Calorimetric Analysis of Photopolymerizations

8 Calorimetric Analysis of Photopolymerizations J. E. MOORE, S. H. SCHROETER, A. R. SHULTZ, and L. D. STANG

Downloaded by VIRGINIA TECH on February 27, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.ch008

General Electric Corporate Research and Development, Schenectady, Ν. Y. 12345

The p r e s e n t h i g h l e v e l o f i n d u s t r i a l a c t i v i t y i n t h e f i e l d o f l i g h t i n i t i a t e d p o l y m e r i z a t i o n s has s t i m u l a t e d r e s e a r c h i n b a s i c p o l y m e r i z a t i o n phenomena i n c l u d i n g p o l y m e r i z a t i o n k i n e t i c s . ( 1 - 4 ) U n f o r t u n a t e l y , p r e v i o u s l y used methods o f d e t e r m i n i n g monomer c o n v e r s i o n s u c h a s d i l a t o m e t r y o r measurements o f u n r e a c t e d monomers are n o t e a s i l y adapted t o t h i n c o a t i n g f i l m s . In a d d i t i o n , t h e p r e s e n c e o f m u l t i f u n c t i o n a l monomers y i e l d i n g n e t w o r k s a t low c o n version i n photopolymerizable formulations a l s o complicates analyses. D i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y (DSC) h a s been used t o i n v e s t i g a t e a d d i t i o n and c o n d e n s a t i o n p o l y m e r i z a t i o n s i n b u l k , s o l u t i o n , and e m u l s i o n s y s t e m s . ( 4 - 8 ) B o t h i s o t h e r m a l and t e m p e r a t u r e s c a n n i n g modes have been e m p l o y e d . In t h e p r e s e n t p r e l i m i n a r y work we d e m o n s t r a t e t h e u t i l i t y o f DSC i n s t u d y i n g t h e k i n e t i c s o f light initiated free radical polymerizations. Differential calor i m e t r y was c h o s e n b e c a u s e i t was a n t i c i p a t e d t h a t d i r e c t , c o n t i n uous m o n i t o r i n g o f t h e r e a c t i o n e x o t h e r m w o u l d g i v e t h e most i n f o r m a t i o n on r a p i d a c r y l a t e p o l y m e r i z a t i o n s . Lauryl a c r y l a t e pro v i d e d a monomer o f low v o l a t i l i t y upon w h i c h r e c e n t c o n v e n t i o n a l k i n e t i c s t u d i e s (9,10) e x i s t e d . 1,6-Hexanediol d i a e r y I a t e , neop e n t y l g l y c o l d i a c r y l a t e and t r i m e t h y l o l p r o p a n e t r i a c r y l a t e were chosen t o determine t h e c a p a b i l i t y o f t h e d i f f e r e n t i a l c a l o r i m e t r i c approach as a p p l i e d t o network-forming photopolymerizations. Experimental Materials. L a u r y l a c r y l a t e was f r e e d o f i n h i b i t o r by t h r e e s e p a r a t o r y f u n n e l e x t r a c t i o n s w i t h 5% s o d i u m c a r b o n a t e s o l u t i o n , t h r e e d e i o n i z e d w a t e r w a s h i n g s , f o l l o w e d by t h r e e c r y s t a l l i z a t i o n s f r o m m e t h a n o l . R e s i d u a l m e t h a n o l and w a t e r were removed by 24 h o u r s p a r g i n g w i t h n i t r o g e n a t room t e m p e r a t u r e . B o t h o r i g i n a l ( i n h i b i t e d ) a n d p u r i f i e d l a u r y l a c r y l a t e (LA) were p o l y m e r i z e d . I , 6 - H e x a n e d i o l d i a c r y l a t e (HDDA), n e o p e n t y l g l y c o l d i a c r y l a t e 90

In Ultraviolet Light Induced Reactions in Polymers; Labana, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

8.

MOORE ET A L .

Calorimetric Analysis of Photopolymerizations

91


(NPGDA), t r i m e t h y l o l p r o p a n e trîacrylate (TMPTA), T r i g o n a l 14 (Noury Chem. C o r p . ; m i x t u r e o f b e n z o i n b u t y l e t h e r s ) , b e n z o y l p e r o x i d e , and t - b u t y l p e r b e n z o a t e were used a s o b t a i n e d f r o m commer c i a l s u p p l i e r s without f u r t h e r p u r i f i c a t i o n . Apparatus. A P e r k i n - E l m e r DSC-IB d i f f e r e n t i a l s c a n n i n g c a l o r i m e t e r was m o d i f i e d by r e p l a c i n g t h e g l a s s windows o f t h e s a m p l e h o l d e r c o v e r w i t h q u a r t z windows. F i g u r e I d i s p l a y s s c h e m a t i c a l l y t h e m o d i f i e d DSC-IB s a m p l e h o l d e r ( A ) , r e f e r e n c e h o l d e r ( B ) , sam p l e h o l d e r c o v e r ( C ) , a r e a d i l y moved s u p e r s t r u c t u r e c o n t a i n i n g n e u t r a l d e n s i t y f i l t e r s (D), s h u t t e r ( E ) , c i r c u l a t i n g water heat f i l t e r ( F ) , a n d a G e n e r a l E l e c t r i c H3T7 medium p r e s s u r e m e r c u r y lamp (G) mounted i n a r e f l e c t o r e n c l o s u r e . T h i s a p p a r a t u s was em p l o y e d i n t h e i n i t i a l s c r e e n i n g s t u d i e s . The m a j o r p o r t i o n o f t h e d a t a r e p o r t e d h e r e were o b t a i n e d w i t h a m o d i f i e d P e r k i n - E l m e r DSC-2 i n s t r u m e n t . T h i s i s we 1 1 - r e p r e s e n t e d by F i g u r e I e x c e p t t h a t t h e i r r a d i a t i o n s u p e r s t r u c t u r e was mounted i n a p r e s s e d - w o o d c o m p o s i t i o n b o a r d p l a t f o r m c o n t a i n i n g a r e c t a n g u l a r q u a r t z window. The b o t t o m o f t h i s p l a t f o r m r e s t e d on a r u b b e r O - r i n g g a s k e t s e a t e d d i r e c t l y on t h e s u r f a c e o f t h e r e f r i g e r a t e d b l o c k ( t u r r e t removed) c o n t a i n i n g t h e s a m p l e h o l d e r s . A t w o pen, s t r i p c h a r t r e c o r d e r ( H i t a c h i model 5 6 ) was c o n n e c t e d t o t h e DSC-2. Procedure. S o l u t i o n s were p r e p a r e d by w e i g h i n g . A l l concen t r a t i o n s o f i n i t i a t o r s a r e s t a t e d as weight i n i t i a t o r per t o t a l weight o f mixture. S i n c e t h e s o l u t i o n s were p r e p a r e d i n a i r , a l l s o l u t i o n s s h o u l d be assumed e s s e n t i a l l y e q u i l i b r a t e d w i t h a i r . The DSC i n s t r u m e n t s were c a l i b r a t e d by t h e h e a t o f f u s i o n o f i n d i u m (6.80 c a l g n H ) . T o t a l i n c i d e n t l i g h t i n t e n s i t y (UV + v i s i b l e ) was d e t e r m i n e d i n t h e DSC-2 a p p a r a t u s by p a i n t i n g t h e s a m p l e . h o l d e r w i t h a t h i n l a y e r o f c a r b o n ( A q u a d a g ) , a d d i n g 10 mg o f U v i n u l N-539 t o c o v e r t h e c a r b o n w i t h a l i q u i d l a y e r , a n d i l l u m i n a t i n g t h e sample h o l d e r ( r e s t o f sample h o l d e r c a v i t y s h i e l d e d ) w i t h t h e r e f e r e n c e h o l d e r and c a v i t y s h i e l d e d f r o m t h e l i g h t . The d i f f e r e n t i a l h e a t i n g r a t e was 3.52 meal s e c " ' o v e r a 0.503 c m a r e a o r 6.99 meal cm" 2 s e c - I f o r t h e H3T7 lamp o n a t y p e LD-S b a l l a s t a t 400W. An empty a l u m i n u m c u p w i t h c r i m p e d - o n l i d was used a s a s e c o n d a r y , w o r k i n g s t a n d a r d f o r s u b s e q u e n t i n t e n s i t y m e a s u r e m e n t s . T h e t o t a l s a m p l e h o l d e r c a v i t y was i l l u m i n a t e d . A f a c t o r o f 2.28 was r e q u i r e d t o c o n v e r t o b s e r v e d meal s e c ' t o i n t e n s i t y i n meal cm~2 s e c " l u s i n g t h i s s e c o n d a r y s t a n d a r d . Long p a s s f i l t e r s show 7\% o f t h e l i g h t e n e r g y l i e s b e l o w 470 nm wave l e n g t h and 58% o f t h e l i g h t e n e r g y l i e s below 380 nm w a v e l e n g t h . The p r i n c i p a l UV l i g h t e n e r g y o u t p u t o f t h e H3T7 l i e s i n t h e t h r e e r e g i o n s a r o u n d 254 nm, 3 1 3 nm, a n d 365 nm. T h e r m a l l y - a c t i v a t e d p e r o x i d e i n i t i a t e d p o l y m e r i z a t i o n s were p e r f o r m e d i n t h e normal s c a n n i n g t e m p e r a t u r e mode (+20° m i n ~ f , 40 mm min"' c h a r t s p e e d ) w i t h t h e u n m o d i f i e d DSC-2 a p p a r a t u s . The s c a n b a s e l i n e was o b t a i n e d by a s e c o n d s c a n o f t h e r e a c t e d ma terial. T e m p e r a t u r e r a n g e 30°-250°C. The c u r v e a r e a i n t e g r a t i o n s 2

