Network Morphology and the Mechanical Behavior ... - ACS Publications

12 Network Morphology and the Mechanical Behavior of Epoxies S. C. MISRA, J. A. MANSON, and L. H. SPERLING

Downloaded by CORNELL UNIV on October 8, 2016 | http://pubs.acs.org Publication Date: December 3, 1979 | doi: 10.1021/bk-1979-0114.ch012

Materials Research Center, Coxe Laboratory #32, Lehigh University, Bethlehem, PA 18015 Since the properties of thermosetting polymers depend on their network structures, the morphology of such polymers has long been a subject of considerable theoretical and practical interest. Thus, i t is well known that the tensile strength of thermoset resins is less than predicted theoretically on the basis of the breakage of primary van der Waal's bonds (1,2), and it was proposed long ago (3) that this discrepancy was due to the rupture of weak regions created during network formation. Indeed, it has been shown (4) that high internal stresses can be developed during curing, especially when the curing rate is low. This view of the role of structural defects is also given credence by modern theories of fracture mechanics (5), which emphasize the concept of the concentration of stress at a flaw. Two-Phase Networks Other investigators (4,6-33) have emphasized a view of the network essentially as a composite, with a high-crosslink-density (essentially spherical) phase (often considered as a microgel) embedded in a less-crosslinked matrix.

In fact, i t i s probably

generally accepted that, regardless of specific details, the curing of thermosets results i n an inhomogeneous, two-phase network. Inhomogeneity has been attributed to incompatibility or to non-optimum curing conditions (8) but has also been proposed to be inherently characteristic of a l l gelling systems (14) . It has also been postulated (7) that these microgels are colloidal i n nature, at least for some epoxy systems. The existence of inhomogeneities has been inferred from results of diverse investigations using techniques such as electron and optical microscopy (13-26,29,30,31,32), thermomechanical measurements (34), d i f f e r e n t i a l swelling (12,35), and d i f f e r e n t i a l scanning calorimetry (36). In contrast, the use of microtomed thin sections and small-angle x-ray scattering f a i l to indicate two-phase structures (37). Dispersed phases have been referred to by such terms as "micelles, "globules," "floccules," "nodules," and "microgels." In this study, the term "microgel" w i l l be used. 11

0-8412-0525-6/79/47-114-157$06.50/0 © 1979 American Chemical Society

Bauer; Epoxy Resin Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1979.


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I t has been suggested by Bobalek et a l . (27), Solomon (33), and Labana et a l . (14) that the two-phase system i s produced by m i c r o g e l a t i o n p r i o r to the formation of the macrogel. Some i n v e s t i g a t o r s (4_5l4_,16) have p o s t u l a t e d that these microgels are l o o s e l y connected to the surrounding matrix, and have a l s o sug gested (12) that these l o o s e connections are developed during the l a t t e r stages of the c u r i n g process. In g e n e r a l , two l e v e l s of s i z e s have been reported i n the l i t e r a t u r e — o n e (type A) ranging from 6 nm to 40 nm, and the other (type B) ranging from 20 pm to 200 pm. I t has a l s o been shown (4) that slow c u r i n g r a t e s r e s u l t i n l a r g e r microgels (type Β ) , which r e s u l t i n a network having a higher Tg, d e n s i t y , and r e s i s t a n c e to etching. The s u r f a c e p r o p e r t i e s of the network depend on the s u r f a c e energy of the mold m a t e r i a l and on the atmospheric environment (7,30). The s i z e and d e n s i t y of microg e l s have a l s o been r e l a t e d to the presence of p l a s t i c i z e r (23), r a d i a t i o n damage (26), prolonged exposure to heat (17) and aging of the r e s i n (24). Studies of i n t e r f a c e s (30,31) have a l s o shown that c e r t a i n substrates such as T e f l o n and s i l i c o n e - c o a t e d sheets g i v e f e a t u r e l e s s s u r f a c e s ; however, subsequent etching of the s u r f a c e r e v e a l s a two-phase s t r u c t u r e . I t was f u r t h e r shown that the m i c r o g e l s i z e decreases w i t h i n c r e a s i n g amounts of c a t a l y s t . Both low t e n s i l e strengths and nodular morphology i n thermo sets have been r e l a t e d to d i f f e r e n c e s i n c r o s s l i n k d e n s i t y (7,10, 14,27,28,32). At the same time, the f a c t that the y i e l d s t r e n g t h of some epoxies i s f a i r l y independent of s t o i c h i o m e t r y has been a t t r i b u t e d to the r o l e of microgels as primary flow p a r t i c l e s (29). Thus, morphology must p l a y an important r o l e i n determin ing network p r o p e r t i e s . However, s u r f a c e morphology should not a f f e c t the mechanical behavior of the network as much as bulk morphology. D i f f u s i o n phenomena, on the other hand, should depend on the morphology of both the outer s u r f a c e and of the bulk. Though there have been s e v e r a l p o s t u l a t e s of morphological changes during the c r o s s l i n k i n g process and of the morphology of the f i n a l network, experimental evidence has been scarce. As part of a comprehensive study (38) o f the e f f e c t s of c r o s s l i n k d e n s i t y on the behavior of epoxies and other polymers, the pre sent morphological i n v e s t i g a t i o n was conducted on epoxies w i t h emphasis on the r o l e s of s t o i c h i o m e t r y , molecular weight of the epoxy prepolymer, and the d i s t r i b u t i o n of molecular weight b e t ween c r o s s l i n k s ( M ) . Bisphenol-A-based prepolymers were used throughout; most were cured w i t h methylene d i a n i l i n e (MDA), a few were cured with a polyamide. c

