Sarah C. Baxter

Application of the Freundlich adsorption isotherm in the characterization of molecularly imprinted polymers

The binding isotherm for a polymer molecularly imprinted with ethyl adenine-9-acetate was observed to obey the common Freundlich isotherm. To test the generality of the Freundlich isotherm with respect to molecularly imprinted polymers (MIPs), a survey of systems from the literature was conducted, revealing that the Freundlich isotherm gives a good mathematical approximation of the binding characteristics for non-covalently imprinted polymers. The utility of the Freundlich isotherm in the calculation of binding parameters, as well as its limitations and implication of an exponential distribution of binding sites in imprinted polymers have been discussed.

Influence of pore geometry on the effective response of porous media

The generalized method of cells (GMC) is used to study the influence of pore geometry on the effective elastic properties and inelastic response of porous materials. Periodic microstructures with four distinct pore geometries are studied and the results for effective elastic properties are compared with several other available models and experimental results. Predictions for the inelastic response of porous alumina are presented for tensile loading, as a function of pore geometry and pore volume fraction, with the inelastic behavior of the bulk material modeled using a unified visco-plasticity theory. All results are presented for discrete pore shape and discrete porosity. It is shown that pore geometry can have a significant influence on both elastic and inelastic response, that pore geometry can be associated with parameters from other models, and that the generalized method of cells is an efficient, flexible and reliable method of analysis for such problems.

Effects of curvilinear anisotropy on radially symmetric stresses in anisotropic linearly elastic solids

It has been known for some time that certain radial anisotropies in some linear elasticity problems can give rise to stress singularities which are absent in the corresponding isotropic problems. Recently related issues were examined by other authors in the context of plane strain axisymmetric deformations of a hollow circular cylindrically anisotropic linearly elastic cylinder under uniform external pressure, an anisotropic analog of the classic isotropic Lamé problem. In the isotropic case, as the external radius increases, the stresses rapidly approach those for a traction-free cavity in an infinite medium under remotely applied uniform compression. However, it has been shown that this does not occur when the cylinder is even slightly anisotropic. In this paper, we provide further elaboration on these issues. For the externally pressurized hollow cylinder (or disk), it is shown that for radially orthotropic materials, the maximum hoop stress occurs always on the inner boundary (as in the isotropic case) but that the stress concentration factor is infinite. For circumferentially orthotropic materials, if the tube is sufficiently thin, the maximum hoop stress always occurs on the inner boundary whereas for sufficiently thick tubes, the maximum hoop stress occurs at the outer boundary. For the case of an internally pressurized tube, the anisotropic problem does not give rise to such radical differences in stress behavior from the isotropic problem. Such differences do, however, arise in the problem of an anisotropic disk, in plane stress, rotating at a constant angular velocity about its center, as well as in the three-dimensional problem governing radially symmetric deformations of anisotropic externally pressurized hollow spheres. The anisotropies of concern here do arise in technological applications such as the processing of fiber composites as well as the casting of metals.

Simulation of local material properties based on moving-window GMC

Abstract When analyzing the behavior of composite materials under various loading conditions, the assumption is generally made that the behavior due to randomness in the material can be represented by a homogenized, or effective, set of material properties. This assumption may be valid when considering displacement, average strain, or even average stress of structures much larger than the inclusion size. The approach is less valid, however, when considering either behavior of structures of size at the scale of the inclusions or local stress of structures in general. In this paper, Monte Carlo simulation is used to assess the effects of microstructural randomness on the local stress response of composite materials. In order to achieve these stochastic simulations, the mean, variance and spectral density functions describing the randomly varying elastic properties are required as input. These are obtained here by using a technique known as moving-window generalized method of cells (moving-window GMC). This method characterizes a digitized composite material microstructure by developing fields of local effective material properties. Once these fields are generated, it is straightforward to obtain estimates of the associated probabilistic parameters required for simulation. Based on the simulated property fields, a series of local stress fields, associated with the random material sample under uniaxial tension, is calculated using finite element analysis. An estimation of the variability in the local stress response for the given random composite is obtained from consideration of these simulations.

The effect of gold nanorods on cell-mediated collagen remodeling

Cardiac fibroblasts, the noncontractile cells of the heart, contribute to myocardial maintenance through the deposition, degradation, and organization of collagen. Adding polyelectrolyte-coated gold nanorods to three-dimensional constructs composed of collagen and cardiac fibroblasts reduced contraction and altered the expression of mRNAs encoding beta-actin, alpha-smooth muscle actin, and collagen type I. These data show that nanomaterials can modulate cell-mediated matrix remodeling and suggest that the targeted delivery of nanomaterials can be applied for antifibrotic therapies.

