Article pubs.acs.org/Macromolecules
Synergistic Toughening of Epoxy Modified by Graphene and Block Copolymer Micelles Tuoqi Li,† Siyao He,‡ Andreas Stein,‡ Lorraine F. Francis,*,† and Frank S. Bates*,† †
Department of Chemical Engineering and Materials Science and ‡Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States S Supporting Information *
ABSTRACT: Binary composites formed by individually mixing exfoliated graphene oxide modified with amineterminated poly(butadiene−acrylonitrile) (GA) and a spherical micelle forming poly(ethylene oxide)-b-poly(ethylene-altpropylene) (OP) diblock copolymer with a thermoset epoxy, and the associated GA/OP/epoxy ternary composites, were prepared and studied as a function of the molecular weight Mc between cross-links. The rigid GA filler dispersed well in the cured epoxies as established by transmission electron microscopy (TEM). The toughening efficacy of GA alone was found to depend strongly on the modifier concentration and the matrix cross-link density with an optimal 1.7-fold increase in the critical strain energy release rate (GIc) over the neat epoxy obtained with a 0.04 wt % loading in the most lightly cross-linked (Mc = 6100 g/mol) material. Addition of 5 wt % OP to this epoxy resin enhanced GIc by a factor of 12. Combining the hard GA and soft OP modifiers at the same loading levels (0.04 and 5 wt %, respectively) resulted in 18 times the GIc of the unmodified material, a 31% improvement over the effect anticipated by simple addition of the fracture properties of the binary composites. Decreasing Mc to 700 g/mol eliminated this synergistic effect while reducing the overall improvement in GIc to just 3 times that of the neat epoxy. Topological features on the fracture surfaces, imaged using a scanning electron microscope (SEM), suggest that the synergistic toughening of the GA/OP/epoxy ternary composite involves concurrent mechanisms operating on different length scales, including micelle cavitation and graphene debonding, resulting in simultaneous shear yielding, crack pinning, and crack deflection.
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INTRODUCTION Epoxy resins, one of the most important classes of thermoset polymers, have widespread industrial applications, including adhesives, protective coatings and paints, encapsulating materials for electronics, and matrix materials for composites.1−5 In fundamental studies, epoxy resins are also employed as cross-linkable selective solvents for modifying block copolymer phase behavior.6−10 Epoxy materials have excellent durability and rigidity along with chemical and thermal stability due to the cross-linked network structure; however, this structure also makes them intrinsically brittle and prone to fracture.11,12 Therefore, neat epoxy thermosets, although rigid and stable, generally suffer from very low fracture toughness. A large body of literature has been devoted to improving the mechanical properties of neat epoxies. The major toughening strategies fall into two categories: (i) modification with soft additives, including liquid rubbers or core−shell rubber particles,11,12 and amphiphlic block copolymers,13−16 and (ii) modification with rigid fillers, including silica particles,17 carbon nanotubes,18−20 clays,21−23 and graphenes.24−27 Soft additives, like liquid rubbers, require relatively high loadings (ca. 20 wt %) to achieve a satisfactory toughness improvement (e.g., the fracture toughness, KIc, increases by 50%). However, the © XXXX American Chemical Society
rubbery phase tends to macrophase separate from the matrix, forming micrometer-sized or even larger domains. Therefore, the resultant epoxy products possess reduced elastic modulus, hardness, and glass transition temperature (Tg), and they are also likely to lose optical transparency, rendering them unfit for many applications, especially coating products.11,12,28,29 On the other hand, amphiphiplic block copolymers can be designed to self-assemble into various nanostructures (e.g., bilayer vesicles, wormlike micelles, or spherical micelles) in the epoxy matrix and provide extraordinary toughening at low loading levels (
