Studies in thermoplastics

A large number of PAL studies on thermoplastic polymers have been carried out over the past 35 year. Once again this section will not attempt to review all the relevant literature. Instead an overview will be presented to illustrate the types of information, which can be obtained from PAS. An comprehnsive review, by Pethrick [45] can be found for the various studies conducted on a varity of thermoplastics. In addition a comphrehensive list of published positron data for a large number of polymers is included as an appendix for this book. 

Many of the early studies done on polymers were carried out on commercial materials. Unfortunately in many cases these materials where not fully characterize in terms of the chemical structure and purity. The result is that data presented by different workers on the same polymers, 

A number of authors have found that the lifetime spectra of semicrystalline polymers are best resolved into three components. In a study of PEEK (poly(ether ether ketone)), Nakanishi et al [14] found that a three component fit was best. They observed that the o-Ps lifetime (r3) did not change with an increasing amounts of crystallinity, but the o-Ps yield (I3) decreases linearly with an increasing amount of crystallininty. It was also demonstrated that I3 extrapolates to 0 at 100% crystallinity. No o-Ps lifetime was observed that might be attributed to the annihilation within the crystalline regions of the polymer. Lind et al [43] found a similar result for polypropylene where the x3 component changed very little with the amount of crystallinity, but the I3 value decreased with an increasing amount of 

The above discussion illustrates a very important aspect of PAL studies in polymers. In order for PAL studies to be meaningful there needs to be a sound theoretical basis for the interpretation of the data and results. The discussion also illustrates the increasing complexity of the underlying theory as more complex systems are studied. It is clear that in order for PAL to be developed as a tool for the characterization of these systems there needs to be simultaneous development and investigation of the underlying theory applicable to these systems. 

Dlubek et al [49] studied a series of metallocene-catalyzed poly a-olefins) with progressively longer chains as the pendant side groups from polypropylene to poly-l-eicosene (20 carbons). Their results show an interesting relationship between the o-Ps lifetime and intensity in the amorphous phase for this series of polymers. They found that the average hole size and o-Ps intensity from the amorphous phase, decreased from polyethylene to polypropylene, followed by a slight increase to poly-1-butene. There was a rapid rise in hole size and intensity to poly-l-dodecene. 

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