Paul J. Phillips

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

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.

The gamma phase of high-molecular-weight polypropylene: 1. Morphological aspects

Abstract A high-molecular-weight polypropylene ( M w = 83000, M w M n = 3.0 ) has been crystallized isothermally as a function of pressure at a constant supercooling of 50°C. Wide-angle X-ray diffraction (WAXD) shows that the X-ray diffraction spectrum of the γ phase is identical to that reported for low-molecular-weight polymers, with the exception that the peaks are broader. Calculations of the γ content using WAXD show the nearly 100% γ phase is formed at 2 k bar, but a mixture of γ and α crystals are formed at lower crystallization pressures. Small-angle X-ray scattering (SAXS) studies show that γ lamellae are approximately one-half the thickness of α lamellae produced at an equivalent supercooling. Morphological studies conducted using transmission electron microscopy of carbon replicas of permanganically etched surfaces confirm the thin nature of the lamellae and demonstrate their inherent waviness. Unlike α crystals, γ lamellae do not form cross-hatched textures. A model is proposed, involving γ α lamellae and epitaxy during the growth process, to account for all the observable morphological data.

The γ-phase of high molecular weight isotactic polypropylene. II: The morphology of the γ-form crystallized at 200 MPa

Abstract The γ-form of isotactic polypropylene, crystallized at elevated pressures, exhibits three distinct classes of spherulites, categorized according to the sign of the birefringence. The results show that at 200 MPa positively birefringent spherulites are observed for crystallization temperatures below 184°C and above 199°C, negatively birefringent spherulites are observed for crystallization in the range 187–198°C and mixed birefringence in the ranges 182–188°C and 196–200°C. In addition, the complex arrangement of the lamellae within these spherulites has been studied using optical and electron microscopy. The results indicate the presence of a hitherto-unreported form of lamellar aggregation, which is best described as feather-like. The birefringence results indicate that the feather-like structure is solely developed by self-epitaxial growth of the γ-lamellae. Melting studies of ‘mixed birefringent’ spherulites show that the epitaxially grown featherlike structures have higher melting points than radial lamellae grown concurrently in the same spherulite, leading to the inference that it is possible to grow substantial amounts of two different types of lamellae concurrently, that are not distinguishable using conventional wide and small angle X-ray analyses.

The triple melting behavior of poly(ethylene terephthalate): Molecular weight effects

The melting behavior of isothermally crystallized PET has been studied using linear heating in a differential scanning calorimeter (DSC). Variables such as crystallization temperature, crystallization time, heating rate, and average molecular weight are the main focus of the study. On the basis of several experimental techniques, a correlation of the melting behavior of PET with the amount of secondary crystallization was found to exist. It was observed that the triple melting of PET is a function of programmable DSC variables such as crystallization temperature, crystallization time, and heating rate. However, in testing the hypothesis that there was a correlation between melting endotherms and secondary crystallization inside spherulites, it was found necessary to use a DSC-independent variable in order to enhance the observed effects. Therefore, on the basis of a crystallization model that involves secondary branching along the edges of parent lamellar structures, it was speculated that an increase in the average molecular weight could affect the triple melting of PET due to an increase of rejected portions of the macromolecules. It was found that the second melting endotherm increased, apparently, at the expense of the third one as the average molecular weight was increased. The second melting endotherm was also found to correlate proportionally with the amount of secondary crystallization inside spherulites. The results support a model of crystallization which basically consists of parent crystals and at least one population of secondary, probably metastable, crystals. This latter structural component must involve excluded portions of the macromolecules that did not crystallize during the isothermal crystallization period of the parent crystals. An increase of molecular weight gives rise to a higher entanglement density which in turn increases the fraction of initially rejected chain sections and therefore the amount of secondary crystallization.

γ-Phase in propylene copolymers at atmospheric pressure

Crystallization studies have been conducted on three copolymers of propylene with ethylene as a function of supercooling at atmospheric pressure. The results confirm the conclusions reached almost 30 years ago that copolymerization causes the γ -phase to be generated at atmospheric pressure for high molecular weight polymers. The greater the amount of comonomer, the greater the ease of formation of the γ -phase. The γ/α ratio is inversely proportional to supercooling, as has been found for isobaric crystallization at elevated pressures. It has been found that the equilibrium melting point decreases with ethylene content in a significant manner.

Nonisothermal melting and crystallization studies of homogeneous ethylene/α-olefin random copolymers

A study of nonequilibrium melting, nonisothermal, and isothermal crystallization behavior of ethylene/1-octene (EO) random copolymers, produced using metallocene catalysts has carried out. As branch (or defect) content increases, the nonisothermal and isothermal crystallization rates, melting temperatures, and heats of fusion decrease. There is also a branch length effect on melting temperature depression, the melting temperature depression of EO random copolymers with hexyl branches were significantly larger than those of ethylene/1-butene (EB) and ethylene/1-propene (EP) copolymers having ethyl and methyl branches, respectively. The melting temperatures of homogeneous random copolymers have been found to be always lower than those of fractions of heterogeneous copolymers, having approximately the same branch content and molecular weight. Hence, defect distribution in copolymer systems is at least as important a parameter as the defect content.

