Radiation Effects on Polymers - American Chemical Society

Chapter 10

Temperature Dependence of the Radiation Chemistry of Polymers 1

R. Wayne Garrett , David J. T. Hill, Tri T. Le, Karen A. Milne, James H. O'Donnell , Senake M. C. Perera, and Peter J. Pomery

Downloaded by COLUMBIA UNIV on February 15, 2015 | http://pubs.acs.org Publication Date: November 12, 1991 | doi: 10.1021/bk-1991-0475.ch010

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Polymer Materials and Radiation Group, Department of Chemistry, University of Queensland, Brisbane, Queensland, 4072 Australia The temperature dependence of main-chain scission and mass loss by depropagation to monomer has been determined for γirradiation of poly(methyl methacrylate), poly(phthalaldehyde), poly(isobutene) and poly(α-methyl styrene). Radiation greatly enhanced the thermal degradation. The yield of main-chain scission also increased markedly with temperature, particularly when the calculation of G(scission) was corrected for the decreasing dose rate due to the mass loss. The behaviour of these polymers was compared with polystyrene and poly(arylene sulfone). The changes in the properties of polymeric materials that occur when they are subjected to high-energy radiation, including 7-rays and electron beams, are a result of chemical reactions initiated by the radiation. The chemical reactions may be classified into (1) main-chain scission, (2) crosslinking, (3) formation of small molecular products, and (4) modification of the chemical structure of the polymer. An objective of fundamental studies of the radiation chemistry of polymers is to determine the rates of these reactions. There is increasing interest in the utilization of polymer materials in radiation environments which are significantly above ambient temperatures, including aerospace applications (i). The deterioration in the properties of polymers may be markedly increased by relatively small rises in temperature. The rates of chemical reactions normally show a positive temperature dependence, which is due to the activation energy barrier and can be described by an Arrhenius relationship. In polymers, the activation energy may show considerable variation and reflect both chemical and physical effects on the reaction. Also, the temperature dependence of reactions in polymers is likely to show discontinuities at first and second order transitions, especially at the glass transition, Tg, and crystalline melting, Tm, temperatures. The reaction rates may be quite different in crystalline and amorphous polymer, and between the glassy and rubbery amorphous states. There is increasing interest in the properties of the mterfacial material between the amorphous and crystalline regions, which may have non-crystalline structure. 1

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Current address: Australian Nuclear Science and Technology Organization, Menai, New South Wales, 2234 Australia Corresponding author 0097-6156/91/0475-0146S06.00/0 © 1991 American Chemical Society

In Radiation Effects on Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


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Radiation Chemistry of Polymers

