The Effects of Radiation on High-Technology Polymers - American

Chapter 3

Spectroscopic Methods in Polymer Studies

Downloaded by UNIV OF NEW SOUTH WALES on August 17, 2015 | http://pubs.acs.org Publication Date: December 13, 1989 | doi: 10.1021/bk-1989-0381.ch003

Kenneth P. Ghiggino Department of Physical Chemistry, University of Melbourne, Parkville 3052, Australia The physico-chemical changes induced in polymers following exposure to radiation can be studied by a range of spectroscopic techniques. Recent developments in instrumentation and data analysis procedures in electronic, vibrational and magnetic resonance spectroscopies have provided considerable new insights into polymer structure and behaviour. The application of these spectroscopic methods in polymer studies are reviewed with emphasis on their u t i l i t y in investigations of radiation effects on macromolecules. Spectroscopic methods are now widely used in the polymer f i e l d as an analytical tool to probe structure and to obtain information on physico-chemical changes occurring in polymers and polymer additives. Spectroscopy u t i l i z e s the interaction of radiation with matter to provide details of molecular energy levels, energy state lifetimes and transition probabilities. This information in turn may be applied in studying chemical structure, molecular environment, polymer tacticity and conformation, and to monitor changes in these properties following external perturbations (e.g., mechanical stress, thermal treatment, radiation exposure). The advantages of spectroscopic measurements over other means of polymer characterization are that they are a non-destructive and rapid means of providing information at a molecular level. Exposure of polymers to ultraviolet and higher energy radiation can lead to extensive physical and chemical modification of polymeric materials. These changes in properties may have both detrimental and beneficial consequences in determining the end-uses of the polymer. Spectroscopy can provide a detailed insight into the mechanisms of polymer modification occurring under irradiation thus enabling control of the final material properties. 0097-6156/89/0381-0027$06.00A) ° 1989 American Chemical Society

In The Effects of Radiation on High-Technology Polymers; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


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EFFECTS OF RADIATION ON HIGH-TECHNOLOGY POLYMERS

There a r e s e v e r a l w e l l w r i t t e n t e x t s d e s c r i b i n g t h e theory and a p p l i c a t i o n o f spectroscopy t o polymer systems (1-3) t o w h i c h t h e r e a d e r i s r e f e r r e d f o r d e t a i l e d information. W i t h i n t h e c o n f i n e s o f t h i s c h a p t e r , some aspects relevant t o r a d i a t i o n e f f e c t s i n polymeric m a t e r i a l s a r e r e v i e w e d w i t h p a r t i c u l a r e m p h a s i s o n new developments i n instrumentation. The v a r i o u s s p e c t r o s c o p i c methods c a n be d i s t i n g u i s h e d by t h e energy o f t h e t r a n s i t i o n s i n v e s t i g a t e d ( c . f . F i g u r e 1) a n d t h i s n o t a t i o n i s employed i n t h e sub-headings discussed below. Surface a n a l y s i s t e c h n i q u e s w i l l n o t be d e s c r i b e d i n d e t a i l i n t h i s a r t i c l e a l t h o u g h m e n t i o n may b e made o f E S C A (Electron Spectroscopy f o r C h e m i c a l A n a l y s i s ) a s one r e c e n t t e c h n i q u e w h i c h may b e u s e d t o a n a l y z e s u r f a c e p r o p e r t i e s . ESCA m o n i t o r s t h e k i n e t i c e n e r g y o f e l e c t r o n s d e t a c h e d from t h e sample f o l l o w i n g i r r a d i a t i o n w i t h an X - r a y s o u r c e . The d e t a c h e d e l e c t r o n s a r i s e f r o m atoms a n d m o l e c u l e s a t t h e s u r f a c e o f t h e p o l y m e r (~2 nm d e p t h ) a n d thus i n f o r m a t i o n concerning chemical m o d i f i c a t i o n s (e.g., p h o t o - o x i d a t i o n ) a t s u r f a c e s c a n be o b t a i n e d ( 1 ) . UV-Visible Absorption

