Development of Radiation Chemistry - American Chemical Society

Chapter 1 Development of Radiation Chemistry

Downloaded by CITY UNIV LONDON on April 20, 2016 | http://pubs.acs.org Publication Date: August 26, 1987 | doi: 10.1021/bk-1987-0346.ch001

G. Arthur Salmon Cookridge Radiation Research Centre, Cookridge Hospital, University of Leeds, Leeds, LS16 6QB, United Kingdom The scientific development of radiation chemistry is reviewed from the discovery in 1895 of x-rays and radioactivity by Roentgen and Becquerel through to the present. The purpose of this article is to review the development of radiation chemistry which began with the discovery of x-rays by Roentgen(l) in 1895 and shortly afterwards of radioactivity by Becquerel(2), which in both cases involved the observation of chemical change in photo graphic plates and luminescence in certain phosphors. Clearly, in the space available, the review will be restricted and subjective, but will, it is hoped, give the general framework in which the subject has developed. The Early Years Very early studies of these radiations by the discoverers and by the Curies, Rutherford and others demonstrated that they were able to ionize the molecules of a gas upon which they acted. Indeed by 1900, the three kinds of rays, α , 3 and γ-rays, emitted by radioactive materials were characterised by their charges and their differing abilities to penetrate and ionize materials. Also, shortly after the discovery of radioactivity Pierre and Marie Curie(3) reported that radiation caused the coloration of glass and the formation of ozone from oxygen. Other chemical effects of radiation were quickly dis covered. For example, Giesel(1900(4)) showed that radiation coloured alkali halides and decomposed water. Becquerel(1901(5)) showed that $- and γ-rays can induce many of the reactions that were known to be caused by absorption of light, such as the conversion of white to red phosphorus and the decomposition of hydriodic acid solutions. Jorissen and Woudstra(1912(6)) showed that the penetrating radiation from radium caused the coagulation of some colloidal solutions and Jorissen and Ringer(1906(7)) demonstrated that hydrogen and chlorine combine at room temperature under the action of these rays. Thus, during the first decade of this century the basic physical properties of ionizing radiations had been established as well as their ability to bring about chemical change. 0097-6156/87/0346-0005S06.00/0 © 1987 American Chemical Society

Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

6

POLYMERS FOR HIGH TECHNOLOGY

Absorption

of

Energy

Quantitative description of the chemical changes initiated by r a d i a t i o n r e q u i r e s an u n d e r s t a n d i n g of the processes by which the r a y s t r a n s f e r e n e r g y t o a s y s t e m a n d k n o w l e d g e o f how much e n e r g y i s transferred. For p a r t i c u l a t e r a d i a t i o n s (α, β , e and e ) the important i n t e r a c t i o n i s i n e l a s t i c c o l l i s i o n s between the p a r t i c l e s and t h e m o l e c u l e s o f t h e medium r e s u l t i n g i n t h e i r i o n i z a t i o n and excitation.


MA/W>

M

+

e

M

(1)

The r a t e o f e n e r g y l o s s f r o m t h e p a r t i c l e p e r u n i t l e n g t h o f t r a c k , o r L i n e a r E n e r g y T r a n s f e r ( L E T ) , was k n o w n t o f o l l o w t h e B r a g g c u r v e w i t h a maximum L E T c l o s e t o t h e e n d o f t h e p a r t i c l e s t r a c k . Bethe (1933(8)) derived t h e o r e t i c a l expression for t h i s quantity for e l e c t r o n s and o t h e r c h a r g e d p a r t i c l e s . For e l e c t r o n s t h i s has the form: 1

4 2ïïNe Ζ

dE dx

r-

2 m ν Ε ο π

1η

m ν ο

-(2 / 1 - 3

2

-

1 +

3 ) 2

1η2

2Κ1-3 ) 2

+

1 +

β

2

+ Ι

(1-/

1-3 ) 2

(2)

2

where ν i s the v e l o c i t y of the e l e c t r o n , m i t s r e s t mass, 3 i s v/c a n d I t h e mean e x c i t a t i o n p o t e n t i a l o f t h e a t o m s o f t h e s t o p p i n g material· F o r x - and γ - r a y s , t h e p h o t o n s i o n i z e t h e m o l e c u l e s of t h e medium by e i t h e r the p h o t o e l e c t r i c effect(P. L e n n a r d , 1902; A. Einstein, 1 9 0 5 ) , the Compton e f f e c t ( 9 ) o r by p a i r p r o d u c t i o n ( 1 0 ) , d e p e n d i n g on t h e p h o t o n e n e r g y and t h e a t o m i c number o f t h e a b s o r b i n g m a t e r i a l . Each of these mechanisms g e n e r a t e s e n e r g e t i c e l e c t r o n s or p o s i t r o n s w h i c h a r e a b l e to b r i n g about f u r t h e r i o n i z a t i o n s and e x c i t a t i o n s as discussed above. Particle Tracks. Thus, i r r e s p e c t i v e of the p a r t i c u l a t e or photon nature of the primary r a d i a t i o n , the net e f f e c t i s the formation of t r a c k s c o n s i s t i n g of i o n i z e d and e x c i t e d m o l e c u l e s . These t r a c k s , a n d t h e i r d e t a i l e d s t r u c t u r e c a n be r e v e a l e d b y t h e C l o u d Chamber i n v e n t e d by W i l s o n i n 1 9 1 1 ( 1 1 ) . For fast electrons (low LET) the t r a c k s m a i n l y c o n s i s t of s p h e r i c a l regions c a l l e d spurs which c o n t a i n from o n e ^ t o f o u r i o n - p a i r s w h i c h a r e s e p a r a t e d i n condensed phases by a b o u t 10 S. F o r more h i g h l y i o n i z i n g p a r t i c l e s s u c h as α - p a r t i c l e s the t r a c k s a r e e s s e n t i a l l y c y l i n d r i c a l columns of i o n i z e d and e x c i t e d molecules. T h u s , by t h e e a r l y t h i r t i e s , the i n t e r a c t i o n of the v a r i o u s forms of r a d i a t i o n w i t h m a t t e r t o form i o n s and e x c i t e d m o l e c u l e s i n t r a c k s was w e l l u n d e r s t o o d a n d p r o v i d e d a f i r m f o u n d a t i o n o n w h i c h t o b a s e models f o r the c h e m i c a l processes which f o l l o w t h i s s t a g e . Gas P h a s e In

