Eight model polyurethane crosslinked coatings were obtained from different compositions of polyisocyanurates of hexamethylenediiso-cyanate (HDI) and isophoronediisocyanate (IPDI) with polypropylenglycol (PPG) and perfluoropolyether macrodiols (PFPE), both having M-n 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 T-g increasing with the density of crosslinking and related to the T-g 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 T-g values. It is found that PAS is sensitive to the phase segregation; furthermore, ortho-positronium lifetimes are correlated to the T-g. From the analysis of PAS spectra the size distributions of the nanoholes forming the free volume was obtained.