and Emission

Spectroscopy

The a b s o r p t i o n a n d e m i s s i o n o f r a d i a t i o n i n t h e n e a r u l t r a v i o l e t (UV) a n d v i s i b l e r e g i o n s o f t h e e l e c t r o magnetic spectrum a r e a s s o c i a t e d w i t h e l e c t r o n i c (and a s s o c i a t e d v i b r o n i c ) t r a n s i t i o n s i n v o l v i n g nand/or n - e l e c t r o n systems o f molecules. S y n t h e t i c and n a t u r a l p o l y m e r s a b s o r b i n t h e UV r e g i o n a n d p a r t i c u l a r l y s t r o n g absorption spectra are recorded f o r polymers containing aromatic and h e t e r o a r o m a t i c groups (e.g., p o l y ( s t y r e n e s ) , p o l y ( v i n y l naphthalenes), p o l y ( v i n y l carbazoles)). P o l y m e r s w i t h c h r o m o p h o r e s e x h i b i t i n g nrc* t r a n s i t i o n s ( e . g . , C=0) e x h i b i t w e a k e r U V - a b s o r p t i o n a n d t h e s e g r o u p s t o g e t h e r w i t h u n s a t u r a t e d carbon-carbon bonds which d e v e l o p d u r i n g r a d i a t i o n damage c a n b e d e t e c t e d b y electronic absorption spectroscopy. The a b s o r p t i o n a n d e m i s s i o n p r o c e s s e s o c c u r r i n g i n o r g a n i c m o l e c u l e s i n c l u d i n g polymers can be d i s c u s s e d q u a l i t a t i v e l y with reference t o the state diagram o f J a b l o n s k i i (Figure 2 ) . A b s o r p t i o n o f r a d i a t i o n by t h e m o l e c u l e l e a d s t o p r o m o t i o n o f an e l e c t r o n from t h e ground s i n g l e t s t a t e (S ) t o a h i g h e r e l e c t r o n i c s t a t e which, t h r o u g h c o n s e r v a t i o n o f s p i n , w i l l a l s o be a s i n g l e t s t a t e S . The e n e r g i e s o f r a d i a t i o n r e q u i r e d t o p r o m o t e e l e c t r o n s t o higher energy s t a t e s and t h e range o f a s s o c i a t e d v i b r a t i o n a l energy l e v e l s , r e s u l t s i n t h e recorded absorption spectrum with t h e i n t e n s i t y d i s t r i b u t i o n o f t h e spectrum r e f l e c t i n g t h e r e l a t i v e probabilities of the transitions. Associated w i t h each e x c i t e d s i n g l e t s t a t e t h e r e i s an e l e c t r o n i c s t a t e o f lower energy i n which t h e e l e c t r o n spins are p a r a l l e l , t h e triplet state. I t s h o u l d be n o t e d t h a t a b s o r p t i o n s p e c t r a a r i s i n g from any e x c i t e d s t a t e o f t h e molecule (e.g., S S absorption, T - T absorption) or other t r a n s i e n t Q

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(cm-1)

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-2

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UV/

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log

(wavelength) ^ (m)

Figure

1.

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I I -6> -5

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techniques

F i g u r e 2. J a b l o n s k i i d i a g r a m illustrating photophysical processes i n a polyatomic molecule.


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species (e.g., f r e e r a d i c a l s , r a d i c a l ions) c a n a l s o be r e c o r d e d i f t h e s t a t e c a n be s u f f i c i e n t l y p o p u l a t e d and these experiments form t h e b a s i s o f t h e f l a s h p h o t o l y s i s technique. The p o s s i b l e f a t e o f e x c i t a t i o n e n e r g y r e s i d i n g i n m o l e c u l e s i s a l s o s h o w n i n F i g u r e 2. The r e l a x a t i o n o f the e l e c t r o n t o t h e i n i t i a l ground s t a t e and accompanying emission of radiation results i n the fluorescence spectrum (S - S ) o r p h o s p h o r e s c e n c e s p e c t r u m {T - S ) . I n addition t o the radiative processes, non-radiative photophysical and photochemical processes can a l s o occur. I n t e r n a l conversion and intersystem c r o s s i n g a r e t h e n o n - r a d i a t i v e p h o t o p h y s i c a l processes between e l e c t r o n i c s t a t e s o f t h e same s p i n m u l t i p l i c i t y a n d d i f f e r e n t s p i n multiplicities respectively. The l i f e t i m e o f t h e e x c i t e d s t a t e w i l l b e i n f l u e n c e d by t h e r e l a t i v e m a g n i t u d e s o f t h e s e non-radiative processes and thus t i m e - r e s o l v e d spectroscopy can provide i n f o r m a t i o n on t h e dynamics o f e x c i t e d s t a t e d e p l e t i o n mechanisms, e.g.,


1

Q

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Q

Rate

Process S

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-» S

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The l i f e t i m e o f t h e s i n g l e t e x c i t e d s t a t e ( t h e fluorescence l i f e t i m e T ) i s of the order of picoseconds t o 100 n a n o s e c o n d s (10" - 1 0 ~ s e c o n d s ) a n d c a n now b e measured a c c u r a t e l y u s i n g p u l s e d l a s e r e x c i t a t i o n methods and o t h e r t e c h n i q u e s . Since t h e r a d i a t i v e t r a n s i t i o n from the lowest t r i p l e t state t o t h e ground state i s formally f o r b i d d e n by s e l e c t i o n r u l e s , t h e phosphorescence l i f e t i m e s can be longer, o f t h e o r d e r o f seconds. The U V - v i s i b l e a b s o r p t i o n a n d e m i s s i o n s p e c t r a a n d e x c i t e d state l i f e t i m e s o f polymers are s e n s i t i v e t o chemical s t r u c t u r e , polymer conformation and molecular environment and thus i n f o r m a t i o n concerning these p r o p e r t i e s i s a c c e s s i b l e by e l e c t r o n i c spectroscopy measurements (4-6). One e x a m p l e o f t h e a p p l i c a t i o n o f such measurements i s g i v e n i n F i g u r e 3 w h i c h i l l u s t r a t e s the p o s s i b l e energy d i s s i p a t i o n pathways which can occur i n a polymer containing aromatic side groups f o l l o w i n g absorption of radiation. F

