Longest lifetime component

Figure 7.19 Lifetime distribution as obtained by MELT for a sample with 23% porogen load (at the threshold to open porosity). The probabilities of lifetime are scale to the maximum of the 0.5 ns component. Lifetime probabilities for 2 to 7 ns are increased by a factor of 10 and for even longer lifetime by a factor of200. The longest lifetime components for a 80% porogen load sample are also shown (dashed).

The diffusion of gases through a polymer matrix is determined by the mobility of gas molecules through the matrix. The diffusion coefficient is therefore, at least partially determined by the free volume size of the polymer. It has been shown, for example, that there is a correlation between the free volume measured by PAL and the diffusivity of carbon dioxide in a seriers of polycarbonates [58], In a study of poly (trimethylsilyl propyne) (PTMSP), which has an extremely high gas permeability and diffusion coefficients, it was found that the lifetime data could be resolved into four components [59]. The longest lifetime component (T4) had a lifetime of... 

One experimental observable that can be obtained easily is the longest lifetime component of the fluorescence decay. This corresponds to k = 1 in eqn. (18). Thus the observed longest lifetime fluorescence lifetime would correspond to... 

In analyzing the PALS spectra, it was found that four distinct lifetime components gave the best fit for all samples. The longest lifetime component, r4 2 to 3 ns, exhibited thermal expansion behavior typical of amorphous polymers, while the second longest lifetime, r3 0.8 ns, was constant, independent of temperature. 

The fluorescence decay is multiexponential.(199 200) This is unequivocal evidence that the wyebutine base can be bound in at least two different conformations with different solvent shielding. Wells and Lakowicz(200) resolved two exponential components. They measured the normalized amplitudes and lifetimes for the wyebutine fluorescence at two different concentrations of added Mg2+ S° = 0.50, t, = 1.7 ns, = 0.50, and t2 = 5.9 ns with no added Mg2+ present, and S°i = 0.16, t, =0.6 ns, S2 = 0.84, and r2 = 6.0ns with 10 nM Mg2+. Since the 6ns component is the longest lifetime present, it must represent the conformation that shields the wyebutine to the greatest extent and is generally believed to involve a 3 stack of bases 34-38.w 199-2011 The fraction of the tRNA in this conformation increases when Mg2 + is added to the solution. This structure is also observed in crystal structures which include Mg2+.(202 204) In the other conformation(s), the wyebutine is more exposed to the solvent. A 5 stack, which does not include bases 37 and 38, is one possibility. The wyebutine base would be more exposed to the solvent and have a shorter fluorescence lifetime as a result. However, both NMR data(205 206) and chemical modification studies(207) are inconsistent with a 5 stack. For the moment, this matter is unresolved. 

Figure 1. Values of the longest lifetime r 3 from a three-component analysis [13] and i3 and r4 from a four-component analysis of the spectra (obtained from our data) vs. crystallinity.

Heat of fusion (AH) of the composites together with the intensities and lifetimes of the two longest positron lifetime components... 

Figure 7. Values of ihe intensity limes lifetime for the two longest-lived components vs. the number of silane layers for the composites with 20 and 50 vol% glass beads.

A number of positron annihilation studies with carbon materials [10-12] have shown the existence of three lifetime components the longest-lived component with a mean lifetime from 1000 to 5000 ps resulted from pick off annihilation of the orthopositronium atoms formed in the samples the intermediate component having a mean lifetime between 350 and 400 ps has been assigned to annihilation of positrons by interaction with the electron density at the surface and near-surface regions, and the shortest-lived component, with mean lifetime from 140 to 225 ps, comes from positron annihilation with 7t-electrons in the bulk of the graphite structure. 

The addition of hexanesulfonate (H) to a y-CD-16 solution in which the dimeric complex (y-CD-16-16-y-CD) is present led to the formation of a three-component complex y-CD-16-H [124], as shown by the decrease of the excimer fluorescence and the biphasic monomer fluorescence decay. The species with the longest lifetime (i.e., the three-component complex with T = 220 ns) was quenched by triethanolamine with lower than 3 X 10 dm mol" s" and by Oj with fc, = 4 x 10 dm mol" s" . For all other forms of the CD-16 complex studied in [124], the corresponding kq are higher, that is, the best protection against the quenching is offered to 11 by the three-component complex. 

Positron annihilation spectroscopy was employed to study the effect of plasticizer addition on free volrrme. The positrorrium atoms may form in amorphous regions of the polymer. In these regiorts, free volrrme exists if orthopositrorrirrm may live for several nanoseconds. The lifetime of the longest hved component, T3, is attributed to the pick-... 

This work investigated PIM-1 membranes in the three states discussed above. Nickel-foil supported NaCl was used as a positron source and stacks of film samples, each about 1mm thick, were placed either side of the source. Annihilation lifetime decay curves were measured with an EG G Ortec fast-fast lifetime spectrometer. Measurements were made both in air and under an inert atmosphere (N2). However, o-Ps lifetimes in air were reduced due to quenching by oxygen, so only results obtained under N2 are discussed here. Results were analysed in terms of a four component lifetime distribution, which allowed obtaining better statistical fit. The two longest lifetimes, T3 and T4, for PIM-1 in... 

Generally, three to four lifetime components are resolved in polymers, and their attribution is as follows. The shortest lifetime component ri with intensity h is attributed to contributions from free positron annihilation (inclusive of p-Ps lifetime). The intermediate lifetime component Z2 with intensity 12 is considered to be due to the annihilation of positrons trapped at defects present in the crystalline regions, or those trapped at the crystalline-amorphous interface boundaries. The longest-lived component T3 with intensity 1, is due to pick-off annihilation of the o-Ps in the free volume cavities present mainly in the amorphous regions of the polymer [42,43]. The simple model of a Ps atom in a spherical potential well of radius R leads to a correlation between o-Ps hfetime and R [70,128-130] ... 

PALS is a nondestmctive technique that provides an effective approach for the study of the free volume in the solid state. It is based on the localization of the positronium (Ps) in the free volume holes because of their repulsion by the surrounding atoms. Generally, the annihilation spectra in polymers consist of three exponentially decaying components that correspond to the three main processes of their annihilation [50]. Each of these processes is characterized by the mean lifetime and the probability of annihilation (intensity). The longest lifetime corresponds to the annihilation process of o-Ps in the free volume holes. This is the process that is used to estimate the free volume in the amorphous polymers. The size of the free volume holes, R, in the polymer can be determined from the measured lifetime, so Ps, using the semiempirical equation... 

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