Studies of the crystallization, melting, and morphology of random ethylene/1-octene copolymers by a combination of differential scanning calorimetry and atomic force microscopy are presented. Two different crystallization mechanisms prevalent in separate temperature ranges are inferred from the effect of cooling rate on the temperature dependence of crystallinity, from the reversibility of crystallization/melting phenomena at the lowest temperatures, and from the temperature dependence of kinetic parameters describing isothermal crystallization and melting. Morphological studies of these copolymers demonstrate the coexistence of two distinct crystalline superstructures (i.e., lamellae and fringed-micellar or chain cluster structures) which we tentatively associate with the two crystallization mechanisms. The multiple melting behavior of these copolymers is associated with the existence of separate morphological entities and is not explained by a mechanism of melting−recrystallization−remelting. Finally, the upward shift of the melting endotherm of secondary crystals (i.e., these formed by the low-temperature mechanism) with longer crystallization times is explained by a decrease in the molar conformational entropy of the remaining amorphous fraction as a result of secondary crystallization.

The applicability of the Hoffman-Weeks (HW) linear extrapolation for the determination of equilibrium melting temperatures of polymers is critically reviewed. In the first paper of this series, it is shown that the linear extrapolation of observed melting temperatures cannot, in general, provide a reliable estimate of the equilibrium melting temperature. A combination of the experimentally observed undercooling dependence of the initial lamellar thickness, l* ) C1/∆T + C2, and the finite lamellar thickness dependent melting temperature depression, as described by the Gibbs-Thomson treatment, provides a venue to the general relationship between the crystallization and observed melting temperatures. It is further shown that, for a constant thickening coefficient, the observed melting temperature must vary nonlinearly with the crystallization temperature. The origin of this nonlinearity lies in the term C2, which is neglected in the classical HW treatment. The principal implications of this study in the context of the Lauritzen-Hoffman theory are the following: (1) the linear extrapolation, when carried out for lamellar crystals exhibiting a constant thickening coefficient, invariably underestimates the equilibrium melting temperature; (2) the extent of the underestimation increases with a decrease in the lamellar thickening coefficient, with an increase in the magnitude of C 2 and with an increase in the range of undercoolings where the crystals are formed; (3) the linear extrapolation always leads to an overestimation of the lamellar thickening coefficient. Finally, a more accurate method is proposed for the determination of equilibrium melting temperatures in cases where the thickening coefficient can be assumed constant.

No abstract

Studies of the temperature dependence of spherulitic growth rates, in the context of the Lauritzen−Hoffman secondary nucleation theory, and investigations of the crystallization time and temperature dependence of the melting behavior, in the context of the nonlinear Hoffman−Weeks (HW) extrapolation, are reported for isotactic polypropylene prepared with Ziegler−Natta catalysts. The quantitative agreement between estimations of the equilibrium melting temperature, T m, through an analysis of spherulitic growth rate data and use of the nonlinear HW approach provides strong support for the latter method. The results of these studies indicate specifically that T m (it-PP, α-phase) is in the vicinity of 212−215 °C, some 30 °C above the most commonly quoted value. This new value of the equilibrium melting temperature of it-PP leads to predictions of the crystallization temperature dependence of the initial lamellar thickness and of the correlation between the lamellar thickness and observed melting temperature, which are in good agreement with the limited morphological information available. The present studies confirm the existence of a regime II/III transition at T x = 139.5 °C. The results of this study furthermore cast some doubts as to the current value of the small-angle X-ray scattering technique for the analysis of morphological parameters in quiescently crystallized isotactic polypropylene.

No abstract