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Polymer Tesring 16 (1997) 7-18

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MATERIAL BEHAVIOUR

Dynamical Mechanical Properties of Polymethylmethacrylate After Exposure to 920 Gamma

Radiation

S. N. Goyanes,a G. M. Benites,a J. J. Gonzalez,” G. H. Rubiolo?b & A. J. Marzocca”

“Laboratorio de Propiedades Mecfinicas de Polimeros y Mat. Compuestos, Dto. de Fisica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pab. 1, Ciudad

Universitaria, (1428), Buenos Aires, Argentina bDto. de Materiales, Comisicin National de Energia Ammica, Av. de1 Libertador 8250 (1429)

Buenos Aires, Argentina

(Received 22 March 1996; accepted 10 April 1996)

ABSTRACT

A study is made of internal friction in commercial-grade polymethylmethacrylate

after y-ray irradiation with doses between 0 and 200 kGy. The influence of y-radi-

ation on the (Y and /3 relaxations is analyzed. Also, this investigation shows that the

structural relaxation a’ is modijed with the irradiation dose. Finally, the results

obtained are discussed considering the changes in the molecular structure produced

by y-radiation. 01996 Elsevier Science Ltd

1 INTRODUCTION

It is well known that polymethylmethacrylate (PMMA) has secondary tran- sitions below the main or glass transition temperature, Tg. This was demon- strated by several researchers in dilatometric, dynamic and dielectric tests.‘,* Those investigators showed that the internal friction (IF), obtained in dynamic mechanical tests, presents three transitions in the temperature region below Tg that are labeled (Y, p and y with decreasing temperature, irrespective of their molecular origin. The (Y transition corresponds to the glass transition.

The temperature at which these transitions appear depends on the frequency of the test. For frequencies of 0.1 and 1 Hz, Tg values of 390 and 398 K,

7

8 S. N. Goyanes et al.

respectively were obtained by previous researchers from the IF peak in PMMA.3-s But this value can change considering the rate of heating of the sample during the experiment and the molecular weight, M,,,, of the. material employed.

The secondary maximum or /3 transition appears as a lower and broad peak in the IF versus temperature diagram and is located near 298 K in experiments reported at about 1 Hz. Considering the chemical structure of PMMA, the molecular mechanism of the /3 maximum is usually associated with the partial rotations of the ester group (COOCH,) about the C-C bond linking the group to the main chain2. The intensity of this relaxation depends, first, on the num- ber of lateral groups for which rotation is possible, and second, on the steric hindrance by adjacent methyl groups in the main chain. The potential barrier is provided by these adjacent methyl groups.

Previous research works on PMMA showed that the most important effect due to y radiation is degradation of the polymeric chain.6-‘0 It was confirmed that the main chain fracture occurs at random, at room temperature, causing a decrease in the molecular weight of the polymer with the increase of the dose and the absence of monomer products.’

According to Todd,6 other chemical changes are promoted by radiation in PMMA. It was proposed that for each scission of the main chain, a lateral group is disrupted with an 83% probability.

Changes in the molecular structure produced by the -y-radiation are reflected in modifications of mechanical and thermal properties. Reported research was generally directed at evaluating results that were obtained through quasi static tests, that is, tensile and compressive,“,‘* creepI and dilatometrici4 tests.

In the present paper we analyze the dynamic mechanical behaviour of y- irradiated PMMA tested at frequencies of the order of 2 Hz for temperatures between 183 K and Tg, and frequencies of the order of 0.2 Hz for temperatures between Tg and 423 K. The results are discussed considering the changes in the molecular structure produced by y-radiation.

