Characterization of Highly Cross-linked Polymers - American

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Downloaded by UNIV OF NORTH CAROLINA on June 20, 2013 | http://pubs.acs.org Publication Date: February 16, 1984 | doi: 10.1021/bk-1984-0243.ch007

Carboxyl-Terminated Butadiene-AcrylonitrileModified Epoxy Resin and Its Graphite Fiber-Reinforced Composite Morphology and Dynamic Mechanical Properties 1

SU-DON HONG , SHIRLEY Y. CHUNG, GEORGE NEILSON, and ROBERT F. FEDORS Applied Mechanics Division, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109

Measurements of dynamic mechanical properties, optical and scanning electron microscopy and small-angle X-ray scattering were carried out to characterize the state of cure, possible phase separation and morphology of both HX-205 and F-185 neat resins and their graphite fiber reinforced composites. HX-205 is a diglycidyl ether bisphenol A (DGEBA) based epoxy resin and F-185 is a rubber-modified epoxy resin containing 86.5 weight % HX-205, 8.1 weight % Hycar 1300 x 9 (a liquid carboxyl-terminated polybutadiene-acrylonitrile, CTBNX) and 5.4 weight % Hycar 1472 (a solid copolymer of butadiene-acrylonitrile having acrylic acid pendant group). The neat resins and the composites were prepared using identical curing cycles. The neat resins as well as the matrix materials in the composites appear to have the same state of cure as characterized by dynamic mechanical properties. The F-185 resin contains CTBN-rich domains with sizes ranging from 50 Å(and possibly smaller) to 20 μm and larger. The F-185 neat resin and the F-185 matrix in the composite both display ductile fracture behavior compared to a brittle fracture of HX-205 neat resin and its composite, indicating a toughening effect of the CTBN inclusions. The morphology of the CTBN domains in the F-185 matrix appear to differ from that in the F-185 neat resin. There are a greater fraction of smaller CTBN domains in the F-185 1

To whom correspondence should be directed. 0097-6156/ 84/0243-0091S06.00/0 © 1984 American Chemical Society In Characterization of Highly Cross-linked Polymers; Labana, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


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HIGHLY CROSS-LINKED POLYMERS

matrix than in the F-185 neat resin. Because CTBN domains in the size range of the order of several hundred angstroms are less effective in improving fracture toughness (6,8), the fact that there are a greater fraction of smaller CTBN particles in the composite matrix may partially explain the reported observations that some of the composites made with the CTBN-modified DGEBA epoxy resin did not show significant improvement in fracture toughness. This study indicates that, when using multiphase resins to make composites, the neat resin and the matrix of the composite may not have similar morphology even when prepared under the same curing program. It has been shown that the fracture toughness of the matrix resin i t s e l f i n a fiber-reinforced composite has a significant effect on the fracture toughness, particularly the interlaminar fracture toughness, of the composite. For instance, the c r i t i c a l strain energy release rate, G J C , of the three matrix resins, i ) tetraglycidyl diaminodiphenyl methane (TGDDM) cured with diaminodiphenyl sulfone (DDS), i i ) diglycidyl ether bisphenol A (DGEBA) cured with dicyandiamide and i i i ) poly (bisphenol-A-diphenylsulfone) i s 0.076 KJ/m (l,2), 0.27 KJ/m (1,2) and 3.2 KJ/m (2), respectively; the GJ_Q for inter laminar fracture for the corresponding graphite cloth-reinforced composite i s 0.36 KJ/m (1), 0.6 KJ/m (1) and 2.2 KJ/m (2)» respectively. Much work has been carried out i n an effort to toughen the epoxy resin by various modifications such as the incorporation of a rubber component, mainly carboxyl-terminated butadiene acrylonitrile (CTBN) polymers (2-10). It was reported that the incorporation of a CTBN elastomer i n diglycidyl ether bisphenol A resin produced more than a 10-fold increase i n the fracture toughness (2-10) of the resin matrix i t s e l f . The fracture toughness of fiber-reinforced composites containing such modified resins, however, has not always been reported to be increased. For example, for interlaminar fracture energy of composites containing a CTBN-modifled DGEBA matrix, McKenna, Mendell, and McGarry (11) reported no measurable effect of CTBN for a glass cloth composite (12), while Scott and Phillips (12) reported a two-fold increase for a non-woven graphite fiber composite and Bascom, Bitner, Moulton and Siebert (1) reported a nearly 8-fold increase for a graphite composite. It was thought that these diverse results were at least in part due to the fact that the CTBN-modifled DGEBA matrix had differing rubber particle sizes as well as size distributions which influenced the shape of the crack-tip deformation zone (1) and hence the fracture toughness of the material. 2

