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INTERPENETRATING POLYMER NETWORKS Introduction An interpenetrating polymer network, IPN, is defined as a combination of two polymers in network form, at least one of which is synthesized and/or cross-linked in the immediate presence of the other (1). An IPN is distinguished from other multipolymer combinations such as Polymer Blends (qv), blocks, and grafts in two ways: (1) An IPN swells, but does not dissolve in solvents; and (2) creep and flow are suppressed (see BLOCK COPOLYMERS; GRAFT COPOLYMERS). The above represents the classical definition of an IPN. The term interpenetrating polymer network was coined before the extent or consequences of phase separation were fully realized. This article covers sequential and simultaneous types of IPNs made in bulk and also includes such materials as IPNs based on latexes and suspension-sized particles; thermoplastic IPNs, which contain physical cross-links in one or both polymers, and hence may be (partly) soluble; and a number of other closely related materials. Some applications of IPNs of note include sound and vibration damping, biomedical applications, natural products and renewable resources, tough and impact-resistant materials, etc. IPN review articles include those by Sperling and Hu (2), Lipatov (3), and Athawale and co-workers (4). There are also papers of note by Srivastava and co-workers (5), and edited books by Klempner and coworkers (6), and Kim and Sperling (7). Two older books review early references, and develop the concepts of IPNs (1,8). Nomenclature. The IPNs have a well-developed nomenclature (9), as do most multicomponent polymeric materials (10). Thus, a cross-linked polymer network is indicated by the prefix net-, and an IPN is indicated by the connective -ipn-. Thus, an IPN of polybutadiene and polystyrene is written as Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

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(net-polybutadiene)-ipn-(net-polystyrene) and if copolymers and/or two-worded mer structures are needed, brackets are employed: {net-poly(styrene-stat-butadiene)}-ipn-{net-poly(ethyl acrylate)}

Synthesis and Structure of IPN and SIN Materials Figure 1 models the structure of interpenetrating polymer networks and related materials. Solid and dotted lines indicate polymer 1 and polymer 2, respectively. The dots indicate cross-links. Structure a indicates a polymer blend, where the two polymers are mixed, but not bonded together anywhere. Structure b indicates a graft copolymer (qv), where polymer 2 is bonded along the side of polymer 1. Structure c indicates a block copolymer (qv), where polymers 1 and 2 are bonded end-on-end. These three structures are all thermoplastic, in that, not being cross-linked, they flow if heated above their glass transition and/or melting temperatures. Structure d illustrates an AB–cross-linked copolymer. In this case, polymers 1 and 2 are bonded to each other. Structure e indicates an IPN, sometimes called a

Fig. 1. Six basic combinations of two polymers, represented by solid and dashed lines, respectively, and the dots representing covalent bonds. (a) A polymer blend, with no bonding between the chains; (b) a graft copolymer, with the dashed polymer bonded to the side of the solid line polymer; (c) a block copolymer, where the chains are bonded end-on-end; (d) an AB-cross-linked copolymer, where one polymer is bonded to the other, but not to itself; (e) an interpenetrating polymer network of two cross-linked polymers; (f) a semi-interpenetrating polymer network, where only one of the polymers is cross-linked.

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full IPN, where both polymers are cross-linked. Structure f illustrates a semi-IPN, where one polymer, but not both, are cross-linked. These last three structures are all thermoset, because at least one polymer network pervades the entire material. Thus, they will not flow on heating. Of course, one may imagine many related structures and/or more complex topologies. By simple analogy, three or more polymers may coexist in any one of the topologies of Figure 1. Some quite different materials include the polyrotaxanes, where rings encircle a polymer chain (11); catenanes (12), where a series of rings form a chain (13); and dendrimers. These latter are hyperbranched, fractallike structures emanating from a central core and containing a large number of terminal groups (14) (see DENDRONIZED POLYMERS). Noting that the polyrotaxanes, catenanes, and dendrimers are usually one-phase polymers, their behavior is quite different from that of any of the structures shown in Figure 1. Figure 2 (15) outlines the two basic methods of IPN synthesis. In Figure 2, the boxes indicate monomers, the X’s indicate cross-links, and the subscripts indicate whether it goes to make up polymer 1 or polymer 2. As in Figure 1, the lines indicate the polymers and the dots indicate cross-links. The P1 and P2 above the arrows indicate polymerization of polymer 1 and polymer 2, respectively. The general procedure to synthesize a sequential IPN calls for mixing monomer 1 and cross-linker 1 and polymerizing to form network 1. Then, monomer 2 and cross-linker 2 are swollen in, and polymerized in situ. A simultaneous interpenetrating network, SIN, as the name suggests, starts with a mix of both monomers and both polymers. These are polymerized simultaneously, but by different routes, such as chain and step polymerization. Thus, while reacting, the two polymers do not interfere with each other, but merely dilute

Fig. 2. Two basic synthetic methods of preparing IPNs, sequentially and simultaneously. (a) Sequential IPNs; (b) simultaneous interpenetrating network (SIN).

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each other. Of course, the rates of the two reactions need not be equal. As explored below, if one polymer or the other reacts faster, different morphologies may result, with concomitant different physical and mechanical behavior patterns. Besides sequential and simultaneous IPN synthetic methods, there are several other methods of preparation: Latex IPNs. The polymers are made via emulsion polymerization, each particle constituting a micro-IPN. Frequently, one monomer and cross-linker are polymerized, followed by addition of a second monomer and cross-linker, which is also polymerized. Depending on the rates of monomer addition relative to the rates of polymerization, various degrees of interpenetrating and/or core-shell morphologies may result. Gradient IPNs. The overall composition and/or cross-link density of the material varies from location to location on the macroscopic scale. One way of preparing these materials involves partial swelling of the first polymer network by the second monomer mix, followed by rapid polymerization before diffusional equilibrium takes place. Thermoplastic IPNs. These materials utilize physical cross-links rather than chemical cross-links. Thus, the materials may be made to flow at elevated temperatures. As such, they are hybrids between polymer blends and IPNs. Such cross-links may involve block copolymers, ionomers, and/or semicrystallinity. Semi-IPN. As illustrated in Figure 1f, a semi-IPN is composed of a cross-linked polymer and a linear polymer. If one is making a sequential synthesis, the terminology semi-I and semi-II IPNs refers to having the first or second polymer cross-linked, respectively. Homo-IPNs. These materials consist of two networks prepared from substantially the same cross-linked polymer.

Special Considerations in IPN and SIN Synthesis. Most of the IPNs and SINs made to date rely on chain and/or step polymerization, although other methods, such as anionic polymerization, have been occasionally employed. Of course, appropriate cross-linkers, initiators, and so forth must be added. For very many IPN and SIN syntheses, except for phase separation aspects, the interaction between the two materials during polymerization has often been ignored. However, to greater or lesser extents in real systems, of course, there is some interaction. A few examples are helpful: (1) In sequential IPNs, if polymer I is based on a diene, and monomer II (such as styrene) is polymerized via free-radical polymerization, some grafting will occur. Of course, this grafting is important for interfacial bonding in such related materials as high impact polystyrene (HIPS) and acrylonitrile– butadiene–styrene (ABS) materials. (2) If polymer I is an acrylic, the labile hydrogen may be abstracted, providing another type of grafting site.

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(3) In a SIN polymerization, the high viscosity of one of the polymers, or its lack of high viscosity as a slow-polymerizing component, may alter the phase morphology or other features. (4) Vitrification of one of the polymers may slow or stop the polymerization of the other component. Some specific investigations of the chemical interactions include the following: Tabka and Widmaier (16,17) undertook the study of the interactions of azobis(isobutyronitrile) (AIBN), free-radical initiators with stannous and stannic organotin polyurethane catalysts. These workers found a synergistic effect of stannous octoate on the AIBN on the radical polymerization (qv) of methyl methacrylate, increasing the reaction rate and shortening the gelation time. While there was a severe deactivation of the tin(II) atom by the AIBN, there was no measurable effect using organotin(IV) stannic catalysts. Michal’chuk and co-workers (18) investigated the changes in mobility during formation on the basis of epoxy and allylic oligomers. Using pulse NMR spectroscopy, these workers concluded that SIN formation results in mutual hindrances to the full conversion of the reactive groups, and a concomitant increase in the number of defects in the IPNs. This results in greater molecular mobility in the final materials than might otherwise be expected. Polymer Blend and IPN Classification Scheme. Of course, all of the various compositions of matter composed of two polymers are related to each other. Figure 3 (19) illustrates how the major kinds of polymeric materials based on two kinds of mers are related to each other. The IPNs are shown here under the occasional grafts heading, because many of the preparations have a few grafts between the two polymers as a consequence of free-radical chemistry, etc. Since one network of these materials is always polymerized in the presence of the other

Fig. 3. A polymer blend classification scheme.

