Extended Time Scales Research Articles

Healthcare in Prisons: reflections on the inertia of systems and the low effectiveness of health policies in Spain

The blockage in implementing transfer of prison health care to regional governments is a problematic social and health care issue, and also brings to light major institutional and political weaknesses. This article considers the power dynamics between administrations and levels, highlighting the subordinate nature of the health care agendas with regard to political and economic ones, as well as the sizeable capacity for blockage created by the environment itself. Within this context, some strategies are proposed to improve the future feasibility of the integration of prison health into the National Health System, based on the awareness of the need to accumulate power and influence, promote service improvements to increase the attractiveness and desirability of prison health, and the acceptance of more extended time scales so that cultural changes may take root.

Computational Design of Multi-component Bio-Inspired Bilayer Membranes

Our investigation is motivated by the need to design bilayer membranes with tunable interfacial and mechanical properties for use in a range of applications, such as targeted drug delivery, sensing and imaging. We draw inspiration from biological cell membranes and focus on their principal constituents. In this paper, we present our results on the role of molecular architecture on the interfacial, structural and dynamical properties of bio-inspired membranes. We focus on four lipid architectures with variations in the head group shape and the hydrocarbon tail length. Each lipid species is composed of a hydrophilic head group and two hydrophobic tails. In addition, we study a model of the Cholesterol molecule to understand the interfacial properties of a bilayer membrane composed of rigid, single-tail molecular species. We demonstrate the properties of the bilayer membranes to be determined by the molecular architecture and rigidity of the constituent species. Finally, we demonstrate the formation of a stable mixed bilayer membrane composed of Cholesterol and one of the phospholipid species. Our approach can be adopted to design multi-component bilayer membranes with tunable interfacial and mechanical properties. We use a Molecular Dynamics-based mesoscopic simulation technique called Dissipative Particle Dynamics that resolves the molecular details of the components through soft-sphere coarse-grained models and reproduces the hydrodynamic behavior of the system over extended time scales.

Computational Design of Multi-component Bio-Inspired Bilayer Membranes

Our investigation is motivated by the need to design bilayer membranes with tunable interfacial and mechanical properties for use in a range of applications, such as targeted drug delivery, sensing and imaging. We draw inspiration from biological cell membranes and focus on their principal constituents. In this paper, we present our results on the role of molecular architecture on the interfacial, structural and dynamical properties of bio-inspired membranes. We focus on four lipid architectures with variations in the head group shape and the hydrocarbon tail length. Each lipid species is composed of a hydrophilic head group and two hydrophobic tails. In addition, we study a model of the Cholesterol molecule to understand the interfacial properties of a bilayer membrane composed of rigid, single-tail molecular species. We demonstrate the properties of the bilayer membranes to be determined by the molecular architecture and rigidity of the constituent species. Finally, we demonstrate the formation of a stable mixed bilayer membrane composed of Cholesterol and one of the phospholipid species. Our approach can be adopted to design multi-component bilayer membranes with tunable interfacial and mechanical properties. We use a Molecular Dynamics-based mesoscopic simulation technique called Dissipative Particle Dynamics that resolves the molecular details of the components through soft-sphere coarse-grained models and reproduces the hydrodynamic behavior of the system over extended time scales.

Exploiting Binding Kinetics of Fluorogen Activating Peptides to Enhance Photostability: Applications to Live Cell Single Molecule Imaging

