Using self-assembled monolayer technology to probe the mechanical response of the fiber interphase-matrix interphase interface

In this paper, a brief review of the fiber-matrix interphase/interface region is given for carbon- and glass-fiber composites. The substructure of the interphase/interface region is discussed in terms of three interphases: (a) the fiber interphase (FI), (b) the sizing interphase (SI), and (c) the matrix interphase (MI), and two interface regions: (a) the FI-SI interface and (b) the SI-MI interface. These substructures are a synthesis of the ideas advanced by Ishida and Koenig and Drzal. The schematic model of interphase deformation behavior originally given by Bascom is reconstructed to include research results from the above researchers. To systematically probe adhesion at the SI-MI interface, functionalized self-assembled monolayers (SAMs) using bonding and non-bonding C₁₁- type trichlorosilanes are prepared using the research of Menzel and Heise, and that of Cave and Kinloch as a guide. Results from this research are compared with short chain bonding and nonbonding silanes prepared by aqueous and non-aqueous deposition processes. The data were interpreted using the mechanisms proposed by Sharpe, Ishida and Koenig, and Drzal and the mathematical equation proposed by Nardin and Ward. For the non-bonding short-chain silane deposited by aqueous deposition, 90% of the adhesion was found to be due to mechanical interlocking, with the remaining adhesion due to physicochemical interactions. For the bonding short-chain silane deposited by aqueous deposition, the interface strength relative to the non-bonding short-chain silane increased by 31%. However the interfacial shear strength (IFSS) of this system was approximately 40% lower than the comparable bonding SAM interface. This difference was interpreted in terms of the propensity of the C₃-alkylamine to form cyclic ring structures in the MI region as described by Ishida, Koenig, et al. The SAM data also indicates that 70-85% of the maximum IFSS is obtained with 25-50% of the surface covered with functional groups. This suggests that steric hindrance, due to the size of the DGEBA molecules, restricts access to the functional groups on the surface. Therefore, only 35% of the surface functional groups are accessible for bonding in the DGEBA/m-PDA epoxy resin system.     

Effect of surface-grafted molecular brushes on the adhesion performance of bonded polymers and composite interfaces

This paper reviews the theoretical principles of the macromolecular design of polymer interface/interphase systems for obtaining maximum adhesion and fracture performance of composite materials and adhesively bonded assemblies. Subsequently, a relatively simple and industry-feasible technology for surface grafting molecular brushes is discussed in detail and supported by a range of experimental examples. It is shown, in agreement with contemporary theory, that the use of chemically attached graft chemicals of controlled spatial geometry and chemical functionality enables a significant increase in the strength and fracture energy of the interphase, to the point of cohesive fracture of the substrate, or that of an adjacent medium such as adhesive, elastomer or matrix material. This occurs even after prolonged exposure of investigated systems to adverse environments such as hot water.     

Determination of the interface strength between two polymer materials by a new curved interface tensile test: Preliminary experimental results

Recently, the authors have proposed a new experimental method for the determination of adhesion strength between two different materials. A curved interface and special arrangement of materials is used for the tensile test of bimaterial specimens to avoid singular stress fields around corners and edges. The main advantage of the test consists in the fact that the strength is determined under conditions of a uniform tensile stress field normal to the interface in the region where debonding starts. The present paper presents experimental results for two bimaterial systems - PMMA/TPE and PC/TPE (two stiff standard polymers with a thermoplastic elastomer). The expected failure behaviour was observed during the experiments, thus enabling the estimation of adhesion strength by using calculated stress concentration factors. The influence of the radius of curvature is discussed in detail.     

