Thermoplastic IPNs

While all of the materials discussed above contain standard covalent crosslinks, it is also possible to utilize a range of physical crosslinking modes. Three distinct types of physical crosslinks may be recognized. 

Interesting enough, each of the above materials behaves as a thermoplastic at elevated temperatures, i.e., they are capable of being processed. Combinations of these materials form the basis for the newest category of IPNs thermoplastic IPNs. Obviously, they offer features heretofore unavailable in IPNs. While the SINs could be processed up to the gelation stage, the thermoplastic IPNs can be processed or reprocessed at any time. 

For the mechanical-blending method, six combinations of blocks, ionomers, and crystalline polymers are possible. The chemical-blending route, which distinguishes the order of polymerization, yields nine possible combinations. 

An example of the synthesis of a chemically blended thermoplastic IPN 85) preparation of a poly (styrene-6-ethylene-co-butylene-6- 

Numerous crystalline/crystalline mechanical blends have been prepared. Very recently, for example, Cruz et studied a series of 

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Typically IPNs exhibit some degree of phase separation in their structure depending on how miscible the component polymers are. However, because the networks are interconnected such phase separation can occur only to a limited extent, particularly by comparison with conventional polymer blends. Polymer blends necessarily have to be prepared from thermoplastics IPNs may include thermosets in their formulation. 

Silicone co-polymer networks and IPNs have recently been reviewed.321 The development of IPNs is briefly described, and the definitions of the main (non-exclusive) classes of the IPNs are cited. Examples of latex IPNs, simultaneous and sequential IPNs, semi-IPNs, and thermoplastic IPNs are provided. The use of silicone-silicone IPNs in studies of model silicone networks is also illustrated. Networks in which siloxane and non-siloxane components are connected via chemical bonds are considered co-polymer networks, although some other names have been applied to such networks. Today, some of the examples in this category should, perhaps, be discussed as organic-inorganic hybrids, or nanocomposites. Silicone IPNs are discussed in almost all of the major references dealing with IPNs.322-324 Silicone IPNs are also briefly discussed in some other, previously cited, reviews.291,306... 

This discussion omits the thermoplastic IPN s, which have physical crosslinks. 

JLnterpenetrating polymer networks (IPNs) are a special class of polymer blends in which the polymers exist in networks that are formed when at least one of the polymers is synthesized or cross-linked in the presence of the other. Classical or true IPNs are based solely on thermosetting polymers that form chemical cross-links. More recently, two classes of thermoplastic IPNs have been developed. Apparent IPNs are based on combinations of physically cross-linked polymers. Semi-IPNs are based on combinations of... 

The silicone-thermoplastic IPN composites exhibit superior mechanical properties (Table IV), and the network structure of the silicone prevents migration. The value of the IPN has been demonstrated in a molded journal bearing for use in high-speed paper-handling equipment. For example, part wear was reduced to a greater extent in a PTFE-lubricated, silicone-nylon... 

If physical crosslinks are used in an IPN system, then we would have a thermoplastic IPN. Up to this time, few IPNs or SINs have been made where both components are elastomeric. One of these rare works involves the crosslinking of cis-l,4-polyisoprene (Elastollan) in thermoplastic polyurethanes (Morthane). ... 

In contrast to the thermoset IPNs, thermoplastic IPNs can be processed in standard extrusion and molding equipment. They are made either by mechanical blending or chemically by sequential polymerization. 

One of the first mechanical thermoplastic IPNs was that prepared by Exxon Chemical and made by melt blending ethylene-propylene (E-P) copolymer or EPDM with polypropylene in the presence of a small amount of peroxide as cross-linking agent. Recently, Shell Chemical... 

A chemically blended thermoplastic IPN was developed by Petrarch Systems (38). It consists of polyurethane, polyamide (40), or... 

Thermoplastic IPN. When physical crosslinks rather than chemical crosslinks are utilized, the materials may flow at elevated temperatures. As such, they are hybrids between polymer blends and IPNs. Such crosslinks commonly involve block copolymers, ionomers, and/or semicrystallinity. 

