Current Position: Head, Global Clinical Operations, Kenvue
Undergraduate Institution: Purdue University
Ph.D. Thesis Research:
Although ionomers were introduced more than four decades ago and have since been used in a variety of commercial applications, little is known about the mechanism by which they flow, though an “ion-hopping” relaxation mechanism is thought to be responsible. The ionic association dynamics are characterized by t, the average length of time an ionic group spends in a particular aggregate before “hopping” to another aggregate, thus allowing relaxation of the polymer chain segment attached to the ionic group. The terminal relaxation time, td, of the ionomer (the time necessary for the polymer chains to disentangle) is increased due to the reduction in the overall diffusion coefficient of the polymer chains caused by the temporary ionic crosslinks.
To understand the dynamics of chain and ion motion in ionomers, a series of commercial ionomers (DuPont Surlyn®) and a complementary model ionomer system have been investigated via rheological and spectroscopic techniques. The objective of this work has been to test the ion-hopping model by measuring the two relaxation times and elucidating how they depend upon the molecular structure. The terminal relaxation time of the polymer chains can be measured using mechanical rheometry, but the ion-hopping time is orders of magnitude faster and requires a different technique. Thus, the rate of ion-hopping has been determined by the use of cation diffusion experiments, in which a finite slab of one ionomer is allowed to diffuse into a matrix of a second ionomer. The diffusion of the cations as they hop from one aggregate to another is monitored via x-ray microanalysis, using a scanning electron microscope (SEM) fitted with an x-ray detector. The diffusivities are then translated into ion-hopping times using the average interaggregate distance as determined by small-angle x-ray scattering.
All ionomers exhibit Newtonian behavior at sufficiently low shear rates. The terminal relaxation time has a strong exponential dependence on the ion content; whereas, the type of neutralizing cation plays only a secondary role upon the observed flow behavior. Comparing t with td, the ion-hopping time is 4 orders of magnitude faster than the relaxation time of the polymer chain for the highly neutralized Surlyn® E/MAA ionomers. The ion-hopping time increases exponentially with increasing ion content implying that the ionic associations become stronger at higher ion contents, and thus may influence resulting polymer chain motion. Therefore, by incorporating this exponential dependence of t into current theories of ionomer dynamics, it is possible to predict the exponential td behavior observed experimentally for the Surlyn® ionomers.