Characterization of Highly Cross-linked Polymers - American

9 Volume Recovery in Aerospace Epoxy Resins

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Effects on Time-Dependent Properties of Carbon Fiber-Reinforced Epoxy Composites ERIC S. W. KONG Department of Materials Science and Engineering, Joint Institute for Surface and Microstructure Research, Stanford University/ΝASA-Ames Research Center, Stanford, CA 94305

Matrix-dominated physical and mechanical properties of a carbon-fiber-reinforced epoxy composite and a neat epoxy resin have been found to be affected by sub-Tg annealing in an inert dark atmosphere. Postcured speci mens of Thornel 300 carbon-fiber/Fiberite 934 epoxy as well as Fiberite 934 epoxy resin were quenched from above T and annealed at 140°C, 110°C or 80°C, for times up to 10 minutes. No weight loss was observed during annealing at these temperatures. Significant variations were found in density, modulus, hardness, damping, mois ture absorption ability, and thermal expansivity. Moisture-epoxy interactions were also studied. The kinetics of aging as well as the molecular aggregation during this densification process were monitored by differential scanning calorimetry, dynamic mechanical analysis, tensile testing, and solid state nuclear mag netic resonance spectroscopy. g

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Time-dependent v a r i a t i o n s have been observed i n mechanical and p h y s i c a l p r o p e r t i e s of polymeric network epoxies and a l s o carbonf i b e r - r e i n f o r c e d composites w i t h epoxy-matrices. These property v a r i a t i o n s are the r e s u l t of d i f f e r e n c e s i n specimen p r e p a r a t i o n c o n d i t i o n s and/or thermal h i s t o r i e s which the m a t e r i a l s have experienced (1-7)· In g e n e r a l , w i t h slower c o o l i n g r a t e s from above the g l a s s temperature, T , and/or i n c r e a s i n g the sub-T annealing time, the d e n s i t y i n c r e a s e s , w h i l e impact s t r e n g t h ( 8 ) , f r a c t u r e energy ( 8 ) , u l t i m a t e e l o n g a t i o n ( 9 ) , mechanical damping (5), creep r a t e s (10), and s t r e s s - r e l a x a t i o n r a t e s (5) decrease. Of p a r t i c u l a r concern i n the p r o c e s s i n g and wide-ranging a p p l i c a t i o n of s t r u c t u r a l epoxies i s the l o s s of d u c t i l i t y of Such m a t e r i a l s on sub-T annealing, i . e . , thermal aging a t temper atures below the g l a s s t r a n s i t i o n of epoxy r e s i n . T h i s sub-T annealing process, more commonly known as " p h y s i c a l aging" (10), i s confirmed to be thermoreversible ( 5 ) . That i s , w i t h a b r i e f g

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0097-6156/84/0243-0125S10.75/0 © 1984 American Chemical Society Labana and Dickie; Characterization of Highly Cross-linked Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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anneal a t temperatures i n excess of the r e s i n T , the thermal h i s t o r y of an aged epoxy can be erased. A subsequent quench from above T would render a "rejuvenated" epoxy. In other words, the polymer e m b r i t t l e s during sub-T annealing. But w i t h an aging h i s t o r y erasure above T , the d u c t i l e behavior can be r e s t o r e d (5,10). I f epoxies a r e to be strong candidates as s t r u c t u r a l m a t r i c e s f o r composite m a t e r i a l s , i t i s of primary importance that an under standing of the nature of t h i s volume recovery process as w e l l as an assessment of the magnitude of i t s e f f e c t s be achieved. To date, there i s a g e n e r a l consensus that the property changes i n g l a s s y polymers on sub-T annealing are the r e s u l t of r e l a x a t i o n phenomena a s s o c i a t e d w i t h the non-equilibrium nature of the g l a s s y s t a t e (11,12) . However, a b a s i c understanding of the changes a t the molecular l e v e l i s s t i l l l a c k i n g . F o r t u n a t e l y , s u b s t a n t i a l progress has been made i n the past few years i n c h a r a c t e r i z i n g the g l a s s y s t a t e from the molecular p o i n t of view by powerful t e c h niques such as procon-decoupled c r o s s - p o l a r i z e d magic-anglespinning (CP/MAS) nuclear magnetic resonance (NMR) spectroscopy (13). Combining such techniques w i t h other conventional i n s t r u mental t o o l s , which measure excess thermodynamic p r o p e r t i e s , i t i s now p o s s i b l e to a s c e r t a i n the changes i n p r o p e r t i e s that can be a t t r i b u t e d to r e l a x a t i o n s of excess thermodynamic s t a t e f u n c t i o n s such as enthalpy and volume. T h i s paper addresses the p e r t i n e n t r e l a t i o n s between excess thermodynamic p r o p e r t i e s and the timedependent behavior of epoxy g l a s s e s . A l s o , an attempt i s made t o d e s c r i b e the molecular nature of t h i s r e l a x a t i o n process. Moisture i s a well-known p l a s t i c i z e r f o r macromolecules (14). S p e c i f i c a l l y , water penetrates i n t o an epoxy network and can lower the g l a s s temperature of the r e s i n (15). In t h i s r e p o r t , moisture has f o r the f i r s t time been u t i l i z e d as a probe to c h a r a c t e r i z e d e n s i f i c a t i o n process during epoxy aging. A l s o , using the same r a t i o n a l e , heavy water d i f f u s e d i n t o the epoxy r e s i n i s used to study the i n t e r a c t i o n s of moisture w i t h the aging polymer by hydrogen-2 (deuterium) NMR spectroscopy. g

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Experimental The epoxy used i n t h i s study was F i b e r i t e 934 r e s i n s u p p l i e d by F i b e r i t e Corporation, Winona, Minnesota, U.S.A. The chemical f o r mulation of t h i s r e s i n i s shown i n F i g u r e 1. The chemical c o n s t i tuents are 63.2% by weight of t e t r a g l y c i d y l - 4,4 -diaminodiphenyl methane (TGDDM t e t r a f u n c t i o n a l epoxy), 11.2% of d i g l y c i d y l orthop h t h a l a t e (DGOP d i f u n c t i o n a l epoxy), 25.3% of the c r o s s l i n k i n g agent 4,4'-diaminodiphenyl s u l f o n e (DDS c r o s s i i n k e r ) , and 0.4% of the boron t r i f l u o r i d e / e t h y l a m i n e c a t a l y s t complex (16,17). The neat epoxy r e s i n was prepared by c a s t i n g . The asr e c e i v e d B-stage m a t e r i a l was subjected to d e g a s i f i c a t i o n a t 85 C i n s i d e a vacuum oven. The softened r e s i n was then t r a n s f e r r e d i n to a preheated s i l i c o n - r u b b e r mold. The c u r i n g schedule was 121°C ,

Labana and Dickie; Characterization of Highly Cross-linked Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


9.

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Volume Recovery in Aerospace Epoxy Resins

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f o r 2 hours, 177°C f o r 2.5 hours, followed by a slow c o o l i n g a t ca. 0.5 C per minute t o room temperature (23 C) . Thornel 300 c a r b o n - f i b e r - r e i n f o r c e d F i b e r i t e 934 epoxy lamin ates ( c a . 60% f i b e r and 40% r e s i n by volume) were f a b r i c a t e d from prepreg tapes manufactured by F i b e r i t e C o r p o r a t i o n . The d e t a i l s of t h i s f a b r i c a t i o n process have been d i s c l o s e d elsewhere 04,j>). With the exception of f i v e specimens (which were t o be tested i n the a s - f a b r i c a t e d c o n d i t i o n ) , a l l specimens were postcured f o r 16 hours a t 250 C, followed by a slow c o o l i n g t o room temperature at a r a t e of 0.5°C per minute. T e s t i n g was then performed on the f i v e as-postcured specimens. The other postcured specimens were heated t o 260°C f o r 20 minutes and then immediately air-quenched to room temperature. F i v e of these quenched specimens were immed i a t e l y t e s t e d , others were sub-Tg annealed i n darkness a t e i t h e r 80. 110 or 140°C ( i n nitrogen) f o r time increments of 10, 1 0 , 1 0 10^ and up to 105 min. The specimens were aged i n darkness i n order t o avoid any chemical aging due t o uv i r r a d i a t i o n . Time zero was taken as the time when a mercury thermometer placed a d j a cent t o the specimens reached the sub-'L, annealing temperature. At each decade of aging time, f i v e spécimens were removed from the environmental chamber and stored at room temperature p r i o r t o testing. In order t o demonstrate the " t h e r m o r e v e r s i b i l i t y " of p h y s i c a l aging, the f o l l o w i n g requenching procedure was c a r r i e d out. Spe c i f i c a l l y , some 10^ min.-aged specimens were heated t o above T f o r 20 minutes (260°C), followed by a i r quenching t o room tempera t u r e . F i v e of these requenched specimens were tested immediately, w h i l e the r e s t were subjected t o " r e a g i n g " i n darkness a t e i t h e r 80. 110 or 140°C i n n i t r o g e n f o r time increments of 10, 1 0 , 1 0 , 10^ and up t o 10^ minutes. At l e a s t f i v e specimens were tested f o r each decade of aging time. Time-dependent s t r e s s - s t r a i n behavior of the neat r e s i n s was s t u d i e d u s i n g an I n s t r o n 1122 t e n s i l e t e s t e r . Dog-bone-shaped epoxy specimens were prepared i n accordance t o ASTM: D1708-66. S t r a i n r a t e used was 5 χ 10"^ sec"^-. Dynamic mechanical a n a l y s i s was performed on 8-ply Thornel 3 0 0 / F i b e r i t e 934 composites that were symmetrically r e i n f o r c e d i n c o n f i g u r a t i o n of ( ± 4 5 ° ) . A dynamic mechanical thermal a n a l y z e r i n t e r f a c e d w i t h a Hewlett Packard 85 computer was k i n d l y s u p p l i e d by P r o f e s s o r R. E. Wetton of Polymer L a b o r a t o r i e s , Loughorough U n i v e r s i t y , Loughborough, United Kingdom. This instrument u t i l i z e d a s i n u s o i d a l bending mode of mechanical deformation on a double c a n t i l e v e r beam (18). Both mechanical d i s p e r s i o n s and dynamic storage modulus were measured i n n i t r o g e n from -100 C t o 300°C a t 1 Hz and 5°C per minute heating r a t e . D i f f e r e n t i a l scanning c a l o r i m e t r y was used to measure both the extent of cure as w e l l as the progress of enthalpy recovery i n the neat epoxy r e s i n . A P e r k i n Elmer DSC-2 d i f f e r e n t i a l scanning calorimeter equipped w i t h a scanning-auto-zero u n i t f o r b a s e l i n e o p t i m i z a t i o n was u t i l i z e d t o measure the heat c a p a c i t y of the 2

