Effect of ZnO nanowire morphology on the interfacial strength of nanowire coated carbon fibers

The ability to tailor interfacial shear strength for a particular fiber and resin system is critical to the development of composite materials that perform optimally in specific applications. One approach to tailor the interface is to introduce a secondary phase between the fiber and matrix, which can act to functionally grade the material properties and enhance load transfer across the interface. This approach has been applied in the past using nanowires, nanotubes, and whiskers and was demonstrated to significantly enhance interface performance. Unfortunately, these processes lack control over the interphase morphology to allow design of the interface for optimal properties. Recently, ZnO nanowires grown on the surface of carbon fibers have demonstrated more than a 110% increase in interfacial strength [1]. Unlike other treatments, this interfacial reinforcement allows precise morphology control. Here, we develop the parameters for the growth of nanowires with varying lengths and diameters and study the influence of the nanowire’s morphology on the interfacial shear strength. ZnO nanowire arrays are grown on carbon fibers, with nanowire diameters ranging from 50 to 200 nm and lengths up to 4 μm. The interfacial shear strength with varying nanowire dimensions is shown to increase by up to 228%, ranging from 45.72 to 154.64 MPa. Unlike existing whiskerization approaches, it is shown that the tensile strength of the ZnO nanowire coated fibers remains constant throughout all growth procedures. The development of an interphase offering control over the interface strength and toughness will provide a means to produce multifunctional composites with optimized performance for multiple applications.     

Jute fibre/epoxy composites: Surface properties and interfacial adhesion

Jute fibres were surface treated in order to enhance the interfacial interaction between jute natural fibres and an epoxy matrix. The fibres are exposed to alkali treatment in combination with organosilane coupling agents and aqueous epoxy dispersions. The surface topography and surface energy influenced by the treatments were characterized. Single fibre pull-out tests combined with SEM and AFM characterization of the fracture surfaces were used to identify the interfacial strengths and to reveal the mechanisms of failure.     

Macroscopic analysis of interfacial properties of flax/PLLA biocomposites

This study presents results from a study of the mechanical behaviour of flax reinforced Poly(l-Lactic Acid) (PLLA) under in-plane shear and mode I interlaminar fracture testing. Slow cooling of the unreinforced polymer has been shown to develop crystalline structure, causing improvement in matrix strength and modulus but a drop in toughness. The in-plane shear properties of the composite also drop for the slowest cooling rate, the best combination of in-plane shear performance and delamination resistance is noted for an intermediate cooling rate, (15.5 °C/min). The values of G_Ic obtained at this cooling rate are higher than those for equivalent glass/polyester composites. These macro-scale results have been correlated with microdroplet interface debonding and matrix characterization measurements from a previous study. The composite performance is dominated by the matrix rather than the interface.     

Increased permittivity nanocomposite dielectrics by controlled interfacial interactions

The use of nanoparticles in polymer composite dielectrics has promised great improvements, but useful results have been elusive. Here, the importance of the interfacial interactions between the nanoparticles and the polymer matrix are investigated in TiO₂ nanocomposites for dielectric materials using surface functionalisation. The interface is observed to dominate the nanocomposite properties and leads to a threefold increase in permittivity at volume fractions as low as 10%. Surface functionalisation of the filler nanoparticles with silanes allows control of this interface, avoiding significant degradation of the other important material properties, particularly electrical breakdown strength, and resulting in a material that is demonstrated successfully as an active material in a dielectric elastomer actuator application with increased work output compared to the pure polymer. Although further permittivity increases are observed when the interface regions have formed a percolation network, the other material properties deteriorate. The observation of percolation behaviour allows the interface thickness to be estimated.     

Influence of the interphase zone on the nanoparticle debonding stress

One of the most appealing features concerned with nanomodification of polymeric resins for structural applications is the perspective of obtaining high toughness even at low nanofiller volume fractions. Such performances are related to the energy dissipated through the damage mechanisms taking place at the nanoscale. Among these, nanoparticle debonding could take an important role either as a mechanism itself or as a trigger for phenomena like plastic void growth or matrix shear yielding. In the present work, a model for the hydrostatic tension related to debonding is presented. The model accounts for some important issues inherently related to the nanoscale with particular reference to the emergence of an interphase surrounding the nanoparticle. Results can be useful in view of a multi-scale modelling of the problem.     

