Hydrophilic Self-Replenishing Coatings with Long-Term Water Stability for Anti-Fouling Applications

Isabel Jiménez-Pardo, Leendert van der Ven, Rolf van Benthem, Gijsbertus de With, A. Esteves
2018 Coatings  
Hydrophilic coatings have recently emerged as a new approach to avoiding the adhesion of (bio)organisms on surfaces immersed in water. In these coatings the hydrophilic character is crucial for the anti-fouling (AF) performance. However, this property can be rapidly lost due to the inevitable damages which occur at the surface, reducing the long-term effectiveness of the AF functionality. We report hydrophilic polycarbonate-poly(ethylene glycol) methyl ether (mPEG) polyurethane coatings with
more » ... ne coatings with tunable hydrophilic properties as well as an excellent and long-term stability in water. The coatings exhibit low protein adhesion values and are able to self-replenish their hydrophilicity after damage, due to the existence of a reservoir of hydrophilic dangling chains incorporated in the bulk. The combination of low T g and sufficient mobility of the mPEG dangling chains (enabled by chains with higher molecular weight) proved to be crucial to ensure autonomous surface hydrophilicity recovery when the coatings were immersed in water. This coatings and design approach offers new possibilities towards high-performance AF coatings with an extended service life-time which can be used in several major applications areas, such as marine and biomedical coatings, with major economic and environmental benefits. Several anti-fouling (AF) strategies have been reported for polymer coatings in which the characteristics of the top surface, i.e., chemical composition and topography, are critical for the effectiveness and long time performance of this surface functionality, as discussed in several literature reviews available [6] [7] [8] [9] [10] . More recently, hydrophilic high surface energy coatings emerged as an interesting option to prevent the adhesion of foulants, most of them making use of the well-known anti-fouling character of polyethylene glycol (PEG)-based derivatives [11] [12] [13] [14] [15] . Although the working principle of this type of coating is still unclear and several mechanisms have been proposed by different authors [9, 16, 17] , it is widely assumed that the use of hydrophilic polymeric surfaces allows the formation of a hydration layer by means of hydrogen bonding between the water molecules and the hydrophilic polymer, which reduces the probability of proteins to adhere to the surface, thus reducing the initial attachment and subsequent accumulation of foulants. However, once the coating is damaged and the surface characteristics (in this case the hydrophilicity) are lost upon wear, degradation or attachment of the first biorganisms, the AF properties are no longer effective and the wet surfaces will become rapidly fouled. Introducing a self-repairing mechanism, which can intrinsically replenish the damaged surface with new hydrophilic AF chemical moieties, would allow a high AF performance level throughout the life-time of the coatings, with major economic and environmental benefits. As previously demonstrated for analogous hydrophobic coatings [18-20], an intrinsic and spontaneous self-replenishing mechanism can be incorporated in coatings by fulfilling some design requirements. The coating should contain: (i) a reservoir of hydrophilic dangling chains chemically bonded to the bulk network; (ii) these dangling chains should be sufficiently mobile, e.g., typically governed by a low T g of the polymer components, to reorient upon creation of new interfaces; and (iii) a proper hydrophilic-hydrophobic balance between all the coating components (i.e., dangling chains and network polymer precursors), which will provide the driving force for the reorientation of the dangling chains towards the air-coating, or in this case, water-coating, interface once damage occurs. To date, most of the self-replenishing systems found in literature are hydrophobic, while the development of self-healing hydrophilic coatings is still scarcely addressed [21] [22] [23] [24] . In one of the few examples, Minko et al. settled guidelines towards the design of materials with long-lasting hydrophilicity and anti-fouling properties. The hydrophilicity and AF properties of PEG 2D surface-grafted and 3D-network grafted films, possessing PEG chains in the surface and inside the network film, were studied and compared. For the measuring time of four weeks used, the 3D-grafting structures demonstrated much higher hydrophilicity stability and fouling resistant properties than the 2D films due to the spontaneous rearrangement of the chains stored inside the film [23]. In a more recent publication self-assembled microgel spheres with grafted hydrophilic chains were synthesized. The films presented oil-repellent and AF properties, and were able to self-repair after induced damage. Also in this case, the self-healing function was attributed to the 3D structure combined with the presence of a reservoir of hydrophilic chains [24] . It should also be noted that, while for some specific applications an easily degradable polymer coating may be required (e.g., for short-term medical implants), for many others, the overall long term stability of the hydrophilic self-healing coatings when immersed in water and the absence of leachable materials (i.e., resulting from bulk or network degradation) are essential, e.g., in the marine field and especially on medical devices in contact with the human skin or body-fluids. Due to their interesting thermal and mechanical properties, aliphatic Poly(carbonates) (PCs) find applications in a wide variety of fields, such as in regenerative medicine, drug delivery and in the coatings industry [25] [26] [27] [28] . Furthermore, PCs typically present longer hydrolytic stability in water when compared to polyesters, are highly transparent to visible light and have a tunable and generally low T g value [29] [30] [31] . Although this low T g can be envisaged as a major drawback for some applications, they are advantageous for preparing protective and functional coatings which are required to interact favorably with water. This type of polymer can confer long-time water stability and high transparency to the coatings while immersed in water, and also the proper mobility in the system for self-replenishing. Additionally, PCs can be easily prepared by Cationic Ring Opening
doi:10.3390/coatings8050184 fatcat:2dqms6mjtfbpxj4fuzpfvq32va