НАНОСТРУКТУРИРОВАННЫЕ СИСТЕМЫ И МАТЕРИАЛЫ
UDK 541.15:541.515:543.422.27
FEATURES OF THE PHASE BEHAVIOR OF GAMMA-IRRADIATED POLYTETRAFLUOROETYLENE POWDER
© 2014 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 27.06.2013 г.
By using the technique of thermomechanical spectroscopy, an amorphous and three crystalline (high melting, intermediate, and low melting forms) blocks of the topological structures of polytetrafluoroetylene (PTFE) powder were characterized and their behavior under y-radiation up to 2420 kGy was explored. The powder has an anisotropic topological structure. Starting from a dose of 4 kGy, the structure is radically changed with the long-range orientation of chains in the intermediate and high melting crystalline blocks of PTFE being replaced by a short range orientation of cluster association structures. The temperatures of glass transition and melting point continuously decreased with an increased dose of irradiation. The influence of y-radiation on the powder and sheet of PTFE are essentially the same, the formation of amorphous character.
DOI: 10.7868/S0023119714020021
Crystallizable polymers, such as polytetrafluoroetylene (PTFE), butyl rubber, and polyethylene, can change their isotropic topological structure under the action of a uniaxial directed mechanical load [1]. Under these conditions, there is an increase in the crystal-linity and a change in the orientation of the vector of the direction of the longitudinal axis of their crystallites with respect to the vector of the load applied to the polymer. As a result, some of the crystallites are irreversibly oriented in the plane normal to this vector, and the topological structure of the polymer transforms from isotropic to anisotropic. The anisotropy of the structure can influence the properties, especially on radiolysis [2].
The axial compression of PTFE powders with an isotropic amorphous-crystal structure that occurs under isobar pressing leads to anisotropies in its properties, including an anisotropic topological structure. The pressed product then undergoes calcination and is ground for use in a final product. The "calcinations" operation consists of heating at a constant ramp up to 380°C and cooling back to a room temperature at the same speed. As a result of the external pressure, most of the macromolecules of the PTFE crystals are focused in a direction perpendicular to the vector of the external hydrostatic pressure. In this direction, a stretching internal pressure arises due to expansion of the polymer as a result of melting of the crystalline phase. This leads to unacceptable products and loss of material. Our preliminary investigations showed that the pressed product has various degrees of topological anisotropy. In some cases, the formed product is exposed to axial cracking and the product is rejected af-
ter calcination. Understanding the relationship between PTFE synthesis methods and its molecular to-pological parameters will provide ways of preventing axial cracking of the calcinated polymer. The anisotro-py of the molecular topological structure formed in the processing of PTFE powder to a finished product can influence the radiation stability of the formed product. There is currently no information about the influence of radiation on the anisotropic structure of the PTFE particles.
Any study of PTFE properties after radiolysis is connected with the difficulties of measuring its molecular-topological parameters. Thermomechanical spectros-copy is a solid phase diagnostic tool for studying the morphology of polymers, which has been used to explore the complex molecular morphology of fluorine containing (co)polymers [3—7]. The phase behavior of PTFE sheet under irradiation up to 2420 kGy [4] as been studied, but the influence ofirradiation on PTFE powder has only been studied for a dose of 140 kGy [3]. The differences in the physical natures of the powder and the sheet do not allow for extrapolation from one to the other for y-radiolysis. In the present work, the influence of the anisotropy of the topological structure on the phase behavior of the PTFE powder is studied after polymer irradiation up to 2420 kGy.
EXPERIMENTAL APPROACH
Materials. The powder was from the Konstantinov Kirovo-Chepetsk Chemical Combine with brand name "Fluoroplast F-4". The polymer was not subjected to further purification.
