Chapter 5 Relations Between Photochemistry and Radiation Chemistry of Polymers
Downloaded by UNIV OF SYDNEY on May 3, 2015 | http://pubs.acs.org Publication Date: August 26, 1987 | doi: 10.1021/bk-1987-0346.ch005
J. E. Guillet Department of Chemistry, University of Toronto, Toronto, Ontario M5S 1A1, Canada
It is often assumed in studies of the radiation chemis try of polymers that because of the very high energies of electron beams, x-rays and γ-rays that a complete randomization of chemical processes and reactivity is observed. However, concurrent experiments involving both exposure to high-energy and deep-UV radiation on the same polymer films show that even with very high en ergy photons such as γ-rays a considerable selectivity of chemical reaction still occurs. This is due to the fact that after the initial absorption step, followed by the emission of a large number of Compton electrons there is a cascading of the energy down the energy scale, which tends to populate upper electronically ex cited states of the molecule. These states themselves have short lifetimes and ultimately the energies appear to be trapped in one of the lower excited states similar to those which can be populated directly by UV. For ex ample, studies of carbonyl containing polymers show that the probability of a carbonyl group reacting is much higher than would be predicted from its mass absorption coefficient. Furthermore, the chemical products appear to be identical to those which are formed from the di rect irradiation of the carbonyl group with UV light. However, when two or more competing reactions can occur out of the excited state of the carbonyl it appears that high energy radiation favors radical reaction over those involving more extensive deformation or changes in the shape of the absorbing molecule. This paper reviews ex periments involving copolymers of a variety of monomers with aromatic and aliphatic vinyl ketones. G values for reaction of the carbonyl chromophore excited by elec trons, γ-rays and soft x-rays will be compared with cor responding quantum yields of processes induced by the direct absorption of UV light. The importance of cage reactions in the dissociation of radical species will be demonstrated. 0097-6156/87/0346-0046$06.00/0 © 1987 American Chemical Society In Polymers for High Technology; Bowden, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
Downloaded by UNIV OF SYDNEY on May 3, 2015 | http://pubs.acs.org Publication Date: August 26, 1987 | doi: 10.1021/bk-1987-0346.ch005
5.
GUILLET
Relations Between Photochemistry
and Radiation
Chemistry
47
This paper i s a review of work c a r r i e d out i n our labora t o r i e s at the University of Toronto i n an attempt to understand the r e l a t i o n s h i p between polymer photochemistry, where e x c i t a t i o n i s provided by UV or v i s i b l e radiation, with processes which occur when the polymer i s excited by much higher energy r a d i a t i o n such as 7-ray or elementary p a r t i c l e s such as neutrons and electrons. The e a r l i e s t studies of r a d i a t i o n chemistry were on polymers which were useful i n the development of nuclear weapons and atomic power i n the e a r l y 1940*s and 50*s. The primary reason f o r such studies was to determine the s t a b i l i t y of polymeric materials f o r technological applications i n these industries. I t was generally assumed at that time, that because of the very large energies as sociated with the i n i t i a l deposition step, very l i t t l e s e l e c t i v e chemical reaction would occur and that a wide v a r i e t y of random bond-breaking processes would occur throughout the polymer l i q u i d or s o l i d . Table I shows the energies associated with various forms of radiation. The 7-ray photon from cobalt-60, f o r example, has an energy 100,000 times greater than that of a t y p i c a l UV pho ton. Much of the e a r l y work has been reviewed elsewhere (1-3).
Table I.
Relation Between Wavelength and Photon Energy
Photon energy, β Wavelength λ (nm)
1250 125 12.5 1.25 0.125 0.0125 0.001
Type of radiation
Infrared Ultraviolet Soft x-rays Soft x-rays Soft x-rays X-rays 7-rays ( CO) 60
eV
1 10 100 1000 10,000 10 1.2 χ 10
kcal
s
e
23 230 2300 2.3 χ 2.3 χ 2.3 χ 2.9 χ
Source: Reproduced w i t h p e r m i s s i o n from Ref. 1985 Cambridge U n i v e r s i t y P r e s s .
kJ
10" 10 10* 10 5
7
10.
96 960 9600 9.6 χ 10 9.6 χ 10.· 9.6 χ 10 12.1 χ: 10 4
6
7
Copyright
As the physics of the process became better understood, i t was r e a l i z e d that the absorption of a 7-ray photon or high-energy p a r t i c l e by a polymer, although o r i g i n a l l y involving the formation of a high-energy ion ultimately resulted i n the production of showers of Compton electrons of much lower energy. These were us u a l l y located i n the tracks or spurs along the path of the o r i g i nal e x c i t a t i o n or those of i t s successor p a r t i c l e s or rays. F i g ure 1 shows a schematic diagram of the events following the ab sorption of a high-energy photon. I t i s f o r t h i s reason that there are many common features between the chemistry of x-rays and 7-rays and processes i n i t i a t e d by electron beams, since the most important energetic intermediates are l i k e l y to involve interac tions of electrons with matter. Furthermore, i t became apparent
American Chemical Society. Library 16th St., Bowden, N.W. M., et al.; In Polymers for1155 High Technology; ACS Symposium Series;Washington, American Chemical D.C. Society: 20036Washington, DC, 1987.
POLYMERS FOR HIGH TECHNOLOGY
Downloaded by UNIV OF SYDNEY on May 3, 2015 | http://pubs.acs.org Publication Date: August 26, 1987 | doi: 10.1021/bk-1987-0346.ch005
48
that although the absorption of the high energy r a d i a t i o n occurred more or l e s s at random and was proportional to the density of the absorbing medium, i t was obvious by even a cursory reading of the l i t e r a t u r e on r a d i a t i o n chemistry, and i n our own studies, that the chemistry d i d not occur at random but that the majority of r e actions occurred at rather s p e c i f i c s i t e s within the polymer material. In our e a r l y work we looked at the Ί-radiolysis of simple a l i p h a t i c ketones [4]. Some r e s u l t s are tabulated i n Table I I . Although the carbonyl group of the ketone represented a minor part of the t o t a l mass of the molecule, products a r i s i n g from the ex c i t e d state of the ketone carbonyl were four times greater than were expected from i t s absorption cross-section. The photochem i s t r y of ketones has been well studied. Bamford and Norrish [5] showed that there are two main reactions: the Norrish type I r e action involving d i r e c t s c i s s i o n of the C - C bond adjacent to the carbonyl group to form two free r a d i c a l s , and the Norrish type II which involves both the s i n g l e t and the t r i p l e t excited states and i s a rearrangement through the intermediate of a 1-4 b i r a d i c a l r e s u l t i n g i n s c i s s i o n of the C - C bond i n α-β p o s i t i o n to form an o l e f i n and a lower ketone. Because these compounds are easy to Η.
Ij
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/ C H
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— C — C H — R ,
2
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C — C H _ — R ~
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C — C H
CH. H
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