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The mechanism of radiation-induced linking phenomena in polyethylene
Vol.22No g-5pp.585-5851983 Printed in Great Britain
/8BIO9585-0B$03.00/O © 1983 Pergamon press Ltd
Radzht. Phys, Chem.
THE MECHANISM OF RADIATION-INDUCED LINKING PHENOMENA IN POLYETHYLENE Joseph Silverman, I F. J. Zoepfl, II J. C. Randall, mem and V. Markovlc mmlm I ii
llm IIII
Laboratory for Radiation and Polymer Science, Institute for Physical Science and Technology, University of Maryland, College Park, MD 20742, USA Pickard, Lowe and Garrick, Inc., Washington, DC Phillips Petroleum Company, Bartlesville, OK
20036, USA
74004, USA
Laboratory for Solid State Physics and Radiation Chemistry, Boris Kidri5 Institute of Nuclear Science, VinSa, Post Office Box 522, 11001 Belgrade, YUGOSLAVIA
INTRODUCTION High-resolutlon solution carbon 13 NMR data on irradiated polyethylene (Randall and others, 1982) provide an important addition to the evidence accumulated by other techniques which have a bearing on the radiation chemistry of polyethylene. It is the purpose of this contribution to suggest a series of reactions that appear to be consistent with the body of experimental results. Our proposed mechanism does not constitute a complete description and further experimental studies will almost certainly lead to additions and revisions; but, despite its potential shortcomings, the mechanism provides a new point of view from which unresolved problems may be addressed.
DISCUSSION Although long-chaln branch formation (Y-linking) by ionizing radiation has been proposed for many years, the e x p e r i m e n t a l evidence based on m e a s u r e m e n t s of gel fraction and elastic modulus (Lyons, 1981) is not strong. The NMR data (Randall and others, 1982), on the other hand, are unambiguous in their confirmation of radlatlon-induced Y-link f o r m a t i o n accompanied by the disappearance of terminal vinyl groups. Therefore, we propose that the principal reaction associated with these observations is the union of a terminal vinyl group with a secondary alkyl radlcal.
CH2
CH
i
12
CH" + CH 2 = CHCH2CH2L,~,
> CH-CH2-CHCH2CH2,uv~
I
(I)
C~2
CH
?
?
12
This reaction was first proposed by Lyons (1967), but has been discussed in only two subsequent publlcations by other authors (Dole, 1979; Ungar, 1981). Since 1967, studies relatinE to the morphology, free radlcal behavior, and the NMR spectra of irradiated polyethylene combine to lend strong support. Morphologlcal studies (Keller and Priest, 1968) of polyethylene single crystals have shown that terminal vinyl groups are excluded from the crystalline core. The data in Ref. (12) (Randall and others, 1982) yield a mean vlnyl concentration of 30 m M for the high-denslty polyethylene NBS 1475; this corresponds to 150 m M in the interlamellar amorphous regio~ ESR experiments (Kusumoto and others, 1971) on irradiated polyethylene single crystals have demonstrated that secondary alk71 radloals tend to be produced on the chain folds at the surface of the crystals prior to their subsequent reactions (presumably from ionic precursors or excitcns produced uniformly in the crystals (Chappas and Silverman, 1980)). The total production of alkyl radicals (G - 4) 583
