Peroxide-based crosslinking of solid silicone rubber, part II: The counter-intuitive influence of dicumylperoxide concentration on crosslink effectiveness and related network structure

Application of elastomers in general demands the conversion of their soluble networks into crosslinked structures. This abrupt change causes several modifications, both in the atomic/molecular level and at the macro-scale. In this study, solid silicone rubber (high molecular weight poly(dimethylsiloxane)), was crosslinked with dicumylperoxide (DCP), a widely used crosslinking agent by the rubber industry. The changes caused by different DCP concentrations were investigated, aiming to bring attention to the molecular transformations, usually neglected when processing-oriented studies are conducted. DCP concentration showed a limited contribution to the network's molecular dynamics, which was found to be mainly dominated by entanglements. The dominance of entanglements over other molecular constraints, like crosslink points, justifies the threshold and counter-intuitive behavior of tensile and hardness properties. However, differences were found in the crystallization ability after crosslinking, when the more crosslink points were introduced, the lower the crystallinity was and the less stable the PDMS crystallites were. In addition to providing a deeper understanding of an industrially applied rubber system n terms of the effective concentration of DCP, and the reasoning behind such concentration, the findings of this study add to the state-of-the-art comprehension of elastomeric networks, and how they behave on a molecular level

Higher dextrose delivery via TPN related to the development of hyperglycemia in non-diabetic critically ill patients

The beneficial effects of total parenteral nutrition (TPN) in improving the nutritional status of malnourished patients during hospital stays have been well established. However, recent randomized trials and meta-analyses have reported an increased rate of TPN-associated complications and mortality in critically ill patients. The increased risk of complications during TPN therapy has been linked to the development of hyperglycemia, especially during the first few days of TPN therapy. This retrospective study was conducted to determine whether the amount of dextrose from TPN in the 1st week in the intensive care unit (ICU) was related to the development of hyperglycemia and the clinical outcome. We included 88 non-diabetic critically ill patients who stayed in the medical ICU for more than two days. The subjects were 65 ± 16 years old, and the mean APACHE (Acute Physiology and Chronic Health Evaluation) II score upon admission was 20.9 ± 7.1. The subjects received 2.3 ± 1.4 g/kg/day of dextrose intravenously. We divided the subjects into two groups according to the mean blood glucose (BG) level during the 1st week of ICU stay: < 140 mg/dl vs ≥ 140 mg/dl. Baseline BG and the amount of dextrose delivered via TPN were significantly higher in the hyperglycemia group than those in the normoglycemia group. Mortality was higher in the hyperglycemia group than in the normoglycemia group (42.4% vs 12.8%, P = 0.008). The amount of dextrose from TPN was the only significant variable in the multiple linear regression analysis, which included age, APACHE II score, baseline blood glucose concentration and dextrose delivery via TPN as independent variables. We concluded that the amount of dextrose delivered via TPN might be associated with the development of hyperglycemia in critically ill patients without a history of diabetes mellitus. The amount of dextrose in TPN should be decided and adapted carefully to maintain blood glucose within the target range

Part-time general surgical training in South Australia: its success and future implications (or: pinnacles, pitfalls and lessons for the future)

BACKGROUND Flexible training options are sought by an increasing number of Australasian surgical trainees. Reasons include increased participation of women in the surgical workforce, postgraduate training and changing attitudes to family responsibilities. Despite endorsement of flexible training by the Royal Australasian College of Surgeons and Board in General Surgery, part-time (PT) training in General Surgery in Australia and New Zealand is not well established. A permanent ‘stand-alone’ PT training position was established at the Royal Adelaide Hospital in 2007 under the Surgical Education and Training Program. This position offered 12 months of General Surgical training on a 0.5 full-time (FT) equivalent basis with pro rata emergency and on-call commitments and was accredited for 6 months of General Surgical training. This paper reviews the PT training experience in South Australia. METHODS De-identified logbook data were obtained from the South Australian Regional Subcommittee of the Board in General Surgery with consent of each of the trainees. Totals of operative cases were compared against matched FT trainees working on the same unit. RESULTS Overall, PT trainees achieved comparable operative caseloads compared with their FT colleagues. All trainees included in this review have subsequently passed the Royal Australasian College of Surgeons Fellowship Examination in General Surgery and returned to FT workforce positions. CONCLUSION This paper presents two validated models of PT training. Training, resource and regulatory requirements and individual and institutional barriers to flexible training are substantial. Successful PT models offer positive and beneficial training alternatives for General Surgical trainees and contribute to workforce flexibility.Susan Neuhaus, Emma Igras, Beverley Fosh and Sarah Benso

Initial Solvent-Driven Nonequilibrium Effect on Structure, Properties, and Dynamics of Polymer Nanocomposites

