11 Effect of Cross-Link Density Distribution on the Engineering 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.ch011
Materials Research Center, Lehigh University, Bethlehem, PA 18015
Fundamental knowledge of relationships between characteristics, synthesis or processing, structure, and mechanical and other properties is required of crosslinked networks used for critical and demanding applications. Thus, epoxy resins, which are widely used for engineering purposes, have received great attention in the past decade. Indeed, many papers have covered many aspects of epoxies such as: stoichiometry (1-5), prepolymer structure (6-8), diluents (4,9,12), fillers (13-22), heat treatment (3,22), and effects of cure conditions (23-25). However, relatively l i t t l e attention has been given to properties such as creep and to the effects of the distribution of crosslink density. Certainly typical commercial epoxy resins usually exhibit a distribution of molecular weights which should result in a distribution of crosslink density in the final network. The broadening of the transition region has been qualitatively attributed to this distribution effect (26). Unfortunately, existing studies were of insufficient scope to permit correlation of the engineering behavior of epoxy networks and the distribution of crosslink density or M (the average molecular weight between crosslinks). Because of its relevance to engineering application, such a study was thought to be of considerable interest. c
With t h i s i n mind, a program was begun to examine the e f f e c t s of c r o s s l i n k i n g and c r o s s l i n k d i s t r i b u t i o n on s e v e r a l aspects o f behavior of high-Tg epoxies (27-31). I t was decided that a thorough c h a r a c t e r i z a t i o n o f v i s c o e l a s t i c behavior i n blends o f two epoxy r e s i n s , each having a q u i t e d i f f e r e n t molecular weight, should enable c o r r e l a t i o n s o f d i s t r i b u t i o n s o f M and other n e t work v a r i a b l e s with the engineering behavior. A c c o r d i n g l y , the c r o s s l i n k d e n s i t y (M ) was v a r i e d by c u r i n g a homologous s e r i e s o f bisphenol-A-based epoxy prepolymers with methylene d i a n i l i n e (MDA). Networks were a l s o prepared at constant c r o s s l i n k d e n s i t i e s by blending low and high-molecular-weight members o f the homologous series. This paper summarizes the f o l l o w i n g p r o p e r t i e s of the networks: the s t a t e of cure, M , dynamic mechanical spectroscopy c
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c
0-8412-0525-6/79/47-114-137$05.00/0 © 1979 American Chemical Society
Bauer; Epoxy Resin Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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EPOXY
RESIN
CHEMISTRY
(DMS), creep, and s t r e s s - s t r a i n and impact behavior. More d e t a i l e d r e s u l t s f o r each t o p i c w i l l be presented and discussed separately.
Downloaded by CORNELL UNIV on October 8, 2016 | http://pubs.acs.org Publication Date: December 3, 1979 | doi: 10.1021/bk-1979-0114.ch011
Experimental M a t e r i a l s . The f o l l o w i n g Epon prepolymers ( S h e l l Chemical Company) were used: Epons 825, 828, 834 ( a l l l i q u i d to semi s o l i d ) ; and 1001, 1002, and 1004 ( a l l s o l i d s ) . Approximate com p o s i t i o n s (degree of polymerization) were determined by g e l per meation chromatography. Bimodal blends were made by mixing Epon 825 with Epon 1004, as shown i n Table 1. Sample p r e p a r a t i o n and the method of c u r i n g have been described i n the preceding paper (29). Table I .
Compositions of Homopolymers and Bimodal Blends
Sample
Mc
E-l E-2 F-l F-2 F-3 E-3 F-4 E-5 F-5 E-7 E-4
Epoxy Resin (wt. % i n blend)
(100) (100) (91) + 1004 (9) (62) + 1004 (38) (60) + 1004 (40) (100) (57) + 1004 (43) (100) (20) + 1004 (80) (100) (100) (with s l i g h t excess of c u r i n g agent) M a t e r i a l s C h a r a c t e r i z a t i o n . Measurements of complex Young's moduli, i n c l u d i n g the storage modulus E , l o s s modulus E", and l o s s tangent tan
