Abstract:

Marine organisms such as plants, algae or small animals can adhere to surfaces of materials that are submerged in ocean. The accumulation of these organisms on surfaces is a marine biofouling process that has considerable adverse effects. Marine biofouling on ship hulls can cause severe fuel consumption increase. Investigations on antifouling polymers are therefore becoming important research topics for marine vessel operations. Antifouling polymers can be applied as coating layers on the ship hull, protecting it against the settlement and growth of sea organisms. Polyethylene glycol (PEG) is a hydrophilic polymer that can effectively resist the accumulation of marine organisms. PEG-based antifouling coatings have therefore been extensively researched and developed. However, the inferior stability of PEG makes it subject to degradation, rendering it ineffective for long-term services. Zwitterionic polymers have also emerged as promising antifouling materials in recent years. These polymers consist of both positively charged and negatively charged functional groups. Various zwitterionic polymers have been demonstrated to exhibit exceptional antifouling properties. Previously, surface characterizations of zwitterionic polymers have revealed that strong surface hydration is critical for their antifouling properties. In addition to these hydrophilic polymers, amphiphilic materials have also been developed as potential antifouling coatings. Both hydrophobic and hydrophilic functional groups are incorporated into the backbones or sidechains of these polymers. It has been demonstrated that the antifouling performance can be enhanced by precisely controlling the sequence of the hydrophobic-hydrophilic functionalities. Since biofouling generally occurs at the outer surface of the coatings, the antifouling properties of these coatings are closely related to their surface characteristics in water. Therefore, understanding of the surface molecular structures of antifouling materials is imperative for their future developments. In this review, we will summarize our recent advancements of antifouling material surface analysis using sum frequency generation (SFG) vibrational spectroscopy. SFG is a surface-sensitive technique which can provide molecular information of water and polymer structures at interfaces in situ in real time. The antifouling polymers we will review include zwitterionic polymer brushes, mixed charged polymers, and amphiphilic polypeptoids. Interfacial hydration studies of these polymers by SFG will be presented. The salt effect on antifouling polymer surface hydration will also be discussed. In addition, the interactions between antifouling materials and protein molecules as well as algae will be reviewed. The above research clearly established strong correlations between strong surface hydration and good antifouling properties. It also demonstrated that SFG is a powerful technique to provide molecular level understanding of polymer antifouling mechanisms.

Key words: Antifouling materials, Sum frequency generation vibrational spectroscopy, Interfacial hydration, Zwitterionic polymer, Salt effect, Protein interaction