Application of Reactive Dye Labeling Technique for Cure

Chapter 34

Application of Reactive Dye Labeling Technique for Cure Characterization of Epoxy Networks

Photophysics of Polymers Downloaded from pubs.acs.org by TUFTS UNIV on 10/04/17. For personal use only.

C. S. P. Sung Institute of Materials Science, Department of Chemistry, University of Connecticut, Storrs, CT 06268 Cure characterization of epoxy network polymers by using reactive dye labels is reported. Two types of reactive dyes, p,p'-diaminoazobenzene (DAA) and p, p'-diaminostilbene (DAS) were used to mimic the reactivities of diamine type curing agents, diaminodiphenyl sulfone (DDS) for the former and methylene dianiline (MDA) for the latter. As cure proceeds, bathochromic shifts in uv-vis absorption spectra with subsequent enhancement of fluorescence intensity are observed due to the reaction of the dyes with the epoxies. In the case of the DAA label, the analyses of the spectral shifts (about 60 nm) provide esti mates of the composition of cure products, the amine reactivity ratio and the activation energy for the amine-epoxy reaction. Strong enhancement of fluores cence (about 100 times) was due to the formation of tertiary amino DAA labels. Thus, fluorescence intensity can be used for sensitive cure monitoring, especially at later stages of cure. For the DAS label, bathochromic shifts of about 20 nm are found in uv spectra due to the conversion of the primary amine groups to tertiary amine groups in DAS. Fluorescence spectra show similar bathochromic shifts with enhanced emission (about 3 times), especially after gelation. Comparison of the fluorescence intensity of DAS with the fully substituted DAS (tt-DAS), a probe molecule, as a function of cure of the matrix indicates that the chemical reaction as well as the medium viscosity contribute to the emission enhancement, particularly at later stages of cure. A calibration curve relating emission intens ity with the extent of epoxide reaction by the IR method has been established to estimate cure extent from emission measurement. 0097-6156/87/0358-0463$06.00/0 © 1987 American Chemical Society


464

PHOTOPHYSICS OF POLYMERS

C h a r a c t e r i z a t i o n o f c r o s s - l i n k e d polymers such as epoxy networks i s required i n order t o e s t a b l i s h structure-property r e l a t i o n s . While a number o f p h y s i c o c h e m i c a l t e c h n i q u e s have been employed t o p r o v i d e a b e t t e r c h a r a c t e r i z a t i o n o f cure p r o c e s s , none o f them can c o n t i n u o u s l y monitor t h e composition o f cure r e a c t i o n products beyond g e l a t i o n . T h e r e f o r e , o u r l a b o r a t o r y has been i n v o l v e d i n r e c e n t y e a r s i n d e v e l o p i n g new methods t o t r a c k c u r e r e a c t i o n p r o d u c t s throughout t h e c u r e p r o c e s s . The approach we have t a k e n i s based on l a b e l l i n g w i t h r e a c t i v e dyes t o mimic t h e c u r e agents and t o use t h e p h o t o c h e m i c a l and p h o t o p h y s i c a l b e h a v i o r o f t h e dyes f o r t h e a n a l y s i s o f t h e c u r e p r o c e s s ( l - U ) . Our approach d i f f e r s from t h e u s u a l one, which m o n i t o r s t h e v i s c o s i t y dependent b e h a v i o r o f f l u o r e s c e n c e n t probes (5)· I n c o n t r a s t t o such p r o b e s , o u r dyes r e a c t w i t h t h e epoxide groups and t r a n s f o r m t h e i r p r i m a r y amine groups i n t o secondary and t e r t i a r y amines as c u r e proceeds. The u v - v i s a b s o r p t i o n and f l u o r e s c e n c e s p e c t r a o f t h e dyes show s y s t e m a t i c changes b y such s u b s t i t u t i o n o f t h e p r i m a r y amine groups, which form t h e b a s i s f o r t h e a n a l y s e s o f t h e cure p r o c e s s . Here we d i s c u s s t h e u s e o f two t y p e s o f such r e a c t i v e dyes, p , p - d i aminoazobenzene (DAA) and ρ,p'-diaminostilbene (DAS), which were used t o mimic t h e r e a c t i v i t i e s o f two diamine t y p e c u r i n g a g e n t s , diamino d i p h e n y l s u l f o n e (DDS) and methylene d i a n i l i n e (MDA). These two c u r i n g agents a r e o f t e n used f o r h i g h temperature epoxy m a t r i c e s f o r g r a p h i t e r e i n f o r c e d composites. f

