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Circumventing boundary effects while characterizing epoxy/copper interphases using nanoindentation
(2017)
Characterization of the size and mechanical properties of interphases is essential when designing multicomponent materials. When nanoindentation is used to investigate the size and mechanical properties of an interphase, a common challenge is that the indenter or the stress zone formed around it are often restricted by the reinforcement, making it difficult to distinguish the mechanical property variations caused by the interphase itself from those caused by the boundary effect. In this work, a testing system was developed that allows determining the indent affected zone and accounting for it in the interphase measurements of an epoxy/Cu system. Using finite element analysis, we confirmed the validity of the proposed system. Nanoindentation was used to investigate the Interphase between copper and two different epoxy systems; amine-cured and anhydride-cured. Nanoindentation results showed that a copper layer that is only 10 nm thick still exhibits a constriction effect on the indentations in its vicinity. The amine-cured epoxy did not show any sign of interphase existence using the introduced method. However,
a soft interphase with a thickness of ~1.7 μm was measured on theanhydride-cured epoxy. Furthermore, we show that the proposed system can be used to determine the interphase thickness as well as its relative mechanical properties regardless of the indentation depth.
This system can be further used for investigating other polymer/metal interphases to better understand the factors influencing them, thus helping engineer the interphase size and properties to enhance composite performance.
The longitudinal and transverse strength of three unidirectional thermoplastic prepreg systems: carbon fiber/polypropylene (CF/PP), polyamide 6 (CF/PA6), and polyphenylene sulfide (CF/PPS) are studied and analytical formulas are proposed for the estimation of matrix and fiber/matrix interface properties from composites properties. Since the matrices are semi-crystalline thermoplastics, the influence of cooling rate on the strength is statistically evaluated. While the 0° tensile strength is found to be independent of the cooling rate, the 90° tensile strength is strongly influenced by the matrix type and cooling rate. The matrix modulus increases as the cooling rate is decreased; the degree of crystallinity also increases. The matrix residual stress, interfacial shear strength, and mode II interlaminar fracture toughness are also found to depend on the cooling rate, with the trends different for different matrices.
In Kooperation zwischen dem Institut für Kunststoffverarbeitung (IKV) und der Bundesanstalt für Materialforschung und -prüfung (BAM) wurde das mechanische Verhalten unter Druck- und Zugbelastung unterschiedlicher morphologischer Strukturen teilkristalliner Thermoplaste (PP und POM) untersucht. Im Ergebnis steigt der E-Modul sowohl der druck- wie auch zugbelasteten Prüfkörper aus spritzgegossenem PP und POM mit gröberem Gefüge an. Probekörper mit scherinduziertem Gefüge zeigen einen hohen Einfluss des Kristallisationsgrads auf den E-Modul.
Due to the different thermal expansion of the constituent materials, cyclic thermal loading of FRP induces alternating stresses in the material at two scales: at the micro scale (level of fibre–matrix-interaction) and at the macro scale (level of the multidirectional laminate). Especially the micro scale effect is not comprehensively investigated yet. Additionally, computational investigations mostly neglect this effect due to the homogenous modelling of the composite material. As this effect is assumed to significantly contribute to the fatigue of FRP at thermal loads, the present paper suggests an experimental and numerical multiscale approach including Experiments at the different involved material scales to separately observe the effects acting at these scales. The approach also includes numerical modelling for each scale to complement the knowledge gained from the Experiments and to create a basis for the consideration of the micro effect even in macroscopic fatigue models treating homogeneous modelled composites. The main focus of the contribution is to bring the overall Approach up for discussion, rather than to present the multiscale modelling details.
The breakage of a fiber and its effect on the strain energy is studied in a model composite, this is, a single fiber embedded in a dogbone specimen. The fracture process is recorded by an acoustic emission device, the crack patterns are monitored under a microscope. A finite element analysis is performed in order to estimate the energy released by different failure scenarios. The abilities and limitations of the acoustic emission analysis in characterizing different failure types in fiber reinforced composites are evaluated.
A mathematical model for the description of the overall crystallization of polymers has been developed which separates the pure geometric spreading of the semicrystalline superstructures from the increasing crystallinity inside these entities. The model uses the Avrami theory for the geometric part and a freely optional function for the increasing local crystallinity. The two functions are combined by a mathematical procedure called convolution. The free option of the local crystallinity function enables this model to permit calculation of this function, which is not accessible to direct measurement. Thus with given Avrami parameters and measured values of the time-dependent overall crystallinity, this function of the local crystallinity (secondary crystallization function) can be calculated. The model has been applied to test data produced by computer simulations.
The effect of transcrystallinity in carbon fiber reinforced poly(p-phenylene sulfide (PPS) composites on the apparent shear strength was investigated with the single fiber pull-out test. Transcrystalline zones around the reinforcing fibers do not seem to improve the adhesion level significantly. Neighbor fibers hinder the formation of the transcrystalline zone and a ductile fracture behavior can be observed. However, the apparent strength level is slightly higher for composites containing such reinforcing neighbor fibers compared with single fiber composite samples. During annealing a brittle interface can be formed in the multifiber composite yielding a higher level of the apparent shear strength.
