Wide-Angle X-ray Diffraction Study of the Phase Behavior of Vinyl Alcohol−Ethylene Copolymers

Abstract

The influence of the composition and thermal history on the type of lattice developed has been studied by wide-angle X-ray diffraction measurements in three vinyl alcohol-ethylene, VAE, copolymers with different compositions, which were crystallized from the melt at various cooling rates. The results are compared with those of the corresponding homopolymers, poly(vinyl alcohol) and lowdensity polyethylene. Both the composition and the cooling rate have been found to determine the crystalline structure of these VAE copolymers. Thus, for the samples slowly crystallized from the melt, a monoclinic lattice is obtained for copolymers VAE71 and VAE68 (with compositions of 71 and 68 mol % VA, respectively) while an orthorhombic lattice has been found for copolymer VAE56 (56 mol % VA). The effect of the cooling rate is very important, since copolymers VAE71 and VAE68 lead to the orthorhombic form when the samples are quenched from the melt. Copolymer VAE56 has been found to crystallize in an orthorhombic lattice under all the crystallization conditions here studied. The polymorphism exhibited by these copolymers is not the usual case for two different unit cells with welldefined lattice parameters: VAE71 and VAE68 show, by the influence of the cooling rate, a continuous change of some of the lattice constants. For instance, the angle β of the monoclinic lattice decreases smoothly from an "equilibrium" value of about 92.2°in the slowly crystallized samples to a final value in the quenched samples of 90°(orthorhombic unit cell). The degree of crystallinity determined from the X-ray diffraction patterns has been found to be in the quenched specimens considerably smaller than that in the slowly crystallized samples, standing in contrast to the constancy of the enthalpy of melting. Moreover, it seems that there are no differences in the melting temperatures between the two crystalline modifications.

Influence of processing on the ethylene-vinyl alcohol (EVOH) properties: Application of the successive self-nucleation and annealing (SSA) technique

“…In order to estimate the crystallinity degree (X c ), the enthalpy of a 100% pure crystalline poly-vinyl alcohol (157.8 J/g) has been taken [8].…”

“…According to Cerrada et al [8], EVOH copolymers show a polymorphic behaviour depending on composition and thermal treatment. Nevertheless, with relative high vinyl alcohol content, the EVOH copolymers crystallize into a monoclinic lattice, similar to that of poly-vinyl alcohol, regardless of the ethylene sequences, which are located as punctual defects into the main crystalline lattice.…”

“…However, the barrier properties, stiffness or tensile strength, drop dramatically in high relative humidity conditions. EVOH copolymers are highly hygroscopic, as a consequence of the hydrophilic OH side groups and their interaction with the absorbed water molecules weakens the inter-and intra-molecular hydrogen bonds, increasing the free volume of the chains [7,8]. To avoid that loss of the barrier capacity, some authors have proposed the addition of organo-modified clays into the EVOH matrix via melt-blending method, using internal mixers and twin-screw extrusion [9,10].…”

The effect of residual acetate groups on the structure and properties of vinyl alcohol-ethylene copolymers

“…The amorphous peak of the different samples was found to be centered at about 2 ϭ 20 degrees, based on our previous work. 2 The error in the crystallinity determinations, when these are expressed as percentage, is estimated to be Ϯ5 units.…”

“…As a consequence of the bulkier nature of acetate groups compared to hydroxyl ones, the crystallization process of VAEAc copolymers could be hindered and this possible feature will be assessed by X-ray diffraction, a technique previously used to establish the crystalline structure of vinyl alcohol-ethylene copolymers. [2][3][4] The structural modifications of these copolymers should affect the thermal and mechanical properties, which will be studied by differential scanning calorimetry, thermogravim-etry, drawing behavior, birefringence measurements, and dynamic mechanical analysis.…”