Permanently grafted icephobic nanocomposites with high abrasion resistance

Article abstract of DOI:10.1039/c6ta03222b

In this work, a series of copolymer/silica nanocomposites are investigated that exhibit excellent anti-icing behavior and can be covalently grafted to any substrate containing C-H bonds with high durability. The copolymers of interest consist of pendant benzophenone, hexafluorobutyl, and a variety of other comonomers that, under mild UV irradiation, can be covalently grafted on a variety of substrates and generate a densely cross-linked network of polymer and well-dispersed nanoparticles. The robustness of thin films was compared in a series of terpolymers with different acrylic comonomer content. Thin films prepared with tert-butyl ester side groups had less backbone chain scission and, therefore, a greater extent of cross-linking than films prepared with n-butyl ester side groups. The iso-butyl acrylate comonomer promotes photoreaction efficiency in terms of kinetic rate and network robustness, leading to films that can sustain high shear forces and abrasion. The anti-icing capability of the composite was investigated using the impact of supercooled water on different substrates. The composite maintains its icephobicity after modified Taber testing with multiple abrasion cycles using a 300 g load, which demonstrates excellent mechanical resistance. In addition, this study has led to rational design rules for copolymers that maximize permanent attachment of different surface functionalities in terms of both grafting density and reaction kinetics.

Full text of DOI:10.1039/c6ta03222b

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Permanently Grafted Icephobic Nanocomposites with High
Abrasion Resistance
Received 00th January 20xx,
Jing Gao, Andrew Martin, Jeremy Yatvin, Evan White and Jason Locklin*
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
In this work, a series of copolymer/silica nanocomposites are investigated that exhibit excellent anti-icing behavior and can
be covalently grafted to any substrate containing C-H bonds with high durability. The copolymers of interest consist of
pendant benzophenone, hexafluorobutyl, and a variety of other comonomers that, under mild UV irradiation, can be
covalently grafted on a variety of substrates and generate a densely cross-linked network of polymer and well-dispersed
nanoparticles. The robustness of thin films was compared in a series of terpolymers with different acrylic comonomer
content. Thin films prepared with tert-butyl ester side groups had less backbone chain scission and, therefore, a greater
extent of cross-linking than films prepared with n-butyl ester side groups. The iso-butyl acrylate comonomer promotes
photoreaction efficiency in terms of kinetic rate and network robustness, leading to films that can sustain high shear forces
and abrasion. The anti-icing capability of the composite was investigated using the impact of supercooled water on
different substrates. The composite maintains its icephobicity after modified Taber testing with multiple abrasion cycles
using a 300 g load, which demonstrates excellent mechanical resistance. In addition, this study has led to rational design
rules for copolymers that maximize permanent attachment of different surface functionalities in terms of both grafting
density and reaction kinetics.
silicon15 or metal13, 16, 19, 20 with ordered nano-scale features by
1. Introduction
etching or photolithography,7 followed by
a top-coat of a
fluorinated self assembled monolayer (SAM) or polymer; or (2)
depositing inorganic nanoparticles (Ag, ZnO, TiO2 , etc.) directly
onto surfaces,2, 10-12 which is likely a more feasible and scalable
process for large scale application. The drawback, however, is that
superhydrophobic surfaces are very fragile due to the
susceptibility of the micro- or nanoscale features to mechanical
abrasion or environmental conditions such as wind and rain.
Superhydrophobic coatings that are attached to the surface via
non-covalent interactions, such as van der Waals forces,
electrostatic interactions, or hydrophobic interactions, are limited
in widespread applications in harsh environments, where
delamination, desorption, and/or dewetting may occur to the
coating.21, 22
Ice accumulation on surfaces is problematic in daily life as it can
cause breakage of power transmission lines, freezing on roads,
and, as any homeowner can attest, buildup of ice inside domestic
freezers.1-3 It also raises potential issues in industrial applications
such as disturbing the aerodynamics of aircraft wings and wind
turbines, resulting in increased maintenance and repair costs
5
every year.4, The mechanism of ice formation under subzero
conditions and the design of ice-repellent coatings have been
areas of intense study for decades.6-10 Such coatings are expected
to either minimize ice adhesion11-14 or delay ice formation on
8, 15, 16
surfaces.2,
Superhydrophobic surfaces, which are
characterized by a high water contact angle (CA) and low water
contact hysteresis (CAH), have been observed to exhibit anti-icing
characteristics.11-13, 17, 18 Due to the small contact area between
the freezing droplet and the rough surface, an air layer is formed
at the interface, which reduces the ice-solid interaction. Two
approaches have been utilized to fabricate anti-icing surfaces with
micro/nanoscale roughness: (1) patterning substrates such as
Benzophenone (BP) photochemistry has been thoroughly
studied and widely used as a photoactive tethering reagent to
functionalize a wide variety of materials.23-27 The extensive use of
BP can be attributed to the following advantages: (1) The BP
moiety attaches to C-H bonds in a wide range of chemical
environments, including commercial plastics and fabrics;28-32 (2)
BP can be manipulated in ambient atmosphere and activated with
mild UV light (345-365 nm), which avoids oxidative damage to
many materials;24 and (3) BP is both thermally and chemically
more stable and synthetically more versatile than most other
tethering functionality, such as sulfonyl azides,33 diazo esters,34
Department of Chemistry, College of Engineering, and the Center for Nanoscale
Science and Engineering, University of Georgia, Athens, Georgia, 30602, United States
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37
aryl azides,35 and diazirines.36, Under irradiation, BP absorbs
photons, resulting in the promotion of an election from n-orbital
of the carbonyl oxygen to the π*-orbital and the formation of
biradicaloid triplet state. The electron-deficient oxygen abstracts a
hydrogen atom from a C-H group in close proximity, followed by
the combination of two carbon radicals and the generation of a
new C-C bond. This photochemistry has been used to permanently
attach polymer films to a broad selection of C-H containing
surfaces in many different applications, such as microfluidic
measured using a DSA 100 drop shape analysis system (KRŰSS)
with a computer-controlled liquid dispenser. Water droplets with
volume of 1 µL were used to measure the static contact angle. The
advancing and receding contact angles were measured by
expanding and retracting the water droplet on the substrate. The
glass transition temperature (Tg) of copolymer was measured by
using
a
differential scanning calorimeter DSC 823e (Mettler
Toledo). Data was stored and manipulated using the software
STARe DB V9.20 (Mettler Toledo). Samples were scanned from -60
°C to 150 °C at a rate of 10 °C/min. Four heating and cooling cycles
were conducted to minimize sample history. Nanoparticle sizing
was determined using a Zetasizer Nano Series (Malvern) with
dynamic light scattering (DLS).
