Isothermal Crystallization and Melting of Isotactic Polypropylene Analyzed by Time- and Temperature-Dependent Small-Angle X-ray Scattering Experiments

Abstract

Structure evolution during isothermal crystallization of isotactic polypropylene and the changes during a subsequent heating to the melt were analyzed by time- and temperature-dependent small-angle X-ray scattering experiments. Results demonstrate that isotactic polypropylene, when forming the α-polymorph, is governed by the same general laws for crystallization and melting as syndiotactic polypropylene. Crystal thicknesses are inversely proportional to the supercooling under a characteristic temperature which is located above the equilibrium melting point of a sample. The crystallization line describing this dependence can be understood as representing the stability limit of a well-defined initial state with higher free energy. Its transformation into the final lamellar two-phase structure provides the stabilization expressed in the difference between the crystallization and melting temperature. Observations and data from other sources can be seen as indicating that (i) crystal thicknesses are independent of the isotacticity and that (ii) the initial state is composed of crystal blocks in planar assemblies, in agreement with corresponding observations on syndiotactic polypropylene. The small-angle X-ray scattering experiments provide a detailed insight into the structure changes during the continuous melting. For high crystallization temperatures no recrystallization occurs. Crystals with different stabilities, all having the same thickness, melt consecutively. Structures produced at low crystallization temperatures are less stable. Here heating leads to repeated melting−recrystallization processes associated with jumplike changes in the length scale of the structure.

Rheooptical Fourier transform infrared spectroscopy of the deformation behavior in quenched and slow‐cooled isotactic polypropylene films

“…Previous WAXS data have shown that these samples would be expected to form monoclinic crystals. 60,63,66,69,71 The stronger dichroism response is consistent with the higher degree of crystallinity. In contrast with the quenched sample, the saturation in the crystalline orientation occurs well beyond 50%.…”

The prediction of mechanical performance of isotactic polypropylene on the basis of processing conditions

“…In agreement with Angelloz et al [23] and Iijima et al [58] the equilibrium melting temperature T 0 m is chosen at 193 C, and used to describe the experimental data sets shown in Fig. 7.…”

“…The conclusion that can be drawn from these results is that a unique relation between T c and l c exists, independent of the degree of stereo defects. Iijima et al used two iPP grades with similar isotacticity, but different molecular weights [58]. They found a relationship T c versus l c that holds for both their isotactic polypropylenes, independent of molecular weight.…”

Nonlinear time-dependent response of isotactic polypropylene

“…Isotactic polypropylene is a semi-crystalline polymer containing three different crystallographic forms: monoclinic α crystallites, hexagonal β structures, orthorhombic γ polymorphs, together with "smectic" mesophase (Iijima and Strobl, 2000). At rapid cooling of the melt (at the stage of injection molding), α crystallites and smectic mesophase are mainly developed, whereas β and γ polymorphs are observed as minority components (Kalay and Bevis, 1997;Al-Raheil et al, 1998) that disappear after annealing above 130 • C (Al-Raheil et al, 1998;Labour et al, 2001).…”