-


UV


92

L I G H T INDUCED REACTIONS IN

POLYMERS

were a c h i e v e d by X e r o x i n g t h e c u r v e s , c u t t i n g , and weighing. A t y p i c a l l i g h t - i n i t i a t e d polymerization involved weighing 7-9 mg o f a c r y l a t e w i t h i n i t i a t o r i n t o t h e s t a n d a r d DSC a l u m i n u m cup. The cup was p l a c e d i n t h e DSC s a m p l e h o l d e r and an empty a l u m i n u m cup was p l a c e d i n t h e r e f e r e n c e h o l d e r . The u s u a l n i t r o gen f l o w w i t h i n t h e c e l l e n c l o s u r e was e s t a b l i s h e d w i t h t h e UV lamp, f i l t e r , and s h u t t e r a s s e m b l y p o s i t i o n e d o v e r t h e h o l d e r s . The g u i l l o t i n e s h u t t e r was o p e r a t e d m a n u a l l y . A h o l d e r t e m p e r a t u r e o f 52°C was used w i t h t h e DSC-IB w h i l e i s o t h e r m a l t e m p e r a t u r e s f r o m 0° t o 80° were employed w i t h t h e DSC-2. A r e c o r d i n g c h a r t s p e e d o f 4 cm min"I was used w i t h t h e DSC-2 w i t h most r u n s o b s e r v e d f o r 15 m i n u t e s . The two pens were s e t a t d i f f e r e n t s e n s i t i v i t i e s so t h a t one t r a c e w o u l d r e m a i n on s c a l e t h r o u g h o u t t h e e x o t h e r m w h i l e t h e s e c o n d t r a c e a m p l i f i e d t h e l a t e r , low r a t e c u r v e s e c t i o n . C u r v e a r e a i n t e g r a t i o n s and d i g i t a l r a t e d a t a were o b t a i n e d by t r a c i n g t h e c u r v e s on a CALMA d i g i t i z e r , s t o r i n g t h e d i g i t a l c o o r d i n a t e d a t a on m a g n e t i c t a p e , and p r o c e s s i n g t h e d a t a f i l e s on a GE 600 s e r i e s d i g i t a l c o m p u t e r . R e s u l t s and

Discussion

The r e s u l t s o f p e r o x i d e i n i t i a t e d and T r i g o n a l 14 p h o t o i n i t i ated p o l y m e r i z a t i o n s of l a u r y l a c r y l a t e (LA), I,6-hexanediol d i a c r y l a t e (HDDA), n e o p e n t y l g l y c o l d i a c r y l a t e (NPGDA), and t r i m e t h y l o l p r o p a n e t r i a c r y l a t e (TMPTA) w i l l f i r s t be p r e s e n t e d . These e x p e r i m e n t s were d e s i g n e d t o o b s e r v e t o t a l h e a t s o f p o l y m e r i z a t i o n u n d e r p r e s c r i b e d c o n d i t i o n s . The r e s u l t s o f more e x t e n s i v e r a t e s t u d i e s on T r i g o n a l 14 p h o t o i n i t i a t e d LA p o l y m e r i z a t i o n s w i l l t h e n f o l low. Mono-, P i - , and T r i - A c r y l a t e P o l y m e r i z a t i o n s : Peroxide Initiation. Table I presents the observed heats of polymerization o f LA, HDDA, NPGDA, and TMPTA c o n t a i n i n g \% b e n z o y l p e r o x i d e +\% t - b u t y l p e r b e n z o a t e upon +20°/min t h e r m a l s c a n n i n g f r o m 30° t o 250°C. The c o m b i n e d p e r o x i d e i n i t i a t o r s y s t e m a s s u r e d a h i g h p r o b a b i l i t y o f r e a c t i o n o f n e a r l y a l l " a c c e s s i b l e " d o u b l e bonds i n t h e n e t w o r k - f o r m i n g monomers. The -19.2 k c a l m o l " l (-80.0 c a l gm"h t o t a l h e a t o f p o l y m e r i z a t i o n f o u n d f o r LA i s c l o s e t o t h e -18.8, -18.6, and -18.6 k c a l mol"' r e p o r t e d f o r m e t h y l , e t h y l , and η-butyl a c r y l a t e s , ( 1 1 , 1 2 ) r e s p e c t i v e l y . The -17.6, - 1 6 . 7 , and -15.4 k c a l p e r mole o f C=C o b s e r v e d f o r HDDA, NPGDA, and TMPTA, r e s p e c t i v e l y , a r e b e l i e v e d t o show t h e p r o g r e s s i v e l y d e c r e a s i n g h e a t s o f p o l y m e r i z a t i o n due t o u n r e a c t e d c a r b o n - c a r b o n d o u b l e bonds on t h e p o l y m e r n e t w o r k s . Mono-, P i , and T r i A c r y l a t e P o l y m e r i z a t i o n s : T r i g o n a l 14 Photoinitiation. The h e a t s o f p o l y m e r i z a t i o n o f t h e monomers a c h i e v e d by p h o t o i n i t i a t i o n ( l i g h t i n t e n s i t y a p p r o x i m a t e l y 5.2 meal cm"2 s e c " ' ) u s i n g T r i g o n a l 14 a t a p p r o x i m a t e l y \% c o n c e n t r a t i o n ( T a b l e I) were o b t a i n e d f r o m r a t e v s . t i m e PSC-2 t r a c e s s i m i l a r t o t h e one shown i n F i g u r e 2 ( c f . s e q . ) . Pue t o t h e r a p i d


