22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Functionalisation of octa-methyl polyhedral oligomeric silsesquioxane (POSS)
by sequential continuous wave and pulsed plasma mode treatments
X. Chen, Z. Chen, L.F. Dumée, X.J. Dai and K. Magniez
Institute for Frontier Materials, Deakin University, AU-3216 Waurn Ponds, VIC, Australia
Abstract: The functionalisation of polyhedral oligomeric silsesquioxane (POSS) with
nitrogen containing functionality is demonstrated for the first time by sequential
Continuous Wave (CW) followed by Pulse (P) plasma mode. Here we show that
octa-methyl POSS can be functionalised in a nitrogen/hydrogen plasma, introducing
terminal nitrogen rich species such as amine and amide. As opposed to wet chemical
functionalisation approach, the plasma method does not require reactive chemicals and is
much more environmental friendly.
Keywords: polyhedral oligomeric silsesquioxane (POSS), plasma functionalisation
1. Introduction
Polyhedral oligomeric silsesquioxane (POSS), a
material that can combine the feature of both organic and
inorganic materials in a silicon cubic cage nano-structure
[1], has been extensively used for a wide range of optical
and electrical, aerospace, biomaterial and nanocomposites applications [1, 2]. However, the synthesis of
reactive functional POSS is generally lengthy, costly and
requires environmentally unfriendly processing routes [2].
Physical plasma treatments are relevant candidates to
design green, up-scalable and efficient functionalisation
systems. Plasma surface functionalisation of carbon
nanotubes and more recently, boron nitride nanotubes has
already been reported [3-5]. Here we extend on this prior
work and this project looks at surface plasma treatment of
octa-methyl POSS with N 2 and H 2 gas mixtures in order
to increase the density of nitrogen functional groups
across the surface of the cage of the POSS materials.
Here, octa-methyl POSS was chosen as a demonstration
compound for its symmetrical and simple structure and
since it is one of the cheapest form of POSS available.
Continues Waves (CW) plasma followed with Pulse (P)
plasma mode were used following a design by Dai et al.
was considered in this project [6], and the resulting
samples were tested by Fourier Transform Infrared
spectroscopy (FTIR), X-ray Photoelectron Spectroscopy
(XPS), and Scanning Electron Microscope (SEM) to
assess the physical and chemical modification across the
POSS for a range of plasma conditions.
2. Experimental method
Octa-methyl POSS materials were purchased from
Hybrid Plastics Inc. The large sized octa-methyl POSS
crystals were first air jet milled to a process finer powders
using a 2 inch Sturtevant micronizer with 620.5 kPa
pressure. The POSS powder was dispersed in acetone (10
g/L) and deposited 4 -5 drops on a cleaned 1.0 cm2 silicon
wafer and dried in an oven at 50 ℃ for 1 min.
P-II-7-4
The POSS particles deposited on the silicon wafers
were plasma treated using plasma polymerization system
with the RF generator (Kurt J. Lesker Co., USA) and the
matching network (EJAT6, Kurt J. Lesker Co., USA) to
generate both continuous (CW) and pulsed (P) plasma
mode with 100% of the RF power transferred to the
plasma. The power was varied between 80 W, 100 W and
130 W. Plasma mode was using CW + P which was
developed by Dai et al [6].
And other conditions such as gas inlet, vacuum
pressure, treatment time, and duty cycle of Pulse plasma
were fixed as N 2 /H 2 gases, 8.0 × 10−2 mbar, CW for 1
min, P plasma for 10 minutes, duty cycle was set as 10%.
Argon gas was used for pre-treatment (100 W CW plasma
for all samples) and purging before and after plasma.
The resulting samples were tested by FTIR Bruker
VERTEX 70 spectrophotometer. Spectra displayed by
transmission mode of KBr discs and a resolution of 4 cm-1
with 128 scans. XPS of the reference and plasma treated
POSS was carried out using a K-Alpha X-ray
photoelectron spectrometer from Thermo Fisher
Scientific. 4-(trifluoromethyl) benzoic anhydride (TFBA)
was used to label the primary amine group introduced
across the samples through the terminal CF 3 groups easily
traceable with XPS analysis [3]. A number of 1 to 2 drops
of TFBA was vaporized to react with the treated samples
for 2 h at room temperature in a sealed vessel. The
amount of primary amine then was calculated as follows:
[NH2 /C]% = �
[F]
3
×
1
[C]−
� × 100
8[F]
3
(1)
SEM was used for surface characterization of the
sample before and after plasma treatment. The surface of
the samples was gold coated with a 3-4 nm thick gold
coating by sputtering to avoid electrical charging during
the milling operations. Micrographs of the particles were
acquired on a JEOL 7800F SEM with an accelerating
1
voltage of 5 kV and a current of 87 pA. The working
distance was 12 mm.
3. Results and discussion
The morphology of the POSS powders across the
different steps of the treatment was first investigated by
SEM. The air jet milling process reduced the size of
particles from 60 µm average to approximately 1 µm to
100 nm (Fig. 1 a and b). The size distribution of the
particles after plasma treatment was however found to be
comparable to that of the samples after air milling
suggesting that no further physical degradation may occur
through the plasma treatments (Fig. 1 c). This result
indicates that the cage structure is likely to be retained at
the microscopic level at least after plasma treatment.
Fig. 1. SEMs of (a) the as purchased octa-methyl
POSS crystal with molecular structure, (b) the
pristine POSS after air jet milling and, (c) the air
jet milled samples after a 100 W plasma
treatment.
