Polymer morphology of CO2‐blown rigid polyurethane foams: Its fractal nature

Small‐angle x‐ray scatting (SAXES) and transmission electron microscopy (TEM) have been applied to study polymer morphologies of CO2,‐blown rigid polyurethane foam samples of varying isocyanate index. The results are consistent with an irregular (meso) phase segregated structure with phase boundaries showing fractal symmetry. Phase segregation persists, througtout the index range, althougt the interfacial surfaces tends to smooothen with increasing isocyanate index to evetually results in a significant degree of energetic ‘sharing’ across the4 phase boundary. The latter can satisfactorily be explained by a free volume double layer (PVDL) model. Mechanistically. Fractal symmetry was attributed to the competition between crossliking and phase segregation tendencies in rigid PU foam polymers. The resulting frozen‐in morphology represents an early stage of the development towards a more regular spinodal phase segregation, as occurring in polyuethane foam systems of lower crosslink density (e.g. flexible foams).