Achieving Stable Patterns in Multicomponent Polymer Thin Films Using Marangoni and van der Waals Forces

Abstract

Liquid−air interfaces can be deformed by surfacetension gradients to create topography, a phenomenon useful for polymer film patterning. A recently developed method creates these gradients by photochemically patterning a solid polymer film. Heating the film to the liquid state leads to flow driven by the patterned surface-tension gradients, but capillary leveling and diffusion of surface-active species facilitate eventual dissipation of the topography. However, experiments demonstrate that using blends of high-and low-molar-mass polymers can considerably delay the decay in topography. To gain insight into this observation, we develop a model based on lubrication theory that yields coupled nonlinear partial differential equations describing how the film height and species concentrations evolve with time and space. Incorporation of a nonmonotonic disjoining pressure is found to significantly increase the lifetime of topographical features, making the model predictions qualitatively consistent with experiments. A parametric study reveals the key variables controlling the kinetics of film deformation and provides guidelines for photochemically induced Marangoni patterning of polymer films.

Controlling Surface Deformation and Feature Aspect Ratio in Photochemically Induced Marangoni Patterning of Polymer Films

“…One reason for the slight discrepancy between model predictions and experiments is likely due to differences between the estimated values of physical constants and their actual values. In addition, the model does not incorporate the effect of any chemical imperfections at the polymer/air interface and neglects van der Waals interactions in the film, which play a critical role in the dynamics of ultrathin liquid films (thicknesses below ∼100 nm). , …”

“…However, as the film thins down close to the substrate, van der Waals interactions become significant . Attractive van der Waals interactions in multicomponent PMP can act against the feature-dissipative mechanisms, effectively locking in features for very long times . Incorporating disjoining pressure in the model for single-component PMP could illuminate the effect of various substrates (including heterogeneous ones) on the patterning process and provide insight into how to further enhance feature aspect ratios.…”

“…The total pressure in the film ( p ) is the sum of contributions from the capillary pressure and the effective intermolecular interactions: where Ca ′ is the reduced capillary number ( Ca ′ = η 1 U /σϵ 3 ), which is defined based on the effective surface tension (σ), and Π* is the disjoining pressure . In our study, we neglect the effects of intermolecular interactions in the film by setting Π* = 0.…”

“…The term on the right-hand side of eq gives the contribution from diffusive transport of the photoproduct while the second term on the left-hand side gives the contribution from convective transport. A detailed derivation of eqs and can be found in the work of Usgaonkar et al…”

“…where Ca′ is the reduced capillary number (Ca′ = η 1 U/σϵ 3 ), which is defined based on the effective surface tension (σ), and Π* is the disjoining pressure. 35 In our study, we neglect the effects of intermolecular interactions in the film by setting Π* = 0. Thus, the film height evolution is governed by contributions from surfacetension-driven capillary leveling (first term on right-hand side of eq 2) and Marangoni flow (second term on right-hand side of eq 2).…”