Tougher and Stiffer

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Science  13 Aug 2004:
Vol. 305, Issue 5686, pp. 919
DOI: 10.1126/science.305.5686.919c

Composites are typically designed so that the addition of a small amount of secondary material, in the form of particles or fibers, enhances specific properties of the primary matrix material. However, when the size of the added particles approaches the nanometer scale, they can alter the crystal structure or bulk morphology, and hence can stabilize metastable or otherwise inaccessible phases.

Shah et al. have examined composites of clay particles (montmorillonite) dispersed into poly(vinylidene fluoride) (PVDF), a commercially important polymer with five known crystalline forms. The α form is most common, but it is the all-trans β form that exhibits the largest piezo- and pyroelectric responses and thus is of the greatest technological interest. Mixing in a surface-modified montmorillonite clay made it energetically favorable for the polymer chains to intercalate between the layers of the clay particles and promoted a transition from ordered α crystallites to disordered, fiber-like β crystallites in the PVDF matrix. In mechanical tests, composites made with this modified clay showed an increase in both toughness and stiffness, whereas in most cases an increase in one property comes at the expense of the other. — MSL

Adv. Mater. 16, 1173 (2004).

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