The strength of composite reinforcement fibres is normally measured on samples of much greater length than the actual residual fibre lengths found in many composite materials. This is due to a number of limitations of the standard techniques which are employed. We present a description of a technique which enables values for the tensile strength of composite reinforcement fibres at short gauge lengths to be obtained. The technique is based on an adaptation of a micro-mechanical test apparatus for fibre pullout measurements. Data is presented which was obtained at gauge lengths of 180380 µm on E-glass and S-2 glass® fibres taken from different chopped reinforcement products. The technique can be used at gauge lengths as short as 20 µm. The data indicates that the values of average fibre strength in these products are significantly below the pristine glass strength values.
The interaction between direct fluorinated carbon fibres and various fluoropolymers (ethylene-chlorotrifluoroethylene, poly vinylidene fluoride, fluorinated ethylene propylene copolymer and tetrafluoroethylene-perfluoro alkoxy vinyl ether copolymer) was studied by means of direct wetting measurements between fibres and the polymer melts and single fibre pull-out tests. The results of both techniques allow the adhesion behaviour between the fibres and the matrices to be predicted. The results obtained show that a low degree of surface fluorination of carbon fibres leads to an improved wettability between the fibres and fluoropolymer melts and this is an indicator for an improved thermodynamic work of adhesion. The apparent interfacial shear strength as measure of practical adhesion, determined by the single fibre pull-out test, increases with increasing degree of surface fluorine content up to a maximum, which depends on the degree of fluorination of the matrix used. The improved interaction between the fibre and the matrix is due to an enhanced compatibility at the fibre/matrix interface.
Triggered biodegradable composites made entirely from renewable resources are urgently sought after to improve material recyclability or be able to divert materials from waste streams. Many biobased polymers and natural fibers usually display poor interfacial adhesion when combined in a composite material. Here we propose a way to modify the surfaces of natural fibers by utilizing bacteria (Acetobacter xylinum) to deposit nanosized bacterial cellulose around natural fibers, which enhances their adhesion to renewable polymers. This paper describes the process of modifying large quantities of natural fibers with bacterial cellulose through their use as substrates for bacteria during fermentation. The modified fibers were characterized by scanning electron microscopy, single fiber tensile tests, X-ray photoelectron spectroscopy, and inverse gas chromatography to determine their surface and mechanical properties. The practical adhesion between the modified fibers and the renewable polymers cellulose acetate butyrate and poly(l-lactic acid) was quantified using the single fiber pullout test.
In recent years, carbon nanotubes (CNTs) grown on fibers have attracted a lot of interest as an additional reinforcing component in conventional fiber-reinforced composites to improve the properties of the fiber/matrix interface. Due to harsh growth conditions, the CNT-grafted fibers often exhibit degraded tensile properties. In the current study we explore an alternative approach to deliver CNTs to the fiber surface by dispersing CNTs in the fiber sizing formulation. This route takes advantage of the developed techniques for CNT dispersion in resins and introduces no damage to the fibers. We focus on unidirectional glass fiber/epoxy macro-composites where CNTs are introduced in three ways: (1) in the fiber sizing, (2) in the matrix and (3) in the fiber sizing and matrix simultaneously. Interfacial shear strength (IFSS) is investigated using single-fiber push-out microindentation. The results of the test reveal an increase of IFSS in all three cases. The maximum gain (over 90%) is achieved in the composite where CNTs are introduced solely in the fiber sizing.
Atmospheric-plasma fluorination was used to introduce fluorine functionalities onto the surface of carbon fibers without affecting their bulk properties. The interfacial adhesion between atmospheric-plasma-fluorinated carbon fibers and poly(vinylidene fluoride) (PVDF) was studied by means of direct wetting measurements and single fiber pullout tests. Measured contact angles of PVDF melt droplets on modified carbon fibers show that short exposure times of carbon fibers to atmospheric-plasma fluorination (corresponding to a degree of surface fluorination of F/C = 0.01 (1.1%)) leads to improved wettability of the fibers by PVDF melts. The apparent interfacial shear strength as a measure of practical adhesion, determined by the single-fiber pullout test, increases by 65% under optimal treatment conditions. The improved practical adhesion is not due to the formation of transcrystalline regions around the fibers or a change of the bulk matrix crystallinity or to an increased surface roughness; it seems to be due to the compatibilization of the interface caused of the atmospheric-plasma fluorination of the carbon fibers.
The feasibility of reinforcing conventional carbon fiber composites by grafting carbon nanotubes (CNTs) onto the fiber surface has been investigated. Carbon nanotubes were grown on carbon fibers using the chemical vapor deposition (CVD) method. Iron was selected as the catalyst and predeposited using the incipient wetness technique before the growth reaction. The morphology of the products was characterized using scanning electron microscopy (SEM), which showed evidence of a uniform coating of CNTs on the fiber surface. Contact angle measurements on individual fibers, before and after the CNT growth, demonstrated a change in wettability that can be linked to a change of the polarity of the modified surface. Model composites based on CNT-grafted carbon fibers/epoxy were fabricated in order to examine apparent interfacial shear strength (IFSS). A dramatic improvement in IFSS over carbon fiber/epoxy composites was observed in the single fiber pull-out tests, but no significant change was shown in the push-out tests. The different IFSS results were provisionally attributed to a change of failure mechanism between the two types of tests, supported by fractographic analysis.
