sealPurdue News

April 1998

Materials show promise for coatings, membranes, drug delivery

WEST LAFAYETTE, Ind. -- Purdue University researchers have developed a new class of materials with a wide variety of potential applications, from coatings to repel liquids to membranes that could be used in wastewater treatment and drug delivery.

The materials are called co-polymer networks, which are "built" from intersecting chains of small molecules linked together to form a larger, mesh-like structure. The two molecular "building blocks," or monomers, used in the new materials are acrylic acid and a derivative of oligoethylene glycol. The properties of an individual material in the class can be varied depending on the relative amounts of the monomers used to prepare it.

"Because these materials are co-polymers, we can control their properties more precisely and over a wider range than we could if they were made of a single type of monomer," says Robert Scott, a Ph.D. candidate at Purdue who helped develop the material. "This level of versatility and control allows for a number of applications."

The new class of materials is unique in that it is the first time materials with such a wide variety of properties have been derived from a combination of these two monomers, says Scott's adviser, Nicholas Peppas, who is the Showalter Distinguished Professor of Biomedical Engineering at Purdue. Peppas has conducted research in polymers for more than 26 years and has developed new materials and polymers for applications that include biomedical applications.

"The most exciting thing about this research is that we've not only developed a class of materials with diverse properties, but we've also come to understand fundamentally, on a molecular level, the basis for those properties," Scott says.

Scott presented information on the new materials in two talks in March at the annual meeting of the American Physical Society in Los Angeles. His research has been funded by the National Science Foundation and the National Institutes of Health.

The new materials, which were developed over the past four years with the help of lab assistant Atsmon Shahar, are particularly suited for separations applications, such as filtering mechanisms used in wastewater treatment, where only certain substances are allowed to pass through the mesh created by the interlacing polymers.

"As we increase the acrylic acid content of the materials, the oligoethylene glycol chains that make up the network move farther apart, increasing the mesh size, which in turn determines what substances can pass through," Scott explains. "By varying the acrylic acid content, as well as other parameters, we can precisely control the size of the molecules we allow through."

Another application Scott has investigated in his lab is the controlled release of substances.

"We've made systems that contain a model drug, and we're studying how the rate of diffusion of that drug out of this polymer varies as we vary the polymer structure," Scott says. "Using the material as a membrane for drug delivery is a particularly appealing application because we have very fine control over what drugs could be released through it and under what conditions."

In addition, the acrylic acid in the materials makes them sensitive to acidity, or pH. The mesh size and diffusive properties vary depending on the pH of the environment -- an important consideration for drug delivery applications, because different parts of the body exhibit different pH levels. For example, a capsule incorporating this material and containing a particular drug might remain "closed" in the mouth and "open" in the stomach to release a drug.

One type of the material also can be made to have a very dense network of molecular chains, which would make it very resistant to liquids, Scott says. "In that case it might make an ideal coating for applications that require a very low permeability to moisture," he says. "We can also modify the properties so that it will absorb various amounts of liquid. We're looking very closely at how we can control its affinity for water." Depending on its affinity for water, the material might have applications in the cosmetics industry in moisturizers, Peppas says.

He says further studies of the materials will be needed before they are ready for industrial or medical use.

Sources: Nicholas Peppas, (765) 494-7944; e-mail,
Robert Scott, (765) 494-3331; e-mail,
Writer: Amanda Siegfried, (765) 494-4709; e-mail,
Purdue News Service: (765) 494-2096; e-mail,

Probing the Network Structure of Highly Crosslinked Ionizable Polymers
Nicholas A. Peppas and Robert A. Scott, School of Chemical Engineering, Purdue University, West Lafayette, IN

Novel ionizable polymer networks have been prepared from oligo(ethylene glycol) multiacrylates and acrylic acid (AA), employing bulk radical photopolymerization techniques. The properties of these materials exhibit a complex dependence on the network structure and composition. Dynamic mechanical analysis and penetrant sorption experiments have demonstrated that the crosslinked structure of the materials depends very strongly on the AA content, although the network chain population is expected to be composed solely of ethylene glycol oligomers. The results suggest that inter-chain interactions are diminished as the AA content is increased, due to the increased spatial separation of oligo(ethylene glycol) chains. The compositional dependence of the glass transition temperature is qualitatively described by a treatment consistent with that employed for polymer blends, and deviations from ideal blend behavior point to the importance of system-specific free volume changes during the radical polymerization process. Hence, the glass transition temperature and other network properties are closely coupled to the polymerization kinetics.

Network Structure and Diffusion in Highly Crosslinked Ionizable Networks
Nicholas A. Peppas and Robert A. Scott, School of Chemical Engineering, Purdue University, West Lafayette, IN

The structural features of novel ionizable polymer networks prepared from oligo(ethylene glycol) (OEG) multiacrylates and acrylic acid (AA) have been examined using diffusion and 13C NMR relaxation studies. The experimental program was designed to probe (i) the composition-dependent network structure of the copolymer networks and (ii) the impact of varying the AA content and the OEG chain length on the polymer chain dynamics. Penetrant diffusion coefficients calculated according to a Fickian treatment of the swelling data indicate an increase in the network mesh size with increasing AA content or with increasing OEG chain length. The shapes of the penetrant uptake curves suggest a coupling of Fickian and relaxation-driven contributions to the overall swelling behavior. Significantly, the effect of increasing the AA content on the characteristic chain relaxation time (as indicated by the location of the inflection point in plots of the fractional penetrant uptake versus the square root of time) is reversed as the OEG chain length is varied, suggesting that chain relaxation is controlled by crosslinking considerations for shorter OEG chains and by compositional considerations for longer OEG chains. Measured compositional effects on solid state 13C NMR relaxation times support our conclusions.

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