EFFECT OF γ IRRADIATION ON THE RELAXATION PROPERTIES OF THE AMORPHOUS PHASE OF PET

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EFFECT OF γ IRRADIATION ON THE

RELAXATION PROPERTIES OF THE AMORPHOUS PHASE OF PET

S. El-Sayed, R. de Batist

To cite this version:

S. El-Sayed, R. de Batist. EFFECT OF γ IRRADIATION ON THE RELAXATION PROPERTIES

OF THE AMORPHOUS PHASE OF PET. Journal de Physique Colloques, 1987, 48 (C8), pp.C8-

501-C8-506. �10.1051/jphyscol:1987878�. �jpa-00227182�

JOURNAL DE PHYSIQUE

Colloque C8, suppl6ment au n 0 1 2 , Tome 48, d6cembre 1987

EFFECT OF Y IRRADIATION ON THE RELAXATION PROPERTIES OF THE AMORPHOUS PHASE OF PET

S. EL-SAYED* and R. DE BATIST*"

Physics Department, SCK/CEN, B-2400 Mol, Belgium

* ~ a t i o n a l Centre for Radiation Research and Technology, Atomic Energy Authority, Cairo, Egypt

* * Also Rijksuniversitair Centrum Antwerpen, B-2020 Antwerpen, Belgium

Resum6

-

L'effet d'une irradiation gamma sur la relaxation mdcanique du poly-Bthylbne-tdrephthalate (PET) a dtd dtudiC sur des feuilles amorphes, prdpardes par compression B l'dtat fondu suivie de trempet couvrant un domaine de fluence jusqu': 300 Mrad. Le spectre d'amortissement a dtB mesurd B differentes frdquences et consiste en trois bandes d'absorption: le pic a B 355 K, le pic

B

?I 210 K et le pic y B 130 K (5 1 Hz). Irradiation jusqu18 30 Mrad conduit B une augmentation du module et B un ddcroissement de l'amortissement. Entre 30 et 300 Mrad, le module ddcroit fortement et le frottement augmente en fonction de la fluence. Ces observations peuvent ctre interprdtdes par une comp6tition entre des effets de r6ticulation et de dggradation.

Abstract

-

The effect of y irradiation on the mechanical relaxation of polyethylene terephthalate (PET) has been studied using amorphous films prepared by melt-pressing and quenching, covering a fluence range up to 300 Mrad. The relaxation spectrum has been measured at various frequencies and is characterized by 3 absorption bands: the a peak near 355 K, the B peak near 210 K and the y peak near 130 K (at 1 Hz). Irradiation up to 30 Mrad leads to an increase in modulus and a decrease in damping. For doses between 30 and 300 Mrad, the modulus decreases markedly whereas the damping increases with increasing fluence. These observations ate interpreted in terms of a competition between cross-linking and chain scission.

INTRODUCTION

Polyethylene terephthalate (PET) is a semicrystalline polymer considered as one of the most commercially important linear polyesters. It is prepared by condensation of terephthalic acid and ethylene glycol. The main repeat unit in the molecule has the structure -CO-0-CH -CH -0-CO-C H

- .

The predominant end groups are -CH2-CH -OH and -COOH. 2 2 6 4

The mechanical and dielectric relaxation behaviour of PET has been studied in 2 the past by Wolf and Schmieder C13, Thompson and Woods C21 and Illers and Breuer 131. The work of Illers and Breuer suggests that PET exhibits 2 broad absorption peaks: the a peak, which corresponds to the glass transition of the mater'al and a very wide B peak. These authors resolved the

B

proce-y by plotting (6Q /6logfIT

-1

taken at 1 Hz ag_a'nst temperature. The quantity (6Q _/6l0gf)~ was obtained from measurements of Q

'

in the range of 0.1 to I0 Hz at constant temperature. This plot yielded three peaks at

-

165"C,

-

105°C and

-

70°C, respectively. The lower temperature peak is more generally called a y peak and is attributed to the hindered rotation of CH2 groups, in analogy with the mechanical results for other polymers containing a sequence of (CH ) groups (crankshaft mechanism as proposed by Schatzki [ 4 1 ) . The two other peaks, 'at

-

^{105OC and}

-

70DC, were ascribed to the motion of COO groups associated with the gauche and trans conformation of the

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987878

C8-502 JOURNAL DE PHYSIQUE

polymer, respectively (Fig. l.a,b). Early studies of the effects of irradiation on PET revealed either cross-linking (Charlesby C6,71) or degradation (Little C81, Todd C91). However, most of these irradiations were carried out in air, which is likely to lead to radiation induced oxidation.