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network, some grafting is hard to avoid, and in many cases is desired. However, an IPN (or SIN) can be distinguished from a graft copolymer (qv) because the morphology (qv) and mechanical behavior are primarily controlled by the crosslinks rather than the grafts.

IPN Morphology and Physical Behavior Most compositions of matter made up of two polymers (see Fig. 1) exhibit some degree of phase separation. This is caused by the very small entropy of mixing two high molecular weight materials, and the very frequent positive heat of mixing of two different chemical structures. In simple polymer blends (qv), the mixing tends to be relatively coarse, although many routes to complex and relatively finely divided morphologies have been developed. Block copolymers (qv) domain sizes are largely controlled by the molecular weight of the individual blocks, and the domain shape by the relative proportions of the two blocks (20,21). Electron Microscopy Studies. In sequential IPNs and semi-IPNs, the size and shape of the polymer II domains are controlled by the cross-link density of polymer I and the relative proportions of the two polymers. Figure 4 illustrates

Fig. 4. Transmission electron microscopy of OsO4 -stained polybutadiene and polystyrene (white) combinations.

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the two-phased morphology of IPNs, semi-IPNs, and polymer blends prepared from styrene-butadiene copolymers (qv) (SBR) and polystyrene (22) (see STYRENE POLYMERS). The double bonds in the butadiene polymers (qv) are stained dark with osmium tetroxide. In this series of transmission electron micrographs, the samples are sliced to about 60 nm thickness, sometimes called salami slices. The upper left photo in Figure 4 is of HIPS, a material known for its toughness (23). This material is a type of polymer blend made by dissolving polybutadiene or SBR in styrene monomer, and polymerizing with stirring. The SBR is first soluble in the styrene monomer, but phase separates after a few percent conversion. Part of HIPS toughness comes from having grafts between the polybutadiene and the polystyrene, and part of the toughness comes from a phase-within-a-phase-withina-phase morphology (24). Shortly after polymerization is initiated, the growing quantity of polystyrene makes the SBR insoluble in the mix. At first, the SBR phase is the continuous phase. On continued polymerization, the growing volume and increasing viscosity of the polystyrene-rich phase causes a phase inversion, helped by the stirring. Of course, the styrene within the polybutadiene droplets is also polymerizing, but cannot get out. It forms a very fine structure, also visible in the upper left photo of Figure 4. HIPS gets its toughness in part from this fine structure, and in part from the grafting that takes place between the SBR and the polystyrene, increasing the interfacial bond strength. (Commercially, block copolymers of polybutadiene and polystyrene are often added to further increase the interfacial bond strength.) The upper right photo has substantially the same composition as that in the upper left, but was made without stirring. This material was first made in 1927 (25). It did not undergo phase inversion, and hence the SBR remains the continuous phase. This material is adhesive in nature, but not commercial. The corresponding sequential IPN morphology of the same overall composition is shown in the lower left photo. In this case, a sheet of SBR was crosslinked, followed by swelling in styrene plus divinyl benzene and polymerizing in situ. The phase domains are of the order of 100 nm, with both phases exhibiting co-continuity. An increased cross-link level for the SBR results in a finer phase domain structure, as shown in the lower right of Figure 4. The two semi-IPNs are shown in the middle left and right photos of Figure 4. The semi-I IPN resembles the full IPN, but the morphology is coarser. The semi-II IPN resembles the upper right morphology. Of significant interest here was that the HIPS exhibited a toughness value of about 80 J/m (1.5 ft·lb/in.) of notch, while the middle left had about 160 J/m (3.0 ft·lb/in.), and the lower left had about 267 J/m (5.0 ft·lb/in.) of notch. The development of the morphology in both the HIPS-type material and the IPNs requires several steps (see Fig. 5) (26). The original polymer I swollen with monomer II solution first has one phase. After a few percent of polymerization, it phase separates via nucleation and growth kinetics. This tends to produce spheroidal-shaped particles, the white material in Figure 5a. (Super-saturated salt solutions often follow nucleation and growth phase separation kinetics.) On further polymerization, the phase separation kinetics follows spinodal decomposition, leading to interconnected cylinders (Fig. 5b) (27–29). Close examination of both the HIPS and IPN morphologies strongly suggests that spinodal

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Fig. 5. Transmission electron microscopy of the polymerization of substantially the same composition as shown in the lower left portion of Figure 4.

decomposition plays a strong role in the final morphology of these materials. (Note that a thin section of a cylinder cut at an angle looks like an ellipsoid.) Figures 5c, 5d, and 5e have something of an appearance of a snowstorm, because the continued cross-linking limits phase separation. Thus, as illustrated in Figure 6, the initial mix first has one phase, then undergoes a region of nucleation and growth. Then, the composition enters the spinodal decomposition region, arriving at the final morphology. Note that if phase separation is more or less complete, the final composition will be a mix of nearly pure polymer I and polymer II, with the final composition being an average. Phase diagrams of IPNs have also been researched. Sophia and co-workers (30) found that the phase diagram of a semi-IPN indicates a lower critical solution temperature for these materials (Fig. 7), similar to that of polymer blends. Seemingly strange, these materials are more mutually soluble at lower temperatures than at higher temperatures, a consequence of their low entropy of mixing.

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Fig. 6. A generalized phase diagram for the polymerization shown in Figure 5. OP = One phase, NG = nucleation and growth, and SD = spinodal decomposition.

Fig. 7. A phase diagram of a polyurethane–poly(vinyl chloride) semi-IPN, showing a lower critical solution temperature.

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Fig. 8. A comparison of two possible morphologies of IPNs, based on (a) {poly(ethyl acrylate)}-ipn-{poly(methyl methacrylate)} and (b) {poly(ethyl acrylate)}-ipn-polystyrene.

The relative miscibility (qv) of the two polymers in IPNs is controlled not only by the thermodynamics of mixing, but to some extend also by the cross-link density and exact method of preparation. Huelck and co-workers (31) learned that while IPNs of poly(ethyl acrylate) and polystyrene exhibited two distinct glass transitions (qv) and two observable phases via transmission electron microscopy, the corresponding {net-poly(ethyl acrylate)}-ipn-{net-poly(methyl methacrylate)} compositions, being isomeric, had a microheterogeneous morphology (see Fig. 8). Ma and Tang (32) showed that increasing cross-link density of SINs of poly(nbutyl acrylate) and epoxy 10:90 compositions tends to increase molecular interpenetration, and thus entanglement between the two networks. ´ Glass-Transition Behavior. Sanchez and co-workers (33) studied the system {net-poly(methyl acrylate)}-ipn-{net-poly(methyl methacrylate)}, which is not chemically isomeric, but close to being so. They varied the cross-linking level of both polymers of these sequential IPNs. When the poly(methyl acrylate) was crosslinked with 10% of ethylene glycol dimethacrylate (EGDMA), dynamic mechanical studies on midrange compositions showed that the system had a single glass transition, showing it to be substantially miscible (see Fig. 9) (33). The poly(methyl acrylate) network was polymerized first with 0.1% (open circles), 0.5% (squares), and 10% (triangles). In each case, the PMMA network contained 10% EGDMA. It must be noted that this apparent miscibility is only mechanical in nature, ´ a product of the high cross-link level of a sequential IPN. Sanchez and co-workers (33) note that the key factor controlling the apparent miscibility in sequential IPNs is the cross-linking density of the network polymerized first. ˜ and co-workers (34) found a similar effect with the In related work, Duenas system poly(butyl acrylate) and poly(butyl methacrylate). At 10% cross-linking level, the forced miscibility was sufficient to allow the glass transitions of the IPNs to follow the Fox equation (35).

