РАДИАЦИОННАЯ ХИМИЯ
UDK 541.15:541.515:543.422.27
EFFECT OF MEV PROTONS ON THE PHASE BEHAVIOUR AND THERMAL STABILITY OF POLYTETRAFLUOROETHYLENE © 2014 S. R. Allayarov*, **, Yu. A. Olkhov*, C. I. Muntele***, D. A. Dixon**, D. Ila***
*Institute of Problems of Chemical Physics of the Russian Academy of Sciences Chernogolovka, Moscow, Russia 142432 E-mail: sadush@icp.ac.ru **Department of Chemistry, The University of Alabama, Tuscaloosa, USA 35487-0336 ***Center for Irradiation of Materials, Alabama A&M University, Normal, Alabama, USA, 35762-1447 Поступила в редакцию 28.02.2013 г.
Thermomechanical spectroscopy analysis was used to study the influence of accelerated protons on the molecular-topological properties of polytetrafluoroethylene (PTFE). The study showed changes in a wide number of polymer parameters as a result of bombardment with 1, 2 and 4 MeV protons at fluences up to 2 x 1015 protons/cm2. The basic topological process occurring under proton bombardment is amorphicity, as found for y-irradiation of PTFE. The flow temperature of bombarded PTFE significantly decreases with increasing the fluxes and energy of the accelerated protons. The general process resulting from proton bombardment is cleavage of C—F bonds, leading to formation of"centered" radicals ~CF2CF • CF2~ and HE The thermal stability ofbombarded PTFE is below than that of virgin polymer. The rate of thermal destruction noticeably increases and the temperature of the initiation of effective thermal decomposition decreases after bombardment. The gaseous products generated during thermal destruction of the bombarded and virgin PTFE are similar.
DOI: 10.7868/S002311971403003X
Polytetrafluoroethylene (PTFE) has a novel mac-romolecular structure due to the closely packed outer fluorine sheath. It has high chemical and thermal stability [1,2], but low stability under y-radiation [3]. This behavior remains a subject of discussion [4—6], because PTFE is often used as an anti-adherent coating and has been investigated for potential use in various radiation dosimeter applications as well as space technologies. An improved understanding of the mechanism of degradation by radiation could enable PTFE to be used more broadly in radiation environments such as space applications.
Analysis showed that the PTFE irradiated with y-rays of 60Co [6] or lasers [7] has lower thermal stability and lower molecular mass than does the original PTFE. The changes in the molecular-topological structure of PTFE occur at low doses of irradiation. Usually PTFE can undergo a chain scission under y-irradiation [3]. However, there is evidence that irradiation of PTFE above 330 to 340°C in vacuum results in a significant improvement in tensile strength and modulus and chain elongation [8—11]. This clearly indicates that cross— linking is occurring, similar to what is observed in the irradiation of polyethylene [3]. At temperatures above 350°C, thermal depolymerization is increasingly accelerated by irradiation and prevails over cross-linking at elevated temperatures [10—12]. Additional interest in PTFE modified under irradiation at elevated tem-
peratures stems from the possibility of improvement of its physicomechanical properties, for example, the use of radiation for improving the properties of the PTFE containing composites. For example, carbon fiber reinforced cross-linked PTFE composites have been fabricated by using an electron beam [13]. They exhibited a very low friction coefficient and improved abrasion resistance when compared to cross-linked PTFE without reinforcement.
The mechanisms of the influence of irradiation on properties of the polymer need to be explored if we are going to use radiation for the modification of PTFE. It is difficult to study the properties of PTFE or other flu-oropolymers due to difficulties in measuring their mo-lecular-topological parameters because of their poor solubility in most solvents. For example, it is necessary to determine the molecular mass (MM) of PTFE before and after its radiolysis to define the degree of radiation destruction of macromolecules. Due to the insolubility of the polymer, the MM of PTFE was usually deduced by indirect methods [1, 2] beginning with heat treatment of the samples. This leads to the thermal destruction of the polymer, which decreases its MM. In addition, a viscous-fluid state of PTFE cannot be obtained, so PTFE cannot be processed into products by using any of the processing methods usually employed for thermoplastics. As a result, the man-
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ufacturing techniques of products from PTFE are complicated [14].
Thermomechanical spectroscopy (TMS) is a solid phase diagnostic tool for studying the morphology of polymers [15] and is based on a thermomechanical analysis (TMA) of the polymer. TMS makes it possible to determine the complex molecular morphology of polymers such as PTFE [5, 6], or the copolymers of tetrafluoroethylene with ethylene [16], hexafluoro-propylene [17] or ®-chlorotetrafluoroethyltrifluo-rovinyl ether [18].