Inefficiency of fluorescent labels imposes key limitations in fluorescence microscopy. Photobleaching, non-specific signal from multiple fluorophores, and lack of multiplexing and orthogonal labels impose major challenges in studying complex cellular systems, both at ensemble and single molecule levels. We have addressed these problems by exploiting the binding kinetics of fluorogen activating peptides that on binding to a non-fluorescent molecule (a fluorogen) exhibit fluorescence. Novel methods in flow cytometry and rational design of peptides enabled the elucidation of peptides and fluorogen pairs with varying off rates. Complexes with fast off rates (koff∼5E-3 s-1) can exchange a bleached fluorogen molecule with a fresh fluorogen molecule in solution. Since the free fluorogen is dark, we can image for extended time scales in the presence of excess fluorogen. For complexes with slow off rates (koff∼5E-5 s−1) and high affinity (Kd<1E-9M), we can target commercially available Streptavidin QDots to proteins on the cell membrane, cytoplasm and the nucleus. We have imaged these proteins at a single molecule level for long time scales by exploiting the exceptional photostability of QDots. We have shown the use of this technique in particular, to the study of the diffusion dynamics of the β-2 adrenergic receptor and a trans-membrane protein on the cell surface simultaneously in real time. We have also developed peptides that non-covalently target cyanine dyes with sub-nanomolar affinities. This solves the problems associated with protein-dye conjugation and enhances the photostability of these dyes up to ten fold. We have demonstrated the use of these peptides in single molecule studies of membrane proteins. Thus by exploiting kinetics, we have developed a genetically encoded toolbox for targeting fluorogens, organic dyes and QDots for the study of proteins on and inside the cell.

Accelerated Molecular Dynamics Simulations with the AMOEBA Polarizable Force Field on Graphics Processing Units

The accelerated molecular dynamics (aMD) method has recently been shown to enhance the sampling of biomolecules in molecular dynamics (MD) simulations, often by several orders of magnitude. Here, we describe an implementation of the aMD method for the OpenMM application layer that takes full advantage of graphics processing units (GPUs) computing. The aMD method is shown to work in combination with the AMOEBA polarizable force field (AMOEBA-aMD), allowing the simulation of long time-scale events with a polarizable force field. Benchmarks are provided to show that the AMOEBA-aMD method is efficiently implemented and produces accurate results in its standard parametrization. For the BPTI protein, we demonstrate that the protein structure described with AMOEBA remains stable even on the extended time scales accessed at high levels of accelerations. For the DNA repair metalloenzyme endonuclease IV, we show that the use of the AMOEBA force field is a significant improvement over fixed charged models for describing the enzyme active-site. The new AMOEBA-aMD method is publicly available (http://wiki.simtk.org/openmm/VirtualRepository) and promises to be interesting for studying complex systems that can benefit from both the use of a polarizable force field and enhanced sampling.

Demonstration of improved sensitivity of echo interferometers to gravitational acceleration

We have developed two configurations of an echo interferometer that rely on standing-wave excitation of a laser-cooled sample of rubidium atoms. Both configurations can be used to measure acceleration $a$ along the axis of excitation. For a two-pulse configuration, the signal from the interferometer is modulated at the recoil frequency and exhibits a sinusoidal frequency chirp as a function of pulse spacing. In comparison, for a three-pulse stimulated-echo configuration, the signal is observed without recoil modulation and exhibits a modulation at a single frequency as a function of pulse spacing. The three-pulse configuration is less sensitive to effects of vibrations and magnetic field curvature, leading to a longer experimental time scale. For both configurations of the atom interferometer (AI), we show that a measurement of acceleration with a statistical precision of 0.5% can be realized by analyzing the shape of the echo envelope that has a temporal duration of a few microseconds. Using the two-pulse AI, we obtain measurements of acceleration that are statistically precise to 6 parts per million (ppm) on a 25 ms time scale. In comparison, using the three-pulse AI, we obtain measurements of acceleration that are statistically precise to 0.4 ppm on a time scale of 50 ms. A further statistical enhancement is achieved by analyzing the data across the echo envelope so that the statistical error is reduced to 75 parts per billion (ppb). The inhomogeneous field of a magnetized vacuum chamber limited the experimental time scale and resulted in prominent systematic effects. Extended time scales and improved signal-to-noise ratio observed in recent echo experiments using a nonmagnetic vacuum chamber suggest that echo techniques are suitable for a high-precision measurement of gravitational acceleration $g$. We discuss methods for reducing systematic effects and improving the signal-to-noise ratio. Simulations of both AI configurations with a time scale of 300 ms suggest that an optimized experiment with improved vibration isolation and atoms selected in the ${m}_{F}=0$ state can result in measurements of $g$ statistically precise to 0.3 ppb for the two-pulse AI and 0.6 ppb for the three-pulse AI.

