5.2 Investigation of the Surface Chemistry of PS 690 -b-PtBA 1210 Films
97
If the films were hydrolyzed in HCl gas phase for 2 h (hydrolysis goes to completion as proved by IR), different topographical and phase images were observed by
AFM as shown in Fig. 5.3e, f. Longer worm-like feature surface was observed, and
also block copolymer microphase separation was more pronounced. These observations may be attributed to the fact that all PtBA groups have been reacted to PAA. Thus
the altered intermolecular interactions lead to an increase in the corresponding FloryHuggins parameter [10]. The measured R a assessed from 1.0 μm
2 was increased to
5.1 nm.
Similar results have been obtained for the films hydrolyzed in trifluoroacetic acid
about 15 min in Fig. 5.3g, h (hydrolysis reached completion as proved by IR above).
The measured R a assessed from 1 μm
2 was about 5.3 nm.
From a comparison of the surface roughness values, we can conclude that the
surface area of the films after hydrolysis in HCl gas and trifluoroacetic acid will
be larger compared to films hydrolyzed in 3 M HCl solution. In addition, the PAA
groups resulting from the hydrolysis of PtBA functional groups can be swollen by
uptake of water from the solution. These properties may yield advantages for such
hydrolyzed films, that is, providing high densities of active functional groups at the
surface in a water-swollen gel-like state. However, such reactive films take 2 h in HCl
gas, whereas 15 min are sufficient if the hydrolysis is performed in trifluoroacetic
acid.
5.2.2 Covalent Coupling of PEG to Activated
PS 690 -b-PtBA 1210 Films
The carboxylic acid generated in the hydrolysis reaction of the PS 690 -b-PtBA 1210
films can be activated with NHS ester moieties using a standard recipe as described in
Chap. 4. The subsequent covalent coupling reaction of the activated NHS ester groups
at and near the surface of PS 690 -b-PtBA 1210 films with amino groups-terminated
PEG 500 -NH 2 from aqueous solution was investigated by FTIR spectroscopy as shown
in Fig. 5.4a. Two new additional absorption bands at 1645 cm
−1 (amide I) and
1544 cm
−1 (amide II) were observed in the spectra. This result showed that the
PEG molecules were covalently grafted on the polymer film through amide linkage
formation (see also Chap. 4). The kinetics of immobilization of PEG-NH 2 molecules
was followed by ellipsometry. Figure 5.4b shows the grafting thickness of the grafted
PEG layer determined by ellipsometry for different reaction times. The maximum
thickness of PEG 500 -NH 2 was ~1.9 nm. A grafting density of PEG of ~2.9 PEG
molecules per nm
2 was thus calculated. This density is about three times higher
than the maximum grafting density of PEG molecules on SAMs (~0.87 nm
−2 , see
Chap. 4).
To confirm the successful surface derivatization, XPS experiments were
performed with a take-off angle of 45° (Fig. 5.5). For neat PS 690 -b-PtBA 1210 films,
only the C 1s peaks at 284.07 eV and O 1s at 532.0 eV [11] were observed. After
97
If the films were hydrolyzed in HCl gas phase for 2 h (hydrolysis goes to completion as proved by IR), different topographical and phase images were observed by
AFM as shown in Fig. 5.3e, f. Longer worm-like feature surface was observed, and
also block copolymer microphase separation was more pronounced. These observations may be attributed to the fact that all PtBA groups have been reacted to PAA. Thus
the altered intermolecular interactions lead to an increase in the corresponding FloryHuggins parameter [10]. The measured R a assessed from 1.0 μm
2 was increased to
5.1 nm.
Similar results have been obtained for the films hydrolyzed in trifluoroacetic acid
about 15 min in Fig. 5.3g, h (hydrolysis reached completion as proved by IR above).
The measured R a assessed from 1 μm
2 was about 5.3 nm.
From a comparison of the surface roughness values, we can conclude that the
surface area of the films after hydrolysis in HCl gas and trifluoroacetic acid will
be larger compared to films hydrolyzed in 3 M HCl solution. In addition, the PAA
groups resulting from the hydrolysis of PtBA functional groups can be swollen by
uptake of water from the solution. These properties may yield advantages for such
hydrolyzed films, that is, providing high densities of active functional groups at the
surface in a water-swollen gel-like state. However, such reactive films take 2 h in HCl
gas, whereas 15 min are sufficient if the hydrolysis is performed in trifluoroacetic
acid.
5.2.2 Covalent Coupling of PEG to Activated
PS 690 -b-PtBA 1210 Films
The carboxylic acid generated in the hydrolysis reaction of the PS 690 -b-PtBA 1210
films can be activated with NHS ester moieties using a standard recipe as described in
Chap. 4. The subsequent covalent coupling reaction of the activated NHS ester groups
at and near the surface of PS 690 -b-PtBA 1210 films with amino groups-terminated
PEG 500 -NH 2 from aqueous solution was investigated by FTIR spectroscopy as shown
in Fig. 5.4a. Two new additional absorption bands at 1645 cm
−1 (amide I) and
1544 cm
−1 (amide II) were observed in the spectra. This result showed that the
PEG molecules were covalently grafted on the polymer film through amide linkage
formation (see also Chap. 4). The kinetics of immobilization of PEG-NH 2 molecules
was followed by ellipsometry. Figure 5.4b shows the grafting thickness of the grafted
PEG layer determined by ellipsometry for different reaction times. The maximum
thickness of PEG 500 -NH 2 was ~1.9 nm. A grafting density of PEG of ~2.9 PEG
molecules per nm
2 was thus calculated. This density is about three times higher
than the maximum grafting density of PEG molecules on SAMs (~0.87 nm
−2 , see
Chap. 4).
To confirm the successful surface derivatization, XPS experiments were
performed with a take-off angle of 45° (Fig. 5.5). For neat PS 690 -b-PtBA 1210 films,
only the C 1s peaks at 284.07 eV and O 1s at 532.0 eV [11] were observed. After