Self‐healing and interfacially toughened carbon fibre‐epoxy composites based on electrospun core–shell nanofibres

Dual components of a self‐healing epoxy system comprising a low viscosity epoxy resin, along with its amine based curing agent, were separately encapsulated in a polyacrylonitrile shell via coaxial electrospinning. These nanofiber layers were then incorporated between sheets of carbon fiber fabric during the wet layup process followed by vacuum‐assisted resin transfer molding to fabricate self‐healing carbon fiber composites. Mechanical analysis of the nanofiber toughened composites demonstrated an 11% improvement in tensile strength, 19% increase in short beam shear strength, 14% greater flexural strength, and a 4% gain in impact energy absorption compared to the control composite without nanofibers. Three point bending tests affirmed the spontaneous, room temperature healing characteristics of the nanofiber containing composites, with a 96% recovery in flexural strength observed 24 h after the initial bending fracture, and a 102% recovery recorded 24 h after the successive bending fracture. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44956.     

Recent progress in interfacial toughening and damage self‐healing of polymer composites based on electrospun and solution‐blown nanofibers: An overview

In this article, we provide an overview of recent progress in toughening and damage self‐healing of polymer–matrix composites (PMCs) reinforced with electrospun or solution‐blown nanofibers at interfaces with an emphasis on the innovative processing techniques and toughening and damage self‐healing characterization. Because of their in‐plane fiber architecture and layered structure, high‐performance laminated PMCs typically carry low interfacial strengths and interlaminar fracture toughnesses in contrast to their very high in‐plane mechanical properties. Delamination is commonly observed in these composite structures. Continuous polymer and polymer‐derived carbon nanofibers produced by electrospinning, solution blowing, and other recently developed techniques can be incorporated into the ultrathin resin‐rich interlayers (with thicknesses of a few to dozens of micrometers) of these high‐performance PMCs to form nanofiber‐reinforced interlayers with enhanced interlaminar fracture toughnesses. When incorporated with core–shell healing‐agent‐loaded nanofibers, these nanofiber‐richened interlayers can yield unique interfacial damage self‐healing. Recent experimental investigations in these topics are reviewed and compared, and recently developed techniques for the scalable, continuous fabrication of advanced nanofibers for interfacial toughening and damage self‐healing of PMCs are discussed. Developments in the near future in this field are foreseen. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2225–2237, 2013     

Electrospinning core‐shell nanofibers for interfacial toughening and self‐healing of carbon‐fiber/epoxy composites

This article reports a novel hybrid multiscale carbon‐fiber/epoxy composite reinforced with self‐healing core‐shell nanofibers at interfaces. The ultrathin self‐healing fibers were fabricated by means of coelectrospinning, in which liquid dicyclopentadiene (DCPD) as the healing agent was enwrapped into polyacrylonitrile (PAN) to form core‐shell DCPD/PAN nanofibers. These core‐shell nanofibers were incorporated at interfaces of neighboring carbon‐fiber fabrics prior to resin infusion and formed into ultrathin self‐healing interlayers after resin infusion and curing. The core‐shell DCPD/PAN fibers are expected to function to self‐repair the interfacial damages in composite laminates, e.g., delamination. Wet layup, followed by vacuum‐assisted resin transfer molding (VARTM) technique, was used to process the proof‐of‐concept hybrid multiscale self‐healing composite. Three‐point bending test was utilized to evaluate the self‐healing effect of the core‐shell nanofibers on the flexural stiffness of the composite laminate after predamage failure. Experimental results indicate that the flexural stiffness of such novel self‐healing composite after predamage failure can be completely recovered by the self‐healing nanofiber interlayers. Scanning electron microscope (SEM) was utilized for fractographical analysis of the failed samples. SEM micrographs clearly evidenced the release of healing agent at laminate interfaces and the toughening and self‐healing mechanisms of the core‐shell nanofibers. This study expects a family of novel high‐strength, lightweight structural polymer composites with self‐healing function for potential use in aerospace and aeronautical structures, sports utilities, etc. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Increasing mechanical strength of electrospun gelatin nanofibers by the addition of aluminum potassium sulfate

Increasing mechanical strength of gelatin‐based materials is required to expand the range of their applications, which is desirable because of biocompatibility, biodegradability, and low cost of gelatin. The effect of aluminum potassium sulfate on preparation and properties of nanofibrous gelatin were investigated. Samples were electrospun from 10M aqueous acetic acid and analyzed by scanning electron microscopy (SEM), Fourier transform infrared microscopy (FTIR), energy‐dispersive x‐ray analysis (EDX), and tensile test. The addition of AlK(SO₄)₂ considerably increases the elastic modulus of the material up to about 10% salt content. The elastic modulus of electrospun gelatin meshes prepared as described in the present work increased from 20 MPa to 70 MPa and the elastic modulus of the fiber material increased from 150 MPa to 620 MPa as the salt content in the fibers increased from 0% to 9.6%. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42431.     