Speed-invariant encoding of looming object distance requires power law spike rate adaptation

Neural representations of a moving object's distance and approach speed are essential for determining appropriate orienting responses, such as those observed in the localization behaviors of the weakly electric fish, Apteronotus leptorhynchus. We demonstrate that a power law form of spike rate adaptation transforms an electroreceptor afferent's response to "looming" object motion, effectively parsing information about distance and approach speed into distinct measures of the firing rate. Neurons with dynamics characterized by fixed time scales are shown to confound estimates of object distance and speed. Conversely, power law adaptation modifies an electroreceptor afferent's response according to the time scales present in the stimulus, generating a rate code for looming object distance that is invariant to speed and acceleration. Consequently, estimates of both object distance and approach speed can be uniquely determined from an electroreceptor afferent's firing rate, a multiplexed neural code operating over the extended time scales associated with behaviorally relevant stimuli.

Several scales of biodiversity affect ecosystem multifunctionality.

Society values landscapes that reliably provide many ecosystem functions. As the study of ecosystem functioning expands to include more locations, time spans, and functions, the functional importance of individual species is becoming more apparent. However, the functional importance of individual species does not necessarily translate to the functional importance of biodiversity measured in whole communities of interacting species. Furthermore, ecological diversity at scales larger than neighborhood species richness could also influence the provision of multiple functions over extended time scales. We created experimental landscapes based on whole communities from the world's longest running biodiversity-functioning field experiment to investigate how local species richness (α diversity), distinctness among communities (β diversity), and larger scale species richness (γ diversity) affected eight ecosystem functions over 10 y. Using both threshold-based and unique multifunctionality metrics, we found that α diversity had strong positive effects on most individual functions and multifunctionality, and that positive effects of β and γ diversity emerged only when multiple functions were considered simultaneously. Higher β diversity also reduced the variability in multifunctionality. Thus, in addition to conserving important species, maintaining ecosystem multifunctionality will require diverse landscape mosaics of diverse communities.

Dedicated Issue ‐ Adaptations to Changing Environments

How do insects adapt to varying environments? Obviously this is a quite simple question, but at the same time it is tough to answer. The appropriate response has to encompass daily, seasonal and more extended time scales as well as many research fields, from ecology, behaviour, morphometrics, biochemistry and physiology to molecular biology. In temperate zones, insects develop unique morphological or physiological characteristics seasonally to overcome harsh biotic and/or abiotic environmental changes. Some organisms have evolved sophisticated systems that allow them to anticipate forthcoming unfavourable seasons. Insects inhabiting Polar Regions have evolved unique mechanisms to withstand severe environmental challenges. These topics were discussed at two recent symposia - ‘Photoperiodic induction of diapause and seasonal morphs’ and ‘New complexities in the regulation of insect diapause and cold hardiness’ held during the XXIV International Congress of Entomology that convened in Daegu, South Korea, August 19–25, 2012. Many of the symposia participants kindly agreed to contribute papers to this Dedicated Issue of Physiological Entomology. We appreciate their efforts, timely submissions and hope that they enjoy the finished product. Thank you to all of the contributors. In the issue, two reviews focus on photoperiodism and the production of seasonal morphs in aphids and butterflies, and a third on surviving cold temperatures as part of the seasonal diapause syndrome, as well as on more intermittent, much shorter time scales. In addition, this issue includes original papers providing examples of recent advances and approaches to the understanding adaptations to environmental fluctuation in a range of insect species. Although the issue cannot be comprehensive for all aspects related to the topic, it covers a number of research themes, such as diapause, temperature tolerance, circadian rhythms, photoperiodism, polyphenism, seasonal morphs, as well as encompassing techniques from molecular biology to physiology. We believe this issue will introduce and enlighten readers on contemporary aspects of environmental adaptations in insects.

