Crystalline-amorphous features semicrystalline

Amorphous stereotactic polymers can crystallise, in which condition neighbouring chains are parallel. Because of the unavoidable chain entanglement in the amorphous state, only modest alignment of amorphous polymer chains is usually feasible, and moreover complete crystallisation is impossible under most circumstances, and thus many polymers are semi-crystalline. It is this feature, semicrystallinity, which distinguished polymers most sharply from other kinds of materials. Crystallisation can be from solution or from the melt, to form spherulites, or alternatively (as in a rubber or in high-strength fibres) it can be induced by mechanical means. This last is another crucial difference between polymers and other materials. Unit cells in crystals are much smaller than polymer chain lengths, which leads to a unique structural feature which is further discussed below. 

Polymers can be either amorphous or semicrystalline in structure. The structure of amorphous materials cannot be described in terms of repeating unit cells such as that of crystalline materials because of nonperiodicity, the unit cell of an amorphous material would comprise all atoms. The physics and chemistry of the amorphous state remain poorly understood in many aspects. Although numerous experiments and theoretical studies have been performed, many of the amorphous-state features remain unexplained and others are controversial. One such controversial problem is the nature of glass-liquid transition. 

A characteristic feature of the material under investigation is its very low microhardness - between 25 and 35 MPa depending on the crystalline modification present. These values are up to 5-6 times lower than those for semicrystalline homo-PBT regardless of the crystalline modification. Moreover, the obtained values for H of PEE (Table 6.3, Fig. 6.5) are about half the amorphous hardness, // , of PBT, being 54 MPa as reported by Giri ef a/. (1997). This means that there should be other factors responsible for the very low H values of the copolymer. 

The semicrystalline, supermolecular structure of the organic carboxylate and the amorphous structure of the sulfonate resins have been studied with x-ray scattering and mechanical relaxation. This work shows no trace of crystallinity in the sulfonates, but the stress-relaxation data suggests the presence of a common structural feature, ion-clustered structure. with regions of high and low ion content. In "Figure 2 is shown the x-ray diffraction patterns depicting the supermolecular structure of perfluorocarboxylate and the sulfonate. Here is shown the amorphous halos in both... 

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. 

The third design feature is the polymer microstructure. Morphology of polymer can influence wear resistance of polymers. For example, in a semicrystalline polymer, both amorphous and crystalline phases coexist. The amorphous phase has been shown by Tanaka (8) to be weaker than the crystalline phase, thus the former wears faster than the latter. In addition to the difference in phases, the size of the spherulites and the molecular profile can also influence the wear rates. Thus, a control of the morphology through crystallization can improve the wear resistance of a polymer such as polytetrafluoroethylene (11). 

SAXS is a powerful tool to study the morphology of semicrystalline systems. The application of this technique is based on the electron density difference between the crystalline and amorphous phases (lamellar structure) in polymer systems. The crystalline (l ) and amorphous (IJ thicknesses can be obtained using this technique. Besides, the distance from one crystalline region to the next provides the size of a lamellar structure, also known as the long period (L). Other morphological features are the interface... 

We shall discuss the assignment of viscoelastic relaxations in a molecular sense to different chemical groups in the molecule, and in a physical sense to features such as the motion of molecules in crystalline or amorphous regions. Because amorphous polymers exhibit fewer structure-dependent features than those that are semicrystalline, we shall use these simpler materials to illustrate some general characteristics of relaxation behaviour. 

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