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.     

Strengthening,toughening, and self-healing for carbon fiber/epoxy composites based on PPESK electrospun coaxial nanofibers

A carbon fiber/epoxy composite modified by electrospun coaxial dicyclopentadiene/poly(phthalazinone ether sulfone ketone) (DCPD/PPESK) nanofibers was successfully fabricated, and the addition of DCPD/PPESK fibrous membranes made the composite have remarkable self-healing ability, and meanwhile effectively improve its mechanical properties. Results of polarization microscope observation and thermogravimetric analysis confirmed liquid DCPD as the healing agent was encapsulated into the PPESK coaxial nanofiber. Three-point bending test was utilized to evaluate the mechanical properties and self-healing effect of the composite. Experimental results indicated that the embedded nanofibers significantly improved the toughness of the composite, and maintained good mechanical properties even at low resin content. Most importantly, the flexural strength of the composite recovered to close to 90% observed 2 h after the bending failure.     

Effect of thermal shock on interlaminar strength of thermally aged glass fiber‐reinforced epoxy composites

Glass fiber/epoxy composites were thermally conditioned at 50, 100, 150, 200, and 250°C for different periods of time and then immediately quenched directly in ice‐cold water from each stage of conditioning temperature. The polymerization or depolymerization by thermal conditioning and the debonding effect by concurrently following thermal shock in polymer composites are assessed in the present study. The short‐beam shear tests were performed at room temperature on the quenched samples to evaluate the value of interlaminar shear strength of the composites. The short conditioning time followed by thermal shock resulted in reduction of shear strength of the composites. The strength started regaining its original value with longer conditioning time. Conditioning at 250°C and thereafter quenching yielded a sharp and continuous fall in the shear strength. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2062–2066, 2006     

Hybrid multi‐scale epoxy composites containing conventional glass microfibers and electrospun glass nanofibers with improved mechanical properties

A mechanically flexible mat consisting of structurally amorphous SiO₂ (glass) nanofibers was first prepared by electrospinning followed by pyrolysis under optimized conditions and procedures. Thereafter, two types of hybrid multi‐scale epoxy composites were fabricated via the technique of vacuum assisted resin transfer molding. For the first type of composites, six layers of conventional glass microfiber (GF) fabrics were infused with the epoxy resin containing shortened electrospun glass nanofibers (S‐EGNFs). For the second type of composites, five layers of electrospun glass nanofiber mats (EGNF‐mats) were sandwiched between six layers of conventional GF fabrics followed by the infusion of neat epoxy resin. For comparison, the (conventional) epoxy composites with six layers of GF fabrics alone were also fabricated as the control sample. Incorporation of EGNFs (i.e., S‐EGNFs and EGNF‐mats) into GF/epoxy composites led to significant improvements in mechanical properties, while the EGNF‐mats outperformed S‐EGNFs in the reinforcement of resin‐rich interlaminar regions. The composites reinforced with EGNF‐mats exhibited the highest mechanical properties overall; specifically, the impact absorption energy, interlaminar shear strength, flexural strength, flexural modulus, and work of fracture were (1097.3 ± 48.5) J/m, (42.2 ± 1.4) MPa, (387.1 ± 9.9) MPa, (12.9 ± 1.3) GPa, and (30.6 ± 1.8) kJ/m², corresponding to increases of 34.6%, 104.8%, 65.4%, 33.0%, and 56.1% compared to the control sample. This study suggests that EGNFs (particularly flexible EGNF‐mats) would be an innovative type of nanoscale reinforcement for the development of high‐performance structural composites. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42731.     

Investigation of mechanical properties of alumina nanoparticle‐loaded hybrid glass/carbon‐fiber‐reinforced epoxy composites

This research work investigates the tensile strength and elastic modulus of the alumina nanoparticles, glass fiber, and carbon fiber reinforced epoxy composites. The first type composites were made by adding 1–5 wt % (in the interval of 1%) of alumina to the epoxy matrix, whereas the second and third categories of composites were made by adding 1–5 wt % short glass, carbon fibers to the matrix. A fourth type of composite has also been synthesized by incorporating both alumina particles (2 wt %) and fibers to the epoxy. Results showed that the longitudinal modulus has significantly improved because of the filler additions. Both tensile strength and modulus are further better for hybrid composites consisting both alumina particles and glass fibers or carbon fibers. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39749.     