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The effects of temperature on the radiation chemistry of polymers has not been studied extensively. Charlesby and Moore (2) found that the yield of scission, G(S), in poly(methyl methacrylate) increasedfrom1.6 at 0 °C to 3.8 at 180 °C., and followed an Arrhenius relationship without any discontinuity at Tg. However, Wundrich (3), in a study of PMMA over a wider temperature range down to 77 K, found three distinct temperature regions with different activation energies. Wundrich also studied polyiisobutene) and obtained similar results (4). Polymers which give high values of G(S) at ambient temperature frequently undergo depropagation to monomer during thermal degradation. Both types of behaviour have been attributed to the large steric strain in these polymers, which usually arises from a quaternary carbon atom in the repeat unit of the main chain and the related low enthalpy of polymerization. Thus, initiation by main-chain scission and degradation by depropagation are both favoured in these polymers. Bowmer and O'Donnell (5) have shown that main-chain scission occurs on irradiation of poly(olefin sulfone)s with a high value for G(S) at temperatures from -196 to 100 °C., and that depropagation to the two monomers increases rapidly above the ceiling temperatureforpropagation/depropagation equilibrium. In this paper we report measurements of the reduction in molecular weight and sample mass for 7-irradiation in vacuum of several polymers which have high G(S) values at ambient temperature, apparently do not undergo crosslinking, and degrade thermally by depropagation to produce mainly monomer. The behaviour of these polymers is compared with polystyrene and poly(arylene sulfone). Experimental Preparation. Poly(methyl methacrylate), PMMA, was prepared byfreeradical polymerization in bulk at 70 °C with AIBN initiator. The polymer was precipitated twice from chloroform into methanol and dried under vacuum with progressive heatingfrom30 to 100 °C. The polymer was ground to a fine powder under nitrogen at 77 K. Poly(styrene) was prepared similarly. Poly(phthalaldehyde), PTA, was provided by Dr Hiroshi Ito of IBM Almaden Research Center and was prepared oy anionic polymerization at -50 °C., and end-capped with acetaldehyde. Poly(a-methyl styrene), PAMS, was made by cationic polymerization at -100 °C with BF3-ether m CH2C12 as initiator. Bisphenol-A polysulfone, PSF, was provided by Union Carbide Corporation (Udel P-1700); it was purified by reprecipitation from chloroform into methanol and dried under vacuum. Irradiation. The polymers were evacuated for 24 hours and sealed in glass ampoules for low temperature experiments. For irradiations at temperatures when mass loss became significant, samples of powdered polymer were contained in small, open tubes which were placed in larger tubes and evacuated continuously during irradiation. The tubes were placeain an aluminium block heater, which maintained a constant temperature within ± 1 °C. The samples were irradiated with " Co 7-rays at about 5 kGy/h in an AECL Gammacell or in the Gatri radiation cell at the Australian Nuclear Science and Technology Research Laboratories for higher temperatures with continuous evacuation. Fricke dosimetery was used to determine the dose rates. u

Characterization. The mass loss was determined by weighing the samples in the small, open sample tubes before and after irradiation. Molecular weights were American Chemical Society. Library 1155on 16th St, N.W. In Radiation Effects Polymers; Clough, R., et al.; DX.Society: 20036 ACS Symposium Series;Washington, American Chemical Washington, DC, 1991.

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RADIATION EFFECTS ON POLYMERS

obtained by gel permeation chromatography using calibration with polystyrene standards and appropriate corrections. The purity of the unirradiated polymers was checked by H and C NMR. A

1 3

Results and Discussion


Poly(styrene). Values for Mn and Mw of the polydisperse polystyrene sample, before and after irradiation to a series of doses, were derivedfromthe GPC chromatograms. The radiation chemical yields for scission, G(S), and crosslinking, G(X) per 16 aJ of energy absorbed, were calculatedfromthe slopes of plots of 1/M versus dose according to equations 1 and 2. Equation 1 applies to any initial molecular weight distribution, MWD, but equation 2 is only valid for a most-probable MWD, when Mw/Mn = 2. l/Mn(D) = 1/Mn(0) + A[G(S) - G(X)]D

(1)

l/Mw(D) = 1/Mw(0) + B[G(S) - 4G(X)]D

(2) 10

n

where the dose, D, is in gray and A = 1.04xl0" , and Β = 5.18xl0" . The variation in G(S)-G(X) with temperaturefrom30 °C to 160 °C is shown in Figure 1. Crosslinking is predominant at 30 °C., but scission increases with increasing temperature. However, there is no significant discontinuity at Tg (100 °C) apparent within the accuracy of the data. Poly(styrene) is known to undergo thermal degradation above 250 °C to yield mainly monomer. Radiation wasroundto enhance the rate of thermal degradation, corresponding to an increase in temperature of about 100 ° C . The rates of mass loss at 292 °C (Thermal) and 240 °C (5 kGy/h) are shown in Figure 2. Analysis of the volatile product and the residue has confirmed the formation of styrene monomer and of dimer, trimer and higher oligomers. It is uncertain whether the oligomers are formed during the depropagation, e.g. through an intra-molecular abstraction (back-biting) reaction, or by polymerization of monomer initiated by irradiation or by radical or ionic species trapped in the polymer. Bisphenol-A Polysulfone. This commercial, high-performance, high-temperature polymer is an amorphous thermoplastic with a suostantial aromatic content in the backbone chain. It differs from polystyrene which has substituent phenyl groups and a lower Tg of 100 °C., compared with 190 °C for PSF. Crosslinking predominates in PSF for irradiation in vacuum at 30 °C., but the G values are comparable to polystyrene, as might be expected. The yields of crosslinking . and scission can be conveniently derivedfromCharlesby-Pinner plots of s + s ' versus 1/D, according to equation 3, where s is the solublefraction,determined by extraction of the irradiated polymer, provided crosslinking occurs by a Hlinking mechanism, and C = 4.82x10 . 1

