Structural origin of the cryogenic relaxations in poly(ethylene terephthalate)

Abstract

synopsisThe effect of molecular organization (crystallinity, orientation) on the internal friction of poly(ethy1ene terephthalate) was studied by means of dynamic mechanical measurements a t temperatures from 300 to 4.2OK, with a free-oscillating torsion pendulum at 1 Hz. It was found that crystallinity decreases the intensity of the composite y relaxation at 21OOK and gives rise to an additional loss maximum B at 26°K. TJniaxial orientation broadens the y relaxation and gives rise to an additional loss peak 6, at 46'K. The 6 and e losses are dependent on molecular organization, occurring only in samples containing aligned, taut chain segments and crystalline structures, respectively. They have a common activation energy of 4 kcal/mole. All three low-temperature relaxations in oriented specimens show pronounced directioiial anisotropy, which, in the y loss, may be due to the preferred orientation of noncrystalline chain segments, while in the 6 and e losses, may be associated with the direction of defect structures. On the basis of the observed behavior of the 6 and e relaxations it is suggested that they may involve motions of defect structures and may thus participate in stress-transfer mechanisms at large deformations.

Oxygen‐barrier properties of copolymers based on ethylene terephthalate

“…The PET homopolymer exhibited a pronounced γ‐relaxation peak at about −75 °C (1 Hz). It is generally thought that the broad peak encompasses several relaxation processes involving both methylene groups and ester linkages 30–32. Most of the polymers in the present study exhibited a γ relaxation at approximately the same temperature as PET.…”

Segmental motions and associated dynamic mechanical thermal properties of a series of copolymers based on poly(hexamethylene terephthalate) and poly(1,4-cyclohexylenedimethylene terephthalate)

“…It was found that the secondary β-relaxation peak of poly(alkylene terephthalate)s is asymmetric and might be composed of two relaxation processes due to the segmental motions of carboxyl and methylene units. 1,28 Recent dynamic mechanical and dielectric analysis has suggested that the secondary β-relaxation of PET consists of two different relaxation processes, showing that the high temperature side of the peak is due to phenylene ring flips and the low temperature side is due to the motion of carboxyl groups. 32 Solid-state NMR relaxation spectra of PBT have also revealed that large amplitude segmental motions of phenylene rings as well as methylene units undergo at the temperatures below the glass transition temperature.…”

Oxygen‐barrier properties of polyethylene terephthalate modified with a small amount of aromatic comonomer

“…It is generally thought that the broad γ‐relaxation peak encompasses several relaxation processes involving both methylene groups and ester linkages 20–22. The complexity of the γ relaxation leads to identification of a lower temperature component with gauche conformations of the glycol and a higher temperature component with trans conformations of the glycol 18, 20, 21.…”

“…It is generally thought that the broad γ‐relaxation peak encompasses several relaxation processes involving both methylene groups and ester linkages 20–22. The complexity of the γ relaxation leads to identification of a lower temperature component with gauche conformations of the glycol and a higher temperature component with trans conformations of the glycol 18, 20, 21. The two relaxations differ somewhat in activation energy23 with E γ for trans conformers (52.1 kJ mol −1 ) higher than E D for diffusion (39.7 kJ mol −1 ) and E γ for gauche conformers (32.8 kJ mol −1 ) slightly lower than E D 3.…”