Structure Formation In Crystalline Polymers In Presence of Nucleating Agents [antikvár]

Chapterl INTRODUCTION The properties of polymers are determined by their chemical composition, molecular structure and morphology. Numerous possibilities exist for modification of their structure and thus their properties. Copolymerisation, chemical modification of the monomers and macromolecules, stereoregular polymerisation are just a few among the techniques offered by chemistry. However, the above mentioned modifications require basic changes in production plants and also in their technology. As a consequence this is not a flexible, rather rarely followed way to modify the structure of the polymers. Beside the chemical structure of the polymer chains, their spatial arrangement (i.e. physical structure), like orientation, supermolecular structure etc., also influence the macroscopic properties of the polymer. This is especially true for crystalline polymers. In some cases their crystalline structure and morphology may influence their mechanical properties more than the chemical composition and structure of the chains. Three different levels of morphology can be distinguished in crystalline polymers. The spatial arrangement of the macromolecules can be described by the unit cell, which is the lowest morphological level in crystalline polymers. According to the spatial arrangement of the chains, the unit cell can be monoclinic, triclinic, hexagonal etc., each corresponds to different crystal forms. It must be also mentioned that the conformation of the polymer chains are strictly determined in the unit cell. For examples, polypropylene macromolecules have a helical and polyethylene chains possess a zigzag conformation in crystalline phase. The formation of a specific crystalline form has its own thermodynamic conditions. The presence of selective nucleating agents can favour a certain crystalline modification.