Polymer Lifetime Measurements

Aaivation energy, 348.9 KJ/mole pre-exponential factor 1.98E+21Arun 

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During the course of these studies, it was found that fluorescence intensity from the polymeric films rapidly decreased on continued excitation in a fluorescence spectrophotometer (ca. 30% loss in 1 min for L). Herein, we (1) elaborate further upon the fluorescence loss studies, (2) provide direct evidence for RET from fluorescence lifetime measurements, and (3) present preliminary findings on the photochemistry of model compounds for polymer 1. The results support the conclusion, from previous studies, that the effectiveness of added stabilizer decreases with time due to formation of a photoproduct(s) from the polymer which competes in RET, and is less able to dissipate the resulting excitation energy.1... 

The measurements were performed in vacuum on dry samples, thus the annihilation technique can be especially useful in the case of soft media (e.g. polymers), prone to swell when filled with a liquid. It is to be noted that positron lifetime measurements can be performed at arbitrary temperature. 

Hayashi has investigated in some detail the ionic photopolymerization of styrene monomers. Free ion lifetimes measured by pulse electrical conductivity measurements were found to agree with those calculated from steady-state conductance measurements. Other studies of interest on radical addition polymerization include the photodimerization of polymers containing thymine bases, diene polymerization by terbium complexes, polymerization of vinyl acetate, and preparation of light-sensitive polyacrylates. 

We have developed a direct and noninvasive method to determine Tg of polymers at the interface with solid substrates [54, 55]. The strategy is to use fluorescence lifetime measurements using evanescent wave excitation. Figure 15a shows PS-NBD, i.e., PS containing the dye 6-[A-(7-nitrobenz-2-oxa-l,3-diazol-4-yl)amino]hexanoic acid (NBD) [55]. The NBD fraction of PS was sufficiently low to avoid self-quenching of the dye. The NBD dye was excited with the second-harmonic generation of a mode-locked titanium sapphire laser equipped with a... 

Algers, J., Suzuki, R., Ohdaira, T., and Maurer, F. H. J., Characterization of free volume and density gradients of polystyrene surfaces by low-energy positron lifetime measurements. Polymer, 45, 4533-4539 (2004a). 

Bandzuch, R, Kristiak, J., Sausa, O., and Zrubcova, J., Direct computation of the free volume fraction in amorphous polymers from positron lifetime measurements, Phys. Rev. B, 611, 8784-8792 (2000). 

Kilburn, D., Wawryszczuk, J., Dlubek, G., Pionteck, J., Hassler, R., and Alam, M. A., Temperature and pressure dependence of the free volume in poly isobutylene from positron lifetime and pressure-volume-temperarnreexperiments,Macromol. Chem. Phys., 207,721-734(2006b). Kim, S. H., Chung, J. W., Kang, T. J., Kwak, S.-Y, and Suzuki, T., Determination of the glass transition temperature of polymer/layered silicate nanocomposites from positronium annihilation lifetime measurements. Polymer, 48, 4271-4277 (2007). 

Figure 3. The experimental setup for the PALS measurements of sorption of liquid vapor in polymers. The positron source together with the sample polymer is contained in one arm of the glassware and the liquid to be sorbed is contained in the other arm. The positron lifetime measurement is performed by detecting the gamma-rays emitted from the source and from positron annihilation in the sample.

The generally recognized and the most reliable method for investigation of free volume in polymers is positron annihilation lifetime spectroscopy (PALS). It was applied for investigation of PTMSN and related polymers. This method is based on the measurement of lifetime spectra of positrons in polymers - lifetimes (ns) and corresponding intensities li (%). Longer lifetimes (or T3 and T4) (so-called o-orthopositronium lifetimes) can be related to the mean size of free volume R. 

The coloration efficiency (tf) was calculated using absorbance chaises (AA) at 560 nm for BBL and at 522 mn for BBB. The tj values for BBB ECDs decreased as the BBB film thickness increased, whereas an opposite trend was observed for BBL ECDs. Therefore, the electrochemical redox reaction probably does not occur efficiently with a thick BBB film. Lifetime measurements were performed by switching potentials fi om neutral to reduced state. ECDs with PEO and PMMA-based polymer electrolytes produced lifetimes with more than 30,000 switching cycles. Despite their better conductivity, PEO-based polymer electrolyte yielded ECD with shorter lifetimes dian those wifii the PMMA-based electrolyte, since an ECD with PMMA lasted for more than 100,000 switching cycles. Passivation of V2OS, such as the formation of PEO-VaOs con und (7), may be possible during cycling and cause fiiis shorter lifetime. 

Provorov et al. [520] have studied a rather extensive group of elastomer additives (accelerators, stabilisers, softeners, fillers, and other ingredients) for possible analysis by fluorescence techniques. No fluorescence lifetime measurements have been applied for discriminating stabilisers in polymers. UV microscopy is another means of measuring the concentration (and distribution) of UV absorbing or fluorescent additives in plastics (cfr. Chp. 5.3.2). 

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