EXPERIMENTAL D i g l y c i d y l e t h e r o f b i s p h e n o l A (DGEBA) was r e c r y s t a l l i z e d from s a t u r a t e d MEK s o l u t i o n b y s e e d i n g i t w i t h p u r i f i e d DGEBA c r y s t a l s and l e a v i n g i t i n t h e f r e e z e r (-15°C) f o r 1-2 weeks. DAA was purchased from Eastman Kodak and r e c r y s t a l l i z e d from t o l u e n e and acetone. DDS and DGEB which were purchased from A l d r i c h were used without p u r i f i c a t i o n . D A S - d i h y d r o c h l o r i d e from A l d r i c h was neu t r a l i z e d w i t h sodium carbonate and r e c r y s t a l l i z e d from methanol t o o b t a i n t h e f r e e amine. I n t y p i c a l cure m o n i t o r i n g s t u d i e s , a s m a l l amount o f DAA (5 ^ Τ mg o r about 0.1% b y weight f o r UV-VIS s t u d i e s and 0.01? b y weight i n most f l u o r e s c e n c e s t u d i e s ) was added t o a s t o i c h i o m e t r i c m i x t u r e o f DGEBA (5.0 g) o r d i g l y c i d y l e t h e r o f b u t a n e d i o l (DGEB 2.98 g ) . DDS (1.825 g) was t h e n added and t h e m i x t u r e was heated w i t h a magnetic s t i r r e r a t 120°C f o r 5 minutes. Two c i r c u l a r q u a r t z p l a t e s were clamped t o g e t h e r w i t h two t h i n M y l a r f i l m s (1.5 m i l ) on t h e edges l e a v i n g a c e n t e r space f o r t h e sample. The clamped q u a r t z p l a t e w i t h M y l a r s p a c e r s were d i p p e d i n t o epoxy heated t o 100°C and t h e sample was drawn i n t o t h e c e n t e r space by c a p i l l a r y a c t i o n . UV-VIS s p e c t r a and f l u o r e s c e n c e s p e c t r a were measured a f t e r c u r i n g i n an oven f o r a s p e c i f i c t i m e and c o o l i n g t h e sample t o room t e m p e r a t u r e . F l u o r e s c e n c e was measured w i t h e x c i t a t i o n a t U50 nm, u s i n g a P e r k i n - E l m e r MPF-66 s p e c t r o m e t e r w i t h a Model 7500 Data S t a t i o n . UV-VIS s p e c t r a were o b t a i n e d w i t h a P e r k i n - E l m e r Diode A r r a y (Model 38U0) System w i t h a Model 7500 Data S t a t i o n . F o r t h e epoxy system o f DGEB and MDA, 0.1% DAS was u s e d , w i t h t h e e x c i t a t i o n a t 372 nm f o r f l u o r e s c e n c e spectra. ( F o r more d e t a i l s on e x p e r i m e n t a l c o n d i t i o n s , r e f e r t o Ref. (3) and (h)).

34.

SUNG

Reactive Dye Labeling Technique

465

RESULTS AND DISCUSSION


L a b e l l i n g b y ρ,ρ'-diaminoazobenzene

(DAA)

