22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Pulsed plasma deposition of N-vinylcaprolactam a robust way to thermoresponsive thin films
M. Moreno-Couranjou1, F. Palumbo2, E. Sardella2, G. Frache1, P. Favia3 and P. Choquet1
1
CRP - Gabriel Lippmann, Science and Analysis of Materials department, LU-4422 Belvaux, Luxembourg
2
Istituto di Metodologie Inorganiche e dei Plasmi, (IMIP-CNR), via Orabona 4, IT-70126 Bari, Italy
3
Dipartimento di Chimica, Università degli Studi di Bari “Aldo Moro”, via Orabona 4, IT-70126 Bari, Italy
Abstract: Thermally responsive thin films have been obtained from N-vinylcaprolactam
(NVCL) fed low pressure plasma process. FT-IR, XPS, ToF-SIMS and MALDI-MS
analyses allowed to confirm the retention of the monomer cyclic structure and accurately
identify different oligomer distributions in the deposited film. The wettability switching
behavior of these smart surfaces is confirmed with WCA measurements.
Keywords: pulsed plasma deposition, thermally responsive, material characterization
1. Introduction
Thermally Responsive Polymers (TRP), such as
NIPAM, are known for their ability to undergo sharp
structural transition in response to temperature changes.
In particular below a critical temperature the polymer is
more hydrophilic than above. As an environmentally
friendly and scalable technique, plasma deposition is an
attractive solution. This technology presents also the
advantage to be a single-step, and substrate independent
route to achieve adherent and homogeneous films. The
plasma deposition of NIPAM thin films has been studied
both at low and atmospheric pressure. However, during
these last years, NIPAM limitations have been pointed out
mostly relying on its toxicity making compulsory an
extensive purification of the materials.
In this work, as an alternative solution to wet chemistry
methods and NIPAM use, N-vinylcaprolactam (NVCL)based thin films have been obtained by means of pulsed
plasma
deposition.
Conventional
poly(Nvinylcaprolactam) (PVCL, Fig. 1), known as a thermoresponsive polymer, is considered suitable for biomedical
applications and proven to be biocompatible. A pulsed
plasma approach has been followed since it is known that
this operation mode can allow a better control of the
polymer structure.
Fig. 1. Chemical structure of N-vinylcaprolactam.
2. Experimental
The coatings were prepared in a home-made parallelplate capacitive reactor. The reactor consists in a
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stainless-steel chamber equipped with two flat electrodes
(150 mm in diameter), 30 mm apart. The upper electrode
with a shower configuration for gas feeding was
connected to a 13.56 MHz radiofrequency power supply
through an impedance matching unit. The substrates
(polished silicon wafers) were placed on the bottom
ground electrode. Process lines and the plasma chamber
were heated at 60 °C to avoid condensation. 5 sccm
argon were added to the feed. The plasma discharge was
run at the pressure of 100 mTorr and 50 W RF power in
pulsed mode alternating ON time in the range 1 - 20 ms
and OFF time between 9 and 180 ms. The coating
morphology was observed, after chromium metallization
by sputter coating, with a Zeiss SUPRA 40 FEG SEM.
Film chemical composition was investigated by Fourier
transform infrared and XPS analysis. Molecular structure
present in the films is investigated by static secondary ion
mass spectrometry and by Atmospheric Pressure - Matrix
Assisted Laser Desorption and Ionization coupled to a
High-Resolution Mass Spectrometer. Static water contact
angle measurements were carried out on a Rame-Hart 100
goniometer equipped with a home-built heating stage
module allowing an accurate control of the temperature in
the 25 - 60 °C range.
3. Results and discussion
Thermally responsive thin films from NVCL were
successfully deposited by pulsed plasma deposition at
50 W with an ONtime of 1 ms and OFFtime of 50 ms.
According to scanning electron microscopy the coating
is pinhole-free and covered homogenously the surface.
By increasing the temperature and measuring the static
water contact angle (WCA) the phase transition of an
NVCL-based plasma polymer was observed. Fig. 2
reports the temperature effect on WCA measurements.
Increasing the substrate temperature from 20 to 44 °C
lead to a continuous WCA increase from 7 to 27°. In
parallel, the LCST of the coating can be estimated to be
around 31 °C, which matches well with observations on
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conventionally polymerized NVCL.
Fig. 2. Temperature dependence of the WCA on the
pulsed plasma polymerized N-vinylcaprolactam.
According to the FT-IR analysis (Fig. 3), filmscontaining amide groups are formed as it is suggested by
the presence of the peaks at 1622 and 1478 cm-1 related to
the C=O and C-N stretch respectively. A very low
intensity-peak around 1730 - 1700 cm-1 points out to the
presence of carboxylic acid, ketone or aldehyde groups.
