Fluorescence Monitoring of Viscosity and Chemical Changes During

Chapter 33

Fluorescence Monitoring of Viscosity and Chemical Changes During Polymerization 1

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F. W. Wang , R. E. Lowry , W . J . Pummer , B. M . Fanconi , and En-Shinn Wu Downloaded by UNIV OF MINNESOTA on May 3, 2013 | http://pubs.acs.org Publication Date: November 30, 1987 | doi: 10.1021/bk-1987-0358.ch033

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Polymers Division, National Bureau of Standards, Gaithersburg, MD 20899 Department of Physics, University of Maryland, Baltimore County, Catonsville, MD 21228

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Three approaches using fluorescent dyes dissolved in epoxy resins were used to determine the viscosity changes during the curing process. First, the intensity of excimer fluorescence from a dye which forms an intramolecular excimer was measured to determine the viscosity changes. In another approach, we used a dye whose fluorescence intensity increases with the increase in the local viscosity, and a second dye whose fluorescence intensity is insensitive to the local viscosity. The ratio of the fluorescence intensities of the two dyes was measured to monitor the cure of epoxy resins. In a third approach, we measured the diffusion coefficient of a fluorescent dye by a photobleaching technique to monitor the curing process. Finally, we used a fluorescence technique to monitor the formation of a polyimide polymer from poly(amide acid). The manufacture of polymer matrix composites involves complex chemical and physical changes that must be adequately controlled to produce desirable products. Monitoring techniques and models to correlate monitoring data to improve processing are therefore key aspects to increasing production rates and product quality. Fluorescence techniques are particularly useful to monitor the change in local viscosity because they are sensitive and can be easily adopted to in-situ, nondestructive monitoring. In a previous paper(1), we described an excimer-fluorescence technique to monitor the polymerization of methyl methacrylate. We show here an application of the excimer-fluorescence technique to monitor the cure of epoxy resins. In addition, we describe the cure monitoring of epoxy resins with the use of two fluorescent dyes, a dye whose fluorescence intensity increases with local viscosity, and another dye which serves as an internal standard with nearly constant fluorescence intensity. This second technique is similar to the ones used by Loutfy(2,3) and by Levy(4). However, to the best of 0097-6156/87/0358-0454$06.00/0 © 1987 American Chemical Society

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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our knowledge, t h e r e has been no p r e v i o u s r e p o r t of the a p p l i c a t i o n of a v i s c o s i t y i n s e n s i t i v e i n t e r n a l s t a n d a r d . S i n c e the use of an i n t e r n a l s t a n d a r d e l i m i n a t e s the d i f f i c u l t i e s i n v o l v e d i n making a b s o l u t e measurements, i t i s a s i g n i f i c a n t s t e p f o r w a r d i n the a p p l i c a t i o n of f l u o r e s c e n c e s p e c t r o s c o p y t o cure m o n i t o r i n g i n the f a c t o r y environment. Furthermore, we r e p o r t a t h i r d t e c h n i q u e t h a t u t i l i z e s the measurement of the d i f f u s i o n c o e f f i c i e n t of a p h o t o b l e a c h a b l e probe t o monitor the cure of an epoxy r e s i n . F i n a l l y , we d e s c r i b e a f l u o r e s c e n c e method t o monitor the f o r m a t i o n of a p o l y i m i d e polymer.

Downloaded by UNIV OF MINNESOTA on May 3, 2013 | http://pubs.acs.org Publication Date: November 30, 1987 | doi: 10.1021/bk-1987-0358.ch033

