Extended-chain crystals. II. Crystallization of polyethylene under elevated pressure

A report on crystallization of polyethylene at elevated pressures to an extended-chain morphology is presented. The crystals have been characterized by electron microscopy and density determination. Pressure, supercooling (temperature), and crystallization time have been varied to find the best conditions for production of perfect crystals. At 10–30°C supercooling completely crystallized polyethylene was obtained between 4.5 and 7 kb crystallization pressure in 1–8 hr. Analysis of fracture surfaces of samples crystallized for different lengths of time shows an increase in size and number of crystal lamellae and an improvement of extended chain crystals in the early stages of crystallization. A further improvement of the less well crystallized material between the lamellae occurs after 15 min of crystallization time.     

Extended-chain crystals. VI. Annealing of polyethylene under elevated pressure

Polyethylene crystals of different degrees of perfection were annealed at 5.1 kb pressure for 20 hr at various temperatures and analyzed by electron microscopy, thermal analysis, and density determination. No annealing took place until the temperature was close to the melting point of the starting material. Up to 235°C increasing solidstate annealing was observed. Mixed crystals of up to 0.989 g/cm³ density and 1500 Å thickness in the chain direction could be produced. At slightly higher temperature recrystallization to extended-chain crystals rather than annealing occurred. The annealing process at atmospheric pressure seems to be similar in nature, but takes much longer for comparable perfection. From a comparison of annealing and crystallization it is concluded that polymer crystallization goes through a stage of internally imperfect order during which most of the observed chain extension occurs. Estimates of this outer imperfect layer of a growing crystal place its depth at 30,000 Å.     

Extended-chain crystals. III. Size distribution of polyethylene crystals grown under elevated pressure

Extended-chain crystals of high molecular weight polymethylene, a polyethylene with a broad molecular weight distribution, and three fractions of polyethylene were grown from the melt under elevated pressure. Comparison of the crystal size distribution in the molecular chain direction (measured on fracture surfaces by electron microscopy) with the molecular weight distribution (measured by gel-permeation chromatography) gave the following results. Up to molecular weight 10,000 all samples showed eutectic separation into fully extended chain crystals of narrow molecular weight distribution. Above molecular weight 10,000 mixed crystals were formed. Under the chosen crystallization conditions larger chain extension was achieved with higher molecular weights. However, an increase in molecular weight by a factor of 1000 led only to a tenfold increase in chain extension. These facts are discussed in the light of a proposed mechanism of crystal growth.     

High-resolution electron microscopy of polyethylene and paraffin crystals: Stability in the electron beam

Uncollapsed polyethylene pyramids (200–1500 Å) in length are irradiated with the electron beam of a 100-kV transmission electron microscope. Their high stability is remarkable compared to the stability of 1–10 μm crystals collapsed on the substrate, usually taken as a reference. Therefore, the maximum magnifications (300,000–750,000 X) of the microscope can be used and high-resolution images can be obtained. No lattice defects can be detected in the images of PE pyramids. Irradiation with D_c > 800 C/m² induces the orthorhombic → hexagonal transition, and slight lattice distortions appear in the high-resolution image of the hexagonal phase. For an irradiation dose D_c ≈ 2400 C/M², the diffraction pattern disappears. Normal C₃₆ orthorhombic and monoclinic paraffins have the same stability as orthorhombic PE and high-resolution patterns are obtained. These exceptional stabilities are discussed in detail. From the diffraction pattern of these uncollapsed pyramids, the fold surfaces of PE pyramids have been indexed as the {111} and paracrystalline distortions in the orthorhombic PE have been measured at low irradiation dose. Along the a and b axes g is ca. 5% and along the chain axis c it is ca. 17%; these values agree with the previous x-ray determinations of PE crystallized from the melt. The large difference between these two distortion factors may be interpreted in terms of packing.     

