Stress relaxation under large step strain for ionomers based on ethylene-co-methacrylic acid copolymer in the melt state

Abstract

Stress relaxation behavior under large step shear strain γ for ethylene-co-methacrylic acid copolymer (EMAA) and its ionomers (EMAA-Zn and EMAA-Na) was studied in the melt state. EMAA ionomers show higher relaxation modulus G (t,γ ) and slower relaxation rate than EMAA. At 140°C the strain dependence of G (t,γ) for EMAA ionomers is very strong. The corresponding damping function h (γ) decreases with γ in two steps around γ=0.6 and γ=4. This suggests that the ion aggregates become small in two steps. However, at 200°C h (γ) monotonically decreases with γ, indicating that the ion aggregates are already small and become smaller monotonically with increasing γ. Compared with EMAA-Na, EMAA-Zn has higher stress relaxation modulus because of strong interactions due to cooperation between the carboxyl acid group and Zn2+.