ESCA was used for determining the surface chemical composition of the coated samples as a function of octadecyl isocyanate reaction time over 60 minutes.
Figure 2A shows the survey spectra of coated pHEMA hydrogels over reaction times of 0, 15, 30, 45, and 60 minutes;
Figure 2B shows their respective atomic surface composition. It was found that the surface composition of the uncoated control samples to be C, 69.3±0.6%, oxygen (O), 30.7±0.6%, and no detectable nitrogen (N), compared with the expected stoichiometric atom percentage of C, 66.6%, and O, 33.3%. After 15 minutes coating reaction, higher C and
n content (C, 82.5±1.4%; O, 13.6±0.9%;
n, 3.9±0.5%) indicates initial hydrophobic coating formation having a surface composition that is significantly different from the uncoated samples. The C content peaks after coating reaction time of 30 minutes: C, 86.7±1.1%; O, 9.9±0.8%;
n, 3.4±1.2%. Composition remains constant for longer reaction times, with no significant difference for 45 minutes (C, 87.3±0.6%; O, 9.4±0.5%;
n, 3.3±0.4%) or for 60 minutes coating time (C, 86.0±0.8%; O, 10.6±0.4%;
n, 3.4±0.6%). These data suggest that after 30 minutes most of the available hydroxyl groups within this depth have reacted with the octadecyl isocyanate, hence the peaking of the C content. Further, the resulting hydrophobic coating is thicker than ESCA's sampling depth capability, >80 Å.
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