Adhesion of epoxy-thermoplastic and polysulfone-LCP matrices to fibres

This work investigated the adhesion strength τ of the joints of polymer blends with fibres. Blends of polysulfone with LC-polyether and epoxy resin (based on DGEBA) with polysulfone, polyetherimide and poly(arylene ether ketone) were taken as matrices. Steel wire, polyamide (nylon-6) and glass fibres were used as substrates. The adhesion strength was determined by the 'pull-out' technique. It was found that incorporation of LCP into polysulfone and incorporation of thermoplastics into epoxy matrix resulted in non-additive relationships between the adhesion strength and modifier (LCP or thermoplastic) content C. In the case of epoxy-polysulfone, epoxy-polyetherimide and polysulfone-LCP matrices, such τ-C dependencies were described by curves showing a maximum. Optimal (maximal) adhesion strength of the blend/fibre joints was obtained at 10 wt% of polysulfone, 15 wt% of polyetherimide and poly(arylene ether ketone) in epoxy resin and 2–5 wt% of LC-polyether in polysulfone. Possible mechanisms of the interface strength enhancement are discussed.     

Use of flow microcalorimetry and dynamic mechanical thermal analysis for probing interphase structure in poly(styrene-b-butadiene-b-styrene)/magnesium hydroxide composites

Interactions between magnesium hydroxide (Mg(OH)₂ particles (both untreated and treated with 16-methyl heptadecanoic acid (isostearic acid)) and low molar mass poly(styrene) (PS) and poly(butadiene) (PB) have been studied by flow microcalorimetry (FMC) and have been related to the interphase structure in poly(styrene-b-butadiene-b-styrene) (SBS)/Mg(OH)₂ composites using dynamic mechanical thermal analysis (DMTA). The FMC studies revealed that both polymers adsorbed strongly onto an untreated magnesium hydroxide surface though the PB showed greater irreversible adsorption from the heptane carrier fluid. Diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) studies on filler samples removed from the FMC cell after the adsorption-desorption cycle confirmed strong polymer filler interaction. Adsorption of the low molar mass samples of PS and PB onto a pre-adsorbed monolayer of isostearic acid on Mg(OH)₂ resulted in a very significant reduction in polymer adsorption activity due to blockage of adsorption sites. DMTA studies revealed that strong adsorption of PS and PB blocks of SBS onto untreated filler in composites containing 60% w/w Mg(OH)₂ gave rise to phase mixing that led to an 18 °C reduction in the T _g of the PS phase relative to that in the unfilled matrix. However, in equivalent composites based on isostearic acid treated filler a smaller reduction (10 °C) was observed, therefore reflecting reduced filler-matrix interaction and reduced phase mixing.     

Effect of Geometry,Loading and Elastic Moduli on Critical Parameters in a Nanoindentation Test in Polymeric Matrix Composites with a Nonhomogeneous Interphase

Fiber nanoindentation models are developed for polymeric matrix composites with nonhomogeneous interphases. Using design of experiments, the effects of geometry, loading and material parameters on the critical parameters of the indentation test such as the load–displacement curve, the maximum interfacial shear and normal stresses are studied. The sensitivity analysis shows that the initial load–displacement curve is dependent only on the indenter type, and not on parameters such as fiber volume fraction, interphase type, thickness of interphase, and boundary conditions. The interfacial tensile radial stresses are not sensitive to indenter type, or to type and thickness of interphase, while the interfacial compressive radial stresses are sensitive mainly to boundary conditions and thickness of interphase; however, the influence of these factors on the interfacial radial stresses can be large. In contrast, the interfacial shear stress is sensitive to all factors, but the influence of the factors is relatively small.     

The effect of interphase adhesion on nanofiller structure in polymer/organoclay nanocomposites

The role of interphase adhesion in the organoclay plate surface-polymer matrix system is shown to be crucial for the degree of dispersion of silicate plates. Quantitative methods for assessing the major structural characteristics of organoclay are discussed. The proposed interpretation of an organoclay packet (tactoid) agrees well with the effective particle model that was elaborated earlier.     

In situ polymer nanocomposites: Effect of nanoparticles on the interfacial region

Composites based on the blends of polyurethane and poly(methyl methacrylate) of various composition were synthesized in situ in the presence of various amounts of nanoparticles (fumed silica). From thermophysical measurements it was found that, during reaction, phase separation and evolution of two phases occur. The temperature transitions in the systems and their positions depend on the blend composition and on various amounts of nanoparticles. Using scanning differential calorimetry from the changing of heat capacity increments the fraction of an intermediate region between two main phases has been estimated. For the first time it was observed that in nanocomposites in the temperature region between two main relaxation transitions, there appears a third transition, which was related to the adsorption layers formed by both components at the interface of the nanoparticles. The appearance of such intermediate regions increases essentially the fraction of an interfacial region in the system.     