Experimental Sample Design. The v a r i o u s formulations are d e s c r i b e d i n Table 1. To examine the e f f e c t of s t o i c h i o m e t r y , S e r i e s A was


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prepared u s i n g d i f f e r e n t proportions o f Epon 828 with MDA ( S h e l l Chemical Co.). To provide a b a s e l i n e f o r examining the e f f e c t s of M a t constant s t o i c h i o m e t r y , S e r i e s E, based on the f o l l o w i n g prepolymers, was prepared using s t o i c h i o m e t r i c amounts of MDA as curing agent: Epon 825, 828, 834, 1001, 1002, and 1004 ( S h e l l Chemical Co.) To vary the d i s t r i b u t i o n of M , S e r i e s F was pre pared to y i e l d r e s i n s which had the same average M values as Epon 828, Epon 834, and Epon 1001. T h i s was done by blending Epon 825 (narrow d i s t r i b u t i o n of molecular weight) with v a r i o u s proportions o f Epon 1004. The d i s t r i b u t i o n s of M thus obtained are e s s e n t i a l l y bimodal, with l i t t l e o v e r l a p . F i n a l l y , to com pare behavior with that of a polyamide-cured epoxy, Sample G - l was prepared by c u r i n g Epon 828 with an e q u i v a l e n t p r o p o r t i o n of Versamid 140 (General M i l l s Chemical Co.). c

c

c


c

Table I .

Compositions of the Samples Prepared i n Epoxy Resin S e r i e s A, E, and F. S e r i e s Ε and S e r i e s F

Series A Sample No. A-7 A-8 A-10 A-ll A-14 A-16 A-18 A-20

Amine/epoxy Ratio 0.7:1 0.8:1 1.0:1 1.1:1 1.4:1 1.6:1 1.8:1 2.0:1

M

c

1523 526 326 370 592 924 1922 CO

(linear)

Sample No. E-l E-2 F-l F-2 F-3 E-3 F-4 E-5 F-5 E-6 E-7

epoxy (wt. %) 825(100) 828(100) 825(91)4-1004(9) 825(62)+1004(38) 825(60)+1004(40) 834(100) 825(57)4-1004(43) 1001(100) 825(20)+1004(80) 1002(100) 1004(100)

Mc

308 326 326 413 419 430 430 740 740 980 1400

C a l c u l a t e d u s i n g B e l l ' s equation (39). 'For reasons discussed by B e l l (39), a c t u a l M values f o r s p e c i mens A-16 to A-20 may be somewhat i n e r r o r . In any case, the e r r o r w i l l not a f f e c t the trends observed as a f u n c t i o n of stoichiometry. c

Resin P r e p a r a t i o n . For systems using l i q u i d epoxy p r e p o l y mers (e.g., Epon 825 and 828) with MDA as the c u r i n g agent, the c u r i n g c y c l e was s i m i l a r to that used by B e l l (39). A f t e r heating to 80°C, the r e s i n and c u r i n g agent were mixed together, evacuated f o r 5 to 15 min. to remove a i r bubbles, and cast and cured as f o l l o w s : 45 min. i n a c i r c u l a t i n g a i r oven a t 60°C, 30 min. a t 80°C, 2.5 h r . a t 150°C, and f i n a l l y slow c o o l i n g to room



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temperature. The mold assemblies comprised clamped 13-cm by 13cm Mylar sheets, separated by 0.5-rom, 1.5-mm, o r 6-mm T e f l o n or ethylene-propylene-copolymer spacers backed by g l a s s p l a t e s . T h i s cure cycle was reported by B e l l to g i v e e s s e n t i a l l y complete c u r i n g , and was a l s o shown (through chemical t i t r a t i o n of the epoxy and amine groups) to be s i m i l a r l y e f f e c t i v e i n t h i s l a b o r a t o r y (38,41,42). S o l i d epoxies were f i r s t melted and then evacuated to remove the entrapped a i r bubbles. In order to avoid a i r entrapment, the c u r i n g agent was mixed in, using a magnetic s t i r r e r , under vacuum. Samples were prepared u s i n g the f o l l o w i n g c u r i n g c y c l e : 1.5 h r . at 100°C, 2.5 hr. a t 150°C, followed by slow c o o l i n g to room temperature. The Versamid-140-cured sample (G-l) was one of the samples prepared by Manson and Chiu (40), using the f o l l o w i n g cure c y c l e : overnight c u r i n g a t room temperature, 2 hr. at 60°C, 2 hr. a t 100°C, and 4 h r . at 140°C. Using these techniques, i t was p o s s i b l e to prepare reproduc i b l e samples s u i t a b l e f o r t e s t i n g , a l b e i t with c o n s i d e r a b l e d i f f i c u l t y i n the case of s o l i d epoxies. E l e c t r o n Microscopy. V a r i o u s etching techniques were t r i e d to study the micro- and macro-structures of MDA-cured epoxy n e t works ("macro" implying a morphological f e a t u r e on the s c a l e of 1 pm or l a r g e r ) . E t c h i n g may be presumed to p r e f e r e n t i a l l y a t t a c k regions of r e l a t i v e l y lower l o c a l c r o s s l i n k d e n s i t y . A f t e r the examination of etched samples under the ETEC Autoscan scanning e l e c t r o n microscope (SEM), i t was concluded that etching f o r 7 h r . with 1 M aqueous Ο ^ Ο β a t 80°C was more e f f e c t i v e than the f o l l o w ing techniques: etching f o r 30 min with HF; etching f o r 15 days with acetone; e t c h i n g f o r 10 hr. w i t h argon a t h i g h v o l t a g e under vacuum. The study of etching time i n d i c a t e d that a t l e a s t one hr. was needed to produce adequate etching with Ο ^ Ο β . Finally, etching times o f 4 hr. and 7 hr. were found to be convenient f o r samples having M s above and below 500, r e s p e c t i v e l y . The gross morphology was examined under the SEM while the f i n e s t r u c t u r e was examined using a P h i l l i p s 300 t r a n s m i s s i o n e l e c t r o n m i c r o scope (TEM), using two-stage carbon-platinum r e p l i c a s of the e t c h ed s u r f a c e s . I t should be noted that one must be very c a r e f u l i n i n t e r p r e t i n g statements about f e a t u r e s seen i n r e p l i c a s . Thus a nodule on a two-stage r e p l i c a a r i s e s from a dimple (not a nodule) on the polymer s u r f a c e . f

