CP620, Shock Compression of Condensed Matter - 2001
edited by M. D. Furnish, N. N. Thadhani, and Y. Horie
© 2002 American Institute of Physics 0-7354-0068-7/02/$ 19.00
TRANSIENT BOND SCISSION OF POLYTETRAFLUOROETHYLENE
UNDER LASER-INDUCED SHOCK COMPRESSION STUDIED BY
NANOSECOND TIME-RESOLVED RAMAN SPECTROSCOPY
Kazutaka G Nakamura1, Kunihiko Wakabayashi2, Ken-ichi Kondo1
Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama,
Kanagawa 226-8503, JAPAN
2
National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba 305-8565, JAPAN
Abstract. Nanosecond time-resolved Raman spectroscopy has been performed to study polymer films,
polytetrafluoroethylene (PTFE), under laser driven shock compression at laser power density of 4.0
GW/cm2. The CF2 stretching mode line of PTFE showed a higher shift (18 cm"1) at delay time of 9.3 ns
due to the shock compression and corresponding pressure was estimated to be approximately 2.3 GPa.
A new vibrational line at 1900 cm"1 appeared only under shock compression and was assigned to the
C=C stretching in transient species such as a monomer (C2F4) produced by the shock-induced bond
scission. Intensity of the new line increased with increasing delay time along propagation of the shock
compression.
EXPERIMENTS
The laser used is a Q-switched Nd:YAG laser
(Continuum Powerlite Plus) with maximum output
of 3 J/pulse at wavelength of 1064 nm. The
second harmonic light (512 nm) was generated by
using a KD*P crystal.
Pulse widths of the
fundamental and second harmonic lights are 10 and
8 ns at FWHM, respectively. Stability of the
output energy was within ±2.5 %. In pump and
probe experiments, the fundamental beam was used
for a pump beam, which generates shock wave, and
the second harmonic beam was used for a probe
beam, which excites Raman scattering [3]. The
second harmonic beam was separated from the
fundamental beam by a dichroic mirror and
introduced into an optical delay line. Delay time
between the pump and probe beams can be
INTRODUCTION
Dynamic and microscopic behavior of
materials is required to investigate in order to
specify a transient state of excitation and relaxation
processes under shock-compression.
Raman
spectroscopy is used to investigate structure and
bond strength of molecules and crystals under
shock compression [1].
Using laser-shock
generation and a pump and probe technique, it is
possible to investigate molecular vibrations under
shock-compression in a time domain of nanosecond
or much shorter [2]. In this paper, we performed
nanosecond time resolved Raman spectroscopy of
laser-shocked poly-tetrafluoroethylene ((C2F2)n,
PTFE) using the pump and probe technique.
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controlled within 25 ns, since the length of the
delay line is about 8 m.
accumulating 1000 laser shots.
Raman
spectroscopy with excitation light of 512-nm detect
a whole volume of the 80-jim PTFE film, since a
penetration depth is larger than 500 (im.
sample
RESULTS AND DISCUSSIONS
Raman measurements
Figure 2 shows a typical example of a Raman
spectrum of the pristine sample (PTFE).
Raman scattering
To spectrometer
Nd: YAG laser 1064mn
shock generation
Raman excitation
probe laser 512nm
Glass
Glass
Al foil
FIGURE 1. This is a schematic drawing of the
The target assembly has a glass confinement
geometry (shown in Fig. 1), which consists of a
back-up glass (100x100x5 mm3), an aluminum foil
(25-um thick), a PTFE film sheet (Dupont-Mitsui
Fluorochemicals Co., 80-jiim thick) and a cover
glass (2-mm thick). The aluminum foil was stick
to the back-up glass with an adhesive. The target
assembly was mounted on an X-Z stage, which was
computer-controlled for synchronizing to the laser
pulses. The pump beam was focused on the
aluminum foil with a spot (1.0 mm(|>). Laser
induced plasma was generated between the sample
and the backup glass and drove a shock wave
through the aluminum foil into the PTFE sample.
The peak pressure driven in the aluminum foil by
laser irradiation in the glass confinement geometry
was estimated by an empirical equation proposed
by Devaux et al [4]. The peak pressure in the
PTFE sample in the present irradiation condition
(4.0 GW/cm2) was calculated to be 2.3 GPa by
using impedance matching.
The probe beam was focused on the rear side of
the target with a diameter of 500 um after passing
through the optical delay line. The energy of the
probe beam was 10 mJ/pulse. Raman scattering
was collected with a camera lens, spectrally
resolved by a polychromater (Kaiser Co.) with a
2400 lines/mm grating and a notch filter (bandwidth
of 350 cm"1), and detected by a CCD camera.
Each Raman spectrum was obtained by
500
1000
1500
2000
Raman Shift (cm"1)
FIGURE 2. This is a typical example of a
Raman spectrum of pristine PTFE.
The observed lines at 291, 381, and 729 cm"1 are
assigned to twisting, bending and symmetric
stretching modes of CF2, respectively. The line at
1379 cm"1 is assigned to a C-C stretching mode.
Overtone and combination modes are observed at
1215 and 1295 cm"1. Broad lines at 550 and 1100
cm"1 are due to the glass substrate.
The observed lines at 291, 381, and 729 cm"1 are
assigned to twisting, bending and symmetric
stretching modes of CF2, respectively. The line at
1379 cm"1 is assigned to a C-C stretching mode.
Overtone and combination modes are observed at
1215 and 1295 cm"1. Broad lines at 550 and 1100
cm"1 are due to the glass substrate.
