Mechanism of strengthening and toughening of a nanostructured styrene-butadiene based block copolymer by oligostyrene-modified montmorillonites

The influence of oligostyrene-modified montmorillonite (OS-MMT) on the morphological, mechanical and fracture mechanical behaviour of a nanostructured styrene-butadiene based block copolymer were studied by using electron microscopy (TEM, SEM), small angle X-ray scattering (SAXS) and uni-axial tensile testing. The crack toughness behaviour of the block copolymer nanocomposites was evaluated with the essential work of fracture approach using a pseudo single specimen method. A high degree of exfoliation of MMT vis-a-vis well distributed tactoids with gallery spacings of ca. 10 nm could be observed. PS-phase specific nano-confinement of silicate tactoids could also be confirmed as indicated by the shift in the glass transition temperature (Tg-PS) of the PS-rich phase to higher temperatures. The Young's modulus and yield stress of the block copolymer nanocomposite (BCP-NC) increased with oligostyrene-MMT content without any compromise in the ductility/toughness. A maximum in the resistance to crack propagation (vwP) at 3 wt.-% of oligostyrene-MMT accompanied by a change in deformation mechanism from homogenous plastic flow (drawing and micro-necking of microdomains) in the BCP-NC with 0-1 wt.-% of OS-MMT to a craze-like deformation (microvoid formation and stretching of polymer fibrils) for the BCP-NC with 3-5 wt.-% of OS-MMT. Above 5 wt.-% OS-MMT content a tough-to-brittle type transition attributed to the mm-sized aggregates in combination with an increased content of rigid PS phase controlling the fracture process as in the BCP-NC with 10 wt.-% nanofillers could be ascertained. Thus, our study demonstrates the possibilities of simultaneous strengthening and toughening of block copolymers by incorporation of modified layered silicates via tuning the dynamics of ""intercalation-exfoliation"" and by phase-specific nano-confinement of silicates in the BCP-NC.