Thermodynamics of crystalline linear high polymers. V. Extended-chain copolymers of polyethylene

Ethylene—propylene and ethylene—butene-1 copolymers with up to 1.7 side groups per 100 carbons have been crystallized at 227°C. and under 4100–4900 atm. pressure. The resulting crystalline polymers are at least partially of extended-chain crystal morphology. Comparison with the same polymers crystallized at atmospheric pressure, which gives folded-chain crystal morphology, revealed: (1) a density higher by 0.008–0.019 g./cm.³ depending on copolymer content; (2) a similar decrease of crystallinity with side group concentration; (3) a similar decrease of the beginning of melting from 125°C. for homopolymer to 65°C. for 1.7 side groups per 100 carbons; (4) a higher (138 ± 0.8°C.) experimental maximum melting point which, in contrast, is independent of copolymer content and seems to vary only with the fraction of low molecular weight material; (5) a decreasing amount of high-melting crystals with increasing copolymer content (72–8%) and an increasing amount of low-melting crystals (27–53%) with increasing copolymer content. In addition, superheating, which reached 5.5°C. for 50°C./min. heating rates, was detected. It was concluded that high-pressure crystallization leads, at least for part of the crystals, to solid solution formation, while atmospheric pressure crystallization does not. Which mode of crystallization is achieved seems kinetically determined. Experimental techniques were dilatometry, DTA, and calorimetry.     

Extended-chain crystals. I. General crystallization conditions and review of pressure crystallization of polyethylene

This is the first paper of a series of reports concerning extended-chain crystals of flexible, linear high polymers. The general conditions for crystal growth are discussed. Polymer crystallization is described as a two-step process: nucleation of each crystallizing molecule to a folded-chain conformation, followed by an increase in fold length in a solid-state reorganization step. This reorganization step is enhanced in the case of polyethylene by crystallization at high temperature under elevated pressure. Mechanical deformation during crystallization is also able to produce extended-chain crystals. The most promising method, however, is crystallization during polymerization. Previous work on crystallization of polyethylene under elevated pressure is critically reviewed.     

Use of electron microscopy to obtain quantitative information about the melting behavior of branched polyethylene

The melting behavior of low-density polyethylene was studied by electron microscopy. The experimental techniques and method of evaluation chosen were such as to deliver quantitative numerical data. The values which could be derived from suitably prepared histograms were the minimum, maximum, average, and most probable values of the crystal thickness d and long-spacing L. All these values were in good agreement with small-angle x-ray scattering results. The histograms also show the distribution functions for d and L, so that effects leading to changes in the average values can be observed in detail. The results provide further support for a model of melting and crystallization proposed in a previous paper.     

Structure analysis of discotic,sanidic and ferroelectric crystals and liquid crystals using electron microscopy

Three types of liquid-crystalline materials with specific molecular structures were investigated using electron diffraction and high resolution imaging. Discotic and smectic liquid crystals were compared with the crystalline state. The superstructure of the sanidic liquid crystals showed effects in the line profiles and contour plots of the electron diffraction patterns.     

Thermal contraction and extension in fibrous crystals of polyethylene

Summary Solution grown polyethylene shish-kebabs have a core, diameter 20–30 nm, which is highly superheatable. When a mat of shish-kebabs is heated to 132 °C the lamellar material melts, leaving birefringent fibres in an isotropic matrix as seen under the polarizing microscope. Microscopic observations reveal that these fibres, aggregates of many shish-kebabs, contract as they are melted and extend on subsequent recrystallization from the partially molten state as the temperature is lowered. This extension is completely reversible on temperature cycling. For fibres originally crystallized at below 96 °C this effect results in crimping on recrystallization and straightening from the crimped state on remelting. This is because these latter fibres are isolated from each other by the lamellar overgrowth and they buckle rather than extending the sample. Analogous reversible length changes and crimping effects were observed also in fibrous crystals produced from the melt.The above observations allow certain conclusions to be reached as regards the nature of the shish-kebab fibres. Thus an estimate of the forces involved in the bucking indicates that the basic unit which buckles is less than 50 nm in diameter in agreement with the single shish-kebab core when this is directly observable. Similar buckling in the melt crystallized fibrous entities indicates a core of similar size, which is not so easily seen. Further, the observed shrinkage behaviour, together with the associated birefringence and calorimetric information collected in this study, can only be accounted for if melting is visualised as the formation of disordered amorphous regions which alternate with perfect crystal regions along the shish-kebab core. This leads to a model equivalent to a linked row of fringed micelles within a given fibre. The chain straightening of the amorphous sections on recrystallization would then be responsible for the fibre extension. It follows further that for the amorphous sections to form in the first instance there must be preexisting centres of imperfections along the fibre. A model of randomly distributed imperfections (visualised as interlocking chain loops) can account for the present observations in a quantitative manner with realistic predictions as regards the lengths of the undisturbed crystal regions.Finally there is an obvious analogy with the crystallization of oriented polymer networks (e.g. stretched rubber) as regards the observed extension —contraction behaviour, providing a link between recent findings and long established experience.With 12 figures and 1 table     

