Dielectric αa- and βa-absorptions in some amorphous and semi-crystalline polymers

Abstract

The dielectricαa- andβa-absorptions in the linear high polymers whose dipoles are short and rigidly attached to their main chains are investigated theoretically and compared with our observed data. Suffix “a” represents that dielectric absorption comes from amorphous part. Examples of this kind are polyvinyl chloride, polyvinyl bromide, polyethylene isophthalate, polyethylene terephthalate and polyoxymethylene. Theαa-absorptions are attributed to the re-orientations of their dipoles due to the segmental micro-Brownian motions of the polymer chains in the amorphous part. Theβa-absorptions seem to be caused by local micro-Brownian motions of the polymer chains. Theβa-absorptions are described by a model of local viscoelastic relaxations of the “dipoles” in the vicinity of their equilibrium directions. Here, the term “dipole” represents the resultant dipole of the permanent dipoles in the motional unit which has about the same order of magnitude as the monomer unit. Thus the various properties ofαa- andβa-absorptions are calculated and agree with our observed values at least semi-quantitatively. In the second section the equations ofαa- andβa-absorptions are derived. The properties of theαa-absorptions are calculated in the third section and compared with our observed data. The magnitudes ofαa-absorptions of the semi-crystalline polymers are smaller than those of the amorphous polymers even in the amorphous samples of the semi-crystalline-polymers. The temperature dependence of the magnitude ofαa-absorption is far gentler in the crystalline polymer compared with the amorphous at temperatures higher than glass transition temperature (Tg). These observed tendencies are explained by our theory. The calculated values of the activation energies ofαa-absorptions are 50 ≈ 150 kcal/mol, the same orders as the observed values. The shape of theαa-absorption is given by the form [e(iω) -e∞]/[e0-g3∞] = [(1+iω π1 (1 +iωπ2)−1/2 at temperatures higher thanTg. This is in good agreement with the observed shape in the amorphous polymer, not only for the order of breadth but also for the order of asymmetry. The shape of theαa-absorption in the semi-crystalline polymer is given by a superposition of the above formula and leads to the same order of breadth as the observed shape, too. The properties ofβa absorptions are calculated in the fourth section with the aid of the above mentioned model. The calculated values of the magnitudeβa-absorptions show fairly good agreement with the observed values too. The temperature dependence of the magnitude ofβa-absorption is discussed. The calculated values of the activation energies of theβa-absorptions are of the order 10 ≈ 20 kcal/mol, in good agreement with the observed values. The shape ofβa-absorption is also given by a superposition of the above formula and leads to the same order of breadth as the observed shape. The order of asymmetry of our theoretical absorption curve agrees with that of the observed curve.