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Redox-responsive polyvinylferrocene-grafted silica nanoparticles have been used for modulating the catalytic activity of surface-attached Grubbs second generation type catalysts for the ring-opening metathesis polymerization (ROMP) of norbornene monomer. A facile and very efficient protocol for the modification of living polyvinylferrocene chains was developed to introduce a suitable functional group for the intended “grafting onto” approach. Grafted particles were characterized by using TEM, SEM/energy dispersive X-ray spectroscopy (EDS), XPS, small-angle X-ray scattering (SAXS), dynamic light scattering (DLS), and cyclic voltammetry, revealing both the presence of redox-responsive polymers and the presence of Grubbs catalyst in the particle exterior. Chemical oxidation protocols for immobilized polymers were applied to deactivate surface-attached catalysts in ROMP protocols, while chemical in situ reduction immediately led to catalyst’s reactivation.

Block ionomer complexes based on sulfonated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SSEBS) and a tertiary amine-terminated poly(ε-caprolactone), denoted as SSEBS-c-PCL, were used to toughen epoxy resin. Well-dispersed spherical microdomains, consisting of a poly(ethylene-ran-butylene) core surrounded by a sulfonated polystyrene shell, were revealed by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) in the cured epoxy blends with 10 wt % SSEBS-c-PCL of various compositions. Structural parameters, core radius (R c), effective hard-sphere radius (R hs), and shell thickness (T s) were obtained by fitting the SAXS data with a core–shell model and, for the first time, correlated with the fracture toughness (critical stress intensity factor K IC and strain energy release rate G IC) of the epoxy blends. K IC and G IC were found to increase with increasing R c and R hs but decrease with T s. The blend containing SSEBS-c-PCL with least PCL, i.e., 2.4 wt %, shows nanostructure of the largest R c and R hs, and smallest T s, displaying highest K IC and G IC. Examination of the fracture surfaces indicates that the increased toughness arises from interfacial debonding of spherical microdomains and plastic expansion of resultant nanovoids, followed by small-scale matrix shear deformation. The correlations between nanostructure parameters and fracture toughness have provided a fundamental understanding of nanostructure toughening of thermosets via an innovative strategy based on block ionomer complexes.

Herein we report a novel approach to toughen epoxy thermosets using a block ionomer, i.e., sulfonated polystyrene-block-poly­(ethylene-co-butylene)-block-polystyrene (SSEBS). SSEBS was synthesized by sulfonation of SEBS with 67 wt % polystyrene (PS). Phase morphology of the epoxy/SSEBS blends can be controlled at either nanometer or micrometer scale by simply adjusting the sulfonation degree of SSEBS. It has been found that there exists a critical degree of sulfonation (10.8 mol %) forming nanostructures in these epoxy/SSEBS blends. Above this critical value, macrophase separation can be avoided and only microphase separation occurs, yielding transparent nanostructured blends. All epoxy/SSEBS blends display increased fracture toughness compared to neat epoxy. But the toughening efficiency varies with the phase domain size, and their correlation has been established over a broad range of length scales from nanometers to a few micrometers. In the nanostructured blends with SSEBS of high sulfonation degrees, the fracture toughness decreases with decreasing size of the phase domains. In the macrophase-separated blends, only a slight improvement in toughness can be obtained with SSEBS of low sulfonation degrees. The epoxy blend with submicrometer phase domains in the range 0.05–1.0 μm containing SSEBS of a moderate degree of sulfonation (5.8 mol %) displays the maximum toughness. This study has clearly clarified the role of phase domain size on toughening efficiency in epoxy thermosets.

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