Banded Spherulites Research Articles

Exceptionally large banded spherulites

This article concerns the crystallization of maleic anhydride from a blend containing 2 wt% of poly(acrylonitrile). High speed photography and temperature measurements during the crystallization as well as X-ray diffraction from the blend after crystallization are consistent with a banded spherulitic morphology. The spacing of the bands, typically 4 to 15 mm, is two orders of magnitude larger than that reported in previous studies of the structure of banded spherulites. The velocity of the growth front is determined to be 33 cm/s when crystallization initiates from a solution supercooled by 28°C. X-ray diffraction patterns recorded using three orthogonal beam propagation directions at different locations in the band structure show (within experimental error) that the blends have the twisting orientation of crystallites characteristic of banded spherulites. Interestingly, the bands caused by spherulitic crystallization of maleic anhydride are imprinted on the polymer phase in the blend. This imprinting as well as the exceptionally large band spacing in the crystallized blend may open the way for studies that would clarify the mechanism of formation of the twisting crystal orientation in banded spherulites.

The morphology of poly(vinylidene fluoride) crystallized from blends of poly(vinylidene fluoride) and poly(ethyl acrylate)

AbstractA combined optical and electron microscopical study has been carried out of the crystallization habits of poly(vinylidene fluoride) (PVF2) when it is crystallized from blends with noncrystallizable poly(ethyl acrylate) (PEA). The PVF2/PEA weight ratios were 0.5/99.5,5/95, and 15/85. Isothermal crystallization upon cooling the blends from the single‐phase liquid region was carried out in the range 135–155°C, in which the polymer crystallizes in the α‐orthorhombic unit cell form. The 0.5/99.5 blend yielded multilayered and planar lamellar crystals. The lamellae formed at low undercoolings were lozenge shaped and bounded laterally by {110} faces. This habit is prototypical of the dendritic lateral habits exhibited by the crystals grown from the same blend at high undercoolings as well as by the constituent lamellae in the incipient spherulitic aggregates and banded spherulites that formed from the 5/95 and the 15/85 blends, respectively. In contrast with the planar crystals grown from the 0.5/99.5 blend, the formation of the aggregates grown from the 5/95 blend is governed by a conformationally complex motif of dendritic lamellar growth and proliferation. The development of these aggregates is characterized by the twisting of the orientation of lamellae about their preferential b‐axis direction of growth, coupled with a fan‐like splaying or spreading of lamellae about that axis. The radial growth in the banded spherulites formed from the 15/85 blend is governed by a radially periodic repetition of a similar lamellar twisting/fan‐like spreading growth motif whose recurrence corresponds to the extinction band spacing. This motif differs in its fan‐like splaying component from banding due to just a helicoidal twisting of lamellae about the radial direction. © 1993 John Wiley & Sons, Inc.

Growth of polyethylene single crystals from the melt: Morphology

Three-dimensional shape of polyethylene single crystals grown from the melt has been studied. Two distinct types of lateral habit have been obtained: lenticular shape (type A) and truncated lozenge (type B) in the range of regime I and II. Electron microscopy has revealed chair-like shape of type B crystal and reconfirmed the planar shape of type A crystal. In the type B crystal, spiral growth has occurred frequently in the {110} sectors and the sense of the handedness of spiral terraces has been maintained. It has been, suggested that the frequens occurrence of spiral growth is responsible for a morphological change (axialite-spherulite) accompanying the regime I–II transition. The origin of the chair-like crystals has been discussed and, a possible mechanism has been suggested for the formation of spiral terraces; the mechanism is based on a distortion caused by the three-dimensional shape of chair-like crystals. It has been found that the chair-like crystals are curved in the opposite way to S-shaped lamellae observed by Bassett and Hodge in banded spherulites. In fact, the present work has led to the recognition of further classes of crystal with curving cross-sections and of distinctions between them. In final analysis, a unifying thread has been identified between lateral habits, growth kinetics and three-dimensional shape of lamellae, in turn, leading to some rationalization of multilayer developments including twisting in banded spherulites, the latter based on existing suggestions in the literature.