Atomic force microscopy of extended-chain crystals of polyethylene

Extended-chain crystals of polyethylene grown at elevated pressure and temperature were analyzed for the first time by atomic force microscopy. It was possible to compare the typical fracture surface striation features with those obtained earlier by electron microscopy. High resolution atomic force microscopy on flat surfaces enabled the recording of an atomic scale regularity that could not be fully indentified. © 1993 John Wiley & Sons, Inc.     

Multiple melting behavior of poly(butylene succinate). II. Thermal analysis of isothermal crystallization and melting process

The multiple melting behavior of poly(butylene succinate) (PBSu) was studied with differential scanning calorimetry (DSC). Three different PBSu resins, with molecular weights (MWs) of 1.1 × 10⁵, 1.8 × 10⁵, and 2.5 × 10⁵, were isothermally crystallized at various crystallization temperatures (T_c) ranging from 70 to 97.5 °C. The T_c dependence of crystallization half‐time (τ) was obtained. DSC melting curves for the isothermally crystallized samples were obtained at a heating rate of 10 K min⁻¹. Three endothermic peaks, an annealing peak, a low‐temperature peak L, and a high‐temperature peak H, and an exothermic peak located between peaks L and H clearly appeared in the DSC curve. In addition, an endothermic small peak S appeared at a lower temperature of peak H. Peak L increased with increasing T_c, whereas peak H decreased. The T_c dependence of the peak melting temperatures [T_m(L) and T_m(H)], recrystallization temperature (T_re), and heat of fusion (ΔH) was obtained. Their fitting curves were obtained as functions of T_c. T_m(L), T_re, and ΔH increased almost linearly with T_c, whereas T_m(H) was almost constant. The maximum rate of recrystallization occurred immediately after the melting. The mechanism of the multiple melting behavior is explained by the melt‐recrystallization model. The high MW samples showed similar T_c dependence of τ, and τ for the lowest MW sample was longer than that for the others. Peak L increased with MW, whereas peak H decreased. In spite of the difference of MW, T_m(L), T_m(H), and T_re almost coincided with each other at the same T_c. The ΔH values, that is crystallinity, for the highest MW sample were smaller than those for the other samples at the same T_c. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2039–2047, 2005     

Crystallization and melting of polyethylene under high pressure. II. Effect of pressure on melting behavior of various types of crystals

The effect of pressure on the melting point and volume of fusion of polyethylene was studied by high-pressure dilatometry. Starting materials were crystallized slowly from the melt under pressures of 1500, 3500, 5130 kg/cm², and 1 atm. It has been shown that the unusual behavior observed at pressures above 4000 kg/cm² is due to crystallization and melting of two kinds of extended-chain crystals differing in thermal stability. These are designated as ordinary extended-chain and highly extended-chain crystals, respectively. The relation between pressure P and melting temperature T_m of folded-chain, ordinary extended-chain, and highly extended-chain polyethylene was determined precisely. At pressures up to about 3000 kg/cm², plots of P against T_m for the crystal forms have almost the same curvature and then become parallel. But at pressures above 4000 kg/cm², ordinary extended-chain crystals show a linear increase of T_m with a constant slope of about 70 atm/deg. Curve for the highly extended-chain crystals changes in slope from 70 to 50 atm/deg at pressures between 3500 and 4300 kg/cm², and then show a sharp increase of T_m with increasing pressure. Experiments show that the meltingpoint curve of the highly extended-chain crystals overlaps that of the ordinary extended-chain crystals at pressures below 4000 kg/cm². Annealing experiments with folded-chain and ordinary extended-chain crystals have been made under high pressure. It is suggested that the formation of highly extended-chain crystals occurs stepwise through the formation and reorganization of ordinary extended-chain crystals from the original folded-chain crystals by a mechanism of partial melting and recrystallization at pressures above 4000 kg/cm².     

