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Long-time behavior of surface properties of microstructures fabricated by multiphoton lithography
(2021)
The multiphoton lithography (MPL) technique represents the future of 3D microprinting, enabling the production of complex microscale objects with high precision. Although the MPL fabrication parameters are widely evaluated and discussed, not much attention has been given to the microscopic properties of 3D objects with respect to their surface properties and time-dependent stability. These properties are of crucial importance when it comes to the safe and durable use of these structures in biomedical applications. In this work, we investigate the surface properties of the MPL-produced SZ2080 polymeric microstructures with regard to the physical aging processes during the post-production stage. The influence of aging on the polymeric microstructures was investigated by means of Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS). As a result, a time-dependent change in Young’s Modulus, plastic deformation, and adhesion and their correlation to the development in chemical composition of the surface of MPL-microstructures are evaluated. The results presented here are valuable for the application of MPL-fabricated 3D objects in general, but especially in medical technology as they give detailed information of the physical and chemical time-dependent dynamic behavior of MPL-printed surfaces and thus their suitability and performance in biological systems.
The coefficient of thermal expansion (CTE) and the thermomechanical properties of the polymers used in superconducting magnets need to be known in order to predict their stress state under the different magnet assembly and operating conditions.
We have measured Young’s moduli of typically used polymers during in situ heat cycles with the dynamic resonancemethod. The dynamic test results are compared with Young’s moduli determined from quasi-static stress–strain measurements at room temperature, 77 K and 4.2 K. A moderate elastic anisotropy is found for the fiber reinforced polymers. CTEs are compared based on dilation experiments. TheCTEs of the fiber reinforced polymers studied are similar to those of copper or steel. In contrast, the pure resins exhibit relatively larger CTEs.
A recently developed theoretical model represents the generalization of the indentation of a sphere into an infinite homogeneous halfspace to the problem of a Hertzian load acting on a halfspace covered with one or more films having different elastic properties. The model allows the analytical calculation of the complete elastic stress field and the deformations within the films and the substrate. Some results of the model shall be confirmed by nanoindentation experiments using an UMIS-2000 nanoindenter into Si3N4/SiO2 and SiO2/ Si3N4 double layers on BK7 glass and Si(100) single crystal. The materials used allow accurate measurements due to their homogeneous, amorphous structure as well as low surface and interface roughness. After the determination of the instrument compliance and the real, depth dependent indenter radius the measured loaddepth data are compared with calculated results. It is shown that measurement results can be correctly interpreted by the model. The onset of plastic deformation is investigated for the same samples by multiple partial unloading experiments with a 4-µm radius diamond sphere. The critical load at which a first deviation from a wholly elastic response occurs is used for a stress calculation with the model. The mechanical behavior of the different film combinations is interpreted by means of the von Mises comparison stress. The measured results, together with the analytical modeling, allow an optimization of the thickness and modulus of the individual layers to get a maximum mechanical stability.
Forcedisplacement curves have been acquired with a commercial atomic force microscope on thin films of poly(n-butyl methacrylate) on glass substrates. Different film thicknesses, from 10 up to 430 nm, were chosen to examine in detail the so called mechanical double-layer topic, i.e., the influence of the substrate on the determination of the mechanical properties of thin films. Taking advantage of the Hertz theory we calculated for all films the contact radius between tip and sample as a function of the applied load. Further Young's modulus of the samples was derived from the experimental data as a function of the applied load and, alternatively, of the deformation. The results of this analysis for 10 different film thicknesses were fitted with several half empirical equations proposed by several researchers. The focus of this work is to evaluate such existing half empirical theories for mechanical double-layers and to show the need for an alternative consistent approach.