Contribution of the cross-linking structure introduced on the surface
hardness of PMMA by low energy ion irradiation
J. Yokoyama, M. Yumoto, T. Iwao
Department of Electrical and Electronic Engineering, Tokyo City - University, Setagaya, Tokyo, Japan
Abstract: Contribution of the introduction of cross-linking structure on the surface
hardness improvement of PMMA by ion irradiation was studied. Relation between the
surface hardness and the quantity of bonding related to the cross-linking structure was
examined by FT-IR (Fourier transform infrared spectrometer) and XPS (X-ray
photoelectron spectroscopy). From these results, it was suggested that introduction of
the cross-linking structure contributed on the surface hardness improvement. It was also
confirmed that the distribution of cross-linking bonds in the direction of depth was
important on the improvement of hardness.
Keywords: PMMA, Ion irradiation, Surface hardness, Cross-linking structure
1. Introduction
PMMA (Poly methyl methacrylate) is a material
which has an excellent transparency, low
birefringence and excellent formability property [1].
However, hardness of PMMA is low. So the
transparency is lost by using for a long time [2].
Then, it is necessary to improve the surface hardness
without losing the excellent bulk properties by
surface modification. Bonds of polymeric materials
are cut by ion irradiation, and new chemical
reactions are introduced. As a result, the surface
hardness should be improved depending on the
species of ion and its energy [3]. The chemical
structure of PMMA is shown in Fig.1. Then, ion
irradiation method with low energy to form
modification layer of 100 nm or less without losing
the optical characteristic was used.
The surface hardness improvement of PMMA by
irradiating nitrogen ions of several 100eV has
already been confirmed as shown in Fig.2 [4].
However, modification of surface hardness
improvement is not clarified enough. There is a
Figure 1. Chemical structure of PMMA.
Figure 2. Change on surface hardness by changing acceleration
voltage.
report that cross-linking structure is introduced by
irradiating ions on the polymeric material [5] [6].
However, the relation between the surface
hardness and number of cross-linking bonds
introduced is not confirmed, because the judging
technique for number of cross-linking bonds without
destruction is not established. Then, the aim of this
study is to clarify the relation between the hardness
and the cross-linking bonds introduced.
In this study, to confirm introduction of crosslinking structure, chemical structure analysis by FTIR (Fourier transform infrared spectrometer) and
chemical composition analysis by XPS (X-ray
photoelectron spectroscopy) were performed. From
these results, the relation between the surface
hardness and the number of cross-linking bonds
introduced was studied. In addition, the distribution
of chemical bonds originating to the cross-linking
Figure 3. Ion irradiation system.
structure in the direction of depth by the take off
angle method was examined.
2. Experiments
Figure 3 shows the ion irradiation system. With
the electrodes wound around the circumference of
the chamber, the capacitive coupling type radio
frequency plasma of 13.56 MHz was generated. A
rotary pump and a turbo molecular pump were used
in order to exhaust below 1.3×10-4 Pa. N2 (purity
99.999 %) was used as an atmosphere gas and
processing atmospheric pressure was set at 1.3×10-2
Pa. Injected power was controlled at 25 W. PMMA
film which thickness is 0.2 nm was used as a sample.
Ion acceleration voltage was varied from 200 V to
1000 V. Exposure dose was adjusted at 1.0×1020
ions/m2 constant.
Surface hardness was measured by using the
nanoindentation method. A magnitude of surface
hardness is calculated by using 13 points which are
subtracted the maximum and minimum from 15
measured points, and the error bar shows a rootmean-square deviation. In order to confirm
introduction of the cross-linking structure were used
the ATR (Attenuated total reflection) method using
FT-IR and also XPS were used.
3. Results and discussion
3. 1. FT-IR measurement
Figure 4 shows the absorption spectrum of
PMMA. Spectrum of C=O (1720 cm-1), C-H (29992900, 1450- 1350 cm-1) and C-O (1270-990 cm-1)
which corresponds to a chemical structure of PMMA
Figure 4. IR spectrum by changing acceleration voltage.
which chemical structure is shown in Fig.1 were
obtained from an untreatment sample. It is observed
that side chains on polymer surface are cut off, and
radicals are formed by irradiating ions onto a sample.
It is expected that increase in C-C bond causing the
cross-linking structure may be introduced by these
radicals.
If the cross-linking structure through nitrogen
atom is introduced, the formation of C-N bond may
be detected. Then, change of an absorbance of C-C
bond and C-N bonds was observed carefully.
The relation between the surface hardness of
PMMA and the number of cross-linking bonds
introduced was examined. The amount of an
increase of C-C and C-N bonds obtained from Fig.4
which is normalized by the absorbance of C-H (1450
cm-1) was calculated. Because the change of
absorbance at C-H (1450 cm-1) after the processing
was small. Fig.5 and Fig.6 show the dependence of
the surface hardness on increase of each peak. As a
result, it is shown that the surface hardness increase
depending on the number increase of C-C and C-N
bonds. The correlation coefficient between the
relative ratio of C-C bond and the surface hardness
became 0.98. The correlation coefficient between the
amount of C-N bond and the surface hardness
became 0.95. From these results, it is confirmed that
there is a strong relationship between the hardness
Figure 5. Surface hardness for relative ratio of C-N.
