Flash Photolysis of Aromatic Diisocyanate-Based Polyurethanes

Chapter 4

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Flash Photolysis of Aromatic Diisocyanate-Based Polyurethanes Charles E. Hoyle, Young G. No, and Keziban S. Ezzell Department of Polymer Science, University of Southern Mississippi, Southern Station Box 10076, Hattiesburg, MS 39406-0076 The laser flash photolysis of aromatic diisocyanate based polyurethanes in solution provides evidence for a dual mechanism for photodegradation. One of the processes, an N-C bond cleavage, is common to both TDI (toluene diisocyanate) and MDI (methylene 4,4'-diphenyldiisocyanate) based polyurethanes. The second process, exclusive to MDI based polyurethanes, involves formation of a substituted diphenylmethyl radical. The diphenylmethyl radical, which readily reacts with oxygen, is generated either by direct excitation (248 nm) or indirectly by reaction with a tert-butoxy radical produced upon excitation of tert-butyl peroxide at 351 nm. The photochemical processes responsible for the ultimate degradation and destructive failure of polyurethanes based on aromatic diisocyanates have been investigated extensively for the past twenty-five years by a large number of research groups (1-7). Schemes I and II summarize proposed pathways for photodegradation of polyurethanes based on toluene diisocyanate (mixture of 2,4-TDI and 2,6-TDI isomers) and methylene 4,4'-diphenyldiisocyanate (MDI). Scheme I for TDI based polyurethanes (exemplified by the 2,4 isomer) depicts a traditional photo-Fries type rearrangement (4). Scheme II, however, shows a dual mechanism for photodegradation of MDI based polyurethanes recently proposed by Gardette and Lemaire (7), i.e., the traditional photoinduced N-C bond cleavage and subsequent reactions as well as a path for formation of hydrogen peroxide on the central methylene carbon of the bisarylcarbamate moiety with the presumption of further reaction to quinoid type products. In order to characterize the intermediates leading to the photo-Fries/cleavage and hydroperoxide products shown in Schemes I and II, laser flash photolysis measurements of solutions of both MDI and TDI based polyurethanes were conducted. The results from this study are interpreted by comparison with transient spectra of an aryl monocarbamate and the bispropyl carbamate of MDI. In addition, a dimethylsilicon analog of the MDI bispropyl carbamate is used to c

0097-6156/89/0381-O043$06.00/0 1989 American Chemical Society

Reichmanis and O'Donnell; The Effects of Radiation on High-Technology Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

44

EFFECTS OF RADIATION ON HIGH-TECHNOLOGY POLYMERS

SCHEME

SCHEME

I I

I

- P h o t o l y s i s o f 2,4-TDI Based P o l y u r e t h a n e s

- D i r e c t and I n d i r e c t P h o t o l y s i s o f MDI Based P o l y u r e t h a n e s


4. HOYLEETAL.

Aromatic Diisocyanate-Based Polyurethanes

45

c o n f i r m t h e m e c h a n i s m l e a d i n g t o h y d r o p e r o x i d e f o r m a t i o n . The t r a n s i e n t s p e c t r a l a n a l y s i s i n t h i s paper s u p p o r t s a n i l i n y l / c a r b o x y l r a d i c a l f o r m a t i o n by N-C bond c l e a v a g e . F i n a l l y , evidence i s p r e s e n t e d f o r d i p h e n y l m e t h y l r a d i c a l f o r m a t i o n i n MDI b a s e d carbamates. EXPERIMENTAL ,

