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Polymer-based electrolytes based on commercially available epoxy resins were prepared through the addition of a liquid electrolyte, a solution of a lithium salt in an ionic liquid. The polymer monoliths were characterized using impedance spectroscopy, 3-point bending test, scanning electron microscopy (SEM) and nitrogen adsorption (BET). The balance of ionic conductivity and flexural modulus is crucially dependent on the relative proportions of epoxy resin to electrolyte. Also the effect of the liquid electrolyte on curing kinetics and processing was assessed by complex viscosity measurements and differential scanning calorimetry (DSC). Only one out of the three resins investigated exhibited a significant acceleration effect.
A simple method of creating defined PMMA and poly (MMA-co-Cz) electrocoatings on carbon fibres is described. The electrodeposition of poly methylmethacrylate (PMMA) onto unsized, unmodified carbon fibres was performed by simple constant current electrolyses of methylmethacrylate (MMA) monomer in dimethylformamide (DMF) solutions and the pure liquid monomer using sodium nitrate and lithium perchlorate as supporting electrolytes. The presence of polymeric coatings successfully attached to the carbon fibres was verified by scanning electron microscopy and photoelectron spectroscopy (XPS).
Performing the electrolysis in dilute MMA in DMF solutions ([MMA] < 5 M) results in the deposition of powder-like polymer on the carbon fibre electrodes. Increasing the MMA concentration in the DMF solution results in a homogeneous PMMA coating of the carbon fibres. The degree of grafting or coating increases with increasing MMA concentration, except when pure MMA is used without solvent.
The adhesive strength between the electrocoated carbon fibres and a PMMA matrix was determined using the single fibre pull-out test. It was found that the interfacial fracture behaviour of all carbon fibre/PMMA model composites is rather brittle. The adhesion strength between the unmodified carbon fibres and the PMMA matrix was equal to the cohesive strength of the polymer matrix itself. Nevertheless, the electrodeposition of thin and homogeneous PMMA coatings resulted in much improved adhesion strengths.
The surface properties of original high strength and preoxidized high modulus carbon fibers were altered by electrocopolymerizing acryl amide and carbazole and therefore depositing a copolymer coating onto the fibers. Scanning electron microscopy and zeta-potential measurements confirmed the presence of a rough but dense and continuous electrocoating with a basic surface character. Therefore, lsquogoodrsquo adhesion behavior between the electrocoated carbon fibers and an epoxy resin matrix should be expected. The interfacial adhesion was measured using the single fiber pull-out and single fiber indentation test. It was shown that only lsquointermediatersquo adhesion was present between the carbon fibers and the electrocoating, but superior adhesion between the coating and epoxy resin exists. The single fiber model composites always failed at the fiber/electrocoating interface. However, as shown by using the indentation test, the interfacial adhesion between fibers and electrocoating can be significantly improved if preoxidized fibers are used as substrate for electropolymerization. A very high tensile strength for the electrocoating can be expected as derived from the single fiber pull-out tests.
It is highly desirable to improve attractive interactions between carbon fibers and unreactive thermoplastic matrices to the possible maximum. This could be achieved by a simple grafting process to create a covalently bonded interface or interlayer, which should result in cohesive interactions between the polymer-grafted fibers and the same matrix material, leading to a better adhesion strength in the obtained composite material. Here, we are describing the grafting of styrene onto unmodified and unsized carbon fibers via free-radical bulk polymerization in the presence of fibers. After grafting, the surface properties of the carbon fiber approach those of pure polystyrene which was proven by contact angle and zeta ({zeta}) potential measurements. As indicated by the water contact angle, the carbon fiber surface becomes more hydrophobic. Scanning electron microscopy (SEM) provides evidence of grafted polymer. This simple procedure results in a continuous polystyrene coating. The fiber diameter increases significantly after polymer grafting. The adhesion and fracture behavior between the original and polystyrene-grafted carbon fibers to a polystyrene (VESTYRON®) matrix was characterized using the single-fiber pull-out test. There is a considerable increase in the measurable adhesion, i.e., the interfacial shear strength IFSS, by almost 300% between the grafted fibers and polystyrene as compared to untreated original fibers. Two planes of interfacial failure could be distinguished; first in the fiber coating interface leading to lower interfacial shear strength and second in the PS-matrix-PS-coating interphase resulting in a higher interfacial shear strength. In addition to the improved adhesion, there are also clear differences in the pull-out behavior between the nongrafted and grafted fibers. After the initial debonding process corresponding to the maximal pull-out force is completed, the pull-out force is increasing again.
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.
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.
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.
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.