Mechanics of Composite Materials, Vol. 34. No. 5. 1998

EFFECT OF ELECTRON IRRADIATION ON THE MECHANICAL AND THERMAL PROPERTIES OF SOME POLYMER MATERIALS

B. A, Ko'zhAmkulov, A. I. Kupchishin~ B. G. Tepikin, and K. ]3. Tlebaev

The effect of electron irradiation on the mechanical properties (deformability) of polyethylene terephthalate (PET), polytetrafluoroethylene ( PTFE ), polyimide ( P I ), and Fenilon ( Fe ) films, as well as the thermal properties (heat conductivity and capacity) of PTFE is studied experimentally. The thickns~ of the films was 40-50 l~m. Mechanical tests showed that polyimide films were more resistant to radiation titan the other films. The investigation of the changes in the thermal properties of PTFE due to electron irradiation revealed that the phase transitions observed at temperatures of 293 and 303 K in unirradiated PTFE were shifted to lower temperature regions.

The effect of ionizing radiation on the properties of polymer materials depends on the features of the macromolecular structure of the initial substance, its phase state, the conditions of irradiation, and other factors. Electron irradiation considerably modifies the l$olymer structure, namely the latter is transformed from the linear to the three-dimensional state, its molecular m~.qs increases or decreases, free radicals are initiated and annihilated, etc. Needless to say that all these factors greatly affect the properties of the polymers [I]. This study deals with the effect of electron irradiation on the mechanical properties of industrially produced polymer films of polyethylene terephthalate (PET), polyJmide (PI), polytetrafluoroethyiene (PTFE), and Fenilon (Fe) of thickness h no less than 50 ~ m and the thermophysical properties of P T F E of thickness h = 2 ram. For the mech~nlcal tests, samples of the films in the form of 10 x 200 m m 2 strips were placed in a special cartridge. The samples were separated by spacers to ensure better removal of heat. For uniform irradiation, the samples were interchanged at regular time intervals. Irradiation was carried out with electrons of 2 M e V up to absorbed doses of 1, 10, 30, and 100 MGy. The temperature conditions for each type of sample was adjusted by electron beam current with appropriate cooling and did not exceed 25-30~ The mechanical characteristics were determined by standard methods after accumulation of specified absorbed doses. Both unirradiated and irradiated polymer films were tested in tension. The mechanical tests were performed with automatic recording of the load-deformation curves. Figure 1 shows the graphs of this dependence for four types of polymer films, whereas the variation of the dependence for the different radiation doses absorbed by the P E T film is given in Fig. 2. The results of processing the mechanical tests are shown in Fig. 3 as the dependence of the rupture stress ~f and relative strain sf in tension on the absorbed dose of electron radiation. The experimental results show that the polyethylene terephthalate film is the strongest of the unirradiated polymer films. In P E T and Fe films, the yield point is very pronounced. As the data in Fig. 2 Abed Alma-Ata State University, Kazakhstan. Translated from Mekhanika Kompozitnykh Materialov, Vol. 34, No. 5, pp. 683-689, 1998. Original articlesubmitted March 19, 1997; revision submitted March 25, 1998.

0191-5665/98/3405-0489520.00 9 1998 Kluwer Academic/Plenum Publishers

489

t6 I p ' kgf

4~ 0

m 10

m 20

30

40

50

Fig. 1. Load--deformation curves for polymer films of PET (O), PI (A), Fe (s and PTFE (O). 16[-p, kgf

12

AI. 0

10

20

30

40

50

Fig. 2. Loadmdeformation curves for PET at different irradiation doses D = 1 (O), 10 (A), 30 (e), and 100 MGy (A). show, the load-deformation curves for the PET film vary significantly with an increase in the dose of radiation: at D = 1 MGy the film becomes fragile, while at D = 3 MGy the rupture stress decreases by a factor of 8. Most of all, irradiation affects the relative failure strain sf in tension, which decreases by two orders of magnitude at the same radiation dose. At D = 100 MGy, the film disintegrates under the action of the electron beam. The Fe film behaves in a peculiar way. Thus, at s= radiation dose of 5 MGy, a certain increase in strength and a decrease in deformation are observed. Probably, at this dose, the crossliD~ing rate of ruptured cb~in~ of polymer molecules prevails over the rate of destruction. With the radiation dose increased to 100 MGy, the rate of destruction apparently becomes higher and reduces the strength of the polymer. The lowest radiation resistance was observed for the PTFE film which failed easily already at D = 10 MGy. The polyimide fl]m was the most radiation resistant, as its mechanical properties did not change up to the absorbed dose 100 MGy. The results of the calculations made on the basis of structural notions and in the framework of the cascade probability method [2] agree well with the experlmentat data obtained in this study. For studying the simultaneous action of a mechanical load and irradiation, the polymer films were irradiated under a load of 40, 50, 60, 70, and 80% of P0 (initial value of the breaking load). Irradlation with and without a load was performed under the same conditions. The polyimide film under a load of P = 4 kgf was irradiated up to D = 300 MGy and the sample did not rupture. The PET film irradiated under a load of 4.3 kgf ruptured at D = 3 MGy. The PTFE film broke at. D = 0.3 MGy under a load of P = 1.3 kgf, while under P = 0.65 kgs it ruptured at D = 0.88 MGy. It is obvious that the strength of the samples irradiated in the loaded state is much lower than that without a load. This is probably due to the fact that in polymer samples irradiated under a load, the concentration of free radicals emerging as a result of broken chemical bonds increases. Since the strength of polymers depends on