devices,38, biosensing,40, antimicrobial coatings,30 patterned
sheets,42, 43 and organic semiconductors.44
39
41
In this article, we describe a robust anti-icing composite that
consists of well-dispersed silica nanoparticles within a cross-
linked, fluorinated polymer network. The surface attachment of
the composite was readily achieved by UV-initiated cross-linking
between BP moieties and surface-bound C-H groups. The
composite demonstrated impressive anti-icing capabilities due to
its fluorinated hierarchical micro/nano-scale surface structure.
The composite also resisted relatively harsh mechanical stress,
which is attributed to the dense covalent network between the
copolymer and nanoparticles, as well as the substrate surface. To
the best of our knowledge, this is the first demonstration of the
covalent immobilization of hierarchical anti-icing coatings onto
surface that can withstand high abrasive forces.
2.3 Synthesis
Methyltriphenylphosphonium Iodide
Triphenyl phosphine (12 g, 45.75 mmol), iodomethane (8.44 g,
59.8 mmol) and toluene (100 mL) were stirred at room
temperature for 16 h under a nitrogen atmosphere. The reaction
mixture was filtered and a pure white solid was collected. The
solid was washed with diethyl ether and air-dried. Yield: 17.17 g
(93%). 1H NMR (500 MHz, CDCl3, δ): 7.84-7.67 (m, 15H, Ar H), 3.25
(d, J = 13.3 Hz, 3H; CH3).
4-Bromostyrene
Methyltriphenylphosphonium iodide (10.66 g, 26.4 mmol) was
suspended in tetrahydrofuran (15 mL) and cooled to 0 °C in a
three-neck round-bottom flask. Potassium tert-butoxide (3.23 g,
28.8 mmol) was dissolved in THF (10 mL) and added drop-wise to
the round-bottom flask. The reaction mixture was stirred at 0 °C
for 20 min and at room temperature for 10 min. 4-
Bromobenzyldeheyde (4.44 g, 24.0 mmol) was dissolved in THF
(10 mL) and added to the flask. The reaction was stirred at room
temperature overnight. The solvent was removed under reduced
pressure. The solid mixture was taken up in diethyl ether, and
washed with water and brine, then dried over magnesium sulfate.
The solvent was removed with a rotary evaporator. The crude
product was purified on a silica gel column using a hexane:ethyl
acetate (4:1) eluent. Yield: 3.97 g (90%). 1H NMR (500 MHz, CDCl3,
δ): 7.44 (d, J = 8.8 Hz, 2H, Ar H), 7.27 (d, J = 8.2 Hz, 2H, Ar H), 6.65
(dd, J = 11.1 Hz, 1H; CH), 5.74 (d, J = 17.6 Hz, 1H; CH2) 5.28 (d, J =
11.1 Hz, 1H; CH2). 1C NMR (300 MHz, CDCl3, δ): 136.61, 135.94,
132.27,128.21, 125.98, 116.54.
2. Experimental Section
2.1 Materials
Triphenylphosphine, iodomethane, potassium tert-butoxide, 2,2’-
azobis(2-methylpropionitrile) (AIBN), 2,2,3,4,4,4-hexafluorobutyl
methacrylate, and n-butyl acrylate (nBA) were obtained from Alfa
Aesar.
4-Bromobenzaldehyde,
were
benzonitrile,
purchased from
and
TCI.
isobutyltrichlorosilane
Tetraethoxysilane was obtained from UTC and magnesium
turnings were obtained from Eastman Organic.
Cetyltrimethylammonium bromide was purchased form Acros
Organic and isobutyl acrylate (iBA) from Sigma-Aldrich. Inhibitor
was removed from the corresponding monomers by passing
through a neutral aluminum column. Silicon wafers with native
oxide were used as substrates. Unless otherwise noted, all
compounds were used as received.
