22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Deposition of amine-rich stable organic coatings by plasma co-polymerisation of
ethylene, 1,3-butadiene and ammonia mixtures
M. Buddhadasa and P.-L. Girard-Lauriault
Plasma Processing Laboratory, Department of Chemical Engineering, McGill University, Montreal, Quebec, Canada
Abstract: It is established that amine-rich films play an important role in influencing cell
processes. Yet, the high surface energy of such films leads to instability in aqueous media an important criterion for bio-applications. We investigate the plasma co-polymerisation of
ethylene, 1,3-butadiene and ammonia mixtures in order to produce water stable, crosslinked films containing high concentrations of nitrogen functional groups.
Keywords: plasma polymer films, stability, amine groups, XPS, chemical derivatisation
1. Introduction
Synthetic polymers are now widely being used in a
number of biomedical applications, such as cell-culture
plates, vascular prosthesis and drug delivery systems.
However, the surfaces of these biomaterials can be
chemically relatively inert in essence having limited
interactions with the surrounding biological environment.
Depositing plasma polymer films is an efficient and
versatile method which allows the surface functionality of
these biomaterials to be modified and tuned in order to
obtain an improved cell-surface interaction.
Plasma polymerisation is a well-established method by
which polymer-like coatings with high and adjustable
densities of chemical functional groups can be readily
deposited on a variety of substrates. By creating such
functional surfaces, one can gain some control over the
interactions between a surface and surrounding cells and
biomolecules. Nitrogen based functional groups, mainly
amines, are considered to play an important role in
influencing cell processes [1, 2]. This is often attributed to
their associated positive charges that, in aqueous solutions
at physiological pH values, attract negatively charged
biomolecules such as proteins, and living cells [1]. High
surface amine content is therefore considered desirable,
but at the same time, most biomedical applications require
long-term stability in aqueous media. While the time
dependent chemical stability to air exposure ("ageing") of
plasma polymer films has been discussed significantly,
water stability, in terms of thickness loss and changes to
surface chemistry, is an area which has been the object of
relatively little attention [3-8].
Several authors have shown that higher the
concentration of amine groups, higher is the solubility of
the coating in water [8-10]. This is attributed to the high
surface energy of amine-rich coatings that lead to strong
affinity towards polar solvents [8]. Work has been
previously conducted to study the stability of coatings
deposited by plasma polymerisation of ethylene and
nitrogen or ammonia mixtures. However, these coatings
still exhibit water instability at high amine concentrations
[11].
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In this study, we investigate a series of thin plasma
polymeric coatings deposited using a mixture of ethylene,
1,3-butadiene and ammonia gases in order to improve the
stability of films containing high concentrations of amine
groups. The butadiene molecule consists of two C-C
double bonds which should facilitate the formation of
highly cross-linked, stable, organic macromolecule. We
chose to focus our study on the effect of the following
plasma parameters: (i) plasma power and (ii) gas mixture
ratio "R" (F NH3 :F HYDROCARBON ), where 'F' stands for flow
rate and the hydrocarbon is either ethylene, butadiene or a
mixture of both, (iii) hydrocarbon (HC) gas flow ratio
"R HC " (F C4H6 :F C2H4 ), on the co-polymerised ethylenebutadiene based films. In all instances, the effect of
plasma parameters on the surface chemistry (i.e. nitrogen
concentration, [N] and amine concentration, [-NH 2 ]) and
water stability of the coatings was investigated. The
development of an empirical model of the effect of
plasma conditions on both the amine content and water
stability will enable the optimisation of a compromise
between these two factors.
2. Experimental Methods
Plasma polymer films were deposited on silicon wafers
using Plasma Enhanced Chemical Vapour Deposition
(PECVD). The substrates, 1cm x 1cm squares of 500 μm
thick (100) p-type silicon wafers, were thoroughly
cleaned ultrasonically in isopropanol prior to deposition.
A cylindrical steel vacuum chamber, approximately 20cm
in diameter and 50cm in height was used to deposit the
coatings. A turbo-molecular pump, backed by a two-stage
rotary vane pump was used to evacuate the chamber to
high vacuum. Gases were then introduced to the chamber
using electronic mass flow controllers, through a "shower
head" gas distributor (~10cm in diameter). The operating
pressure, during plasma deposition was set constant at
80Pa by a throttling gate valve, in combination with a
capacitance
pressure
gauge.
The
low-pressure
capacitively coupled r. f. (13.56 MHz) glow discharge
was generated via an impedance matching network
connected to a 10cm diameter powered electrode in the
1
centre of the chamber, with the walls of the chamber
acting as the grounded electrode. The distance between
the bottom of the shower head and the powered electrode
was ~4cm.
