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.     

Thickness dependence of elastic modulus of polycarbonate interphase

The oligomer of bis-phenol A (oligo-PC) with M _w = 1300 and bis-phenol A polycar-bonate (PC) with M _w = 20 000 were deposited onto E-glass surface using SiCl₄ as the grafting and cross-linking agent. Thickness of the deposited layers was varied from 30 to 10⁶ nm and the layers were investigated as prepared and after thermal annealing at 245°C for 10 min in the air. Vibrational piezoelectric resonator technique and the speed of Rayleigh wave measurement were used to determine elastic moduli of the ultra thin layers deposited on flat E-glass substrate as a function of their thickness. In all cases, increase of the Young modulus of the interphase, E _i, with decreasing layer thickness, t _i, was observed. At a given thickness, the E _i of PC layer was significantly lower than that for the oligo-PC layer. Thermal annealing of the deposited PC layer resulted in a significant increase of its E _i compared to the as received layer. No significant change was observed for oligo-PC interphases. Increase of the shear strength of the interface, τ _a, with reducing interphase thickness, t _i, was observed. The observed increase of E _i with the decreasing t _i was ascribed to the reduction of the molecular mobility of chains near solid surface compared to their mobility in the bulk. Most probably, the observed increase of E _i after thermal annealing of PC was caused by rearrangement of both segment density distribution in individual PC coils near the solid surface and cooperative rearrangements of multiple PC chains. Since the oligomers attached to the surface attained presumably more regular extended conformations with lower conformation entropy compared to the PC random coils, the effect of thermal annealing was negligible. In agreement with theoretical predictions, increase of E _i at the same extent of interfacial interactions resulted in the observed increase of the τ _a measured using the single embedded fiber test.     

Hydrolytically Stable Interphase on Alumina and Glass Fibers via Hydrosilylation

The poor hydrolytic stability of silane interphase greatly limits the use of fiber reinforced composites (FRC) in demanding applications in which the FRC part is permanently exposed to a moist environment such as in prosthetic dentistry and orthodontics. To improve hydrolytic stability of the interphase between the matrix composed of a blend of triethyleneglycol dimethacrylate (TEGMA) and bisphenol A glycidylmethacrylate (Bis–GMA) monomers and glass or alumina oxide fibers, a two-step hydrosilylation procedure was employed. The process consisted of creating hydride intermediate on the fiber surface followed by hydrosilylation reaction attaching the unsaturated organic monomer (Bis–GMA) forming stable –Si–C bonds. Infrared spectroscopy (FTIR) confirmed formation of the hydride intermediate on the surface and then, attachment of the appropriate organic compound in the second step. The amount of deposited interphase and its stability was significantly enhanced compared to standard silanization treatment. Fracture surfaces were observed by scanning electron microscopy (SEM) before and after environmental exposure proving that the most stable interfacial bond was obtained with the two-step treated fibers. It was concluded that hydrosilylation provides a viable alternative to silanization for both glass and ceramic fibers in composites intended for applications requiring enhanced hydrolytic stability of the composite parts.     

Interphase formation and fiber matrix adhesion in carbon fiber reinforced epoxy resin: influence of carbon fiber surface chemistry

The chemistry and morphology of the carbon fiber surface are important parameters which govern the properties of the interfacial region and the adhesion between carbon fibers and polymeric matrix in carbon fiber reinforced polymers. In the presented paper the surface chemistry of the fibers is varied while the surface morphology is left unchanged. We analyze chemical functionality and morphology of carbon fiber surfaces showing different degrees of activation, together with the adhesion of these fibers to an epoxy matrix and the width of the interfacial region between fiber and matrix. An increase of the oxygen and nitrogen concentration of the fiber surface, in particular in form of carboxyl functional groups, results in a significant increase of interfacial shear strength. Also the width of the interphase, as determined by scanning force microscopy in nanomechanical mode, depends on the activation degree of the carbon fibers. However, no direct correlation between interphase width, surface chemistry and fiber matrix adhesion is found, suggesting no direct influence of interphase width on adhesion properties.     

Improvement in damage tolerance of CFRP laminate using the hybridization method

Flexible epoxy resin was used as an interphase matrix to improve the transverse properties of carbon fiber reinforced plastic (CFRP) laminates. The use of flexible interphase is considered to act as the energy absorber and prevent the fiber/matrix interface cracking. The purposes of the present study are fabrication and characterization of CFRP laminates with flexible interphase. The effect of the flexible interphase on the damage tolerance of CFRP laminates was investigated. It was confirmed that transverse strength was improved and transverse crack progress was suppressed in the laminates with flexible interphase.     