2

2

2

2

2

In Characterization of Highly Cross-linked Polymers; Labana, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


7. HONG E T AL.

Butadiene-Acrylonitrile-Modified Epoxy Resin

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The enhanced toughness of the CTBN-modifled DGEBA epoxy was the r e s u l t of the presence of d i s c r e t e CTBN-rich r e g i o n s , which p r e c i p i t a t e d from the r e s i n mixture during p o l y m e r i z a t i o n . These regions c o n s i s t of r e l a t i v e l y s o f t p a r t i c l e s of s i z e s ranging from s e v e r a l hundred angstroms to 10 urn and l a r g e r (2-9), depending on the type of carboxyl-terminated rubber used. The toughness was s i g n i f i c a n t l y a f f e c t e d by the p a r t i c l e s i z e . For r e s i n s c o n t a i n i n g small p a r t i c l e s of s i z e s l e s s than about 0.5 urn, the samples f a i l e d by shear band formation and were only s l i g h t l y tougher than the unmodified r e s i n (6,8). When the r e s i n s contained p a r t i c l e s of s i z e s 1 Mm or l a r g e r , the samples deformed by a combination o f , i ) a d i l a t a t i o n a l deformation of the rubber i n c l u s i o n a t the crack t i p (7,8,11), i i ) the e l o n g a t i o n of the rubber p a r t i c l e s (10) and i i i ) l o c a l i z e d shear deformation of the epoxy matrix (6,10)» These deformation mechanisms lead to the development of a l a r g e p l a s t i c zone a t the crack t i p . The p l a s t i c zone diameters of CTBN-modifled DGEBA c o n t a i n i n g l a r g e CTBN par t i d e s a r e t y p i c a l l y of the order of 20-40 urn compared with about 1 μιη f o r the unmodified epoxies (V)· The l a r g e p l a s t i c zone a t the crack t i p c o n t r i b u t e s to the l a r g e i n c r e a s e i n f r a c t u r e toughness. I t a l s o was reported that a bimodal d i s t r i b u t i o n of CTBN p a r t i c l e s i z e s c o n t r i b u t e s to a greater f r a c t u r e toughness than does a unimodal d i s t r i b u t i o n (8). The p a r t i c l e s i z e s and s i z e d i s t r i b u t i o n of the rubber i n c l u s i o n s i n a CTBN-modified epoxy can be a f f e c t e d by both the c u r i n g c o n d i t i o n s and the chemistry and composition of the s t a r t i n g r e s i n mixture. I t i s known that the phase s e p a r a t i o n behavior of CTBN-modified epoxy, which has a d i r e c t i n f l u e n c e on the s i z e s and s i z e d i s t r i b u t i o n of the i n c l u s i o n , i s a f f e c t e d by i ) r e a c t i v i t y and s e l e c t i v i t y of the f u n c t i o n a l groups of rubber and hardener (6,8,9,13), i i ) the s o l u b i l i t y parameter of rubber which i s r e l a t e d to the a c r y l o n i t r i l e content i n the rubber (4,14), i i i ) i n i t i a l molecular weight of the rubber ( 4 , 6 ) , i v ) c o n c e n t r a t i o n of rubber and hardener (4,9,13,14) and v) a d d i t i o n of a m o d i f i e r such as bisphenol A ( 8 ) . The c u r i n g temperature a l s o i n f l u e n c e s the phase s e p a r a t i o n because of the temperature dependence of m i s c i b i l i t y of the CTBN-epoxy misture (14) and of the temperature dependence of copolymerizat i o n of the r e s i n mixture. Block copolymerization of the CTBN component w i l l favor formation of CTBN p a r t i c l e s . When the g r a p h i t e f i b e r s are impregnated with the r e s i n t o f a b r i c a t e the composite, the morphology of the CTBN i n c l u s i o n s may a l s o be f u r t h e r i n f l u e n c e d by the presence of g r a p h i t e f i b e r . Depending on the f i b e r manufacturing process and surface t r e a t ment, the surfaces of g r a p h i t e f i b e r s may have r e a c t i v e chemical groups (15,16) which w i l l i n f l u e n c e the cure k i n e t i c s of the r e s i n and, consequently, p o s s i b l y change the morphology of CTBN inclusion.


