Lamellar and interlamellar structure in melt-crystallized polyethylene. II. Lamellar spacing,interlamellar thickness,interlamellar density,and stacking disorder

Measurements of the small-angle scattering power and the degree of crystallinity in melt-crystallized high-density polyethylene have been used to evaluate the “amorphous” density in situ by the relation, where V is the irradiated volume and ?(S) is the “slit-smeared” absolute intensity. The amorphous density is a function of sample history and is always higher than the extrapolated melt density. After slit-height correction, and within the experimental error, the ratio of the two observed long periods is 2:1 at all temperatures (25--126°C). The lamellar thickness and the average interlamellar spacing are obtained from the degree of crystallinity and the first corrected long period. At increasing temperatures between 25°C and 110°C, the lamellae become thinner while the interlamellar zone expands by almost half. Over this range the changes are reversible with temperature. Above 110°C, both the lamellae and the interlamellar region expand with temperature. The thickening is partially reversible upon recooling. Other results obtained include measurements of stacking disorder and of microstructural changes with crystallization temperature and with time at ambient temperature.     

Electron microscopy investigations of the morphology of a polyester-polyurethane

In the current paper the morphology of a slightly crosslinked polyester-polyurethane (hard-segment content ca. 33 wt.-%) is studied by means of transmission electron microscopy (TEM). A new staining technique based on ruthenium tetroxide was applied. This new technique allows the unambiguous visualization of the typical microdomain structure of the polyurethane and also of a wide size spectrum of globular hard-segment-rich domains which result from phase separation in the early stage of preparation. The hard-segment-rich microdomains have a size between 1.5 and 10 nm while the globular domains span a size spectrum from 10 to 2000 nm.     

Electron microscopy and quantitative polymer morphology

The lamellar nature of semi-crystalline polymers can now be studied systematically using transmission electron microscopy, especially following permanganic etching. In combination with thermal analysis and selective extraction followed by molecular characterisation, this approach allows the distribution of lamellae, thermal stability and molecular constitution to be mapped in real space. Extension of this methodology has allowed the extraction of individual melt-crystallized lamellae from bulk polymer. Their study is adding to understanding of fundamental processes of crystal growth.     

Electron microscopy of oriented high-density polyethylene: Comparison with small-angle X-ray scattering

The morphology of cold-drawn, rolled and annealed high-density polyethylene was investigated by transmission electron microscopy of stained sections. From the electron micrographs, a model of the structure was developed and the scattering pattern calculated. This was then compared with the corresponding small-angle X-ray scattering (SAXS) pattern, in order both to aid in the interpretation of SAXS patterns of oriented polymers, and to assess the effects of staining with chlorosulphonic acid on the morphology.     

Effect of crystallization time on the properties of melt-crystallized linear polyethylene

Linear polyethylene was crystallized isothermally from the melt. Specimens were removed at different crystallization times and quenched to room temperature. The density, static mechanical properties, and small-angle x-ray scattering (SAXS) behavior of these specimens were measured at room temperature. The density and Young's modulus increased with crystallization time, whereas the upper yield point decreased with crystallization time. SAXS data showed that a zero-angle peak gradually disappeared as crystallization time increased. Concurrently, the breadth of the SAXS peaks, the Bragg angle, and the integrated intensity decreased. Changes in the ratio of second- and first-order peak intensities were also noted. On the basis of the SAXS and density data, it was concluded that a competition exists between the thickening of existing crystals and the creation of new crystallites between the older ones. At relatively low crystallization times, numerous new crystals can form during quenching to room temperature, whereas quenching after prolonged crystallization primarily results in the additional thickening of existing crystals. No change in the density of the amorphous material is found. A model is given whereby the upper yield stress is coupled to these morphological changes through a stress concentration effect caused by a decreased population of chains connecting adjacent crystallites. The tie-chain population change occurs by their elimination as crystallites disappear.     