Figure 3 shows a typical example of the
nanosecond time-resolved Raman spectra detected
at delay times of 9.3, 14.7, 17.6 and 20.6 ns after
the pump beam irradiation. After the shock
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generation, a new peak appeared at around 1900
cm"1 and its intensity increases as increase of delay
time. However this new peak is not observed in
the Raman spectrum of the recovered sample,
which is identical to that of the pristine PTFE.
The new peak, therefore, is due to the transient
species generated under the shock compression.
The CF2 stretching peak at delay time of 9.3 ns
shows higher shift of 18 cm"1.
The
pressure-induced shift has been reported in
experiments of hydrostatic compression [5]. The
relationship between the pressure-induced shift (S
[cm"1]) and the pressure (P [GPa]) is well fitted by
linear equation: S=6.97P+1.93.
Using this
equation, the present shift (18 cm-1) corresponds to
pressure of 2. 3GPa, which is comparable to the
ablation-pressure.
400
Ab initio Calculation
In order to assign the new Raman line (1900
cm"1), ab initio calculation was performed. The
most conceivable candidate is a stretching mode of
carbon double bonds (OC), because the sample
consists of C and F atoms. Ab initio calculation
was performed for CnFm (n<ll, m<23) molecules
and radicals containing the C^C bond. The
calculation was performed with a Gaussian98W
program [6] and the Hartree-Fock (RHF and UHF)
level and 6-31G basis sets were used. PTFE was
modeled with a Ci0F22 molecule. The frequency
was calculated after the full geometry optimization
C/5
•4—1
'c
300
e)
200
c
03
E
<o
100
100
500
1000
1500
C8F18
V)
2000
"c
Raman shift (cm )
.a
80
CO
FIGURE 3. This a time-resolved Raman
spectra of laser-shocked PTFE: a) 0 ns, b) 9.3
ns, c) 14.7 ns, d) 17.6 ns, and e) 20.6 ns.
S
60
(/)
C
S
"c
-
The increase of the new peak can be explained in
connection with the propagation of shock waves.
Since Raman spectroscopy probes a whole volume
of the PTFE, the observed Raman spectrum is made
up by the superposition of the scattering from both
volumes under shock compression and in front of
the shock wave. By propagation of the shock
wave inside the film along with the delay time, the
volume under the shock compression increases and
the intensity of the new peak increases.
40
CO
E
CO
a:
20
800
1200
1600
2000
Raman shift (cm )
FIGURE 4. These are calculated Raman spectra
obtained by ab initio calculation (HF 6-31G).
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and the scale factor of 0.89, which is systematically
used for Hartree-Fock level calculations [7]. The
CgFig is not so big for a model of polymer, but the
calculated Raman lines represent well the
experimentally obtained ones and the Raman
spectrum of a larger molecule such as CioF22 is
comparable with that of CgFig.
Figure 4 shows the calculated Raman spectra.
The pristine PTFE is well reproduced by CgFjg.
The highest Raman frequency is a C-C stretching
mode at 1409 cm"1. In radicals such as C4F9, the
highest peak is also the C-C stretching. In the
long-chain radicals with a C=C bond such as C4F8,
the highest frequency is the C=C stretching (1792
cm"1) but this is smaller than the observed frequency.
However, the C2F4 has the C-C stretching at 1902
cm"1. The transiently observed Raman line at 1900
cm"1 is assigned to the C=C stretching of C2F4
monomer,
which
is
generated
under
shock-compression.
It is known that the degradation of PTFE leads to
the formation of C2F4 monomer with high yield.
The bond scission of C8F18 was also calculated by
modeling: C 8 F I8 ->2(C4F9) ->2(C2F5+C2F4). Since
the bond energy of C-F is higher than C-C, the C-C
bond is broken. The calculation shows that the
first step is endothermic (3.04 eV) but the second
step is exothermic (2.99 eV).
If a C-C bond
scission is induced, the successive scission may
occur. Graham proposed the mechanical chemical
reaction and shock-induced bond scission [8].
Therefore the observed Raman line at 1900 cm"1
may be due to the C2F4 monomer transiently
generated by shock-induced bond scission.
ACKNOWLEDGEMENTS
This work has been supported by the CREST
(Core Research for Evolutional Science and
Technology) program organized by the Japan
Science and Technology Corporation (1ST).
REFERENCES
1. GI. Pangilinan and Y.M. Gupta, J. Phys. Chem. 98,
4522(1994).
2. S.A. Hambir , J. Franken, D.E. Hare, E.L. Chronister,
BJ. Baer, and D.D. Dlott, J. Appl. Phys. 81, 2157
(1997).
3. K. Wakabayashi, K.G Nakamura, K. Kondo, and M.
Yoshida, Appl. Phys. Lett. 75, 947 (1999).
4. D. Devaux, R. Fabbro, L. Tollier, and E. Bartnicki, J.
Appl. Phys. 74,2268(1993).
5. C. Wu and M. Nicol, Chem. Phys. Lett. 21, 153
(1973).
6. Gaussian 98, Revision A.6, M. J. Frisch et al.,
Gaussian, Inc., Pittsburgh PA, 1998.
7. J. B. Foresman and A. Frisch, in Exploring Chemistry
with Electronic Structure Methods (Gaussian Inc.
1993).
8. R.A. Graham, J. Phys. Chem. 83, 3048 (1979).
CONCLUSION
Nanosecond time-resolved Raman spectroscopy
has been performed to study PTFE polymer, under
laser-driven shock compression at 2.3 GPa (laser
power density of 4.0 GW/cm2). A new vibrational
line at 1900 cm"1 appeared only under shock
compression and was assigned to the C=C
stretching in transient species such as a monomer
(C2F4) produced by the shock-induced bond
scission.
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