Structure and stability of helices in square-well homopolymers

Recently, it has been demonstrated [Magee et al., Phys. Rev. Lett. 96, 207802 (2006)] that isolated, square-well homopolymers can spontaneously break chiral symmetry and freeze into helical structures at sufficiently low temperatures. This behavior is interesting because the square-well homopolymer is itself achiral. In this work, we use event-driven molecular dynamics, combined with an optimized parallel tempering scheme, to study this polymer model over a wide range of parameters. We examine the conditions where the helix structure is stable and determine how the interaction parameters of the polymer govern the details of the helix structure. The width of the square well (proportional to lambda) is found to control the radius of the helix, which decreases with increasing well width until the polymer forms a coiled sphere for sufficiently large wells. The helices are found to be stable for only a window of molecular weights. If the polymer is too short, the helix will not form. If the polymer is too long, the helix is no longer the minimum energy structure, and other folded structures will form. The size of this window is governed by the chain stiffness, which in this model is a function of the ratio of the monomer size to the bond length. Outside this window, the polymer still freezes into a locked structure at low temperature, however, unless the chain is sufficiently stiff, this structure will not be unique and is similar to a glassy state.Comment: Submitted to Physical Review

What's in a WordTM? When one electronic document format standard is not enough

Purpose - The purpose of this paper is to examine the history of the standardisation of two largely overlapping electronic document formats between 2005 and 2008, and its implications for future IT standards development. Design/methodology/approach - The document format controversy is researched as an exemplary case study of the institutional rivalries, perspectives and strategic interests at play in standardisation processes. The study adopts a methodological lens of discursive institutionalism in order to explain how actors assume and perform a variety of roles during the controversy. It consults a range of documentary sources, including media commentary, corporate press releases and blog posts, financial reports and technical specifications. Findings - The study shows that: first, intentions to increase competition in the office software market through the standardisation of document formats led to a standards "arms race"; second, this further entrenched the position of a single market actor; and third, the resulting public debate nevertheless has reinvigorated the push for genuinely open standards. Social implications - Information technology standards are often touted as mechanisms for increasing the competitiveness of a market, thereby benefitting consumers and the greater public. In the presence of dominant institutional actors, efforts to standardise can, perversely, undermine this benefit. Increased public scrutiny through online media offers a potential remedy. Originality/value - This research presents a novel account of the controversy over the document format standardisation process, understood through the lens of discursive institutionalism. It also shows the increasing and potentially putative role of online media in the development of ITstandards generally

How much do disasters cost? A comparison of disaster cost estimates in Australia

Extreme weather events in Australia are common and a large proportion of the population are exposed to such events. Therefore, there is great interest as to how these events impact Australia's society and economy, which requires understanding the current and historical impact of disasters. Despite global efforts to record and cost disaster impacts, no standardised method of collecting and recording data retrospectively yet exists. The lack of standardisation in turn results in a range of different estimates of economic impacts. This paper examines five examples of aggregate disaster loss and impacts of natural disasters in Australia, and comparisons between them reveal significant data shortcomings. The reliability of data sources, and the methodology employed to analyse them can have significant impacts on conclusions regarding the overall cost of disasters, the relative costs of different hazards (disaster types), and the distribution of losses across Australian states. We highlight difficulties with time series comparisons, further complicated by the interdependencies of the databases. We reiterate the need for consistent and comparable data collection and analysis, to respond to the increasing frequency and severity of disasters in Australia

Teaching an Engineering Systems Doctoral Seminar: Concepts and Structure

This paper describes the current state of the evolving engineering systems doctoral seminar in the Engineering Systems Division (ESD) at MIT. This subject is required for all first year engineering systems doctoral students. It is intended to bring them into the culture of ESD, and to establish a common base on which subsequent subjects can build. It is the first of three required subjects that make up the core of the ESD doctoral program. The seminar is intended to provide students with the foundations and context of engineering systems, largely focused on providing an appreciation for the many facets of socio-technical complexity. We discuss the seminar’s pedagogy, learning objectives, assignments and readings, and provide insights gained from teaching the course

Death Bed of Abraham Lincoln. Died April 15th 1865.