Lamellar thickening of polyethylene under high pressure

Single crystal mat (SCM) samples of polyethylene (PE) were prepared from dilute solution of p-xylen, then they were annealed at pressures of 200 and 500 MPa. Lamellar thickness of the original and annealed SCM samples was measured by small-angle X-ray scattering method. Orientation of the molecular chain in those SCM samples was investigated by wide-angle X-ray diffraction pattern. From these X-ray measurements, annealing temperature dependence of the lamellar thickness, i.e., lamellar thickening, under high pressure was obtained. Melting process of the SCM samples was also investigated at 200 and 500 MPa by high pressure differential thermal analysis. Then correspondence between the lamellar thickening and the melting process was studied. The lamellar thickness increases markedly with approaching to the melting temperature of the orthorhombic crystal even in the high pressure region where the high pressure phase (hexagonal phase) appears. The annealing temperature dependence curve of the lamellar thickness at 200 MPa can be superimposed on the curve at 500 MPa by shifting the curve along the temperature scale by 47 K. Large scale lamellar thickening occurs in the orthorhombic crystal phase in the high pressure region. The formation process of extended-chain crystal is discussed. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys, 35: 535–543, 1997     

Structure and morphology of nylon 46 lamellar crystals: Electron microscopy and energy calculations

A detailed electron microscopy study of the structure and morphology of lamellar crystals of nylon 46 obtained by crystallization from solution has been carried out. Electron diffraction of crystals supported by X‐ray diffraction of their sediments revealed that they consist of a twinned crystal lattice made of hydrogen‐bonded sheets separated 0.376 nm and shifted along the a‐axis (H‐bond direction) with a shearing angle of 65°. The interchain distance within the sheets is 0.482 nm. These parameters are similar to those previously described for nylon 46 lamellar crystals grown at lower temperatures. A combined energy calculation and modeling simulation analysis of all possible arrangements for the crystal‐packing of nylon 46 chains, in fully extended conformation, was performed. Molecular mechanics calculations showed very small energy differences between α (alternating intersheet shearing) and β (progressive intersheet shearing) structures with energy minima for successive sheets sheared at approximately 1/6 c and 1/3 c. A mixed lattice composed of a statistical array of α and β structures with such sheet displacements was found to be fully compatible with experimental data and most appropriate to describe nylon 46 lamellar crystals. Annealing of the crystals at temperatures closely below the Brill transition induced enrichment in β structure and increased chain‐folding order. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 41–52, 2000     

Lamellar and interlamellar structure in melt-crystallized polyethylene. I. Degree of crystallinity,atomic positions,particle size,and lattice disorder of the first and second kinds

An x-ray diffraction method for the simultaneous determination of crystallinity (including intracrystalline defects), effective Debye-Waller factors, and atomic positions has been developed and applied to semicrystalline polyethylene. It was found that this material unambiguously constitutes a two-phase system. Measurements of intracrystalline lattice disorder in the chain direction and perpendicular to the chain direction show these to be in the ratio 1:2.5. Lattice disorder was principally of the first kind. Paracrystalline disorder in the [110] direction was less than 2.4% at all experimental conditions. Results include measurements of degree of crystallinity, particle size, space group, and unit cell parameters and variation of these quantities with crystallization temperature, ambient temperature, and time.     

Morphological partitioning of chain ends and methyl branches in melt-crystallized polyethylene by 13C-NMR

The partitioning of methyl and vinyl ends between the crystalline and noncrystalline regions of polyethylene has been investigated using ¹³C-NMR in the solid state. The polyethylene samples, which were crystallized from the melt, varied in molecular weight, polydispersity, crystallization rate, and comonomer content. For the limited set of samples considered, the ratio of crystal to overall end concentration is independent of those variables. This ratio takes the values of 0.75 and 0.60 for the methyl and vinyl ends, respectively. When the crystalline fraction of these samples is taken into account, 50–75% of the total saturated ends and 42–63% of the total vinyls reside in the crystal. For an ethylene/propylene copolymer, 21–27% of the methyl branches were determined to be in the crystal. This level of incorporation puts methyl branches in a position intermediate between chain ends and ethyl branches.     

