Morphology, toughness mechanism, and thermal properties of hyperbranched epoxy modified diglycidyl ether of bisphenol A (DGEBA) interpenetrating polymer networks

New hyperbranched poly(trimellitic anhydride-triethylene glycol) ester epoxy (HTTE) is synthesized and used to toughen diglycidyl ether of bisphenol A (DGEBA) 4,4'-diaminodiphenylmethane (DDM) resin system. The effects of content and generation number of HTTE on the performance of the cured systems are studied in detail. The impact strength is improved 2-7 times for HTTE/ DGEBA blends compared with that of the unmodified system. Scanning electron microscopy (SEM) of fracture surface shows cavitations at center and fibrous yielding phenomenon at edge which indicated that the particle cavitations, shear yield deformation, and in situ toughness mechanism are the main toughening mechanisms. The dynamic mechanical thermal analyzer (DMA) analyses suggest that phase separation occurred as interpenetrating polymer networks (IPNs) for the HTTE/DGEBA amine systems. The IPN maintains transparency and shows higher modulus than the neat epoxy. The glass transition temperature (T-g) decreases to some extent compared with the neat epoxy. The T. increases with increase in the generation number from first to third of HTTE and the concentrations of hard segment. The HTTE leads to a small decrease in thermal stability with the increasing content from TGA analysis. The thermal stability increases with increase in the generation number from first to third. Moreover, HTTE promotes char formation in the HTTE/DGEBA blends. The increase in thermal properties from first to third generation number is attributed to the increase in the molar mass and intramolecular hydrogen bridges, the increasing interaction of the HTTE/ DGEBA IPNs, and the increasing crosslinking density due to the availability of a greater number of end hydroxyl and end epoxide functions. Copyright (c) 2008 John Wiley & Sons, Ltd.