Crystalline-amorphous features spherulite

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. 

In the case of a semicrystalline polymer such as PP, the microstructural features are likely to appear at the scale of the spherulites (typically 5-100 pm in diameter) or even closer at the scale of the long period of the lamellar stacks (10-100 nm). In order to accede to the latter details, it was shown previously (48) that etching of the polished surface with oxidizing acids engraves the amorphous interstices and let the crystalline morphology appear lamellae, or at least stacks of lamellae, become visible. 

The third design feature is the polymer microstructure. Morphology of polymer can influence wear resistance of polymers. For example, in a semicrystalline polymer, both amorphous and crystalline phases coexist. The amorphous phase has been shown by Tanaka (8) to be weaker than the crystalline phase, thus the former wears faster than the latter. In addition to the difference in phases, the size of the spherulites and the molecular profile can also influence the wear rates. Thus, a control of the morphology through crystallization can improve the wear resistance of a polymer such as polytetrafluoroethylene (11). 

The process uses crystallizable polymers, of which the most important in PET. The first step is to injection mould (hence the name) a parison, or preform as it is more usually termed here. The preform is closed at the bottom and is considerably shorter and thicker than the final bottle. It is rapidly cooled (quenched) by using chilled water to cool the injection mould and this ensures that it is in its amorphous condition, i.e. no crystalline structure. Next it is reheated with infra-red elements to above its Tg, about 90-100 C for PET and enters the bottle mould and the mould is closed. The blow pin enters and pushes the soft preform downwards almost simultaneously the blow occurs, compressed air blowing the material outwards. The result is biaxial orientation - downwards from the movement of the blow pin, outwards from the action of the expanding air. The orientation induces crystallization, but in the form of lamellar crystals rather than spherulitic ones. This type of crystallization is strain-induced, and is characteristic of synthetic fibres and film, e.g. Melinex. It gives a transparent product with enhanced physical properties, both important for bottling carbonated drinks. The alternative name for the process is the stretch-blow process. Its main feature as a process is the control of the crystallinity of the polymer at its different stages. 

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