John M. Wiest

Formation mechanism and composition distribution of FePt nanoparticles

Self-assembled FePt nanoparticle arrays are candidate structures for ultrahigh density magnetic storage media. One of the factors limiting their application to this technology is particle-to-particle compositional variation. This variation will affect the A1 to L10 transformation as well as the magnetic properties of the nanoparticles. In the present study, an analysis is provided for the formation mechanism of these nanoparticles when synthesized by the superhydride reduction method. Additionally, a comparison is provided of the composition distributions of nanoparticles synthesized by the thermal decomposition of Fe(CO)5 and the reduction of FeCl2 by superhydride. The latter process produced a much narrower composition distribution. A thermodynamic analysis of the mechanism is described in terms of free energy perturbation Monte Carlo simulations.

Mesoscale constitutive modeling of magnetic dispersions.

A constitutive model for dispersions of acicular magnetic particles has been developed by modeling the particles as rigid dumbbells dispersed in a solvent. The effects of Brownian motion, anisotropic hydrodynamic drag, a steric force in the form of the Maier-Saupe potential, and, most importantly, a mean-field magnetic potential are included in the model. The development is similar to previous models for liquid-crystalline polymers. The model predicts multiple orientational states for the dispersion, and this phase behavior is described in terms of an orientational order parameter S and an average alignment parameter J; the latter is introduced because the magnetic particles have distinguishable direction due to polarity. A transition from isotropic to nematic phases at equilibrium is predicted. Multiple nematic phases-both prolate and oblate-are predicted in the presence of steady shear flow and external magnetic field parallel to the flow. The effect of increasing magnetic interparticle interactions and particle concentration is also presented. Comparisons with experimental data for the steady shear viscosity show very good agreement.

Birefringence in strong flows of polymer solutions

Abstract The constitutive relation for the refractive index of dilute solutions of flexible polymer molecules is obtained, and calculations of the intrinsic birefringence exhibited in uniaxial elongational flow are presented. The polymer molecules are modeled as chains of beads connected by finitely extensible nonlinear elastic springs under the Peterlin approximation. It is found that at very low elongation rates there is very little birefringence, but there is a critical elongation rate above which there is substantial birefringence. This critical elongation rate scales as the inverse square of the number of beads in the chain. At this elongation rate the individual springs in the chains are stretched and oriented with the flow, but the chains have not uncoiled. It is found that the stress-optic relation holds for elongation rates below the critical elongation rate. However, the stress-optic coefficient is not constant above the critical rate. In the instantaneous inception of the flow above the critical elongation rate, the stress optic coefficient initially takes on the value that it would have below the critical elongation rate, but it decays to a much lower value as the strain increases.

Accelerated aging studies and the prediction of the archival lifetime of optical disk media

Data archivists expect information storage media to have a lifetime greater than ten years. Furthermore they desire the ability to predict when the media will fail in order to plan for its replacement. Archival lifetime predictions are based on accelerated aging studies, where the media are subjected to conditions of high temperature and high humidity. The rate of failure is measured and the data extrapolated to obtain rates of failure under ambient conditions. This extrapolation is reasonable provided the degradation process is activated and the Arrhenius relationship holds. However this may not be the case for the complicated materials packages in optical data storage media. A primary concern for the polymeric materials is any phase transition, such a glass transition or a beta relaxation, that may occur at temperatures between ambient and the accelerated aging conditions. It is not clear how one extrapolates through those transitions. These phase transitions can give rise to large changes in the rates of diffusion for water, oxygen and other agents of degradation. Furthermore, for polymers, such as polycarbonate, the mode of failure is often hydrolysis and the degradation products can catalyze further hydrolysis, an autocatalytic degradation. The polymer degradation will change the phase transition temperatures. The degradation products may also plasticize the polymer, causing further changes in diffusion rates. We provide here a simple analysis of accelerated aging techniques and discuss other factors that may be involved.

