Photocurable amphiphilic perfluoropolyether/poly(ethylene glycol) networks for fouling-release coatings

Article abstract of DOI:10.1021/ma102271t

We demonstrate a facile way of cross-linking hydrophobic perfluoropolyethers, PFPEs, with a series of hydrophilic poly(ethylene glycol)s, PEGs, to prepare a range of amphiphilic networks for use as fouling-release coatings. The PFPE matrix of the networks endows the coating with a low surface energy while the PEG is added to weaken fouling adhesion. It is therefore envisioned that the coating surfaces of these optically transparent and mechanically robust films will display hydrophobicity leading to nonfouling and fouling release characteristics. Two kinds of functionalized PEG oligomers have been cross-linked with reactive, dimethacryloxy-functionalized PFPE oligomers to form a range of amphiphilic networks: (i) a monomethacryloxy-functionalized PEG macromonomer (454 g/mol) (PEG454-MA) which was used to yield blends with flexible PEG chains on the surface as well as in bulk and (ii) a dimethacryloxy-functionalized PEG (550 g/mol) (PEG550-DMA) which results in PEG chains that are relatively more restricted in the network blends and serve as an added difunctional cross-linker for the network along with the dimethacryloxy-functionalized PFPE. The PFPE/PEG cross-linked networks coated on a substrate show very low swelling characteristics in water when PEG454-MA comprises not more than 10 wt % of the overall composition or when PEG550-DMA is used and does not comprise more than 30 wt % of the overall composition. The PFPE/PEG454-MA coatings having PEG chains with one untethered chain end were found to display relatively high spore and barnacle release performance in comparison to PFPE/PEG550-DMA coatings which have the PEG chains in a more restricted network topology.

Full text of DOI:10.1021/ma102271t

878 Macromolecules 2011, 44, 878–885
DOI: 10.1021/ma102271t
Photocurable Amphiphilic Perfluoropolyether/Poly(ethylene glycol)
Networks for Fouling-Release Coatings
Yapei Wang,† Douglas E. Betts,† John A. Finlay,§ Lenora Brewer,^ Maureen E. Callow,§
James A. Callow,§ Dean E. Wendt,^ and Joseph M. DeSimone*,†,‡
†Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599,
United States, ‡Department of Chemical & Biomolecular Engineering, North Carolina State University,
Raleigh, North Carolina 27695, United States, §The University of Birmingham, Birmingham, B15 2TT, U.K.,
and ^Cal Poly, San Luis Obispo, Biological Sciences Department, San Luis Obispo, California 93407, United
States
Received November 10, 2010; Revised Manuscript Received January 12, 2011
ABSTRACT: We demonstrate a facile way of cross-linking hydrophobic perfluoropolyethers, PFPEs, with
a series of hydrophilic poly(ethylene glycol)s, PEGs, to prepare a range of amphiphilic networks for use as
fouling-release coatings. The PFPE matrix of the networks endows the coating with a low surface energy
while the PEG is added to weaken fouling adhesion. It is therefore envisioned that the coating surfaces of
these optically transparent and mechanically robust films will display hydrophobicity leading to nonfouling
and fouling release characteristics. Two kinds of functionalized PEG oligomers have been cross-linked with
reactive, dimethacryloxy-functionalized PFPE oligomers to form a range of amphiphilic networks: (i) a
monomethacryloxy-functionalized PEG macromonomer (454 g/mol) (PEG454-MA) which was used to
yield blends with flexible PEG chains on the surface as well as in bulk and (ii) a dimethacryloxy-functionalized
PEG (550 g/mol) (PEG550-DMA) which results in PEG chains that are relatively more restricted in the
network blends and serve as an added difunctional cross-linker for the network along with the dimethacry-
loxy-functionalized PFPE. The PFPE/PEG cross-linked networks coated on a substrate show very low
swelling characteristics in water when PEG454-MA comprises not more than 10 wt % of the overall
composition or when PEG550-DMA is used and does not comprise more than 30 wt % of the overall
composition. The PFPE/PEG454-MA coatings having PEG chains with one untethered chain end were
found to display relatively high spore and barnacle release performance in comparison to PFPE/PEG550-
DMA coatings which have the PEG chains in a more restricted network topology.