The electrically ruptured area of crosslinked polyethylene

The electrically ruptured areas of failed crosslinked polyethylenes have been investigated using optical microscopy and Fourier transform infrared spectroscopy (FTIR). The formation of well-defined banded spherulites was observed around the ruptured areas. Since spherulites do not occur in crosslinked polyethylenes, the formation of spherulites in failed specimens is considered to be related to breakage of the network during the breakdown process. FTIR results are shown to support this suggestion, revealing that more than oxidation occurred during the breakdown process. This observation contradicts some well-established breakdown mechanisms, such as thermal breakdown. It is also shown that, if the initiation of electrical breakdown involves network rupture, the properties of crosslinked polyethylene could be enhanced by designing less reactive crosslink points. >

Stability of spherulite growth rate

AbstractSpherulite growth rates in thin films of isotactic polypropylene, poly (methylene oxide), and poly (ethylene adipate) have been studied by light microscopy. The growth rate of the parts of spherulites formed from pockets of occluded melt is decreased drastically. The decrease is associated with the negative pressure buildup inside the pockets due to the density change as the melt is converted to spherulites. Material subjected to negative pressure shows a higher melting point of 3–4'C if the pressure is released prior to melting. Slight periodic changes of the growth rate were detected during crystallization of banded spherulites. No changes in growth rate were observed for two spherulites about to impinge or for spherulites growing near a solid obstacle.

Molecular structure, crystallization behavior, and morphology of fractions obtained from an extrusion grade high‐density polyethylene

AbstractAn extrusion‐grade of high density polyethylene (HOPE) (3 ethyl groups per 1000 carbons) has been divided into 16 fractions by preparative GPC and selective p‐xylene extraction. The fractions, with molecular weights ranging from 900 to 1,000,000, have been studied by IR spectros‐copy, DSC, WAXS, polarized microscopy, and small‐angle light scattering (SALS), The average degree of chain branching (percent C2H5) is 0.5 percent for the part of the sample having a molecular weight lower than 10,000 and it decreases monotonically with increasing molecular weight, finally approaching 0.1 percent C2H5. A crystallinity depression with respect to linear PE equivalent to 20 percent/(percent C2H5) is recorded for all samples except for the very low molecular weight samples for which the crystallinity depression is much larger (30 to 35 percent/ (percent C2H5)). The unit cell volume increases with increasing percent C2H5, presumably due to the inclusion of ethyl groups in the crystals as interstitlals at 2gl kinks. The concentration of ethyl groups in the crystals (ϵc) unanimously follows the relationship: ϵc(percent) = 0.32 + 0.25 log(percent C2H5) except for the low molecular weight fractions which have significantly lower values for ϵc. Our admittedly speculative explanation for this major discrepancy between high and low molecular weight samples is based on the idea that segments with ethyl groups close to chain ends have a greater difficulty in crystallizing than segments containing ethyl groups located at positions far from the chain ends. The fractions obtained from the extrusion‐grade HDPE show a solidification temperature depression with respect to linear PE which can only be explained by the presence of chain branches in these samples. The depression is particularly pronounced for the low molecular weight samples as is expected from the data on molecular structure. Well‐developed non‐banded spherulites are observed in rapidly cooled (crystallized at about 35 K supercooling), low molecular weight samples (6,000 < Mw < 8,000)from the extrusion‐grade HDPE in contrast to the axialites observed in linear PE of the same molecular weight and thermal treatment. This discrepancy in morphology has been related to the presence of ethyl groups in the extrusion grade HDPE fractions. Higher molecular weight samples (20,000 < Mw < 1,000,000)from the extrusion‐grade HDPE and linear PE both display well‐developed banded spherulites of similar nature as is expected due to the similarity in molecular structure of the two sets of sample.

On isolated lamellae of melt-crystallized polyethylene

The growth, isolation and examination of individual melt-grown crystals of linear polyethylene is reported. Monolayers grown at 130°C have continuously curved elliptic prism faces (despite being grown under ‘regime I’ conditions) and fold surfaces close to {2 0 1}. Such monolayers, showing a sector boundary along their length, and distinct sectors differing very slightly in orientation across this boundary, have been revealed in dark-field electron microscopy. This is evidence for regularity in chain folding. A more frequent form is when a central screw dislocation is present near the centre of a lamella. The spiral terrace developing from this shows a remarkable habit in which adjacent layers splay apart, but growth soon stops, leaving the upper and lowermost layers plano-convex in outline. In every case, the planar outline is of a prism face making an acute angle with the previous layer. This is a consequence of growth having only occurred around screw dislocations of the appropriate hand: right-handed for a (201) surface. It is suggested that this habit, illustrating how spiral development selects screw dislocations of a consistent hand, may be the missing element in understanding the growth of banded spherulites in polyethylenes via systematic arrays of similar screw dislocations.

