Custom-made Molecular Networks
Copolymers, which consist of two different base materials, show different properties depending on their composition. They can therefore be used in many different areas - from water treatment to the targeted release of medication in certain body regions. The materials are referred to as a copolymer network. They are made up of chains of small molecules that overlap and form a tissue-like structure. The two monomers in the new material, which was developed at Purdue University, are acrylic acid and a derivative of oligoethylene glycol. The properties of materials made from it vary depending on the relative ratio of the monomers that make it up.
"Because these materials are copolymers, we can control their properties more precisely and vary them over a wider range than if they were made from a single type of monomer," says Robert Scott, who is involved in the development of the material was."This level of versatility and control allows for a staggering number of applications."
The new class of materials is unique because it is the first time that materials with so many different properties have been obtained from a combination of these two monomers, explains Scott's supervisor Nicholas Peppas, who has been studying polymers for over 26 years.
"The most interesting thing about this research is that not only have we developed a class of materials with diverse properties, but now we understand fundamentally - on a molecular basis - the basis for these properties," explains Scott. At the annual meeting of the American Physical Society on March 16, 1998 in Los Angeles, he presented the new substances in two lectures.
The molecular nets are particularly suitable as sieves for separation techniques in which only certain substances are allowed to pass through the polymer."As we increase the amount of acrylic acid in the materials, the oligoethylene glycol chains, which are responsible for crosslinking, move further apart and the mesh gets larger, which in turn determines which substances can pass through," explains Scott. "By varying the amount of acrylic acid and other parameters, we can precisely determine the size of the molecules that we allow through."
Another application Scott has explored in his lab is the controlled delivery of substances. "We have created systems containing a model drug and are currently investigating how the rate of diffusion of that drug out of the polymer changes with modifications in the polymer structure," he says. "Using the material as a membrane for drug delivery is a particularly attractive application because we can control in great detail which drugs are allowed to pass through the membrane and under what conditions."
Due to the acrylic acid, the copolymers also react to the pH of their environment. Mesh size and diffusion properties vary with pH – an important point in drug delivery applications since different parts of the body have different pH values. A capsule made of this material that contains a specific drug can remain "closed" in the mouth but "open" in the stomach to release the drug.
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