s + sV2 = G(S)/2G(X) + C/[G(X)Mn(0)D]

(3)

The variation of the solublefractionwith dose for irradiation of PSF under vacuum at 35,125 and 220 °C is shown in Figure 3. The gel dose decreases substantially over this temperature range. The ratio of scission to crosslinking is indicated by the limiting solublefraction.It is remarkable that crosslinking increases markedly above Tg and scission decreases to zero, which is in contrast to the behaviour of polystyrene. It is likely that the onset of depropagation above Tg in polystyrene stabilizes main chain scission, whereas depropagation does not occur m PSF and the chain radicals have greater mobility for


z



GARRETT ET AL.

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Temperature / °C Figure 1. Temperature dependence of the net rate of scission, G(S) - G(X), in polystyrene for 7-irradiation in vacuum.

20.0 h

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Time / hours Figure 2. Radiation enhancement of the thermal degradation of polystyrene; mass loss with time at 240 °C (radiation), and 292 °C (thermal).


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recombination reactions, including crosslinking. Crosslinking could also occur by a Y-linking mechanism, which would be enhanced above Tg. Poly(methyl methacrylate). Irradiation of PMMA at ambient temperatures causes main-chain scission. Values of G(S) have been reported in the range 12, and apparently crosslinking does not occur. Charlesby and Moore (2) have found that G(S) increases with irradiation temperatures above ambient as shown in Figure 4. Wundrich (3) have reported increasing values of G(S)from77 Κ with discontinuities at second order phase transitions. We have measured the mass of small, powdered samples of PMMA during irradiation under vacuum and found that there was negligible mass loss up to 116 °C., but that it increased rapidly at higher temperatures. Above 200 °C the rate of depropagation was extremely rapid and may well have been determined by physical factors. Thermal degradation starts to become significant above about 250 °C., so that the enhancement by irradiation is very significant at 200 °C. Measurements have been made by GPC of the change in molecular weight of these PMMA samples during irradiation. Corrections nave to be made for the decreasing dose to the sample on account of the loss of sample through depropagation. G(S) has been found to increase very rapidly above 150 °C., and zip lengths are short compared with the degree of polymerization (DP) of the polymer. Poly(phthalaldehyde). The ceiling temperature for the propagation/depropagation equilibrium of the reaction phthalaldehyde poly(phthalaldehyde) is -40 °C in the liquid state. Therefore, the polymer molecules must be end-capped with acetaldehyde; otherwise the polymer undergoes slow depropagation on standing at ambient temperature. The polymer has been investigated by Ito and Willson (6) as a resist for microelectronic applications in which photolysis or radiolysis of a sensitizer catalyses depropagation to monomer. Irradiation of poly(phthalaldehyde) in the absence of a sensitizer causes main-chain scission and the new chain ends undergo depropagation. Measurements of molecular weight (determined by GPC) and weight loss have been reported for 7-irradiation at ambient temperature (6). In the present work, the effect of irradiation temperature on G(S), G(-monomer) and the zip length for depropagation have been investigated. The decrease in Mn with dose from irradiation at 25 °C of low mol. wt. PTA are shown in Figure 5 and compared with the results of Ito and Willson for PTA of higher molecular weight. PTA is inherently unstable even at sub-ambient temperatures and therefore any radiation or thermal formation of propagating radicals will lead to depropagation. Consequently, the polymer is very sensitive to radiation enhancement of thermal degradation. Polyisobutene. PIB is a polymer with a low Tg of -70 °C and a ceiling temperature of 175 °C. It is believed to undergo only main-chain scission on irradiation. Wundrich (4) has determined values of G(S), assuming that crosslinking does not occur, from solution viscosity measurements, for irradiation over a range of temperatures. We have used GrC to derive Mn and Mw values of samples irradiated from 77 Κ to 350 K, and derived G(S)fromequation 1. Poly (a-methyl styrene): Compared with poly(methyl methacrylate) and poly(isobutene), poly(a-methyl styrene), PAMS, has a relatively low ceiling temperature or 60 °C for polymerization of the pure liquid monomer at 1



