How to make a fluoropolymer sticky!
Hadi Izadi and Alexander Penlidis
Department of Chemical Engineering, Institute for Polymer Research, University of Waterloo,
Waterloo, ON, Canada N2L 3G1
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13
The non-reactivity along with low surface energy lead to very weak adhesion and friction of
fluoropolymers, making them ideal candidates for fabrication of non-sticky and releasing
surfaces. However, possessing low dielectric constants and high electronegativite fluorine-based
units, fluoropolymers can become highly charged at the surface upon contact with other
materials via the so-called contact electrification phenomenon, leading to formation of
electrostatic interactions at the surface. The electrostatic forces developed via contact
electrification (the same incident that generate charges over a piece of amber when it is rubbed
against wool) can generate large work of adhesion which is even comparable to the fracture
energies of ionic-covalent materials.1-3 In view of that and considering that an efficient
triboelectric charging requires enhanced actual area of contact, for the first time, we have
fabricated a novel electrostatic-based dry adhesive by developing high aspect-ratio (AR)
nanopillars on a rigid fluoropolymer (Teflon AF).4-6 Due to the extremely high AR of nanopillars
which are terminated at the tip with a peculiar (fluffy) sheet-like nanostructure (inspired by the
sticky fibrillar structure of gecko lizard foot pads) (see Figure 1), the fabricated dry adhesive can
come into an ultimate contact, leading to enhanced adhesion and friction. Unlike gecko and other
synthetic dry adhesives, which require efficient van der Waals interactions upon contact, the
fabricated dry adhesive is the only one of its kind which relies for the most part on electrostatic
interactions.4-6
Figure 1. SEM image of bi-level Teflon AF nanopillars (45°-view). Extremely high AR Teflon AF nanopillars (200 nm in
diameter) are terminated at the tip with a unique fluffy nanostructure. The magnified image is from the top-view.6
In this research, bi-level nanopillars of various heights and hierarchy features (as a dry adhesive)
were fabricated from Teflon AF 1600 (DuPont) by using anodic aluminum oxide (AAO)
membranes (0.2 µm pore diameter, 60 µm thick, pore density 25-50 %; Whatman Inc.) as the
mold. As shown in Figure 2, the polymer granules were placed over the AAO membrane and,
1
subsequently, the polymer and the mold were heated from the bottom to reach the constant
desired "heating temperature" above the glass transition temperature of the polymer (T g = 160
°C).7 From the top, the polymer was cooled down by setting the "cooling temperature" at a
temperature much lower than the glass transition temperature of the polymer.
Stainless
steel
cover
T ≈ TC
Polymer
Cooling
AAO
membrane
Heating
13
24 h
AAO pore wall
Cooling
Polymer
granules
Heating
T = TH
Hotplate
Polymer
backing layer
Brush-like
fingered
nanostructure
20
Figure 2. Schematic of the procedure for fabrication of bi-level Teflon AF nanopillars. The precursor film of the polymer
melt, which was formed at the forehead of the developing polymer melt confined within AAO nanopores, fingered over
the pore walls. Upon removal of the mold, the collapse and entanglement of the developed brush-like nanostructure led to
formation of a fluffy nanostructure on top of the base Teflon AF nanopillars.
