Molecular aggregation state of ultrathin polar polymer films

Polyurethane (PU) is one of the most important polar polymers. This strong polarity might affect the aggregation structure for the PU ultrathin films because the effect of surface and interface of the ultrathin films is quite effective with decreasing film thickness. In this study, we investigated the relationship between aggregation structure and film thickness using atomic force microscope (AFM) and Fourier transform infrared (FT-IR) spectroscopy. The PU was synthesized with poly(oxytetramethylene) glycol (PTMG), 4,4'-diphenylmethane diisocyanate (MDI) and 1,4-butanediol (BD) by a prepolymer method. The PU ultrathin films were prepared onto a hydrophilic silicon wafer from the PU tetrahydrofuran solution by a spin coating method. Microphase-separated structure was clearly observed for all PU films, which the hard segment domains were surrounded by the soft segment matrix. For the thick film (>200 nm), the size of the hard segment domains was ca. 20 nm. This is almost same as bulk one. On the other hand, the domain size for the thin film with 8 nm thickness was ca. 10 nm. 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.