the

Radiation Chemistry

1920* s

and

30 s 1

most

i n the studies

20 s T

were

and

30 s T

confined

to

the

gas


phase


1.

SALMON

Development

of Radiation

7

Chemistry

and used α - p a r t i c l e s from r a d o n as the r a d i a t i o n s o u r c e . The r e s u l t s w e r e e x p r e s s e d a s M/N v a l u e s , t h e n u m b e r o f m o l e c u l e s c o n v e r t e d per i o n - p a i r formed i n the g a s . S . C . L i n d and h i s c o - w o r k e r s p l a y e d an i m p o r t a n t p a r t i n much o f t h i s w o r k . I n a number o f c a s e s , e . g . , the p o l y m e r i s a t i o n of a c e t y l e n e , i t was f o u n d t h a t M/N v a l u e s c o n s i d e r a b l y exceeded u n i t y . L i n d ( 1 2 ) a n d M u n d ( 1 3 ) p r o p o s e d t h a t M/N v a l u e s g r e a t e r t h a n u n i t y c o u l d be a c c o u n t e d f o r by t h e n e u t r a l m o l e c u l e s f o r m i n g c l u s t e r s a r o u n d t h e i o n s and on n e u t r a l i z a t i o n of the c l u s t e r i t was b e l i e v e d t h a t t h e e n e r g y l i b e r a t e d c o u l d be u s e d t o cause c h e m i c a l change i n a l l the m o l e c u l e s f o r m i n g the c l u s t e r . A n o t e a b l e landmark i n the development o f t h e s u b j e c t was the p u b l i c a t i o n i n 1936 of two p a p e r s by E y r i n g , H i r s c h f e l d e r and T a y l o r ( 1 4 , 1 5 ) i n w h i c h they c r i t i c a l l y d i s c u s s e d the mechanisms of t h e r a d i a t i o n i n d u c e d c o n v e r s i o n o f o r t h o to p a r a h y d r o g e n and the s y n t h e s i s and d e c o m p o s i t i o n of h y d r o g e n b r o m i d e . In these papers they considered i) the nature of the i n i t i a l i o n i z a t i o n processes, l a y i n g s t r e s s on t h e i n f o r m a t i o n a v a i l a b l e from mass-spectrometric studes, i i ) possible ion-molecule reactions i i i ) electron-attachment processes, iv) i o n - n e u t r a l i z a t i o n to form n e u t r a l r e a c t a n t s and v) the r o l e of i o n - c l u s t e r i n g . In g e n e r a l , they concluded that the r e a c t a n t s r e s p o n s i b l e f o r the b u l k of the c h e m i s t r y were n e u t r a l f r e e radicals that participated i n free r a d i c a l chain reactions. They a l s o showed t h a t i o n - m o l e c u l e r e a c t i o n s were expected t o be fast r e a c t i o n s and were i m p o r t a n t i n d e t e r m i n i n g the n a t u r e o f t h e c a t i o n s w h i c h were n e u t r a l i z e d . T h u s , t h e s e a u t h o r s i n t r o d u c e d many o f the p r i n c i p l e s w h i c h form the b a s i s of p r e s e n t day r a d i a t i o n c h e m i c a l thinking. Influence

of

The

Second World

War

The p r o j e c t t o b u i l d t h e a t o m i c bomb i n t h e S e c o n d W o r l d W a r , the Manhattan project, had a very great influence on all nuclear r e s e a r c h , a n d r a d i a t i o n c h e m i s t r y was no e x c e p t i o n d u e t o t h e g r e a t e r number of p e r s o n s i n v o l v e d i n s t u d y i n g the c h e m i c a l e f f e c t s of radiation. R a d i a t i o n s o u r c e s a l s o became m u c h m o r e p o w e r f u ^ a n d m o r e r e a d i l y a v a i l a b l e as n o n - n a t u r a l r a d i o i s o t o p e s , such as Co, were produced i n atomic r e a c t o r s . Also at t h i s time, r a d i a t i o n chemists c o n c l u d e d t h a t i t was p r e f e r a b l e t o e x p r e s s r a d i a t i o n - c h e m i c a l y i e l d s i n terms of the e n e r g y a b s o r b e d by a s y s t e m , r a t h e r t h a n as a M/N-value. T h i s was b e c a u s e f o r c o n d e n s e d p h a s e s y s t e m s w h i c h w e r e p l a y i n g an i n c r e a s i n g l y important r o l e i n r a d i a t i o n c h e m i s t r y , i t i s i m p o s s i b l e t o m e a s u r e N , t h e number o f i o n s f o r m e d , and v a l u e s were based a r b i t r a r i l y on gas phase W - v a l u e s , i . e . the e n e r g y r e q u i r e d t o f o r m a n i o n p a i r i n t h e g a s p h a s e . T h u s , t h e G - v a l u e was d e f i n e d a s t h e n u m b e r o f m o l e c u l e s c o n v e r t e d p e r 100 eV a b s o r b e d b y t h e s y s t e m . This definition has held upto the present although recent r e c o m m e n d a t i o n by i n t e r n a t i o n a l b o d i e s a r e f o r t h e d e f i n i t i o n t o be m o d i f i e d to conform to the S . I . System of u n i t s . On t h i s b a s i s t h e G - v a l u e c a n be d e f i n e d a s t h e n u m b e r o f m o l e s o f p r o d u c t f o r m e d o r r e a c t a n t consumed p e r J o u l e a b s o r b e d and G-value/mol j "

