Puncture Reversal of Polyethylene Ionomers - Mechanistic Studies
Ionomers are polymers that contain ionic groups in relatively low concentrations along the polymer backbone. These ionic groups, in the presence of oppositely charged ions, form aggregates that lead to novel physical properties of the polymer. React-A-Seal and Surlyn are poly(ethylene-co-methacrylic acid) (EMAA) ionomer-based materials and Nucrel is the EMAA acid copolymer neutralized to produce Surlyn. React-A-Seal, Surlyn, and Nucrel recover into their original shapes following a high impact puncture at velocities ranging from 300 to 1200 ft/s ("self-healing"). This self-healing process may be of great benefit in space applications where structures are exposed to matter impacts. A thermal IR camera indicated a temperature increase to 98°C for Nucrel 925, Surlyn 8940, React-A-Seal, and Surlyn 8920 after initial penetration. To understand and generalize the observed phenomena, questions concerning the mechanism of the puncture resealing must be answered. One suggestion is that the elastic character of the melt created by the puncture drives the self-healing. This inference is based on the observed temperature rise of ~3°C above the melting temperature of the samples (~95°C) during the impact.
With the expectation of gaining additional insight into the self-healing phenomenon, a thermodynamic and viscoelastic investigation was conducted using primarily DSC and DMA. Surlyn and React-A-Seal showed the characteristic order-disorder transition at ~52°C that has been reported in literature. Master curves were constructed from the creep isotherms for the four EMAA samples. An aging study was performed to investigate the irreproducibility and "tailing effect" observed in the creep data. The aging study indicated that, with increased aging time and temperature, changes in the polyethylene matrix lead to complexities in morphology resulting in changes in the magnitude and shape of the creep curves.
As a result of the thermodynamic, viscoelastic, and high-speed impact experiments it has been theorized that self-healing can occur in Nucrel 925, Surlyn 8940, React-A-Seal, and Surlyn 8920 because of two features, ionic aggregation and complex flow behavior.