323 research outputs found
3D printing of optical materials: an investigation of the microscopic properties
3D printing technologies are currently enabling the fabrication of objects
with complex architectures and tailored properties. In such framework, the
production of 3D optical structures, which are typically based on optical
transparent matrices, optionally doped with active molecular compounds and
nanoparticles, is still limited by the poor uniformity of the printed
structures. Both bulk inhomogeneities and surface roughness of the printed
structures can negatively affect the propagation of light in 3D printed optical
components. Here we investigate photopolymerization-based printing processes by
laser confocal microscopy. The experimental method we developed allows the
printing process to be investigated in-situ, with microscale spatial
resolution, and in real-time. The modelling of the photo-polymerization
kinetics allows the different polymerization regimes to be investigated and the
influence of process variables to be rationalized. In addition, the origin of
the factors limiting light propagation in printed materials are rationalized,
with the aim of envisaging effective experimental strategies to improve optical
properties of printed materials.Comment: 8 pages, 3 figure
Financing for public Veterinary Services to ensure that they meet international standards
Summary It is vital that public Veterinary Services develop the skills to address the challenging questions posed by national Treasury officials, who exercise an important mandate to ensure that expenditures are efficient and effective in serving the public interest. Examples of such questions are considered in this paper, along with systems and strategies for preparing for such reviews, including the use of the Performance of Veterinary Services (PVS) evaluation tools of the World Organisation for Animal Health. Some lessons and observations are drawn from PVS Evaluation and PVS Gap Analysis missions in nine countries
Neurosurgical Applications of Magnetic Resonance Diffusion Tensor Imaging
Magnetic Resonance (MR) Diffusion Tensor Imaging (DTI) is a rapidly evolving technology that enables the visualization of neural fiber bundles, or white matter (WM) tracts. There are numerous neurosurgical applications for MR DTI including: (1) Tumor grading and staging; (2) Pre-surgical planning (determination of resectability, determination of surgical approach, identification of WM tracts at risk); (3) Intraoperative navigation (tumor resection that spares WM damage, epilepsy resection that spares WM damage, accurate location of deep brain stimulation structures); (4) Post-operative assessment and monitoring (identification of WM damage, identification of tumor recurrence). Limitations of MR DTI include difficulty tracking small and crossing WM tracts, lack of standardized data acquisition and post-processing techniques, and practical equipment, software, and timing considerations. Overall, MR DTI is a useful tool for planning, performing, and following neurosurgical procedures, and has the potential to significantly improve patient care. Technological improvements and increased familiarity with DTI among clinicians are next steps
Impact of size effects on photopolymerization and its optical monitoring in-situ
Photopolymerization processes are exploited in light exposure-based 3D printing technologies, where either a focused laser beam or a patterned light sheet allows layers of a UV curable, liquid pre-polymer to be solidified. Here we focus on the crucial, though often neglected, role of the layer thickness on photopolymerization. The temporal evolution of polymerization reactions occurring in droplets of acrylate-based oligomers and in photoresist films with varied thickness is investigated by means of an optical system, which is specifically designed for in-situ and real-time monitoring. The time needed for complete curing is found to increase as the polymerization volume is decreased below a characteristic threshold that depends on the specific reaction pathway. This behavior is rationalized by modeling the process through a size-dependent polymerization rate. Our study highlights that the formation of photopolymerized networks might be affected by the involved volumes regardless of the specific curing mechanisms, which could play a crucial role in optimizing photocuring-based additive manufacturing
Mechanics of Hydrogenated Amorphous Carbon Deposits from Electron-Beam-Induced Deposition of Paraffin Precursor
Many experiments on the mechanics of nanostructures require the creation of rigid clamps at specific locations. In this work, electron-beam-induced deposition(EBID) has been used to depositcarbonfilms that are similar to those that have recently been used for clamping nanostructures. The film deposition rate was accelerated by placing a paraffin source of hydrocarbon near the area where the EBIDdeposits were made. High-resolution transmission electron microscopy, electron-energy-loss spectroscopy, Raman spectroscopy, secondary-ion-mass spectrometry, and nanoindentation were used to characterize the chemical composition and the mechanics of the carbonaceous deposits. The typical EBIDdeposit was found to be hydrogenated amorphous carbon (a-C:H) having more sp2- than sp3-bonded carbon.Nanoindentation tests revealed a hardness of âŒ4GPa and an elastic modulus of 30â60GPa, depending on the accelerating voltage. This reflects a relatively soft film, which is built out of precursor molecular ions impacting the growing surface layer with low energies. The use of such deposits as clamps for tensile tests of poly(acrylonitrile)-based carbon nanofibers loaded between opposing atomic force microscope cantilevers is presented as an example applicatio
Local mechanical properties of electrospun fibers correlate to their internal nanostructure.