The quality of interfacial interaction is dictated by the surface chemistry of the carbon fibres and the composition of the matrix. The composition of poly(vinylidene fluoride) (PVDF) was modified by the addition of maleic anhydride grafted PVDF. The surface properties of the various matrix formulations were characterised by contact angle and electrokinetic measurements. Carbon fibres were modified by industrial electrochemical oxidation and oxidation in nitric acid, or the use of a traditional epoxy-sizing of industrially oxidised fibres. The surface composition, morphology and wetting behaviour of the carbon fibres was characterised. The interaction between modified PVDF and the carbon fibres was studied by direct contact angle measurements between PVDF melt on single carbon fibres and by single fibre pull-out tests. The best wetting and adhesion behaviour was achieved between PVDF containing 5 ppm grafted maleic anhydride (MAH) and epoxy-sized carbon fibres. The addition of MAH-grafted PVDF to the unmodified PVDF caused the apparent interfacial shear strength to increase by 184%. The apparent interfacial shear strength of this fibrematrix combination allowed for the utilisation of 100% of the yield tensile strength of PVDF.
Thin coatings of poly(acrylic acid) (PAA) and poly(hydroxyethylmethacrylate) (PHEMA) were deposited onto carbon fibers by means of the electrospray ionization (ESI) technique in ambient air. These high-molecular weight polymer layers were used as adhesion promoters in carbon fiberepoxy resin composites. Within the ESI process, the carbon fibers were completely enwrapped with polymer in the upper 10 plies of a carbon fiber roving. As identified with scanning electron microscopy also shadowed fibers in a bundle as well as backsides of fiber rovings were pinhole-free coated with polymers (electrophoretic effect'). Under the conditions used, the layers have a granular structure. Residual solvent was absent in the deposit. PAA and PHEMA films did not show any changes in composition and structure in comparison with the original polymers as analyzed by X-ray photo-electron spectroscopy and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Single-fiber pullout tests of coated fibers embedded in epoxy resin showed significantly increased interfacial shear strength. It is assumed that chemical bonds between carbon fiber poly(acrylic acid) and epoxy resin contribute significantly to the improved interactions.
Characterisation of the fibre/matrix interface in reinforced polymers by the push-in technique
(1997)
The push-in test is the only micromechanical test method that is not restricted to artificial fibre/matrix arrangements, but allows the in situ characterisation of interfaces in composites fabricated and stressed under realistic conditions. However, with the application of this method to reinforced polymers some problems arise both in the mathematical model for evaluating test data and in the practical performance of the test. Because in some cases the deformation of the relatively compliant polymeric matrices cannot be neglected, an extension of the existing model is required. For this purpose, the elastic energy of the material around the debonded part of the fibre is estimated and included in the energy-balance analysis. Because of the small diameter of the fibres usually used for reinforcing polymers, a test apparatus was designed which ensures a high positioning accuracy in the xy plane as well as in the z direction. In order to minimise thermal and mechanical influences, the microscope for fibre selection and the force sensor/indenter are directly connected together and the apparatus is designed to be stiff in all components. A solid-state bending joint guarantees very precise control of the axial movement. Three examples of the application of this easy-to-handle and low-cost test apparatus are presented briefly in the paper: assess fibre/matrix combinations, measures to improve the interfacial adhesion and the influence of water on the interface.
High-performance fibre-reinforced polymer composites are important construction materials based not only on the specific properties of the reinforcing fibres and the flexible polymer Matrix but also on the compatible properties of the composite interphase. First, oxygen-free (a-CSi:H) and oxygen-binding (a-CSiO:H) plasma nanocoatings of different mechanical and tribological properties were deposited on planar silicon dioxide substrates that closely mimic E-glass. The nanoscratch test was used to characterize the nanocoating adhesion expressed in terms of critical normal load and work of adhesion. Next, the same nanocoatings were deposited on E-glass fibres, which were used as reinforcements in the polyester composite to affect its interphase properties. The shear properties of the polymer composite were characterized by macro- and micromechanical tests, namely a short beam shear test to determine the short-beam strength and a single fibre push-out test to determine the interfacial shear strength. The results of the polymer composites showed a strong correlation between the short-beam strength and the interfacial shear strength, proving that both tests are sensitive to changes in fibre-matrix adhesion due to different surface modifications of glass fibres (GF).
Finally, a strong correlation between the shear properties of the GF/polyester composite and the adhesion of the plasma nanocoating expressed through the work of adhesion was demonstrated.
Thus, increasing the work of adhesion of plasma nanocoatings from 0.8 to 1.5 mJ·m−2 increased the short-beam strength from 23.1 to 45.2 MPa. The results confirmed that the work of adhesion is a more suitable parameter in characterising the level of nanocoating adhesion in comparison with the critical normal load.
