Per Magnus Kristiansen

Engineered Water Highways in Fuel Cells: Radiation Grafting of Gas Diffusion Layers

A novel method to produce gas diffusion layers with patterned wettability for fuel cells is presented. The local irradiation and subsequent grafting permits full design flexibility and wettability tuning, while modifying throughout the whole material thickness. These water highways have improved operando performance due to an optimized water management inside the cells.

Iso- and variothermal injection compression moulding of polymer micro- and nanostructures for optical and medical applications

The surfaces of medical and optical polymer products are being increasingly functionalized with micro- and nanostructures using mass replication methods like injection moulding. We compared the filling behaviour and replication quality of such structures using four different moulding processes with two polymers of different viscosity and wetting behaviour. For this purpose, we replicated three representative 2D and 3D micro- and nanostructures into polymethylmethacrylate and amorphous polyamide by isothermal and variothermal injection moulding with and without compression, respectively, using the same mould. The parallel compression phase reduced the internal pressure in the cavity, leading to fewer demoulding issues but without significant influence on the replication fidelity. Variothermal heating of the mould in combination with a polymer of low viscosity and good wetting behaviour was favourable especially for filling of high aspect-ratio structures. For microstructure replication, melt viscosity and no-flow temperature were clearly more relevant than wetting as flow resistance and frozen layer formation are the main reasons for incomplete filling at this length scale. In nanostructures, the capillary effect becomes increasingly dominant depending on the surface energy of the polymer.

High-throughput fabrication of compact and flexible bilayer nanowire grid polarizers for deep-ultraviolet to infrared range

The authors present the design and fabrication of a bilayer metallic wire-grid polarizer with a period of 80 nm on a flexible polymeric substrate optimized for broadband operation ranging from the infrared down to the deep-ultraviolet range. Their high-throughput fabrication over large areas is realized by nanoimprint lithography by producing the imprint master stamps using extreme ultraviolet interference lithography. Optical measurements show that the fabricated bilayer polarizer covers a broad spectral range, starting from wavelength of 280 nm. Transverse magnetic transmission of 70% and an extinction ratio of 30 dB were realized.

Nanoimprint lithography process chains for the fabrication of micro- and nanodevices

Abstract. The nanoimprint lithography (NIL) process with its key elements molding and thin film pattern transfer refers to the established process chain of resist-based patterning of hard substrates. Typical processes for mass fabrication are either wafer-scale imprint or continuous roll-to-roll processes. In contrast to this, similar process chains were established for polymeric microelements fabricated by injection molding, particularly when surface topographies need to be integrated into monolithic polymer elements. NIL needs to be embedded into the framework of general replication technologies, with sizes ranging from nanoscopic details to macroscopic entities. This contribution presents elements of a generalized replication process chain involving NIL and demonstrates its wide application by presenting nontypical NIL products, such as an injection-molded microcantilever. Additionally, a hybrid approach combining NIL and injection molding in a single tool is presented. Its aim is to introduce a toolbox approach for nanoreplication into NIL-based processing and to facilitate the choice of suitable processes for micro- and nanodevices. By proposing a standardized process flow as described in the NaPANIL library of processes, the use of establish process sequences for new applications is facilitated.

3D filling simulation of micro- and nanostructures in comparison to iso- and variothermal injection moulding trials

Flow simulations can cut down both costs and time for the development of injection moulded polymer parts with functional surfaces used in life science and optical applications. We simulated the polymer melt flow into 3D micro- and nanostructures with Moldflow and Comsol and compared the results to real iso- and variothermal injection moulding trials below, at and above the transition temperature of the polymer. By adjusting the heat transfer coefficient and the transition temperature in the simulation it was possible to achieve good correlation with experimental findings at different processing conditions (mould temperature, injection velocity) for two polymers, namely polymethylmethacrylate and amorphous polyamide. The macroscopic model can be scaled down in volume and number of elements to save computational time for microstructure simulation and to enable first and foremost the nanostructure simulation, as long as local boundary conditions such as flow front speed are transferred correctly. The heat transfer boundary condition used in Moldflow was further evaluated in Comsol. Results showed that the heat transfer coefficient needs to be increased compared to macroscopic moulding in order to represent interfacial polymer/mould effects correctly. The transition temperature is most important in the packing phase for variothermal injection moulding.

