Studying properties of polymer X-ray lenses

The focusing properties and radiation resistance of refractive polymer X-ray lenses have been experimentally studied on an ESRF synchrotron. The axisymmetric X-ray lenses with parabolic profile have been manufactured by “imprinting” a high-precision stamp into a photocurable polymer. Results of measurements of the characteristics of lenses for 9.9-keV X-rays with an intensity of 10¹³ ph/(mm² s) are presented and discussed.     

Mechanical modeling of viral capsids

As revealed by techniques of structural biology and single-molecule experimentation, the protein shells of viruses (capsids) are some of nature’s best examples of highly symmetric multiscale self-assembled structures, with impressive mechanical properties of strength and elasticity. Mechanical models of viral capsids built “from the bottom up,” i.e., from all-atom models in the context of molecular dynamics and normal mode analysis, have chiefly focused on unforced vibrational capsid dynamics. Due to the size of viral capsids, which can contain several hundred thousand atoms, the computer power needed for these types of methods has only recently reached the level required for all-atom simulations of entire viral capsids. Coarse-grained normal mode analysis has provided a simplified means of studying the unforced vibrational dynamics of viral capsids. Recent focus on “top-down” mechanical models of viral capsids based on two- and three-dimensional continuum elasticity have provided a theoretical complement to single molecule experiments such as atomic force microscopy, and have advanced the fundamental understanding of the forced mechanics. This review serves to assess the current state of modeling techniques for the study of the mechanics of viral capsids, and to highlight some of the key insights gained from such modeling. In particular, a theme is established of a link between shape—or geometry—and the global mechanical properties of these hierarchical multiscale biological structures.     

Nano fountain pen manufacture of polymer lenses for nano-biochip applications

Polymer microlenses have been manufactured by delivering droplets of a monomer mixture to a glass substrate using a nano fountain pen (NFP). Subsequent UV polymerization yielded microlenses with optical properties that were controlled by varying the deposition time of the monomer solution. Using this approach, it is possible to strategically place single microlenses at predefined positions with very high accuracy, an ability which may prove very useful for nano-biochip applications, as demonstrated.     

Mechanical behaviour of polymer concrete systems

The mechanical behaviour of epoxy and polyester polymer concrete systems was studied under different loading conditions at various temperatures, resin content, and glass fibre content. While polymer content varied between 10 and 20% of the total weight of polymer concrete, the fibre content was limited to 4% by weight. The temperature was varied between 22 and 110°C, depending on the glass transition temperature of the resin. Compared to vibration, the compaction method of preparation reduces the void content and enhances the strength and modulus of polymer concrete. The compressive and flexural strength and stiffness of the polymer concrete systems increase up to a certain limit of polymer content at which they exhibit maximum strength and stiffness. They subsequently decrease or remain almost constant with further increase in polymer content. The strength and stiffness of polymer concrete are very much dependent on the temperature. The stiffness model, based on inclusion theory, yields satisfactory results for the three-phase polymer concrete. Using this model, the compression and flexural modulus of polymer concrete can be predicted from the properties of the constituents and their composition. Incremental strength and stiffness models developed in this study are effective in predicting the increase in strength and stiffness of glass-fibre-reinforced polymer concrete.     

An efficient algorithm for propagating fluid-driven fractures

This paper presents an efficient finite element algorithm for propagating fluid-driven fractures in pressure sensitive geomaterials. Fluid flow in the fracture is modelled by lubrication theory. Rock deformation is assumed to be elastoplastic and dilatent. A cohesive model based on the softening behaviour of rocks is employed as the propagation criterion. A special continuation method based on the volume of injected fluid in the fracture is used for direct coupling of the fluid-flow with rock deformation and for driving the solution during propagation. Sample results are provided for the problem of hydraulic fracturing to demonstrate the efficiency of the proposed algorithm. Received 5 February 1999     

Mechanical behavior of glass polymer multilayer composites

To identify the best reinforcement condition for development of tough glass polymer multi-layer composites (GPMLC) with high failure strain, two such model composite structures were developed. Soda–lime–silica glasses of two different thicknesses viz (A—1.01 mm and B—1.17 mm) were used as the matrix layers. The A-glass and B-glass based GPMLC samples were prepared by a novel, low pressure lamination technique applied to the alternating planar structure of the matrix and reinforcing phases. These GPMLC materials were fabricated with and without a thin sprayed layer of kerosene, between the glass layer and the reinforcing layer in the interface where; the interface was either epoxy (a thermosetting resin) or polyvinyl butyral (PVB, a thermoplastic resin). The GPMLC samples which exhibited stepped load—displacement behaviour in the most pronounced fashion, had the thermoplastic resin at the interface. Most of these GPMLC samples had a thin layer of kerosene intentionally introduced between the glass layer and the reinforcing polymer layer such that a weak interface is obtained. Fractographic evidence suggested that more of controlled delaminaton cracking occurs in such samples. Apart from the chemical nature of the reinforcing polymer phase, the interfacial layer thickness (h _i ) and the interfacial shear stress ( _xy ) were found out to have significant influence on the specific failure load and the failure stress of the current glass polymer multi-layer composites.     

