ХИМИЯ ВЫСОКИХ ЭНЕРГИЙ, 2013, том 47, № 2, с. 95-102
РАДИАЦИОННАЯ ХИМИЯ
UDK 541.15:541.515:543.422.27
MOLECULAR-TOPOLOGY OF POWDER OF THE y-IRRADIATED TETRAFLUOROETHYLENE/ETHYLENE COPOLYMER
© 2013 г. Yu. A. Olkhov1, S. R. Allayarov1, V. G. Nikolskii2, D. A. Dixon3
1Institute of Problems of Chemical Physics of the Russian Academy of Sciences Chernogolovka, Moscow, Russia 142432
E-mail: sadush@icp.ac.ru 2Institute of Chemical Physics of the Russian Academy of Sciences Moscow, Russia 119991 3Department of Chemistry, The University of Alabama Tuscaloosa, AL 35487-0336 Поступила в редакцию 31.09.2012 г. В окончательном виде 23.10.2012 г.
The molecular-topological structure of y-irradiated commercial copolymers of tetrafluoroethylene with ethylene (CTE) were explored in the coaxial mode, which has the vectors of CTE loading and compression pressure in the same plane, and in the perpendicular mode with the two vectors perpendicular to each other. Instead of the amorphous-crystal structure observed with the coaxial orientation of the vectors, the CTEs are completely amorphous in their perpendicular orientation. Radiolysis with a dose of 140 kGy leads to the formation of a crosslinked structure for the CTEs. On irradiation, the crystals are transformed to an amorphous state and CTE can form a fully amorphous structure after 600 kGy of irradiation.
DOI: 10.7868/S0023119713020129
The copolymer of ethylene with tetrafluoroethylene has a number of excellent properties distinguishing it from other fluorine containing polymers [1—4] for example, higher mechanical durability, hardness, elasticity, wear resistance, and chemical and radiation stability. The mechanical properties of CTE depend less on the temperature than do those for polytetrafluo-roethylene (PTFE) or the tetrafluoroethylene/hexaflu-oropropylene copolymer (FEP), two of the most common perfluorinated (co)polymers. The thermostability of CTE is close to that of FEP and somewhat below that of PTFE. In contrast with PTFE, CTE can be processed into products by using the broadly practiced thermoplastic extrusion method, which reduces the cost of processing CTE. The electronic properties of CTE are also less than those of PTFE, but CTE has a higher radiation stability than does PTFE. The combination of properties makes CTE a very useful material for atomic energy applications and in other technological areas where radiation is present.
Under pressure, the powders of the amorphous-crystal fluorine containing (co)polymers can lose their isotropic structures transforming into anisotropic structures because of changes in the orientation of the crystals [5]. This behavior can influence the molecular-topology properties of remolded powders and may affect their operational properties, in particular their properties after radiation damage [6]. Despite studies of the irradiation of CTE [7—9], the influence of structural anisotropy on the behavior of CTE has not been
reported. In addition, prior reports [10—12] on the phase behavior of CTE are not in complete agreement.
Due to the poor solubility of many (co)polymers containing fluorine, an investigation of their molecular-topology is difficult. For such cases, thermome-chanical spectroscopy (TMS) [13] is a useful analytical tool. TMS yields a wide number of (co)polymer parameters, such as the molecular mass distribution (MMD) and the interchain behavior (temperature of transformation, linear thermal expansion, free volume, topological blocks). In the present work the influence of y-irradiation on the phase behavior of CTE powder is explored by the TMS method.
EXPERIMENTAL APPROACH
Materials. The powders of the studied copolymers were from the Konstantinov Kirovo-Chepetsk Chemical Combine (Russia) with brand names "Fluoroplast F-40" (CTE-1) and "Fluoroplast F-40LD" (CTE-2). The copolymers were not subject to post purification.
Thermomechanical analysis (TMA). The methodology of the TMS method used to study fluorine containing (co)polymers and the symbol notation used in the text have been described [13] including the study of granules of CTE [14] and of CTE powder [5]. The features of the powders of fluoropolymers explored by TMS have been described [5, 6]. The methodology for the study of granules or films of (co)polymers is different from the method used for the powder analysis. The investigation of powders by TMS assumes their one-
dimensional compression under optimized pressure. In this case, the copolymer may lose its isotropic structure and can transform into anisotropic forms. The results show that both the degree of crystallinity and the orientation of the crystals change. Some parts of the crystals are oriented perpendicular to the compression pressure vector. Thus, the degree of crystal-linity 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 ^ and ^ are defined in terms of the coaxial and perpendicular orientation of the vectors, respectively. They serve as a measure of the structural anisotropy.