In this work, a closed loop recycling process is investigated, which allows polymerised bulk thermoplastic matrix (Elium 150) from production waste (also referred to as recyclate) to be reused as additive in composite manufacturing by vacuum assisted resin infusion (VARI) of virgin Elium 150 monomer. It is shown that this process can save up to 7.5 wt% of virgin material usage in each processing cycle. At the same time, the thermal stability and stiffness of the composite increases with the proportion of recyclate introduced. Contemporarily, the shear and bending properties have also been observed to improve. Gel permeation chromatography (GPC) showed that the changes observed are due to an increase in molecular weight with the recyclate content. In particular, a correlation between the molecular weight and the shear properties of the composite was discovered using single fibre push-out tests.
Among the strategies to produce healable thermosetting systems is their modification by the addition of thermoplastic particles. This work investigates the influence of poly(ethylene-co-methacrylic acid) (EMAA) on fibermatrix interfacial properties of a glass fiber reinforced epoxy matrix composite. Epoxy-EMAA interactions were evaluated using differential scanning calorimetry (DSC) and infrared spectroscopy. The effects of EMAA on the epoxy network formation were evidenced by changes in glass transition temperature, cure kinetics and alteration of chemical groups during cure. Interfacial shear strength (IFSS) measurements obtained by single fiber pull-out tests indicate similar interfacial properties for pure and EMAA modified epoxy. Additionally, the potential for self-healing ability of an EMAA modified epoxy was demonstrated. However, IFSS after a healing cycle for the EMAA modified epoxy was lower as compared to the pure epoxy, because of the lower fiber-EMAA interfacial shear strength. So, thermoplastic healing agents has not only to fill cracks in the matrix material, but also have to be optimized regarding its interface properties to the reinforcing fibers.
Mechanical and microstructural characterisation of multifunctional structural power composites
(2013)
Although the inherent anisotropy of polymer composites has presented daunting technical challenges, these materials now offer engineers considerable opportunities for efficient structural design. More recently, the advent of multifunctional composites which can fulfill more than one role within a system has attracted considerable interest, providing designers with exciting opportunities to innovate. Of particular interest here are structural power composites, which simultaneously carry mechanical load whilst storing/delivering electrical energy. Although the development of these composites is highly challenging, often with conflicting constituent requirements, the STORAGE consortium has had considerable success in the development of these materials for automotive applications.
The focus of this paper is structural supercapacitors, the basic architecture of a single cell of which is shown in Fig. 1. This entails two carbon fibre woven lamina (electrodes) which sandwich a glass fibre woven lamina (separator), all of which is embedded within a multifunctional matrix (electrolyte). This architecture has been the focus of the research to date, leading to components such as that shown in Fig.1 having been fabricated. This paper reports on the mechanical properties and microstructures of the different reinforcement and matrix combinations for structural supercapacitors.
The research presented here attempts to assess the potential for re-using carbon fibre (CF) fabrics recovered from recycling infusible acrylic thermoplastic carbon fibre reinforced polymer composites (CFRPs) in a universal manner, i.e. by combining with a wide variety of matrices to manufacture 2nd generation composite laminates by resin infusion. The 2nd generation composites have been compared in terms of bulk and interfacial properties against counteparts processed with virgin carbon fibre fabric infused with the same matrices. Generally, an increase in damping (tanδ) was observed in all 2nd generation composites, which can be attributed to a residual thin thermoplastic layer present on the recovered fibres. The interfacial adhesion of the 2nd generation Composites was investigated by shear tests and scanning electron micsoscopy, and also appears to be less influenced by the type of matrix.
After a delamination process in a C-fiber reinforced polymer composite, the identification of the polymer residue of poly-(phenylene sulphide) on carbon fiber surfaces gives important information on the composite failure mechanism. Using scanning force microscopy (SFM) in its lateral force imaging mode or with the stiffness imaging mode, it is almost impossible to distinguish the two composite components owing to a very low material-based contrast of the local friction and compliance. The new technique using contrast based on a local AC current measurement allows a clear identification of the conducting carbon fiber and the non-conducting polymer coverage.
Electrolyte systems that can carry mechanical load while allowing for high levels of ionic conductivity are an important prerequisite for structural power storage devices. Introduction of structural power storage into the variety of consumer products will allow saving in weight and volume. Moreover, using a supercapacitor/battery system in hybrid electric vehicles (HEV), the supercapacitor part will extend the battery lifetime by protecting it from the high peak currents. To successfully produce structural power storage requires the development
of multifunctional electrolytes where one has to simultaneously maximize mechanical properties and ionic conductivity.
An exciting challenge is to create unduloid-reinforcing fibers with tailored dimensions to produce synthetic composites with improved toughness and increased ductility. Continuous carbon fibers, the state-of-the-art reinforcement for structural composites, were modified via controlled laser irradiation to result in expanded outwardly tapered regions, as well as fibers with Q-tip (cotton-bud) end shapes. A pulsed laser treatment was used to introduce damage at the single carbon fiber level, creating expanded regions at predetermined points along the lengths of continuous carbon fibers, while maintaining much of their stiffness. The range of produced shapes was quantified and correlated to single fiber tensile properties. Mapped Raman spectroscopy was used to elucidate the local compositional and structural changes. Irradiation conditions were adjusted to create a swollen weakened region, such that fiber failure occurred in the laser treated Region producing two fiber ends with outwardly tapered ends. Loading the tapered fibers allows for viscoelastic energy dissipation during fiber pull-out by enhanced friction as the fibers plough through a matrix. In these tapered fibers, diameters were locally increased up to 53%, forming outward taper angles of up to 1.8°. The tensile strength and strain to failure of the modified fibers were significantly reduced, by 75% and 55%, respectively, ensuring localization of the break in the expanded region; however, the fiber stiffness was only reduced by 17%. Using harsher irradiation conditions, carbon fibers were completely cut, resulting in cottonbud fiber end shapes. Single fiber pull-out tests performed using these fibers revealed a 6.75-fold increase in work of pull-out compared to pristine carbon fibers. Controlled laser irradiation is a route to modify the shape of continuous carbon fibers along their lengths, as well as to cut them into controlled lengths leaving tapered or cotton-bud shapes.