FIG. l.a,b

Molecular conformations in PET C51

a. The crystalline "trans"

conformation

b. The amorphous "gauche"

conformation

In the present work the aim was to study the effect of y-irradiation on the mechanical relaxation properties of the amorphous phase of PET, with a view to contributing to solving this controversy.

EXPERIMENTAL

Specimen preparation

Amorphous PET films were prepared by melt pressing commercial PET pellets (Janssen Chemical, B-2340 Beerse, Belgium) in a hydraulic press. The pellets were put in a square mold between two metal sheets covered with aluminium foil and transferred to the press which had been brought to 270°C (the melting point of PET is 260°C). After 30 min. the assembly was removed from the press and quenched in cold water. To dissolve the aluminium foils, the PET films were treated with a 5 % NaOH aqueous solution for 30 min.

The resulting PET samples, having a thickness of 1 mm, were washed carefully and dried and cut to parallelepiped shape with 4 cm length and 6 mm width. Fig. 2 shows the results of differential scanning calorimetry measurements for as-received pellets and melt-pressing prepared films. It can be seen that the crystalline fraction present in the pellets (melting point 232°C) has been completely removed by the preparation of the film.

Internal friction measurements

For studying the internal friction of the material, two types of system were used, operating at two distinct frequency ranges.

a) The flexural resonance technique was applied using equipment developed in our laboratory. In this technique the specimen is maintained in resonance by means of an electromechanical closed loop system. The frequency of vibration depends on the specimen geometry and the system covers a range of frequencies from approximately 20 to 2000 Hz. Cooling is achieved by filling a liquid nitrogen reservoir in contact with the specimen, and the measurements are carried out during inertial warming to room temperature (over a period of several hours).

The specimen is fixed at one end and the other end is free between two electrodes used for driving and detecting vibration; oscillation is assured by electrostatic coupling between the driving electrode and the specimen. The damping of the specimen changes with the variation of the external conditions such as the temperature. This means that the driving voltage must also be changed in order to keep the vibration amplitude constant. In this case the driving voltage is used as a measure of the internal friction.

The damping is proportional to the components of the driving voltage required for maintaining the amplitude of oscillation constant at Ed and is given by the following relation:

FIG. 2

DSC data for deter- mining melting points

in PET

A: semicrystalline PET pellets, as-received B: PET films after

melt-pressing and quenching in cold water

TEMPERATURE I C I -1 E E

Q

^{= K}₂ ^S

w E

0 d

where Q-l is the damping, E and El are t$ components of the driving voltage (static bias voltage and gplitude of 1 harmonic) and w represents the resonance frequency.

The K factor has to be determined by the free decay me hod. By interrupting

-i

the feedback circuit the logarithmic decrement (or Q ) is determined by counting the number of periods needed for the vibration amplitude to decay between two prefixed values.

The specimen and the electrodes are enclosed in an evacuated chamber. The detection voltage Ed and the driving components (E ,E ) are measured with a multichannel digital voltmeter. The resonance frequt?ncy is registered using a 1 frequency meter. Fig. 3 gives a typical example of the change of the damping and of the resonance frequency as a function of temperature in the range 100

-

^{300 K.}

b) Secondly, the internal fricgion spectrum has also been measured using the

"Micr~mecanal~ser" of the Materials Science Department at the ULB University of Brussels. This set-up is based on an inverted torsion pendulum driven in harm nic oscillation at a constant frequency ranging between very low and low (lo-' Hz to 1 Hz). The detection occurs by using reflection of a light beam from a mirror moving with the specimen. A low drift amplifier yields a voltage proportional to the angular strain of the specimen. The specimen is surrounded with a cryostat and a furnace to allow variation in temperature between 100 K and 670 K and so designed as to keep the temperaLyre gradient over the specimen negligibly small. The damping is calculated as Q from the phas_elrelationship between driving stress and strain. Fig. 4 represents a typical Q spectrum as well as the change of the modulus over a range of temperature between 100 K and 400 K, at a fixed frequency of 1 Hz. Further experiments have been done using different values of frequency, i.e. 0.01 and 0.1 Hertz.

Y Irradiation of the specimens

Gamma -9radiation of the specimens was carried out at room temperature under vacuum (10

-

Pa) to avoid irradiation ind~ted oxidation. Doses ranging from 0 to 300 Mrad were obtained using either the Co cell of S.C.K.1C.E.N. Mol with a dose rate of 60 ~ r a d f h , or the spent fuel irradiation facility yielding a wider spectrum of gamma energies with an average dose rate of 15.5 Mradfh.