2.2 Instrumental Methods
UV-vis spectroscopy was performed on
a
Cary Bio
4-Vinylbenzophenone (4VBP)
spectrophotometer (Varian). The UV light sources were a Compact
UV lamp (UVP) and FB-UVXL-1000 UV Crosslinker (Fisher Scientific)
with bulbs of wavelength at 254 nm for small (1 × 2 cm) and large
(3.6 × 4.2 cm) substrates, respectively. The substrates were held a
certain distance from the light source during irradiation to obtain
power of 7.5 mW cm-2. Surface topography was analyzed with a
FEI Inspec F FEG scanning electron microscopy at 20 keV. Infrared
spectroscopy studies of polymer coated films were carried out
using a Thermo-Nicolet model 6700 spectrometer equipped with a
variable angle grazing angle attenuated total reflection (GATR-
ATR) accessory (Harrick Scientific). Water contact angles were
Magnesium turnings (0.65 g, 26.6 mmol) and a flake of iodine
were dissolved in dry THF (5 mL) in a three-neck round-bottom
flask under nitrogen atmosphere at 0 °C. 4-Bromostyrene (4.48 g,
24.5 mmol) was dissolved in anhydrous THF (5 mL) and added to
the flask drop-wise from an additional funnel. The reaction
mixture was stirred overnight at room temperature. A catalytic
amount of copper (I) bromide and benzonitrile (2.27 g, 22.0 mmol)
were added by syringe, and the reaction was stirred for 16 h.
Sulfuric acid (15 mL, 15% V/V) was added to quench the reaction,
followed by extraction with diethyl ether (50 mL). The organic
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layer was washed with water and brine, dried over magnesium
sulfate and concentrated with a rotary evaporator. The crude
product was purified by column chromatography (hexane:ethyl
acetate, 4:1). Yield: 2.23 g (49%). 1H NMR (500 MHz, CDCl3, δ):
7.81 (d, J = 7.6 Hz, 2H, Ar H), 7.80 (d, J = 8.4 Hz, 2H, Ar H), 7.59 (t, J
= 7.6 Hz, 1H, Ar H), 7.51 (d, J = 8.3 Hz, 2H, Ar H), 7.49 (t, J = 7.8 Hz,
2H, Ar H), 6.79 (dd, J = 10.8 Hz, 1H; CH), 5.90 (d, J = 17.6 Hz, 1H;
A silicon wafer (3.6 x 4.2 cm) was pre-treated with a self-
assembled monolayer (SAM) of isobutyltrichlorosilane (iBTS) using
solution deposition of iBTS in toluene solution (0.1 mM) overnight.
The terpolymer-nanoparticle composite was prepared by mixing
12 mg of polymer and 6 mg of nanoparticle in toluene/acetone
(500/100 µL) and sonicating the mixture for 5 min. The suspension
was then applied on the alkylated silicon wafer by drop casting.
After air-drying, the coated substrate was irradiated with UV light
(254 nm, 7.5 mW cm-2) for 30 min to covalently graft the polymer-
nanoparticle composite to the substrate surface using the pendant
benzophenone moiety on the polymer.
1
CH2), 5.41 (d, J = 11.2 Hz, 1H; CH2). C NMR (300 MHz, CDCl3, δ):
116.59, 128.27, 129.09, 129.92, 130.53, 132.31, 135.98, 136.65,
137.72, 141.53, 196.11.
Terpolymer
The polymer was synthesized by free radical polymerization of
vinyl benzophenone (0.16 g, 0.79 mmol), hexafluorobutyl
methacrylate (1.18 g, 4.72 mmol), and n-butyl or isobutyl acrylate
(0.10 g, 0.79 mmol) in toluene (3 mL) with AIBN (0.0038 g, 0.0024
2.4 Abrasion Test
The robustness of the surface-bound copolymer-nanoparticle
coating on the substrate was evaluated by a modified Taber test
using eraser rubbing. A pencil (# 2, HB, Ticonderoga) latex free
eraser was pressed against the coated substrate mounted to a
scale with 300 g force and rubbed back and forth (considered as
one rubbing cycle) in a straight line over the surface. After a given
amount of rubbing cycles, the damage of the coating was
examined by measuring the static contact angle and supercooled
water impaction.
mmol) as the initiator in
a Schlenk flask under nitrogen
atmosphere. The degassed mixture was stirred at 65 °C for 16 h.
The resulting polymer was precipitated in cold ethanol, filtered,
and dried under vacuum. Yield: 0.31 g (67%). 1H NMR (500 MHz,
CDCl3, δ): 7.73 (bs, 4H); 7.60 (bs, 1H); 7.48 (bs, 2H); 7.14 (bs, 2H);
4.91 (bs, 6H); 4.34 (bs, 12H); 2.62 (bs, 1H), 1.89 (bs, 19H); 1.48 (bs,
2H); 1.26 (bs, 2H); 1.01 (bs, 6H) 0.937(bs, 10H); 0.72 (bs, 4H). The
number average molecular weight (Mn) of two copolymers was
~137 kDa. Đ of the copolymers that contains n-butyl and isobutyl
acrylate were 1.9 and 2.0, respectively.
2.5 Icing Experiments
Freezing rain was simulated under a laboratory condition by
impinging supercooled water on the samples at subzero
temperatures. The preparation of supercooled water, the cold
substrate, and decanting water onto the copolymer-nanoparticle
coating were conducted in an explosion-proof lab freezer
(temperature: -18 °C, relative humidity: 40%). Supercooled water
was prepared by carefully cooling still, pure water (18 MΩ) in the
freezer for 2 h. Bare and coated substrates were placed in freezer
in advance, followed by pouring supercooled water (100 mL) from
a 20 cm height. The anti-icing effectiveness of the coating was
evaluated by visualizing ice-formation upon impact of supercooled
water and recorded by photography and video. The anti-icing
capability of the coating was examined before and after abrasion.
