PSI - Issue 17

Michał Kwietniewski et al. / Procedia Structural Integrity 17 (2019) 154 – 161 Michał Kwietniewski / Structural Integrity Procedia 00 (2019) 000 – 000

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cross-linking (crosslinking) that stiffens the polymer structure significantly reducing its ability to deformations Porejko et al. (1974). The specificity of interatomic and intermolecular forces occurring in polymers causes that the influence of spatial construction on the properties of the polymer is extremely important. The types of spatial construction of polymers were schematically shown in Fig. 8 as:

a

b

Fig. 6. (a) C2H4 ethylene molecule, (b) polyethylene macroparticle (C2H4)n; Hiemenz and Lodge (2007).

Fig. 7. Styrene and acrylonitrile copolymer owing butadiene macromolecules implanted in the main chain, which create domains with high elasticity, providing resistance to cracking of hard S-A material; Hiemenz and Lodge (2007).

In addition to pure polymers (consisting of one type of mers), in practice combinations of various polymer chains are often used, as are alloys in metals. Examples of "polymer alloys" are copolymers, e.g. ABS (which chains consist of durable but brittle acrylonitrile A and styrene S molecules, with grafted branches of elastic butadiene B, making the material less brittle (Fig. 7). Atoms and molecules are linked in strong polymer chains by strong covalent bonds, but the van der Waals' weak physical bonds act between the chains, allowing the chains to move easily relative to each other under load - hence the good deformability of linear-structured polymers. Chains can also be linked to each other by strong covalent cross-linking (crosslinking) that stiffens the polymer structure significantly reducing its ability to deformations as reported in Porejko et al. (1974). The specificity of interatomic and intermolecular forces occurring in polymers causes that the influence of spatial construction on the properties of the polymer is extremely important. The types of spatial construction of polymers were schematically shown in Fig. 8 as: • linear partially ordered (semi-crystalline) (Fig. 8a) and completely chaotic (amorphous), • branched (Fig. 8b) or • poorly (Fig. 3c) or strongly (figure 3d) cross-linked. Partial chain ordering (crystallinity), occurring only in linear polymers without branches, e.g. polyethylene (PE), polypropylene (PP), polyamides (PA), (see Table 1), has a beneficial effect on the strength of the polymer, because it enables better packaging chains in space and strong approximation of macromolecules, which results in the formation of stronger bonds between the chains. The result is better strength, stiffness and higher softening temperature (heat resistance). Technological operations that increase the degree of polymer crystallization are therefore deliberately applied. For example, the polyethylene production process can be modified so that up to 90% of the crystalline regions are obtained (which gives a high density). Such a polymer is called high density polyethylene (HDPE), in contrast to less durable and rigid low-density polyethylene (LDPE). The spatial structure of polyethylene chains with chaotic