584
J. SILVERMAN et a~.
produced by a 10 kGy dose leads to a m a x i m u m m e a n concentration of 4 mM. I n t r a l a m e l l a r radical recombination reactions are difficult because of the large spacing between chains in the orthorhombic crystal and the large strain energy involved; they can occur more easily at loose extended folds. Interlamellar radlcal-radical linking reactions are also difficult except by means of cilia extending from the ordered regions. Reaction between radicals at the crystal surface and those in the amorphous region is a reasonable possibility but much less so than the simple, obvious exothermlc union of the radical with the abundant vinyls. In pointing out the prominent role of the radical-vinyl reaction, we do not reject the possibility of H-link f o r m a t i o n by the direct union of two secondary alkyl radicals. Although they are not observed in the NMR study (Randall and others, 1982), owing perhaps to limitations of experimental detection, Reaction (I) initiates a short-chain reaction whose likely termination mechanism is H-llnk formatiom However, a detailed understanding of the radical disappearance mechanism remains uneertaim Reference (12) (Randall and others, 1982) reports a significant increase in saturated end groups f o l l o w i n g irradiation of high-denslty polyethylene. This is a c c o m p a n i e d by an increase in molecular weight and a broadening of the dlstributiom This evidence of seission, even for room-temperature irradiations, is probably caused by beta cleavage• ~vCH2CHCH2CH2vuv~>~CH2CH=CH2
÷ "CH2tn#u~
(2)
This scission mechanism is a well-known degradation reaction at high temperatures. The production of a secondary radical at a chain fold introduces strain energy that could lead to this reaction in the crystalline regiom The primary alkyl radical produced in Reaction (2) is a transient species. It has not been observed in the ESR spectra of irradiated PE even at 77 K. Radtslg and Butyagin (1967) produced it by mechanical fracture of deep-frozen PE and observed its rapid disappearance at temperatures of 150 K with no change in spin concentration. Apparently, the primary radicals were converted to secondary radicals. This step not only accounts for an increase in saturated end groups, but also provides an additional source of vinyls. A method of vinyl production is required to fit the observations of Dole, Fallgatter, and Katsuura (1966) that the vinyls do not vanish even at high doses. However, Reaction (2) produces the vinyl at the cost of a scission. There is an additional possible mechanism for vinyl production by a reaction observed in the high pressure polymerization of ethylene, namely, a 1,5 backbiting reaction:
.,/,,,""-CHCH2CH2CH2CH3 ~CH2CH2CH2CH2CH2 ~, "k,,,,,,,CH=CH2 . ~H2OH2OH3
(31
This not only produces additional vinyl unsaturation but also leads to volatile products that would escape detection both in gel fraction measurements and in solution NMR experiments (Bowmer and O'Donnell, 1977). Clearly, backbiting is favored by increasing temperatures, but is an unlikely contributor to the radiation chemistry of polyethylene at room temperature. Reference (12) (Randall and others, 1982) provides NMR data on the thermal degradation of polyethylene at 500 K in vacuum; they show a significant yield of both vinyls and butyl side chains. Still another mechanism of possible significance is the union of a secondary alkyl radical with the primary radical produced in Reaction (2):
| c~2 c~2 , CH 12 I
u~.~CH2CHCH2CH2uvu + "CH2CH2CH2u~L
(g)
>~CH2CHCH2CH2~
This reaction would produce a Y-link without the consumption of a vinyl unit, and could also account for the disappearance of some of the radicals.
Radiation induced linking phenomena in polyethylene
585
As indicated above, the primary radical has not been observed by ESR in irradiated polyethylene even at cryogenic temperatures. Fop it to play a significant role in Reaction (4) at room temperature and flt this condition, the radical would have have a significant lifetime at room temperature. Intermoleoular or intramoleoular hydrogen atom abstraction processes are competitive processes, but they are not particularly rapi~ The methyl radical abstraction reaction with n-hexane has a rate constant of 120 M-Is -I, and the ethyl radical abstracts hydrogen from n-heptane with a rate constant of 4 M'ls "1 (Burns and Barker, 1965); this suggests a lifetime for the primary radical of polyethylene by this process that is perhaps a millisecond and possibly much longer. The 1,2 isomerization of the primary radical is also a slow process as evidenced by the small yield of methyl side groups in the commercial high pressure polymerization of ethylene by a radlcal mechanism. The ratio of vinyl disappearance to Y-link formation reported in Ref. (12) (Randall and others, 1982) is approximately 2:1. The substantial uncertainty in the absolute values for the yields of each of the species arises in good measure from systematic errors that do not apply in calculation of the ratio. Thus the value observed for this ratio requires explanation. An interesting possibility is the short chain cationic polymerization proposed by Chang, Yang, and Wagner (1959).
