Novel method to measure the shear strength of exfoliated montmorillonite/polymer nanocomposite films

Shear fracture of exfoliated montmorillonite/poly(methyl acrylate‐co‐methyl methacrylate) (MMT/P(MA‐co‐MMA)) nanocomposite films fabricated via soap‐free emulsion polymerization was studied using a unique test fixture that is capable of controlling the inclined angle between the loading axis and the shear fracture plane. It was found that fracture of the films depended on both shear and normal stresses and could be described by the Mohr–Coulomb criterion. The effects of MMT content on the intrinsic shear strength (i.e. the shear strength at zero normal stress) τ_s were also examined. The obtained τ_s increased from 1.66 to 2.78 MPa as 1 wt% MMT was incorporated into P(MA‐co‐MMA) and then increased almost linearly to 5.32 MPa when the content of MMT increased to 15 wt%. Also, the fracture‐surface morphology of the composite film depended on both the MMT content and the inclined angle (and hence the normal‐to‐shear stress ratio). For low MMT contents and low normal‐to‐shear stress ratios, striations normal to the shear direction appeared on the fracture surface. As the MMT content or the stress ratio increased, the fracture surface became rough accompanied by the presence of interfacial micro‐cracking between MMT nanoplatelets and the matrix. With further increase in MMT content and high stress ratio, the MMT nanoplatelets were compressed into large flakes on the fracture surface. Copyright © 2011 Society of Chemical Industry