1

= 1.037

χ 10~

7

χ G-value/molecules

(100

eV)


1

POLYMERS FOR HIGH T E C H N O L O G Y

8 Aqueous

Solutions

The F r e e R a d i c a l H y p o t h e s i s . A l t h o u g h s t u d i e s on aqueous s o l u t i o n s had c o n t i n u e d t h r o u g h the 3 0 s , n o t a b l y by H . F r i c k e and h i s c o - w o r k e r s , d e t a i l e d i n t e r p r e t a t i o n o f d a t a was p r e v e n t e d b y t h e l a c k of a c l e a r h y p o t h e s i s f o r the n a t u r e of the s p e c i e s r e s p o n s i b l e f o r the c h e m i s t r y . I t was a p p r e c i a t e d t h a t f o r s o l u t i o n s t h e b u l k o f t h e e n e r g y was a b s o r b e d b y t h e s o l v e n t a n d t h i s l e d t o t h e c o n c e p t of indirect action(16,17) whereby a c t i v a t e d species d e r i v e d from the solvent reacted w i t h the s o l u t e , but the nature of the activated s p e c i e s was u n k n o w n . H o w e v e r i n 1 9 4 4 , W e i s s ( 1 8 ) i n E n g l a n d r e v i v e d a v e r y e a r l y p r o p o s a l by D e b i e r n e ( 1 9 ) t h a t i r r a d i a t i o n of water y i e l d e d Η-atoms a n d O H - r a d i c a l s ,


f

H

2

0 V ^ V

H * + OH*

(3)

and he i n d i c a t e d , f o r example, how t h i s h y p o t h e s i s e x p l a i n e d the f o r m a t i o n o f h y d r o g e n p e r o x i d e when a e r a t e d w a t e r was i r r a d i a t e d a n d a l s o the o x i d a t i o n and r e d u c t i o n of v a r i o u s m e t a l i o n s i n i r r a d i a t e d water. T h i s p r o p o s a l was v a l u a b l e i n p r o v i d i n g a c o n c r e t e m e c h a n i s m f o r e x p l a i n i n g t h e r a d i a t i o n c h e m i s t r y o f w a t e r , b u t i t was b a s e d o n t h e i n v a l i d a s s u m p t i o n t h a t t h e same g e n e r a l p r i n c i p l e s a p p l y t o l i q u i d s as t o g a s e s , and t h a t d i s s o c i a t i o n p r o c e s s e s are the i m p o r t a n t r e s u l t of i r r a d i a t i n g m a t e r i a l s . T h i s ignores the profound i n f l u e n c e that s o l v e n t s t r u c t u r e and p o l a r i t y c a n have on the r e c o m b i n a t i o n of i o n s , especially electrons. H o w e v e r , i n 1953 S a m u e l a n d M a g e e ( 2 0 ) p r o d u c e d a model f o r the primary event i n water which suggested that the e l e c t r o n s g e n e r a t e d i n t h e i o n i z a t i o n e v e n t t r a v e l a b o u t 20A f r o m t h e p o s i t i v e ion while being thermalised. At t h i s d i s t a n c e they would s t i l l be w i t h i n t h e s t r o n g C o u l o m b i c f i e l d o f t h e c a t i o n a n d w o u l d be p u l l e d back t o i t l e a d i n g to i t s n e u t r a l i z a t i o n and t h e f o r m a t i o n of H* a n d 0 H . Thus t h i s model p r o v i d e d s t r o n g s u p p o r t f o r t h e W e i s s h y p o t h e s i s w h i c h c o n s e q u e n t l y had a v e r y s t r o n g i n f l u e n c e on the thinking of radiation chemists until almost 1960 even though S t e i n ( 2 1 ) a n d P l a t z m a n ( 2 2 ) h a d p o s t u l a t e d i n 1952 a n d 1953 t h a t t h e e l e c t r o n c o u l d be s o l v a t e d a n d p a r t i c i p a t e i n r e a c t i o n s w i t h s o l u t e s . In f a c t , P l a t z m a n had c o n s i d e r e d the f o r m a t i o n and f a t e of the h y d r a t e d e l e c t r o n i n some d e t a i l , b u t h i s i d e a s w e r e p r e s e n t e d a t a n informal conference a n d d i d n o t make a n i m m e d i a t e i m p a c t on the r a d i a t i o n chemical community. The formation of the hydroxyl radical has been amply demonstrated, b u t i t i s now a c c e p t e d that the major route to its f o r m a t i o n i s by the i o n - m o l e c u l e r e a c t i o n (4). e