Scanning electron microscopy of oriented high density polyethylene

Summary The microfibrillar and lamellar morphologies in cold-drawn and cold-drawn/annealed high-density polyethylene sheets were observed by means of scanning electron microscopy. Differences in contrast on fracture surfaces for cold-drawn sheet are interpreted in terms of a preferential orientation of inter-microfibrillar tie molecules in the plane of the sheet brought about by the drawing mechanism. In annealed, cold-drawn sheet, stacks of lamellae were observed which showed twinned orientations of inclined lamellae. This roof-top structure is interpreted in terms of shear within the individual microfibrils during micronecking, and corresponds to the well-known 4-point small-angle X-ray pattern for this type of specimen. Light etching with fuming nitric acid was necessary in order to resolve the individual lamellar texture.With 9 figures     

Direct observation of polyethylene molecules with electron microscopy

Individual polyethylene molecules have been imaged in the electron microscope. Preparative difficulties are overcome by the following procedures. (1) The polymer is dissolved in n-hexadecane at 130°C; (2) the solution is deposited on a cooled substrate by spraying in an atmosphere of cold nitrogen; (3) the deposited polymer molecules were shadowed by platinum. Molecular weights obtained are in good agreement with those from light scattering.     

Longitudinal growth of polymer crystals from flowing solutions V.: Structure and morphology of fibrillar polyethylene crystals

Summary Longitudinal fibrillar polyethylene crystals grown from xylene solutions subjected to simple shear flow have been investigated by various X-ray diffraction and electron microscopic techniques. The macrofibers with diameters in the micron range are composed of elementary fibrils of two basically different types. They may consist of either striated fibrils of the shishkebab type or smooth fibrils. The latter morphologies were obtained when the crystallization was carried out at temperatures above approximately 114 °C and the fibrillar crystals were allowed to grow longitudinally at the surface of the rotor of a Couette type instrument at a sufficiently high speed.The average spacings between the lamellae of the shish-kebabs as derived from the maximum in the small-angle X-ray scattering curve correspond remarkably well to the values obtained from transmission electron micrographs. The lateral dimensions of the elementary fibrils were acquired from equatorial line-broadening analysis of electron diffraction spots, wide-angle X-ray diffractograms, dark-field images and Guinier plots.They varied from 260 Å for fibrillar polyethylene crystals grown at 103 °C to approximately 150 Å for fibers formed at 118 °C The lateral crystallite size was also found to diminish at higher growth rates. Under the latter conditions the growth is accompanied by an extensive coil deformation as reflected by the presence of a considerable amount of the triclinic crystal modification.Dark-field electron microscopy indicates that the lengths of the crystallite blocks are of the order of 250–350 Å for fibers grown at 103 °C and 450 Å and longer for fibers formed at 118 °C. These values are in agreement with those obtained from the broadening of the (002) electron diffraction spots.

