Surface composition of biopolymer blends Biospan-SP/Phenoxy and Biospan-F/Phenoxy observed with SFG, XPS, and contact angle goniometry

The surface compositions of two biopolymer blends, Biospan-SP/Phenoxy (BSP/ PHE) and Biospan-F/Phenoxy (BF/PHE), have been studied using sum frequency generation (SFG), X-ray photoemission spectroscopy (XPS), and contact angle goniometry. BSP and BF are polyurethanes capped with poly(dimethylsiloxane ) (PDMS) and fluoroalkyl (-(-CF2-)(n)-) as end groups, respectively. With c ontact angle goniometry, the surface tensions of pure BSP, BF, and PHE were found to be 26, 16, and 45 dyne/cm. For each of the blends, the polymer co mponent with a lower;surface tension (BSP or BF, not PHE) tends to segregat e to the surface. Our SFG experiment shows that the surface concentration o f the surface-active component increases sharply as its bulk concentration increases. For BSP/PHE land BF/PHE) in air, the surface of the polymer blen d is fully covered by BSP land BF) at a bulk concentration of 3.5 wt % (and 1 wt %). The contact angle measurements and the XPS studies yield compatib le results. Comparison of results for BSP/PHE, BS/PHE (published before(1)) , and BF/PHE polymer blends shows that the lower the surface energy of the surface-active component (surface tension: BF < BS ( BSP), the easier it is for the component to segregate to the surface (the minimum bulk concentrat ion to saturate the surface is BF (1 wt %) < BS (1.7 wt %) < BSP (3.5 wt %) ). After exposure to water, SFG spectra indicate that the surface layer of a polymer blend could be restructured. For BSP (3.5 wt %)/PHE, the hydropho bic end groups of BSP submerge while the hydrophilic polyurethane backbone emerges. For BF (1 wt %)/PHE, PHE emerges at the surface after exposure to water, but for BF (5 wt %)/PHE, the BF component dominates the surface in b oth air and water. Our results demonstrate the bifunctionality of polymer b lends and show that the surface chemistry of polymer blends may be dominate d by a minor component, while the mechanical stability of the polymer is co ntrolled by the major component.