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polymeric network g l a s s e s . Each d i s c - l i k e , 0.8mm t h i c k specimen of diameter 5mm was measured from 50 t o 280°C i n n i t r o g e n a t a heating r a t e of 10°C/min. Each specimen was scanned two times (160°C c o o l i n g r a t e from 280 t o 50°C a f t e r the f i r s t scan). The enthalpy recovery measurements were made by superimposing the f i r s t and the second scans f o r each specimen using a d a t a - a n a l y s i s method suggested by M. G. Wyzgoski (19). D e n s i t y measurements were made a t 23°C on s p h e r i c a l neat r e s i n s of 5mm diameter using the f l o t a t i o n method i n accordance to ASTM: D-1505. The d e n s i t y g r a d i e n t column (model DC1) was s u p p l i e d by Techne Incorporated, P r i n c e t o n , New Jersey. Calcium n i t r a t e s o l u t i o n column was s e t up which could measure d e n s i t y that ranges from 1.210 t o 1.290. Hardness measurements were made on 50θΧ go Id-dec orated epoxy square p l a t e s (2.5cm. by 2.5cm., 2mm t h i c k ) using a L e i t z miniload micro-hardness t e s t e r , supplied by E r n s t L e i t z Company, Midland, Ontario, Canada. A load of 200gm was a p p l i e d t o the specimen. Experiments were done i n accordance t o ASTM D-785 and ASTM D-1706 t e s t procedures. Thermal mechanical a n a l y s i s was performed on 2.5mm t h i c k neat epoxy d i s c s of 6mm diameter using a P e r k i n Elmer TMS-2 a n a l y z e r . The expansion mode was u t i l i z e d i n order t o study the thermal expansion behavior of the network epoxies. Each specimen was measured from 50°C t o 260°C a t 5°C per minute h e a t i n g r a t e i n helium atmosphere. S i m i l a r t o the DSC experiment described ear l i e r , each specimen was scanned twice from 50°C t o 260 C. A f t e r the f i r s t scan, a c o o l i n g r a t e of 160°C per minute was u t i l i z e d to quench the system from 260°C t o 50°C. The f i r s t and second scans were then superimposed a t the high-temperature "rubbery" domain i n order t o measure the volume recovery during sub-Tg annealing. Thermal e x p a n s i v i t y was measured a t the l i n e a r expan s i o n regions below and above the epoxy g l a s s temperature. Moisture s o r p t i o n k i n e t i c s by neat epoxies were measured using g r a v i m e t r i c a n a l y s i s u s i n g a M e t t l e r balance which was accurate t o ±0.05mg. This technique was d e s c r i b e d i n d e t a i l e l s e where (20). Another method was used t o monitor the s o r p t i o n k i n e t i c s of heavy water d i f f u s i n g i n t o neat epoxies. T h i s t e c h nique involved the use of s o l i d s t a t e hydrogen-2 NMR spectroscopy. By the use of the normalized f r e e i n d u c t i o n decay (FID) NMR s i g n a l one can r e a d i l y determine the amount of heavy water sorbed by the epoxy specimen. C y l i n d r i c a l - s h a p e d 20mm-long epoxy specimens of 5mm diameter were immersed i n heavy water a t 23 C f o r 2 months and 40°C f o r 1 month b e f o r e the NMR experiment. The hydrogen-2 NMR experiment i n v o l v e d l o c a t i n g the non-spinning heavy-watersaturated s o l i d polymer i n a magnetic f i e l d of 5 T e s l a w h i l e p u l s i n g the m a t e r i a l w i t h a r a d i o frequency of 30.7 MHz. This technique was used t o study moisture-epoxy i n t e r a c t i o n s a t the molecular l e v e l (14). In order t o study the molecular aggregation during the volume r e l a x a t i o n of network epoxies, CP /MAS carbon-13 ( n a t u r a l abundance)



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NMR was u t i l i z e d . The Hartman-Hahn c r o s s - p o l a r i z a t i o n technique (21) was used w i t h a c r o s s - c o n t a c t time of 1 msec f o r t r a n s f e r of proton p o l a r i z a t i o n t o carbon n u c l e i . The proton-decoupling was achieved a t the r a d i o frequency of 56.4 MHz. Carbon-13 14.2 MHz s p e c t r a were measured i n a 1.4 T e s l a magnetic f i e l d . Room tempera t u r e (23°C) experiments were performed a t 54.7° MAS a t 1 KHz. The b r i t t l e , aged epoxies posed some experimental d i f f i c u l t i e s i n using higher spinning r a t e s (e.g., 4 KHz) a t which the spectrum would have a higher r e s o l u t i o n . The probe was constructed u s i n g a d o u b l e - t u n e d / s i n g l e - c o i l c i r c u i t r y . The spinner was constructed using an Andrew-type r o t o r d r i v e n by compressed a i r . A Bruker WM-500 NMR Spectrometer was used t o study the carbon13 resonances f o r epoxy components (TGDDM and DDS) d i s s o l v e d i n deuterated chloroform ( C D C L 3 ) . TGDDM or DDS components were d i s solved i n s o l v e n t - c o n t a i n i n g 10mm NMR tube. 125 MHz carbon-13 NMR s p e c t r a were measured a t 23°C u s i n g a superconducting magnetic f i e l d of 11.7 T e s l a . R e s u l t s and D i s c u s s i o n In previous communications (4>7.»9.*22), we reported the importance of p h y s i c a l aging processes i n a f f e c t i n g time-dependent changes i n mechanical p r o p e r t i e s of TGDDM-DDS network epoxies and t h e i r c a r b o n - f i b e r - r e i n f o r c e d composites. Recently, by means of a t r a n s p o r t experiment, we have demonstrated the time-dependent " f r e e volume c o l l a p s e " i n neat, f u l l y - c r o s s l i n k e d TGDDM-DDS epoxies by water d i f f u s i o n experiments 03,23). I n a d d i t i o n , the mechanical damping and s t r e s s r e l a x a t i o n r a t e s of such epoxies were observed to decrease, w h i l e t e n s i l e modulus of the c a r b o n - f i b e r - r e i n f o r c e d epoxies was suggested t o i n c r e a s e by our s t r e s s - r e l a x a t i o n s t u d i e s (22) . In t h i s paper, r e s u l t s from v a r i o u s i n s t r u m e n t a l techniques w i l l be d i s c u s s e d and c r i t i c a l l y reviewed. Thermal a n a l y s i s per formed by d i f f e r e n t i a l scanning c a l o r i m e t r y (DSC) of a s - c a s t epoxy i n d i c a t e d the m a t e r i a l was not f u l l y c r o s s l i n k e d — an exotherm w i t h a maximum peak temperature a t 263°C was detected during the f i r s t scan from room temperature t o 300°C, using a h e a t i n g r a t e of 20 C per minute. Because i t i s obvious that continued "chemical aging" such as an i n c r e a s e i n c r o s s l i n k d e n s i t y can change the p h y s i c a l and mechanical p r o p e r t i e s of an epoxy, i t was important to t h i s study that a l l p o s s i b i l i t i e s of continuous chemical aging be e l i m i n a t e d t o permit a f u l l e v a l u a t i o n of the e f f e c t of the p h y s i c a l aging phenomenon on mechanical behavior. The F i b e r i t e 934 epoxies and t h e i r composites were g i v e n a p o s t c u r i n g treatment of 16 hours a t 250°C i n n i t r o g e n . A f t e r p o s t c u r i n g , DSC confirmed that the epoxy matrix was i n a f u l l y - c u r e d s t a t e w i t h a r e g u l a r s t e p - f u n c t i o n i n c r e a s e i n heat c a p a c i t y a t a Tg range of 180 C t o 270 C. T h i s r e s u l t was a l s o confirmed u s i n g dynamic mechanical a n a l y s i s and F o u r i e r transform i n f r a r e d spectroscopy (24).


