Equilibrium Melting Temperature Determination of Semicrystalline Polymers through Nonlinear Hoffman-Weeks Extrapolation and Secondary Crystallization of Ethylene/Styrene Copolymers


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Virginia Tech


The applicability of the conventional Hoffman-Weeks (HW) linear extrapolation for the determination of the equilibrium melting temperatures of semicrystalline polymers is critically reviewed. It is shown that the linear extrapolation of observed melting temperatures cannot, in general, provide a reliable estimate of the equilibrium melting temperature. A more rigorous nonlinear HW analysis is proposed in this dissertation, which yields more accurate estimates of the equilibrium melting temperatures for semicrystalline polymers. The proposed nonlinear HW analysis is successfully applied to the cases of isotactic polypropylene and poly(ethylene oxide). The predicted initial lamellar thickness as a function of the crystallization temperature matches well with experimental results and/or literature values. Results based on the nonlinear HW analysis are consistent with those obtained from the analysis of the temperature dependence of the crystal growth rates. The general applicability of the Lauritzen Hoffman (LH) secondary nucleation theory is also addressed for isotactic polypropylene and poly(ethylene oxide). While the LH theory provides an excellent account of the temperature dependence of spherulitic growth rates and ratio of nucleation constants for different regimes, it appears not to yield a meaningful value for the substrate length, L, for poly(ethylene oxide).

In a second project, the effects of structural and topological constraints on the morphology, melting and crystallization behavior of ethylene/styrene copolymers are investigated. During cooling from the melt, the longest ethylene sequences crystallize into lamellae in the primary crystallization process, while the shorter ethylene sequences are suggested to form fringed micelles in the secondary crystallization process. Kinetic studies indicate that secondary crystallization is characterized by an Avrami exponent of ½ which is consistent with a one dimensional, diffusion controlled growth. The increase in the melting temperature of secondary crystals with crystallization time is tentatively explained by a decrease in the molar conformational entropy of the remaining amorphous fraction as a result of secondary crystallization, although the possible role of an increase of crystal lateral dimensions with time cannot be rigorously ruled out.



Equilibrium Melting Temperature, Secondary Crystallization