Analysis and computation of thermal residual stresses at the fiber/matrix interface

Because of the importance of thermal residual stresses in composite materials, our study aims to compute them by the finite element method. Numerical analysis shows that these stresses need to be taken into account. The interface is affected by these stresses, particularly in the free edge. The discontinuity of the normal stresses along the interface and the shear value at the free edge influence the composite material behaviour during its use (e.g. the composite used as a patch for repairing a crack).     

Influence of material processing and interface on the fiber fragmentation process in titanium matrix composites

The objective of this work is to study the effect of composite processing conditions on the nature of the fiber–matrix interface in titanium matrix composites and the resulting fragmentation behavior of the fiber. Titanium matrix, single fiber composites (SFCs) were fabricated by diffusion bonding and tensile tested along the fiber axis to determine their interfacial load transfer characteristics and the resulting fiber fragmentation behavior. Two different titanium alloys, Ti-6Al-4V (wt%) and Ti-14Al-21Nb (wt%), were used as matrix material with SiC (SCS-6) fibers as reinforcement. The tensile tests were conducted at ambient temperature and were continuously monitored by acoustic emission. It was observed that the Ti-6Al-4V/SCS-6 composite system exhibited a greater degree of fiber–matrix interfacial reaction, as well as a rougher interface, compared to Ti-14Al-21Nb/SCS-6 composites. Acoustic emissions during tensile testing showed that most of the fiber fractures in Ti-6Al-4V/SCS-6 occurred at strains below ～5% and the fragmentation ceased at ～10% strain corresponding to specimen necking. In contrast, the Ti-14Al-21Nb/SCS-6 composite deformed without necking and fiber fractures occurred throughout the plastic range until final fracture of the specimen at about 12% strain. The markedly different fragmentation characteristics of these two composites were attributed to differences in the fiber–matrix interfacial regions and matrix deformation behavior.     

Filler treatment effects on the weathering of talc-, CaCO3- and kaolin-filled polypropylene hybrid composites

Talc, calcium carbonate (CaCO₃), and kaolin hold considerable promise in the development of polymer composites for good mechanical properties and stability. Comparative studies on the usage of these minerals as single fillers in polypropylene (PP) have shown varying degrees of reinforcement due to their differences in terms of particle geometry, surface energy and affinity towards the matrix polymer. In this study, comparisons were made in terms of mechanical, thermal and weatherability properties between hybrid-filler PP composites (i.e. PP filled with either talc–CaCO₃ or talc–kaolin hybrid filler combinations), with particular attention directed towards the effect of surface modification of the fillers. The talc/CaCO₃ hybrid composites have shown exceptional performance in terms of flexural and impact properties. The contribution of talc in the talc–kaolin hybrid composite system has been significant in terms of enhancing the overall tensile and flexural properties. The ability of silane and titanate coupling agents in boosting the resistance of the composites to severe damage and degradation due to natural weathering has been shown.     

Amino Functionalization of MWNTs and Their Effect on ILSS of Hybrid Nanocomposites

Multi-walled carbon nanotubes (MWNT) were oxidized by treatment with a mixture of sulfuric and nitric acids to introduce carboxyl groups on their surfaces. Triethylene tetraamine (TETA) was then grafted onto the oxidized MWNTs via a thionyl chloride route to obtain the amino-functionalized MWNTs (f-MWNT). The presence of amino functional groups on the MWNTs was confirmed using FT-IR, and scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to compare the morphology of pristine MWNT (p-MWNT) and f-MWNT. Both the p-MWNT and f-MWNT were dispersed in epoxy resin using ultrasonic agitation and the suspensions were injected into E-glass fiber woven fabric using a specialized vacuum assisted resin transfer molding (VARTM) process in which a flow flooding chamber (FFC) was used to re-direct the suspension flow. Control samples were fabricated using the same E-glass fiber mat and unmodified epoxy resin following the same procedure. Compression shear testing (CST) was performed on all the manufactured samples to determine their Inter laminar shear strength (ILSS). Results show 41% increase in ILSS for hybrid composites containing p-MWNTs and a 61% increase for samples containing f-MWNTs relative to the control samples without MWCNT.     