Solid-state morphology,structure, and tensile properties of polyethylene/polyamide/nanoclay blends: Effect of clay fraction

The effect of nanoclay fraction on the linear and non-linear tensile properties of a polyethylene/polyamide 12 blend with droplet morphology was investigated. All ternary blends were prepared at a fixed polyamide (PA) weight fraction of 20%, and at clay volume fractions varying from 0.5 to 2.5% relative to PA. Scanning electron microscopy and transmission electron microscopy were used to characterize the morphology of the blends and the clay interphase structure. The nanoclay content was shown to strongly influence both linear and non-linear tensile properties. Young's modulus, elongation at yield, yield strength, tensile strength and elongation at break as a function of clay fraction were studied and discussed in terms of morphological changes and strain-induced structural reorganization of the clay interphase.     

Morphology of melt-crystallized linear polyethylene fractions and its dependence on molecular weight and crystallization temperature

Replicas and thin-section electron microscopic studies were made of fractions of linear polyethylene covering the molecular weight range 2.78 × 10⁴ to 6.0 × 10⁶ for a variety of crystallizing conditions. Lamellar crystallites were found under all circumstances; and the supermolecular structure, or crystalline morphology, is in agreement with that previously reported from an analysis of the small-angle light-scattering patterns of the same samples under similar crystallization conditions. Details of the crystalline microstructure are also described, which range from truncated hollow pyramids which degenerate as the molecular weight or the undercooling are increased. From these results, it is possible to describe the mechanism of formation of polyethylene spherulites.     

Structure and morphology of heavily deformed single crystals of a polydiacetylene. I. Electron microscopy and x-ray diffraction

The structure and morphology of heavily deformed single crystals of a diacetylene polymer have been studied using a combination of x-ray diffraction and electron microscopy. Crystals have been deformed by both rolling and hammering. The crystals remain intact during deformation and can be reduced in thickness by a factor of over 5 in directions perpendicular to their chain axes. It is found that the chain orientation is maintained during both hammering and rolling. A greenish-colored surface skin develops during both types of deformation but the structure of the interior of the crystals depends upon the type of deformation employed. The interior of the hammered crystals consists of crystal blocks ca. 50 μm thick formed by cleavage perpendicular to the chain direction whereas the rolled crystals tend to be fibrous with no evidence of molecular fracture. The possible deformation mechanisms which have given rise to the different structures have been discussed.     

Lamellar thickness of polyethylene single crystal isothermally crystallized in dilute solution

Summary Polyethylene single crystals were formed isothermally from 0.1% solution in tetralin (at 77.2, 84.8, and 89.8°C) and n-hexadecane (106.6°C), and those lamellar thicknesses were investigated as a function of crystallization time. These crystals have revealed no change or, if any, a little change in the lamellar thickness. Even for the crystals formed at considerably high temperature, 106.6°C, at which the segmental motion in a cyrstal begins to become active, the increase in those lamellar thicknesses was of the order of 10 Å. When polyethylene fractions with different molecular weight were crystallized at the same temperature, those lamellar thicknesses have shown nearly the same values. The results observed in this study are in striking contrast to those from isothermal crystallization in bulk, as observed byHoffman andWeeks. The constancy of the thickness of solution-grown crystals is interpreted as that the longitudinal translation of chains and chain ends in the interior of a crystal, necessary to thickening, will have been restrained on account of a considerably lower crystallization temperature than that in bulk. The lamellar thickness observed is presumed to have nearly equal value to the thickness of the growth nucleus as derived from the kinetic theory byLauritzen andHoffman.