490

200 I O'f, MPa

8.

MGy

0

.

.

.

I00.0

30.0

160 Of, o,6

b

120

80 ,qp

T D, MGy

1" 0

30.0

100.0

Fig. 3. Dependence of rupture stress and relative strain in tension on the absorbed dose for polymer films of PI (O),

pE~v (A), ~ F E (O), and Fe (e). the presence of strained chemical bonds, the action of the radiation leads to degradation of the polymers, i.e., the rupture of all (including stressed) bonds. The accumulation of such ruptures in a degraded polymer with an increase in the radiation dose leads to an increase in deformation and thus accelerates the process of degradation. I n v e s t i g a t i o n of t h e e f f e c t of electron i r r a d i a t i o n on the phase t r a n s i t i o n s observed in polytetrafluoroethylene at room temperature is of special interest. It is known that the heat capacity and conductivity are highly sensitive to the phase transitions observed in linear polymers at certain temperatures. In [3], the results of measuring the heat and thermal conductivity of PTFE in the range of room temperatures are given and the presence of two phase transitions is established: one of them occurs at a temperature of 293 K and is very prouounced while the other observed at 303 K is weaker. X-ray structural investigations showed that the observed anomalies in the behavior of the dependences of the thermophysical characteristics on the temperature were associated with transitions of the "order-disorder ~ type. The qualitative theory of the phase transitions observed in the PTFE at room temperature is described in [4] on the basis of the theory of the heat capacity of phase transitions suggested by Bmgg and Williams. For a better understanding of the processes occurring in PTFE at room temperature, it is necessary to examine the effect of ionizing radiation on phase transitions. In some studies (see, for example, [5, 6]) it was found that y-irradiation of PTFE with doses of 0.05 and 1.9 MGy shifted the phase transition observed in nonirradiated PTFE at T = 293 K toward the low temperature region by 1 and 13 K, respectively. The investigators in [7] did not detect any phase transitions

491

/

~., W/m-K

o. o I_

d~

"

0.20~ 80

I 160

I 240

I

m T, K

320

Fig. 4. Temperature dependence of heat conductivity of P T F E at D = 0 (A), 0.04 ([3),and 1 M G y (0). The relative error is 6 = 1.4% . in PTFE, whereas y-irradiation with doses of 1.17 and 1.26 M G y considerably improved the heat conductivity. The absence of phase transitions in [7] was probably due to the insufficient sensitivity of the equipment used. The results of investigating the effect of radiation on the thermophysical properties of P T F E obtained by different researchers are rather contradictory; moreover, there are no studies on the effect of electron irradiation on the thermophysical properties of PTFE. Therefore, the effect of electron irradiation on phase transitions in P T F E was studied by measuring the thermophysical characteristics of the heat conductivity and capacity. The samples of P T F E were industrially produced tablets of diameter d = 30 ram and thickness h = 2 ram, with a density of p = 2132 kg/m 3 and degree of crystallinity of 65%. The samples were irradiated by a linear electron accelerator of 6 MeV. The temperature during irradiation was mainlined by a special Cooling device in the range of 25 to 30~ The heat conductivity ~. and capacity C were measured on an experimental setup using the nonstationary method of the heated disk. The limit of the basic relative error of the measurements was 1 % for the heat conductivity and 3 % for the heat capacity. The thermophysical characteristics were measured in the temperature range of 80 to 330 K at 5 K intervals. In the region of phase transitions, the measurements were carried out in 1 K increments, while the relative error of the measurements was 8 = 1.4% for the heat conductivity and 1.7% for the heat capacity. Figures 4 and 5 show the experimental curves of the temperature dependence of the heat conductivity and capacity of P T F E unirradiated and irradiated with electrons up to doses of 0.04-1 MGy. As follows from the experimental curves, the heat conductivity of the P T F E irradiated with doses of 0.04 and 1 M G y up to the appearance of a phase transition increases monotonically with an increase in the temperature. This testifies that the irradiated PTFE is a partly crystalline structure whose increased heat conductivity depends on the ratio of crystalline and amorphous phases. In addition, the curves of the heat conductivity of the PTFE irradiated with doses of 0.04 and 1 MGy, over the whole temperature interval, lie higher than those of the nonirradiated PTFE. The increase in the heat conductivity of the PTFE irradiated with doses 0.04 and 1 MGy at the same temperature can be explained by the increased crystallinity. This is because during radiation degradation, the fragments of molecules attach to each other, thus forming small crystallites. The increase in the crystallinity in PTFE for small radiation doses was observed in [5, 7]. The phase transition observed in PTFE at a temperature of 293 K during electron irradiation is shifted toward the region of low temperatures. At an absorbed dose of 0.04 MGy, the shift constitutes 1 K, while at 492