Macro-mechanical modeling of blast-wave mitigation in foams. Part II: reliability of pressure measurements

A phenomenological study of the process occurring when a plane shock wave reflected off an aqueous foam column filling the test section of a vertical shock tube has been undertaken. The experiments were conducted with initial shock wave Mach numbers in the range \(1.25\le {M}_\mathrm{s} \le 1.7\) and foam column heights in the range 100–450 mm. Miniature piezotrone circuit electronic pressure transducers were used to record the pressure histories upstream and alongside the foam column. The aim of these experiments was to find a simple way to eliminate a spatial averaging as an artifact of the pressure history recorded by the side-on transducer. For this purpose, we discuss first the common behaviors of the pressure traces in extended time scales. These observations evidently quantify the low frequency variations of the pressure field within the different flow domains of the shock tube. Thereafter, we focus on the fronts of the pressure signals, which, in turn, characterize the high-frequency response of the foam column to the shock wave impact. Since the front shape and the amplitude of the pressure signal most likely play a significant role in the foam destruction, phase changes and/or other physical factors, such as high capacity, viscosity, etc., the common practice of the data processing is revised and discussed in detail. Generally, side-on pressure measurements must be used with great caution when performed in wet aqueous foams, because the low sound speed is especially prone to this effect. Since the spatial averaged recorded pressure signals do not reproduce well the real behaviors of the pressure rise, the recorded shape of the shock wave front in the foam appears much thicker. It is also found that when a thin liquid film wet the sensing membrane, the transducer sensitivity was changed. As a result, the pressure recorded in the foam could exceed the real amplitude of the post-shock wave flow. A simple procedure, which allows correcting this imperfection, is discussed in detail.

Stable isotope series from elephant ivory reveal lifetime histories of a true dietary generalist

Longitudinal studies have revealed how variation in resource use within consumer populations can impact their dynamics and functional significance in communities. Here, we investigate multi-decadal diet variations within individuals of a keystone megaherbivore species, the African elephant (Loxodonta africana), using serial stable isotope analysis of tusks from the Kruger National Park, South Africa. These records, representing the longest continuous diet histories documented for any extant species, reveal extensive seasonal and annual variations in isotopic--and hence dietary--niches of individuals, but little variation between them. Lack of niche distinction across individuals contrasts several recent studies, which found relatively high levels of individual niche specialization in various taxa. Our result is consistent with theory that individual mammal herbivores are nutritionally constrained to maintain broad diet niches. Individual diet specialization would also be a costly strategy for large-bodied taxa foraging over wide areas in spatio-temporally heterogeneous environments. High levels of within-individual diet variability occurred within and across seasons, and persisted despite an overall increase in inferred C(4) grass consumption through the twentieth century. We suggest that switching between C(3) browsing and C(4) grazing over extended time scales facilitates elephant survival through environmental change, and could even allow recovery of overused resources.

Neural Mechanisms of Short-term Plasticity in the Human Visual System

Following circumscribed retinal damage, extensive reorganization of topographically organized visual cortical areas has been demonstrated in several species of mammals (including humans). Although reorganization is often studied over extended time scales, neural response properties change within seconds of retinal deafferentation. Understanding the mechanisms underlying these short-term effects is essential for developing a complete picture of representational plasticity. One approach to the study of short-term plasticity has been to use an artificial scotoma, a stimulus-induced analog of a retinal scotoma, as a model. Here, we use event-related potentials in an artificial scotoma paradigm to examine 2 aspects of short-term plasticity in the human visual system. First, we investigated the changes within visual representations temporarily deprived of patterned visual input by probing the inner boundaries of an artificial scotoma. We found an enhanced early sensory P1, consistent with a reduction in inhibition (disinhibition), a proposed mechanism of short-term visual plasticity. Second, we investigated mechanisms through which representations of surrounding space invade a visually deprived area by probing the outer boundaries of an artificial scotoma. In this case, a later visual component, the N1, was enhanced, suggesting that feedback may provide a source of unmasked, or invading, activity to visually deprived representations.

