Mechanics of elastomeric molecular composites.

A classic paradigm of soft and extensible polymer materials is the difficulty of combining reversible elasticity with high fracture toughness, in particular for moduli above 1 MPa. Our recent discovery of multiple network acrylic elastomers opened a pathway to obtain precisely such a combination. We show here that they can be seen as true molecular composites with a well-cross-linked network acting as a percolating filler embedded in an extensible matrix, so that the stress-strain curves of a family of molecular composite materials made with different volume fractions of the same cross-linked network can be renormalized into a master curve. For low volume fractions (<3%) of cross-linked network, we demonstrate with mechanoluminescence experiments that the elastomer undergoes a strong localized softening due to scission of covalent bonds followed by a stable necking process, a phenomenon never observed before in elastomers. The quantification of the emitted luminescence shows that the damage in the material occurs in two steps, with a first step where random bond breakage occurs in the material accompanied by a moderate level of dissipated energy and a second step where a moderate level of more localized bond scission leads to a much larger level of dissipated energy. This combined use of mechanical macroscopic testing and molecular bond scission data provides unprecedented insight on how tough soft materials can damage and fail.

Arctic marine phytobenthos of northern Baffin Island.

Global climate change is expected to alter the polar bioregions faster than any other marine environment. This study assesses the biodiversity of seaweeds and associated eukaryotic pathogens of an established study site in northern Baffin Island (72° N), providing a baseline inventory for future work assessing impacts of the currently ongoing changes in the Arctic marine environment. A total of 33 Phaeophyceae, 24 Rhodophyceae, 2 Chlorophyceae, 12 Ulvophyceae, 1 Trebouxiophyceae, and 1 Dinophyceae are reported, based on collections of an expedition to the area in 2009, complemented by unpublished records of Robert T. Wilce and the first-ever photographic documentation of the phytobenthos of the American Arctic. Molecular barcoding of isolates raised from incubated substratum samples revealed the presence of 20 species of brown seaweeds, including gametophytes of kelp and of a previously unsequenced Desmarestia closely related to D. viridis, two species of Pylaiella, the kelp endophyte Laminariocolax aecidioides and 11 previously unsequenced species of the Ectocarpales, highlighting the necessity to include molecular techniques for fully unraveling cryptic algal diversity. This study also includes the first records of Eurychasma dicksonii, a eukaryotic pathogen affecting seaweeds, from the American Arctic. Overall, this study provides both the most accurate inventory of seaweed diversity of the northern Baffin Island region to date and can be used as an important basis to understand diversity changes with climate change.

Interactions affecting the mechanical properties of macromolecular microsphere composite hydrogels.

Macromolecular microsphere composite (MMC) hydrogel is a kind of tough hydrogel fabricated by using peroxidized macromolecular microspheres as polyfunctional initiating and cross-linking centers (PFICC). The contribution of chemical cross-linking (covalent bonding) and physical cross-linking (chain entanglement and hydrogen bonding) to the mechanical properties are understood by testing the hydrogels, which were swollen in water or aqueous urea solutions to different water contents. The as-prepared MMC gels exhibited moderate moduli (60-270 kPa), high fracture tensile stresses (up to 0.54 MPa), high extensibilities (up to 2500%), and high fracture energies (270-770 J m(-2)). The moduli of the swollen gels decrease dramatically, but there are no significant changes in fracture tensile strength and fracture strain, even slight increases. More interestingly, the swollen gels show much-enhanced fracture energies, higher than 2000 J m(-2). A gradual decrease in the hysteresis ratio and residual strain is also found in the cyclic tensile testing of the hydrogels that were swollen to different water contents. The covalent bonding determines the tensile strength and fracture energy of the MMC gels, whereas the physical entanglement and hydrogen bonding among the polymer chains contributes mainly to the modulus of the MMC gels, and they are also the main reason for the presence of hysteresis in the loading-unloading cycles.

Managing scientific diving operations in a remote location: the Canadian high Arctic.

Global climate change is expected to alter the Arctic bioregion markedly in coming decades. As a result, monitoring of the expected and actual changes has assumed high scientific significance. Many marine science objectives are best supported with the use of scientific diving techniques. Some important keystone environments are located in extremely remote locations where land-based expeditions offer high flexibility and cost-effectiveness over ship-based operations. However, the extreme remoteness of some of these locations, coupled with complex and unreliable land, sea and air communications, means that there is rarely quick access (< 48 h) to any specialized diving medical intervention or recompression. In 2009, a land based expedition to the north end of Baffin Island was undertaken with the specific aim of establishing an inventory of the diversity of seaweeds and their pathogens that was broadly representative of a high Arctic marine environment. This account highlights some of the logistical considerations taken on that expedition; specifically it outlines the non-recompression treatment pathway that would have been adopted in the event of a diver suffering decompression illness.

Polymer entanglement density and its influence on interfacial friction.

The entanglement density of amorphous glassy polymers is well known to control their fracture mechanisms under tensile loading. There have been some reports indicating substantial deformation of a glassy polymer's surface region when exposed to interfacial friction. It is shown conclusively here that there is a direct correlation between the entanglement density of a glassy polymer and the deformation mechanisms that facilitate sliding friction. This correlation was shown experimentally by studying the topography of polymer surfaces following a single sliding pass by an inorganic glass sphere. Four different polymers were studied, including polystyrene cross linked to different degrees. It is also shown that permanent plastic deformation accompanies interfacial friction, and, furthermore, that the type of deformation is a direct function of the respective polymer's entanglement density. In contrast, no difference in the observed friction force could be attributed to the entanglement density of the respective polymers. The findings can be explained by the state- and rate-dependent friction model.