c

I s o l a t i o n of M i c r o g e l s . The f o l l o w i n g experiments were made to i s o l a t e m i c r o g e l s during the process of c u r i n g and from the etchant s o l u t i o n : 1. The formation of m i c r o g e l s during the c u r i n g process was v e r i f i e d experimentally on samples E-2 and E-5 by d i s s o l v i n g them i n acetone, before g e l a t i o n . Sample E-2, which has a longer g e l a t i o n time than E-5, was d i s s o l v e d a f t e r c u r i n g a t 60°C f o r 30 min and a t 80°C f o r another 15 min whereas sample E-5 was


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d i s s o l v e d a f t e r c u r i n g f o r only 10 min a t 100°C. The s o l u t i o n was d i l u t e d to a c o n c e n t r a t i o n of 100 ppm, and e l e c t r o n microscope g r i d s were made and examined under the TEM. 2. Etched samples were removed, the etching s o l u t i o n was d i l u t e d with d e i o n i z e d water to a very low c o n c e n t r a t i o n , and e l e c t r o n microscope g r i d s were prepared by platinum shadowing. S i m i l a r l y , a l e s s d i l u t e s o l u t i o n (from the etching of sample E5) was placed on a c l e a n microscope s l i d e , allowed to dry, and examined under the SEM. Results F i g u r e 1 shows t y p i c a l macrostructures, with f e a t u r e s i z e s from ca. 10 ym to 40 ym, revealed by p r o g r e s s i v e etching by the aqueous ( ^ 0 3 . The e f f e c t s of d i f f e r e n t v a r i a b l e s on morphology are discussed below. E f f e c t of Stoichiometry. Examination of the etched surfaces under the SEM (Figures 2 and 3) revealed a two-phase s t r u c t u r e . Dimples were observed on the surface, the dimple s i z e increased with d e v i a t i o n from equivalent stoichiometry. Samples having an excess of epoxy appeared to have fewer dimples than samples having a s i m i l a r M but excess amine. The s i z e s of the dimples v a r i e d from ca. 10 ym to 70 ym, depending on the percent excess of r e a c t a n t s , i n general agreement with s i z e s reported by Cuthr e l l (7) and Selby and M i l l e r (37). I t was a l s o observed that above an amine/epoxy r a t i o of 1.6/1 the morphological d i f f e r ences were n e g l i g i b l e . The two-stage r e p l i c a s examined with the TEM a l s o revealed the existence of two phases on a much f i n e r s c a l e (Figure 4). The r e p l i c a s showed nodules on the s u r f a c e s , corresponding to dimples on the polymer s u r f a c e s . The dimple s i z e s i n these samples were i n the range of 25 nm to 50 nm except f o r the case of 100% excess amine (A-20) where a l a r g e d i s t r i b u t i o n of s i z e s , ranging from 20 nm to 200 nm, was observed. The d i s t r i b u t i o n of phase s i z e tends to broaden with i n c r e a s i n g proportions of excess amine; the average s i z e a l s o tended to i n c r e a s e with the p r o p o r t i o n o f excess amine. On the other hand, an i n c r e a s e i n the amount of excess epoxy d i d not show s i g n i f i c a n t changes from the morphology observed a t equal s t o i c h i o m e t r y . These observations d i f f e r from those of Racich and Koutsky (31), who found that the domain s i z e decreased with i n c r e a s i n g amounts of c a t a l y s t and with the presence of saturated vapor of the c u r i n g agent. They argued that the c r o s s l i n k d e n s i t y i n c r e a s e s with i n c r e a s i n g amounts of c a t a l y s t , r e s u l t i n g i n smaller domain s i z e s . The present r e s u l t s i n d i c a t e that, as reported by Racich and Koutsky, the domain s i z e tends to increase with a decrease i n c r o s s l i n k d e n s i t y , but only i n the excess-amine case; the domain i s not s i g n i f i c a n t l y a f f e c t ed by changes i n c r o s s l i n k d e n s i t y f o r the excess-epoxy case. c



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Figure 1.

