4 research outputs found
Structural dependence of MEH-PPV chromism in solution.
The chromism observed in the MEH-PPV polymer
in tetrahydrofuran (THF) solution is discussed as a
function of the structural morphology of the backbone
chains. To evaluate this phenomenon, we carried out simulations
employing a hybrid methodology using molecular
dynamics and quantum mechanical approaches. Our
results support the hypothesis that the morphological order–
disorder transition is related to the change from red to blue
phase observed experimentally. The morphological disorder
is associated with total or partial twisted arrangements in the
polymer backbone, which induces an electronic conjugation
length more confined to shorter segments. In addition, the
main band of the MEH-PPV UV–Vis spectrum at the lower
wavelength is related to the blue phase, in contrast to the red
phase found for the more planar backbone chains
Polarized emission from stretched PPV films viewed at the molecular level.
We present a study on the photoluminescence (PL) of thin films of poly-(p-phenylene vinylene) (PPV),
non-stretched and uniaxially stretched. The experimental study was carried out using linear polarized
light as the excitation beam, oriented either parallel or perpendicular to the stretch axis (S). The results
showed that when the excitation light source has polarization perpendicularly oriented to the stretch
direction, the emitted PL presents maximum intensity in the orientation S, and a minimum in the
direction orthogonal to S. In order to understand this interesting phenomenon, we employ theoretical
simulations at the atomistic level. We use classical molecular dynamics to simulate amorphous PPV
films, non-stretched and stretched, from which we find a tendency of overall alignment of PV units to S,
and of local clustering in herring-bone and p-stacking partial symmetries. Our study of optical activity of
these kinds of clusters, performed through a quantum semi-empirical method, allows us to explain this
polarization conversion behavior, and indicates the possibility of using underivatized PPV as the active
layer for polarized electroluminescent devices
Synthesis and application of a new carboxylated cellulose derivative. Part III: Removal of auramine-O and safranin-T from mono- and bi-component spiked aqueous solutions
In the third part of this series of studies, the adsorption of the basic textile dyes auramine-O (AO) and safranin-T (ST) on a carboxylated cellulose derivative (CTA) were evaluated in mono- and bi-component spiked aqueous solutions. Adsorption studies were developed as a function of solution pH, contact time, and initial dye concentration. Adsorption kinetic data were modeled by monocomponent kinetic models of pseudo-first- (PFO), pseudo-second-order (PSO), intraparticle diffusion, and Boyd, while the competitive kinetic model of Corsel was used to model bicomponent kinetic data. Monocomponent adsorption equilibrium data were modeled by the Langmuir, Sips, Fowler-Guggenhein, Hill de-Boer, and Konda models, while the IAST and RAST models were used to model bicomponent equilibrium data. Monocomponent maximum adsorption capacities for AO and ST at pH 4.5 were 2.841 and 3.691 mmol g−1, and at pH 7.0 were 5.443 and 4.074 mmol g−1, respectively. Bicomponent maximum adsorption capacities for AO and ST at pH 7.0 were 1.230 and 3.728 mmol g−1. Adsorption enthalpy changes (ΔadsH) were obtained using isothermal titration calorimetry. The values of ΔadsH ranged from −18.83 to −5.60 kJ mol−1, suggesting that physisorption controlled the adsorption process. Desorption and re-adsorption of CTA was also evaluated