Figure 3 (a) Schematic diagram of degradation and hydrolysis of PMx By . Reproduced with permission.[44] Copyright 2019 American Chemical Society. (b) Hydrolysis and degradation of copolymer. Reproduced with permission.[46] Copyright 2019 American Chemical Society. (c) Schematic diagram of the preparation and degradation process of HPMx E. (d) Mass loss of HPMx E after immersion in ASW at 25 °C. Reproduced with permission.[47] Copyright 2021 Royal Society of Chemistry. (e) Schematic diagram of degradation and hydrolysis of PTAx . (f) Fluorescence images of P. sp. on PTA5 surface with and without a 3-day pre-hydrolysis. Reproduced with permission.[51] Copyright 2021 American Chemical Society.
We synthesized hyperbranched degradable copolymer (HPMx E) with via RROP of MDO, vinyl acetate (VAc) and diethylene glycol divinyl ether (DEGDVE) (Figure 3c).[47] Since the vinyl monomers and MDO are close in reactivity ester bonds can randomly distribute along polymer chain, so that the degradation happens everywhere on the surface. The coating had high adhesion strength after crosslinking under UV. Figure 3d shows that mass loss of the coating increased with the ester units. The coating with 44 mol% ester units had the highest mass loss after a 14-day immersion in ASW, much higher than the coating without ester units (0.4 mg·cm−2). The final degradation product could dissolve in seawater, so it has limited impact on the marine environment. Antibacterial assay demonstrates that the copolymers with 50 mol% DEGDVE had good antibacterial ability. That is due to the presence of DEGDVE, forming a hydration layer with water and hindering bacterial adhesion.
We also prepared a hyperbranched copolymer via reversible addition fragmentation chain transfer (RAFT) polymerization of MMA, TCB and divinyl-functional PCL (PCL-vi2).[48] Owing to the hyperbranched structure, the hyperbranched PCL had a crystallinity lower than liner PCL. The fragmentation of branching points would make the final degradation product smaller. The existence of TCB further improved the hydrophilicity, self-renewal capacity and effectively inhibited the adhesion of fouling organism. Thanks to the continuous degradation of PCL and the hydrolysis of TCB, the self-renewal performance and long-term antifouling ability of coating were guaranteed even in static marine environments.
We studied the effect of branching points structure on self-renewal and antifouling properties of hyperbranched copolymer. Three kind of divinyl monomers including methacrylic anhydride (MAAH), N, N′-adipic bis(diacetone acrylamide hydrazone) (DAA2H), and ethylene glycol dimethacrylate (EGDMA) were used.[49] It shows the degradation rate and mass loss of the polymer increased with the branching point content. For the same branching point content, the more breakable sites in branching points, the greater the mass loss (EGDMA<MAAH<DAA2H). Namely, the degradation rate of the hyperbranched copolymer can be regulated by varying the content or structure of branching points. Antibacterial and anti-diatom assays demonstrate that the coatings have excellent antifouling performance due to branching points degradation and zwitterions produced by hydrolysis. We synthesized vinyl-functional 4-bromo-2-(4-chlorophenyl)-5-trifluoromethyl-1H-pyrrole-3-carbonitrile (Econea -vi) as antifouling group, and introduced it to hyperbranched polymer via RAFT polymerization.[50]With the function of contact killing, the grafted Econea could effectively inhibit biofouling. On the other hand, Econea could be released and degrade in seawater through the breaking the copolymer chain.
Based on previous research, we proposed a strategy called the ”kill-resist-renew-trinity”.[51] For this purpose, we synthesized a dual-functional monomer with TCB and TCPM, and copolymerized it with MAAH to prepared a hyperbranched polymer (PTAx ) with fouling killing, resisting and releasing functions (Figure 3e). Even before hydrolysis, TCPM with contact killing property provided the coating antibacterial ability. After hydrolysis, TCPM was released from the coating to further improve antifouling performance. Meanwhile, it generated zwitterion inhibiting the adhesion of bacteria. The dynamic surface could remove the dead bacteria attached to the surface. As shown in Figure 3f, the hyperbranched polymer with fouling killing, resisting and releasing functions had excellent anti-biofouling performance. Moreover, the hyperbranched copolymer would eventually degrade into small molecules without formation of microplastics.
4. Conclusions
In conclusion, Dynamic Surface Antifouling (DSAF) provides a general strategy to defeat marine biofouling. We have developed main chain degradable polymer based antifouling materials. They exhibit good anti-biofouling performances and mechanical properties both on static and dynamic conditions. The surface erosion of degradable polymers has linear kinetics, the service life can be regulated by the coating thickness, and the release of the antifoulants can be well controlled. The main chain degradation yields low molecular weight molecules instead of microplastics, so the polymer materials are relatively eco-friendly. By adjusting the molecular structure of the polymer, the antifouling performance, service life and the mechanical properties can be regulated. DSAF materials are promising in marine antifouling.
Acknowledgement
The financial support of the National Key Research and Development Program of China (2022YFB3806403), National Natural Science Foundation of China (52273073, U2241286, and 52003082), Guangdong Basic and Applied Basic Research Foundation (2023B1515020025), the China Postdoctoral Science Foundation (2022M721179) and Ministry of Science and Technology of China is acknowledged.
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