Morphology crystal lamellae

The volume inside the semicrystalline polymers can be divided between the crystallized and amorphous parts of the polymer. The crystalline part usually forms a complicated network in the matrix of the amorphous polymer. A visualization of a single-polymer crystallite done [111] by the Atomic Force Microscopy (AFM) is shown in Fig. 9. The most common morphology observable in the semicrystalline polymer is that of a spherulitic microstructure [112], where the crystalline lamellae grows more or less radially from the central nucleus in all directions. The different crystal lamellae can nucleate separately... 

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. 

As with single crystal lamellae, there are a range of complex morphologies that can be observed, but we wish to focus on just a few key features of the ordinary common or garden spherulites. Figure 8-58 showed an actual micrograph of spherulites in the pro-... 

As pointed out above, the semicrystalline polymer can be considered as a two-phase composite of amorphous regions sandwiched between hard crystalline lamellae (Fig. 4.2(a)). Crystal lamellae ( c) are normally 10-25 nm thick and have transverse dimensions of 0.1-1 pm while the amorphous layer thickness, a, is 5-10 nm. As mentioned in the previous section, melt-crystallized polymers generally exhibit a spherulitic morphology in which ribbon-like lamellae are arranged radially in the polycrystalline aggregate (Bassett, 1981). Since the indentation process involves plastic yielding under the stress field of the indenter, microhardness is correlated to the modes of deformation of the semicrystalline polymers (see Chapter 2). These... 

The complicated morphology of crystalline polymer solids and the coexistence of crystalline and amorphous phases make the stress and strain fields extremely nonhomogeneous and anisotropic. The actual local strain in the amorphous component is usually greater and that in the crystalline component is smaller than the macroscopic strain. In the composite structure, the crystal lamellae and taut tie molecules act as force transmitters, and the amorphous layers are the main contributors to the strain. Hence in a very rough approximation, the Lennard-Jones or Morse type force field between adjacent macro-molecular chain sections (6, 7) describes fairly well the initial reversible stress-strain relation of a spherulitic polymer solid almost up to the yield point, i.e. up to a true strain of about 10%. 

Crystalhzation studies in blends of iPP/POE reveal that the crystallization process of iPP is affected by the addition of POE and vice versa. It has been demonstrated how the POE promotes the nucleation and crystal growth processes of iPP, this effect being more appreciable at low POE concentration (< 10 wt% POE). Analysis of the crystallization kinetics of the iPP crystals isothermally crystallized at different temperatures in blends of iPP/POE is supported by the morphological observations (lamellae, dendritic, and eventually spherulitic texmres) through optical microscopy. 

As discussed above (Section 8.2.2), crystal lamellae are composed of folded chains. No means however exist for direct observation of these folds so that the detailed morphology of the fold surface must be inferred from indirect evidence. Three models for folded surface morphology have been proposed (Figure 8.6). 

Pig. 1. Schematic illustration of random stacks of amorphous and crystal lamellae in the PVDF polymer (a) the morphology after the film is melt cast (b) after orientation of the film by mechanically stretching several times its original length and (c) after depositing metal electrodes and poling through the film thickness. 

Figure 23.28. Model diagram of the structural relationship between a-crystal lamellae and 7-crystal lamellae (the morphology corresponding to the oriented area in Figure 23.27)...

Supaphol et al. [216-218] reviewed a number of hypotheses that have been proposed to explain the occurrence of multiple melting endotherm phenomena. In the studies on isothermal crystallization under quiescent conditions (i.e., crystallization is only a function of temperature), the multiple melting behavior of these semicrystalline polymers have been attributed to the following four phenomena (1) the presence of two (or more) crystal modifications in the sample (2) the presence of two (or more) crystalline morphologies (3) the presence of two populations of crystal lamellae of different thicknesses and (4) the simultaneous melting, recrystallization, and re-melting of the lamellae initially formed at the crystallization conditions. 

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