CHARACTERIZATION OF METALLOCENE-MADE POLYPROPYLENE WITH NARROW DISTRIBUTION OF MOLECULAR WEIGHT

Metallocene based polypropylene (mPP) with very narrow distribution of molecular weight was peroxide-degraded to materials of four different molecular weights including the original mPP labeled MET1–MET3 and MET0, respectively. Double bonds formed after peroxide-degradation was proved on material surfaces by FTIR-ATR (attenuated total reflection of Fourier-transformed infrared spectroscopy). The decreasing molecular weight led to gradual decrease of the tensile strength, tensile modulus as well as the strain and to the decrease of the temperature of thermal decomposition. Confocal laser scanning microscopy (CLSM) of chemically etched surfaces of MET0–MET3 revealed supramolecular structure of commonly occurred structure (radical spherulites) but also supramolecular structure of form (sheaf-like structure). The latter was proved by XRD together with the fact that the content of form decreases with decreasing molecular weight. The mentioned structure differences were not visible on DSC curves because the amount of structure was small and melting temperature, temperature of crystallization and the degree of crystallinity remained the same for all four types of mPP. The structure of the original materials was also characterized after isothermal crystallization performed on differential scanning calorimetry (DSC) and under polarizing optical microscope (POM). The first was performed at 120–126 °C and the latter at 130 °C (Tic). The materials obtained on DSC always contained the structure and its amount increased with increasing Tic whereas higher content of form was always in MET0 with respect to MET3. The structure was proved by XRD and also by DSC heating run followed immediately after the isothermal process. The latter revealed two endotherms belonging to melting of and forms. The presence of form was on the surfaces proved by CLSM. The formation of structure was in-situ observed on POM and the amount of it decreased with decreasing molecular weight. The spherulite growth rate increased with decreasing molecular weight whereas the rate of crystalline portion expressed as half-time of crystallization decreased with decreasing molecular weight