= Ethylene-vinyl acetate
= EVA
{c}
{synonym}
\b[Ethylene-vinyl acetate], abbreviated \b[EVA], is a <copolymer> made from the <monomers> <Ethylene> and <vinyl acetate>. Its chain contains ethylene-derived units $-\mathrm{CH_2}-\mathrm{CH_2}-$ and vinyl-acetate-derived units $-\mathrm{CH_2}-\mathrm{CH}(\mathrm{OCOCH_3})-$. 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>.<PubChem Compound database by National Center for Biotechnology Information>[CID 6325, ethylene, and CID 7904, vinyl acetate: molecular formulas and connectivity]{ref}<ExxonMobil EVA 2018 Series product datasheet>[p. 1, product identification as an ethylene vinyl acetate copolymer]{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 $M_{\mathrm E}=28.05\,\mathrm{g\,mol^{-1}}$ and $M_{\mathrm{VA}}=86.09\,\mathrm{g\,mol^{-1}}$. Thus
$$
w_{\mathrm{VA}}=\frac{xM_{\mathrm{VA}}}{xM_{\mathrm{VA}}+(1-x)M_{\mathrm E}},\qquad
x=\frac{w_{\mathrm{VA}}/M_{\mathrm{VA}}}{w_{\mathrm{VA}}/M_{\mathrm{VA}}+(1-w_{\mathrm{VA}})/M_{\mathrm E}}.
$$
A 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.<PubChem Compound database by National Center for Biotechnology Information>[CID 6325, Molecular Weight 28.05 g/mol; CID 7904, Molecular Weight 86.09 g/mol]{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.<ExxonMobil EVA 2018 Series product datasheet>[p. 1, Resin Properties, Vinyl Acetate Content: 18.2 wt%; Notes: typical properties are not specifications]{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.<Laser capture microdissection by Michael R. Emmert-Buck and others>[Science 274, 998–1001, Abstract: film, carbon-dioxide laser activation and focal adhesion]{ref}
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