Chapter 1
Pulse Radiolysis Studies of Polymers Seiichi Tagawa
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Research Center for Nuclear Science and Technology, University of Tokyo, 2-22 Shirakata-Shirane, Tokai-mura, Ibaraki, 319-11 Japan Recent or picosecond (PS) and nanosecond (NS) electron beam (EB), ion beam (IB), and synchrotron radiation (SR) pulse radiolysis studies on polymers are reviewed. Transient absorption and emission spectra of short-lived reactive intermediates such as electrons, radical anions and cations, excited states and neutral radicals have been observed in irradiated solid polymers and polymers solutions, such as polyethylene model compounds and ethylene-propylene copolymers, polystyrene and related compounds including resists, and sigma-conjugated polymers such as polysilanes and polygermanes. Very recent progress in our picosecond EB pulse radiolysis systems, especially new LL (linac-laser) twin system, is also shown. IB and SR pulse radiolysis systems (1 ns and subnanosecond time resolution for emission spectroscopy, respectively) are developed very recently and applied to polymers. The history of pulse radiolysis studies on polymers is also reviewed shortly. A large number of papers on radiation effects on polymers for various fields have been published over the last 40 years [1,2,3,4]. However, the complexity of reactions of reactive intermediates is so large that studies on the detailed reaction mechanisms are very d i f f i c u l t . In principle, various kinds of ions and excited states are initially formed by ionization and excitation of polymers. Beginning with these species, further new reactive intermediates such as ions and radicals are often formed. Final products are produced through a lot of reactions of various reactive intermediates. Direct measurements of short-lived reactive intermediates are essential to make clear the detailed reaction mechansims of i r r a d i a t e d polymers. A pulse radiolysis technique, which can detect s h o r t - l i v e d reactive intermediates d i r e c t l y , is very powerful method to study detailed reaction mechanisms of reactive intermediates. Although several research groups published papers on pulse radiolysis studies of polymer solutions, few groups studied pulse radiolysis of s o l i d polymers and no clear transient absorption due to polymer ions and excited states was observed in irradiated s o l i d polymers before 1983 [5-9]. Very recently clear transient
0097-6156/91/0475-0002S08.25/0 © 1991 American Chemical Society
Clough and Shalaby; Radiation Effects on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
1. TAGAWA
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Pulse Radiolysis Studies of Polymers
a b s o r p t i o n s p e c t r a o f p o l y m e r i o n s and e x c i t e d s t a t e s have been o b s e r v e d i n i r r a d i a t e d s o l i d p o l y m e r s [ 1 0 - 1 3 ] . New t y p e s o f p u l s e r a d i o l y s i s t e c h n i q u e s s u c h a s i o n beam [ 1 4 16] a n d s y n c h r o t r o n r a d i a t i o n [17,18] p u l s e r a d i o l y s i s have a l s o been a p p l i e d t o p o l y m e r s , e s p e c i a l l y t o LET e f f e c t s t u d i e s on p o l y m e r s .
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The present paper mainly d e s c r i b e s recent progress i n e l e c t r o n beam, i o n beam a n d s y n c h r o t r o n r a d i a t i o n p u l s e r a d i o l y s i s s t u d i e s on p o l y m e r s . 2.
OUR PULSE RADIOLYSIS SYSTEMS APPLICATIONS TO POLYMERS
2.1
Electron Beam Pulse Radiolysis
AND
THEIR
TYPICAL
A 35 MeV S-band (2856MHZ) e l e c t r o n l i n e a r a c c e l e r a t o r (linac) with c a p a b i l i t y of producing a picosecond single e l e c t r o n p u l s e ( p u l s e w i d t h ; 10 p s ) has been o p e r a t e s s i n c e 1977 [ 1 9 ] . The picosecond pulse r a d i o l y s i s with o p t i c a l emission s p e c t r o s c o p y [20] has been a p p l i e d t o p o l y m e r s i n s o l u t i o n s [21]. The t i m e r e s o l u t i o n i s a b o u t 10 ps f o r t h e e m i s s i o n spectroscopy. P i c o s e c o n d a n d n a n o s e c o n d e l e c t r o n beam p u l s e r a d i o l y s i s w i t h o p t i c a l a b s o r p t i o n s p e c t r o s c o p y h a s been a p p l i e d t o polymers such as r e s i s t s [10,22], p o l y s t y r e n e and r e l a t e d polymers [12,23,24], ethylene-propylenediene terpolymer [ 2 5 ] , e t h y l e n e - p r o y r e n e c o p o l y m e r s [ 1 3 ] , epoxy r e s i n [ 1 1 ] , p o l y s i l a n e s [26-30] and p o l y g e r m a n e s [ 2 8 ] . The s t a n d a r d a b s o r p t i o n s p e c t r o s c o p y s y s t e m i s composed o f a Xe light source, a monochromator, a fast response photodiode, a t r a n s i e n t d i g i t i z e r o r sampling o s c i l l o s c o p e w i t h a f a s t p u l s e a m p l i f i e r a n d a c o m p u t e r s y s t e m [32] . F i g . 