1 2

7



3. GHIGGINO


31

L i g h t i n i t i a l l y a b s o r b e d b y one chromophore on t h e p o l y m e r c h a i n may v e r y r a p i d l y b e t r a n s f e r r e d t o a n e i g h b o u r i n g chromophore by t h e process o f n o n - r a d i a t i v e energy t r a n s f e r (4-6). I n t h i s way, e x c i t a t i o n e n e r g y c a n migrate along t h e polymer chain u n t i l i ti s trapped a t s i t e s o f lower energy which subsequently undergo f u r t h e r photophysical/photochemical processes. The l o w e n e r g y s i t e s may b e e x c i m e r s ( e x c i t e d s t a t e d i m e r complexes(£)) or s p e c i e s w i t h lower e x c i t e d s t a t e e n e r g i e s which have been d e l i b e r a t e l y i n c o r p o r a t e d i n t h e polymer. Since t h e emission from s i n g l e e x c i t e d chromophores, excimer s i t e s and acceptor groups a r e s p e c t r a l l y d i s t i n g u i s h a b l e , f l u o r e s c e n c e techniques c a n be used t o characterize the excited state species present i n the polymer. I n Figure 4 t h e fluorescence spectra recorded f r o m d i l u t e s o l u t i o n s o f p o l y ( s t y r e n e ) ( P S ) a n d a 1:1 a l t e r n a t i n g copolymer o f styrene and maleic anhydride are shown. F o r PS, f l u o r e s c e n c e f r o m t h e e x c i t e d a r o m a t i c p h e n y l c h r o m o p h o r e s i s o b s e r v e d ( f l u o r e s c e n c e maximum 2 9 0 nm) t o g e t h e r w i t h a b r o a d e m i s s i o n f r o m e x c i m e r s i t e s i n t h e p o l y m e r ( f l u o r e s c e n c e maximum 3 3 0 n m ) . H o w e v e r i n t h e a l t e r n a t i n g c o p o l y m e r no e x c i m e r f l u o r e s c e n c e i s d e t e c t e d demonstrating t h a t under these c o n d i t i o n s excimer energy t r a p s i t e s i n PS must f o r m b e t w e e n a d j a c e n t p h e n y l g r o u p chromophores along t h e polymer c h a i n . Recent developments i n l a s e r technology and f a s t d e t e c t i o n m e t h o d s now a l l o w t h e k i n e t i c b e h a v i o u r o f t h e e x c i t e d s t a t e species a r i s i n g from a b s o r p t i o n o f r a d i a t i o n by p o l y m e r s t o b e s t u d i e d on t i m e - s c a l e s down t o t h e p i c o s e c o n d r e g i o n (£). An example o f a t i m e - r e s o l v e d f l u o r e s c e n c e spectrometer w h i c h c a n be u s e d t o study such u l t r a f a s t phenomena i s i l l u s t r a t e d i n F i g u r e 5 ( 2 ) . The c o m m e r c i a l l y a v a i l a b l e l a s e r s o u r c e i s a model o c k e d argon-ion l a s e r synchronously pumping a c a v i t y dumped d y e l a s e r . T h i s l a s e r s y s t e m p r o d u c e s t u n a b l e light p u l s e s , e a c h p u l s e w i t h a t i m e d u r a t i o n o f a b o u t 10 picoseconds, a n d w i t h p u l s e r e p e t i t i o n r a t e s u p t o 80 m i l l i o n l a s e r pulses/second. The l a s e r p u l s e s a r e u s e d t o e x c i t e t h e sample under study and t h e r e s u l t i n g sample f l u o r e s c e n c e i s s p e c t r a l l y d i s p e r s e d t h r o u g h a monochromator and d e t e c t e d by a f a s t p h o t o m u l t i p l i e r tube (or i n some c a s e s a s t r e a k c a m e r a ( i i ) ) . In t h e t i m e - c o r r e l a t e d s i n g l e photon counting t e c h n i q u e d e p i c t e d i n F i g u r e 5, e l e c t r o n i c p u l s e s synchronised with the l a s e r pulses are used t o i n i t i a t e a v o l t a g e - t i m e ramp i n t h e t i m e - t o - a m p l i t u d e c o n v e r t e r (TAC) w h i l e e l e c t r o n i c pulses a r i s i n g from fluorescence photons i n c i d e n t on t h e p h o t o m u l t i p l i e r t u b e t e r m i n a t e t h e v o l t a g e r a m p . T h e a m p l i t u d e o f t h e v o l t a g e p u l s e f r o m t h e TAC, w h i c h i s s t o r e d i n a memory o f a m u l t i c h a n n e l analyser o p e r a t i n g i n p u l s e h e i g h t a n a l y s i s mode, w i l l b e d i r e c t l y r e l a t e d t o t h e time between e x c i t a t i o n o f t h e sample by the l a s e r pulse and t h e d e t e c t i o n o f a f l u o r e s c e n c e p h o t o n . C o l l e c t i o n o f many s u c h e v e n t s a t a f i x e d e m i s s i o n w a v e l e n g t h r e s u l t s i n a f l u o r e s c e n c e d e c a y c u r v e w h i c h may


32



migration down polymer chain

absorption

acceptor emission

excimer .emission F i g u r e 3. Energy r e l a x a t i o n pathways i n a polymer c o n t a i n i n g aromatic side groups f o l l o w i n g absorption l i g h t . (Reproduced w i t h p e r m i s s i o n from R e f . 21. C o p y r i g h t 1987 C h e m i s t r y i n A u s t r a l i a . )

of

INTENSITY (arb units)

280

320 360 WAVELENGTH (nm)

400

F i g u r e 4. F l u o r e s c e n c e s p e c t r a o f (a) p o l y ( s t y r e n e ) a n d (b) s t y r e n e / m a l e i c a n h y d r i d e a l t e r n a t i n g c o p o l y m e r i n t e t r a h y d r o f u r a n a t 20°C. E x c i t a t i o n w a v e l e n g t h : 2 6 5


3.