Materials. M e t h y l e n e 4 , 4 - d i p h e n y l d i i s o c y a n a t e (MDI, Mobay) was r e c r y s t a l l i z e d from cyclohexane. Toluenediisocyanate ( T D I — r e p r e s e n t s m i x t u r e o f 2,4- and 2 , 6 - i s o m e r s i n 8 0 / 2 0 r a t i o ) , p t o l u i d i n e ( A l d r i c h ) and a n i l i n e ( A l d r i c h ) were p u r i f i e d by vacuum d i s t i l l a t i o n b e f o r e use. Diphenylmethane, t e r t - b u t y l p e r o x i d e (TBP), 4 - b r o m o a n i l i n e , b u t y l l i t h i u m i n hexane, and e t h y l c h l o r o f o r m a t e , were o b t a i n e d from A l d r i c h and used a s r e c e i v e d . Spectrograde t e t r a h y d r o f u r a n (THF) and benzene from B u r d i c k and J a c k s o n were used as r e c e i v e d . P o l y ( t e t r a m e t h y l e n e e t h e r g l y c o l ) w i t h MW 1000 was o b t a i n e d from p o l y s c i e n c e s and dehydrated under a rough vacuum a t 50 °C f o r 24 h. P r e p a r a t i o n o f model compounds and p o l y u r e t h a n e s . A procedure f o r the p r e p a r a t i o n o f t h e n o n - s i l i c o n c o n t a i n i n g m o n o c a r b a m a t e a n d biscarbamate models has been r e p o r t e d p r e v i o u s l y ( 8 ) . I n o r d e r t o o b t a i n t h e s i l i c o n c o n t a i n i n g m o d e l compound, 5.0 g o f b i s ( 4 aminophenyl) d i m e t h y l s i l a n e [ s y n t h e s i z e d a c c o r d i n g t o P r a t t , e t . a l ( 9 ) ] was added t o 7.0 g e t h y l c h l o r o f ormate a t room t e m p e r a t u r e . A s a l t i m m e d i a t e l y formed. The m i x t u r e was r e f l u x e d f o r 15 min. The s o l u t i o n was then c o o l e d , vacuum f i l t e r e d , and r e c r y s t a l l i z e d from e t h a n o l t o y i e l d 1.0 g o f a w h i t e powder: MP 162-3 °C; A n a l . 20 26°4 2 C * ^ » 62.17; H, 6.70; N, 7.23; Found C, 62.01; H, 6.79; N, 7.42. B o t h o f t h e s i m p l e p o l y u r e t h a n e s (TDI-PU and MDI-PU) were s y n t h e s i z e d a c c o r d i n g t o a w e l l known s o l u t i o n p o l y m e r i z a t i o n t e c h n i q u e ( 1 0 ) . The p o l y u r e t h a n e e l a s t o m e r (MDI-PUE) was prepared by a pre-polymer method ( 1 1 ) . c

H

N

S i

1

c

Instrumentation The l a s e r f l a s h p h o t o l y s i s u n i t c o n s i s t s o f a Lumonics HyperEx Excimer L a s e r p h o t o l y s i s s o u r c e , a n A p p l i e d P h o t o p h y s i c s x e n o n lamp/monochromator/PMT/auto-offset probe, a T e k t r o n i x 7912 t r a n s i e n t d i g i t i z e r , a m i c r o - P D P 11 c o m p u t e r f r o m D i g i t a l Equipment C o r p o r a t i o n , and an A p p l i e d P h o t o p h y s i c s c o n t r o l u n i t . The l a s e r was o p e r a t e d i n t h e charge on demand mode and t h e xenon lamp s o u r c e ( r i g h t - a n g l e arrangement) was momentarily p u l s e d t o a c h i e v e a h i g h i n t e n s i t y which was f l a t f o r about 200 / i s . The l a s e r was o p e r a t e d a t e i t h e r 2 4 8 nm ( K r F ) o r 3 5 1 nm ( X e F ) . Nominal o u t p u t s were 80 mJ/pulse a t 248 nm and 60 mJ/pulse a t 351 nm. UV a b s o r p t i o n s p e c t r a o f t r a n s i e n t i n t e r m e d i a t e s were c o n s t r u c t e d p o i n t - b y - p o i n t from decay p l o t s t a k e n a t s p e c i f i e d wavelength i n t e r v a l s . For t h e k i n e t i c decay s t u d i e s , the d a t a were a n a l y z e d u s i n g a s o f t w a r e package from A p p l i e d Photophysics.


