ХИМИЧЕСКАЯ ФИЗИКА, 2014, том 33, № 8, с. 65-68
UDC 541.15; 541.515; 543.422.27
ХИМИЧЕСКАЯ ФИЗИКА ПОЛИМЕРНЫХ МАТЕРИАЛОВ
THE PHASE BEHAVIOR OF GAMMA AND LASER IRRADIATED POLYTETRAFLUORINE © 2014 S. R. Allayarov1*, Yu. A. Olkhov1, V. G. Nikolskii2, P. N. Grakovich3, D. A. Dixon4
institute ofproblems of chemical physics of the Russian academy of sciences 2Institute of chemical physics of the Russian academy of sciences 3Institute of mechanics of metal-polymer systems of National academy of sciences of Belarus 4Department of chemistry, The University of Alabama, USA *E-mail: sadush@icp.ac.ru Received 24.12.2013
By using thermomechanical spectroscopy, an amorphous and three crystalline (high melting, intermediate, and low melting forms) blocks of the topological structures of the polytetrafluoroetylene (PTFE) were characterized and their behavior under y-radiation up to 2420 kGy was explored. The influence of y-radiation on the powder and sheet of PTFE is essentially the same leading to formation of amorphous character. The glass transition and melting point temperatures of PTFE continuously decreased with increasing dose of irradiation. On exposure to continuous CO2 laser radiation, PTFE degrades at a high rate and its clusters have a fibrous form. Initial y-irradiation of PTFE enhances the laser ablation process.
Keywords: polytetrafluoroethylene, powder, sheet, y-irradiation, thermomechanical analysis, anisotropic to-pological structure.
DOI: 10.7868/S0207401X14080020
1. INTRODUCTION
Polytetrafluoroethylene is an important polymer with many applications due to its unique properties
[1—3]. There is a considerable body of research dedicated to the radiolysis ofPTFE [4]. The inconsistent data of the high chemical and thermal stability of PTFE [1—3] and its very low radiation stability under y-ray irradiation [4], even though it has the lowest yield of the radiation degradation, was discussed previously [5].
Because of its high thermal stability and chemical resistance, PTFE has been the subject of a number of studies of it as a laser ablation target [6]. However, the available results are insufficient to determine the mechanism of laser induced processes in polymers in general, and, in PTFE in particular. Issues in the study of PTFE by y- or laser-irradiation are due to the difficulties of measuring its molecular-topological parameters before and after irradiation. These studies are further complicated by its low solubility and the difficulty of analyzing the products of irradiation. Thermomechanical spectroscopy (TMS) is a technique for determining the polymer molecular-topographical structure [7]. The TMS studies yielded a wide number of polymer parameters, such as the molecular mass distribution and the interchain behavior (temperature of transformation, linear thermal expansion, free volume, topological blocks). Thermomechanical spec-
troscopy has been used to study the influence of a 140 kGy dose of irradiation on PTFE powder [8] and other forms of PTFE [9, 10].
In the present work, TMS is used to investigate the molecular-topological characteristics of the sheet and powder of PTFE before and after y-irradiation up to 2420 kGy as well as after laser ablation.
2. EXPERIMENTAL APPROACH
The samples of PTFE were from the Konstantinov Kirovo-Chepetsk Chemical Combine (Russia) with brand names "F-4" and "F-4D". The polymers were not subjected to any post purification. The methodology of the TMS method used to study fluorine containing polymers and the symbol notations used in the text have been described previously [11] for PTFE sheets [9] and powders [8].
Radiolysis of the PTFE was carried out in glass ampoules in air or in a vacuum with 60Co y-rays on a Gammatok-100 source at an absorbed dose rate of 2.7 • 10-4 kGy/s. The laser ablation experiments were carried out in the vacuum unit detailed previously [6]. An LGN-703 continuous C02 laser with an emission power of 35—40 W was used.
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Fig. 1. The dependence of the amorphous region of the PTFE (1, 2, 3), the glass temperature (4, 5, 6) and the fusion temperature (7, 8, 9) in the PTFE powder (1, 2, 4, 5, 7, 8) and the PTFE sheet (3, 6, 9) and on a irradiation dose. The TMCs of powders were tested in the co-axial (1, 4, 7) and perpendicular (2, 5, 8) orientation of the vectors.