Formation of aminosilane-oxide interphases

The aim of this work is to understand the interactions and interphase formation mechanisms between a liquid aminosilane oligomer (γ-APS) and glass, steel or gold surfaces as a function of the pH of the liquid aminosilane. When basic liquid γ-APS (pH 11.4) is applied onto a gold coated substrate, no interphase is detected. Similarly, when liquid γ-APS controlled at pH 8 is applied onto steel or glass substrates and cured, properties are the same as the bulk ones. In contrast, when the liquid γ-APS (pH 11.4) is applied onto steel or glass substrates and cured, an interphase, with chemical, physical and mechanical properties quite different from those of the bulk oligomer, is created between the substrate and the oligomer. Using various analytical techniques (DSC, FTIR, ICP, SEM, AFM, nano-indentation and XPS) it was shown that the amino-silane chemically reacts with and dissolves the oxide or hydroxide layers. Then metallic ions diffuse through the organic layer to form a complex, assumed to be of coordination type with the amine function of the oligomer molecule. These organometallic complexes are insoluble at room temperature and crystallize in the form of sharp needles. The Young's modulus of the resulting crystal is equal to approximately 5 GPa, i.e.over two orders of magnitude higher than that of the silane. In other words, these organometallic complexes act as a short fiber in a matrix.     

Interphase Modelling of Human Osteoblasts Spread on Pure Titanium Surface Covered with TiO2 Nanotubes

The purpose of the present work was to define some parameters of compatibility between the titanium nanotubes surface and the human cells layer. The nanotube–matrix interphase provided the necessary information concerning the possibility of compatibility between a titanium prosthesis and human tissue. An important point of view was the biomechanical response of the interphase created by the surfaces involved (interface modelling). The viscoelastic hybrid interphase model developed for the determination of the interphasial stress and strain fields as well as for the prediction of the stiffness variation within the area of the interphase material has been applied in the special case of a substrate containing human osteoblasts and the pure titanium surface covered with titanium nanotubes. The results showed that in order to achieve a strong fixation of the metal to the tissue while at the same time to develop a good environment for the development of the osteoblast cells, extreme values corresponding to both, perfect adhesion or zero adhesion, should be excluded. It was found that the optimum condition is achieved for nanotube–matrix adhesion factor 'k' of intermediate value.     

Finite element prediction of debonding onset in short fiber reinforced polymers incorporating a hybrid interphase region

A robust finite element procedure for investigating damage evolution in short fiber reinforced polymeric composites under external loads is developed. This procedure is based on an axisymmetric unit cell composed of a fiber, surrounding interphase and bulk matrix. The hybrid interphase concept involves a degraded material phase, the extent of which is material and property dependent. One of the most significant features of the model relies on establishment of variable adhesion conditions between the primary material phases. The unit cell is discretized into linearly elastic elements for the fiber and the matrix and interface elements which allow debonding in the fiber–matrix interface. The interface elements fail according to critical stress and critical energy release rate criteria. The tension and shear aspects of failure are uncoupled, although the resulting nonlinear problem is solved implicitly utilizing quasi-static incremental loading conditions. Final failure resulting from saturation and breakage is modeled by the vanishing interface element technique. Details of the propagation of interface cracks and the initiation of debonds are also observed and discussed for various shapes of fiber end. Numerical results reveal an intense effect of the fiber-end geometry on the initial fiber–matrix de-cohesion. The present finite element procedures can generate meaningful results in the analysis of fiber-reinforced composites.     

Exploring Microstructures and Interphase Properties of Surface- Grafted Diblock Copolymers in a Homopolymer Melt by Self-Consistent Field Theory Simulations

Polymeric self-consistent field theory is used to investigate microstructures and interphase properties of diblock copolymers grafted onto solid surfaces in a homopolymer melt. The calculations show that the grafted diblock copolymers can self-assemble into hemispherical microstructures at low grafting densities of the diblock copolymers. The morphology transforms into hemicylinder-like and sandwich-like lamellar microstructures with an increase in the chain-grafting density. The effective thickness of the grafted block layer and the interphase width between the homopolymer melt and the grafted copolymers strongly depend on the physicochemical parameters of the system, such as the composition of the grafted copolymer, the chemical incompatibility between the different components, the length ratio of grafted copolymer to homopolymer, and the grafting density of the diblock copolymers. In addition, the above computational results of microphase-separated structures and interphase properties are qualitatively compared with our previous experimental observations. The comparison indicates that our theoretical results not only reproduce the general feature of the experimental observations, but also elucidate the internal structural information and complement the findings in the region of high grafting densities of diblock copolymers.     