Preparation and evaluation of nano‐epoxy composite resins containing electrospun glass nanofibers

In this study, electrospun glass (structurally amorphous SiO₂) nanofibers (EGNFs) with diameters of ～ 400 nm were incorporated into epoxy resin for reinforcement and/or toughening purposes; the effects of silanization treatment (including different functional groups in silane molecules) and mass fraction of EGNFs on strength, stiffness, and toughness of the resulting nano‐epoxy composite resins were investigated. The experimental results revealed that EGNFs substantially outperformed conventional glass fibers (CGFs, with diameters of ～ 10 μm) in both tension and impact tests, and led to the same trend of improvements in strength, stiffness, and toughness at small mass fractions of 0.5 and 1%. The tensile strength, Young's modulus, work of fracture, and impact strength of the nano‐epoxy composite resins with EGNFs were improved by up to 40, 201, 67, and 363%, respectively. In general, the silanized EGNFs with epoxy end groups (G‐EGNFs) showed a higher degree of toughening effect, while the silanized EGNFs with amine end groups (A‐EGNFs) showed a higher degree of reinforcement effect. The study suggested that electrospun glass nanofibers could be used as reinforcement and/or toughening agent for making innovative nano‐epoxy composite resins, which would be further used for the development of high‐performance polymer composites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

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     

Toughening of epoxy resin with functionalized core‐sheath structured PAN/SBS electrospun fibers

Core‐sheath structured electrospun fibers with styrene‐butadiene‐styrene (SBS) block copolymer as a rubbery core and polyacrylonitrile (PAN) as a hard sheath were prepared by coaxial electrospinning, and used to improve the toughness of epoxy resin. The surface of the fibers was aminated by reacting PAN with diethylenetriamine to improve the interfacial interaction between the fibers and epoxy. Scanning and transmission electron microscopies confirm the core‐sheath structure of the PAN/SBS fibers. The Charpy impact energy is increased by the addition of electrospun fibers. When the content of aminated core‐sheath fibers is 4 wt %, the Charpy impact energy is increased by 150%. Dynamic mechanical analysis shows that the glass transition temperature of epoxy is not decreased by the addition of core‐sheath fibers. The high impact resistance is attributed to the rubbery core of the fibers that can absorb and dissipate impact energy, and the chemical bonding between the fibers and epoxy. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41119.     

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     

Preparation and characterization of electrospun PLA/nanocrystalline cellulose‐based composites

Biodegradable nanocomposites of Nanocrystalline Cellulose (NCC) and electrospun poly‐(lactic acid) were prepared via a new mixing technique. Dispersion of hydrophilic NCC in hydrophobic PLA was improved through aqueous mixing and freeze drying of perfectly suspended NCC with PLA nanofibers. Freeze drying produced aerogels with good mechanical integrity. The aerogels were further processed via hot pressing. Resulting composites displayed an improvement in mechanical properties, which was greatest at temperatures below the glass transition temperature of PLA. The optimum compositions were found to be in the 0.5–3% NCC (by weight) range. Experiments performed also showed that due to electrospinning, the crystallinity of the PLA slightly increased and this is accompanied by a decrease in its glass transition temperature. Furthermore, adding NCC to the electrospun PLA matrix did not alter the crystallinity of the final composite. The composites investigated proved their potential to be used in packaging and tissue engineering applications. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3345–3354, 2013     

Preparation and characterization of electrospun ultrafine polyamide‐6 nanofibers

We report on the preparation and characterization of ultrafine polyamide‐6 nanofibers by the electrospinning technique. The effect of electrospinning on the formation of ultrafine polyamide‐6 nanofiber structure was examined. The morphological and structural characterizations and thermal properties of the ultrafine polyamide‐6 nanofibers were investigated in comparison with bulk polyamide‐6 pellets. In order to accurately characterize the ultrafine polyamide‐6 nanofiber structure by direct identification of mass resolved components, we performed matrix‐assisted laser desorption ionization time‐of‐flight (MALDI‐TOF) mass spectrometry. Field emission scanning electron microscopy images revealed the presence of ultrafine polyamide‐6 nanofibers bound between the main fibers. The diameter of the polyamide‐6 nanofibers was observed to be in the range 75–110 nm, whereas the ultrafine structures consisted of regularly distributed very fine nanofibers with diameters of about 9–28 nm. The MALDI‐TOF spectra showed the presence of protonated and sodiated ions that were assigned to polyamide‐6 chains. Copyright © 2011 Society of Chemical Industry     

