Semicrystalline polymers lamellae

Figure 8.6. A diagrammatic view of a semicrystalline polymer showing both chain folding and interlamellar entanglements. The lamellae are 5-50 nm thick (after Windle 1996).

The lamellae within a semicrystalline polymer are often aligned with one another. On a local scale, neighboringlamellae tend to be stacked, so that their lateral planes are parallel, as shown in Fig. 7.6. On a longer scale, lamellae can arrange themselves into extended stacks, known as cylindrites , or radially in three dimensions, to form spherulites , as illustrated schematically in Fig. 7.7 a) and b) respectively. 

For instance, crystalline lamellae in an amorphous matrix (semicrystalline polymer materials), hard domains in a soft matrix (thermoplastic elastomers)... 

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... 

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. ... 

In semicrystalline polymers such as polyethylene, yielding involves significant disruption of the crystal structure. Slip occurs between the crystal lamellae, which slide by each other, and within the individual lamellae by a process comparable to glide in metallic crystals. The slip within the individual lamellae is the dominant process, and leads to molecular orientation, since the slip direction within the crystal is along the axis of the polymer molecule. As plastic flow continues, the slip direction rotates toward the tensile axis. Ultimately, the slip direction (molecular axis) coincides with the tensile axis, and the polymer is then oriented and resists further flow. The two slip processes continue to occur during plastic flow, but the lamellae and spherullites increasingly lose their identity and a new fibrillar structure is formed (see Figure 5.69). 

Cavitation is often a precursor to craze formation [20], an example of which is shown in Fig. 5 for bulk HDPE deformed at room temperature. It may be inferred from the micrograph that interlamellar cavitation occurs ahead of the craze tip, followed by simultaneous breakdown of the interlamellar material and separation and stretching of fibrils emanating from the dominant lamellae visible in the undeformed regions. The result is an interconnected network of cavities and craze fibrils with diameters of the order of 10 nm. This is at odds with the notion that craze fibrils in semicrystalline polymers deformed above Tg are coarser than in glassy polymers [20, 28], as well as with models for craze formation in which lamellar fragmentation constitutes an intermediate step [20, 29] but, as will be seen, it is difficult to generalise and a variety of mechanisms and structures is possible. 

Crystalline lamellae are the basic units in the microstructures of solid semicrystalline polymers. The lamellae are observed to be organized into two types of larger structural features depending on the conditions of the bulk solidification process. 

The phenomenon of strain hardening in polymers is a consequence of orientation of molecular chains in the stretch direction. If the necked material is a semicrystalline polymer, like polyethylene or a crystallizable polyester or nylon, the crystallite structure will change during yielding. Initial spherulitic or row nucleated structures will be disrupted by sliding of crystallites and lamellae, to yield morphologies like that shown in Fig. 11-7. 

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... 

Semicrystalline polymers may crystallize from solution, as well as from the melt, in the form of chain folded lamellar crystals. The high spatial resolution of ATM enables one to assess lamellar thicknesses from images of these lamellar crystals in edge-on and flat-on orientation. As discussed in this section, images of flat-on oriented lamellae are particularly suitable for a quantitative determination of lamellar thicknesses. 

For instance, crystalline lamellae in an amorphous matrix (semicrystalline polymer materials), hard domains in a soft matrix (thermoplastic elastomers) several sharp peaks of colloidal crystals are observed in the SAXS, the unit cell can be determined. In this case peak profile analysis can be carried out using the methods discussed in Sect. 8.2.5... 

Orientation of semicrystalline polymers below the melting point is often referred to as "cold drawing." Although some stress crystallization does occur, the process primarily involves the transformation of existing crystalline structures. A widely accepted model of the deformation mechanism is that provided by Peterlin (Figure 5) (41). Prior to necking, the crystal lamellae which... 

The addition of a second non-crystallizable component to a crystallizable matrix can cause drastic variations of important morphological and structural parameters of the semicrystalline phase, such as the shape, size, regularity of sphemlites and intersphemlitic boundary regions, lateral dimensions of the lamellae, etc. These factors may greatly influence the mechanical behavior and, in particular, the fracture mechanisms, and thus are of great importance, especially when the toughening of semicrystalline polymer blends is considered. 

The present study has shown that low crystallinity ePP crystallizes from the melt into well-defined morphologies. This smdy presented definitive evidence that this class of materials, when crystallized isothermally from the melt, exhibits morphologies that are reminiscent of classical semicrystalline polymers. The presence of lamellae, crosshatching, and spherulites was revealed by high resolution tapping mode AFM and optical microscopy. The nonisothermally crystallized ePP specimens also display the hierarchical ordering as seen in the case of iPP. 

In the case of a semicrystalline polymer such as PP, the microstructural features are likely to appear at the scale of the spherulites (typically 5-100 pm in diameter) or even closer at the scale of the long period of the lamellar stacks (10-100 nm). In order to accede to the latter details, it was shown previously (48) that etching of the polished surface with oxidizing acids engraves the amorphous interstices and let the crystalline morphology appear lamellae, or at least stacks of lamellae, become visible. 

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