596 research outputs found
Homolytic bond strengths and formation rates in half-sandwich chromium alkyl complexes: Relevance for controlled radical polymerization
(Chemical Equation Presented) Radicals in check: The steric properties of the aryl substituents in chromium ÎČ-ketiminato complexes can be tuned to achieve reversible radical trapping of a growing poly(vinyl acetate) radical chain (see scheme; V-70 = radical initiator, VOAc = vinyl acetate). © 2008 Wiley-VCH Verlag GmbH & Co. KGaA
Phase Behavior of Polyelectrolyte Block Copolymers in Mixed Solvents
We have studied the phase behavior of the poly(n-butyl
acrylate)-b-poly(acrylic acid) block copolymer in a mixture of two miscible
solvents, water and tetrahydrofuran (THF). The techniques used to examine the
different polymers, structures and phases formed in mixed solvents were static
and dynamic light scattering, small-angle neutron scattering, nuclear magnetic
resonance and fluorescence microscopy. By lowering the water/THF mixing ratio
X, the sequence unimers, micron-sized droplets, polymeric micelles was
observed. The transition between unimers and the micron-sized droplets occurred
at X = 0.75, whereas the microstructuration into core-shell polymeric micelles
was effective below X = 0.4. At intermediate mixing ratios, a coexistence
between the micron-sized droplets and the polymeric micelles was observed.
Combining the different aforementioned techniques, it was concluded that the
droplet dispersion resulted from a solvent partitioning that was induced by the
hydrophobic blocks. Comparison of poly(n-butyl acrylate) homopolymers and
poly(n-butyl acrylate)-b-poly(acrylic acid) block copolymers suggested that the
droplets were rich in THF and concentrated in copolymers and that they were
stabilized by the hydrophilic poly(acrylic acid) moieties.Comment: 11 pages, 12 figures, to appear in Macromolecule
Parametric Analysis of an Active Winglet Concept for High Aspect Ratio Wing Using CFD/CSM Computations
This paper presents a parametric analysis of an active winglet concept applied to a high
aspect ratio wing. The technology studied here only consists in a single degree of freedom
wing-tip whose only the cant angle deflection can be controlled. The main parameters under
study are the hinge line location and its orientation with respect to the longitudinal axis of
the aircraft. High-fidelity aerodynamic and structural computations are used to assess the
impact of the device on both drag and loads. The influence of cant angle deflections on flutter
characteristics is also evaluated. First a "wing only" configuration is studied and the results
are compared with complete aircraft computations to take into account the contributions due
to the trim. It is shown that the hinge line parameters highly influence the drag evolution
with cant angle but with limited impact on the minimum area - in which we are interested in.
Loads are significantly impacted by both cant variations and hinge line geometry. Regarding
dynamic characteristics, the mode sequence is dependent on the cant deflection and massively
impacts flutter onset
Poly(N-2-(methacryloyloxy)ethyl pyrrolidone)-poly(benzyl methacrylate) diblock copolymer nano-objects via RAFT alcoholic dispersion polymerisation in ethanol
N-2-(methacryloyloxy)ethyl pyrrolidone (NMEP) is a highly polar monomer that is much more readily copolymerised with methacrylic monomers than its structural analogue, N-vinyl pyrrolidone. The RAFT solution polymerisation of NMEP has been conducted in ethanol at 70 °C while varying the degree of polymerisation (DP) from 70 to 400. Reducing the CTA/initiator molar ratio from 10.0 to 3.0 when targeting PNMEP100 led to a three-fold rate enhancement and increased the final monomer conversion from 72% to 98%. Targeting DPs of 200 or greater led to lower monomer conversions. DMF GPC analysis of a series of PNMEPx homopolymers confirmed a linear increase in molecular weight with conversion and relatively low dispersities (Mw/Mn 90%) were obtained and electron microscopy studies indicated that a range of diblock copolymer morphologies, e.g. spheres, worms and vesicles, were produced by polymerisation-induced self-assembly (PISA). Finally, a PNMEP47-PBzMA243 diblock copolymer was synthesised via a convenient 'one-pot' protocol whereby a PNMEP47 macro-CTA was first prepared at 97% conversion, followed by in situ RAFT dispersion polymerisation of BzMA to produce diblock copolymer nano-objects. TEM analysis of aliquots taken during the diblock copolymer synthesis indicated a gradual evolution in copolymer morphology from spherical micelles after 1 h to a pure vesicle phase after 8 h via intermediate mixed phases (including worms)
Phosphorus-containing gradient (block)copolymers via RAFT polymerization and post-polymerization modification
Reversible additionâfragmentation chain transfer (RAFT) copolymerization of styrene (St) and 4â(diphenylphosphino)styrene (DPPS) is explored to establish the statistical distribution of the phosphineâfunctional monomer within the copolymer. RAFT copolymerization of St and DPPS at a variety of feed ratios provides phosphineâfunctional copolymers of low dispersity at moderate monomer conversion (Ă 60%). In all cases, the fraction of DPPS in the resulting polymers is greater than that in the monomer feed. Estimation of copolymerization reactivity ratios indicates DPPS has a strong tendency to homopolymerize while St preferentially copolymerizes with DPPS (rDPPS = 4.4; rSt = 0.31). The utility of the copolymers as macroâRAFT agents in block copolymer synthesis is demonstrated via chain extension with hydrophilic acrylamide (N,Nâdimethylacrylamide (DMAm)) and acrylate (poly(ethylene glycol) methyl ether acrylate (mPEGA), and di(ethylene glycol) ethyl ether acrylate (EDEGA)) monomers. Finally, access to polymers containing phosphine oxide and phosphonium salt functionalities is shown through postpolymerization modification of the phosphineâcontaining copolymers
