Positron annihilation and phase separation phenomena in polyurethane crosslinked coatings with different polyether segments

Eight model polyurethane crosslinked coatings were obtained from different compositions of polyisocyanurates of hexamethylenediisocyanate (HDI) and isophoronediisocyanate (IPDI) with polypropylenglycol (PPG) and perfluoropolyether macrodiols (PFPE), both having Mn 1000. The polyurethanes were characterized by Differential Scanning Calorimetry (DSC) and Positron Annihilation Spectroscopy (PAS). The DSC analysis showed that PPG based polyurethanes are monophasic materials having one glass transition temperature Tg increasing with the density of crosslinking and related to the Tg of the starting polyisocyanurate. On the other hand PFPE containing polyurethanes are polyphasic with segregation of fluorinated moiety and presence of urethane phases with highly diversified Tg values. It is found that PAS is sensitive to the phase segregation; furthermore, ortho-positronium lifetimes are correlated to the Tg. From the analysis of PAS spectra the size distributions of the nanoholes forming the free volume was obtained.