This is the final version of the article. Available from the publisher via the DOI in this record.The properties of polymeric nanofibers can be tailored and enhanced by properly managing the structure of the polymer molecules at the nanoscale. Although electrospun polymer fibers are increasingly exploited in many technological applications, their internal nanostructure, determining their improved physical properties, is still poorly investigated and understood. Here, we unravel the internal structure of electrospun functional nanofibers made by prototype conjugated polymers. The unique features of near-field optical measurements are exploited to investigate the nanoscale spatial variation of the polymer density, evidencing the presence of a dense internal core embedded in a less dense polymeric shell. Interestingly, nanoscale mapping the fiber Young's modulus demonstrates that the dense core is stiffer than the polymeric, less dense shell. These findings are rationalized by developing a theoretical model and simulations of the polymer molecular structural evolution during the electrospinning process. This model predicts that the stretching of the polymer network induces a contraction of the network toward the jet center with a local increase of the polymer density, as observed in the solid structure. The found complex internal structure opens an interesting perspective for improving and tailoring the molecular morphology and multifunctional electronic and optical properties of polymer fibers.V. Fasano and G. Potente are acknowledged for confocal and
SEM images, respectively. The authors also gratefully thank S.
Girardo for high-speed imaging of the polymer jet and E. Caldi
for assistance in the SNOM measurements. We gratefully
acknowledge the financial support of the United States-Israel
Binational Science Foundation (BSF Grant 2006061), the
RBNI-Russell Berrie Nanotechnology Institute, and the Israel
Science Foundation (ISF Grant 770/11). The research leading
to these results has received funding from the European
Research Council under the European Unionâs Seventh
Framework Programme (FP/2007-2013)/ERC Grant Agreement
306357 (ERC Starting Grant âNANO-JETSâ)
Enhanced Electrospinning of Active Organic Fibers by Plasma Treatment on Conjugated Polymer Solutions
Realizing active, light-emitting fibers made of conjugated polymers by the electrospinning method is generally challenging. Electrospinning of plasma-treated conjugated polymer solutions is here developed for the production of light-emitting microfibers and nanofibers. Active fibers from conjugated polymer solutions rapidly processed by a cold atmospheric argon plasma are electrospun in an effective way, and they show a smoother surface and bead-less morphology, as well as preserved optical properties in terms of absorption, emission, and photoluminescence quantum yield. In addition, the polarization of emitted light and more notably photon waveguiding along the length of individual fibers are remarkably enhanced by electrospinning plasma-treated solutions. These properties come from a synergetic combination of favorable intermolecular coupling in the solutions, increased order of macromolecules on the nanoscale, and resulting fiber morphology. Such findings make the coupling of the electrospinning method and cold atmospheric plasma processing on conjugated polymer solutions a highly promising and possibly general route to generate light-emitting and conductive micro- and nanostructures for organic photonics and electronics
Organizing Engineering Work - A Comparative-Analysis
This article analyzes the organization of engineering work in six industrial capitalist countries. It identifies four major models for the organization of engineering work; the engineering profession did not succeed in achieving professional âclosureâ in any of the six countries under review. A review of the historical evolution of the organization of engineering work in each of the six countries reveals that engineering has been shaped by a complex interaction among the profession itself, employers, the state, labor, and preindustrial forces. However, none of the national variations on the four models for organizing engineering labor is stable or without internal contradiction because of the ambiguous âintermediateâ position of engineers
Mechanical Properties of Glassy Polyethylene Nanofibers via Molecular Dynamics Simulations
The extent to which the intrinsic mechanical properties of polymer fibers depend on physical size has been a matter of dispute that is relevant to most nanofiber applications. Here, we report the elastic and plastic properties determined from molecular dynamics simulations of amorphous, glassy polymer nanofibers with diameter ranging from 3.7 to 17.7 nm. We find that, for a given temperature, the Youngâs elastic modulus E decreases with fiber radius and can be as much as 52% lower than that of the corresponding bulk material. Poissonâs ratio Îœ of the polymer comprising these nanofibers was found to decrease from a value of 0.3 to 0.1 with decreasing fiber radius. Our findings also indicate that a small but finite stress exists on the simulated nanofibers prior to elongation, attributable to surface tension. When strained uniaxially up to a tensile strain of Δ = 0.2 over the range of strain rates and temperatures considered, the nanofibers exhibit a yield stress Ïy between 40 and 72 MPa, which is not strongly dependent on fiber radius; this yield stress is approximately half that of the same polyethylene simulated in the amorphous bulk.DuPont MIT AllianceDuPont (Firm) (Young Professor Award
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