Design and Replication of Micro- and Nanotextured Polymer Surfaces: A Height-to-Pitch Criterion to Predict Hydrophobicity Enhancement

Langmuir ,   https://dx.doi.org/10.1021/acs.langmuir.6c03714

The present study examines how the replication of micro- and nanotextures on polymer surfaces enhances their hydrophobic properties and defines a geometric parameter required to obtain functional surfaces at an industrial scale. Nickel shims were fabricated by lithographic techniques, featuring cylindrical textures (20 μm in diameter, 20 μm in height) arranged in hexagonal arrays with pitches ranging from 10 to 60 μm. Both positive (pillars) and negative (holes) polarities were produced, some of which exhibited hierarchical textures. These textures were then replicated on polystyrene (PS) surfaces via microinjection molding (μIM) and hot embossing (HE). The main results showed that the texturing enhances hydrophobicity, even on the shims themselves, despite the nickel’s high surface energy, which is intrinsically “hydrophilic” (over a flat surface, θYoung = 64°). The gain is greatest for positive structures, which reach static contact angles with water (sCAW) of approximately 140°. For the replication of these structures onto PS, μIM showed limitations: incomplete filling of the shim cavities, particularly for negative structures, resulted in reduced structure heights and thus prevented the attainment of higher contact angles. Hot embossing, by contrast, successfully produced taller structures and yielded the highest contact angle (sCAW ∼ 140°), corresponding to a mixed Wenzel/Cassie–Baxter wetting regime. The novelty of this work lies in an analysis that reveals discrepancies between theoretical predictions and experimental sCAW values. A new geometric criterion was established: the height-to-pitch ratio (h/P) must exceed 1 to reach its maximum hydrophobicity for a given system (structure configuration and intrinsic hydrophobicity of the materials). This threshold was validated on several polymers, including polystyrene, polycarbonate (PC), and fluorinated cycloolefin polymer, using a Boltzmann fit. These results demonstrate that the height-to-pitch ratio can serve as a design parameter for producing functional surfaces with enhanced hydrophobic properties through industrial processes, without chemical modification. This work thus provides a pragmatic design guideline for substantially enhancing the hydrophobicity of polymers by texture replication.