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Upon infiltration of the Teflon AF melt into the AAO nanopores at elevated temperatures, like
other polymers, the polymer melt spread over the pore walls (because of the large difference in
surface energy of the polymer and the mold) and formed a thin nanoscopic precursor film at the
forehead of the developing polymer melt. However, due to the very low surface energy of the
polymer (15.7 mJ/m2),7 the thermocapillarity-driven stresses at the contact line of the precursor
film have led to instability of the precursor film and its fingering over the pore walls (Figure 2).46
The very soft nanostructure of the developed brush-like hierarchical level, after solidification of
the polymer and dissolving the mold in NaOH solution (1.25 M), instantly collapsed during the
drying step, resulting in formation of an exceptional fluffy nanostructure on top of the base
nanopillars of ~200 nm in diameter (see Figure 1). As drying continued, the sheet-like
nanostructure, with sufficiently high density and large size, held the tip of the base nanopillars
away from each other and, accordingly, hindered the lateral collapse at the tips during the drying
step (see Figure 1).4-6
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In the current fabrication method, by enhancing the "heating temperature" and keeping the
"cooling temperature" constant (i.e., increasing the temperature gradient), the size (length and
width) of the hierarchical fingered brushes was decreased while their density increased. In
addition to size and density of the fingered brushes, adjusting the heating temperature also
altered the infiltration depth of the polymer melt and, accordingly, the height of the fabricated
nanopillars. Particularly, by setting the "heating temperature" to 270, 300, 330, or 360 °C,
nanopillars with heights of approximately 5.5, 16, 37, or 45 µm were fabricated, respectively (for
fabrication of all samples, the "cooling temperature" was kept constant at the room
temperature).5,6 At the low density of the top terminating layer (corresponding to the lowest
temperature gradient), due to the high AR of nanopillars (AR = 27.5), the nanopillars collapsed
at the tip and formed an island-like structure (see Figure 3A). However, by increasing the density
of the hierarchical level on top (see Figures 3B and 3C), extremely high AR nanopillars (AR =
80 and 185, respectively) were fabricated. For taller pillars (AR = 225), although the terminating
2
nanostructure was very dense, due to its small size, the top layer could not hold itself together
and the characteristic self-sticking of pillars at the walls led to the bundling of pillars into an
island-like structure, but this time at the cost of shrinkage of the top nanolayer.5,6 It is worthwhile
mentioning that keeping the "heating temperature" constant while changing the "cooling
temperature" allows the fabrication of bi-level Teflon AF nanopillars of identical heights but
with terminating nanostructures of distinct geometrical properties. However, for the sake of
brevity, the cases where the "cooling temperature" was changed will not be discussed in the
current report.
D
20
B
C
13
A
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Figure 3. SEM images of bi-level Teflon AF nanopillars of different geometrical properties; nanopillars with different
heights of approximately A) 5.5 µm, B) 16 µm, C) 37 µm, and D) 45 µm, have been fabricated by setting the "heating
temperature" to 270, 300, 330, and 360 °C, respectively ("cooling temperature" was kept constant at the room
temperature). The images show the top-view of the samples while the magnified images are from the 45°-view.6
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The adhesive and frictional properties of the fabricated nanopillars as well as those of the flat
Teflon AF (as the control sample) were characterized by indentation and load-drag-pull (LDP)
tests, respectively. The tests were performed using an 8 mm in diameter hemispherical fused
silica (ISP Optics Corp.) probe. The same as with flat control samples, negligible pull-off forces
were detected for the tallest nanopillars (sample D; ~45 μm tall), which were attributed to the
collapse and bundling of the nanopillars and accordingly the very small actual area of contact
(see Figure 4).5,6,8-10 However, for one level shorter nanopillars (sample C; ~37 μm tall) which
did not collapse at the tip because of the presence of the terminating layer on top, substantial
pull-off forces were detected while still lower than those for sample B with nanopillars of ~16
μm tall. Even though sample B had a lower surface area available for contact in comparison to
sample C (as detected by water contact angle measurement tests), it showed a remarkably
superior adhesion performance, most likely due to a lower chance of buckling during loading
because of the lower height compared to the 37 μm tall pillars.5,6,8-10 Sample B reached the
astonishing adhesion strength value (pull-off force per unit surface area) of ~1.6 N/cm2, while
sample C showed a lower efficiency (i.e., adhesion strength of ~1.1 N/cm2), although both
3
performed better than a gecko foot pad (see Figure 4).5,6 As the height decreased further (sample
A, ~5.5 μm tall), it was observed that the pillars showed remarkable adhesive properties at low
preloads, while at higher preloads their efficiency in generating adhesion compared to more
flexible taller pillars was lower, as expected.5,6,8 The 5.5 μm tall nanopillars showed very high
achievable adhesion strength of ~1.3 N/cm2 at a very small preload of ~12 mN.
