Polyethylene spherulites

Figure 5 Electron micrograph of a portion of melt crystallised polyethylene spherulite by transmission electron microscopy (TEM) showing lamellae. Reproduced from Ref. [3] with permission of John Wiley Sons, Inc.

The existence of crystal lamellae in melt-crystallised polyethylene was independently shown by Fischer [28] and Kobayashi [39]. They observed stacks of almost parallel crystal lamellae with amorphous material sandwiched between adjacent crystals. At the time, another structure was well known, the spherulite (from Greek meaning small sphere ). Spherulites are readily observed by polarised light microscopy and they were first recognised for polymers in the study of Bunn and Alcock [40] on branched polyethylene. They found that the polyethylene spherulites had a lower refractive index along the spherulite radius than along the tangential direction. Polyethylene also shows other superstructures, e.g. structures which lack the full spherical symmetry referred to as axialites, a term coined by Basset et al. [41]. 

Fig.8 Schematic drawing of the morphological hierarchy of a polyethylene spherulite...

Fig. 5.17 Early stages in the growth of spherulites (a) the sheaf-life stage in the growth of a polyethylene spherulite and (b) the beginnings of radial growth in a spherulite of poly(4-methylpentane). ((a) Reprinted by permission of Kluwer Academic Publishers (b) Cambridge University Press 1981.)...

Figure 8.15. Polyethylene spherulite between crossed polars showing Maltese cross and...

A critical comparison of experimental patterns from polyethylene spherulites with those predicted for perfect truncated spherulites leads to... 

Figure 5.64 is an electron micrograph of the rephca of a fracture surface of a melt-grown polyethylene spherulite. It gives another proof that polymer sphemlites can be made of lamellae. In addition, it shows that there is a regular twist that permits an... 

Electron micrograph of a polyethylene spherulite grown from the melt showing a lamellar morphology... 

Fig. 7.7. Detailed view of polyethylene spherulites, showing lamellae, together with interlamellar and interspherulitic fibrils (from R. A. Fava in Macromol. Rev. 5 (1971), p. 2)...

Spherulites of aliphatic hydrocarbon polymers such as polyethylene generally have their molecular chains with the highest refractive index oriented perpendicular to the radial growth direction. This is the case with the polypropylene spherulites shown in Fig. 2.9a, which were crystallized at 140 °C and consequently contain very few cross-hatched lamellae, so in this respect they are like polyethylene spherulites. The lower refractive index in this view is radial in direction, giving a yellow colour where the radius is parallel to the slow direction of the tint plate these are therefore termed negative spherulites. 

Figure 1.15 A model of the lamellar arrangement in a polyethylene spherulite. The small diagrams of the a, b, c axes show the orientation of the unit cell at various points. (Adapted from Takayanagi, M. (1963) Viscoelastic properties of crystalline polymers. Memoirs of the Faculty of Engineering Kyushu Univ., 23, 1. Copyright (1963) Kyushu University.)...

The direction of growth of polyethylene spherulites is always close to [010], i.e. the radius of the spherulite is parallel to the crystallographic b axis. Other polymers have other growth directions, e.g. monoclinic isotactic polypropylene grow fastest along the a axis. 

Electron microscopy has also revealed the twist of the crystals in the spherulites that was suspected from optical microscopy. Surface replicas taken from spherulitic polymers have clearly shown that the lamellae twist but the reason for this twisting is not yet understood. A possible structure of the twisted lamellae in polyethylene spherulites is illustrated schematically in Fig. 4.16. 

Figure 5.30. Electron micrography of a polyethylene spherulite at the beginning of growth. [Courtesy of B. Lotz, ICS-CNRS, Strasbourg (France).]...

Figure 3.68 Scanning electron microscope image of a polyethylene spherulite emerging from the free surface of a molten film the spherulite center is below the surface. Clearly evident is the band spacing S = 4.5 pm. Furthermore, the lamellae exhibit a C shape that reflects the direction of twist. The lower right sector has a right-hand twist, while the larger upper left portion has left twisting lamellae. From Lotz and Cheng [23] with permission from Elsevier.

Assuming the unaxial deformation of the polyethylene spherulite to follow an affine deformation hypothesis keeping its volume constant, as has been postulated by several authors,the number of crystallites orienting in a range from 6 to (0 -hd0 ) is given by equation (35), where Nq is the total number of crystallites within a spherulite and X is the extension ratio of the spherulite along the X3 axis. 

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