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NANOSYSTEMS: PHYSICS, CHEMISTRY, MATHEMATICS, 2018, 9 (1), P. 29-32
Stability at high temperature and decomposition kinetics of the fullerene dimers and photopolymers
K. P. Meletov1*, J. Arvanitidis2, D. Christofilos2, G. Kourouklis2, V.A. Davydov3
1Institute of Solid State Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow region, Russia 2 Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece 3Institute of High Pressure Physics, Russian Academy of Sciences, 42092 Troitsk, Moscow region, Russia
PACS 78.30.Na DOI 10.17586/2220-8054-2018-9-1-29-32
The decomposition kinetics of the fullerene dimers and photo-oligomers was studied at elevated temperature by Raman scattering. The polymeric content decreases exponentially with the thermal treatment time while the decay time constant decreases at higher temperatures. The activation-type behavior is well described by the Arrhenius law that gives the activation energy Ea = (1.71 ± 0.06) eV/molecule for the dimers and Ea = (0.87 ± 0.06) eV/molecule for the C60 photopolymer.
Keywords: fullerene polymers, Raman spectroscopy, decomposition kinetics.
Received: 14 June 2017 Revised: 22 September 2017
1. Introduction
The photopolymerization of C6o occurs in thin films or surfaces of bulk samples resulting in rather disordered oligomers (C60)n that appear via the [2 + 2] cyclo-addition mechanism [1-3]. The treatment of C60 under various conditions of high pressure and high temperature (HPHT) results in the formation of bulk polymers [4,5]. The lowering of the C60 molecular symmetry leads to the splitting and softening of phonon bands and the Raman examination of polymers is based on the behavior of the Ag (2) pentagon pinch (PP) mode frequency which decreases from 1468 cm-1 for the C60 monomer to 1463, 1458, 1446 and 1408 cm-1 for the dimers, linear orthorhombic, planar tetragonal and rhombohedral polymers, respectively [5,6]. Fullerene polymers are stable at ambient conditions but revert to the monomeric state at elevated temperature [7,8]. The differential scanning calorimetry (DSC) of the HPHT polymers shows transitions in the range 170 - 280 °C which depend on the polymeric phase and somewhat on the scanning rate, indicating that the process is controlled by kinetics. The decomposition kinetics of polymers was studied by X-ray thermal expansion and Raman measurements [7,9]. In the case of photopolymers, the small amount of the photo- transformed material on the surfaces of samples makes the DSC and X-ray measurements inappropriate, so the Raman spectroscopy becomes a unique tool in the study of the photopolymer decomposition. Here, we report the Raman study of the decomposition kinetics of the C60 dimers and photopolymers. The intensity of the polymer-related PP-mode decreases exponentially with the thermal treatment time. Decomposition becomes faster at elevated temperatures and the activation energy EA, obtained from the Arrhenius dependence of the exponential decay time constant on temperature, is (1.71 ± 0.06) and (0.87 ± 0.12) eV for dimers and photopolymers, respectively.
2. Experimental details
The dimeric samples, prepared by HPHT treatment of pristine C60, were a mixture 4:1 of C60 dimer and monomer [10]. The C60 photopolymer was formed under laser irradiation at the wavelength Aexc = 785 nm and 180 ^W power. The photopolymer decomposition was studied by Raman probing at Aexc = 785 nm with 18 ^W power under continuous thermal treatment up to 400 min at an elevated temperature. The Raman measurements were performed in the back-scattering geometry using a LabRam HR spectrometer and a high temperature cell with a quartz window equipped by a temperature controller unit [11].
3. Results and discussion
The dimeric samples are sensitive to visible light and may be subjected to further photopolymerization during long acquisition Raman experiments. On the other hand, the spectra recorded at Aexc = 785 nm and laser power of 80 ^W do not show any changes for irradiation time up to 200 min. The kinetics of dimer decomposition was studied at these conditions under thermal treatment at temperatures ranging from 130 - 177 0C. The Raman spectra were measured in the frequency region of the PP-mode with fixed acquisition time, consecutively, during continuous thermal treatment, using a new fresh sample for each temperature.
The inset of Fig. 1 depicts the evolution of the Raman spectra of the C60 dimers upon thermal treatment at 155 0C. In the bottom spectrum, recorded at ambient conditions, the PP-mode component of dimers is dominant, while subsequent spectra recorded at 155 0 C exhibit an intensity increase of the monomer component at the expense of the dimer one. The top spectrum recorded after 97 min of thermal treatment is dominated by the PP-mode component of the monomer while the dimer component becomes negligible. Open circles in Fig. 1 represent the fractional intensity of the dimer PP-mode as a function of the thermal treatment time, whereas the dotted line is the fit of the data by the exponential decay function:
D(t) = Do exp(-t/T), (1)
where D(t) and D0 are the current and the initial contents of dimers and t is the decay time constant.