The icing experiment was repeated 3 times for each sample.
Surface Modified Silica Nanoparticle
An ammonia solution (10.8 g, 29% w/w) was added to deionized
water (442 mL) and heated to 50 °C, followed by addition of
cetyltrimethyl ammonium bromide (0.279 g) with rapid stirring.
The solution was cooled to room temperature and
tetraethoxysilane (1.394 mL) was added. The solution was stirred
vigorously for 2 h. The water was removed by centrifugation and
the white solid was dried in a vacuum oven at 60 °C for 16 h. Yield:
0.40 g. The silica nanoparticles were suspended in toluene (40
mL). Isobutyltrichlorosilane (7.662 g) was then added to the
suspension and stirred rapidly for 16 h. The resulting suspension
was centrifuged and supernatant was discarded. The solid was
washed by resuspending in THF (30 mL), vortexing and
centrifugation. This process was repeated three times. The white
solid was dried in a vacuum oven overnight.
3. Results and discussion
3.1 Cross-linking Kinetics of Terpolymers
The terpolymers, p(BP/F/nBA) and p(BP/F/iBA), which contain
benzophenone (BP), hexafluorobutyl (F), and butyl side chains
(BA), were prepared by free-radical polymerization of the three
Preparation
Composite
of
Surface-Bound
Terpolymer-Nanoparticle
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Scheme 1. Outline of the Synthesis and Chemical Structure of the
Terpolymers
monomers (Scheme 1). The terpolymer compositions were
checked by NMR spectroscopy, which revealed that the polymer
composition matched the pendant group feed ratio (BP : F : BA = 1
: 6 : 1). The hexafluorobutyl side-chain constitutes the majority of
the total polymer pendant groups due to hydrophobicity
requirement of the polymer. Fluorinated materials possess high
hydrophobicity because fluorine has a low polarizability due to its
dense electron cloud, which results in weak van der Waals
interaction between fluorocarbons and water.45 The
benzophenone moiety acts as a cross-linker between the polymer
and any organic interface through hydrogen abstraction followed
by C-C recombination under UV irradiation. Carbon radical
stability affects the hydrogen abstraction rate, which further
governs the crosslinking kinetics. Previous studies have
demonstrated that β-fluorine substitution gives rise to radical
destabilization relative to the ethyl radical,46 so a more reactive
hydrogen is necessary to enhance the cross-linking efficiency. The
butyl side-chain was chosen to provide this reactive hydrogen.
Two copolymers containing n-butyl and isobutyl acrylate (nBA,
iBA) were prepared in order to compare the cross-linking kinetics,
mechanical strength, and hydrophobicity between the copolymers
with a varying reactivity of abstractable hydrogens.
Fig.
1
BP conversion of p(BP/F/iBA) and p(BP/F/nBA) is
monitored via UV/vis absorption of the n-π* peak at 260 nm at
experimental temperature of 20 °C and 50 °C. Inset: UV-vis
spectra of p(BP/F/iBA) at 20 °C with increasing irradiation time.
where ꢎꢏ is the characteristic energy for BP conversion and
the constant absorbance at infinite energy. The decay of the
benzophenone triplet is first order,47,
ꢍ is
48
accordingly the rate
constant
ꢐ is determined as
ꢊ
ꢐ ꢇ ꢊ ꢑ
ꢋ
(2)
where t is the irradiation time. The ꢎꢏ and
ꢐ of the two polymers
at two different irradiation temperatures are reported in Table 1.
