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Chapter 25
Characterization of Bisbenzocyclobutene High-Temperature Resin and Bisbenzocyclobutene Blended with a Compatible Bismaleimide Resin Lisa R. Denny, Ivan J . Goldfarb, and Michael P.
Fair
1
Air Force Wright Aeronautical Laboratories, Materials Laboratory (AFWAL/MLBP), Wright-Patterson Air Force Base, OH 45433
Cured Bisbenzocyclobutene (BCB) terminated resin systems exhibit good mechanical properties with 70% to 85% retention of properties at 260°C and high thermal stability. The Materials Laboratory has studied these materials for use as high temperature structural matrix resins in composites. They are well suited for this use since they do not require the use of catalysts and cure without the evolution of volatiles. The samples studied were a BCB terminated aromatic imide oligomer and a BCB terminated imide monomer blended with a compatible bismaleimide (BMI) resin. Neat properties studied on both resin systems included the thermal and rheological properties of the uncured specimens subsequently used to determine appropriate cure conditions. Thermal and mechanical properties of the cured materials are also discussed. This laboratory has been involved in the synthesis and characterization o f new h i g h temperature r e s i n systems f o r use as structural matrix materials i n composites at use temperatures exceeding the c a p a b i l i t i e s of c u r r e n t l y a v a i l a b l e r e s i n s . There are a number o f c r i t e r i a w h i c h t h e s e r e s i n s must meet t o be s u i t a b l e f o r t h i s u s e , i n c l u d i n g good m e c h a n i c a l p r o p e r t i e s and t h e r m a l s t a b i l i t y i n the cured specimens as w e l l as l o w m o i s t u r e a b s o r p t i o n . The resin i t s e l f must e x h i b i t a r e a s o n a b l e s h e l f l i f e and r e l a t i v e ease in processing. A s e r i e s o f new b i s b e n z o c y c l o b u t e n e (BCB) t e r m i n a t e d resins currently being studied by t h i s l a b o r a t o r y appear t o be promising for t h i s use. These r e s i n s a r e s t a b l e a t room t e m p e r a t u r e and p o l y m e r i z e upon h e a t i n g w i t h o u t the use of c a t a l y s t s and c u r e via an a d d i t i o n r e a c t i o n w i t h o u t the e v o l u t i o n of v o l a t i l e s . The properties exhibited by t h e cured neat resins are i n d i c a t i v e of those necessary for a composite matrix. 1
C u r r e n t address: Materials Science Department, Pennsylvania State U n i v e r s i t y , U n i v e r s i t y Park, P A 16802 T h i s chapter not subject to U . S . copyright P u b l i s h e d 1987 A m e r i c a n C h e m i c a l Society
25.
D E N N Y E T A L .
Bisbenzocyclobutene
High-Temperature
Resin
367
A v a r i e t y of BCB t e r m i n a t e d r e m i n d have been s y n t h e s i z e d i n this laboratory, a review of which i s also being given i n this symposium . The c h a r a c t e r i z a t i o n o f two o f t h o s e r e s i n systems will be p r e s e n t e d h e r e . The f i r s t r e s i n t o b e d i s c u s s e d i s a BCB terminated i m i d e o l i g o m e r (BCB o l i g o m e r ) w i t h t h e s t r u c t u r e shown below. c
l
X
)
ÇF / 3
ο
ο
ο
ο
ο
ÇF \ 3
ο
ο η =0,1,2.3...
BCB OLIGOMER The synthesis and proposed cure mechanisms of this resin are described i n reference 2. While the cure mechanism o f t h e BCB terminated r e s i n i s n o t y e t known, i t i s speculated that i t reacts via one o f two d i f f e r e n t routes. Initially the s t r a i n e d four member r i n g of the benzocyclobutene undergoes a thermally induced ring opening. The opened r i n g s then r e a c t with one a n o t h e r by a linear type addition to form a network type of structure o r by cycloaddition to form l i n e a r polymer c h a i n s . An i l l u s t r a t i o n of the proposed p o l y m e r i z a t i o n mechanism o f b e n z o c y c l o b u t e n e (BCB) t e r m i n a t e d r e s i n s i s shown b e l o w .
Proposed Benzocyclobutene
(BCB) T e r m i n a t e d R e s i n C u r e M e c h a n i s m
The s t r u c t u r e formed through a l i n e a r a d d i t i o n of the molecules i s shown a t t h e t o p o f t h e i l l u s t r a t i o n . The b e n z o c y c l o b u t e n e g r o u p s on either end o f t h e BCB t e r m i n a t e d resin molecules r e a c t w i t h two o t h e r BCB g r o u p s a s s h o w n t o f o r m a p o l y ( o - x y l y e n e ) strands. With BCB r e a c t i v e g r o u p s p r e s e n t a t b o t h e n d s o f t h e r e s i n m o l e c u l e s , a double s t r a n d o f p o l y ( o - x y l y e n e ) i s formed which i s b r i d g e d by the aromatic imide chains. The second proposed p o l y m e r i z a t i o n m e c h a n i s m shown at the bottom of the i l l u s t r a t i o n i n v o l v e s a c y c l o a d d i t i o n r e a c t i o n . The strained four member b e n z o c y c l o b u t e n e r i n g undergoes a thermally induced r i n g opening as d e s c r i b e d p r e v i o u s l y , t h e opened r i n g t h e n reacts with one o t h e r opened r i n g t o form an e i g h t member r i n g between t h e two m o l e c u l e s . As shown t h i s m e c h a n i s m r e s u l t s i n t h e linear p o l y m e r m o l e c u l e s w i t h no b r a n c h o r n e t w o r k f o r m a t i o n . The BCB terminated resins may a c t u a l l y undergo a c o m b i n a t i o n of both reactions during the cure.
368
CROSS-LINKED POLYMERS
The o t h e r BCB r e s i n s y s t e m t o be d i s c u s s e d i s a b l e n d o f a BCB terminated resin synthesized in this laboratory and a bismaleimide (BMI) resin commercially available from Aldrich Chemical Co. T h e two c o m p o n e n t s w e r e b l e n d e d i n a 1:1 m o l a r m i x t u r e and t h e s t r u c t u r e o f e a c h i s shown b e l o w .