Fig. 1. Evolution of the Raman spectra of the C60 dimers under thermal treatment at 155 0C
Figure 2 illustrates the decomposition kinetics of dimers, which is relatively slow at 135 0C with t = 419 min, while it becomes very fast at the highest temperature of 177 0C with t = 6.9 min. The activation energy of the dimer decomposition reaction can be described by the Arrhenius equation:
t (T ) = A exp(EA /hs T), (2)
where EA is the energy barrier between the dimeric and the monomeric states, ks is the Boltzmann constant, T is the treatment temperature and t the decay time constant. The constant A, related to the characteristic phonon frequency, is measured in time units [9]. The corresponding Arrhenius plot is included in the Fig. 2 inset. The experimental data exhibit a good linear dependence in logarithmic scale, yielding an activation energy of Ea = (1.71 ± 0.06) eV/molecule. This value is close to the EA = (1.75 ± 0.05) eV/molecule obtained from the X-ray thermal expansion studies of powdered dimeric samples, synthesized by a solid-state mechanochemical reaction of C60 with potassium cyanide, as well as to the activation energy EA = (1.9 ± 0.1) eV/molecule of the 1D and 2D polymers decomposition [9]. It is interesting to compare these data with the decomposition kinetics of fullerene photooligomers (C60)n. The Raman spectra of the photopolymerized fullerene prepared at Aexc = 532 nm and 60 ^W laser power are shown in the left inset of Fig. 3. The initial spectrum recorded in the region of the Ag(2) PP-mode is typical for the C60 monomer while the following spectra recorded with 60 min exposure under
Stability at high temperature and decomposition kinetics of the fullerene dimers and photopolymers
31
Fig. 2. Decomposition kinetics of the C60 dimers
0 200 400 600 800
Exposure time, min.
Fig. 3. Raman spectra and photopolymerization kinetics of the C60 fullerite
continuous illumination show gradual decrease of the PP-mode intensity of the monomer and increase of the new PP-modes intensity. The intensity distribution between the new modes shows that photo-oligomers are mainly linear chains and conjugated linear chains. Open circles depict the dependence of the photopolymer content on the irradiation time whereas the dotted line is the fit by the exponential growth function:
P(t)= Psat{1 - exp(-t/T)}, (3)
where P(t) and Psat are the current and the saturated contents of the photopolymer, t is the time constant that linearly decreases with laser power (right inset).
The stability of the photo-oligomers, prepared on the surface of the C60 crystals at Aexc — 785 nm and laser power 180 ^W, was studied by Raman spectroscopy with Aexc — 785 nm, 18 ^W and short acquisition time to prevent further polymerization and ensure accurate measurements.
Figure 4 illustrates the kinetics of the photo-oligomer decomposition. The relatively slow decomposition at 130 °C with t — 109 min becomes very fast at 170 °C with t — 9.6 min. The activation energy of the decomposition can be estimated from the Arrhenius plot included in the Fig. 4 inset. The experimental data exhibit a good
linear dependence in logarithmic scale, yielding an activation energy of EA = 0.87 ± 0.06 eV/molecule. Note that a value of 1.25 eV/molecule was obtained earlier in the Raman study of the C60 photopolymer decomposition [7]. The Arrhenius data allows the estimation of the photopolymer complete decomposition temperature (decrease of its content to 1 % within 15 min as in DSC experiments [8]). Despite of the twice as large activation energy of the C60 dimers, their complete decomposition temperature is ^ 180 0 C, lying close to that of ^ 190 0 C for the photopolymers due to the giant difference in the A constant of the Arrhenius dependence. Finally, the recent study of the photopolymerization in the fullerene layers of the {Pt(dbdtc)2 }-C60 complex shows that the activation energy of the layered photopolymer decomposition is EA = 1.12 ± 0.11 eV/molecule while the temperature of its complete decomposition is ^ 130 0C [12]. Note, that the 2D polymers appear to be the most stable among the polymeric forms of C60, characterized by a complete decomposition temperature of ^ 260 0 C [8].
Fig. 4. Time dependencies of the C60 photopolymer content under treatment at various temperatures and their fit by exponential decay functions (dotted lines). Inset: Arrhenius dependence of the decay time constant on temperature
Acknowledgements
The authors are thankful to the Center of Interdisciplinary Research and Innovation of the Aristotle University of Thessaloniki (KEAEK) for the access to the Raman instrumentation. K.P.M. acknowledges the main financial support by the Federal Agency for Scientific Organizations, the partial support by the Russian Foundation for Fundamental Research, grant 15-02-01495, the RAS Presidium Program "Physics of compressed matter", as well as the hospitality of the Aristotle University of Thessaloniki.
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