We investigate the impact of the non-uniform spatio-temporal conversion, intrinsic to photopolymerisation, in the context of light-driven 3D printing of polymers. The polymerisation kinetics of a ...series of model acrylate and thiol-ene systems, both neat and doped with a light-absorbing dye, is investigated experimentally and analysed according to a descriptive coarse-grained model for photopolymerisation. In particular, we focus on the relative kinetics of polymerisation with those of 3D printing, by comparing the evolution of the position of the conversion profile (
) to the sequential displacement of the object stage (
). After quantifying the characteristic sigmoidal monomer-to-polymer conversion of the various systems, with a combination of patterning experiments, FT-IR mapping, and modelling, we compute representative regimes for which
is smaller, commensurate with, or larger than
. While non-monotonic conversion can be detrimental to 3D printing, for instance in causing differential shrinkage of inhomogeneity in material properties, we identify opportunities for facile fabrication of modulated materials in the
-direction (i.e., along the illuminated axis). Our simple framework and model, based on directly measured parameters, can thus be employed in photopolymerisation-based 3D printing, both in process optimisation and in the precise design of complex, internally stratified materials by coupling the
-stage displacement and frontal polymerisation kinetics.
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Burnout is a psychological syndrome that is very common among medical residents. It consists of emotional exhaustion (EE), depersonalization (DP) and reduced personal accomplishment (PA).
To estimate ...burnout among different medical residency specialties.
A systematic review with meta-analysis was performed following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. A search of bibliographic databases and grey literature was conducted, from inception to March 2018. The following databases were accessed: Embase, PubMed, Web of Science, Google Scholar and Scopus, and 3,575 studies were found. Methodological quality was evaluated by Agency for Healthcare Research and Quality Methodology Checklist for Cross-Sectional/Prevalence Study. In the final analysis, 26 papers were included. Their references were checked for additional studies, but none were included.
4,664 medical residents were included. High DP, EE and low PA proportions were compared. Specialties were distributed into three groups of different levels of burnout prevalence: general surgery, anesthesiology, obstetrics/gynecology and orthopedics (40.8%); internal medicine, plastic surgery and pediatrics (30.0%); and otolaryngology and neurology (15.4%). Overall burnout prevalence found for all specialties was 35.7%.
The prevalence of burnout syndrome was significantly higher among surgical/urgency residencies than in clinical specialties.
CRD42018090270.
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Organic solvent nanofiltration (OSN) membranes with ultrathin separation layers down to 35 nm in thickness fabricated from a polymer of intrinsic microporosity (PIM‐1) are presented. These membranes ...exhibit exceptionally fast permeation of n‐heptane with a rejection for hexaphenylbenzene of about 90%. A 35 nm thick PIM‐1 membrane possesses a Young's modulus of 222 MPa, and shows excellent stability under hydraulic pressures of up to 15 bar in OSN. A maximum permeance for n‐heptane of 18 Lm−2h−1bar−1 is achieved with a 140 nm thick membrane, which is about two orders of magnitude higher than Starmem240 (a commercial polyimide‐based OSN membrane). Unexpectedly, decreasing the film thickness below 140 nm results in an anomalous decrease in permeance, which appears to be related to a packing enhancement of PIM‐1, as measured by light interferometry. Further, thermal annealing of the membranes formed from PIM‐1 reveals that their permeance is preserved up to temperatures in excess of 150 °C, whereas the permeance of conventional, integrally skinned, asymmetric polyimide OSN membranes decreases significantly when they are annealed under the same conditions. To rationalize this key difference in response of functional performance to annealing, the concept of membranes with intrinsic microporosity (MIMs) versus membranes with extrinsic microporosity (MEMs) is introduced.
Ultrathin PIM‐1 membranes are fabricated and applied to organic solvent nanofiltration. A 140 nm thick PIM‐1 membrane shows a permeance value for heptane 90 times higher than Starmem240 (a commercial polyimide‐based organic solvent nanofiltration membrane). Its mechanical response is found to be robust to nanofiltration pressures of about 10 bar. The performance of even thinner membranes deteriorates due to enhanced packing upon further confinement.
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Methylammonium lead iodide perovskite can make high-efficiency solar cells, which also show an unexplained photocurrent hysteresis dependent on the device-poling history. Here we report quasielastic ...neutron scattering measurements showing that dipolar CH3NH3(+) ions reorientate between the faces, corners or edges of the pseudo-cubic lattice cages in CH3NH3PbI3 crystals with a room temperature residence time of ∼14 ps. Free rotation, π-flips and ionic diffusion are ruled out within a 1-200-ps time window. Monte Carlo simulations of interacting CH3NH3(+) dipoles realigning within a 3D lattice suggest that the scattering measurements may be explained by the stabilization of CH3NH3(+) in either antiferroelectric or ferroelectric domains. Collective realignment of CH3NH3(+) to screen a device's built-in potential could reduce photovoltaic performance. However, we estimate the timescale for a domain wall to traverse a typical device to be ∼0.1-1 ms, faster than most observed hysteresis.
