Ionomer Melt Rheology and Processing
While ionic associations lie at the root of the desirable material properties exhibited by ionomers, they also greatly increase the viscosity and elasticity relative to comparable nonionic polymers. Indeed, such materials are frequently termed "reversible networks" because the ionic associations act as temporary crosslink points. Rather than being permanent crosslinks, these ionic associations can relax by an ion-hopping mechanism, where ionic groups and their associated segment of polymer chain backbone "hop" from one aggregate to another, as shown schematically below:
Schematic of the ion-hopping mechanism in ionomers. Top to bottom represents a time sequence. Curved line in the center represents a segment of a polymer chain bearing a single ionic group, which "hops" from the left aggregate to the right aggregate.
This ion-hopping mechanism eventually permits relaxation of the entire polymer chain, though slowly. Therefore, standard melt rheological techniques can be used to probe the dynamics, provided the low deformation rates necessary can be accessed. In some cases, the terminal (Newtonian) region is evident in master curves prepared from dynamic frequency sweeps (see figure below), but more commonly, creep measurements must be conducted on a controlled-stress rheometer to access the Newtonian regime.
Dynamic viscoelastic master curves, prepared through time-temperature superposition, for an ethylene-methacrylic acid ionomer partially neutralized with Na+.
We have complemented these melt rheological measurements with studies of cation diffusion, using both electron spin resonance (ESR) spectroscopy and x-ray microanalysis. The cation diffusion experiments measure the ion-hopping time t , while rheometry yields the chain's terminal relaxation time td. The ratio td/ t depends on molecular weight and achitecture, but is typically greater than 103.
Besides commercial E/MAA ionomers, we have also synthesized model sulfonated and carboxylated ionomers by post-functionalization of narrow-distribution styrene-ethylene-butene terpolymers. The synthetic route to the sulfonated ionomers is shown below:
Multistep synthetic route to model sulfonated ionomers: randomly-sulfonated, narrow-distribution styrene-ethylene-butene (SEB) terpolymers. At 40 wt% styrene, SEB has a glass transition temperature of -15oC, permitting melt rheological measurements to be conducted over a broad temperature range.
Measurements of td and t in these materials revealed an unexpected and strong influence of functionalization level on t , a feature not considered in theoretical models. Moreover, these materials fail to obey time-temperature superposition and show unexpectedly low plateau moduli, due to the rapid relaxation of the “outer portions” of these linear chains—from the chain end to the first ionic group.
Ethylene copolymers, including ionomers, are commonly converted to sheet by the process of film blowing. Blown films frequently show a strong anisotropy in their tear behavior, meaning that (as with newspaper), it is often much easier to propagate a tear in one direction than another. This anisotropy may be desired or undesired, depending on the application, but control over this anisotropy is always essential. Through a detailed investigation of ethylene-methacrylic acid ionomers and related ethylene homopolymers and copolymers, wherein process conditions were systematically varied, we showed that in the E/MAA copolymers and ionomers (but not ethylene homopolymers), it was possible to prepare films from the same resin which had a preference to tear in the machine direction, or in the transverse direction, or show no preference--all by varying the process conditions. The preferred tear direction was directly reflected in the WAXS patterns from the blown films (see figure below).
WAXS patterns for three blown films of a single ethylene-methacrylic acid ionomer. Red arcs or rings correspond to the (110) reflection of polyethylene; machine direction is vertical, transverse direction horizontal. From left to right, the three films were processed at increasing blow-up ratio (BUR).
The orientation is controlled by the threadlike crystal nuclei from which chain-folded crystallites grow; these nuclei are generated as the film exits the die, is inflated into the bubble, and cooled. For small blow-up ratios (BUR), the nuclei are aligned in the machine direction, while at high BUR, the nuclei are “flipped” to lie along the transverse direction. The extent of orientation is directly correlated with the melt viscosity of the material; since the film blowing operation is essentially a strain-controlled process, this orientation-viscosity relationship is as the stress-optical rule would predict, though the orientation is measured in the solid state, not in the melt. This means that raising the neutralization level, which raises the viscosity, will yield a more anisotropic film for the same blowing conditions, a prediction we have confirmed experimentally. This sort of flexibility--the preparation of films with very different tear behavior from a single resin through variations in processing--demonstrates the versatility of these materials.
Supported by DuPont Packaging and Industrial Polymers
Recent/Current People and Projects:
Li-Bong Lee *04 - Origin of Directional Tear in Blown Films of Ethylene Copolymers
Neena Tierney *01 - Chain and Ion Dynamics in Random Ionomer Melts
Selected Recent Publications:
L.-B.W. Lee, R.A. Register, and D.M. Dean, “Origin of Directional Tear in Blown Films of Ethylene/Methacrylic Acid Copolymers and Ionomers”, J. Polym. Sci. B: Polym. Phys., 43, 97-106 (2005).
N.K. Tierney, S.T. Trzaska, and R.A. Register, “Matched Random Ionomers: Carboxylate vs. Sulfonate”, Macromolecules, 37, 10205-10207 (2004).
N.K. Tierney and R.A. Register, “Ion-Hopping in Ethylene-Methacrylic Acid Ionomer Melts as Probed by Rheometry and Cation Diffusion Measurements”, Macromolecules, 35, 2358-2364 (2002).