Positron studies of vapour absorption and desorption by melamine–formaldehyde resin

The lifetime spectra of positrons annihilating in vapour-absorbing polymers were measured in order to gain information about the change of dimensions and number of free volume holes during absorption and desorption. Absorption of water, methanol and ethanol vapours in melamine–formaldehyde resin leads to the shortening of ortho-positronium lifetime that can be explained by predominant filling larger holes by absorbed molecules, leaving smaller ones empty and accessible for ortho-positronium. Removal of molecules by outpumping the vapour increases the lifetime. It was found that the intensity of positronium formation does not follow the rule of proportionality to the number of free volume sites.