Fluorescence polarization and rheological studies of the poly(N-vinyl-2-pyrrolidone) hydrogels produced by UV radiation

Abstract Poly(N-vinyl-2-pyrrolidone) hydrogels produced by direct ultraviolet irradiation of PVP aqueous solution leads to crosslinking through pyrrolidinone moiety photolysis. Generally, hydrogel physical properties, like crosslinking density, pore size, swelling capacity, storage and loss moduli are obtained by swelling and rheological tests. However, relations between anisotropy obtained by fluorescence polarization studies and these properties have not been addressed for hydrogel systems. In this work we show that there is a correlation between the data obtained from anisotropy and rheological experiments, since both of them are related with crosslinking density of the hydrogels. These results reveal that fluorescence polarization spectroscopy is a promising tool for understanding the structure of hydrogels.

Characterization of the second- and third-order nonlinear optical susceptibilities of monolayer MoS2_2 using multiphoton microscopy

We report second- and third-harmonic generation in monolayer MoS$_\mathrm{2}$ as a tool for imaging and accurately characterizing the material's nonlinear optical properties under 1560 nm excitation. Using a surface nonlinear optics treatment, we derive expressions relating experimental measurements to second- and third-order nonlinear sheet susceptibility magnitudes, obtaining values of $|\chi_s^{(2)}|=2.0\times10^{-20}$ m$^2$ V$^{-1}$ and for the first time for monolayer MoS$_\mathrm{2}$, $|\chi_s^{(3)}|=1.7\times10^{-28}$ m$^3$ V$^{-2}$. These sheet susceptibilities correspond to effective bulk nonlinear susceptibility values of $|\chi_{b}^{(2)}|=2.9\times10^{-11}$ m V$^{-1}$ and $|\chi_{b}^{(3)}|=2.4\times10^{-19}$ m$^2$ V$^{-2}$, accounting for the sheet thickness. Experimental comparisons between MoS$_\mathrm{2}$ and graphene are also performed, demonstrating $\sim$3.4 times stronger third-order sheet nonlinearity in monolayer MoS$_\mathrm{2}$, highlighting the material's potential for nonlinear photonics in the telecommunications C band.Comment: Accepted by 2D Materials, 28th Oct 201

Surface characterization of photodegraded poly(ethylene terephthalate). The effect of ultraviolet absorbers

Abstract The surface characterization of photodegraded poly(ethylene terephthalate) was investigated. Films obtained by bi-axial extrusion were exposed in a laboratory weathering chamber for periods of up to , 1100 h of irradiation. Samples prepared with and without an ultraviolet (UV) absorber were tested by infrared, UV -visible and fluorescence spectroscopy. The results indicated that the unstabilized films are very susceptible to the degradation effects causing a large deterioration, especially in surface layers. The presence of an ultraviolet light absorber effectively reduced the formation of carboxyl end-groups at the surface as well as in the bulk of the films. In the case of samples with UV absorber the fluorescence data showed a barrier imposed by this additive in the formation of the monohydroxy-terephthalate. Scanning electron microcopy of fracture surfaces showed that film ductility is highly reduced after exposure.

First Observation of an Undular Mesospheric Bore in a Doppler Duct

On 1 October 2005, during the SpreadFEx campaign, a distinct mesospheric bore was observed over S˜ao Jo˜ao do Cariri (7.4 S, 36.5 W), Brazil by using airglow allsky imagers. The event appeared both in the OI5577 and OH emissions, forming a well extended wave front which was followed by short waves from behind. Simultaneous wind and temperature data obtained by the meteor radar and the TIMED/SABER satellite instrument revealed that the bore event occurred during the Doppler ducting condition in the emission layers

Mesospheric Gravity Waves Observed Near Equatorial and Low-Middle Latitude Stations: Wave Characteristics and Reverse Ray Tracing Results

Gravity wave signatures were extracted from OH airglow observations using all-sky CCD imagers at four different stations: Cachoeira Paulista (CP) (22.7° S, 45° W) and São João do Cariri (7.4° S, 36.5° W), Brazil; Tanjungsari (TJS) (6.9° S, 107.9° E), Indonesia and Shigaraki (34.9° N, 136° E), Japan. The gravity wave parameters are used as an input in a reverse ray tracing model to study the gravity wave vertical propagation trajectory and to estimate the wave source region. Gravity waves observed near the equator showed a shorter period and a larger phase velocity than those waves observed at low-middle latitudes. The waves ray traced down into the troposphere showed the largest horizontal wavelength and phase speed. The ray tracing results also showed that at CP, Cariri and Shigaraki the majority of the ray paths stopped in the mesosphere due to the condition of m2\u3c0, while at TJS most of the waves are traced back into the troposphere. In summer time, most of the back traced waves have their final position stopped in the mesosphere due to m2\u3c0 or critical level interactions (|m|→∞), which suggests the presence of ducting waves and/or waves generated in-situ. In the troposphere, the possible gravity wave sources are related to meteorological front activities and cloud convections at CP, while at Cariri and TJS tropical cloud convections near the equator are the most probable gravity wave sources. The tropospheric jet stream and the orography are thought to be the major responsible sources for the waves observed at Shigaraki

Overview and Summary of the Spread F Experiment (SpreadFex)

We provide here an overview of, and a summary of results arising from, an extensive experimental campaign (the Spread F Experiment, or SpreadFEx) performed from September to November 2005, with primary measurements in Brazil. The motivation was to define the potential role of neutral atmosphere dynamics, specifically gravity wave motions propagating upward from the lower atmosphere, in seeding Rayleigh-Taylor instability (RTI) and plasma bubbles extending to higher altitudes. Campaign measurements focused on the Brazilian sector and included ground-based optical, radar, digisonde, and GPS measurements at a number of fixed and temporary sites. Related data on convection and plasma bubble structures were also collected by GOES 12, and the GUVI instrument aboard the TIMED satellite