The previous section showed how IPNs and related materials can be synthesized. The several synthetic methods, such as sequential, simultaneous, latex, and thermoplastic IPN formation, will result in different morphologies. One of the main advantages of IPN synthesis relates to the ease of promoting dual phase continuity, i.e., for a... 

Another advantage of using IPNs involves its thermosetting characteristics. By definition, IPNs will not flow when heated. A partial exception is the thermoplastic IPNs, which behave crosslinked at ambient temperatures, but flow at elevated temperatures. While some IPNs are tough, impact-resistant plastics, the crosslinking permits many other types of applications, such as sound and vibration damping, biomedical, adhesives and coatings uses, etc. (see Section 6.5). 

Thermoplastic IPNs are defined above as being hybrid materials between the IPNs and the mechanical blends. Many of these materials consist of polypropylene, a semi-crystaUine polymer (the crystallinity contributing to the physical crosslinking of the system), and EPDM. The EPDM is... 

The important points developed in this section are that sequential IPN synthesis tends to make dual phase continuous materials. Eor both sequential and simultaneous syntheses, a metastable phase diagram can be developed to study the kinetics of phase separation and gelation, so that better control of the morphology can be attained. The thermoplastic IPNs depend on equal volume and viscosity ratios to attain the dual phase continuity. 

Figure 6.5. An SEM fracture surface of an extracted thermoplastic IPN, 25/75 PA-ll/SEBS. The SEES phase was extracted.

While the field of IPNs is relatively young compared with this of polymer blends, the number of appfications are growing rapidly. In 1979, the total number of scientific papers was about 125 there were about 75 patents. Today, about that many are published each year. Major advances include thermoplastic IPNs, renewable resource materials, biomedical materials, and non-linear optical materials. 

The thermoplastic IPNs utihze physical cross-hnks, rather than chemical crosshnks. Usually, these materials wiU flow when heated to sufficiently high temperature (hence the terminology thermoplastic), but behave as thermosets at ambient temperature, with IPN properties, often possessing dual phase continuity. Most often, physical crosshnks are based on triblock copolymers (thermoplastic elastomers being the leading material), ionomers, or semi-crystalhne materials. 

Table 6.9. Selected plastic-elastomer thermoplastic IPNs by trade name ...

Any combination of the above two types of polymer can form thermoplastic IPNs in which some degree of dual-phase continuity is attained. Some of the thermoplastic IPNs have been claimed to be tough materials (44, 45). Though tribological properties of these new materials have not been investigated, they should be of... 

Rimplast materials from Petrarch are silicone/thermoplastic IPNs that combine the warpage and wear resistance properties of silicone with nylon, thermoplastic polyurethane, or styrene/butadiene block copolymers. The combination of properties these IPNs possess make them suitable for high quality, high tolerance gear and bearing applications that can benefit from the addition of internal lubrication and isotropic shrinkage of silicone to the fatigue endurance and chemical resistance of crystalline resin, while processability still remains the same. 

Recent research on IPN s has emphasized thermoplastic IPN s based on physical crosslinks, and the factors controlling the variation of domain sizes in sequential IPN s. Most recently, decross linking and extraction studies on sequential IPN s has led to an improved understanding of the dual phase continuity sometimes present in these materials. The sequential IPN system poly(n-butyl acrylate)/polystyrene is emphasized. 

In the case of thermoplastic IPN s, the crosslinks are of a physical, rather than a chemical, covalent nature. Important types of physical crosslinks include the hard blocks of a multiblock copolymer, ionomeric sites, or crystalline regions in semicrystalline polymers. Frequently, the polymers exhibit some degree of dual phase continuity. In all such cases, the thermoplastic IPN s behave as thermosets at use temperature, but as thermoplastics at some more elevated temperature. 

Fig. 1. A chemically blended thermoplastic IPN from an SEES triblock copolymer and neutralized styrene-methaerylie acid copolymer.

Thermoplastic IPN Polymer alloy, containing two or more polymers in a co-continuous network form, each physically cross-linked. The cross-linking originates in crystallinity, ion cluster formation, presence of hard blocks in copolymers, etc. 

SEES dissolved in styrene, methacryiic acid, and isoprene and then polymerized Thermoplastic IPN, with superior mechanical properties Siegfried et al. 1984... 

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