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Abstract
The tensile and torsional moduli, as well as the glass transition temperature, Tg of poly(ethylene terephthalate)(PET) are substantially higher than those of polycaproamide (nylon-6, N6). After considering and discarding various potential reasons for these differences, it is shown that the difference in all these properties between PET and N6 may be attributed to the existence of terephthaloyl residues in PET and to two important consequences: One is that, due to their shape and π-electron attraction, the tcrephthaloys tend to pack in stacks even in the amorphous phase, and these stacks show tendencies to aggregate in –nodules— or –embryonic nanoparticles—. Resistance to separation of stacked terephthaloys is one contributor to the higher properties of PET. The second consequence of the terephthaloys in PET is the very large volume such groups must sweep in space when they perform translational and other motions in space. This holds true when such motions are cooperative, and much more so when the motions are performed independently. Here, too, the resistance to the translational and orientational motions of very large volume terephthaloyl residues is much larger than in the case of N6 where much smaller volumes of chain fragments may be involved in the same motions. The stronger resistance to motion is reflected by the higher moduli and Tg of PET. All these features are amplified in the semi-crystalline polymers, leading to further elevation of these properties of both PET and N6.