Surface-patterned micromechanical elements by polymer injection molding with hybrid molds

Hybrid molds enable the fabrication of polymeric parts with features of different length scales by injection molding. The resulting polymer microelements combine optical or biological functionalities with designed mechanical properties. Two applications are chosen for illustration of this concept: As a first example, microelements for optical communication via fiber-to-fiber coupling are manufactured by combining two molds to a small mold insert. Both molds are fabricated using lithography and electroplating. As a second example, microcantilevers (μCs) for chemical sensing are surface patterned using a modular mold composed of a laser-machined cavity defining the geometry of the μCs, and an opposite flat tool side which is covered by a patterned polymer foil. Injection molding results in an array of 35 μm-thick μCs with microscale surface topographies. In both cases, when the mold is assembled and closed, reliefs are transferred onto one surface of the molded element whose outlines are defined by the micromold cavity. The main advantage of these hybrid methods lies in the simple integration of optical surface structures and gratings onto the surface of microcomponents with different sizes and orientations. This allows for independent development of functional properties and combinations thereof.

Bilayer wire-grid polarizers for DUV to IR fabricated using EUV interference and nanoimprint lithography

We present the design of a bilayer metallic wire-grid polarizer (WGP) optimized for operation in the deep-ultraviolet (DUV) region, and their high-throughput fabrication of over large areas by nanoimprint lithography (NIL). The master imprint stamps were fabricated using our newly developed scanning exposure strategy with extreme ultraviolet interference lithography (EUV-IL). Optical measurements show that the fabricated bi-layer polarizer covers a broad spectral range, starting from wavelength of 280 nm. TM transmission of 50%, and an extinction ratio of 20 dB (102) were realized.

Anisotropy in polyetheretherketone films

Optical measurements reveal the preferential orientation of nanostructures within polymer films, which results from the fabrication process including mechanical and thermal treatments. As the wavelength of the incident light is generally much larger than the characteristic dimensions of the molecular arrangement in semi-crystalline or amorphous polymers, the optical signal originates not directly from the nanostructure of the polymers. Linear dichroism measurements were correlated with synchrotron radiation-based x-ray scattering data on commercially available polyetheretherketone (PEEK) thin films (12 to 50 μm). Annealing changed the structure of amorphous films to semi-crystalline ones associated with the measured linear dichroism. The intensity of the measured anisotropic signal depended on the film thickness. While for wavelengths between 450 and 1100 nm the transmission was higher when the polarizer was parallel to the machine direction, for larger wavelengths maximum transmission was observed with the polarizer perpendicular to the machine direction indicating excitations parallel and perpendicular to the PEEK molecule axis, respectively. Annealing PEEK films at temperatures between 160 and 240°C decreased the transmission at 540 nm by a factor of two, whereas the anisotropy remained constant. x-ray scattering revealed strongest anisotropy for a periodicity of 15 nm in the machine direction of the cast film extrusion process. The long-range order of amorphous and semi-crystalline entities can explain the x-ray scattering data and the related optical anisotropy of casted PEEK films.

Nanometer-size anisotropy of injection-molded polymer micro-cantilever arrays

Understanding and controlling the structural anisotropies of injection-molded polymers is vital for designing products such as cantilever-based sensors. Such micro-cantilevers are considered as cost-effective alternatives to single-crystalline silicon-based sensors. In order to achieve similar sensing characteristics,structure and morphology have to be controlled by means of processing parameters including mold temperature and injection speed. Synchrotron radiation-based scanning small- (SAXS) and wide-angle x-ray scattering techniques were used to quantify crystallinity and anisotropy in polymer micro-cantilevers with micrometer resolution in real space. SAXS measurements confirmed the lamellar nature of the injection-molded semi-crystalline micro-cantilevers. The homogenous cantilever material exhibits a lamellar periodicity increasing with mold temperature but not with injection speed. We demonstrate that micro-cantilevers made of semi-crystalline polymers such as polyvinylidenefluoride, polyoxymethylene, and polypropylene show the expected strong degree of anisotropy along the injection direction.

Enhanced thermal stability of a polymer solar cell blend induced by electron beam irradiation in the transmission electron microscope

We show by in situ microscopy that the effects of electron beam irradiation during transmission electron microscopy can be used to lock microstructural features and enhance the structural thermal stability of a nanostructured polymer:fullerene blend. Polymer:fullerene bulk-heterojunction thin films show great promise for use as active layers in organic solar cells but their low thermal stability is a hindrance. Lack of thermal stability complicates manufacturing and influences the lifetime of devices. To investigate how electron irradiation affects the thermal stability of polymer:fullerene films, a model bulk-heterojunction film based on a thiophene-quinoxaline copolymer and a fullerene derivative was heat-treated in-situ in a transmission electron microscope. In areas of the film that exposed to the electron beam the nanostructure of the film remained stable, while the nanostructure in areas not exposed to the electron beam underwent large phase separation and nucleation of fullerene crystals. UV-vis spectroscopy shows that the polymer:fullerene films are stable for electron doses up to 2000kGy.