2 EXPERIMENTAL

For this study, commercially available polymethylmethacrylate was used. Samples were exposed to y-radiation at room temperature using a 6oCo cell of the Comisicin National de Energia Atomica (CNEA, Argentina). Doses of 50, 100, 150 and 200 kGy were employed. In order to characterize the molecu- lar weight distribution of the specimens, gel permeation chromatograms (GPC) of PMMA were obtained using Shimadzu L-6A liquid chromatograph system with RID-6A refractive index detector, and Shimpack GPC 802-803-804-805- 807 as the columns at 30°C. THF was used as the elute. The measured values

Properties of polymethylmethacylate after 6oCo y-radiation 9

of molecular weight for the unirradiated and irradiated samples are shown in Table 1.

The internal friction, IF, was measured using a torsion, free-decay pendu- lum.15 This pendulum is completely instrumented both in its control and data acquisition systems, and it is commanded by a PC-AT.

The samples used in the tests were cylindrical in shape, with a usable length of 25 mm and diameter of 3 mm. The length of the samples was selected in such a way that the oscillation frequencies at room temperature were of about 2 Hz, with a dispersion among samples of less than 0.2 Hz. The tests were made in argon atmosphere at a pressure of 60 Torr. The temperature was sensed by means of two Chromel-Alumel thermocouples, one of which was placed on the probe at the fixed clamp (inferior clamp), and the other was left in argon at 5 mm of the upper part of the probe. A temperature ramp of 0.4 K/min was chosen, in order to ensure thermal homogeneity throughout the probe.

3 RESULTS

From the molecular weight data of Table 1, the number of scissions per chain, II, produced by gamma radiation can be evaluated as9

IZ = M = M,J.O4xlO-‘GT n

where Mn0 is the molecular weight of the unirradiated material, M, the molecu- lar weight after irradiation, T the irradiation dose in kGy and G the average number of reactions (scissions) per 100 eV of absorbed energy.

TABLE 1 Variation of the Molecular Weight with the y-Irradiation Dose

Dose (kGy) M, (g/mol) M, (g/mol)

0 1.878 lo6 3.496 lo6 50 1.417 105 2.652 105 100 5.750 lo4 1.244 105 150 4.303 104 8.541 lo4 200 3.205 lo4 5.942 lo4

10

The variation of n slope value, it results estimations reported irradiation.‘.”

S. N. Goymws et al.

with the dose can be appreciated in Fig. 1; from the in G = I .49. This value falls within the range of other

in literature for PMMA subjected to gamma

Figure 2 shows the experimental curves, IF and angular frequency, w, obtained from the free oscillation tests with the torsion pendulum for all the analyzed doses. Above the glass transition temperature, the frequency is kept approximately constant as the temperature grows. This kind of behaviour is characteristic of viscoelastic materials at the ‘plateau’ zone.16 Immediately below Tg, the transition zone begins and the frequency increases by almost an order of magnitude.

Since the measured frequency is not constant throughout the peak, it does not seem appropriate to consider the location of the IF peak maximum as the value of the glass transition temperature. Our estimation of Tg was the lower temperature of the range where the frequency remains constant (see Table 2).

Both the (Y and p relaxations are conveniently seen on the logarithmic scale of Fig. 2. The location and the intensity of the p relaxation are influenced by the (Y transition. Then, in order to obtain that contribution to the internal fric- tion, it is necessary to subtract the (Y contribution. At first order this could be fitted as a Debye’s peak. As an example the difference between the original IF curve and the cx contribution, in the range below 7’,, is given in Fig. 3 for the unirradiated sample and the sample irradiated at 50 kGy. In this plot the presence of another transition between the p and Q transitions is more discern- ible. As is mentioned in the literature, this relaxation called CX’ is mainly observed in dilatometric and thermal ana1ysis.3*‘4,17 Only in recent research has this transition been reported in dynamic tests on PMMA.”

60

40

c

20

Fig.

