Exposure to the positron source

One of the main factors which needs to be considered in PAL analysis of polymers, is the affect which prolonged exposure to the positron source has on the lifetime parameters. It has been found that on prolonged exposure to a positron source, the o-Ps lifetimes are largely unchanged, but that there are significant variations in the o-Ps intensities for some polymers. Examples of these effects for a wide variety of polymers can be found polypropylene (PP), polyethylene (PE) [71], polystyrene [72], polycarbonates [73] poly(a-olefins) [49], poly(vinlyacetate) [74], poly(methyl methacrylate) [74] and a number of copolymers [75]. 

Dlubek et al [49] also showed an interesting relationship between the observed decrease in the o-Ps intensity and the temperature of the measurement in relation to the Tg for a series of poly (a-olefins). They found that the rate of decrease of the o-Ps decreased, but the saturation levels increased, the more the Tg is below the temperature of the measurements. 

At very low temperatures (typically below 150K) the o-Ps intensity increases as a function of exposure time for a large number of polymers. This increase in the o-Ps yields at very low temperature is explained by the reaction of free positrons with trapped electrons produced by the previously injected positrons [77], 

The variation of I3 on e+ exposure illustrates that after prolonged exposure the I3 values can no longer be considered to be related to the number of free volume holes and therefore equation 6 cannot be used. These effects have also lead a number of authors to completely discount the reliability of equation 6 as a measure of the free volume fractions [49, 75, 78, 79]. Other authors have suggested that Eq. 6 may still be used provided the samples are rejuvenated at high temperatures between each measurement [67], 

In summary the effects of exposure to the e+ source have several very important consequences for polymer studies, particularly in those where the polymer is exposed to the source for extended periods (for examples in temperature dependent studies)  

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In summary, it is clear that the o-Ps lifetime determined via the PALS technique provides accurate information on the apparent mean size of the nanoholes, which comprise the free volume in amorphous polymers. It also seems well established (see Chapter 11) that provided that the noise level in the PALS spectrum is sufficiently reduced, the distribution of o-Ps lifetimes can be obtained, which generates information regarding nanohole-size distribution. Concerns have been raised about the utility of the o-Ps intensity, I3, to characterize the number density of nanoholes and hence the fractional free volume via Eq. (12.2), because the value of I3 can be influenced significantly by the presence of species that inhibit or enhance positronium formation. We feel that we can utilize I3 values to evaluate fl actional free volumes via Eq. (12.2), provided either that the sample is rejuvenated by heating above Tg prior to measurement, and/or experiment indicates that the value of I3 remains constant within experimental error, during the time of exposure to the positron source. 

Figure 10.6 The dependence on exposure time to a positron source of the o-Ps intensity in polystyrene at different temperatures [72].

X-rays are obtained by the conventional method where accelerated electrons are used to bombard a metal anode. X-rays of higher intensity can be obtained from synchrotrons where they are emitted by accelerating electrons or positrons. The higher X-ray intensity of the synchrotron source is helpful in obtaining larger numbers of diffraction spots or e flections in shorte r exposure times. 

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