3. RESULTS AND DISCUSSION
Comparison of the data of thermomechanical analysis shows that y-irradiation has a similar influence on the molecular-topological structure of PTFE in the powder or sheet forms. The radiation destroys the crystalline regions and increases the amorphous character of the PTFE. At high doses, the PTFE was transformed to the state formed by only the chain segments between junctions in a pseudo-network of the amorphous phase. In the case of irradiated PTFE powder, the crystalline regions were transformed to cluster regions, unlike the case of PTFE sheet, where the crystalline regions are transformed to the amorphous phase. The curves of the dependence of the amount of the amorphous portion amount in the PTFE powder on the
irradiation dose are given in Fig. 1, curves 1 and 2. The samples reach >90% of amorphous character at doses >250 kGy. An increase in the rate of amorphous character formed is observed in the perpendicular orientation of vectors (curve 2 in Fig. 1). Irradiation of the PTFE sheet leads to a decrease in amorphous character only at doses up to 4 kGy (curve 3 in Fig. 1). Thus, the irradiation of PTFE results in amorphous character. The glass transition temperature of irradiated PTFE powder decreases with increasing irradiation dose (Fig. 1, curves 4 and 5). The rate of decrease is larger in the irradiated powder of PTFE than in irradiated PTFE sheet (Fig. 1, curve 6). Molecular flow of the irradiated PTFE powder can occur after melting of the crystal modifications or after transformation into
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Fig. 2. Dependence of the intensity, averaged over 20 s, of PTFE ablation with 40-W laser radiation upon the dose of preliminary y-irradiation: fibrous fraction (1), gaseous fraction (2) and integral ablation flux (3).
the high-elasticity state, if the irradiated PTFE consists only of the chain segments between junctions in a pseudo-network of amorphous region. Therefore, the observed changes in the physical state of PTFE are significantly influencing the flow temperature of irradiated PTFE (Fig. 1, curves 7, 8). Decrease of the fusion temperature on radiolysis arises both in the sheet of PTFE (curve 9) and in the powder (curves 7 and 8).
The dependence of the average MM of the PTFE powder after irradiation is similar to that after radioly-sis of PTFE sheet. The radiation chemical yield of degradation (Gd) of irradiated PTFE powder is Gd « « 0.03 macromolecules per 100 eV of irradiation dose. The average molecular mass of initial PTFE
sheet Mw « 1415 • 104 is close to that of the initial PTFE powder tested in co-axial orientation of vectors
Mw « 1077 • 104.
Effect of preliminary y-irradiation on the laser ablation of PTFE ("F-4")
The dependence of the laser ablation rate on the dose of preliminary y-irradiation of PTFE is illustrated in Fig. 2. At low doses below 4 kGy, a rise in the formation rate of both components (a gas and fibrous product) with dose was observed. At higher doses, the intensity of the release of the fibrous fraction (Fig. 2, curve 1) undergoes a smooth decline, which continues until a dose of22—32 kGy, where it reaches an absolute minimum; for some samples, the minimum is lower in value than that for the unirradiated polymer. The intensity of gas evolution increases until ~20 kGy and remains unchanged within the limits of experimental error («10%) with an increase in the dose of preliminary irradiation up to a maximum of 120 kGy (Fig. 2, curve 2). The integral ablation intensity (Fig. 2, curve 3)
increases with the dose until ~20 kGy and, then, remains almost unchanged up to a dose of ~33 kGy. At higher doses, the integral intensity of ablation increases due to the increase in the rate of evolution of the fibrous fraction. Thus, the formation of fibers by the laser ablation of PTFE is facilitated by its preliminary y-irradiation over most of the dose range examined.
4. CONCLUSIONS
The isotropic and anisotropic states of the PTFE powder before and after y-irradiation were investigated. The powder samples reach >90% of amorphicity at doses >250 kGy. Decrease of the fusion temperature on radiolysis arises both in the sheet of PTFE and in the powder. The influence of irradiation on the PTFE powder and sheet are essentially the same, leading to the development of amorphous character. The impact of high-intensity laser radiation on a polymer in vacuum is accompanied by releasing of gaseous products of degradation and, in some cases, of clusters of the partially destroyed polymer. PTFE exhibits an abnormal behavior in this process as exposure to continuous CO2 laser radiation leads to a high rate of degradation forming clusters with a fibrous form.
ACKNOWLEDGMENTS
The research was partially supported financially by RFBR research project No. 13-03-12164 ofi_m and by found OCHNM-8 of RAS.
D.A. Dixon is indebted to the Robert Ramsay Endowment of the University of Alabama for partial support.
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REFERENCES
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2. Wall L.A. Fluoropolymers. N.Y.—London—Sydney-Toronto: John Wiley and Sons Inc., 1972.
3. Panshin J.A., Malkevich S.G., Dunaevskaja S. Fluoro-plastics (Ftoropla
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