Isotactic polypropylene (iPP) nanocomposites containing 0.1, 0.2, 0.4 and 0.6 wt % of silver nanoparticles (nAg) were prepared by melt compounding in a one-screw extruder. Selected samples were isothermally recrystallized at different temperatures 50, 75, 100, 125, 130 and 135 °C, and under non-isothermal conditions. The size of the silver particles dispersed in the iPP matrix were estimated by ultraviolet/visible spectroscopy (UV-VIS). The effects of nAg additions on the structure of the iPP were studied by wide angle X-ray scattering (WAXS) and differential scanning calorimetry (DSC) techniques. The results clearly indicate that the addition of nAg induces the formation of the b-iPP polymorph. It is shown that the relative amount of the b form in the recrystallized nanocomposites depends on both the concentration of silver nanoparticles and thermal conditions of recrystallization.

Molecular modelling (MM) was used to explain the mechanism of formation of polymorphic b form of isotactic polypropylene (iPP) at the surface of the silver nanoparticles (nAg). Geometrical optimization of iPP chains and unit cell of Ag systems was made with the use of MM+ force field in vacuum. According to the results, the arrangement of methyl groups in the (110) contact plane implies a lateral packing of helices with a periodicity a » 19 Å, which is a characteristic feature of the complicated packing of helices in b iPP. Partial charges calculated on the basis of the hybrid potential BLYP revealed the possibility of electrostatic interactions between hydrogens from methyl groups and silver atoms at the edge of unit cell. Analysis of the optimized iPP/Ag system confirmed that the formation of the polymorphic b iPP can be explained by the epitaxial mechanism.