Micrographs of Cr O -etched surfaces of Epon 1004-MDA (Specimen E-7). Etching times: (A) 2 hr; (B) 4 hr; (C) 7 hr; (D) 11 hr 2

s



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Figure 2. Scanning electron micrographs of 7-hr-etched samples having different stoichiometric ratio of MDA to Epon 828: (A) 0.7/1; (B) 0.8/1; (C) 1/1; (D) 1.1/1



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Figure 3. Scanning electron micrographs of 7-hr-etched samples having different stoichiometric ratio of MDA to Epon 828: (A) 1.4/1; (B) 1.6/1; (C) 1.8/1; (D) 2.0/1



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Figure 4. Transmission electron micrographs of replicas of 4-hr-etched samples having different stoichiometric ratio of MDA to Epon 828: (A) 0.7/1; (B) 0.8/1; (C) 1.1/1; (D) 1.4/1; (E) 1.6/1; (F) 2.0/1



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E f f e c t of Molecular Weight of the Prepolymer. When the c r o s s l i n k d e n s i t y was changed by changing the molecular weight of the epoxy prepolymers, a two-phase s t r u c t u r e , s i m i l a r to that found f o r the case of v a r i a b l e s t o i c h i o m e t r y , was observed even at the macro l e v e l (Figures 5 and 6 ) . A marked d i f f e r e n c e i n morphology was observed i n networks prepared from l i q u i d r e s i n s as compared to those from s o l i d or s e m i - s o l i d r e s i n s . The l i q u i d - r e s i n (Epon 825, 828) samples had dimples which had a d i s t r i b u t i o n of s i z e ranging from 5 ym to 25 ym. In a d d i t i o n to dimples, small r i d g e s were a l s o seen i n both agglomerated and i s o l a t e d forms. In samples from s e m i - s o l i d and s o l i d r e s i n s (Epon 834, 1001, 1002 and 1004), e t c h - r e s i s t a n c e (and, hence, presumably h i g h l y c r o s s l i n k e d ) s h e l l s were observed throughout the c r o s s - s e c t i o n (Figure 7), encapsulating the l e s s c r o s s l i n k e d m a t e r i a l . These shell-and-core-type s t r u c t u r e s were present i n these samples i n a range of s i z e s from 2.5 ym to 25 ym. I t was found that the range narrowed but that the average s i z e increased with an increase i n the molecular weight of the prepolymer. T h i s t y p i c a l morphology was a l s o observed i n samples that were solvent cast from acetone s o l u t i o n s and cured a f t e r most of the acetone evaporated. Since i t has been shown that c u r i n g does not take place i n the presence of acetone i n the Epon-MDA system (39), i t can be assumed that the acetone-cast samples were s i m i l a r to the bulk-cured samples. Indeed no d i f f e r e n c e s i n mechanical proper t i e s i n bulk and acetone-cast samples were observed (41, Ch. 6). Etched under s i m i l a r c o n d i t i o n s , s o l v e n t - c a s t samples showed the same shell-and-core-type morphology (Figure 7d), i n d i c a t i n g that t h i s t y p i c a l morphology was not due to inadequate mixing. A l l the samples made from s o l i d or s e m i - s o l i d prepolymers had the same f i n e s t r u c t u r e , with the s i z e of the discontinuous phase i n the range from 15 nm to 25 nm, whereas the samples made from l i q u i d prepolymers had s l i g h t l y l a r g e r discontinuous phases i n the range from 25 nm to 50 nm. E f f e c t of the D i s t r i b u t i o n of Molecular Weight i n the P r e polymer . The gross morphologies (both at macro and micro l e v e l s ) i n the bimodal blend samples ( S e r i e s F) appeared to be a p p r o x i mately s i m i l a r to those of the counterparts prepared from the S e r i e s Ε commercial r e s i n s (see F i g u r e 8 f o r a t y p i c a l compari son) . E f f e c t of High-Molecular-Weight Curing Agent. Sample G-l (cured w i t h Versamid 140) a l s o showed the shell-and-core-type morphology (Figure 9). The s i z e s of the shell-and-core s t r u c t u r e (range from 2.5 ym to 25 ym) and the discontinuous phase were l a r g e r than those cured w i t h MDA, probably because t h i s sample had a higher M . F i g u r e 9 shows a s h e l l ( e n c i r c l e d i n black) which was not etched open by the chromic a c i d a t the time the sample was removed. A semi-open s h e l l can a l s o be seen which looks s i m i l a r to a dimpled nodule (though of a much l a r g e r s i z e ) c


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Figure 5. Scanning electron micrographs of 7-hr-etched networks prepared from equivalent stoichiometric amounts of Epon-MDA resins: (A) Epon 825; (B) Epon 828; (C) Epon 834; (D) Epon 1001; (E) Epon 1002; (F) Epon 1004



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Figure 6. Transmission electron micrographs of replicas of networks prepared from equivalent stoichiometric ratios of Epon-MDA resins etched for 4 hr: (A) Epon 825; (B) Epon 828; (C) Epon 834; (D) Epon 1001; (E) Epon 1002; (F) Epon 1004



