Nanoscale repetitive impact testing of polymer films

The deformation of polymer films under repetitive contact at high strain rates was investigated using nanoscale impact testing. Four systems were studied: (i) rubber-modified acrylonitrile-butadiene-styrene (ABS) (0–25 wt% rubber), (ii) uniaxially and biaxially drawn poly(ethylene terephthalate) film; (iii) poly(ethylene oxide)–clay nanocomposites, and (iv) nylon 6–organoclay nanocomposites. The initial results suggest that the technique has much potential in evaluating the fatigue behavior of thinner polymer films and coatings that are unsuitable for conventional methods designed for bulk samples. The extent of impact-induced deformation may be used as a measure of ductility because ductile failures are associated with significant plastic deformation before failure whereas brittle failures usually involve little plastic deformation. The nano-impact technique provides valuable highly localized information about deformation under high strain rate, which is complementary to low strain rate tests such as nanoindentation and nano-scratch. The technique has been shown to be sensitive to nano-/microstructural variations in ABS–rubber film when Berkovich indenters and low impact forces were used. The impact behavior of the nanocomposites is only significantly worse than that of the virgin polymers at the highest clay loading studied (15 wt%). This could be a factor when assessing the suitability of novel nanocomposite materials for applications where toughness is important. On ABS film, there is only an approximate correlation between the plastic work function determined from nanoindentation and the rubber loading in the film while the correlation between the rubber loading and nano-impact data is clear, suggesting that the dynamic test is a more useful predictor of thin polymer film toughness than the slow-loading quasi-static indentation test.