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In this paper, we report the results of a morphological characterization of a DGEBA-based epoxy resin, a CTBN-modified DGEBA resin and their corresponding graphite fiber-reinforced composites. Experimental


Materials. The compositions of the unmodified base epoxy resin, trade name Hexcel 205 (HX-205), and the CTBN-modified epoxy resin, trade name F-185, are summarized i n Table I. The chemical structure of CTBNXis as follows: HOOC

COOH

where x, y, ζ and m depend on the molecular weight and the a c r y l o n i t r i l e content. Hycar CTBNX 1300 χ 9 (B. F. Goodrich Chemical Company) i s a terpolymer which has a nominal molecular weight of 3500, and Hycar 1472 i s a higher molecular weight (260,000) ter polymer of butadiene, acrylonitrile and acrylic acid having carboxyl pendant groups randomly distributed along the polymer backbone. Hycar 1300 χ 9 has an a c r y l o n i t r i l e content of 18% and, for Hycar 1472, the acrylonitrile content i s 26% (17).

Table I. Compositions of HX-205 and F-185 Resins HX-205 Component

F-185 Approx. Wt. %

Component Approx. Wt. %

EPOXIDES (Diglycidyl Ether of Bisphenol A) (Epoxidized Novolac, Epox. Eq. Wt 165)

73

HX-205

86.5

20

Hycar 1300 χ 13

8.1

7

Hycar 1472

5.4

DIPHENOLS (Bisphenol-A) (Tetrabromobisphenol-A)

CATALYST (Dicyandiamide)



7. HONG ET AL.


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Two f i b e r - r e i n f o r c e d composites, designated GD-31 and GD-48, made from C e l i o n 6000 g r a p h i t e f i b e r were a l s o used f o r the t e s t i n g . Both composites are 6-ply laminates ( t h i c k n e s s a p p r o x i mately 0.045 inch) with u n i d i r e c t i o n a l f i b e r layup. The matrix corresponding to GD-31 i s F-185 and that f o r GD-48 i s HX-205. The r e s i n content i n both composites i s about 37% by weight. The p o r o s i t y of the composites was c h a r a c t e r i z e d by u l t r a s o n i c C-scans. The t e s t specimens of no measurable p o r o s i t y were used. The r e s i n specimens and the composite laminates were cured i n a h y d r a u l i c press at 250°F and 75 p s i f o r one hour, and subsequently postcured at the same temperature i n the absence of pressure f o r another two hours. A d d i t i o n a l c u r i n g f o r up to 16 hours i n the case of HX-205 and F-185 r e s i n s showed no measurable changes i n dynamic mechanical p r o p e r t i e s . Microscopy. The p o l a r i z e d o p t i c a l micrographs of t h i n f i l m s of HX-205 and F-185 neat r e s i n s were obtained using a Z e i s s u l t r a phot microscope equipped with a p o l a r i z e r and an a n a l y z e r . Thin f i l m s , approximately 100 microns t h i c k , were prepared by t h i n s e c t i o n i n g the r e s i n sheet with a razor blade at room temperature. The domains were observable because of l i g h t s c a t t e r i n g as a r e s u l t of r e f r a c t i v e index mismatch between the rubber domain and the epoxy matrix, as w e l l as to s t r e s s - i n d u c e d b i r e f r i n g e n c e produced by the thermal s t r e s s imposed on the domains· An ISI model 60Â scanning e l e c t r o n microscope was used to examine the morphology of the f r a c t u r e s u r f a c e s . Both the neat r e s i n s and the composite laminates were notched at room temperature with a razor blade. The samples were then immersed i n l i q u i d n i t r o g e n and f r a c t u r e d i n a i r immediately a f t e r removal from l i q u i d n i t r o g e n . The neat r e s i n s were f r a c t u r e d by bending the samples with p l i e r s and the laminates were f r a c t u r e d along the f i b e r by opening up the notched cracks with p l i e r s . Small-Angle X-Ray S c a t t e r i n g . The small-angle X-ray s c a t t e r i n g (SAXS) measurments were c a r r i e d out on a conventional Kratky instrument (made by Anten Paar) having a sample to d e t e c t o r - s l i t d i s t a n c e of 208 mm. Entrance s l i t s of 0.030 and 0.060 mm were used. N i c k e l - f i l t e r e d Cuka r a d i a t i o n was employed, which was measured with a s c i n t i l l a t i o n counter i n c o n j u n c t i o n with a pulse -height a n a l y z e r . A microcomputer was employed f o r automatic stepwise c o l l e c t i o n and a n a l y s i s of the s c a t t e r i n g data. The s c a t t e r i n g curves shown i n t h i s paper are the experimental curves a f t e r c o r r e c t i o n f o r p a r a s i t i c s c a t t e r i n g . A l s o no c o r r e c t i o n was made f o r d i f f e r e n c e s i n SAXS i n t e n s i t i e s between samples due to d i f f e r e n c e s i n X-ray t r a n s m i s s i o n , since a l l samples were about the same thickness (0.6 mm), the measured t r a n s m i s s i o n ( I / I ) values f o r samples F-185, HX-205, GD-31 and GD-48 were 0.56, 0.46, 0.66 and 0.63, r e s p e c t i v e l y . 0




