Physical and Chemical changes induced by 70 Mev carbon ions in

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PHYSICAL AND CHEMICAL CHANGES INDUCED BY 70 MEV
CARBON IONS IN POLYMETHYL METHACRYLATE (PMMA)
c
H. S. VIRKa, P. S. CHANDIb, A. VARADA RAJULUc
a
Department of Physics, Guru Nanak Dev University,India-143005
b
Department of Physics, Khalsa College, Amritsar,India- 143005
Department of Polymer Science and Technology, S K University,India
ABSTRACT. Polymethyl methacrylate (PMMA) is one of the promising class of
polymers which finds application in the field of telecommunication for fabrication of
optical components such as splitters, as well as material for ion beam lithography as a
photo-resist in the semi-conductor industry, and tribological applications. Physical and
chemical response of 70 MeV carbon ion irradiated PMMA has been studied by using
UV- visible and FTIR spectroscopy and XRD technique. Pelletron machine at Nuclear
Science Centre, New Delhi supplied 12C+5 ion beam current of 4 pna and a fluence
ranging from 3.1  1011 to 1.2  1014 ion cm-2 for irradiation of polymer samples under a
vacuum in the order of 4  10-6 Torr. Ion beam energy and thickness of the target were so
chosen that only the electronic energy loss was responsible for material modification of
PMMA. Recorded UV-visible spectra shows an anomalous behaviour of irradiated
polymer with chain scission and cross-linking competing simultaneously with increase of
ion beam fluence. The FTIR analysis reveals a general decrease in intensity of recorded
peaks on irradiation. X-ray diffraction pattern reveals an increase in amorphous content
of the irradiated PMMA. However, a decrease of 8% in crystallite size has been observed
after irradiation.
Keywords: PMMA polymer; carbon ions; radiation effects; UV and FTIR spectroscopy;
X-ray diffraction.
INTRODUCTION
The vigorous development of polymer science and the extensive utilization of polymeric
materials in all fields of technology has led, in recent years, to the enhanced interest in
the various problems of the physics and chemistry of polymers. The interest in the
irradiation of polymers with swift heavy ions came initially from their ability to register
nuclear tracks and applications of polymers as particle track detectors [1]. More recently,
the polymer membranes and ion track filters have been widely described in literature [25].
Polymethyl methacrylate (PMMA) is one of the promising class of polymers
which can be used for the expanding optical networks in the field of telecommunication
for fabrication of optical components such as splitters [6]. It is an excellent material
because it is easy to structure and has the desired optical properties. However, it has one
limitation: its temperature stability is limited to 800C due to its glass transition
temperature (Tg) of 1050C. The modified forms of PMMA have been prepared and

e-mail: [email protected] (H.S.Virk)
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studied for a broader field of applications focusing on the optical properties and ion
induced chemical changes [6].
PMMA, also known as a positive photo-resist for its degradation upon irradiation,
has been the subject of more investigations in radiolysis than many other polymers. This
was partly due to a growing interest in the application of PMMA in ion beam lithography
in the semiconductor industry [7] and partly due to its non-gelling nature [8]. Davenas et
al. [9] also found that PMMA transformed from a positive resist into a negative resist for
high fluence irradiations. Both cross-linking and scission occur simultaneously during
irradiation of polymers, but the relative magnitude depends upon polymer structure. The
irradiation effects of low LET radiation sources (e-beams, 60Co -rays) and high LET
ions (He+, Ar+) on cross-linking and scission mechanisms of PMMA have been studied
extensively [10].
Tribological characterisation of 15N+-ion implanted PMMA films has also been
reported [11]. Varada Rajulu et al. [12,13] have investigated the chemical response of
PMMA-blend films using 28Si ion beams. The present investigation reports the physical
and chemical changes induced by 70 MeV carbon ions in PMMA.
EXPERIMENTAL METHOD
Polymethyl methacrylate (PMMA) polymer foils with a thickness of 125m used in this
experiment were purchased from M/S Goodfellow (U.K). The repeating unit of PMMA
molecular structure is shown in figure 1. The samples of size (1x1 cm2) were prepared
for irradiation in as-received condition without further treatment. Polymer samples were
mounted on a vacuum shielded vertical sliding ladder and irradiated in the generalpurpose scattering chamber under vacuum in the order of 4x10-6 Torr. The 70 MeV 12C+5
ion beam with a beam current of 4 particle-nano-Ampere, delivered from the 15 UD
Pelletron accelerator at the Nuclear Science Centre, New Delhi was used. The ion beam
fluence was measured by integrating the ion charge on the sample ladder, which was
insulated from the chamber. The fluence was varied in the range 3.1x1011-1.2x1014 ions
cm-2. In order to expose the whole target area, the beam was scanned in the x-y plane.
The range of the incident ion of 147m, as estimated by SRIM [14], was higher than the
thickness of the polymer film. The ion beam energy and the thickness of the target were
so chosen that only the modification due to electronic energy loss affected the exposed
sample.
