Authors: D. Liu, H-J Sue, Z.J. Thompson, F.S. Bates, M.A. Hillmyer
Affilation: Exponent, Inc., United States
Pages: 597 - 600
Keywords: epoxy toughening, block copolymer, micelles, self-assembly
A model diglycidyl ether of bisphenol A (DGEBA) epoxy resin was modified with a poly(ethylene-alt-propylene)-b-poly(ethylene oxide) (PEP-PEO) block copolymer to improve fracture toughness. Two distinct nanoscaled morphologies were achieved in the cured systems: spherical micelles (Figure 1) and worm-like micelles (Figure 2). The diameters of the spherical and worm-like micelles are both around 15 nm. Epoxies containing both nanostructures exhibit substantially improved fracture toughness at a very low loading (5 wt%). Toughening mechanisms were carefully investigated using the double-notch four-point-bending (DN-4PB) technique and microscopy tools. In the system with spherical micelles, it is found that the nanosized particles cavitate and then induce shear yielding of the epoxy matrix, which mainly accounts for the increase in fracture toughness. In the system with worm-like micelles, the toughening appears to be due to crack bridging, interfacial debonding, and limited shear yielding. The unique nanoscaled structures and interfacial characteristics of PEP-PEO are key to the significantly improved mechanical properties, without compromises to epoxy Tg and modulus. The implication of the present finding for designing toughened polymers will also be discussed.