8.


MOORE E TA L .

93

< ·> J F Ε L

_U

Figure 1. Schematic of modified DSC-IB apparatus: (A) sample holder, (B) reference holder, (C) sample holder cover with quartz windows, readily moved superstruc ture with (D) neutral density filters, (E) guillotine shutter, (F) circulating water heatfilter,and (G) G. Ε. H3T7 medium pressure mercury lamp in a reflector enclosure

,


0 He

Β

Α

TABLE I H e a t s o f P o l y m e r i z a t i o n o f A c r y l i c Monomers Thermally-activated Peroxide I n i t i a t i o n . S c a n 30°-250°C. P h o t o - a c t i v a t e d T r i g o n a l 14 I n i t i a t i o n . * * 40° and oCPC. I n i t i a t o r Cone. l 0 C ( g m gm-1) 2

Temp. °c

ca1

- Δ Hp gm~ '

kcal moHc=C

Monomer

Initator

LA (C-13) (C-5) (C-19)

peroxides TR-14 TR-14 TR-14

1 +1 0.989 1.01 1.13

scan 30 40 40

80.0 73. 71. 74.

19.2 17.5 17.0 17.8

HDDA

peroxides TR-14 TR-14 TR-14 TR-14

1 +1 1.312 1.000 1.312 1.000

scan 40 40 40 60

155.4 99. 106. 106. 147.

17.6 11.2 12.0 12.0 16.6

NPGDA

peroxides TR-14 TR-14 TR-14

1 +1 1.236 1.236 1.236

scan 40 40 60

157.5 76. 78. 91.

16.7 8.1 8.3 9.7

TMPTA

peroxides TR-14 TR-14 TR-14 TR-14

1 +1 1.045 1.045 1.045 1.045

scan 40 40 40 60

156.1 94. 94. 88. 103.5

15.4 9.3 9.3 8.7 10.2

**400W H3T7 lamp; no f i l t e r , a p p r o x i m a t e l y

5.2 meal cm"^ s e c "



94

UV


POLYMERS

i n i t i a l p o l y m e r i z a t i o n r a t e s o f t h e m u l t i f u n c t i o n a l monomers, sam p l e w e i g h t had t o be r e s t r i c t e d t o 2-3 mg. The 40° p h o t o a c t i v a t e d , T r i g o n a l 14 i n i t i a t e d r u n s y i e l d e d p o l y m e r i z a t i o n h e a t s o f -17.4, - 1 1 . 7 , - 8 . 2 , and -9.1 k c a l p e r m o l e o f C=C f o r LA, HDDA, NPGDA, and TMPTA, r e s p e c t i v e l y . A s s u m i n g Δ H =- 19.2 k c a l p e r mole o f C=C f o r c o m p l e t e c o n v e r s i o n , t h e LA, HDDA, NPGDA, and TMPTA p h o t o p o l y m e r i z a t i o n c o n v e r s i o n s a t 40° were 9 1 $ , 6\%, 43%, and 47%. Based on t h e i r p a r t i c u l a r p o l y m e r i z a t i o n h e a t s i n t h e t h e r m a l s c a n s , t h e LA, HDDA, NPGDA, and TMPTA photopoIymerizations a t 40° were 9\%, 66%, 49%, and 59%, r e s p e c t i v e l y , o f t h e i r " u l t i mate" p o s s i b l e c o n v e r s i o n s . The m u l t i f u n c t i o n a l a c r y l a t e s e x h i b i t i n c r e a s e d c o n v e r s i o n d u r i n g p h o t o p o l y m e r i z a t i o n a t 60°. B a s e d o n -19.2 k c a l p e r mole o f C=C f o r c o m p l e t e c o n v e r s i o n , t h e 60° p h o t o p o l y m e r i z a t i o n c o n v e r s i o n s o f HDDA, NPGDA, and TMPTA a r e 86%, 5\%, and 53%, r e s p e c t i v e l y . A g a i n , b a s e d o n t h e i r p a r t i c u l a r p o l y m e r i z a t i o n h e a t s i n t h e t h e r m a l s c a n s , t h e HDDA, NPGDA, a n d TMPTA p h o t o p o I y m e r i z a t i o n s a t 60° were 94%, 5 8 , a n d 66%, r e s p e c t i v e l y , of t h e i r " u l t i m a t e " conversions. The i n c r e a s e d m o b i l i t y o f t h e n e t w o r k s b r o u g h t a b o u t by r a i s i n g t h e t e m p e r a t u r e p r o b a b l y a c c o u n t s f o r t h e i n c r e a s e d c o n v e r s i o n o f C=C bonds i n g o i n g f r o m 40° t o 60°. Two f u r t h e r c o n c l u s i o n s may be drawn. F i r s t , t h e g r e a t e r f l e x i b i l i t y and r e s u l t a n t c o n f i g u r a t i o n p o s s i b i l i t i e s o f HDDA n e t w o r k u n i t s r e l a t i v e t o NPGDA n e t w o r k u n i t s p e r m i t f o r t h e f o r m e r s l i g h t l y h i g h e r C=C bond c o n v e r s i o n s i n t h e t h e r m a l s c a n s and m a r k e d l y h i g h e r c o n v e r s i o n s a t 40° and 60° i n t h e p h o t o p o I y merizations. S e c o n d l y , t h e p h o t o p o l y m e r i z a t i o n o f TMPTA a t 40° and 60° i s s l i g h t l y more e f f i c i e n t t h a n t h a t o f NPGDA i n s p i t e o f a l o w e r " u l t i m a t e " c o n v e r s i o n o f TMPTA s u g g e s t e d by t h e t h e r m a l scan. I t i s i n t e r e s t i n g t o s p e c u l a t e on t h e p o s s i b i l i t y t h a t t h e p o l y m e r n e t w o r k p r o d u c e d by t h e TMPTA p o l y m e r i z a t i o n p r o v i d e s g r e a t e r a c c e s s i b i l i t y f o r r e a c t i o n o f t h e p e n d a n t d o u b l e bonds a t 40° a n d 60° t h a n t h a t w h i c h e x i s t s i n t h e NPGDA n e t w o r k . L a u r y l A c r y l a t e P o l y m e r i z a t i o n by P h o t o - a c t i v a t e d T r i g o n a l 14 Initiation. A more e x t e n s i v e p r e l i m i n a r y s t u d y o f t h e c a p a b i l i t y o f c a l o r i m e t r y t o f o l l o w r a p i d p h o t o p o l y m e r i z a t i o n r e a c t i o n s was made o f t h e k i n e t i c s o f LA p o l y m e r i z a t i o n i n i t i a t e d by p h o t o - a c t i v a t e d TR-14. F i g u r e 2 i s a t r a c i n g o f an i s o t h e r m a l e x o t h e r m c u r v e o f LA c o n t a i n i n g TR-14 d u r i n g c o n t i n u o u s i r r a d i a t i o n by a 400W H3T7 medium p r e s s u r e m e r c u r y lamp. A t h i g h l i g h t i n t e n s i t i e s and h i g h TR-14 c o n c e n t r a t i o n s no i n d u c t i o n p e r i o d i s s e e n . Differential h e a t i n g i s noted a t about 2 s e c a f t e r s h u t t e r opening; t=0 i s t a k e n a t t h i s p o i n t . The d i f f e r e n t i a l h e a t i n g peak ( e x o t h e r m peak) i s seen t o o c c u r a t about t=7 s e c w i t h subsequent d e c l i n e a s monomer ( a n d i n i t i a t o r ) c o n c e n t r a t i o n d i m i n i s h e s . The u p p e r c u r v e i s t h e b a s e l i n e o b t a i n e d by r e p e a t i n g t h e i r r a d i a t i o n c y c l e a f t e r 15 min i r r a d i a t i o n . The " r a t e a t peak" i s used i n t h e f o l l o w i n g data p r e s e n t a t i o n a s a c o n v e n i e n t comparator o f r a t e s . The sub sequent computer treatment o f t h e data r e v e a l s f o r t h i s run t h a t


