CHARACTERIZATION OF THE PARTICLE-MATRIX INTERFACE IN RUBBER-MODIFIED EPOXY BY ATOMIC-FORCE MICROSCOPY

Atomic force microscopy (AFM) was used to study the interphase regions in rubber-toughened epoxy polymers. The nature of the interphase region was varied by either the adducting of reactive oligomers or by crosslinking the shell polymer on core/shell latex particles. The adducted reactive oligomers were comprised of carboxyl-terminated, butadiene-actrylonitrile copolymers (CTBN) prereacted with either (1) a low molecular weight diglycidyl ether of bisphenol A-based epoxy, which results in an interphase with increased crosslink density, or (2) a high molecular weight epoxy based on the diglycidyl ether of propylene glycol (DEGP), which results in an interphase with decreased crosslink density. The second type of rubber particles is custom-made submicron core/shell latex particles of a poly(butadiene-co-styrene)[P(BS)] core with an acrylate shell. Two acrylate shells were (1) PMMA/AN shell containing 25% acrylonitile and (2) a similar PMMA/AN with 5% divinyl benzene. The toughness of these blends was characterized using linear elastic fracture mechanics. The features on the fracture surfaces were examined using both AFM and FESEM (field emission scanning electron microscopy). AFM was able to detect features not observed in SEM and also to quantify all of the fracture surface features. In particular, the height-to-width ratio of the rim surrounding cavitated particles provided a useful means for determining the ductility of the interphase region. Attempts were made to determine the size of the interphase region using the frictional mode and the tip-adhesion forces. Unfortunately, the results of both approaches are inconclusive at the present time; this is most likely due to the deformation surrounding the rubber particles detected in the fast fracture regions. (C) 1995 John Wiley & Sons, Inc.