GARRETT ET AL.


1.0

ω 0.5

-

ο I 220°C

1 2

0

1 4

1 6

Dose / MGy

Figure 3. Dose dependence of the soluble fraction of bisphenol-A polysulfone for irradiation at 35,125 and 220 °C.

0

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200

300

400

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Temperature / Κ

Figure 4. G(S) values for 7-irradiated poly(methyl methacrylate) in the temperature range -196 to 200 °C. (•) Charlesby and Moore - ref. 2; (A) Wundrich - ref. 3; ( · ) present work.



152


atmosphere pressure (7). The Tg is 180 °C. The thermally-induced depropagation of PAMS has been studied extensively (8-ll) and reported to occur at a significant rate at temperatures greater than 230 "C., with the only product of the depropagation being monomer. However, by comparison with the thermally-induced depropagation, the radiation-inducea depropagation has received little attention. Here we report a study of the radiation chemistry of PAMS at three temperatures (25 °C., 80 °C and 163 °C). The mass loss and molecular weight of polymer after various periods of irradiation have been measured and the yields of chain scission and loss of monomer have been determined. The polymer (initially 50 mg, Mn=60,000 g/mol) was irradiated under vacuum (dose rate = 2.8 kGy/h) at each temperature. The mass of the polymer was monitored over a period of approx. 30 hours. At the two lowest temperatures, no significant loss in mass was observed during the period of irradiation, but at the highest temperature, the mass loss was approx. 25 %. GC/MS studies showed that the mass loss was caused by depropagation of the polymer to monomer. The variation in the mass of polymer was found to follow first-order kinetics, as demonstrated in Figure 6. The yields for loss of monomer, G(-M), can be calculated from a plot of mass loss versus absorbed dose (Figure 7), to yield a value of 320 ± 45 at 163 °C., indicative of a chain reaction. Because no significant depropagation occurs at 25 °C and 80 °C., the value of G(S) can be calculated from the linear relationship between 1/Mn and absorbed dose. These plots are shown in Figure 8 and yield values for G(S) of 0.29 ± 0.01 at 25 °C and 0.48 ± 0.06 at 80 °C. At 163 °C., the value of G(S) cannot be determined directlyfromthe 1/Mn versus dose plot, because the mass of polymer varies during irradiation. A second complication arises as a result of the depropagation process, because a chain scission within a zip length, z, of a chain end will not produce an increase in the number of polymer chains present in the sample. In order to deal with this problem, the variation in the molecular weight of the polymer (Mn and Mw) and the loss in mass have been analysed over a range of absorbed dose (0-50 kGy) using a Monte-Carlo simulation of chain scission and depropagation. The details of this simulation are discussed elsewhere (12). The Monte-Carlo simulation returns values for G(S) and z, which were found to be 1.8 and 400, respectively, at 163 °C. In Figure 9 the values for G(S) and ζ are plotted against the irradiation temperature. The results clearly show that both G(S) and ζrisesubstantially as the temperature increase above 80 °C. Poly(a-methyl styrene) can thus undergo radiation-induced depropagation at temperature approximately 100 deg. lower than that at which thermally-induced depropagation occurs at a significant rate. Conclusions Radiation enhances the rate of thermal degradation of polymers which undergo depropagation to monomer to an extent equivalent to a temperature increase greater than 100 °C. Complementary measurements of mass loss and reduction m molecular weight enable values to be derived for the rates of main-chain scission, G(S), and monomer loss, G(-monomer), provided that appropriate corrections are made for the decreasing dose rate due to the continuing mass loss. It is then possible to determine the average zip length for depropagation. The very high G(S) values obtained at high temperatures suggest tnat radiation energy supplements thermal energy in the initiation process.