XPS was used in quantitative analysis of elements and
identification of the functional groups in the material
surface. As seen across the XPS spectra shown in Fig. 2,
the silicon and oxygen bands obtained across the POSS
materials before and after plasma treatment appear
unchanged which the peak for both control and treated in
Fig. 2 a maintained same characteristics. This signifies
that the Si – O and Si – C bonds were not affected by the
plasma [7] and additionally, O1s spectra present same
characteristics for both control and treated sample, which
(a)
(b)
2
P-II-7-4
affects the concentration of amine groups. Moreover, the
N1s band indicate the presence of nitrogen containing
species after plasma treatment in Fig. 2 b, while the
control sample does not appear any peak in N1s, but
treated sample contains a clear peak for supporting the
presence of nitrogen species. Thus, there were nitrogen
containing functional groups introduced in the sample
surface after plasma treatment.
The reactivivity of the derivatization reaction between
the plasma treated POSS and the TFBA was then assessed
in order to determine and quantify the primary amine
(a)
groups (-NH 2 ) from the surface of plasma treated sample.
During the derivatization reaction with TFBA, the NH 2
groups were converted to imine C=N-C groups after
elimination of the terminal –C=O across the TFBA. The
amount of primary amine groups were therefore assessed
by the other terminal CF 3 in TFBA in XPS. As seen in
Fig. 2 c and for the 80 W plasma treated samples react
with TFBA, the C1s band is composed of peaks at 296.5
eV and 293.5 eV which can be attributed to the C-F and
C-F 3 groups. This provide evidence that TFBA was
covalently linked with the primary amine groups present
across the plasma treated samples. The primary amine
percentage was calculated from Equation (1) and,
resulting [NH2 /C] content of 1.7%.
Further chemical analysis by FTIR were performed to
confirm the XPS results. The spectra for the series of
(b)
samples are shown in the Fig. 3. The untreated
POSS
material should display absorption bands at 1075 – 1135
cm-1 and 780 cm-1 corresponding to the structural groups
from the POSS cage (Si-O-Si) [7, 8]. The spectra in Fig 3
present those two peaks which support the result from
XPS that the cage structure was well maintained.
Furthermore, new bands at 1630 cm-1 and 3400-3500 cm-1
are visible for
Fig. 2. XPS spectra for for control and 130W treated
samples: (a) O1s and Si2p; (b) C1s and N1s; (c) XPS
spectra for C1s after react with TFBA, sample was 80W
plasma treated
Fig. 3. FTIR for treated and untreated samples
provides supporting evidence that of the silicon cage
structure within the crystalline structure was not
physically damaged. The octa-methyl POSS powders
were treated by N 2 /H 2 gas plasma, and the bands for C1s
and N1s indicate the appearance of nitrogen rich species
(Fig. 2 b). As seen across the C1s band two peaks 285.4
eV and 288.9 eV are present after plasma treatment which
can be attributed to C – N (amine group), and CO – NH
(amide group) [3-5]. The amide groups’ formation were
due to the air exposure after plasma treatment which
P-II-7-4
the plasma treated samples which may be attributed to
presence of the amine and amide groups previously
discussed [9]. These again are in good agreement with
XPS results, showing that nitrogen rich species were
introduced in the sample after plasma.
4. Conclusions
The collective outcomes of the XPS, FTIR, and SEM
results provide evidence that the POSS core cage structure
3
was maintained after plasma treatment. XPS and FTIR
data demonstrated the presence of nitrogen containing
species formed across the materials’ surface after plasma
treatment,
thus
confirming
the
successful
functionalisation route. Furthermore, the amount of
nitrogen functional groups were identified primarily as
amines and amides groups. Last, the successful
functionalisation with TFBA, as a labelling agent, was
performed demonstrating that up to 1.7% of primary
amines were formed by using the combined CW and P
mode.
5. Future works
From those results, the functionalisation by plasma work
on octa-methyl POSS is technically possible which still
need conditions control. Hence, to extend the work, firstly
is to define a most efficient condition for introducing
amine groups, which treatment power, time, pressure,
duty cycle of Pulse mode are considered. After we find a
critical condition, the work will extend to stirring plasma
system which intend to produce a higher amount of
sample for functionalisation. Furthermore, using
alternative gases to have different functional groups
introduced such as carboxylic functionalities by using
oxygen gas could be achieved. Using other POSS is also a
consideration for the future work, because other POSS
like octa-phenyl POSS has much more usability such as
emitting layer of OLED (organic light emission device).
Moreover, the time stability of the groups in air and their
potential decay will be further examined in the future.
6. Acknowledgement
The authors acknowledge Xin Liu for FTIR training,
Marion Wright for technical support, Gayathri Devi
Rajmohan for XPS measurements in RMIT, and the XPS
facility at RMIT.
7. Reference
[1]
C. HartmannThompson, Editor. 2011, Springer:
Dordrecht. p. 1-420.
[2]
Cordes, D.B., P.D. Lickiss, and F. Rataboul,
Chemical Reviews, 2010. 110(4): p. 2081-2173.
[3]
Dai, X.J., et al., Plasma Processes and Polymers,
2009. 6(8): p. 490-497.
[4]
Chen, Z., et al., Plasma Processes and Polymers,
2012. 9(7): p. 733-741.
[5]
Chen, Z., et al., Composites Part A: Applied
Science and Manufacturing, 2014. 56(0): p. 172-180.
[6]
Li, L., et al., Plasma Processes and Polymers,
2009. 6(10): p. 615-619.
[7]
Garea, S.A. and H. Iovu, 2012, Wiley-VCH
Verlag GmbH & Co. KGaA. p. 115-142.
[8]
Ramírez, C., et al., European Polymer Journal,
2008. 44(10): p. 3035-3045.
[9]
Coates, J., 2006, John Wiley & Sons, Ltd.
4
P-II-7-4