Hybrid deterministic and stochastic approach for efficient atomistic simulations at long time scales

We propose a hybrid deterministic and stochastic approach to achieve extended time scales in atomistic simulations that combines the strengths of molecular dynamics (MD) and Monte Carlo (MC) simulations in an easy-to-implement way. The method exploits the rare event nature of the dynamics similar to most current accelerated MD approaches but goes beyond them by providing, without any further computational overhead, (a) rapid thermalization between infrequent events, thereby minimizing spurious correlations, and (b) control over accuracy of time-scale correction, while still providing similar or higher boosts in computational efficiency. We present two applications of the method: (a) Vacancy-mediated diffusion in Fe yields correct diffusivities over a wide range of temperatures and (b) source-controlled plasticity and deformation behavior in Au nanopillars at realistic strain rates (${10}^{4}/$s and lower), with excellent agreement with previous theoretical predictions and in situ high-resolution transmission electron microscopy observations. The method gives several orders-of-magnitude improvements in computational efficiency relative to standard MD and good scalability with the size of the system.

Microfluidic-Based Trap for Single Cell Micromanipulation and Analysis

We present a microfluidic tool for free-solution confinement and analysis of single cells using a recently developed hydrodynamic trapping technique. The hydrodynamic trap is a microfluidic-based device that enables confinement and manipulation of single “target” cells in concentrated and complex biological samples. In this work, we apply the hydrodynamic trap to confine single Escherichia coli cells in free solution for extended time scales. Using optical microscopy, we observe single E. coli cells growing and dividing both at room temperature and at 370C by incorporating a heat exchanger into the microfluidic device. We characterize cell proliferation in a variety of media conditions to ensure viability of single cells for long time scales, which is achieved by the gentle and non-perturbative action of hydrodynamic trapping at a fluid stagnation point. Our initial studies demonstrate that the hydrodynamic trap is a useful non-perturbative method for single cell trapping and may be used to study gene expression dynamics in real-time with precise control of the surrounding medium. Cell confinement is achieved using a free-solution hydrodynamic trap, which is constructed using a cross-slot microfluidic device where two opposing streams converge at the junction of orthogonal channels, thereby generating a flow field with a stagnation point. Using automated control of the stagnation point position with an on-chip valve, we confined cells to within 1 µm of the trap center for analysis. Overall, we anticipate that the microfluidic-based hydrodynamic trap will provide an ideal platform to study cellular regulation and gene network dynamics in single living cells for extended time scales.

Hydrodynamic Trap for Single Cells and Micro- and Nanoparticles

Over the past twenty-five years, a diverse set of particle trapping and micromanipulation techniques have been developed to elucidate biological and biophysical mechanisms of proteins, nucleic acids, enzymes and cells. Many of the existing trapping methods rely on optical, magnetic or electric fields which are potentially perturbative to biological function. In this work, we present a novel flow-based confinement and manipulation method called the “hydrodynamic trap” which is based solely on hydrodynamic forces generated in a microfluidic device. The hydrodynamic trap is a non-contact confinement technique based on a stagnation point flow created at the junction of two perpendicular microchannels. In this way, the hydrodynamic trap enables free-solution trapping, manipulation, stretching and sorting of objects ranging from single molecules to individual cells. We successfully demonstrate trapping of single micro- and nanoscale particles (as small as 100 nm), single DNA molecules, and single cells for extended time scales with high resolution (within 1 μm for micron-sized particles) [1]. Trap stiffness was determined to be in the range κ=10−4 -10−3 pN/nm, which compares favorably to magnetic and electrophoretic tweezers. Hydrodynamic trapping is feasible for any particle with no specific requirements on the material composition or the chemical/physical nature (optical, magnetic, surface charge) of the trapped object. The hydrodynamic trap inherently enables confinement of a single target object in dilute or concentrated particle or cell suspensions, due to the semi-stable nature of trapping potential. In summary, the hydrodynamic trap provides a new platform for observation of molecules, cells and particles without surface immobilization and offers the ability to vary the surrounding medium conditions of the trapped object in real-time.[1] M. Tanyeri, E. M. Johnson-Chavarria, and C. M. Schroeder, “Hydrodynamic Trap for Single Particles and Cells” Applied Physics Letters, 2010, 96(22).