GHIGGINO


33

+


mode locked A r laser

sample

cavity dumper

dye laser

synchronized trigger pulse

[ p u m p j — I dye

scanning monochromator / PMT detection system

TAC

MCA

data processing computer (VAX)

data acquisition computer

20.0 12.0 4.0 TIME (nsec) 320

360 400 440 480 WAVELENGTH (nm)

F i g u r e 5. Schematic diagram of a time-resolved f l u o r e s c e n c e s p e c t r o m e t e r u s i n g a p i c o s e c o n d l a s e r as an e x c i t a t i o n source. Inset diagram: intensity/time/ wavelength surface f o r poly(acenaphthalene) i n benzene a t 20°C. E x c i t a t i o n w a v e l e n g t h 295 nm. (Reproduced w i t h p e r m i s s i o n f r o m R e f . 2 1 . C o p y r i g h t 1987 C h e m i s t r y i n Australia.)



34


be a n a l y z e d t o e x t r a c t t h e f l u o r e s c e n c e d e c a y parameters (£). A l t e r n a t i v e l y a c c u m u l a t i o n o f f l u o r e s c e n c e e v e n t s a t a number o f e m i s s i o n w a v e l e n g t h s and a t v a r i o u s t i m e s a f t e r e x c i t a t i o n can produce t h r e e - d i m e n s i o n a l hypers u r f a c e s d i s p l a y i n g t h e s p e c t r a l , t e m p o r a l and i n t e n s i t y information simultaneously (Figure 5). The i n s e t d i a g r a m i n F i g u r e 5 d e p i c t s t h e k i n e t i c behaviour of f l u o r e s c e n c e from s i n g l e e x c i t e d chromophores ( f l u o r e s c e n c e maximum 340 nm) a n d e x c i m e r sites ( f l u o r e s c e n c e maximum 400 nm) f o l l o w i n g e x c i t a t i o n o f a s o l u t i o n of poly(acenaphthalene) with laser pulses a t 295 nm. S u c h h y p e r s u r f a c e s p r o v i d e a n o v e r v i e w o f t h e e x c i t e d state processes o c c u r r i n g i n polymers f o l l o w i n g i r r a d i a t i o n and c a n show w h e r e t h e e n e r g y initially absorbed by t h e polymer f i n a l l y r e s i d e s and t h e r a t e s o f v a r i o u s energy d i s s i p a t i o n pathways. A b s o r p t i o n , f l u o r e s c e n c e and p h o s p h o r e s c e n c e measurements have been a p p l i e d t o t h e s t u d y o f r a d i a t i o n e f f e c t s on a w i d e r a n g e o f p o l y m e r s a n d p o l y m e r a d d i t i v e s (1-7) . C h e m i l u m i n e s c e n c e i s a f u r t h e r example o f the a p p l i c a t i o n o f e l e c t r o n i c s p e c t r o s c o p y measurements t o polymer d e g r a d a t i o n s t u d i e s . The w e a k v i s i b l e r a d i a t i o n e m i t t e d f r o m p o l y m e r s u n d e r g o i n g t h e r m o - o x i d a t i v e d e g r a d a t i o n has been a t t r i b u t e d t o e m i s s i o n from e l e c t r o n i c a l l y e x c i t e d o x i d a t i o n p r o d u c t s and t h u s chemiluminescence p r o v i d e s a means o f d e t e c t i n g and m o n i t o r i n g t h e i n c i p i e n t s t a g e s o f c e r t a i n p o l y m e r decomposition processes. Infrared