46


RESULTS AND DISCUSSION In order t o demonstrate t h e use o f l a s e r f l a s h p h o t o l y s i s i n e l u c i d a t i o n o f t h e MDI based p o l y u r e t h a n e p h o t o l y s i s mechanism, t h r e e p o l y u r e t h a n e s , two a r y l b i s c a r b a m a t e models, an a r y l monocarbamate model, and an a r o m a t i c amine were s e l e c t e d . Two o f t h e p o l y u r e t h a n e s a r e based on MDI w h i l e t h e t h i r d i s based on TDI ( m i x t u r e o f 2,4 and 2,6 i s o m e r s i n 80/20 r a t i o ) . The MDI based p o l y u r e t h a n e s a l l have the same b a s i c carbamate r e p e a t u n i t . The MDI e l a s t o m e r (MDI-PUE) i s s o l u b l e i n t e t r a h y d r o f u r a n (THF). The s i m p l e p o l y u r e t h a n e (MDI-PU) based on MDI and 1,4-butanediol i s used i n the tert-butoxy abstraction reactions since i t does not contain a polyether backbone. (See page 47 f o r structures of polymers and models.) The r e s u l t s and d i s c u s s i o n s e c t i o n i s d i v i d e d i n t o two p a r t s . The f i r s t p a r t d e a l s w i t h d i r e c t l a s e r f l a s h p h o t o l y s i s o f t h e MDIPUE polymer and a p p r o p r i a t e s m a l l m o l e c u l e models. The t r a n s i e n t s p e c t r a g e n e r a t e d by d i r e c t e x c i t a t i o n o f t h e p o l y u r e t h a n e a r e i n t e r p r e t e d by c o n s i d e r a t i o n o f t h e p r i m a r y p h o t o c h e m i c a l r e a c t i o n s o f t h e carbamate m o i e t y . The second p a r t d e s c r i b e s r e s u l t s o b t a i n e d by p r o d u c t i o n o f a r a d i c a l t r a n s i e n t s p e c i e s which i s c a p a b l e o f a b s t r a c t i n g l a b i l e hydrogens from t h e p o l y u r e t h a n e . This latter p r o c e d u r e r e p r e s e n t s an a l t e r n a t i v e method f o r p r o d u c t i o n o f t h e t r a n s i e n t s p e c i e s which were o b t a i n e d by d i r e c t e x c i t a t i o n . L a s e r F l a s h P h o t o l y s i s a t 248 nm o f TDI-PU, MDI-PUE, and Model Compounds. F i g u r e s 1 and 2 show t h e t r a n s i e n t a b s o r p t i o n s p e c t r a o f MDI-PUE (5.5 X 10-3 / ) TDI-PU (2.3 X 10"3 g/dL) i n THF a t a 2.0 /is d e l a y a f t e r p u l s i n g w i t h a k r y p t o n f l u o r i d e excimer l a s e r ( A = 2 4 8 nm) i n a i r and n i t r o g e n s a t u r a t e d samples. Both s p e c t r a have common peaks i n n i t r o g e n s a t u r a t e d s o l u t i o n s (shown by a r r o w s ) a t 310 nm, 330-360 nm ( b r o a d ) , and above 400 nm ( b r o a d , d i f f u s e absorbance).. The MDI-PUE s a m p l e h a s a n a d d i t i o n a l and q u i t e d i s t i n c t i v e peak a t 370 nm. I n t h e presence o f a i r , t h e peak a t 370 nm f o r MDI-PUE i s c o m p l e t e l y e x t i n g u i s h e d , w h i l e t h e sharp peaks a t 310 nm f o r TDI-PU and MDI-PUE and t h e broad band above 400 nm a r e o n l y m a r g i n a l l y quenched by oxygen. I t s h o u l d be noted t h a t a t t h e e x c i t a t i o n w a v e l e n g t h employed, the absorbance o f MDI-PUE i s 1.1 w h i l e t h e absorbance o f t h e s o l v e n t THF i s 0.3. T h i s i s a c o n d i t i o n d i c t a t e d by polymer s o l u b i l i t y c o n s i d e r a t i o n s and c h o i c e o f e x c i t a t i o n wavelength. We a r e c o n f i d e n t t h a t t h e s p e c t r a l r e s u l t s f o r t h e p h o t o l y s i s a t 248 nm a r e d e r i v e d from r a d i c a l s g e n e r a t e d by d i r e c t e x c i t a t i o n , a s opposed t o r a d i c a l a b s t r a c t i o n by s o l v e n t r a d i c a l s , s i n c e t h e k i n e t i c c u r v e s i n d i c a t e no delay i n r a d i c a l formation of the t r a n s i e n t s . In o r d e r t o i n t e r p r e t t h e r e s u l t s f o r MDI-PUE and TDI-PU, t h e l a s e r f l a s h p h o t o l y s i s measurements o f s e v e r a l model systems were performed. The t r a n s i e n t s p e c t r a o f t h e p - t o l u i d i n y l r a d i c a l , r e c o r d e d upon l a s e r f l a s h p h o t o l y s i s ( X = 2 4 8 nm) o f p - t o l u i d i n e (1.4 X 10-4 M i n THF), has a d i s t i n c t maximum a t a p p r o x i m a t e l y 310 nm and a broad, d i f f u s e absorbance above 400 nm ( F i g u r e 3 ) . The r e s u l t s f o r p - t o l u i d i n e a r e i n agreement w i t h p r e v i o u s l y r e p o r t e d s p e c t r a f o r a n i l i n y l type r a d i c a l s (12,13). Comparing t h e t r a n s i e n t s p e c t r a f o r p - t o l u i d i n e w i t h TDI-PU and MDI-PUE, i t i s q u i t e o b v i o u s t h a t t h e pt o l u i d i n y l r a d i c a l i n THF ( F i g u r e 3) i s e s s e n t i a l l y i d e n t i c a l t o t h e 300-330 nm and > 400 nm p o r t i o n s o f t h e t r a n s i e n t s p e c t r a o f MDI-PUE and TDI-PU i n F i g u r e s 1 and 2. g