The nature of the ion beam induced changes has been analysed using the UVvisible spectrophotometer (UV-160-Shimadzu) in the wavelength range of 200-800 nm.
The Fourier transform infrared (FTIR) spectroscopy was performed in the absorption
mode using Nicolet –Avatar 320 FT-IR in the range of 4000-400 cm-1. The preliminary
structural studies were carried out by an X-ray powder diffractometer (Rigaku D Max
IIC) with Cu-K radiation (1.5418 Å) for a wide range of Bragg angles 2 (502 600)
at the scanning rate of 50/min. All the physical and chemical measurements were carried
out at room temperature of 24-260C.
RESULTS & DISCUSSION
The energy lost by the swift heavy ion in a solid is mainly dominated by two mechanisms
known as electronic and nuclear stopping. The electronic energy loss is dominant for
ions with high energy and involves the energy transfer to the target electrons. The SRIM
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calculation [14] indicates that 99.95% of energy lost by 70 MeV 12C+5 ions in 125 m
thick PMMA sheet is due to electronic energy loss. The electronic stopping power of the
beam, (dE/dx)e is 2.938  102 keV/m. The irradiation doses deposited in the PMMA at
different fluences, 3.1 1011, 3.7 1012, 1.8 1013, 5.6 1013 and 1.2 1014 ions cm-2 are
1.1, 13.5, 66.0, 205.5 and 440.4 Joules, respectively.
3.1 Optical response
The absorption of light energy by polymeric materials in the ultraviolet and visible
regions involves promotion of electrons in ,  and n- orbitals from the ground state to
higher energy states which are described by molecular orbitals [15]. High-energy ion
beam interaction with PMMA produces cross-linking while low energy radiation sources
(e-beams and -rays) result in the production of chain scission [10].
The results of optical absorption studies with UV-visible spectrophotometer
carried out on virgin and irradiated samples are shown in Fig. 2 (A-E). The optical
absorption spectrum of virgin sample shows maximum absorbance at 200 nm with a
narrow peak at 260 nm and a broad peak at 337 nm. The optical spectra of irradiated
samples in figure 2 (B-E) show some interesting features. At a fluence of 3.11011
ions/cm2 (Fig. 2B), the absorption decreases over the whole range of wavelengths from
200 to 800 nm, with a maximum decrease of 35% at 200 nm. When the fluence is
increased to 3.7  1012 ions/cm2, the optical absorption is again increased over the full
spectrum (Fig. 2C) as if the polymer sample has recovered its pristine form. Further
irradiation to 1.8 x1013and 5.6  1013 ions/cm2 produces almost identical patterns (Fig. 2
D-E) with sharp peaks at 300, 345 and 411 nm with low absorption in low wavelength
region and high absorption in the high wavelength region. There is a complete reversal
of the properties of virgin polymer after irradiation. We have never observed such a
strange behaviour in the polymer studies, viz. polyimides and polycarbonates, in our
laboratory [16-17]. The periodic nature of radiation effects in PMMA needs further
investigation using low LET and high LET radiation sources. Other workers have also
observed such an anomalous behaviour of PMMA [9,10,18,19].
3.2 Chemical response (FTIR spectroscopy)
Chemical response or the nature of chemical modifications can be studied through the
characterization of the vibration modes determined by infrared spectroscopy. Figure 3
(A-C) represents the FTIR spectra of the virgin (A) and two of the four irradiated samples
(B&C) using the fluence of 3.11011 and 3.71012 ions cm-2, respectively.
Figure 3 shows clearly the general decrease in intensity of the peaks of irradiated samples
compared to the virgin sample. There is a maximum absorption (0% transmittance) in
the range of 2800-3000 cm-1 corresponding to C-H stretching vibrations. The peak
around 1716.56 cm-1 (Fig. 3A) corresponding to C=O vibrations in the virgin sample is
slightly shifted to 1705.44 and 1720.12 cm-1 in the irradiated samples (Fig. 3 B-C). A
regular trend in the variation of the peak positions is not observed with increase of
fluence. This type of anomaly has been observed in the UV-vis spectra also (Fig. 2) .
The peak at 1455.92 cm-1 which corresponds to the CH2 symmetric bending and the peak
around 1380 cm-1 corresponding to CH2 asymmetric bending, which is overlapped in the
above peak, are not showing any appreciable change at higher fluence of 3.7x10 12 ions
cm-2. This indicates that gas evolution (either methane or hydrogen) is not taking place
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after irradiation. The band around 3000 cm-1 corresponding to CH2 is also not showing
any change after irradiation, indicating that CH2 groups are also intact. The peak at 1637
cm-1 corresponding to C=O is also not showing any change indicating that C=O groups
are not abstracted by the ionic radiation. The bands corresponding to the C-O-C
stretching groups around 1260 and 1150 cm-1 have not been observed in both the virgin
and irradiated samples giving rise to an anomalous behaviour of the PMMA.