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Fig. 9. Sequential IPNs of poly(methyl acrylate) and poly(methyl methacrylate) as a function of cross-linking level with EGDMA.

In block copolymers composed of random-coiled polymers, the molecular weight of the blocks and their relative proportions controls the domain size and shape (36,37). A somewhat similar relationship exists in IPN. Domain Size Theory. Assuming spherical domains, Yeo and co-workers (38) derived equations for the domain size in sequential IPNs. The domain diameter of polymer II, DII , was related to the interfacial tension γ , the absolute temperature T times the gas constant R, and the concentration of effective network chains, cI and cII , occupying volume fractions vI and vII , respectively: 4γ RT(AcI + BcII )

(1)

 1
1/3 4/3 3vI − 3vI − vI ln vI 2vII

(1a)

 1
2/3 ln vII − 3vII + 3 2

(1b)

DII = where A= and B=

The quantity γ is experimentally determined from the work to create a unit interface. For many polymer/polymer interfaces, the value is in the range of 20–60 J/µm2 (39). Although equation 1 was derived for spheres, it yields a reasonable estimation of the cylindrical diameter of spinodally phase-separated IPNs.

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Phase Continuity in Sequential IPNs. The question has been repeatedly raised: Do IPN morphologies really exhibit dual phase continuity? Evidence for dual phase continuity in sequential IPNs was examined by Widmaier and Sperling (40,41). A series of sequential IPNs were prepared from poly(n-butyl acrylate) and polystyrene. Two cross-linkers were used, divinyl benzene (DVB), which forms ordinary covalent cross-links, and acrylic anhydride (AA), which forms labile crosslinks. The AA cross-links were cut by soaking the samples in a 10% aqueous ammonium hydroxide solution for about 12 hours. Some of the samples had the AA cross-links in the acrylic component and some of them had it in the styrenic component. After de–cross-linking one component or the other, the materials were subjected to a Soxhlet extractor. The remaining materials were examined by scanning electron microscopy. The density of the remaining material was also determined. For both types, nearly the whole amount of de–cross-linked polymer could be extraced, indicating a low level of chemical grafting between the two polymers, and/or little entrapped material. When polymer II (polystyrene) was de–crosslinked and extracted, one still recovered polymer I (the acrylic) without substantial damage as shown by its density, equilibrium swelling degree, and scanning electron micrographs. When polymer network I was de–cross-linked and extracted, the remaining network II presented a porous structure as indicated by reduced density and scanning electron microscopy. The void regions, corresponding to the location of the poly(n-butyl acrylate) before extraction, appeared interconnected. However, the extent of continuity of polymer II depended on its concentration. For midrange compositions, there was good evidence for dual phase continuity. However, when the amount of polymer II was decreased below 30%, the sample crumbled by itself, indicating a macroscopically discontinuous second phase. At a microscopic level, though, some degree of connectivity could still be detected. In summary, greater than 20–30% of polymer network II, its phase domain structure was continuous. Throughout the composition range studied, polymer network I was continuous. The foregoing finding was important especially in the light of transmission electron micrograph studies, resulting in thin salami slices of osmium tetroxide stained samples. The observation of polymer network II could be interpreted as spherical domains in many such cases, but suggestive of ellipsoidal structures in others. The above discussion leads to the conclusion that the spherical and/or ellipsoidal structures, being two-dimensional, are better interpreted as interconnecting structures containing spheroidal and other morphologies (26,42). The modulus evidence for dual phase continuity will be presented below. Modulus vs Composition. The modulus of IPNs as a function of composition yields information about their relative phase continuity. The first such study on polyacrylate/polyurethane latex interpenetrating elastomeric networks (IENs) was carried out by Matsuo and co-workers (Fig. 7) (43). Note that in the midrange of composition, the modulus begins to follow the parallel Takayanagi model (44). This suggests that the stiffer acrylic phase is becoming a continuous phase, as predicted from percolation models of three-dimensional mosaics of compressed spheroidal structures. Yeo and co-workers (Fig. 10) (45) compared several models of phase continuity with the behavior of {net-poly(butyl acrylate)}-ipn-(net-polystyrene). The

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Fig. 10. Sequential IPNs of poly(butyl acrylate) and polystyrene as a function of composition. A: Upper–Lower Bound Model; B: Hashin–Shtrikman Model; C: Davies Model; D: Budiansky Model; and E: Coran–Patel Model. dyne/cm2 = 0.1 Pa.

S-shaped curve has the greatest slope near the 50/50 composition. The Budiansky model (46,47) was the only model to follow the S-shaped curve. Ramis and co-workers (48) made a similar study of SINs prepared from unsaturated polyester–styrene mixes with MDI-poly(propylene glycol). When the storage modulus, E , was plotted through a range of compositions, the Budiansky equation (47) was again found to fit better than several other relationships. Phase Separation and Gelation Studies. A phase diagram of linear poly(ethylene oxide) (LPEO) and divinyl benzene (DVB) cross-linked poly(methyl methacrylate) as a function of polymerization of the latter was also researched by duPrez and co-workers (49) (Fig. 11). The temperature of measurement was also a variable, but the curve separating the one-phased system from the two-phased system did not change much in the narrow temperature range covered. A time–temperature-transformation (TTT) diagram (50,51) study on a poly(ether sulfone)-ipn-net-epoxy semi-II IPN was carried out by Kim and coworkers (Fig. 12) (52). Note the several regions delineated by the progressive polymerization and cross-linking of the epoxy. Several steps in the transformation during polymerization were identified: (1) (2) (3) (4) (5)

Onset of phase separation Gelation of the epoxy Fixation of the phase-separated morphology End of phase separation Vitrification (glass forming) of the epoxy phase.

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Fig. 11. A ternary phase diagram of the semi-IPN based on methyl methacrylate, poly(methyl methacrylate), and linear poly(ethylene oxide).

These workers also found that the lower critical solution temperature for the material was at 265◦ C. The development of SINs requires three steps: gelation of polymer I, gelation of polymer 2, and phase separation of 1 from 2. Since these three events may occur

Fig. 12. A time–temperature-transformation (TTT) cure diagram for the system poly(ether sulfone)-ipn-epoxy.

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in any order depending on concentration, cross-linker level, initiator, temperature, etc, there are 3! = 6 different morphologies and physical behavior patterns that may be observed. This led to the development of a series of metastable phase diagrams for (net-polyurethane)-ipn-{poly(methyl methacrylate)} SINs (see Figs. 13 and 14) (53). The term metastable derived from the realization that none of the IPNs or SINs are truly at thermodynamic equilibrium relative to phase separation. Two figures were utilized to illustrate the phenomena more clearly. Since both monomers and both polymers needed to be represented, a tetrahedron structure was utilized, where the four pure components occupy the four vertices. The four triangular faces of both figures represent the ternary systems delineated by the compositions indicated in the vertices. Since the methyl methacrylate mix contained 0.5% tetraethylene glycol dimethacrylate as a cross-linker, it gelled after about 8% conversion, indicated by the G1 -U-PU plane in Figure 13. The phase separation curve for the ternary system MMA-PMMA-“U” on polymerization of only the methyl methacrylate is indicated by the points C-D-A-E. In a similar fashion, the phase separation curve for the MMA-PMMA-PU system, rear triangle in Figure 13, is shown by the points J-B-K-L. The entire tetrahedron volume is divided into two regions: one phase-separated and the other single-phased, separated by C-D-A-E-L-K-B-J. Tracing the sail-like characteristics of this surface aids in following the separation of the two regions of space. The curvilinear line A-B indicates the intersection of the PMMA gelation plane with that of the phase

Fig. 13. A tetrahedron metastable phase diagram for the SIN polymerization of poly(methyl methacrylate) and polyurethane.

