Investigation of size effect through in-situ SEM testing of polystyrene micropillars

Size effects in micron sized pillars loaded in compression, fabricated out of metal single crystals with FIB milling has been reported by several authors. The size effect manifests in a significant amplification of the flow stress with decrease in diameter. Amorphous glassy polymers are widely used in micro-structures, such as in many micro-electro-mechanical system (MEMS) based devices in electronics and biological applications. But their mechanical behaviours at these length scales have not been adequately explored. In this work, we show that micropillars of an amorphous material, polystyrene (PS), also exhibit significant size effects under compression. The pillars are fabricated by methods that do not involve focused ion beam (FIB) milling and are therefore, free of ion damage. Micropillars of diameters ranging from 38 to 190 μm have been fabricated using a microfabrication technique and they are deformed in-situ in a FESEM under compression. The stress strain plots are obtained by fitting the load displacement behaviour using well calibrated constitutive model. Results suggest that the yield stress of PS intrinsically depend on the diameter.