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

using “grafting to” approach. The contact angle data of OTS modified surface has confirmed the
formation of highly hydrophobic surface. The CTS surface has an intermediate value of contact
angle, and it is attributable to incomplete surface coverage of the CTS silane due to the steric
hindrance of the trimethyl group in the molecule. The CTS modified surface is therefore a two-
phase heterogeneous surface: one component is CTS silane (assuming pure CTS has the same
contact angle as OTS) and the other is pure silica. Using a Cassie approximation,[6, 7] the CTS
coverage can be calculated.
cos θtotal = f1 cos θCTS + f2 cos θsilica (2.1)
In Equation 2.1, f1 is the surface coverage of CTS silane and f2 is the surface coverage of
silica. Assuming θCTS ≈ θOTS, f1 is estimated to be ~ 0.48, which means only half of the CTS
silane treated surface is actually grafted with CTS.
Table 2.1. Water contact angle and ellipsometry thickness data of SAM surfaces.
Contact Angle (º)
Surface Advancing Receding Thickness(Å)
PEG 33 ± 0.5 14 ± 1.5 14 ± 2
CTS 70.5 ± 0.5 60.5 ± 0.5 3 ± 1
OTS 109 ± 1 93 ± 4 28 ± 2
Clean glass 14 ± 1.5 0 NA
2.3.2 Diffusion coefficients
A typical set of FRAP images are presented in Figure 2.2(a), and the experimental
data(symbols) and simulated data (lines) of fluorescence intensity profile versus radial position
as a function of time are presented in Figure 2.2(b).
Diffusion coefficients of Prodan on various surfaces under dry nitrogen and wet nitrogen
with controlled humidity are summarized in Table 2.2.
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