Thermal expansion of irradiated polyoxymethylene

Eur. Polym. J. Vol. 23, No. 3, pp. 207 211, 1987 Printed in Great Britain

0014-3057/87 $3.00+0.00 Pergamon Journals Ltd

T H E R M A L E X P A N S I O N OF I R R A D I A T E D POLYOXYMETHYLENE H. N. SUBRAMANYAM a n d S. V. SUBRAMANYAM Department of Physics, Indian Institute of Science, Bangalore 560 012, India (Received 8 July 1986)

Abstract--The thermal expansion coefficient of gamma irradiated polyoxymethylene has been measured in the temperature range 80-340 K by using a three terminal capacitance technique. The radiation induced changes are measured by recording the i.r. spectra of the irradiated samples. The change in crystallinity caused by irradiation is measured by an X-ray technique. The thermal expansion coefficient increases with radiation dose below 170 K due to the predominant effect of degradation. Above 170 K, this trend reverses and the expansion coefficient decreases with radiation dose due to the increased crystallinity caused by irradiation.

INTRODUCTION Investigation of the thermal properties o f irradiated polymers is i m p o r t a n t b o t h scientifically a n d technologically. While there have been a few studies on the specific heat a n d thermal conductivity of irradiated polymers, no studies have been reported on their thermal expansion [1]. We have investigated the t h e r m a l properties of a few technologically imp o r t a n t polymers such as poly(methyl methacrylate), polystyrene, polytetrafluoroethylene and polyoxymethylene, as a function of radiation dose at low temperatures. This p a p e r presents our results of thermal expansion studies on irradiated polyoxymethylene ( P O M ) in the range 80-340 K. P O M is a semicrystalline polymer; its r a d i a t i o n chemistry has been reviewed by Dole [2]. P O M is f o u n d to undergo drastic d e g r a d a t i o n u n d e r the influence of radiation, confirmed by viscometric m e a s u r e m e n t s [3-5], i.r. studies [5-6], gas chrom a t o g r a p h y [5-6] and m e a s u r e m e n t of mechanical properties [3-5]. Jaffe a n d W u n d e r l i c h [7], irradiating an extended chain crystal of P O M in air at a dose rate of 0.1 M r a d / m i n , f o u n d by index of refraction, wide angle X-ray and D T A studies t h a t the anisotropy of the crystal is reduced d u r i n g irradiation. The wide angle X-ray diffraction p a t t e r n s showed n o change of crystal structure, even after long exposure to X-rays. This implies that the dose of radiation employed affected only the defect structure of POM. Similar results were o b t a i n e d by Suzuki et al. [8] employing D T A technique. Kusy a n d T u r n e r [9] investigated in detail the cryoscopy of g a m m a irradiated P O M a n d f o u n d t h a t the heat of fusion increased with irradiation indicating an increase in crystallinity. There have been no studies on the specific heat, thermal conductivity a n d thermal expansion of irradiated POM. EXPERIMENTAL Irradiation procedure Commercial grade POM ('Polyacetal' supplied by Polypenco Ltd, England) was used. Samples of length and

diameter each 1 cm, cut from a rod are irradiated in air with gamma rays from a Co 6° source at a dose rate of" 0.26 Mrad/hr at room temperature. The irradiation is done in the gamma chamber at Cotton Technological Research Laboratory, Bombay to various integral doses from 0 to 50Mrad, in steps of 10Mrad. At doses above 50Mrad, POM, is found to develop cracks and to crumble. i.r. Spectrum

The i.r. spectra of the samples in powder form with nujol are taken with 'Shimatzu' i.r. Spectrophotometer. The spectrum for the unirradiated sample and for that irradiated to 50Mrad are shown in Fig. l, to indicate the extent of degradation of the samples. The intensity of the absorption bands at 1100 and 940cm -~, arising from C-O C antisymmetric stretching and C-O-C symmetric stretching respectively, become enhanced during irradiation as a result of degradation [10]. The absorption bands due to CH, bending vibration at 1460 cm-I and CH 2 wagging vibration at 1380cm -L superpose over nujol absorption peaks and register an increase in intensity [10]. The vibration of the C O group seems to be responsible for the absorption band at 1240 cm-~ with intensity also increasing under irradiation [11]. The absorption bands at 630 and 720cm -~ seem to arise from the crystalline and amorphous regions and the increase in intensity of the former with the reduction in the latter indicate an increase in crystallinity. Crystallinity measurements