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Stress-Strain Analysis T e n s i l e t e s t s were performed on neat epoxy r e s i n s i n the f o l l o w i n g c o n d i t i o n s : a s - c a s t , as-postcured, as-quenched, and aged a t decade increments from 10 to 10^ minutes a t 140°C i n n i t r o g e n w h i l e stored i n darkness. A summary of the observed r e s i n s t r e s s s t r a i n behavior i s shown i n F i g u r e 2. As can be seen, the epoxy polymer was found t o be extremely s e n s i t i v e t o thermal h i s t o r y . The as-cast specimens e x h i b i t e d the h i g h e s t v a l u e of u l t i m a t e t e n s i l e - s t r e n g t h (UTS) and by f a r the g r e a t e s t values of s t r a i n to-break ( ε ) and toughness. Toughness here i s d e f i n e d as the area under the s t r e s s - s t r a i n curve, which i s d i f f e r e n t from the dynamic toughness values obtained from impact t e s t s . As reported e a r l i e r i n the carbon/epoxy composites i n v e s t i g a t i o n s (4,23), the p o s t c u r i n g treatment r e s u l t e d i n a s i g n i f i c a n t r e d u c t i o n i n these mechanical p r o p e r t i e s . T h i s e f f e c t i s undoubtedly due to the c r o s s l i n k i n g r e a c t i o n s i n the thermoset. Oddly enough, the postcured specimens g i v e n an air-quench from above Tg e x h i b i t e d a l o s s i n s t r e n g t h , d u c t i l i t y and tough ness s i g n i f i c a n t l y g r e a t e r than that of the as-postcured specimens ( F i g u r e 2). This o b s e r v a t i o n was unexpected, based on the f r e e volume concept. A r a p i d quench w i l l r e s u l t i n a l a r g e r d e v i a t i o n from the e q u i l i b r i u m g l a s s y s t a t e ; thus, a r e l a t i v e l y l a r g e amount of f r e e volume w i l l be f r o z e n i n t o the epoxy. Because more f r e e volume can be i n t e r p r e t e d t o mean higher c h a i n m o b i l i t y and shorter molecular r e l a x a t i o n time, an i n c r e a s e i n f r e e volume was a n t i c i p a t e d t o r e s u l t i n an i n c r e a s e i n epoxy t e n s i l e p r o p e r t i e s i n s t e a d of a severe decrease. Quenched specimens given a b r i e f thermal annealing a t 140 C f o r 10 minutes were found to e x h i b i t toughness s i m i l a r t o that observed f o r as-postcured specimens (Table 1 ) . Even though the s t r e n g t h f o r 10 minutes annealed specimens was not t o t a l l y r e s t o r e d compared to the as-postcured specimens, the d u c t i l i t y was much improved. One e x p l a n a t i o n f o r these observations i s the presence of r e s i d u a l thermal s t r e s s e s , which can develop i n the b u l k m a t e r i a l as the r e s u l t of r a p i d thermal changes. The s k i n and the core of the b u l k epoxy would experience d i f f e r e n t c o o l i n g r a t e s during the air-quench, and r a p i d c o o l i n g of the specimen would not permit the time-dependent r e l a x a t i o n of these s t r e s s e s . The f a c t that a b r i e f thermal annealing r e s u l t s i n r e s t o r a t i o n of epoxy t e n s i l e p r o p e r t i e s suggests that the r e s i d u a l thermal s t r e s s e s have been removed and have not caused i r r e v e r s i b l e damage i n specimen. Thermal annealing a t 140°C i n an i n e r t dark atmosphere r e s u l t ed i n decreases i n s t r e n g t h , d u c t i l i t y , and toughness, as seen i n F i g u r e 2 and Table 1. These changes are a t t r i b u t e d to p h y s i c a l aging processes o c c u r r i n g i n the g l a s s y polymer. As an a d d i t i o n a l check to assure that compositional changes were not o c c u r r i n g i n the polymer w i t h thermal exposure, specimen weights were followed. β




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1.6 ± 0 . 5 5

13.30 ±1.11

10^ min 413K (140°C) N 2 atm

2.00

8666

0.07

1.2 ± 0 . 5 5

11.66 ±1.19

3

10 min 413K (140°C) N 2 atm

SUB-Tg ANNEALING

1.80

10399

0.06

1.0 ± 0 . 5 0

9.50 ± 1.01

4

10 min 413K (140°C) N 2 atm

Table I . Mechanical p r o p e r t i e s o f F i b e r i t e 934 epoxy as a f u n c t i o n of thermal h i s t o r y and sub-T annealing time.


134

HIGHLY CROSS-LINKED

POLYMERS

No r e s o l v a b l e weight change was observed i n any of the aged specimens. The e f f e c t s of p h y s i c a l aging a t 140°C on UTS and e of F i b e r i t e 934 epoxies a r e shown i n F i g u r e s 3 and 4, r e s p e c t i v e l y . The e f f e c t s of thermal h i s t o r y on the d u c t i l i t y of network epoxies are summarized i n F i g u r e 5. The decreases i n UTS and e appear to be l i n e a r as a f u n c t i o n of l o g a r i t h m i c aging time (see F i g u r e s 3 and 4 ) . Toughness and y i e l d s t r e n g t h (o*y) a l s o decreased w i t h time (Table 1 ) . The modulus (E) v a r i e d somewhat e r r a t i c a l l y , but was roughly constant. B


B

Dynamic Mechanical A n a l y s i s Dynamic mechanical a n a l y s i s of polymeric m a t e r i a l s , i n c l u d i n g epoxies (25-43), i s an e s t a b l i s h e d t o o l i n measuring the mechanic a l d i s p e r s i o n peaks as w e l l as other parameters such as the dynami c storage modulus of the macromolecules. Wetton has reported some e f f e c t s by sub-Tg annealing on the modulus and damping peaks a low-Tg epoxies (44). I n t h i s i n v e s t i g a t i o n of high-performance/ high-Tg epoxy-matrix composites, we have observed a decrease i n damping and an i n c r e a s e i n dynamic storage modulus of the composi t e as a f u n c t i o n of p h y s i c a l aging time ( 5 ) . F i g u r e 6 shows a s p e c i f i c example of a 140°C/10^ min.-aged carbon/epoxy composite having a g l a s s t r a n s i t i o n maximum peak temperature near 242°C. The onset of the T i s near 175°C. This composite, which i s ±45° c a r b o n - f i b e r - r e i n f o r c e d , shows a dynamic storage modulus of the epoxy m a t r i x i n the g l a s s y - s t a t e of ca. 15 GPa. At the onset of the g l a s s - t o - r u b b e r t r a n s i t i o n (see F i g u r e 6 ) , the modulus drops g r a d u a l l y from 15 GPa (175°C) t o about 3 GPa (300°C) as the rubbery p l a t e a u i s reached. With p h y s i c a l aging a t 140°C i n n i t r o g e n / d a r k atmosphere, the dynamic storage modulus i s v e r y s e n s i t i v e to aging time. The modulus increased from 13 GPa (10 min.-aged) t o 18 GPa f o r samples aged up t o 1()5 min. a t 140°C (see F i g u r e 7) . These r e s u l t s agree w i t h o b s e r v a t i o n s made i n the s t r e s s r e l a x a t i o n experiments reported e a r l i e r (5>) i n which the epoxy t e n s i l e modulus increased w i t h sub-Tg a n n e a l i n g . The mechanical d i s p e r s i o n peaks i n low-Tg epoxies such as Epon 828 r e s i n have been the subject of numerous s t u d i e s (26,2831,35-38,42). The a peak can undoubtedly be a t t r i b u t e d t o the l a r g e - s c a l e c o o p e r a t i v e segmental motion of the macromolecules. The B r e l a x a t i o n near -55°C, however, has been the s u b j e c t of much controversy (29,36). One p o s t u l a t e d o r i g i n of the d i s p e r s i o n peak i s the " c r a n k s h a f t mechanism (29,42,45) a t the j u n c t i o n p o i n t of the network epoxies (Figure 8 ) . The "crankshaft motion" f o r l i n e a r macromolecules was f i r s t proposed (46-49) as the molecular o r i g i n f o r secondary r e l a x a t i o n s which i n v o l v e d r e s t r i c t e d motion of the main c h a i n r e q u i r i n g a t l e a s t 5 and as many as 7 bonds (50). This k i n d of c r a n k s h a f t r o t a t i o n needs an energy of a c t i v a t i o n of g

11


KONG


135


9.

Figure 4. D u c t i l i t y of F i b e r i t e 934 epoxy as a f u n c t i o n of log sub-T aging at 140 °C.