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Fig. 14. Continuation of Figure 13, illustrating the gelation planes of both the poly(methyl methacrylate), G1 and polyurethane, G2 .

separation sail-like surface. It represents the critical line along which there is simultaneous gelation of the PMMA and phase separation of the PU from that of the PMMA. Thus, reactions moving to the left of this curve will have the PMMA gel before phase separation, while reactions to the right of A-B phase separation before gelation. Figure 14 illustrates the gelation plane of the polyurethane, G2 -MMAPMMA, as well as the G1 gelation plane of the PMMA (also shown in Figure 13). The intersection of the two planes, G1 -G2 , indicates the line of simultaneous gelation of both polymers. The line G1 -G2 also intersects the line A-B of Figure 13, not shown. This intersection represent a triple critical point. Here, both polymers simultaneously gel and phase-separate. This defines an ideal SIN synthesis. All SIN and IPN polymerizations must begin somewhere along the edge of the line MMA-“U”, and end along the edge line PMMA-PU. However, the lines need not be straight or even single lines. When phase separation ensues, the line divides into two portions, ending at points J and L of Figure 13. Since the polymerization rates can be controlled, and in general will not be equal, and the cross-linker levels independently control the gelation planes, all six possibilities of SIN morphologies can be envisioned. Also, as an extreme, a sequential IPN synthesis can also be traced on these figures. The semi-IPNs will clearly lack one of the gelation planes. A polymer blend polymerization will have only the phase separation curvilinear plane, without either gelation plane. Thus, Figures 13 and 14 provide a kind of map to determine how and why different morphologies and concomitant physical and mechanical behavior patterns emerge.

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Of course, these maps serve as a guide to bring about specific morphologies and concomitant behavior profiles of the IPNs. Following the work of Mishra and co-workers (53), Dean and co-workers (54,55) investigated the SIN syntheses of dimethacrylate- and epoxy-based materials by near-infrared and other methods. The rates and order of the polymerizations were systematically altered. The final conversion of the dimethacrylate was limited by the vitrification or topological restraint of the SIN. Whether one or two glass transitions are observed depends on the relative rates and order of the polymerizations. Of course, the relative reaction rates of the two polymers in SINs may be controlled by the quantity of cross-linker, temperature, initiator level, concentration of the two monomers, level of light intensity, time and completeness of phase separation, etc. The above assumes that there is no interference between the two reactions, except for the mutual dilution effect.

Thermoplastic IPNs Thermoplastic IPNs are hybrids between the polymer blends and the IPNs. Often, physical cross-links are used to bond the chains in one or both polymers. In other cases, one of the phases actually has chemical cross-links, but it constitutes a discontinuous phase so as to allow flow. In this latter case, the discontinuous case is often cylindrical in shape, highly elongated. The most important commercial thermoplastic elastomers are based on polypropylene (PP) and ethylene–propylene–diene monomer (EPDM) elastomer (see ETHYLENE-PROPYLENE ELASTOMERS). The diene portion of the elastomer is cross-linked, frequently by dynamic vulcanization, ie, chemical cross-linking during melt shearing. The PP/EPDM thermoplastic elastomer was modeled by Kresge (56) to show the morphology of the material after crystallization (see Fig. 15). To allow for extraction of the elastomer, he used EPM, lacking in chemical cross-linking. The PP clearly exhibits phase continuity, resembling an open-celled sponge with micronsized porosity. Careful examination of the micrograph indicated that the voids were also continuous. (Otherwise, the EPM could not have been extracted.) Therefore, the material formed an interpenetrating two-phased structure. However, it is probable that the increase in viscosity of the EPDM phase, brought about by cross-linking, would reduce the continuity of the elastomer phase. Coran and Patel (57) point out several examples of the improvements achieved: (1) (2) (3) (4) (5) (6)

Reduced permanent set Greater ultimate mechanical strength Greater fatigue resistance Improved fluid resistance to hot oils Greater high-temperature stability Greater stability of the phase morphology in the melt; the phase domain size does not increase.

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Fig. 15. A scanning electron micrograph of a model thermoplastic IPN based on a binary (70/30) blend of EPM and polypropylene. The EPM phase was removed, then the sample fractured under liquid nitrogen to illustrate the dual phase continuity of the material.

(7) Greater green strength, ie, greater melt strength (8) More reliable fabricability The PP/EPDM materials are most famous for being tough, energy-absorbing materials, which led them to be used as the plastic component of automotive bumpers. Davison and Gergen prepared thermoplastic IPNs based on block copolymers and semicrystalline polymers (see Table 1) (58). Here, a hydrogenated version of the styrene–butadiene–styrene triblock copolymer was utilized, ie, SEBS, where the EB stands for ethylene-butylene (saturated 1,2-polymerized butadiene mers). Table 1. Composition of Co-Continuous Interlocking Network Phases Composition by parts Component 25,000-100,000-25,000 SEBS Poly(butylene terephthalate) Shellflex 790 extending oil Tuffto 6050 oil Polypropylene Irganox 1010 antioxidant Dilaurothiodipropionate antioxidant TiO2

A

B

100 100 100 – 10 0.2 0.5 5

100 70 – 50 10 0.2 0.5 5

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Fig. 16. Various morphologies illustrating the development of dual-phase continuity. (a) Dispersed sphere, clearly discontinuous in space; (b) dispersed fibers or cylinders. Figures (c) and (d) together illustrate dual-phase continuity, where both phases are interlocked in a kind of three-dimensional puzzle.

By blending these polymers, taking care to match melt viscosities and volume fractions (see eq. 2 below), dual-phase continuity can be achieved (see Fig. 16) (59). In Figure 16a, droplets of material are dispersed in a continuous matrix, a morphology observed in rubber-toughened plastics. Figure 16b illustrates dispersed fibers, observed in liquid crystal polymers and some thermoplastic elastomers. If the fibers or cylinders are infinitely long, they exhibit one-dimensional phase continuity. Figures 16c and 16d constitute the important figures that model the materials in Table 1. These two figures together depict a co-continuous morphology, where the two phases are interlocked or interpenetrating. Note that three polymers are presented in Table 1. The major components are the SEBS and the poly(butylene terephthalate). The third polymer is the polypropylene, used as a compatibilizer. It appears at the interfaces between Figures 16c and 16d. Thus, three co-continuous phases are formed in this material! These materials are useful as under-the-hood electrical insulators because they have substantially constant modulus between the glass transition of the EB center block and the melting temperature of the poly(butylene terephthalate). The requirements for both dual phase continuity and phase inversion for polymer blends and thermoplastic IPNs alike require the specification of the melt viscosity and the volume fraction of each component. Some melt shearing is assumed. These concepts were formulated quantitatively by Paul and Barlow (60)

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and Jordhamo and co-workers (61): V1 η2 × =X V2 η1

(2)

where if X > 1, phase 1 is continuous X ≈ 1, dual phase continuity or phase inversion X < 1, phase 2 is continuous In equation 2, V represents the volume fraction and η represents the melt viscosity of phase 1 or 2, as per subscript. Equation 2 is true in the limiting case for zero shear. Utracki (62) proposed somewhat more complex relationships for finite shear rates. However, in many experimental and industrial situations, equation 2 provides a reasonable approximation, as reviewed by Jordhamo and co-workers (see Fig. 17) (61). Note especially the dual phase continuity region in Figure 17. If the shearing action is stopped while the composition is within this region, it may remain dual phase continuous. Here, the phase continuity of a number of materials was examined and plotted. It must be noted that dual-phase continuity and/or phase inversion took place over about a factor of 4 in the data. Thus, some aspect of dual phase continuity exists via equation 2 as X varies from about 0.5 to about 2. Thermoplastic IPN technology was also explored by Han and co-workers (63). In this case, thermoplastic IPNs based on polypropylene (PP) and EPDM were prepared using supercritical solution technology. Propane was used as the supercritical fluid. The synthesis steps were as follows:

Fig. 17. Phase continuity and inversion diagram for polymer blends and IPNs during polymerization and under shear.