0 50 100 150 200

Dose [kGy]

1. Number of main chain ruptures II plotted against irradiation dose.

Properties of polymethylmethacrylate after 6oCo y-radiation 11

0.0 - a

ii? -0.8 -

5 -1.2 -

-1.6 - cu n

0.0 t

0 b

-0.4

-0.8

3 -1.2

-1.6

-2.0

16

12

I. I, 1.1. 1.1 * 0 1% 200 250 300 350 400

TWI

Fig. 2. Internal friction (M’) and frequency (0) curves obtained from the free oscillation tests for the following doses: (a) 0 kGy; (b) 50 kGy; (c) 100 kGy; (d) 150 kGy; (e) 200 kGy.

The curve of Fig. 3 can be adjusted empirically by the contribution of two Gaussian curves as it is shown in the plot. It is then easy to obtain the tempera- ture and intensity of each peak. The same procedure was employed for all the doses analyzed. We obtained a temperature for the p peak near to 300 K, independent of the irradiation dose; however, the intensity of this relaxation, ZFp, seems to decrease with the dose level as is shown in Fig. 4.

In Fig. 5 the temperature and intensity of the (Y’ relaxation are given. In this case a decrease in the peak temperature at higher doses is observed.

12 S. N. Goyanes et al.

0.0

5 -0.0

g -1.2

-1.6

-2.0

1

1 8. t. I. I

C

I 200 260 300 360 400

TWI

0.0 -

-0.4 -

0 0 d

g

-0.6 -

3 -1.2 -

-1.6 -

-2.0 - L . I. I. I.

150 200 260 300 360 400

-I

I

16

12

6 E 'i;; ,'

4

0

-0.6 -

I . I . 1 I * 1 . I . lo 160 200 260 300 360 400

TWI

Fig. 2. Continued.

Properties of polymethylmethacylate after 6oCo y-radiation

TABLE 2

13

Glass Transition Temperature for Different Molecular Weights

M, C&01) Tg (K)

1.878 IO6 400 1.417 105 399 5.750 104 398 4.303 104 395 3.205 lo4 393

0.09

0.06

g d

0.03

O.O(1

unirradiated

) 200 250 300 350 400

0.09 - SkGy

o.ooL . I I 6

150 200 250 300 350 ’

TWI

0

Fig. 3. Difference A(F) in the internal friction between the original IF curve and the a! contri- bution. The dotted lines are the Gaussian contribution of the CY’ and p transitions to the empiri-

cal fit (full line).

14 S. N. Goynnes et al.

1.00

0.99

0.95

Dose [kGy]

Fig. 4. Dependence of the intensity of the IF at the /3 peak with the irradiation dose, nor- malized to the unirradiated value.

1.8 v I I , I I 0 IF, I lFao

. A $ - A T.. I T.., 1.00

1.4 -

A

A

- 0.99

g 1.2- 0 Q?

. 2 - 0.98 QG d

l.O- 0

0 - 0.97 A

0.8 - 0

1 I I I I -0.96 0 50 100 150 200

Dose [kGy]

Fig. 5. Dependence of the temperature of the IF a' peak and its intensity, normalized to the u&radiated value, with the irradiation dose.

4 DISCUSSION

The principal consequence of y-irradiation is the rupture of the main chain which causes a decrement in the molecular weight (cf. Fig. l), and a conse-

Properties of polymefhylmefhacrylafe after To y-radiation 15

quent decrease in Tg. In Fig. 6, a plot of Tg as a function of the molecular weight is given. In this case we prefer to represent the molecular weight instead of the dose. In the same figure, results obtained by Marzocca et al. I4

in dilatometric tests on the same material are also plotted. The dilatometric tests were performed at a higher heating rate (10 K/min), so if we wanted to compare these results with those from dynamical tests, we should make a correction considering the different heating rate used in the dynamical tests. To do this, we used the relationship proposed by Schwartz:18

TX = T,, + alog t 0 (2)

where Tgo is the glass transition temperature measured at a heating rate v0 and T, is measured at the heating rate v, and a is a constant (a =3.3 K for PMMA). Then we obtained a new set of corrected data points that are given in Fig. 6.