Adhesion of giant unilamellar vesicles on double-end grafted DNA carpets

We have recently shown that the bio-mimetic adhesion of Giant Unilamellar Vesicles on carpets of lambda-phage DNAs, grafted by one end to the substrate, leads to DNA scraping and stapling. As the lipid adhesion patch is built, outward forces stretch the DNA, while adhesion patch formation staples the chains into frozen conformations, trapped between the GUV membrane and the substrate. Analysis of the scraped and stapled DNA conformations provides a wealth of information about the membrane/polymer interactions at play during the formation of a bio-adhesive contact zone. In this paper we report new phenomena revealed by scraping and stapling phenomena associated with the bio-mimetic adhesion of Giant Unilamellar Vesicles on carpets of lambda-phage DNAs that were grafted to the substrate by both ends. In particular, the peculiar shapes of stapled DNA observed in this case, suggest that the membrane exerces not only outward radial forces during patch formation, but is is also able to confine the DNA molecules in the orthoradial direction.     

Localized grafting through chemical lift-off

In this work we present two techniques that provide localized functionalization of the surface of materials. Both lead to localized grafted thin organic films (10-200 nm). The localization is brought by a chemical lift-off process, which relies on patterned weakly bonded films as sacrificial layers, combined with electrochemical (SEEP) or chemical (GraftFast©) processes which provides the final robust pattern on the surface. Both grafting processes, which were recently described, take advantage of the redox activation of diazonium salts associated with vinylic monomers in aqueous solution, and lead to similar grafted polymer films. Thanks to the high difference in adhesion between the grafted polymer and the patterned sacrificial layer (either an ink or weakly bound self-assembled monolayers), the latter may be easily removed, which unveils uncovered areas of the substrate.     

Interfacial region in blends of linear polymers formed in situ

Blends of linear polyurethane and poly(methyl methacrylate) were obtained by the simultaneous curing of the mixture of two monomers. It was shown that the blends obtained in situ are two-phase systems in which two phases enriched in one of the blend components are separated by an intermediate region, the interphase. From the DSC data the compositions of two phases were estimated. It was observed that introduction of a filler leads to the appearance of an additional temperature transition lying between glass transition temperatures of the two phases. The fraction of the interphase was calculated from the calorimetric data. The introduction of a filler increases this fraction. This may be considered as some improving of compatibility of the two components in the presence of a filler.     

Application of a nonlinear boundary condition model to adhesion interphase damage and failure

In an earlier paper [J. Sadler, B. O'Neill, and R. Maev, J. Acoust. Soc. Am. 118, 51-59 (2005)], a set of generalized boundary conditions were proposed, based on a thin layer (thickness < wavelength) model of the acoustic interface. In this paper, the model is extended to cover the more pathological nonlinearity of the adhesion interphase-that is, the critically important thin layer where bonds are formed between adhesive and substrate. First, the boundary conditions are shown to be sufficiently general to cope with all manner of interphase nonlinearity, including unilateral cases such as clapping or slipping. To maintain this generality, an analytic time domain solution is proposed based on expansion in terms of the layer thickness rather than the conventional expansion in terms of harmonics. Finally, the boundary conditions are applied to an interphase failure model based upon basic continuum damage mechanics principles. It is proposed that such a model, which can predict the evolution of the interphase damage under stressful conditions, may allow a proper prediction of the ultimate adhesion strength based on nonlinear parameters measured nondestructively with ultrasound.     

EPR imaging study of paramagnetic centre distribution in thiokol-epoxy hermetics

The distribution of paramagnetic centres in carbon black filler in the interphase layer of the thiokol-epoxy hermetics on the border of brass or glass substrate was studied using EPR-imaging method. It was shown that the relative content of radicals decreases near the hermetic-“rigid” surface contact border. The thickness of the layer with a low concentration of radicals is estimated as 0.5±0.3 mm. The inhomogeneous distribution of radicals is more obvious in the case of hermetic hardening on a brass surface. These results are explained by a catalytic acceleration of the thiokol-epoxy polymerization reaction in the region of hermetic-metal surface contact.     

