Nanoparticle effects on spherulitic structure and phase formation in polypropylene crystallized at moderately elevated pressures: The influence on fracture resistance

We report here for the first time the changes in spherulitic structure and phase formation that occur in polypropylene in the presence of nanoclay on crystallization at high pressures. To delineate the effects of pressure and nanoclay on the crystallization behavior, polypropylene with and without nanoclay were studied under identical experimental conditions. The two important findings are (i) the spherulite size and morphology of polypropylene in the absence and presence of nanoclay is a function of the crystallization pressure and (ii) at constant crystallization pressure, the presence of nanoclay reduced the spherulite size from 155 μm to 19 μm at 0.1 MPa. Another important observation was the evidence of γ-phase in polypropylene with nanoclay at pressures between ∼0.1 MPa and 59 MPa. In neat polypropylene, increasing crystallization pressure changed the deformation process from vein-type crazing to fibrillation, while in polypropylene-containing 4 wt.% clay, an increased tendency towards fibrillated fracture with increase in crystallization pressure was observed. At the present time, the observations imply the following possibilities for future considerations: (a) nanoclay changes the equilibrium state of the polymer chain or its conformation and (b) nanoclay provides a surface that is favorable for possible epitaxial growth of γ-phase because the lattice mismatch is <10%. The observed nanoparticle and pressure-induced effects, namely dramatic reduction in spherulite size and γ-phase appears to be beneficial for impact strength with consequent change in the micromechanism of plastic deformation.