Biopolymers are very efficient for significant applications ranging from tissue engineering, biological devices to water purification. There is a tremendous potential value of cellulose because of ist being the most abundant biopolymer on earth, swellability, and functional groups to be modified. A novel, highly efficient route for the fabrication of mechanically stable and natural hydrogels is described in which cellulose and glycine are dissolved in an alkaline solution of NaOH and neutralized in an acidic solution. The dissolving temperature and the glycine amount are essential parameters for the self-assembly of cellulose chains and for Tuning the morphology and the aggregate structures of the resulting hydrogels. Glycine plays the role of a physical crosslinker based on the Information obtained from FTIR and Raman spectra. Among the prepared set of hydrogels, CL5Gly30 hydrogels have the highest capacity to absorb water. The prepared CL5Gly30 gels can absorb up to seven times their dry weight due to its porous 3-D network structure. CL5Gly10 hydrogel exhibits 80% deformation under 21 N force executed. The method developed in this article can contribute to the application of heavy metal adsorption in aqueous solutions for water purification and waste management.
Epoxy R-Glass Fiber-Reinforced Polymer (GFRP) composite plates were hydrothermally aged at 60 ◦C for 23, 75, and 133 days. The water content reached 0.97 wt%, 1.45 wt% and 1.63 wt%, respectively. The studied GFRP matrix was inert to hydrolysis or chain scission, allowing for investigation of irreversible changes in the fiber-matrix interphase due to hydrothermal aging upon re-drying. During each period, a subset of the specimens was removed from the water bath and dried in a chamber. The weight loss upon drying was explained with epoxy leaching (impurities), sizing-rich interphase hydrolysis, glass fiber surface hydrolysis, accumulated degradation products escaping, and water changing state from bound to free. The influence of hydrothermal aging on the fiber-matrix interfacial properties was investigated. Lower interfacial strength of hydrothermally aged (wet)
samples was attributed to plasticization of the epoxy, plasticization and degradation of the sizing-rich interphase (including formation of hydrolytic flaws), and hydrolytic degradation of the glass fiber surface. The kinetics of epoxy-compatible epoxysilane W2020 sizing-rich interphase hydrolysis provided an estimate of ca. 1.49%, 4.80%, and 8.49% of the total composite interphase degraded after 23, 75, and 133 days, respectively. At these conditions, the interface lost 39%, 48%, and 51% of its strength. Upon re-drying the specimens, a significant part of the interfacial strength was regained. Furthermore, an upward trend was observed, being 13%, 10% and 3% strength, respectively; thus, indicating a possibility of partial recovery of properties.
The present work provides a comparative study on the interface and adhesion properties of surface modified single glass fibers embedded in an acrylate matrix. To facilitate a covalent bonding at the fibermatrix interface, the fibers are functionalized with selected organosilanes that comprise either passive (unsaturated C¼C bonds of methacrylate moieties) or photoactive functionalities (photocleavable bis(acyl)phosphane oxide groups). Immobilization of the functional silanes is carried out by a classic silanization reaction involving a condensation reaction across the surface hydroxyl groups of the inorganic glass fibers. The change of the physico-chemical properties of the fibers due to desizing and subsequent surface modification is monitored by X-ray photoelectron spectroscopy and zeta potential measurements. In addition, scanning electron microscopy is used to follow the changes in surface morphology. After the modification step, the desized and modified single fibers are embedded in a photocurable acrylate resin formulation. By performing single fiber pull-out tests, maximum pull-out force, friction strength and apparent interfacial shear strength are determined as a function of the coupled silanes. The results reveal that the attached organosilanes lead to a significant increase in adhesion strength, whilst the performance of the photo-cleavable organosilane is superior to the passive methacryl-functional derivative.
Carbon nanotube-grafted carbon fiber polymer composites: Damage characterization on the micro scale
(2017)
Multiwall carbon nanotubes (CNTs) e carbon fibers (CFs)hybrid materials were produced by directly growing CNTs on CFs by means of chemical vapor deposition. For the latter, the oxidative dehydrogenation reaction of C2H2 and CO2 was applied, which allows growing CNTs without damaging the CF surface. Uni-directional nano-engineered carbon fiber reinforced composites (nFRCs) were fabricated by impregnating these hybrid materials with epoxy. The nFRCs subjected to single fiber push-out tests revealed a decrease of the interfacial shear strength (IFSS) of about 36% compared to the carbon fiber composites without CNTs. By means of transverse three-point bending tests performed on pre-notched composite beams inside a scanning electron microscope, the fracturing behavior parallel to the fibers was studied in-situ. The nFRCs showed significantly reduced fiber/matrix debonding while CNTs pull-out, CNTs bridging as well as matrix failure occurred. These results demonstrate that the presence of CNTs in nFRCs affects the stress distribution and consequently the damage Initiation as well as the damage propagation. The presence of CNTs suppresses the stress concentration at the fiber/Matrix interface and reduces the debonding of CFs from the matrix. However, our results indicate that the stress concentration shifts towards the CNTs' ends/matrix interface and causes promoted matrix failure leading to lower IFSS.