Effect of chemical modifications of the pineapple leaf fiber surfaces on the interfacial and mechanical properties of laminated biocomposites

Natural fiber reinforced renewable resource based laminated composites were prepared from biodegradable poly(lactic acid) (PLA) and untreated or surface-treated pineapple leaf fibers (PALF) by compression molding using the film stacking method. The objective of this study was to determine the effects of surface treatment of PALF on the performance of the fiber-reinforced composites. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) were used to aid in the analysis. The mechanical properties of the PLA laminated composites were improved significantly after chemical treatment. It was found that both silane- and alkali-treated fiber reinforced composites offered superior mechanical properties compared to untreated fiber reinforced composites. The effects of temperature on the viscoelastic properties of composites were studied by dynamic mechanical analysis (DMA). From the DMA results, incorporation of the PALF fibers resulted in a considerable increase of the storage modulus (stiffness) values. The heat defection temperature (HDT) of the PALF fiber reinforced PLA laminated composites was significantly higher than the HDT of the neat PLA resin. The differential scanning calorimeter (DSC) results suggest that surface treatment of PALF affects the crystallization properties of the PLA matrix. Additionally, scanning electron microscopy (SEM) was used to investigate the distribution of PLA within the fiber network. SEM photographs of fiber surface and fracture surfaces of composites clearly indicated the extent of fiber–matrix interface adhesion. It was found that the interfacial properties between the reinforcing PALF fibers and the surrounding matrix of the laminated composite are very important to the performance of the composite materials and PALF fibers are good candidates for the reinforcement fiber of high performance laminated biodegradable biocomposites.     

Influence of Grafting Density of Diblock Copolymers on Interfacial Assembly Behavior and Interfacial Shear Strength of Glass Fiber/Polystyrene Composites

To investigate the influence of the grafting density and the molecular structure of block copolymers on the interfacial assembly behavior and interfacial shear strength, macromolecular coupling agents, hydroxyl-terminated poly(n-butyl acrylate-b-styrene) (HO-P(BA-b-S)) were synthesized by atom transfer radical polymerization, and then chemically anchored on the glass fiber surfaces to form a well-defined monolayer. The phase separation and 'hemispherical' domain morphologies of diblock copolymer brushes at the polystyrene/glass fiber interface were observed. The interfacial assembly morphology differs with changes in the grafting density of diblock copolymers. When the grafting density is greatest, the highest height difference of the hemispherical domain and the largest surface roughness are achieved, as well as the best interface shear strength. It was also found that the copolymer brush with a PBA block of the polymerization degree (Xn) about 77 is the optimal option for the interfacial adhesion of PS/GF composites. Thus, the grafting density and molecular structure of diblock copolymers determines the interfacial assembly behavior of copolymer brushes, and therefore the interfacial shear strength.     

Influence of chemical treatments on the interfacial adhesion between sisal fibre and different biodegradable polymers

The influence of chemical treatments on the interfacial adhesion of sisal fibres and biodegradable matrices were studied in the present work. For that purpose, four different polymers were used: polycaprolactone (PCL), cellulose acetate, MaterBi Z (a commercial starch/polycaprolactone blend) and MaterBi Y (a commercial starch/cellulose derivatives blend). Alkaline and acetylation treatments were performed on sisal fibres. Properties were determined by means of tensile tests, adhesion measurements and contact angle determination. The interfacial shear strength was correlated with the hydrophilic character of the material.     

The influence of the cooling rate on interface segregation

The influence of the cooling rate on interface segregation is discussed theoretically. Criteria are obtained for a “freeze-in” of the segregation level assuming a state of equilibrium before quenching. The predictions of the theory are found to be in agreement with the work of Burton, Helms and Polizzotti on the difference in segregation levels between elevated and ambient temperature. Since the assumption of a dilute solid solution is used, the validity of the results is restricted to either very low bulk concentration or to elements which are not too strongly surface-active.     

