Long-time crystallization kinetics in zinc-neutralized ethylene–methacrylic acid ionomers

The classic paper by Flory regarding the crystallization of random copolymers presents a theory where fractional crystallization depends only on molar substitution level if the comonomer cannot be incorporated into the crystal structure of the crystallizable comonomer. A rather large number of indirect experiments on ethylene copolymers since that publication nearly 60 years ago are consistent with this hypothesis; for example the crystallinity content in ethylene copolymers depends to a first approximation only on the level of molar substitution and not the identity of the comonomer. Ethylene–methacrylic acid copolymer ionomers offer a unique opportunity to test this hypothesis directly, since it is possible to vary crystallization kinetics simply by varying the neutralization cation or neutralization level, without changing the comonomer content or its distribution along the polymer backbone. After annealing times as long as 6 months, it was found that the constraints caused by the phase-separating ionic groups lead to significantly lower crystallinity levels versus the case where such constraints were not present. Although disproving Floryʹs theory in this particular case, the result was not surprising since Floryʹs original paper certainly did not consider the case where the other monomer can phase separate. One surprising result of this study was that the rates of formation of secondary crystals relative to the crystallization amount at infinite time was independent of neutralization level for the phase-separated ionomers; in other words the Avrami constants n and K were independent of neutralization level for the secondary crystals.