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Zusammenfassung Es wurden Polyäthylen-Einkristalle isotherm aus 0,1%iger Lösung in Tetralin bei 77,2, 84,8 und 89,8 °C und aus n-Hexadecane (106,6 °C) hergestellt, und es wurden die Lamellendicken als Funktion der Kristallisationszeit untersucht. Diese Kristalle haben keine Änderung gezeigt oder, wenn, dann nur eine sehr geringfügige Änderung der Dicke. Sogar für Kristalle bei beträchtlich höherer Temperatur, erzeugt bei 106,6 °C, bei der die Segmentbeweglichkeit in den Kristallen aktiv zu werden beginnt, war der Anstieg der Lamellendicken nur von der Größenordnung 10 Å. Wenn Polyäthylenfraktionen von verschiedenen Molekulargewichten bei derselben Temperatur kristallisiert wurden, zeigten die Lamellendicken nahezu dieselben Werte. Die beobachteten Ergebnisse sind in schlagendem Kontrast zu denjenigen isothermer Kristallisation aus der Masse, wie sie beiHoffman undWeeks beobachtet wurden. Die Konstanz der Dicke von lösungsgewachsenen Kristallen wird damit interpretiert, daß die longitudinale Translation der Ketten und Kettenenden im Inneren eines Kristalls, notwendig zum Dicken-Wachstum, infolge der beträchtlich langsameren, niedrigeren Kristallisationstemperaturen als bei Kristallisation in der Masse stark behindert ist. Die beobachteten Lamellendicken haben nahezu den gleichen Wert wie die Dicke des Wachstumskeims, wie er sich aus der kinetischen Theorie vonLauritzen undHoffman ableilet.

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Electron microscopy of drawn polypropylene

Extremely thin polypropylene films formed by evaporation of dilute solutions floating on water, thin films deposited on Mylar or on carbon-coated Mylar, and bulk samples were deformed; after etching with aqua regia or chromic acid, the surfaces were studied by electron microscopy of surface replicas. At small draw ratio, microfibrils with lateral dimensions of about 200 Å, originating in micronecks at crack boundaries of the original crystal lamellae, were obtained in isolated areas exhibiting maximum local strain separated by large regions of much less deformed material. With increasing draw ratio the necked regions grow, the old structure gradually being reduced to smaller and smaller islands until it disappears completely. The inhomogeneity of strain in adjacent bundles of microfibrils creates a great many longitudinal voids with more or less disoriented microfibrils bridging the gaps. The regular arrangement of crystalline blocks of rather uniform length and width can be occasionally seen on surface replicas of drawn samples, and much better on dark-field electron micrographs of drawn and annealed thin membranes. In the latter case the blocks are very uniform and have similar dimensions along and perpendicular to the axis of the microfibril. The evidence from the electron micrographs, together with previous small-angle x-ray scattering data, supports Peterlin's molecular model for plastic deformation of crystalline polymers.     

Electron microscopic study of deformation in polyethylene

Thin films of polyethylene prepared from blends of fractionated polymer and a linear hydrocarbon (n-C₃₂H₆₆) have been used to study the role of intercrystalline links in the deformation of semicrystalline polymer under uniaxial stress. These links have been found to be strong and virtually inextensible elements of the structure. It is shown that they are firmly attached to the chain-folded lamellar crystals they bridge (both within the same spherulite and across boundaries between adjacent spherulites) and that, by concentrating applied stress, they commonly induce these lamellae to begin yielding in regions close to their points of attachment. Where there are many closely spaced links the stress is distributed fairly evenly, and drawing is relatively smooth and uniform. With more sparsely distributed links, however, stresses tend to be concentrated at widely separated points; deformation then tends to be severe and highly localized, often resulting in failure of the material upon drawing. There are indications that stress is also transmitted between chain-folded lamellae in ways other than by intercrystalline links. One such way is by means of chain ends and molecular loops that emerge from the surfaces of these crystals and are embedded in interlamellar material. Experiments in which the deformed films were subsequently heated confirm earlier conclusions that extended chains in drawn polymer may undergo refolding during annealing.     

Electron microscopy studies of nickel dimethylglyoximate

Nickel dimethylglyoximate crystals precipitated from homogeneous solution are well formed and convenient for microscopic study. The crystals are precipitated in solution as long needles with a rectangular cross-section. The direction of the long axis, i.e., the direction of the long dimension of the needle, is the c-axis ; the shortest axis is either the a- or b-axis. When the single crystal was slowly heated in the electron microscope, it began to sublime at 120° with changes occurring in its surface structure. Above 400° the Debye-Scherrer diffraction rings appeared as the crystal changed to nickel oxide.