Melt viscoelasticity of polypropylenes prepared under different polymerization regimes

Four samples of polypropylene (PP) prepared under different polymerization regimes were characterized by wide-angle (WAXS) and small-angle (SAXS) x-ray diffraction, by specific heat capacity in the temperature interval from - 50 to 220°C (DSC), and by linear viscoelasticity at several fixed temperatures in the interval from 120 to 220°C. Storage G'(ω) and loss G(ω) shear moduli in the melt state (measured in the frequency window spanning about three decades) were treated to derive the relaxation times spectra h(T) using the NLREG computer program based on Tikhonov's method of nonlinear regularization. Molecular characteristics were derived from the melt viscoelastic properties of three crystallizable (isotacticity index above 95%) PP samples exhibiting Newtonian melt flow behavior at low angular frequencies. The anomalous viscoelastic behavior in the first heating run of the elastomeric PP in the temperature interval below ca. 170°C, combined with the relevant WAXS, SAXS, and DSC data, was considered as an evidence for the existence of a spatial network of microcrystallites formed by lateral aggregation of stereoregular sequences which were, however, too short for the development of chain-folded, lamellar crystals typical for semi-crystalline homopolymers. The normal viscoelastic behavior during the subsequent cooling run from a structureless melt state suggested a very slow kinetics of microcrystallinity development. The results obtained demonstrate a high potential of viscoelastic measurements for structural characterization of poorly crystallizable, elastomeric polymers.