Experimental investigation of moisture diffusion in short‐glass‐fiber‐reinforced polyamide 6,6

Moisture diffusion in polyamide 6,6 (PA66) and its short glass fiber‐reinforced composites has a great influence on their mechanical properties and service lives under hydrothermal environments. Hence, the moisture diffusion in neat PA66 and its composites was studied comprehensively in this study with the general Fickian model. To systematically investigate the effects of the fiber content, humidity, temperature, and humidity–temperature coupling effect on the diffusion coefficient and equilibrium concentration, gravimetric experiments for the PA66 composites were carried out under different hydrothermal conditions. The results show that the equilibrium moisture concentration depended on the relative humidity and fiber content but only depended weakly on temperature. The diffusion velocity was affected by the three aforementioned factors with different trends. The analysis of variance demonstrated that the humidity–temperature coupling effect played an important role in the diffusion process. The regression analysis gave the corresponding quadratic regression equations. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42369.     

Densifying and strengthening of electrospun polyacrylonitrile‐based nanofibers by uniaxial two‐step stretching

The effects of alignment of polyacrylonitrile (PAN) nanofibers and a two‐step drawing process on the mechanical properties of the fibers were evaluated in the current study. The alignment was achieved using a high‐speed collector in electrospinning synthesis of the nanofibers. Under optimal two‐step drawing conditions (e.g., hot‐water and hot‐air stretching), the PAN nanofiber felts exhibited large improvements in both alignment and molecular chain‐orientation. Large increase in crystallinity, crystallite size, and molecular chain orientation were observed with increasing draw ratio. Optimally, stretched PAN‐based nanofibers exhibited 5.3 times higher tensile strength and 6.7 times higher tensile modulus than those of the pristine one. In addition, bulk density of the drawn PAN nanofibers increased from 0.19 to 0.33 g/cm³. Our results show that fully extended and oriented polymer chains are critical in achieving the highest mechanical properties of the electrospun PAN nanofibers. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43945.     

Dielectric studies of hyperbranched aromatic polyamide and polyamide‐6,6 blends

The objective of this work was to study the interactions between polyamide‐6,6 (PA‐6,6) and hyperbranched (HB) polyamide with different functional end groups. The investigation focused on the thermal, dielectric, and viscoelastic properties of two kinds of HB polyamides, with amine and alkyl end groups, prepared by a one‐pot process, in a polyamide‐6,6 matrix. Thermal analysis (by TGA and DSC) allowed us to observe decomposition and glass‐transition temperatures of these polymers. The melting point, crystallization temperature, and crystallinity ratio remained practically independent of HB content. Dielectric relaxation spectroscopy (DRS) showed two secondary relaxation (γ and β) and one primary (α) relaxation in the HB polymers and in the blends similar to those observed in polyamide‐6,6 with comparable activation energies and distribution parameters. An increase of the glass‐transition temperature was observed, showing a reinforcement of the polymer matrix and a decrease of the molecular mobility of the polyamide chains when the percentage of amine‐terminated HB polyamide increased in the matrix. DRS results found on the alkyl‐terminated HB polymer blend were indistinct from those of the polyamide‐6,6 matrix. Viscoelastic experiments confirmed the results observed in DRS. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1522–1537, 2005     

Freeze–thaw resistance of unidirectional‐fiber‐reinforced epoxy composites

The freeze–thaw resistance of unidirectional glass‐, carbon‐, and basalt‐fiber‐reinforced polymer (GFRPs, CFRPs, and BFRPs, respectively) epoxy wet layups was investigated from ?30 to 30°C in dry air. Embedded optic‐fiber Bragg grating sensors were applied to monitor the variation of the internal strain during the freeze–thaw cycles, with which the coefficient of thermal expansion (CTE) was estimated. With the CTE values, the stresses developed in the matrix of the FRPs were calculated, and CFRPs were slightly higher than in the BFRP and GFRP cases. The freeze–thaw cycle showed a negligible effect on the tensile properties of both GFRP and BFRP but exhibited an adverse effect on CFRP, causing a reduction of 16% in the strength and 18% in the modulus after 90 freeze–thaw cycles. The susceptibility of the bonding between the carbon fibers and epoxy to the freeze–thaw cycles was assigned to the deterioration of CFRP. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effects of surface modification of carbon nanofibers on the mechanical properties of polyamide 1212 composites