96

Dynamic Mechanical P r o p e r t i e s . A Servohydraulic Instron Model 1322 was u t i l i z e d to measure both the storage and l o s s modulus as a f u n c t i o n of temperature. A sine wave deformation mode at a frequency of 3.5 Hz and a s t a t i c s t r a i n of 0.3% with a superposed dynamic s t r a i n of + 0.1% was employed i n the t e s t . A l o c k - i n a m p l i f i e r , EG&G Model 5422, was used to measure the storage and l o s s modulus. A command s i g n a l from a d i g i t a l f u n c t i o n generator to c o n t r o l the c y c l i c motion of the ramp of the Instron was used as the reference s i g n a l to the l o c k - i n a m p l i f i e r . The dynamic s t r a i n was f i r s t measured by balancing the phase d i f f e r e n c e between the s i g n a l f o r s t r a i n and the reference s i g n a l using the phase adjustment of the l o c k - i n a m p l i f i e r . Subsequently the i n phase f o r c e component and the out-of-phase force component from which the storage and l o s s moduli as w e l l as the tan δ were c a l c u l a t e d , were d i r e c t l y measured. The composite specimens used f o r t e s t i n g were 16-ply and 6-ply u n i d i r e c t i o n a l laminates cut so that the f i b e r o r i e n t a t i o n was perpendicular to the s t r e t c h i n g d i r e c t i o n . The dynamic mechanical p r o p e r t i e s so measured represent p r i m a r i l y the response of the matrix. Results and D i s c u s s i o n Figures 1 and 2 show the storage modulus E* and tan δ f o r HX-205 and F-185 neat r e s i n s as a f u n c t i o n of temperature. E* f o r HX-205 decreases g r a d u a l l y with an i n c r e a s e i n temperature and does not show any t r a n s i t i o n i n d i c a t i v e of secondary molecular r e l a x a t i o n u n t i l the temperature reaches the g l a s s t r a n s i t i o n temperature, which i s approximately 60°C. E f o r F-185, on the other hand, shows a t r a n s i t i o n s t a r t i n g at about -50°C accompanied by an increase i n tan δ. The tan δ continues to i n c r e a s e u n t i l the temperature reaches the g l a s s t r a n s i t i o n temperature at which point the tan δ increases d r a s t i c a l l y . Figure 3 shows the comparison of the tan δ vs temperature p l o t s f o r HX-205 and F-185. F-185 shows an enhanced tan δ at temperatures above -50°C, and when the temperature reaches 25°C, the tan δ f o r F-185 s t a r t s to increase d r a s t i c a l l y even though HX-205 and F-185 appear to have the same g l a s s t r a n s i t i o n temperature. The g l a s s t r a n s i t i o n temperature f o r Hycar 1300 χ 9 i s -49°C and i s approximately -24°C f o r Hycar 1472 as reported by the manufacturer (18). F-185 does not show the separate t r a n s i t i o n peaks corresponding i n d i v i d u a l l y to Hycar 1300 χ 9 and Hycar 1472 i n the tan δ vs temperature p l o t as would be expected i n a completely phase-separated system. I t appears that Hycar 1300 χ 9 and Hycar 1472 do not form a pure rubber phase, but r a t h e r the rubber phase i s blended with the epoxy r e s i n to form CTBN-rich domains. The appearance of extensive blending of epoxy r e s i n with CTBN i s probably due to the h i g h a c r y l o n i t r i l e content of Hycar 1300 χ 9 and Hycar 1472. Wang (14), Sultan and McGarry (19), and Manzione, Gillham and McPherson (20) have reported that CTBN copolymer with a higher a c r y l o n i t r i l e content tends to mix 1



HONG ET AL.

σ

ι

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0.2

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1

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The storage modulus E and tan 6 f o r HX-205 neat r e s i n measured at 3.5 Hz.