TMA is carried out by penetration of a quartz hemispherical probe into the copolymer. The dynamics of its interaction with a polymer surface have been described [15, 16]. One of the measured values is the change of the linear size of a sample between a substrate and the probe. A polymer sample should have continuity of structure in the whole 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 copolymer.
0.2—1.0 g of CTE powder was compressed under an optimized pressure of 200—250 kg cm-2. A DP36 press (Germany, "Carl Zeis Jena") was used. The diameter of the metallic form used for pressing is 6 mm and meets the #14 Russian standard for surface treatment. Pressing proceeded 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 degree/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 a rate of 5 degree/min.
The accuracy and reproducibility of the TMS method has been analyzed [17]. 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 in the molecular mass (MM) and free volume fraction determinations are less than or equal to ±10%. The data were reproducible within the error limits of ±5 to ±10%, but in some cases, the errors can be as large as ±20% due to heterogeneity of the materials and differences in their thermal and stress history.
Irradiation of the samples of CTE was performed in glass ampoules sealed in air with 60Co y-rays from a Gammatok-100 source at an absorbed dose rate of 140 Gy/sec.
RESULTS AND DISCUSSION
The thermomechanical curves (TMC) ofpowder of copolymers CTE-1 (a, b, c, d) and CTE-2 (e, f, g, h) before (a, b, e, f) and after y-irradiation (c, d, g, h) are presented in Figure 1. The TMC of the powder of copolymer CTE-1 tested in the perpendicular orientation of vectors is characteristic of diblock and fully amorphous polymer (Figure 1a). The copolymer expands with speed a1 = 10.42 x 10-5 degree-1 and its amorphous structure is retained from — 100°C to 16°C. At 16°C, the glass transition of the amorphous region occurs and the transition band of the TMC (line BC) is formed.
The form of the transition band shows the multimodal MMD function of the chain segments between the junctions in the pseudo-network of the copolymer amorphous region (Figure 2, curve 1). Their average values are equal respectively to the average numerical
Mgn = 524000 and the average weight Mgw = 730000 of the molecular mass (MM). The principal fracturing behavior of the network structure of this region is the existence of chain entanglements ("topological junctions"), which play the role of junctions of the network. The abnormal (negative) values of the slope of a curve in the plateau of high elasticity expansion (curve CD, a3 = -10.42 x 10-5 degree-1) can be used to describe the topological junctions. The amount (tytop) of junctions can be calculated from the parity relationship (equation (1))
ф1ор = 0.50 - 0.045[(аз/а2) - 1]-1P,
(1)
where P is the thermomechanical loading to the sample, a2 = 29.0 x 10-5 degree-1, and qtop = 0.52. Except for the topological junctions, the cluster junctions of branching participate in the formation of the network in the amorphous region with a value of Tcl = 106°C. The MM of the macromolecule fragments forming in the cluster region is proportional to AT = Tcl - Tf, where Tf is the flow temperature of the copolymer. The flow of copolymer is begun at Tf = 207°C. The calculated values of the average weight MM of the cluster portion is Mg„ ~ Mgw = 70000 with a weight ratio 9cl = 0.17.
The CTE-1 tested in the perpendicular mode is fully amorphous and the topological junctions are the main component of its pseudo-network junctions. The topo-logical junctions constitute more than half of the total pseudo-network (tytop = 0.52). Therefore, it can be assumed that the actual value of MM of the amorphous copolymer is likely to exceed 2 to 3 times the above calculated value Mgw = 730 000 and is thus Mgn ~ 2000000. Because of the low MM (70000) and low weight ratio (0.17) of the cluster portion, the average MM of the copolymer has to be near to Mw ~ 2000000.
The TMC of CTE-1 powder in the coaxial orientation of vectors is presented in Figure 1b. One amor-
T
100 A B
0 a2C
100 200
cd ai là^Kp^
! I I
300 (c)
M
Tg Tm Tm T
-100 0 100 200 300 400
„ (d)T
100 A
100
Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.