Two-Dimensional Fourier Transform Raman Correlation Spectroscopy Study of Composition-Induced Structural Changes in a Series of Ethylene/Vinyl Acetate Copolymers

Generalized two-dimensional (2D) FT-Raman correlation spectroscopy has been applied to the study of the composition-induced structural changes in eleven different ethylene/vinyl acetate (EVA) copolymers with vinyl acetate (VA) content varying from 6 to 42 wt %. The eleven copolymer samples have been divided into four sets according to their composition range, and 2D correlation analysis has been applied to detect the characteristic bands for each set. Throughout the composition range of EVA copolymers, the increase of VA content always causes the crystalline lamellae to shrink, the amorphous interlamellar layers to expand, the all-trans −(CH2)−n conformers to decrease, and the gauche conformers to increase in population. In particular, for the EVA with 7 wt % VA, small addition of VA comonomers acts as spacers between adjacent methylene segments and converts the orthorhombic unit cell to the anisotropic unit cell, manifested by the correlation splittings at (1438 cm-1, 1415 cm-1) and (1446 cm-1, 1434 cm-1), respectively. For the EVA with 26 wt % VA, small addition of VA comonomers mainly shortens the length of continual methylene segments and shifts the orthorhombic band from 1415 to 1419 cm-1. The 2D correlation analysis has identified the cause-effect relationship for structural events occurring both at the supermolecular phase scale and at the segmental submolecular level.