Quantification of oxygenated species on a diamond-like carbon (DLC) surface

This paper discusses the use of chemical derivatization methods for surface chemical composition analysis of diamond-like carbon (DLC) films synthesized through plasma-enhanced chemical vapor deposition with X-ray photoelectron spectroscopy (XPS). The main challenge in applying chemical derivatization reactions for titration of organic functional groups on the DLC surface is that sub-surface oxygenated species are not accessible to the derivatizing agent. As a simple approximation, a functional group that can be quantified unambiguously with XPS can be used as an internal reference to estimate the accessible-to-inaccessible ratio, and this information can be used to estimate the total amount of other functional groups from the chemical-derivatization-assisted XPS analysis. The use of this principle to obtain the surface composition of hydroxyl, ether, carbonyl, and carboxyl groups in the oxidized surface region of the DLC film was demonstrated.     

Melt brushes of diblock copolymer

Using self-consistent field theory (SCFT), we investigate the morphologies formed by a melt brush of AB diblock copolymers grafted to a flat substrate by their B ends. In addition to a laterally uniform morphology, SCFT predicts three ordered morphologies exhibiting different periodic patterns at the air surface: a hexagonal array of A-rich dots, an alternating sequence of A- and B-rich stripes, and a hexagonal pattern of B-rich dots. When the phase diagram of the tethered film is plotted as a function of A/B incompatibility, N , and diblock composition, f , it resembles the bulk phase diagram with the periodic phases converging to a mean-field critical point at weak segregation. The periodic-phase region in the phase diagram shrinks with increasing grafting density and expands when the air surface acquires an affinity for the grafted B blocks.     

Glassy carbon - A promising substrate material for pulsed laser deposition of thin Li1+xMn2O4−δ electrodes

The spinel LiMn₂O₄ is a promising candidate for future battery applications. If used as a positive electrode in a battery, the charging capacity of such a battery element is limited by the formation of a solid electrolyte interphase like layer between the electrolyte and the spinel. To study the electrolyte-electrode interaction during electrochemical cycling, spinel thin films are deposited as model electrodes on glassy carbon substrates by pulsed laser ablation. The obtained polycrystalline oxide thin films show a well defined surface morphology and are electrochemical active. Adhesion of these thin films on glassy carbon is in general poor, but can be improved considerably by a surface pretreatment or adding a thin metallic coating to the substrate prior deposition. The best adhesion is obtained for films deposited on argon plasma pretreated as well as Pt coated glassy carbon substrates. During the electrochemical characterization of Li_1.06Mn₂O_3.8 thin film electrodes, no additional reactions of the substrate are observed independent of the used electrolyte. The best cycle stability is achieved for films on Pt coated glassy carbon substrates.     

X-ray photoemission spectroscopy (XPS) analysis of Fe82B18-Al2O3 granular systems

X-ray photoemission spectroscopy (XPS) has been used to investigate the surface composition (down to some hundreds of Å) of granulars consisting of Fe₈₂B₁₈ small particles (?～40–70 Å) dispersed in an alumina insulating matrix.Fully oxidized iron and boron are found in the outermost region of samples while in depth analysis, achieved by Ar⁺ ion etching, provides evidence of both metallic and oxidized states for the two elements.Aluminate forms, suggestive of an interaction at the surface of the alloy particle and alumina, are also present in the sub-surface region and the interaction extent is quantitatively evaluated as a function of the particle size.     

Film thickness effects on the distribution of high-molecular-weight heterotelechelic polymers

High-molecular-weight heterotelechelic deuteriopolystyrene, NDPSF, possessing an amine functional group at one end of the chain and a fluorocarbon group at the other was tethered to a silicon substrate by its amine functional group. These layers were coated with an unfunctionalised polystyrene matrix, HPS, such that the total film thickness covered a range from 2.2 to 9 times the radius of gyration of NDPSF. The detailed distribution of the polymers after annealing for times much greater than the reptation period of either of the components, was obtained using neutron reflectometry. No evidence for bridging of the two interfaces was found for the thicker films, but the finite concentration of the NDPSF polymer observed for the thinnest films may be due to bridging since the energy gain of the fluorocarbon end is just greater than the loss due to configurational entropy losses. A linear increase in the ellipsometric thickness of the excess of NDPSF at the substrate was discovered and we attribute this to the NDPSF slowly being leached out of the layer initially at the substrate followed by diffusion into the bulk of the film. The concentration profiles obtained are consistent with hindered relaxation of the large NDPSF molecules, when they are tethered at the substrate or at the vacuum surface. Received 21 August 2001 and Received in final form 7 January 2002