Introduction
these elastomers is certainly regarded as a better choice. Poly
(ethylene glycol), PEG, is well-known to inhibit protein as well as
The conventional approach to biofouling prevention in marine
craft applications has been to use antifouling paints and coatings,
which function through the release of toxins or other biocides in
the immediate vicinity of a ship’s hull through an ablative or
leaching process.1-3 The use of suchtechnology, while admittedly
effective, has proven to be responsible for increases in the levels of
organotin and other toxic compounds in or around dry docks,
harbors and shipping lanes.4,5 Therefore, the development of
minimally adhesive, mechanically durable, easy to apply, non-
toxic, fouling-release coatings as responsible and practical alter-
natives to the currently used antifouling technologies is being
urgently pursued.6-12 On one hand, it is suggested that surface
energy and mechanical properties are the key factors to determine
how the coatings can resist fouling attachment.13,14 In this regard,
elastomers with a low surface energy and a low Young’s modulus
such as silicones1,15-21 and fluorinated polymers22 have been
evaluated as good candidates to facilitate fouling release. In an
effort to increase the fouling-release efficiency of these materials,
antifouling polymers as well as biocidal substances have been
blended into them.23-27 However, the inclusion of biocidal units
increases the toxicity of the marine coatings which limits their
wide application.28 Rather than the inclusion of biocidal compo-
nents, the combination of environmentally benign materials in
cell adsorption due to electrostatic repulsion and a hydration
effect at the interface.29-33 Self-assembled monolayers of PEG or
cross-linked PEG hydrogel coatings havebeenreported to exhibit
good protein resistance.34-36 However, since hydrophilic PEG
swells in water, these PEG-based coatings are not very effective
for use as marine coatings because they lack durability. A new
concept to blend PEG with low surface energy fluoropolymers to
maintain the nonfouling property of the coatings as well as
increase the coating stability has been exploited by the research
groups of Wooley,37-39 Ober,40-42and Galli.43 To adequately
blend PEG into a fluorinated matrix without serious phase
separation resulting, Wooley et al. demonstrated a stepwise
condensation polymerization to synthesize a hyperbranched
fluoropolymer-poly(ethylene glycol) network for fouling release
coatings.37-39 The evaluation of settlement and release assays of
sporelings of the green alga Ulva, indicate this hyperbranched
network exhibited good fouling-release performance. The Ober
and Galli groups separately reported side-chain block copolymer
consisting of grafted ethoxylated fluoroalkyl segments that can
release both sporelings of Ulva and diatoms.40-43
Previously we have reported the use of PFPE polymers as
a unique class of high performance coating materials that
have extremely low surface tensions (8-18 mN/m), a tunable
modulus, and excellent thermal and chemical stabilities.44,45
Building upon these materials we have also established the utility
*Corresponding author. E-mail: desimone@unc.edu.
pubs.acs.org/Macromolecules
Published on Web 01/25/2011
r 2011 American Chemical Society

Article
Macromolecules, Vol. 44, No. 4, 2011 879
removed under vacuum at room temperature for 12 h. The
successful functionalization of the TSPA on the glass slides was
confirmed by X-ray photoelectron spectroscopy.
Preparation of Cross-Linked PFPE/PEG Blends. PEG454-
MA and PEG550-DMA were first passed through an alumina
chromatographic column (1 cm ꢀ3 cm) to remove the inhibitor.
The PEG454-MA was then added to the desired amount of the
PFPE-DMA precursor in a range of composition weight ratios
of 5/95, 10/90, 15/85, 20/80, 25/75, 30/70, 50/50, and 70/30 to
form a transparent homogeneous mixture. To the blend was
then added 0.2 wt % of the photo initiator HCPK which was
dissolved by vortexing. Since PEG550-DMA is not well mis-
cible with PFPE-DMA, Solkane (1/10, v/w) was employed as
the cosolvent to enhance the miscibility of PEG550-DMA with
the PFPE-DMA. After vortexing for 5 min, colorless and clear,
homogeneous liquid blends of PFPE-DMA and PEG550-
DMA with composition weight ratios of 5/95, 10/90, 30/70, 50/50,
and 70/30 were obtained. In the same way, 0.2 wt % HCPK
photo initiator was dissolved in the mixture of PEG550-DMA
and PFPE-DMA. The cross-linked PFPE/PEG films for me-
chanical, UV-vis spectroscopy, and environmental scanning
electron microscope (ESEM) analysis were made of liquid
PFPE/PEG blends fully cured in a PDMS mold (1 mm ꢀ
35 mm ꢀ75 mm) by UV irradiation (Electronlite UV curing
chamber model no. 81432-ELC-500, λ = 365 nm, irradiated for
10 min under N2 purge). To covalently attach the PFPE/PEG
films onto the glass substrate, the PFPE/PEG blends were cast
onto TSPA-modified glasses (1 mm ꢀ35 mm ꢀ75 mm) and were
exposed to UV irradiation for 10 min under N2 purge. Finally,
the films composed of PFPE/PEG550-DMA were kept under
vacuum for 24 h to remove any residual Solkane solvent.
Ultraviolet-Visible Spectroscopy. UV-vis transparency me-
asurements were performed on a Shimadzu UV-3600 UV-vis
spectrophotometer over the wavelength range of 300-700 nm.
Thin films of ca. 1 mm were measured for optical transparency
with the percent transmittance at 550 nm.
Figure 1. Molecular structures of the building blocks for PFPE/PEG
blends.
of methacryloxy-functionalized PFPE materials copolymerized
with methacryloxy-fuctionalized PEG segments using photoirra-
diation to generate a new family of amphiphilic networks.46
Herein we report the use of this family of amphiphilic PFPE/
PEG blends as nontoxic UV-curable, fouling-release coatings
(Figure 1). Two kinds of PEGs were utilized for incorporation
into amphiphilic networks with PFPEs: flexible monofunctional
PEG and a restricted difunctional PEG. The antifouling/fouling
release performance of the coatings was assessed by laboratory
assays with barnacles and the green alga Ulva.
Experimental Section
Materials. 1,1,1,3,3-Pentafluorobutane (Solkane) was pur-
chased from Micro-Care. The 1.5 kg/mol PFPE-diol (Fluo-
rolink D10-H) was purchased from Solvay Solexis. Tetrabutyl-
tin diacetate (DBTDA), R-hydroxycyclohexyl phenylketone
(HCPK), 2-isocyanatoethyl methacrylate (IEM), poly(ethylene
glycol) monomethacrylate (PEG454-MA, 454 g/mol), poly-
(ethylene glycol) dimethacrylate (PEG550-DMA, 550 g/mol),
3-aminopropyl trimethoxysilane, acryloyl chloride, triethyla-
mine, and all solvents were purchased from Sigma-Aldrich.