The copolymers of tetrafluoroethylene with hexafluoropropylene and perfluoromethoxyethylene have been bombarded with 1 MeV protons to test their use for radiation dosymetry [19, 20]. The gas emission during the proton bombardment and the optical absorption spectra of the bombarded copolymers were studied. The gas emission from films of PTFE bombarded with 1 MeV protons has been reported [21]. However, the molecular — topological parameters of the bombarded fluoropolymers were not explored, so a more complete mechanism of the processes occurring under proton bombardment was not developed.
In the present work, TMS was used to explore the molecular topological properties of PTFE bombarded with MeV protons. The polymer was also irradiated by y-beams of 60Co to compare this type of radiation damage with that of accelerated protons.
EXPERIMENTAL
We used mechanical grade PTFE sheet and virgin electrical grade film of Teflon® PTFE (McMaster-Carr Supply Company, Atlanta, Georgia). The polymer was not subjected to secondary purification. The molecular topology of the polymers was investigated by the TMS method [15-17]. TMS was carried out by penetration of the polymer by a quartz hemispherical probe. The dynamics of the interaction of the probe with a polymer surface have been described [22].
The polymer samples were placed in the chamber of the standard thermoanalyzer "UIP-70M" and cooled to -100°C at a rate 5 deg/min. The polymer was maintained at this temperature for 5 min and then the probe loaded with a force of 0.5 g. Finally, the sample was heated at a rate of 5 deg/min. The accuracy and reproducibility of the TMA method have been analyzed [23]. The accuracy of the temperature measurements in the thermostatic chamber of the instrument is ±0.05°C. The errors of MM and free volume fraction are less than or equal to ±10%. The data were reproducible within the error limits of ±5 to ±10%, but in some cases, the error limits can be as large as ±20% due to heterogeneity of the materials and differences in their thermal and stress history.
The standard synchronous thermal analyzer STA 409C Luxx (German firm NETZSCH) interfaced with a quadruple mass-spectrometer (QMS 403C Ar-
eolas) was used for a simultaneous estimation of change of the weight (TG) and a quantitative measurement of thermal effects (DSC) using a set temperature program. The measurements were made in an Ar atmosphere and aluminum pans were used as crucibles. The temperature was measured with an S (Pt/Pt-Rh) thermocouple. The dependence of the sensitivity of the thermocouple on temperature was calibrated using the melting points of 6 pure reference metals. The quadruple mass-spectrometer is interfaced with the thermoanalyzer by means of a flexible quartz capillary which can be heated up to 300°C. This prevents the possible condensation of reaction products and minimizes loss of material before it reaches the mass spectrometer. Masses up to m/e = 300 can be measured. The ion impact energy was 70 eV.
The PTFE samples (20 x 20 mm2) were bombarded with 1, 2 and 4 MeV protons from the Pelletron accelerator at the Alabama A&M University at fluences of 1 x 1015 and 2 x 1015 protons/cm2. The proton beam was scanned over the entire surface of the samples using a random pattern for uniformity of beam density. The current was kept below 300 nA to avoid macros-cale sample heating during bombardment. Residual gas analyses (RGA, Stanford Research Systems, Model 200) were conducted in real time during the bombardment. The residual pressure in the sample chamber was kept below 1.33 x 10-4 Pa at all times. y-Irradiation of the polymer was carried out with 60Co y-rays on a Gammatok-1000 source at an absorbed dose rate of 2.7 x 10-4 kGy/s.
The Raman scans were taken with a Jobin-Yvon HR800 UV Confocal Microscope using the excitation line of 632.81 nm from a HeNe laser with approximately 12 mW of power at the sample. The shifts were detected using a Peltier cooled CCD detector.
EPR spectra of the bombarded polymers were recorded using a PS100X spectrometer (NPP "Adani" Company). After proton bombardment, the bombarded samples were transported from the target of the Pelletron accelerator into a box cooled to -78.5°C by dry ice. The time of this transport is less than 2 seconds. During this time the samples were in contact with air. The bombarded samples were kept in the cooled box until the EPR measurements were made at 27°C. In the case of the ESR study of polymer irradiated with y-rays, the samples of PTFE in SK-4B tubes were evacuated to a low pressure (0.13 Pa) and were irradiated at room temperature. After, these irradiated samples were stored at -78.5°C prior to the ESR measurements.
RESULTS AND DISCUSSION
Molecular topological properties of PTFE sheet. The thermomechanical curves of the PTFE sheet before (a) and after bombardment with accelerated protons (b, c, d) and after y-irradiation (e) are given in Figure 1.
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