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Figure 7. Scanning electron micrographs of Epon 1001-MDA sample (E-5) etched for 7 hr: (A) and (B) sample cross section; (C) sample surface; and (D) surface of solvent-cast sample



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Figure 8. Scanning electron micrographs of Series F networks along with their Series Ε counterparts etched for 7 hr: (A) Epon 828; (B) blend (Epons 825 and 1004) equivalent to Epon 828; (C) Epon 1001; (D) blend (Epons 828 and 1004) equivalent to Epon 1001



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Figure 9. Scanning electron micrographs of Epon 828 resin cured with Versamid 140 (network comprising 25% ghss beads by volume; etched for 7 hr)


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shown by Racich and Koutsky (31). To check i f the shell-and-core-type s t r u c t u r e observed i n s e m i - s o l i d and s o l i d prepolymers was due to f a s t c u r i n g , Epon 828 ( l i q u i d prepolymer) and MDA ( i n s t o i c h i o m e t r i c a l l y e q u i v a l e n t amounts) were cured at 150°C f o r 4 hr. T h i s sample, a f t e r being etched as described e a r l i e r , d i d not show any d i f f e r e n c e i n morphology from the sample cured through the r e g u l a r c u r i n g c y c l e . I t t h e r e f o r e appears that t h i s p e c u l i a r morphology depends on the molecular weight of the prepolymer and not on the r a t e of c u r i n g or inhomogeneous mixing. I s o l a t e d M i c r o g e l s . The e l e c t r o n micrographs (Figures 10, 11a and l i b ) c l e a r l y show the i n d i v i d u a l microgels (ranging i n s i z e from 20 nm to 200 nm) and c l u s t e r s of m i c r o g e l s that were etched out from the network s u r f a c e . The appearance of these microgels i s i n good agreement w i t h the observations made on r e p l i c a s and i n d i c a t e s that sample E-2 (prepared from l i q u i d prepolymers) had l a r g e r m i c r o g e l s ( s i z e s from 50 nm to 150 nm) as compared to sample E-5 (prepared from s o l i d prepolymer, s i z e s from 25 nm to 60 nm). These micrographs prove that the dimples, observed through r e p l i c a s , on the s u r f a c e s of the epoxy networks were due to the e t c h i n g of the weak connections o f the microgels or c l u s t e r s of m i c r o g e l s and not to the etching of m a t e r i a l having a low average c r o s s l i n k d e n s i t y . The platinum shadows i n F i g u r e s 11a and l i b i n d i c a t e that the microgels are s o l i d and n e a r l y s p h e r i c a l i n shape (a c o n f i g u r a t i o n having minimum s u r f a c e energy) and can pack i n a hexagonal a r r a y . S i m i l a r l y the e l e c t r o n micrographs ( F i g u r e s 11c and l i d ) show the existence of m i c r o g e l s even before g e l a t i o n (darker phase) i n the s i z e range from 30 nm to 100 nm. Thus, i n both cases d i s c r e t e m i c r o g e l p a r t i c l e s were observed, i n d i c a t i n g that microgel formation takes p l a c e before g e l a t i o n and that the m i c r o g e l s are d i s p e r s e d and l o o s e l y connected to each other and i n some cases ( n o n - s t o i c h i o m e t r i c compositions) to a weaker continuous phase. These r e s u l t s a l s o show that the discontinuous phase (though etched out f i r s t as clumps) i s composed of microgels that are stronger than the continuous phase. I t i s the connections between the aggregates that are weak. The s i z e and number of these g e l s probably continues to i n c r e a s e u n t i l the v i s c o s i t y becomes high enough f o r p h y s i c a l g e l a t i o n to take p l a c e . Discussion General Proposed Model f o r Network Formation. As mentioned e a r l i e r , e l e c t r o n microscopic evidence shows that m i c r o g e l p a r t i c l e s form p r i o r to g e l a t i o n (Figures 10 and 11), both with low and h i g h molecular weight epoxies and with epoxy and amine-rich systems. T h i s o b s e r v a t i o n i s i n accord with the suggestions of other i n v e s t i g a t o r s (14,27,28). Some i n v e s t i g a t o r s have observed



12.

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Figure 10. Scanning electron micrographs of isolated secondary microgels from Epon 828-MDA sample (E-2)



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Figure 11. Transmission electron micrographs of isolated primary microgeh: (A) and (B) gel particles etched out by Cr O solutions at70°C after 4 hr; (C) and (D) microgels formed before gelation of the epoxy resins. [A and C—E-2; Β and D—E-5] 2

s



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ET

AL.


