A study of internal friction in polypropylene (PP) filled with nanometer-scale CaCO3 particles

The internal friction and relative elastic modulus of polypropylene (PP) filled with nanometer-scale calcium carbonate (nm-CaCO3) particles in different contents (0, 4, 7, and 15 vol.%) are measured in the temperature range 150–400 K with a torsion pendulum. The α peak associated with the glass transition and a small peak (α′ peak) associated with the pre-melting process in crystalline parts of PP was observed around 290 and 370 K, respectively. At temperatures lower than 270 K, no peaks were observed. With increasing content of nm-CaCO3 particles, the apparent activation energy of the α peak decreases, and after passing a minimum of 4.7 eV at 4 vol.% of nm-CaCO3, it increases. In contrary to this behavior, the peak temperature has a maximum of 289 K at the same filler fraction. Correspondingly, the highest tensile and flexural strength of PP were obtained around this content. These observations may be understood through the influences of fillers on the degree of crystallization of PP and on the mobility of molecules of PP.