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Richard Register

Richard A. Register

Eugene Higgins Professor of Chemical and Biological Engineering
Chair, Department of Chemical and Biological Engineering

S.B., Chemistry, Massachusetts Institute of Technology, 1983
S.B., Massachusetts Institute of Technology, 1984
M.S.C.E.P., Massachusetts Institute of Technology, 1985
Ph.D., University of Wisconsin, 1989

Room: A217 Engineering Quad
Phone: 609-258-4691
Email: register@princeton.edu

Webpage: Polymer Research Laboratory

Honors and Awards

  • Fellow, American Institute of Chemical Engineers, 2014
  • Fellow, American Chemical Society, 2012
  • Graduate Mentoring Award, Princeton University, 2008
  • Charles M.A. Stine Award, American Institute of Chemical Engineers, 2002
  • Fellow, American Physical Society, 2001
  • Young Investigator Award, National Science Foundation, 1992
  • Unilever Award, American Chemical Society, 1992

Concurrent University Appointments

  • Executive Committee, Princeton Institute for the Science and Technology of Materials

Publications

Research Areas

Research Interests

Polymeric materials containing multiple components can self-assemble into supramolecular structures on length scales ranging from nanometers to microns, useful in products as diverse as encapsulant gels and laptop displays. Our research focuses on the synthesis, processing, structure, properties, and applications of such complex polymers. An active in-house program in each of these aspects facilitates cross-fertilization and rapid materials development, while research group members gain a broad background in materials science and technology. Current examples follow.

Block copolymers comprise two or more different monomer units, strung together in long sequences rather than randomly distributed (e.g., a diblock copolymer comprising one run of polystyrene and one of polyisoprene). Repulsions between unlike blocks yield self-assembled mesophases having intricate nanometer-scale structure, with topology and dimensions tunable through composition and molecular weight. We synthesize well-defined block copolymers of diverse chemistry through "living" polymerization techniques (anionic, ring-opening metathesis, controlled free-radical). These materials possess rich phase behavior, since the mesophase can be altered through changes in pressure or temperature, or through the addition of other molecular or macromolecular components (such as solvents, nanoscale particles, other polymers or block copolymers). Block copolymer melts exhibit intriguing rheological phenomena, including “shear-melting”of cubic mesophases, where the viscosity drops by 3-4 orders of magnitude at a critical stress, and flow-induced orientation of non-cubic mesophases.

Templating polymer crystallization. The structure and properties of block copolymers can be enriched by incorporating a second self-assembling mechanism, such as block crystallization. We have effectively confined crystals inside nanoscale block copolymer microdomains, templating the crystals' size and orientation, as shown in the figure below.


Right: transmission electron microscopy image of a cylinder-forming block copolymer (insert), viewed down the cylinder axis. Each cylinder (black) contains a single ribbon-like crystal (white) running down the cylinder axis. Left: small-angle -x-ray scattering patterns show that the hexagonal packing of the cylinders, formed through self-assembly in the melt, is preserved in the crystallized solid.

Remarkably, crystal confinement can be achieved even when the polymer forming the confining matrix is fluid, provided the interblock segregation strength is sufficiently large. Recently, we have developed synthetic routes to block copolymers containing high-crystallinity blocks (both linear polyethylene and hydrogenated polynorbornene), using ring-opening methathesis polymerization and catalytic hydrogenation, as well as novel diblocks from a combination of metathesis and anionic polymerizations. In such materials, we have demonstrated that crystal thickness and melting point can be controlled through the block architecture, and we are continuing to explore the structure and properties of these high-crystallinity materials.

Nanolithography. We have also exploited block copolymers as surface templates, in a process we term “block copolymer nanolithography” (with Paul Chaikin, Physics) wherein an ultrathin block copolymer film is employed as a contact mask. This process can yield entire wafers covered with regular arrays of compound semiconductor (GaAs, InGaAs) "quantum dots" or metal (Ni, Au) nanoparticles (1011 dots per cm2) or patterns of parallel metal nanowires (300,000 per cm). To obtain the long-range order necessary for an addressable array, we are studying the defect annihilation and microdomain orientation processes in these films, as well as developing novel techniques to guide the orientation of these nanoscale domains over centimeter distances. Examples are shown in the figure below.


Structures produced through block copolymer nanolithography. The holes/dots in each image are 20-30 nm in diameter. Left: GaAs quantum dotes (TM-AFM image). Center: Si(100) wafer patterned over its entire area with holes (FEG-SEM image). Right: Au dots (TM-AFM image).

Ionomers contain a small amount of bound ionic functionality, such as carboxylic or sulfonic acid groups neutralized with a metal cation, whose aggregation produces dramatic material property changes. We combine rheology and x-ray microanalysis to study chain and ion motion, the small- and large-scale components of the "sticky reptation" process by which ionomers relax, and have synthesized model ionomers by anionic polymerization to achieve rigorous control of molecular architecture and chain length. The incorporation of polymer crystallization alongside ionic aggregation, as in polyethylene-based ionomers, further diversifies material properties. We are particularly interested in how crystal morphology and crystallization kinetics, and ultimately material properties, can be manipulated through these ionic associations.

Electroluminescent polymers, which emit light when passing current, could form the basis for bright, large-area flat-panel displays. With Professors Jim Sturm (Electrical Engineering) and Mark Thompson (USC), we have developed a range of materials with brightnesses approaching that of a fluorescent lamp, using dye-doping to cover the full red-green-blue color spectrum. These systems exhibit rich photophysics, with exciplexes and electroplexes controlling energy transfer to the dyes. While several known polymers are good hole-transporters, efficient devices require a balance of hole and electron densities. We are currently synthesizing block copolymers from monomers individually optimized for hole and electron transport, to produce a self-assembled nanoscale structure with separate but adjoining carrier transport pathways, and an extremely high internal surface area for carrier recombination.