(4) A l l e n ( 2 3 ) has i n d i c a t e d t h a t M. B u r t o n and J . F r a n c k h e l d the f r e e r a d i c a l h y p o t h e s i s of w a t e r r a d i o l y s i s d u r i n g t h e i r w a r - t i m e work and t h a t t h e y c o n s i d e r e d ' O H t o be g e n e r a t e d b y r e a c t i o n (4). The H y d r a t e d E l e c t r o n . However, d u r i n g the l a t e 1950*s results became a v a i l a b l e o n a q u e o u s s o l u t i o n s w h i c h c o u l d n o t be r e c o n c i l e d w i t h the major r e d u c i n g s p e c i e s b e i n g the hydrogen atom(24-28) and a l s o C z a p s k i and Schwarz(29) and D a i n t o n and c o - w o r k e r s ( 3 0 , 3 1 ) proved


1.

SALMON

Development

of Radiation

9

Chemistry

t h a t the r e d u c i n g species c a r r i e d u n i t negative charge. The i n v e n t i o n o f p u l s e r a d i o l y s i s b y Boag and H a r t ( 3 2 , 3 3 ) and K e e n e ( 3 4 ) l e d t o the o b s e r v a t i o n i n i r r a d i a t e d aqueous s o l u t i o n s of a broad a b s o r p t i o n with ^ = 7 2 0 n m , w h i c h was i d e n t i f i e d a s b e i n g d u e t o t h e h y d r a t e d electron, e q. m a x


a

This species is essentially an electron stabilised by the surrounding water molecules. I t has been the s u b j e c t of detailed theoretical studies(35), b u t c a n be c o n s i d e r e d a s a n e l e c t r o n i n a s p h e r i c a l p o t e n t i a l w e l l c o n s i s t i n g of solvent molecules. Specific s h o r t - r a n g e s o l v a t i o n e f f e c t s a r e t h o u g h t t o be i m p o r t a n t a s w e l l a s long-range p o l a r i z a t i o n forces. With the d i s c o v e r y of the h y d r a t e d e l e c t r o n and the newly a v a i l a b l e t e c h n i q u e o f p u l s e r a d i o l y s i s r a p i d a d v a n c e s w e r e made i n the u n d e r s t a n d i n g of the p r o c e s s e s w h i c h o c c u r i n i r r a d i a t e d aqueous solutions. For an e x c e l l e n t review of the c u r r e n t s t a t e of knowledge see r e f e r e n c e 36. The c h e m i s t r y o f t h e h y d r a t e d e l e c t r o n h a s b e e n r e v i e w e d i n d e t a i l b y H a r t a n d A n b a r ( 3 7 ) , b u t i t s r e a c t i o n s c a n be summarised a s : i)

redox eg.

ii)

M

Z

+

+

electron eg.

iii)

reactions e" aq

RCHO + e~ aq

o

reactions eg.

M

-*

H

+

(

Z

dq

1

)

(4)

+

reactions RCH0~

N +

0

I

e" aq

w i t h Brjamsted

+ e"

"

ions

(5)

attachment

N.O + e~ Ζ aq ClCH C0~ 2 2

and i v )

-

attachment

dissociative eg.

with metal

+

+ 0~ ->

(6)

Cl~ +

e

CH C0~ 2 2

(7)

o

acids

Η*

(8)

Solvated Electrons i n Organic Media. T h e r e w a s , o f c o u r s e , no r e a s o n t o s u p p o s e t h a t w a t e r was t h e o n l y l i q u i d c a p a b l e o f s o l v a t i n g t h e e l e c t r o n and o v e r a v e r y few y e a r s p u l s e r a d i o l y s i s e x p e r i m e n t s de m o n s t r a t e d t h e e x i s t e n c e o f s o l v a t e d e l e c t r o n s , e~, i n many l i q u i d s · (Table I). Liquid water alcohols ammonia amines dialkylamides ethers HMPA hydrocarbons

_e~ i n V a r i o u s

A

Liquids

max 720 580-820 1500 >1700 1500-1800 1900-2300 ^2300 >2000

(Ref) (32-34) (38) (39) (40,41) (41) (42) (43) (44)


POLYMERS FOR HIGH T E C H N O L O G Y

10

The s t a b i l i t y o f t h e s o l v a t e d e l e c t r o n i n t h e v a r i o u s s o l v e n t s varies g r e a t l y depending on the r e a c t i v i t y of the e l e c t r o n w i t h the s o l v e n t . I n l i q u i d ammonia s o l v a t e d e l e c t r o n s a r e s t a b l e f o r l o n g p e r i o d s i n the b l u e s o l u t i o n s of a l k a l i m e t a l s i n ammonia(45). In water,_^eâq d e c a y s s l o w l y b y r e a c t i o n ( 9 ) w i t h a r a t e c o n s t a n t o f 16 M s (46) e" + H 0 aq ζ