Cure Composition from UV-VIS S t u d i e s : F i g . 1 compares UV-VIS s p e c t r a o b t a i n e d as a f u n c t i o n o f c u r e t i m e i n DGEB-DDS and DGEBADDS a t l60°C. I n b o t h s e t s o f s p e c t r a , s i g n i f i c a n t r e d s h i f t s o f DAA d e r i v a t i v e s a r e observed as t h e cure t i m e i n c r e a s e s . The c o n v e r s i o n o f DDS t o a t e r t i a r y amine a l s o r e d - s h i f t s t h e a b s o r p t i o n o f t h e epoxy m a t r i x , r e s u l t i n g i n a s h i f t o f t h e minimum p o i n t o f t h e s p e c t r a from 360nm t o 380nm. I n t h e DGEBA-DDS epoxy, t h e m a t r i x g e l s a f t e r about 50 minutes o f cure and v i t r i f i e s a f t e r 150 minutes o f cure a t t h i s cure temperature a c c o r d i n g t o t h e TimeTemperature-Transformation (Τ-Τ-Τ) diagram (6). A f t e r v i t r i f i c a t i o n , t h e c u r e r e a c t i o n i s supposedly quenched. As a conse quence, t h e UV-VIS s p e c t r a i n t h e DGEBA-DDS m a t r i x does n o t show much change a f t e r v i t r i f i c a t i o n (See F i g . l ( l ) d c o r r e s p o n d i n g t o 300 min. cure t i m e ) . I n c o n t r a s t , no v i t r i f i c a t i o n o c c u r s a t l60°C i n t h e DGEB-DDS epoxy s i n c e i t s maximum T i s o n l y about 80°C. I n g

t h i s epoxy, t h e cure r e a c t i o n has been pushed f u r t h e r as i n d i c a t e d by a d d i t i o n a l r e d s h i f t s shown i n F i g . l ( l l ) d . The d i s a p p e a r a n c e o f t h e epoxy r i n g as m o n i t o r e d b y IR p r o v i d e s f u r t h e r support f o r t h i s observation. I n o r d e r t o i n s u r e t h a t t h e s e s p e c t r a l s h i f t s a r e due t o cure r e a c t i o n s and not t o t h e changes i n t h e m a t r i x ( e . g . p o l a r i t y change as a f u n c t i o n o f c u r e ) , we r a n UV-VIS s p e c t r a o f f u l l y s u b s t i t u t e d DAA ( t h e 5th compound i n T a b l e I ) i n t h e DGEBA-DDS epoxy as a f u n c t i o n o f c u r e a t l60°C. Any s p e c t r a l s h i f t i n t h i s case would be due t o t h e m a t r i x change s i n c e i t cannot r e a c t any f u r t h e r w i t h epoxide and i n f a c t , o n l y a n e g l i g i b l e (ωμ ω ό: ω

3£ Lt_Z UJ

> 500

550

600

650

700


WAVELENGTH (nm)

DGEB

500

550

600

650

W A V E L E N G T H (nm) 1

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 ρ,p -diaminoazobenzene i n a s t o i c h i o m e t r i c m i x t u r e o f DGEBA-DDS ( I ) and DGEB-DDS ( I I ) as a f u n c t i o n o f cure time a t 160°C ( e x c i t a t i o n a t 456 nm). Repro duced from Ref. 4. C o p y r i g h t 1986 American Chemical S o c i e t y . ) r

•

A.

Δ

DGEB 12

DGEBA 10

— Ε c

/ /|40°C I60°C/

I80°C

8

Ο — -

1 4

600

*

*

#

/ */

400 %/ χ

200

Q 0

1

, 2

1 3

L o g (time)

F i g u r e 9. F l u o r e s c e n c e i n t e n s i t y a t 418 nm as a f u n c t i o n o f cure time a t 140°C and 120°C, r e s p e c t i v e l y , i n DAS (0.1%) l a b e l l e d DGEB-MDA epoxy. (Reproduced w i t h p e r m i s s i o n from Ref. 3. Copy r i g h t 1987 B u t t e r w o r t h & Co. [ P u b l i s h e r s ] L t d . )

PHOTOPHYSICS OF POLYMERS

478

800

1 14ÛC OAS 2 140C

tt-DAS

600


400

200

L o g (time)

F i g u r e 10. F l u o r e s c e n c e i n t e n s i t y a t 418 nm as a f u n c t i o n o f cure time a t 140°C, comparing DAS and tt-DAS i n DGEB-MDA epoxy. (Reproduced w i t h p e r m i s s i o n from Ref. 3. C o p y r i g h t 1987 B u t t e r w o r t h & Co. [ P u b l i s h e r s ] L t d . ) 400.

