Impact strength of polymer-clay nanocomposites

The first reports of polymer-clay nanocomposites sprang out of industrial research from the motor industry in 1987. Since then, academic research has flourished and far outstrips industrial implementation of these new materials; approximately 700 papers were published in 2007. Much of this work, although it emphasises mechanical properties, neglects the characterisation parameter that industry regards as critical; impact strength. The expression 'toughness' has become ambiguous and inferences from the area under the tensile stress-strain curve (conducted at low strain rates) can directly contradict impact testing results. The mechanical, barrier and thermal stability properties of polymers are usually enhanced by incorporation of nano-clays, often to a remarkable extent and these improvements are well-documented. It seems to be important now to take a closer look at progress in retaining toughness when a polymer is converted to a nanocomposite and what matters is sensitivity to defects and energy absorption under high strain rate conditions. This is a lacuna that might be holding back the deployment of nanocomposites. This Review shows how difficult it is to identify the factors contributing to retention of toughness. Most of the polyamide studies report slight reductions in impact strength or very modest increases, explicable in terms of micro-cavitation as observed in the electron microscope. There is a growing view that clay reinforcement is only effective in retaining toughness above T-g. In other polymers, it is difficult to predict how nano-reinforcements will affect impact strength because of the confounding effects of crystallinty, spherulite size, preferred orientation or processing variables.