Standard glass (1 mm ꢀ35 mm ꢀ75 mm) was purchased from
Fisher.
Synthesis of PFPE-DMA. Perfluoropolyether-dimetha-
cryloxy-functionalized PFPE (PFPE-DMA) was synthesized
according to our previous work.46,47 Briefly, PFPE diol (1.5 kg/
mol) was first dissolved in 1,1,1,3,3-pentafluorobutane and
reacted with a 2.05:1 molar ratio of IEM at 45 ꢀC for 24 h,
using 0.1 wt % DBTDA as a catalyst. The solution was then
passed through a chromatographic column filled with alumina.
After the solvent was evaporated, the product was filtered
through a 0.2 μm poly(ether sulfone) filter to yield a clear,
colorless, viscous oil.
Dynamic Mechanical Thermal Analysis. DMTA measure-
ments were performed using a 210 Seiko Dynamic mechanical
spectrometer, operating at a fixed frequency of 1 Hz in tension
mode. The temperature was varied from -150 to þ150 ꢀC with a
heating rate of 2 ꢀC/min.
Atomic Force Microscopy. The surface topography of the
coating films was imaged using an Asylum Research MFP3D
AFM in tapping mode with silicon cantilevers from Mikro-
masch USA with a resonance frequency of 160 kHz, spring
constant of 5.0 N/m, and radii of less than 10 nm.
Synthesis of N-(3-(trimethoxysily)propyl)acrylamide (TSPA).
The synthesis was conducted as previously reported in the
literature.12 Into 50 mL of tetrahydrofuran (THF) were dis-
solved 3-aminopropyl trimethoxysilane (2.74 g, 15.3 mmol) and
triethylamine (2.32 g, 22.9 mmol). The mixture was cooled to
0 ꢀC in an ice-water bath and then a solution of acryloyl chloride
(1.50 mL, 18.3 mmol) in 20 mL THF was added dropwise. After
stirring at 0 ꢀC for 3 h, the suspension was filtered and then the
solvent was removed under vacuum to yield a brown oil. The
product was then washed with cold water and extracted with
chloroform. The organic layer was dried over anhydrous
Na2SO4 followed by evaporation of the solvent to afford the
product. 1H NMR (CDCl3, 400 MHz, ppm): δ 6.26 (1H, d, J=
17.2 Hz, CH2), 6.06-6.13 (1H, m, CH), 5.61 (1H, d, J=10.0 Hz,
CH2), 3.55 (9H, s, OCH3), 3.31-3.36 (2H, m, CH2), 1.66-1.62
(2H, m, CH2), 0.68 (2H, t, J=8 Hz, CH2).
Mechanical Properties. Stress-strain measurements were
performed at ambient temperature on an Instron model 5566
system using a 10 kN load cell at a crosshead speed of
10 mm/min. An extensometer of 3 mm gauge length was used
to measure the strain accurately. Young’s modulus was deter-
mined from the stress-strain curves. Four replicates were
performed for each sample.
Water Swelling. The free-standing cured PFPE/PEG films
and those coated on glass were soaked in deionized water for
24 and 48 h. The weight percent swelling ratio was determined by
using the following equation: wt % swelling=100% * (Wwet - Wdry)/
Wdry. By accurately measuring the length (L), width (W), and
thickness (D) changes of each sample before and after water
immersion, the volume percent swelling ratio was confirmed by
the equation: Vol%swelling =100%(LwetWwetDwet - LdryWdry
-
Ddry)/LdryWdryDdry, where the subscripts dry and wet corre-
spond to the samples before and after water immersion, respec-
tively.
X-ray Photoelectron Spectroscopy. XPS analysis was per-
formed using a Kratos Axis Ultra equipped with a monochro-
mated Al Kr X-ray source. Photoelectrons at pass energies
ranging from 20 to 80 eV were collected with a concentric
hemispherical analyzer and detected with a delay line detector.
Preparation of Glass Substrates. Fisher brand standard glass
slides (1 mm ꢀ 35 mmꢀ75 mm) were boiled in piranha solution
(H2SO4:H2O2=7:3) at 90 ꢀC for 30 min, and then washed with
deionized water. After being fully dried in an oven at 110 ꢀC
overnight, the glass sides were soaked in a N-(3-(trimethoxy-
silyl)propyl)acrylamide, TSPA, ethanol solution (1/100, v/v) for
12 h. Nonreacted excess TSPA on the glass surface was washed
away with ethanol and then the residual solvent on the slides was

880 Macromolecules, Vol. 44, No. 4, 2011
Wang et al.
Static Contact Angle and Surface Energy Measurement. Static
contact angles were measured using a KSV Instruments LCD
CAM 200 optical contact angle meter at room temperature. All
measurements were carried out with drops that had a total
volume of 10 μL on the surface of each cured film coating on
glass using a 1000 μL screw-top syringe. The polar (γsp) and
dispersive (γsd) components of the surface free energy of the
PFPE/PEG coatings on the substrates were determined accord-
ing to the Owens-Wendt-Kaelble (OWK) method:48,49
1=2
ðγSd γLd Þ þ ðγpSγLp Þ
1=2
γL ¼ 2
1 þ cos θ
where θ is the static contact angle; γL is the surface tension of the
probe liquid; γdL and γLp are the dispersive component and the
polar component of the liquid surface tension, respectively. By
measuring the contact angles of two different probe liquids (one
polar and one nonpolar) on each surface, the overall solid
surface tension can be calculated based on the OWK equation.