175

these microgels i n s i z e s smaller than 0.5 ym while others have reported l a r g e r than 5 ym. The present r e s u l t s i n d i c a t e the presence of microgels at both l e v e l s ; a lower l e v e l (10 nm to 50 nm) and a higher l e v e l (2 ym to 50 ym). I t i s , t h e r e f o r e , probable that the systems studied by other i n v e s t i g a t o r s a l s o had microg e l s at both l e v e l s . In order to d i f f e r e n t i a t e between the two s i z e l e v e l s , the term primary microgels i s used to r e f e r to microgels i n the s i z e range from 10 nm to 50 nm, whereas secondary microgels r e f e r s to s i z e s l a r g e r than 1 ym. In view of the morphological and property data at hand, a. model f o r network formation i s proposed. The p r i n c i p a l p o i n t s to consider i n c l u d e the f o l l o w i n g questions: the b a s i c morphologic a l u n i t s , the phase c o n t i n u i t y , and the c r o s s l i n k d e n s i t y and other p r o p e r t i e s of the v a r i o u s e n t i t i e s . The proposed mechanism which e s s e n t i a l l y comprises a s y n t h e s i s of the mechanisms proposed by Bobalek et a l . (27) , Solomon (33) and Labana et a l . (14), considers that the formation of the macrogel takes place i n three s e q u e n t i a l steps (Figure 12): formation of the primary microgels, formation of the secondary m i c r o g e l s , and formation of the macrog e l . F o l l o w i n g Labana et a l . (14), i t i s a l s o proposed that the secondary microgels and the macrogel are not completely coherent (that i s , they have some unconnected dangling c h a i n s ) . Formation of the Primary M i c r o g e l s . The r e a c t i v i t i e s of the primary and secondary amine groups i n MDA are approximately i n the r a t i o of 1.4/1 (39). The primary amine groups should, t h e r e f o r e , r e a c t f i r s t to give a l i n e a r s t r u c t u r e . Due to a combinat i o n of an exothermic heat of r e a c t i o n with poor heat t r a n s f e r , the temperature should r i s e i n the v i c i n i t y of these molecules, r e s u l t i n g i n the r e a c t i o n of the secondary amine groups. Indeed, i t has been shown that the primary and secondary amine groups react almost simultaneously even when t h e i r r e a c t i v i t i e s d i f f e r by a f a c t o r of two (43), The growth of the n u c l e i continues u n t i l the r e a c t i v e polymer molecules can d i f f u s e to the r e a c t i v e s i t e s of the n u c l e i . During t h i s process s e v e r a l new n u c l e i are a l s o developed. Therefore, a d i s t r i b u t i o n of s i z e of the primary microgels would be expected at a l l times during the c u r i n g proccess. T h i s phenomenon has been p r e d i c t e d s t a t i s t i c a l l y by Labana et a l . (14), who termed i t a n u c l e a t i o n process resembling the one that occurs i n c r y s t a l l i z a t i o n . Formation of the Secondary M i c r o g e l s . A f t e r a c e r t a i n conc e n t r a t i o n has been reached, the primary microgels and the growing n u c l e i begin to i n t e r a c t with each other and g i v e r i s e to new n u c l e i f o r the secondary m i c r o g e l s . These n u c l e i grow i n part due to c a p i l l a r y f o r c e s which w i l l encourage p h y s i c a l s i n t e r i n g and i n part due to the r e a c t i o n of unreacted f u n c t i o n a l groups i n the primary m i c r o g e l s . Thus, the secondary microgels are not as coherent as the primary m i c r o g e l s . The s i z e of a p a r t i c u l a r secondary microgel would depend on the c o n c e n t r a t i o n of the



EPOXY

PRIMARY NUCLEI

GROWING NUCLEI

PRIMARY MICROGEL

SECONDARY .NUCLEI

SECONDARY MICROGEL

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MACROGEL

MACROGEL

Figure 12.

Proposed model for network formation


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177


primary m i c r o g e l s near the secondary nucleus. A s i z e d i s t r i b u t i o n i n the secondary m i c r o g e l s should a l s o be expected. Formation of the Macrogel ( F i n a l Network). At a c r i t i c a l s o l i d s c o n c e n t r a t i o n (-74% f o r monodispersed spheres arranged i n a hexagonal packing), the secondary m i c r o g e l s pack together and the experimental g e l point i s observed. At t h i s stage, phase i n v e r s i o n takes p l a c e d u r i n g which the secondary m i c r o g e l s become the matrix and the unreacted or p a r t i a l l y reacted prepolymers become the dispersed phase. By the time of complete r e a c t i o n , the secondary m i c r o g e l s become l o o s e l y connected to each other with the help of the i n t e r s t i t i a l prepolymer or due to s e l f d i f f u s i o n of r e a c t i v e dangling groups. Thus, the i n t e r c o n n e c t i o n s between the secondary m i c r o g e l s would be weaker than those between the primary m i c r o g e l s . The p r o p e r t i e s o f the f i n a l n e t work should be a f f e c t e d by the coherence of the network. In n e t works prepared by n o n - s t o i c h i o m e t r i c compositions, the secondary microgels would be embedded i n a matrix of lower c r o s s l i n k dens i t y but s t i l l connected to each other through chemical bonds. Based on the above model, the morphological d i f f e r e n c e s and mechanical behavior observed i n the samples of d i f f e r e n t s e r i e s are i n t e r p r e t e d below. Morphological D i f f e r e n c e s . In epoxy prepolymers, the r e a c t i v i t y i n c r e a s e s with the number of hydroxyl groups present i n the prepolymer c h a i n (44) . Thus, an i n c r e a s e i n the molecular weight of the epoxy prepolymer would not only reduce i t s d i f f u s i v i t y but would a l s o i n c r e a s e i t s r e a c t i v i t y , r e s u l t i n g i n a reduced time of m i c r o g e l formation. T h i s combined e f f e c t would r e s u l t i n a smaller s i z e of the primary m i c r o g e l s , as observed experimentally. In the case of n o n - s t o i c h i o m e t r i c compositions, the s i z e of the primary m i c r o g e l s should be expected to be governed both by the v a l u e of M and by the s t r u c t u r e , because the d i f f u s i v i t y and the r e a c t i v i t i e s of the prepolymers should be almost the same at a l l s t o i c h i o m e t r i e s . In the Epon 828/MDA system ( S e r i e s A), an excess i n amine c o n c e n t r a t i o n should i n i t i a l l y y i e l d many l i n e a r molecules whose l e n g t h would depend on the amount of excess amine. The premature r e a c t i o n of the secondary amines of these long molecules would r e s u l t i n l a r g e r m i c r o g e l s . Thus, i n the case of amine excess, the s i z e of primary m i c r o g e l s , as observed experimentally, should i n c r e a s e w i t h i n c r e a s i n g amounts of excess amine. In the case of excess epoxy, a branched s t r u c ture having four epoxy molecules attached to one amine molecule i s formed (39). Thus, the s i z e of the m i c r o g e l s formed at 100% epoxy-excess would be much smaller than those formed at 100% amine excess. The experimental r e s u l t s showed a s i m i l a r trend. For systems having a slower r a t e of r e a c t i o n (such as samples of S e r i e s A, sample E - l , sample E-2, and sample F-2, which were made from l i q u i d prepolymers) the higher d i f f u s i v i t y c