Photoinitiation studies have indicated that radical stability of the
alkyl pendent group governs the rate of reactivity of BP.49 As ketyl
radical stability increases with carbon substitution, rate constants
for hydrogen abstraction increase in the order of primary,
secondary and tertiary. Consequently, iBA is expected to have
higher reactivity than nBA due to the tertiary proton. However,
when irradiated at room temperature, the ꢎꢏ of iBA is higher than
nBA by 1.8 J cm-2. This is likely explained by the segmental motion
difference between the two polymer backbones at room
temperature. The DSC traces and glass transition temperature (Tg)
of two copolymers are given in Fig. S1 and Table 1. Under
irradiation, the actual surface temperature of the quartz substrate
measured by an infrared thermometer is 31.1 °C. Thus segmental
chain movement of p(BP/F/nBA) may occur since the local heating
caused by UV irradiation is sufficient to allow the polymer to be
raised above its Tg (29.2 °C) during crosslinking. On the other hand,
p(BP/F/iBA), with a Tg considerably higher than room temperature
(35.4 °C), does not have sufficient free-volume to encounter
surrounding C-H groups in the solid state, i.e. hydrogen
The cross-linking kinetics of copolymers p(BP/F/nBA) and
p(BP/F/iBA) were investigated by UV-vis spectroscopy on an iBTS
functionalized quartz substrate. The polymer film was deposited
on the alkylated quartz by drop-casting with polymer solution (10
μL, 10 mg/mL in CHCl3). Upon absorption of 254 nm light, the
promotion of one electron from a nonbonding orbital to the
antibonding π* orbital of the carbonyl group on the BP yields a
biradicaloid triplet state where the electron-deficient oxygen n
orbital interacts with surrounding weak C-H δ bonds, resulting in H
abstraction to complete the half-filled n orbital. The two resulting
carbon radicals then combine to form a new C-C bond. This
process is monitored by the absorbance decrease in n - π*
transition of BP at 260 nm, as shown in the inset of Fig. 1. The
conversion of BP in p(BP/F/iBA) and p(BP/F/nBA) as function of
irradiation energy (E, J/cm-1) was determined by plotting the
decay of this peak (Fig. 1) at room temperature and elevated
temperature. Initial absorbance was normalized to 1 for both
polymers. The data can be described by a single-exponential
decay:
Table 1. Irradiation characteristics and glass transition temperatures
of the terpolymers used in this study
ꢎꢏ (J/cm2)
ꢐ (s-1)
Copolymer
BP/F/iBA
Irradiation
Temp (°C)
T (°C)
g
25 °C
6.4
1.17
35.4
29.2
50 °C
25 °C
50 °C
4.0
4.6
4.5
1.88
1.63
1.67
⁄
ꢊꢋ
ꢀꢁꢂꢃꢄꢅꢆ ꢇ ꢈꢉꢊ ꢌ ꢍ
(1)
BP/F/nBA
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abstraction is limited. This assumption is further confirmed by
remained on the substrate surface with a macroscopically smooth
topography. The composite was then irradiated for 45 min to
ensure complete BP photo-cross-linking. When the nanoparticles
are in the composite, the BP moiety of the copolymer reacts non-
selectively with the C-H groups on the silane-treated silicon
substrate surface, the nanoparticle surface and the pendent butyl
side-chain of polymer itself, resulting in formation of a fluorinated
polymeric network embedded with NPs, covalently attached to
the substrate surface.
plotting the BP decay and calculating ꢎꢏ and
ꢐ at 50 °C, which is
well above Tg of both polymers. Under this condition, both
polymers have sufficient segmental motion and the reaction rate
is governed by the stability of the radical. It should be noted that
the difference of ꢎꢏ and
ꢐ between two types of butyl side chain is
not remarkable (only by 0.5 J cm-2 and 0.21 s-1, respectively). A
plausible explanation is likely that the nBA pendant group contains
more 2° hydrogens than iBA. Additionally, although the iBA
contains a 3° hydrogen, the reactivity of the 3° C-H is only one
order of magnitude higher than 2° C-H. For this reason, the BP
cross-linking reaction rate of iBA and nBA are comparable.
The completion of the photochemical cross-linking of the
terpolymer was also indicated by GATR-FTIR. Fig. 2 shows the IR
spectra of a p(BP/F/iBA) film coated on silicon wafer before (A)
and after (B) UV irradiation. In Fig. 2(A) and (B), the ester C-O and
C=O stretches are assigned at 1187 and 1746 cm-1. The peak at
1284 cm-1 is due to C-F stretching frequency of the hexafluorobuyl
pendant group. The C-C ring vibration is assigned to 1607 cm-1.
The peak at 1658 cm-1 in Fig 2(A) represents the C=O stretch of BP
chromophore. A significant reduction of this peak, as seen in Fig.
Scheme 2. Covalent attachment of a copolymer/nanoparticle
composite to C-H containing substrates.
3.2.1 Surface Morphology
2(B), is
a support for the photodegradation of the BP
Fig. 3 demonstrates the surface morphology of the UV-cured
composite on a silicon substrate. In the SEM images (Fig. 3A),
many asperities are seen across the surface, which indicates
microscale surface roughness. When magnified (Fig. 3B), the
protrusions can be observed to consist of aggregates of
nanoparticles, which imply nano-scale roughness. The diameter of
the nanospherical feature is ~ 70 nm based on SEM, which is ~10
nm higher than the average size of the unfunctionalized silica
nanoparticle measured by DLS (60 nm, Fig. S2). The micro/nano-
scale roughness is also observed by the SEM side view image of
the cross section of the composite coated on substrate in Fig 3C.
There are numerous nanoparticles embedded in a thin layer of
polymer, which are also adhered to the substrate surface. It is
noteworthy that in lieu of being completely embedded in a
polymer matrix, which would result in a smooth topology, the
concentration of nanoparticles is large enough that the nanoscale
features of the particles are retained and exposed. Fig. 3D shows
the surface profile of the composite coated substrate at the micro-
scale (using surface profilometry) with spike-like protrusions and
valleys with a width of 25 μm and average roughness of 4.0 μm.
Herein, the composite coated substrate shows a surface structure
of hierarchical multiscale (micro and nano) roughness. This surface
structure directly determines its hydrophobicity and anti-icing
capability as discussed below.
chromophore and formation of the covalent grafting after
irradiation.
Fig. 2 FTIR spectra of p(BP/F/iBA) film on SiO2 (A) before and (B)
after UV exposure.The peak labeled at 1658 cm-1 is assigned to
the C=O stretch of BP chromophore.