Mechanical properties of glass polymer multilayer composite

The preliminary experimental studies on the comparative behaviour of the deformation processes involved in the failure of a commercial, 0.3 mm thick, 18 mm diameter soda-lime-silica glass disks (G) and multilayered glass disk-epoxy (GE) as well as glass disk-epoxy-E-glass fabric (GEF) composite structures are reported. The failure tests were conducted in a biaxial flexure at room temperature. The epoxy was a commercial resin and theE-glass fabric was also commercially obtained as a two-dimensional weave ofE-glass fibres to an area density of about 242 g m⁻². The multilayered structures were developed by alternate placement of the glass and reinforcing layers by a hand lay-up technique followed by lamination at an appropriate temperature and pressure. Depending on the number of layers the volume fraction of reinforcement could be varied from about 0.20 for the GE system to about 0.50 for the GEF system. It was observed that the specific failure load (load per unit thickness) was enhanced from a value of about 60 N/mm obtained for the glass to a maximum value of about 100 N/mm for the GE composites and to a maximum of about 70 N/mm for the GEF composite system. Similarly, the displacements at failure (δ) measured with a linear variable differential transformer (LVDT) were also found to be a strongly sensitive function of the type of reinforcement (GE or GEF) as well as the number of layers.     

Multiscale modeling of carbon nanotube reinforced polymer composites

This article examines the effect of interfacial load transfer on the stress distribution in carbon nanotube/polymer composites through a stress analysis of the nanotube/matrix system. Both isostrain and isostress loading conditions are investigated. The nanotube is modeled by the molecular structural mechanics method at the atomistic level. The matrix is modeled by the finite element method, and the nanotube/matrix interface is assumed to be bonded either perfectly or by van der Waals interactions. The fundamental issues examined include the interfacial shear stress distribution, stress concentration in the matrix in the vicinity of nanotube ends, axial stress profile in the nanotube, and the effect of nanotube aspect ratio on load transfer.     

Stochastic multi-scale modeling of CNT/polymer composites

A full stochastic multi-scale modeling technique is developed to estimate mechanical properties of carbon nanotube reinforced polymers. Developing a full-range multi-scale technique to consider effective parameters of all nano, micro, meso and macro-scales and full stochastic implementation of integrated modeling procedures are the novelties of the present research. The length, orientation, agglomeration, curvature and dispersion of carbon nanotubes are taken into account as random parameters. It is proven that random distribution of carbon nanotube length and volume fraction can be replaced with corresponding mean values. The results of predictions are in a very good agreement with published experimental observations.     

Mechanical properties of natural fibre reinforced polymer composites

During the last few years, natural fibres have received much more attention than ever before from the research community all over the world. These natural fibres offer a number of advantages over traditional synthetic fibres. In the present communication, a study on the synthesis and mechanical properties of new series of green composites involving Hibiscus sabdariffa fibre as a reinforcing material in urea-formaldehyde (UF) resin based polymer matrix has been reported. Static mechanical properties of randomly oriented intimately mixed Hibiscus sabdariffa fibre reinforced polymer composites such as tensile, compressive and wear properties were investigated as a function of fibre loading. Initially urea-formaldehyde resin prepared was subjected to evaluation of its optimum mechanical properties. Then reinforcing of the resin with Hibiscus sabdariffa fibre was accomplished in three different forms: particle size, short fibre and long fibre by employing optimized resin. Present work reveals that mechanical properties such as tensile strength, compressive strength and wear resistance etc of the urea-formaldehyde resin increases to considerable extent when reinforced with the fibre. Thermal (TGA/DTA/DTG) and morphological studies (SEM) of the resin and biocomposites have also been carried out.     

