LOW-SHEAR MELT RHEOLOGY OF PARTIALLY-NEUTRALIZED ETHYLENE-METHACRYLICACID IONOMERS

We examine the melt rheology of a series of ethylene-methacrylic acid (E/MAA) ionomers, all based on the same E/MAA copolymer, with an empha sis on the low-shear-rate Newtonian region. All ionomers show Newtonia n behavior at sufficiently low shear rates. The zero-shear viscosity e ta(0) is a strong function of neutralization level and is significantl y higher for Zn ionomers than for Na ionomers at the temperatures and neutralization levels examined. The recoverable compliance J(e)(0) is independent of cation type and neutralization level, indicating that t he ionic associations bear no stress during steady flow in the termina l region. Therefore, the individual ionic associations have lifetimes much shorter than the terminal relaxation time of the polymer chain. W e probe the effect of unneutralized acid groups on eta(0) by removing these groups through esterification. Unneutralized acid groups substan tially lower the viscosity of Na ionomers but have no discernable effe ct in Zn ionomers. An ''acid-cation exchange'' mechanism is proposed t o explain these findings.