8.

MOORE

ET

AL.


95


\0% o f t h e t o t a l p o l y m e r i z a t i o n h e a t (69.4 c a l g i r H f o r t h i s sam p l e ) i s o b s e r v e d p r i o r t o t h e peak. It i s possible to extrapolate the rate vs. conversion curve t o zero conversion ( c f . s e c t i o n Monomer C o n c e n t r a t i o n E f f e c t ) , b u t f o r t h e p r e s e n t t h i s i s n o t done. The r e a d e r ' s a t t e n t i o n i s d i r e c t e d t o t h e f a c t t h a t t h e s e are very r a p i d p o l y m e r i z a t i o n s being described here. 50% o f t h e t o t a l h e a t a p p e a r s by t=25 s e c i n t h e run r e p r e s e n t e d by F i g u r e 2. D a t a o b t a i n e d f r o m c u r v e s s u c h a s t h e one shown i n F i g u r e 2 a r e p r e s e n t e d i n T a b l e s II t h r o u g h V. The dependence o f LA p o l y m e r i z a t i o n r a t e on t e m p e r a t u r e , i n i t i a t o r c o n c e n t r a t i o n , l i g h t i n t e n s i t y , and monomer c o n c e n t r a t i o n (conversion) i s discussed in the f o l l o w i n g s e c t i o n s . Temperature E f f e c t . T a b l e II l i s t s d a t a on LA p o l y m e r i z a t i o n r a t e a t t h e e x o t h e r m peak i n t h e t e m p e r a t u r e r e g i o n 0°-80°C a t two l i g h t i n t e n s i t i e s . A \% TR-14 c o n c e n t r a t i o n was u s e d . The d a t a a r e p r e s e n t e d g r a p h i c a l l y i n F i g u r e 3. For the high l i g h t i n t e n s i t y s e r i e s a low a p p a r e n t a c t i v a t i o n e n e r g y o f 0.63 k c a l mol"' i s o b s e r v e d f r o m t h e t e m p e r a t u r e d e p e n d e n c e o f t h e r a t e a t e x o t h e r m peak. ( T h i s a c t i v a t i o n energy i s c a l c u l a t e d from t h e c a l gm"' s e c " I r a t e s r a t h e r t h a n f r o m mol I"I s e c " ' r a t e s . Use o f t h e l a t t e r would d e c r e a s e t h e c a l c u l a t e d a c t i v a t i o n energy s i n c e about o n e - t h i r d o f t h e r a t e i n c r e a s e w o u l d be d i m i n i s h e d by t h e s p e c i f i c volume i n c r e a s e i n t h e 50° t e m p e r a t u r e i n t e r v a l . Of c o u r s e , t h e i n i t i a t o r c o n c e n t r a t i o n on a mol I " * b a s i s w o u l d a l s o d e c r e a s e about 5%.) The l o g ( r a t e a t p e a k ) v s . I/T d a t a ( S e r i e s 11) a t low l i g h t i n t e n s i t y a p p e a r c o m p l e x ( F i g u r e 3 ) . No a t t e m p t w i l l be made a t t h i s time t o e x p l a i n the observed negative apparent a c t i v a t i o n e n e r g y f o r t h e r e a c t i o n i n t h e 0°-30° t e m p e r a t u r e r a n g e . I n i t i a t o r Concentration Effect. Table III presents data f o r f i v e s e r i e s o f p h o t o p o l y m e r i z a t i o n s o f LA u s i n g v a r i o u s c o n c e n t r a t i o n s o f TR-14. The s e r i e s d i f f e r f r o m one a n o t h e r i n i n c i d e n t l i g h t i n t e n s i t y a n d / o r t e m p e r a t u r e . A l s o , S e r i e s I I I was p e r formed w i t h LA monomer s t i l l c o n t a i n i n g f r e e - r a d i c a l i n h i b i t o r . F i g u r e 4 r e p r e s e n t s t h e d a t a o f S e r i e s IV, V, and VI run i n t h e m o d i f i e d DSC-2 a p p a r a t u s p l u s a n o n - t a b u l a t e d s e r i e s on i n h i b i t e d monomer i n t h e DSC-IB a p p a r a t u s . S e r i e s III data (not p l o t t e d ) w o u l d l i e s l i g h t l y b e l o w and p a r a l l e l t o t h e S e r i e s V d a t a . One n o t e s t h a t t h e d a t a (C=.00l t o .01) a t h i g h i n t e n s i t y i l l u m i n a t i o n may be a p p r o x i m a t e d by s t r a i g h t l i n e r e l a t i o n s i n F i g u r e 4. The slope of these l i n e s y i e l d s (rate at peak)* C . The ( r a t e a t peak) α C ' r e p r e s e n t s t h e d a t a f o r S e r i e s I I I and V (400W lamp o p e r a t i o n ) l e s s w e l l and o v e r a more l i m i t e d c o n c e n t r a t i o n r a n g e . The same power r e l a t i o n a p p e a r s t o a p p r o x i m a t e t h e low i n t e n s i t y d a t a ( S e r i e s V I ; 400W lamp o p e r a t i o n w i t h n e u t r a l d e n s i t y f i l t e r ) o n l y i n t h e r e g i o n n e a r C = .01. 0 e