Crystallinity in chemically crosslinked low density polyethylenes: 3. Morphology of the XLPE-2 system

Studies of the morphology of low density branched polyethylene crosslinked using 2% dicumyl peroxide have been carried out using transmission electron microscopy of replicas of surfaces etched with permanganic reagent. Under optimal etching conditions detailed information regarding the morphologies of bulk crystallized samples of low crystallinity can be obtained. The morphology of crosslinked polyethylene is compared with that of the original branched polyethylene as a function of crystallization temperature. Branched polyethylene crystallizes in the form of banded spherulites from 88°C to 103°C with a temperature dependent band periodicity. Introduction of a small amount of crosslinking produces profound effects with banded sheaf or bundle-like morphologies resulting. The absence of lamellar banding in the sheaves of the gel fraction of the crosslinked polyethylene is attributed to pre-existent tie-molecules which are an inherent feature of network junctions in a crosslinked system. In the case of a typical unextracted crosslinked polyethylene, the morphology is found to vary with crystallization temperature. D.s.c. studies reveal that the thermal behaviour of XLPE-2 is not too different from that of OPE. The cause of multiple melting behaviour in this class of material is discussed and it is suggested that in addition to other processes of reorganization, hindrance to solid state thickening may be important. A method of monitoring different stages of lamellar melting for semicrystalline polymers was developed involving annealing at elevated temperatures in the presence of chlorosulphonic acid.

Twisting orientation and the role of transient states in polymer crystallization

A possible connection is suggested and explored between non-planer crystal habits in banded polymer spherulites and disordered chain folding in polymers crystallized relatively rapidly from the melt. It is proposed that, when lateral growth faces and fold surfaces are not orthogonal (because chain stems are tilted with respect to the lamellar normal), different degrees of disorder develop at opposite fold surfaces. Resulting differences in surface stress give rise to bending moments, but these are likely to be of transient existence. It is shown that, on this basis, an appealingly simple rationale can be developed to account for the complex and hitherto puzzling observations of Bassett and Hodge on polyethylene spherulites, including S-bending and non-uniform axial twisting in lamellae, and also an empirical correlation between these deformations. Much depends, however, upon interactions between interleaved crystals and upon relaxation of bending moments. Existing evidence in support of the rationale is outlined. Implications with respect to polymers other than polyethylene, and to kinetics of crystallization in general, are discussed briefly. Calculations concerning axial twisting under the influence of surface stresses suggest that the twisted crystals incorporate twist boundaries, possibly formed by aggregation of dislocations generated during the growth of what must initially be relatively disordered crystals. The ‘chiral’ factor determining handedness of twisting in a given crystal is the direction in which chain stems tilt with respect to the lamellar normal.

On the morphology of melt-crystallized polyethylene - I. Lamellar profiles

The morphology of melt-crystallized polyethylene has been surveyed, and it has been revealed that all specimens are highly lamellar. Classifi­cation of the textures is possible in terms of the lamellar profiles in b -projection, i. e. down the radii of spherulites. For polyethylene crystal­lized as the orthorhombic phase only three types of lamellar profile have been observed for readily accessible growth conditions. There are ridged sheets of alternating {201} facets, planar {201} sheets and S-shaped sheets which are, to a first approximation, derivable from {201} platelets by shear of the lateral regions displacing {100} parallel to c . It is possible, moreover, to distinguish between the first-forming or dominant lamellae which lay down the framework of the texture and the later-forming or subsidiary lamellae which fill the framework. Dominant lamellae may have ridged, planar or S-shaped profiles but subsidiary lamellae appear only ridged or planar. Ridged dominant sheets form under régime I conditions as a consequence of distinct facets on the growth front. The association of ridges and growth facets indicates regularity in folding. S-shaped sheets are usually dominant in régime II growth. They appear to have been deformed from planar {201} sheets by forces associated with the forma­tion of banded spherulites.

Lamellar morphologies in melt-crystallized polyethylene

The lamellar morphologies of linear polyethylene crystallized from the melt in vacuo have been surveyed. It is shown that spherulites form by the outward growth of dominant lamellae along b, with low angle branching, with the intervening spaces subsequently filled by subsidiary lamellae. There are three principal habits of dominant lamellae: ridged sheets of alternating {201} facets, planar {201} sheets and S- or C-shaped sheets which can be derived from {201} sheets by suitable shear. These occur in increasing order of growth rate, caused either by increasing molecular mass or decreasing crystallization temperature. The most widespread morphology is one of S-shaped dominant lamellae with narrow {201} subsidiary platelets; these are found, e.g., in banded spherulites. It can be shown, at least in certain circumstances, that longer molecules tend to form the dominant lamellae and shorter ones the subsidiary. Even in quenched samples the lamellar texture is not homogeneous. Here and elsewhere it has a lateral scale which can be measured by the widths of dominant lamellae, but this varies much less than the Keith and Padden parameter δ to which it has been thought to be related.

An investigation of the microstructure and mechanical properties of high density polyethylene spherulites

Scanning transmission electron microscopy and related techniques have been used to characterize the micromorphologies of high-density polyethylene spherulites produced under different crystallization conditions. The experimental techniques used in the investigation are summarized, as are some relationhips between spherulite morphology, crystallization conditions and mechanical properties. A preliminary model for the micromorphology of banded polyethylene spherulites is proposed.

