Pressure-induced coordination-structural change around zinc(II) in zinc(II)-neutralized ethylene-methacrylic acid ionomers. 1. Infrared spectroscopic studies

The coordination structure around zinc(II) in a 60% neutralized zinc(II) salt of ethylene-methacrylic acid (E-MAA) ionomer was investigated by infrared (IR) spectroscopic studies. It was found that the IR carboxylate antisymmetric stretching (v(as)(COO-)) band profile, and thus the coordination structure around zinc(II), strongly depends on the pressure (P) applied at 403 K in the melt: When the sample was placed at 403 K under vacuum (i.e., P = 0), two v(as)(COO-) bands at 1624 and 1538 cm(-1) were observed and ascribed to hexacoordination structures of zinc(II) carboxylates, but increasing P intensified a sharp v(as)(COO-) band at 1585 cm(-1), assigned to a tetracoordination structure. A remarkable feature of this pressure-induced coordinational change is that even exposure to atmospheric pressure (P = 0.1 MPa) is enough to change the coordination structure significantly. The coordinational change was found to be dependent on the temperature at which the pressure was applied below the melting temperature (similar to 357 K) of the polyethylene crystalline region, which is considered to act as a physical cross-link, thus constraining the transformation of the coordination structure.