136

-150 -85 -20 45 110 175 240 305 370 435 500 SAMPLE TEMPERATURE, °C

Figure 6. Dynamic mechanical a n a l y s i s of 10" min.-aged (- 45 ) « Thornel 3 0 0 / F i b e r i t e 934 composite showing the l o s s tangent and the dynamic storage modulus.


s

9. KONG


50 1 0 min

45

5

- - - - -

40

140°C A N N E A L I N G T300/F934

30


10^ min 10 min

35 h 25

(±45°) 8-PLY

20h

2 S

COMPOSITES

15 10 h 5

-150 - 8 5 - 2 0 45 110 175 240 305 370 435 500 SAMPLE T E M P E R A T U R E , °C

Figure 7. The i n f l u e n c e of p h y s i c a l aging time on the dynamic storage modulus of Thornel 3 0 0 / F i b e r i t e 934 composites.

2

c

3

/O-

CHo

JUNCTION POINT OH I ^ Λ / Ν -CH -CH -CH 2

OH 2

CH 2

^

C H * C H - Ν/ 2

o=s=o

Figure 8. Proposed "crankshaft motion" at the j u n c t i o n point of a c r o s s - l i n k e d TGDDM-DDS epoxy.



138


the order of 11 to 15 Kcal/mol and most l i k e l y r e q u i r e s c r e a t i o n of f r e e volume i n order that the crankshaft may r o t a t e (50). F i g u r e 9 shows the e f f e c t s of thermal h i s t o r y on the mechani c a l d i s p e r s i o n peaks i n F i b e r i t e 934 epoxy composites. The asf a b r i c a t e d m a t e r i a l s show by f a r the l a r g e s t damping (Τβ spans from -100 C to c a . 20°C f o r t h i s epoxy system). P o s t c u r i n g com p l e t e s the c r o s s l i n k i n g and r e s u l t s i n a s i g n i f i c a n t l y lowerdamping composite. P h y s i c a l aging a f f e c t s s i g n i f i c a n t l y the mechanical damping i n both the α and β r e l a x a t i o n peaks. F i g u r e 10 shows the decrease i n the β l o s s peak as a f u n c t i o n of aging time a t 140 C i n n i t r o gen. T h i s g r a d u a l decrease i n damping can be explained by a r e l a x a t i o n model i n which the epoxy network loses m o b i l i t y and f r e e volume d u r i n g i t s asymptotic approach towards the e q u i l i b r i u m g l a s s y s t a t e ; as a r e s u l t , the a b i l i t y to d i s s i p a t e energy i s reduced. T h i s i s a s i g n i f i c a n t observation i n view of the f a c t that the area under the secondary mechanical d i s p e r s i o n peak i s o f t e n c o r r e l a t e d w i t h the impact r e s i s t a n c e of the polymer (51). Upon requenching from above T and re-aging such m a t e r i a l , the thermoreversible nature of p h y s i c a l aging can be demonstratively shown. The e f f e c t of 140°C aging on the β-transition i n the epoxy matrix of requenched specimens i s shown i n F i g u r e 11. With sub-Tg annealing at 140°C the maximum peak temperature of T tended to s h i f t t o higher temperatures. For example, the v a l u e was 242.0°C f o r 10 min.-aged samples. I n the two aging/ reaging experiments, T s h i f t e d to 253.0°C f o r both 1 0 min.-aged and reaged samples. Tg, however, appeared to be l e s s s e n s i t i v e to p h y s i c a l aging time and the β maximum peak temperature stayed at about -55°C. g

g

5

g

D i f f e r e n t i a l Scanning C a l o r i m e t r y DSC was u t i l i z e d to study both the s t a t e of the cure (extent of c r o s s l i n k i n g ) as w e l l as the k i n e t i c s of enthalpy r e l a x a t i o n i n network epoxies (5,12,52,53). DSC r e s u l t s confirmed a f u l l y c r o s s l i n k e d epoxy network having no exotherm a t temperatures up to 280°C. In an e a r l i e r communication ( 5 ) , we reported enthalpy r e l a x a t i o n s t u d i e s a t 140°C aging. In t h i s paper, both 110°C and 80°C sub-T annealing data are presented f o r neat-epoxy aging. F i g u r e 12 shows the DSC scans of f u l l y - c u r e d epoxy samples that were quenched from above Τ and then subjected t o aging at 110 C. The f u l l l i n e i s the f i r s t scan, and the dotted l i n e represents a second scan taken immediately a f t e r c o o l i n g from the i n i t i a l scan. The f o l l o w i n g observations were made: 1. The enthalpy r e l a x a t i o n peak appears near the onset of the t r a n s i t i o n from the g l a s s y s t a t e to the rubbery s t a t e . This peak appears a f t e r only 10 min. of aging at 110 C. 2. During sub-T annealing, the r e l a x a t i o n peak s h i f t s to higher temperature and grows i n magnitude. g



9.

KONG


.0011 -100

1

1

-40

1

-J-

20 80 TEMPERATURE, °C

140

139

1

200

Figure 9. Secondary mechanical d i s p e r s i o n peaks of Thornel 300/ F i b e r i t e 934 composites as influenced by t h e i r specimen thermal history. ir

.0011 -100

1 -40

1

1

20 80 TEMPERATURE, °C

1

1

140

200

Figure 10. The influence of p h y s i c a l aging time on the secondary loss peaks of Thornel 300/ F i b e r i t e 934 composites.


140



•1r

.0011 -100

1

1

-40

20

1

1

1

80 140 200 TEMPERATURE, °C

1

260

1

320

Figure 11. The i n f l u e n c e of requenching followed by reaging on the secondary loss peaks of Thornel 3 0 0 / F i b e r i t e 934 composites.

60 80 100 120 140 160 180 200 220 240 260 280 TEMPERATURE, °C THERMAL HISTORY: QUENCHED AND AGED IN NITROGEN HEATING RATE: 10°C/min FIRST SCAN RESCAN

Figure 12. The i n f l u e n c e of 110 °C p h y s i c a l aging on the endothermic enthalpy r e l a x a t i o n peak of neat F i b e r i t e 934 epoxies.


9.

141


KONG

3.

T h i s recovery phenomenon i s thermoreversible. Upon re-aging m a t e r i a l that i s cooled from above Tg, the r e l a x a t i o n peak w i l l reappear and grow w i t h annealing time (see F i g u r e 13)· Compared to the 140°C enthalpy r e l a x a t i o n data reported e a r l i e r ( 5 ) , the 110°C aging k i n e t i c s are d e f i n i t e l y slower. A s e r i e s of 80°C sub-Tg annealing experiments were a l s o performed u s i n g s i m i l a r postcured-and-quenched specimens. "Aging peaks were again observed f o r 80°C annealing even though t h i s time the magnitude of the r e l a x a t i o n peak was much s m a l l e r compared t o 110°C aging data. In 80°C aging, the peak temperature s h i f t e d from the 100°C (10 min.) to 125 C ( 1 0 min.). I n the case of 110°C aging, that peak temperature s h i f t e d from 130°C to 180°C (10 min. to 10-* min. a g i n g ) . In an e a r l i e r r e p o r t , we n o t i c e d a s h i f t from 160°C (10 min.-aged) t o 210°C ( 1 0 min.-aged) f o r 140°C sub-Tg annealing ( 5 ) . As mentioned e a r l i e r , the r e l a x a t i o n enthalpy was measured by superimposing the f i r s t and second DSC scans f o r each specimen. F i g u r e 14 shows the r e l a x a t i o n - e n t h a l p y l o s s versus l o g a r i t h m i c sub-T annealing time a t 140°, 110° and 80°C. There i s c l e a r l y a l i n e a r r e l a t i o n s h i p between the enthalpy r e l a x a t i o n process and the l o g a r i t h m i c aging time. F i g u r e 14 demonstrates that aging k i n e t i c s slow down as the temperature increment (T - T ) , i n c r e a s e s , i . e . , the recovery process i s a thermally-stimulated phenomenon which r e q u i r e s seg mental m o b i l i t y of the polymer i n i t s g l a s s y s t a t e . The lower the sub-T annealing temperature, T , the slower i s the aging k i n e t i c s . Since there e x i s t s a l i n e a r r e l a t i o n s h i p between ΔΗ (decrease i n enthalpy) and aging time, we can determine the a c t i v a t i o n energy of the enthalpy r e l a x a t i o n process assuming the Arrhenius equation h o l d s . F i g u r e 15 shows the Arrhenius p l o t . From the s l o p e , we can estimate the a c t i v a t i o n energy to be 5.9 Kcal/mol. This a c t i v a t i o n energy i s v e r y c l o s e to the t y p i c a l hydrogen bond d i s s o c i a t i o n energy f o r a m a j o r i t y of hydrogen-bonded systems (54-56). I t i s suggested that during the r e s i n c o n t r a c t i o n ( d e n s i f i c a t i o n ) process i n volume r e l a x a t i o n s , hydrogen bonds may be broken and re-formed. The a c t i v a t i o n energy f o r epoxy polymer r e l a x a t i o n of 5.9 Kcal/mol. estimated from the Arrhenius a n l a y s i s i s a low v a l u e compared t o enthalpy r e l a x a t i o n i n i n o r g a n i c g l a s s e s such as the 2°3 y reported by Moynihan et a l (57) (ΔΗ a c t i v a t i o n energy values of the order of 90 Kcal/mol). This simply suggests that the r e l a x a t i o n mechanisms i n the epoxy polymer-networkglasses (PNG) may proceed by d i f f e r e n t mechanisms than do s t r u c t u r a l r e l a x a t i o n s i n i n o r g a n i c g l a s s e s . The low v a l u e estimated may a l s o suggest that r e l a x a t i o n mechanism at 140 C versus those a t 110°C and 80°C i n t h i s epoxy system may be d i f f e r e n t from each other.