Figure 7. N1s spectrum.
Table 1. Percentage of each bond
Figure 6. Surface hardness for relative ratio of C-C.
and the number of C-N and C-C bonds. However,
the result is obtained by the ATR methods.
Accordingly, it is difficult to decide the number of
bonds introduced quantitatively.
3. 2. XPS measurement
It have been confirmed that density of nitrogen
had a peak around the projection range of each
acceleration voltage by using XPS [7]. The N1s
spectrum is shown in Fig.7. The spectrum was
divided into 2 peaks by the deconvolution as shown
in Fig.7, one peak corresponds to the C-N-C bond.
On the other hands, magnitude of chemical shift of
N ≡C and C-N=C is almost the same. Consequently,
the N1s spectrum was not able to be separated into
two peaks. From the Fig.7, it is confirmed that
nitrogen injected into the sample reacts with C atom
composing PMMA. Moreover, it is expected that the
cross-linking structure may be formed by the C-N-C
bond.
Next, the contribution of the cross-linking
structure including nitrogen atom on the surface
hardness is examined. Table 1 shows the ratio of
each bonding against all elements obtained by the
waveform separation. As a result, it is shown that the
ratio of the C-N-C bond and N ≡ C and/or C-N=C
bond increases with the acceleration voltage.
Moreover, when the acceleration voltage is higher,
an increase of C-N-C bond is remarkable than that of
N ≡ C and/or C-N=C bonds. Here, the result of the
surface hardness to the ratio of the C-N-C bond is
shown in Fig.8. As a result, it is clear that the
surface hardness increases depending on the ratio of
the C-N-C bond. The correlation coefficient between
the hardness and percentage of C-N-C bond is 0.93.
Therefore, it is confirmed that a strong correlation is
obtained between the C-N-C bond and the surface
hardness. It is also suggested that cross-linking
structure contribute to the surface hardness
improvement. The depth profile of the C-N-C bond
which is expected to cross-linking structure was
obtained. Table 2 shows the ratio of each bond in a
range of depth. In the table, a rang of depth was
calculated by the escape depth using the take off
angle method. Fig.9 shows the profile of C-N-C
bond obtained under the condition that acceleration
voltage is 200V. The peak appears at the similar
depth of the projected range of ions which is 1.9 nm.
From the result, it is expected that many cross-
Figure 8. Dependence of surface hardness on percentage of
C-N-C bond
Table 7. Depth profile of percentage of each bond
From the result of XPS analysis, the C-N-C bond
was detected after ion irradiation. As a result, the
introduction of the cross-linking structure that
contained nitrogen was suggested. Moreover, it was
confirmed that there was a strong correlation
between the surface hardness improvement and the
number of C-N-C bond. A peak position of C-N-C
bond existed around the projection range of inject
ion. Therefore, it was concluded that C-N-C bond
introduced up to the depth of the projected range of
ions contributed to the surface hardness
improvement of PMMA.
References
[1] http://www.cmcbooks.co.jp/report.html
[2] F.Ide and H.Terada ： “ Optical fiber and
Opticalmaterial”, Kyoritsu publication, pp.12-13 (1987)
(in Japanese)
[3]Edited by T.Takahashi ： “ Electron and ion beam
engineering”， pp.200-214(1995) (in Japanese)
[4] Y.Sakurabayashi, T.Masaki, T.Iwao and M.Yumoto：
“ Surface Hardness Improvement of PMMA by Low
energy Ion Irradiation and Electron Irradiation”, Trans.
IEE of Japan ， Vol.129 ， No.4, pp.293-298 (2009) (in
Japanese)
[5] H.Schonhorn and R.H.Hansen：“Surface Treatment
of Polymers for Adhesive Bomding”，J.Appl.Poly.Sci，
Vol.11，pp.1461-1474 (1967)
Figure 9. Depth profile of C-N-C bond.
linking bonds are introduced around the projected
range.
4. 4.
Summary
[6] J.Zhang, J.Kang, P.Hu and Q.Meng ： “ Surface
modification of poly(propylene carbonate) by oxygenion
implantation ” ， Applied Surface Science ， Vol.253 ，
pp.5436-5441 (2007)
[7] J.Yokoyama, T.Iwao and M.Yumoto ： “ Surface
Modification of PMMA by Low-Energy Ion Irradiation－
Influence of modifying layer thickness on surface
hardness improvement－”, J. IEED of Japan，Vol.53，
No.4, pp.16-21 (2010) (in Japanese)
The purpose of this study is to confirm the
contribution of the cross-linking structure introduced
on the improvement of the surface hardness.
From the result obtained by FT-IR, it was
confirmed that number of C-C bond increased by ion
energy. Moreover, number of C-N bond increased
depending on the ion irradiation, too. From these
result, it was suggested that the introduction of the
cross-linking structure contributed to the hardness
improvement.
Department of Electrical and Electronic Engineering,
Tokyo City – University, Setagaya, Tokyo, Japan
Junya Yokoyama
[email protected]