+

o

*H + 0H~

(9)


w h i l e t h e a n a l o g o u s r e a c t i o n i n e t h a n o l has a h a l f - l i f e o f 9 ys a t room t e m p e r a t u r e ( 4 7 ) . In o r g a n i c amides l i f e t i m e s of the order of microseconds have been observed(48) despite the presence of the carboxyl group. Ionic

processes

i n Non-Polar

Media

F o l l o w i n g the r e a l i s a t i o n t h a t the r e a c t i o n s of the h y d r a t e d e l e c t r o n p l a y e d an i m p o r t a n t r o l e i n the r a d i a t i o n c h e m i s t r y of l i q u i d water i t was n o t l o n g b e f o r e e v i d e n c e was s o u g h t , a n d f o u n d , t h a t the e l e c t r o n and t h e c o u n t e r c a t i o n c o u l d be i n v o l v e d i n c h e m i c a l r e a c t ions i n non-polar liquids before they underwent neutralisation. S c h o l e s and S i m i c ( 1 9 6 4 ( 4 9 ) ) showed t h a t on i r r a d i a t i o n o f solutions o f n i t r o u s o x i d e i n h y d r o c a r b o n s n i t r o g e n was f o r m e d i n t h e d i s s o c iative attachment reaction analogous to r e a c t i o n (6). Similarly, B u c h a n a n a n d W i l l i a m s ( 1 9 6 6 ( 5 0 ) ) a t t r i b u t e d t h e f o r m a t i o n o f HD i n γ - i r r a d i a t e d s o l u t i o n s o f C2R5OD i n c y c l o h e x a n e t o t h e t r a n s f e r o f a p r o t o n from the C ^ H ^ i o n to C ^ O D ( r e a c t i o n 10):

C

6 12 H

+

+

C

2 5° H

e~ + C H O D H 2

D

'

+

5

C

*

D

+

6 12 H

C

-> *

6 11 H

#

+

C

2

H

5

O

D

H

+

(

1

C H 0 H + D* 2

C

H

+

H D

)

(11)

5

6 ll"

0

(

1

2

)

The s c a v e n g i n g o f e l e c t r o n s a n d c a t i o n s i n n o n - p o l a r m e d i a c a n be e x p l a i n e d i f i t i s assumed t h a t the t h e r m a l i s a t i o n o f the e l e c t r o n i s a s t o c h a s t i c process g i v i n g r i s e to a d i s t r i b u t i o n of i o n - p a i r s e p a r a t i o n s w i t h a s i g n i f i c a n t n u m b e r o f p a i r s w i t h s e p a r a t i o n s > 200 A . Most of the e l e c t r o n s d r i f t back towards the c a t i o n under the i n f l u ence of the Coulombic f i e l d and undergo r e c o m b i n a t i o n w i t h their geminate p a r t n e r . While d r i f t i n g under the i n f l u e n c e of t h e i r mutual f i e l d e i t h e r o f t h e p a r t n e r s may e n c o u n t e r a s o l u t e w i t h w h i c h t h e y can r e a c t . The d y n a m i c s o f t h i s r e c o m b i n a t i o n a n d s c a v e n g i n g process a r e , of c o u r s e , determined by the d i f f u s i o n c o e f f i c i e n t s o f t h e two c h a r g e d s p e c i e s and hence on t h e i r d e g r e e of l o c a l i z a t i o n . Since r e a c t i o n of the charges w i t h s o l u t e s i s i n c o m p e t i t i o n w i t h t h e i r recombination i n the Coulombic f i e l d , the dependence of the y i e l d of p r o d u c t from the scavenging r e a c t i o n i s s t r o n g l y dependent on the s o l u t e c o n c e n t r a t i o n . The e m p i r i c a l e q u a t i o n due t o W a r m a n , Asmus and S c h u l e r ( 5 1 ) has proved t o be p a r t i c u l a r l y u s e f u l in t r e a t i n g s c a v e n g i n g d a t a , where G(P) i s the s c a v e n g i n g y i e l d , G the free-ion y i e l d (see below) and G ^ the y i e l d of i o n - p a i r s which undergo g e m i n a t e r e c o m b i n a t i o n i n trie a b s e n c e o f a s c a v e n g e r .

G(P)

= G . . + G .{

-±f-

a

i)


(13)

1.

SALMON

Development

of Radiation

Chemistry

11

Due t o B r o w n i a n m o t i o n , a s m a l l f r a c t i o n o f t h e i o n - p a i r s e s c a p e r e c o m b i n a t i o n a n d become h o m o g e n e o u s l y d i s t r i b u t e d . This y i e l d of i o n - p a i r s i s c a l l e d e i t h e r the f r e e - i o n y i e l d , G^^ o r the escaped yield, G and i s g i v e n by e

g

c

G , II

«

f

G„. t l

Γ

ο

n(r)

exp(-r

c

/r).dr

where G £ i s the t o t a l i o n i z a t i o n y i e l d , i n i t i a l i o n - p a i r s e p a r a t i o n s and r , t h e is e /4πε ε k ^ T . E x p r e s s i o n ( 1 4 ) i s o n l y of a s i n g l ? î - ï o n - p a i r s p u r , but s e r v e s as The r e c e n t r e v i e w b y Warman(52) gives factors governing the behaviour of i r r a d i a t e d non-polar systems. t


c

(14)

n(r) i s the d i s t r i b u t i o n of Onsager c r i t i c a l d i s t a n c e , s t r i c t l y v a l i d for the case a u s e f u l model d e s c r i p t i o n . a detailed account of the electrons and cations in