30
Adhesion strength
preload
1.6
25
1.4
1.2
20
1
13
Gecko
Preload (mN)
Maximum adhesion strength (N/cm²)
1.8
15
0.8
0.6
10
0.4
5
0.2
0
0
Sample A
(AR=27.5)
Sample B
(AR=80)
Sample C
(AR=185)
Sample D
(AR=225)
20
Figure 4. The adhesion strength and the corresponding preload for bi-level nanopillars with different heights of ~5.5, 16,
37, and 45 μm (samples A, B, C, and D, respectively); the red dashed line shows the normal adhesion strength of natural
gecko (1 N/cm2).6
Due to the geometrical properties of the shortest nanopillars (i.e., low density of the hierarchical
level), these nanopillars can make an effective sidewall contact with the fused silica probe (see
Figure 3A), most likely leading to their observed improved effective shear strength as observed
in LDP tests;5,6 sample A reached the remarkable shear strength value of ~12 N/cm2 at a 25 mN
preload (Figure 5), ~20% higher than that of a natural gecko (i.e., 10 N/cm2).
16
A
B
C
12
D
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Effective shear strength (N/cm²)
14
Gecko
10
8
6
4
2
0
0
5
10
15
Preload (mN)
20
25
30
Figure 5. Effective shear strength at the start of the dragging step in LDP tests for bi-level Teflon AF nanopillars with
different heights of ~5.5, 16, 37, and 45 μm (samples A, B, C, and D, respectively) at nominal preloads of 5, 10, 17.5, and
25 mN. The red dashed line shows the shear strength of natural gecko (10 N/cm2) reported in the literature.6
Even though for mid-sized nanopillars the elevated density of the top terminating layer hindered
the sidewall contact, the sheet-like structure of the top layer helped these nanopillars to still
generate large shear strength values (up to ~3 and 7 N/cm2 for samples B and C, respectively).5,6
In comparison to these nanopillars, the bundled 45 μm tall nanopillars (sample D) reached a
relatively moderate effective shear strength (up to ~4 N/cm2), most likely due to their ability to
4
generate minor sidewall contact with the probe (see Figure 5), even though they still had a low
actual surface area because of the bundling effect.5,6,8,9
13
In summary, we reported the fabrication of bi-level Teflon AF fibrillar nanostructures of various
geometrical properties, as a new generation of dry adhesives. These nanostructures work based
on electrostatic interactions at the surface rather than van der Waals forces (based on which most
other dry adhesives are operating). By means of a novel fabrication method, the nanopillars were
terminated at their top by a unique sheet-like nanostructure, effectively hindering the selfsticking of high-aspect-ratio and high-density nanopillars. The generated adhesion and friction
strengths by these nanopillars not only matched those of gecko (a typically used upper limit from
nature) but also surpassed them by ~60 and 20%, respectively.
References
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1. H.T. Baytekin, A.Z. Patashinski, M. Branicki, B. Bay-tekin, S. Soh, B.A. Grzybowski,
Science, 2011, 333, 308-312.
2. C. Liu and A.J. Bard, Nature Materials, 2008, 7, 505-509.
3. R.G. Horn and D.T. Smith, Science, 1992, 256, 362-364.
4. H. Izadi, B. Zhao, Y. Han, N. McManus, A. Penlidis, Journal of Polymer Science Part B:
Polymer Physics, 2012, 50, 846-851.
5. H. Izadi, M. Golmakani, A. Penlidis, Soft Matter, 2013, 9, 1985-1996.
6. H. Izadi and A. Penlidis, Proceedings of the 36th Annual Meeting of the Adhesion Society,
2013, March 3-6, Daytona Beach, FL, USA.
7. J. Scheirs, Modern Fluoropolymers: High Performance Polymers for Diverse Applications,
1997, John Wiley & Sons, Chichester.
8. C. Greiner, A. del Campo, E. Arzt, Langmuir, 2007, 23, 3495-3502.
9. N.J. Glassmaker, A. Jagota, C.Y. Hui, J. Kim, Journal of the Royal Society Interface, 2004,
1, 23-33.
10. A.K. Geim, S.V. Dubonos, I.V. Grigorieva, K.S. No-voselov, A.A. Zhukov, S. YU.
Shapoval, Nature Materials, 2003, 2, 461-463.
5
Outline
y
Introduction
y
Fabrication
y
Adhesion and Friction
y
Current and Future Work
How to Make a Fluoropolymer Sticky!
Hadi Izadi and Alexander Penlidis
Institute for Polymer Research (IPR), Department of Chemical Engineering, University of Waterloo
Waterloo, ON, Canada
13
May 08, 2013
1
2
20
Introduction
y
Biological
Bi
l i l adhesives:
dh
h i
Green dock beetle
Introduction
y
4
Introduction
Bi
l i l adhesives:
dh
h i
Biological
Fibrillar adhesive
Gorb et al. Bioinspir. Biomim. 2007
3
IP
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Introduction
y
trong adhesion
adhe
Total strong
esiv
Bio-inspired adhesives:
PUA
PS
P
S
Kim
Ki
K
iim
m ett al
al.. Adv.
