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.     

Silane-modified polymers as interphases in glass-fibre-reinforced composites: 2. Grafting and crosslinking of interphase materials

_—This paper presents an interphase engineering technique suitable for grafting silane-modified polymers onto glass fibres to be used in composites with enhanced impact tolerance. The silane-modified polymers include ethylene polymers grafted with γ-methacryloxypropyltrimethoxysilane (MPS) and a copolymer of butyl acrylate (BuA) and MPS. The grafting of functionalized interphase materials onto glass fibres is performed in solution. By changing the concentrations of the solutions, different amounts of polymer can be deposited on the fibres. Water crosslinking of the polymer gives the possibility of producing stabilised interfacial polymer coatings over a range of thicknesses. It is concluded that acidic conditions (1) promote the grafting of silane-modified polymers on glass fibres and (2) for a given reaction time, increase the amount of crosslinked polymer in the interphase, i.e. yield more stable interphases. It is also likely that preserving acidic conditions at the fibre/polymer interface is important for maintaining bonding across the interface. It is shown that polystyrene/glass-fibre composites having SEBS at the interface are promising candidates for high-impact-tolerance composites.     

Wave propagation in 2D elastic composites with partially debonded fibres by the null field approach

Time-harmonic plane wave propagation in a two-dimensional (2D) elastic matrix with partially debonded elastic fibres of nonclassical cross-section is investigated. The modified null field approach, taking into account the asymptotic behaviour of the solution at the interface crack-tips, is exploited to obtain the numerical results for a single scatterer. The effective medium approach based on Foldy's approximation is applied to estimate the average dynamic parameters of the composites containing randomly distributed partially debonded fibres of dilute concentration. Numerical results concern the longitudinal wave dispersion and attenuation owing to scattering by both randomly oriented and aligned fibres. The effects of the fibre shape, debonding (interface crack) size and direction of wave incidence on the effective P-wave velocity and attenuation coefficient are analysed.     

Modulus, confinement, and temperature effects on surface capillary wave dynamics in bilayer polymer films near the glass transition

We report relaxation times (τ) for surface capillary waves on 27-127?nm polystyrene (PS) top layers in bilayer films using x-ray photon correlation spectroscopy. At ～10?°C above the PS glass transition temperature (T_{g}), τ tracks with underlayer modulus, being significantly smaller on softer substrates at low in-plane scattering wave vector. Relative to capillary wave theory, we also report stiffening behavior upon nanoconfinement of the PS layers. At PS T_{g}+40?°C, both effects become negligible. We demonstrate how neighboring polymer domains impact dynamics over substantial length scales.     

Investigation of failure mechanisms in polymer composites by simultaneous measurement of ultra-small-angle scattering and acoustic emission during the deformation. I. Method

Failure mechanisms in polymer composites were investigated with the method of ultra-small-angle X-ray scattering using synchrotron radiation and acoustic emission analysis simultaneously. The composites were prepared from polystyrene filled with glass beads with different coatings. The two-dimensional scattering patterns were analyzed by means of a cylinder model, while the curves of the acoustic emission signals per stress or amplitude interval were calculated and fitted by a Weibull distribution function.     

INVESTIGATION OF STATIC AND DYNAMIC FRACTURE TOUGHNESS OF SHORT CERAMIC FIBER REINFORCED POLYPROPYLENE COMPOSITES

Short ceramic fiber reinforced polypropylene composites have been investigated to determine their static and dynamic fracture toughness for different reinforcing fiber contents. The composites were reinforced with fibers produced by a carding technique combined with needle-punching. Static fracture toughness (K _c) was measured on single-edge notched tensile (SEN-T) specimens, while dynamic fracture toughness (K _d) was tested by impact strength Charpy specimens. Specimens in both cases were cut transverse (T) and in longitudinal (L) directions. Test results show that dynamic fracture toughness is larger than the static one. During loading of SEN-T specimens the burst-type acoustic emission (AE) signals were monitored. From AE signals it can be concluded that the main damage form is the pull-out in the T specimens, and debonding in L ones. These results were supported by scanning electron microscopy micrographs taken from fracture surfaces.     

Thermophysical characterization of the deformation behavior of polyethylene/kaolin composites

The deformation behavior at room temperature of injection-molded and blow-molded samples of high-density polyethylene (HDPE) differing in orientation (with respect to the melt flow direction) and in filler content (untreated and surface-treated kaolin, respectively) were characterized by the stretching calorimetry technique. Samples with longitudinal and transversal orientations were examined. Strain-softening and strain-hardening phenomena were observed, respectively, in the range of inelastic strains above the yield point. The depression of yield stresses for the filled composites compared to those for the corresponding neat polymers is associated with the onset of debonding of the matrix polymer from the filler surface. The analysis of the energy balance of the debonding process suggests that the fraction of a debonded interface is smaller in samples containing a coupling agent and larger filler particles.     

