Macromolecules 1998, 31, 8230-8242
Equilibrium Melting Temperature and Undercooling Dependence of the Spherulitic
Growth Rate of Isotactic Polypropylene
Jiannong Xu, Srivatsan Srinivas, and Herve´ Marand*
Virginia Polytechnic Institute and State University,
Department of Chemistry and Department of Materials Science and Engineering,
Blacksburg, Virginia 24061-0212
Pawan Agarwal
Exxon Chemical Company, Baytown Polymer Center, Baytown, Texas 77522-5200
Received May 11, 1998; Revised Manuscript Received September 17, 1998
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,
Tm, 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 Tm (it-PP, a-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 Tx = 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.