EXPERIMENTA L MONOMERS. Amine hardener, 4 , 4 ' - m e t h y l e n e - b i s - ( c y c l o h e x y l a m i n e ) (PACM), was d i s t i l l e d under reduced p r e s s u r e and s t o r e d under dry argon. I t was melted under dry argon b e f o r e v.se. Epoxy r e s i n , d i g l y c i d y l e t h e r of b i s p h e n c l A (DGEBA), had an epoxy e q u i v a l e n t weight of a p p r o x i m a t e l y 175 and was used w i t h o u t f u r t h e r purification. SYNTHESIS 0F_ POLYIMIDE. The p o l y i m i d e was prepared from 2,2b i s ( 3 , 4 - d i c a r b o x y p h e n y l ) h e x a f l u o r o p r o p a n e d i a n h y d r i d e (American Hoechst 6 F ) and 2 , 2 - b i s [ 4 ( 4 - a m i n o p h e n o x y ) p h e n y l ] h e x a f l u o r o p r o p a n e (Morton T h i o k o l 4BDAF). Both 6F and 4BDAF are s o l u b l e i n dry glyme at 25°C. 4BDAF (0.5g, 9.6xlC" mol) was d i s s o l v e d i n 3 ml of d r y glyme at 25°C w i t h s t i r r i n g i n a 25 ml g l a s s - s t o p p e r e d f l a s k . When d i s s o l u t i o n was completed ( u s u a l l y w i t h i n 3 m i n u t e s ) , 0.42g ( 9 . 6 x l 0 " mol) of s o l i d 6F i n s m a l l p o r t i o n s {O.lg each) was added t o the s o l u t i o n of 4BDAF at 25°C. W i t h i n 5 minutes a f t e r the 6F was added, s t i r r i n g was impeded by the i n c r e a s e d s o l u t i o n v i s c o s i t y . A f t e r .15 minutes of manually s w i r l i n g the c o n t e n t s , the s o l u t i o n v i s c o s i t y decreased s u f f i c i e n t l y t o a l l o w normal s t i r r i n g t o proceed. The r e a c t i o n was stopped a f t e r 44 hours at 25°C. T h i s s o l u t i o n (26% s o l i d s ) of the p o l y ( a m i d e a c i d ) i n glyme was used t o prepare f i l m s f o r f l u o r e s c e n c e s p e c t r o s c o p y as d e s c r i b e d below. A few drops of the p o l y ( a m i d e a c i d ) s o l u t i o n were spread on a c l e a n q u a r t z s l i d e by drawing a wedge of the s o l u t i o n beneath another c l e a n s l i d e . Room temperature s o l v e n t e v a p o r a t i o n and a l l heat t r e a t m e n t s were c a r r i e d out i n a i r . The f i l m was cured f o r 0.5 hour at each of seven temperatures r a n g i n g from 80° t o 350°C, w i t h oven warmup and c o o l i n g down times of up t o 0.5 hour each. F r o n t s u r f a c e f l u o r e s c e n c e from the same f i l m r e g i o n was measured at room temperature a f t e r each heat t r e a t m e n t . F i l m t h i c k n e s s averaged 13pm. 1

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C e r t a i n commercial m a t e r i a l s and equipment are i d e n t i f i e d i n t h i s paper t o s p e c i f y a d e q u a t e l y the e x p e r i m e n t a l p r o c e d u r e . In no case does such i d e n t i f i c a t i o n i m p l y recommendation or endorsement by the N a t i o n a l Bureau of S t a n d a r d s , nor does i t i m p l y n e c e s s a r i l y the best a v a i l a b l e f o r the purpose.


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FiyP.?ESCENT._^ An e x c i m e r - f o r m i n g probe, l , 3 - b i s - ( l pyrene)propane (8PP), was o b t a i n e d from a commercial source and used w i t h o u t f u r t h e r p u r i f i c a t i o n . A v i s c o s i t y - s e n s i t i v e probe, 1( 4 - d i m e t h y l a m i n o p h e n y l ) - 6 - p h e n y l - l , 3, 5-hexatric?ne (DKA-DPH) , was d i s s o l v e d i n DGEBA by r o t a t i n g the r e s i n at 45°C f o r s e v e r a l h o u r s . An i n t e r n a l s t a n d a r d , 9 , 1 0 - d i p h e n y l a n t h r a c e n e (DPA), at a c o n c e n t r a t i o n of 2 x l 0 ~ mol/1 was added t o the r e s i n - h a r d e n e r mixture.


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?MÇTION_.,ÇEL!L. The r e a c t i o n c e l l was made of a s i l i c o n e rubber gasket sandwiched between two pyrex cover s l i d e s . The c e l l was pressed a g a i n s t t h e f l a t f a c e of a c y l i n d r i c a l h e a t e r , which was p r o p o r t i o n a l l y c o n t r o l l e d t o w i t h i n 1°C. Two s m a l l h o l e s were bored near t h e top of the rubber gasket f o r i n s e r t i n g a thermocouple and f o r f i l l i n g the c e l l . c

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y. :S_>ÎONITORING. I n a t y p i c a l experiment, 0.5 ml of the epoxy r e s i n c o n t a i n i n g a probe at a c o n c e n t r a t i o n of 5 x l 0 mol/1 was put i n t o a stoppered s y r i n g e kept at 40°C. The amine hardener was then r a p i d l y mixed w i t h the epoxy r e s i n t o form a m i x t u r e c o n t a i n i n g the same e q u i v a l e n t s of the hardener and the r e s i n . F i n a l l y , the r e s i n m i x t u r e was i n j e c t e d i n t o the r e a c t i o n c e l l which had been heated to 60°C. Cure m o n i t o r i n g w i t h the probe BPP was c a r r i e d out i n t h e manner p r e v i o u s l y d e s c r i b e d i n Reference 1. The sample was i r r a d i a t e d a t 345 nm and the monomer and excimer f l u o r e s c e n c e i n t e n s i t i e s at 377 nm and 488 nm were measured as a f u n c t i o n of time. When DMA-DPH was used as a probe and DPA was used as an i n t e r n a l s t a n d a r d , they were e x c i t e d at 420 nm and 345 nm, respectively. - 5

