Crystallisation morphology

The P(3HB)/PLA blend is one of the most studied blends, which exhibits mechanical properties that are intermediate between the individual components. Although PLA and P(3HB) are biodegradable polymers synthesized from renewable resources, their potential applications are hampered due to their brittleness and the formation of very large spherulities. P(3HB)/PLA blends were studied as early as 1996 to explore their miscibility, crystallisation, morphology, mechanical properties and biodegradation behaviour. P(3HB)/PLA blends with different compositions (100/0, 80/20, 60/40, 40/60, 20/80 and 0/100, wt%) were prepared by casting a film from a common solvent, chloroform, at room temperature. ... 

By its physical essence the heterophase fluctuation type, proposed in paper [43], is similar to local order domain (cluster) in the cluster model of the amorphous state structure of polymers [7, 8]. The last model application allows a quantitative description of such heterophase fluctuations and, as a consequence, analysis of the change in crystallisation morphology and nucleation mechanism [46]. In papers [47, 48] such analysis was carried out on the example of PCP and LDPE orientational crystallisation. 

Table 1.3 The different exponents and expressions for the rate constant in the Avrami equation for different crystallisation morphology...

Copolymerisation also affects morphology under other crystallisation conditions. Copolymers ia the form of cast or molded sheets are much more transparent because of the small spheruHte size. In extreme cases, crystallinity cannot be detected optically, but its effect on mechanical properties is pronounced. Before crystallisation, films are soft and mbbery, with low modulus and high elongation. After crystallisation, they are leathery and tough, with higher modulus and lower elongation. 

The density of the polymer will clearly depend on the density of the soft phase (usually low), and the density of the hard phase (generally higher with crystallisable polar blocks) and the ratio of the soft and hard phases present. It will also clearly depend on the additives present and to some extent on the processing conditions, which may affect the crystalline morphology. 

This rule of thumb does not apply to all polymers. For certain polymers, such as poly (propylene), the relationship is complicated because the value of Tg itself is raised when some of the crystalline phase is present. This is because the morphology of poly(propylene) is such that the amorphous regions are relatively small and frequently interrupted by crystallites. In such a structure there are significant constraints on the freedom of rotation in an individual molecule which becomes effectively tied down in places by the crystalhtes. The reduction in total chain mobility as crystallisation develops has the effect of raising the of the amorphous regions. By contrast, in polymers that do not show this shift in T, the degree of freedom in the amorphous sections remains unaffected by the presence of crystallites, because they are more widely spaced. In these polymers the crystallites behave more like inert fillers in an otherwise unaffected matrix. 

Crystallisation of isotactic PP from homogeneous solution in supercritical propane yielded open-cell foams of high surface area. Their morphology usually consisted of microspheres with a dense core and a porous periphery... 

From the crystallographic point of view selenium benzoylacetone crystallises in elongated, six-sided plates, with a dome termination of 64°. The extinction is straight and a positive bisectrix emerges normal to the plate, whether acute or obtuse has not been determined with certainty. In any case, the optic axial plane contains the morphological direction of elongation. 

Abstract The morphology of polyethylene has been an important theme in polymer science for more than 50 years. This review provides an historical background and presents the important findings on five specialised topics the crystal thickness, the nature of the fold surface, the lateral habit of the crystals, how the spherulite develops from the crystal lamellae, and multi-component crystallisation and segregation of low molar mass and branched species. 

It is not intended to present a comprehensive review of the extensive literature on polyethylene morphology. Several themes have been selected on the basis of novelty and importance in the author s eyes. The historical review, which is part of Sect. 1, presents the old discoveries in a brief form and it also serves as an introduction to the more specialised themes presented in the subsequent sections. The link to the theory of polymer crystallisation— historically closely related to discoveries made within the polyethylene morphology field—is briefly discussed. 

The morphology of a polyethylene blend (a homopolymer prepared from ethylene is a blend of species with different molar mass) after crystallisation is dependent on the blend morphology of the molten system before crystallisation and on the relative tendencies for the different molecular species to crystallise at different temperatures. The latter may lead to phase separation (segregation) of low molar mass species at a relatively fine scale within spherulites this is typical of linear polyethylene. Highly branched polyethylene may show segregation on a larger scale, so-called cellulation. Phase separation in the melt results in spherical domain structures on a large scale. 

The morphology and crystallisation behaviour of a series of binary blends based on a low molar mass linear polyethylene (Mw=2500 g mol-1 Mw/Mn= 1.1) and two higher molar mass branched polyethylenes... 

FIG. 19.1 Morphological models of some polymeric crystalline structures. (A) Model of a single crystal structure with macromolecules within the crystal (Keller, 1957). (B) Model of part of a spherulite (Van Antwerpen, 1971) A, Amorphous regions C, Crystalline regions lamellae of folded chains. (C). Model of high pressure crystallised polyethylene (Ward, 1985). (E) Model of a shish kebab structure (Pennings et al., 1970). (E) Model of paracrystalline structure of extended chains (aramid fibre). (El) lengthwise section (Northolt, 1984). (E2) cross section (Dobb, 1985). 

Table 19.2 gives a survey of the morphology of polymer crystallisation. The survey is self-explanatory it demonstrates an almost continuous transition from the pure folded chain to the pure extended-chain crystallite. 

In practice, many fabrication processes take place under non-isothermal, non-quiescent and high-pressure conditions. Mechanical deformation and pressure can enhance the crystallisation as well as the crystal morphology, by aligning the polymer chains. This leads to pressure-induced crystallisation and to flow-induced or stress-induced crystallisation, which in fact is the basis for fibre melt-spinning (see Sect. 19.4.1)... 

This development started with an observation of Pennings and Kiel (1965) that, when dilute solutions of polyethylene were cooled under conditions of continuous stirring, very fine fibres were precipitated on the stirrer. These fibres had a remarkable morphology a fine central core of extended CH2-chains, with an outer sheath of folded chain material. Electron microscopy revealed a beautiful "shish kebab" structure (see Fig. 19.16). Shish kebabs have also been observed in experiments without any stirring. For example, by washing polyethylene powder with xylene (Jamet and Perret, 1973) and by crystallising nylon 4 from a glycerol/water mixture (Sakaoku et al., 1968). 

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