o

7.

G. A. Doshchinskii and V. I. Maksak, "Experimental study of plastic deformations under a complex loading," Inzh. Zh. Mekh. Tverd. Tela, 118-122 (1966). A. K. Malmeister, "Fundamentals of the strain-locality theory (Review i)," Mekh. Polim., No. 4, 12-27 (1965).

CREEP OF POLYOLEFINS* B. A. Averkin, V. P. Volodin, and S. B. Gut

UDC 539.376:678

The deformation behavior of various polyolefins on extension at a constant rate has been studied previously [i-4]. These were low-pressure polyethylene (LPPE), high-pressure polyethylene (HPPE), and copolymers of ethylene and propylene containing two (EPC-2) and seven (EPC-7) parts by weight propylene. It turned out that in their deformation properties the enumerated polyolefins belong to a single class of substances. The main structural parameter which determines the behavior of the various polyolefins proved to be the branching of their macromolecules, which is responsible for the density of the substance (the degree of crystallinity). It was natural to study the deformation behavior of these same polyolefins under other deformation conditions, and first to study the creep process, which has great practical importance. In addition to the enumerated polymers, we have also studied a medium-pressure polyethylene (MPPE). The density of the polyolefins p~o studied, as measured at room temperature~ was varied over wide limits: 917 kg/m 3 (HPPE), 943 kg/m 3 (EPC), 949 kg/m 3 (LPPE), and 969 kg/m 3 (MPPE). All the polyolefins had a finely spherulitic structure. The mean diameter of the spherulites, as determined by the method of low-angle polarized light scattering, was about 3~. The polyolefin specimens were prepared by the procedure described in [i]; they were lengths of filaments 1-2 mm in diameter and not less than 50 mm long, with conical thickenings at the ends, formed on gentle pressure of the end of the specimen to a hot surface. These thickenings fitted tightly into the conical holes of the apparatus clamps. This method of sample fastening ensures the least effect of the clamps on measurement of specimen deformation during the creep process. Control experiments showed that the polyolefins were subject to processes of further crystallization with only slight change in their specific volume due to thermal expansion or as a result of loading. Thus, when MPPE specimens were kept in a thermostat at 40°C for 5 min, their density changed by 0.7%. The same increase in density was also observed when the specimen was deformed by 2%. The maximum change in the density of the MPPE was I~8% [annealing for 5 min at IO0°C). Polyolefins are able to retain a nonequilihrium state for a long time at room temperature: specimens of MPPE stored for a year nevertheless increased their density by 0.5% when they were annealed for 5 min at lO0aC. The density value reached remains practically unchanged, at least over a period of 3 h, if the temperature does not exceed 80-90°C. At 100°C, after approximately i h, the density of MPPE began to rise again, as a result of processes of thermooxidative degradation. Therefore~ all the investigated polyolefin samples were preliminarily annealed at 100QC for 5 min, and the experimental time at temperatures above room temperature was limited to a period of 3 h at 80-90°C or to I h at 100°C. Creep of polyolefins was investigated on an LPI universal assembly, the combined scheme of which (only for the creep regime) is shown in Fig. io A specimen of the polymer I under study was fastened in the clamps 2. The upper clamp was connected in a hinged way to the movable plate of capacity sensor 3 of a dynamometric device. The lower clamp was fastened to rack and pinion 4, which was movable by reversible motor 5. The load on the specimen is *A report presented at the Third All-Union Conference on Polymer Mechanics, Riga, 1976. M. I. Kalinin Leningrad Polytechnic Institute. Translated from Mekhanika Polymerov, No. I, pp. 22-26, January-February, 1978. Original article submitted August 25, 1976.

0032-390X/78/1401-0017507.50

© 1978 Plenum Publishing Corporation

17

23

-

Fig. i.

Combined scheme of setup.