Rheological behavior and microstructure of magnetic dispersions with varying nonmagnetic particle content

Abstract We examine the frequency, stress, and strain dependencies of the dynamic shear moduli of magnetic dispersions containing mixtures of varying amounts of magnetic cobalt-modified γ-Fe 2 O 3 and nonmagnetic cobalt-modified α-Fe 2 O 3 particles, and we interpret these results in terms of a network structure in the dispersions. The storage modulus exhibits a plateau, providing evidence for such a network. The critical strain amplitude indicating the transition from linear to nonlinear behavior is independent of magnetic particle fraction. This implies that the topology of the network structure formed by magnetic interactions between the particles is independent of nonmagnetic particle content. However, the critical stress amplitude indicating the transition to nonlinear behavior does depend on magnetic particle fraction. This indicates that the strength of the network depends on nonmagnetic particle content. The loss modulus exhibits a minimum when considered as a function of frequency. This minimum can be rationalized in terms of network elasticity at low frequencies and local viscous behavior at high frequencies. Furthermore, the ‘network elasticity’ extracted from small-amplitude oscillatory measurements scales the magnetic particle fraction dependence of shear stress–shear rate data for steady shear flow onto a single master curve.

Molecular weight distributions from linear viscoelastic measurements

A method is presented whereby the molecular weight distribution of a molten polymer can be determined from measured values of the storage and loss moduli (the response of the system to small amplitude oscillatory shear flow). The Curtiss-Bird theory for the constitutive behavior of polymer melts is used. This results in an integral equation relating the moduli to the molecular weight distribution. The method that we describe here uses a regularization scheme with quadratic programming to extract the MWD from the components of the moduli (or, equivalently, from the components of the complex viscosity). We verify the applicability of the scheme by considering concocted “data” for given molecular weight distributions. The scheme is also used to predict the molecular weight distributions of polystyrene from complex viscosity data that have been presented in the literature. Variations between the predicted and measured molecular weight distributions can be attributed to the inadequacy of the underlying molecular theory at large frequencies. The predicted average molecular weights obtained using the truncated dynamic data are in reasonable agreement with the experimental values.

Nonequilibrium energetic effects in the inception of shear flow of dilute polymer solutions

The inception of adiabatic steady shear flow of a dilute polymer solution modeled as Hookean dumbbells is considered. The effects of temperature variations in the flow are included explicitly in the constitutive equation. These temperature variations are caused in the flow by the conversion of mechanical energy (work) into thermal energy through the process known as viscous dissipation. However, in a polymeric liquid, part of the mechanical work is also stored by the material as elastic energy, and we account for this energy by including the nonequilibrium nature of the internal energy function. The model predicts significant temperature increases in the material upon the inception of shear flow. Stress overshoots and long-time decay are also predicted in contrast to the monotonic stress growth predicted for the isothermal system.

Inception of shear flow for ferromagnetic dispersions

Rheological data are presented for shear flows of concentrated dispersions of acicular ferromagnetic particles; such dispersions are widely used in the manufacture of data storage materials. In steady shear flow these dispersions are strongly shear thinning. Upon the inception of shear flow, they show stress overshoots for both the shear stress and the primary normal stress difference. The data can be rationalized in terms of a temporary junction network, resulting from the strong magnetic interactions between the particles, if the network consists of relatively weak magnetic links connecting strongly bound flocs of particles. Application of a shear deformation results in disruption of the initially sample spanning network. The overshoot in the shear stress occurs when the flocs become arranged in the shear planes. The overshoot in the primary normal stress difference occurs as a result of deformation of the strongly bound flocs.

Relaxation following uniaxial extension of dilute polymer solutions

How polymer molecules uncoil and stretch under the influence of an elongational or stretching flow has been the subject of considerable study and discussion. Many experimental studies have been conducted to try to achieve such flows for dilute polymer solutions, but the recent development of filament stretching devices has made it possible to examine the flows unequivocally. Furthermore, these devices allow examination of the processes by which the polymer molecules relax upon cessation of the flow. Here, we present the predictions of the finitely extensible nonlinear elastic chain model (under the Peterlin approximation) for both the stretching and the relaxation. Calculations of conformational, rheological, and optical properties are given. It is found that the process by which the molecules relax, which is governed primarily by segment relaxation and Brownian motion, is quite different from that by which the molecules stretch. Whereas the stretching process passes through several distinct stages, the r...