Figure 2 (a) The mass loss of the PU coatings as a function of immersion time in ASW at 25 °C. (b) Time dependence of the cumulative release of DCOIT from the PU coatings. Reproduced with permission.[22] Copyright 2017 Elsevier. (c) The POM images of pure PCL and PCL/clay (1. PCL; 2. PCL/clay; 3. PCL/DCOIT; 4. PCL/clay/DCOIT). Reproduced with permission.[26] Copyright 2014 Royal Society of Chemistry. (d) Illustration of the antiprotein mechanism of PCL-PU-DEM coatings. Reproduced with permission.[29] Copyright 2015 American Chemical Society. (e) Schematic diagram of degradation of PU-Sx coatings. (f) Mass loss of PU-Sx coatings as a function of immersion time in ASW at 25 °C. Reproduced with permission.[30] Copyright 2014 American Chemical Society. (g) Mass loss of PLA-PU coatings as a function of immersion time in ASW at 25 °C. (h) Release rate of butanolide from PLA-PU-x as a function of soaking time in ASW at 25 °C. (i) Density distribution of instantaneous 3D velocity on PLA-PU-x coating surface for P. sp. . Reproduced with permission.[32] Copyright 2020 American Chemical Society
Butanolide is a natural compound and highly effective antifoulant with low toxicity.[23,24] We used degradable PCL-PU80 to carry butanolide. QCM-D measurements showed that PCL-PU80 degraded much faster in natural seawater (NSW) than in ASW, indicating enzymes secreted by marine microorganisms can improve the degradation rate. With the butenolide content above 5 wt%, PCL-PU80 coating can effectively inhibit bacterial adhesion. However, because the self-renewal rate was slower, the release of butenolide decreased. The coating surface was covered by biofouling in 3 months. Introduction of rosin to PCL-PU80 can reduce the crystallinity and accelerate its erosion. For the same content (5 wt%) of butenolide, PCL-PU80 with rosin had better antifouling performance than that without rosin since the former has a higher and stable release rate. Clearly, both the dynamic surface and release of butenolide play critical roles in antibiofouling. On the other hand, as the seawater temperature rises, the release rate of butanolide increased. This is understandable because the mobility of butanolide (T m=23 °C) would increase with temperature. Note that biofouling accumulation also accelerates with temperature yielding more enzymes. So, the system is enzyme and temperature responsive. We also used degradable PU to carry other natural antifoulants of fungal isolates from the depths of South China Sea.[25] PCL-PU coating with Aspergillus westerdijkiae had excellent antifouling performance after 4-month immersion in seawater.
Besides crystallinity, spherulite size also influences the degradation rate. We developed PCL/clay composite coating via solution mixing process.[26] DSC measurements showed that the crystallinity of composite was close to that of PCL without clay, which makes sure that the coating had enough mechanical strength. We observed smaller spherulites in the composite compared to pure PCL by using polarizing optical microscope (POM) (Figure 2c), which accelerates enzymatic and hydrolytic degradation. We added DCOIT to the composite to further improve its antifouling performance, which was released along with PCL degradation. The combination of dynamic surface and stable release rate of DCOIT enables the composite to have good antifouling performance for up to 10 months in seawater.
Another approach to reduce spherulite size is to blend different biodegradable polymers together. We prepared PCL/PBS blend by mixing them in solution.[27] Because they inhibit each other in crystal growth, the blend had spherulite notably smaller than those of the two homopolymers. Namely, it had increased amorphous interfacial area. After a 4-month immersion in ASW, the mass loss of blend coating is higher than that of pure PCL. The blend was also used as controlled release system of DCOIT with a stable release rate in ASW (correlation coefficient >0.99).
