| The objective of this page is to briefly describe some of the historical events responsible for the discovery and development of IPNs as a class of heterogeneous polymeric materials. This discussion is far from exhaustive and people interested this subject may read more about it in the references given below. |
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Aylsworth never called this material an IPN, though. The concept of polymeric materials was not proposed for another 6 years and popular belief held that no molecules could be made with a molecular weight greater than 4,000 grams/mole. In 1920 Herman Staudinger proposed the existence of chain-like polymeric structures with molecular weights that far exceeded 5000 grams/mole. The idea was met with criticism at first but finally gained exceptance. Therefore Aylsworth really had no idea what he had really made. He had no knowledge of polymer networks or the complex relationships between heterogeneous polymer systems.
IPNs were not studied as a class of hetergeneous materials until the late 1950's - early 1960's. There had been a couple of patents filed on polymeric materials that were later identified as IPNs but these researchers were only interested in the properties of the materials made, not understanding the why these materials had the properties that they did. J. Millar was the first researcher to systematically investigate the properties of IPN materials. He was interested in ion-exchange materials (like for water-softening) based on polyelectrolytes, like sulfonated polystyrene. In order to keep the styrene from dissolving in water, it is crosslinked with divinylbenzene, which is shown below. Millar prepared samples of crosslinked polystyrene and swelled them in solutions of styrene and divinylbenzene.He then free radically polymerized the monomers within the swollen network to form a polystyrene network within another polystyrene network. This kind of IPN is called a homo-IPN, since both networks are made of the same material. The IPN material displayed a lower degree of swelling than other crosslinked polystyrene ion-exchange materials, which helps the material to selectively exchange ions based on ion size.
Millar was the first person to use the term "Interpenetrating Polymer Network". The term was used to describe what what he thought the material looked like - two different networks intermeshed with each other on a molecular level, as shown in the Figure on the main page of Ye Olde IPN Shop. However, the named was used before there was an understanding of the morphology of IPNs and there is almost never mixing of the two components on a molecular level. This will be discussed in more detail in the section of Ye Olde IPN Shoppe called the Anatomy of an IPN . Unfortunately, even though it has been known for a long time that these materials don't have the kind of morphology that we originally thought they did, people still use the term interpenetrating network. The name has become so entrenched in the literature that there is little hope of a more appropriate name being used for these materials.
While the idea of an IPN was published in 1960, it took about another 10 years before research on IPNs began to flourish. Many different types of IPNs were synthesized in a relatively short amount of time between 1969 and 1977.
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