Unusual structures and dynamic properties found in polymer nanocomposites (PNCs) are often attributed to immobilized (adsorbed) polymers at nanoparticle/polymer interfaces, which are responsible for reducing the intrinsic incompatibility between nanoparticles and polymers in PNCs. Although tremendous efforts have been made to characterize the presence of immobilized polymers, systematic understanding of the structure and dynamics under different processing conditions is still lacking. Here, we report that the initial dispersing solvent, which is not present after producing PNCs, drives these non-equilibrium effects on polymer chain dynamics at interfaces. Employing extensive small angle scattering, proton NMR spectroscopy, and rheometry experiments, we found that the thickness of immobilized layer can be dependent on initial solvent, changing the structure and the properties of the PNC significantly. In addition, we show that the outcome of the initial solvent effect becomes more effective at particle volume fractions where the immobile layer begins to interact. The incorporation of nanoparticles into a polymer matrix, thus creating polymer nanocomposites (PNCs), is regarded as a general strategy to enhance the physical properties of neat polymers ???[1-4]. However, the intrinsic incompatibility between nanoparticles and polymers requires the effective control of polymer???nanoparticle interactions at the interface. Polymers can be chemically grafted or physically adsorbed onto the particle surface, creating an immobilized layer, which is believed to control the resulting structures and properties of PNCs ???[5-9]. Many attempts have been made, therefore, to develop a stable immobilized layer by changing the chemical structure of particles/polymers, and to characterize governing parameters such as grafting/adsorption density, the sizes of the polymers/particles, and their compositions ???[10-19]. Few studies, however, have reported on non-equilibrium effects present during the processing of PNCs ???[4,20-24]. While PNC production involves complicated yet dynamic processes such as initial dispersion in solvents, mixing with polymers, solvent evaporation, and drying, the relaxation time of polymers in the presence of nanoparticles may significantly increase, suggesting that the polymers and particles may not reach their equilibrium structures in experimentally accessible processing times, becoming kinetically trapped ???[25,26]. In this letter, we report that when the initial dispersing solvent is varied, PNCs may not reach their equilibrium state, resulting in a dramatic change in particle dispersion, polymer dynamics, and rheological properties. We composed PNCs with poly(ethylene glycol) (PEG) and silica nanoparticles using either ethanol or water as casting solvents. Employing extensive small angle X-ray scattering (SAXS), NMR free induction decay (FID), double-quantum (DQ), and rheometry experiments, we found that the initial solvent influences (i) the initial/final particle microstructure, (ii) the dynamics of the immobilized layers, and (iii) the resulting physical properties of the PNCs, even though the solvent was thoroughly evaporated and thus not present in the final state of the PNCs

Intermediate motions as studied by solid-state separated local field NMR experiments

In this report, the application of a class of separated local field NMR experiments named dipolar chemical shift correlation (DIPSHIFT) for probing motions in the intermediate regime is discussed. Simple analytical procedures based on the Anderson-Weiss (AW) approximation are presented. In order to establish limits of validity of the AW based formulas, a comparison with spin dynamics simulations based on the solution of the stochastic Liouville-von-Neumann equation is presented. It is shown that at short evolution times (less than 30% of the rotor period), the AW based formulas are suitable for fitting the DIPSHIFT curves and extracting kinetic parameters even in the case of jumplike motions. However, full spin dynamics simulations provide a more reliable treatment and extend the frequency range of the molecular motions accessible by DIPSHIFT experiments. As an experimental test, molecular jumps of imidazol methyl sulfonate and trimethylsulfoxonium iodide, as well as the side-chain motions in the photoluminescent polymer poly[2-methoxy-5-(2(')-ethylhexyloxy)-1,4-phenylenevinylene], were characterized. Possible extensions are also discussed. (c) 2008 American Institute of Physics

NMR spectroscopy of polymer nanocomposites

Nuclear magnetic resonance (NMR) spectroscopy is a technique of utmost significance when dealing with the characterization of the polymers and their nanocomposites. This chapter focuses on the influence of this particular technique in analyzing the filler dispersion, functionalization, interfacial interactions, etc. existing in nanocomposites. In light of the very recent discoveries to utilize one- and two-dimensional solid-state NMR methods in structural biology (to investigate the structure and function of nucleic acids or proteins) and the pharmaceutical industry (to identify new drugs), its resolution and selectivity on the structure and dynamics of a wide range of polymers and nanocomposites are discussed. All kinds of NMR techniques allow semiquantitative monitoring of changes in segmental dynamics near the polymer-filler interface and bonding within the polymer chain and are discussed in this chapter.Scopu

A Methodology for Using Workforce Data to Decide Which Specialties and States to Target for Graduate Medical Education Expansion

OBJECTIVE: To outline a methodology for allocating graduate medical education (GME) training positions based on data from a workforce projection model. DATA SOURCES: Demand for visits is derived from the Medical Expenditure Panel Survey and Census data. Physician supply, retirements, and geographic mobility are estimated using concatenated AMA Masterfiles and ABMS certification data. The number and specialization behaviors of residents are derived from the AAMC's GMETrack survey. DESIGN: We show how the methodology could be used to allocate 3,000 new GME slots over 5 years—15,000 total positions—by state and specialty to address workforce shortages in 2026. EXTRACTION METHODS: We use the model to identify shortages for 19 types of health care services provided by 35 specialties in 50 states. PRINCIPAL FINDINGS: The new GME slots are allocated to nearly all specialties, but nine states and the District of Columbia do not receive any new positions. CONCLUSIONS: This analysis illustrates an objective, evidence‐based methodology for allocating GME positions that could be used as the starting point for discussions about GME expansion or redistribution