and

Raman

Spectroscopy

A quantum d e s c r i p t i o n o f t h e s t r e t c h i n g and b e n d i n g v i b r a t i o n s o f m o l e c u l a r bonds r e s u l t s i n t h e assignment o f v i b r a t i o n a l energy l e v e l s a s s o c i a t e d w i t h each e l e c t r o n i c s t a t e o f f u n c t i o n a l g r o u p s i n p o l y m e r s (1). Since bond f o r c e c o n s t a n t s and atomic masses determine t h e v i b r a t i o n a l f r e q u e n c i e s , t h e e n e r g y l e v e l s w i l l be c h a r a c t e r i s t i c of the chemical groups present i n the polymer. T r a n s i t i o n s between these v i b r a t i o n a l energy l e v e l s c a n b e i n v e s t i g a t e d u s i n g i n f r a r e d a n d Raman spectroscopies. I n f r a r e d (IR) s p e c t r o s c o p y has b e e n w i d e l y u s e d i n polymer s t u d i e s f o r the assignment of molecular s t r u c t u r e and f o r m o n i t o r i n g changes i n t h e arrangement o f c h e m i c a l bonds (1-8-111. I n f o r m a t i o n c o n c e r n i n g c o n f o r m a t i o n , t a c t i c i t y and c r y s t a l l i n i t y may a l s o b e o b t a i n e d (1). Vibrational t r a n s i t i o n s a c c e s s s i b l e t o IR s p e c t r o s c o p y a r e g o v e r n e d by t h e s e l e c t i o n r u l e t h a t t h e r e must be a change i n d i p o l e moment d u r i n g e x c i t a t i o n o f t h e p o l y m e r v i b r a t i o n s . T h u s s y m m e t r i c v i b r a t i o n s w h i c h a r e d e t e c t e d b y Raman s p e c t r o scopy a r e i n a c c e s s i b l e t o IR a b s o r p t i o n measurements. P o l a r g r o u p s , s u c h a s c a r b o n y l (C=0) a n d h y d r o x y l (OH), h a v e a s t r o n g g r o u n d s t a t e d i p o l e moment a n d s h o w s t r o n g IR a b s o r p t i o n s a t c h a r a c t e r i s t i c f r e q u e n c i e s . The IR s p e c t r u m c a n t h u s be u s e d as a ' f i n g e r p r i n t of molecular 1



3. GHIGGINO


35

s t r u c t u r e and, s i n c e t h e p o s i t i o n s o f v i b r a t i o n a l frequencies are s e n s i t i v e t o neighbouring chemical groups, c o n c l u s i o n s concerning l o c a l environment can be made. A c o n s i d e r a b l e v o l u m e o f p o l y m e r I R s p e c t r a h a v e been recorded and p u b l i s h e d (12). Both c o n v e n t i o n a l d i s p e r s i v e element IR s p e c t r o m e t e r s a n d F o u r i e r T r a n s f o r m i n f r a r e d (FTIR) i n t e r f e r o meter based instruments a r e a v a i l a b l e t o r e c o r d IR s p e c t r a of polymers. In the conventional instruments a d i s p e r s i v e element, such as a g r a t i n g o r p r i s m i s used t o measure t h e frequencies a t which i n f r a r e d r a d i a t i o n i s absorbed by t h e sample. An i n t e r f e r o m e t e r ( F i g u r e 6) c o n s t i t u t e s t h e b a s i s o f t h e FTIR instrument. In the interferometer the IR r a d i a t i o n i s s p l i t i n t o two p a t h s and, a f t e r r e f l e c t i o n f r o m m i r r o r s (one m o v a b l e ) , t h e beams a r e r e c o m b i n e d a t t h e beam s p l i t t e r . When t h e p a t h l e n g t h s f o l l o w e d b y t h e two beams a r e i d e n t i c a l a l l w a v e l e n g t h s o f r a d i a t i o n i n c i d e n t o n t h e beam s p l i t t e r a d d c o h e r e n t l y a n d r e s u l t i n t h e maximum f l u x a t t h e d e t e c t o r . A t o t h e r p o s i t i o n s o f the movable m i r r o r d e s t r u c t i v e i n t e r f e r e n c e o f each w a v e l e n g t h a t t h e beam s p l i t t e r o c c u r s a n d t h e f l u x a t t h e d e t e c t o r w i l l d e c r e a s e . T h e i n t e r f e r o g r a m (F ( x ) ) p r o d u c e d by r e c o r d i n g t h e r a d i a t i o n f l u x as t h e m i r r o r u n d e r g o e s t r a n s l a t i o n a l movement h a s t h e f o r m o f a damped o s c i l l a t i o n corresponding t o : F(x)

=

j A ( v ) c o s (27Cv) d v

where A(v) i s t h e s p e c t r a l i n t e n s i t y d i s t r i b u t i o n c o n t a i n i n g t h e s p e c t r o s c o p i c i n f o r m a t i o n . The s p e c t r o s c o p i c data i s e x t r a c t e d by a F o u r i e r t r a n s f o r m o f t h e i n t e r f e r o g r a m (XI) . The i n s t r u m e n t i s s i n g l e beam a n d a b l a n k must a l s o be measured a n d s u b s e q u e n t l y subtracted. A computer c a r r i e s out t h e F o u r i e r t r a n s f o r m c a l c u l a t i o n s , performs v a r i o u s c o n t r o l f u n c t i o n s and manipulates t h e data f o r d i s p l a y and i n t e r p r e t a t i o n . FTIR instruments o f f e r advantages i n speed and h i g h e r s i g n a l - t o - n o i s e r a t i o s compared t o d i s p e r s i v e IR s p e c t r o m e t e r s . These advantages combined w i t h t h e f a c i l i t y f o r e x t e n s i v e data p r o c e s s i n g have seen t h e FTIR t e c h n i q u e f i n d i n c r e a s i n g a p p l i c a t i o n s i n polymer studies (11). IR s p e c t r o s c o p y h a s p r o v e d most u s e f u l i n s t u d y i n g chemical m o d i f i c a t i o n s o f polymers induced by e x t e r n a l f a c t o r s i n c l u d i n g r a d i a t i o n damage ( l 1 3 ) . Oxidation i s an i m p o r t a n t d e g r a d a t i o n p a t h w a y f o l l o w i n g e x p o s u r e t o both heat and r a d i a t i o n . The s t r o n g l y p o l a r f u n c t i o n a l g r o u p s w h i c h a r e t h e p r o d u c t s o f o x i d a t i v e damage ( e . g . , OH, C=0) a r e r e a d i l y d e t e c t e d b y I R s p e c t r o s c o p y a n d t h u s t h i s t e c h n i q u e c a n be used t o f o l l o w t h e e a r l y s t a g e s o f degradation. A t t e n u a t e d t o t a l r e f l e c t i o n (ATR) o f I R r a d i a t i o n (1) may a l s o b e u s e d t o m o n i t o r s u r f a c e modifications during degradation. An e x a m p l e o f t h e a p p l i c a t i o n o f I R s p e c t r o s c o p y i s in the photooxidation of poly(propylene) ( l 3). During the e a r l y s t a g e s o f o x i d a t i o n a b s o r p t i o n due t o a l d e h y d e s f