d L

a

n

d

ex

ex



HOYLE ET AL.

POLYMERS MDI-PU

TDI-PU ^HCO—f-CH^-CW

MDI-PUE MDI + 1,4-Butanediol + PolyCtetramethylene ether glycol) (MW=1000)

MODELS PrQ CH 2

BP-MDI

p-Toluidine

SiMe -MDI

Propyl N-tolylcarbamate

2

American Chemical Society Library 1155 16th st. N.W. f

Reichmanis and O'Donnell; The Effects ofD.C. Radiation on High-Technology Polymers Washington, 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

48


CM


4. HOYLE ET AL.


49

B e f o r e a s s i g n i n g t h e 310 nm and 400 nm bands i n t h e MDI-PUE and TDI-PU t r a n s i e n t s p e c t r a ( F i g u r e s 1 a n d 2 ) . t h e l a s e r flash p h o t o l y s i s o f p r o p y l N - t o l y l c a r b a m a t e ( 1 . 3 X 10~* M) was r e c o r d e d i n THF ( =2b8 ) » The t r a n s i e n t spectrum o f p r o p y l N - t o l y l c a r b a m a t e i s v e r y s i m i l a r t o t h e spectrum r e p o r t e d f o r p r o p y l N-phenylcarbamate (13). F o r p r o p y l N - t o l y l c a r b a m a t e , t h e peaks a t 310 nm, 330-360 nm and > 400 nm ( F i g u r e 4) a r e i d e n t i c a l t o t h r e e o f t h e f o u r peaks i n t h e t r a n s i e n t spectrum o f MDI-PUE and t h e t h r e e peaks i n TDI-PU. Thus, by analogy w i t h p - t o l u i d i n e , t h e 310 nm ( s h a r p peak) and 400450 nm band i n t h e t r a n s i e n t spectrum o f p r o p y l N - t o l y l c a r b a m a t e can be a t t r i b u t e d t o t h e p - t o l u i d i n y l r a d i c a l . These r e s u l t s a r e r e a d i l y t r a n s l a t e d i n t o assignment o f t h e 310 nm and 400-450 nm bands i n t h e MDI-PUE a n d TDI-PU t r a n s i e n t s p e c t r a ( F i g u r e s 1 and 2) t o pt o l u i d i n y l type r a d i c a l s . A n i l i n y l type r a d i c a l s have been p r e v i o u s l y i m p l i c a t e d a s an i n t e r m e d i a t e i n t h e p h o t o l y s i s o f a r y l carbamates and t h e i r presence i s c e r t a i n l y expected ( 4 - 6 ) . The broad band absorbance between 330 and 360 nm may be due, a t l e a s t i n p a r t , t o f o r m a t i o n o f t h e o r t h o photo-Fries product. C o n t r i b u t i o n s from o t h e r s p e c i e s may a l s o be i m p o r t a n t i n t h i s r e g i o n and f i n a l assignment depends on k i n e t i c s t u d i e s i n p r o g r e s s . The f i n a l peak under c o n s i d e r a t i o n i n t h e t r a n s i e n t spectrum ( F i g u r e 1) o f MDI-PUE i s t h e sharp peak a t 370 nm which i s q u i t e s e n s i t i v e t o oxygen. F i g u r e 5a shows t h e t r a n s i e n t a b s o r b a n c e spectrum ( X = 2 4 8 nm) o f t h e b i s p r o p y l carbamate o f MDI ( d e s i g n a t e d BP-MDI) i n d i c h l o r o m e t h a n e (2.7 X 1 0 ~ M). (We r e c e n t l y r e p o r t e d a s i m i l a r spectrum f o r BP-MDI i n THF ( 1 3 ) ) . As i n t h e case o f MDI-PUE ( F i g u r e 1 ) , t h e t r a n s i e n t spectrum o f BP-MDI i n F i g u r e 5a has bands