Oxygen permeability in thermoplastic polyurethanes

The thermal and oxygen transport properties of a series of thermoplastic polyurethanes (TPUs) based on 4,4′‐methylene diisocyanate (MDI) and 1,4‐butanediol (BD) as hard segments, and poly(tetramethylene glycol) (PTMG) or poly(butylene adipate) (PA) as soft segments, are studied. Oxygen permeabilities (P) of both polyester‐based and polyether‐based TPUs increase with decreasing hard segment fractions. Oxygen solubility (S) and diffusivity (D) can be derived from permeation curves. S correlates with the amount of excess free volume as determined by the difference between glass‐transition and testing temperatures (i.e., the degree of super cooling) and decreases with the increased T_g in polyester‐based TPUs. The intensity of low temperature gamma transition reflects the activation energy for D; the higher the intensity is, the lower D is annealed TPU samples exhibited higher oxygen permeabilities as well as lower storage moduli at room temperature, despite modest increases in overall crystallinity. Dedensification of the soft segment phase during annealing/crystalline phase growth is the most likely explanation for loss of mechanical and barrier properties after annealing as partially confirmed by Fourier transform infrared spectroscopy. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012