The effect of tensile drawing on the structure and relaxation processes in vinyl alcohol–ethylene copolymers

The influence of the tensile drawing on the structure and dynamic mechanical relaxations of vinyl alcohol–ethylene (VAE) copolymers with higher content in the former counit is studied. The structure of VAE copolymers is considerably modified by drawing, which produces a disordered crystalline form, as revealed by WAXD and DSC measurements. The structural changes have significant effects on the viscoelastic relaxations of the stretched samples. A remarkable increase in the storage modulus is observed in the direction parallel to the draw direction in detriment to those found in specimens cut transverse and diagonally to the draw direction. The drawn copolymers show an additional α′ relaxation, attributed to motions within the crystalline regions, taking place at temperatures higher than that of the α relaxation (glass transition). A clear anisotropy tan δ45>tan δ0≈tan δ90 is exhibited in this α′ relaxation similar to that observed in oriented LDPE where the chain axes are aligned in the draw direction while the lamellae are inclined at 45° to the draw direction, and the pattern of anisotropy thus suggests chain shear as a mechanism for the loss process. The α′ process in the VAE copolymers is thus assigned to chain shear while the α and β processes are assigned to interlamellar shear.