A “Layered Look” for Spherical Nanoparticles in Semicrystalline Polymers

In 1993, researchers at Toyota Central R&D reported that 5 wt % of an exfoliated clay could double the roomtemperature modulus of nylon-6, a semicrystalline engineering plastic, and raise its heat distortion temperature by more than 80 °Call without sacrificing toughness. That work stimulated a worldwide effort to synthesize and study polymer–matrix nanocompositesand it also highlighted the importance of high filler aspect ratio, and good filler dispersion, in achieving these properties. In this issue of ACS Central Science, Zhao et al. demonstrate a more general route to nanoparticle reinforcement of semicrystalline polymers, achieving similar increases in modulus, again with no reduction in toughnesswith spherical nanoparticles, rather than plate-like clay layerssimply by controlling the relative rates of particle diffusion and crystal growth. The nylon-6-clay composites developed at Toyota were made by an in situ polymerization of the nylon-6 monomer, after it had swollen an organically modified clay. While this approach succeeds in exfoliating (and thereby thoroughly dispersing) the clay layers, such exfoliation is difficult to achieve for most polymer−filler combinations, and in situ polymerizations can be difficult to execute at the commercial scale. One would prefer to simply blend together preformed nanoparticles and polymer, much as one adds carbon black or silica to rubber to reinforce it for tire treads. And over the past few decades, a variety of effective approaches to graft spherical nanoparticles with polymer chains have been developed, to enable these particles to disperse well in a range of polymer matrices. But to produce significant stiffness increases in plastics (vs rubbers) requires high loadings of such spherical nanoparticlesmuch more than the ∼5 wt % required when the filler is a plate-like clay sheet. Indeed, nature recognized long ago the value of platelike architectures, which it employs in seashell nacre (mother-of-pearl): an organic–inorganic composite showing both excellent stiffness and toughness, arising from its structure on multiple length scales and complex deformation mechanisms. Semicrystalline polymers also form a layered nanoscale structure, consisting of alternating crystalline and amorphous layers with a typical periodicity of 10–50 nm. If the nanoparticles could be selectively sequestered within the amorphous layers, then even modest overall nanoparticle loadings would translate to high local nanoparticle fractions, rendering those amorphous layers stiffmuch like exfoliated clays. This is precisely what Zhao et al. have achieved.