r


36


- 1

- 1

(1735 c m ) a n d k e t o n e s ( 1 7 2 0 c m ) a r e a p p a r e n t w h i l e a t l a t t e r t i m e s c a r b o x y l i c a c i d s (1710 cm ) c a n be d e t e c t e d . I n p o l y ( e t h y l e n e ) h y d r o p e r o x i d e s ( 3 5 5 0 cm" ) a r e o b s e r v e d d u r i n g e a r l y stages o f i r r a d i a t i o n w h i l e FTIR has r e v e a l e d an i n c r e a s e i n v i n y l e n d g r o u p s , c a r b o n y l s a n d t r a n s v i n y l i d e n e d o u b l e b o n d s (11). C o r r e l a t i o n s have been noted between p h y s i c a l changes i n t h e polymer and c h a i n s c i s s i o n processes d e t e c t e d by IR spectroscopy d u r i n g photodegradation of poly(ethylene) (13). The a d v e n t o f l a s e r s h a s a s s i s t e d i n t h e d e v e l o p m e n t o f Raman s p e c t r o s c o p y a s a m e a n s o f r e c o r d i n g v i b r a t i o n a l spectra o f polymers and other molecular systems. Raman s p e c t r o s c o p y i s based on i n e l a s t i c l i g h t s c a t t e r i n g and uses monochromatic r a d i a t i o n i n t h e v i s i b l e region as t h e e x c i t a t i o n source. A n a l y s i s o f r a d i a t i o n scattered from a sample o f molecules i n d i c a t e s t h e presence o f f r e q u e n c i e s which a r e s p e c t r a l l y s h i f t e d t o lower energies (Stokes l i n e s ) and h i g h e r e n e r g i e s (anti-Stokes l i n e s ) compared t o the incident r a d i a t i o n . The s p e c t r a l l y s h i f t e d l i n e s a r i s e due t o t h e t r a n s f e r o f v i b r a t i o n a l q u a n t a between t h e i n t e r a c t i n g r a d i a t i o n a n d t h e medium. The o b s e r v e d t r a n s i t i o n s a r e governed by t h e s e l e c t i o n r u l e that t h e r e i s a change i n p o l a r i z a b i l i t y d u r i n g t h e m o l e c u l a r v i b r a t i o n and thus I R - i n a c t i v e t o t a l l y symmetric v i b r a t i o n s may b e o b s e r v e d . In order t o discriminate the Raman s p e c t r a l l i n e s f r o m t h e s t r o n g T y n d a l l s c a t t e r i n g o f the i n c i d e n t r a d i a t i o n , h i g h l y monochromatic r a d i a t i o n a v a i l a b l e from l a s e r sources ( t y p i c a l l y argon-ion o r k r y p t o n l a s e r s ) i s p r e f e r r e d a n d d o u b l e o r t r i p l e monochromators a r e often required t o achieve t h e necessary spectral resolution. R e s o n a n c e Raman s c a t t e r i n g may a l s o be o b s e r v e d i f t h e f r e q u e n c y o f t h e e x c i t i n g r a d i a t i o n c o r r e s p o n d s c l o s e l y t o an e l e c t r o n i c a b s o r p t i o n bond. In t h i s c a s e , t h e Raman l i n e s a r i s i n g f r o m c o u p l i n g o f t h e v i b r a t i o n s w i t h t h e e l e c t r o n i c t r a n s i t i o n a r e much s t r o n g e r t h a n o r d i n a r y Raman s c a t t e r i n g . - 1


