Isothermal and non-isothermal crystallization of PP: effect of annealing and of the addition of HDPE

Abstract

In this report, we present the results of our investigation into the crystallization behaviour of polyproplene. The crystallization process was followed by hot-stage optical microscopy, differential scanning calorimetry, and dynamic mechanical methods. As well, nuclear magnetic resonance spin-spin relaxation techniques were employed to probe the morphology of the polypropylene. Nucleation in this isotactic polypropylene was athermal and appeared to be heterogeneous. The dependence of the onset temperature of crystallization on the annealing temperature and on the annealing time indicated that PP nuclei were able to survive the melting process. Nuclear magnetic resonance spin-spin relaxation experiments indicated that the PP melt contained a significant proportion of regions of high segment density. It was postulated that these ‘ordered’ regions acted as nucleation sites for PP crystallization, and that the number and size of these regions was determined by the annealing time and temperature. Addition of HDPE to PP resulted in melting-point depression and plasticization of the PP phase at lower HDPE contents. HDPE was able to penetrate the PP phase sufficiently at lower HDPE contents to reduce the number and size of regions of high segment density, thereby delaying the nucleation and subsequent crystallization of the PP phase.