Structural features semi-crystalline polymers

The discussions above focus on the small strain as a response of polymer materials to the small stress. Large stress brings large strain and even destroys the inherent structure of the solid materials, causing permanent deformation. Under the constant strain rates, the stress-strain curve reflects the structural and viscoelastic characteristic features of materials. For polymer materials, there occur five typical curves, as illustrated in Fig. 6.18 (1) hard and brittle, such as PS and PMMA, eventually brittle failure (2) hard and tough, such as Nylon and PC, most of semi-crystalline polymers. 

Semicrystalline polymers exhibit a melting transition temperature (Tm), a glass transition temperature f Tg) and crystalline order, as shown by. . lay and electron scattering. The fraction of the crystalline material is determined by x-ray diffraction, heat of fusion and density measurements. Major structural units of semi-crystalline polymers are the platelet-like crystallites, or lamellae, and the dominant feature of melt crystallized specimens is the spherulite. The... 

The semi-crystalline structure of polymers is characterized by special features. Firstly, the crystallites are embedded in an amorphous matrix, resulting in a two-phase morphology. Secondly, most polymers form folded-chain crystals in which the chains fold back into the same crystallite. Thirdly, several crystallites stack up and form superstructures known as sphemlites [8], Fig. 1.2. Finally, chains wander from one crystallite to the next one, thereby connecting them to each other. The chain segment between two adjacent crystals is known as tie-molecule [9], Tie-molecules act as stress-transmitters [10-12]. Thus they play an important role in mechanical properties of semi-crystalline polymers. 

What is the structure of a semi-crystalline polymer Essentially all semi-crystaUine polymers behave as a two-phase system one being the amorphous fraction and the other being the crystalline fraction. If the sample were fully crystalline, then no relaxations due to the amorphous phase would be observed [14]. The only thermal features would be a gradual... 

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. 

The crystallization of isotactic polypropylene (iPP) is affected by many factors, such as macromolecular characteristics [82], crystallization conditions (temperature, cooling rate), and stress (shear, elongational fields) [83-85]. In fiber reinforced iPP matrix composites, the crystalline morphology of the polymer is influenced by the fibers that can act as nucleating agents affecting the crystallization process [86-88]. TranscrystaUinity is a well-known structural feature in pol rmers, which occurs as the result of dense nucleation of the semi-... 

Polyether ether ketone (PEEK) is a linear, aromatic, semi-crystalline thermoplastic possessing excellent thermal stability, chemical resistance and mechanical properties under engineering applications. It has the repeating unit structure, 0—Ph—O—Ph—CO Ph—, wherein "Ph" is the 1,4-phenylene unit. Its true scientific name is poly (oxy-1, 4-phenylene-oxy-l, 4-phenylenecarbonyl-l, 4-phenylene). The polymer is featured in a wide range of applications including transportation, energy, industrial, electronics, semiconductor and medical. 

When the structural features of crystalline polymers are examined beyond the level of the unit cell, it is very important that their semi-crystalline character... 

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 semi-crystalline, we shall use these simpler materials to illustrate some general characteristics of relaxation behaviour. 

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