Coating Conditioning Prior to Ulva Spore Settlement and
Growth. Coatings were leached in deionized water for 24 h prior
to testing. Following leaching, the coatings were immersed in
seawater for 1 h before the start of the experiment.40
Figure 2. (a) Illustration of the attachment of TSPA onto glass sub-
strate. (b) XPS analysis of unmodified and TSPA-modified glass
substrates demonstrating the successful functionalization of the surface.
Settlement of Spores. Zoospores were obtained from mature
Ulva plants as described previously.50 In brief, zoospores were
allowed to settle (attach) to test surfaces placed in individual
compartments of quadriperm dishes (Greiner) to which 10 mL
of zoospore suspension (1.0 ꢀ 106 mL-1) were added. After
incubation in the dark at ∼20 ꢀC for 1 h, the slides were gently
washed in seawater to remove unattached zoospores.
Attachment Strength of Sporelings. Samples with attached
spores were grown in enriched seawater medium for 7 days to the
stage of sporelings (young plants).51 Sporeling biomass was
determined in situ by measuring the fluorescence of the chlor-
ophyll contained within the cells with a Tecan fluorescence plate
reader. The biomass was quantified in terms of relative fluores-
cence units (RFU). The RFU value for each slide is the mean of
70 point fluorescence readings.51
Figure 3. Cured 95:5 PFPE/PEG454-MA coatings on (a) TSPA-
modified glass and (b) bare glass.
Scheme 1. Schematic Illustration for the Preparation of Cured PFPE/
PEG Coatings on Substrates
The strength of attachment of sporelings was assessed using a
water jet apparatus with individual slides of each treatment
being exposed to increasing water pressures.51 Biomass remain-
ing within the area exposed to the water jet was assessed using
the fluorescence plate reader. The percentage removed was
calculated from readings taken before and after water jetting.
Percentage removal was plotted versus impact pressure and
from these curves, the critical impact pressure to remove 50%
of the biomass was determined.
Settlement and Removal of Barnacles. To determine the
toxicity of the coatings to brine shrimp, all of the coatings were
soaked for 6 days in 100 mL of seawater and at 72 h intervals the
leachate was removed and replaced with 100 mL of fresh sea-
water. Samples of the leachate from the coatings were used to
conduct assays of survivorship with approximately 100 nauplii
larvae of Artemia sp. (brine shrimp). The larvae were exposed to
the coating leachate and their survival was monitored after 2
days. The survival of the larvae in each coating leachate was
compared to leachate from a glass slide control.
Settlement Assay of Barnacle Cypris Larvae. Balanus amphitrite
cypris larvae were obtained from the Duke University Marine
Laboratory. A 400 μL drop of seawater containing 20 to 40, 2-4-
day old barnacle cypris larvae was placed on the surface of each
coating replicate in a covered Petri dish. The larvae were then
placed in an incubator at 25 ꢀC with a 12 h light/dark cycle and
allowed to settle for 48 h. At the end of the initial assay period the
numbers of individuals that successfully attached and metamor-
phosed were counted. Larvae that did not settle by the end of the
24 h period were observed for signs of abnormal behavior to assess
any compromises to normal physiological function. Settlement on
each coating formulation was compared to settlement on the glass
and T2 controls.52,53
Removal Assay of Barnacles. Post settlement, newly meta-
morphosed juveniles were transferred to growth chambers and
kept in an incubator at 25 ꢀC with a 12 h light/dark cycle where
they were fed a mixture of the unicellular alga Dunaliella
tertiolecta and the diatom Skeletonema costatum for 2 weeks,
and then a mixture of D. tertiolecta, S. costatum, and naupliar
larvae of Artemia sp. for an additional week. Barnacles were
then transferred to a 16-L aquaria tank in an automated rack
system with temperature, salinity and pH monitors and a
programmed 10% daily water change. Barnacles in the tank
were fed a 500-mL flask of Artemia sp. three times per week for
another 4-6 weeks, which is the time it took the juvenile
barnacles to reach a basal plate diameter of 3-5 mm, the
minimum size necessary to conduct force gauge tests. Prior to

Article
Macromolecules, Vol. 44, No. 4, 2011 881
Table 1. Water-Uptake and Percent Swelling of PFPE/PEG454-MA Cross-Linked Networks of Different Weight Ratiosa
blends (weight ratio)
PFPE/
PEG454-
MA (95:5)
PFPE/
PEG454-
MA (90:10)
PFPE/
PEG454-
MA (85:15)
PFPE/
PEG454-
MA (80:20)
PFPE/
PEG454-
MA (70:30)
PFPE/
PEG454-
MA (50:50)
PFPE/
PEG454-
MA (30:70)
PFPE
in H2O 24 h
in H2O 48 h
weight increase
(%)
volume swelling
(%)
weight increase
(%)
volume swelling
(%)
0.09
0.22
3.16
0.28
3.91
0.56
4.07
0.72
5.77
2.34
22.15
3.32
8.02
29.10
9.36
21.79
46.64
21.82
44.20
67.34
86.12
66.41
86.34
151.82
215.93
150.11
211.48
1.68
0.12
2.62
18.47
31.97
a Cross-linked networks indicated in italics had the lowest percent swelling and water-uptake values and therefore were the focus of further study.
