Current issues in research on structure–property relationships in polymer nanocomposites

Epoxy resins have been used as structural materials since the late 1940s. Despite their desirable properties such as high strength, excellent creep resistance, and good adhesion, they suffer from low fracture energy. Rubber modification as a major toughening approach to overcome the inherent brittleness of epoxy polymers was introduced during the early 1970s. Since then, a large number of investigations have been conducted to elucidate different aspects of rubber-toughened epoxies. The present work is a critical review of the field focusing on the important parameters affecting rubber-toughening. The studies reviewed are classified in five categories including roles of matrix ductility, rubber concentration, blend morphology, particle cavitation, and particle/matrix interface. It has been tried to provide an in-depth view of the state-of-the-art knowledge in the field and to direct future studies towards exploring new approaches for toughening of epoxy polymers.

Rubber-Toughened Epoxies: A Critical Review

The synthesis and self-assembly of poly(lactide)-b-poly(acrylic acid) and poly(lactide)-b-poly(dimethylaminoethylacrylate) block copolymers by a combination of ring-opening polymerization and reverse-addition fragmentation chain transfer (RAFT) polymerization is reported. Self-assembly of block copolymers containing enantiopure homochiral poly(lactide), PLA, by a simple direct dissolution methodology results in core-crystallization to afford micelles with a cylindrical morphology. Amorphous atactic PLA cores and conditions that did not promote crystallization resulted in spherical micelles. Cylindrical micelles were characterized by transmission electron microscopy (TEM) with cryo-TEM, small angle neutron scattering (SANS) and angular dependent dynamic light scattering (DLS) proving that the cylindrical morphology was persistent in solution. Manipulation of the assembly conditions enabled the length and dispersity of the resultant cylindrical micelles to be controlled.

Cylindrical micelles from the living crystallization-driven self-assembly of poly(lactide)-containing block copolymers

Controlling the interface interaction of polymer/filler is essential for the fabrication of high-performance polymer composites. In this work, a core–shell structured hybrid (SiO2–GO) was prepared and introduced into an epoxy polymer matrix as a new filler. The incorporation of the hybrid optimized the modulus, strength and fracture toughness of the composites simultaneously. The ultrathin GO shells coated on silica surfaces were regarded as the main reason for the enhancement. Located at the silica-epoxy interface, GO served as an unconventional coupling agent of the silica filler, which effectively enhanced the interfacial interaction of the epoxy/SiO2–GO composites, and thus greatly improved the mechanical properties of the epoxy resin. We believe this new and effective approach that using GO as a novel fillers surface modifier may open a novel interface design strategy for developing high performance composites.

Enhanced epoxy/silica composites mechanical properties by introducing graphene oxide to the interface.

Polyactide (PLA) was blended with an ethylene/n-butyl acrylate/glycidyl methacrylate (EBA-GMA) terpolymer and a zinc ionomer of ethylene/methacrylic acid (EMAA-Zn) copolymer. The phase morphology of the resulting ternary blends and its relationship with impact behaviors were studied systematically. Dynamic vulcanization of EBA-GMA in the presence of EMAA-Zn was investigated by torque rheology, and its cross-link level was evaluated by dynamic mechanical analysis. Reactive compatibilization between PLA and EBA-GMA was studied using Fourier transform infrared spectroscopy. The dispersed domains in the ternary blends displayed a “salami”-like phase structure, in which the EMAA-Zn phase evolved from occluded subinclusions into continuous phase with decrease in the EBA-GMA/EMAA-Zn ratio. An optimum range of particle sizes of the dispersed domains for high impact toughness was identified. Also, the micromechanical deformation process of these ternary blends was also investigated by observation of the impact-fra...

Interaction of Microstructure and Interfacial Adhesion on Impact Performance of Polylactide (PLA) Ternary Blends

An amphiphilic poly(ethylene-alt-propylene)-b-poly(ethylene oxide) (PEP-PEO) block copolymer (BCP) was blended with a bisphenol A-based epoxy resin formulation and self-assembled into a wormlike micelle structure. With an incorporation of 5 wt % of the BCP material, the fracture toughness was improved by >100% over the neat epoxy. The morphology and mechanical properties of this BCP-modified epoxy were investigated using transmission electron microscopy, dynamic mechanical analysis, tensile tests, and fracture toughness measurements. Toughening mechanisms from the wormlike micelle-modified material were investigated using the double-notch four-point-bending technique, and the results are compared with data obtained from the same epoxy thermoset formulation containing a BCP that self-assembled into spherical micelles. Elongated cylindrical micelles produce improved toughness, which is interpreted on the basis of a combination of mechanisms including crack tip blunting, cavitation, particle debonding, limit...

Toughening of Epoxies with Block Copolymer Micelles of Wormlike Morphology

An amphiphilic block copolymer toughener was incorporated into a liquid epoxy resin formulation and self-assembled into well-dispersed nanometer scale spherical micelles with a size of about 15 nm. The nanosized block copolymer at 5 wt % loading can significantly improve the fracture toughness of cured epoxy thermosets without reduction in modulus at room temperature and with only a slight drop in glass transition temperature. The toughening mechanisms were investigated, and it was found that the 15 nm size block copolymer micelles could cavitate to induce matrix shear banding, which mainly accounted for the observed remarkable toughening effect. Other mechanisms, such as crack tip blunting, may also play a role in the toughening. A discussion of the possible reasons responsible for the observed attractive mechanical property improvements due to the block copolymer modification is given. Implications of the present finding for designing toughened polymers are also discussed.

Nanocavitation in Self-Assembled Amphiphilic Block Copolymer-Modified Epoxy

Effect of crosslink density on fracture behavior of model epoxies containing block copolymer nanoparticles

Strain rate effect on toughening of nano-sized PEP–PEO block copolymer modified epoxy

Curable compositions, cured compositions, and methods of forming the same, including an epoxy resin, a curing agent, an amphiphilic toughening agent, and an inorganic nanofiller, wherein the toughening agent forms a second phase having at least one dimension being on the nanometer scale.

George Jacob

Papers

Toughening of Epoxies with Block Copolymer Micelles of Wormlike Morphology

Nanocavitation in Self-Assembled Amphiphilic Block Copolymer-Modified Epoxy

Effect of crosslink density on fracture behavior of model epoxies containing block copolymer nanoparticles

Strain rate effect on toughening of nano-sized PEP–PEO block copolymer modified epoxy

Amphiphilic block copolymers and inorganic nanofillers to enhance performance of thermosetting polymers