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If. GARRETT ET AL.

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Dose / k G y Figure 5. The decrease in M n of poly(phthalaldehyde) with radiation dose for initial high and low molecular weight polymer. *(•) high mol. wt. - ref. 6; ( · ) low mol. wt. - present work.

0.00

-0.05

-0.10

-0.15

-0.20 h

-0.25 25

Time /

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Figure 6. Mass loss of poiy(a-methyl styrene) irradiated under vacuum at a temperature of 163 ° C ; examination of first-order kinetics.



154

30

w

20 h

o


.2?

Absorbed Dose / kGy

Figure 7. Percent mass loss of poly(a-methyl styrene) with absorbed dose of radiation under vacuum at a temperature of 163 °C. 25

1 5

I

1

1

1

1

'

'

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Dose / k G y

Figure 8. The variation in the value of 1/Mn for poly(a-methyl styrene) absorbed dose of radiation under vacuum at temperatures ot ( · ) c2 (V)80°C.



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GARRETT ET AL.

300.0


350.0

400.0

450.0

155

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Temperature / Κ

Figure 9. Temperature dependence of the values of (·) scission, G(S), and (•) zip length, z, for 7-irradiation of poly (a-methyl styrene) in vacuum.

Acknowledgments The authors thank the Australian Research Council, and the Australian Institute of Nuclear Science and Engineering for supporting their research and the Australian Nuclear Science and Technology Organization for providing irradiation facilities, Mr David F. Sangster for advice, Dr Hiroshi Ito for providing the sample of PTA and Dr U.C. Willson for encouragement. Literature Cited 1. O'Donnell, J.H. In The Effects of Radiation on High-Technology Polymers; Reichmanis, E.; O'Donnell, J.H., Eds.; Symposium Series 381; Amer. Chem. Soc.; Washington, DC, 1989, p 1. 2. Charlesby, Α.; Moore, N. Int. J. Appl. Rad. Isotopes 1964, 15, 703. 3. Wundrich, K.J. Polym. Sci., Polym. Chem.Ed.1964, 11, 1293. 4. Wundrich, K. Europ. Polym. J. 1971, 10, 341. 5. Bowmer, T.N.; O'Donnell, J.H.J. Macromol. Sci. Chem. 1982, A17, 243. 6. Ito, H.; Willson, C.G. Polym. Eng. Sci. 1983, 23, 1012. 7. Kilroe, J.G.; Weale, K.E.J. Chem. Soc. 1960, 3849. 8. Brown, D.W.; Wall, L.A.J. Phys. Chem. 1958, 62, 848. 9. Bywater, S.; Blank, P.E.J. Phys. Chem. 1965, 69, 2987. 10. Roestamsjah, L.A.; Wall, L.A.; Florin, R.; Aldridge, M.H.; Fettes, L.J.J. Res. Natl. Bureau Stand. 1978, 83, 371. 11. Guaita, M.; Chiantore, O. Polym.Degrad.Stab. 1985, 11, 167. 12. Garrett, R.W.; Hill, D.J.T.; Le, T.;O'Donnell,J.H.; Pomery, P.J. In Polymers for Microelectronics Science and Technology, Tabata, Y; Mita, I.; Nonogaki, S., Eds.; VCH Publishers; Federal Republic of Germany, 1990, in press. R E C E I V E D March 22,

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Radiation Effects on Polymers - American Chemical Society

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