Chain entanglement in thin freestanding polymer films.

When a thin glassy film is strained uniaxially, a shear deformation zone (SDZ) can be observed. The ratio of the thickness of the SDZ to that of the undeformed film is related to the maximum extension ratio, lambda, which depends on the entanglement molecular weight, M(e). We have measured lambda as a function of film thickness in strained freestanding films of polystyrene as a probe of M(e) in confinement. It is found that thin films stretch further than thick films before failure, consistent with the interpretation that polymers in thin films are less entangled than bulk polymers, thus the effective value of M(e) in thin films is significantly larger than that of the bulk. Our results are well described by a conceptually simple model based on the probability of finding intermolecular entanglements near an interface.

Effects of contact time and polarity level on adhesion between interacting surfaces.

We have measured the rate at which adhesion develops between two surfaces that interact by hydrogen bonding. A poly(dimethylsiloxane) elastomer lens with a slightly oxidized surface was pushed against a polystyrene-based copolymer substrate that contained acid groups. The interaction was measured on both forming and breaking the contact using the JKR technique. The toughness of the joint, Gc, increased considerably with increasing acid content in the substrate while the apparent work of adhesion, W, measured while making the contact, decreased with increasing acid content. This decrease in W implies that the acid groups caused repulsion between the surfaces when they were not in contact, but the increase in Gc shows that they formed bonds after contact was made. The rate of increase of Gc with time was found to depend on the acid content in the substrate, but for moderate acid levels, Gc was found to saturate at values that varied approximately linearly with acid content. For 8% acid and 47% acid substrates, the rate of adhesion development over a period of 24 h could be fitted by a model assuming (i) the toughness increased linearly with areal density of bonds between the substrate and the elastomer and (ii) the rate of reaction between the substrate and the elastomer varied as the cube of the density of unreacted acid groups. This cube law may possibly be explained by the kinetics of motion of the elastomer chains on the substrate.

Selective anterior fusion of thoracolumbar/lumbar curves in adolescents: when can the associated thoracic curve be left unfused?

STUDY DESIGN: A retrospective multicenter study was conducted to investigate patients with a major thoracolumbar/lumbar adolescent idiopathic scoliosis and an associated minor thoracic curve treated with an anterior instrumentation and fusion of the lower curve. OBJECTIVE: To establish criteria for determining when such curves can be successfully treated by an anterior only procedure of the lower curve with acceptable spinal balance and residual thoracic curve. SUMMARY OF BACKGROUND DATA: Anterior spinal instrumentation techniques have been proved effective for the management of isolated thoracolumbar/lumbar scoliosis with small compensatory thoracic curves. The success of a selective anterior fusion when the associated thoracic curve had some structural changes in a small preliminary study was variable and was the stimulus for this study. METHODS: A multicenter study involved 49 adolescent patients with a major thoracolumbar/lumbar curve in which the associated minor thoracic curve measured between 30 degrees and 55 degrees. In all the patients, the apical vertebra of the lower curve lay outside the midsacral line, and the thoracic apical vertebra fell outside a line dropped from the center of C7. Multiple radiographic parameters were evaluated. The Risser sign, height, weight, onset of menses, and closure of the triradiate cartilages were studied to access the patients' maturity. All the patients were observed at least 2 years. Patients were considered to have a satisfactory result if the thoracic curve at the final follow-up assessment measured 40 degrees or less, if balance and sagittal alignment were reasonable, and if additional procedures were not required. RESULTS: At final follow-up assessment, two groups emerged. Group 1 (n = 43) had satisfactory results. The preoperative thoracic curve in this group averaged 40 degrees and 26 degrees after surgery. The lumbar curve averaged 56 degrees before surgery and 22 degrees after surgery. Group 2 (n = 6) had unsatisfactory results. The average thoracic curve was 49 degrees before surgery 54 degrees after surgery, whereas the lumbar curve averaged 59 degrees before surgery and 27 degrees after surgery. Three of these patients underwent posterior thoracic instrumentation and fusion. CONCLUSIONS: Statistical analysis showed that a successful surgical outcome was dependent on both the structural changes in the thoracic curve and the patient's maturity. The thoracolumbar/lumbar-thoracic (TL/L:T) Cobb ratio in combination with the degree of the thoracic curve on lateral bending was the best predictor among the structural indexes. Of 44 patients with a TL/L:T Cobb ratio of 1.25 or greater and/or a thoracic curve, which bent out to 20 degrees or less, 42 had a satisfactory result. The best predictor among the maturity indexes was closure of the triradiate cartilages. Of 43 patients in whom the triradiate cartilages were closed, 42 had satisfactory results. When this data is combined, the outcome for the thoracic curve can be reasonably predicted.

Stable sulfur isotope analysis of SO2 pollution impact on vegetation.

The δ34S value of SO2 emitted by natural gas refineries is about +25, which is higher than that for non-industrial sulfur sources in our study areas. Terrestrial mosses absorb SO2 from the atmosphere and have a δ34S value which is directly related to the degree of SO2 stress to which they are subjected. The δ34S values for conifer needles are lower than for mosses at the same collection site, which indicates that trees obtain sulfur from both atmospheric and soil sources.Potted conifers were transferred to sites differing in their degree of SO2 stress. This difference is reflected by the change of δ34S values of their needles. SO2 absorbant pot covers, such as charcoal and moss, reduce the amount of airborne sulfur which is available to tress. Moss also may reduce SO2 absorbed by soils in forest stands. We have used analysis of δ34S values to (1) help define SO2 dispersion patterns; (2) reveal the rates at which plants accumulate this pollutant; and (3) associate suspected SO2 injury more closely to an emission source.