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 Tg 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. (c) 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012