Institute of Fundamental Technological Research

The analysis of WAXS profiles for various polyethylenes indicates that the proper description of a structure needs the introduction of a kind of ‘third phase’ in addition to the classical crystalline and amorphous phases. The structure of the additional phase is intermediate between that of the amorphous and crystalline phase. With increasing branch content and molecular weight the intermediate phase becomes more similar to the structure of amorphous phase. The experimental evidence for the intermediate phase is derived not only from the crude approximation of WAXS profiles based on the two phase model but also from the unexpected behavior of the parameters of amorphous halo during crystallization. When crystallization is started, an analysis based upon two-phase model results in an apparent increase of the diffraction angle and width of amorphous halo with time above the values anticipated from the range before the start of crystallization. This is caused by the fact that the amorphous fitting function tries to cover a peak of the intermediate component that appears between morphous halo and (110) reflection of crystalline phase. The conventionally applied two-phase model leads to several serious errors in determination of structural parameters of both phases. The analysis of crystallization kinetics using three-phase model provides additional information on the nature of crystallization itself.

Polyethylene, Crystallization, Intermediate phase

Crystallization of polypropylene (PP) can result in formation of different crystal modifications depending on external conditions. The mechanisms of formation of various crystal modifications in polypropylene are still under discussion.We have investigated non-isothermal melt crystallization of isotactic polypropylene at cooling rates ranging from 1 up to 180,000 K/min using two types of differential scanning calorimeters-standard device Perkin–Elmer DSC Pyris-1 and ultra-fast calorimeter. Additional results were obtained by means of wide angle X-ray scattering and optical microscopy. At cooling rates below 6000 K/min there is only one exothermic peak corresponding to simultaneous crystallization ofalpha andbeta modifications. At cooling rates higher than 6000 K/min there is additional low temperature DSC peak corresponding to formation of mesomorphic phase. At the rates higher than 36,000 K/min there is no trace of formation of any ordered phase. In our opinion this complex behavior observed during crystallization of polypropylene can be explained using the concept of metastable phases. An increase ofbeta content in samples with quinacridone pigment has been observed only at very low cooling rates, corresponding to high temperatures of crystallization and low homogeneous nucleation rate.

Polypropylene, Crystallization, Utra-fast calorimetry, Crystallographic modifications, Metastable phases