Synchrotron radiation refraction topography for characterization of lightweight materials

The employment of synchrotron radiation for refraction topography of materials has considerable advantages over standard x‐ray sources. The much higher beam intensity and the parallel and monochromatic radiation provide faster measurements and better angular and spatial resolution. X‐ray refraction techniques image the inner surface and interface concentration of micro‐structured materials. This effect of x‐ray optics is additional to small‐angle scattering by diffraction, when the scattering objects reach micrometre dimensions. We have developed x‐ray refraction techniques within the last decade in order to meet the growing demands for improved non‐destructive characterization of high‐performance composites, ceramics and other low‐density materials. Sub‐micron particle dimensions, the pore size of ceramics, the crack density distribution and single fibre debonding within damaged composites can be measured and visualized by computer‐generated interface topographs. For this purpose investigations are now being performed at the new hard x‐ray beamline of the Federal Institute for Materials Research and Testing (BAM) at BESSY, Berlin. This BAMline provides monochromatic radiation of photon energies from 5 to 60 keV from a double multilayer and/or a double‐crystal monochromator. A separate instrument is dedicated to the further development and application of synchrotron radiation refraction (SRR) topography. Different from conventional small‐angle scattering cameras with collimating slits and pinholes, scattering angles down to a few seconds of arc are selected by a single‐crystal analyser, similar to a Bonse–Hart diffractometer. A 20 µm spatial resolution of the scattering micro‐structures is achieved by a CCD camera with a fluorescent converter. First SRR topographs of aircraft composites [carbon fibre‐reinforced plastics (CFRP), carbon fibre‐reinforced ceramics (C/C), metal matrix ceramics (MMC)] will be reported. Copyright © 2004 John Wiley & Sons, Ltd.     

Mechanical properties of flat braided composite with flexible interphase

Textile composites have been used extensively as industrial materials because of the excellent mechanical properties resulting from the continuously oriented fiber bundle. In a study of the mechanical properties, it is important to consider the fiber/matrix interface property as for other composite materials. In a recent study, the fiber/matrix interface is regarded as an interphase that has its own material constants and thickness; consequently, the mechanical properties of a composite can be controlled by specifically designing the interphase. In this study, we applied this concept to braided composites with flexible resin as interphase for the purpose of designing the interphase. In a static tensile test, though there were no improvements in Noncut specimens (normal braided composites), but a Cut specimen (each side of the Noncut specimen was cut) with flexible interphase was improved in fracture load and displacement. The observation of the specimen edge was carried out and it was confirmed that the progress of debonding at the fiber bundle intersection was interrupted by a flexible interphase, and a matrix crack did not occur in the Cut specimen with flexible interphase. In a fiber bundle pull-out test, it was confirmed that debonding progressed not into the fiber/resin interface but into the flexible interphase in the specimen with flexible interphase, and the interfacial property at the fiber bundle intersection was improved.     

原位聚合纳米Ni/聚苯乙烯复合材料及其热力学性能

 磁性聚苯乙烯(PS)材料有利于实现惯性约束聚变靶丸在辐射场中的无接触支撑。采用液相还原法制备了粒度为75~200 nm的Ni球形粒子,用原位聚合的方法制备了纳米Ni/PS复合材料。利用X射线衍射仪、傅氏转换红外线光谱分析仪、扫描电子显微镜及热重差示扫描量热仪分别研究了所得纳米Ni的特性、复合材料的结构及掺杂量对纳米Ni/PS复合材料的热力学行为的影响。研究结果表明：纳米Ni的掺杂能提高玻璃转变温度;纳米Ni的掺杂可以增大PS热分解焓变,在掺杂质量分数为1%~2%之间焓变达到最大;纳米Ni的掺杂降低了纳米Ni/PS复合材料热分解的比热容。     

Modified null field method for elastic wave scattering from a partially debonded fiber-SH-waves

Debonded fibers influence the macro-mechanical behavior of fiber-reinforced composite materials. Debonded fibers contribute to the initiation and growth of cracks at the fiber/matrix interface. To examine such problems, the scattering of elastic SH-waves (problem of anti-plane strain) from debonded fibers is studied with a numerical method which can handle the mixed boundary conditions on the partially bonded fiber. A modification of the null field of T-matrix method is developed for this purpose. The modification is achieved by the introduction of a mathematical surface. The simultaneous solution of the integral representations of the field scattered by the mathematical surface and the actual fiber surface give rise to sufficient equations that permit solution of the debonded fiber problem. The scattering cross-section and the far field amplitude are calculated as a function of frequency, fiber properties and debonding area. This will be of interest in structural applications where such cross-sections can be used to compute the dynamical effective properties of damaged composites.     

