Positron annihilation spectroscopy for chemical analysis: A novel probe for microstructural analysis of polymers☆

Positron annihilation : A probe for nanoscale voids and free volume ?

Measurements of average free volume hole sizes, 〈vf〉, and the fractional free volumes, fps, in vulcanized cis-polyisoprene (CPI), high-vinyl polybutadiene (HVBD), and their 50 : 50 blend were made via determination of orthopositronium annihilation lifetimes. The results are compared to corresponding data on the uncured materials. On crosslinking, 〈vf〉 decreases in the rubbery state but remains essentially unchanged in the glass. This is consistent with the expectation that the crosslinks greatly restrict the thermal expansion of the chains above the glass transition temperature (Tg) but have less influence on the packing density in the glass. Scaling relationships between 〈vf〉, fps, the thermal expansion coefficient αf = dfps/dt, and Tg are examined. We find that 〈vf〉g, the hole volume at Tg, and fps,g, the fractional free volume at Tg, each increase significantly with increasing Tg. This behavior is consistent with previous observations reported in the literature and has been interpreted as a manifestation of the kinetic character of the glass transition. High-Tg polymers need a larger free volume to pass into the liquid state. The change in expansion coefficient on passing from the glass to the liquid, Δαf = αf,l − αf,g, increases slowly with Tg, as predicted by free volume theory. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2754–2770, 1999

Positron annihilation lifetime studies of changes in free volume on cross-linking cis-polyisoprene, high-vinyl polybutadiene, and their miscible blends

The effect of cross-linking on the free-volume properties of poly(diethylene glycol bis(allyl carbonate)) networks was studied using a series of networks with progressively decreasing density of cross-links. The networks were prepared by bulk copolymerization of ethylene glycol bis(allyl carbonate) with an increasing amount of allyl ethoxyethyl carbonate. The mean free-volume hole radius (r) and the hole volume (v) as well as the hole radius and hole volume density distributions were estimated from positron lifetime measurements. It was found that with increasing concentration of monoallyl comonomer, the mean hole size increases from r = 0.250 nm (v = 0.066 nm3) for the pure diallyl component to 0.300 nm (0.114 nm3) for the 75% concentration of the monoallyl comonomer. Similarly, the hole size distributions shift to larger values. The local free-volume properties were correlated with the glass transition temperature and the specific volume. The comparison of the hole volume with the specific volume allowe...

Effect of cross-linking on the free-volume properties of diethylene glycol bis(allyl-carbonate)polymer networks: a positron annihilation lifetime study

Nanoscopic properties of silica filled polydimethylsiloxane by means of positron annihilation lifetime spectroscopy

Positronium annihilation spectroscopy (PAS) has been used to study the microstructural properties of amine-cured epoxy polymers. We have determined the free-volume “hole” sizes in these polymers by comparing the observed ortho-positronium lifetimes with the known lifetime–free volume correlation for low-molecular-weight systems. The free volumes for four epoxies with different crosslink densities are found to vary significantly over the temperature range between −78° and 250°C. The free-volume holes for these polymers are found to range from 0.025 to 0.220 nm3. Two important transition temperatures were found: one corresponds to the glass transition temperature Tg determined by differential scanning calorimetry (DSC), and the other occurs about 80–130°C below Tg. The sub-Tg transition temperature is interpreted tentatively as being where hole size reaches dimensions adequate for positronium trapping or else the onset temperature for local mode or side-chain motions. These two transition temperatures plus two additional onset temperatures are found to be correlated with crosslink densities calculated from stoichiometry.

D. P. Ames

Papers

Positronium annihilation in amine‐cured epoxy polymers