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TEMPERATURE (°C)

Figure

2.

1

The storage modulus Ε and tan δ f o r F-185 neat r e s i n measured at 3.5 Hz.



98


more r e a d i l y before cure with epoxy because of c l o s e r matching of s o l u b i l i t y parameters. The p l o t s of storage modulus and t a n δ as a f u n c t i o n of temperature f o r the composites GD-48 and GD-31, shown i n Figures 4-6, are very s i m i l a r to the corresponding p l o t s f o r the HX-205 and F-185 neat r e s i n s . Thus the dynamic mechanical p r o p e r t i e s c h a r a c t e r i z a t i o n i n d i c a t e s that the HX-205 m a t e r i a l both as neat r e s i n and the matrix i n GD-48 composite as w e l l as F-185 neat r e s i n and matrix i n GD-31 composite have s i m i l a r s t a t e s of cure. For both F-185 neat r e s i n and matrix extensive mixing of CTBN rubber and epoxy r e s i n occurs. Figure 7 shows the p o l a r i z e d o p t i c a l micrographs f o r HX-205 and F-185 r e s i n s . The F-185 r e s i n has a l a r g e number of i n c l u s i o n s of s i z e s greater than 40 urn. These i n c l u s i o n s which are shown as the white regions are probably the r u b b e r - r i c h r e g i o n s . In a d d i t i o n , there are many smaller domains which probably represent both r u b b e r - r i c h domains as w e l l as inhomogeneous regions i n the epoxy-rich phase. These inhomogeneous regions i n the epoxy phase are a l s o present i n the HX-205 neat r e s i n . Figure 8 shows the SEM micrographs of f r a c t u r e surfaces of both HX-205 and F-185 neat r e s i n s . The f r a c t u r e surface of HX-205 i s very smooth, i n d i c a t i v e of t y p i c a l b r i t t l e f r a c t u r e behavior. On the other hand, F-185 has a very rough f r a c t u r e s u r f a c e , i n d i c a t i n g that the r e s i n was h i g h l y s t r a i n e d before f r a c t u r e occurred. There are a l s o some c r a t e r s which appear t o represent the s e p a r a t i o n of s p h e r o i d a l rubber domains from the matrix. SEM micrographs of i n t e r l a m i n a r f r a c t u r e surfaces of both the HX-205/graphite f i b e r composite (GD-48) and the F-185/graphite f i b e r composite (GD-31) are shown i n Figure 9. The GD-48 laminate gave a r e l a t i v e l y c l e a n f r a c t u r e with no s i g n of the r e s i n being s t r a i n e d before f r a c t u r e occurred. On the other hand, the GD-31 laminate e x h i b i t e d a very rough f r a c t u r e surface with i n d i c a t i o n s that some regions of the matrix were h i g h l y s t r a i n e d before f r a c t u r e . Figure 10 shows a d d i t i o n a l SEM micro graphs of the f r a c t u r e surfaces of GD-31. This f i g u r e shows more c l e a r l y the domains resembling the s e p a r a t i o n of rubber p a r t i c l e s from the matrix. In the GD-31 laminate, furthermore, there a r e i n d i c a t i o n s that the f r a c t u r e may have propagated from one p l y t o the adjacent p l i e s as shown i n Figure 11. The branching of cracks from one p l y to the adjacent ones has been reported ( 1 ) . The c h a r a c t e r i s t i c s of f r a c t u r e surfaces of F-185 neat r e s i n and those of F-185 matrix i n the composites are s i m i l a r to those reported i n the l i t e r a t u r e (1)· The f a c t that the f r a c t u r e surfaces of F-185 neat r e s i n and F-185 matrix i n the composites show t y p i c a l d u c t i l e f r a c t u r e behavior, while the unmodified HX-205 shows b r i t t l e f r a c t u r e behavior, seems to i n d i c a t e the toughening e f f e c t of F-185 as a r e s u l t of i n c o r p o r a t i o n of CTBN rubber. The f r a c t u r e energies of these m a t e r i a l s are being



HONG ET AL.

0.3h

Ο HX-205 F-185

Δ


Δ

*>0.2

Δ Ο

0.1 ΔΔΔ

-150

Figure 3.

-100

-50 0 TEMPERATURE, °C

50

100

Comparison of tan δ for HX-205 and F-185 neat resins replotted from data in Figures 1 and 2.

TEMPERATURE ( C)

Figure 4.

The storage modulus E* and tan δ for composite GD-48 measured at 3.5 Hz. The matrix i s HX-205.