In this study, the effects of carbon nanofiber (CNF) surface modification on mechanical properties of polyamide 1212 (PA1212)/CNFs composites were investigated. CNFs grafted with ethylenediamine (CNF‐g‐EDA), and CNFs grafted with polyethyleneimine (CNF‐g‐PEI) were prepared and characterized. The mechanical properties of the PA1212/CNFs composites were reinforced efficiently with addition of 0.3 wt % modified CNFs after drawing. The reinforcing effect of the drawn composites was investigated in terms of interfacial interaction, crystal orientation, crystallization properties and so on. After the surface modification of CNFs, the interfacial adhesion and dispersion of CNFs in PA1212 matrix were improved, especially for CNF‐g‐PEI. The improved interfacial adhesion and dispersion of CNFs in PA1212 matrix was beneficial to reinforcement of the composites. Compared with pure PA1212, improved degree of crystal orientation in the PA1212/CNF‐g‐PEI (CNF‐g‐EDA) composites was responsible for reinforcement of mechanical properties after drawing. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41424.     

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.     

Influence of fiber modification on interfacial adhesion and mechanical properties of pineapple leaf fiber‐epoxy composites

Three kinds of surface treatment, that is, the alkalization (5% w/v NaOH aqueous solution), the deposition of diglycidyl ether of bisphenol A (DGEBA) from toluene solution (1% w/v DGEBA), and the alkalization combined with the deposition of DGEBA (5% w/v NaOH/1% w/v DGEBA) were applied to modify interfacial bonding and to enhance mechanical properties of pineapple leaf fiber (PALF) reinforced epoxy composites. The fiber strength and strain were measured by single fiber test and the fiber strength variation was assessed using Weibull modulus. Furthermore, a fragmentation test was used to quantify the interfacial adhesion of PALF‐epoxy composite. It was verified that the interfacial shear strength of modified PALFs was substantially higher than that of untreated PALF by almost 2–2.7 times because of the greater interaction between the PALFs and epoxy resin matrix. The strongest interfacial adhesion was obtained from the fibers that had been received the alkalization combined with DGEBA deposition. Moreover, the flexural and impact properties of unidirectional PALF‐epoxy composites were greatly enhanced when reinforced with the modified PALFs due to an improvement in interfacial adhesion, particularly in the synergetic use of 5% NaOH and 5% NaOH/1% DGEBA. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

The influence of various kinds of PA12 interlayer on the interlaminar toughness of carbon fiber‐reinforced epoxy composites

Modifying the impact toughness of carbon fiber‐reinforced epoxy composites by introducing thermoplastic inserts in the interlaminar layer is state‐of‐the‐art. This article compares the introduction of thermoplastics in continuous and discontinuous form. Test plate samples were produced using unidirectional noncrimp carbon fabrics with two different aircraft resin systems: HEXFLOW RTM6 (Hexcel) and Cycom 890 RTM (Cytec). In addition, Polyamide 12 (PA12) was laid in the interlaminar layer in the forms of two different laid scrims, as powder or as nonwoven fabric (NWF). The performance of the resulting combinations was assessed by testing the samples in Mode I and II interlaminar fracture toughness (GIc and GIIc), interlaminar shear strength (ILSS), and compression strength after impact (CAI). The results show that in nearly all the tests a fine‐mesh laid scrim performs similarly to a NWF with twice the weight per surface area. They show furthermore that the curing dynamics of the resin systems together with the melting characteristics of the thermoplastic during processing have an important effect on the performance of the test samples. Hardening of the resin before the PA12 reaches its melting point hinders the compacting of the thermoplastic. This limits the reduction in the original thickness of the insert, leading to an increase in the sample thickness and, thus, reducing the fiber volume content. Otherwise, the discrete arrangement of the laid scrim has positive effects on the material properties of the composite at elevated temperatures, considerably reducing the falloff in ILSS resulting from the temperature‐dependent Young's modulus of PA12. POLYM. COMPOS., 36:1249–1257, 2015. © 2014 Society of Plastics Engineers