sheets. This led us to discover that impurities can be inadvertently incorporated during the electrospinning process from something as simple as a short piece of silicone tubing on a syringe pump. These findings should help advance the field of electrospinning by demonstrating the importance of spectroscopic characterization of materials synthesized by this technique. Copyright © 2006 John Wiley & Sons, Ltd.     

Perceived color of undoped electrospun polyacrylonitrile nanofibers

This work reveals influence of electrospinning of polyacrylonitrile–N ,N‐dimethylformamide solution of different concentrations on nanofiber web color parameters, molecular structure, and heat stability. It is found that fiber diameters depend on concentration through the power law relationship; however, the medium concentration‐based web is characterized by a green–yellow hue, representative of the chromophore color; while, the solvent‐rich and solvent‐poor solution‐based webs give rise to Stokes shifts and ultraviolet‐blue emission bands, attributed to fluorescence. The chromophore structure, present in the neat powder, undergoes changes as a result of electrospinning reflected by the enamine‐to‐ketonitrile conversion and the fraction of C?N conjugation. Blue‐shifting of the C?N conjugation is indicative of a reduction of the π‐electron system, which is coincident with the decreased color saturation value but observed only in the nanofibers prepared from the medium concentration solution. A decrease in the glass transition and an increase in cyclization temperatures also support these findings. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55 , 1278–1285     

Thermo-mechanical behavior of electrospun thermoplastic polyurethane nanofibers

Analysis of the thermo-mechanical behavior of electrospun thermoplastic polyurethane (TPU) block co-polymer nanofibers (glass transition temperature ∼−50 °C) is presented. Upon heating, nanofibers began to massively contract, at ∼70 °C, whereas TPU cast films started to expand. Radial wide-angle X-ray scattering (WAXS) profiles of the nanofibers and the films showed no diffraction peaks related to crystals, whereas their amorphous halo had an asymmetric shape, which can be approximated by two components, associated with hard and soft segments. During heating, noticeable changes in the contribution of these components were only observed in nanofibers. These changes, which were accompanied with an endothermic DSC peak, coinciding with the start of the nanofibers contraction, can be attributed to relaxation of an oriented stretched amorphous phase created during electrospinning. Such structure relaxation becomes possible when a portion of the hard segment clusters, forming an effective physical network, is destroyed upon heating.