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and lower r e a c t i v i t y r e s u l t i n s l i g h t l y l a r g e r primary microgels. However, the loose connections of the adjacent secondary microgels can be r e a d i l y attacked, r e s u l t i n g i n the etching of the secondary microgels and thus formation of dimples on the surface (as was observed e x p e r i m e n t a l l y ) . The s e m i - s o l i d and s o l i d r e s i n s are composed of high-M and low-M prepolymer components; the amount of low-M component decreases with the average M of the prepolymer (41, Ch. 3). The more r e a c t i v e h i g h M components form the microgels that get dispersed i n the low-M component, which r e a c t s l a t e r to give a high c r o s s l i n k d e n s i t y s h e l l . A decrease i n the p r o p o r t i o n of the low-M component should i n c r e a s e the s i z e but decrease the number of the s h e l l and-core-type of s t r u c t u r e as was found experimentally. Dynamic Mechanical Spectroscopy. As pointed out above, the heterogeneous morphology, observed i n the present study, i m p l i e s network flaws (weak connections between the primary and secondary m i c r o g e l s ) . However, the low c y c l i c s t r a i n s a p p l i e d i n dynamic mechanical t e s t s d e t e c t only the e f f e c t of the b a s i c network s t r u c t u r e and not the network flaws. Therefore, as observed experimentally (38,41), dynamic mechanical spectroscopy should not i n d i c a t e heterogeneity i n the samples. Soluble Content. The morphological s t u d i e s i n d i c a t e d an increase i n the dimple s i z e of M . A dimple a c t u a l l y represents a b i t of m a t e r i a l extracted from the network. This observation i s i n good agreement with the e x t r a c t i o n r e s u l t s (41,42) that showed an i n c r e a s e i n s o l u b l e content with M . c

c

Mechanical Behavior. At present, i t i s not p o s s i b l e to r e c o n c i l e a l l the d i v e r s e dependences on s t o i c h i o m e t r y . However, c l e a r l y there i s no a p r i o r i reason f o r trends i n a l l p r o p e r t i e s to be r e l a t e d . By a m o d i f i c a t i o n of the G r i f f i t h equation (45) a

u

=

( 2 ES / a )

1

/

2

(1)

where o = u l t i m a t e t e n s i l e strength, Ε = Young's modulus, S = f r a c t u r e energy, and a. = c h a r a c t e r i s t i c flaw s i z e . Thus o at constant Ε depends on how S v a r i e s with a.; S may change while o does not, i f changes i n a_ compensate f o r changes i n S. For the Epon 828/MDA system, both B e l l (46) and Kim et a l . (45) have shown that o i s almost independent of s t o i c h i o m e t r y . Further more, i t has a l s o been shown that impact toughness i s a t a maxi mum not a t 100% s t o i c h i o m e t r y but at a 1/1.4 epoxy/amine r a t i o (37,45,46). The impact strength f o r the case of excess epoxy was a l s o reported to be higher than that at equal stoichiometry. Under these circumstances, i t has been shown that the apparent c r i t i c a l flaw s i z e a i s a l i n e a r f u n c t i o n of the amine/epoxy r a t i o , changing from 32 ym to 141 ym (45). u