Table 2. Water-Uptake and Percent Swelling in Water of PFPE/PEG550-DMA Cross-Linked Networks with Different
Weight Ratiosa
blends (weight ratio)
PFPE/PEG550
DMA (95:5)
-
PFPE/PEG550
DMA (90:10)
-
PFPE/PEG550
DMA (70:30)
-
PFPE/PEG550
DMA (50:50)
-
PFPE/PEG550-
DMA (30:70)
in H2O 24 h
in H2O 48 h
weight change (%)
volume change (%)
weight change (%)
volume change (%)
0.28
0.36
0.26
0.33
0.45
1.08
0.49
2.31
2.13
2.45
2.44
2.31
4.69
10.89
3.74
8.49
19.24
6.92
10.86
18.04
a Cross-linked networks indicated in italics had the lowest percent swelling and water-uptake values and therefore were the focus of further study.
measured by the area of the basal plate. The photographs of
the basal plates were used to determine their area using NIH’s
ImageJ. The average CRS for each coating was compared
against the control groups. If the barnacle broke and its basal
plate was partially removed, the remaining basal plate was
photographed again and the exact percentage remaining after
removal testing was calculated.
Results and Discussion
Using UV-initiated free radical polymerization techniques,
dimethacryloxy-functionalized PFPE-DMA materials were
cross-linked into networks with monomethacryloxy-functionalized
PEG454-MA or dimethacryloxy-functionalized PEG550-DMA
in the presence of a photoinitiator. Interestingly, it was found that
PFPE-DMA is miscible with PEG454-MA forming completely
clear miscible liquid solutions. However, PEG550-DMA did not
form a miscible solution with PFPE-DMA. The resulting milky
mixture was observed to quickly macro-phase separate after
vortexing. However it was found that by incorporating 10%
(v/w) of the cosolvent Solkane both the PFPE-DMA and
PEG550-DMA formed a miscible solution. Upon addition of
0.2% HCPK photoinitiator and exposure to UV-irradiation for
10 min under N2 purge these solutions were observed to cure as
fully cross-linked elastomers. Typically, by casting the photocur-
able blends in a 1 mm ꢀ35 mm ꢀ75 mm PDMS mold, a series of
size specific PFPE/PEG films were obtained.
Figure 4. Stability of the cured PFPE/PEG coatings on TSPA treated
glass substrates against water immersion.
The PFPE/PEG blends were cured on glass substrates.
However delamination from the substrate was found to be a
frequent problem for these low surface energy PFPE blends.
Therefore, in order to minimize delamination between the
cured film and the glass slide, a layer of N-(3-(trimethoxy-
removal, a digital photograph was taken of each barnacle basal
plate using a Canon EOS 10D camera attached to an Olympus
SZX12 dissecting microscope. Each slide was then clamped into
silyl)propyl)acrylamide, TSPA, was deposited on the substrate
the force-gauge apparatus, which consisted of an IMADA ZP-
surface via the hydrolysis of the trimethoxysilyl groups by
11 digital force gauge mounted on an IMADA SV-5 motorized
the hydroxyl groups on the glass surface. The methacrylamide
test stand and a custom chamber that houses the slide immersed
moiety was then able to be covalently incorporated with PFPE/
in seawater; thus allowing the testing of the force necessary to
PEG blends when exposed to UV-irradiation minimizing the
delamination issues. Successful modification of the glass sub-
strate was verified by XPS analysis as indicated by the
appearance of the N1s peak at 398 eV (Figure 2). Casting and
photocuring a film of a PFPE/PEG blend onto the TSPA
remove the barnacles in shear and in situ. The barnacles were
removed with the force gauge with the force applied in shear at a
rate of 4.5 N s-1and the maximum force was recorded. If
barnacles removed wholly from the coating, the critical removal
stress, CRS, (N/mm2) was calculated by dividing the force

882 Macromolecules, Vol. 44, No. 4, 2011
Wang et al.
Figure 5. Surface topographies of the cured PFPE/PEG coatings on the substrates by AFM observation (a-e). Height images are 5 ꢀ 5 μm .
