Surface Studies of Polymer Blends by Sum Frequency Vibrational Spectroscopy, Atomic Force Microscopy, and Contact Angle Goniometry

The surface composition, surface structure, and wetting properties of a polymer blend were studied by a combination of surface-sensitive techniques: IR + visible sum-frequency generation vibrational spectroscopy (SFG), atomic force microscopy (AFM), and contact angle goniometry. The polymer blend studied is composed of Biospan-S (BS) and a phenoxy base polymer (BP). BS is a polyurethane, capped with poly(dimethylsiloxane) (PDMS) end groups, and has a water contact angle of 94°. BP is more hydrophilic with a water contact angle of 74°. The SFG study shows that the more hydrophobic BS component likes to segregate to the blend surface in air and approaches a full surface coverage when the BS bulk concentration is only 1.7 wt %. The surface density of BS decreases with the decrease of its bulk concentration from 1.7 wt % and becomes undetectable at the surface, at a bulk concentration of 0.17 wt %. The surface free energy of the blend varies accordingly, as seen from the contact angle measurements. The AFM study also shows that the blend surface is predominantly occupied by BP for BS bulk concentrations lower than 0.17 wt % and by BS for BS bulk concentrations higher than 1.7 wt %. It reveals a domain pattern in the intermediate region when BS and BP coexist on the surface. The combined information obtained by SFG, AFM, and contact angle goniometry provides a more detailed understanding of interfacial phenomenon of blended macromolecular systems at the molecular level.