assigned by calibrated spring 6. The gap between the plates of capacity sensor 3 is maintained unchanged during the process of operation, due to a servo system consisting of LCgenerator 7, frequency discr~m~uator 8, amplifier 9, and reversible motor 5. Together with the capacity sensor 3 and spring 6, it represents the scheme of an absolutely rigorous, highsensitivity dynamometer. As a result of the action of the force assigned by the initial tension in spring 6 on the specimen, a deformation in creep will be observed, which is recordable by the deformation sensor i0 and the recording potentiometer Ii. To study creep of the specimen at constant stress, the slides of rheocord potentiometers 13 and 14 are fastened to the rack-and-pinions 4 and 12; these are arms of Wheatstone bridge 15. An increase in specimen length during the creep process leads to an imbalance in bridge 15. The imbalance signal, amplified by amplifier 16, causes rotation of the reversible motor 17, which, in turn, decreases the tension in spring 6, and restores the balance in bridge 15. On appropriate selection of initial conditions, the product FZ is constant, where F is force and I is specimen length; that is, the stress will be constant at an unchanged specimen volume. The least recordahle displacement of the lower specimen clamp is 0.01 m , and the accuracy of force measurement is i0 -s N. Constancy of the FZ product is maintained with an a=curacy of 0.2%. The thermal chamber 18 permits one to conduct experiments in the temperature range from --i00 to 150°C. The temperature is maintained with an accuracy up to 0.5@C by the use of a servo system consisting of amplifier 19, reversible motor 20, and autotransformers 21 and 22. The emf difference between the thermocouple and the reference voltage of potentiometer 23 serves as the controlling signal. The cold junctions of the thermocouples are in thermostat 24. Temperature is measured and recorded by recording potentiometer 25. The uniformity of specimen deformation during the creep process was checked in specially set up experiments. Using an aniline dye which had no effect on the behavior of the specimen, a series of lines was laid off on the specimen, and these were photographed during the process of specimen creep. We show the results of one of these experiments in Fig. 2. Along the ordinate axis is laid off the local deformation of the specimen; along the abscissa, the coordinate of local deformation is read off from the immobile clamp. T h e parameter of the curves is time. It is evident that the creep deformation is uniform only at small deformations (in all the cases studied, on change in stress from 10 ~ to 10 s N/m s ), not exceeding 5-8%. Therefore, when such a deformation had been reached by the specimen, the experiments were stopped. Later on, the buildup of creep deformation in individual sections of the specimen begins to take place at a greater rate. It is precisely in these sections that a neck is TABLE 1 Polyolefin MPPE LPPE EPC H PPE

18

To, °C

Vo "lOS, mS/kg

-9 -19 -20 -50

! ,039 1,044

t,032

1,048

tO0 ~0

I" I Oi°, m2/N nitrogen,

60

~"

~f

20 I

,o

,b

Fig.

,o

4o

-~° l mm

I,l0s, see

f

-20

so

_--

oxygen

i

0

)

I

,

2

Fig. 2. Distribution of local deformation over sample length. time in minutes.

0

,0

Fig. 3 Numbers on curves denote

Fig. 3. Dependence of compliance on time: i) HPPE, T = --30°C; 2) EPC, T = 0°C; 3) MPPE, T = 0°C; 4) MPPE, T = 70°C; 5) LPPE, T = 60aC; oo = 55°i0 s N/m2; 6) MPPE, T = 100=C, Oo = 40"105 N/m 2. The moment of oxygen admission is marked by a vertical line. The section of the creep curve in nitrogen medium is depicted by a dashed line. formed with time. In the case of LPPE, for example, this usually took place at a total specimen deformation of about 20%, almost regardless of the magnitude of the stress; that is, neck formation during the creep process, just as in the case of extension at constant velocity [2], is associated with the deformation of the specimen, and not with the stress. There may be several weak points in a specimen; we often observed two, more rarely three, weak points. The importance of weak points in specimens of polyolefins during the creep process may really not be so small, even in the case where deformation seems to be uniform. Actually, the reproducibility of polyolefin stretching diagrams may reach 1-2% [1-4], while the reproducibility of creep curves for the same specimens has never been better than 5%, in spite of careful experimentation. The creep of polyolefins* was studied in air, oxygen, and nitrogen atmospheres at a constant stress of 40.i0 s N/m 2, in the temperature range 20-I00°C. It turned out that the 3-h creep curves at room temperature only depend little on the medium composition. When the temperature is raised, the rate of increase in creep deformation in a nitrogen or air medium proves to be greater than in the oxygen medium. In Fig. 3 we give the compliance dependence l(t) = ~(t)/ao [where ~(t) is the deformation in creep and Oo is a constant stress] for MPPE at 100°C. Initially the polymer creep was observed in a nitrogen atmosphere, and after 70 min the nitrogen was replaced by oxygen. It is evident that thereupon the compliance decreased, and 5 min after the admission of oxygen it had reached a certain practically constant value, which was maintained for almost 1.5 h. If one keeps a sample under the same conditions without load and measures its density, it turns out that the density will increase by 0.1%. The phenomena observed reflect the development of thermooxidative degradation processes in the polymer. These processes take place somewhat more slowly even in an air medium: at 100°C the retardation of the creep process in an air medium becomes noticeable i h after the start of the experiment. The density of polyolefins, of course, changes most greatly upon change in temperature. This occurs, first, because of thermal expansion of the polymer, and second, because of changes in its structure. In Fig. 4 we show dependences of compliance on specific volume V as measured by the method of hydrostatic suspension in isopropyl alcohol. It is evident that the compliance of all the polyolefins studied rises, not monotonically, with increase in specific volume. Up to a certain critical value of specific volume Vo the compliance increases slightly and its values for the various polyolefins are close to another another. After Vo is reached,

*We have not given the form of the creep curves, since qualitatively they are similar to the curves for polyethylene, which have been repeatedly described in the literature [5].