An AFM study of the effect of chemical treatments on the surface microstructure and adhesion properties of flax fibres

The mechanical properties of fibre-reinforced polymer composites are largely dependant on the adhesion between the matrix and the fibre. In order to enhance the interaction between flax fibres and unsaturated polyester resins, raw fibres were chemically modified using sodium hydroxide, sodium hydroxide plus acetic anhydride and formic acid-based treatments. The physical properties of the modified fibres were investigated by means of the atomic force microscopy. At first, the morphological analysis of the surfaces shows that after the chemical treatments, the fibres surface appear to be less heterogeneous in topology and smoother. Nonetheless, no significant roughness difference was found between the different treatments. Secondly, adhesion forces measurements were performed between a standard AFM silicon nitride tip and the fibres. The adhesion forces were found to vary according to the chemical treatment. The sodium hydroxide-based treatment was found to increase the adhesion force between the fibre and the AFM tip whereas the lowest adhesion force was found for the formic acid- based treated fibre. These results were attributed to the different hydrophilic character of the modified fibres. Due to the importance of the water layer adsorbed on the fibres, the adhesion forces between the AFM tip and the different samples are found to be mainly dominated by capillary forces in relation with the fibre's surface hydrophilicity.     

The influence of interface disorder on the electronic structure of a quantum well: A theoretical study

We present a theoretical analysis of the lowest electronic states in a quantum well whose thickness takes the random values of n−1, n or n+1 monolayers. We calculate the average electronic propagator using a two dimensional muffin-tin version of the coherent potential approximation. For scarce but large “terraces”, the electronic structure is strongly perturbed and the density of states at zero wave vector exhibits three peaks. For abundant but small terraces the nominal n-monolayers electronic structure is recovered. This should allow one to characterize, from luminescence experiments, the statistics of interface disorder in different growth conditions.     

Theoretical calculations on the adhesion, stability, electronic structure, and bonding of Fe/WC interface

The adhesion, stability, electronic structure, and bonding of Fe/WC interfaces were studied using first-principles calculations. The preferred stacking sequence is HCP structure that Fe atoms continue the natural stacking sequence of the bulk WC. For two different interfaces with HCP stacking geometry (C-HCP and W-HCP), the work of adhesion of the optimized Fe/WC interfaces are 9.7 J m⁻² for C-HCP and 5.1 J m⁻² for W-HCP, respectively. The effects of the interface on the electronic structures of both the metal Fe and ceramic WC are mainly localized within the first and second layers of the interface. C-HCP interface has strong covalency and W-HCP interface is dominated by metallic bonds. The magnetic moments of Fe atoms at interface are decreased in both interfaces. Calculations of the interfacial energies provide theoretical evidence for the excellent wear behaviors of Fe/WC composites. Besides, the chemical bonding properties for the interfacial atoms are also discussed in this paper based on Milliken population method.     

Effects of hygrothermal ageing and thermal shock on reliability of interfacial adhesion between black oxide coated copper substrate and epoxy resin

Black oxide is a conversion coating applied onto the Cu substrate to improve the interfacial adhesion with polymeric adhesives. A comprehensive study was made to characterize the black oxide coating and the corresponding interfacial adhesion with various types of polymeric resin, aiming to optimize the oxide processing conditions. The reliability of adhesion performance of the coating was evaluated before and after accelerated hygrothermal ageing, such as temperature cycling, pressure cooker test, and moisture sensitivity test followed by thermal shock. The moisture resistance of the substrate with black oxide coating was much higher than the bare Cu substrate, during both the moisture absorption and desorption processes. Thermal cycling alone did not change significantly the adhesion performance of any of the substrates studied. Pressure cooker test was detrimental to adhesion performance of oxide coated Cu substrates. Nevertheless, the residual interfacial bond strengths were consistently much higher for the black oxide coated substrates than the bare Cu surface. Significant delamination occurred between the bare Cu and the moulding compound after the moisture sensitivity test followed by thermal shock, whereas there was virtually no delamination on the black oxide coated samples under the same ageing condition, confirming the higher reliability of interfacial adhesion performance for the latter.