FLUORESCENT The d i f f u s i o n c o e f f i c i e n t of t h e photobleachabïe probe, 1 , 1 - d i h e x y l - 3 , 3 , 3 ' , 3 ' t e t r a m e t h y l i n d o c a r b o c y a n i n e p e r c h l o r a t e [ D i I C e ( 3 ) ] , was determined by t h e FRAP method(5,6,7) d e s c r i b e d below. The c o n c e n t r a t i o n of the probe i n the m i x t u r e c o n t a i n i n g t h e same e q u i v a l e n t s of t h e epoxy r e s i n and t h e amine hardener was 2 x l 0 ~ mol/1. The r e s i n m i x t u r e , sandwiched between a microscope s l i d e and a cover s l i p , was p l a c e d on a stage t h e r m o s t a t i c a l l y c o n t r o l l e d t o w i t h i n 0.5°C. The t h i c k n e s s of t h e r e s i n m i x t u r e was about 5pm. I n a FRAP e x p e r i m e n t ( 6 ) , a s m a l l a r e a of the sample i s i l l u m i n a t e d w i t h a weak beam of e x c i t i n g l i g h t ( m o n i t o r i n g beam). The f l u o r e s c e n c e from t h i s area i s r e c o r d e d as F. At a p r e d e t e r m i n e d time, t = 0, the sample i s momentarily i l l u m i n a t e d w i t h a s t r o n g l a s e r beam ( b l e a c h i n g beam) t o cause an i r r e v e r s i b l e b l e a c h i n g of f l u o r o p h o r e . F o l l o w i n g the b l e a c h i n g , the f l u o r e s c e n c e i s again m o n i t o r e d by t h e m o n i t o r i n g beam. The f l u o r e s c e n c e , F ( t ) , i s i n i t i a l l y v e r y weak, but g r a d u a l l y i n c r e a s e s as the f r e s h f l u o r e s c e n t m o l e c u l e s d i f f u s e i n t o t h e bleached a r e a , and e v e n t u a l l y r e c o v e r s t o i t s o r i g i n a l i n t e n s i t y . From t h e r a t e of t h e r e c o v e r y of f l u o r e s c e n c e i n t e n s i t y , t h e d i f f u s i o n c o e f f i c i e n t can be determined. 1

