Positron Annihilation Spectroscopy Polymer Blends and Miscibility

Positron Annihilation Spectroscopy Polymer Blends and Miscibility 

Characterization of Polymer Blends Miscibility, Morphology, and Interfaces, First Edition. 

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I 27 Positron Annihilation Spectroscopy Polymer Blends and Miscibility... 

Gomaa, E., Microstructure and miscibility of acrylonilrile-butadiene rubber/ethylene-propylene-diene monomer blends studied by positron annihilation spectroscopy, J. Appl. Polym. ScL, 105, 2564-2570 (2007). 

Liu, J., Jean, Y. C., and Yang, H., Free-volume hole properties of polymer blends probed by positron annihilation spectroscopy miscibility. Macromolecules, 28, 5774-5779 (1995). 

Positron Annihilation Spectroscopy (PALS) can investigate the free volume existing between polymer chains. The lifetime of particles (positrons) injected into a sample can thus provide information on the void structure existing in polymers and polymer blends. For immiscible polymers, free volume existing at the interface due to poor adhesion can be detected by PALS. In miscible polymers, densification due to favorable interactions may be capable of determination. This technique involves the injection of positrons into a polymeric system from a radioisotope capable of emitting positrons, such as Na. The positrons (positively charged electrons) combine with electrons to annihilate or to form a bound state called a positronium (Ps). If the spins of the positron and electron are antiparaUel, para-positroniums (pPS) with a lifetime of 0.125 ns are formed. If the spins of the positron and electrons are parallel, an orthopositronium (oPs) is formed with a lifetime of 1-5 ns. The oPs hfetime, Ts, is related to the free volume cavity in which the oPs is formed [388,389]. 

Positron annihilation spectroscopy (PALS) is a technique of free volume determination in polymers that involves the injection of subatomic particles and the measurement of their decay times. This technique can be very sensitive to the degree of miscibihty and free volume behavior of polymer blends. The concept of free volume is important to understand polymer characteristics in the glassy state. For instance, PALS was used to evaluate the free volume sites of thermotropic hquid crystalline polymer blends. These blends presented smaller and fewer free volume sites than expected from a weighted average due to their intrinsic affinity. This is interesting because in contrast to thermoplastic blend results, the degree of blend miscibility alters free volume behavior as a function of blend composition [106]. In addition, the order and the dynamics in the mesophase can be accessed by nuclear magnetic resonance [107]. 

Recent developments have been in the area of microthermal analysis using thermal conductivity with thermal diffiisivity signals or AFM to visualize specific areas or domains in the material and perform localized thermal analysis studies (183,184). Relaxational behavior over time and temperature is related to changes in free volume of the material. Positron annihilation lifetime spectroscopy (PALS) measurements of positron lifetimes and intensities are used to estimate both hole sizes and free volume within primarily amorphous phases of polymers. These data are used in measurement of thermal transitions (185,186) structural relaxation including molecular motions (187-189), and effects of additives (190), molecular weight variation (191), and degree of crystallinity (192). It has been used in combination with DSC to analyze the range of miscibility of polymethyl methacrylate poly(ethylene oxide) blends (193). 

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