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.