11

5

5

a

a

B

s

s

t

e

m

a s




60 80 100 120 140 160 180 200 220 240 260 280 TEMPERATURE, °C HEATING RATE: 10°C/min FIRST SCAN RESCAN

Figure 13. The e f f e c t of 110 °C reaging on the enthalpy r e l a x a t i o n peak of as-requenched F i b e r i t e 934 epoxies.


9. KONG

143


Density The d e n s i t y o f the cured epoxy was followed as a f u n c t i o n of i t s thermal h i s t o r y . Postcuring caused the l a r g e s t decrease i n d e n s i t y , from 1.290 gm/cm f o r as-cast epoxy t o 1.230 gm/cm f o r as-postcured/slowly-cooled epoxy. The decrease i n room tempera t u r e - d e n s i t y can be explained p a r t i a l l y by escape of unreacted DDS c r o s s l i n k e r ( d e n s i t y = 1.380 gm/cm ) during p o s t c u r i n g . Aherne e t a l . (58) a l s o observed a decrease i n d e n s i t y w i t h Epon 828 epoxy p o s t c u r i n g , b u t argued from a free-volume explanation f o r the observation. A p r i o r i , one would assume on the b a s i s o f c r o s s l i n k d e n s i t y that a postcured system (presumably with a higher c r o s s l i n k density) would have a higher d e n s i t y . This may a c t u a l l y be the case a t the postcure temperature. A t 23°C, however, G i l l h a m et a l . (59) have i n d i c a t e d that the h i g h e r - c r o s s l i n k e d system may be quenched f u r t h e r from the h y p o t h e t i c a l e q u i l i b r i u m g l a s s y s t a t e r e s u l t i n g i n a higher value of f r e e volume, i . e . , lower epoxy d e n s i t y (59). With an air-quench, the d e n s i t y of the f u l l y - c r o s s l i n k e d epoxy drops from 1.230 to 1.215 gm/cm . With s u b - T annealing, an i n c r e a s e of 0.82% i n the r e s i n d e n s i t y was observed d u r i n g the 140°C aging. This f i t s the " f r e e volume c o l l a p s e " model i n which the r e s i n d e n s i f i e s . F i g u r e 16 summarizes these observations. 3

3


3

g

Hardness The hardness of a m a t e r i a l i s r e l a t e d t o i t s r e s i s t a n c e t o s c r a t c h i n g or denting. The hardness value f o r the cured epoxy i s very dependent on i t s thermal h i s t o r y and i s a l s o a time-dependent parameter during p h y s i c a l aging. M-scale of the Rockwell hardness index i s r e p o r t e d . As-case epoxy has a low hardness value of M70±3. With p o s t c u r i n g , the value increased to M97±3. This repre sents a 39% i n c r e a s e o f hardness with p o s t c u r i n g . With an a i r quench, the epoxy hardness dropped s l i g h t l y f o r about 4% to M93±3. This i s reasonable s i n c e the excess trapped f r e e volume i n the a s quenched epoxy may w e l l s o f t e n the system. With aging a t 140 C, the epoxy hardened from M93±3 t o M110±3 f o r 1 0 min. aged m a t e r i a l (an 18% i n c r e a s e ) . With a requenching of 10 min.-aged epoxy, a drop of 7% i n the hardness i s observed, probably due to a r e s t o r a t i o n of f r e e volume i n the r e s i n , as manifested i n a softened hardness index of M92.5±2.5. Hence, once again, i s demonstrated the t h e r m o - r e v e r s i b i l i t y of the p h y s i c a l aging process. F i g u r e 17 summarizes the observations. 4

Thermal Mechanical A n a l y s i s Thermal mechanical a n a l y s i s (TMA) was u t i l i z e d by Ophir (60) to study the d e n s i f i c a t i o n of Epon 828 epoxy. The g l a s s t r a n s i t i o n temperature can be e a s i l y c h a r a c t e r i z e d by a s l o p e change as the r e s i n t r a n s i t s from the g l a s s y s t a t e t o the rubbery s t a t e (see


144



5.90 Kcal/mole

Figure 15. Arrhenius a n a l y s i s of the enthalpy loss data of Figure 14. 1.290 1.280 1.270 1.260 1.250 1.240] 1.230 1.22011.210

AS-CAST

f

10

10

2

10

3

10

4

10

5

min

POSTCURED QUENCHED

AGED 140°C/NITROGEN

THERMAL HISTORY

Figure 16. Density of neat F i b e r i t e 934 epoxy as a f u n c t i o n of thermal h i s t o r y .



9

KONG


Figure 17. Hardness of neat F i b e r i t e 934 epoxy as a of thermal h i s t o r y .

145

function


146


F i g u r e 18) · Hence, i n g l a s s y m a t e r i a l , i t i s t y p i c a l l y r e p r e s e n ted by two thermal e x p a n s i v i t y parameters, one below T (glassy thermal e x p a n s i v i t y ) and one above Τ (rubbery thermal expansiv ity) · F i g u r e 18 shows the thermal expansion behavior of a f u l l y c r o s s l i n k e d epoxy as a f u n c t i o n of aging time a t 140°C sub-T annealing. By superimposing the f i r s t scan ( f o r aged m a t e r i a l ) and the second scan ( f o r as-quenched m a t e r i a l ) a t the h i g h temperature rubbery r e g i o n , i t i s p o s s i b l e to monitor the develop ment of aging i n the r e s i n , i . e . , the progress of the d e n s i f i c a t i o n process. As shown i n F i g u r e 18, the aged g l a s s t y p i c a l l y has a l e s s e r volume i n the glassy s t a t e as compared to the as-quenched s t a t e . I t i s obvious from the data that the longer the aging time, the l a r g e r i s the amount of volume l o s t due t o sub-T annealing a t 140 C. This o b s e r v a t i o n a l s o f i t s w e l l i n t o the "free-volumec o l l a p s e " model d i s c u s s e d e a r l i e r . F i g u r e 19 shows the thermal expansion behavior of requenched epoxies. Upon r e a g i n g , the d e n s i f i c a t i o n process was again measurable. Data shown i n F i g u r e 19, t h e r e f o r e , supports the thermoreversible nature i n p h y s i c a l aging. By a n a l y z i n g the l i n e a r p o r t i o n of the thermal expansion curves below and above Tg, i t i s p o s s i b l e t o c a l c u l a t e the expan s i v i t y of each specimen taking i n t o account i t s i n d i v i d u a l t h i c k ness. Through such a n a l y s i s , s i g n i f i c a n t v a r i a t i o n s were observed i n the thermal e x p a n s i v i t y of the cured epoxy both below and above its T . F i g u r e 20 shows the e x p a n s i v i t y v a r i a t i o n s as a f u n c t i o n of thermal h i s t o r y . As-cast epoxy has a value of 5.43 χ 10~$ C" (below Ία). E x p a n s i v i t y below Tg decreased w i t h p o s t c u r i n g t o 5.20 χ 10~5 °C~1, which i s reasonable because p o s t c u r i n g r e s u l t e d i n a network which has higher c r o s s l i n k - d e n s i t y and hence, l e s s e r m o b i l i t y . Quenching, which introduced a thermal shock and a l s o r e s i d u a l thermal s t r e s s e s , caused the epoxy to be l e s s expansible below Tg (4.98 χ 10"-* C~*) i n s p i t e of the increased f r e e volume through quenching. I n t h i s experiment, s i m i l a r to the r e s u l t s suggested by the s t r e s s - s t r a i n a n a l y s i s , r e s i d u a l thermal s t r e s s e s seem to o v e r r i d e the importance of f r e e volume c o n s i d e r a t i o n s i n a f f e c t i n g the g l a s s y e x p a n s i v i t y of the as-quenched r e s i n . With 10 minutes of sub-IL anneal at 140°C, the thermal expan s i v i t y below Tg decreased t o ^ . 7 8 χ 10"^ ° C ~ . T h i s parameter decreased throughout the 140°C aging experiment. A f t e r 105 min utes of aging, the v a l u e decreased to 4.30 χ 10*" ° C ~ . The f r e e volume decrease e v i d e n t l y d i c t a t e s the thermal e x p a n s i v i t y i n the g l a s s y s t a t e during sub-Ί^ annealing. To erase r e s i n h i s t o r y , aged samples were quenched from above Τ . With the requenching, g l a s s y s t a t e e x p a n s i v i t y was r e s t o r e d to h i g h v a l u e of 5.22 χ 10" * ° C ~ % comparable to the as-postcured v a l u e . Requenching o b v i o u s l y has introduced a s i g n i f i c a n t l y g

β

8


g

g

g

1

0

1

5

1


9. KONG



Υ ιό* min

•

•

60

80

»

I

I

I

1

—I

1

L_

100 120 140 160 180 200 220 240 TEMPERATURE, °C

Figure 18. Thermal expansion behavior of neat F i b e r i t e 934 epoxies as i n f l u e n c e d by aging h i s t o r y at 140 C.