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(1) PP and EPDM were dissolved in propane; a homogeneous solution was formed. (2) t-Butyl peroxide was injected. (3) EPDM was cross-linked for one hour at 175◦ C and 65 MPa. (4) The polymers could be precipitated by two independent methods: a. Reducing the pressure below the cloud point, followed by lowering the temperature; or b. Isobaric cooling to ambient temperature, then reducing the pressure. (5) Residual solvent was vented. (6) Dried thermoplastic IPN was recovered. The morphologies using the 4a route were fibrous in nature (64); those utilizing route 4b resulted in a porous spheroidal structure for the PP phase (65). In all cases, the morphology was significantly finer than that available with the then commercial routes of preparing PP/EPDM thermoplastic IPNs. Siegfried and co-workers (66,67) investigated thermoplastic IPNs based on SEBS triblock copolymers with a polystyrene ionomer. Two subclasses of thermoplastic IPNs were identified: those prepared by a sequential polymerization method, and those prepared by mechanically blending separately synthesized polymers. In each case, a polystyrene acid sulfate was neutralized dynamically in the melt with a concentrated solution of sodium hydroxide. The water steamed off rapidly under the reaction conditions. The first subclass resulted in lower melt viscosities, but more nearly equal dual-phase continuity was achieved with the second subclass. The subject of thermoplastic IPNs was recently reviewed by Sperling (68).

Latex IPNs Latex IPNs, by definition, have their origin in emulsion polymerization. Several types of latex IPNs exist. If one blends two kinds of latex particles, followed by film formation and cross-linking of both polymers, the material is called an interpenetrating elastomeric network, IEN. Usually, IENs form a three-dimensional mosaic structure (69,70). Latex IPNs are often made by sequential polymerization of two (or more) cross-linked polymers utilizing emulsion polymerization (71). For core-shell latex IPN synthesis, first a cross-linked seed latex of polymer 1 is synthesized (2). Then, a second monomer and cross-linker are added to the system, usually with no added surfactant. Often, a starved polymerization route is employed, ie, the rate of polymerization equals or exceeds the rate of monomer addition. This reduces the swelling of the seed latex by the monomer 2 mix, producing a two-layer latex having a spherical core, and an overlaying shell. Obviously, multiple shells can be added for different purposes. Some possible morphologies of latex IPNs having three polymers are illustrated in Figure 18 (72). Note that in Figure 18a, the monomer B mix first dissolves

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Fig. 18. Some possible latex IPN particle morphologies. (a) IPN core/shell particles; (b) separated core/shell particles; and (c) multilayered particles.

in polymer A, then polymerizes and phase separates to form internal domains. Then, polymer network C forms a shell around the A-B core. If the glass transitions of the three polymers are sufficiently different, and the three polymers are phase-separated, then one obtains three glass transitions, as illustrated in Figure 19 (73). In this case, the three polymers were obtained by blending two latexes having different cores but the same shell, as in Figure 18b. G and G represent the dynamic shear modulus. Both latexes have a core of poly(butadiene-stat-styrene), with some of the latexes having a SAN shell and some having a shell of poly(ethylhexyl methacrylate-stat-styrene). In both cases, the polymerizations were carried out under starved conditions. Silverstein and co-workers (74), Silverstein and Narkis (75), and Nemirovski and Narkis (76) studied the morphology and rheological behavior of latex IPNs and related materials (see Fig. 20) (75). After water evaporation, some degree of phase continuity of individual phase domains is achieved in this case. All of these compositions were based on polyacrylates and polystyrene. After polymerizing and drying, the latex particles do not interdiffuse to form a monolithic solid because of the cross-links in each latex particle. However, being moderately soft, they flow together as modeled in Figure 20 (75). The solid black spheres represent polystyrene domains, and the cross-hatched material represents an acrylic

Fig. 19. The dynamic mechanical behavior of a mix of two latexes or the morphologies represented by Figure 18b.

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Fig. 20. Model of a latex IPN.

network. The rheological behavior of these materials resembles the flow behavior that might be expected for a room full of underinflated beach balls.

Sound and Vibration Damping Behavior Sound and vibration damping are important throughout the world today for the automotive industry, appliances, aircraft, tall buildings, submarine technology, etc. However, the frequencies of the sound or vibration differ greatly, from a fraction of a Hertz for tall buildings to 103 –104 Hz for cars and appliances. This is important, because the WLF equation shows that the glass transition increases some 6–7◦ C with each decade of frequency increase (77). One of the most important methods of damping involves polymeric materials in or near their glass transitions temperatures where the loss tangent tan δ and/or the loss modulus E are high. Many IPNs exhibit a microheterogeneous morphology. When the domains are of the order of 10–20 nm, the whole system becomes substantially all interphase material, ie, it is extensively but incompletely mixed, with different microregions having different compositions (see Fig. 8) (31). In the transmission electron micrograph of Figure 8a, the microheterogeneous morphology is made from the isomeric pair, poly(ethyl acrylate) and poly(methyl methacrylate). When polystyrene is used as the second polymer, the cellular structure in Figure 8b emerges. Whereas an immiscible IPN pair will have two distinct glass transitions, the microheterogeneous morphology often results in one broad glass transition, covering the temperature range between both homopolymer glass transitions. Thus, more or less constant E or tan δ can be obtained.

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Fig. 21. Damping effectiveness as a function of temperature for several commercial materials compared to an IPN, composition A.

There are two basic engineering mechanisms for the use of damping polymers: (1) Extensional damping, where a polymer is placed as a free layer on the resonating vibrating system; and (2) Constrained layer damping, where a stiff material is placed on the top of the damping polymer to increase the shearing action. Sometimes a simple sandwich arrangement is used, where two thin layers of sheet metal (the bread) have a thin layer of polymer between them (the jelly). The damping of an IPN constrained layer system based on an acrylic/methacrylic IPN and an epoxy constraining layer is compared with several other commercial homopolymers and statistical copolymers in Figure 21 (78). Hitachi Chemical has a high temperature sound and vibration damping material based on vinyl–phenolic compositions (79).

Natural Products/Renewable Resource-Based IPNs Of increasing importance in modern society are polymeric materials that either originate from plants or animals, are naturally degradable, or both. One of the major approaches utilizes triglyceride oils with functionality other than double bonds. For example, most of the edible oils such as corn oil are triglycerides containing one or more double bonds in the side chains. Usually, the double bonds are structurally cis. An exception is tallow, which is a saturated oil. Castor Oil-Based IPNs. Those triglyceride oils with functionality other than double bonds such as hydroxyl or carbonyl groups are generally considered inedible, although they are sometimes used for medicinal purposes. Prime among

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these materials is castor oil, containing one hydroxyl group per side chain (see eq. 3):

(3) In medicinal applications, the hydroxyl group irritates the stomach lining. Chemically, the hydroxyl group can react to form urethanes or esters, and hence form a polymer network. Polyurethane foam rubber frequently contains large amounts of reacted castor oil, for example. The glass transition of castor oil polyesters are of the order of −50◦ C (80). If styrene (or other vinyl monomer) plus cross-linker is swollen in and polymerized, an IPN is formed. The products tend to be tough, leathery materials. Research on castor oil-based IPNs began in 1974 as a cooperative research program between the Universidad Industrial de Santander, Bucaramanga, Colombia, and Lehigh University in the United States (81,82). Castor oil hydroxyl groups can be easily reacted with either isocyanate groups to form polyurethanes, or with carboxyl groups to form polyesters. The synthesis scheme for forming polyester-based SINs from castor oil is illustrated in Figure 22 (80). Here, sebacic acid (itself a derivative of castor oil) is reacted with

Fig. 22. A synthesis scheme for castor oil-polyester based SINs with polystyrene being the second component. = polyester is the continuous phase; = polystyrene is the continuous phase; and = Stirring.