0

3 5

The c o m p l e x i t y o f t h e i n i t i a t o r m i x , f i l m t h i c k n e s s v a r i a t i o n , and o x y g e n p r e s e n c e i n t h e s e p h o t o p o l y m e r i z a t i o n s does n o t


U V L I G H T INDUCED REACTIONS

IN

POLYMERS

TIME (SEC)


0

60

120

180

240

300

Figure 2. Photopolymerization exotherm trace. Rate of sample heat change vs. time at 40°C. Photoactivated Trigonal 14 initiation of lauryl acrylate polymerization.

TABLE II Temperature E f f e c t Lauryl A c r y l a t e Photopolymerization R a t e a t Peak -dH/dt ( c a l gm"' s e c " ' )

Series*

Run

T(°C)

1

T-l T-2 Î-3 T-4 T-5

10 30 40 50 60

2.29 2.47 2.50 2.69 2.68

T-6 T-7 T-8 T-9 T-10 T-l 1 T-12

0 10 20 30 40 60 80

0.615 0.601 0.586 0.563 0.573 0.601 0.729

II

*Series

Series

2

I

8Ô0+ w a t t s ; i n t e n s i t y 16.7 meal c m " T-14 I02c = 0.989 gm gm"'

II

400 w a t t s , 0.1 n e u t r a l d e n s i t y f i l t e r ; i n t e n s i t y 0.88 meal cm-2 s e c " I ; T-14 I0 C = 0.989 gm gm"' 2


1

sec" ;

MOORE

Calonmetric Analysis of Photopolymerizations

ET AL.

TABLE III I n i t i a t o r Concentration E f f e c t Lauryl Acrylate Photopolymerization Half Reaction Total Heat Time, f "ΔΗ 5 ( ca 1 gm 1 ) (sec)

Run C-l C-2 C-3 C-4 C-5 C-6 C-7

0.101 0.205 0.405 0.604 1.01 2.01 4.03

0.80 1.19 1.68 1.88 2.05 2.11 2.42

34. 47. 57. 71 . 71 . 74. 79.

30 28 25 29 25 26 26

C-8 C-9 C-10 C-l I C-l 2 C-l 3

0.0101 0.0375 0.159 0.352 0.634 0.989

0.023 0.59 1.74 2.33 2.79 3.34

17. 48. 59. 68. 73.

25 21 18 17 17

C-14 C-l 5 C-l 6 C-17 C-18 C-19 C-20 C-21

107 140 257 257 569 13 2.06 4.59

1.01 0.92 I .56 I .59 2. 18 2.55 2.65 2.55

39. 39. 60. 54. 69. 74. 77. (83.)

28 32 29 24 25 22 21 25

VI

C-22 C-23 C-24 C-25 C-26 C-27 C-28 C-29 C-30 C-31 C-32 C-33 C-34

0.107 0. 140 0. 159 0.257 0.352 0.569 0.634 0.989 1.13 1.13 2.06 4.59 4.59

0.057 0.046 0.132 0.262 0.313 0.515 0.451 0.546 0.570 0.637 0.693 0.588 0.612

5.5 4.6 14.6 30. 35. 54. 42. 52. 52. 57. 65. 68. 70.

64 68 82 90 91 101 79 86 85 85 95 122 I 16

VII

C-35 C-36 C-37 C-38

0.0375 0.159 0.352 0.634

(0) 0.182 0.352 0.432

(0) 23. 41 . 47.

I 16 120 122

2

Series* III


Rate a t Peak -dH/dt (cal am sec"')

Trigonal 14 I0 C (gm gm"1)

IV

*Series I 11 Series IV Series V Series VI Series VII

-1

11

2

400 watts; i n t e n s i t y 6.6 meal cm" s e c - l ; T=40°C; inhibited monomer 800+ watts; i n t e n s i t y 16.7 meal cm" sec"'; T=30°C 400 watts; i n t e n s i t y 6.3 meal cm" s e c - l ; T=40°C 400 watts; 0.1 neutral density f i l t e r ; i n t e n s i t y 0.89 meal cm~2 sec"!; T=40°C 400 watts; 0.1 neutral density f i l t e r ; intensity 0.89 meal cm- sec"I; 6 0 ° C 2

2

2


U V LIGHT INDUCED REACTIONS IN

POLYMERS

2.0 RATE AT PEAK (CAL GUT'SEC ) -1

1.0 0.8

ο


0.6

2.8

2.6

3.0

3.2

3.4

3.6

3.8

I0 /T 3

Figure 8. Temperature dependence of —dH/dt exotherm rate at peak for lauryl acrylate containing 0.989 gm gm' of Trigonal 14 (cf. Table II). Upper points at 16.7 meal cm~ sec total light intensity; lower points at 0.88 meal cm' sec' total light intensity.

1

2

1

2

10.0 h

1

1

1

—

1

' ι · · "I

oc 0.1

J_jj

0.001

0.010

TRIGONAL 14 CONCENTRATION (GM/GM MIXTURE)

Figure 4. Dependence of —dH/dt exotherm rate at peak on initial Trigonal 14 concentration in photo polymerizations of lauryl acrylate. ·, 800 -f- W, inhib ited monomer, Τ = 52°C, DSC-IB; 0,800 + W, Τ = 80° (Series IV); 3 , 4O0W, Τ = 40°C (Series V); €, 4O0W, 0.1 neutral densityfilter,Τ = 40°C (Series VI).



8.

MOORE

ET AL.

