From Contradictory Polymers, Scientists Develop Nanocoatings with Optimal Properties for Optics

Materials scientists at Kiel University in Germany have synthesized nanoscale gradient copolymers, that possess distinct contradictory properties, to create thin-film polymer coatings that could be used to coat sensor devices and MEMS technology. The material coatings could also be applied to aircraft and glass fronts, making the surfaces of those objects easier to “de-ice.”

Additional applications for the thin-film polymer coatings exist with molecular machines that transfer mechanical processes to the nanoscale.

A material’s contrasting properties (for example, simultaneously hard and soft) are useful in developing gradual antireflective lenses. In nature, these appear in the human eye, as well as in mussels — the animal is able to adhere to hard surfaces without being detached by the force administered by current, thanks to the soft tissue inside their shells helping the animal affix to hard surfaces. Mussels form elastic adhesive threads that become harder toward their endpoints, due to a protein combination that changes evenly from end to end within the fiber.

Mimicking that phenomenon, the Kiel-based research team, led by Stefan Schroder, a Ph.D. candidate at the Chair for Multicomponent Materials (and first author of a study describing the research), combined two materials with different properties at the nano level in an attempt to develop its own set of gradient thin films — thin materials with similarly merging properties. The team used polytetrafluoroethylene, or Teflon, and the polymer pV3D3. Where Teflon, with its water-repelling (hydrophobic) and nonstick properties, is difficult to apply to other surfaces, pV3D3 possesses strong properties of adhesion.

The researchers demonstrated a strong bond between the materials, without compromising either material’s differing properties, achieving a coating with a water-repellent upper-facing side and a well-adhering lower side.

Despite the research team’s effective demonstration, a controlled polymer coating process requires vapor decomposition; polymers on their own cannot be vaporized or sputtered without decomposition. Vapor deposition (sputtering) processes for coating ceramic materials and metals have been applied in large-scale industrial settings for decades, and the researchers opted to expand on an existing deposition technique known as initiated chemic vapor deposition, or iCVD, to grow a polymer on a substrate surface.

Using an elaborate process, the research team has joined two polymers at the nanoscale in a flowing process: The transition from pV3D3 to Teflon (PTFE) in the scanning electron microscope image of the gradient layer is marked here as the transition from red to blue. Courtesy of Kiel University.

The scientists specifically used the iCVD process to first create a thin polymer layer, and, simultaneously, bond two polymers in a gradual transition. After introducing a monomer to the process, the researchers added the “starting” material for Teflon deposition, and proceeded to continuously increase its concentration. By lowering the monomer’s deposition at the same time, the team formed a polymer film on the substrate, with a transition from a pure pV3D3 polymer to a pure Teflon film.

The researchers ultimately synthesized a polymer gradient that was 21 nm thick. A human hair, for comparison, is about 50,000 nm thick.

Especially for applications in optics, coatings of only a few nm in width are essential, so as to not impair the optical properties of windows or lenses, for example, said Thomas Strunskus, a research associate and member of the research group.

A first round of projects with industrial partners from the coating and air-conditioning technology sectors are in preparation, Strunskus said.

The research was published in Materials Today (www.doi.org/10.1016/j.mattod.2020.02.004).