РАДИАЦИОННАЯ ХИМИЯ
541.15:541.515:543.422.27
THE PHASE BEHAVIOR OF THE GAMMA-IRRADIATED
POLYTETRAFLUOROETHYLENE SHEET © 2013 S. R. Allayarov*, Yu. A. Olkhov*, D. A. Dixon**, D. E. Nikles**
*Institute of Problems of Chemical Physics of the Russian Academy of Sciences Chernogolovka, Moscow, Russia 142432 **Department of Chemistry, The University of Alabama Tuscaloosa, USA 35487-0336 E-mail: sadush@icp.ac.ru Поступила в редакцию 07.03.2013 г. В окончательном виде 13.05.2013 г.
Four phases, an amorphous phase and three crystalline forms (high-melting, intermediate and low-melting modifications) were identified in the sheet of polytetrafluoroethylene (PTFE) by thermomechanical analysis. After y-irradiation, the PTFE showed a substantial change both in the molecular relaxation characteristics in blocks and in the PTFE block composition itself at a dose of 1 kGy. The crystalline phase of PTFE transforms from the triblock to the diblock structure. The high temperature amorphous block (cluster) appears. At 560 kGy doze of irradiation, the PTFE sheet is completely amorphous. The glass transition temperature and melting point of the polymer continuously decreased with an increased dose of y-radiation. The development of amorphous character is the basic result of the radiolysis. The radiation chemical yield of degradation of the irradiated PTFE sheet is 0.03 macromolecules per 100 eV of a dose irradiation.
UDK
DOI: 10.7868/S0023119713060021
Polytetrafluoroethylene remains an irreplaceable polymer in many applications due to its unique properties. It possesses the highest thermal and chemical stability among synthetic polymers. However, the melting temperature is too close to its thermal decomposition temperature and therefore it cannot be formed by conventional polymer processing technologies, such as injection molding or melt extrusion. This has led to a search for ways of decreasing the processing temperature of PTFE by decreasing its melting point or by synthesizing copolymers having properties similar to PTFE [1]. However, a copolymer has not been founds that possesses all of the properties characteristic of PTFE.
The melting point of PTFE can be decreased by decreasing the molecular weight; however this leads to degradation of the polymer properties. One way to decrease the fusion temperature of PTFE by decreasing its molecular mass may be radiolysis of the initial PTFE polymer. However, the radiolysis and the resulting changes of the physical and chemical properties of the initial PTFE has to be made under conditions where the unique properties of the PTFE molecules are not being lost. Aa search for the optimum conditions of irradiation of PTFE, which would allow using irradiation for its primary processing with aim to process it after its irradiation by means of the usual methods used for other plastics, has not been investigated previously.
The features of PTFE arise from the unique structure of the macromolecules, possessing closely packed fluorine, with the chain often in a helical configuration in some phases. Consequently, oxygen, acids or alkalis, and also bacteria or other microorganisms cannot react with C—C bonds of the PTFE macromolecules. As a result, PTFE is chemically stable and is also stable against the influences of all known microorganisms.
There is a considerable body of research dedicated to PTFE [1—8]. Very few solvents will dissolve PTFE, and its solvent resistance is an asset, although its lack of solubility precludes many of the tradition polymer characterization techniques such as molecular weight determination by GPC. PTFE has a high chemical and thermal stability [1, 2] but very low radiation stability [3]. Ifwe understood the mechanism of degradation by radiation, it may be possible to use PTFE in space applications.
The study of PTFE radiolysis is connected with the difficulties of measuring its molecular-topological parameters before and after irradiation. Studies of PTFE radiolysis are complicated by its low solubility and the difficulty of analyzing the radiolysis products. The existing methods of polymer molecular heterogeneity are based on the properties of dilute solution. Such methods allow the analysis only of soluble polymers. For insoluble polymers such as PTFE, these methods
do not apply. Thermomechanical spectroscopy (TMS) is a technique for determining the polymer molecular-topographical structure [9, 10]. It is based on the thermomechanical analysis (TMA) of a polymer and allows complex polymer molecular-topographical testing. The functionalities of TMS have been expanded to the quantitative analysis of crystal fraction in amorphous-crystal polymers [11] and fluorine containing polymers [12—14]. We used TMA to study the influence of 140 kGy [14] and 500 kGy [15] of a dose of irradiation on the PTFE powder. In the present work, TMS is used to investigate the molecular-topological characteristics of the sheet PTFE before and after y-irradiation up to 2420 kGy.
