All materials are not equal

The models that relate lifetimes to pore sizes contain no material dependence. However, it is easy to imagine that the interaction of positronium with the pore wall does indeed depend on the wall material. 

It turns out that the positronium signal from the pores is very small and quite comparable to MSSQ with less than 10% porogen. Either positronium rarely forms and traps the pores, or the positronium-wall interaction is much stronger and causing pick-off annihilation within a few bounces, rather than thousands of bounces as in the case of MSSQ. 

The Doppler shifts of positron annihilations in the sample provide a more sensitive measure of this effect. The plasma treatment oxidizes the rather open structure of HSSQ. Oxygen atoms are included in the network. These have high momentum electrons in p-orbitals, which lower the sharpness of the annihilation line [22], 

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