Ph.D. Thesis Research:

Ionomers are polymers that incorporate a small fraction of ionic functionality along the polymer backbone.  In practice, the backbone is usually comprised of hydrocarbon or fluorocarbon units. The most commercially successful ionomers are those derived from the random copolymers of ethylene (E) and methacrylic acid (MAA). This particular series of ionomers has been marketed under the trade name of Surlyn® for approximately four decades.

Since E/MAA ionomers are largely composed of ethylene units, it is not surprising that some of the physical properties of these materials are reminiscent of those of low-density polyethylene (LDPE). However, there are considerable differences in many of the mechanical and optical properties of these ionomers which make them desirable for use in a wide range of applications. For example, the high optical clarity and toughness of E/MAA ionomers motivates their use as packaging films. High abrasion and cut resistance are desirable properties in the sporting goods market, where E/MAA ionomers are used as protective coatings for golf balls, ski boots, and bodyboards.

Improvements in the physical properties of E/MAA ionomers are due to the formation of nanometer-size ionic aggregates in the polymer matrix. Since these ionomers are semicrystalline, the morphology is complex and consists of polyethylene crystals, amorphous chain segments and ionic aggregates. It is known that the mechanical properties of E/MAA ionomers may be altered by changing material and processing parameters, though connections remain largely empirical. The goal of this research is to elucidate the relationship between structure, processing, and non-linear mechanical properties in E/MAA ionomers. So far, our focus is on the yield stress (s_y) and toughness.

We have found that, in addition to polyethylene crystal plasticity, one must consider the active mechanical relaxations to understand the mechanical yielding of E/MAA copolymers and ionomers. To this end, we have developed a model for s_y of these materials (and, most likely, any semicrystalline ethylene/a-olefin copolymer) as a function of temperature and strain rate. Yield stress data, collected at various strain rates and temperatures, are fit using nonlinear least-squares regression (in my case, the Levenberg-Marquardt algorithm). The resulting model fit allows s_y to be determined at any strain rate and temperature, even in experimentally-inaccessible regimes. The only caveats are that the microstructure must be preserved (i.e., no crystal melting) and all relevant incomplete relaxation modes must be taken into account

We find that, while polyethylene crystal plasticity is important for both E/MAA copolymers and ionomers, incomplete amorphous chain relaxation on the time scale of the deformation experiment is the key to boosting yield strength. In the case of the E/MAA copolymers, the yield stress increases rapidly below the effective glass transition temperature. Upon neutralization, however, the regions around the ionic aggregates become vitrified, leading to a significant boost in s_y, especially at very low strain rates.

We also find that large-scale, permanent deformation does not occur at the yield point (at around 5% strain) as described above. In fact, virtually all strain (up to roughly 100%) initially applied to E/MAA copolymers and ionomers is recovered after a few days at room temperature. True irrecoverable deformation does not seem to occur until one passes the so-called “second yield point”, usually visible as a broad bump centered around 100% strain in a standard tensile test. Using small- and wide-angle x-ray scattering techniques, we have shown that the second yield point corresponds to a coarse-slip process in which large polyethylene crystals are broken up into smaller bits. This process destroys the “memory” of chain conformation preserved upon crystallization, leading to permanent, irrecoverable deformation.