Characterization of the aramid/epoxy interfacial properties by means of pull-out test and influence of water absorption

Single fiber pull-out tests were carried out to investigate the influence of water absorption on the interfacial properties of aramid/epoxy composite. The fiber/matrix interfacial strength was severely decreased between 4 and 7 week immersion time in deionized water at 80 °C, and thereafter showed a plateau. This change with immersion time did not correspond with that of the water gain of the pull-out specimens, because the water gain did not reflect the one in the fiber/matrix interface. As a result of the degradation of the fiber/matrix interfacial strength, the pulled-out fiber surfaces of 7, 10 and 13 week wet specimen were smooth. In situ observations of interfacial crack propagation by a video microscope and an analysis of acoustic emission (AE) signals showed that AE signals obtained during the pull-out process were classified into four types according to fracture modes. AE signals detected at final unstable crack propagation and fiber breakage had high amplitude and long duration.     

Effect of oxygen plasma treatment of PPTA and PBO fibers on the interfacial properties of single fiber/epoxy composites studied by Raman spectroscopy

The interfacial micromechanics of single poly(p-phenylene terephthalamide (PPTA) and poly(p-phenylene benzobisoxazole (PBO) fibers embedded in an epoxy resin has been investigated by determining the interfacial shear stress distributions along the fiber length. The effects of an oxygen plasma treatment on the interfacial shear stress of the fiber-epoxy systems are analyzed. Raman spectroscopy was used to map the stress distributions along the fiber when the composite is subjected to a small axial tensile strain (3.5% for PPTA and 2.5% for PBO). The quality of the interface or adhesion was improved after the surface treatment, supporting the ability of plasma oxidation to enhance the adhesion of high-performance fibers to epoxy resins. The tensile behavior of fiber-reinforced systems was different in each case. PPTA reinforcements underwent fragmentation, likely by fiber microfailure, whereas debonding or bridging is the most probable fragmentation mechanism in the case of PBO.     

Direct measurements of interfacial shear strength of multi-walled carbon nanotube/PEEK composite using a nano-pullout method

This paper presents a proposal of a simple and easy method to evaluate the interfacial shear strength (IFSS) of CNT-dispersed polymer composites. An individual multi-wall carbon nanotube (MWNT) was pulled out from a MWNT-dispersed/PEEK composite using a nano-pullout testing system installed in an SEM. The tensile load was measured using the elastic deformation of an AFM cantilever. The pull-out length was controlled by making a through-thickness hole near the specimen edge using a focused ion beam (FIB) system. The IFSS of a MWNT/PEEK composite was measured as 3.5-14 MPa, which agrees with the IFSS estimated from the macroscopic stress-strain behavior of the MWNT/PEEK composites.     

Measurement of the critical aspect ratio and interfacial shear strength in MWNT/polymer composites

This paper presents a bulk composite method for determining the critical aspect ratio and relative interfacial shear stress (ISS) for multiwalled carbon nanotube (MWNT)/polymer composites. Through a modified pullout test and fragmentation test, it was found that the critical aspect ratio was 300 and decreased by a factor of 3 due to surface modification, and that MWNTs at an angle of greater than 60° to the loading direction failed in bending instead of pulling out of the matrix. Finite element analysis was used to determine the critical bending shear strength and MWNT modulus. The obtained bending shear strength was used in a mechanics model developed to provide bounds for the ISS in the experimental composite system. The calculated ISS for as-received nanotube falls between 4.8 and 13.7 MPa, and for surface treated nanotube falls in the range of 11.1 and 38.3 MPa. These values are consistent with the ISS reported for carbon fiber/polymer composites and also show that the ISS almost triples due to chemical modification of the MWNT surface.     

Development of aligned-hemp yarn-reinforced green composites with soy protein resin: Effect of pH on mechanical and interfacial properties

Unidirectional hemp yarn-reinforced green composites were fabricated with soy protein concentrate (SPC) resin processed at various pH values. To preserve the yarn alignment during the fabrication of green composites, hemp yarn was wound onto a metal frame with slight tension and precured SPC resin was applied to the yarns. Effects of pH values on the tensile properties of the SPC resin and hemp yarn/SPC resin interfacial shear strength (IFSS) were investigated. Increasing pH of the SPC resin from 7 to 12 decreased its fracture stress and Young’s modulus from 13.1 MPa and 357.5 MPa to 8.1 MPa and 156.2 MPa, respectively. At the same time fracture strain and moisture content increased from 31.5% and 15.65% to 53·4% and 19.30%, respectively, indicating resin plasticization. However, hemp yarn/SPC resin IFSS increased from 17.7 MPa at pH 7 up to 28.0 MPa at pH 10, after which it decreased. The fracture toughness of the composites increased up to pH of 10 but further increase in pH reduced the toughness. SEM photomicrographs showed fracture surfaces of hemp yarn-reinforced green composites that indicated better resin/fiber interaction at pH of 10 than 7 or 12.     

