Polyethylene terephthalate unit cell

Polyethylene terephthalate crystallizes very slowly into only one stable crystalline form, containing monoclinic unit cells. To maximize its physical strength, high crystallinities must... 

In fibres of some polymers, made under certain conditions, the crystalline regions are found to be tilted with respect to the fibre axis in a well-defined crystallographic direction. This is a very valuable feature, because the diffraction patterns of specimens in which this type of orientation occurs are of precisely the same form as tilted crystal diffraction patterns of single crystals rotated round a direction inclined to a principal axis. The unit cell cannot be obtained directly, for 90° oscillation tilted crystal photographs are required for direct interpretation, but unit cells obtained by trial can be checked by the displacements of diffraction spots from the layer lines this is a severe check, and consistent displacements would leave no doubt of the correctness of a unit cell. This procedure played an effective part in the determination of the unit cell of polyethylene terephthalate (Daubeny, Bunn, and Brown, 1954). 

Polyethylene terephthalate is known to crystallize in a triclinic crystal structure with unit-cell dimensions as follows [8] ... 

Normally, one would not usually consider that the change from an amorphous or glassy state to a crystalline or ordered state is readily discerned by infrared absorption. With some polymers, such as polyethylene terephthalate (PET), this change in structure gives rise to diffefrences in the infrared spectrum that can be used for quantitative analysis of samples for crystalline content. With PET, a band at 10.2 was shown by Cobbs and Burton [ ] to be a function of crystallinity and variously annealed samples were run for infrared absorption and density. The density of amorphous PET is 1.33, while the density for crystalline polymer is 1.47 according to x-ray unit cell measurements. From the density-infrared absorbance samples it was then possible to construct a calibration curve of absorbance versus percent crystallinity. 

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