Defect-assisted photoluminescence intensity enhancement in poly(p-phenylene vinylene) films probed by time-resolved photoluminescence

We report on picosecond time resolved photoluminescence measurements in poly(p-phenylene vinylene) (PPV) films of thickness which exceeds the penetration depth of the exciting laser, irradiated by laser in air. The photoluminescence (PL) intensity increases during the irradiation process despite the increasing density of photo-oxidation induced carbonyl groups, usually acting as quenching centers. We study the PL enhancement and the dynamics of the PL decay for different irradiation conditions and different sample thicknesses. The PL decay surprisingly shows a reduced importance of defects trapping as the defect concentration increases, thus explaining the PL enhancement. We explain our results in terms of a chain shortening due to carbonyl incorporation and formation of an energy profile that extends and migrates into the film, enabling efficient spectral diffusion of excited carriers into nondegraded PPV segments by Förster energy transfer

Influence of light guide tip used in the photo-activation on degree of conversion and hardness of one nanofilled dental composite

The aim of this study was to evaluate the degree of conversion and hardness of a dental composite resin Filtek (TM) Z-350 (3M ESPE, Dental Products St. Paul, MN) photo-activated for 20 s of irradiation time with two different light guide tips, metal and polymer, coupled on blue LED Ultraled LCU (Dabi Atlante, SP, Brazil). With the metal light tip, power density was of 352 and with the polymer was of 456 mW/cm(2), respectively. Five samples (4 mm in diameter and 2mm in thickness-ISO 4049), were made for each Group evaluated. The measurements for DC (%) were made in a Nexus-470 FT-IR, Thermo Nicolet, E.U.A. Spectroscopy (FTIR). Spectra for both uncured and cured samples were analyzed using an accessory of reflectance diffuse. The measurements were recorded in absorbance operating under the following conditions: 32 scans, 4 cm(-1) resolution, 300-4000 cm(-1) wavelength. The percentage of unreacted carbon double bonds (% C=C) was determined from the ratio of absorbance intensities of aliphatic C=C (peak at 1637 cm(-1)) against internal standard before and after curing of the sample: aromatic C-C (peak at 1610 cm(-1)). The Vickers hardness measurements (top and bottom surfaces) were performed in a universal testing machine (Buehler MMT-3 digital microhardness tester Lake Bluff, Illinois USA). A 50 gf load was used and the indenter with a dwell time of 30 s. The data were submitted to the test t Student at significance level of 5%. The mean values of degree of conversion for the polymer and metal light guide tip no were statistically different (p = 0.8389). The hardness mean values were no statistically significant different among the light guide tips (p = 0.6244), however, there was difference between top and bottom surfaces (p < 0.001). The results show that so much the polymer light tip as the metal light tip can be used for the photo-activation, probably for the low quality of the light guide tip metal.CAPES BrazilCoordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES

Giant Magnetoresistive Phosphoric Acid Doped Polyaniline–Silica Nanocomposites

The phosphoric acid doped conductive polyaniline (PANI) polymer nanocomposites (PNCs) filled with silica nanoparticles (NPs) have been successfully synthesized using a facile surface initiated polymerization method. The chemical structures of the nanocomposites are characterized by Fourier transform infrared (FT-IR) spectroscopy. The enhanced thermal stability of the PNCs compared with that of pure PANI is observed by thermogravimetric analysis (TGA). The dielectric properties of these nanocomposites are strongly related to the silica nanoparticle loading levels. Temperature dependent resistivity analysis reveals a quasi 3-dimensional variable range hopping (VRH) electrical conduction mechanism for the synthesized nanocomposite samples. A positive giant magnetoresistance (GMR) is observed with a maximum value of 95.5% in the PNCs with a silica loading of 20.0 wt % and 65.6% for the pure PANI doped with phosphoric acid. The observed MR is well explained by wave function shrinkage model by calculating the changed localization length (ξ), density of states at the Fermi level (N(EF)), and reduced average hopping length (Rhop). The effects of particle size on the properties including thermal stability, dielectric properties, temperature dependent resistivity, electrical conduction mechanism, and GMR of the nanocomposites are also studied. © 2013 American Chemical Society