With
respect to the increasing need for fully characterizing surface-tethered
polymer brushes, the capacity of quantitative IR-Fourier transform
infrared (FTIR) spectroscopy using a multiple-internal-reflection
Si prism as the attenuated total reflection (ATR) element is investigated
to directly characterize the surface chemical modifications occurring
during a surface-initiated controlled polymerization. A simple two-step
strategy is used involving first the covalent grafting of atom transfer
radical polymerization (ATRP) initiators on a hydrogenated silicon
surface and the subsequent polymerization of styrene. Three prefunctionalized
surfaces designated Si-Br1, Si-Br2, and Si-Br3 are obtained by different
procedures. The initiator grafting densities obtained by quantitative
IR are 1.7 ± 0.3 nm–2 for Si-Br1, 1.5 ±
0.3 nm–2 for Si-Br2, and 0.9 ± 0.2 nm–2 for Si-Br3. After the polymerization of styrene under the same experimental
conditions (grafting from without sacrificial initiators) and a careful
Soxhlet rinse to remove physisorbed polymers formed in solution, almost
no polymerization is observed using Si-Br1 with a value of the density
in polymerized styrene units of 12 ± 2 nm–2, which is probably due to the chelating effect of the amino linkers
used for grafting the initiators in Si-Br1. In contrast, the densities
in styrene units are 54 ± 11 nm–2 using Si-Br2
and 141 ± 28 nm–2 using Si-Br3. The degree
of polymerization (DP) has been evaluated by measuring the polymer
thickness (by ellipsometry and atomic force microscopy, AFM) and using
a scaling law relating the latter to DP for dry polymer brushes. High
DP values of 200 and 1000 are found in the case of Si-Br2 and Si-Br3,
respectively. The fraction of active polymerization initiators is
found to be 15–18%, independent of the initiator surface density.
In contrast, polymerization kinetics appear affected by steric hindrance
and conformational disorder among grafted initiators. This approach
for determining surface densities of grafted initiators and grafted
polymer chains and DPs is fully generalizable to any other polymer
system.