Grafting antifouling groups to degradable polymer can improve its antifouling performance. Through click reaction and polymerization, we synthesized PCL-PU with N-(2,4,6-trichlorophenyl) maleimide (TCPM) as lateral chains.[28] Due to the large phenyl groups, TCPM reduced the crystallinity of polymer and accelerated degradation process in ASW, which is beneficial for surface renewal and resisting marine organisms. In addition, we also grafted 2-(dimethylamino) ethyl methacrylate (DEM), an acrylate monomer with fouling resistant group, to PCL-PU to regulate its crystallinity (Figure 2d).[29] The crystallinity of PCL-PU decreased with DEM content. Due to the hydrophilic nature and full deprotonation of DEM in seawater, the anti-adhesion properties against marine bacteria increased with DEM content. Therefore, we can adjust the surface renewal rate of degradable PU by changing the structure of backbone and lateral groups to meet different application requirements.
We synthesized triisopropylsilyl acrylate (PTIPSA) oligomers with two hydroxyls at one end and grafted them onto PCL-PU so that the hydrolyzable side chains were introduced to degradable main chain (Figure 2e).[30,31] As PTIPSA content increased, the water contact angle (WCA) after immersion decreased significantly. The mass loss of copolymer also increased with PTIPSA content, so it is related to the surface renewal or antifouling ability (Figure 2f). We used scanning electron microscopy (SEM) to investigate the effect of PTIPSA hydrolysis on coating surface morphology. As the soaking time increased, the coating surface roughness slightly varied, so its effect could be neglected. PU-S40 (contain 40 wt% of PTIPSA) exhibited excellent antifouling performance in a 3-month marine field test even without antifoulants. After carrying DCOIT, it exhibited better antifouling performance and longer service life. Polyurethanes with various polyester (PLA, PEA) segments and different length of PTIPSA side chains were also examined. The degradation rate increased with density of ester bonds no matter whether they were in the main chain or lateral chains.
We prepared PLA-PU containing hydrolyzable TIPSA side groups via thiol-ene click reaction.[32] Since polyurethane has excellent mechanical properties, PLA-PU showed a high adhesion strength on flexible substrate surface. We adjusted the degradation rate by changing the structure of soft segment and TIPSA content (Figure 2g), and used it as a carrier of butanolide. By regulating the degradation rate, we achieved controlled release of butenolide (Figure 2h). The system could have a long-term antifouling performance. We observed and recorded the movement trajectory of P. sp. on the coating surface by DHM. The PLA-PU-0 without butanolide was used as the control sample. The adhered number (Nb ) of P. sp. on control sample was higher than that on PLA-PU-0 with butanolide. For the same butenolide content, as TIPSA content increased,Nb of P. sp. decreased, further indicating that both dynamic surface and release of antifoulants would affect antifouling performance (Figure 2i).
Hyperbranched polymers with three-dimensional random structures are a homologue of dendritic macromolecules. Due to their special branched molecular structure and abundant terminal groups, they usually exhibit high solubility, chemical reactivity and excellent film-forming properties, which enables them to be used in antifouling coatings. Compared with linear degradable polymers, hyperbranched polymer can degrade in its main chain and branching points, leading to lower molecular weight degradation products. Thus, degradable hyperbranched polymers are more eco-friendly.
We prepared hyperbranched PCL (h-PCL) by using CL, glycidol and 3-isocyanatopropyltriethoxysilane.[33] The degradable PCL segments in h-PCL lead to dynamic surface to inhibit the attachment of fouling organisms, whereas the siloxane endows h-PCL with fouling release ability. To further improve the antifouling performance of h-PCL coating, we introduced an amphiphilic triblock copolymer as crosslinking agent.[34] The anti-adhesion performance to bacteria and diatom markedly increased with the amphiphilic triblock copolymer content.