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FI?. 3

Q- ( . ) and resonance frequency (

-

^{) as a}

function of temperature using the flexural resonance technique.

Notice the peak at 233 K and the y peak at 170 K

Fig. 4

Modulus and Q-I as a function of temperature at a frequency of 1 Hz using the inverted torsion pendulum. Notice the a peak at 358 K and the B peak at 210 K

+I25 +225 325

T e m p LKI RESULTS AND DISCUSSION

From Figs. 3 and 4, one may distinguish three broad, more or less well-defined relaxation bands. Following established nomenclature, these are: the a peak, occurring near 355 K for a frequency of 1 Hz; the 6 peak, occurring near 210 K and near 230 K for a frequency of 1 Hz and of 260 Hz, respectively; and the y peak occurring near 170 K for a frequency of 300 Hz. As mentioned before, the a peak corresponds to the glass transition of the material and has been related to the micro-Brownian motion in the amorphous phase, The B peak may be due to the reorientation of hydroxyl groups (10) in the non-crystalline phase of the polymer, or, following Illers and Breuer [31, to. the motion of COO groups. The y process is ascribed to the hindered motion of the methylene sequence of the chain in both the cr'ystalline and the non-crystalline regions of the polymer. The activation enthalpy for the process has been determined from our data using an Arrhenius graph of log frequency versus inverse peak temperature and was found to be equal to 0.69 eV (Fig. 5), which compares well with the valge of 0.74 eV found by Illers and Breuer;

the corresponding value of f is 3.9 x 10 Hz.

FIG. 5

Arrhenius graph for the B relaxation amorphous PET

The effect of Y irradiation on the relaxation behaviour in the B peak region is illustrated in Fig. 6. Up to a gamma irradiation dose of 30 Mrad, the polymer exhibits a decrease in B peak damping accompanied by an increase in the modulus. In addition, the glass transition temperature increases from 80°C for an unirradiated specimen to 86OC after irradiation. For higher doses, the modulus starts to

decrease again whereas the damping increases. Furthennore, infra-red measurements have shown a reduction of the absorption bands related with the benzene rings in specimens irradiated to 300 Mrad. These results are consistent with the idea that both cross-linking and chain scission occur simultaneously during irradiation. At the lower doses, (up to 30 Mrad), cross-linking is predominant and leads to a

FIG. 6. Variation with Y dose of (A) and modulus (B) in the B peak region, measured at 1 Hz

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decrease in free volume resulting in a stiffening of the polymer chain and hence an increase of T

.

These effects lead to an increase of the modulus. The observed decrease in flgpeak damping, which is at least in part due to relaxation of the -COOH groups, would then suggest that cross-linking occurs in the vicinity of these -COOH groups, thus reducing their mobility.

For the higher radiation doses, chain scission appears to take over, leading to the decrease in modulus and to an increase in the concentration of -COOH groups (as suggested by IR absorption) and thus an increase of the

fl

peak height.

Acknowledgements

The authors thank Prof. J.C. Bauwens (Universitg Libre de Bruxelles) for the use of the Micromecanalyser and for frequent discussions, Prof. H. Reynaerts (Catholic University of ieuven) for help "ith specimen prkparation and general discussions, Messrs. W.,Timmermans, M. Segers and Mrs. R. Vercauter for-the thermal analysis and Mrs. M.J. Webers for skilful text processing.

References

[I] K. Wolf and K. Schmieder, Symp. Intern. Chim. Macromol., Suppl. Ric. Sci., 3 (1955)

C21 A.B. Thompson and D.W. Woods, Trans. Faraday Soc. 52, 1383 (1956) C31 K. Illers and H. Breuer, J. Colloid Sci.

18,

1 (19n)

[41 T.F. Schatzki, Meeting of Am. Chem. Soc., Div. Polymer Chem., Atlanta City (19651, Polymer Reprints

5,

646 (1965)

[51 P.C. Schmidt and F.P. Gay, Angew. Chem.

E ,

638 (1962) [61 A. Charlesby, Nature

171,

167 (1953)

[ 7 ] A. Charlesby, in: Atomic Radiation and Polymers", Pergamon Press, N.Y., p. 348

(1960)

[81 K. Little, Nature

173,

680 (1954) [9] A. Todd, Nature

174,

613 (1954)

[lo] W. Reddish, Trans. Faraday Soc.

46,

459 (1950)