Influence of Physical Aging on the Molecular Motion and Structural Relaxation in Poly(ethylene terephthalate) and Related Polyesters

Abstract

A combination of dielectric relaxation, dynamic mechanical thermal analysis, and positron annihilation measurements is reported on melt-cast films of poly(ethylene terephthalate), poly(ethylene naphthalate), and their copolymers. The effects of change in the chemical structure of the polyesters are rationalized in terms of increased restriction of the mobility of the polar segments of the chain by the incorporation of the bulky naphthalate structure. All the quenched samples exhibit physical aging when raised to elevated temperatures, the rate depending on the degree of undercooling used in the aging experiments. The free volume surprisingly does not change significantly with temperature and leads to the suggestion that the reduction in the dielectric permittivity is a consequence of a reduction in the mobility of the local segments. The increased storage modulus is also consistent with a reduction in mobility. This proposal is further confirmed by the observation of a good correlation between the rates and extents of the physical aging as detected by dynamic mechanical and dielectric relaxation measurements. Similarities in the activation energies of the β relaxation process for all the polymers investigated indicate that the dipole relaxation processes have a common origin and can be ascribed to motion of the linking polar entity. This study implies that aging is accompanied by an increase in ordering within these polyesters.

Diffusion mechanism for physical aging of polycarbonate far below the glass transition temperature studied by means of dielectric spectroscopy

“…In view of the common observation for all aging experiments, that C(t) exclusively undergoes a decrease with time, we may conclude that the C(t) response is governed by effect (ii). This was shown in the past to be dominant during physical aging of amorphous PET and related polyesters [13]. In the present study, similarly the dielectric permittivity decreases with the aging time as a consequence of the densification of the system.…”

Crossover of aging dynamics in polymer glass: From cumulative aging to noncumulative aging

“…In the case of PMMA, the dielectric susceptibility ǫ ′ (and ǫ ′′ ) decreases with aging time during the isothermal aging process [10,12]. This decrease can be observed also in other polymeric systems such as polycarbonate [24] and poly(ethylene terephthalate) [25]. Hence, the decrease in the dielectric susceptibility during isothermal aging process is a characteristic property common for physical aging of various polymeric systems, although the microscopic origin of this decrease is not fully understood.…”

Dielectric study of the glass transition of PET/PEN blends

“…The relative content of PET and PEN also affects the α relaxation. Particularly, there is a noticeable reduction in the dielectric strength due to a decrease in the mobility of local segments as the content of PEN increases [8]. With regards to other relaxations, a subglass relaxation β * can be observed in PEN and copolymers but not in PET [6,9].…”