The rheology of water-soluble polyelectrolytes at intermediate and high concentrations is controlled by entanglement, hydrophobic, and electrostatic interactions, whose influences are difficult to ...isolate. We investigate the rheology of semidilute solutions of sodium carboxymethyl cellulose (NaCMC) with molecular weight M w ≃ 2.5 × 105 g/mol and varying degree of substitution (DS) as a function of polymer concentration in various solvent media: salt-free water (long-ranged electrostatic interactions), 0.5 M aqueous NaCl (screened electrostatics), and 0.5 M aqueous NaOH (screened electrostatics, diminished hydrophobic interactions) in order to selectively examine the role played by these different interactions. Decreasing DS is found to decrease solubility and induce partial aggregation and eventual gelation. In salt-free and 0.5 M NaCl solution, NaCMC with DS ≃ 1.2 exhibits hydrophilic polyelectrolyte and neutral polymer in good solvent behavior, respectively. Decreasing DS to ≃0.7–0.8 leads to hydrophobic behavior in both media, becoming weak gels at high concentrations. In 0.5 M NaOH (pH = 13.5) the viscosities of solutions with different DS become identical when plotted against the overlap parameter, which we interpret as resulting from the solubilization of unsubstituted cellulose blocks. Small-angle neutron scattering (SANS) data indicate that the polymer conformation is not strongly affected by hydrophobic interactions. By varying DS, ionic strength, and pH, we demonstrate the tuning of NaCMC–solvent interactions, controlling separately the electrostatic and hydrophobic effects on the solution rheology.
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Shear flow applied to bicontinuous microemulsions is expected to induce a transition to lamellae via the suppression of surfactant monolayer fluctuations. Compared to the ...topologically analogous L3 (sponge) phase, composed of surfactant bilayers, this transition is likely to occur at much higher shear rates.
We examine the flow response of a model bicontinuous microemulsion, D2O/n-octane/C10E4 by coupling microfluidics with small-angle neutron scattering (SANS), attaining wall shear rates in excess of 105 s−1. The reduction of probed sample volumes down to ∼10 nL allows the spatial mapping of the structural and orientation changes within the microchannel, as a function of the flow field components.
With increasing flow rate, we observe a gradual increase in scattering anisotropy, accompanied by a decrease of the microemulsion domain size along the main flow orientation. A consistent description of the degree of anisotropy was obtained when considering the velocity gradient along the scattering plane perpendicular to the flow. We discuss the flow dependence of the effective bending rigidity, rationalizing a strong influence of shear on thermal membrane fluctuations. Assuming a similar shear dependence for the saddle splay modulus, the bicontinuous-to-lamellar transition can be attributed to the gradual disappearance of inter-lamellar passages.
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We report a general light processing strategy for organic solar cells (OSC) that exploits the propensity of the fullerene derivative PC60BM to photo-oligomerize, which is capable of both stabilizing ...the polymer:PC60BM active layer morphology and enhancing the device stability under thermal annealing. The observations hold for blends of PC60BM with an array of benchmark donor polymer systems, including P3HT, DPP-TT-T, PTB7, and PCDTBT. The morphology and kinetics of the thermally induced PC60BM crystallization within the blend films are investigated as a function of substrate and temperature. PC60BM nucleation rates on SiOx substrates exhibit a pronounced peak profile with temperature, whose maximum is polymer and blend-composition dependent. Modest illumination (<10 mW/cm2) significantly suppresses nucleation, which is quantified as function of dose, but does not affect crystalline shape or growth, in the micrometer range. On PEDOT:PSS substrates, thermally induced PC60BM aggregation is observed on smaller (≈100 nm) length scales, depending upon donor polymer, and also suppressed by light exposure. The concurrent thermal dissociation process of PC60BM oligomers in blend films is also investigated and the activation energy of the fullerene–fullerene bond is estimated to be 0.96 ± 0.04 eV. Following light processing, the thermal stability, and thus lifetime, of PCDTBT:PC60BM devices increases for annealing times up to 150 h. In contrast, PCDTBT:PC70BM OSCs are found to be largely light insensitive. The results are rationalized in terms of the suppression of PC60BM micro- and nanoscopic crystallization processes upon thermal annealing caused by photoinduced PC60BM oligomerization.
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9.
After Society Pina-Cabral, João; Bowman, Glenn
2020, 2020-07-01, Volume:
39
eBook
In the early 1980s, when the contributors to this volume completed their graduate training at Oxford, the conditions of practice in anthropology were undergoing profound change. Professionally, the ...immediate postcolonial period was over and neoliberal reforms were marginalizing the social sciences. Analytically, the poststructuralist critique of the notion of 'society' challenged a discipline that dubbed itself as 'social'. Here self-ethnography is used to portray the contributors' anthropological trajectories, showing how analytical and academic engagements interacted creatively over time.
The combined effects of illumination and thermal annealing on the morphological stability and photodimerization in polymer/fullerene thin films are examined. While illumination is known to cause ...fullerene dimerization and thermal stress their dedimerization, the operation of solar cells involves exposure to both. The competitive outcome of these factors with blends of phenyl‐C61‐butyric acid methyl ester (PCBM) and polystyrene (PS), supported on PEDOT:PSS is quantified. UV–vis spectroscopy is employed to quantify dimerization, time‐resolved neutron reflectivity to resolve the vertical composition stratification, and atomic force microscopy for demixing and coarsening in thin films. At the conventional thermal stress test temperature of 85 °C (and even up to the PS glass transition), photodimerization dominates, resulting in relative morphological stability. Prior illumination is found to result in improved stability upon high temperature annealing, compatible with the need for dedimerization to occur prior to structural relaxation. Modeling of the PCBM surface segregation data suggests that only PCBM monomers are able to diffuse and that illumination provides an effective means to control dimer population, and thus immobile fullerene fraction, in the timescales probed. The results provide a framework for understanding of the stability of organic solar cells under operating conditions.
The competitive effects of light and thermal stress on polymer:fullerene blend morphology are investigated. While light promotes fullerene dimerization, elevated temperatures cause dimer decomposition; evidently solar cells experience both. Time‐resolved neutron reflectivity and atomic force microscopy are employed to examine demixing and stratification in thin films and provide a framework for rationalizing stability under these innate environmental conditions.
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