According to Bueche, l9 T, is related to 44, as follows:

where Tg, is the glass transition temperature corresponding to an infinite mol- ecular weight and C is a constant depending upon the polymer density and the free volume contributed by chain ends.

410

400

390

z

b* 380

370

1 I I I I

u-----o___ --------- q

----D-___

--n---___ ---____

0 1xlo-5 2x10-5 3x1o-5 4x1o-5 5x1o-5

l/M, [mG31

Fig. 6. Measurements of T, for different irradiation doses: dynamic (O), dilatometric’4 (+) and dilatometric corrected by heating rate (0).

16 S. N. Goyanes et al.

Both dilatometric and mechanical dynamic tests, at the same heating rate, were fitted using eqn (3) with the constants given in Table 3. From these results, we can conclude that the difference in the parameters Tgr and C are due to the test frequencies. As is known, higher frequencies shift Tg to higher values2’ and our results confirm this.

Now we consider the changes in the secondary transition produced by gamma radiation. Heijboer 2 analyzed the dependence on frequency of second- ary loss maxima originated from different kinds of molecular motion in several polymers. He introduced an Arrhenius equation that correlates a number of experimental data:

lnv = lnv, - ;F m

where E, is the activation energy, T,,, the temperature of the maximum loss, R the gas constant, v the frequency of the test and V, approx. 1013 Hz, a reasonable frequency for molecular vibration.

Using eqn (4) with our experimental data at different irradiation doses, we obtain a value of E, =76 kJ/mol that is similar to the value of 77.8 kJ/mol reported by Muzeau et al. for unit-radiated PMMA.’

According to Heijboer, 2 the location of the p maximum is determined by the local intramolecular barrier. The fact that this temperature does not change with the irradiation dose leads us to the idea that the intramolecular barrier could be not affected by the dose.

In Fig. 4 we can appreciate that ZFp decreases with the irradiation dose. Since the relaxation intensity is governed by the amount of ester groups with capability of rotation2 our data agree with the accepted theory that y-radiation produces scissions of the main chain with a high probability of remotion of the ester lateral group.6

The QI’ transition of PMMA, according to Muzeau et al.,’ is not a true relaxation peak, i.e. it is not an intrinsic property of the polymer. This tran- sition is normally associated to local relaxation in backbone of the molecule. A recent work in dilatometric tests in the same material shows that gamma

TABLE 3 Fitting Parameters for Eqn (3)

Test method T,qx (K) C (K g/mol)

Dilatometric Dynamic

381 3.47 lo5 401 2.29 IO5

Properties of polymethylmethacylate after Yo y-radiation 17

radiation produces a decrement in the intensity of this transition.14 In Fig. 5 we find that low doses of y-radiation magnifies this relaxation, while at higher doses the intensity of this transition decreases. This effect is followed by a shift of the peak to lower temperatures as it happens with Tg. To elucidate the mechanisms responsible for this behaviour further work must be done using doses lower than 50 kGy and several frequencies.

5 CONCLUSIONS

An investigation of internal friction of y-irradiated PMMA shows that the (Y relaxation temperature appears to shift to lower values with the increase of the irradiation dose, while the temperature of the p relaxation does not change. According to our experience, the intensity of the p relaxation decreases with the increase of the irradiation dose, showing the remotion of the ester lat- eral group.

Finally the presence of the (Y’ relaxation, that is very low in unirradiated samples, appears important at doses of 50 kGy and decreases at higher doses in commercial-grade PMMA.

ACKNOWLEDGEMENTS

This work was partially supported by the University of Buenos Aires, Argentina, and the Fundaci6n Antorchas (Buenos Aires, Argentina). The authors want to thank Lit. G. Femgndez (CITIP, INTI, Argentina) for his assistance in the GPC measurements and Dr E. Smolko (CNEA, Argentina) for the y-radiation of the samples.

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