Adv M
ate
ter.
er. 2
200
20
0 9
Mater.
2009
Carbone
C
Ca
Car
bo
bon
on
ne and
an
nd
d Pierro
P rro Small 2012
Pie
PDMS
Substrate
Many weak intera
interactions
PU
Mur
urphy
r
Murphy
et al. ACS Appl. Mater. Interfaces 2009
5
Glassmaker et al. PNAS 2007
6
Introduction
Introduction
Fluoropolymers (e.g., Teflon)
nergy
Very low surface energy
Contact Electrification
Low electronegativity
Insulating
In
nssu
ula
lati
t in
ngg
Surface 1
Strong
Str
ttrron
ong
o
ng
n
g
ele
lect
le
ctr
c
ttro
ost
ssttat
ati
a
ttiic iinteractions
nt ractions
nte
electrostatic
Surface 2
13
Weak
ons
van der Waals interactions
7
Introduction
20
Introduction
Contact Electrification
Surface 1
Contact Electrification
Surface 1
Electrostatic Force
Surface 2
9
10
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R
Surface 2
8
Introduction
Real contact area
Surface charge density
mpliant su
Compliant
surface
ulating m
Insulating
material
Terminating
nanostructure
Fabrication
High aspect-ratio
(AR) nanopillars
Backing layer
11
12
Fabrication
y
Fabrication
Replica-molding
y
Replica-molding
Ń Polymer: Teflon AF
Ń Mold: Anodic Aluminum Oxide (AAO) Membrane
x steel
Pore
Stainless
cover x Pore
diameter: 200 nm
length: 60 ȝm
Stainless steel
cover
Teflon AF
granules
AAO
membrane
Teflon AF
granules
24 h
Heating
Heating
24 h
AAO
membrane
Hotplate
13
Hotplate
Heating
Heating
13
Low temperature
AAO pore wall
Heating
Low temperature
Polymer
Polymer
Polymer
15
IP
Fabrication
16
Fabrication
5.5 ȝm
Sample A
Terminating
nanostructure
Sample B
Sample C
Sample D
45 ȝm
R
Heating
AAO pore wall
Polymer
Fingering of the
High thin-film
temperature
AAO pore wall
High temperature
Replica-molding
AAO pore wall
y
37 ȝm
Replica-molding
16 ȝm
y
Fabrication
20
Fabrication
14
High aspect-ratio
(AR) nanopillars
Backing layer
270 °C
17
300 °C
330 °C
360 °C
18
Fabrication
13
Adhesion & Friction
Izadi et al. Soft Matter 2013
19
20
20
Adhesion & Friction
Adhesion: Indentation tests
Bi-level
Teflon AF
nanopillars
Adhesion & Friction
21
22
IP
R
Load
Fused silica
probe (8 mm)
Unload
y
Adhesion & Friction
Maximum adhesion strength (N cmʚ²)
1.8
1.6
1.4
1.2
1
Gecko
0.8
0.6
0.4
Adhesion & Friction
0.2
0
Sample A
(AR=27.5)
Sample B
(AR=80)
Sample C
(AR=185)
Izadi et al. Soft Matter 2013
Sample D
(AR=225)
23
24
Adhesion & Friction
y
Adhesion & Friction
Friction: Load-Drag-Pull (LDP) tests
y
Start of dragging:
16
A
B
Pull
Load
Fused silica
probe (8 mm)
Effective shear strength (N cmʚ²)
14
Drag
C
12
D
Gecko
10
8
6
4
2
Bi-level
Teflon AF
nanopillars
13
0
0
5
10
15
Preload (mN)
20
25
30
Izadi et al. Soft Matter 2013
25
26
20
Current & Future Work
y
Current & Future Work
Current work
Stainless steel
cover
Teflon
on
n AF
AF
gra
gr
g
rra
anul
n les
nu
granules
Alumina
Alu
um
min
mi
iin
na
mem
m
em
e
mbra
br ne
e
membrane
Cooling
C
Co
oo
olliin
ng
Cooling
Co
C
oling
6h
Heating
He
H
ea
attin
i g
Heating
27
IP
R
Hotplate
H
Ho
Hot
plate
pla
te
28
Current & Future Work
Future work
Ń I. Underwater adhesion
12
10
THANK YOU!
