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There is considerable interest in the physics of soft condensed matter, including structural issues such as solid-state phase transitions. These materials, which typically exhibit no long-range order, are not only of fundamental interest but also have tremendous commercial implications, especially in the case of polymers.
Positron methods show promise of significant impact in this area due to their ability to probe open volume sizes and distributions in these materials. As an example, shown below in the figure are positron annihilation data obtained from an amorphous syndiotactic polystyrene sample, as a function of temperature. The sample was heated and then cooled, slowly. In the data we observe the following features: upon initial heating there is a change in open volume as the material relaxes (irreversible), until we reach 70 degrees, the temperature T g of the transition from the glassy phase to the rubbery phase. Above T g we see a slope due to (reversible) thermal expansion of the sample. At 120 degrees we see an abrupt decrease in free volume, due to the transition from amorphous to crystalline phases. On cooling we again observe a ‘kink' in the line, this time at the glass transition temperature of the crystalline phase.
An example to illustrate the industrial relevance of studying the microstructure of polymers is provided by gas-permeable membranes. Desirable properties for these materials include high permeability, high selectivity (i.e. for gas separation) and mechanical and thermal stability. Techniques for developing high-performance membranes fall into two categories: (i) synthesis of structures with the required properties, and (ii) post-synthesis modification, using a variety of techniques including pyrolysis, fluorination, and irradiation. Typically there is a trade-off between permeability and selectivity, so increasing one decreases the other. It has been shown however that by using ion irradiation to modify the material it is possible to achieve gains in both permeability and selectivity. The mechanism by which this can occur is not understood and is a promising subject for further study.
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