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RESULTS AND DISCUSSION F i g u r e 1 g i v e s t h e change i n t h e i n t e n s i t y r a t i o FM/FD as a f u n c t i o n of cure time f o r 1 , 3 - b i s - ( 1 - p y r e n e ) p r o p a n e (BPP) d i s s o l v e d i n a m i x t u r e c o n t a i n i n g t h e same e q u i v a l e n t s of t h e epoxy r e s i n and the amine h a r d e n e r . Here, FM and Fn a r e , r e s p e c t i v e l y , t h e f l u o r e s c e n c e i n t e n s i t y of t h e monomer a t 377 nm and t h a t o f t h e excimer a t 488 nm. As c r o s s - l i n k i n g proceeded, t h e v i s c o s i t y i n c r e a s e d owing t o t h e growth i n m o l e c u l a r weight and t h i s l e d t o an i n c r e a s e i n t h e i n t e n s i t y r a t i o . A f t e r 45 minutes of t h e c u r e , t h e r e was a s m a l l decrease i n t h e i n t e n s i t y r a t i o due t o p h o t o d e g r a d a t i o n of t h e probe. The c o m p l i c a t i o n due t o p h o t o d e g r a d a t i o n can be e l i m i n a t e d by r e d u c i n g t h e exposure o f t h e r e s i n t o t h e e x c i t i n g UV r a d i a t i o n . The use o f an o p t i c a l m u l t i c h a n n e l a n a l y z e r i s one approach t o reduce exposure t i m e s . Owing t o t h e l a c k of s e n s i t i v i t y o f t h e BPP probe a t t h e l o n g e r cure t i m e s , we have examined t h e use of o t h e r types of v i s c o s i t y - s e n s i t i v e probe m o l e c u l e s . Figure 2 gives the e x c i t a t i o n and e m i s s i o n s p e c t r a of t h e v i s c o s i t y s e n s i t i v e dye DMA-DPH i n d i g l y c i d y l e t h e r of b i s p h e n o l A (DGEBA). The e x c i t a t i o n and t h e e m i s s i o n s p e c t r a of t h e i n t e r n a l s t a n d a r d DPA a r e s i m i l a r t o t h e ones p u b l i s h e d by Berlman(8). F i g u r e 3 g i v e s t h e f l u o r e s c e n c e i n t e n s i t y , Fp, o f DMA-DPH a t 480 nm (the upper curve) and t h e f l u o r e s c e n c e i n t e n s i t y , F R , o f DPA at 415 nm ( t h e lower curve) as a f u n c t i o n of cure time f o r a m i x t u r e c o n t a i n i n g t h e same e q u i v a l e n t s of t h e epoxy r e s i n and t h e amine h a r d e n e r . The f l u o r e s c e n c e i n t e n s i t y a t t h e frequency o f t h e DMA-DPH e m i s s i o n i n c r e a s e d f i v e f o l d w h i l e t h a t a t t h e DPA peak emission increased only s l i g h t l y . (The apparent i n c r e a s e i n t h e DPA f l u o r e s c e n c e was m a i n l y due t o t h e i n c r e a s e i n t h e f l u o r e s c e n c e of i m p u r i t i e s i n t h e epoxy r e s i n . At t h e e x c i t a t i o n wavelength o f 345 nm, t h e f l u o r e s c e n c e i n t e n s i t y of t h e r e s i n m i x t u r e alone a t 415nm i n c r e a s e d by about 80%. However, t h i s i n c r e a s e c o n t r i b u t e d to t h e much s m a l l e r apparent i n c r e a s e i n t h e DPA f l u o r e s c e n c e because a t t h e b e g i n n i n g o f t h e c u r e , t h e f l u o r e s c e n c e i n t e n s i t y o f DPA was f o u r times l a r g e r than t h a t of t h e r e s i n alone.) I n F i g u r e 4, we show t h e i n t e n s i t y r a t i o F P / F R as a f u n c t i o n o f cure t i m e . The i n t e n s i t y r a t i o i n c r e a s e d s t e a d i l y w i t h cure t i m e , r e a c h i n g a p l a t e a u v a l u e a t 70 minutes. T h i s i n t e n s i t y r a t i o i s not s e n s i t i v e to t h e inhomogeneity or t h e d e f o r m a t i o n of t h e sample. We can t h e r e f o r e use t h i s r a t i o t o monitor t h e cure of samples which shrink during polymerization or contain r e i n f o r c i n g f i b e r s or particles. F i g u r e 5 shows t h e l o g a r i t h m o f t h e t r a n s l a t i o n a l d i f f u s i o n c o e f f i c i e n t o f t h e probe D i l C e ( 3 ) as a f u n c t i o n of cure time when the r e s i n was cured at 45°C, 60°C, and 75°C. I n a l l c a s e s , t h e t r a n s l a t i o n a l d i f f u s i o n c o e f f i c i e n t increased s l i g h t l y at the b e g i n n i n g of t h e cure when t h e sample temperature was r a i s e d from 22°C t o t h e cure temperature because of t h e decrease i n t h e r e s i n v i s c o s i t y w i t h t h e i n c r e a s e i n t e m p e r a t u r e . As c r o s s - l i n k i n g proceeded, t h e v i s c o s i t y i n c r e a s e d owing t o t h e growth i n m o l e c u l a r w e i g h t , and t h i s was r e f l e c t e d by t h e decrease i n t h e t r a n s l a t i o n a l diffusion coefficient.


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Cure Time, Minutes

F i g u r e 1. R a t i o of monomer f l u o r e s e n c e i n t e n s i t y a t 377nm ( F M ) to excimer f l u o r e s c e n c e i n t e n s i t y at. 488nm (Fo) f o r 1 , 3 - b i s (1-pyrene)propane i n epoxy as a f u n c t i o n of cure time. The e x c i t a t i o n wavelength was 345 nm.

350

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F i g u r e 2. E x c i t a t i o n and e m i s s i o n s p e c t r a of l - ( 4 dimethylaminophenyl)-6-phenyl-l,3,5-hexatriene(DMA-DPH) i n d i g l y c i d y l e t h e r of b i s p h e n o l A (DGEBA). The e x c i t a t i o n and e m i s s i o n wavelengths were 405 nm and 498 nm, r e s p e c t i v e l y .


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WANG ET AL.

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Fluorescence Monitoring of Viscosity and Chemical Changes During

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