Semicrystalline polymers spherulites

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. 

SEM micrographs of two members of these polymers (HB and HBIB-50) are shown in Figure 7 to provide further evidence for superstructure on the micron level within the solution cast films. One can directly observe the surface of the spherulitic structure of the HB homopolymer as well as in that of the copolymer HBIB-50. Clearly, the level of structure (-5 pm) is well above that of the individual domains of either HB or HI and reflects the possible primary nucleation and subsequent growth behavior common to spherulitic semicrystalline polymers. The Hv patterns shown in... 

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... 

It is of course important to note that the overall rate of crystallization is not only determined by the growth rate of the spherulites, but also by the amount of nuclei being present in the system. This possibility is used as an effective method to influence the total crystallization rate of commercial polymeric materials in a controlled manner and to influence the size of spherulites and thus the physical properties of finished articles made from semicrystalline polymers. 

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. ... 

Fourier transform infrared imaging allows for the examination of semicrystalline polymers via their dichroism.31 Typical images of the center of a PEG (Mw35k, Tc 35°C) spherulite are shown in Fig. 5.15. These images were generated by plotting the baseline corrected absorbance intensity of the 1343 cm-1 CH2 wagging... 

The effects of morphology (i.e., crystallization rate) (6,7, 8) on the mechanical properties of semicrystalline polymers has been studied without observation of a transition from ductile to brittle failure behavior in unoriented samples of similar crystallinity. Often variations in ductlity are observed as spherulite size is varied, but this is normally confounded with sizable changes in percent crystallinity. This report demonstrates that a semicrystalline polymer, poly(hexamethylene sebacate) (HMS) may exhibit either ductile or brittle behavior dependent upon thermal history in a manner not directly related to volume relaxation or percent crystallinity. 

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. 

Semicrystalline polymers are impact resistant if their glass transition temperatures are much lower than the test temperature. The impact strength of such materials decreases with increasing degree of crystallinity and particularly with increased size of supercrystallinc structures like spherulites. This is because these changes are tantamount to the progressive decrease in the numbers of tie molecules between such structures. 

Figure 14.4 Diagram of the change of a spherulite during the cold drawing of a semicrystalline polymer. (From Ref. 25.)...

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... 

In semicrystalline polymers, fillers may act as reinforcement, as well as nucle-ation agents. For example in PP, nanoscale silica fillers may nucleate the crystallization resulting in spherulites that show enrichment in particles in the center of the spherulite (Fig. 3.64). For a quantitative analysis of, e.g., filler sizes and filler size distributions, high resolution imaging is necessary and tip convolution effects [137-140] must be corrected for. The particles shown below are likely aggregates of filler particles considering the mean filler size of 7 nm [136]. 

It is a semicrystalline polymer " with a helical polymer chain and a pseudohexagonal crystal. Crystal growth is spherulitic and consists of folded chains. The large size... 

Figure 4. Crystalline morphologies of semicrystalline polymers for different processing conditions (a) crystallized from a quiescent melt (spherulitic structures) (b) crystallized under moderate stress...

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