3.2 Terpolymer/Nanoparticle Composites
Scheme
2 illustrates the covalent attachment strategy for
3.2.2 Surface Hydrophobicity
copolymer/nanoparticle composites to C-H containing surfaces
upon UV irradiation. The surface-bound copolymer/surface-
alkylated nanoparticle composite was prepared by drop-casting
the polymer/NP solvent suspension on alkyl-functionalized silicon
wafers and performing photochemical irradiation. In order to
obtain a uniform dispersion of the polymer and nanoparticles, the
mixture was sonicated vigorously in toluene/acetone co-solvent
and then applied to the substrate surface by drop casting. Upon
solvent evaporation, the resulting polymer/nanoparticle mixture
The hydrophobicity of the composite coated surface (mpolymer/mNP
= 2) is characterized by a static CA and CAH of 138° and 5°,
respectively. When a droplet resides on top of a rough surface,
two wetting states, the Wenzel state and Cassie-Baxter state, can
occur. In the Wenzel state, the droplet impales the grooves and
asperities and the contact angle is defined as follows:
cos ꢒ∗ ꢇ ꢓ cos ꢒ
(3)
ꢔ
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Fig. 3 Morphology study of p(iBP/F/iBA)/nanoparticle composite on alkylated SiO2 substrates. (A) Representative top view SEM images.
(B) Zoom-in SEM image of an asperity. (C) Side view SEM image of coated substrate cross section. (D) Surface profile thickness of the
coated surface. (E) Schematic cross-sectional profile of water in contact with a composite coated surface that illustrates the Cassie-
Wenzel transition.
where ꢒ∗ and
respectively. ꢔ, known as the roughness factor, is the ratio of the
ꢒ
are the CA on rough and flat surfaces,
formed instantly when the supercooled water impacted the
surface. A full layer of ice is shown on the control whereas no ice
was formed on the coated substrate. Only some accumulated
water remains on the surface due to the low tilt angle in the
experiment. The substrates were warmed back to room
temperature and air-dried. The process of striking substrate with
supercooled water, warming and drying was considered as one
icing/deicing cycle. The icing/deicing process was repeated three
times on the same substrate and no ice was formed on the
composite coated surface each time. According to previous
reports, multiple icing/deicing cycles caused gradual loss of
effective icing prevention, as a result of damage of nano-asperities
over the expansion of water upon freezing13. This is not the case
with the crosslinked composite, which has reliable anti-icing
capability towards striking of supercooled water after multiple
icing/deicing cycles, due to its excellent mechanical durability as
described in the following section.
ꢓ
actual area of the solid surface to the apparent area. In Cassie-
Baxter state, the droplet sits on top of the asperities, forming “air
pockets” between the liquid and solid surface. In this case, the
contact angle is defined by the following equation:
∗
ꢖ
ꢗ
cos ꢒ ꢇ ꢕ 1 ꢌ cos ꢒ ꢘ 1
(4)
A
where
ꢕ is the area fraction of liquid-solid interface.
superhydrophobic surface that consists of hierarchical micro- and
nano-scale roughness, combined with low surface energy
a
moiety, satisfies the Cassie-Baxter model.9, 50 Based on the surface
image and profile shown above, Fig. 3E demonstrates the wetting
behavior of the water droplet on these polymer/nanoparticle
composites. The composite dual-scale roughness prohibits water
droplets to wet the spaces between spikes of both micro- and
nanostructure. Consequently, the water-solid contact area is
small, due to an extremely small
ꢕ
value (
ꢕ
ꢇ ꢕꢄꢚꢃꢏꢂ ꢜ
ꢙꢚꢛꢃꢅꢃꢏꢙꢚꢏꢅꢆ
ꢕꢁꢅꢁꢂ). According to Equation 4, the CA of such surface is large.
Because of the air entrapment inside the grooved texture, a
heterogeneous interface composed of solid and air is generated.
As a result, the adhesion force between water and solid is
extremely low, which accounts for the observed low hysteresis.
Previous studies have been focused on two aspects to design an
anti-icing coating: elimination of ice adhesion and delay or
prevention of ice nucleation. It also has been shown that wetting
hysteresis directly relates to ice adhersion.11 In this work, the
3.2.3 Ice Prevention Towards the Impact of Supercooled Water
One of the methods to examine the anti-icing capability of
surfaces involves impacting the tested surface with supercooled
water.10, 15, 51 In our study, the uncoated and composite coated
silicon wafers were tilted at an angel of 5° from horizontal and
impinged by a stream of supercooled water. The effectiveness was
evaluated by observation of ice generation on the substrate. The
videos that recorded the icing experiment in the freezer are
provided in Movie S1 and the photos of resulting substrates after
ice formation are shown in Fig. 4. In Fig. 4A, the left substrate is
the control (bare SiO2) and the right substrate is the wafer coated
with the polymer/particle composite after UV irradiation, where
the opaque coating is apparent. On the uncoated substrate, ice
Fig. 4 Optical images demonstrating the anti-icing property of
p(BP/F/iBA)/nanoparticle composite coated silicon wafer before
(A) and after (B) abrasion.
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composite coated surface with a CA of ~140° and a CAH of 5° is
speculated to be wetted in the Cassie-Wenzel transition state,
where water is partially pinned in the grooves or asperities. This
mixed state explains the fact that the composite coated surface
was slightly wetted after striking with supercooled water (Fig. 4).