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Zusammenfassung Dieser Bericht befaßt sich mit der Untersuchung von longitudinalen Polyäthylen-Faserkristallen, gewachsen aus Xylollösungen in einer Scherströmung, mit Hilfe verschiedener Röntgen-Diffraktions- und elektronenmikroskopischer Techniken. Die Makrofasern mit Durchmessern von einigen Mikron enthalten Elementar-Fibrillen von 2 grundsätzlilch verschiedenen Typen. Sie enthalten entweder gestreifte Fibrillen, die wie Schaschlik-Strukturen erscheinen, oder glatte Fibrillen.Die letztgenannten glatten Fibrillen wurden erhalten, wenn die Kristallisation bei Temperaturen oberhalb ungefahr 114 °C ausgeführt wurde und die Faserkristalle die Möglichkeit hatten, longitudinal an der Oberfläche des Rotors eines Apparates vom Couette-Typ zu wachsen, wobei der Rotor genügend rasch rotierte.Der mittlere Abstand zwischen den Lamellen der Schaschlik-Struktur, berechnet aus dem Maximum der Weitwinkel-Streuungskurve, stand in sehr guter Übereinstimmung mit den Werten der elektronenmikroskopischen Beobachtungen im Durchlicht. Die lateralen Dimensionen wurden aus der Verbreiterung der Reflexe erhalten, aus Weitwinkel-Aufnahmen Bowie aus Dunkelfeld- und Guinier-Aufnahmen.Die Abstände waren 260 Å für Polyäthylen-Faserkristalle, gewachsen bei einer Temperatur von 103 °C, und ungefähr 150 Å für Fasern entstanden bei 118 °C. Die laterale Kristallitgröße verringerte sich bei höheren Wachstumsgeschwindigkeiten. Unter den letztgenannten Verhältnissen ist das Wachstum verbunden mit größerer Knäuelverformung, was sich durch die Anwesenheit eines erheblichen Anteils von trikliner Kristallmodifikation bemerkbar macht.Dunkelfeld-elektronenmikroskopische Untersuchung hat gezeigt, daß die Länge der Kristallanteile von einer Größenordnung von 250–350 Å für Fasern, gewachsen bei 103 °C, and 450 Å und länger für Fasern entstanden bei 118 °C ist. Diese Werte stehen in guter Übereinstimmung mit den Werten, erhalten aus Verbreiterung der (002) Elektronen-Beugungsbilder.

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With 13 figures and 2 tables     

Equilibrium and non-equilibrium melting of branched polyethylene relating to defect incorporation within crystals

New equilibrium melting point data, for polyethylene containing chain defects, are tested in the light of random copolymer predictions. A simplified expression for the melting point depression of random copolymers containing small amounts of non-crystallizable units is derived. Non-equilibrium melting data for rapidly quenched polyethylene samples are also reported. The fusion enthalpyH(X), and the surface free energy _e for crystals containing defects are evaluated using crystallinity, equilibrium meltingtemperatures and X-ray long period data. It is shown that increasing defect penetration within crystals induces a decrease ofH(X) withX in accordance with theoretical predictions. Finally _e is, similarly, shown to decrease with increasing number of chain defects attached to the crystal surface.     

Further electron microscope observations on polyethylene III. Smectic intermediate state during melting and crystallization

While it was possible to demonstrate in the first part of this paper [1] that the granular structure in an LPE melt created by short-time staining with chlorosulfonic acid is an artifact, it was demonstrated in the second part [2] that an artifact can actually be useful. It makes it possible to differentiate between the mobile melt and a very thin layer of fixed melt on the crystalline lamellae which corresponds to the switchboard model.This third part reports the discovery of a smectic type of liquid crystal intermediate state both in the melting and in the crystallization processes, which many authors regarded as impossible because of the flexibility of the molecules in polyethylene.Extracts presented at the 32nd Hauptversammlung der Kolloid-Gesellschaft und Berliner Polymeren Tage 1985, 2–4 October 1985 in Berlin     

Crystallization and melting of polyethylene under high pressure. III. Mixed crystallization of two kinds of extended-chain crystals

Thermal behaviors of extended-chain crystals of polyethylene formed during various crystallization processes and conditions under about 5000kg/cm² were studied by high-pressure dilatometry and differential scanning calorimetry. The experiments indicate that by isothermal crystallization at small undercoolings for prolonged periods, the products show two endothermic peaks in the melting region of the usual extended-chain crystals. This means of the presence of a bimodal lamellar thickness distribution in the extended-chain crystals. A phase diagram has been made for pressures up to 5000 kg/cm². The experimental results confirm the existence of two kinds of extended-chain crystals, i.e., ordinary extended-chain and highly extended-chain crystals, as suggested previously by the authors.