1

Further d e t a i l s o f t h e theory and a p p l i c a t i o n o f Raman s p e c t r o s c o p y i n p o l y m e r s t u d i e s c a n b e f o u n d e l s e w h e r e (1,9) . H o w e v e r , v i b r a t i o n a l f r e q u e n c i e s o f f u n c t i o n a l groups i n polymers can be c h a r a c t e r i z e d from t h e s p a c i n g o f t h e Raman l i n e s a n d t h u s information complementary t o IR a b s o r p t i o n s p e c t r o s c o p y c a n be obtained. In addition, since v i s i b l e r a d i a t i o n i s used the t e c h n i q u e c a n be a p p l i e d t o aqueous media i n c o n t r a s t to IR spectroscopy, allowing studies o f synthetic p o l y e l e c t r o l y t e s and biopolymers t o be undertaken. Conformation and c r y s t a l l i n i t y o f polymers have a l s o been s h o w n t o i n f l u e n c e t h e Raman s p e c t r a (1) w h i l e t h e p o s s i b i l i t y o f s t u d y i n g s c a t t e r i n g from s m a l l sample v o l u m e s i n t h e f o c u s s e d l a s e r b e a m (-100 [im d i a m e t e r ) c a n p r o v i d e i n f o r m a t i o n on l o c a l i z e d changes i n c h e m i c a l structure. One new t e c h n i q u e o f p o t e n t i a l i m p o r t a n c e t o t h e study o f t h e i n t e r a c t i o n o f r a d i a t i o n with polymers i s t i m e - r e s o l v e d Raman s p e c t r o s c o p y ( 1 4 1 5 ) . I n t h e s e r



3. GHIGGINO

37


experiments e x c i t e d s t a t e s and t r a n s i e n t species a r e produced d u r i n g e x c i t a t i o n o f t h e sample by a s h o r t l a s e r pulse. T h e Raman s c a t t e r i n g i n d u c e d b y a s e c o n d p r o b e p u l s e i n c i d e n t on t h e sample a f t e r a f i x e d t i m e d e l a y c a n be u s e d t o c h a r a c t e r i z e t h e v i b r a t i o n a l f r e q u e n c i e s a n d hence t h e s t r u c t u r e o f t h e i n t e r m e d i a t e s p e c i e s . The a p p l i c a t i o n o f mode-locked t u n a b l e dye l a s e r s has a l l o w e d t h e t e c h n i q u e t o b e e x t e n d e d down t o t h e p i c o s e c o n d time region. Transient species produced f o l l o w i n g absorption o f p i c o s e c o n d l i g h t p u l s e s b y b a c t e r i o r h o d o p s i n (a p r o t e i n complex c o n t a i n i n g t h e r e t i n a l chromophore i n t h e p u r p l e membrane o f H a l o b a c t e r i u m H a l o b i u m ) h a v e r e c e n t l y b e e n undertaken (G.Atkinson, U n i v e r s i t y o f A r i z o n a , personal communication,1987) although t h e a p p l i c a t i o n o f such measurements t o s y n t h e t i c macromolecular systems has y e t to be f u l l y i n v e s t i g a t e d .

Magnetic Resonance Spectroscopy E l e c t r o n s a n d n u c l e i h a v e a m a g n e t i c moment a s s o c i a t e d w i t h a n g u l a r momenta o f t h e p a r t i c l e s . In the presence o f a magnetic f i e l d t h e degeneracy o f d i s c r e t e energy l e v e l s a s s o c i a t e d w i t h t h e m a g n e t i c moment i s r e m o v e d a n d a b s o r p t i o n and emission o f r a d i a t i o n between these energy l e v e l s may b e o b s e r v e d (l 16), The e n e r g y d i f f e r e n c e between t h e q u a n t i z e d l e v e l s depends on t h e m a g n e t i c f i e l d strength and i s given by: r

A E = h v = g\iB w h e r e g i s a c o n s t a n t ( g = 2 f o r f r e e e l e c t r o n s ; g = 0.1 t o 6 f o r many n u c l e i , e . g . , g = 5 . 5 8 5 4 f o r a p r o t o n ) , | l i s t h e B o h r m a g n e t o n (|ig) f o r e l e c t r o n s a n d t h e n u c l e a r m a g n e t o n (p, ) f o rn u c l e i and B i s t h e magnetic f i e l d f l u x density. The t e c h n i q u e o f s t u d y i n g t h e a b s o r p t i o n o f r a d i a t i o n by unpaired e l e c t r o n s i n a magnetic f i e l d i s c a l l e d E l e c t r o n S p i n - R e s o n a n c e (ESR) s p e c t r o s c o p y w h i l e the study o f t h e resonance frequencies f o r n u c l e i i s c l a s s i f i e d a s N u c l e a r M a g n e t i c R e s o n a n c e (NMR) spectroscopy. Under e x t e r n a l magnetic f i e l d s t r e n g t h s o f a b o u t 1 T e s l a , ESR s p e c t r o s c o p y r e q u i r e s e n e r g i e s i n t h e m i c r o w a v e r e g i o n (-10 GHz) t o i n i t i a t e t r a n s i t i o n s w h i l e , s i n c e t h e i n t e r a c t i o n between n u c l e i and t h e magnetic f i e l d i s m u c h w e a k e r , t h e NMR t e c h n i q u e u s e s l o w e r e n e r g y r a d i o waves i n t h e 1 t o 5 metre band. E x p e r i m e n t a l l y t h e sample i s p l a c e d i n a s t r o n g magnetic f i e l d and, r a t h e r than t h e frequency being scanned a t a constant f i e l d s t r e n g t h t o detect absorption of r a d i a t i o n , i n p r a c t i c e t h e frequency o f e x c i t i n g r a d i a t i o n i s kept constant and t h e magnetic f i e l d f l u x i s varied. B o t h E S R a n d NMR s p e c t r o s c o p y h a v e f o u n d widespread a p p l i c a t i o n i n polymer s t u d i e s and s e v e r a l excellent texts describing the techniques are a v a i l a b l e e

N

N

N

(1,17-19).



