406 research outputs found
Implanted muon spin spectroscopy on 2-O-adamantane: a model system that mimics the liquid
The transition taking place between two metastable phases in 2-O-adamantane, namely the [Formula: see text] cubic, rotator phase and the lower temperature P21/c, Z  =  4 substitutionally disordered crystal is studied by means of muon spin rotation and relaxation techniques. Measurements carried out under zero, weak transverse and longitudinal fields reveal a temperature dependence of the relaxation parameters strikingly similar to those exhibited by structural glass[Formula: see text]liquid transitions (Bermejo et al 2004 Phys. Rev. B 70 214202; Cabrillo et al 2003 Phys. Rev. B 67 184201). The observed behaviour manifests itself as a square root singularity in the relaxation rates pointing towards some critical temperature which for amorphous systems is located some tens of degrees above that shown as the characteristic transition temperature if studied by thermodynamic means. The implications of such findings in the context of current theoretical approaches concerning the canonical liquid-glass transition are discussed.Postprint (author's final draft
Stationary Dynamic Displacement Solutions for a Rectangular Load Applied within a 3D Viscoelastic Isotropic Full Space-Part II: Implementation, Validation, and Numerical Results
Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento CientÃfico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de NÃvel Superior (CAPES)In part I of the present article the formulation for a dynamic stationary semianalytical solution for a spatially constant load applied over a rectangular surface within a viscoelastic isotropic full-space has been presented. The solution is obtained within the frame of a double Fourier integral transform. These inverse integral transforms must be evaluated numerically. In the present paper, the technique to evaluate numerically the inverse double Fourier integrals is described. The procedure is validated, and a number of original displacement results for the stationary loading case are reported.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento CientÃfico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de NÃvel Superior (CAPES)UFMS-FoundationFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento CientÃfico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de NÃvel Superior (CAPES
Drug-Biopolymer Dispersions: Morphology- and Temperature- Dependent (Anti)Plasticizer Effect of the Drug and Component-Specific Johari–Goldstein Relaxations
Amorphous molecule-macromolecule mixtures are ubiquitous in polymer technology and are one of the most studied routes for the development of amorphous drug formulations. For these applications it is crucial to understand how the preparation method affects the properties of the mixtures. Here, we employ differential scanning calorimetry and broadband dielectric spectroscopy to investigate dispersions of a small-molecule drug (the Nordazepam anxiolytic) in biodegradable polylactide, both in the form of solvent-cast films and electrospun microfibres. We show that the dispersion of the same small-molecule compound can have opposite (plasticizing or antiplasticizing) effects on the segmental mobility of a biopolymer depending on preparation method, temperature, and polymer enantiomerism. We compare two different chiral forms of the polymer, namely, the enantiomeric pure, semicrystalline L-polymer (PLLA), and a random, fully amorphous copolymer containing both L and D monomers (PDLLA), both of which have lower glass transition temperature (Tg) than the drug. While the drug has a weak antiplasticizing effect on the films, consistent with its higher Tg, we find that it actually acts as a plasticizer for the PLLA microfibres, reducing their Tg by as much as 14 K at 30%-weight drug loading, namely, to a value that is lower than the Tg of fully amorphous films. The structural relaxation time of the samples similarly depends on chemical composition and morphology. Most mixtures displayed a single structural relaxation, as expected for homogeneous samples. In the PLLA microfibres, the presence of crystalline domains increases the structural relaxation time of the amorphous fraction, while the presence of the drug lowers the structural relaxation time of the (partially stretched) chains in the microfibres, increasing chain mobility well above that of the fully amorphous polymer matrix. Even fully amorphous homogeneous mixtures exhibit two distinct Johari–Goldstein relaxation processes, one for each chemical component. Our findings have important implications for the interpretation of the Johari–Goldstein process as well as for the physical stability and mechanical properties of microfibres with small-molecule additives
Search for weakly interacting sub-eV particles with the OSQAR laser-based experiment: results and perspectives
Recent theoretical and experimental studies highlight the possibility of new
fundamental particle physics beyond the Standard Model that can be probed by
sub-eV energy experiments. The OSQAR photon regeneration experiment looks for
"Light Shining through a Wall" (LSW) from the quantum oscillation of optical
photons into "Weakly Interacting Sub-eV Particles" (WISPs), like axion or
axion-like particles (ALPs), in a 9 T transverse magnetic field over the
unprecedented length of m. No excess of events has been
detected over the background. The di-photon couplings of possible new light
scalar and pseudo-scalar particles can be constrained in the massless limit to
be less than GeV. These results are very close to the
most stringent laboratory constraints obtained for the coupling of ALPs to two
photons. Plans for further improving the sensitivity of the OSQAR experiment
are presented.Comment: 7 pages, 7 figure
High resolution measurements of carbon monoxide along a late Holocene Greenland ice core: evidence for in situ production
We present high-resolution measurements of carbon monoxide (CO)
concentrations from a shallow ice core of the North Greenland Eemian Ice
Drilling project (NEEM-2011-S1). An optical-feedback cavity-enhanced
absorption spectrometer (OF-CEAS) coupled to a continuous melter system
performed continuous, online analysis during a four-week measurement campaign.
