Plasma-enhanced atomic layer deposition: A gas-phase route to hydrophilic, glueable polytetrafluoroethylene

Article abstract of DOI:10.1039/c4cc09474c

This communication reports an approach based on plasma-enhanced atomic layer deposition of aluminium oxide for the functionalization of polytetrafluoroethylene (PTFE or Teflon ) surfaces. Alternating exposure of PTFE to oxygen plasma and trimethylaluminium causes a permanent hydrophilic effect, and a more than 10-fold improvement of the glueability of PTFE to aluminium. This journal is

Full text of DOI:10.1039/c4cc09474c

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Plasma-enhanced atomic layer deposition:
a gas-phase route to hydrophilic, glueable
polytetrafluoroethylene†
Cite this: Chem. Commun., 2015,
1, 3556
5
Received 26th November 2014,
Accepted 22nd January 2015
a
a
a
a
Amit K. Roy, Jolien Dendooven, Davy Deduytsche, Kilian Devloo-Casier,
b
b
a
Kim Ragaert, Ludwig Cardon and Christophe Detavernier*
DOI: 10.1039/c4cc09474c
www.rsc.org/chemcomm
This communication reports an approach based on plasma-enhanced
Kemell et al. reported surface modification of planar PTFE
atomic layer deposition of aluminium oxide for the functionalization substrates via thermal atomic layer deposition (ALD) of metal
9
of polytetrafluoroethylene (PTFE or ‘‘Teflon’’) surfaces. Alternating oxides. PTFE substrates were exposed to alternating pulses of a
exposure of PTFE to oxygen plasma and trimethylaluminium causes metalorganic precursor and water vapour resulting in Al
a permanent hydrophilic effect, and a more than 10-fold improvement TiO deposition. Unfortunately, the adhesion of the deposited
of the ‘‘glueability’’ of PTFE to aluminium.
2 3
O or
2
metal oxides was poor on PTFE as the ALD layers were easily
removed during a Scotch tape test. This poor adhesion might
Polytetrafluoroethylene (PTFE) commercially known as ‘‘Teflon’’ be due to the absence of covalent bonds between the deposited
has excellent properties such as chemical inertness, thermal oxide and the PTFE substrate, as no active functional groups
1
stability, low dielectric constant, and low water absorption. PTFE are available on the PTFE surface to initiate a ligand exchange
can sustain harsh environmental conditions. These properties type ALD process. In addition, it is impossible to cleave the
render PTFE an interesting material for corrosion protective covalent C–F bond during a thermal ALD process using water
coatings, composite membranes in fuel cells, microelectronic vapour. Therefore, thermal ALD is likely initiated by physical
2
,3
packaging, and biomedicine and space applications. In spite adsorption of precursor vapour and/or by anchoring at defect
of this wide range of applications, the low surface energy of sites, leading to weak bonds at the oxide/PTFE interface.
PTFE presents a technical challenge because it leads to poor
This communication reports the successful synthesis of
adhesion with other materials (such as metals and ceramics, stable hydrophilic and glueable PTFE surfaces via plasma-
1
,4
and other polymers).
enhanced (PE-)ALD of Al O . In this process, the PTFE substrate
2 3
Plasma treatment is a well-known, relatively fast and easy, is sequentially exposed to oxygen plasma and trimethylaluminum
technology for modifying polymer surfaces, especially for (TMA) vapour. It is shown that the oxygen plasma breaks the C–F
1
,5
inducing wetting transitions and improved adhesion. A major bond, thus making active surface species that can react with TMA
5
,6
drawback is however the aging effect. The active polymer during the subsequent precursor pulse. In this way a strong
surface species created by the plasma often decompose or get covalent bond is established between the Al coating and the
removed over time during environmental exposure. As a conse- PTFE substrate. Also, because Al is an inorganic material that
2 3
O
2 3
O
quence, plasma-treated polymers may return to their original is stable in air, the PE-ALD approach does not suffer from aging
state in terms of surface composition and wettability. Oxygen effects like the traditional plasma treatments.
plasma generally induces hydrophilic effects on hydrophobic
Thermal and PE-ALD of Al O on planar PTFE substrates
2 3
polymers. However, for PTFE, it can also increase the hydro- were compared for different numbers of ALD cycles. The
2,7
phobicity. Careful optimization of the oxygen plasma operating depositions were performed as explained in the ESI.† Energy
parameters is therefore essential for the realization of a hydro- dispersive X-ray (EDX) spectroscopy of the aluminium content
8
philic effect on PTFE substrates.