L o c a l i z e d and Q u a s i - F r e e E l e c t r o n s . The m o b i l i t y o f e l e c t r o n s in n o n - p o l a r m e d i a r a n g e s f r o m 5 χ 10 ~ 3 cm 2 v ~ l s*~l i n l i q u i d h y d r o g e n t o v a l u e s a s h i g h a s 2 2 0 0 c m ^ v " ^ s~* i n l i q u i d x e n o n ( s e e reference 35). F o r common h y d r o c a r b o n s t h e v a l u e r a n g e f r o m 0 . 0 1 3 cm v ~ ^ s ~ ^ for trans-decalin(53) t o 100 cm ^ v " s for tetramethylsilane(54) · These v a l u e s a r e t o be compared w i t h t h a t f o r e q o f 2 χ 1 0 ~ 3 m 2 v~^ s ~ * ( q u o t e d i n r e f e r e n c e 37). A s i m p l i f i e d model(55(1972)) which e x p l a i n s the wide range of v a l u e s , i s t h a t the e l e c t r o n i n n o n - p o l a r condensed media can e x i s t i n two s t a t e s , a l o c a l i z e d o r t r a p p e d s t a t e whose m o b i l i t y is c o m p a r a b l e w i t h t h a t of s m a l l m o l e c u l a r i o n s and a q u a s i - f r e e state the m o b i l i t y of which i s of the order of t h a t i n l i q u i d xenon. In t h e q u a s i - f r e e s t a t e t h e e l e c t r o n moves f r e e l y f o r much o f i t s t i m e i n the u n i f o r m p o t e n t i a l between m o l e c u l e s and o n l y b r i e f l y i n t e r a c t s w i t h the p o t e n t i a l w e l l s u r r o u n d i n g m o l e c u l e s . Thus t h e l o c a l i z e d and quasi-free states can be considered to exist in dynamic e q u i l i b r i u m and t h e m o b i l i t y o f t h e e l e c t r o n c a n be a p p r o x i m a t e d b y 1

a

U(e") where

y(e

quasi-free

)

= y is

exp(A H ^ / ^ T )

the m o b i l i t y

state,

states.

d £

C

a n d 'ΔΗ

of the

(15)

the e l e c t r o n , enthalpy

the m o b i l i t y

difference

between

in the

the two

1

Of c o u r s e , e l e c t r o n s i n n o n - p o l a r media o n l y d i s p l a y a b s o r p t i o n s p e c t r a i f they spend a major f r a c t i o n of t h e i r e x i s t e n c e i n a l o c a l i z e d s t a t e , as i s the case w i t h n - h e x a n e . The

Influence

of

the P o l a r i t y

of

t h e Medium

The a b o v e d i s c u s s i o n s t r e s s e s t h e k e y r o l e o f s o l v e n t p o l a r i t y a n d s t r u c t u r e i n d e t e r m i n i n g the subsequent b e h a v i o u r of the ionic species generated i n the primary p r o c e s s e s . Thus, i n water w i t h i t s h i g h d i e l e c t r i c c o n s t a n t the b u l k of the e~ and Η 0 escape the C o u l o m b i c f i e l d and t h e s p u r p r o c e s s e s depend o n l y on t h e i r random diffusion. In n o n - p o l a r media i n the absence of e l e c t r o n or charge scaveng ing solutes, most e l e c t r o n s and i o n s w i l l undergo f a s t geminate c h a r g e r e c o m b i n a t i o n l e a d i n g t o n e u t r a l f r e e r a d i c a l s , o r i n some media to l o n g - l i v e d e x c i t e d s t a t e s , which react to g i v e the f i n a l products. 3

+


12


Ionic

Processes

in

Solids

C o n c u r r e n t l y w i t h the v i e w t h a t r e a c t i o n s of e l e c t r o n s and p o s i t i v e i o n s p l a y an i m p o r t a n t p a r t i n the r a d i a t i o n c h e m i s t r y of l i q u i d s , i t was b e i n g d e m o n s t r a t e d , n o t a b l y b y H a m i l l ( 5 6 ) a n d h i s c o - w o r k e r s o v e r the p e r i o d 1 9 6 2 - 1 9 6 6 , t h a t c h a r g e t r a p p i n g , m i g r a t i o n and r e a c t i o n w e r e a l s o i m p o r t a n t i n t h e r a d i a t i o n c h e m i s t r y o f many s o l i d s y s t e m s , e s p e c i a l l y at low temperatures. As a r e s u l t of t h e s e studies, ^ - i r r a d i a t i o n o f l o w t e m p e r a t u r e s o l i d s h a s become p e r h a p s t h e m o s t v e r s a t i l e method of s t u d y i n g the s p e c t r o s c o p i c p r o p e r t i e s of r a d i c a l a n i o n s and c a t i o n s .