This analytical setup generated stable measurements of CO concentrations
with an external precision of 7.8 ppbv (1σ), based on repeated
analyses of equivalent ice core sections. However, this first application of
this measurement technique suffered from a poorly constrained procedural
blank of 48 ± 25 ppbv and poor accuracy because an absolute
calibration was not possible. The NEEM-2011-S1 CO record spans 1800 yr and
the long-term trends within the most recent section of this record
(i.e., post 1700 AD) resemble the existing discrete CO measurements from the
Eurocore ice core. However, the CO concentration is highly variable (75–1327 ppbv
range) throughout the ice core with high frequency (annual scale), high
amplitude spikes characterizing the record. These CO signals are too abrupt
and rapid to reflect atmospheric variability and their prevalence largely
prevents interpretation of the record in terms of atmospheric CO variation.
The abrupt CO spikes are likely the result of in situ production occurring
within the ice itself, although the unlikely possibility of CO production
driven by non-photolytic, fast kinetic processes within the continuous
melter system cannot be excluded. We observe that 68% of the CO spikes
are observed in ice layers enriched with pyrogenic aerosols. Such aerosols,
originating from boreal biomass burning emissions, contain organic
compounds, which may be oxidized or photodissociated to produce CO within
the ice. However, the NEEM-2011-S1 record displays an increase of
~0.05 ppbv yr<sup>−1</sup> in baseline CO level prior to 1700 AD (129 m
depth) and the concentration remains elevated, even for ice layers depleted
in dissolved organic carbon (DOC). Thus, the processes driving the likely
in situ production of CO within the NEEM ice may involve multiple, complex
chemical pathways not all related to past fire history and require further investigation
SKI-1 and Furin Generate Multiple RGMa Fragments that Regulate Axonal Growth
SummaryThe nervous system is enormously complex, yet the number of cues that control axonal growth is surprisingly meager. Posttranslational modifications amplify diversity, but the degree to which they are employed is unclear. Here, we show that Furin and SKI-1 combine with autocatalytic cleavage and a disulfide bridge to generate four membrane-bound and three soluble forms of the repulsive guidance molecule (RGMa). We provide in vivo evidence that these proprotein convertases are involved in axonal growth and that RGMa cleavage is essential for Neogenin-mediated outgrowth inhibition. Surprisingly, despite no sequence homology, N- and C-RGMa fragments bound the same Fibronectin-like domains in Neogenin and blocked outgrowth. This represents an example in which unrelated fragments from one molecule inhibit outgrowth through a single receptor domain. RGMa is a tethered membrane-bound molecule, and proteolytic processing amplifies RGMa diversity by creating soluble versions with long-range effects as well
Nose temperature and anticorrelation between recrystallization kinetics and molecular relaxation dynamics in amorphous morniflumate at high pressure
We probe the dielectric response of the supercooled liquid phase of Morniflumate, a drug with anti-inflammatory and antipyretic properties, studying in particular the pressure and temperature dependence of the relaxation dynamics, glass transition temperature Tg, and recrystallization kinetics. Tg increases by roughly 20 K every 100 MPa at low applied pressure, where the ratio Tg/Tm has a constant value of ~0.8 (Tm = melting point). Liquid Morniflumate displays two dielectric relaxations: the structural a relaxation associated with the collective reorientational motions, which become arrested at Tg, and a secondary relaxation likely corresponding to an intramolecular dynamics. The relaxation times of both processes scale approximately with the inverse reduced temperature Tg/T. Near room temperature and under an applied pressure of 50 MPa, supercooled Morniflumate recrystallizes in a characteristic time of few hours, with an Avrami exponent of 1.15. Under these conditions, the recrystallization rate is a nonmonotonic function of temperature, displaying a maximum at around 298 K, which can be taken to be the optimum crystal growth temperature Tnose. The ß relaxation becomes kinetically frozen at ambient temperature under an applied hydrostatic pressure higher than 320 MPa, suggesting that the Morniflumate glass should be kinetically stable under these conditions.Peer ReviewedPostprint (author's final draft
Tuning the Kinetic Stability of the Amorphous Phase of the Chloramphenicol Antibiotic
We employ broadband dielectric spectroscopy to study the relaxation dynamics and crystallization kinetics of a broad-spectrum antibiotic, chloramphenicol, in its supercooled liquid form. Two dynamic processes are observed: the structural a relaxation, which becomes kinetically frozen at Tg = 302 ± 1 K, and an intramolecular secondary relaxation. Under isothermal conditions, the supercooled drug displays interconversion between different isomers, followed by recrystallization. Recrystallization follows the Avrami law with Avrami exponent n = 1.3 ± 0.1, consistent with a one dimensional growth of crystalline platelets, as observed by electron microscopy. Exposure to humid atmosphere and subsequent heating to high temperature is found to degrade the compound. The partially degraded sample displays a much lower tendency to crystallize, likely because the presence of the degradation products results in spatial frustration. This sample exhibits enhanced conductivity and an additional relaxation, intermediate to the ones observed in the pure sample, which likely corresponds to the noncooperative dynamics of the main degradation product. We find that dispersing the antibiotic in polylactic acid results in an amorphous sample which does not crystallize at room temperature for relatively long times.Peer ReviewedPostprint (published version
A calibration method for broad-bandwidth cavity enhanced absorption spectroscopy performed with supercontinuum radiation
An efficient calibration method has been developed for broad-bandwidth cavity enhanced absorption spectroscopy. The calibration is performed using phase shift cavity ring-down spectroscopy, which is conveniently implemented through use of an acousto-optic tunable filter (AOTF). The AOTF permits a narrowband portion of the SC spectrum to be scanned over the full high-reflectivity bandwidth of the cavity mirrors. After calibration the AOTF is switched off and broad-bandwidth CEAS can be performed with the same light source without any loss of alignment to the set-up. We demonstrate the merits of the method by probing transitions of oxygen molecules O-2 and collisional pairs of oxygen molecules (O-2)(2) in the visible spectral range
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