a t 310 nm ( s h a r p p e a k ) , 330-360 nm (broad peak), 370 nm ( s h a r p peak) and above 400 nm (weak, d i f f u s e broad band a b s o r b a n c e ) . Only t h e p e a k a t 3 7 0 nm i s r e a d i l y q u e n c h e d b y o x y g e n . From t h e i n v e s t i g a t i o n s o f P o r t e r and Windsor ( 1 4 ) , i t i s w e l l known t h a t p h o t o l y s i s o f d i a r y l m e t h a n e s r e s u l t s i n a C-H bond c l e a v a g e a n d formation o f diarylmethyl r a d i c a l s . I n t h e case o f diphenylmethane, the d i p h e n y l m e t h y l r a d i c a l has a v e r y sharp absorbance maximum a t 330-360 nm. S u b s t i t u t i o n a t t h e para p o s i t i o n s , such a s i s t h e case w i t h BP-MDI and t h e biscarbamate group i n t h e MDI-PUE polymer, would be expected t o r e d s h i f t t h e a b s o r p t i o n maximum. Thus, t h e peak a t 370 nm i n F i g u r e s 1 and 5a c a n be t e n t a t i v e l y p o s t u l a t e d t o a r i s e from t h e absorbance o f a 4 , 4 - d i s u b s t i t u t e d d i p h e n y l m e t h y l r a d i c a l . In o r d e r t o p r o v i d e c o r r o b o r a t i v e e v i d e n c e f o r t h e assignment o f the 370 nm peak t o a d i p h e n y l m e t h y l r a d i c a l , a d i p h e n y l d i m e t h y l s i l i c o n a n a l o g o f t h e BP-MDI m o d e l compound was prepared ( s e e s t r u c t u r e f o r t h i s compound w h i c h i s d e s i g n a t e d S i M e 2 ~ M D I ) . O b v i o u s l y , f o r m a t i o n o f a d i p h e n y l m e t h y l r a d i c a l upon p h o t o l y s i s o f SiMe2-MDI i s i m p o s s i b l e . Accordingly, the transient absorption spectrum r e c o r d e d f o r t h e l a s e r f l a s h p h o t o l y s i s ( x = 2 4 8 nm) o f SiMe2-MDI ( F i g u r e 5b) i n d i c h l o r o m e t h a n e (2.6 X 1 0 ~ M) has peaks a t 310 nm ( s h a r p ) , 330-360 nm ( b r o a d ) , and above 400 nm ( b r o a d , weak d i f f u s e ) , b u t no sharp peak a t 370 nm. A p p a r e n t l y , a l l o f t h e s t r u c t u r a l f e a t u r e s e x h i b i t e d by t h e SiMe2~MDI t r a n s i e n t spectrum a r e found i n t h e t r a n s i e n t spectrum o f BP-MDI, save t h e 370 nm peak. T h i s c e r t a i n l y p r o v i d e s a d d i t i o n a l e v i d e n c e f o r a s s i g n i n g t h e 370 nm peak t o a s u b s t i t u t e d d i p h e n y l m e t h y l r a d i c a l . The next s e c t i o n d e a l s w i t h t h e u s e o f i n d e p e n d e n t l y generated tert-butoxy transient s p e c i e s which a r e capable o f a b s t r a c t i n g n m

ex

ex

5

f

e x

5


50


300

350

400 WAVELENGTH[nm]

500

450

F i g u r e 3. T r a n s i e n t a b s o r p t i o n spectrum ( 2 . 0 /is) o f 1.4 X 10-4 M p - t o l u i d i n e i n n i t r o g e n s a t u r a t e d THF (X =248 nm). EX

o

< PQ O CO PQ

Flash Photolysis of Aromatic Diisocyanate-Based Polyurethanes

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