A method for scale‐up of co‐electrospun nanofibers via flat core‐shell structure spinneret

An approach to the scale‐up of co‐electrospinning via a flat core‐shell structure spinneret has been developed in this study. The spinneret with a flat surface involves shell‐holes and core‐needles. Electric field simulation reveals that the flat core‐shell spinneret configuration creates a more uniform electric field gradient. Experimental study shows that in comparison with the conventional needle co‐electrospinning, core‐shell nanofibers produced by this new designed setup are finer and of better morphology. Composite nanofibers with special morphologies can be fabricated by modifying the structure of this spinneret. The production rate of the core‐shell nanofibers can be enhanced by increasing the hole and needle number of the spinneret. This novel design is expected to provide a promising method towards the massive production of core‐shell nanofibers. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41027.     

Preparation and hemocompatibility of electrospun O‐carboxymethyl chitosan/PVA nanofibers

Chitosan was deacetylated and carboxymethylated to prepare O‐carboxymethyl chitosan (CMC) for further electrospinning. CMC was characterized using FTIR, NMR, and chemical titration, indicating a degree of carboxymethylation of 51.4%. CMC was electrospun together with poly(vinyl alcohol) (PVA) to prepare membranes composed of nanofibers. The electrospinning conditions were optimized. The CMC/PVA membrane obtained at the conditions of 15.2 g/mL CMC 50 mL, 8 g/mL PVA 5 mL, 25 kV, and a distance of 23 cm, had nanofibers without beads, with diameters of 70–200 nm. The mats were crosslinked by glutaraldehyde before platelet adhesion measurement. The nanofibrous structure remained after crosslinking while the wettability decreased. CMC/PVA mats with higher CMC amount and fewer beads, had fewer adherent platelets and less platelets aggregation showing better hemocompatibility. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43565.     

Fabrication and electrical characterization of electrospun polyacrylonitrile‐derived carbon nanofibers

Carbon nanofibers were produced from a polyacrylonitrile/N,N‐dimethylformamide precursor solution by an electrospinning process and later pyrolysis at temperatures ranging from 500 to 1100°C in an N₂ atmosphere for about 1 h. The morphological structure of the nanofibers was studied with scanning electron microscopy. Scanning electron microscopy images of carbonized polyacrylonitrile nanofibers without a gold coating showed that the carbonized polyacrylonitrile nanofibers possessed electrical properties. The thermal behavior of the nanofibers was studied with thermogravimetric analysis. An indirect four‐point‐probe method was used for the measurement of the conductivity of nanofiber mats. The conductivity increased sharply with the pyrolysis temperature. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Enhancement of the mechanical properties and thermostability of poly(vinyl alcohol) nanofibers by the incorporation of sodium chloride

The mechanical properties and thermostability of poly(vinyl alcohol) (PVA) nanofiber mats have been obviously improved by the incorporation of sodium chloride (NaCl). The tensile properties including tensile strength and modulus of membranes with an addition of 1.0 wt % NaCl increased from 2.51 to 4.22 MPa and 33.0 to 176.30 MPa, respectively, more than 160 and 700% of those of the electrospun pure PVA membranes. Moreover, thermogravimetric analysis showed that the initial decomposition temperature (T_i) and the half decomposing temperature (T_50%) of PVA nanofibers with the addition of NaCl were at least 26 and 59 °C higher than that of pure PVA nanofibers, respectively, indicating a strong interaction between the PVA and the salt ions. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45981.     

Effects of surfactants on the microstructures of electrospun polyacrylonitrile nanofibers and their carbonized analogs

