1981 Volume 37 Issue 10 Pages T387-T414
A phenomenological theory separating an isochronal data of temperature dispersion of the complex dynamic modulus function of PET into α, β, and γ relaxation mechanisms (αI, αII, β, γI, γII, and γIII) was proposed on the basis of the following three assumptions: (1) additivity for assessing the contribution of each relaxation mechanism to the viscoelastic functions; (2) validity of the time-temperature superposition hypothesis within each relaxation mechanism independently of other relaxation mechanisms; (3) symmetric loss modulus function of each relaxation mechanism with respect to reduced logarithmic frequency, and, in turn, to reciprocal of absolute temperature.
The temperature dispersion of loss modulus function for each (j-th) relaxation mechanism was represented by a Gaussian type function characterized by its height Aj, breadth C*j, and integrated intensity ΔEj, and the viscoelastic anisotropy in relation to molecular orientation was discussed in terms of the correlation coefficients between the viscoelastic constants, Aj, C*j and ΔEj, and the second and fourth moments of orientation distributions of crystalline and noncrystalline structural units for various types of stretched and annealed specimens of a quenched PET.
The highest correlation to the molecular orientations was found for ΔEj to result in the following conclusions: the β mechanical dispersion of PET being related not merely to the orientation relaxation of noncrystalline chain segments but also to the orientation relaxation of certain crystalline textures, if any; the αI mechanical dispersion of PET being related to the rotational relaxation of certain crystal grains probably around the crystal α axis owing to the nature of the (100) crystal plane to be the most easily glidable plane; and the γ mechanical dispersion (γII and γIII) being related to localized molecular distortions in noncrystalline chains.