1 shows t h e b l o c k d i a g r a m o f t h e s y s t e m a n d t h e formation process of polymethylpropylsilane r a d i c a l anion through the reaction of p o l y s i l a n e with the solvated e l e c t r o n i n t e t r a h y d r o f u r a n (THF) m o n i t o r e d by t h e s y s t e m . The t i m e - r e s o l u t i o n i s a b o u t 700 p s f o r t h e t r a n s i e n t d i g i t i z e r s y s t e m a n d 60 p s f o r t h e s a m p l i n g o s c i l l o s c o p e system. An a b s o r p t i o n s p e c t r o s c o p y s y s t e m w i t h a t i m e r e s p o n s e o f a b o u t 50 ps h a s a l s o d e v e l o p e d by u s i n g a s t r e a k camera w i t h a g a t e o p t i o n [ 3 3 ] . R e c e n t l y a new a b s o r p t i o n s p e c t r o s c o p y s y s t e m w h i c h h a s a v e r y h i g h t i m e r e s o l u t i o n o f 20 p s , h a s been d e v e l o p e d by u s i n g two p a r a l l e l l i n a c s [ 3 4 ] . The system, w h i c h i s c a l l e d " t w i n l i n a c p i c o s e c o n d p u l s e r a d i o l y s i s s y s t e m , " h a s been a p p l i e d t o p o l y e t h y l e n e model compounds [13] and p o l y s i l a n e s [27] . F i g . 2 shows t h e l a y o u t o f t h a t t w i n l i n a c s y s t e m [34] . F i g . 3 shows t h e f o r m a t i o n process of the p o l y m e t h y l p r o p y l s i l a n e through t h e r e a c t i o n of p o l y s i l a n e w i t h t h e s o l v a t e d e l e c t r o n i n THF m o n i t o r e d by t h a t t w i n l i n a c system [27]. Clough and Shalaby; Radiation Effects on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
RADIATION EFFECTS ON POLYMERS
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time / nsec LINAC CONTROLLER
TRANSIENT 0I6ITIZER
OR SAMPLING OSCILLOSCOPE
Fig. 1. a, The block diagram of the pulse radiolysis system with absorption spectroscopy. (Reproduced with permission from reference 32. Copyright 1983 Pergamon.) b, Optical density change of 93 mM polymethylpropylsilane solution in tetrahydrofuran after the pulse, which was monitored at 365 nm. (Reproduced from reference 26. Copyright 1987 American Chemical Society.) ΪΛ
_
PhoKxJdectoc Monodtromotor Lcod Shield
GRID MOOUUOR
Ccrentov Ugh!
I—Phose shifters for
Time Scorning
Fig. 2. Layout of the new twin linac system. (Reproduced with permission from reference 34. Copyright 1986 Elsevier Sequoia.) 2.2 New p i c o s e c o n d
Pulse Radiolysis
(LL Twin System)
Very r e c e n t l y LL ( L i n a c - L a s e r ) t w i n p i c o s e c o n d pulse r a d i o l y s i s s y s t e m has been d e v e l o p e d [ 3 5 . 3 6 ] . Conventional t w i n l i n a c system uses Cerenkov l i g h t as a monitor light, w h i c h i s n o t i n t e n s e enough i n w a v e l e n g t h r e g i o n h i g h e r t h a n 500 nm. B u t many i m p o r t a n t r e a c t i v e i n t e r m e d i a t e s have a b s o r p t i o n a t h i g h e r t h a n 500 nm. New L L t w i n p i c o s e c o n d p u l s e r a d i o l y s i s system uses p u l s e d l a s e r diode s y n c h r o n i z e d Clough and Shalaby; Radiation Effects on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
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TAGAWA
Puke Radiolysis Studies of Polymers
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0.5 1 1.5 t i m e / ns Fig. 3. Primary process of the formation of polymethylpropylsilane radical anion observed with the twin linac system. (Reproduced from reference 27. Copyright 1988 American Chemical Society.) w i t h l i n a c microwave, w h i c h i s i n t e n s e i n n e a r i n f r a - r e d and infra-red region. Both c o n v e n t i o n a l t w i n l i n a c system and new L L t w i n s y s t e m a r e t h e s t r o b o s c o p i c t y p e methods. The p r i n c i p l e o f b o t h systems i s shown as shown i n F i g . 4. B o t h systems u s e 10 p i c o s e c o n d e l e c t r o n p u l s e s f r o m t h e l i n a c as i r r a d i a t i o n sources. As a n a l y z i n g l i g h t , t h e t w i n l i n a c s y s t e m a n d t h e L L t w i n s y s t e m u s e C e r e n k o v l i g h t and l a s e r pulses, respectively. The s y s t e m i s shown i n F i g . 5. The s e t u p o f new L L t w i n s y s t e m i s much e a s i e r t h a n t h e c o n v e n t i o n a l t w i n l i n a c system. F i g . 6 shows t h e d e c a y o f t h e r a d i c a l c a t i o n o f n-dodecane a s t h e model compound o f p o l y e t h y l e n e i n i r r a d i a t e d n e a t dodecane m o n i t o r e d by t h e new LL t w i n s y s t e m . 2.3 I o n Beam P u l s e R a d i o l y