The crystallinity changes caused by irradiation are monitored by an X-ray technique. A Philips model PW-1050/70 diffractometer using Ni filtered CuK, radiation is employed for measuring X-ray crystallinity. The diffraction patterns for the samples irradiated to various doses are shown in Fig. 2. POM gives a well defined peak at an angle of 20 = 22.77. The ratio of the area of the crystalline scattering peak to the total scattering area (crystalline plus amorphous scattering) is taken as an index of crystallinity. The variation of X-ray crystallinity as a function of radiation dose is shown in Fig. 3. The crystallinity increases from 49 to 70% due to an increase of radiation dose from 0 to 50 Mrad. The increase is rapid in the region 0-30 Mrad and slow in the region 30-50 Mrad. The rapid increase for low doses is due to the predominant effect of crystallization in the amorphous regions as a result of degradation. Main chain scissions produced by irradiation reduce the molecular entanglements and relieve intramolecular stress both of which increase molecular mobility. The increase in mobility allows the 207

208

H . N . SUBRAMANYAMand S. V. SUBRAMANYAM

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~POM (0 Mrad) .... POM ( 5 0 Mrad )

'I i

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I

1500

1000

v

I 500

Wovenumber (era- I )

Fig. 1. i.r. Spectrum.

polymer molecules in the amorphous phase to crystallize. At high doses, the rate of increase of crystallinity is less because the rate of the crystallization process decreases with depletion of the amorphous phase. Further, at higher doses, the crystallization process is countered by the introduction of defects in the crystalline phase such as branches, crosslinks and chain ends which tend to lower crystallinity.

Thermal expansion apparatus The thermal expansion coefficient of the samples are measured by using the three-terminal capacitance technique, described in detail elsewhere [12]. In this technique, the changes in the length of the sample are converted into changes in electrical capacitance which is then measured by using a ratio transformer bridge to a resolution of one part per million. The three-terminal capacitance cell used for the purpose is shown in Fig. 4, 1 and 2 are the two plates of the capacitor. Plate 2 is surrounded by a guard ring 6 which is earthed. Plate 1 is connected to the "high" side of the ratio transformer bridge and plate 2 to the "low" side. Plate 1 rests on the top of the sample 3 by means of three symmetrically placed springs 4. By raising or lowering the sample platform 5, the capacitance gap can be varied. By adjusting the nuts 7, the parallelism of the capacitor gap can be adjusted. Mica sheets of thickness 0.05 mm are used to insulate plate 2 from the guard ring 6 and also plate 1 from the body of the cell. The cell is suspended inside a copper chamber 8 which is filled with helium exchange gas at a pressure of about 1 torr. The copper chamber is mounted inside another evacuated metal chamber which is immersed in a liquid N 2 Dewar. The capacitance is measured by using a six decade ratio transformer bridge developed in our laboratory to a resolution of one part per million [13]. A Brookdeal lock-in amplifier, Model 9503, is used as a null detector of the bridge. It has a sensitivity of I p V and incorporates an oscillator which provides a sinusoidal signal of 10 V rms and 5 Hz to excite the bridge. The temperature of the sample is measured and controlled to 0.1 K by using Lake Shore Cryotronics digital

J/

j

50 Mrod

l Mrad

10 Mrad

0 Mrad

Fig. 2. X-ray diffraction pattern.

209

Thermal expansion of irradiated P O M

a sensor for the controller. A 2 5 ~ heater element is wrapped around the chamber 1. Thermal gradients across the sample are measured by two differential copperconstantan thermocouples T~, T2, that across the capacitance cell by T~ and T 4 and that across the copper chamber by T~ and T 6.

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Fig. 3. Variation of crystallinity with radiation dose. thermometer/controller Model DRC-84C. The Pt resistance thermometer P~ m o u n t e d just below the sample platform measures the sample temperature. Another Pt resistance thermometer P2 mounted on the copper chamber serves as

The two circular faces of the cylindrical samples are polished to render them flat and parallel to each other. The sample is mounted in the capacitance cell and cooled to liquid N 2 temperature slowly over a period of 12hr. The sample is heated in steps of 6 K, by using the temperature controller. For every stabilized temperature, stable over 45 min as indicated by no thermal gradients across the sample and the cell, the ratio transformer bridge is balanced and the reading is taken to give 1/C directly. Thus the values of T and 1/C are recorded in the temperature range 8(~340 K for samples irradiated to 0, 10, 20, 30, 40 and 50 Mrad. The thermal expansion coefficient is calculated from the formula

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7

T~ 7

8

VIllA

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Fig. 4. Three-terminal capacitance cell.

