Phosphorescence lifetime in polymer relaxation studies: Phase resolution method.

We describe in this work a simple experimental set up to perform time dependent luminescence experiments in time scales from mill-seconds to seconds, based on the phase resolution of the emission signal. This system is composed by modulation of a continuous light source with an external chopper controlled by a lock-in amplifier. We exemplified the utility of the system in studies of phosphorecence emission using benzophenone dissolved in polystyrene and since the phosphorescence intensity and lifetime are temperature-dependent processes, we also studied polymer relaxation processes in the temperature range from 20 to 400K. A software that drives the spectrofluorimeter and controls both the cryosystem and the lock-in amplifier was developed.21333233

Phosphorescence Lifetime In Polymer Relaxation Studies: Phase Resolution Method [aquisição De Tempos De Vida De Fosforescência Com Resolução De Fase E Sua Aplicação Ao Estudo De Relaxações Em Polímeros]

We describe in this work a simple experimental set up to perform time dependent luminescence experiments in time scales from mili-seconds to seconds, based on the phase resolution of the emission signal. This system is composed by modulation of a continuous light source with an external chopper controlled by a lock-in amplifier. We exemplified the utility of the system in studies of phosphorecence emission using benzophenone dissolved in polystyrene and since the phosphorescence intensity and lifetime are temperature-dependent processes, we also studied polymer relaxation processes in the temperature range from 20 to 400K. A software that drives the spectrofluorimeter and controls both the cryosystem and the lock-in amplifier was developed.213332336Birks, J.B., (1970) Photophysics of Aromatic Molecules, , Willey-Interscience, LondonSakurovs, R., Ghiggino, K.P., (1981) Aust. J. Chem., 34, p. 1367Rabek, J.F., (1982) Experimental Methods in Photochemistry and Photophysics, , John Willey & Sons, AvonO'Connor, D., Phillips, D., (1984) Time-Correlated Single Photon Counting, , Academic Press, LondonBailey, E.A., Rollefson, G.K., (1954) J. Chem. Phys., 21, p. 1315McGown, L.B., Bright, F.V., (1984) Anal. Chem., 56, pp. 1400A(1989) Manual de Operação do Amplificador Lock-in SR530, , Stanford Research SystemsTalhavini, M., Atvars, T.D.Z., (1995) Quím. Nova, 18, p. 298Sabadini, E., (1988), Tese de MestradoUNICAMPTalhavini, M., (1995), Dissertação de MestradoUNICAMPGuillet, J.E., (1988) Advances in Photochemistry, 14, p. 91. , Volman, D. H.Hammond, G. S. and Gollnick, K. (editores:), Wiley, New YorkHalary, J.L., Monnerie, L., (1986) Photophysical and Photochemical Tools in Polymer Science, 182, p. 589. , NATO ASI Series, Winnik, M. (editor), D. Riedel, New YorkRusakonics, R., Byer, G.W., Leermakers, P.A., (1971) J. Am. Chem. Soc., 93, p. 3263Hikosaka, Y., Hikida, T., (1996) Chem. Phys., 203, p. 137Zevenhuijzen, D., Van Der Werf, R., (1977) Chem. Phys., 26, p. 27

Photostability of xanthene molecules trapped in poly(vinyl alcohol) (PVA) matrices

Photobleaching reactions of fluorescein and Rose Bengal molecules dissolved in PVA matrices were studied as a function of the temperature. We used and compared two theoretical approaches to describe these photobleaching kinetics. Photobleaching quantum yield calculations were done using an adapted model (B-model) for polychromatic excitation that allowed us to estimate the average number of photons absorbed by each dye molecule before its bleaching. The other model uses exponential functions allowing us to calculate rate constants and activation energy. A slower photobleaching process was obtained for Rose Bengal sorbed in PVA matrices compared with fluorescein. We also obtained that the Rose Bengal photobleaching process is always single-exponential kinetic independent of the polymer relaxation process. Nevertheless, fluorescein photobleaching reaction is bi-exponential process at higher concentrations and it is strongly dependent on the polymer relaxation processes. These results were interpreted in terms of the different efficiencies among the processes involving quenching of the lowest triplet state. (C) 1999 Elsevier Science S.A. All rights reserved.120214114

Aquisição de tempos de vida de fosforescência com resolução de fase e sua aplicação ao estudo de relaxações em polímeros Phosphorescence lifetime in polymer relaxation studies: phase resolution method

<abstract language="eng">We describe in this work a simple experimental set up to perform time dependent luminescence experiments in time scales from mili-seconds to seconds, based on the phase resolution of the emission signal. This system is composed by modulation of a continuous light source with an external chopper controlled by a lock-in amplifier. We exemplified the utility of the system in studies of phosphorecence emission using benzophenone dissolved in polystyrene and since the phosphorescence intensity and lifetime are temperature-dependent processes, we also studied polymer relaxation processes in the temperature range from 20 to 400K. A software that drives the spectrofluorimeter and controls both the cryosystem and the lock-in amplifier was developed

The role of the triplet state on the photobleaching processes of xanthene dyes in a poly(vinyl alcohol) matrix

This work presents the results for the temperature dependence of the photobleaching reactions of Fluorescein and Rose Bengal dissolved in poly(vinyl alcohol) in the presence of diethylenetriamine, potassium dichromate and 2-aminoethanethiol hydrochloride. These samples were subjected to a continuous irradiation with a 150 W mercury high pressure are lamp at a wavelength coincident with the maximum of the dye absorption. The photobleaching processes were described by a double exponential and a mono-exponential function for Fluorescein and Rose Bengal, respectively. Apparent activation energies were calculated using the Arrhenius plots and these Values were strongly affected by the presence of additives. Different additives are employed in the studies of the photobleaching reactions of Fluorescein and Rose Bengal in poly(vinyl alcohol) matrices: (a) diethylenetriamine is a quencher of all excited states; (b) potassium dichromate is an electron acceptor complexed with the polymer chains and inhibited the photobleaching; (c) 2-aminoethanethiol hydrochloride is an efficient quencher of the triplet state. The role of these additives upon the photoreaction is discussed. (C) 2001 Elsevier Science B.V. All rights reserved.1394170018719