EXPERIMENTAL
Commercial PTFE sheet of type 'F-4' from Kon-stantinov Kirovo-Chepetsk Chemical Combine (Russia) was used. It is the product of PTFE pressing followed by "calcination". Irradiation of the PTFE was carried out in glass ampoules in air with 60Co y-rays on a Gammatok-100 source at an absorbed dose rate of 2.7 x 10-4 kGy/s.
The polymer of 0.2—1.0 g was compressed under the optimized pressure of 200—250 kg cm-2. The DP36 press made in Germany by "Carl Zeis Jena" was used. The diameter of the metallic form used for pressing is 6 mm and meets the No 14 Russian standard for surface treatment. Pressing was carried out at room temperature. The resulting pellet was placed in the chamber of the standard thermoanalyzer "UIP-70M" and cooled to -100°C at a rate of 5 deg/min. It was maintained at this temperature for 5 min and then the probe was loaded with a force of 0.5 g. Finally the sample was heated at the rate of 5 deg/min. The PTFE sheet was investigated by TMS. A review of the methodology is described in Refs. [9, 10, 13, 16]. TMA was carried out by penetration of a quartz hemispherical probe into the polymer. The dynamics of its interaction with a polymer surface has been described [17]. One of the measured values is a change of the linear size of a sample between a substrate and the probe. A polymer sample should have a continuous structure in the entire sample volume. It may have any shape, but it must have two plane-parallel sides separated by tens of microns up to several mm depending on the sensitivity of the measuring equipment and the temperature-expansion coefficient of the polymer. The accuracy and reproducibility of the TMS method was analyzed previously [18]. The accuracy of the temperature measurements in the thermostatic chamber of the instrument is ±0.05°С. The accuracy of deformation measurement is ±5 nm. The errors of the molecular mass (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, it 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 from the German firm NETZSCH, interfaced with a quadrupole mass-spectrometer QMS 403C Areolas was used for the simultaneous estimation of change of weight (TG) and quantitative measurement of thermal effects (DSC) using a set temperature program. Measurements were performed in an atmosphere of Ar and aluminum pans were used as crucibles. A thermocouple of S (Pt/Pt—Rh) type was used to measure the temperature. The dependence of the sensitivity of the thermocouple on temperature was calibrated by determining the melting points of 6 pure reference metals. The quadrupole mass-spectrometer is interfaced with a thermoanalyzer by means of a flexible quartz capillary which can be heated up to 300°C. This prevents the possible condensation of reaction products. The device is able to analyze more than 60 lines due to different mass numbers in the range of m/e <300). The electron impact energy was 70 eV.
EPR spectra were recorded with a PS100X spectrometer and their simulation was achieved with the EPRTOOLS program of the NPP "Adani" Company. Samples of PTFE in SK-4B EPR tubes were evacuated to a low pressure (0.13 Pa) at —196°C prior to irradiation with y — rays of -60Co.
RESULTS AND DISCUSSION
The thermomechanical curves (TMK) of the PTFE sheet before (a) and after radiolysis (b, c) are given in Figure 1. Analysis of these curves yield a1 and a2, the coefficients of linear thermal expansion in the glassy and high-elasticity slate; ak, a'k, a'k, the coefficients of linear thermal expansion at the fusion temperature of the low-melting, intermediate and high-melting crystalline portion; 7g, for the glass transition temperature of the amorphous region; 7m, 7^, and TI^, fusion temperatures for the low-melting, intermediate and high-melting crystalline portions; 7m the beginning temperature of molecular flow, T^ beginning temperature of a high — elasticity state; Vf the free
geometrical volume; Mgn, Mgw, Mn, M^, average numerical and average weight mass in a pseudo-network structure of an amorphous region and high-temperature amorphous ("cluster") region; Mcr, Mcr1, and Mcr2, the molecular mass in the low-melting, intermediate and high melting crystalline regions; Mcr, Mcr1,
and Mcr2, average weight molecular mass ofblocks; Ka, Kcl the polydispersity factors in the amorphous and the cluster regions; and ycl, ycr, , Vr, the weight fractions of the amorphous region, cluster region, and
(а) (b)
-100 100 300 500 T, °C 3 4 5 6 Log Mc.
Fig. 1. Thermomechanical curves of the PTFE sheet before (a) and after radiolysis in air with 140 kGy (b) and 2300 kGy (c). Function of MMD of the chain segments between junctions in a pseudo — network of amorphous region of the PTFE sheet (d) before (1) and after irradiation with a dose 140 kGy (2) and 2300 kGy (
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