Producing Polymers

ORNL and UT researchers design, synthesize, and characterize new kinds of polymers.

Among the greatest challenges facing chemistry, physics, and materials science today is the controlled synthesis of mesoscopic materials—particles with characteristic dimensions of 1 to 100 nanometers—with well-defined structures and properties, and the assembly of these materials into macroscopic functional devices.

Enzyme bound inside 30-nm silica pore to increase enzyme stability. Courtesy Pacific Northwest National Laboratory.

One way to address these challenges is the "bottom-up" approach used by nature: Start with single molecules (monomers) of controlled size, shape, and functionality, such as amino acids, and assemble them to produce a new material with unique properties, such as an enzyme. These macromolecules are called polymers—many (poly) monomers.

Polymers derive many of their properties from the number and kind of monomers connected together, a phenomenon related to the polymer's length and molecular weight. Natural polymers, such as cellulose, starch, proteins, and DNA, and synthetic polymers, such as polyethylene in milk jugs, polyvinylchloride in drain pipes, and polystyrene in plastic peanuts and cups, are used every day. Researchers still hope, however, to create new, complex, multi-functional materials that possess greatly enhanced mechanical, optical, catalytic, chemical, or electrical properties.

Scientists working in the Macromolecular Complex Systems research theme at the Center for Nanophase Materials Sciences focus on the design, synthesis, and characterization of polymers with well-controlled, and often complex, architectures. A research area of particular interest is block copolymers, in which two different types of polymers are chemically bound together. Two different polymers will usually not mix, but because they are bound together they cannot separate, as would a mixture of oil and water. The two polymers instead separate on the nanoscale, forming tiny domains that may be spherical, cylindrical, layered, or bi-continuous, depending primarily on their composition.

Block copolymers have everyday applications as adhesives, rubbers, and plastics. ORNL researchers are seeking to expand these applications by exploiting the nanoscale ordering in these materials. Examples of these applications include improved drug delivery systems that control the rate of drug release or carry the drug to a specific site in the body; materials for regeneration of living tissue, such as cartilage cells for arthritic knees; sensors for detecting specific chemical or biological agents; lightweight, flexible materials that can harvest energy from sunlight; and "super-elastomers," materials that can be reversibly stretched to 20 times their initial length.

Some of these applications take advantage of the ability of certain polymer molecules of designed architecture to change their shape, or to associate or dissociate with other polymer molecules, based on a change in their environment.

Nanotubes inside a polymer lend electrical conductivity to the composite.

Nature utilizes such effects extensively in living organisms, so scientists seek to mimic nature in materials research. ORNL researchers have developed a block copolymer, derived in part from a natural amino acid, which can associate with, or dissociate from, another polymer in response to a change in temperature or pH. At temperatures above 32ºC and pH greater than 4, the individual polymer chains aggregate to form micelles, electrically charged particles combining water-hating and water-loving polymers that might be used to deliver hydrophobic drugs.

Working with visiting professors from Germany and Massachusetts, they have developed super-elastomers. Together, they are taking advantage of the unique molecular topologies available at Oak Ridge to create specialized polymerization techniques available only in a few laboratories worldwide. These nanoscale pioneers have created a centipede architecture, where many glassy polystyrene side chains are connected to a rubbery polyisoprene backbone. This complex shape gives rise to super-elasticity as a result of enhanced coupling of the rubbery backbone with dispersed, nanosized glassy domains of polystyrene.

A new ORNL capability is the synthesis of deuterated polymers. By substituting deuterium for ordinary hydrogen, a particular polymer or portion of the polymer can be made to scatter neutrons strongly, while not affecting the polymer's other properties. This capability makes neutron scattering a particularly useful technique for studying the structure and properties of polymers and multicomponent polymer mixtures. However, custom synthesis of well-defined, well-characterized monomers and polymers labeled appropriately with deuterium is required. Recent workshops sponsored by the National Science Foundation highlighted the critical need for custom-deuterated synthesis facilities convenient to neutron scattering facilities.

Upon completion of the Spallation Neutron Source (SNS) and the upgrades to the High Flux Isotope Reactor (HFIR) by 2006, Oak Ridge National Laboratory will assume world leadership in neutron scattering. Located adjacent to the SNS and close to HFIR, ORNL's nanocenter envisions a synergistic relationship with these facilities. Nanocenter researchers seek to fulfill a critical national need by working closely with neutron scientists to create custom-synthesized materials (polymers, lipids, surfactants) that are optimized for a particular neutron scattering experiment.

In addition to synthetic capabilities, ORNL's nanocenter will have a broad range of state-of-the-art equipment for characterizing the structure, composition, molecular weight, and thermal and physical properties of polymers. These instruments include gel permeation chromatography with light scattering and viscosity detectors; high-resolution nuclear magnetic resonance spectroscopy; near-infrared spectroscopy; matrix-assisted laser desorption/ionization time-of-flight mass spectrometry; simultaneous static and dynamic light scattering; spectroscopic ellipsometer, differential scanning calorimetry and thermogravimetric analysis; transmission electron microscopy and scanning electron microscopy; atomic force microscopy, and rheometers.

The characterization made possible by these instruments will be especially important when trying to control the structure at the nanoscale and to understand the nanoscale physics and chemistry. In a "jump-start" project, students from Professor Timothy Long's group at Virginia Tech sought help in characterizing the novel polymers they synthesized. These polymers can associate in aqueous media and, therefore, have potential for use in controlled gene or drug delivery.

In the Nanomaterials Theory Institute headed by Peter Cummings, the Center for Nanophase Materials Sciences has world-class expertise and facilities for theory, simulation, and modeling of soft materials such as synthetic polymers and biomaterials. Researchers from both the center and the institute are working together to identify opportunities where theory and modeling can favorably impact user programs, complement experimentation, and aid in the design of experiments.—Phillip Britt, ORNL's Chemical Sciences Division, and Professor Jimmy Mays, UT-ORNL Distinguished Scientist

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