Micromechanics based random material property fields for particulate reinforced composites

Particle reinforced metal matrix composites offer a number of advantages over continuously reinforced composites. They generally can be made using conventional metal-working processes and often fabricated to near net shape. Like continuously reinforced composites though, the potential exists to tailor these materials for higher specific stiffnesses, greater strength and improved fracture properties over their homogeneous counterparts. Their effective use requires an accurate characterization, which is made difficult by a three-dimensional (3D) random microstructure. A micromechanics based moving window technique, used to develop material property fields associated with the random 3D microstructure of a particulate reinforced composite, is described in this paper. The resulting sample material property fields are computationally tractable and have a direct link to the composite microstructure. The method can be used to generate material property fields for elastic or inelastic properties. Statistical and probabilistic descriptions of these property fields can subsequently be used to simulate the material and characterize the variability of the material response. The method is illustrated in this paper by generating fields for selected elastic moduli developed from a numerically simulated microstructure.

Light scattering from gold nanorods: tracking material deformation

We demonstrate the use of optical patterns, produced by resonant Rayleigh scattering from gold nanorods, as markers by which local deformations can be measured using image correlation techniques. While the use of optical data, in this case from dark-field microscopy, to generate deformational field information (displacements and strains) is not new, the use of the light scattered from gold nanorods as the correlated pattern is new, and has the potential to enable smaller scale measurements even over large deformations. We find excellent agreement between the measured and theoretical deformation and strain fields for two sample polymers with gold nanorod markers. The gold nanorod surface can be modified to make biocompatible nanomaterials, which will be useful for examining mechanical effects in biological tissue.

Effects of scale and interface on the three-dimensional micromechanics of polymer nanocomposites:

Nanocomposite materials hold the power to revitalize and revolutionize the field of composite materials. Nanoscaled, even common materials can exhibit strikingly different material properties from the bulk counterparts. If these properties can be accessed at the bulk scale, not only can materials be better tailored to suit various applications, but the possibility of designing multi-functional materials expands exponentially. In this study, the Generalized Method of Cells (GMC) micromechanics model is used to model 3D nanoscale composite architecture, including an interfacial region between the included and matrix phases, and predict the effective viscoelastic properties of a gold nanorod, polymer matrix, nanocomposite. Scale is introduced by referencing the dimensions of the interface to those of the nanorods. Comparisons are made of micromechanical response based on volume fraction and number density, highlighting the scale effects resulting from the high surface area to volume ratio of nanoparticles. Effective composite viscoelastic properties were developed, for static creep, for varying interfacial elastic stiffnesses. These experiments suggest that an elastically stiff interface greatly increases the stiffness of the polymer in response to an ‘instantaneous’ step load, reduces the rapid creep response, and results in a rapid leveling off of the time-dependent strain curves. The response of the composite to increasing stiffness of the interface region eventually reaches a plateau or threshold value, where further increases in the stiffness of the interface produces negligible increases in stiffness, or further reduction in creep response.

End effects for anti-plane shear deformations of sandwich structures

The purpose of this research is to further investigate the effects of material inhomogeneity and the combined effects of material inhomogeneity and anisotropy on the decay of Saint-Venant end effects. Saint-Venant decay rates for self-equilibrated edge loads in symmetric sandwich structures are examined in the context of anti-plane shear for linear anisotropic elasticity. The problem is governed by a second-order, linear, elliptic, partial differential equation with discontinuous coefficients. The most general anisotropy consistent with a state of anti-plane shear is considered, as well as a variety of boundary conditions. Anti-plane or longitudinal shear deformations are one of the simplest classes of deformations in solid mechanics. The resulting deformations are completely characterized by a single out-of-plane displacement which depends only on the in-plane coordinates. They can be thought of as complementary deformations to those of plane elasticity. While these deformations have received little attention compared with the plane problems of linear elasticity, they have recently been investigated for anisotropic and inhomogeneous linear elasticity. In the context of linear elasticity, Saint-Venants principle is used to show that self-equilibrated loads generate local stress effects that quickly decay away from the loaded end of a structure. For homogeneous isotropic linear elastic materials this is well-documented. Self-equilibrated loads are a class of load distributions that are statically equivalent to zero, i.e., have zero resultant force and moment. When Saint-Venants principle is valid, pointwise boundary conditions can be replaced by more tractable resultant conditions. It is shown in the present study that material inhomogeneity significantly affects the practical application of Saint-Venants principle to sandwich structures.

Age-dependent expression of collagen receptors and deformation of type I collagen substrates by rat cardiac fibroblasts.

Little is known about how age influences the ways in which cardiac fibroblasts interact with the extracellular matrix. We investigated the deformation of collagen substrates by neonatal and adult rat cardiac fibroblasts in monolayer and three-dimensional (3D) cultures, and quantified the expression of three collagen receptors [discoidin domain receptor (DDR)1, DDR2, and β1 integrin] and the contractile protein alpha smooth muscle actin (α-SMA) in these cells. We report that adult fibroblasts contracted 3D collagen substrates significantly less than their neonate counterparts, whereas no differences were observed in monolayer cultures. Adult cells had lower expression of β1 integrin and α-SMA than neonate cultures, and we detected significant correlations between the expression of α-SMA and each of the collagen receptors in neonate cells but not in adult cells. Consistent with recent work demonstrating age-dependent interactions with myocytes, our results indicate that interactions between cardiac fibroblasts and the extracellular matrix change with age.