On lamellar organization in banded spherulites of polyethylene

On lamellar organization in banded spherulites of polyethylene

Spherulitic crystallization in biopolymers

Banded spherulites with diameters of the order of 150 μm have been grown by evaporation from solutions of the branched polysaccharide, dextran, in DMSO, formamide and water. The average size of the spherulites depends on molecular weight, evaporation temperature, and the solvent used. Activation energies for spherulitic growth were measured as a function of molecular weight and solvent environment. Two distinct types of spherulite were obtained depending only on the molecular weight: negative, banded spherulites from the high molecular weight dextran; and positive, unbanded spherulites from the low molecular weight dextran.

MORPHOLOGIES OF THIN SPHERULITIC FILMS OF POLYETHYLENE

In order to examine the structure of interlamellar tie fibrils found by Keith et al. (J. Polymer Sci., A-2, 4, 267 (1966)), the mixtures of polyethylene and paraffine were crystallized at various temperatures from melt into thin films on various substrates, and the resultant textures were studied by an electron microscope before and after paraffine was extracted by xylene. When a micro cover glass was used as the substrate, sheaf-like crystals in the order of 3μ were observed, suggesting that the substrate facillitated the nucleation in the thin film. When carbon and mica sheets were used as the substrate, the ordinary two dimensional spherulites with macro fibrils grown radially and the so-called banded spherulites were observed in the thicker region of the film. In the very thin region of the film were observed ribbon like crystals, which have not been found so far. They are fibrilar but can be considered as monolayer lamellae laid on the substrate. The lack of twisting of the lamellae suggests again some effects of the substrate. Under the same conditions as those used by Keith et al., the open structure of the spherulite they found was not observed. When polethylene/C32H66 ratio was 1:2 and the mixture was crystallized under higher supercoolings, the open texture similar to the Keith's was observed. However, the interlamellar fibrils was not observable but the fibril like materials seen among the lamellae were in fact composed of small irregular shaped lamellae. In view of the pronounced effect of the substrate on the morphology of the resultant spherulites, it seems rather questionable that the interlamellar fibrils found by Keith et al. are formed in bulk crystalline polymers.

Scattering of Light by Two-Dimensional Spherulites

The low-angle light scattering for anisotropic disks (representing two-dimensional polymer spherulites) is calculated for the cases of (1) the optic axis lying in the plane of the disk and tilted at the angle β to the radius, (2) the optic axis rotating cylindrically about the radius through the angle ω while making a constant angle β to the radius where (a) ω rotates randomly and (b) ω varies linearly with the distance r from the center of the disk, and (3) affine deformation of the disk to an ellipsoid. For case (1) the scattering pattern is shown to rotate azimuthally through the angle β from that previously described where β=0° in agreement with experimental observations on polytetrafluoroethylene. Case (2) is shown to predict an azimuthal dependence of the wide-angle scattering maxima of a type experimentally observed for banded spherulites. Case (3) predicts a deformation of the scattering pattern of a type observed for deformed polyethylene spherulites.

Banded Spherulites of Hippuric Acid

IT has long been known that under certain conditions many substances crystallize in the form of spherulites showing concentric extinction bands between crossed polars. The explanation of this effect, originally put forward by Michel-Levy and Munier Chaimas1 for chalcedony, and later confirmed by other workers2–5 for a number of substances of low molecular weight, was that the structure of the spherulite has a twist around any given radius, and that the bands correspond to those orientations in which an optic axis is parallel to the light beam. Thus, Gaubert5 obtained spherulites of hippuric acid in which the radial twist was around the β-axis of the indicatrix, a fact which he established conoscopically, that is, by a study of the interference figures along the radii. More recently there has been a revival of interest in banded spherulites, since they are formed by many polymers (for example, polythene and terylene). These cases have been the subject of much investigation by, among others, Keller6, and Point7, who have shown convincingly, from the optical effects of tilting the preparations with respect to the direction of the light, that the spherulites have a radially twisted structure as for the low molecular weight substances referred to above.

Investigations on banded spherulites

AbstractThe nature of banded spherulites was further investigated. The principal material used was a polyester, namely, polytrimethylene glutarate, but use was made also of nonpolymeric materials. A variety of observations on extinction effects with the Universal Stage conclusively confirmed the twisted crystal model of banded spherulites. It is shown that banding observed in the form of other than extinction effects need not indicate a zone structure. Such bands can be due to diffraction effects or pseudopleochroism following from the twisted structure, examples of such effects being quoted. In some cases, genuine rhythmic distribution of matter was observed, and it is shown that these effects arise in two‐dimensional spherulites only, and can still follow from the twisted morphology under the influence of a substrate. Direct observations of a helicoidal morphology are also quoted.