Photo-polymerized and thermal-polymerized silicon hydrogels with different surface microstructure and wettability

Abstract Thermal-initiation and photo-initiation are widely used for the preparation of hydrogels. But few studies were found to explore the difference between the hydrogel prepared by these two methods, especially the surface wettability. In this study, both methods were adopted for the synthesis of SNHM (SiMA-NVP-HEMA-MAA) silicone hydrogels. An interesting phenomenon was found that the surface of silicone hydrogels prepared by thermal polymerization was significantly more hydrophilic than the surface made by photo-initiation polymerization. Surface chemical groups on both hydrogels were characterized by attenuated total reflection Fourier transform infrared (ATR-FTIR) and the results showed no obvious difference. Furthermore, no significant distinction presented in the composition of the surface of the hydrogel either analyzed by X-ray photoelectron spectroscopy (XPS). However, atomic force microscope (AFM) images revealed difference, where the surface prepared by thermal polymerization was covered with nanoscale hills, while the surface by photo polymerization was supplied with a lot of nanoscale pits. The distinct wettabilities caused by different surface morphologies can be explained by Wenzel model and Cassie-Baxter model. This study concluded that the polymerization methods can lead to distinguish surface morphology, so as to surface wettabilities, which should be chosen thoughtfully in future studies.