Indeed, according to Fig. 3, it can be inferred that at
10 ms of ON time, the carbonyl band becomes broader as
the OFF time is decreased. In particular for a OFF time
of 180 ms ketone and acid absorption bands are barely
visible at 1708 and 1726 cm-1, respectively. Finally, it
has been observed that, despite pulsing the discharge,
working with a 100 W peak power induced a strong
monomer fragmentation as the FT-IR spectra present a
broad peak around 1730 cm-1.
To get some insights into the surface chemistry of the
deposited film, XPS analyses were carried out confirming
nitrogen content. In accordance with FT-IR results, the
XPS C1s signal shows 5 contributions: i) hydrocarbon
(CHx) at 285.0 eV, ii) secondary shift carboxyl
(C-COOH) or secondary shift amide (C-C(O)-N) at
285.8 eV, iii) alcohol or ether (C-OH/R)) at 286.6 eV,
iv) ketone, aldehyde (C=O) and amide (O=C-NHxR3-x)
at 287.8eV, and v) acid, ester (COOH (R)) at 289.1 eV.
The analysis of the coating by ToF-SIMS led to the
detection of a significant signal from the protonated ion
[C8H13NO+H]+ corresponding to the monomer at
m/z = 140.12. Hence, this signal likely indicates that the
surface contains a significant amount of intact monomer,
whose structure has not been damaged during the plasma
polymerization process. Interestingly, peaks at m/z
259.17, 398.27 and 537.37 have also been detected. The
mass difference of 139 between each fragment suggests
the formation of monomer-based oligomers.
Concerning AP-MALDI-HRMS analysis, the spectra,
reported in Fig. 4, reveal the presence of an ion at
m/z = 140.10, related to the monomer, along with several
distributions of oligomers presenting the general formula
[R-(C8H13NO)n-R’ + X]+ with R and R’ being the end
groups and X being the charge carrier ( proton or sodium
adduct).
Fig. 4. AP-MALDI-HRMS spectra of the pulsed plasma
polymerized N-vinylcaprolactam.
Fig. 3. Normalized FT-IR spectra of ppVCL films
deposited at 50 W in continuous mode and for different
pulsed discharges presenting a 10 ms ton and toff values
ranging from 50 to 180 ms, and a ppVCL film deposited
at 100 W in a 10:90 ms pulsed mode.
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Hence,
the
presence
of
the
[C8H8N(C8H13NO)nH+H]+ oligomer series (with n up to 3) in
the film was confirmed as it is shown in the spectra with
the corresponding peaks labeled An (1 < n < 3).
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Furthermore, two other main oligomer distributions were
detected with high intense signals: i) the
[H-(C8H13NO)nH+H]+ oligomer distribution with
2 < n < 6 (i.e., Bn peaks in Fig. 4), and ii) the
[H-(C8H13NO)n-OH+H]+ oligomer series with 2 < n < 5
(i.e., Cn peaks in Fig. 4). Further MS/MS experiments
were carried out on different ions from the A, B and C
families. Systematically, a fragment ion corresponding to
a neutral loss of 113.084 amu and related to a loss of
caprolactam ring (C6H11NO) was detected. This result
clearly highlights that an efficient NVCL plasma
deposition through the vinyl bond was achieved with
retention of monomer cyclic structure, and thus
responsible for the TRP response of the coating.
AP-MALDI-HRMS results coupled with the other
collected data confirm that the structure of the deposited
coating can account for the continuous polymer phase
transition reported in Fig. 2.
4. Conclusions
In conclusion, N-vinylcaprolactam appears as an
innovative and interesting alternative monomer to
NIPAM for the plasma deposition of a smart thermoresponsive thin film.
It has been shown that a
polymerization through the monomer vinyl bond can be
ignited and controlled with specific cold plasma
conditions as different oligomer distributions-containing
intact monomer as repeated unit have been detected in the
film. Furthermore, the combination of ToF-SIMS with
AP-MALDI-HRMS appears as a powerful analytical
method that provides significant information about the
chemical structure of the plasma polymerized film
surface.
5. Acknowledgements
The authors would like to thank the Luxembourgish
‘Fonds National de la Recherche’ (FNR) for financial
support through the THERMOFILM project.
In
particular, Dr M. Moreno-Couranjou wishes to thank the
FNR for supporting her visiting scientist position at the
University of Bari (Italy). The Regione Puglia is also
acknowledged for funds provided through the "Reti di
Laboratorio" project "Apulian Industrial Plasma Lab”
(LIPP). The authors wish to thank Savino Cosmai for its
skills and valuable technical assistance.
Finally,
Dr J. Guillot and Dr. N. Desbenoit are hereby gratefully
acknowledged for XPS and ToF-SIMS analyses,
respectively.
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