Types of IPNs

The Point of the Page...

The purpose of this page is to outline the enormous variety of IPNs structures that can be formed by varying the components of the system and the chemistry used to form the IPN. There are two classification schemes used to describe IPNs. Both schemes are necessary to completely describe the synthesis of any IPN.

IPN Classification by Chemistry

Sequential IPNs

An IPN formed by polymerizing the first mixture of monomer, crosslinking agent, and initiator or catalyst to form a network. The network is swollen with the second combination of monomer and crosslinking agent and polymerized to form an IPN.

Click here to see a movie depicting this process.

Simultaneous Interpenetrating Networks (SINs)

An IPN formed by polymerizing two different monomer and crosslinking agent pairs together in one step. The key to the success of this process is that the two comonents must polymerize by reactions that will not interfere with one another. This is often accomplished by polymerizing one network by a condensation reaction, while the other network is formed by a free radical reaction.
Simultaneous IPN scheme

IPNs formed by a simultaneous reaction of components are inherently simpler to make since there is only one reaction required to make the IPN, while the sequential method requires two reactions. If the monomers are chosen correctly, SINs can be injection molded, which is a real big advantage over the sequential IPNs. Sequential IPNs must be either compression molded or synthesized in bead form. The swelling of the network in the sequential method can get a little messy too. These factors make simultaneous IPN reactions a little more appealing to those folks who are looking to commercialize IPN materials.

The materials produced by the two methods are not exactly the same either. Due to the intermediate swelling step in the sequential process, the first network formed usually has an extended chain conformation. The networks in SINs usually have relaxed conformations. One consequence of this difference is that sequential IPNs ususually swell less than SINs.

IPN Classification by Structure

Full-IPNs

This is the type of IPN which has been used in the description of IPNs in Ye Olde IPN Shoppe thus far. It is comprised of two networks that are ideally juxaposed, which generates alot of entanglements and interaction between the networks. A picture of the molecular structure of this material is given below on the left. Since it is difficult to see the structure of the two networks, an alternative geometric structure is given on the right. The geometric depiction shows the ideal intermeshing of the two networks. This type of network can be by either a sequential or simultaneous process.
Structure of a Full-IPN

Homo-IPNs

This IPNs are a special type of full-IPNs, where both polymers used in the networks are the same. They are usually sequential IPNs. They were one of the first types of IPNs to be commercialized and they are used as model materials for theoritical work.

Semi- or Pseudo-IPNs

One of the components of these IPNs has a linear structure instead of a network structure. A molecular and geometric depiction of this structure is given below. The linear component changes some of the properties of the IPN. Some of these IPNs can be extruded if the linear component is making up a majority of the material. One thing to keep in mind is that the linear component of the IPN can be removed from the network if the material is swollen in the appropriate solvent. These types of IPNs can be formed by either a sequential or simultaneous process. To further complicate the issue, IPNs of this type that are made by a sequential process are called semi-IPNs and the ones made by a simultaneous process are pseudo-IPNs.
Structure of a Semi-IPN

Latex IPNs

One of the problems with most IPNs is that they can't be molded after they are formed since they are thermosets. One way of getting around this problem is to use a latex IPN. These IPNs are formed by an emulsion polymerization. The morphology of IPN depends upon how the IPN components are polymerized. Both monomers can be added at once, which will tend to give you a more uniform morphology in the particles (a simultaneous IPN formation). The monomers can also be added in stages. For instance monomer 1 can be polymerized to form a latex and monomer 2 can then be added (a sequential IPN formation). Depending on how fast monomer 2 diffuses into the latex, one can get either a homogeneous incorporation of the monomer into the latex or most of monomer 2 may react near the surface of the latex particle. If most of monomer 2 reacts near the surface, one has a core-shell morphology. A TEM picture of a core-shell morphology is shown below. The finished latex particles can be collected and used as they are or they can be used as a coating. To read more about how IPN latexes are used in coatings, click here

Thermoplastic IPNs

As the name implies, these IPNs are moldable, can be extruded, and can be recycled. At least one component of these IPNs is usually a block copolymer, like SBS rubber . The othe component is typically a semi-crystalline or glassy polymer. These IPNs have completely thrown the idea of chemical crosslinks out the window and use physical crosslinks, like thermoplastic elastomers. Typical physical crosslinks arise from ionic groups, crystallinity, or glassy domains. A TEM picture of a thermoplastic IPN comprised of polystyrene and polybutadiene is shown below. Companies have used these IPNs in a number of applications, since they can be extruded.

A Few Final Comments...

The extremely diverse number of IPNs that can be formed has lead to the development of a nomenclature system for unambiguously classifying these materials. It is rather complex but is gradually emerging in the literature and gaining acceptance among researchers investigating IPNs. Those with sadomasochist tendancies who are interested in reading more about the nomenclature system can read about it in the references given in the reference page of Ye Olde IPN Shoppe.

How about taking a little quiz on the material on this page to see how well you learned it.

References

Sperling, L.H. An Overview of Interpenetrating Networks in Polymeric Materials Encyclopedia; Joseph C. Salamone, Ed. Vol 5; CRC Press: Boca Raton, FL, 1996.