CalofimeMc Analysis of Photopolymerizations

99

encourage d e t a i l e d a n a l y s i s o f t h e i n i t i a t o r c o n c e n t r a t i o n e f f e c t . However, t h e r a p i d d e c r e a s e i n t h e peak r a t e w i t h l o w e r i n g o f C b e l o w 0.001 a p p e a r s a s s o c i a t e d w i t h i n i t i a t o r l o s s t o s i d e r e a c t i o n s , most l i k e l y t o d e p l e t i o n due t o o x y g e n p r e s e n c e . A t v e r y low C a n e x t e n d e d i n h i b i t i o n p e r i o d i s n o t e d i n t h e i s o t h e r m a l DSC p h o t o p o l y m e r i z a t i o n s l e a d i n g i n e x t r e m e i n s t a n c e s , e . g . , r u n C-35, t o no o b s e r v e d p o l y m e r i z a t i o n . T h e a c c e n t u a t e d d r o p - o f f a t low l i g h t i n t e n s i t y ( S e r i e s V I ) i n t h e peak r a t e w i t h i n i t i a t o r c o n c e n t r a t i o n d e c r e a s e i s n o t r e a d i l y e x p l i c a b l e . We p r e s e n t l y a s c r i b e t h e d e c r e a s e i n t h e r a t e a t peak a t h i g h TR-14 c o n c e n t r a tions t o optical thickness effects. T h i s aspect r e q u i r e s study w i t h b e t t e r f i l m t h i c k n e s s geometry, s i n g l e i n i t i a t o r s p e c i e s , and l i m i t e d wavelength ranges. Figure 5 d i s p l a y s t h e v a r i a t i o n o f t o t a l heat o f r e a c t i o n w i t h i n i t i a l i n i t i a t o r c o n c e n t r a t i o n f o r S e r i e s I V , V, V I , a n d VII. T h i s e f f e c t i s most a s s u r e d l y a s s o c i a t e d w i t h i n i t i a t o r d e p l e t i o n throughout t h e continuous i r r a d i a t i o n . Some enhancement of t o t a l c o n v e r s i o n a t given i n i t i a t o r c o n c e n t r a t i o n i s noted i n r a i s i n g t h e p o l y m e r i z a t i o n t e m p e r a t u r e f r o m 40° ( S e r i e s V I ) t o 60° ( S e r i e s V I I ) . A l a r g e decrease i n t o t a l heat o f r e a c t i o n i s noted i n g o i n g f r o m t h e 16.6 meal cm~2 s e c - I a n d 10 meal cm" 2 s e c " ' i l l u m i n a t i o n s t o t h e 0.89 meal cm-2 s e c " ' i l l u m i n a t i o n s . Light Intensity Effect. The e f f e c t o f l i g h t i n t e n s i t y upon fhe p o l y m e r i z a t i o n r a t e a t peak i s d i s p l a y e d by t h r e e s e r i e s o f r u n s ( c f . T a b l e I V ) . The s e r i e s d i f f e r s l i g h t l y i n i n i t i a l TR-14 c o n c e n t r a t i o n and t e m p e r a t u r e . Figure 4 presents these data a s log ( r a t e a t p e a k ) v s . l o g ( i n t e n s i t y ) . The r a t e s p l o t t e d have been c o n v e r t e d t o r a t e s a t C = 0.0100 and T=3I3°K (40°C) by u s i n g ROj^OO = r C χ ( C / . 0 I ) 0 . 3 5 +3l6(T"' -313"'). C o n s i d e r i n g t h e 30f o l d i n t e n s i t y r a n g e 0.48 meal c m s e c " ' t o 14.3 meal c m " s e c " ' ( F i g u r e 6) and f i r s t a v e r a g i n g t h e r a t e a t peak v a l u e s a t g i v e n i n t e n s i t i e s , we f i n d t h a t t h e l e a s t - s q u a r e s s t r a i g h t l i n e t h r o u g h the r e s u l t i n g t e n data p o i n t s y i e l d s a slope corresponding t o ( r a t e a t peak) α ( i n t e n s i t y ) * T h i s i s near t h e expected de pendence o f p o l y m e r i z a t i o n r a t e upon t h e s q u a r e r o o t o f t h e l i g h t i n t e n s i t y f o r s i m p l e i n i t i a t o r p h o t o l y s i s and r a d i c a l + r a d i c a l termination o f k i n e t i c chains i n solution. x

e

- 2

2

0

Monomer C o n c e n t r a t i o n E f f e c t . In b u l k p o l y m e r i z a t i o n s s u c h a s t h o s e c o n d u c t e d i n t h e p r e s e n t s t u d y , t h e dependence o f p o l y m e r i z a t i o n r a t e o n monomer c o n c e n t r a t i o n c a n be d e t e r m i n e d o n l y on t h e b a s i s o f t h e dependence o f r a t e o n t h e e x t e n t o f r e a c t i o n . Re d u c t i o n o f t h e r a t e v s . t i m e DSC t r a c e s t o d i g i t a l d a t a f i l e s p e r m i t s c o m p u t e r c a l c u l a t i o n o f r e a c t i o n r a t e a s a f u n c t i o n o f mono mer c o n v e r s i o n . A computer program which y i e l d s p r i n t - o u t o f t h e r a t e and t i m e a t g i v e n f r a c t i o n s o f t h e t o t a l heat r e l e a s e a l l o w s computation o f t h e order o f r e a c t i o n with respect t o carbon-carbon d o u b l e bond c o n c e n t r a t i o n . A s s u m i n g -80.0 c a l gm"' r e p r e s e n t s t h e


100

UV L I G H T INDUCED REACTIONS IN

POLYMERS

TOTAL HEAT REACTION °l" (CALGM-1, 4

Figure 5. Total heat of reaction, — ΔΗ , plotted against initial Trigonal 14 concentration, C, for lauryl acrylate polymerizations (cf. Table III). ·, Series IV; d , Series V; O, Series VI; €, Series VII.


Ρ

10'C (GM/GM MIXTURE)

TABLE IV

Lauryl

Series*

Run

Light Intensity Effect Acrylate Photopolymerization

Light Intensity (mca1 c m " s e c " ' ) 2

R a t e a t Peak -dH/dt ( c a l gm"' s e c " ' )

l-l 1-2 1-3 1-4 1-5

0.69 1.42 3.52 9.0 14.3

0.70 1.00 1.91 2.26 3.32

IX

1-6 1-7 1-8 1-9 1-10 l-l 1

0.34 0.34 1.53 1.53 14.3 14.3

0.074 0.059 0.727 0.763 2.74 3.02

X

1-12 1-13 1-14 1-15 1-16 1-17 1-18 1-19

VI 11

*Series VIII Series Series

IX X

0.107 0.214 0.48 0.48 0.84 2.80 4.34 4.34

0.007 0.187 0.197 0.504 0.80 1 .59 1.96 1.93 2

800+ w a t t s ; TR-14 I 0 C = 1.00; 52°C; i n h i b i t e d monomer; DSC-IB 800+ w a t t s ; TR-14 I 0 C = 0.989; 30°C 400 w a t t s ; TR-14 I 0 C = 1.13; 40°C 2

2


8.

MOORE


ET AL.