In this study, the influence of surfactants on the processability of electrospun polyacrylonitrile (PAN) nanofibers and their carbonized analogs was investigated. The surfactants employed in this effort are Triton X‐100 (nonionic surfactant, SF‐N), sodium dodecyl sulfate (SDS) (anionic surfactant, SF‐A), and hexadecyltrimethylammonium bromide (HDTMAB) (cationic surfactant, SF‐C). Interactions between electrospun PAN and the surfactants, reflected in effects on as‐spun and carbonized nanofiber morphologies and microstructures, were explored. The results show that uniform nanofibers are obtained when cationic and anionic surfactants (surfactant free and nonionic surfactants) are utilized in the preparation of electrospun PAN. In contrast, a bead‐on‐a‐string morphology results when the aniconic and cationic surfactants are present, and defect structure is enhanced with cationic surfactant addition. Moreover, fiber breakage is observed when the nonionic surfactant Triton X‐100 is employed for electrospinning. After carbonizaition, the PAN polymers were observed to have less ordered structures with addition of any type of surfactant used for electrospinning and the disorder becomes more pronounced when the anionic surfactant is utilized. Owing to the fact that microstructure defects create midband gap states that enable more electrons to be emitted from the fiber, an enhancement of electron emission is observed for PAN electrospun in the presence of the anionic surfactant. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3726–3735, 2013     

Optical sensor based on fluorescent PMMA/PFO electrospun nanofibers for monitoring volatile organic compounds

The development of polymeric materials with superior electrical and/or optical properties is highly demanded for designing optical gas sensors, where conjugated polymers play an important role due to their π‐electron conjugation. However, usually the low processability and high cost of these materials hinder technological applications. Here we report on a simple route to develop highly fluorescent electrospun nanofibers of poly(methyl methacrylate) (PMMA) containing low contents of polyfluorene (PFO). The PMMA_PFO nanofibers were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, and thermogravimetric analysis, while the luminescence properties changes were evaluated by exposing the PMMA_PFO nanofibers to distinct volatile organic compounds (VOCs) including ethanol, toluene, tetrahydrofuran, acetone, dichloromethane, and chloroform. The changes in luminescence properties, specifically fluorescence quenching, of PMMA_PFO nanofibers were analyzed in terms of conformational changes from glassy‐phase to β‐phase of PFO when the nanofibers were exposed to the VOCs. The developed nanostructured platform showed a suitable response to detect chloroform, with linear responses in the concentration range from 10 to 300 ppm and from 350 to 500 ppm and limits of detection of 47.9 and 15.4 ppm, respectively. The results suggest the PMMA_PFO electrospun nanofibers are highly potential materials for optical gas sensor applications based on luminescence quenching. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46128.     

Mechanical properties of individual fiber segments of electrospun lignocellulose‐reinforced poly(vinyl alcohol)

The influence of lignocellulosic nanofibers (LCNF) additive on the inherent mechanical properties of submicron electrospun poly(vinyl alcohol) (PVA) fibers is reported. LCNF with a diameter of 25 ± 15 nm and a length of 220 ± 90 nm obtained from hemp shives were dispersed in aqueous PVA solutions to produce homogeneous nanocomposite fibers with 0, 5, and 10 w/w % LCNF loads in solid PVA. Tensile tests on mats show that LCNF additive causes up to sevenfold increase in stiffness and significant decrease in elongation at yield. AFM‐based 3‐point bending tests on single LCNF‐doped fibers reveal up to 11.4 GPa Young's modulus in the diameter range of 300 to 500 nm, indicating a 2.4 times increase compared to neat PVA fibers. Mechanical properties of both neat and LCNF‐doped PVA fibers are found to be strongly size‐dependent at lower fiber diameters, with Young's modulus values exceeding 100 GPa at below 100 nm diameters. The results can be explained by extensive restructuration of hydrogen bonding network due to the LCNF additive. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44361.     

Statistical analysis of nanofibers alignment in magnetic‐field‐assisted electrospinning including an alignment percentage formula

Magnetic‐field‐assisted electrospinning (MFAES) is a simple and effective method to align polymer nanofibers. In this method, further research is needed to identify alignment mechanism. Hence, this article includes statistical analysis of affecting factors to investigate alignment mechanism in MFAES. Tip to target distance, magnets distance, voltage, and collection time, which are recognized as the most effective factors on nanofibers alignment, were applied in design of experiments. Central composite method was applied to get required experiments with designed expert 8 software. A response surface was proposed with regression coefficient of 97%. Then, the common physics concepts and statistical results were used to discuss the affecting mechanism of the electric and magnetic fields on the electrospinnig jet and the nanofibers alignment. Field emission scanning electron microscopy images were used to characterize the nanofibers alignment and calculate overall alignment percentage using a proposed statistical combinatorial weighted percentage formula. MFAES method, used in this research, achieved 95.3% polyacrylonitrile‐aligned nanofibers. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41179.     