2 3
revealed that the amount of Al O deposited by PE-ALD was
much higher than by thermal ALD (Fig. 1a). Possible explanations
for this are a higher deposition rate on PTFE for PE-ALD
compared to thermal ALD, as also observed on a Si reference
wafer (0.13 vs. 0.10 nm per cycle), and/or a faster nucleation of
the PE-ALD process on PTFE.
a
Department of Solid State Sciences, Faculty of Sciences, Ghent University,
Krijgslaan 281/S1, B-9000 Gent, Belgium.
E-mail: Christophe.Detavernier@UGent.be
b
Centre for Polymer and Material Technologies, Faculty of Engineering and
Architecture, Ghent University, Technologiepark 915, B-9052 Zwijnaarde, Belgium
The native hydrophobic PTFE substrates had a water contact
angle of 1031 (Fig. 1b, 0 cycles). Treatment with oxygen plasma
†
Electronic supplementary information (ESI) available: Experimental section.
See DOI: 10.1039/c4cc09474c
3
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Fig. 2 Demonstration experiment on the ‘‘glueability’’ of uncoated and
PE-ALD coated PTFE sheets. (a) Schematic representation of the experiment
geometry. (b–d) The uncoated PTFE sheet peels off during filling of the
plastic bottles with blue-coloured water, while the strong adhesion between
the PE-ALD modified PTFE sheet and the aluminium plate allows complete
filling of the bottle.
sheets were glued on an aluminium plate using a commercial
adhesive (Loctite 401 from Henkel) as indicated in Fig. 2a.
Empty 1.5 liter plastic bottles were fixed to the PTFE sheets and
slowly filled with coloured water. While the uncoated PTFE
sheet peeled off from the aluminium plate when approximately
1/4 of the bottle was filled with water (Fig. 2c), the adhesion
between the PE-ALD modified PTFE sheet and the aluminium
plate was strong enough to allow the complete filling of the
plastic bottle (Fig. 2d). This experiment illustrates the significant
improvement in ‘‘glueability’’ of the PTFE substrate induced by
the PE-ALD treatment.
To quantify the adhesion strength, peeling tests were carried
out using an Instron 5565 set up, and the obtained peeling
2
forces are presented in Fig. 3. PTFE samples of 20 Â 20 mm
were glued to an aluminium block using the above-mentioned
commercial adhesive. The peeling tests were performed at an
angle of 1801 using a speed of 0.5 mm s . As reference, a
À1
Fig. 1 (a) EDX intensity of aluminium against the number of (PE-)ALD
cycles before and after Scotch tape adhesion test. (b) Water contact angle
against the number of (PE-)ALD cycles or plasma exposures.
Scotch tape was attached to the aluminium plate and the peel-off
force was determined using the same peeling angle and speed.
The peel-off force for the uncoated PTFE sheets was 0.90 Æ
0
.40 N whereas for Scotch tape a value of 3.75 Æ 0.55 N was
pulses (without intermediate TMA pulses) increased the contact
angle to 1111 due to an even stronger hydrophobic behaviour.
In contrast, when the oxygen plasma pulses were alternated with
TMA exposures in PE-ALD, the water contact angle decreased
and a steady value of 191 was reached after 200 PE-ALD cycles,
proving that the PTFE surface became hydrophilic. After
measured. For the thermal and PE-ALD treated PTFE sheets,
800 cycles of thermal ALD, the water contact angle was only
reduced to 401, indicating that PE-ALD is more effective to
induce a hydrophilic effect on PTFE. The hydrophilic thermal
and PE-ALD samples were stable upon storage for 6 months in
ambient air.
2 3
The adhesion of the Al O coatings to the PTFE surface was
evaluated by the Scotch tape test. The thermal ALD layers did
not pass the test as most of the aluminium content was
removed (Fig. 1a). This result is in agreement with the previous
9
work by Kemell et al. In contrast, the PE-ALD deposited Al O
2
3
coatings remained intact after removal of the Scotch tape,
allowing us to conclude that the coating/substrate adhesion
was significantly improved by using a plasma-activated process.
The ability for gluing of native PTFE and PE-ALD modified
Fig. 3 Average maximum load in peeling tests against the number of
(
PE-)ALD cycles. The values obtained for uncoated PTFE and Scotch tape
PTFE was compared in a demonstration experiment. Both PTFE as reference are also indicated.