Thus, instead of reaching a perfectly non-wetting Cassie-Baxter
state in which the frozen droplets are easily removed with a small
input of thermal energy,15 the delay of ice nucleation is likely the
more important factor that leads to the icephobic nature of this
coating. Cao et al. systematically investigated the effect of the size
of the spherical roughness on icing probability.10 They successfully
demonstrated that the ratio of the free energy barrier between
polymer/nanoparticle composition ratio inceases to 2.8 (Fig. 6B),
the individual particles start to become buried and submerged in
the polymer matrix, which leads to a smoother surface on the
nanoscale. The morphology change due to the higher polymer
content results in a low CA (122°) and high CAH (22°). The
different morphologies of two composites lead to different
behavior in icing experiments (Movie S1 and Movie S2). Upon
striking of the surface with supercooled water, the composite with
polymer/NP ratio of 2 repells the water and prevents icing. In
comparison, though inhibiting instant ice formation, the
composite with ratio of 2.8 provide much less water repellancy.
The remaining water on surfaces can nucleate in sub-zero
conditions. Therefore, the proper composition of p(iBP/F/iBA)/NP
is essential to enable the optimum amount of nanoscale
roughness, which aids in water repellency and icephobicity.
heterogeneous and homogeneous nucleation,
0 and 1), estimates the anti-icing capability of surfaces with nano-
spherical roughness. The mathematic derivation to calculate and
experimental parameters are found in the Supporting Information.
Under our experimental conditions of icing, is plotted versus
radius of nanoparticle, in Fig. 5. is found to drop dramatically
as increases from 10 to 110 nm. When is above 1 μm, the size
of nanoparticle has little influence on . This curve implies that
ꢕ (values between
ꢕ
ꢕ
ꢝ
ꢕ
ꢝ
ꢝ
ꢕ
within a 10 to 110 nm particle radius, the size of the surface
exposed nano-spherical features is the defining parameter in
terms of likelihood of preventing ice nucleation. Specifically, the
smaller the nanoparticles, the greater the nucleation energy
barrier, and lower tendency towards icing on the surface. The
radius of our surface exposed nanoparticle is 35 nm based on
Fig. 6 SEM images of p(BP/F/iBA)/nanoparticle composites
coated on SiO2 substrates with different polymer/nanoparticle
mass ratios. The ratios of polymer/nanoparticle are (A) 2 and (B)
2.8, respectively.
SEM, and
ꢕ is 0.862 according to Fig. 5, suggesting a relatively high
nucleation free energy barrier. For these reasons, the composite
coating has excellent icephobicity towards impinging supercooled
water.
3.3 Robustness of Polymer/Nanoparticle Composites against
Abrasion
3.3.1 On Alkylated Glass
The robustness of BP induced covalent bonding to substrate
surface was evaluated by conducting an eraser abrasion test
(modified Taber testing) on BP containing and non-BP containing
polymer/nanoparticle composites. P(BP/F/iBA) was used as the
polymer in the tested samples and a homopolymer that only
contains the fluorinated hydrocarbon, poly(hexafluorobutyl
methacrylate), was used as the control. An eraser with 300 g load
was rubbed across the two composite coated samples on silicon
substrates 60 times. Optical pictures and water CA of the resultant
coating after abrasion testing are shown in Fig. 7. The control
poly(hexafluorobutyl
methacrylate)/nanoparticle
composite
Fig. 5 Relationship of the ratio of free energy barrier between
heterogeneous and homogeneous nucleation on spherical surface
(f) versus nanoparticle radius (R).
3.2.4 Composites with different polymer/NP ratios
The effect of different terpolymer/nanoparticle composition on
morphology and anti-icing capability was also invesitigated by
comparing
the composties with a polymer (p(iBP/F/iBA))
Fig. 7 Abrasion study of polymer/nanoparticle composites coated
SiO2 substrates. Optical images of substrates coated with (A)
p(hexafluorobutyl methacrylate) and (B) p(BP/F/iBA)/
nanoparticle composite after one cycle of abrasion. Inset: image
of water droplet on the substrate.
nanoparticle mass ratio of 2 and 2.8 (w/w). Fig. 6 illustrates the
morphology of the two composties when coated on alkylated SiO2
substrates. With the composite containing a polymer/nanoparticle
ratio of 2 (Fig. 6A), numerous spherical particles are uniformely
exposed, which provides nanoscale roughness. However, as the
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Fig. 8 Robustness of p(BP/F/iBA) and p(BP/F/nBA) nanocomposites against abrasion: SEM images of (A) p(BP/F/iBA) and (B)
p(BP/F/nBA) nanocomposites coated SiO2 surfaces after 28 abrasion cycles (dashed box: abraded area); (C) Plot of mechanical abrasion
cycles and static water CA after every other abrasion cycle for p(BP/F/iBA) and p(BP/F/nBA) coated composites.