210

H.N. SUBRAMANYAMand S. V. SUBRAMANYAM 200 --i,--

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180

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530

(K)

Fig. 5. Variation of Al/1 with temperature.

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(1)

where L, is the length of the sample, Lc is the length of the cell which contributes to the differential expansion of the gap, ~c the expansivity of the cell which is determined by a calibration experiment using A1 and Ge as standard reference materials and which is described elsewhere [12]. The derivative d/dT(l/C) is calculated by fitting the data of Tvs I/C into a cubic spline and then differentiating by a computer program. The accuracy in the measurement of ~5 is about 4%. The relative expansion AI/I of the sample is calculated from the formula A1

(Lc'~(AI~ _(0.15625X]A

(2)

RESULTS AND DISCUSSION

The variation of the relative expansion A1/I for POM samples irradiated to 0 and 5 0 M r a d as a function of temperature is shown in Fig. 5. The

variation of the thermal expansion coefficient, ~, of POM irradiated to 0, 10, 30 and 50Mrad, as a function of temperature is shown in Fig. 6. The variation of ~ of POM as a function of dose at a few representative temperatures (80, 240, 3 0 0 K and 330 K) is shown in Fig. 7. The thermal expansion curves in Fig. 6 indicate two transitions--a sharp transition around 2 8 4 K and another around 200 K. The dynamic mechanical measurements have also indicated the existence of three relaxational transitions in POM; an ~ transition at 400 K, a/~ transition at 260 K and a 7 transition at 205 K [14]. There is wide disagreement in the reported transition temperatures depending on the nature of the sample used and the techniques employed. The fl transition is found to vary in the temperature range 253-323 K depending on the frequency of measurement [14]. The transition around 200 K is associated with the unfreezing of segmental motion in POM, which results from the transition of the amorphous regions from the glassy to the rubbery state. It is believed that the transition at 284 K is

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Thermal expansion of irradiated POM

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of a glass is only slightly greater than that of a corresponding crystalline phase below Tg, where the amorphous regions are in the glassy state, expansivity is independent of crystallinity. But above Tg, a large increase in expansivity with decreasing crystallinity is expected as the expansion for a liquid or rubbery phase is much larger than that for the corresponding crystalline phase.

x

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I 30

I 50

Radiation dose, Mrad

Fig. 7. Variation of ~ with radiation dose.

"rudimentary residue" of the glass transition process

[15]. F r o m Fig. 7, it can be seen that ~ of P O M increases with dosage below 170 K. The increase in ~ is about 27% for a 50 Mrad dose at 80 K. Around the temperature region 170-200 K, this trend reverses and begins to decrease with radiation dose. The decrease in ~ is about 15% at 2 4 0 K and 10% around 300 and 330 K. These variations in ~ with dose can be explained as due to the radiation induced degradation and crystallinity changes. The decrease in a below 170K is due to the predominant effect of radiation induced degradation. During the scission of polymer chains, covalent bonds are broken and the end groups formed at the scission point will have Van der Waal interactions. As the thermal expansion coefficient depends on the strength of interactions of the constituent units, ~ is larger by two orders of magnitude for a solid whose molecules have Van der Waal interaction than for a covalently bonded solid. Hence, due to a relative increase in Van der Waal bonds caused by the radiation induced scissioning of polymer chains, increases with radiation dose. The decrease in ~ with radiation dose above 200 K is due to the effect of increase in crystallinity. The effect of crystallinity on thermal expansion is predominant only above the glass transition temperature, Tg [16, 17]. Since the expansion coefficient

Acknowledgements--The authors thank the Director of CTRL, Bombay for providing the irradiation facilities. They are also grateful to DST and ISRO-IISc Space Technology Cell for financial assistance. One of the authors (HNS) is grateful to UGC for a fellowship under Faculty Improvement Programme. The assistance of Messrs Govendaraju, Jayanna and Satish is gratefully acknowledged. REFERENCES

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