Interface Stability of Microencapsulated-Paraffin Filled Epoxy Composites: Effect of Methylation on Melamine–Formaldehyde Shell Material

Microcapsules containing phase change materials (microPCMs) can be filled in polymeric matrix forming smart temperature-controlling composites. The aim of this study was to investigate the composite interface stability of microPCMs/epoxy composites. Paraffin was applied as the phase change materials. Methanol–melamine–formaldehyde (MMF) and melamine–formaldehyde (MF) were both used to form the shell of microPCM by in-situ polymerization. The investigation focused on studying the effect of methylation and interface stability. The MF- and MMF-prepolymers were both successfully used to fabricate microcapsules with globe shapes and smooth surface. Thermal stability tests proved that the MMF-shell microPCMs/epoxy composites had a better interface stability than MF-shell microPCMs/epoxy composites. Mechanical properties also indicated that the MMF-shell microPCMs/epoxy composites had less tensile strength destruction after the thermal treatment. These results mean the MMF-resin enhanced the interface bonding stability though improving the interface molecule entanglement. As the MMF molecular has relative flexible ability and can adjust its conformation under change in environmental conditions, the interface will decrease the microstructure defects such as cracks, gaps and debonding.     

Thermodynamic and structural properties of polystyrene/C_(60) composites: A molecular dynamics study

To tailor properties of polymer composites are very important for their applications.Very small concentrations of nanoparticles can significantly alter their physical characteristics.In this work,molecular dynamics simulations are performed to study the thermodynamic and structural properties of polystyrene/C_(60)(PS/C_(60)) composites.The calculated densities,glass transition temperatures,and coefficient of thermal expansion of the bulk PS are in agreement with the experimental data available,implying that our calculations are reasonable.We find that the glass transition temperature Tg increases accordingly with an added concentration of C_(60) for PS/C_(60) composites.However,the self-diffusion coefficient D decreases with increase of addition of C_(60.)For the volumetric coefficients of thermal expansion(CTE) of bulk PS and ps/C_(60) composites,it can be seen that the CTE increases with increasing content of C_(60) above Tg(rubbery region).However,the CTE decreases with increasing content of C_(60) below Tg(glassy region).     

Effects of the embedding kinetics on the surface nano-morphology of nano-grained Au and Ag films on PS and PMMA layers annealed above the glass transition temperature

The morphology evolution of nano-grained Ag and Au films deposited on polystyrene (PS) and poly(methyl methacrylate) (PMMA) polymeric layers were studied, using the atomic force microscopy technique, when annealed above the polymers glass transition temperature. The main effects on the morphology changes were identified with those concerning the embedding kinetics of the Ag and Au nanoparticles in the PS or PMMA layers. The embedding process of the nanoparticles follows as a consequence of the long-range mobility of the polymeric chains above the glass transition temperature. In particular, the dependence of the nanoparticles mean height and surface density on the annealing time at various temperatures was quantified. The analyses of these behaviors allowed us: (1) to distinguish the overall embedding process in a first stage in which a thin wetting layer of the polymer coats the nanoparticles followed by a true embedding process of the nanoparticles into the polymer layer; (2) to evaluate the characteristic coating time for the Ag and Au nanoparticles in the PS and PMMA in the first stage; (3) to evaluate the characteristic embedding velocity for the Ag and Au nanoparticles in the PS and PMMA in the second stage; (4) to derive the activation energies for the embedding process of the Ag and Au nanoparticles in PS and PMMA; (5) to identify the embedding statistics of the Ag and Au nanoparticles in PS and PMMA with a “failure” Weibull statistics.     

Investigation of craze development using small-angle X-ray scattering of synchrotron radiation

The development of crazes in polycarbonate was investigated with the method of ultra-small-angle X-ray scattering (USAXS) of synchrotron radiation. Measurements at different temperatures and with different draw rates were carried out. The two-dimensional scattering patterns were analyzed by means of a fibrillar model of the craze. The geometrical parameters of the craze as a function of the macroscopic draw ratio were determined by using a curve-fitting procedure. From the measured values of the diameter and the mean distance of the fibrils, it is possible to calculate the volume fraction v_f of the fibrils directly. Additional scattering caused by submicrocracks is discussed.     