3.2. Degradable Polyacrylates
Polyacrylates have found wide applications in adhesives and coatings. Traditional antifouling coating is composed of polyacrylate with hydrolyzable side groups in seawater. However, only hydrolysis cannot enable the antifouling ability enough, especially on static conditions. Polyacrylate with degradable main chain and hydrolysable side groups is expected to have excellent antifouling ability, but its synthesis is a challenge. This is because it is synthesized via copolymerization of cyclic monomers and vinyl monomers, which generally does not happen. In 2012, our group found anionic hybrid copolymerization of cyclic monomers and vinyl monomers in the presence of organic catalyst phosphazene base (t -BuP4)[35,36], making the synthesis of degradable polyacrylates possible. We synthesized random copolymer of CL and MMA[37], hyperbranched PCL with glycidyl methacrylate as branching agent[38]and poly(CL-co-methacrylic acid)[39]. To enhance the fouling resistance, we combined LA with 2-(2-methoxyethoxy) ethyl methacrylate (MEO2MA) to synthesize a random copolymer that has good enzymatic degradation and antifouling performance.[40]
We prepared poly(ester-co-acrylate) with low crystallinity and good degradability by using radical ring opening polymerization (RROP) of cyclic ketene acetals (CKA) and vinyl monomers. For example, we synthesized amorphous polymer of methylene-1,3-dioxepane (MDO) and MMA by RROP.[41] It exhibited excellent film-forming properties and mechanical properties. The antifouling ability of the copolymer increased with the ester density. Acting as a controlled release carrier for DCOIT, it showed good antifouling performance with long service life.
We synthesized poly(ester-co-acrylate) via RROP of MDO, MMA and tributylsilyl methacrylate (TBSM),[42] which has degradable backbone and hydrolyzable lateral base. The degradable backbone accelerated the erosion process of copolymer and decreased the swelling degree, which ensured a stable release rate when it was used as DCOIT carrier in ASW. Even under the static immersion in seawater, the copolymer coating had good antifouling performance.
As regarding these copolymers, the ratio of polyester to silyl methacrylate plays a crucial role in optimizing polymer properties. To understand the effect of the silyl methacrylate group on coating properties, we synthesized degradable copolymers by reacting MDO with various silyl acrylates (bis(trimethylsiloxy)methylsilyl methacrylate (MATM2), triisopropylsilyl methacrylate (TIPSM), and TBSM).[43] Our study showed that the hydrolysis rate of silyl methacrylate decreased with the steric hindrance. After 7-week immersion in ASW, MDO-MATM2 and MDO-TBSM exhibited a significant reduction of weight (90% and 86%), whereas MDO-TIPSM held its weight. Actually, the rapid hydrolysis of silyl methacrylate groups enabled the surface more hydrophilic with higher water absorption, promoting the breaking of the polymer backbone. By adjusting the molecular structure, we can also regulate the degradability and hydrolysability of polymer.
We have made attempts to improve the antifouling ability of degradable polyacrylates. N-methacryloyloxymethyl benzisothiazolinone (BIT) monomer with antifouling function was introduced by copolymerizing with MDO and MMA (Figure 3a).[44] After a 90-day immersion in ASW, the PM49B0 coating (with 49 wt% ester units) showed pronounced mass loss. In the laboratory tests, the PM0B47coating (with 47 wt% BIT) has the best anti-adhesion performance in relation. However, marine field test shows that the PM23B28 (with 28 wt% BIT) coating has much better antifouling performance. The facts further indicate that both surface self-renewal and BIT release play critical roles in anti-biofouling.
In principle, zwitterionic polymers can be used in antifouling. However, the high hydrophilicity enables them to absorb water and swell readily in seawater, lowering their mechanical strength. Their high polarity also limits their copolymerization with non-polar monomers, so they are difficult to chemically modified. Particularly, non-degradable zwitterionic polymers have limited antifouling ability. To improve the properties, we synthesized tertiary carboxybetaine ester (TCB) which can convert into hydrophilic zwitterions after hydrolysis.[45] Then, we prepared poly(ester-co-acrylate) with TCB and MDO via RROP. The ester bonds in the copolymer can degrade in seawater to form a dynamic surface. As the surface is renewed, the side groups are continuously hydrolyzed to generate zwitterions, so that the copolymer coating has a long-term antifouling ability. To regulate the generation rate of zwitterions, tertiary carboxybetaine triisopropylsilyl ester ethyl acrylate (TCBSA), a hydrolysis-induced zwitterions monomer, was used to synthesize a degradable polymer with self-generating zwitterionic (Figure 3b).[46] The resulting polymer coating can generate hydrophilic zwitterionic in seawater with good mechanical strength, indicating that the hydrolysis only occured on the surface. The roughness and mass loss increased with TCBSA content. Because of the self-generating zwitterionic, the copolymer could effectively restrain the attachment and growth of biofouling.