200

EXTENT OF REACTION F i g u r e 11. C o r r e l a t i o n o f f l u o r e s c e n c e i n t e n s i t y a t 418 nm w i t h the e x t e n t o f epoxy r e a c t i o n (E,-^)by IR method a t 140°C c u r e . (Reproduced w i t h p e r m i s s i o n from Ref. 3. C o p y r i g h t 1987 B u t t e r w o r t h & Co. [ P u b l i s h e r s ] L t d . )

34. S U N G

Reactive Dye Labeling Technique

e s p e c i a l l y i n c u r e d epoxy, can be e x p l a i n e d b y a h i g h e r b a r r i e r t o t h e r o t a t i o n around t h e CH = CH bond. ( 3 )

479 activation

Acknowledgment The support o f t h i s r e s e a r c h b y t h e Army R e s e a r c h O f f i c e ( C o n t r a c t No. DAAG 29-85-K-0055) and t h e N a t i o n a l S c i e n c e F o u n d a t i o n , P o l y mers Program (Grant No. DMR 82-05897) i s acknowledged. W i t h g r e a t p l e a s u r e , I acknowledge s e v e r a l c a p a b l e c o l l a b o r a t o r s (Dr. I . J . C h i n , Ms. E. Pyun, Mr. R. M a t h i s e n , Dr. H. L. Sun.)


References 1. Chin, I. J.; Sung, C. S. P. Macromolecules, 1984, 17, 2603. 2. Sung, C. S. P.; Chin, I.-J.; Yu, W. C. Macromolecules, 1985, 18, 1510. 3. Sung, C. S. P.; Mathisen, R. Polymer, 1987, in press. 4. Sung, C. S. P.; Pyun, E.; Sun, H. L., Macromolecules, 1986 19, 2922. 5. Wang, F. W.; Lowry, R. E.; Fanconi, Β. M. ACS Polym. Mater. Sci. & Eng. Proceedings, 1985, 53-2, 180. 6. Enns, J. B. and Gillham, J. Κ., Polymer Characterization Ed. by Craver, C. D., Advances in Chemistry Series #203, ACS, 1983. 7. Byrne, C. Α.; Hagnauer, G. L. and Schneider, N. S., Polym. Comp., 1983,4,206. 8. Dusek, K.; Ilavsky, M. and Lunak, S., J. Polym. Sci., Symp. No. 53, 1975, 29. 9. Bidstrup, S. A. and Macosko, C. W., Proceedings of ANTEC '84, SPE 1984, 278. 10. Lunak, S. and Dusek, K., J. Polym. Sci., Symp. No. 53, 1975, 45. 11. Miller, D. R. and Macosko, C. W., Macromolecules, 1980, 13, 1063. 12. See references cited in Table 1 of Charlesworth J., J. Polym. Sci., Polym. Chem., 1980, 18, 621. 13. Zukas, W. X.; Schneider, N. S. and MacKnight, W. J., Polym. Materials Sci. & Eng. Preprint, 1983, 49-2, 588. 14. Bell, J. P., J. Polym. Sci., A-2, 1970, 8, 417. 15. Morgan, R. J.; Happe, J. A. and Mones, E. T., Proc. 28th SAMPE Symp., April 12-14, 1983, 596. 16. Prime, R. Β., Thermal Characterization of Polymeric Materials, Ed. Turi, Ε. Α., Academic Press: 1981, Fig. 29, p. 479. 17. We can calculate k by expressing [b] in terms of [pp], followed by integration. However, i t does not give as r e l i able values as based on [ξ ]. l

a

18. Riccardi, C. C.; Adabbo, Η. E. and Williams, R. J. J., J. Appl. Polym. Sci., 1984, 29, 2481. 19. For a review, Lakowicz, J. R., Principles of Fluorescence Spectroscopy, Plenum: NY, 1983; Chap. 7. RECEIVED September 10, 1987

Application of Reactive Dye Labeling Technique for Cure

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