Spherulites structural characteristics

While some structures show seemingly independent spherulitic structures on the surface, we know from other studies that these structures are connected to one another and to the more amorphous regions overall giving a material with a characteristic flexibility and strength. In general, chains are shared with adjacent areas allowing a sharing of stress or strain factors. 

A preferentially and a sheaf-like aggregation with random in-plane orientation are observed for the thinner films (thicknesses of 0.1, 0.2 and 0.4 pm in panels a-c). By contrast thick films (0.6 pm and thicker, panel d) show a morphology that resembles the well known (bulk) spherulitic form with a banded structure, characteristic of linear polyethylene crystallized from the melt at moderately high undercooling. 

Another characteristic feature of polyethyleneterephthalate is almost plane configuration of chains and presence of two centres of symmetiy on each repeat unit [188], These two features cause the ability of PETP to crystallization [189], Crystallization of PETP flows with formation of spherulite structure leading to polymer turbidity. 

Examination of thin sections of semicrystalline polymers reveals that the crystallites themselves are not arranged randomly, but form regular birefringent structures with circular symmetry. These structures, which exhibit a characteristic Maltese cross-optical extinction pattern, are called spherulites. Although spherulites are characteristic of crystalline polymers, they have also been observed to form in low-molar-mass compoimds that are crystallized from highly viscous media. 

HDPE is also linear and usually has no branches and a narrow polydispersity index. However, in some versions branches are introduced intentionally and the polydispersity index broadened to modify the HDPE properties. The polydispersity index for this version of HDPE is similar to that for LDPE (5.3). The differences between these three PEs are a result of morphological and structural differences between the materials. LDPE, LLDPE, and HDPE all contain the characteristic PE crystalline lamellae and spherulitic structures (shown in Figure 2) in their crystalline phases. 

Most of crystalline polymer materials exhibit multi-scale hierarchical structures. At the scale of 0.1 nm, the polymer chains contain regular sequences. At the scale of 0.5 nm, they form stable helical conformations, which then pack together in a compact parallel fashion to make the periodic lattice structure, with the unit cell at the scale of 1 nm. At the scale of 10 run, the folded-chain lamellar crystals are formed for the flexible polymer chains. At the scale of micrometers or larger, the lamellae further assemble into spherulites. Such hierarchical structural characteristics at varying length scales of polymer morphologies are illustrated in Fig. 10.7. 

The structure found here is very similar to that characteristic of the ethylene-butene copolymers (Figure 7a). We conclude that we have a very similar mechemism for gel formation. This conclusion is supported by the weU-developed, overlapping spherulitic structures that can be observed in the polarized light micrographs of the undried gels formed by this polymer. Theories that are based on other type structures and mechanisms will need to be reexamined. 

Furthermore, the crazes in PP show other similar characteristics to those of amorphous polymers. They grow apparently normal to the direction of major tensile stress which somewhat deviates from the tensile direction because of spherulitic structure. There are similar environmental effects on craze initiation (see also Environmental stress cracking of polypropylene in this book). Crazing is also an important source of toughness in toughened PP alloy systems such as propylene-ethylene block copolymers. 

Under certain conditions, the lamellae of the TPEEs organize into a spherulitic structure, which is characteristic structure of the common semi-crystalline polymers. Based on the results of different methods, it was concluded that crystallization occurs by chain folding through which a spherulitic superstructure is formed. A well developed spherulitic crystalline superstructures, with diameters of about 5-20 pm, can be formed depending on the crystallization conditions [33]. Also, the soft, amorphous phase is embedded between radial crystalline fibrils of the hard segment spherulites. Under some conditions, other structures such as dendrites are developed. 

In general terms PEEK shows the typical lamellar and spherulitic structures which are characteristic of many polymers. The crystalline structure of PEEK can be described at a number of different scales. 

The major feature of polymers that have been bulk crystallized under quiescent conditions are polycrystalline structures called spherulites. These are roughly spherical supercrystalline structures which exhibit Maltese cross-extinction patterns when examined under polarized light in an optical microscope. Spherulites are characteristic of semicrystalline polymers and are also observed in low-molecular-weight materials that have been crystallized from viscous media. Spherulites are aggregates of lamellar crystallites. They are not single crystals and include some... 

Figure 7 shows that the micro-injection molded polyethylene parts exhibit typical skin-core morphology similar to that observed for conventional injection molding parts. While the interface between the skin layer and the transitional shear zone is apparent, the interface between the transitional shear zone and the spherulitic core is hard to locate. The skin layer probably has shish-kebab structural characteristics. The Kebabs , which are crystalline lamellae, fill the crystalhzed space. Fibrous crystals, or the Shishs , are ahgned parallel to the injection direction. They penetrate those lamellae. The fibrillar structure follows the direction of the flow, as shown in Figure 7. The transitional shear zone may be thought of as crystalline ribbons that branch and fill crystallized space with some loss of orientation. Crystallization occurring at the sites of both the skin layer and the transitional shear zone is significantly influenced by shear or elongational stress history. On the other hand, the influence of shear on the crystalhzation occurring in the spherulitic core is negligible. The crystalline structure...

The individual spherulite lamellae are bound together by tie molecules that are present in more than one spherulite. Sometimes these tie segments form intercrystalline links, which are threadlike structures, that are important in developing the characteristic good toughness found in semicrystalline polymers. They act to tie together the entire assembly of spherulites into a more or less coherent package. ... 

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