Acetate Availability and its Influence on Sustainable Bioremediation of Uranium-Contaminated Groundwater

Field biostimulation experiments at the U.S. Department of Energy's Integrated Field Research Challenge (IFRC) site in Rifle, Colorado, have demonstrated that uranium concentrations in groundwater can be decreased to levels below the U.S. Environmental Protection Agency's (EPA) drinking water standard (0.126 μM). During successive summer experiments – referred to as “Winchester” (2007) and “Big Rusty” (2008) - acetate was added to the aquifer to stimulate the activity of indigenous dissimilatory metal-reducing bacteria capable of reductively immobilizing uranium. The two experiments differed in the length of injection (31 vs. 110 days), the maximum concentration of acetate (5 vs. 30 mM), and the extent to which iron reduction (“Winchester”) or sulfate reduction (“Big Rusty”) was the predominant metabolic process. In both cases, rapid removal of U(VI) from groundwater occurred at calcium concentrations (6 mM) and carbonate alkalinities (8 meq/L) where Ca-UO2-CO3 ternary complexes constitute >90% of uranyl species in groundwater. Complete consumption of acetate and increased alkalinity (>30 meq/L) accompanying the onset of sulfate reduction corresponded to temporary increases in U(VI); however, by increasing acetate concentrations in excess of available sulfate (10 mM), low U(VI) concentrations (0.1–0.05 μM) were achieved for extended periods of time (>140 days). Uniform delivery of acetate during “Big Rusty” was impeded due to decreases in injection well permeability, likely resulting from biomass accumulation and carbonate and sulfide mineral precipitation. Such decreases were not observed during the short-duration “Winchester” experiment. Terminal restriction fragment length polymorphism (TRFLP) analysis of 16S rRNA genes demonstrated that Geobacter sp. and Geobacter-like strains dominated the groundwater community profile during iron reduction, with 13C stable isotope probing (SIP) results confirming these strains were actively utilizing acetate to replicate their genome during the period of optimal U(VI) removal. Gene transcript levels during “Big Rusty” were quantified for Geobacter-specific citrate synthase (gltA), with ongoing transcription during sulfate reduction indicating that members of the Geobacteraceae were still active and likely contributing to U(VI) removal. The persistence of reducible Fe(III) in sediments recovered from an area of prolonged (110-day) sulfate reduction is consistent with this conclusion. These results indicate that acetate availability and its ability to sustain the activity of iron- and uranyl-respiring Geobacter strains during sulfate reduction exerts a primary control on optimized U(VI) removal from groundwater at the Rifle IFRC site over extended time scales (>50 days).

Studies of the growth, evolution, and self‐aggregation of β‐amyloid fibrils using tapping‐mode atomic force microscopy

Amyloid peptide (Aβ) is the major protein component of plaques found in Alzheimer's disease, and the aggregation of Aβ into oligomeric and fibrillic assemblies has been shown to be an early event of the disease pathway. Visualization of the progressive evolution of nanoscale changes in the morphology of Aβ oligomeric assemblies and amyloid fibrils has been accomplished ex situ using atomic force microscopy (AFM) in ambient conditions. In this report, the size and the shape of amyloid β(1-40) fibrils, as well as the secondary organization into aggregate structures were monitored at different intervals over a period of 5 months. Characterizations with tapping-mode AFM serve to minimize the strong adhesive forces between the probe and the sample to prevent damage or displacement of fragile fibrils. The early stages of Aβ growth showed a predominance of spherical seed structures, oligomeric assemblies, and protofibrils; however the size and density of fibrils progressively increased with time. Within a few days of incubation, linear assemblies and fibrils became apparent. Over extended time scales of up to 5 months, the fibrils formed dense ensembles spanning lengths of several microns, which exhibit interesting changes due to self-organization of the fibrils into bundles or tangles. Detailed characterization of the Aβ assembly process at the nanoscale will help elucidate the role of Aβ in the pathology of Alzheimer's disease.