The mechanism of crystallization of linear polyethylene, and its copolymers with octene, over a wide range of supercoolings

Abstract The crystallization behavior of a linear polyethylene has been studied using conventional isothermal hot stage microscopy and with the Ding–Spruiell method of rapid cooling. When studied at rapid cooling rates the polymer generates its own pseudo-isothermal crystallization temperatures, in agreement with Ding–Spruiells studies on other systems, permitting experiments to be carried out isothermally at a temperature as low as 90°C, thus extending the range of supercooling available from the 9°C (120–129°C) typical of conventional experimentation to 29°C (90–129°C). The points generated using both the isothermal and the rapid cooling techniques form a single consistent trend, as in polypropylene. In conventional crystallization experiments it was found, as expected, that the spherulite growth rates conformed to the regime I–regime II scheme, already well established for this polymer. When analyzed using a secondary nucleation approach all three regimes are found to exist and to adequately describe the data. The regime II–regime III transition temperature was found to occur at 120.6°C. The crystallization behavior of a series of ethylene-octene copolymers synthesized using metallocene catalysts has also been studied using the same experimental methodologies. In conventional crystallization experiments it was found, as expected, that the spherulite growth rates varied with octene content and molecular weight. When studied at rapid cooling rates, at the lowest temperatures of crystallization, the spherulite growth rates of all of the copolymers studied merge with the growth rate curve of the linear polyethylene and are virtually indistinguishable. The results indicate a major breakdown of all current theories of polymer crystallization, in that the overriding equation involving the relation between crystallization rate, lamellar thickness, surface free energy and supercooling appears to be superceded in the copolymers by some hitherto unrecognized process or law. The underlying physics behind this conclusion needs to be elucidated, but appears to be consistent with the formation of a partially ordered intermediate of 3–4 stems in size on the growth face under all conditions of growth in linear polyethylene and its copolymers. The degree of disorder in the cluster is believed to be strongly dependent on supercooling, permitting incorporation of hexyl groups into the intermediate. Subsequent ordering of the cluster produces the ultimate crystal packing and ejection of hexyl groups and other impurities. The rate of formation of the cluster is suggested to be the rate controlling step in secondary nucleus formation.

Crystallization of ethylene–octene copolymers at high cooling rates

Abstract The crystallization behavior of a series of ethylene–octene copolymers synthesized using metallocene catalysts was studied using the Ding–Spruiell method of rapid cooling. In conventional crystallization experiments it was found, as expected, that the spherulite growth rates varied with octene content and molecular weight. When studied at rapid cooling rates the polymers generate their own pseudo-isothermal crystallization temperatures, in agreement with Ding–Spruiell’s studies on other systems, however, at the lowest temperatures of crystallization, the spherulite growth rates of all of the copolymers studied merge and are virtually indistinguishable. The results indicate that there is a major change of crystallization mechanism under these conditions, of considerable relevance to polymer processing operations.

Crystallization kinetics of fractions of branched polyethylenes. 2. Effect of molecular weight

Abstract Three series of cross-fractionated branched polyethylenes having molecular weights in the ranges 20 000, 60 000 and 110 000 and branch contents ranging from zero to 32 hexyl branches per 1000 C atoms have been studied. Both linear and bulk growth rates have been determined using optical methods. The two lower molecular weight groups show a regime I-regime II transition, which translates to lower temperatures as branching is increased. The highest molecular weight series shows either a regime II-regime III transition or simply regime III. The transition temperature also translates to lower temperatures as branching increases. The rate of secondary nucleation is reduced by branching, but the rate of reptation is reduced by increasing molecular weight causing the aforementioned effects. It is suggested that the appearance of the regime II-regime III transition is caused by a change in nucleation mechanism.

The Equilibrium Melting Points of Random Ethylene-Octene Copolymers: A Test of the Flory and Sanchez-Eby Theories

Ethylene/l-octene copolymers produced with metallocene catalysts are believed to have a homogeneous comonomer content with respect to molecular weight. Two series of copolymers of different molecular weights with a 1-octene content ranging from 0 to 39 branches per 1000 carbon atoms were studied. The influence of branch content on structure and melting behavior as well as on isothermal and nonisothermal bulk crystallization was studied. In this article, the equilibrium melting temperatures of ethylene/l-octene random copolymers is the focus. The principal techniques used were thermal analysis and small-angle X-ray scattering. The use of Hoffman-Weeks plots to obtain the equilibrium melting temperatures of ethylene/l-octene random copolymers resulted in nonsensical high values of the equilibrium melting point or showed behavior parallel to the T m = T c line, resulting in no intercept and, hence, an infinite equilibrium melting point. The equilibrium melting temperatures of linear polyethylenes and homogeneous ethylene/ l-octene random copolymers were determined as a function of molecular weight and branch content via Thompson-Gibbs plots involving lamellar thickness data obtained from small-angle X-ray scattering. This systematic study made possible the evaluation of two equilibrium melting temperature depression equations for olefin-type random copolymers, the Flory equation and the Sanchez-Eby equation, as a function of defect content and molecular weight. The range over which the two equations could be applied depended on the defect content after correction for the effect of molecular weight on the equilibrium melting temperature. The equilibrium melting temperature, T m 0 (n, p B ), of the ethylene/l-octene random copolymers was a function of the molecular weight and defect content for low defect contents (p B ≤ 1.0%). T m 0 (n, p B ) was a weak function of molecular weight and a strong function of the defect content at a high defect content (p B ≥ 1.0%). The Flory copolymer equation could predict T m 0 (n, p B ) at p B ≤ 1.0% when corrections for the effect of molecular weight were made. The Sanchez-Eby uniform inclusion model could predict T m 0 (n, p B ) at a high defect content (1.6%≤p B ≤ 2.0%). We conclude that some defects were included in the crystalline phase and that the excess free energies (18-37 kJ/mol) estimated in this study were within the theoretical range.