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A flexible, dense, defect-free, highly adhesive, and highly dissociation energy-rich protective coating is essential to enhance the atomic oxygen (AO) resistance of polymeric materials in a low Earth orbit (LEO). In this work, a dense, defect-free hybrid HMDSO/SiO 2 thin film coating with compositional gradients on the surface of polyimide was synthesized using vacuum-ultraviolet (VUV) irradiation. The effects of VUV irradiation on the morphology, optical transmittance, and chemical components of plasma-polymerized HMDSO (pp-HMDSO) thin-film coatings deposited on the polyimide surface were investigated in depth. There were no defects such as cracks and holes in the surface morphology of pp-HMDSO films after VUV irradiation, but the surface roughness increased slightly, and the corresponding optical transmittance decreased slightly. The chemical components of pp-HMDSO films were changed in the depth direction starting from the top of the surface, forming hybrid HMDSO/SiO 2 thin films with compositional gradients. The component gradient HMDSO/SiO 2 composite coating further enhanced the atomic oxygen resistance of the polyimide due to the surface layer of the UV-modified coating enriched with high dissociation energy SiO x material. Therefore, this work provides a facile UV-induced synthesis method to prepare dense, defect-free, and highly dissociation energy-rich protective gradient coatings, which are promising not only for excellent AO protection in LEO but also for potential application in water-oxygen barrier films.

Plasma polymerization has emerged as an appealing technique for surface modification because of its advantages over a variety of conventional techniques, including ease-of-use and the possibility to modify nearly any substrate. One of the main challenges of plasma polymer-based surface modification, however, is having control over the coating chemistry, as plasma deposition generates a diversity of chemical structures. Therefore, this study presents an alternative plasma-based method for the fabrication of coatings that contain selective functionalities. In a first step, hexamethyldisiloxane (HMDSO) plasma polymerization is performed in a medium-pressure dielectric barrier discharge (DBD) to deposit polydimethylsiloxane (PDMS)-like coatings. In a second step, this coating is exposed to an air plasma in a similar DBD setup to introduce silanol groups on the surface. These groups are used in a third and final step as anchoring points for grafting of (3-aminopropyl)triethoxysilane (APTES) and (3-bromopropyl)trichlorosilane (BrPTCS) to selectively introduce amino or bromo groups, respectively. X-ray photoelectron spectroscopy (XPS) and water contact angle (WCA) measurements indicated that the first two steps were successful. Moreover, the coating could be synthesized on three different surfaces, namely, glass, ultrahigh-molecular-weight polyethylene, and polytetrafluoroethylene, indicating the wide applicability of the developed procedure. Afterward, XPS also proved that the APTES and BrPTCS grafting resulted in the formation of a coating containing primary amines and alkyl bromides, respectively, in combination with an organosilicon matrix containing silanol groups as remaining reactive groups, proving the successful synthesis of selective functional plasma-based coatings. The intermediate air-plasma-activation step was demonstrated to be necessary for successful and stable grafting of the final layer. In conclusion, this study established a general procedure for the development of coatings with selective functionality that can be applied on a wide variety of substrates for, e.g., biosensor applications, biomolecule, or polymer immobilization or for the synthesis of antibacterial coatings.

In this work, the water condensation performance of methylated silica-nanospring (SN)-coated horizontal aluminum tubes is assessed. Coated samples with varying nanospring mat thicknesses, from 784 to 2902 nm, were studied, which exhibited static contact angles and CA hysteresis values of 155° and 16°, respectively. Dropwise condensation and increased droplet shedding were observed on these coated tubes. Video analysis determined that tubes with 15 and 20 min SN growth times experienced an 84% increase in the condensate removal rate over the baseline. Moreover, with a hybrid wettability consisting of alternating regions of SN and bare aluminum, a 96% increase in condensate removal was experienced. Additionally, the average droplet departure size was reduced on these SN-coated tubes. SEM imaging and XEDS analysis were also performed on the tubes and revealed that the coating was reasonably durable having withstood the condensation environment. Moreover, the coated tubes were shown to exhibit the same XEDS spectra both before and after testing.

Thin non-perfluoroalkoxy superhydrophobic coatings are desirable for heat exchangers because of their lower thermal resistance and reduced environmental concerns. Coatings requirements must also include robustness and longevity and facilitate high defrosting rates in refrigeration applications to warrant their adoption and use. Methyl-functionalized silica nanosprings (SN) possess water droplet static contact angles above 160° with contact angle hysteresis values as low as 6.9° for a sub-micrometer-thick coating. The methyl functional groups render the silica surface hydrophobic, whereas the geometrical and topographical characteristics of the nanosprings make it super-hydrophobic. Results show that SN are capable of removing 95% of the frost from the surface at a lower temperature than the base aluminum substrate. The sub-micrometer SN coating also decreases the time to defrost by ≈1.5 times and can withstand more than 20 frosting-defrosting cycles in a high humidity environment akin to real working conditions for heat exchangers.