Elsevier

Polymer

Volume 39, Issue 16, 1 June 1998, Pages 3735-3744
Polymer

The γ-phase of high molecular weight isotactic polypropylene: III. The equilibrium melting point and the phase diagram

https://doi.org/10.1016/S0032-3861(97)10121-5Get rights and content

Abstract

The equilibrium melting point of polypropylene has been determined as a function of pressure. For pressures of crystallization below 0.5 kbar the melting point observed is characteristic of the α-phase, whereas for crystallization pressures above 0.9 kbar the values are typical of the γ-phase. The principal technique used, to be reported in this paper, was the Hoffman Weeks plot of melting point versus crystallization temperature. Unlike the α-phase, the γ-phase does not show significant levels of abnormal lamellar thickening and the use of the Hoffman Weeks plot is accurate, correlating well with results from small angle X-ray scattering studies. Results demonstrate that the equilibrium melting point of the γ-phase, when extrapolated back to atmospheric pressure, is similar to that of α-polypropylene. The heat of fusion has been determined using the Clapeyron equation and the phase diagram constructed using the Gibbs equation. Reasons for the relative stability of the phases are proposed.

Introduction

Although first studied in the 1960s1, 2, 3, 4, 5, 6, 7, 8the γ-form of isotactic polypropylene is not very well understood. Additional, more in-depth studies of the form were initiated as result of the need for high pressure studies of regime transitions, the existence of modern polypropylenes and the suggestion of a completely new type of crystal structure to account for its X-ray diffraction spectrum.

Early studies had associated the formation of the γ-phase with chemical heterogeneity in the polypropylene chain caused by atacticity or by copolymerization[9]. More recent studies[10]have demonstrated that the γ-phase is produced at elevated pressures from high molecular weight homopolymers and that it has the same diffraction patterns as the low molecular weight polymers crystallized at atmospheric pressure. This study has also confirmed that the γ-phase is not the result of some unexpected degradation reaction at elevated pressures. The analysis of Turner-Jones[9]considered some copolymers of propylene with ethylene (as well as other comonomers) and found that the presence of a comonomer enhanced the formation of the γ-phase. The polymers that were available for study at that time contained atactic material, and there was no a priori way of separating the effects of atacticity from the effects of copolymerization, which would be complementary. More recent studies, conducted by Mezghani and Phillips[11]using variable amounts of ethylene content in >98% isotactic propylene copolymers, not only confirmed the results of Turner-Jones but also indicated that the amount of the γ-form is proportional to the ethylene content and to the crystallization temperature. Accordingly, the amount of the γ-form is higher at low supercoolings.

Since its description 6 years ago12, 13the orthorhombic γ-phase of polypropylene has been an enigma. Its structure, being composed of sheets of parallel chains juxtapositioned next to one another so that non-parallel chains are generated normal to the sheets, is unique to polymer science12, 13, 14. Unexplained is the fact that the α-phase is normally encountered at atmospheric pressure. It was reported in the first paper of this series that as the pressure of crystallization is increased the proportion of the γ-phase present increases from zero at atmospheric pressure to close to 100% at 2 kbar[10]. In that paper a model was proposed for crystallization in which the two crystals deposit within the same lamellae through an epitaxial process. A question not yet answered is why it should require an elevated pressure for the γ-phase to be formed.

When varying the pressure only the α- and γ-forms are observed3, 10. As the crystallization pressure increases from atmospheric the γ-form begins to form, and to coexist with the α-form, until it becomes dominant at 2 kbar[10]. Furthermore, it appears from experiment that the lower the supercooling the higher is the amount of the γ-form produced at a specific pressure.

On the spherulitic level, no studies of pure γ-form had been reported until the publication of Part II of this series[15]. It has now been shown that the γ-form exhibits both positive and negative birefringence. However when α- and γ-forms are present in the same sample the morphological features become complex. The optical studies of microtomed sections as reported by Campbell, Phillips, and Lin[10]show no evidence of Maltese cross formation when less than 10% of the material is in the γ-form, whereas when more than 60% γ-form is present, a clear Maltese cross exists. In addition, optical and electron microscope studies of etched specimens reveal no cross-hatching in a specimen with >60% γ-crystals. Studies of the pure γ-form crystallized at 200 MPa[15]have shown that the birefringence changes from positive to negative to positive as supercooling is increased. It was also shown that spherulites grown at low supercoolings take the form of large "featherlike" structures, apparently caused by massive self-epitaxy.

The melting point of the γ-form is mostly reported in the range from 125 to 150°C for low molecular weight samples. In the case of pressure-crystallized samples, with high molecular weight isotactic polypropylene (iPP), the melting occurs above 150°C. In this paper are described experiments conducted to determine the pressure dependence of the equilibrium melting points of both phases. They have been sufficiently successful for the equilibrium melting point of the γ-phase at atmospheric pressure to be extrapolated with acceptable precision. It is demonstrated that the melting point of the γ-phase is slightly higher than that of the α-phase. The heat of fusion has also been obtained from the Clapeyron equation. The relative stability of the two crystals has been determined, as has the phase diagram. It is demonstrated that the α-phase is the most stable phase at atmospheric pressure because of the low value of the heat of fusion of the γ-crystal.

Section snippets

Materials

iPP was supplied by Exxon Corporation and had an isotactic pentad content of 90.7% and structural irregularities of 1.26%, as characterized by Dr L. Mandelkern. In order to correlate results it is necessary to assume that most crystallizing molecules are close to 99% isotactic, whereas some of the molecules are highly atactic. During crystallization the atactic molecules will be rejected from the lamellar growth front: consequently, their effects on lamellar thickness will be negligible. Since

Melting of the γ-form

The DSC melting curves at atmospheric pressure of samples crystallized at 200 MPa (2.0 kbar) and at different crystallization temperatures are shown in Fig. 1. The samples were heated from 50°C to 200°C at a heating rate of 10°C min-1.

The transformation of the γ-form to the α-form has been reported by several authors12, 14, 18. All studies have indicated that this phenomenon occurs at temperatures above 140°C and is time dependent. For example, according to Campbell[18], when the WAXD was

Formation of the γ-phase at elevated pressures

As discussed earlier, the γ-form of polypropylene can be generated by several methods. It is easily produced by crystallization at elevated pressures. As the crystallization pressure increases the γ-form increases from zero content at atmospheric pressure, coexisting with the α-form, until it becomes dominant at 200 MPa (2 kbar)[10]. The results of WAXD experiments of samples prepared at 125 MPa (1.25 kbar) and at different isothermal crystallization temperatures show that the lower the

Conclusions

The equilibrium melting points of the γ-form at elevated pressures were determined from lamellar thickness studies as well as from short time crystallization procedures and found to be quite similar. The melting of iPP at high pressures was affected by the amount of γ-phase present. This phenomenon limited to low supercoolings the range of temperatures from which the equilibrium melting point could be determined.

A plot of the equilibrium melting point versus pressure showed a linear

Acknowledgements

Support for this research by the Polymers Program of the National Science Foundation under grants DMR9107675 and DMR9408678 is gratefully acknowledged.

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