u

u

u



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Whether the c r i t i c a l flaw s i z e corresponds to the aggregates f i r s t formed p r i o r to phase i n v e r s i o n or whether i t corresponds to the i n c l u s i o n s i s , of course, not known. Indeed, the r e a l i t y of a flaw corresponding to the c a l c u l a t e d value of a. i s not proven. Nevertheless, the c l o s e agreement between m i c r o s c o p i c evidence of the present study and the computed flaw s i z e (45) i s s t r o n g l y suggestive of a c o r r e l a t i o n . Though the modulus, E, does not decrease ( i n f a c t i n c r e a s e s s l i g h t l y (45,46) w i t h an i n c r e a s e i n amine content, the o v e r a l l p l a s t i c deformation should i n c r e a s e due to the p l a s t i c i z i n g character of the amine. Hence, S i n c r e a s e s (E i t s e l f may increase due to enhanced c o n t i n u i t y of the network). At the same time, the flaw s i z e i n c r e a s e s a l i t t l e but not as f a s t as S so that t e n s i l e s t r e n g t h a c t u a l l y increases a l i t t l e . But with an epoxy excess, the decreased flaw s i z e (corresponding to smaller aggregates) more than balances out the lower value of S so that the s t r e n g t h again i n c r e a s e s . However, the toughness otherwise seen a t high amine contents i s v a s t l y reduced by the high l o a d i n g r a t e i n impact; while with epoxy excess, the smaller flaw s i z e i s a b l e to d e l a y f r a c t u r e . In the case i n which the M i s v a r i e d by v a r y i n g the molecu l a r weight of the epoxy r e s i n a t equal s t o i c h i o m e t r y ( S e r i e s Ε ) , the u l t i m a t e t e n s i l e s t r e n g t h i s independent of M but the impact strength decreases with M (38,41). When the molecular weight of the epoxy i s i n c r e a s e d , a d i f f e r e n t behavior occurs. The inherent S of the epoxy components i s low compared to that of amine and, hence, the t e n s i l e s t r e n g t h does not i n c r e a s e and impact s t r e n g t h decreases. c

c

c

The Glass T r a n s i t i o n Temperature. Samples having a bimodal d i s t r i b u t i o n of molecular weight ( S e r i e s F) showed the same Tg as t h e i r counterparts having a broad d i s t r i b u t i o n i n the high (> 700) and the low (< 400) ranges of M „ At intermediate M *s, the Tg was found to be lower i n samples having a bimodal d i s t r i bution (42). Though the morphology of the bimodal and broad d i s t r i b u t i o n was the same, the p r o p e r t i e s of the former w i l l be determined by the dominant component i n the f i n a l network. I f t h i s i s a high-molecular-weight r e s i n (which i s more r e a c t i v e ) , t h i s network may be expected to dominate and Τ w i l l be low, perhaps because the i n i t i a l f a s t r e a c t i o n r e s u l t s i n g r e a t e r incoherence. At intermediate compositions, a j u d i c i o u s balancing may occur and r e s u l t i n a more coherent network. Thus p r o p e r t i e s may be governed by the low-molecular-weight component. The f a c t that small-angle X-ray s c a t t e r i n g and stained or unstained microtomed t h i n s e c t i o n s f a i l to i n d i c a t e a two-phase s t r u c t u r e (37,41) a l s o supports the present model, which e x p l a i n s that heterogeneity i n networks i s p r i m a r i l y due to incoherence on a l o c a l s c a l e and not to major v a r i a t i o n s i n average c r o s s l i n k density. c

c

β


180


Although t h i s d i s c u s s i o n i s c l e a r l y s p e c u l a t i v e , i t does e x p l a i n some of the behavior noted and suggests ideas capable of t e s t i n g i n the l a b o r a t o r y .


Conclusions From the present study and the s t u d i e s of other i n v e s t i g a t o r s (discussed e a r l i e r ) , i t i s c l e a r that c r o s s l i n k e d networks i n many epoxy r e s i n s and i n some alkyd r e s i n s a r e heterogeneous i n nature. According to the proposed model, small primary microgels ranging i n s i z e from 10 nm to 100 nm a r e formed much before the i n s e t of p h y s i c a l g e l a t i o n . These primary m i c r o g e l s agglomerate together through weak connections to produce secondary m i c r o g e l s ranging from 0.5 ym to 50 ym. The secondary m i c r o g e l s c o a l e s c e together and the experimental g e l point i s observed. The f i n a l c r o s s l i n k e d network i s produced a f t e r complete r e a c t i o n of the unreacted prepolymers l e f t i n the i n t e r s t i c e s of the coalesced secondary m i c r o g e l s . As compared to primary m i c r o g e l s , these secondary m i c r o g e l s a r e g e n e r a l l y connected to each other through "weaker" or l e s s coherent l i n k s . In networks prepared through n o n - s t o i c h i o m e t r i c compositions, the secondary m i c r o g e l s a r e embedded i n a matrix of lower c r o s s l i n k d e n s i t y but s t i l l con nected to each other through chemical bonds. The coherence o f the f i n a l network (macrogel) depends on the d e n s i t y s t r e n g t h of the connections between the secondary m i c r o g e l s which i n turn governs i t s t e n s i l e p r o p e r t i e s . In the case i n which the prepolymers have a bimodal d i s t r i bution of molecular weight, the p r o p e r t i e s of the network a r e governed by the component that i s more dominant i n the m i c r o g e l s . Though s u f f i c i e n t experimental evidence i s not p r e s e n t l y a v a i l a b l e , the o p t i m i z a t i o n o f c u r i n g c o n d i t i o n s should a l s o p l a y an important r o l e i n a c h i e v i n g maximum coherence i n the network. Ac kno wl ed g em en t s The authors wish to acknowledge support f o r the f i r s t part of t h i s work through AFMC C o n t r o l No. F33615-75C-5167. A d d i t i o n a l support from the M a t e r i a l s Research Center and the Ford Motor Fund i s a l s o much a p p r e c i a t e d .

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RECEIVED May 21, 1979.


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