Rate dependent finite deformation stress-strain behavior of an ethylene methacrylic acid copolymer and an ethylene methacrylic acid butyl acrylate copolymer

The large strain deformation behaviors of an ethylene methacrylic acid (EMAA) copolymer and an ethylene methacrylic acid butyl acrylate (EMAABA) copolymer are evaluated and compared in compression over nearly eight orders of magnitude in strain rate, from 10−4 to almost 104/s. Transition regimes are quantified using dynamic mechanical analysis. The stress-strain behavior of these copolymers exhibits a relatively stiff initial behavior followed by a rollover to a more compliant response. The low strain modulus, the rollover stress and the large deformation stress-strain behavior are strongly dependent on strain rate. The proximity of the material glass transition to the room temperature test conditions results in a substantial change in the nature of the rate sensitivity of the stress-strain behavior as one moves over the range of strain rates. The mechanical behavior of the EMAA is contrasted to that of a corresponding EMAABA terpolymer and to its sodium-neutralized counterpart (EMAABANa). The nature of the rate sensitivity of the room temperature stress-strain behavior of EMAA transitions from a behavior near the glassy end of the leathery regime at low rates to a near glassy behavior at high rates. The butyl acrylate content in the EMAABA lowers the glass transition temperature and leads to a more compliant mechanical behavior (reduced initial stiffness, reduced rollover stress, reduced post-rollover stress level) at room temperature. The EMAABA behavior transitions from a rubbery-like behavior at the lowest rates to a leathery-like behavior at the highest rates. Upon sodium neutralization, the overall stiffness and flow stress levels are enhanced likely due to the presence of the ionic aggregates; the glass transition of EMAABANa is broadened in comparison to the EMAABA, giving a rate dependent room temperature behavior that transitions through the leathery regime with increasing strain rate. A constitutive model that separately accounts for the distinct deformation resistances of the crystalline domains and the amorphous domains is able to capture the changes in rate dependent deformation behavior of the EMAA copolymers studied herein. The crystalline domains provide resistance to flow across a wide window in rate and temperature whereas the amorphous domains provide increasing resistance as the strain rate is increased and the material effectively transitions through the glass transition regime, providing a mechanism for changing rate sensitivity.