101

t o t a l Δ Η o f LA, .the r e s i d u a l h e a t o f p o l y m e r i z a t i o n a t t i m e t , H ( t ) = δθ.0 - / x ( d H / d t ) - d t , i s p r o p o r t i o n a l t o t h e u n r e a c t e d monomer c o n c e n t r a t i o n i n t h e s y s t e m a t t h a t t i m e . F i g u r e s 7 and 8 are p l o t s o f t h e exotherm r a t e s , - ( d H / d t ) v s . H ( t ) on l o g - l o g c o o r d i n a t e s f o r t h e S e r i e s IV a n d S e r i e s V d a t a . τ a t which t h e p o i n t s a r e shown a r e t o> o.3> "*"(). 4' + *> ' w h i c h 0.2, 0.3, 0.4, e t c . , a r e t h e f r a c t i o n s o f total h e a t f o r e a c h r u n . T h u s , t h e upper f o u r p o i n t s on t h e t o p c u r v e o f F i g u r e 7 r e p r e s e n t f o r r u n C - l 3 t h e r a t e s o f r e a c t i o n a t 0.2 χ 73 = 14.6, 0.3 χ 73=21.9, 0.4 χ 73 = 29.2, a n d 0.5 χ 73 = 36.5 c a l gm" f o r -H c u m u l a t i v e = " £ x ( d H / d t ) d t , χ = .2, .3, .4, .5. I f t h e r e were l i t t l e i n i t i a t o r d e p l e t i o n and o t h e r f a c t o r s remained c o n s t a n t , one would e x p e c t t h e c u r v e s i n F i g u r e s 7 a n d 8 t o be p a r a l l e l s t r a i g h t l i n e s whose s l o p e s w o u l d be t h e o r d e r o f r e a c t i o n w i t h r e s p e c t t o mon omer c o n c e n t r a t i o n . O b v i o u s l y , c u r v a t u r e e x i s t s i n t h e p l o t s and the curve slopes i n t h e e a r l y stages o f conversion appear t o i n c r e a s e a s t h e i n i t i a l i n i t i a t o r c o n c e n t r a t i o n i s lowered. Despite t h e s e d e v i a t i o n s we may e x p e c t t h e s l o p e s o f t h e c u r v e s i n F i g u r e s 7 a n d 8 a t low c o n v e r s i o n and h i g h i n i t i a l i n i t i a t o r c o n c e n t r a t i o n s t o represent nearly the order o f reaction with respect t o monomer c o n c e n t r a t i o n . Table V presents the r e s u l t s o f least-squares f i t t i n g o f s t r a i g h t l i n e s t o t h e 0.2, 0.3, 0.4, a n d 0.5 f r a c t i o n a l h e a t p o i n t s o f t h e l o g - l o g p l o t s ( o f F i g u r e s 7 a n d 8) o f p o l y m e r i z a t i o n rates (-dH/dt) and r e s i d u a l h e a t s o f r e a c t i o n f o r S e r i e s I I I , IV, and V d a t a . T h e s l o p e s , B, p r o g r e s s i v e l y i n c r e a s e a s i n i t i a l i n i t i a t o r concentrations decrease. However, r e s t r i c t i n g o u r c o n s i d e r a t i o n t o t h e t e n r u n s i n w h i c h - A H >68 c a l gm"' t h e a v e r a g e Β i s 1.65. F o r t h e s i x r u n s ( S e r i e s I V a n d S e r i e s V) f o r w h i c h -ΔΗ > 68 c a l gm"' and no f r e e r a d i c a l i n h i b i t o r was i n i t i a l l y p r e s e n t t h e a v e r a g e Β i s 1.57. We t h e r e f o r e c o n e i u d e , w i t h c o n s i d e r a b l e r e s e r v a t i o n , t h a t -dH/dt Ql .6 f l/\ where M i s t h e monomer c o n c e n t r a t i o n . ρ

T

R

x

t x

R

x

χ

T

e

c

n

Q

1


+

+ x

p

o

r

Summary. T h e f o r e g o i n g s e c t i o n s l e a d t o t h e c o n c l u s i o n t h a t f o r t h e p h o t o - a c t i v a t e d T r i g o n a l 14 p o l y m e r i z a t i o n o f l a u r y l a c r y l a t e , t h e r a t e o f p o l y m e r i z a t i o n may be a p p r o x i m a t e l y e x p r e s s e d a s -dM/dt

= Κ, χ (-dH/dt) 0

= K C 2

3 5

|°-

5

3

M

L

6

e*

3

,

6

/

T

(I)

where C i s t h e T r i g o n a l 14 c o n c e n t r a t i o n , I i s t h e i n c i d e n t l i g h t i n t e n s i t y , OQ i s t h e l a u r y l a c r y l a t e monomer c o n c e n t r a t i o n , a n d Τ i s t h e absolute temperature. This expression holds best f o r t h e h i g h e r l i g h t i n t e n s i t i e s , 0.001 £ C < 0.010 r a n g e , a n d M / M Q >_ 0.5 r e g i o n . A l t h o u g h t h e f o r e g o i n g i n f o r m a t i o n was o b t a i n e d o n l a u r y l a c r y l a t e i n i t i a l l y e q u i l i b r a t e d w i t h a i r and i n i t i a t e d by a com p l e x c o m m e r c i a l p h o t o - i n i t i a t o r , i t i s s t i l l p r o f i t a b l e t o compare


UV L I G H T INDUCED REACTIONS I N P O L Y M E R S

102 10.0

-i—ι—ι ι ι ι ι ι

RATE AT PEAK (CAL GM" SEC")


1

1

QJI I I 1 I I I I _l I I I I I I I I #

10

INCIDENT LIGHT INTENSITY (MCAL CM" SEC' ) 2

1

Figure 6. Exotherm rate at peak, —dH/dt (peak), vs. incident light intensity for photoactivated Trigonal 14 initiated lauryl acrylate polymerizations (cf. Table IV). ·, Series VIII; O, Series IX and Series X.

EXOTHERM RATE (CAL GM" SEC"' ) 0.6| 1

Figure 7. Exotherm rates, (—dH/dt) , vs. residual polymerization heats, H (t ), for Series TV lauryl acrylate polymeriza tions. Points correspond to 0.2, 0.3, 0.4, etc., fractional conversions for each run based on its total —ΔΗ . tx

R

Ρ

x

10

20 RESIDUAL


60 80

8.

MOORE

E T AL.

Cahrimetric Analysis of Photopolymerizations


4.0h

20

40

60 80

RESIDUAL HEAT (CAL GM' ) 1

Figure 8. Exotherm rates, (—dU/dt) , vs. residual polymerization heats, H (t ), for Series V lauryl acrylate polymerizations. Points correspond to 0.2, 0.3, 0.4, etc., fractional conversions for each run based on its total — Δ Η . tx

R

x

Ρ


103

104

U V L I G H T INDUCED REACTIONS IN

POLYMERS

TABLE V P o l y m e r i z a t i o n R a t e Dependence on R e s i d u a l P o l y m e r i z a t i o n H e a t Lauryl A c r y l a t e Photopolymerization TR-14 I0 ( 3 (gm gm" h

-Δ H ( c a l grn ')