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Fig. 23. Stress–strain curves for several castor oil (CO), polyester (PE), polyurethane (PU), and polystyrene network (PSN) materials. A: COPEN; B: COPEUN; C: COPUN; D: 40/60 COPEN/PSN; E: 40/60 COPEUN/PSN; F: 40/60 COPUN/PSN; and G: 40/60 COPUN/PSN.

castor oil. Note that stirring is required after phase separation to obtain phase inversion. A number of polyester, polyurethane, and mixed structures were made into IPN. Stress–strain curves on these compositions are illustrated in Figure 23 (83). In some cases, the castor oil was first reacted with a diacid, then finished with an isocyanate to form a urethane. Here, COPEN stands for castor oil polyester network, U stands for urethane, and EU stands for the mixed ester–urethane derivative. PSN stands for polystyrene network. Compositions 1, 2, and 3 are relatively soft and weak. Compositions 4 and 5 are somewhat higher modulus, more energy-absorbing materials. Composition 6 is much tougher. The sequential IPN, composition 7, is identical to composition 6, except that the sequential mode of IPN formation replaced the SIN mode. The yield point in the stress–strain curve of composition 7 suggests greater phase separation with the polystyrene being a more continuous phase. It must be pointed out that composition 6 is tough enough to make long-lasting shoe heels. Castor oil has also been made into semi-IPNs with poly(ethylene terephthalate) (PET). Typical time–temperature reaction conditions are shown in Figure 24 (84). Since the castor oil is an ester as well as the PET, significant bond interchange takes place at elevated temperatures, as indicated. If the material is heated too long, a statistical copolymer will result. Under the given reaction conditions, the mix goes from a two-phase mix to a one-phase mix. On cooling, it phase separates again. Tan (85) investigated SIN compositions based on castor oil polyurethanes and styrene–acrylic mixes. Under these conditions, there is considerable grafting. Abrasion resistance to jetting sand was very high, leading to a coating application of the Ge Zhou Ba hydroelectric power station dam in the desert region of western China (86). Other triglyceride oils that are being investigated as IPN monomers include vernonia oil and lesquerella oil. Vernonia oil, a naturally epoxidized oil,

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Fig. 24. A typical time–temperature cure cycle illustrating bond interchange and phase behavior of castor oil/urethanes and poly(ethylene terephthalate) semi-IPNs. Axes not drawn to scale.

was combined with poly(ethylene terephthalate) to make semi-IPNs (87). Lesquerella oil comes from a wild flower growing in the desert of southwest United States and Mexico. It, too, makes tough IPNs (88). It is thought that lesquerella could be grown where no crop exists today. Other functionalized oils of interest include crambe, epoxidized linseed oil, and epoxidized soybean oil (89). Natural Rubber Latex-Based IPNs. Another kind of natural product IPN is based on natural rubber latexes. After processing, this rubber is in wide use today for truck and aircraft tires (see RUBBER, NATURAL). While still in the latex form, these latexes can be treated just like their synthetic counterparts and used as the core material for IPN production. An interesting material, prepared by polymerizing methyl methacrylate in the rubber latex, is called Hevea Plus MG (90–92). This makes an impact-resistant composition somewhat resembling ABS. Polystyrene as well as PMMA was used variously as the second polymer. Notched Izod impact strengths were increased over those of the pure plastics by a range of 10–15, reaching over 30 kJ/m2 . Polysaccharide-Based IPNs. Cellulose (qv) and cellulose derivatives have been the largest tonnage single polymer sold in the world since the beginning of civilization until the present. Kamath and co-workers (93) prepared IPNs and related AB–cross-linked copolymers using compositions such as allyl cellulose cinnamonate and cross-linked acrylics, polystyrenes, or vinyl acetates. An improved thermal stability was noted. Hydrogel materials prepared from cellulose esters and polyacrylamide have also been investigated (94) (see HYDROGELS). Combinations of both polysaccharides and castor-oil–based polyurethanes have also been investigated. Semi-IPNs of castor-oil–based polyurethanes and nitrokonjac glucomannan (NKGM) were investigated by Gao and Zhang (95). The konjac glucomannan is a natural polysaccharide, and was nitrated for these studies. Possible substitution of the semi-IPN for some petrochemical products was mentioned, along with biodegradability. Films made from the semi-IPN were found to be highly transparent, with a breaking strength of some 30 MPa.

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Table 2. History of IPNs and Related Materials Event Vulcanization of rubber First IPN Macromolecular Hypothesis Graft Copolymers Homo-IPNs Block copolymers HIPS and ABS Block copolymer surfactants Interpenetrating polymer network term used Thermoplastic elastomers AB–Cross-linked copolymers Interphase characteristics of IPNs Sequential IPNs Latex IENs SINs Thermoplastic IPNs Modern IPN nomenclature

Investigators

Year

Ref.

Goodyear Aylsworth Staudinger Ostromislensky Staudinger and Hutchinson Dunn and Melville Amos, McCurdy, and McIntire Lunsted Millar

1844 1914 1920 1927 1951 1952 1954 1954 1960

96 97 98 25 99 100 23 101 102

Holden and Milkovich Bamford, Dyson, and Eastmond Lipatov and Sergeeva Sperling and Friedman Frisch, Klempner, and Frisch Sperling and Arnts Davison and Gergen Kahovec and co-workers

1966 1967 1967 1969 1969 1971 1977 1997

103 104 105 106 107 108 109 9

History of Interpenetrating Polymer Networks and Related Materials The patent literature shows that IPNs were invented over and over again, beginning in 1914 (see Table 2). Each time, the idea was lost and reinvented. Aylsworth (97) was the first person known to invent an IPN. In 1914 he combined the then new phenol–formaldehyde compositions with natural rubber and sulfur. Of some historical interest is that Jonas Aylsworth was Thomas Edison’s chief chemist (110). Edison had just switched from the cylinder-type record to the platter, using the new phenol–formaldehyde materials. However, these phenol– formaldehyde materials were brittle, and the first phonograph records had to be made very thick to prevent breakage. In effect, by adding natural rubber and sulfur, Aylsworth made not only the first IPN, but also the world’s first rubber-toughened plastic. This was decades before rubber-toughened styrenics were commercial. The West Orange, NJ, Edison Museum has evidence that Aylsworth’s composition was used for toughening phonograph records from 1914 to 1929, stopping only when Edison left the phonograph record business. Then, the idea was apparently lost. Aylsworth’s patent (97), however, does not mention IPNs and polymer networks, or even use the term polymer. Note that that date was six years prior to H. Staudinger’s enunciation of the macromolecular hypothesis, which was, of course, the first statement that certain colloids had the structure of long chains. Several other independent discoveries of IPNs followed. J. J. Staudinger (son of H. Staudinger) and Hutchinson have a 1951 patent (with a 1941 application date!) to smooth the surfaces of transparent plastic sheeting for esthetic purposes (99). These men took sheets of cross-linked polystyrene or PMMA and swelled the

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sheets with the same monomer mix as the first network. This stretched the sheets, and after polymerization of the second monomer mix the waviness or surface imperfections of the original material were reduced. These were the first homoIPNs. The next independent invention of IPNs was by Solt in 1955 (111). Solt invented anionic–cationic ion exchange resins with the idea of having both charges on the same suspension-sized particle. The idea was that the exchange efficiency would be improved if both charges were in juxtaposition, but still separated in space. Indeed, the charges must be separated, even by a fraction of a nanometer, to prevent coacervation. The term interpenetrating polymer network was first coined by Millar in 1960 (102). These were polystyrene–polystyrene homo-IPN suspension-sized particles. The idea was that some particles, the fines, were too small. On adding more monomer and cross-linker, larger and more satisfactory particles could be made. Early works by Shibayama and Suzuki (112), Lipatov and Sergeeva (105), Frisch and co-workers (107), and Sperling and Friedman (106) set the pace for the modern development, founding the field of IPNs (1). In Table 2, there is an apparent gap of two decades (1977–1997) between thermoplastic IPNs and the modern IPN nomenclature; in fact many people were busy characterizing a number of new materials and developing their applications, as described above. Application-oriented papers and patents are delineated below.