Functionalization of electrospun nanofibers of natural cotton cellulose by cerium dioxide nanoparticles for ultraviolet protection

Nanofibers of natural cotton cellulose with a degree of polymerization above 10,000 were prepared by electrospinning; they were then functionalized with a rare‐earth nano‐oxide material of cerium dioxide (CeO₂) by means of the hydrothermal method to obtain the designated properties. The morphology, structure, and properties of the as‐obtained nanocomposite fibers were characterized by scanning electron microscopy, transmission electron microscopy, energy‐dispersive spectroscopy, X‐ray diffraction, Fourier transform infrared spectroscopy, and ultraviolet (UV)–visible spectrophotometry. The results show that hydrothermally grown CeO₂ nanoparticles exhibited a polycrystalline cubic fluorite structure and could be dispersed uniformly on the surface of the cellulose nanofiber. The strong interface and electrostatic interactions between the nanoparticles and nanofibers effectively prevented nanoparticle fall‐off. The modified natural cotton cellulose nanofibers showed excellent protection against UV radiation because of the function of the CeO₂ particles. Such cellulose nanocomposite materials could have potential applications in UV protection for data‐storage or memory devices. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1524–1529, 2013     

Characteristics and 3D formation of PVA and PEO electrospun nanofibers with embedded urea

The behavior of electrospun polyvinyl alcohol (PVA) and polyethylene oxide (PEO) nanofibers embedded with urea is studied as a function of various process parameters. Our results show that three‐dimensional nanofiber networks can be obtained when high concentrations of urea in the solution are used during electrospinning. The nanofibers are characterized using both scanning electron microscope (SEM) and Fourier transform infrared spectroscopy (FTIR). The stability of the nanofiber as a function of electric field has also been studied. The successful formation of three‐dimensional nanofiber networks can open new trends toward applications in fertilizers containing nanofibers in the nanoagricultural field. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39840.     

Influence of processing conditions on polymorphic behavior,crystallinity, and morphology of electrospun poly(VInylidene fluoride) nanofibers

The polymorphism and crystallinity of poly(vinylidene fluoride) (PVDF) membranes, made from electrospinning of the PVDF in pure N,N‐dimethylformamide (DMF) and DMF/acetone mixture solutions are studied. Influence of the processing and solution parameters such as flow rate, applied voltage, solvent system, and mixture ratio, on nanofiber morphology, total crystallinity, and crystal phase content of the nanofibers are investigated using scanning electron microscopy, wide‐angle X‐ray scattering, differential scanning calorimetric, and Fourier transform infrared spectroscopy. The results show that solutions of 20% w/w PVDF in two solvent systems of DMF and DMF/acetone (with volume ratios of 3/1 and 1/1) are electrospinnable; however, using DMF/acetone volume ratio of 1/3 led to blockage of the needle and spinning process was stopped. Very high fraction of β‐phase (～79%–85%) was obtained for investigated nanofiber, while degree of crystallinity increased to 59% which is quite high due to the strong influence of electrospinning on ordering the microstructure. Interestingly, ultrafine fibers with the diameter of 12 and 15 nm were obtained in this work. Uniform and bead free nanofiber was formed when a certain amount of acetone was added in to the electrospinning solution. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42304.     

Development,optimization, and characterization of electrospun poly(lactic acid) nanofibers containing multi‐walled carbon nanotubes

Electrospinning of poly (L ‐D ‐lactic acid) (PLA) was investigated with the addition of multi‐walled carbon nanotubes (MWNT) for development of a scaffold for tissue engineering. Through this experiment, it was determined that the optimal concentration of PLA with weight average molecular weight (M_w) 250,000 g/mol is ～20 wt % as indicated by scanning electron microscopy. This concentration produces fibers with no beading or film formation. The preferred solvent system is a combination of chloroform and dimethyl formamide to alleviate the volatile action of chloroform. The optimum processing parameters for PLA are an electric field of 1 kV/cm which was determined by a surface response plot to minimize fiber diameter based on the applied voltage, working distance, and addition of MWNT. Fourier Transform infrared spectroscopy has indicated the removal of the solvent system. With the addition of MWNT, the fiber diameter was drastically reduced by 70% to form fibers with a mean diameter of 700 nm. This is believed to be due to an increased surface charge density for the MWNT/polymer solution. Transmission electron microscopy validated the alignment of the MWNT within the fibers. MWNT loading exhibited an increase in the conductance of the scaffold and the tensile modulus at an optimal loading level of 0.25 wt %. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Magnetic‐field‐assisted electrospinning highly aligned composite nanofibers containing well‐aligned multiwalled carbon nanotubes