CONCLUSIONS Our proposed mechanism ties radiation-lnduced gel formation to w e l l - k n o w n reactions of thermal degradation modified by considerations of temperature and morphology. Since chemically- and thermally-inltiated gel formation involves the production of an alkyl radical as an early precursor, the same reactions (with the exception of the cationic vinyl polymerization) may apply.
REFERENCES
(1)
Bowmer, T. N., and J. H. O'Donnell (1977).
Polvm., 18, 1032.
(2)
Burns, W.G., and R. Barker (1965). In G. Porter Kinetics, Vol. 3, Pergamon Press, Oxford, UK, P. 303.
(3)
Chang, P. C., N. C. Yang, and C. D. Wagner (1959).
(4)
Chappas, W. J., and J. Silverman (1980).
(5)
Dole, M. (1979).
(6)
Dole, M., M. Fallgatter, and K. Katsuura (1966).
(7)
Keller, A., and D. J. Priest (1968).
(8)
Kusumoto, N., T. Yamamoto, and M. Takayanagi (1971). J. Polym. Sol., A-2, 9, 1173.
(9)
Lyons, B. J. (1967). 672.
(ed.), Progress
in Reaction
J. AmeP. Chem. Soo., 81, 2060.
~adlat. Phys. Chem., 16, 437.
Polvm. ~last. Technol. ~dlg., 1~, 41. J. Phys. Chem., 70, 62.
J. Maoro~l. Soi.-Phvs., Part ~, 2, 479.
In Amer. Chem. 3o0., Div. Polym. Chem., Polvm. Prenrints, 8,
(10)
Lyons, B. J. (1981). In Radiation Processin~ for Plastics and Rubber, Rubber and Plastics Institute, London~ UK, p. 5.1.
(11)
Radtsig, V. A., and P. Yu. Butyagin (1967).
(12)
Randall, J. C., F. J. Zoep£1, and J. Silverman (1982). ceedin2s of the Fourth International Meetine on ~ Yugoslavia.
(13)
Ungar, G. (1981).
Polvm. Sol., ~SSR. 9, 2883.
J. Materials Soi., 16, 2635.
In V. Markovic (ed.), ProProoesslne, Dubrovnik,
/8BIO9585-0B$03.00/O © 1983 Pergamon press Ltd
Radzht. Phys, Chem.
THE MECHANISM OF RADIATION-INDUCED LINKING PHENOMENA IN POLYETHYLENE Joseph Silverman, I F. J. Zoepfl, II J. C. Randall, mem and V. Markovlc mmlm I ii
llm IIII
Laboratory for Radiation and Polymer Science, Institute for Physical Science and Technology, University of Maryland, College Park, MD 20742, USA Pickard, Lowe and Garrick, Inc., Washington, DC Phillips Petroleum Company, Bartlesville, OK
20036, USA
74004, USA
Laboratory for Solid State Physics and Radiation Chemistry, Boris Kidri5 Institute of Nuclear Science, VinSa, Post Office Box 522, 11001 Belgrade, YUGOSLAVIA
INTRODUCTION High-resolutlon solution carbon 13 NMR data on irradiated polyethylene (Randall and others, 1982) provide an important addition to the evidence accumulated by other techniques which have a bearing on the radiation chemistry of polyethylene. It is the purpose of this contribution to suggest a series of reactions that appear to be consistent with the body of experimental results. Our proposed mechanism does not constitute a complete description and further experimental studies will almost certainly lead to additions and revisions; but, despite its potential shortcomings, the mechanism provides a new point of view from which unresolved problems may be addressed.
DISCUSSION Although long-chaln branch formation (Y-linking) by ionizing radiation has been proposed for many years, the e x p e r i m e n t a l evidence based on m e a s u r e m e n t s of gel fraction and elastic modulus (Lyons, 1981) is not strong. The NMR data (Randall and others, 1982), on the other hand, are unambiguous in their confirmation of radlatlon-induced Y-link f o r m a t i o n accompanied by the disappearance of terminal vinyl groups. Therefore, we propose that the principal reaction associated with these observations is the union of a terminal vinyl group with a secondary alkyl radlcal.
