NANOMECHANICAL TESTING OF FREESTANDING POLYMER THIN FILMS

摘要

menon will be discussed in relation to current models of polymer deformation mechanisms. A new approach for tensile testing of freestanding polymer thin films has been developed to investigate nanomechanical phenomena with precise control of strain rate, environmental and in situ TEM imaging capabilities. Several techniques for mechanical testing of polymer thin films have been reported previously, but there is a lack of consensus regarding size-dependent mechanical properties1"3. The technique described here is derived from a nanomechanical tensile testing platform known as at Push-to-Pull (PTP) device (Figure 1) using a novel sample preparation approach. A free-standing specimen is placed across the tensile actuation gap of the PTP device such that it can be mounted at the end of a specialized TEM holder for quantitative in situ tensile testing or to a specialized mount was designed to enable PTP experiments to be performed using a stand-alone nanoindenter. With this adaptation, all of the capabilities of ex situ nanoindentation are accessible to PTP tensile testing; which includes environmental control (temperature and humidity), DMA, and a wide range of strain rates. Polystyrene was chosen as a model system for direct comparison with alternative testing techniques. While polystyrene is traditionally thought of as a brittle polymer at room temperature, our initial testing of thin sections has revealed extreme ductility (Figure 1). Ductility in polystyrene thin films has been previously reported in literature1"3, but only to elongations of less than 7% before fracture. Initial results using the PTP device have shown extreme ductility in polystyrene, with strains exceeding 100% without fracture. Our results appear to be independent of strain rate in the range tested; unlike the yield stress, which shows a strong strain-rate dependence. The origin of this nanomechanical pheno