100


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100

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τ. °c F i g u r e 5.

The storage modulus Ε* and tan δ f o r composite GD-31 measured at 3.5 Hz. The matrix i s F-185.

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GD-31 COMPOSITE*! |Δ GD-48 ΖΛ IsLf-HB

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Comparison of tan δ f o r composite GD-48 and GD-31 r e p l o t t e d from data i n Figures 4 and 5.


7. HONG ET AL.



HX-205

Figure 7.

F-185

P o l a r i z e d o p t i c a l micrographs obtained from t h i n f i l m s of HX-205 and F-185 neat r e s i n s . The m a g n i f i c a t i o n i s 224. The s i z e of the l a r g e s t domains i s about 45 ym. HX-205

Figure 8.

100 /jum

F-185

SEM micrographs of f r a c t u r e surfaces of HX-205 and F-185 neat r e s i n s . The marker shown on the micrographs i s 10 ym. The specimens were cooled i n l i q u i d n i t r o g e n and then f r a c t u r e d i n a i r immediately a f t e r removing from the l i q u i d nitrogen.


102



HX-205/Graphite F i b e r Composite (GD-48)

F i g u r e 9.

F-185/Graphite F i b e r (GD-31)

Composite

SEM micrographs of the i n t e r l a m i n a r f r a c t u r e surfaces of the composites GD-48, whose matrix i s HX-205, and GD-31, whose matrix i s F-185.

Figure 10. SEM micrographs of the i n t e r l a m i n a r f r a c t u r e Surfaces of GD-31 composite. The domains resembling the separation of CTBN p a r t i c l e s are c l e a r l y shown i n the micrograph. The marker i n d i c a t e s 10 ym.



7. HONG ET AL.


103

Figure 11. SEM micrograph of the i n t e r l a m i n a r f r a c t u r e surface of GD-31 composite at lower m a g n i f i c a t i o n . The cracks are shown to branch from one p l y to adjacent p l i e s . measured and w i l l be reported i n the f u t u r e to c o r r e l a t e with the morphology c h a r a c t e r i z a t i o n . The d e t a i l s of the f r a c t u r e surfaces of F-185 neat r e s i n and those of F-185 matrix i n the composites do not appear a l i k e . However, the f i n e d i f f e r e n c e i n the f r a c t o g r a p h i c appearances of F-185 neat r e s i n and F-185 matrix i n the composites can be due to morphological d i f f e r e n c e of the CTBN-rich domains as w e l l as other f a c t o r s such as s l i g h t d i f f e r e n c e s i n f r a c t u r i n g c o n d i t i o n s and the presence of f i b e r , etc. In order to determine whether or not the morphology of the CTBN-rich domains i n the neat r e s i n and i n the composite matrix i s s m a l l e r , small-angle X-ray s c a t t e r i n g c h a r a c t e r i z a t i o n was c a r r i e d out. Results of small-angle X-ray s c a t t e r i n g on both HX-205 and F-185 neat r e s i n s as w e l l as t h e i r corresponding composite are shown i n Figure 12. In the s c a t t e r i n g angle range 0.7 χ 10"^ to 40 χ 10~3 r a d i a n s , the F-185 neat r e s i n has a higher s c a t t e r i n g i n t e n s i t y , by a f a c t o r of about 10 i n the lower angle r e g i o n , than does the HX-205 neat r e s i n . T h i s i n d i c a t e s t h a t , i n a d d i t i o n to the l a r g e r domains observed by o p t i c a l microscopy and SEM, there are smaller r u b b e r - r i c h domains having s i z e s of the order of 100Â to s e v e r a l thousand angstroms present i n the CTBN-toughened neat r e s i n . A comparison of the s c a t t e r i n g p r o f i l e s f o r both the F-185 neat r e s i n and the GD-31 composite i n d i c a t e s that both have n e a r l y i d e n t i c a l s c a t t e r i n g i n t e n s i t y i n the r e g i o n of s c a t t e r i n g angle lower than 4 χ 10"^ r a d i a n s . Considering the f a c t that the amount of F-185 m a t e r i a l i n the GD-31 i s only about h a l f of that i n the F-185 neat r e s i n i n terms of volume, the s c a t t e r i n g i n t e n s i t y per u n i t volume from the



104

10,000. Of

I Ç

F-186/RUN 30

Δ HX-206/RUN 30 φ GD-31/RUN 30 9

GD-48/RUN 30

Ο F-186/RUN 31

1000. oh

•Δ HX-206/RUN31 d

QD-31/RUN31

-Ο QD-48/RUN31 Θ F-186/RUN 32 Δ HX-206/RUN32


•

GD-31/RUN32

Θ GD-48/RUN 32

loaoh HX-205 Δ

ιαομ GD48: GD-31:

