Crystallinity - Thermoplastic polymers, elastomers and additives

The crystalline state of the polymers is characterized by a three-dimensional order of at least a part of the chains e, independently of the details of the structure. They are known as crystalline seeds as only partially crystallizing giving a mixed structure characterized by a crystalline melting temperature Tf and a glass transition temperature Tg always much lower than the Tf. The degree of crystallinity of polymers, which due to their regular structure and the flexibility of their chains have a greater tendency to crystallize, depends on the crystallization conditions. A crystalline resin to solidify its chains seek to form structures called crystallites. The crystalline resins are opaque since there is no space for light to pass between the molecules.

The difference between the crystallinity of the low molecular weight compounds and that relative to the high molecular weight compounds consists essentially in the fact that, while in the first, the elementary cell generally comprises more molecules or more ions or more atoms ( depending on whether it is a molecular solid, or ionic or a metallic element), in polymers, the same molecule includes more elementary cells and, sometimes, even more crystals; in fact, as the crystallinity of the polymers is never complete but partial, and therefore the crystalline regions alternate with the amorphous zones, generally the same molecule comprises crystalline parts and amorphous parts, in several successive sections. This leads to a marked irregularity of the crystals, which are usually indicated by the name of crystallites. Therefore, for a polymer, the degree of crystallinity is defined as the ratio between the weight of a substance in crystalline form and the total weight and is generally expressed as a percentage. The degrees of crystallinity vary from zero or from a few units, in many polymers, until reaching very high percentages for polymers with linear non-branched structure. The degree of crystallinity also depends on the same type of configuration of the mechanical thermal history of the sample. Especially in the case of long linear chains, rolling, stretching and extrusion, especially if carried out at suitable temperatures, at which the mobility of the macromolecules is high, tend to be oriented parallel to each other. The permanence at a sufficiently high temperature (annealing), favoring the macrobrown movements of the macromolecules, instead tends to disorient them, while the cooling, especially if it is abrupt (tempered), tends to stabilize the preexisting structure, both in the case of that this is mainly amorphous as messy. which instead is crystalline and oriented.