Polymer Brushes for Molecular Transport - Paul Braun Research ...

3.3.1 XPS characterization
XPS was used to confirm both the formation of the surface initiator on the silicon
substrate and the successful polymerization of PNIPAAm. XPS was collected from the freshly
cleaned silicon wafer, the APS-treated silicon wafer, the surface initiator, and the PNIPAAm
brushes. In the spectrum of the clean silicon wafer, there was only one weak C 1s peak, likely
due to organic contamination. After treatment with APS, both C 1s and N 1s peaks were
observed and the intensity of C 1s was greater than on the bare substrate. Subsequent to grafting
the initiator to the APS-treated substrate, the C 1s peak was at 285 eV, the N 1s peak was at
400.1 eV and the Br 3d peak was at 68.8 eV. The N 1s and Br 3d peaks are presented in Figure
3.2.
After polymerization, the chemical composition of a 54 nm thick PNIPAAm brush was
determined using XPS (Figure 3.3). The oxygen:nitrogen:carbon molar ratio was determined to
be 11.3:12.0:76.4, which agrees with the expected value ratio of 12.5:12.5:75.0 for PNIPAAm
(Figure 3.3(a)). The high resolution C 1s spectrum was fit with the 4 peaks expected for
PNIPAAm, resulting in a very close match to the experimentally observed spectrum (Figure
3.3(b)). The sp 2 hybridized carbon atom in the carbonyl group, labeled IV in Figure 3.3(b), is at
287.9 eV and has an integrated molar ratio of 15%, which is very close to the expected value
(16.7%). The sp 3 hybridized carbon peak at 285.0 eV was divided into three components: peak I
at 284.9 eV corresponding to the two CH3- groups in the isopropyl group and the -CH2- in the
PNIPAAm backbone; peak II at 285.3 eV attributable to the -CH- unit in the PNIPAAm
backbone; peak III at 286.2 eV corresponding to the -CH- unit adjacent to the -NH- group. A
molar ratio of 3:1:1 for the three components, I:II:III, yielded a match to the overall shape of the
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