Lithograph depicting Abraham Lincoln on his deathbed surrounded by his family,cabinet members, and others. The image is surrounded by a black memorial drape.https://scholarsjunction.msstate.edu/fvw-prints/1204/thumbnail.jp

Death Bed of Abraham Lincoln. Died April 15th 1865.

Lithograph depicting Abraham Lincoln on his deathbed surrounded by his family,cabinet members, and others. The image is surrounded by a black memorial drape.https://scholarsjunction.msstate.edu/fvw-artifacts/1362/thumbnail.jp

The Volatile Composition of the Split Ecliptic comet 73P/Schwassmann-Wachmann 3: A Comparison of Fragments C and B

The composition of fragments C and B of the Jupiter-family comet 73P/Schwassmann-Wachmann 3 (SW3) was investigated in early April of 2006 at IR wavelengths using high-dispersion echelle spectroscopy. Both fragments were depleted in ethane, and C was depleted in most forms of volatile carbon. In particular, fragment C shows a severe depletion of CH_(3)OH but a "normal" abundance of HCN (which has a similar volatility). Thermal processing is a possible explanation, but since fragment B is perhaps sublimating fresher material because of the frequent outbursts and fragmentation, the observed depletions might have cosmogonic implications. The chemistry of the volatile ices in SW3, like in the Oort Cloud comet C/1999 S4 (LINEAR), may be associated with sublimation of icy mantles from precometary grains followed by subsequent gas-phase chemistry and recondensation

The Lennard-Jones-Devonshire cell model revisited

We reanalyse the cell theory of Lennard-Jones and Devonshire and find that in addition to the critical point originally reported for the 12-6 potential (and widely quoted in standard textbooks), the model exhibits a further critical point. We show that the latter is actually a more appropriate candidate for liquid-gas criticality than the original critical point.Comment: 5 pages, 3 figures, submitted to Mol. Phy

Detecting human impacts on the flora, fauna, and summer monsoon of Pleistocene Australia

International audienceAll of Australia's largest mammalian vertebrates became extinct 50 to 45 ka (thousand years ago), shortly after human colonization. Between 60 and 40 ka Australian climate was similar to present and not changing rapidly. Consequently, attention has turned toward plausible human mechanisms for the extinction, with proponents for over-hunting, ecosystem change, and introduced disease. To differentiate between these options we utilize isotopic tracers of diet preserved in eggshells of two large, flightless birds to track the status of ecosystems before and after human colonization. ?13C preserved in their eggshells monitor a bird's dietary intake in the weeks to months before egg-laying. More than 500 dated eggshells from central Australia of the Australian emu (Dromaius novaehollandiae), an opportunistic, dominantly herbivorous feeder, provide a continuous 140 kyr dietary ? 13C reconstruction. More than 350 dated eggshells from the same region of the heavier, extinct, giant bird Genyornis newtoni define its dietary intake from 140 ka until its extinction about 50 ka. Additional dietary records for both species were developed from two distant regions. Dromaius eggshell dietary ?13C reveals an unprecedented reduction in the bird's food resources about 50 ka, coeval in all three regions, suggesting conversion at that time of a tree/shrub savannah with occasionally rich grasslands to the modern desert scrub. We speculate that ecosystem collapse across the arid and semi-arid zones is a consequence of systematic burning by early humans. Genyornis diet everywhere is more restricted than in co-existing Dromaius, implying a more specialized feeding strategy. These data suggest that generalist feeders, such as Dromaius, were able to adapt to a changed vegetation regime, whereas more specialized feeders, such as Genyornis, became extinct. The altered vegetation may have also impacted Australian climate. Changes in the strength of climate feedbacks linked to vegetation and soil type (moisture recycling, surface roughness, albedo) may have weakened the penetration of monsoon moisture into the continental interior under the new ecosystem. Climate modeling suggests such a shift may have reduced monsoon rain in the interior by as much as 50%