CH2
CH
i
12
CH" + CH 2 = CHCH2CH2L,~,
> CH-CH2-CHCH2CH2,uv~
I
(I)
C~2
CH
?
?
12
This reaction was first proposed by Lyons (1967), but has been discussed in only two subsequent publlcations by other authors (Dole, 1979; Ungar, 1981). Since 1967, studies relatinE to the morphology, free radlcal behavior, and the NMR spectra of irradiated polyethylene combine to lend strong support. Morphologlcal studies (Keller and Priest, 1968) of polyethylene single crystals have shown that terminal vinyl groups are excluded from the crystalline core. The data in Ref. (12) (Randall and others, 1982) yield a mean vlnyl concentration of 30 m M for the high-denslty polyethylene NBS 1475; this corresponds to 150 m M in the interlamellar amorphous regio~ ESR experiments (Kusumoto and others, 1971) on irradiated polyethylene single crystals have demonstrated that secondary alk71 radloals tend to be produced on the chain folds at the surface of the crystals prior to their subsequent reactions (presumably from ionic precursors or excitcns produced uniformly in the crystals (Chappas and Silverman, 1980)). The total production of alkyl radicals (G - 4) 583
584
J. SILVERMAN et a~.
produced by a 10 kGy dose leads to a m a x i m u m m e a n concentration of 4 mM. I n t r a l a m e l l a r radical recombination reactions are difficult because of the large spacing between chains in the orthorhombic crystal and the large strain energy involved; they can occur more easily at loose extended folds. Interlamellar radlcal-radical linking reactions are also difficult except by means of cilia extending from the ordered regions. Reaction between radicals at the crystal surface and those in the amorphous region is a reasonable possibility but much less so than the simple, obvious exothermlc union of the radical with the abundant vinyls. In pointing out the prominent role of the radical-vinyl reaction, we do not reject the possibility of H-link f o r m a t i o n by the direct union of two secondary alkyl radicals. Although they are not observed in the NMR study (Randall and others, 1982), owing perhaps to limitations of experimental detection, Reaction (I) initiates a short-chain reaction whose likely termination mechanism is H-llnk formatiom However, a detailed understanding of the radical disappearance mechanism remains uneertaim Reference (12) (Randall and others, 1982) reports a significant increase in saturated end groups f o l l o w i n g irradiation of high-denslty polyethylene. This is a c c o m p a n i e d by an increase in molecular weight and a broadening of the dlstributiom This evidence of seission, even for room-temperature irradiations, is probably caused by beta cleavage• ~vCH2CHCH2CH2vuv~>~CH2CH=CH2
÷ "CH2tn#u~
(2)
This scission mechanism is a well-known degradation reaction at high temperatures. The production of a secondary radical at a chain fold introduces strain energy that could lead to this reaction in the crystalline regiom The primary alkyl radical produced in Reaction (2) is a transient species. It has not been observed in the ESR spectra of irradiated PE even at 77 K. Radtslg and Butyagin (1967) produced it by mechanical fracture of deep-frozen PE and observed its rapid disappearance at temperatures of 150 K with no change in spin concentration. Apparently, the primary radicals were converted to secondary radicals. This step not only accounts for an increase in saturated end groups, but also provides an additional source of vinyls. A method of vinyl production is required to fit the observations of Dole, Fallgatter, and Katsuura (1966) that the vinyls do not vanish even at high doses. However, Reaction (2) produces the vinyl at the cost of a scission. There is an additional possible mechanism for vinyl production by a reaction observed in the high pressure polymerization of ethylene, namely, a 1,5 backbiting reaction:
.,/,,,""-CHCH2CH2CH2CH3 ~CH2CH2CH2CH2CH2 ~, "k,,,,,,,CH=CH2 . ~H2OH2OH3
(31
This not only produces additional vinyl unsaturation but also leads to volatile products that would escape detection both in gel fraction measurements and in solution NMR experiments (Bowmer and O'Donnell, 1977). Clearly, backbiting is favored by increasing temperatures, but is an unlikely contributor to the radiation chemistry of polyethylene at room temperature. Reference (12) (Randall and others, 1982) provides NMR data on the thermal degradation of polyethylene at 500 K in vacuum; they show a significant yield of both vinyls and butyl side chains. Still another mechanism of possible significance is the union of a secondary alkyl radical with the primary radical produced in Reaction (2):
| c~2 c~2 , CH 12 I
u~.~CH2CHCH2CH2uvu + "CH2CH2CH2u~L
(g)
>~CH2CHCH2CH2~
This reaction would produce a Y-link without the consumption of a vinyl unit, and could also account for the disappearance of some of the radicals.
