Local free volume in ethylene-vinyl acetate copolymers:: A positron lifetime study

We report on the local free-volume properties in a series of commercial ethylene-vinyl acetate (EVA) copolymers ranging from PE to PVA, obtained via positron lifetime spectroscopy. The samples were characterized by DMA (dynamic mechanical analysis), DSC (differential scanning calorimetry), WAXS (nide-angle x-ray scattering), and density measurements. For the analysis of the lifetime spectra, the routines MELT (maximum entropy for lifetime analysis) and LIFSPECFIT were used. Three or four lifetimes are resolved. depending on the composition of the specimen. The longest lifetime is attributed to o-Ps (positronium) annihilation from free-volume holes of the amorphous phase and is used to estimate the mean volume, v, of these holes. By using crystallinity data (DSC, WAXS), the specific volume of the amorphous phase V-a is estimated from the experimental total specific volume. By assuming a linear superposition of the specific bulk volumes V-b of PE (polyethylene) and PVA (polyvinyl acetate), the fractional (free) hole volume h = (V-a - V-b)/V-a in EVA (ethylene-vinyl acetate) copolymers is estimated. We find that v and h remain constant for VA contents between 0 and similar to 45% and decrease thereafter linearly down to the values for PVA. This behavior correlates well with the nonclassical (constant) behavior of T-beta (DR/LA) and T-g (DSC), which is attributed to a correlation between the crystallinity and the chemical and physical structure of the amorphous phase. For higher concentrations of VA, T-beta and T-g increase linearly with increasing VA content. From the relation N = h/v, a number density of holes of N = 0.67 +/- 0.05 mm(-3) was estimated, which does not change with composition of EVA copolymers.