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.