Plasma process parameters for deposition of various
coatings are listed in Table 1. Here, "PPE:N" and
"PPB:N" correspond to N-rich Plasma Polymerised (or
Co-polymerised in the case of "PCEB:N") Ethylene and
Butadiene based films. "F T " refers to the total flow rate of
hydrocarbons.
Table 1. Plasma process parameters
10W
PPB:N
F HC = 5sccm
F NH3 = 5, 10, 15, 20
(R = 1, 2, 3, 4)
PPE:N
PCEB:N
F T = 10sccm
F NH3 = 40sccm
(R = 4)
R HC = 2:8, 4:6, 5:5,
6:4, 8:2
20W
F HC = 10sccm
F NH3 = 10, 30, 50
(R = 1, 3, 5)
F HC = 10sccm
F NH3 = 10, 20, 40
(R = 1, 2, 4)
R HC = 2:8, 5:5, 8:2
X-ray Photoelectron Spectroscopy (XPS) analyses were
performed at most 30h after deposition, in a Thermo
Scientific
K-Alpha
XPS
instrument,
using
monochromated Al Kα X-rays, producing photons of
1486eV. Wide scans with step size 1eV, pass energy
160eV, dwell time 200ms and in the range 1200 to -10eV
were acquired for each sample.
A selective derivatisation procedure with 4Trifluoromethylbenzaldehyde (TFBA) followed by XPS
analysis was performed no later than 25h after deposition,
to determine the surface concentration of primary amines.
Here, the TFBA is covalently linked via imine bonds and
thus, the [-NH 2 ] can be calculated using the fluorine
concentrations, [F], obtained by XPS, using the formula
presented in Truica-Marasescu et. al. [1]. The reaction
was carried out in a small glass jar into which 4 to 5 drops
of TFBA were dripped onto a ~1cm thick layer of glass
beads. The samples were placed on a microscope slide,
which was then placed on the layer of glass beads,
thereby avoiding direct contact with the TFBA liquid. The
glass jar was then flushed with argon and placed in an
oven at 45˚C for 3h, allowing sufficient time to convert
all near-surface amine groups [1, 11, 12]. It is noteworthy
that recent work strongly suggests that TFBA is selective
not only towards primary amines but also reacts with
imine groups on the coating [13]. Imines are usually
generated during plasma polymerisation due to high
fragmentation of monomer molecules and subsequent rearrangement that occur under high plasma powers.
Studies also show that N-H groups are the dominant
functionality and that C=N accounts to only ~1/5th of
nitrogen functional groups formed at low plasma powers
2
such as 25W [14]. Since in our study, films were
deposited under mild plasma conditions (10W and 20W)
it can be assumed that the concentration of amines formed
during deposition was more significant compared to that
of imine groups formed. Moreover, surface charge studies
conducted by Babaei et. al. [15] from this laboratory,
show that an increase in R leads to a clear increase in
positive charge on these films. Therefore, it can be safely
assumed that amine groups being responsible for this
positive charge contribute to a significant portion of N
functional groups that are present on the surface.
Stability was evaluated as the percentage loss in film
thickness after immersion in reverse osmosis water for
24h. Here, the coating deposited on the silicon wafer was
scratched down to the substrate, and the resulting step
height (thickness) was measured using a Dektak
profilometer.
3. Results and discussion
Fig. 1 shows a comparison of the deposition rates for
PPE:N and PPB:N films, wherein it is evident that
butadiene based films have a higher rate of deposition
than ethylene based films. This can be attributed to the
presence of two C-C double bonds in the butadiene
molecule, which by dissociation of the pi bonds more
easily react in the plasma polymerisation process, and
also that butadiene contains twice as much carbon as
ethylene for the same volumetric flow rate. As reported
previously [1, 7, 8], the deposition rate, r, of PPE:N's
decreased with increase in R and a similar trend was
observed with the PPB:N films. It is suggested that as the
[NH 3 ] in the plasma is increased, ammonia which acts as
an etchant for organic materials [16, 17], promotes
"ablation" (chemical sputtering), thereby resulting in a
decrease in r [8]. However, the higher deposition rates of
butadiene compared to ethylene based films, even at high
R values such as 5, can be considered an advantage of
PPB:N films.
Fig. 2 shows the [N] of PPE:N and PPB:N films as a
function of R. Nitrogen content for both film types
increased with increase in R as previously reported for LPPE:N films [1]. Yet, it is evident that ethylene based
films contained almost as four times more N as butadiene
based films for coatings deposited at the same R value.
Nevertheless, from the [-NH 2 ] values, shown by Fig. 3,
one can see that despite the significant difference in [N],
the [-NH 2 ] of both types of films seem to be comparable.
