научная статья по теме THERMO-STIMULATED LUMINESCENCE OF GAMMA–IRRADIATED POLYTETRAFLUOROETHYLENE Химия

Текст научной статьи на тему «THERMO-STIMULATED LUMINESCENCE OF GAMMA–IRRADIATED POLYTETRAFLUOROETHYLENE»

ХИМИЯ ВЫСОКИХ ЭНЕРГИЙ, 2014, том 48, № 2, с. 94-99

РАДИАЦИОННАЯ ХИМИЯ

UDK 541.15:541.515:543.422.27

THERMO-STIMULATED LUMINESCENCE OF GAMMA-IRRADIATED

POLYTETRAFLUOROETHYLENE

© 2014 V. G. Nikolskii*, S. R. Allayarov**, D. A. Dixon***

*Institute of Chemical Physics of the Russian Academy of Sciences Moscow, Russia 119991 **Institute of Problems of Chemical Physics of the Russian Academy of Sciences Chernogolovka, Moscow, Russia 142432 ***Department of Chemistry, The University of Alabama Tuscaloosa, AL 35487-0336

E-mail: sadush@icp.ac.ru Поступила в редакцию 19.07.2013 г.

The thermo-stimulated luminescence of y-irradiated commercial polytetrafluoroethylene (PTFE) was explored. There are up to four intense maxima in the luminescence curves of different formulations of PTFE. The position and number of maxima remain constant at doses up to 30 kGy. The differences of the glow curves in the different copolymers may be the result of impurities capable of reacting with ions or radicals formed under irradiation and by differences in the polymer's topological structures.

DOI: 10.7868/S0023119714020082

Polytetrafluoroethylene (PTFE) has a number of excellent properties and is widely used in a broad range of applications [1—5]. The operating properties of PTFE are determined by its phase transition processes. There is a substantial amount of information on the phase transitions in PTFE as measured by different methods. The data available in literature can be divided into three temperature regimes: (i) low-temperature, (ii) 250-320 K and (iii) 400-450 K. The low temperature (i) transitions in PTFE were observed at 160 K [6], 186 K [7], and 176 K [8] by calorimetry [6] and by a torsion pendulum with frequency of 1 Hz [7]. The transitions have been attributed to the beginning of rotation of small segments in the amorphous and para-crystalline phases as a result of the polymer glass transition. The mechanical relaxation process at 208 K was attributed to relaxation between the y- and the P-processes in irradiated PTFE [9]. Double transitions in the range of250-320 K (ii) were attributed to the beginning of the low-temperature motion in the amorphous segments of PTFE macromolecules [10]. Such transitions were also observed at 233 K and 258 K and other transitions at 290-300 K were attributed to transitions in the crystal structure due to the beginning of torsional fluctuations in the crystalline chains [11]. Two transitions were detected at 285 K and 310 K in samples of PTFE with a degree of crystallinity 4060%. The existence of transitions at similar temperatures (294 K, 303 K) was earlier observed by using a dilatometric technique [12]. Thus, the transitions displayed in PTFE near ambient temperature were attributed to the phase transitions. The phase transitions in PTFE heated up to 400-450 K (iii) were observed by a number of groups [7, 10, 11]. It has been suggested that there are two first order transitions at 353 K and 373 K in the crystalline region of PTFE, and two sec-

ond order transitions at 293 K and 413 K in amorphous regions of the polymer.

Analysis of the literature data shows that there are conflicting results about the phase transitions in PTFE. These conflicts are probably connected with the semi-modularity of its topological structure [3], which varies noticeably even at small doses of gammairradiation [13] or due to the different ways of manufacturing PTFE [14].

The critical temperature of the operational properties of PTFE is determined by the interval between the glass transition (Tg) and fusion temperatures (T). Below the glass transition temperature, PTFE loses elasticity and above the fusion temperature, the polymer loses spatial volume. There is considerable confusion concerning the value of the glass transition temperature of semi-crystalline PTFE with a range from 141 K [15] to 393 K [16]. The existence of such a large difference may be connected with the kinetic character of the glass transition and its dependence on experimental conditions and on the physical structure of the polymer.

