Resumen de Functional structured surfaces scaled up via roll-to-roll nanoimprint technology
Bioinspired nanostructured surfaces can bestow materials with a number of advanced functionalities, such as the superhydrophobic character inspired on the lotus leaf, the dry adhesion of the gecko foot, or the antireflective capabilities of the moth eyes.
A practical, low-cost, versatile, and easy-scalable approach for the fabrication of these bioinspired functional micro/nanostructures in polymers is nanoimprint lithography (NIL). This technology involves transferring nano or micro patterns by mechanical embossing from a mold to a thermoplastic film in the case of thermal NIL (T-NIL), or to a resin curable by UV light in the case of UV-NIL. The process can be scaled up via roll-to-roll (R2R) nanoimprint technology, where the mold is incorporated into a roller to carry out a continuous imprint process, and as a result to produce large areas of structured surfaces.
This thesis aim is to develop the NIL process to fabricate moth-eye inspired antireflective (AR) nanostructures in transparent films at laboratory scale, and to go one step further by the scaling up of the fabrication technology employing R2R NIL. The moth-eye like nanotopography consists of hexagonal arrays of nanocones with dimensions in the nanometer scale. Produced at a large-scale, this antireflective technology can be particularly useful to improve the efficiency in photovoltaic devices as light trapping structures to improve the vision in screens or in optical components by reducing the light reflection on their surface.
A major concern for the practical application of these surfaces is their poor mechanical resistance and their short-term durability when exposed to outside environmental conditions of UV radiation, humidity, and temperature, leading to the fast degradation of the material and therefore of the surface functionality. As such, another important objective of this thesis is to improve the physical and UV resistance of the moth-eye AR surfaces to allow for their use in real applications.
Towards this aim, in the first part of the thesis, the addition of titanium dioxide (TiO2 or titania) nanoparticles as reinforcing elements of the moth-eye AR topography made of polymethyl methacrylate (PMMA) is studied. Highly transparent PMMA/TiO2 surface nanocomposites using T-NIL are prepared, and the impact of the nanofillers on the mechanical and thermal properties is investigated. Being TiO2 an excellent photocatalyst, the incorporation of TiO2 nanoparticles to the surface also provides for additional functionalities such as self-cleaning behavior, which is very useful particularly for solar devices.
An additional approach studied to improve the performance of the AR surfaces is the coating of the AR topography with a conformal thin film of titania. The coating, while preserving the antireflective properties, protects the nanostructures against mechanical damage and improves substantially their thermal stability. Likewise, the titania coated AR surfaces shows self-cleaning properties.
To improve the functional surfaces durability, a more weather resistant polymer (poly vinylidene fluoride or PVDF) is investigated replacing the PMMA as nanocomposite matrix, whereby further improvement on durability and weathering resistance is demonstrated.
In the second part of the thesis, the fabrication process of AR nanostructured surfaces scaled up by roll-to-roll NIL processing is developed, increasing significantly the fabrication throughput. The influence of the processing parameters on the imprinted AR features and its final functional performance is studied and adjusted in order to improve the mechanical stability concomitantly with the optical performance.
Finally, roll-to-roll NIL processing is also applied to produce other functional surfaces, such as microstructured Fresnel lenses, used in concentrator photovoltaic devices. Further to imprint these optical components using R2R UV-NIL technology, a double UV-NIL process is developed to incorporate the moth-eye AR topography on the reverse side of the Fresnel array, improving the optical efficiency by reducing the reflection losses.
In summary, this thesis describes the development of multifunctional antireflective surfaces based on the moth-eye polymer nanostructures reinforced with active nanofillers to achieve improved mechanical, thermal, and weathering properties, providing in addition self-cleaning functionalities. The fabrication of the AR surfaces and Fresnel lenses using the mass production roll-to-roll NIL format is demonstrated, and therefore the validity of the processing technology for the manufacturing of structured functional surfaces suitable for real applications is substantiated.