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À1
were observed around 1155 and 1215 cm (bottom Fig. 4c). The
À1
relatively sharp peak around 1155 cm can be ascribed to CF
2
À1
symmetric stretching, while the broad peak around 1215 cm is
1
0
À1
due to asymmetric stretching. The weak peak at 637 cm is
related to the wagging vibration mode of C–F bonds. The effect
of an oxygen plasma pulse on the PTFE surface was evaluated by
subtracting the FTIR spectrum of the native PTFE sample from
the spectrum measured on the treated surface. This so-called
difference spectrum showed negative absorbance for both the
positions of stretching and wagging vibration modes (top
Fig. 4c). This result indicates the dissociation of C–F bonds
during the oxygen plasma pulse and explains the superior
properties of the PE-ALD treated PTFE samples.
In conclusion, this work reported PE-ALD as an easy and
elegant method to functionalize PTFE substrates. While oxygen
plasma pulses resulted in an increased hydrophobic behaviour,
alternating pulses of oxygen plasma and TMA resulted in an
2 3
Al O coating firmly attached to the PTFE substrate, inducing a
stable hydrophilic effect. Because the degree of hydrophilicity
increased with the number of deposited cycles, the presented
PE-ALD approach allows for the controlled tuning of this surface
property. Furthermore, an average peeling strength higher than
À1
600 N m
was measured between aluminium and PE-ALD
modified PTFE, using a commercial adhesive. This represents
a more than 10-fold increase compared to native PTFE samples.
This work was supported by the European Research Council
Fig. 4 (a) SEM image of an Al
ALD cycles. An uncoated region is indicated. (b) SEM image of an Al
2
O
3
layer deposited on PTFE using 400 thermal
layer
2 3
O
deposited on PTFE using 300 PE-ALD cycles. (c) FTIR spectrum for native (ERC starting grant no. 239865), by the Special Research Fund
PTFE (bottom) and difference spectrum after oxygen plasma treatment (top).
BOF of Ghent University (GOA 01G01513), and by the Research
Foundation – Flanders (FWO). J.D. is a postdoctoral fellow of
we obtained values of 1.66 Æ 0.51 N and 12.60 Æ 0.91 N, the FWO.
respectively. The PE-ALD treatment thus resulted in a significant,
ca. 14-fold, increase of the PTFE/superglue/Al adhesion strength.
Scanning electron microscopy (SEM) images of the thermal
and PE-ALD coated PTFE substrates are presented in Fig. 4a
Notes and references
1
E. T. Kang and Y. Zhang, Adv. Mater., 2000, 12, 1481.
and b, respectively. The Al O layer obtained via the PE-ALD
2 N. Vandecasteele, B. Nisol, P. Viville, R. Lazzaroni, D. G. Castner and
2
3
F. Reniers, Plasma Processes Polym., 2008, 5, 661.
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process had a granular structure suggesting island type growth
Fig. 4b). During this process, nucleation likely started on
3
(
2
various spots of the substrate due to the random activation of
the PTFE surface by the oxygen plasma radicals. The thermally
deposited Al O layer appeared smooth in SEM, although large
2 3
(c) G. Subodh, C. Pavithran, P. Mohanan and M. T. Sebastian,
J. Eur. Ceram. Soc., 2007, 27, 3039; (d) A. I. Cassady, N. M. Hidzir
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4
uncovered parts, as confirmed by local EDX measurements,
were also observed (Fig. 4a). These uncoated regions possibly
originated from damage of the coating during sample handling
after deposition (given the poor adhesion of the Al O layer for
5
6
2
3
the thermal ALD process).
In situ FTIR spectroscopy was carried out to elucidate the
faster nucleation and improved adhesion strength of the PE-ALD
modified PTFE sheets. Spectra were recorded on 60 mm thick
porous PTFE substrates in transmission mode using a Vertex 70v
8 S. Zanini, R. Barni, R. D. Pergola and C. Riccardi, J. Phys. D: Appl.
Phys., 2014, 47, 325202.
9
M. Kemell, E. F ¨a rm, M. Ritala and M. Leskel ¨a , Eur. Polym. J., 2008,
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4
set up from Bruker. For native PTFE, two typical strong peaks 10 W. D. Wilde, Thin Solid Films, 1974, 24, 101.
3
558 | Chem. Commun., 2015, 51, 3556--3558
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