Mechanical characterization of iPP injection molded samples on multiple length scale

The mechanical characterization at different length scales, from macro to nanometer scale, represents a very intriguing possibility to analyze the effect of developed morphologies on the properties of a molded polymer material. In this work, isotactic polypropylene molded samples, obtained with different processing conditions, have been mechanically characterized on different lengths scales. Dynamic Mechanical Analysis (DMA), micro Indentation and HarmoniX Atomic Force Microscopy (AFM) tests have been carried out to characterize injection molding iPP samples from millimeter to nanometer scale. Optical microscopy and AFM analyses show a complex morphology composed ? by a thin layer of globular elements at the sample surface, spherulites in the sample core and fibrils in the intermediate zone. The well organized and oriented structures, i.e. the fibrils, are characterized by higher values of mechanical modulus, whereas the globules, less organized structures, show very small elastic modulus values, close to the characteristic values observed in quiescent amorphous phase. Intermediate values of elastic modulus are observed in the sample core, where well developed spherulitic structures are characterized by both higher crystallinity and lower level of orientation. The elastic modulus obtained by HarmoniX AFM technique are in good agreement with the values obtained by Indentation tests in the same sample positions, confirming the complementarity of these techniques for a complete multi-scale sample characterization. The cross section modulus distributions nicely compare with DMA results.