without BP crosslinker was removed after only one cycle of
rubbing, which left the bare substrate with CA of 83°, whereas the
BP containing copolymer composite remained, and the surface
retained its superhydrophobicity with CA of 140°. This implies that
BP moiety can act as a “photo-reactive glue” that combines the
polymer and nanoparticles together with adherence to the
substrate surface through crosslinking. In comparison, the
physisorbed non-BP composite provides no mechanical resistance
towards abrasion. Additionally, for the surface-bound composite,
as the exterior of the coating is abraded and lost, more deeply
embedded nanoparticles are exposed at the interface, and the
nano-scale roughness remains intact. Hence, the BP-containing
composite presents mechanical resistance to friction. Anti-icing
capability of p(BP/F/iBA) nanoparticle composite was tested after
30 abrasion cycles. The video of the icing experiment on the
abraded coatings is shown in movie S2 and the resultant substrate
is shown in Fig. 4B. No ice formation is observed, demonstrating
that the composite not only has excellent anti-icing capability, but
composite decreases by only 5° (from 134° to 129°), retaining
its hydrophobicity. The robustness difference is likely explained
by the occurrence of β-scission of the polymer backbone during
cross-linking. It is suggested that β-scission is one of the
pathways
a carbon radical can undergo after hydrogen
abstraction.52 If β-scission occurs on backbone, it causes chain
scission in the polymer. For a polyacrylate that contains 3°
hydrogens on the polymer backbone, hydrogen abstraction
occurs most favorably on the polymer main chain, leading to
higher amounts of chain scission and resultant loss in the
mechanical integrity of the coating. However, if there are
additional 3° hydrogens on the pendent groups (as is the case
with an isobutyl side chain), the probability of hydrogen
abstraction from the backbone is smaller, where the pendant
3° hydrogens “dilute” the β-scission probability. Additionally, it
is likely that isobutyl groups are more accessible for
abstraction, and hinder access to the polymer backbone more
than n-butyl side chains. A similar conclusion was reached by
Christensen et al. in a recent study on polymer gelation.53
also
a mechanical robustness that survives harsh abrasion.
However, it was observed that some liquid droplets remained on
the abraded surface due to the slight decline in surface
hydrophobicity (~5° CA decrease after abrasion (Fig. 8(C)).
3.3.2 On Plastic
To investigate the versatility of the p(BP/F/iBA)/nanoparticle
coating on commercially available plastics, we photochemically
modified a polypropylene (PP) sheet with the composite by drop-
casting using the same material composition described above. The
PP substrates were irradiated to covalently graft the
copolymer/nanoparticle to the plastic surface via hydrogen
abstraction from the PP surface. After UV curing, the uncoated
and coated PP substrates were challenged against the icing test
using the icing experimental method described above. Fig. 9
shows the anti-icing capability of composite coated PP before (A)
and after (B) eraser abrasion (300 g, 30 cycles). In both (A) and (B),
ice is observed on the pristine PP substrate (left). No ice formation
occurred on the composite coated PP substrate (right) before and
after undergoing 30 abrasion cycles. These results demonstrate
A comparison of robustness between the terpolymers of
p(BP/F/iBA) and p(BP/F/nBA) within the nanoparticle
composite was also performed using eraser abrasion testing
and subsequent SEM imaging (Fig. 8). The morphology of
abraded surfaces coated with p(BP/F/iBA) and p(BP/F/nBA)
nanocomposites are presented in Fig. 8A and Fig. 8B,
respectively. For p(BP/F/iBA) composite, only the top layer of
the nanoparticles was removed after treated with 28 abrasion
cycles. The embedded nanoparticles within the polymeric
network were exposed and the surface nanoscale roughness
was preserved. In contrast, p(BP/F/nBA) nancomposite was
completely removed from the surface after undergoing the
same number of abrasion cycles, indicating the adhesive failure
of the composite. Furthermore, the CAs were measured over
continuous abrasion cycles to characterize the changes of the
hydrophobicity for two polymer/nanoparticle composites (Fig.
8C). It was observed that the CA of p(BP/F/nBA) composite
drops by ~30° (from 128° to 96°, which is close to the original
contact angle of the isobutyl SAM on silicon wafer) after 28
abrasion cycles. On the other hand, the CA of p(BP/F/iBA)
that
the
covalent
immobilization
of
durable
Fig. 9 Optical images demonstrating the anti-icing property of
a p(BP/F/iBA)/nanoparticle composite coated polypropylene
substrate before (A) and after (B) abrasion.
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14 A. J. Meuler, J. D. Smith, K. K. Varanasi, J. M. Mabry, G. H.
McKinley and R. E. Cohen, ACS Appl. Mater. Interfaces,
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copolymer/nanoparticle composite is applicable to a variety of
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15 L. Mishchenko, B. Hatton, V. Bahadur, J. A. Taylor, T.
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22 P. M. Hansson, L. Skedung, P. M. Claesson, A. Swerin, J.
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In conclusion, we have demonstrated an efficient and easy-to-
implement approach to covalently attach an anti-icing composite
on to any plastic substrate or surface that contains C-H bonds. The
copolymer contains pendant benzophenone, hexafluorobutyl, and
isobutyl side chains. When the polymer is combined with silica
nanoparticles, a composite that consists of a densely cross-linked
network is immobilized on a substrate surface via covalent bonds
upon UV irradiation. The optimized copolymer/nanoparticle
composite exhibits fast attachment and good mechanical
durability when subjected to abrasion. No ice formation occurred
on the coated surface when subjected to supercooled water for
both abraded and non-abraded coatings. This simple, one step
photochemical attachment of icephobic coatings is promising and
scalable for domestic and industrial applications. In addition, we
have observed that incorporation of 3° hydrogens on pendent
groups of BP containing polymer reduces the backbone chain
scission occurrence and improves the overall crosslinking
possibility and mechanical strength of the coatings after
irradiation. This study provides some design principles for
polymer/nanoparticle composites that have excellent anti-icing
capability along with robust attachment and mechanical
durability.
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Acknowledgements
The authors gratefully acknowledge financial support from
Georgia Research Alliance and the GRA Ventures Program.
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