Structural electrolytes were prepared using a fully formulated commercially available high performance epoxy resin (MTM57) and an ionic liquid based electrolyte: lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) dissolved in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI). Through a systematic study, the composition of the formulations was found to have a greater effect than the curing temperature on the morphology and properties of the resulting structural electrolytes. The presence of lithium salt is essential to form a structurally homogeneous electrolyte. Bicontinuous morphologies containing continuous (coarse) epoxy networks surrounded by connected spherical epoxy nodules were obtained with different length scales upon varying the lithium salt concentration. Increasing the LiTFSI concentration improved the miscibility of MTM57 with the electrolyte and decreased the characteristic length scale of the resulting bicontinuous microstructure. The properties of the structural electrolytes correlated with the morphology, showing increased Youngs modulus and decreased ionic conductivity with increasing lithium salt concentration. The miscibility of the epoxy system with the electrolyte was also improved by substitution of EMIM-TFSI with an equal weight of an aprotic organic solvent, propylene carbonate (PC); however, the window of PC concentrations which resulted in structural electrolytes with bicontinuous microstructures was very narrow; at PC concentrations above 1 wt %, gel-like polymers with no permanent mesoporosity were obtained.
The introduction of carbon nanotubes (CNTs) modifies bulk polymer properties, depending on intrinsic quality, dispersion, alignment, interfacial chemistry and mechanical properties of the nanofiller. These effects can be exploited to enhance the matrices of conventional microscale fibre-reinforced polymer composites, by using primary reinforcing fibres grafted with CNTs. This paper presents a methodology that combines atomic force microscopy, polarised Raman spectroscopy, and nanoindentation techniques, to study the distribution, alignment and orientation of CNTs in the vicinity of epoxy-embedded micrometre-scale silica fibres, as well as, the resulting local mechanical properties of the matrix. Raman maps of key features in the CNT spectra clearly show the CNT distribution and orientation, including a parted morphology associated with long grafted CNTs. The hardness and indentation modulus of the epoxy matrix were improved locally by 28% and 24%, respectively, due to the reinforcing effects of CNTs. Moreover, a slower stress relaxation was observed in the epoxy region containing CNTs, which may be due to restricted molecular mobility of the matrix. The proposed methodology is likely to be relevant to further studies of nanocomposites and hierarchical composites.
Structural supercapacitor electrolytes based on bicontinuous ionic liquid-epoxy resin systems
(2013)
'Structural electrolytes' retain the desirable mechanical characteristics of structural (epoxy) resins whilst introducing sufficient ionic conductivity to operate as electrolytes in electrochemical devices. Here, a series of ionic liquid–epoxy resin composites were prepared to identify the optimum system microstructure required to achieve a high level of multifunctionality. The ionic conductivity, mechanical properties, thermal stability and morphology of the cured epoxy based structural electrolytes were studied as a function of phase composition for three fully formulated high performance structural epoxy systems. At only 30 wt% of structural resin and 70 wt% of ionic liquid based electrolyte, stiff monolithic plaques with thicknesses of 2–3 mm were obtained with a room temperature ionic conductivity of 0.8 mS cm-1 and a Young's modulus of 0.2 GPa. This promising performance can be attributed to a long characteristic length scale spinodal microstructure, suggesting routes to further optimisation in the future.
We invented the first non-metallic, self-adhesive and dry biosignalling electrode. The PEDOT polymer electrode changes its aggregate state and conductivity by a light curing procedure. The electrode can be applied as a gel underneath hair without shaving. With the aid of blue light, the electrode can be hardened within a few seconds at the desired location on the scalp. The cured polymer electrode is highly conductive and can be applied on a very small location. Unlike other EEG electrodes, our electrode does not lose conductivity upon drying. Furthermore, our electrode strongly bonds to Skin and does not require any additional adhesive. Short circuits due to an outflow of gel are prevented with this technique. Therefore, the PEDOT polymer electrode is extremely well suited for applications that, up to now, have been challenging, such as non-invasive EEG recordings from awake and freely moving animals, EEG recordings from preterm babies in the neonatal intensive care unit or long-term recordings in the case of sleep monitoring or epilepsy diagnostics. We addressed two technical questions in this work. First, is the EEG recorded with polymer electrodes comparable to a standard EEG? Second, is it possible to record full-band EEGs with our electrodes?
