Ethylene-vinyl acetate

Ethylene-vinyl acetate, abbreviated EVA, is a copolymer made from the monomers Ethylene and vinyl acetate. Its chain contains ethylene-derived units $-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2-$ and vinyl-acetate-derived units $-\mathrm{C}{\mathrm{H}}_2-\mathrm{CH}(\mathrm{OCOC}{\mathrm{H}}_3)-$. The polymerization joins the carbon skeletons while retaining the acetate side group. Consequently ethylene-vinyl acetate denotes a family of compositions and chain lengths, rather than one small molecule with a unique molar mass.^[ref][ref]

A useful distinction is between the mole fraction of incorporated vinyl acetate units and their mass fraction. Write $x=N_{\mathrm{VA}}/(N_{\mathrm{VA}}+N_{\mathrm{E}})$. Ignoring the contribution of chain ends for sufficiently long chains, the unit molar masses are $M_{\mathrm{E}}=28.05\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$ and $M_{\mathrm{VA}}=86.09\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$. ThusA chain population with 20% of its units derived from vinyl acetate has about 43.4% of its mass in those units. Conversely, 20% by mass corresponds to only about 7.53% of the units. The difference comes from the roughly threefold unit-mass ratio. Here $x$ describes the incorporated chain composition; incorporated composition versus feed composition in EVA is a separate issue from this arithmetic conversion.^[ref]

The manufacturer's EVA 2018 datasheet illustrates actual reporting practice: its typical vinyl acetate content is 18.2% by mass. That value characterizes the named product and does not define every ethylene-vinyl acetate material.^[ref]

A particularly revealing application is laser capture microdissection. The 1996 report placed a transparent thermoplastic ethylene-vinyl acetate film on a histological section. A carbon dioxide laser pulse activated a chosen region of the film, whose local adhesion transferred selected cells for subsequent analysis. This connects a polymer processing response to spatially selective sampling of tissue; the report's film is not identified with the commercial grade cited above.^[ref]