Polymers The h i s t o r y o f t h e r a d i a t i o n c h e m i s t r y o f p o l y m e r s i s t h e s u b j e c t o f a n o t h e r l e c t u r e i n t h i s symposium by P r o f e s s o r Chapiro. However, s i n c e t h e m a i n theme o f t h e s y m p o s i u m i s a i m e d a t t h e u s e o f e l e c t r o n beams i n l i t h o g r a p h y f o r t h e m a n u f a c t u r e o f e l e c t r o n i c d e v i c e s , i t i s a p p r o p r i a t e to r e f e r b r i e f l y to the r a d i a t i o n c h e m i s t r y of polymeric materials· The f i r s t s y s t e m a t i c s t u d y of the i r r a d i a t i o n of polymers was u n d e r t a k e n by D o l e and Rose d u r i n g t h e p e r i o d 1947-1949. These workers discovered that i r r a d i a t i o n of polyethylene caused some d e g r a d a t i o n to low m o l e c u l a r weight p r o d u c t s and the i n t r o d u c t i o n of u n s a t u r a t i o n i n the polymer c h a i n s , b u t by f a r the most exciting d i s c o v e r y was t h a t c r o s s l i n k s w e r e f o r m e d b e t w e e n p o l y m e r c h a i n s a n d t h i s had a p r o f o u n d e f f e c t on the s t r e s s - s t r a i n c u r v e s and the c o l d drawing properties of polyethylene(57-59). In 1952 Charlesby(60) p u b l i s h e d t h e f i r s t o f many p a p e r s o n t h e e f f e c t s o f r a d i a t i o n o n polymers. The r a p i d d e v e l o p m e n t o f t h e f i e l d u p t o 1960 i s reviewed i n books by C h a r l e s b y (61) and C h a p i r o ( 6 2 ) . The m a j o r s t u d i e s of h i g h p o l y m e r s y s t e m s have concentrated m a i n l y on r e l a t i n g t h e c h e m i c a l changes o f c r o s s l i n k i n g and c h a i n d e g r a d a t i o n to the m o d i f i c a t i o n of the p h y s i c a l p r o p e r t i e s of the polymer. Most s t u d i e s have u t i l i s e d f r e e r a d i c a l mechanisms to e x p l a i n c r o s s l i n k i n g and d e g r a d a t i o n of p o l y m e r s . S i n c e many polymers are p o l y o l e f i n e s a n d n o n - p o l a r , i t i s t o be e x p e c t e d that most of the charges formed i n the i n i t i a l i o n i z a t i o n event will undergo r a p i d r e c o m b i n a t i o n to y i e l d r a d i c a l s . However, the d e t a i l e d m e c h a n i s m s c o n t r o l l i n g t h e m o t i o n o f t h e r a d i c a l s w h i c h a l l o w them t o come t o g e t h e r t o f r o m a c r o s s l i n k i s s t i l l t h e s u b j e c t o f m u c h u n c e r t a i n l y (63). A l s o for halogenated polymers d i s s o c i a t i v e electron c a p t u r e processes analogous to r e a c t i o n (7) are i m p o r t a n t i n d e t e r mining the i n i t i a l l o c a l i s a t i o n of the f r e e r a d i c a l s . Recent developments i n the f i e l d i n c l u d e the a p p l i c a t i o n of the picosecond p u l s e r a d i o l y s i s technique to the study of the processes o c c u r r i n g i n p o l y m e r s ( 6 4 ) and the use of pulse radiolysis i n conjunction with l i g h t s c a t t e r i n g measurements to s t u d y t h e i r d e g r a d a t i o n (65). The use of r a d i a t i o n to modify the p h y s i c a l p r o p e r t i e s of p o l y m e r s h a s become a v e r y i m p o r t a n t i n d u s t r y w i t h p r o d u c t s s u c h a s electrical cables with i n s u l a t i o n capable of withstanding high temperatures and h e a t - s h r i n k a b l e p o l y e t h y l e n e . However, of direct r e l e v a n c e t o t h i s s y m p o s i u m was t h e r e c o g n i t i o n i n t h e e a r l y 1 9 7 0 ' s t h a t e l e c t r o n beam i r r a d i a t i o n o f polymer f i l m s could provide an important l i t h o g r a p h i c t o o l for the manufacture of microelectronic components. For c o n s i d e r a t i o n of the g e n e r a l p r i n c i p l e s of these processes see, f o r example, references (66) and ( 6 7 ) . The p r o d u c t s required in this field are complex requiring both microscopic


1. SALMON

Development of Radiation Chemistry

13


resolution of elaborate patterns and proceedures for producing both p o s i t i v e and negative r e s i s t s . Thus the emphasis has been on b r i n g i n g about h i g h e f f i c i e n c y d e g r a d a t i o n or c r o s s l i n k i n g of s p e c i a l i s e d polymers. One of the r a t i o n a l e s behind the use of electron beams for t h i s purpose i s that the effective wavelength of electrons i s so small that d i f f r a c t i o n effects i n the formation of patterns are n e g l i g i b l e . However, i t i s probable that the resolution l i m i t w i l l prove to be determined by the thermalization length of the secondary electrons generated i n the primary i o n i z a t i o n events. Current Trends In the course of t h i s lecture I have attempted to outline the development of r a d i a t i o n chemistry from i t s beginnings. The main h i s t o r i c a l theme has been the e l u c i d a t i o n of the extent to which neutral free r a d i c a l s as opposed to i o n i c species contributed to the o v e r a l l chemistry. Our present understanding leads to the view that the i n t e r p l a y between free r a d i c a l and i o n i c processes i s v e r y dependent on the system being considered, p a r t i c u l a r l y i t s d i e l e c t r i c properties and the presence, or absence, of solutes which can react with electrons or cations. Since the r a d i a t i o n chemistry of water and aqueous solutions i s w e l l understood and the y i e l d s of the primary s p e c i e s are w e l l characterised, the r a d i o l y s i s of aqueous solutions has become perhaps the most v e r s a t i l e means of studying a wide range of free r a d i c a l and redox processes i n s o l u t i o n , e s p e c i a l l y when combined w i t h the nanosecond time-resolution which i s routinely a v a i l a b l e with pulse radiolysis. It i s outside the scope of t h i s review to consider the wide range of processes which have been studied using these techniques and the reader i s referred to the paper by Buxton(68) for further information. The study of the very fast processes that follow on the absorption of r a d i a t i o n i n organic systems i s a very active f i e l d with pulse r a d i o l y s i s with picosecond time r e s o l u t i o n being one of the major t o o l s . This technique, the l a t e s t version of which employs twin l i n e a r accelerators(69), has time resolution of about 20 ps. These methods are being used to investigate the fast recombination of charges, the formation of excited states and free r a d i c a l s , mainly i n hydrocarbon media, but have also recently been applied to the study of r a d i a t i o n effects i n polymers(70). In general, detailed knowledge of the r a d i a t i o n chemistry of organic l i q u i d s i s r e s t r i c t e d to the lower alcohols and some hydrocarbons and information on other systems i n very sparse. This i s one of the reasons why pulse r a d i o l y s i s i n organic solvents has not yet f u l f i l l e d i t s p o t e n t i a l for a p p l i c a t i o n to the study of general chemical problems, as has been the case for aqueous systems. L i t e r a t u r e Cited 1. Roentgen, W.C. Sitzungosberichte der Physikalisch-Medizinischen Gesellschaft zu Wurzburg, 1895. 2. Becquerel, H. Compt. rend. 1896, 122, 420. 3. Curie, P . ; Curie, M. Compt. rend. 1899, 129, 823. 4. G i e s e l , F. Verh. Peut. Phys. Ges. 1900, 2, 9. 5. Becquerel, H. Compt. rend. 1901, 133, 709. 6. J o r i s s e n , W . P . ; Woudstra, H.W. Z e i t . Chem. u . I n d u s t r i e d . K o l l o i d e . 1912, 10, 280. 7. J o r i s s e n , W.P.; Ringer, W.E. Berichte 1906, 39, 2093. Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.