In situ observation of interfacial debonding process induced by matrix crack propagation in two-dimensional unidirectional model composites

Interfacial debonding behavior is studied for unidirectional fiber reinforced composites from both experimental and analytical viewpoints. A new type of two-dimensional unidirectional model composite is prepared using 10 boron fibers and transparent epoxy resin with two levels of interfacial strength. In situ observation of the internal mesoscopic fracture process is carried out using the single edge notched specimen under static loading. The matrix crack propagation, the interfacial debonding growth and the interaction between them are directly observed in detail. As a result, the interfacial debonding is clearly accelerated in specimens with weakly bonded fibers in comparison with those with strongly bonded fibers. Secondary, three-dimensional finite element analysis is carried out in order to reproduce the interfacial debonding behavior. The experimentally observed relation between the mesoscopic fracture process and the applied load is given as the boundary condition. We successfully evaluate the mode II interfacial debonding toughness and the effect of interfacial frictional shear stress on the apparent mode II energy release rate separately by employing the present model composite in combination with the finite element analysis. The true mode II interfacial debonding toughness for weaker interface is about 0.4 times as high as that for a stronger interface. The effect of the interfacial frictional shear stress on the apparent mode II energy release rate for the weak interface is about 0.07 times as high as that for the strong interface. The interfacial frictional shear stress and the coefficient of friction for weak interface are calculated as 0.25 and 0.4 times as high as those for strong interface, respectively.     

Growth of interfacial debonding in notched two-dimensional unidirectional composite under stress and displacement controls

A simplified calculation method for study of the growth of interfacial debonding between elastic fiber and elastic matrix ahead of the notch-tip in composites under displacement and stress controlled conditions was presented based on the shear lag approach in which the influences of residual stress and frictional shear stress at the debonded interface were incorporated. The calculation method was applied to a model two-dimensional composite. An outline is given of the difference and similarity in the growing behavior of the debonding between the displacement and stress controls, and of the influences of the residual stresses, frictional shear stress, the nature of the final cut component (fiber or matrix) and sample length on the debonding behavior.     

Enhancing the interfacial strength of glass/epoxy composites using ZnO nanowires

This paper investigated the application of ZnO nanowires (ZnO NW) to enhance the interfacial strength of glass/epoxy composites. ZnO NW were grown on glass fibers by hydrothermal method, tensile properties of bare and ZnO NW coated fibers were measured by single fiber tensile testing, wettability of fiber with resin was studied by contact angle measurements and finally the interfacial strength and mechanisms were determined by single fiber fragmentation testing of glass/epoxy composites. The surface coverage of ZnO NW on glass fibers was fairly uniform without formation of major clusters. The coating of ZnO NW slightly reduced the tensile strength and improved the tensile modulus of fibers. Wettability tests showed reduction in contact angles for ZnO NW coated fibers because of enhanced wetting and infiltration of epoxy resin into nanowires. In fragmentation testing of microcomposites, smaller and concentrated interfacial debonding zones for ZnO NW coated fibers indicated good stress transfer and strong interfacial adhesion. A new form of crossed and closely spaced stress patterns were observed for nanowires of high aspect ratios. The interfacial strength of ZnO NW coated fibers increased by at least 109% and by 430% on average, which was attributed to the increased surface area and mechanical interlocking provided by ZnO NW.     

Effects of residual stress and interfacial frictional shear stress on interfacial debonding at notch-tip in two-dimensional unidirectional composite

A calculation method based on the shear lag approach was presented to get an approximate estimate of influences of residual stresses and frictional shear stress at the debonded interface on the interfacial debonding behavior at the notch-tip along fiber direction in two-dimensional unidirectional double-edge-notched composites. With this method, the energy release rate for initiation and growth of debonding as a function of composite stress were calculated for some examples. The calculation results showed in outline how much the tensile and compressive residual stresses in the matrix and fiber along fiber direction, respectively, act to hasten the initiation and growth of the debonding when the final cut element in the notch is matrix, while they act to retard them when the final cut element is fiber, and how much the frictional shear stress at the debonded interface reduces the growth rate of the debonding.     

The Optical Properties of Polydisperse Glass Microsphere Ensembles in the Infrared Spectral Region

Semitransparent composites with glass microspheres are promising modern materials. In particular, they contribute to the increase of the efficiency of heat-insulating coatings. An experimental study of the transmission and scattering of infrared radiation (at wavelengths from 2 to 25 μm) by a sodium borosilicate glass microsphere powder is presented. The spectra, angular-scattering patterns, and the degree of polarization of scattered radiation were studied.