Radiation induced linking phenomena in polyethylene
585
As indicated above, the primary radical has not been observed by ESR in irradiated polyethylene even at cryogenic temperatures. Fop it to play a significant role in Reaction (4) at room temperature and flt this condition, the radical would have have a significant lifetime at room temperature. Intermoleoular or intramoleoular hydrogen atom abstraction processes are competitive processes, but they are not particularly rapi~ The methyl radical abstraction reaction with n-hexane has a rate constant of 120 M-Is -I, and the ethyl radical abstracts hydrogen from n-heptane with a rate constant of 4 M'ls "1 (Burns and Barker, 1965); this suggests a lifetime for the primary radical of polyethylene by this process that is perhaps a millisecond and possibly much longer. The 1,2 isomerization of the primary radical is also a slow process as evidenced by the small yield of methyl side groups in the commercial high pressure polymerization of ethylene by a radlcal mechanism. The ratio of vinyl disappearance to Y-link formation reported in Ref. (12) (Randall and others, 1982) is approximately 2:1. The substantial uncertainty in the absolute values for the yields of each of the species arises in good measure from systematic errors that do not apply in calculation of the ratio. Thus the value observed for this ratio requires explanation. An interesting possibility is the short chain cationic polymerization proposed by Chang, Yang, and Wagner (1959).
CONCLUSIONS Our proposed mechanism ties radiation-lnduced gel formation to w e l l - k n o w n reactions of thermal degradation modified by considerations of temperature and morphology. Since chemically- and thermally-inltiated gel formation involves the production of an alkyl radical as an early precursor, the same reactions (with the exception of the cationic vinyl polymerization) may apply.
REFERENCES
(1)
Bowmer, T. N., and J. H. O'Donnell (1977).
Polvm., 18, 1032.
(2)
Burns, W.G., and R. Barker (1965). In G. Porter Kinetics, Vol. 3, Pergamon Press, Oxford, UK, P. 303.
(3)
Chang, P. C., N. C. Yang, and C. D. Wagner (1959).
(4)
Chappas, W. J., and J. Silverman (1980).
(5)
Dole, M. (1979).
(6)
Dole, M., M. Fallgatter, and K. Katsuura (1966).
(7)
Keller, A., and D. J. Priest (1968).
(8)
Kusumoto, N., T. Yamamoto, and M. Takayanagi (1971). J. Polym. Sol., A-2, 9, 1173.
(9)
Lyons, B. J. (1967). 672.
(ed.), Progress
in Reaction
J. AmeP. Chem. Soo., 81, 2060.
~adlat. Phys. Chem., 16, 437.
Polvm. ~last. Technol. ~dlg., 1~, 41. J. Phys. Chem., 70, 62.
J. Maoro~l. Soi.-Phvs., Part ~, 2, 479.
In Amer. Chem. 3o0., Div. Polym. Chem., Polvm. Prenrints, 8,
(10)
Lyons, B. J. (1981). In Radiation Processin~ for Plastics and Rubber, Rubber and Plastics Institute, London~ UK, p. 5.1.
(11)
Radtsig, V. A., and P. Yu. Butyagin (1967).
(12)
Randall, J. C., F. J. Zoep£1, and J. Silverman (1982). ceedin2s of the Fourth International Meetine on ~ Yugoslavia.
(13)
Ungar, G. (1981).
Polvm. Sol., ~SSR. 9, 2883.
J. Materials Soi., 16, 2635.
In V. Markovic (ed.), ProProoesslne, Dubrovnik,