Thickness Dependence of Contact Angles in Multilayered Ultrathin Polymer Films

Abstract

The wetting behavior of ultrathin polymer films (<100 nm) is systematically studied via dynamic contact angle measurements on a model bilayer system of cross-linked polystyrene (PS) and poly­(methyl methacrylate) (PMMA) to deconvolute surface energy contributions from material property changes due to confinement and long-range influence from underlayers, i.e., wetting transparency. We find that ultrathin PMMA films show no detectable material property changes and block the influence from PS underlayers when surface wetting is dominated by short-range hydrogen bonding, while suggestive wetting transparency is observed (<10 nm thick) for systems in which the wetting behavior is dominated by van der Waals (vdW) forces. In contrast to PMMA, the wettability of ultrathin PS (<100 nm thick) is dominated by changes in material properties, such as density and refractive index, and wetting transparency is not observed. Comparison between a vdW interface potential calculation and experimental data suggests that the cooperative nature of polymers may lead to a large (∼10 nm) critical thickness for wetting transparency.

Open Questions that Can Bridge Intermolecular Interactions and Macroscopic Wetting/Dewetting Behaviors of Thin Films

“…[74] It is also reported that T g -confinement effects are pronounced in crosslinked systems when compared to analogous linear polymers. [92] These pronounced effects in the crosslinked system support the large critical thicknesses for contact angle dependence on crosslinked ultrathin PS/PMMA films observed by Petek et al [36] This review invites future work to combine the additive vdW potential with the cooperative nature of polymers to aid in the understanding of ultrathin polymer film wettability. Experimentation that assesses variables known to impact the cooperative motion of polymers, such as bulky side groups [91] or fragility, [93,94] would be insightful to determine if the unexplained changes in contact angle and surface energy presented within are truly driven by cooperativity.…”

“…[85] We suspect that this temperature difference can also cause conflicting trends in the thickness dependence of surface energy. For example (Figure 8a), PS films spun on Si wafers (Si/SiO x /PS) investigated with surface capillary spectra at 110 °C exhibited decreasing surface energy, [62] while the same system showed multiple reports of increasing surface energy with a decrease in film thickness [29,34,36] when the surface energy is calculated from the contact angle measurements conducted at room temperature. In addition, the thickness-dependence of surface energy is different depending on temperature, even though all data were collected above T g (Figure 3b).…”

“…Adding to this complexity, Petek et al.’s work suggested such material properties change for PS, but not PMMA (Figure 7). [ 36 ] In a critical argument, they also illustrated that the predicted vdW potential trends and critical thicknesses (≈1 nm) remain relatively constant and unable to reflect experimental data (≈10 nm), even when the Hamaker constants are varied by unrealistic orders of magnitude ( Figure ). The application of Equations () and () to achieve these predicted trends indicates that the vdW potential model should be modified beyond adjustments in Hamaker constants.…”

“…Adding to this complexity, Petek et al's work suggested such material properties change for PS, but not PMMA (Figure 7). [36] In a critical argument, they also illustrated that the predicted vdW potential trends and critical thicknesses (≈1 nm) remain relatively constant and unable to reflect exper- Reproduced with permission. [36] Copyright 2022, American Chemical Society.…”

“…It remains to be seen whether this application of dewetting theory can be extrapolated to wetting theory to accurately capture the trends and critical length scales observed for thickness‐dependent wetting behavior. [ 36 ]…”