Relationship between film thickness and chain structure of ultrathin polyurethane films

Polyurethanes (PUs) with three hard segment contents (20, 34 and 45 wt%) were synthesized with poly(oxytetramethylene) glycol (PTMG), 4,4′-diphenylmethane diisocyanate (MDI) and 1,4-butanediol (BD) by a prepolymer method. PU thin and ultrathin films were prepared onto a hydrophilic silicon wafer by a spin coating method. The change in microphase separation of PU thin and ultrathin films was investigated by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and grazing incidence small angle x-ray scattering (GISAXS) measurement as a function of film thickness. Microphase-separated structure was clearly observed for all PU films, in which the hard segment domains were surrounded by the soft segment matrix. With decreasing film thickness, interdomain spacing between hard segment domains decreased. This seems to be simply related to a decreasing space. For the first time, we successfully obtained the experimental data, which the phase-separated domain size decreased with decreasing film thickness.