Bi-soft segment polyurethane membranes: Surface studies by X-ray photoelectron spectroscopy

Polyurethane membranes containing two soft segments, poly(propylene oxide) and polybutadiene, were synthesized by the reaction of a polypropylene oxide-based tri-isocyanate-terminated prepolymer (PU) with a polybutadiene diol (PBDO). The soft segments of these polyurethanes are phase-separated in the bulk to different degrees according to the variation of the PBDO/PU ratio. The membrane surfaces were investigated by X-ray Photoelectron Spectroscopy (XPS). The results show that the surface is enriched in PU. This surface enrichment was found to increase with the increase of PBDO content and therefore, with the mixing of the two soft segments in the bulk. Moreover, segregation at the surface of urethane/urea hard segments was found to reach a maximum at PBDO/PU equal to 0.33 and to decrease for larger PBDO/PU ratio. The membrane with highest surface segregation of urethane/urea groups presents the highest value of surface energy and the highest polarity confirming the migration of the hard segments to the surface.