Flat (Dry)
Flat (Under water)
Nanopillars (Dry)
8
Ń II. Fingering
instability mechanism
Pull-off Force (mN)
y
Nanopillars (Under water)
6
4
2
Ń III. Contact
mechanics model
x Van0 0der Waals
forces
+ electrostatic
interactions
10
20
30
40
50
60
Preload (mN)
Izadi et al. J. Polym. Sci., Part B: Polym. Phys. 2012
29
30
Contact Electrification
Introduction
Surface charge density
area
Contact Contact
Electrification
y
‫– = ݁ܿݎ݋ܨ ܿ݅ݐܽݐݏ݋ݎݐ݈ܿ݁ܧ‬
ܽܿ‫ ߪ ݊݋‬2
2ߝ0 ߝ‫ݎ‬
Permittivity of free space
Permittivity of separating medium
Surface 1
0.8
After contact
0.008
0.006
0.004
0.002
y
Electrostatic Force
I. Rubbed with fused silica
Surface 2
0.6
O
0.4
Cu
0.2
II. Immersed in 1 mM CuSO4
0
0
550
600
650
700
750
800
Wavelength (nm)
850
0
900
1
2
3
Energy (keV)
4
5
6
Izadi et al. Soft Matter 2013
13
y
0.01
Before contact
Counts × 1000
Absorbance
0.012
C F
1
0.014
31
Adhesion & Friction
20
Adhesion & Friction
120
y100I. Van
Pull-off force (mN)
60
Sample B
50
80
60
40
Ń Pull-off force §1 mN
Ń Pull-off force is load-independent
0
120 0
20
200
300
400
y80II. Electrostatic
0
60
40
20
500
600
Preload (mN)
700
800
900
1000
5
10
15
20
25
30
35
40
900
1000
Ń Large pull-off forces
Ń Pull-off force is load-dependent
0
Pull-off
Force
R
0
IP
1.8
Sample
Sam
Sa
S
am
a
mp
ple
pl
lle
eB
Maximum adhesion strength (N cmʚ²)
Sample A
400
500
600
Preload (mN)
700
800
Adhesion & Friction
10 mN
5
300
34
50 mN
5 mN
200
33
8
6
100
Displacement (ȝm)
Adhesion & Friction
25 mN
interactions
Sample
Sam
S
Sa
am
ampl
ple
p
le C
4
3
Sample D
2
1
1.6
30
Adhesion strength
preload
25
1.4
1.2
1
20
Gecko
15
0.8
0.6
10
0.4
5
0.2
0
0
0
Flat Teflon AF
10
0
20
30
20
30
Pillar height (ȝm)
Izadi et al. Soft Matter 2013
40
40
0
Sample A
(AR=27.5)
50
35
Preload (mN)
0
-10
7
100
100
10
Pull-off force (mN)
der Waals forces
20
30
Pull-off force (mN)
Normal force (mN)
40
32
Sample B
(AR=80)
Sample C
(AR=185)
Izadi et al. Soft Matter 2013
Sample D
(AR=225)
36
Adhesion & Friction
Adhesion & Friction
y
Start of dragging
Dragging
Start of dragging:
A
0
10
20
30
40
50
60
70
Time (s)
-5
-10
-15
Izadi et al. Soft Matter 2013
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Effective shear strength (N cmʚ²)
5
C
12
D
Gecko
10
8
A
6
4
2
13
0
0
37
y
Unloading:
Sample A Sample B
8
25 mN
17.5 mN
7
10 mN
6
5 mN
5
4
Sample C
Sample D
3
2
1
0
10
20
30
Pillar height (ȝm)
40
50
R
0
5
10
IP
Izadi et al. Soft Matter 2013
39
15
Preload (mN)
20
Izadi et al. Soft Matter 2013
20
Adhesion & Friction
Pull-off force (mN)
Force (mN)
10
0
B
14
Fx (Flat)
Fz (Flat)
Fx (sample C)
Fz (sample C)
15
B
DC
16
Unloading
20
25
30
38