For example, in Fig. 3, the PPE:N deposited at 20W
(denoted by blue squares) and R=4, and the PPB:N
deposited at same power (denoted by blue spots) and R=5
have approximately the same [-NH 2 ] of 5.7%. The same
two films, observed in Fig. 2, show [N] of 42 and 17%,
respectively. This observation indicates that butadiene
based films have higher amine selectivity, [NH 2 ]/[N], as
compared to ethylene based films. Furthermore, the same
films described above, observed from Fig. 4,
demonstrates that the PPE:N was completely unstable in
water, whereas the PPB:N had a thickness loss of only
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~15%. This can be attributed to the more unsaturated
structure of butadiene which has led to a high degree of
cross-linking and polymerisation in the plasma polymer
deposited.
Fig. 3. Amine concentration (at. %), determined by
chemical derivatisation XPS, as a function of gas mixture
ratio, at 10W and 20W for PPE:N and PPB:N films.
Fig. 1. Deposition rate (nm/min), r, as a function of gas
mixture ratio for PPB:N and PPE:N films.
Fig. 4. Percentage loss in film thickness as a function of
gas mixture ratio for PPB:N and PPE:N films at 10W and
20W.
Fig. 2. Nitrogen concentration (at. %), determined by
XPS, as a function of gas mixture ratio at 10W and 20W
for PPE:N and PPB:N films.
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Fig. 5. Amine concentration (at. %) as a function of
hydrocarbon gas flow ratio, at 10W and 20W, with R=4
and F T =10sccm for PCEB:N films.
3
Fig. 6. Percentage loss in film thickness as a function of
hydrocarbon gas flow ratio at 10W and 20W, with R=4
and F T =10sccm for PCEB:N films.
The novel butadiene based films deposited have,
henceforth, proved to behave differently to those
deposited from ethylene. This was a motivation to next
investigate the effect of plasma co-polymerising these two
hydrocarbons in presence of ammonia in the hope of
obtaining coatings with both high [N] and high stability in
aqueous media. Fig. 5 and 6 show results of PCEB:N
films as a function of R HC . As the concentration of
butadiene in the plasma (or R HC ) was increased, as shown
in Fig. 5, a decrease in [-NH 2 ] was observed, suggesting
that butadiene hinders the incorporation of amine groups
in the coating. This can be due to a higher requirement of
activation energy which could be related to resonance
stabilisation of conjugated double bonds in the
carbocation of the 1,3-butadiene molecule [18]. An indepth study of the reaction chemistry in the plasma must
be done to arrive at a more detailed explanation for this
observation. A higher concentration of amines was
observed at 20W than at 10W. This suggests that for the
given pressure and gas flow rate, 10W was not sufficient
to consume all the available monomers [19] to incorporate
a maximum amount of amine groups into the film. The
stability results of PCEB:N's, shown in Fig. 6, re-confirm
that the addition of butadiene to the plasma, owing to its
more unsaturated structure, improves the stability of the
resulting films. The negative loss in film thickness is
considered as a result of swelling.
4. Conclusions
Ethylene and butadiene based films show a similar
behaviour in response to variation of plasma parameters:
power and R. In both cases, deposition rates decrease with
increase in R and higher deposition rates at higher power
were observed. Nitrogen content increased with R for
both films, but ethylene based films clearly incorporated
much more nitrogen than the ones deposited from
butadiene. Nevertheless, PPB:N films show better
selectivity toward amine groups and yet retain a good
stability in aqueous media. Plasma co-polymerisation of
both hydrocarbons has led to a series of films which reconfirm the effects of butadiene in the properties of the
coatings deposited.
Use of 1,3-butadiene as a hydrocarbon in the plasma
polymerisation process has indeed led to more stable
films compared to films deposited utilising only ethylene
as the hydrocarbon gas. The optimum films were found to
be a PPB:N and a PCEB:N film which had [-NH 2 ] of
7.2% and 5.6% respectively, with no loss in film
thickness. The amine content values obtained were not
considerably better than those previously published by
Ruiz et. al. [11], however this can be attributed in the
well-acknowledged significant inter-laboratory variability
in plasma related processes and analysis methods.
Nevertheless butadiene containing films clearly
4
performed better than ethylene based films within the
scope of our study and the marked increase in stability of
those films renders them worthy candidate for the
development of surface engineering solutions for
biomedical applications.
5. Acknowledgements
This research is being supported by grants from Natural
Sciences and Engineering Research Council of Canada
(NSERC), Fonds de Recherche Nature et Technologies
Quebec (FQRNT), Plasma Quebec and the McGill
Engineering Doctoral Award (MEDA).
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