Multifunctional composites which can fulfil more than one role within a system have attracted considerable interest. This work focusses on structural supercapacitors which simultaneously carry mechanical load whilst storing/delivering electrical energy. Critical mechanical properties (in-plane shear and in-plane compression performance) of two monofunctional and four multifunctional materials were characterised, which gave an insight into the relationships between these properties, the microstructures and fracture processes. The reinforcements included baseline T300 fabric, which was then either grafted or sized with carbon nanotubes, whilst the baseline matrix was MTM57, which was blended with ionic liquid and lithium salt (two concentrations) to imbue multifunctionality. The resulting composites exhibited a high degree of matrix heterogeneity, with the ionic liquid phase preferentially forming at the fibres, resulting in poor matrix-dominated properties. However, fibre-dominated properties were not depressed. Thus, it was demonstrated that these materials can now offer weight savings over conventional monofunctional systems when under modest loading.
Manufacturing and deformation behavior of alumina and zirconia helical springs at room temperature
(2023)
Ceramic helical springs with identical dimensions were produced by hard machining from alumina, alumina toughened zirconia (ATZ), and tetragonal zirconia polycrystals (TZP) stabilized with different oxides. According to the results of the spring constant determination under deformation rates of 3 mm/min, the deformation behavior of all ceramic springs obeys to Hook’s law. However, variation of the deformation rate, tests under constant load, and spring recovery behavior revealed differences in the deformation behavior of alumina, TZP, and ATZ springs. Alumina springs exhibited time-independent deformation in all tests. In contrast, anelastic deformation at room temperature was demonstrated in all springs containing TZP. This deformation is completely reversible over a period of several days. Anelastic behavior is particularly pronounced in Y-TZP springs, whereas Ce-TZP springs exhibit comparatively very low but still reliably detectable anelasticity. Oxygen vacancies in the TZP ceramic are considered the most likely explanation for the anelastic behavior of TZP springs at room temperature.
The present study aims at the development of photodegradable epoxy based materials comprising o-nitrobenzyl ester links that undergo well defined bond cleavage in response to UV irradiation. New bi-functional epoxy based monomers bearing o-nitrobenzyl ester groups are synthesized and thermally cured with an anhydride hardener to yield photosensitive polymers and duromers. The UV induced changes in solubility are exploited for the preparation of positive-type photoresists. Thin patterned films are obtained by photolithographic processes and characterized by microscopic techniques. The results evidence that sensitive resist materials with good resolution and high contrast behavior can be accomplished. Along with resist technology, the applicability of o-nitrobenzyl chemistry in the design of recyclable polymer materials with thicknesses in the millimeter range is evaluated. By monitoring the thermo-mechanical properties upon UV illumination, a distinctive depletion of storage modulus and glass transition temperature is observed with increasing exposure dose. Additionally, single fiber pull-out tests are carried out revealing a significant decrease of the interfacial adhesion at the fiber-matrix interface due to the phototriggered cleavage reaction.
Plasma polymers and electrospray-ionization (ESI) polymer layers are compared for most efficient adhesion promotion in carbon fiber-epoxy resin composites. The ultra-thin ESI layers (2–30 nm) of commercial poly(acrylic acid) and poly-(hydroxyethylmethacrylate) produce an significant increase of adhesion measured by single-fiber pull out tests.
However, plasma Treatment has also advantages, such as simultaneous activation of the fiber substrate. Chemical structure and composition are rather far from the regular structure of commercial polymers as deposited by ESI processing.
Poly (ether ether ketone) (PEEK) has a high continuous service temperature, excellent mechanical properties, and good solvent and abrasion resistance, which can be further improved through the addition of carbon nanotubes (CNTs). CNT-PEEK nanocomposites are promising matrices for continuous carbon fibre composites; powder processing can mitigate the high melt viscosities in these systems. In this study, model single fibre (hierarchical) composites were produced by embedding sized and desized carbon fibres in nanocomposite CNTPEEK powders followed by single fibre pull-out tests to assess interfacial characteristics. Carbon fibre-PEEK interfacial shear strength is typically 40–45 MPa. Increasing CNT loadings increased fibre-matrix interfacial shear strength linearly up to ~70 MPa at 5.0 wt%, which was attributed to the CNT-based mechanical modification of the PEEK matrix. Apparent interfacial shear strength was inversely correlated with the embedded fibre length irrespective of carbon fibre sizing or CNT loading, indicating brittle fracture of the fibre-matrix interface.
Pulled out carbon fibres were still coated with the matrix, which indicated strong adhesion at the interface in all samples, likely related to a transcrystalline region. Adhesion was, however, negatively affected by the presence of epoxy sizings. Frictional shear strength was independent of embedded fibre length and CNT content for all samples.
The bond between polymer fibers and the surrounding cementitious matrix is essential for the development of concrete reinforcement. The single fiber pull-out test (SFPT) is the standard characterization technique for testing the bond strength. However, the different phases of debonding cannot be distinguished by the SFPT. This study investigates the debonding of different polymer fibers from the surrounding cementitious matrix with a modified SFPT and proposes methods to change the SFPT setup to generate more valuable information on the debonding mechanism. The SFPT was equipped with linear variable differential transformers (LVDT), digital image correlation (DIC) and acoustic emission (AE) analysis. The results demonstrate that the modified SFPT allows a better understanding of the different phases of debonding during fiber pull-out. Furthermore, bond strength values calculated by different methods reveal that the chemical bond of the investigated polymers is not different as reported by previous studies. Deformation measurements performed using LVDTs and DIC are suitable measuring techniques to characterize the debonding mechanism in SFPT. A correlation between recorded AE and debonding phases was not found.
