Current Position: Assistant Professor of Chemical Engineering, Bucknell University
Advisor: Prof. R.A. Register
Undergraduate Institution: University of Pennsylvania
Ph.D. Thesis Research:
Recently, there has been something of a renaissance in the study of semicrystalline polymers, particularly polyolefins (such as polyethylene and polypropylene), as the advent of new polymerization chemistries and technologies remind us how incomplete our understanding of such material is. At the same time, interest in "microstructured" and "nanostructured" materials has exploded, with the promise that control of material structure at microscopic (and perhaps multiple) length scales will produce new or improved material properties. Polymers are perhaps the quintessential nanostructured material, as their macromolecular architecture (size on the order of 10 - 100nm) intrinsically makes them denizens of this length scale. Crystallization is one method by which polymers can self-assemble into a nanoscale structure (stacks of crystalline lamellae alternating with amorphous layers). Moreover, crystallizability is easily "built into" a macromolecule side-by-side with other features which induce self-assembly, allowing the rich structural diversity which comes from having multiple self-organizing mechanisms at work.
One such mechanism is ionic aggregation, arising from the association of metal salt groups covalently incorporated into a nonpolar chain. Because electrostatic interactions are strong relative to those normally encountered in polymers, even very small ionic aggregates (order 1 nm) are stable, yielding structures an order of magnitude smaller than crystallizaiton produces. Polymers containing a small amount of ionic comonomers are termed "ionomers"; the best-known (and commercially most important) ionomers are drived from ethylene-methacrylic acid copolymers, by neutralizing some or all of the methacrylic acid units with a metal cation (e.g., Na+ or Zn2+). Selected materials in this category are marketed by DuPont under the tradename Surlyn®. Because these materials have ethylene as the majority monomer, they still have "polyethylene-like" crystallinity, though there are important differences. The changes in key mechanical and optical properties upone ionomerization make Surlyn the material of choice for a variety of applications where nonionic polyethylene performs dismally. For example, the cut and abrasion resistances of Surlyn are essential to its use in golf ball covers, while its transparency is vital to its use as a packaging material and, in many cases, as a molding resin. These property differences, in turn, must arise from underlying structural (morphological) differences in Surlyn ionomers as compared with polyethylene; but this connection is not understood, so material development has proceeded by trial and error.
The current project is a comprehensive study of the structure-processing-property relationships in Surlyn-type ionomers: to understand how different material and processing variables affect material structure, and how the sturcture in turn influences key material properties. With this understanding, it should be possible to design a material optimized for performance in a particular area, and to identify those material parameters which chould be further manipulated to yield property values outside the current spectrum. Some of the material processing variables likely to be important are:
- cation type (e.g., Na, Zn, Mg ... and mixed cations) and level
- content of acid comonomer
- branch content (controls crystallinity and melting temperature)
- specimen thermal history (quenched, annealed, slow-cooled)
- presence of additives/plasticizers (e.g., stearate salts, surfactants which resemble "ionomer oligomers")
For example, it is known that the solid-state properties of ionomers made from the same ethylene-methacrylic acid copolymer depend on the cation type, and that in some cases synergy can be achieved by using a mixture of two different cations. En route to developing such a structure-processing-property understanding in these systems, this project would employ a range of experimental techniques broadly useful in studying the physics of polymeric materials. The polyethylene-like crystallinity which Surlyn-type ionomers exhibit is certainly a key factor in dictating their properties. Therefore, structural measurements which probe the crystallites are essential: wide- and small-angle x-ray scattering (WAXS and SAXS) are most appropriate here. Thermal analysis, particularly the new technique of dynamic differential scanning calorimetry, is also potentially quite powerful in probing the various crystal populations. The presence or absence of crystal superstructures ("spherulites"), which influence optical clarity, can be discerned using small-angle light scattering (SALS), which we can conduct in-real time on heating or cooling with a CCD camera. These structural measurements will then be used as the basis for interpreting differences in mechanical response.