HX-206/GRAPHITE F-18S/GRAPHITE

LOI αϊ

ι

1 ιοαο

ι

ι.ο

ιο.ο

2θ, χ ΙΟ" RADIAN 3

Figure 12. The s c a t t e r i n g i n t e n s i t y of X-ray as a f u n c t i o n of s c a t t e r i n g angle f o r HX-205 and F-185 neat r e s i n s f o r the composites GD-48 and GD-31. The run number i n d i c a t e s measurements c a r r i e d out at d i f f e r e n t s c a t t e r i n g angle range. F-185 i n the composite i s much higher than that from the F-185 neat r e s i n . In order to analyze the c o n t r i b u t i o n t o s c a t t e r i n g i n t e n s i t y from the CTBN component, a theory r e c e n t l y developed by Wu (21) w i l l be u t i l i z e d . The theory shows that i n the high angle r e g i o n where the Porod law i s a p p l i c a b l e , the angledependent s c a t t e r i n g i n t e n s i t y f o r a multiphase system can be expressed as f o l l o w s :

i(h) * i ( h ) e

—I h

4

(

P i

-

P

j

J

)

2

S

(1)

±j J

4it where h • λ s i n θ, θ i s the s c a t t e r i n g angle and λ the wave length. I i s the s c a t t e r e d i n t e n s i t y o f a s i n g l e e l e c t r o n , p i i s the e l e c t r o n density of phase i and S-y i s the t o t a l i n t e r face area between phases i and j w i t h i n the s c a t t e r i n g volume. To obtain the c o n t r i b u t i o n t o the s c a t t e r i n g i n t e n s i t y from the CTBN component one may subtract the s c a t t e r i n g i n t e n s i t y due t o the epoxy phase from the s c a t t e r i n g i n t e n s i t y o f F-185. There f o r e f o r the neat r e s i n one obtains e


7. HONG ET AL. I

F

H

Butadiene-Acrylonitrile-Modified Epoxy Resin (h) - I F - 1 8 5 '

V

105

^-205 (2)

h where e^ represents the epoxy phase and R represents the CTBN phase, ν « 0.43 i s the volume f r a c t i o n of epoxy r e s i n i n F-185. For the composite, one obtains


I

1

h

h

(h) = lGD-3l( > - lGD-48< ) + - le W

1*

I

L \

p

[< e

4

J

v

Σ

ΗΧ-205

" P ) S R ] R Λ D

2

E

A

(3)

where v which i s taken equal t o 0.08 takes i n t o account the f a c t that there i s a smaller amount of HX-205 r e s i n i n the GD-31 composite than i n the GD-48 composite. The use of equation (3) i s based on the assumption that the g r a p h i t e f i b e r s i n the composite form a macroscopic domain, which seems reasonable s i n c e the diameter of the f i b e r i s of the order of 10 μ; consequently equation (3) contains no c o n t r i b u t i o n to the s c a t t e r i n g i n t e n s i t y from the presence of f i b e r . F i g u r e 13 shows the p l o t s of Ipn ( h ) , which represents the neat r e s i n , and I ^ ( h ) , which represents the corresponding composite. I t i s c l e a r that the s c a t t e r i n g i n t e n s i t y a t a g i v e n s c a t t e r i n g angle f o r the F-185 i n the composite i s much higher than that f o r the F-185 neat r e s i n . Since the s c a t t e r i n g i n t e n s i t y was measured i n the s c a t t e r i n g range where the s c a t t e r i n g i s produced mainly by domains ranging i n s i z e from approximately 50 Â to 1500 Â, the r e s u l t s i n d i c a t e that there a r e more smaller CTBN domains i n the F-185 matrix of the composite than i n the F-185 neat r e s i n . Since CTBN p a r t i c l e s of s e v e r a l hundred angstroms are not very e f f e c t i v e i n improving the f r a c t u r e toughness ( 6 ) , the F-185 matrix of the composite w i l l be c h a r a c t e r i z e d by a lower f r a c t u r e toughness. In c o n c l u s i o n , the HX-205 and F-185 neat r e s i n s and the corresponding composites (GD-48 using HX-205 as matrix and GD-31 u s i n g F-185 as matrix) appear t o have the same s t a t e o f cure as c h a r a c t e r i z e d by dynamic mechanical p r o p e r t i e s . I t appears that the CTBN-rich domains i n F-185 neat r e s i n and F-185 matrix i n the composite are e x t e n s i v e l y mixed with DGEBA epoxy r e s i n . The F-185 r e s i n has CTBN-rich domains with s i z e s ranging from 50 Â or smaller to 40 urn and l a r g e r . The F-185 m a t e r i a l both as neat r e s i n and matrix show a d u c t i l e f r a c t u r e behavior, i n d i c a t i n g a toughening e f f e c t due to i n c o r p o r a t i o n of CTBN rubber. The morphology of the CTBN domains i n the F-185 matrix, as determined by small-angle X-ray s c a t t e r i n g , appears to be d i f f e r e n t from that i n the neat r e s i n . There i s a l a r g e r f r a c t i o n of smaller