Preparation and characterization of foamed polyurethane/silicone rubber/graphite nanocomposite as radio frequency wave absorbing material: The role of interfacial compatibilization

A novel method for the preparation of radio frequency (RF) wave absorber polyurethane foam (PU) has been developed by impregnation of PU foam in n-hexane solution of room temperature vulcanizing (RTV) silicone rubber (SR) hybridized with graphite nanosheets (GNs) called doping solution. Extent of the GNs dispersion was optimized by the incorporation of a specific type of bifunctional compatibilizer. Insulator to conductive transition threshold as well as electromagnetic wave absorption characteristics of the fabricated nanocomposites was shown to be dependent upon the compatibilizer functionality. All PU/SR/GN nanocomposites generated from bifunctional compatibilizer exhibited higher electrical conductivity with enhanced permittivity implying enhanced formation of conductive networks by GN platelets. Permittivity of the PU/SR/GN nanocomposite based on bifunctional compatibilizer showed to be higher than uncompatibilized counterpart. Electromagnetic reflection loss behavior of the PU/SR/GN nanocomposites exhibited a non-linear correlation with the electrical conductivity. Although all PU/SR/GN prepared nanocomposites exhibited electromagnetic wave reflection loss behavior, but this revealed to be affected by the GN level as well as the size and dispersion state of the graphite nanosheets.     

Moisture absorption by epoxy/montmorillonite nanocomposite

The peculiarities of moisture absorption of epoxy–nanoclay composite are estimated in the paper. Second Fick’s law of diffusion was used to predict moisture diffusivity and equilibrium moisture content using accelerated analytical procedure. It was experimentally confirmed that sorption process in NC passes more slowly than in pure epoxy resin, for the highest filler content diffusivity reduces about half of diffusivity as for epoxy resin. The deviation from mixture rule was obtained for the equilibrium moisture content and the estimation of interphase content in composite was undertaken. It was determined that the higher content of interphase consistently leads to greater moisture absorption.     

Strengths of C/C composites under tensile, shear, and compressive loading: Role of interfacial shear strength

Various strengths of carbon–carbon composites (C/Cs) are comprehensively reviewed. The topics reviewed include tensile, shear, compressive, and fatigue strength as well as fiber/matrix interfacial strength of C/Cs. When data are available, high temperature properties, including creep behavior, are presented. Since C/Cs have extremely low fiber/matrix interfacial strength τ_d, the interfacial fracture plays important roles in all of the fracture processes dealt in this review. The low τ_d was found to divide tensile fracture units into small bundles, to seriously degrade both shear and compressive strength, and to improve fatigue performance. In spite of the importance of the interfacial strength of C/Cs, techniques for its evaluation and analysis are still in a primitive stage.     

Comparative study on the optical, surface mechanical and wear resistant properties of transparent coatings filled with pyrogenic and colloidal silica nanoparticles

We applied two kinds of silica nanoparticles, i.e. colloidal and pyrogenic ones, to improve the performance of transparent coatings on polymer substrates. The urethane-acrylate oligomer was mixed with varied concentrations of silica nanoparticles, spin-coated onto polycarbonate substrate and finally cured by ultraviolet rays. The resultant thickness of the coatings can be controlled in the range of 20-30 μm. The transmission electron microscopy revealed that both silica nanoparticles presented different dispersion states, i.e. mono-dispersion for the colloidal nanoparticles and floc-like dispersion for the pyrogenic ones. In comparison with the colloidal nanoparticles filled coatings, the pyrogenic ones exhibited much improved modulus, hardness and wear resistance, but slightly decreased optical properties such as transmittance, haze and gloss. The nanoparticle morphology, amorphous structure, dispersion state and particle-matrix interfacial bonding relating to these properties were discussed in the present study.     

Study on interfacial adhesion strength of single glass fibre/polypropylene model composites by altering the nature of the surface of sized glass fibres

Polypropylene (PP) compatibly sized glass fibres (GFs) were treated with boiling water and toluene, respectively, to reveal the interactions of water and toluene with different components in the sizing of sized GF and their influences on the interfacial adhesion strength of GF/PP model composites. Compared to control GF/PP model composites, about 30% increase of interfacial adhesion strength was achieved for composites with water-treated GF, whereas a small decrease of interfacial adhesion strength was revealed for composites with toluene-treated GF. X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, Zeta-potential measurement, and water contact angle measurement demonstrated that the boiling water-treated GFs posses a more polar and hydrophilic surface with homogeneously distributed derivatives of 3-aminopropyltriethoxysilane, which is related to a higher interfacial adhesion strength for water-treated GF/PP model composites. In contrast, hot toluene-treated GFs led to a more hydrophobic surface with low molar mass PP and surfactants enriching on the outermost surface.     

