Role of Localized Network Damage in Block Copolymer Toughened Epoxies

Abstract

The underlying mechanisms responsible for the toughening of block copolymer modified thermoset epoxies are not completely understood. A current theory targets cavitation of the rubbery cores in dispersed micelles as the key event that triggers shear yielding, resulting in enhanced toughness. To evaluate this hypothesis, we prepared spherical micelle forming block copolymers with rubbery cores (prone to cavitation) and glassy cores (unable to cavitate). Surprisingly, both systems enhance fracture toughness, although the rubbery core micelles outperform the glassy core counterparts. This finding challenges previous deductions regarding the toughening mechanism. We propose that the mechanical integrity of the region immediately surrounding the micelle core is compromised by the presence of the corona blocks, facilitating local deformation of the matrix. We speculate that the compliant nature of the rubber amplifies this effect.

Effect of block copolymer concentration and core composition on toughening epoxies

“…13 Recent simultaneous deformation and scattering experiments have confirmed the two relevant ideas for our discussion: cavitation in rubbery nanodomains and lack of void formation in epoxies modified with glassy core micelles. 14 However, in a previous paper 18 we reported that micelle core cavitation is not a necessary condition for toughening, as glassy core micelles also toughened epoxies, despite their inability to cavitate. It is important to note that the origins of toughness in epoxies modified with glassy core micelles are fundamentally different from the toughening mechanism in epoxies modified with other rigid particles.…”

“…18 This paper expands upon that work by better understanding the differences in spherical micelle core composition and additive concentration on the toughening of epoxy thermosets. In particular, the M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 4 effect of PEO homopolymer addition on microstructure and mechanical properties of the epoxy matrix is studied to provide insight to the corona, and fractographic analysis is carried out to explore further the differences between the fracture behavior of epoxies modified by glassy and rubbery cores.…”

“…% RuO 4 aqueous solution for 8 -10 minutes; based on our experience with epoxies, we believe the ruthenium preferentially stains PEO/matrix interface. 16,18,22 Specimens were examined in an FEI Technai T12 or an FEI Technai G2 Spirit BioTWIN transmission electron microscope with 120 kV accelerating voltage. All images were processed using Digital Micrograph software (Gatan).…”

Bio-based epoxy resin toughening with cashew nut shell liquid-derived resin

“…When high levels of the rubbery phase are required, rubber toughening may also result in materials with reduced modulus, lower glass transition temperatures and increased resin viscosities. 18 Diblock copolymer toughening of epoxy resins 23 enables toughening of thermoset systems without significant sacrifices in the modulus or glass transition temperature. 24 Further, by altering diblock copolymer structure by manipulating the chain segment block lengths, the toughening characteristics of the resulting thermoset materials can be controlled.…”

The change in the environment of the immiscible block stabilizes an unexpected HPC phase in a cured block copolymer/epoxy blend

“…Block copolymer (BCP)/epoxy blends have been the subject of a large number of recent studies where the focus was placed on the role of the BCP as a processing aid [1,2], as a template for the self-assembly of different type of nanoparticles [3][4][5], or as a toughening agent [6][7][8][9][10][11][12][13][14][15][16][17][18][19]. The nanostructuration of the BCP in the epoxy matrix requires an immiscible block, either initially [20,21] or during polymerization [22,23], and another block that keeps its miscibility up to high or full conversions.…”