7.4 Strategy C: Local Deprotection of PS 690 -b-PtBA 1210 Films
137
7.4.3 Mechanism of Local Hydrolysis
The fluorescence microscopic images represent the local chemical composition and
thus indicate to which extent the hydrolysis proceeded during the μCP step. From
the analysis of the observed features and fluorescence emission intensities, as shown
above, the presence and local concentration of the organic acid, which induces the
hydrolysis, can be qualitatively estimated.
Initially, the oxidized PDMS stamp was covered with a liquid layer of trifluoroacetic acid. The acid spread over the stamp indicates a contact angle close to 0°.
The acid filled the circular depressions in the stamp and may have also formed a
liquid film of not well-known thickness on the stamp. From the volume of the drop
and the stamp design, an initial layer thickness of ~200 μm was calculated. Since
trifluoroacetic acid is volatile [17], it thus evaporated rapidly in these experiments.
For the discussion of the observations and the formulation of a mechanism of the
local hydrolysis, there are three different cases that need to be taken into account.
These cases can be identified based on the following observations and arguments:
(1) The observation of the strong fluorescence emission from circular areas that
were not in conformal contact with the stamp (Fig. 7.7a) leads to the conclusion
that the depressions are still acid-filled after an evaporation time of 30 s. The
observed circular areas of high fluorescence emission intensity are consistent
with a hydrolysis induced by liquid acid present in the circular pits of the stamp.
(2) Ultimately, for very long evaporation time, no liquid acid will be present on
the stamp surface, but the PDMS is likely swollen with acid, at least in the
near-surface region.
(3) For intermediate evaporation time, and thus lower acid coverage, a thinner
film of acid can be expected to remain on the stamp surface, when the stamp
is brought into contact with the polymer film.
To develop the optimum printing conditions, the three cases identified will be
discussed in the following section:
1. When the stamp, including the depressions, is covered with liquid trifluoroacetic
acid, the ink may spread across the surface upon establishing contact between
the stamp and substrate. Acid transport via diffusion can likely be ignored. In
this case, hydrolysis occurs not only in the area of conformal contact between
the PDMS stamp and the polymer, where a thin acid film may be present (or
where acid may diffuse from the stamp into the film), but predominantly in
the areas where the voids (depressions) in the stamp are located. As shown
schematically in Fig. 7.10, these voids may serve as a reservoir of liquid acid.
Depending on the amount of acid on the stamp, the depressions may be more
or less filled (Fig. 7.10a/b). Because of the locally high concentration of acid in
both cases, the degree of hydrolysis of the tBA functional groups in the circular
area will be high compared to the matrix.
Further, in the case of partially filled depressions, the wetting of trifluoroacetic
acid can be expected to lead to the formation of a meniscus (Fig. 7.10b), which will
137
7.4.3 Mechanism of Local Hydrolysis
The fluorescence microscopic images represent the local chemical composition and
thus indicate to which extent the hydrolysis proceeded during the μCP step. From
the analysis of the observed features and fluorescence emission intensities, as shown
above, the presence and local concentration of the organic acid, which induces the
hydrolysis, can be qualitatively estimated.
Initially, the oxidized PDMS stamp was covered with a liquid layer of trifluoroacetic acid. The acid spread over the stamp indicates a contact angle close to 0°.
The acid filled the circular depressions in the stamp and may have also formed a
liquid film of not well-known thickness on the stamp. From the volume of the drop
and the stamp design, an initial layer thickness of ~200 μm was calculated. Since
trifluoroacetic acid is volatile [17], it thus evaporated rapidly in these experiments.
For the discussion of the observations and the formulation of a mechanism of the
local hydrolysis, there are three different cases that need to be taken into account.
These cases can be identified based on the following observations and arguments:
(1) The observation of the strong fluorescence emission from circular areas that
were not in conformal contact with the stamp (Fig. 7.7a) leads to the conclusion
that the depressions are still acid-filled after an evaporation time of 30 s. The
observed circular areas of high fluorescence emission intensity are consistent
with a hydrolysis induced by liquid acid present in the circular pits of the stamp.
(2) Ultimately, for very long evaporation time, no liquid acid will be present on
the stamp surface, but the PDMS is likely swollen with acid, at least in the
near-surface region.
(3) For intermediate evaporation time, and thus lower acid coverage, a thinner
film of acid can be expected to remain on the stamp surface, when the stamp
is brought into contact with the polymer film.
To develop the optimum printing conditions, the three cases identified will be
discussed in the following section:
1. When the stamp, including the depressions, is covered with liquid trifluoroacetic
acid, the ink may spread across the surface upon establishing contact between
the stamp and substrate. Acid transport via diffusion can likely be ignored. In
this case, hydrolysis occurs not only in the area of conformal contact between
the PDMS stamp and the polymer, where a thin acid film may be present (or
where acid may diffuse from the stamp into the film), but predominantly in
the areas where the voids (depressions) in the stamp are located. As shown
schematically in Fig. 7.10, these voids may serve as a reservoir of liquid acid.
Depending on the amount of acid on the stamp, the depressions may be more
or less filled (Fig. 7.10a/b). Because of the locally high concentration of acid in
both cases, the degree of hydrolysis of the tBA functional groups in the circular
area will be high compared to the matrix.
Further, in the case of partially filled depressions, the wetting of trifluoroacetic
acid can be expected to lead to the formation of a meniscus (Fig. 7.10b), which will