Table 3. Summarized Characterizations of PFPE/PEG Cross-Linked Networks Including Static Contact Angle by Using Water and Hexadecane,
Surface Energy, Young’s Modulus, Surface Roughness, and Transparency
coatings
PFPE/
PEG454-
MA (95:5)
PFPE/
PEG454-
MA (90:10)
PFPE/
PEG550-
DMA (95:5)
PFPE/
PEG550-
DMA (90:10)
PFPE/
PEG550-
DMA (70:30)
PFPE
contact angle (water)
contact angle (hexadecane)
surface energy (mN/m)
modulus (MPa)
surface roughness (nm)
transparency (%)
109.5 ( 2.0
67.7 ( 0.7
14.2
13.20 ( 0.66
0.89
97.2
111.3 ( 0.6
67.9 ( 1.4
13.9
8.89 ( 0.50
1.25
96.1
112.3 ( 2.4
69.3 ( 0.6
13.3
6.13 ( 0.57
1.36
94.9
106.3 ( 2.1
69.5 ( 1.7
14.4
12.57 ( 1.71
1.19
95.9
108.4 ( 2.3
68.7 ( 1.5
14.1
11.86 ( 1.39
1.35
94.7
109.6 ( 1.1
68.9 ( 1.4
13.9
8.47 ( 0.53
2.27
91.4
When 10 wt % or less of the PEG454-MA was incorporated into
the blends, the films displayed high water-resistance with a low
degree of swelling. In similar fashion, the PFPE/PEG550-DMA
blends were seen to remain relatively unchanged when the
PEG550-DMA content was not more than 30 wt % in the
blends, as shown in Table 2. Cured films with 30 wt % or less
PEG content were found to maintain integrity on the substrates
after 24 h water immersion, as shown in Figure 4. However, for
example, the coating with 50 wt % PEG550-DMA content was
observed to fracture and peel off the substrate due to a large,
swelling-induced stress. Typically, the stable PFPE/PEG coating
films, including PFPE/PEG454-MA (95:5, 90:10) and PFPE/
PEG550-DMA (95:5, 90:10, 70:30), were selected as candidates
for the subsequent fouling-release studies.
The surface topographies of the cured PFPE/PEG blends were
investigated by AFM. As shown in Figure 5a-e, AFM results
indicate that the gradual addition of PEG has a slight affect
on the cured surface structure, suggesting phase separation
existing between the PFPE and PEG. This increase in surface
roughness and the existence of microphase separation is particu-
larly clear in the 70:30 PFPE/PEG550-DMA sample, as seen in
Figure 5e. AFM measurement of the surface roughness of the
various cured PFPE/PEG blends compared to a cured neat
PFPE-DMA sample reveal an increase in roughness with
increasing PEG content (Table 3). The neat PFPE sample has a
surface roughness of less than 1 nm while the 70:30 PFPE/
PEG550-DMA sample has a roughness of 2.27 nm. Interest-
ingly, the roughness appears to be irrespective of the PEG
functionality as the 95:5 PFPE/PEG-MA and PFPE/PEG-
DMA samples have roughly the same roughness (1.25 and 1.19
nm, respectively) as do the 90:10 PFPE/PEG-MA and PFPE/
PEG-DMA samples (1.36 and 1.35 nm, respectively). The AFM
observations are in agreement with the optical transparency
results. As shown in Table 3, the optical transparency of the
resulting films was quantitatively elucidated by measuring the
transmittance at 550 nm. The cured films of the PFPE/PEG
blends showed high transmittance, but a slight decrease is
observed upon the increased addition of PEG, indicative of
immiscibility between the PFPE and PEG components in these
blends resulting in microscale phase separation.46
Figure 6. Critical water surface pressures for 50% removal of sporel-
ings from PFPE/PEG coatings. The points are derived from detach-
ment curves and read off the lines as 50% values, consequently an error
bar cannot be put on the data. The data show the relative ranking of
each sample in terms of fouling-release.
modified substrate resulted in a robust coating that was firmly
attached to the substrate (Scheme 1). After curing, the residual
Solkane in the PFPE/PEG-DMA coatings was removed under
vacuum at room temperature overnight. Taking the 95:5
PFPE/PEG454-MA as an example, the cross section of Figure
3a shows that the film has been tightly bound to the TSPA-
modified glass substrate. Conversely, as shown in Figure 3b, clear
delamination was observed when the film was coated on bare
glass, demonstrating the benefit of using TSPA-functionalized
glass to enhance the attachment of the PFPE/PEG blend films to
the substrate.
Low water-uptake and high stability of coatings on substrates
are needed for marine applications. Since PEG is hydrophilic,
as the PEG content in the PFPE/PEG coatings is increased the
coatings will increasingly swell and offer poor water resis-
tance when the PEG content is above some critical amount. To
address this issue, the water uptake and the swelling degree of
each cured PFPE/PEG coating were determined. As shown in
Table 1, the water uptake and volume swelling degree of PFPE/
PEG454-MA gradually increase as more PEG454-MA was
incorporated into the blends. When the PEG454-MA content
reached 15 wt %, the films were found to swell greatly with an
increase of 22.15% in volume after 24 h of immersion in water.
The product of the surface energy of a given material and its
Young’s modulus has been reported to be a good predictor of the

Article
Macromolecules, Vol. 44, No. 4, 2011 883
Figure 7. Percentage of basal plate remaining as a function of the basal plate area prior to removal. Coatings that exhibited 100% remaining basal plate
lie on the same line as the glass control. Silastic T2 is shown as a representative foul-release surface; data for T2 were collected during a different
experiment.
through the selection of either mono- or difunctional PEG and
varying its weight ratio in the blend with PFPE-DMA. Since the
modulus and surface energy values for 95:5 PFPE/PEG454-MA
and 70:30 PFPE/PEG550-DMA are similar, the product values
are also approximately same and therefore these materials are
expected to exhibit similar fouling release performance.