Composite nanofiber meshes of well‐aligned polyacrylonitrile (PAN)/polyvinylpyrrolidone (PVP) nanofibers containing multiwalled carbon nanotubes (MWCNTs) were successfully fabricated by a magnetic‐field‐assisted electrospinning (MFAES) technology, which was confirmed to be a favorable method for preparation of aligned composite nanofibers in this article. The MFAES experiments showed that the diameters of composite nanofibers decreased first and then increased with the increase of voltage and MWCNTs content. With the increase of voltage, the degree of alignment of the composite nanofibers decreased, whereas it increased with increasing MWCNTs concentration. Transmission electron microscopy observation showed that MWCNTs were parallel and oriented along the axes of the nanofibers under the low concentration. A maximum enhancement of 178% in tensile strength was manifested by adding 2 wt % MWCNTs in well‐aligned composite nanofibers. In addition, the storage modulus of PAN/PVP/MWCNTs composite nanofibers was significantly higher than that of the PAN/PVP nanofibers. Besides, due to the highly ordered alignment structure, the composite nanofiber meshes showed large anisotropic surface resistance, that is, the surface resistance of the composite nanofiber films along the fiber axis was about 10 times smaller than that perpendicular to the axis direction. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41995.     

Enhancement of interlaminar fracture toughness of carbon fiber–epoxy composites using polyamide‐6,6 electrospun nanofibers

In this study, carbon fiber–epoxy composites are interleaved with electrospun polyamide‐6,6 (PA 66) nanofibers to improve their Mode‐I fracture toughness. These nanofibers are directly deposited onto carbon fabrics before composite manufacturing via vacuum infusion. Three‐point bending, tensile, compression, interlaminar shear strength, Charpy impact, and double cantilever beam tests are performed on the reference and PA 66 interleaved specimens to evaluate the effects of PA 66 nanofibers on the mechanical properties of composites. To investigate the effect of nanofiber areal weight density (AWD), nanointerlayers with various AWD are prepared by changing the electrospinning duration. It is found that the electrospun PA 66 nanofibers are very effective in improving Mode‐I toughness and impact resistance, compressive strength, flexural modulus, and strength of the composites. However, these nanofibers cause a decrease in the tensile strength of the composites. The glass‐transition temperature of the composites is not affected by the addition of PA 66 nanofibers. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45244.     

Hierarchical-structure-dependent high ductility of electrospun polyoxymethylene nanofibers

Two types of electrospun polyoxymethylene nanofibers with rough and smooth surface morphologies [rough fibers (RFs) and smooth fibers (SFs), respectively] were successfully prepared via the control of the electrospinning voltages. Mechanical tensile tests showed that the RF nonwoven mats exhibited a much higher elongation (440%) than the SFs (180%) without sacrifices in the stiffness and strength. Scanning electron microscopy characterization revealed that the large ductility of a single RF resulted from its unique multiple-necking mode, which was induced by its rippled structural features. In the meantime, the large ductility led to a high molecular orientation under tension and further improved the strength and toughness of the RF nonwoven mats. In comparison, the SF behaved in a single-necking deformation mode, and this led to a rapid rupture behavior. This surface-morphology-dependent mechanical behavior helped us to deeply understand the relationship between the structure and properties and should guide the development of high-performance materials for load-bearing applications. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47086.     

Carbonized electrospun polyacrylonitrile nanofibers as highly sensitive sensors in structural health monitoring of composite structures

Electrospun polyacrylonitrile (PAN) nanofibers were stabilized at 280°C for 1 h in an ambient condition, and then carbonized at 850°C in inert argon gas for additional 1 h in order to fabricate highly pure carbonous nanofibers for the development of highly sensitive sensors in structural health monitoring (SHM) of composite aircraft and wind turbines. This study manifests the real‐time strain response of the carbonized PAN nanofibers under various tensile loadings. The prepared carbon nanofibers were placed on top of the carbon fiber pre‐preg composite as a single layer. Using a hand lay‐up method, and then co‐cured with the pre‐preg composites in a vacuum oven following the curing cycle of the composite. The electric wires were connected to the top surface of the composite panels where the cohesively bonded conductive nanofibers were placed prior to the tensile and compression loadings in the grips of the tensile unit. The test results clearly showed that the carbonized electrospun PAN nanofibers on the carbon fiber composites were remarkably performed well. Even the small strain rates (e.g., 0.020% strain) on the composite panels were easily detected through voltage and resistance changes of the panels. The change in voltage can be mainly attributed to the breakage/deformation of the conductive network of the carbonized PAN nanofibers under the loadings. The primary goal of the present study is to develop a cost‐effective, lightweight, and flexible strain sensor for the SHM of composite aircraft and wind turbines. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43235.