Mauritz - Interpenetrating Networks Organic-Inorganic Interpenetrating Networks
An interpenetrating polymer network (IPN) is any material containing two polymers, each in network form. The three conditions for eligibility as an IPN are: (1) the two polymers are synthesized and/or crosslinked in the presence of the other, (2) the two polymers have similar kinetics, and (3) the two polymers are not dramatically phase separated. 1 Of course, these are loosely held guidelines. IPNs that have only one polymer crosslinked (where the polymers are synthesized separately) or where the polymers have vastly different kinetics are still considered to be IPNs. IPNs are distinguishable from blends, block copolymers, and graft copolymers in two ways: (1) an IPN swells but does not dissolve in solvents, and (2) creep and flow are suppressed 1.
Several kinds of IPN architectures exist (See Figure 1). These systems differ mainly because of the number and types of crosslinks that exist in the system. A non-covalent semi-IPN is one in which only one of the polymer systems is crosslinked. A
Figure 1: IPN Materials
non-covalent full IPN is one in which the two separate polymers are independently crosslinked. A covalent semi-IPN contains two separate polymer systems that are crosslinked to form a single polymer network. This covalent semi-IPN is similar to a non-covalent IPN because one of the polymer systems can be crosslinked without networking with the second linear system. However, the two systems tend to be networked for better property development. These covalent semi-IPNs are developed with organic-inorganic composite materials.
Non-Covalent IPN Materials
A wide variety of organic-inorganic non-covalent IPN materials have been formulated in an effort to improve material properties. Inorganic incorporation into polymers ranging from polyacrylates and polyesters to polyimides and nylons have been attempted.x The problem with the non-covalent systems, which can also be a problem with the covalent systems, is the lack of a effective interface. This problem could stem from several factors including surface energy phenomena and lack of molecular interactions between phases. Figure 1 in Organic-Inorganic Associations shows several polymers that can interact with the inorganic phase. These polymers are proposed to hydrogen bond with the inorganic phase, creating an interface between the two materials (See Figure 2). However, the key to having non-covalent organic-inorganic materials is not only utilizing a polymer that can have hydrogen bonding between the two phases but also to have low loading of the inorganic phase. Low loading of the inorganic phase will result in an increase in the overall material properties without sacrificing the interfacial bonding. 
Figure 2: Non-Covalent IPN Materials Covalent IPN Materials
A variety of polymers and copolymers have been synthesized incorporating reactive silicon alkoxides along the backbone of the polymer (See Figure 3). Silicon alkoxide incorporation into the polymer backbone can be accomplished with many monomers and through various synthetic means (See Figure 4). For example, silicon alkoxides can be incorporated into a polymer backbone via free-radical polymerization through a vinyl moiety, via a condensation reaction with an organic moiety on the silicon alkoxide monomer, or via a post-reaction (such as a hydrosilylation reaction). Through covalent attachment of the reactive silicon alkoxides, polymer-polymer interfacial
Figure 3: Covalent IPN Materials problems were hypothesized to be reduced. However, covalent IPN materials can have similar problems with the interface as the non-covalent materials. Again, similar to the non-covalent systems, a general lack of cohesiveness between the two phases can exist at molecular weight loadings higher than 10%. This problem with the gross phase
Figure 4: Some Monomers Utilized in Covalent IPN Materials separation at the interface is under investigation by researchers. Utilization of a variety of intermolecular bonding forces seems to improve upon the overall separation problems of the material. For example, by utilizing a polymer with a covalently bound silicate material that can also hydrogen bond with the organic polymer backbone creates more opportunities for better interfacial interactions.
References:
1. Sperling, L. H. "Interpenetrating Polymer Networks and Related Materials", Plenum Press, 1981, Chpt. 1. Written by: Sandra Young (Partially From Her Research Prospectus)
Questions or comments about this page can be mailed to Dr. Ken Mauritz at Kenneth.Mauritz@usm.edu