19

300 J'lOJ°'m2/N

// //

250

2ooi

//

ooi i

/

Fig. 4. Dependence of compliance on specific volume at ao = 55-105 N/m 2 for HPPE (I), EPC (2), LPPE (3), and MPPE (4), and one V -- Vo difference (5).

/

[o0

] Qo6

~/'J~j -~,o3

0

v-I0 3, r~/.gf O.O3

O,O6

the compliance begins to increase sharply, and a break is observed on the curve. The dependences of compliance on specific volume are identical in all the polyolefins studied and can be combined if the dependence of compliance is constructed as a function of the V --Vo difference (curve 5, Fig. 4).* Values of the critical specific volume and the temperatures corresponding to it are given in Table i. From the foregoing it follows that the creep of the various polyoleflns at the same value of V-- Vo, that is, at corresponding temperatures, should develop in an identical manner with time. This is actually the fact. In Fig. 3 we give creep curves for various polyolefins at corresponding temperatures. An essentially complete agreement of the creep curves for the various polyolefins is seen (within the limits of experimental error). Hence, it follows that the critical specific volume and the critical temperature are important factors which determine the behavior of the polyolefins studied during the creep process. The values of Vo or To distinguish the behavior of various polyolefins which are substances of a single class. In a study of the temperature dependence of the stretch diagrams of the same polyolefins [4], the conclusion was drawn that there is a critical temperature, called the structural hardening temperature, on attainment of which the deformation behavior of polyolefins changes abruptly. The temperature values given in Table 1 and in [4] approximately coincide; consequently, in both cases we have to do with the very same process which is manifested in a definite temperature range close to the structural hardening temperature of each polyolefin on attainment of a definite value of the specific volume for each polyolefin also. The structural hardening temperature of crystalline polyolefins depends on the time of action on the polymer. The smaller the time, the larger values Vo and To will have. For example, in the case of HPPE at t = 0 sec, Vo = 1.06 and To = --20°C; when t = 5-I0 s sec, Vo = 1.05 and To = --50°C. This indicates that the structural hardening temperature and the critical specific volume reflect the presence of a relaxational transition in crystalline polyolefins. One might attempt to connect up To with the glass temperature of the amorphous phase of a crystalline polyolefin. However, first of all, the very concept of an amorphous phase in crystalline polymers is unclear at present; and second, the glass temperature values known from the literature (if only for HPPE), vary from --II0 to --20°C according to various references. Therefore the nature of the relaxation transition observed in polyolefins should be analyzed in considerably greater detail.

*The temperature dependence of the compliance of polyolefins is described by similar curves, but the inflection is less sharply expressed. Moreover, the curves for various polyolefins cannot be combined with one another by such a simple method.

20

CONCLUSIONS i. In their deformation properties the polyolefins studied are substances of a single class even under creep conditions. The principal factor which distinguishes the behavior of one polyolefin from another is the critical values of the specific volume Vo. At an identical value of V -- Vo (at corresponding temperatures) the creep process is identical in polyolefins of different density. 2. Change in the specific volume of polyolefins takes place upon the action, not only of temperature, but also of stress, and also as a result of thermooxidative processes, which exert a strong effect on the creep of these substances. LITERATURE CITED i. 2.

3. 4.

5.

Yu. A. Antsupov, V. P. Volodin, and E. V. Kuvshinskii, "Analysis of the cold drawing of polyolefins using stretching diagrams," Mekh. Polim., No. 3, 509-514 (1968)~ B . A . Averkin, Yu. A. Antupov, V° P. Volodin, and E. V. Kuvshinskii, "Cold drawing of crystalline polyolefins. Effect of drawing rate and temperature," Mekho Polim., No. 3, 404-409 (1969). Yu. A. Antsupov, V. P. Volodin, and E. V. Kuvshinskii, "Uniform monoaxial stretching of polyethylenes. Effect of deformation rate," M e ~ . Polim., No. 6, 987-992 [1972). Yuo A. Antsupov, V. P. Volodin, G. N. Kosterina, and E. V. Kuvshinskii, "Uniform monoaxial stretching of polyethylenes. Effect of temperature," Mekh. Polim., No. 3, 540544 (1974). Yu. M. Molchanov, Physical and Mechanical Properties of Polyethylene, Polypropylene, and Polyisobutylene [in Russian], Riga (1966), p. 439.

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