ATLAS2030 Pediatric Gait Exoskeleton: Changes on Range of Motion, Strength and Spasticity in Children With Cerebral Palsy. A Case Series Study
Background: Cerebral Palsy (CP), the most common motor disability in childhood, affects individual's motor skills, movement and posture. This results in limited activity and a low social participation. The ATLAS2030 exoskeleton is a pediatric device that enables gait rehabilitation for children with neurological or neuromuscular pathologies with gait pathology. Purpose: To study changes in relation to range of motion (ROM), strength and spasticity in children with CP after using the ATLAS2030 gait exoskeleton. Methods and Participants: Three children (mean age 8.0 ± 2.0), two girls and one boy, two of them with GMFCS IV and one with GMFCS III, received robot-assisted gait training (RAGT) with ATLAS2030 for one month. Results: The average time of exoskeleton use was 54.7 ± 10.4 min in all sessions, and all participants were able to perform all exercises. The strength of all muscle groups was increased after the 10 sessions for the participants assessed and the limited ROM in the sagittal plane (hip and knee extension and ankle dorsiflexion) decreased after the use of the exoskeleton compared to the initial state. Spasticity was reduced at the end of the sessions after the use of the exoskeleton compared to their initial state. Conclusion: The ROM, spasticity and strength were improved after RAGT with ATLAS2030 exoskeleton in these children with CP. However, further studies with larger samples should be carried out to confirm our findings.This work has been partially funded by Agencia Estatal
de InvestigaciĂłn through Grant PID2019-110492RBI00 and from Comunidad de Madrid through Grant
IND2018/TIC9618. AP Acknowledges support from
Comunidad de Madrid through Grant IND2017_TIC-7698.
JR acknowledges support from ConsejerĂa de EducaciĂłn
e InvestigaciĂłn de la Comunidad de Madrid through
Grant PEJ-2018-AI_TIC-11333. CC acknowledges support
from ConsejerĂa de EducaciĂłn e InvestigaciĂłn de la
Comunidad de Madrid through Grant PEJ-2019-AI_TIC15202. GP acknowledges support from Agencia Estatal de
InvestigaciĂłn (Ministry of Science and Innovation) through
Grant DI-16-08731. EGarcé acknowledges support from
Comunidad de Madrid through Grant IND2018/TIC9618.
MD acknowledges support from Agencia Estatal de
InvestigaciĂłn (Ministry of Science and Innovation) through
Grant PTQ2018-010119.
We would like to thank Fernando Aneiros (Marsi Bionics S.L., Madrid, Spain) for his support reviewing the paper. We would also like to thank the participants and their families for their commitment during the development of the study.Peer reviewe
Polymer-Nanoparticle Complexes : from Dilute Solution to Solid State
We report on the formation and the structural properties of supermicellar
aggregates also called electrostatic complexes, made from mineral nanoparticles
and polyelectrolyte-neutral block copolymers in aqueous solutions. The mineral
particles put under scrutiny are ultra-fine and positively charged yttrium
hydroxyacetate nanoparticles. Combining light, neutron and x-ray scattering
experiments, we have characterized the sizes and the aggregation numbers of the
organic-inorganic complexes. We have found that the hybrid aggregates have
typical sizes in the range 100 nm and exhibit a remarkable colloidal stability
with respect to ionic strength and concentration variations. Solid films with
thicknesses up to several hundreds of micrometers were cast from solutions,
resulting in a bulk polymer matrix in which nanoparticle clusters are dispersed
and immobilized. It was found in addition that the structure of the complexes
remains practically unchanged during film casting.Comment: 18 pages, 11 figures, 2 table
Chain transfer kinetics of acid/base switchable n-aryl- n-pyridyl dithiocarbamate RAFT agents in methyl acrylate, n-vinylcarbazole and vinyl acetate polymerization
This is an accepted manuscript of an article published by American Chemistry Society in Macromolecules on 14/05/2012, available online: https://doi.org/10.1021/ma300616g ©American Chemical Society. The accepted version of the publication may differ from the final published version.The structures of the "Z" and "R" substituents of a RAFT agent (Z-C(S)S-R) determine a RAFT agent's ability to control radical polymerization. In this paper we report new acid/base switchable N-aryl-N-pyridyl dithiocarbamates (R = -CH 2CN, Z = -N(Py)(Ar)) which vary in substituent at the 4-position of the aryl ring and the use of these to control molecular weight and dispersity. In their protonated form, the new RAFT agents are more effective in controlling polymerization of the more activated monomer, methyl acrylate (MA), whereas in their neutral form they provide more effective control of the polymerization of less activated monomers, N-vinyl carbazole (NVC) and vinyl acetate (VAc). For each polymerization, the apparent chain transfer coefficient (C trapp) shows a good correlation with Hammett parameters. Dithiocarbamates with more electron-withdrawing aryl ring substituents have the higher C trapp. This demonstrates the influence of polar effects on C trapp and supports the hypothesis that the activity of these RAFT agents is determined by the availability of the lone pair of the dithiocarbamate nitrogen.The authors gratefully acknowledge the Capability Development Fund of CSIRO Materials Science and Engineering for financial support.Published versio
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