(Invited) Polymers Wettability Control through Plasma Surface Engineering

Abstract

Wettability is one of the key properties of a solid material defining its functionality and a niche of applications. The morphology of the surface and chemical compositions are two main characteristics, determining the surface wetting behaviour [1–3]. By controlling the microstructures and surface chemistry, the degree of surface wetting can be manipulated to the level desired by technological applications that take advantage of interactions between liquids and the solid. Metals, ceramics, and polymers with superhydrophobic properties are extensively used in anti-biofouling [4], and self-cleaning application [5]. Some hydrophilic surfaces can be used for improving cell adhesion to artificial materials [6-7]. Among all materials, the polymers and their wetting behaviour are of particular interest to the industry.Nowadays there is a large set of techniques that are capable of controlling polymers surface wettability for on-demand practical applications: i.e. chemical vapour deposition [8], wet chemistry approach [9] and laser processing. However, these methods either tend to cause environmental issues or irreversible structural damages of the materials or too expensive to be sued on large scale. Therefore, an environmentally friendly, large scale and low-cost wetting control method that does not result in bulk damage, would result in improvement of industrial applications. A possible solution to this wetting control problem is atmospheric-pressure plasma (APP), especially the plasma generated in open-air due to the benefits of solvent-free treatment, requiring no vacuum systems and suitable for in-line processes. In the current work, we will give a comprehensive overview of different atmospheric pressure plasma processes capable to change the surface properties of the polymers with little or no change of the bulk. Two main approaches: (i) plasma activation introducing oxygen-containing groups into the material surface; (ii) plasma polymerization directed to the change of the surface composition will be highlighted and background will be explained.Recently our team developed a new approach based on the use of a combination of plasma activation and plasma polymerization, two different plasma techniques in a single process to achieve different surface wetting properties from hydrophilic to hydrophobic, with the high long-term stability of the coatings in water. Such a type of research approach realized in one plasma source was not yet applied for wettability control and has very promising application potential in the industrial processing of polymers. For surface engineering, and easy to scale-up the radio frequency (RF) plasma system was adopted to perform both plasma activation and plasma polymerization on PET substrate in the atmospheric pressure in the open air. Different characterization methods including WCA measurements, Fourier-transform infrared spectroscopy (FT-IR), XPS, and atomic force microscopy (AFM) were applied to get an insight into surface chemistry and morphology and the effect of the combination of the plasma activation with plasma polymerization.The developed approach has shown the capability of stable coatings deposition with the use of acrylic acid, HMDSO or fluorine-containing precursors PFDA. The method to improve wettability and deposit highly stable hydrophobic coatings on polymer surface was tested in modification of um size membranes (Nylon and polypropylene). Appropriate engineering of the surface of membranes with the use of acrylic acid-based layers allows controlling the membrane hydrophilic behaviour. Such membranes can be used in numerous applications including an attractive field of water/oil separation. Pristine membranes with pores size above 30 um are not capable of effective separation of oil from water in two-phase systems or stabilized emulsions. However, we have shown that plasma deposition of coatings on the membranes can drastically improve their performance in separation tests. With the new approach of surface engineering, it was possible to achieve fast and repeatable separation of oil that is of high importance in industry, water treatment processes and removal of oil spill from seawater. Acknowledgements This work is supported by Vlaio project HBC.2019.0157.