3.3 Ultrathin PS n -b-PtBA m Polymer Films
55
3.3.1 Characterization of Polymer Thin Film Surface
Composition
The reactivity of the t-butyl ester groups in thin films of PS n -b-PtBA m (shown
in Scheme 3.4) is potentially influenced by the local chemical composition and
structure of the surface of the films. A skin layer of PtBA can be expected for
microphase-separated block copolymer films owing to the lower surface tension of
PtBA compared to PS (γ PtBA = 31.2 mN/m; γ PS = 40.7 mN/m). To validate the
existence of a skin layer, the surfaces of spin-coated films were investigated by CA
and angle-dependent X-photoelectron spectroscopy (XPS) measurements.
Static CA was measured with water (H 2 O) and diiodomethane (CH 2 I 2 ) as probe
liquids on PS n -b-PtBA m films (thickness ~90 nm) (Table 3.3). The values obtained
for PtBA and PS 690 -b-PtBA 1210 , as well as PS 2091 -b-PtBA 1054 , were identical within
the experimental error, while neat PS films showed lower CA. These data agree well
with the differences in surface tension of the blocks and indicate that PtBA is exposed
on the PS 690 -b-PtBA 1210 and PS 2091 -b-PtBA 1054 block copolymer films. In contrast,
films of PS 88 -b-PtBA 35 , which do not exhibit microphase separation (see below),
showed intermediate CA values.
More definite evidence for skin layer formation, in particular for films of PS 690 -
b-PtBA 1210 , was obtained in XPS scans with a take-off angle of 45° (Fig. 3.7).
We observed only the expected elements, i.e., carbon and oxygen (except for PS
homopolymer, which showed only carbon). The C 1s peak corresponding to neutral
carbon was observed at 284.0 eV. The C 1s peak observed at 287.8 eV for the PtBA
containing polymers was assigned to the carbonyl carbon. For PS also the C 1s shakeup peak corresponding to π–π* transition of the benzene ring was observed at 290.2–
291.0 eV.
The oxygen/carbon ratio for PS 690 -b-PtBA 1210 derived from the integrated intensities of the corresponding O 1s and C 1s peaks, as summarized in Table 3.4, is within
Scheme 3.4 Schematic of hydrolysis of PS n -b-PtBA m diblock copolymer to yield PS n -b-PAA m
Table 3.3 Static CA
measured with water and
diiodomethane as probe
liquids on spin-coated films
of various polymers on
oxidized silicon (film
thickness ~90 nm)
Sample
θ H2O [deg]
θ CH2I2 [deg]
PS 690 -b-PtBA 1210
89 ± 2
23 ± 2
PS 2091 -b-PtBA 1054
89 ± 2
23 ± 2
PS 88 -b-PtBA 35
88 ± 2
20 ± 2
PtBA
89 ± 2
23 ± 2
PS
86 ± 2
15 ± 2
55
3.3.1 Characterization of Polymer Thin Film Surface
Composition
The reactivity of the t-butyl ester groups in thin films of PS n -b-PtBA m (shown
in Scheme 3.4) is potentially influenced by the local chemical composition and
structure of the surface of the films. A skin layer of PtBA can be expected for
microphase-separated block copolymer films owing to the lower surface tension of
PtBA compared to PS (γ PtBA = 31.2 mN/m; γ PS = 40.7 mN/m). To validate the
existence of a skin layer, the surfaces of spin-coated films were investigated by CA
and angle-dependent X-photoelectron spectroscopy (XPS) measurements.
Static CA was measured with water (H 2 O) and diiodomethane (CH 2 I 2 ) as probe
liquids on PS n -b-PtBA m films (thickness ~90 nm) (Table 3.3). The values obtained
for PtBA and PS 690 -b-PtBA 1210 , as well as PS 2091 -b-PtBA 1054 , were identical within
the experimental error, while neat PS films showed lower CA. These data agree well
with the differences in surface tension of the blocks and indicate that PtBA is exposed
on the PS 690 -b-PtBA 1210 and PS 2091 -b-PtBA 1054 block copolymer films. In contrast,
films of PS 88 -b-PtBA 35 , which do not exhibit microphase separation (see below),
showed intermediate CA values.
More definite evidence for skin layer formation, in particular for films of PS 690 -
b-PtBA 1210 , was obtained in XPS scans with a take-off angle of 45° (Fig. 3.7).
We observed only the expected elements, i.e., carbon and oxygen (except for PS
homopolymer, which showed only carbon). The C 1s peak corresponding to neutral
carbon was observed at 284.0 eV. The C 1s peak observed at 287.8 eV for the PtBA
containing polymers was assigned to the carbonyl carbon. For PS also the C 1s shakeup peak corresponding to π–π* transition of the benzene ring was observed at 290.2–
291.0 eV.
The oxygen/carbon ratio for PS 690 -b-PtBA 1210 derived from the integrated intensities of the corresponding O 1s and C 1s peaks, as summarized in Table 3.4, is within
Scheme 3.4 Schematic of hydrolysis of PS n -b-PtBA m diblock copolymer to yield PS n -b-PAA m
Table 3.3 Static CA
measured with water and
diiodomethane as probe
liquids on spin-coated films
of various polymers on
oxidized silicon (film
thickness ~90 nm)
Sample
θ H2O [deg]
θ CH2I2 [deg]
PS 690 -b-PtBA 1210
89 ± 2
23 ± 2
PS 2091 -b-PtBA 1054
89 ± 2
23 ± 2
PS 88 -b-PtBA 35
88 ± 2
20 ± 2
PtBA
89 ± 2
23 ± 2
PS
86 ± 2
15 ± 2