Mechanical properties of carbon nanotubes and their polymer nanocomposites

More than 10 years have passed since carbon nanotubes (CNT) have been found during observations by transmission electron microscopy (TEM). Since then, one of the major applications of the CNT is the reinforcements of plastics in processing composite materials, because it was found by experiments that CNT possessed splendid mechanical properties. Various experimental methods are conducted in order to understand the mechanical properties of varieties of CNT and CNT-based composite materials. The systematized data of the past research results of CNT and their nanocomposites are extremely useful to improve processing and design criteria for new nanocomposites in further studies. Before the CNT observations, vapor grown carbon fibers (VGCF) were already utilized for composite applications, although there have been only few experimental data about the mechanical properties of VGCF. The structure of VGCF is similar to that of multi-wall carbon nanotubes (MWCNT), and the major benefit of VGCF is less commercial price. Therefore, this review article overviews the experimental results regarding the various mechanical properties of CNT, VGCF, and their polymer nanocomposites. The experimental methods and results to measure the elastic modulus and strength of CNT and VGCF are first discussed in this article. Secondly, the different surface chemical modifications for CNT and VGCF are reviewed, because the surface chemical modifications play an important role for polymer nanocomposite processing and properties. Thirdly, fracture and fatigue properties of CNT/polymer nanocomposites are reviewed, since these properties are important, especially when these new nanocomposite materials are applied for structural applications.     

Mechanical and conductive properties of metal fibre-filled polymer composites

Data are presented which show that the critical volume loading of a metallic filler needed to induce electrical conductivity in a polymer matrix can be substantially reduced by adding the metal as randomly dispersed fibres. Higher aspect ratio fibres will induce conductivity at lower volume loadings. Composites exhibiting resistivities below 200 ohm cm have been prepared with as little as 7.7 volume percent aluminium fibres having an aspect ratio of 24 : 1. At such low filler loadings the mechanical properties of the composite are similar to those obtained with an identical loading of milled glass fibres. While aluminium is poorly wet by most virgin polymers, this characteristic can be improved by polymer modification and surface treatments. The thermal properties of these composites are shown to follow predictions based on Nielsen's theory.     

Mathematical modeling of the process of plane polymer film formation

The approximate solution is considered of a nonlinear nostationary problem of heat conduction with conjugate boundary conditions.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 58, No. 5, pp. 862–866, May, 1990.     

Thermomechanical modeling of polymer nanocomposites by the asymptotic homogenization method

There is much current interest to incorporate nano-scale fillers into polymer matrices to achieve potentially unique properties. Compared with traditional microcomposites, a nanocomposite has a significant large ratio of interface area to volume that results in improved thermomechanical properties. Desired thermomechanical properties of polymer nanocomposites, to achieve the ever-increasing performance requirements, can be obtained by tailoring their microstructures. To this end, computational analyses of the relations between the thermomechanical properties, e.g., Young’s modulus, shear modulus, Poisson’s ratio, yield strength, coefficient of thermal expansion and coefficient of thermal conductivity, in different directions and the microstructures of polymer nanocomposites are performed. The asymptotic homogenization method based on the finite element analysis is used to model the thermomechanical behaviors of different polymer nanocomposites with periodic microstructures. The effects of adding silica, rubber, and clay nanoparticles to epoxy resin as a polymer matrix are analyzed. Mixtures of the nano-particles which differ in volume fraction, material type, size and/or geometry are considered. Some predictions of the thermomechanical properties are compared with experimental data in order to verify the applied modeling technique as an effective design tool to tailor optimal microstructures of polymer nanocomposites.     

Thermo-optical modeling of an intrinsically heated polymer fiber Bragg grating

A fully concatenated thermo-optical model is presented to predict the thermo-optical behavior of an intrinsically heated polymer fiber Bragg grating (PFBG). Coupled-mode theory and heat-conduction theory are first used to determine the axial heat generation and temperature distribution of a PFBG and the transfer matrix method (TMM) is subsequently employed to predict its thermo-optical behavior. The validity of the TMM is corroborated experimentally using an externally heated glass fiber Bragg grating (FBG) with an axially decaying temperature field. The verified model is utilized to investigate the thermo-optical behavior of a poly(methyl methacrylate) (PMMA) FBG. The counteracting thermally driven changes in the refractive index and the grating pitch, respectively, are found to be of comparable magnitude and to result in very modest net shifts in the Bragg wavelengths despite the considerable temperature changes induced by the absorption of the incident light.     

Diffractive optical elements in hybrid lenses: modeling and design by zone decomposition

We propose to model hybrid optical systems (i.e., lenses with conventional and diffractive optical elements) as multiaperture systems in which the images formed by each zone of the diffractive optical element should be summed up coherently. This new zone decomposition concept is explained and compared with the standard diffraction-order expansion with the help of a hybrid triplet example.     

A micromechanics approach for damage modeling of polymer matrix composites

A new model for damage evolution in polymer matrix composites is presented. The model is based on a combination of two constituent-level models and an interphase model. This approach reduces the number of empirical parameters since the two constituent-level models are formulated for isotropic materials, namely fiber and matrix. Decomposition of the state variables down to the micro-scale is accomplished by micromechanics. Phenomenological damage evolution models are then postulated for each constituent. Determination of material parameters is made from available experimental data. The required experimental data can be obtained with standard tests. Comparison between model predictions and additional experimental data is presented.