Actual or Proposed IPN Applications A number of commercial IPN materials and their applications are summarized in Table 3. The value of IPNs to society, of course, lies in their usefulness. IPNs can and are being made into rubber-toughened plastics and a special value seems to lie in their capability of forming tough but flexible materials. IPNs often have another unusual property: that of forming co-continuous, very finely divided phases. These tough but flexible materials have suggested biomedical applications. Biomedical Applications. Dillon (114) synthesized IPN membranes from polytetrafluoroethylene (PTFE) and poly(dimethyl siloxane) (PDMS) (see PERFLUORINATED POLYMERS, POLYTETRAFLUOROETHYLENE; SILICONES). These flexible membranes are used for a variety of medical purposes, especially seconddegree burn care. These materials are commercially available under the trade name Silon. The PDMS component rapidly transports body fluids away from the burn site, while the PTFE provides mechanical strength. The PTFE is also waterproof, so that when the wound area is washed, water beads up on the film. An additional valuable feature is that the films are highly transparent, so that the doctor can observe the wound area easily, making early treatment of any infection or other problem possible. ´ Salmer´on Sanchez and co-workers (115) prepared sequential IPNs based on {net-poly(ethyl acrylate)-ipn-{net-poly(hydroxyethyl acrylate)}. These two polymers differ primarily in the presence or absence of a hydroxyl group. Investigation via differential scanning calorimetry indicated three qualitatively different kinds of thermograms depending on the initial quantity of water present. The water present is primarily associated with the poly(hydroxyethyl acrylate), but the

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Table 3. Selected Commercial IPN Materialsa Manufacturer

Trade Name

Shell Chemical Co. LNP Plastics

Kraton IPN Rimplast

DSM N.V.

Kelburon

Uniroyal (Reichhold Chemical Co.) Rohm & Haas Monsanto BF Goodrich

Composition

Application Automotive parts Gears or medical

TPR

SEBS–polyester Silicone rubber–nylon or PU PP–EP or rubber–PE EPDM–PP

– Santoprene Telcar

Anionic–cationic EPDM–PP EPDM–PP or PE

Exxon Cook Composites

Vistalon Acpol

Dentsply International Hitachi Chemical Bio Med Sciences

Trubyte, Bioform – Silon

EPDM–PP Acrylic–urethane– polystyrene Acrylic–based

Ion-exchange resins Tires, hoses, belts, and gaskets Tubing, liners, and wire and cable insulation Paintable automotive parts Sheet molding compounds

a Updated

Vinyl-phenolics PDMS–PTFE

Automotive parts Auto bumper parts

Artificial teeth Damping compounds Burn dressing, scar abatement

from Table 10.5 in Ref. 113.

poly(ethyl acrylate) component clearly interferes with water transport and freezing. Whereas this was a basic study of hydrogel behavior, other systems examined below have several potential applications, particularly to the biomedical field (see HYDROGELS). Polymer gels based on poly(N-isopropylacrylamide) [P(N-iPAAm)] in aqueous media undergo a temperature-induced collapse of the chains from a perturbed random coil to a globular structure known as a coil-to-globular transition. This is macroscopically observable as a sudden decrease of the degree of swelling at a sharp first-order phase transition known as a lower critical solution temperature (LCST) at 32◦ C. The P(N-iPAAm) gels exhibit the sharpest transition of the class of thermosensitive alkylacrylamide polymers examined (116) (see ACRYLAMIDE POLYMERS). Since this LCST occurs between room temperature and body temperature, there has been great interest in this and related materials from a biomedical point of view. In order to adjust the LCST to meet various proposed objectives, a number of copolymers and IPNs of P(N-iPAAm) and have been prepared. Zhang and Peppas (117) examined sequential IPNs of poly(methacrylic acid) (PMAA) and P(N-iPAAm) hydrogels as a function of pH and temperature. pH is critically important to studies involving PMAA, since the carboxyl group is ionized at higher pH but in the acid form below about 5.5. The IPNs had a swelling/deswelling transition in a range that included that of pure P(N-iPAAm), 32◦ C, but was broader. An important possible use of such IPNs involves drug delivery. A number of model drugs was examined by Zhang and Peppas (117). As expected, higher

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permeability was observed for the smaller model drug compounds (see CONTROLLED RELEASE TECHNOLOGY). At low water contents, the LCST depends on the water concentration for P(NiPAAm) (118), being constant for high swelling ratios (as most often needed), but shifting to higher temperatures when the initial water concentration is below 50%. Hydrophilic poly(acrylic acid) (PAA), when added in IPN form, tends to increase the LCST. In the case of P(N-iPAAm) and PAA sequential IPNs, the association between the complementary binding sites produces complexes that decrease the ability of the hydrogel to swell in water. For the various copolymers and IPNs, a major effect was the broadening of the transition. The first-order LCST of P(N-iPAAm) leads to many possible applications in the biomedical field. Wu and Jiang patented an IPN hydrogel based on {netpoly(N-isopropylacrylamide)}-ipn-{net-gelatin} (119). These materials also swell greatly at room temperature, but can be made to undergo a rapid reduction in volume at blood temperature, 37◦ C. When formed into a tube, these hydrogels can be placed over the cut ends of a blood vessel. The material shrinks rapidly as the material increases in temperature from room temperature to body temperature, grasping both ends of the blood vessel and allowing blood to be temporarily transported in a normal manner. There are two ways of bending light. The most common is to present the light with a material surface at an angle to the direction of the light’s travel. However, light waves can also be bent by changing the refractive index internally in the material. This latter fact has been used to prepare gradient IPN compositions in soft contact lenses. By using computer-controlled laser polymerization of polymer II in gradient IPNs (polymers of two different refractive indices), it is possible to have improved astigmatism control (120,121). Another class of IPNs are the homo-IPNs, made of two more or less identical polymers. An example are the homo-IPNs prepared from cross-linked poly(methyl methacrylate) (PMMA) (122,123). In this case, densely cross-linked material of suspension-sized PMMA particles are prepared. These are mixed with a very similar monomer mix, and polymerized. Some, but not all, of the second monomer mix swells into the suspension particles. Homo-IPNs of PMMA are used commercially for artificial teeth, the kind that come out at night (122,123). One trade name is Bioform IPN (Table 3). In one example, suspension-sized particles of cross-linked PMMA were mixed with linear PMMA, MMA monomer, cross-linker, and initiator. After polymerization, the teeth have two superior properties. First, because a suspension polymerization route is used, the dentist can grind the teeth to a fine powder. This is important when fitting the new teeth to the opposing set of teeth. They can be ground better because the interface between the suspension-sized particles and the surrounding phase is slightly weaker than the bulk material. A single network of a similar composition may char under the dentist’s drill. Second, a densely cross-linked polymer is required. Salad oil, butter, and similar materials are excellent plasticizers (qv) for linear PMMA, which cause the teeth to swell. Smart Materials. Eck and co-workers (124) developed a smart material for solar energy management. It seems that the use of solar energy to heat water in some tropical areas results in water that is too hot. Usually, the water pipes to