American Chemical Society Library 1155 16th St. N. w. Labana and Dickie; Characterization of Highly Cross-linked Polymers Washington, D. C.Society: 20036 ACS Symposium Series; American Chemical Washington, DC, 1984.

147


148


-J

60

«

80

1

1

1

1

1

100 120 140 160 180 TEMPERATURE, °C

I

L _ J

200 220 240

Figure 19· Thermal expansion behavior of neat F i b e r i t e 934 epoxies as i n f l u e n c e d by requenching (erasure of thermal h i s t o r y ) and subsequent reaging at 140 °C.



9.

KONG


/

/

β

/

//

β

10min Ι Ο 140°C

2

10

*

3

10

4

10

5

^?10min10 Ι Ο Ι Ο if. REAGED 2

3

4

ΙΟ

149

5

THERMAL HISTORY

Figure 20. G l a s s y - s t a t e thermal e x p a n s i v i t y (60-160 °C) of F i b e r i t e 934 epoxies as a f u n c t i o n of thermal h i s t o r y .



150


l a r g e r amount of f r e e volume i n the r e s i n and thus made the r e s i n more expansible (see F i g u r e 20). Reaging the r e s i n a t 140°C again caused a decrease i n g l a s s y s t a t e e x p a n s i v i t y b u t the decrease was l a r g e r than i n the f i r s t round of aging. E v a l u a t i n g the decrease f o r 10^ minute data i n the two s e r i e s o f aging i n d i c a t e d that the aging k i n e t i c s was pro bably s i m i l a r . F i g u r e 21 shows the thermal e x p a n s i v i t y of the epoxy above i t s Tg as a f u n c t i o n o f thermal h i s t o r y . Rubbery-state expansiv i t y i s g e n e r a l l y an order o f magnitude l a r g e r compared t o the g l a s s y - s t a t e e x p a n s i v i t y (Table 2). As-cast epoxy has an expan s i v i t y above Tg o f 3.22 χ 10"^ °C~^-. With p o s t c u r i n g and quench ing, t h i s parameter tends t o i n c r e a s e due t o the i n t e r p l a y o f f r e e volume v a r i a t i o n s and f a c t o r s i n v o l v i n g r e s i d u a l thermal s t r e s s e s (see F i g u r e 21) . As i s c l e a r l y i n d i c a t e d by the data i n F i g u r e 21, e x p a n s i v i t y above T f o r epoxies tend t o i n c r e a s e w i t h aging a t 140°C. T h i s i n c r e a s e i n e x p a n s i v i t y i n the rubbery-state probably can be traced t o a "catching-up-process" f o r volume l o s t d u r i n g p h y s i c a l aging. At temperatures above Tg, there i s enough thermal energy for the system t o reach e q u i l i b r i u m . Since volume was l o s t during p h y s i c a l aging, the h i g h temperatures provide the thermal energy to recover f o r the " l o s t volume". The longer the g l a s s i s sub j e c t e d to aging, the higher would be i t s tendency t o recover the l o s t volume, hence, manifested i n a l a r g e r v a l u e of thermal expan s i v i t y above T . An i n c r e a s e from 3.14 χ 1 0 ~ ° C " to 5.11 χ 10* ° C ~ (62.7% increase) was observed. With requenching and reaging, e x p a n s i v i t y above T again i n c r e a s e s w i t h reaging time (96.6% i n c r e a s e ) , demonstrating once again the t h e r m o r e v e r s i b i l i t y o f p h y s i c a l aging. Table 2 summar i z e s the r e s u l t s of the TMA i n v e s t i g a t i o n s . g

4

1

g

1

g

Moisture S o r p t i o n K i n e t i c s 140°C aged epoxy glasses were subjected t o 40°C/98% r e l a t i v e hu m i d i t y moisture p e n e t r a t i o n . F i g u r e 22 shows the r e s u l t s of t h i s t r a n s p o r t experiment. We observed both a decrease of i n i t i a l s o r p t i o n k i n e t i c s as w e l l as a decrease of e q u i l i b r i u m s o r p t i o n l e v e l as a f u n c t i o n of aging time. T h i s supports the idea that during sub-T annealing, the r e s i n c o n t r a c t s and d e n s i f i e s , r e s u l t i n g i n decreased f r e e volume. In another s e r i e s of d i f f u s i o n experiments, as-postcured epoxies were f i r s t immersed i n 23°C heavy water f o r 2 months. Then the temperature of the epoxy/heavy water i n t e r a c t i n g system was increased t o 40°C. The continuous i n f l u x o f heavy water i n t o the epoxy can be e a s i l y monitored by deuterium NMR spectroscopy. The f r e e induction-decay s i g n a l as normalized by the specimen weight showed an i n c r e a s e as a f u n c t i o n of s o r p t i o n time (see F i g u r e 23). For example, a 2 month room-temperature s o r p t i o n r e s u l t s i n an epoxy having 2.10% of moisture (determined by gravimetry) . With g



151


KONG

Figure 22. Moisture absorption behavior of f u l l y - c r o s s - l i n k e d F i b e r i t e 934 epoxy as i n f l u e n c e d by 140 C sub-T aging.



152


Table I I . Thermal e x p a n s i v i t y of F i b e r i t e as a f u n c t i o n of thermal h i s t o r y .

THERMAL HISTORY AS-CAST AS-POSTCURED AS-QUENCHED

140°CAGED

5

1

10 102 103 10* 10 min

AS-REQUENCHED 102 10 10* 10 min 3

5

epoxies

α( ABOVE Τα) Χ 10 K " (ca. 2 0 0 - 2 4 0 ° C ) 5

1

32.2 37.7 52.9

5.43 5.20 4.98

5

140°C REAGED

ûf(BELOW Τα) Χ 10 Κ " (ca.60-160°C)

934 neat

DEI CREASE 4.40 1 WITH 4.32 I AGING 4.30 •

314 1 . 32.4 1 CREASE 33.9 1 WITH 40.1 A AGING 51.1 •

5.22

38.8

! DE5 00 * CREASE 4.82 1 WITH 4.56 ± AGING 3.88

2*2 33.3 34.5 36.6 51.5

4 7 8 4

5

6

1

Ι

3

3

Ύ

I

N

I IN1 CREASE • WITH A AGING Ύ



9. KONG


153

667 h of a d d i t i o n a l s o r p t i o n a t 40°C, 3.14% of moisture r e s i d e d i n the epoxy. Correspondingly, the s o r p t i o n k i n e t i c s f o r NMR FID s i g n a l showed an i n c r e a s e from 275 ( a r b i t r a r y u n i t s ) f o r 2 months/ 23°C d i f f u s i o n to c a . 990 w i t h a d d i t i o n a l 667 h of d i f f u s i o n a t 40°C (Figure 23). I n general, the epoxy-water gravimetry e x p e r i ment and the epoxy-heavy water deuterium NMR agree w e l l w i t h each other. In a d d i t i o n to d e t e c t i n g the heavy water content, we a l s o u t i l i z e d the deuterium NMR technique to study epoxy/moisture i n t e r a c t i o n s . F i g u r e 24 shows the 4.65 ppm nuclear magnetic resonance of f r e e l y - t u m b l i n g heavy water. F i g u r e 25 shows the NMR spectrum of deuterium as i t r e s i d e s i n s i d e an as-quenched epoxy a f t e r immersion i n heavy water f o r 2 months a t 23°C and then 1 month a t 40°C. In F i g u r e 25, we can c l e a r l y see the sharp component (with l i t t l e broadening) that i s due to i s o t o p i c a l l y tumbling or l e s s bound heavy water. The wide broadening of the deuterium resonance could be due to deuterium oxide trapped by the hydrogen bonds of epoxy r e s i n . I t i s p o s s i b l e that the deuterium of heavy water may exchange w i t h the protons i n the epoxy (61) so the broadening may only r e f l e c t deuterons that have exchanged and now r e s i d e i n the epoxy network. Experiments u s i n g deuterated r e s i n can c l a r i f y this point. In the TGDDM-DDS epoxies, hydrogen bonds may be formed among the p o l a r groups. F i g u r e 26 summarizes such p o s s i b l e hydrogen bonding p o s s i b l e s . To propose one example, h y d r o x y l groups may hydrogen bond i n the f o l l o w i n g f a s h i o n .

j

Resin

0 ^

^0

{ Resin

|

H S i m i l a r to water, i t i s a l s o known that heavy water i s h i g h l y a s s o c i a t e d by hydrogen bonding. I t i s very l i k e l y that the heavy water hydrogen bonding system d i s r u p t s the epoxy hydrogen-bonding system ( F i g u r e 27), thereby causing s w e l l i n g i n the moisturesaturated -res i n network (6,20,62) The epoxy-heavy-water i n t e r a c t i n g model i s proposed i n F i g u r e 28 i n which some less-bound moisture molecules would r e s i d e i n v o i d s ( f r e e volume) hence g i v i n g r i s e t o the sharp NMR component, whereas some moisture would be trapped by the epoxy hydrogen bonds, r e s u l t i n g i n the NMR broad component. From the NMR broadening, i t can be estimated that the c o r r e l a t i o n time f o r heavy water among the epoxy hydrogen bonds i s i n the order of 10 sec. E a r l i e r we reported that p h y s i c a l aging a f f e c t s the " s w e l l i n g e f f i c i e n c y " and d i f f u s i v i t y of epoxy as moisture i s transported i n t o the network ( 6 ) . In c o n j u n c t i o n w i t h t h i s e a r l i e r communica t i o n ( 6 ) , we can now summarize the f i n d i n g s f o r i n t e r a c t i o n s between moisture and aging epoxies: m



154


EPOXY POSTCURED 250°C/16h IN NITROGEN ROOM TEMP. IMMERSION IN D 0 2 MONTHS ROOM TEMP. IMMERSION IND 0 40°C IMMERSION IN D 0 2

2

2

TIME OF IMMERSION AT 40°C, hr

Figure 23. Heavy water absorption bg F i b e r i t e 934 epoxy as a f u n c t i o n of immersion time at 40 C as monitored by deuterium NMR spectroscopy.

