Figure 1 (a) The ratio of subdiffusive motion and active motion of both bacteria near the coating surfaces. (b) Force vs apparent separation curves of P. sp. and mean normalized adhesion force (γ ) of both bacteria on the coating surfaces. (c) Density distribution of instantaneous 3D velocity and distance from the surface for E. coli on coating surfaces. Reproduced with permission.[14] Copyright 2017 American Chemical Society.
Polymer erosion usually results in mass loss of polymer and renewal of coating surface. It contains water intrusion and removals of polymers via main chain degradation and hydrolysis of lateral groups. The erosion including surface erosion and bulk erosion leads to a dynamic coating surface. Generally, the former occurs only on the surface with stable self-renewal rate, whereas the later occurs throughout the coating.
Both SPC and degradable polymers are DSAF materials because they can form a continuously changing surface in seawater. However, they are quite different from each other in structure and performance. SPC has a hydrophobic and non-degradable backbone. The hydrolysis of the lateral groups is an equilibrium reaction, and a strong water flow is needed to remove the hydrolytic polymer chains. That is why SPC coating has poor antifouling performance under static conditions. SPC coatings generally do not exhibit linear thickness decrease over time, so the service life is difficult to control. Besides, SPC coating yields microplastics because of the non-degradable backbone.
In contrast, degradable polymer, e.g., degradable polyester undergo main chain cleavage spontaneously on surface in contact with seawater,[17,18] yielding low molecular weight products which readily disperse into seawater, and they would eventually degrade into carbon dioxide and water. As a result, degradable polyester coating would have much less negative impact on ecology. On the other hand, the degradation of polyester backbone is non-equilibrium reaction, the removal of the degraded moieties on the coating surface does not need external force. Accordingly, the coating has good antifouling performance even on static conditions. The surface erosion exhibits linear kinetics, so the service life can be controlled by the coating thickness.
3. Degradable Polymers
3.1. Degradable Polyurethanes
Aliphatic polyesters such as PCL, poly(butylene succinate) (PBS) and poly(L-lactic acid) (PLA) can enzymatically or hydrolytically degrade with main chain cleavage, leading to a renewable surface.[19,20] In principle, they can act as anti-biofouling materials. However, their low adhesion strength, low degradation rate, and poor film formation properties make it impossible. They must be chemically modified for this use.
We first synthesized PCL-based polyurethane (PU) with lysine ethyl ester diisocyanate and 1,4-butanediol. Yet, PCL-PU was highly crystallized and thus degraded slowly. To further lower crystallinity, we synthesized copolymer of CL and glycolide (GA) to use as the soft segment of PU.[21] Differential scanning calorimetry (DSC) measurements showed that such a copolymer had lower crystallinity compared with PCL and PGA homopolymers. This is because the copolymerization destroys the regularity of molecular chains and hinder the crystallization. The increase of amorphous area makes it more possible that enzyme and water molecules can contact and react with the polymer chains, so the enzymatic and hydrolytic degradations of the copolymer happen. The increase of GA content can accelerate the degradation process of copolymer. The copolymer with 10 mol% GA exhibited best antifouling performance in a 3-month marine field test.
We introduced dihydroxy-terminated poly(ethylene adipate) (PEA), poly(1,4-butylene adipate) (PBA) or poly(1,6-hexamethylene adipate) (PHA) to regulate the surface renewal rate of degradable polyurethane.[22] Our study demonstrated that when PU content was 80 wt%, the degradation rate and mass loss of PEA-PU is higher than that of PBA-PU and PHA-PU in artificial seawater (ASW) (Figure 2a). Namely, the probability of main chain cleavage increased with the ester density, so the degradability could be readily regulated. On the other hand, hydrogen bonding between the urethanes and polar substrate significantly improved the adhesion strength (4 MPa), which is favorable to long-term application in marine environments.
As we know, an antifouling system usually consists of antifoulants and polymer resin, where the latter serves as the carrier of the former. For a traditional system with polymer having non-degradable main chain, antifoulant usually is released quickly in the initial stage, but slows down thereafter. As a result, it has a short service time. In contrast, biodegradable polymer linearly degrades in seawater, so it can be well used as controllable release carrier for antifoulants. We have investigated the release of organic antifoulants carried by degradable polyurethane. PEA/PBA/PHA-based polyurethane were used to carry antifoulant 4,5-dichloro-2-octyl-isothiazolone (DCOIT), which can be released with a stable and sustainable release (Figure 2b). Anyhow, the combination of dynamic surface and stable release of antifoulants enable the coating to exhibit excellent antifouling performance in marine field test.