106


10,000.0

1000.0

D Q ΔΙ

Λ •

BETWEEN F-185 AND HX-205 BETWEEN GD-31 AND GD-48

100.0

10LO

ν

1.0

0.1

0.1

1.0

10.0

100.0

1000.0

2Θ, χ 10 RADIAN 3

F i g u r e 13. The s c a t t e r i n g i n t e n s i t y d i f f e r e n c e as a f u n c t i o n of s c a t t e r i n g angle between F-185 and HX-205 neat r e s i n s , and between GD-31 and GD-48 composites.


7. HONG ET AL.


107


sizes of CTBN domains existing i n the F-185 matrix as compared to the corresponding F-185 neat resin. Because CTBN domains i n the size range of the order of several hundred angstroms are less effective in increasing fracture toughness (6,8), this fact may partially explain the reported observation that some composites made with the CTBN-modified DGEBA epoxy resin did not show significant improvement i n fracture toughness. It i s emphasized that the neat resin as well as the corresponding matrix prepared from the identical resin material may not have similar morphology even when prepared using the same curing program. Acknowledgment s The authors are grateful to Dr. Norman Johnston of NASA Langley Research Center for providing the specimens. The research presented i n this paper was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

W. D. Bascom, J. L. Bitner, R. J. Moulton and A. R. Siebert, Composites, January 1980, 9. W. D. Bascom, R. J. Moulton, Ε. H. Rowe and A. R. Siebert, Org. Coat. Plast., Preprint, 1978, 39, 164. F. J. McGarry, Proc. Roy. Soc. London, 1970, A319, p. 59. Ε. H. Rowe, A. R. Siebert and R. S. Drake, Mod. Plast., 1970, 417, 110. J. N. Sultan, R. C. Laible and F. J. McGarry, Appl. Polymer Symp., 1971, 16, 127. J. N. Sultan and F. J. McGarry, Polym. Eng. Sci., 1973, 13, 29. W. D. Bascom, R. L. Cottington, R. L. Jones and P. Peyser, J. Appl. Polymer Sci., 1975, 19, 2545. C. K. Riew, Ε. H. Rowe, and A. R. Siebert, ACS ADVANCES IN CHEMISTRY, 1976, SERIES No. 154, p. 326. C. B. Bucknall and T. Yoshii, Brit. Polym. J., 1978, 10, 53. W. D. Bascom and D. L. Hunston, Plastic and Rubber Insti tute, London, Preprints, 1978, 1, p. 22. G. B. McKenna, J. F. Mendell and F. J. McGarry, Soc. Plastic Industry, Ann. Tech. Conf., 1974, Section 13-C. J. M. Scott and D. C. Phillips, J. Mat. Sci., 1975, 10, 551. A. C. Meeks, Polymer, 1974, 15, 675. T. T. Wang and H. M. Zupko, J. Appl. Polym. Sci., 1981, 26, 2391. F. Hopfgarten, Fiber Sci. Technol., 1978, 11, 67. G. E. Hammer and L. T. Drzal, Applications of Surface Science, 1980, 4, 340. R. Drake and A. Siebert, SAMPE Quarterly, July 1975, 6, No. 4.


108


18. A. Siehert, Private communication. 19. J. N. Sultan and F. J. McGarry, Research Report R68-8, School of Engineering, Massachusetts Institute of Technology (1968). 20. L. T. Manzione, J. K. Gillham and C. A. McPherson, ACS Pre prints, Div. Org. Coat. Plast. Chem., 1979, 41, 364. 21. W. L. Wu, Polymer, 1982, 23, 1907.


RECEIVED September 14, 1983


Characterization of Highly Cross-linked Polymers - American

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