4.66 ppm

FREE D 0 2

50

40

30

20

10 δ, ppm

-10

-20

-30

-40

Figure 24. Deuterium NMR spectrum of i s o t r o p i c a l l y - t u m b l i n g heavy water molecules.


9.


KONG

155


AS-QUENCHED 934 RESIN D2O IMMERSED 2 MONTHS AT 23° C 1 MONTH AT 40°C

^ 400

300

200

100

^

^

^

0 -100 -200 CHEMICAL SHIFT, (δ), ppm

-300

-400

-500

-600

Figure 25. Deuterium NMR spectrum of heavy water absorbed by an as-quenched F i b e r i t e 934 epoxy. /

c-Qt-

-H-N:

_ A_

I DIGLYCIDYL ORTHOPHTHALATE | | DPS | M 1.2% OF F934)

o-s-o

I

DPS

HO-| TGPDM]

ITGDDM |-0-H

Ο — I TGDDM |

I

\-Q

HO-|

TGDDM|

JDGOPl

I ·.

/

O-S-0

H-N:

|PDS|

fBosl HYDROGEN BOND

Figure 26. Various hydrogen bonding p o s s i b i l i t i e s by p o l a r groups i n F i b e r i t e 934 epoxy (N.B.: D i g l y c i d y l orthophthalate i s given the acronym DGOP).


156


pry,

/Η Ο \ CH, CH

2

C

H

2

HYDROGEN BOND

Ν

CH

I ο I

2


, CH.

2 — C H Ο

CH —CH 2

/ CH-O—Η \

/

Ο

/CH

2

CH

-CH

/

Figure 27. Hydrogen-bonded network of F i b e r i t e 934 epoxy (N.B. 7T e l e c t r o n s may a l s o p a r t i c i p a t e i n hydrogen bonding).

D 0 RESIDES IN VOIDS/FREE VOLUME

FREELY TUMBLING "D2O

2

D 0 RESIDES IN BETWEEN ΕΡΟΧΥ-ΟΗ·«·ΗΟ • j? i: HYDROGEN BONDS 2 (CORRELATION TIME * 10" *c ) 2

7

Figure 28. Proposed model f o r "heavy water-epoxy i n t e r a c t i o n s " (N.B.: Heavy water may form aggregates i n the voids or d i s r u p t the epoxy hydrogen bonds).



9. KONG 1. 2. 3. 4.

As epoxies d e n s i f y , the amount of moisture uptake decreases. As epoxies d e n s i f y , the " s w e l l i n g e f f i c i e n c y " (6,10) of aged epoxy i s i n c r e a s e d . D i f f u s i v i t y (6) decreases w i t h p h y s i c a l aging. Moisture can e i t h e r r e s i d e i n the f r e e volume or d i s r u p t s the epoxy hydrogen bonds.

Proton-Decoupled Downloaded by CHINESE UNIV OF HONG KONG on November 2, 2016 | http://pubs.acs.org Publication Date: February 16, 1984 | doi: 10.1021/bk-1984-0243.ch009

157

CP/MAS NMR and S o l u t i o n C-13 NMR

For the f i r s t time, proton-decoupling, c r o s s - p o l a r i z a t i o n , and magic angle s p i n n i n g NMR techniques are a p p l i e d t o study the F i b e r i t e 934 TGDDM-DDS system. F i g u r e 29 shows a carbon-13 spec trum of a s - c a s t epoxy. I n the spectrum, the aromatic carbons ( r e s i d i n g i n downfield between 100 t o 150 ppm) can c l e a r l y be r e s o l v e d from the a l i p h a t i c s (20 t o 80 ppm). By i n t e g r a t i o n , the p o p u l a t i o n of a l i p h a t i c and aromatic carbons was shown t o be roughly the same ( F i g u r e 29). In order to study p h y s i c a l aging a t the molecular l e v e l , CP/MAS NMR was used t o study the epoxy d e n s i f i c a t i o n process. F i g u r e 30 shows the s p e c t r a l d i f f e r e n c e between a s - c a s t , 10 min.aged and 10-> min.-aged epoxies. P o s t c u r i n g and aging c l e a r l y has r e s u l t e d i n many s p e c t r a l changes (see F i g u r e 30). Of g r e a t e s t i n t e r e s t was the o b s e r v a t i o n that the sharp and h i g h e s t aromatic resonance a t 127 ppm ( a s - c a s t epoxy) tended t o s h i f t downfield w i t h aging. With aging to 10^ min., t h i s resonance peak s h i f t e d to c a . 131 ppm. We can i n t e r p r e t t h i s downfield s h i f t of aromatic resonance t o molecular aggregation o f the phenyl r i n g s i n the r e s i n causing " r i n g - c u r r e n t e f f e c t s " (63) during volume r e l a x a t i o n i n the epoxy r e s i n s . 125 MHz carbon-13 s p e c t r a of TGDDM, DDS, and DGOP i n deuter ated chloroform s o l u t i o n s are shown i n F i g u r e s 31-33, r e s p e c t i v e l y . The s p e c t r a show sharp components o f resonance peaks f o r the aromatics between 110 and 150 ppm and f o r the a l i p h a t i c s between 40 and 80 ppm. The l a r g e peak a t 77 ppm i s due to C D C I 3 carbon-13 resonance. I n F i g u r e 32, the doublets a t 45 ppm and 50 ppm can be a t t r i b u t e d to conformational isomers a r i s i n g from an "umbrella l i k e " i n v e r s i o n a t the pyramidal-bonded n i t r o g e n atom. Conclusions For the f i r s t time, i t i s now p o s s i b l e t o c h a r a c t e r i z e the molecu l a r aggregation during p h y s i c a l aging i n network epoxies by spec t r o s c o p i c technique. Other important f i n d i n g s are summarized: as p h y s i c a l aging of the polymer network g l a s s proceeds, - Damping decreases - U l t i m a t e mechanical p r o p e r t i e s decrease - S t r e s s r e l a x a t i o n r a t e s (and creep r a t e s ) decrease - Moisture s o r p t i o n decreases - Moisture d i f f u s i v i t y decreases



158


127 ppm \ HARTMANN-HAHN CROSS POLARIZATION I P2 1.50 msec 1 D3 - 4.50 usee I D4 - 50.00 msec D5 « 2.00 sec

180

160

140

120

100

80 ppm

60

40

20

0

Figure 29. Proton-decoupled CP/MAS carbon-13 NMR spectrum of an as-cast F i b e r i t e 934 epoxy showing a l s o the i n t e g r a t i o n of the aromatic (downfield) and a l i p h a t i c ( u p f i e l d ) carbons.


KONG


159


9.

Figure 30. The e f f e c t of p h y s i c a l aging on the s p e c t r a l changes i n proton-decoupled CP/MAS carbon-13 NMR measurements.



160


Figure 31. 125 MHz carbon-13 NMR spectrum of TGDDM i n deuterated chloroform s o l u t i o n .


9. KONG


ι Λ Aw-^.


,1 150

161

140

142

138

134

130

118

126

114

110

Figure 32. Proton-decoupled 125 MHz carbon-13 NMR spectrum of DDS c r o s s - l i n k i n g agent i n deuterated chloroform s o l u t i o n .

C-0-CH " 2

3 C-O-CHo5 /il / ° CA. 220 ppm FOR CAR BON Y L CARBON

Λ

[6

2

DEUTERATED CHLOROFORM (SOLVENT)

1 170 165 160 155 150 145 140 135 130 125 120 115 110 105 100 95 90 85 CHEMICAL SHIFT, δ, ppm

75

V

1 3

70 65 60 55 50 45 40

Figure 33. Proton-decoupled 125 MHz carbon-13 NMR spectrum of DG0P i n deuterated chloroform s o l u t i o n . Downfield carbonyl carbon resonance not shown.


