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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(amorphous) and ordered (crystalline) arrays was shown in Fig. 8e, and the distribution of carbon and hydrogen atoms in the ordered area in Fig. 8f.

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e f Fig. 8. Schematic representation of polymer structure types: (a) linear, partly crystalline; (b) branched; (c) with cross-links (slightly cross linked); (d) strongly cross-linked; (e and f) - spatial structure of polyethylene chains with chaotic (amorphous) and ordered (crystalline) arrays (e), distribution of carbon and hydrogen atoms in ordered (crystalline) region of polyethylene chain (f); Białkowska (2009) . The following factors influence the tendency of polymers to crystallize which was propose by Wirpsza and Białkowska (2002): • complexity of monomer molecules - the most easily crystallized are linear polymers in which there are no large side groups or branches, • cooling speed - during slow cooling there is more time for the chains to be sorted out, • heating - heating the amorphous polymer just below the melting temperature causes the supply of thermal energy enabling the nucleation and growth of crystals, • degree of polymerization - crystallization of polymers with long chains is more difficult, • deformation - slow plastic deformation of the polymer in the temperature range between the glass transition temperature (Tg) and melting temperature (Tm) facilitates crystallization, as they occur during the deformation of the straightening of the chains. Polymers such as thermosets and elastomers have a spatially cross-linked structure resulting from the reaction with a cross-linking agent (a hardener in the case of duroplasts and a vulcanizing agent in the case of elastomers). The crosslinking reaction consists in the interconnection of the macromolecule chains with covalent cross-links (Fig. 8c, d). The crosslinking influences the limitation of the possibility of the chains moving relative to each other, i.e. deformation of the material, increasing its stiffness and strength. An example of cross-linking is the reaction that takes place in a liquid resin, e.g. epoxy, after addition of a hardener (e.g., a two-component epoxy adhesive). The material becomes irreversibly stiffened what was proved in Białkowska and Wirpsza (2002). In the literature it can be found that special additives can be used during cross-linking to change the chain structure. In the paper by Skarja and Woodhouse (2000) a series of degradable polyurethanes of variable soft segment chemistry and content were synthesized and characterized with the use of amino acid ‐ based diester chain extender was used to confer degradability and both polycaprolactone diol (PCL) and polyethylene oxide (PEO) as soft segments. In addition, the diisocyanate component was a potentially nontoxic diisocyanate (2,6 ‐ diisocyanato