14


8. Bethe, H.A. Handbuch der Physik; Julius Springer: Berlin, 1933; vol. 23. 9. Compton A.H. Phys. Rev. 1923, 21, 483-502. 10. Blackett, P.M.S.; Occhialini, G.P.S. Proc. Roy. Soc. (London) 1933, A139, 699-727. 11. Wilson, C.T.R. Proc. Roy. Soc. (London) 1923, A104, 192. 12. Lind, S.C. The Chemical Effects of Alpha-Particles and Electrons; Chemical Catalog Co., Inc.: New York, 1928. 13. Mund, W. L'Action chimique des rayons alpha on phase gazeuze; Herman et Cie.: Paris, 1935. 14. Eyring, H.; Hirschfelder, J.O.; Taylor, H.S. J. Chem. Phys. 1936, 4, 479. 15. Eyring, H.; Hirschfelder, J.O.; Taylor, H.S. J. Chem. Phys. 1936, 4, 570. 16. Risse, O. Ergeb. Physiol. 1930, 30, 242. 17. Fricke, H. Cold Spring Harbor Symopsium 1934, 2, 241. 18. Weiss, J. Nature 1944, 153, 748. 19. Debierne, A Ann. Physique (Paris) 1914, 2, 241. 20. Samuel, A.H.; Magee, J.L. J. Chem. Phys. 1953, 21, 1080. 21. Stein, G. Disc. Faraday Soc. 1952, 12, 227. 22. Platzmann, R.L. Physical and Chemical Aspects of Basic Mechanisms in Radiobiology; U.S. National Research Council: 1933, No. 305, 22. 23. Allen, A.O. In The Chemical and Biological Action of Radiations; Haissinsky, M., Ed.; Academic Press: London, 1961; Vol. 5, p.14. 24. Hart. E.J. J. Am. Chem. Soc. 1954, 76, 4312. 25. Hart, E.J. Radiation Res. 1955, 2, 33. 26. Hayon, Ε.; Weiss, J. Proc. 2nd Int. Conf. Peaceful Uses of Atomic Energy, 1958, 29, 80. 27. Baxendale, J.H.; Hughes, G. Z. Phys. Chem. (Frankfurt), 1958, 14, 306. 28. Barr, N.F.; Allen, A.O. J. Phys. Chem. 1959, 63, 928. 29. Czapski, G.; Schwarz, H.A. J. Phys. Chem. 1962, 66, 471. 30. Collinson, E.; Dainton, F.S.; Smith, D.R.: Tazuke, S. Proc. Chem. Soc. 1962, 140. 31. Dainton, F.S.; Watt, W.S. Proc, Roy. Soc. (London), 1963, 275A, 447. 32. Boag, J.; Hart, E.J. J. Am. Chem. Soc. 1962, 84, 4090. 33. Boag, J.; Hart, E.J. Nature, 1963, 197, 45. 34. Keene, J.P. Nature, 1963, 197, 47. 35. Kestner, N.R. In Electron-Solvent and Anion-Solvent Interactions; Kevan, L. and Webster, B.C., Eds.; Elsevier: Amsterdam, 1976, Chapter 1. 36. Buxton, G.V. In The Study of Fast Processes and Transient Species by Electron Pulse Radiolysis; NATO Advanced Study Institute Series; Baxendale, J.H. and Busi, F., Eds.; Dordrecht: Holland, 1982, p.241. 37. Hart, E.J.; Anbnar, M. The Hydrated Electron; WileyInterscience: New York, 1970. 38. Taub, I.Α.; Harter, D.A.; Sauer, M.C.; Dorfman, L.M. J. Chem. Phys. 1964, 41, 979, 39. Compton, D.M.J.; Bryant, J.F.; Cesena, R.A.; Gehman, B.L. In Pulse Radiolysis; Ebert, M.; Keene, J.P.; Swallow, A.J.; Baxendale, J.H., Eds.; Academic Press: London & New York, 1965, p.43. 40. Belloni, J.; de la Renaudiere, J.F. Nature Phys. Sci. 1971, 232, 173. Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.


1. SALMON

Development of Radiation Chemistry

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Development of Radiation Chemistry - American Chemical Society

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