The fouling release performance of the PFPE/PEG coatings
was evaluated by studying the release of sporelings (young plants)
the green fouling seaweed Ulva. After 7 days of growth, a green
lawn of sporelings covered all of the test surfaces, the amounts of
biomass on all the PFPE/PEG coatings being similar (data not
shown). Since the sporelings grew normally on all of the PFPE/
PEG coatings with no signs of toxicity, these coating films
could be considered as environmentally benign coatings. To
determine the strength of attachment of sporelings to each type
Figure 8. ATR-FTIR spectra: (a) PFPE coating; (b) 90:10 PFPE/
PEG454-MA coating; (c) 90:10 PFPE/PEG550-DMA coating.
coating fouling release performance against certain organ-
isms.13,14 The surface energy of each cured PFPE/PEG blend
was evaluated using static contact angles by the Owens-
Wendt-Kaelble (OWK) method and Young’s modulus was
calculated from the slope of the stress-strain curve obtained
using an Instron. In the OWK method, by measuring two static
contact angles of two liquids, including one polar solvent (e.g
water) and one apolar solvent (e.g., hexadecane) whose γd and γp
values are known, the surface free energy can be calculated. Since
the relative content of PEG was kept low in the blends, all the
cured films exhibited low surface energies ranging from 13.3 to
14.4 mN/m as a result of the high content of low surface energy
PFPE in the matrix. The modulus of the films was observed to
significantly decrease as the PEG454-MA content was increased
because the degree of cross-linking was reduced by incorporation
of the monofunctional PEG454-MA. As shown in Table 3,
compared to the Young’s modulus of pure PFPE-DMA of 13.20
MPa, the addition of 10 wt % PEG454-MA to the blend was
seen to decrease the Young’s modulus to 6.13 MPa. The incor-
poration of PEG550-DMA into the PFPE/PEG blends was also
seen to decrease the Young’s modulus of the PFPE-DMA, but
not as dramatically as seen when incorporating PEG-MA due to
the difunctionality of the PEG-DMA macromonomer.46 For
instance, when 10 wt % of PEG550-DMA was incorporated
into the network the blend had a Young’s modulus of 11.86 MPa,
in comparison the 90:10 PFPE/PEG454-MA blend has a
modulus of 6.13 MPa. The product of surface energy and
Young’s modulus of the cured PFPE/PEG blends can be tuned
of PFPE/PEG coating, ease of removal was assessed using a
water jet apparatus with individual slides of each treatment being
exposed to increasing water jet pressures. The critical water
pressures (Figure 6) to remove 50% of the biofilms were
calculated from plots of percentage removal versus water impact
pressure. In the PFPE/PEG454-MA blends the strength of
attachment of the sporelings decreased as the PEG454-MA
content increased from 5 to 10%, i.e., a lower critical impact
pressure was needed to removed 50% of the biomass. Similarly,
the strength of attachment of the sporelings to the PFPE/
PEG550-DMA blends also decreased as the PEG content of
the coating increased from 5-30 wt %. At the highest loading of
30 wt % PEG550-DMA, the sporelings had a similar attach-
ment strength to that the pure PFPE and the 95:5 PFPE/
PEG454-MA coating. The best fouling-release performance
was observed for the 90:10 PFPE/PEG454-MA coating.
Additionally, the PFPE/PEG454-MA coatings were also
found to display better barnacle removal performance than the
PFPE/PEG550-DMA coatings. In fact, of the experimental
coatings tested, PFPE/PEG454-MA (90:10) showed the best fouling-
release performance, although not as good as PDMSe standards.
It should be noted that none of the leachate samples from the
coatings were found to be toxic to brine shrimp, supporting the
possibility that PFPE/PEG blends can be employed as environ-
mentally benign materials for fouling release. Upon removal,
most of the barnacles that were removed from the test coatings
broke leaving part or all of the basal plate attached to the coat-
ing surface. When breakage occurs, the force measured is the

884 Macromolecules, Vol. 44, No. 4, 2011
Wang et al.
Figure 9. DMTA spectra: (a) cured PFPE/PEG454-MA blends of different weight ratios; (b) cured PFPE/PEG550-DMA blends of different weight
ratios.
cohesive force of the barnacle to itself and not the force of
adhesion of the barnacle to the surface; therefore critical removal
stress (CRS) can not be calculated. Berglin et al. suggest that the
remaining fraction of the basal plate left on a surface appears
to be a function of barnacle bioadhesive bond strength and
that it could be used as a measure of the efficacy of foul-
release coatings.52 As shown in Figure 7, coatings of PFPE/
PEG454-MA perform relatively well compared to our other
experimental coatings. For example, PFPE/PEG454-MA have
a lower average percentage of basal plate remaining than do
PFPE and PFPE/PEG550-DMA, which often left behind the
entire basal plate on attempted removal; a result similar to glass
controls, to which barnacles readily attached with great tenacity.
In addition to having a lower basal plate percentage remaining
compared to our other coatings, PFPE/PEG454-MA have a
steeper negative slope, which further supports better fouling
release performance, because the shift in mode of failure upon
removal occurs at smaller sizes. For comparison, the fouling-
release control surface, Silastic T2, had a very low percentage of
basal plate remaining and the slope of line is much steeper than
our experimental coatings (Figure 7). The average CRS for
animals on Silastic T2 is 0.149 N/mm2.
