Toughening of epoxy thermosets with nano‐sized or micron‐sized domains of poly(ethylene oxide)‐b‐poly(butadiene‐co‐acrylonitrile)‐b‐poly(ethylene oxide) triblock copolymers synthesized using room temperature ester coupling reaction

Abstract

Poly(ethylene oxide)-b-poly(butadiene-co-acrylonitrile)-b-poly(ethylene oxide) (PEO-b-PBN-b-PEO) triblock copolymers with three different compositions were synthesized from poly(ethylene glycol) methyl ethers and carboxylic acid-terminated poly(butadiene-co-acrylonitrile) (CTBN) by ester coupling reaction at room temperature. The PEO-b-PBN-b-PEO was incorporated into anhydride cured epoxy thermosets to improve the fracture toughness by the formation of either nano-sized spherical micelles or micron-sized vesicles. The polymer chemical structure was confirmed by Fourier transform infrared spectroscopy, nuclear magnetic resonance, and gel permeation chromatography. The morphology of PEO-b-PBN-b-PEO within the epoxy thermosets was investigated using a transmission electron microscope, an atomic force microscope, and a scanning electron microscope. Also, we conducted impact testing and plane-strain fracture toughness testing to evaluate the fracture toughness in terms of the impact strength and the critical stress intensity factors (K IC) for the modified epoxy thermosets. The results revealed that all the PEO-b-PBN-b-PEO triblock copolymers are more effective in the toughening of epoxy thermoset compare to CTBN. We found that the 5 wt% PEO-b-PBN-b-PEO modified epoxy thermoset containing micron-sized vesicles exhibited the highest K IC , which was 3.23 times as high as the K IC of pristine epoxy thermoset. Besides, the glass transition temperature remained and the tensile modulus did not reduce remarkably when the amount of PEO-b-PBN-b-PEO added into epoxy was 5 wt%.

Effects of Block Copolymer Terminal Groups on Toughening Epoxy-Based Composites: Microstructures and Toughening Mechanisms

“…As shown in Figure 8, during the fracture process, the dispersed phase can induce shear deformation of the matrix. Additionally, microcracking and crack deflection caused by the island phase can improve fracture toughness [34,36,43]. Further analysis of the fracture surface of the carboxyl-terminated BCP system showed that after the crosslinking and curing reactions, a significant number of island- phase structures of different sizes were distributed in the epoxy resin matrix.…”

“…As shown in Figure 8, during the fracture process, the dispersed phase can induce shear deformation of the matrix. Additionally, microcracking and crack deflection caused by the island phase can improve fracture toughness [34,36,43]. A comparison of the fracture surface results (Figure 9) at room and high temperatures showed that the main difference was that the matrix exhibited more bulk deformation during high-temperature testing at low magnification.…”

“…However, the addition of these tougheners often decreases the storage modulus and glass transition temperature (Tg). Among the many toughening studies related to epoxy resin, research on nanodomain toughening agents in multi-BCPs has attracted the most attention [26][27][28][29][30][31][32][33][34][35][36]. Owing to their long-range ordered nanostructures, BCPs can generate dispersed nanodomains in epoxy resin, which can improve toughness while maintaining Tg and the storage modulus [37][38][39][40][41].…”