8.3 Domain-Selective Coupling on Solvent-Treated Polymer Films
153
sequential derivatization. The results are illustrated in Fig. 8.4, where representative force volume images are shown. In Fig. 8.4a and b, the microdomain structure
of the solvent-treated film and the low pull-off forces measured on top of these
domains (compared to the PS matrix) are displayed. The domains of low pull-off
force observed corresponded systematically to the islands in the height image, as
indicated by the circles. After hydrolysis with trifluoroacetic acid, the force images
showed a significant increase in pull-off force (Fig. 8.4c and d). The islands showed
higher pull-off force compared to the PS matrix. Finally, after the covalent immobilization of fluoresceinamine or n-butyl amine (following the activation with NHS)
on the films, domains with relatively low pull-off forces were observed.
Very similar observations were made in AFM friction measurements, as shown in
Fig. 8.5. While for the unreacted block copolymer films, following the solvent treatment, low friction forces were observed in areas associated with the circular domains
of PtBA, the friction forces were higher compared to the PS matrix after hydrolysis
with trifluoroacetic acid. After coupling fluoresceinamine (or n-butyl amine), the
friction forces of the matrix were larger than those observed inside the domains.
Based on these observations, we attribute the changes in local pull-off and friction
forces on the islands (with respect to the matrix) to the different surface composition
induced by the chemical modification [21]. PS, as matrix for all the cases, has a
surface tension of γ = 40.9 mN/m. This value is higher than that of the PtBA block
(γ = 30.7 mN/m), but lower than that of the PAA block. Fluoresceinamine and n-butyl
amine can be expected to be hydrophobic [22], thus leading to the lowest surface
energy surfaces. Consequently, the changes of pull-off forces F on the islands and
domains can be ranked according to this ranking of surface tensions [23]:
F(n-butyl amine) ≈ F(fluoresceinamine) ≤ F(PtBA) < F (PS) << F (PAA)
A more detailed insight into the surface derivatization in this heterogeneous system
was obtained from a quantitative analysis of the FV images using a custom-made
software [20]. As shown in Figs. 8.6 and 8.7, the pull-off force maps were locally
not homogeneous.
While we observed a multimodal pull-off force distribution (and a qualitatively
very similar distribution of the adhesion hysteresis, data not shown) with pulloff forces around 40 nN for the hydrolyzed films, the derivatization with fluoresceinamine inside the PtBA nanodomains led to a broadening of the distribution.
In this case the maximum of the distribution was shifted to ~7 nN and a long tail
was observed toward higher pull-off force values. Also, for the fluoresceinaminederivatized films, the distribution of the adhesion hysteresis looked qualitatively very
similar to the pull-off force distribution.
These pull-off force distributions reflect on the one hand the surface chemistry,
i.e., the different relative coverages of the functionalized polymer film surface (PtBA
phase vs. PS phase and fluoresceinamine-derivatized areas vs. PS phase, respectively), on the other hand, they show the lateral heterogeneity of the different surface
chemistries. The differences between PtBA and PS are small, although the highest
153
sequential derivatization. The results are illustrated in Fig. 8.4, where representative force volume images are shown. In Fig. 8.4a and b, the microdomain structure
of the solvent-treated film and the low pull-off forces measured on top of these
domains (compared to the PS matrix) are displayed. The domains of low pull-off
force observed corresponded systematically to the islands in the height image, as
indicated by the circles. After hydrolysis with trifluoroacetic acid, the force images
showed a significant increase in pull-off force (Fig. 8.4c and d). The islands showed
higher pull-off force compared to the PS matrix. Finally, after the covalent immobilization of fluoresceinamine or n-butyl amine (following the activation with NHS)
on the films, domains with relatively low pull-off forces were observed.
Very similar observations were made in AFM friction measurements, as shown in
Fig. 8.5. While for the unreacted block copolymer films, following the solvent treatment, low friction forces were observed in areas associated with the circular domains
of PtBA, the friction forces were higher compared to the PS matrix after hydrolysis
with trifluoroacetic acid. After coupling fluoresceinamine (or n-butyl amine), the
friction forces of the matrix were larger than those observed inside the domains.
Based on these observations, we attribute the changes in local pull-off and friction
forces on the islands (with respect to the matrix) to the different surface composition
induced by the chemical modification [21]. PS, as matrix for all the cases, has a
surface tension of γ = 40.9 mN/m. This value is higher than that of the PtBA block
(γ = 30.7 mN/m), but lower than that of the PAA block. Fluoresceinamine and n-butyl
amine can be expected to be hydrophobic [22], thus leading to the lowest surface
energy surfaces. Consequently, the changes of pull-off forces F on the islands and
domains can be ranked according to this ranking of surface tensions [23]:
F(n-butyl amine) ≈ F(fluoresceinamine) ≤ F(PtBA) < F (PS) << F (PAA)
A more detailed insight into the surface derivatization in this heterogeneous system
was obtained from a quantitative analysis of the FV images using a custom-made
software [20]. As shown in Figs. 8.6 and 8.7, the pull-off force maps were locally
not homogeneous.
While we observed a multimodal pull-off force distribution (and a qualitatively
very similar distribution of the adhesion hysteresis, data not shown) with pulloff forces around 40 nN for the hydrolyzed films, the derivatization with fluoresceinamine inside the PtBA nanodomains led to a broadening of the distribution.
In this case the maximum of the distribution was shifted to ~7 nN and a long tail
was observed toward higher pull-off force values. Also, for the fluoresceinaminederivatized films, the distribution of the adhesion hysteresis looked qualitatively very
similar to the pull-off force distribution.
These pull-off force distributions reflect on the one hand the surface chemistry,
i.e., the different relative coverages of the functionalized polymer film surface (PtBA
phase vs. PS phase and fluoresceinamine-derivatized areas vs. PS phase, respectively), on the other hand, they show the lateral heterogeneity of the different surface
chemistries. The differences between PtBA and PS are small, although the highest