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be heated are in a panel covered by glass, and exposed to the sun. The material of Eck and co-workers makes use of a lower critical solution temperature (see Fig. 7 for example) with an IPN based on {net-poly(propylene oxide)}-ipn-{netpoly(styrene-stat-2-hydroxymethyl acrylate)}. A film of this material is placed on the underside of the glass. At low temperatures, the film is clear, transmitting the light. When the temperature exceeds a certain value, the film clouds up, increasing reflection back into the atmosphere. A polymer blend of the same or similar composition cannot be used because the phase domains that will form will become large enough to take many hours to diffuse back to a single phase. The IPN forms domains of only some tens of nanometers, allowing diffusion back and forth between one phase and two phases in minutes. Interestingly, this appears to be the only application suggested for the LCST of polymer blends or IPNs. Pervaporation Membranes. Pervaporation membranes for separation of ethanol and water and other types of solutions are being investigated by Kim and his students (125,126). Some of these membranes are anionic/cationic IPNs. Starting with a 10% ethanol feed, after three stages the retentate is 94% ethanol. Water vapor passes through the membrane as the pervaporate. Other Applications. Another new material, developed by Lipatov and Karbanova (127) and Bukhbinder and Kosjakov (128) flies under the designation gradans. Cylindrical glass or polymer fibers are widely used to transmit light or other electromagnetic energy for communication purposes, optoelectronics, integrated optics, and medical engineering. A major problem is that the electromagnetic energy tends to leak out of the surface of the fiber, shortening the useful path. If the fiber is made so that its refractive index has an increasing gradient from the center to the surface, the energy will be bent back towards the center as it moves through the fiber. One way to accomplish this is through making a gradient IPN, with the second monomer making a polymer of higher refractive index than the first. Some of the materials mentioned in Table 3 have already been discussed. The reader will recognize the Silon and Bioform materials described above. Kraton IPN is also discussed above (see Figs. 16c and 16d and Table 1). Several of the materials manufactured are based on EPDM–PP thermoplastic IPNs. Current Literature Suggestions and Applications. In 2004, the production of IPN-based papers is averaging about 100 papers a year. This contrasts to the fact that up to 1979, there was a total of about 125 scientific papers and about 75 patents issued on the subject. The number of IPN-related patents has also increased accordingly. Many of the most recent papers mention possible applications (see Table 4). Again, possible medical applications stand out the most. Kim and Kim describe a most interesting material in Reference 134 of Table 4. The need for synthetic blood vessel sections is great. A blood vessel must be both flexible and strong. However, a major problem with materials has been that the inner surface may abrade red blood cells and cause adsorption of materials such as the fibrinogens and platelets, important in coagulating blood. (The lifetime of red blood cells passing various surfaces is inversely related to the modulus of the material of contact.) Fibrinogen is one of the most important globular proteins circulating in the blood, playing a central role in the regulation of hemostasis

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Table 4. Actual and Proposed IPN Applications from Select Recent Literature Composition Biomedical/Pharmaceutical Poly(ε-caprolactone) semi-homo-IPNs Poly(N-isopropylacrylamide-stat-acrylic acid) + grafted polyacrylamide copolymers Acrylic resin IPNs Poly(acrylic acid) + cross-linked gelatin Segmented polyurethane + poly(2-methacryloyloxyethyl phosphoryl chloride) Polyurethane + polystyrene with pendant poly(ethylene oxide) chains Chitosan + poly(ethylene oxide) semi-IPN Sodium alginate + cross-linked gelatin or egg albumin Poly(vinyl alcohol) + poly(dimethyl siloxane) hydrogels Chondroitin + poly(acrylic acid) semi-II IPN Polyurethane + polystyrene IPNs Poly(hydroxyethyl methacrylate) + poly(ethylene glycol) semi-IPN Polypropylene + polyacrylamide mix Poly(vinyl alcohol) + poly(2-hydroxyethyl methacrylate) Poly(2-hydroxyethyl methacrylate) + chitosan Polysiloxane + poly(vinyl alcohol) with ferromagnetic derivate Electrical Polystyrene or poly(methyl methacrylate) + modified epoxy Castor oil-based polyurethane + poly(methyl methacrylate) + polyaniline Polyaniline + poly(vinyl alcohol) semi-IPN Poly(vinyl alcohol) + chitosan hydrogel Polyaniline + poly(vinyl acetate) semi-II IPN Polyaniline + melamine-urea Poly[methoxyoligo(oxyethylene) methacrylate] + poly(methyl methacrylate) Poly(sodium acrylate) + poly(N-isopropyl acrylamide) semi-II IPN

Application

Ref.

Bone implant composites with hydroxyapatite Engineered cartilage, polymer scaffolds

129

Acrylic false teeth Hydrogel drug release systems. Blood compatibility, no platelet adhesion

131 132 133

Blood compatibility; adsorption of fibrinogens and platelets suppressed. Drug release Controlled release of medication

134

Drug release

137

Controlled drug release

138

Adhesion of endothelial cells Antithrombogenic material

139 140

Controlled release of drugs Prosthetic disc nucleus

141 142

Lysozyme adsorption

143

Immobilize antigens

144

Underfill adhesive for electronic applications Electrical conducting properties

145

Electrical conducting materials Bends in electric field; smart material Electrical conductivity

147 148

Electrical conductivity Electrochemical conductivity

150 151

Intelligent hydrogels, thermovolume sensitive, heavy metal ion recovery

152

130

135 136

146

149

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Table 4. (Continued) Composition Poly(vinyl alcohol) + poly(diallyl dimethyl ammonium chloride) Damping Vegetable oils Polyester + epoxy with carbon black Poly(dimethyl siloxane) + polyacrylates + polymethacrylates Polysiloxanes + poly(methyl methacrylate) Ion exchange Sulfonated polystyrene + poly(vinyl chloride) semi-I IPNs Poly(hydroxyethyl methacrylate) + chitosan Poly(vinyl chloride) + polystyrene with cation exchange groups General Polyimides + alkoxysilane dye Epoxy + unsaturated polyester Poly(vinyl alcohol) grafted with acrylamide + sodium alginate Poly(acrylic acid) + poly(butyl methacrylate-stat-methyl methacrylate) Natural rubber + polystyrene Carboxymethyl cellulose + poly(ethylene glycol) + cross-linked polyacrylamide Unsaturated polyester + poly(2-hydroxyethyl methacrylate)

Application

Ref.

Samples bend in electric field

153

Damping materials Heat-resistant damping High temperature damping

154 155 156

Sound and vibration damping

157

Cation exchange membranes

158

Removal of heavy metal ions from 159 aquatic systems Cation exchange membrane 160

Second-order nonlinear optics Rubber-toughened plastics Pesticide release

161 162 163

Pervaporation membrane

164

High transport membranes Controlled release of KNO3 agricultural fertilizer Improved flame resistance

165 166 167

and thrombosis. It participates in blood coagulation and facilitates adhesion and aggregation of platelets. The result might be thrombosis in the patient. Mother Nature makes the inner lining of blood vessels very soft, so that the probability of coagulation is far reduced. The hydrophilic pendant poly(ethylene oxide) (PEO) chains, described in Reference 134, stick out from the inner side of the blood vessel, creating a soft gel environment. The presence of the pendant PEO chains significantly suppresses the adhesion of fibrinogens and platelets on the surface of the polymeric blood vessel by steric repulsion. In these IPNs, the blood compatibility increases with both the length of the pendent chains and the density of grafting. Thus, the polyurethane and polystyrene, main components of the IPN, provide both the strength and the flexibility, and the grafted PEO provides a soft, gelatinous inner surface, mimicking Mother Nature. Besides possible use as artificial blood vessels, there is a wide range of blood-handling equipment now in use for surgery, etc, as possible applications. A similar effect arises in the materials described in Reference 133 of Table 4. Segmented polyurethanes, of course, are a type of block copolymer. These

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materials are very strong elastomers, which are in wide use as the elastic fiber in undergarments, bathing suits, etc. The poly(2-methacryloyloxyethyl phosphoryl chloride) forms a highly swollen, soft gel partly exposed on the IPN blood vessel surface. However, there are possible applications to optoelectronics, agriculture, and other topics as well. References 148, 152, and 153 in Table 4 mention smart materials, particularly those that bend in an electric field. These are widely being investigated as sensors of various kinds.

Concluding Comments In a certain sense, the term interpenetrating polymer networks is a misnomer, since most of these materials phase separate, rather than being mutually soluble on the molecular level. One could argue that the phases interpenetrate, and/or that true molecular interpenetration takes place at the interphase. However, the term describes the general synthesis and structure of the materials. The field of interpenetrating polymer networks now takes its place parallel to the fields of polymer blends, grafts, and blocks. Indeed, most of these materials are useful because they undergo some kind of phase separation. Many IPNs seem to work best when the degree of phase separation is only partial or the size of the domains is in the tens of nanometer range. Those IPNs composed of an elastomer and a plastic to make flexible materials seem to be the most unique among these materials.

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L. H. SPERLING Lehigh University

IONOMERS.

See Volume 6.

ISOBUTYLENE POLYMERS.

See BUTYL RUBBER.

ISOCYANATE-DERIVED POLYMERS.

See Volume 6.