A

Β

C-13 C-12 C-ll C-10

0.989 0.634 0.352 0.159

73 68 59 48

-4.476 -5.156 -6.182 -8.835

1.34 1.47 1.67 2.20

C-21 C-20 C-19 C-18 C-l7,16 C-15

4.59 2.06 1.13 0.569 0.257 0.140

83 77 74 69 57 39

-5.618 -5.756 -5.463 -7.278 -9.631 -14.12

1.56 1.60 1.52 1.90 2.37 3.29

C-7 C-6 C-5 C-4 C-3 C-2 C-l

4.03 2.01 1.01 0.604 0.405 0.205 0.101

79 74 71 71 57 47 34

-5.789 -6.824 -7.019 -7.146 -9.318 -10.817 -16.729

1.59 1.81 1.83 1.84 2.31 2.57 3.83

2


Run

In R -R

p

= A + Β ΙηΔΗ ( r e s i d u a l ) Ρ = r a t e ( c a l gm"' s e c " 1 )

Ρ A H ( r e s i d u a i ) = 80.0 - (-ΔΗ c u m u l a t i v e ΔH cumulative

1

[ c a l gm" ])

= i n t e g r a t e d heat t o chosen f r a c t i o n o f t o t a l heat

A and Β o b t a i n e d f r o m l e a s t - s q u a r e s l i n e a c c o r d i n g t o a b o v e e q u a t i o n u s i n g 0.2, 0.3, 0.4, and 0.5 f r a c t i o n a l h e a t d a t a p o i n t s . Β = 1.65 a v e r a g e f o r t h e t e n r u n s h a v i n g

- Δ Η >68 c a l gm"'.



8.

MOORE ET A L .


105

t h e c a l o r i m e t r i c s t u d y o f p o l y m e r i z a t i o n r a t e w i t h more c o n v e n t i o n a l d i l a t o m e t r i c s t u d i e s . (9,10) O u r p r e s e n t d a t a a r e i n a g r e e m e n t w i t h t h e o b s e r v a t i o n o f no Trommsdorf e f f e c t i n LA b u l k p o l y m e r i z a t i o n . ( 9 ) O u r d e t e r m i n a t i o n o f a p p r o x i m a t e monomer o r d e r 1.6 f o r t h e v e r y r a p i d p h o t o p o l y m e r i z a t i o n s i s h i g h e r t h a n t h e 1.4 o r d e r f o u n d p r e v i o u s l y f o r b u l k p o l y m e r i z a t i o n and n e a r t h e 1.6 o r d e r f o u n d f o r s o l u t i o n p o l y m e r i z a t i o n . (£) I t s h o u l d be r e c o g n i z e d t h a t t h e r a t e s o f p o l y m e r i z a t i o n i n t h e p r e s e n t work a r e a s much a s 3 0 0 t i m e s t h e r a t e s i n t h e d i l a t o m e t r i c s t u d y . * The e f f e c t o f i n i t i a t o r d e p l e t i o n on t h e computed monomer o r d e r i n t h e present treatment i s t o increase t h e apparent order. T h e 0.35 o r d e r i n T r i g o n a l 14 c o n c e n t r a t i o n c o r r e s p o n d s q u a l i t a t i v e l y w i t h t h e 0.45 o r d e r i n i n i t i a t o r c o n c e n t r a t i o n f o u n d i n t h e d i l a t o m e t r i c work. I t i s i n t e r e s t i n g t h a t a l t h o u g h t h e T r i g o n a l 14 o r d e r i s a p p r o x i m a t e l y 0.35, t h e o r d e r i n l i g h t i n t e n s i t y i s n e a r 0.5. The p r e s e n t p r e l i m i n a r y s t u d y o f r a p i d p h o t o p o l y m e r i z a t i o n r e a c t i o n s by d i f f e r e n t i a l c a l o r i m e t r y shows t h i s method t o have great potential u t i l i t y . Future s t u d i e s i n v o l v i n g simpler r e a c t i o n s y s t e m s and b e t t e r c o n t r o l l e d r e a c t i o n c o n d i t i o n s w i l l d i s p l a y t h e power a n d s o p h i s t i c a t i o n o f t h e d i f f e r e n t i a l c a l o r i m e t r i c t e c h nique i n photochemical research.

Abstract Differential calorimetry has been applied to the study of rapid photopolymerizations. This new technique holds great promise for basic and applied research on photopolymerization and other photochemical reactions. The method requires only a few milligrams of sample, can be used on network-forming systems, and can approx imate actual conditions of thin f i l m and coating technologies. Lauryl acrylate polymerizations i n i t i a t e d by a photo-acti vated mixture of benzoin butyl ethers (Trigonal 14) were performed in Perkin-Elmer model DSC-IB and DSC-2 apparata modified by attach ment of a heat-filtered medium pressure mercury lamp. Within spec i f i e d variable l i m i t s , the rate of polymerization may be approxi mated by the relation R = const. |0.53 0.35[ ]|.6 -316/T. | is light intensity; C i s initiator concentration; [M] i s monomer con centration; Τ i s absolute temperature. Peroxide i n i t i a t e d polymerizations (scan 30°-250°C) of lauryl acrylate (LA), 1,6-hexanediol diacrylate (HDDA), neopentyl glycol diacrylate (NPGDA), and trimethylol propane t r i a c r y l a t e (TMPTA) revealed total polymerization heats per mole of C=C of 19.2, 17.6, 16.7, and 15.4 kcal, respectively. Photoactivated Trigonal 14 i n i t i a t e d polymerizations at 40° yielded total polymerization heats per mole of C=C of 17.4, 11.7, 8.2, and 9.1 kcal for LA, HDDA, NPGDA, and TMPTA, respectively. At 60°C the photopolymerization heats for the latter three monomers increased to 16.6, 9.7, and 10.2 kcal per mole of C=C, respectively. p

C

Μ

e


UV

106


POLYMERS


Literature Cited 1. McGinnis, V.D., Holsworth, R.M. and Dusek, D.M., A.C.S. Coat ings and Plastics Preprints, (1974), Vol. 34, (No. I ) , p. 669 2. Osborn, C.L. and Sander, M.R., i b i d . , p. 660. 3. Schroeter, S.H., Moore, J.E. and Orkin, O.V., i b i d . , p. 751. 4. McGinnis, V.D. and Dusek, D.M., J . Paint Tech., (1974), 46, 23. 5. Faru, R.A., Polymer, (1968) 9, 137. 6. Horie, K., Mita, I. and Kambe, H., J . Poly. S c i . , (1968), Vol. 6, (A-1), 2663. 7. Burrett, K.E.J, and Thomas, H.R., B r i t . Polymer J . , (1970), Vol. 2, p. 45. 8. Willard, P.E., Polym. Eng. and S c i . , (1972), Vol.12, (No. 2), 120. 9. Scott, G.E. and Senogles, E, J . Macromol. Sci.,-Chem., (1970), A4, 1105. 10. Scott, G.E. and Senogles, E., i b i d . , (1974), A8, 753. 11. Joshi, R.M., Makromol. Chem., (1963), 66, 114. 12. Joshi, R.M., J . Polymer S c i . , (1962), 56, 313.


Calorimetric Analysis of Photopolymerizations

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