The product of surface energy and Young’s modulus has been
found to not be an accurate predictor for the fouling release
performance of these amphiphilic PFPE/PEG coatings, since the
product of surface energy and Young’s modulus of the PFPE/
PEG454-MA (95:5) and PFPE/PEG550-DMA (70:30) materi-
als are approximately same but the 95:5 PFPE/PEG454-MA
coating had much better barnacle release performance. It is
anticipated that the free PEG chains are flexible in the PFPE/
PEG454-MA blends which can migrate out to the surface in a
hydrophilic environment and therefore provide enhanced bio-
fouling performance. But the more densely cross-linked PEG
chains in the PFPE/PEG550-DMA blends are restricted due to
the difunctionality of the oligomers and therefore are not able to
migrate to the surface as easily. In light of this hypothesis, it can
be understood why the PFPE/PEG454-MA blends perform
much better in comparison to the PFPE/PEG550-DMA coat-
ings as the flexible PEG454-MA is more likely to migrate to the
surface to weaken fouling attachment. In addition to surface
chemistry segregation, the product of the surface energy and
Young’s modulus also does not account for differences in surface
roughness. This is believed to be the reason why the 70:30 PFPE/
PEG550-DMA sample, though possessing the highest PEG
content also has the roughest surface of all the materials tested,
and hence does not perform any better in biofouling testing than
the smooth PFPE-DMA sample.
90:10 PFPE/PEG454-MA is much stronger than that of 90:10
PFPE/PEG550-DMA blend. The higher intensity of PFPE/
PEG454-MA of Figure 8b indicates the existence of flexible
PEG chains on this coating surface. On the other hand, the more
restricted PEG chains of the PFPE/PEG550-DMA coating has
a weaker IR intensity at 2980 cm-1 indicating that there is less
PEG on the surface, as shown in Figure 8c.
The feature of flexible PEG segment in the PFPE/
PEG454-MA blend was additionally interrogated by DMTA
of the bulk materials. Two transitions were found for the cured
PFPE-DMA, the glass transition (Tg) at 37.2 ꢀC was assigned to
the cross-linked methacrylate end group domains and a second-
ary relaxation peak at -95.4 ꢀC was assigned to the PFPE
fluorocarbon domains of the main chains located away from
cross-links. As shown in Figure 9a, for the blends of PFPE-
DMA with PEG454-MA, the methacrylate groups of the PFPE
segments are miscible with the methacrylate groups of PEG
segments and formed a polymethacrylate domain after cross-
linking, corresponding to the broad Tg from 0 to 35 ꢀC, which
gradually shifts to a lower temperature with increasing PEG in
the composition ratio. The decrease in Tg upon addition of
PEG454-MA indicates that the side chain of PEG moves freely
which should be induced by the flexible PEG grafting branches.
However, for the blends of PFPE with PEG550-DMA
(Figure 9b), in which the side PEG chains of polymethacrylate
are well cross-linked in the networks which actually did not enhance
the freedom of main chain of polymethacrylate, the Tg nearly
maintained unchanged upon gradual addition of PEG550-DMA.
Conclusion
Herein we have presented a facile approach for the preparation
of amphiphilic hydrophilic/fluorophilic cross-linked networks
via photocuring for fouling-release coating applications. A range
of networks composed of elastomeric perfluoropolyether (PFPE)
and poly(ethylene glycol), in which the PFPE was cross-linked
with PEG through the copolymerization of methacryloxy-func-
tionalized end groups. These coatings maintained a low surface
energy and displayed low swelling characteristics as a function of
water when less than 10% PEG454-MA or 30% PEG550-
DMA is blended, promising high stability for potential applica-
tion. In particular, the PFPE/PEG454-MA (10%) coating
displayed better fouling-release performance when evaluated
against Ulva and removal of juvenile barnacles in comparison
to all the PFPE/PEG550-DMA coatings, which is attributed to
the flexibility of PEG454-MA chains allowing the PEG to
migrate to the surface of the PFPE/PEG454-MA blend. With-
out complicated synthesis, the functionalized PFPE blends with
monomethacryloxy-functionalized PEG were found to be well
suited as photocurable coatings on the substrates. Since the
PFPE/PEG coatings are nontoxic and the liquid precursors are
easily applied it is hoped that this new family of photocurable
The presence of PEG on the PFPE/PEG454-MA surface was
confirmed by ATR-FTIR as shown in Figure 8. The absorbance
at 2980 cm-1 is ascribed to the stretching vibration of -CH2- of
the PEG segments. Typically, the IR intensity at 2980 cm-1 of

Article
Macromolecules, Vol. 44, No. 4, 2011 885
amphiphilic blends can be applied as minimally adhesive, mech-
anically durable, nonfouling/fouling-release coatings in marine
applications. Although the fouling-release attributes of these
coatings are not as good as the PDMSe standard, this research
is providing an understanding on how to tune fouling-release
properties based on PEG flexibility. Highly efficient fouling-
release coatings are being pursued by tuning bulk stiffness, PEG
length, surface segregation, and surface roughness.
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Acknowledgment. This work was supported by the Office of
Naval Research under Grant No. N00014-02-1-0185 as well as
the STC program of the National Science Foundation for shared
facilities. J.A.C. and M.E.C. also acknowledge support from
ONR under Grant No. N00014-08-1-0010. Dr. Zhaokang Hu,
Dr. Meredith Hampton, and Dr. Stuart Williams of UNC-CH
are acknowledged for their helpful discussion on PFPE materials.
Dr. Carrie Donley of UNC-CH performed the XPS experiments
at the Chapel Hill Analytical & Nanofabrication Laboratory
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