38


ESR s p e c t r o s c o p y r e q u i r e s t h e p r e s e n c e o f u n p a i r e d e l e c t r o n s i n t h e sample and thus i t f i n d s a p p l i c a t i o n i n the study o f t r i p l e t e x c i t e d s t a t e s , n e u t r a l free r a d i c a l s a n d r a d i c a l i o n s w h i c h may b e f o r m e d i n p o l y m e r s f o l l o w i n g exposure t o r a d i a t i o n . The d e g e n e r a t e e n e r g y l e v e l s o f u n p a i r e d e l e c t r o n s a r e s p l i t i n t o two i n t h e presence o f a m a g n e t i c f i e l d and a s i n g l e r e s o n a n c e a b s o r p t i o n might be expected. However, t h e r e a r e o f t e n i n t e r a c t i o n s between t h e m a g n e t i c moments o f o t h e r n e i g h b o u r i n g n u c l e i (e.g., protons) and t h e e l e c t r o n l e a d i n g t o h y p e r f i n e s p l i t t i n g s i n t h e a b s o r p t i o n s p e c t r u m (12). I n t h i s case, t h e e l e c t r o n and n u c l e a r s p i n s i n t e r a c t and impose s l i g h t l y d i f f e r e n t e n e r g y l e v e l s on t h e o r i g i n a l e n e r g y s p l i t t i n g a r i s i n g from t h e e f f e c t o f t h e m a g n e t i c f i e l d on t h e electron. Thus d i f f e r e n t r e s o n a n t f r e q u e n c i e s w i l l be o b s e r v e d a n d t h e number a n d i n t e n s i t y o f t h e a b s o r p t i o n bands i n t h e spectrum can p r o v i d e i n f o r m a t i o n about t h e chemical environment o f t h e unpaired e l e c t r o n (1,11). For example, p o l y ( m e t h y l methacrylate) exposed t o h i g h e n e r g y o r UV r a d i a t i o n g i v e s a n i n e l i n e E S R s p e c t r u m , a s d e p i c t e d i n F i g u r e 7. Analysis of this spectrum has i n d i c a t e d t h a t t h e l i k e l y s t r u c t u r e o f t h e free radical responsible i s (H): ~ C H - C" ( C H ) C O O C H . 2

often

3

3

In p o l y ( o l e f i n e s ) the metastable a l l y l r a d i c a l i s o b s e r v e d b y ESR t e c h n i q u e s f o l l o w i n g i r r a d i a t i o n : ~

—

=

CH CH—HC

.

The c a p a b i l i t y t o d e t e c t s u c h s p e c i e s b y E S R s p e c t r o s c o p y p r o v i d e s a means t o a n a l y s e t h e m e c h a n i s m s o f polymer breakdown under i r r a d i a t i o n (17,19). I n a d d i t i o n , c e r t a i n compounds u s e d t o p h o t o s t a b i l i z e p o l y m e r s a g a i n s t UV r a d i a t i o n a c t b y s c a v e n g i n g t h e r e a c t i v e r a d i c a l s t o f o r m more s t a b l e r a d i c a l s p e c i e s ( e . g . , h i n d e r e d phenoxy r a d i c a l s ) and thus t h e performance o f these s t a b i l i z e r s c a n b e a s s e s s e d b y ESR m e t h o d s (12). The s p e c i e s p r e s e n t i n p o l y m e r s t h a t c a n b e s t u d i e d by ESR a r e o f t e n h i g h l y r e a c t i v e , s h o r t - l i v e d a n d a r e p r e s e n t i n l o w c o n c e n t r a t i o n s . However, d e v e l o p m e n t s i n i n s t r u m e n t a t i o n have o f f e r e d improvements i n s e n s i t i v i t y a n d , c o m b i n e d w i t h more r e l i a b l e i n t e r p r e t a t i o n o f d a t a (1), t h e i n c r e a s i n g a p p l i c a t i o n o f t h i s method o f polymer c h a r a c t e r i z a t i o n i n s t u d y i n g r a d i a t i o n e f f e c t s on p o l y m e r s can be expected. I n c o n t r a s t t o ESR s p e c t r o s c o p y , w h i c h c a n o n l y b e u s e d t o s t u d y s p e c i e s w i t h u n p a i r e d e l e c t r o n s , NMR spectroscopy i s applicable to the investigation of a l l polymer samples. N u c l e i w i t h non-zero t o t a l n u c l e a r s p i n (e.g., H, C , F , N ) w i l l h a v e a m a g n e t i c moment w h i c h w i l l i n t e r a c t w i t h an e x t e r n a l m a g n e t i c f i e l d r e s u l t i n g i n q u a n t i z e d energy l e v e l s . T r a n s i t i o n s between these energy l e v e l s f o r m t h e b a s i s o f NMR s p e c t r o s c o p y . ^-H a n d C 1

l 3

1 9

1 4

1 3


3.

GHIGGINO

39



BEAM SPLITTER

LIGHT SOURCE

MOVABLE MIRROR

The Effects of Radiation on High-Technology Polymers - American

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