Surfaces with reversible hydrophilic/hydrophobic characteristics on cross-linked poly(N-isopropylacrylamide) hydrogels

摘要

Temperature-sensitive surfaces were prepared by grafting paly(N-isopropylacrylamide) (PNIPAAm) hydrogel on the surface of silicone wafers. The silicone wafer substrates were modified by organosilane (vinyltriethoxylsilane). They were further reacted with N-isopropylacrylamide (NIPAAm), using N,N'-methylenebisaerylamide as the cross-linking agent, to generate the cross-linked PNIPAAm layer on the surface of the substrate. The surfaces modified by the cross-linked PNIPAAm layer were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, and attenuated total reflectance-Fourier transform infrared (FTIR) spectroscopy. The reversible hydrophilic/hydrophobic properties of the surface were evaluated by the dynamic contact angle. The morphology of the surface modified by a crosslinked PNIPAAm layer can be turned fr-om "sea island" to "mountain valley" by changing the molar ratio of NIPAAm to BisAAm and by varying the polymerization time. A completely hydrophilic surface (advancing contact angle = 0 degrees) was observed below 25 degrees C, and the surface became extremely hydrophobic (advancing contact angle 92 degrees) above 40 degrees C. The sensitivity of the surfaces to temperature change can be improved by increasing the cross-linking density of the polymer layer and varying the polymerization time. The water meniscus height in a capillary tube, whose wall was coated by a cross-linked PNIPAAm layer, went up or down as the temperature changed to below or above the lower critical solution temperature of PNIPAAm. The differences in the water meniscus heights are 10 and 5 mm for a capillary tube with a diameter of 2 and 3 mm, respectively, corresponding to a change in temperature from 23 to 50 degrees C. The temperature-sensitive characteristics, which produce remarkable and rapid changes of surface properties, make this technology applicable for use as actuators, modulators, sensors, and switches. [References: 31]