Surface Modification of Polymers with Self-Assembled Molecular Structures:  Multitechnique Surface Characterization

A simple, one-step procedure for generating ordered, crystalline methylene chains on polymeric surfaces via urethane linkages was developed. The reaction of dodecyl isocyanate with surface hydroxyl functional groups, catalyzed by dibutyltin dilaurate, formed a predominantly all-trans, crystalline structure on a cross-linked poly(2-hydroxyethyl methacrylate) (pHEMA) substrate. Allophanate side-branching reactions were not observed. Both X-ray photoelectron spectrocopy and time-of-flight secondary ion mass spectrometry show that the surface reaction reached saturation after 30 min at 60 °C. Unpolarized Fourier transform infrared-attenuated total reflection showed that, after 30 min, the stretching frequencies, νCH<INF>2,asym</INF> and νCH<INF>2,sym</INF>, decreased and approached 2920 and 2850 cm^-1, indicative of a crystalline phase. The distance between two hydroxyl groups is roughly 4 Å. A tilt angle of 33.5° ± 2.4° was estimated by dichroic ratios measured in polarized ATR according to the two-phase and Harrick thin film approximations. The findings reported here are significant in that the possibilities for using structures similar to self-assembled monolayers (SAMs) are expanded beyond the rigid gold and silicon surfaces used through most of the literature. Thus, SAMs, biomimetics for ordered lipid cell wall structures, can be applied to real-world biomedical polymers to modify biological interactions. The terminal groups of the SAM-like structure can be further functionalized with biomolecules or antibodies to develop surface-based diagnostics, biosensors, or biomaterials.