162

HIGHLY CROSS-LINKED POLYMERS -

Density increases Dynamic modulus i n c r e a s e s Hardness increases Moisture-epoxy i n t e r a c t i o n i n c r e a s e s ( i . e . , s w e l l i n g increases) - G l a s s y - s t a t e e x p a n s i v i t y decreases; rubbery-state e x p a n s i v i t y increases


Aclmowledgments T h i s research has been supported by Grant NCC 2-103 from NASA to Stanford U n i v e r s i t y . The use of the Southern C a l i f o r n i a Regional NMR F a c i l i t y i s g r a t e f u l l y acknowledged. T h i s f a c i l i t y i s sup ported by NSF Grant No. CHE 79-16324. The author would l i k e to thank Dr. L. M u e l l e r , Mr. J . L a i , Mr. R. Kamdar and Ms. S. Lee f o r t h e i r t e c h n i c a l support and a l s o Dr. T. Sumsion and Mr. M. Adamson f o r t h e i r v a l u a b l e comments and c o n s t r u c t i v e reviews.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

13. 14. 15.

Ophir, Ζ. H.; Emerson, J. Α.; Wilkes, G. L., J. Appl. Phys. 1978, 49, 5032. Kaiser, J. Makromol. Chem., 1979, 180, 573. Kong, E. S. W.; Wilkes, G. L.; McGrath, J. E.; Banthia, A. K.; Mohajer, Y.; Tant, M. R., Polym. Eng. Sci., 1981, 21, 943. Kong, E. S. W., J. Appl. Phys., 1981, 52, 5921. Kong, E. S. W., Composites Technol. Rev., 1982, 4, 97. Kong, E. S. W.; Adamson, M. J., Polymer Commun., 1983, 24, 171 Kong, E. S. W.; Lee, S. M.; Nelson, H. G., Polym. Composites, 1982, 3, 29. Chang, T. D.; Brittain, J. O., Polym. Engin. Sci., 1982, 22, 1228. Kong, E. S. W., Contemporary Topics in Polymer Science, 1983, 4, ed. by J. Bailey and T. Tsuruta, Plenum Press, New York. Struik, L. C. Ε., "Physical Aging in Amorphous Polymers and Other Materials"; Elsevier: Amsterdam, 1978; p. 16. Kovacs, A. J., Fortschr. Hochpolym. Forsch., 1963, 3, 394. Petrie, S. Ε. B., "Polymeric Materials: Relationships Between Structure and Mechanical Behavior"; ed. by E. Baer and S. V. Radcliffe, Amer. Soc. Metals, Metals Park, Ohio, 1975; p. 55. Schaefer, J.; Stejskal, E. O., J. Amer. Chem. Soc., 1976, 98, 1031. Rowland, S. P. (Ed.), "Water in Polymers"; Amer. Chem. Soc. Symp. Series 127; American Chemical Society; Washington, DC, 1980. Moy, P.; Karasz, F. E., Polym. Eng. Sci., 1980, 20, 315.



9. KONG


163

16. May, C. Α.; Fritzen, J. S.; Whearty, D. Κ., "Exploratory Development of Chemical Quality Assurance and Composition of Epoxy Formulations", Lockheed Missiles and Space Company, Air Force Technical Report: AFML-TR-76-112, 1976. 17. Hadad, D. K.; Fritzen, J. S.; May, C. Α., "Exploratory Development of Chemical Quality Assurance and Composition of Epoxy Formulations", Lockheed Missiles and Space Company, Air Force Technical Report: AFML-TR-77-217, 1977. 18. Wetton, R. E.; Croucher, R. G.; Fursdon, J. W. M., Polym. Prepr., Amer. Chem. Soc., Div. Poly. Chem., 1981, 22 (1), 256. 19. Wyzgoski, M. G., J. Appl. Polym. Sci., 1980, 25, 1455. 20. Adamson, M. J., J. Mater. Sci., 1980, 15, 1736. 21. Fukushima, E.; Roeder, S. B. W., "Experimental Pulse NMR", Addison-Wesley: Reading, Massachusetts, 1981; p. 284. 22. Kong, E. S. W., "Epoxy Resins - II"; Bauer, R. W., A.C.S. Symp. Series, 22, American Chemical Society: Washington, DC 1983 Chapter 9, p. 171-191. 23. Kong, E. S. W., "Physical Aging in Graphite/Epoxy Composites" Science of Advanced Materials and Processing Series, S.A.M.P.E. National Meeting, 1983, 28, 838. 24. Kong, E. S. W., unpublished data. 25. Heijboer, J., Annals New York Acad. Sci., 1976, 279, 104. 26. Takahama, T.; Geil, P. H., J. Polym. Sci., Phys. Ed., 1982, 20, 1979. 27. Bailey, R. T.; North, A. M.; Pethrick, R. Α., "Molecular Motion in High Polymers", Oxford University Press, Oxford, United Kingdom, 1981; p. 287. 28. Kaelble, D. H., J. Appl. Polym. Sci., 1965, 9, 213. 29. May, C. Α.; Weir, F. E. Soc. Plast. Eng. Transactions, 1962, 7, 207. 30. Browning, C. E., Polym. Eng. Sci., 1978, 18, 16. 31. Murayama, T.; Bell, J. P., J. Polym. Sci., A-2, 1970, 8, 437. 32. Kalfoglou, Ν. K.; Williams, H. L., J. Appl. Polym. Sci., 1973, 17, 1377. 33. Cook, W. D.; Delatycki, O., J. Polym. Sci., Polym. Phys. Ed., 1975, 13, 1049. 34. Murayama, T., "Dynamic Mechanical Analysis of Polymeric Material", Elsevier: Amsterdam, 1978. 35. Wyzgoski, M. G., J. Appl. Polym. Sci., 1980, 25, 1443. 36. Willbourn, Α. Η., Transactions Faraday Soc., 1958. 37. Kenyon, A. S.; Nielsen, L. E., J. Macromol. Sci. Chem., 1969, A3, 275. 38. Chang, T. D.; Carr, S. H.; Brittain, J. O., Polym. Eng. Sci., 1982, 22, 1205. 39. Nielsen, L. E., Soc. Plast. Engin., 1960, 16, 525. 40. Read, B. E.; Dean, G. D., "The Determination of Dynamic Properties of Polymers and Composites", Adam Hilger: Bristol, United Kingdom, 1978.




164

41. von Kuzenko, M.; Browning, C. E., Amer. Chem. Soc., Preprints Org. Coat. Plast. Chem., 1979, 40, 694. 42. Keenan, J.; Seferis, J. C.; Quinlivan, J. T., J. Appl. Polym. Sci., 1979, 24, 2375. 43. Hata, N.; Yamaguchi, R.; Kumanotani, J., J. Appl., 1973, 17, 2173. 44. Wetton, R. E., Anal. Proc., Anal. Div. Royal Soc., Chem., 1981, October, 416. 45. Bank. L.; E l l i s , B., Polym. Bull., 1979, 1, 377. 46. Schatzki, T. F., J. Polym. Sci., 1962, 57, 496. 47. Wunderlich, B., J. Chem. Phys., 1962, 37, 2429. 48. Boyer, R. F., Rubber Rev., 1963, 34, 1303. 49. Pogany, G. A., Polymer, 1970, 11, 66. 50. Roberts, G. E.; White, E. F. T., "The Physics of Glassy Polymers", ed. by R. N. Haward, Wiley: New York, 1973, p. 153. 51. Meier, D. J. (ed.), "Molecular Basis of Transitions and Relaxations", Midland Macromolecular Monographs, Volume 4, Gordon and Breach Science Publishers, 1978. 52. Wyzgoski, M. G., Polym. Eng. Sci., 1976, 16, 265. 53. Matsuoka, S.; Bair, H. E., J. Appl. Phys., 1977, 48, 4058. 54. Hoeve, C. A. J., "Water in Polymers", ed. by S. P. Rowland, Amer. Chem. Soc. Symp. Series No. 127, American Chemical Society: Washington, DC, 1980, p. 135. 55. Joesten, M. D.; Schaad, L. J., "Hydrogen Bonding", Dekker: New York, 1974. 56. Pimentel, G. C.; McClellan, A. L., "The Hydrogen Bond", Freeman: San Francisco, 1960. 57. Moynihan, C. T.; Macedo, P. B.; Montrose, C. J.; Gupta, P. K.; DeBolt, Μ. Α.; D i l l , J. F.; Dom, Β. E.; Drake, P. W.; Easteal, A. J.; Elterman, P. B.; Moeller, R. P.; Sasabe, H.; Wilder, J. Α., Ann. New York Acad. Sci., 1976, 279, 15. 58. Aherne, J. P.; Enns, J. B.; Doyle, M. J.; Gillham, J. Κ., Amer. Chem. Soc. Org. Coat. Appl. Polym. Sci. Proc., 1982, 46, 574. 59. Gillham, J. Κ., Private communications, 1983. 60. Ophir, Z., Ph.D. Thesis, Princeton University, Princeton, NJ, 1979. 61. Jelinski, L. W.; Dumais, J. J.; Stark, R. E.; Ellis, T. S.; Karasz, F. E., Macromolecules, 1983, in press. 62. Garcia-Fierro, J. L.; Aleman, J. V., Macromolecules, 1982, 15, 1145. 63. Levy, G. C.; Nelson, G. L., "Carbon-13 Nuclear Magnetic Resonance for Organic Chemists", Wiley: New York, 1972. RECEIVED October 13, 1983


Characterization of Highly Cross-linked Polymers - American

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