This work presents a further development of the methods of simultaneous determination of temperature and axial strain using a single fibre Bragg grating (FBG). The reflected spectrum of composite-embedded FBG sensors has been analyzed in order to separate temperature and load effects. We found out that during the curing process of the laminates, a superstructure has been introduced on the FBG. Analyzing this effect, a temperature and strain separation was implemented by simply calculating and comparing the integrated intensity of the spectral response signal. A mathematical four-parametric model has been developed to calculate the reflected spectrum of a superstructured fibre Bragg grating. The mathematically achieved data has been evaluated with experimentally determined data from fibre Bragg gratings embedded in glass-fibre reinforced composite materials.
Self-healing polymers have been developed to improve durability and reduce costs associated with maintenance during service. The addition of thermoplastics to thermosets to produce mendable polymers appears as a promising selfhealing technique. In this study, poly (ethylene-co-methacrylic acid) (EMAA) was added to epoxy resin and the effects of EMAA addition on epoxy properties were evaluated. Specimens with two different contents of thermoplastic and particles sizes were manufactured. A two-level full factorial experimental design was used to evaluate the effect of particle size and particle content on properties of epoxy modified with addition of EMAA. Tensile tests and dynamic mechanical analysis (DMA) were used and the evaluated responses were tensile strength, modulus of elasticity, and glass transition temperature (Tg). X-ray computed tomography (XCT) was used to investigate particle size and concentration after manufacturing. It was found that the particle concentration has greater effects on stress–strain behavior of epoxy while Tg was not significantly affected by neither of the analyzed entrance variables.
AbstractTwo‐photon polymerization (2PP) additive manufacturing (AM) utilizes feedstocks of ceramic nanoparticles of a few nanometers in diameter, enabling the fabrication of highly accurate technical ceramic design with structural details as small as 500 nm. The performance of these materials is expected to differ from conventional AM ceramics, as nanoparticles and three‐dimensional printing at high resolution introduce new microstructural aspects. This study applies 2PP‐AM of yttria‐stabilized zirconia to investigate the mechanical response behavior under compressive load, probing the influence of smallest structural units induced by the line packing during the printing process, design of sintered microblocks, and sintering temperature and thereby microstructure. We find a dissipative mechanical response enhanced by sintering at lower temperatures than conventional. The pursued 2PP‐AM approach yields a microstructured material with an increased number of grain boundaries that proposedly play a major role in facilitating energy dissipation within the here printed ceramic material. This microplastic response is further triggered by the filigree structures induced by hollow line packing at the order of the critical defect size of ceramics. Together, these unique aspects made accessible by the 2PP‐AM approach contribute to a heterogeneous nano‐ and microstructure, and hint toward opportunities for tailoring the mechanical response in future ceramic applications.
AbstractTwo‐photon polymerization (2PP) additive manufacturing (AM) utilizes feedstocks of ceramic nanoparticles of a few nanometers in diameter, enabling the fabrication of highly accurate technical ceramic design with structural details as small as 500 nm. The performance of these materials is expected to differ from conventional AM ceramics, as nanoparticles and three‐dimensional printing at high resolution introduce new microstructural aspects. This study applies 2PP‐AM of yttria‐stabilized zirconia to investigate the mechanical response behavior under compressive load, probing the influence of smallest structural units induced by the line packing during the printing process, design of sintered microblocks, and sintering temperature and thereby microstructure. We find a dissipative mechanical response enhanced by sintering at lower temperatures than conventional. The pursued 2PP‐AM approach yields a microstructured material with an increased number of grain boundaries that proposedly play a major role in facilitating energy dissipation within the here printed ceramic material. This microplastic response is further triggered by the filigree structures induced by hollow line packing at the order of the critical defect size of ceramics. Together, these unique aspects made accessible by the 2PP‐AM approach contribute to a heterogeneous nano‐ and microstructure, and hint toward opportunities for tailoring the mechanical response in future ceramic applications.
Fluorescence imaging microscopy is an essential tool in biomedical research. Meanwhile, various fluorescent probes are available for the staining of cells, cell membranes, and organelles. Though, to monitor intracellular processes and dysfunctions, probes that respond to ubiquitous chemical parameters determining the cellular function such as pH, pO2, and Ca2+ are required. This review is focused on the progress in the design, fabrication, and application of photoluminescent nanoprobes for sensing and imaging of pH in living cells. The advantages of using nanoprobes carrying fluorescent pHindicators compared to single molecule probes are discussed as well as their limitations due to the mostly lysosomal uptake by cells. Particular attention is paid to ratiometric dual wavelength nanosensors that enable intrinsic referenced measurements. Referencing and proper calibration procedures are basic prerequisites to carry out reliable quantitative pH determinations in complex samples such as living cells. A variety of examples will be presented that highlight the diverseness of nanocarrier materials (polymers, micelles, silica, quantum dots, carbon dots, gold, photon upconversion nanocrystals, or bacteriophages), fluorescent pH indicators for the weak acidic range, and referenced sensing mechanisms, that have been applied intracellularly up to now.