The enhanced coercivity for the magnetite/silica nanocomposite at room temperature

Magnetite/silica nanocomposite was synthesized by a facile solvothermal processing at 150 °C for about 10 h. X-ray diffraction (XRD) analysis revealed the effect of annealing on the crystallinity of silica. Transmission electron microscopy (TEM) images showed the good dispersion of magnetite in the silica matrix. Magnetic properties of the nanocomposite were characterized by vibration sample magnetometer (VSM), and the enhanced coercivity was explained by the intrinsic anisotropy of the particles enhanced by the interparticle dipolar fields.     

Feasibility study of a tungsten wire-reinforced tungsten matrix composite with ZrOx interfacial coatings

Brittleness problem imposes a severe restriction on the potential application of tungsten as high-temperature structural material. In this paper, a novel toughening method for tungsten is proposed based on reinforcement by tungsten wires. The underlying toughening mechanism is analogous to that of fiber-reinforced ceramic matrix composites. Strain energy is dissipated by debonding and frictional sliding at engineered fiber/matrix interfaces. To achieve maximum composite toughness fracture mechanical properties have to be optimized by interface coating. In this work, we evaluated six kinds of ZrO_x-based interface coatings. Interfacial parameters such as shear strength and fracture energy were determined by means of fiber push-out tests. The parameter values of the six coatings were comparable to each other and satisfied the criterion for crack deflection. Microscopic analysis showed that debonding occurred mostly between the W filament and the ZrO_x coating. Feasibility of interfacial crack deflection was also demonstrated by a three-point bending test.     

Surface-modification of cellulose nanowhiskers and their use as nanoreinforcers into polylactide: A sustainably-integrated approach

In this study, bionanocomposites based on renewable cellulose nanowhiskers (CNWs) as nanofillers and renewable poly(l-lactide) (PLA) as polymeric matrix were successfully and straightforwardly prepared using melt-extrusion technique. Due to the affinity difference between hydrophobic PLA and hydrophilic CNWs, the surface of CNWs was directly modified in aqueous suspension (pH = 5.4 – citrate buffer) at ambient temperature in the presence of functional trialkoxysilanes bearing various organic moieties (alkyl, amino, and (meth)acryloxy). The surface-functionalization of CNWs was first investigated using methacryloxy-based trialkoxysilane as model. The influence of parameters such as the amount of silane agents and the post-treatment conditions were optimized on the surface-modification of methacryloxy-modified CNWs. FT-IR, TEM, WAXS and XPS analyses provided further evidences about the efficiency of the surface-modifications of CNWs. In a subsequent step, the chemically modified CNWs were successfully incorporated into PLA by melt-extrusion in the absence of solvent, without any alterations of their nanostructure after melt-processing. The thermal/mechanical properties of the resulting bionanocomposites were determined and shown to be enhanced when silanized CNWs were used as nanofillers.     

Effect of TiO2 and nanoclay on the properties of wood polymer nanocomposite

High density polyethylene (HDPE), low density polyethylene (LDPE), polypropylene (PP) and poly(vinyl chloride) (PVC) with Phragmiteskarka wood flour (WF) and polyethylene-co-glycidyl methacrylate (PE-co-GMA) was used to develop wood polymer composite (WPC) by solution blending method. The effect of addition of nanoclay and TiO₂ on the properties of the composite was examined. The exfoliation of silicate layers and dispersion of TiO₂ nanopowder was studied by X-ray diffractometry and transmission electron microscopy. The improvement in miscibility among polymers due to addition of compatibilizer was studied by scanning electron microscopy (SEM). WPC treated with 3 phr each of clay and TiO₂ showed an improvement in thermal stability. Mechanical, UV resistance and flame retarding properties were also enhanced after the incorporation of clay/TiO₂ nanopowder to the composites. Both water and water vapor absorption were found to decrease due to inclusion of nanoclay and TiO₂ in WPC.     

Fiber/epoxy interfacial shear strength measured by the microdroplet test

In this study, we analyzed the interfacial shear strength between epoxy and carbon fibers, measured from microdroplet specimens adhered onto a single carbon fiber. Microdroplet specimens exhibited different results of the interfacial strength according to the range of embedded fiber lengths and the calculation methods. The shear stress distributions along the interface were calculated by using the finite element analysis of the droplet, sphere and cylinder model types. The stress analysis showed that a larger shear stress concentration arose along the interface for the droplet model than for the cylindrical one. A compensation factor for the conventional shear lag model is suggested to deduce the real interfacial shear strength on the basis of the average octahedral shear stress calculated from the distortional energy for a unit volume.