PTFE is a typical semi-crystalline polymer [3, 13, 14] and an "apparent double glass transition" [17] may be present in PTFE. At present, there is evidence for two or more incompatible phases or domains in the PTFE matrix which are large enough to each have a characteristic Tg of their own. They may be physically or chemically linked to each other. There are examples of real double or even more phase transitions in PTFE as determined by the thermo-mechanical spectrometry (TMS) method [13, 14]. The TMS analysis showed that the topological structure of PTFE is a semi-block polymer and the pseudo-network structure of its amorphous region is formed from the different forms

TSL data and TMS analysis of the different brands of PTFE

TSL data TMS data

Trade mark T Kb 1 max, K modec T, Kd

state state T ? m тЛ T m TÎU T m Tf

F-4 Film 160a 181 280 290a Powder [13] 287 609 641 733 784

(II) 290 618 638 703 785

Sheet [13] 289 563 630 718 776

PF-4EN Film 157 176 ATmax = 20-30) 296

F-4D Film 150 ± 5a 176 210 ± 10a 330 ± 10a Powder [20] 287 620 721 776

(II) 284 603 705 503e 738

Sheet [20] 294 581 682 769

F-4D FUM Film 176 192 (ATmax = 8) 182 (ATmax = 8) 283

F-4D SKL Film 162± a 180 ± 5a 296 ± 1

PF-4D SF Film 132 (AJmax = 20) 190a 292a 360a

Aflon Film 160 214 256

F-701N Film 153 ± 5a 183 ± 2 (ATmax= 15-25) 283 ± 2 293 ± 3a

PF-4KO Film 167 280

Teflon ® Sheet [19] 290 592 614 653e 756

Teflon 6C Powder [32] 286 597 640 775

(II) 289 601 625e 724

PTFE Sheet [19] 290 615 639e 746

a Maximum is weakly expressed.

b The values of temperature of separate maxima ( Tmax) and their half-width (ATmax) shown on the TSL glow curves.

c The analysis of powders is carried out under two analytical conditions, coaxial (||) and perpendicular (±), where the vector of the compression pressure and the vector of loading during the release of the polymer deformation in the thermo-analyzer can be in the same plane (coaxial) or in perpendicular planes (±).

d Temperature of glass transitions of the amorphous region ( Tg). Temperatures of fusion beginning of low-melting ( Tm ), intermediate ( T^m ) and high-melting crystalline portions ( TT^1 ). Beginning temperature of molecular flow (Tf). e Temperature of glass transitions of the cluster regions.

of the crystallized fragment macromolecules, which play a role in interchain branching [14]. Four structures: an amorphous and three crystalline topological forms (high, intermediate, and low melting) are identified by TMS of the powder [13, 18] and the sheet of PTFE [13]. The values of the temperatures corre-

sponding to the phase transitions of the topological blocks determined by TMS are listed in Table.

The term "apparent double glass transition" [17] assumes the existence of an upper Tg, presumably arising from the amorphous polymer under constraints of the crystalline regimes, and a lower Tg, presumably

arising from the amorphous polymer completely or relatively free from constraints due to the crystalline regimes. Then as a result of several crystalline modifications of PTFE, several Tg's may exist for the semiblock matrix of PTFE. The possibility of the existence of several glass transition temperatures may be one of the reasons for the differences in the glass transition temperatures available in literature as determined by different methods. This issue is further explored in the current work.

Thus, using of the TMS method were identified several crystal portions and amorphous region in topological structure of PTFE [14] and was explored their transformation under gamma—irradiation [13, 18], at the proton bombardment [19] and during the laser ablation [20]. Established legitimacies, in particular, the temperature intervals of the phase (glassy, rubberlike and viscous-fluid) transformations allow to to improve operational properties of the PTFE. However, TMS method is being developed for measurements above 173 K and allows to determine the molecular topological parameters of the polymer from 173 K.

Thermo-stimulated luminescence (TSL) is an useful analytical tool to investigate the polymers [21, 22, 23]. The intensity of TSL glows can be increased as a result of a phase transformation due to increasing the molecular mobility, which can promote escape of active centers from traps. The intensity of the TSL changes in the temperature regions where the polymer undergoes structural transformations. Therefore, TSL can be used to determine the low temperature phase transitions of polymers. In the present work, the ther-mo-stimulated luminescence method is used to explore the low-temperature transitions in different brand names of PTFE.

EXPERIMENTAL APPROACH

Materials. The samples of PTFE were from the Konstantinov Kirovo-Chepetsk Chemical Combine (Russia) with brand names "F-4", "F-4D", "F-4EN", "F-4D FUM", "F-4D SKL" and "F-4D SF". The used samples of PTFE with brand names "Aflon" and "F-701" are produced in the Netherlands and Japan, respectively. The polymers were not subjected to post purification.

Thermo-stimulated luminescence analysis. A standard device "TLG" was used to measure the TSL curve of PTFE. The samples of PTFE with brand names "F-4", "F-4D", "Aflon" and "F-701" are powders. They were pressed at 573 K to form films for the TSL measurements. Disks with diameter 8 mm and thickness 500—1000 microns were cut from the pressed polymers. Then, the samples placed on the bottom of a cylindrical sample pan made of duralumi-num and densely pressed them to the bottom of the pan by means of a metal cover with apertures for an output of light. The weight of such pans together with

the sample did not exceed 500 mg. Pans were placed in glass ampoules and were pumping out at 293 K down to 0.14 Pa to release gases dissolved in the sa

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.

Показать целиком