Relating morphology to nanoscale mechanical properties: from crystalline to mesomorphic iPP

A nanoindentation technique using an atomic force microscope (AFM) was applied to characterize the mechanical behaviour of several isotactic polypropylene (iPP) samples. The samples were solidified from the melt with a CCT (continuous cooling transformation) procedure spanning a wide range of cooling rates thanks to a fast quenching apparatus developed by the authors. The influence of instrumental parameters on the nanoscale mechanical properties (indentation depth, Young's modulus) shows that for modulus determination one has to rely on simpler methods of force curve analysis based on trace curve alone. Structure homogeneity up to the scale of macroscopic samples used to evaluate elastic moduli allowed a successful comparison of these values with those determined by AFM, which showed that an increase in cooling rate leads to a significant decrease in the material's mechanical response. AFM can thus provide correlations between operating conditions and mechanical properties and can be used for analysing the structure distribution and for mapping properties on a sub-micron scale.