Self-consistent modelling of Mercury’s exosphere by sputtering, micro-meteorite impact and photon-stimulated desorption

A Monte-Carlo model of exospheres ( Wurz and Lammer, 2003) was extended by treating the ion-induced sputtering process, photon-stimulated desorption, and micro-meteorite impact vaporisation quantitatively in a self-consistent way starting with the actual release of particles from the mineral surface of Mercury. Based on available literature data we established a global model for the surface mineralogy of Mercury and from that derived the average elemental composition of the surface. This model serves as a tool to estimate densities of species in the exosphere depending on the release mechanism and the associated physical parameters quantitatively describing the particle release from the surface. Our calculation shows that the total contribution to the exospheric density at the Hermean surface by solar wind sputtering is about 4×10 7 m –3, which is much less than the experimental upper limit of the exospheric density of 10 12 m –3. The total calculated exospheric density from micro-meteorite impact vaporisation is about 1.6×10 8 m –3, also much less than the observed value. We conclude that solar wind sputtering and micro-meteorite impact vaporisation contribute only a small fraction of Mercury’s exosphere, at least close to the surface. Because of the considerably larger scale height of atoms released via sputtering into the exosphere, sputtered atoms start to dominate the exosphere at altitudes exceeding around 1000 km, with the exception of some light and abundant species released thermally, e.g. H 2 and He. Because of Mercury’s strong gravitational field not all particles released by sputtering and micro-meteorite impact escape. Over extended time scales this will lead to an alteration of the surface composition.

Separating stress and time dependent effects in the temperature dependent creep of polyethylene nanocomposites

AbstractThe nonlinear time dependent creep of linear‐low density polyethylene (LLDPE) reinforced with montmorillonite layered silicate was investigated. A previous study related the time/stress dependence of creep compliance of the material at room temperature using the Burger and Kohlrausch‐Williams‐Watts models. Using both the creep and recovery compliance curves, we employ the Schapery formulation to study the relationship between deformation, time, stress, and temperature of LLDPE nanocomposites. Smooth mastercurves are constructed using time–temperature–stress superposition principles. The stress and temperature‐related creep constants and shift factors were determined for the material using the Schapery nonlinear viscoelastic equation. The prediction results confirm the enhanced creep resistance of nanofillers even at extended time scales and low temperatures. POLYM. ENG. SCI., 50:1646–1657, 2010. © 2010 Society of Plastics Engineers

Durable Glass for Thousands of Years

The durability of natural glasses on geological time scales and ancient glasses for thousands of years is well documented. The necessity to predict the durability of high‐level nuclear waste (HLW) glasses on extended time scales has led to various thermodynamic and kinetic approaches. Advances in the measurement of medium‐range order in glasses has led to the understanding that the molecular structure of a glass, and thus the glass composition, controls the glass durability by establishing the distribution of ion exchange sites, hydrolysis sites, and the access of water to those sites. During the early stages of glass dissolution, a “gel” layer resembling a membrane forms through which ions exchange between the glass and the leachant. The hydrated gel layer exhibits acid/base properties, which are manifested as the pH dependence of the thickness and nature of the gel layer. The gel layer ages into clay or zeolite minerals by Ostwald ripening. Zeolite mineral assemblages (higher pH and Al3+rich glasses) may cause the dissolution rate to increase, which is undesirable for long‐term performance of glass in the environment. Thermodynamic and structural approaches to the prediction of glass durability are compared versus Ostwald ripening.