600/C, J/kg.K

j

~

r

3OO 2OO

100 80

, K

160

240

320

Fig. 5. Temperature dependence of the heat capacity of PTFE. Notation as in Fig. 4. 8 = 1.7%.

1 MGy, the phase transition is shifted by 13 K. The poorly pronounced phase transition taking place in unirradiatsd P T F E at 303 K is not observed in irradiated PTFE. The shift of the phase transitions in PTFE under electron irradiation results from the increased rate of thawing of segmental mobility around the main chain. Since the final rate of the change in the form of the chain macromolecule depends on the interaction between the elements of the chain, which requires certain activation energies for its development, upon irradiation the activation energy decreases, and then the flexibilityof the macromolecules becomes higher than before irradiation at the same temperature. Therefore, irradiationaccelerates the process of disordering of the system that leads to shifting of the phase transition.All this agrees well with the advanced theory of phase transitions [4]. It is characteristic of the heat capacity of an irradiated PTFE that it depends linearly on the temperature up to the appearance of a phase transition. However, the heat capacity at doses of 0.04 and 1 M G y decreases (see Fig. 5). Obviously, this has to do with the ratio between the crystalline and amorphous phases. The region of the phase transition on the temperature curves of the heat capacity of irradiated PTFE at doses of 0.04 and 1 M G y is also shifted toward low temperatures by I and 13 K, respectively.In our opinion, this displacement is caused by a decrease in the average length of the PTFE chain due to destruction. Conclusion. In this study, the experimental dependences of the load P on the relative elongation for industrially produced polymer films of PET, PI, Fe, and PTFE, as well as the dependences of P on for P E T at radiation doses of D = 1, 10, 30, and 100 M G y are obtained. Moreover, the rupture stress and relative elongation for polymer films in tension are shown as a function of the radiation dose. The experimental results agree well with the theoretical calculations [2]. The temperature dependences of the heat capacity and conductivity of P T F E irradiated with electrons at doses of 0-1 M G y in the temperature range of 80-330 K are analyzed. It is shown that the character of the changes in the temperature dependences of the thermophysical properties of P T F E irradiated with different doses is similar to that of unirradiated PTFE. It is established that at doses of 0.04 and 1 MGy, the absolute value of the heat conductivity increases, while that of the heat capacity decreases. It is found that electron irradiation of PTFE at 293 K leads to s h i f t i n g of the phase transition to the region of low temperatures by 1 and 13 K at radiation doses of 0.04 and 1 MGy, respectively.

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C. E. Yermatov, A. I. Kupchishin, and B. A. Kozhamkulov, UA study of mechanical properties of polymers irradiated under load," Cryst. Latt. Def. Amorph. Mat., 13, Nos. 3-4, 153-155 (1987). R.C. Steere, "Detection of polymer transitions by measurement of thermal properties," J. Appl. Polym. Sci., 107, No. 11, 1673-1685 (1966). A.I. Kupchishin, B. G. Tepikin, and K. B. Tlebaev, ~Effect of electron irradiation on phase transitions in polytetrafluoroethelene (Teflon),~ in: Proc. 14th Int. Conf. Application Acceleration in Research and Industry, Denton, Texas, U S A (1996), p. 139. K.-L. Hsu, D. E. Kline, and J. N. Tomlinson, ~Thermal conductivity of irradiated polytetrafluoroethylene,~ J. Appl. Polym. Sci., 9, 3567-3574 (1965). W . R . Licht and D. E. Kline, "Specific volume of irradiated polytetr~luoroethylene, ~ J. Polym. Sci., A2, 4673-4678 (1964). B.A. Briskman, V. D. Bondarev, and V. P. Savina, ~Effect of y-irradiation on heat conductivity and density of some polymers," Plast. Massy, No. 7, 7-10 (1973).