Thermomechanicalanalysis (TMA). The methodology of the TMS method used to study fluorine containing polymers and the symbol notations used in the text have been described previously [6] including PT-FE sheets [4] and powders [3]. The features of the powders of fluoropolymers explored by TMS have been described [5, 6]. TMS was also applied to study the influence of the synthetic procedure (initiator, temperature, pressure, conversion, emulsifier, bivalent iron salts) on the topological and molecular structure of PTFE powder [8]. This prior study 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). The investigation of powders by TMS assumes their one dimensional compression under optimized pressure. In this case, the polymer may lose its isotropic structure and transform into anisotropic forms. The results show that both the degree of crystallinity and the orientation of the crystals change. Some portions of the crystals are oriented perpendicular to the compression pressure vector. Thus, the degree of crystallinity can be determined under two analytical conditions, coaxial (||) and perpendicular (±), where the vector of the compression pressure and the vector of loading during the release of the copolymer deformation in the thermoanalyzer can be in the same plane (coaxial) or in perpendicular planes. The crystallinity is defined in terms of the coaxial and perpendicular orientation of the vectors, which serve as a measure of the structural anisotropy.
A polymer powder of 0.2—1.0 g was compressed under an optimized pressure of 200—250 kg cm-2 (DP36 press from Carl Zeis Jena, Germany). The diameter of the metallic pressing form is 6 mm and meets the No 14 Russian standard for surface treatment. Compression 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. The sample was then heated at the rate of 5 deg/min. TMA was performed by penetration of a quartz hemispherical probe into the polymer. The dynamics of its interaction with a polymer surface has been described [9] and the accuracy and reproducibility of the TMS method has been analyzed [10]. The accuracy of the temperature measurements in the thermostatic chamber of the instrument is ±0.05°C. The accuracy of the deformation measurement is ±5 nm. The errors of the molecular mass (MM) and free volume fraction are < ±10%. The data were reproducible within the error limits of ±5 to ±10%, but in some cases, the error can be as large as ±20% due to heterogeneity of the materials and differences in their thermal and stress history.
EPR spectra. EPR spectra were recorded with a PS100X spectrometer and were simulated with the EPRTOOLS program from NPP Adani. 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.
Y-radiation. Irradiation of the PTFE was carried out in glass ampoules in air with 60Co gamma-rays on a Gammatok-100 source at an absorbed dose rate of 2.7 x 10-4 kGy/s.
RESULTS AND DISCUSSION
The TMA of crystallizable polymers allows for the quantitative determination of the ratio of isotropic to anisotropic transformations in their structure. This determination is based on differences in the deformability of polymer during the melting of its crystallites [11] and the detection of deformability at different orientations of the vector of the axial direction of the crystallites with respect to the vector ofload applied during TMA, either coaxial (||) or perpendicular (±). The ratio of the degree
of crystallinity Zan = (Z^cr - )/r is a measure of the structural anisotropy. The crystallinity r and
are defined by the coaxial and perpendicular orientation of vectors, respectively.
Samples of PTFE sheet have isotropic structures. The industrial processing by calcination leads to the transformation of its anisotropic structures before calcination into isotropic structures after calcination. This is confirmed by the TMCs being identical in the coaxial and perpendicular orientation of vectors used during the PTFE powder pressing for production of this PTFE sheet (Fig. 1a).
The topological anisotropy of the structure of the PTFE powder pressed by 20 MPa to form the tablets using the uniaxial external hydrostatic pressure were determined. Fig. 1 shows the TMC of these samples tested in the co-axial (a) and perpendicular (b) orientations of the vectors and measured in the temperature range from -100 to 550°C. The character of the TMCs of the PTFE powder tested in the both orientations does not change, and is characteristic of semiblock, amorphous-crystal polymers containing one amorphous and three crystal (low melting, intermediate melting, and high melting) blocks. Parameters of their topological structures are listed in Table 1. However, the weight distribution between the amorphous and crystal blocks of the PTFE powder does depend on the testing orientation. The crystallinity is always
more in the coaxial mode (E^r) than in the perpendicular
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