3.3 Structural response
X-ray diffraction pattern of the virgin (A), and one of the irradiated (1.8 x 1013 ions cm-2)
PMMA samples (B) is shown in figure 4 (A-B). The diffraction pattern indicates that the
pristine PMMA is semicrystalline in nature with dominating amorphous content in it.
The 6 peaks to be seen in the X-ray spectrum of the virgin sample are summarized in
Table 1. The two sharp peaks are shown in the virgin sample at 2 = 21.220 and 25.560,
with lattice spacing d = 4.183 and 3.482A0, respectively. However, in case of the
irradiated sample, a truly significant change in the diffraction pattern has been observed
with one sharp peak only, corresponding to 2 = 21.260 and d = 4.175A0 (Table 2). The
disappearance of five X-ray peaks for irradiated sample indicates a clear increase in the
amount of amorphous content of PMMA.
The crystallite sizes of the virgin and the irradiated PMMA have been calculated
(Table 3) using Scherrer’s equation [20]
b = K/L cos,------------------------(1)
where b is full width at half maxima (FWHM) in radians, , the wavelength of X-ray
beam (1.5418A0) and K, a constant which is assumed to be 1 [21]. The crystallite size of
irradiated sample decreases by 8%.
CONCLUSIONS
UV- vis spectroscopic analysis of irradiated PMMA samples reveals an
anomalous behaviour with fluence, which may be due to both degradation (scission) and
cross-linking of polymer chains competing simultaneously. FTIR analysis reveals that
ionic radiation results in decrease of intensity of some of the prominent peaks without
causing significant changes in their position. X-ray diffraction pattern reveals a
characteristic increase in the amorphous nature of the irradiated PMMA. The crystallite
size of irradiated sample shows a decrease of 8%. However, further experiments are
necessary to establish the true nature of anomalies observed in the physical and chemical
behaviour of PMMA.
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REFERENCES
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Phys. Lett. 42 (1983) 457.
[8] M. Dole, The Radiation Chemistry of Macromolecules, Academic Press, NY, 1973.
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(1985) 513.
[10] E.H. Lee, G.R. Rao, L.K. Mensur, Radiat. Phys. Chem. 55 (1999) 293.
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(Eds. J.S. Williams, R.G. Elliman and M.C. Ridgway), Elsevier, Amsterdam, 1996.
[12] A. Varada Rajulu, R.L. Reddy, K.M. Raju, D.K. Avasthi, K. Asokan, Nucl. Instr.
Meth. B 156 (1999) 195.
[13] A. Varada Rajulu, R.L. Reddy, D.K. Avasthi, K. Asokan, Radiat. Effects & Defects
in Solids, 152 (2000) 57.
[14] J.F. Ziegler, SRIM-97 The Stopping Range of Ions in Matter, IBM Research, NY,
USA, 1997.
[15] J.R. Dyer, Applications of Absorption Spectroscopy of Organic Compounds,
Prentice Hall Inc., NJ, USA, 1994.
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[18] T.M. Hall, A.Wagner, L.F.J Thompson, Appl. Phys. 53 (1982) 3997.
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Table1. X –Ray diffraction data of Virgin PMMA.
S.No.
1.
2.
3.
4.
5.
6.
2
9.84
12.8
13.9
21.22
25.56
36.06
D(A0)
8.981
6.91
6.365
4.183
3.482
2.488
Intensity
123
138
133
1147
1565
111
Width(A0)
0.84
0.63
0.72
1.05
1.83
0.99
I/I0
8
9
8
73
100
7
Table2. X-Ray diffraction data of irradiated PMMA (fluence 1.8x1013ions cm-2)
S.No.
1.
2
21.26
d(A0)
4.175
Intensity
600
Width(A0)
1.14
I/I0
100
Table3. Crystallite size of virgin and irradiated PMMA
Virgin
2
Cos
21.22
0.9829
Width, b
(radians)
0.0183
Crystallite
size, L (A0)
85.7
Irradiated
2
Cos
21.26
.9828
Width, b
(radians)
0.0199
Crystallite size,
L (A0)
78.8
Figure Captions:
Fig. 1. Structural formula of PMMA
Fig. 2. The optical absorption spectra of PMMA irradiated with 70 MeV 12C+5 ion
beam: (A) virgin, (B) 3.1  1011 , (C) 3.7  1012 , (D) 1.8  1013, (E) 5.6  1013 ions cm2.
Fig. 3(a). FTIR spectra of PMMA virgin sample.
Fig. 3(b). FTIR spectra of PMMA irradiated at a fluence of 3.1  1011 ions cm-2.
Fig. 3(c). FTIR spectra of PMMA irradiated at a fluence of 3.7  1012 ions cm-2.
Fig. 4. X-ray diffraction pattern of (A) virgin sample and (B) irradiated one at the
fluence of 1.8  1013 ions cm-2.
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