Dye-polymer interactions controlling the kinetics of fluorescein photobleaching reactions in poly(vinyl alcohol)

This work presents results for the temperature dependence of the photobleaching reactions of Fluorescein dissolved in poly(vinyl alcohol) (PVA) irradiated by a continuous 150 W Hg high pressure are lamp. We observed that the photobleaching process efficiency is dependent on: lamp power, dye concentration and temperature. Although the photobleaching processes can only be described by a single-exponential function for lower concentration samples, a biphasic model must be employed for intermediate concentrations, resulting in one slower and one faster processes, while, for higher concentration samples, even multiple-exponential functions are unable to fit the experimental data. This complex behavior is probably due to the co-existence of many different types of energy transfer and photochemical processes with different rate constants. Therefore, a model based on a sequential distribution of site occupation by the dye molecules in the matrix, controlled by dye-polymer interaction strengths, is proposed to explain the complex-exponential kinetic behavior. Moreover, photobleaching process efficiency is somewhat increased by temperature and, above the PVA glass transition temperature, T-g, (350 K), it follows a first order kinetics, fitted by a mono-exponential function. (C) 1998 Elsevier Science S.A. All rights reserved.1141657

Characterization of static and dynamic properties of a low-density polyethylene. Evidence for translocation of fluorescent probes upon stretching and determination of relaxation processes at the mesoscopic level

Fluorescence from anthracene guest molecules (AN) has been used to study secondary alpha, beta and gamma relaxation processes in unstretched and stretched low density polyethylene (LDPE) in the temperature range 15-400 K. The LDPE was characterized by several different techniques. It contains about 4 vinylidene, vinylene and vinyl type C=C bonds and about 320 CH3 groups per 10 000 CH2 units. The unstretched polymer is 31% crystalline and has a melting point, T-m, of 109 degrees C. The 5x stretched polymer is 51% crystalline and T-m = 110 degrees C. Although the degree of crystallinity of the films suffers a large change upon macroscopic stretching, the melting temperature (determined by d.s.c. and fluorescence spectroscopy) and the relaxation temperatures do not. However, plots of total fluorescence intensity or fluorescence intensity ratios at different wavelengths vs temperature indicate that dynamic processes involved with the excited singlet states of anthracene are dependent upon polymer stretching. These results, in combination with others obtained previously, suggest that stretching reduces the average amount of free volume at guest sites and induces a translocation of anthracene molecules toward sites at the crystalline-amorphous interfaces. Copyright (C) 1996 Elsevier Science Ltd.37194365437

Translocation of fluorescent probes upon stretching low-density polyethylene films. Comparison between 'free' and covalently-attached anthryl groups

Fluorescence from (9-anthryl)methyl groups (AN-C) attached covalently to polymer chains has been used to study secondary alpha-, beta- and gamma-relaxation processes in two types of stretched and unstretched low-density polyethylene (LDPE) with different degrees of crystallinity and frequencies of branches in the temperature range 55-400 K. Plots of total fluorescence intensity or fluorescence intensity ratios at different wavelengths versus temperature indicate that the efficiency of radiationless deactivation processes is very sensitive to film stretching (i.e. micromorphological changes induced by macroscopically applied stress) and to secondary relaxation processes of LDPE (i.e. micromorphological changes induced by temperature). Also, since the efficiencies of intermolecular radiative energy transfer processes in LDPE suffered by AN-C and 'free' anthracene (ANH), doped into native films, are much different, the mode of 'reporter' incorporation into a film and its treatment thereafter can establish differing local concentration gradients. The results suggest that, although stretching reduces, on average, the amount of free volume available at guest sites and partially orients both AN-C and ANH probes, the AN-C (due to their being constrained to their original positions of incorporation) are more influenced by polymer relaxation processes, and the excited singlet states of both probes are more sensitive to temperature in stretched films than in unstretched ones. Overall, comparisons between the behaviour of the covalently-attached and free lumophores provide insights into how external perturbations, such as stress and temperature changes, affect specific aspects of polymer microstructure and mobility and what is the influence exerted by subtly differing environments on the photophysical properties of the probe. (C) 1998 Elsevier Science Ltd. All rights reserved.39143221323

The role of the triplet state on the photobleaching processes of xanthene dyes in a poly(vinyl alcohol) matrix

Abstract This work presents the results for the temperature dependence of the photobleaching reactions of Fluorescein and Rose Bengal dissolved in poly(vinyl alcohol) in the presence of diethylenetriamine, potassium dichromate and 2-aminoethanethiol hydrochloride. These samples were subjected to a continuous irradiation with a 150 W mercury high pressure arc lamp at a wavelength coincident with the maximum of the dye absorption. The photobleaching processes were described by a double exponential and a mono-exponential function for Fluorescein and Rose Bengal, respectively. Apparent activation energies were calculated using the Arrhenius plots and these values were strongly affected by the presence of additives. Different additives are employed in the studies of the photobleaching reactions of Fluorescein and Rose Bengal in poly(vinyl alcohol) matrices: (a) diethylenetriamine is a quencher of all excited states; (b) potassium dichromate is an electron acceptor complexed with the polymer chains and inhibited the photobleaching; (c) 2-aminoethanethiol hydrochloride is an efficient quencher of the triplet state. The role of these additives upon the photoreaction is discussed