Homo-IPNs

Homo-IPNs are interpenetrating networks made up of polymers that are identical but retain their specific characteristics. An example is an epoxy/epoxy IPN. 

The term interpenetrating polymer networks was coined by John Millar in 1960, who prepared homo-IPNs (an IPN with both polymers identical) of polystyrene [Millar, I960]. Millar knew about Solt s work, and his objective was to increase the size of suspension particles of polystyrene intended for ion-exchange applications. 

All interpenetrating polymer networks utilize two different polymers. The exception involves the homo-IPNs, where both polymers are identical [Millar, 1960 Siegfried et al., 1979]. While these polymers may be synthesized by any of the known methods of polymer synthesis, some methods clearly work better in given objectives than others. The principal kinetic methods used are chain and step polymerization. 

A comprehensive review of the synthesis and properties of IPNs is beyond the scope of this book, and in this section we will deal only with the properties of IPN networks formed by styrene—DVB copolymers. These IPNs were named Millar IPNs after the author who was the first to prepare this novel type of networks as early as in 1960 [225]. These materials are also named homo-IPNs, either keeping in mind the fact that they are formed by chemically identical polymers or sometimes assuming a homogeneous character of their physical structure. 

The materials are called homo-IPN s if polymers I and II are chemically identical. 

Because of the special swelling and mutual dilution effects encountered in sequential IPN s, special equations were derived for their rubbery modulus and equilibrium swelling. The new equations were used to analyze polystyrene/polystyrene homo-IPN swelling and rubbery modulus data obtained by four different laboratories. In the fully swollen state, there was no evidence for IPN related physical crosslinks, but some data supported the concept of network I domination. In the bulk state, network I clearly dominates network II because of its greater continuity in space. The analysis of the data concerning the possible presence of added physical crosslinks in the bulk state yielded inconclusive results, but this latter is of special interest for modern network theories. 

While the Flory-Rehner equation is not expected to fit homo-IPN data, it is of interest to note where the data lies with respect to the theoretical diagonal. In Figure 1, the data of Millar, and Shibayama and Suzuki tend to lie above the line, while Thiele and Cohen and Siegfried et al s. data tend to lie below it. Yet the Thiele-Cohen equation, applied to the same data in Fig. 2, showed a marked improvement, pulling the data towards the theoretical line whether it originally lay above or below the Flory-Rehner line in Fig, 1, The modified Thiele-Cohen equation, employed in Fig. 3,... 

While there is some evidence for new physical crosslinks, unfortunately the data do not permit a reasonable conclusion either way. Only the modulus data of Shibayama and Suzuki indicates the presence of any added physical crosslinks contributing to the rubbery modulus. Siegfried, Manson, and Sperling s data. Fig. 5b, indicate fewer physical crosslinks in the homo-IPN than in the corresponding single networks, because the data lies to the left of the line. 

An interpenetrating polymer network (IPN) is defined as a combination of two crosslinked polymers, at least one of which has been synthesised [98] and/or crosslinked in the immediate presence of the other. From the topological point of view, IPNs are closely related to pol)nner blends and to block, graft and crosslinked copolymers. From the synthesis point of view, IPNs can be classified, broadly, into two general types (a) sequential IPNs where a polymer network is formed which is then swollen by the monomer, plus a crosslinking agent and an activator, which is then polymerised in situ to form the second network and (b) simultaneous IPNs (SIPN) where the components necessary to form both networks are mixed and polymerised, at the same time, by non-competing mechanisms. If one of the two polymers is linear (uncrosslinked), a semi-IPN results. A homo-IPN results if both the network polymers are identical in chemical composition [98]. 

Over the years, the interest in IPNs for biomedical applications has increased. One trick being developed is to have one network with low cross-link density and the other with high cross-link density. The result has been shown to yield stronger, more fracture-resistant materials. As will be shown below, this idea works even if the two polymers are otherwise identical, i.e., homo-IPNs. Also, many of the new, proposed materials are versions of hydrogels. Since these materials are intended for improvement or replacement of body parts, many are studied at body pH 7.4. 

In brief, the monograph will proceed from the basic to the applied. Since the IPNs are members of the larger polyblend field elements of polymer blends will be introduced next, followed by a proposed nomenclature. Then the subfield of homo-IPNs will be taken up, as model materials. The main theme of this work will be found in Chapters 5-8. 

Homo-IPN an IPN where both polymers are identical. This term replaces the earlier notation Millar IPNs because it is more descriptive of the materials. 

In important aspects, this chapter will describe a reexamination of the homo-IPN data by Siegfried et The results will be scrutinized in the... 

Clark employed poly(dimethyl siloxane) (PDMS) homo-IPNs to make improved adhesives. Three separate linear PDMS chains were mixed, each with reactive groups. Polymers I and II reacted to form a network, yielding a semi-IPN. The remaining linear polymer provided the adhesive properties. After adhering the two required surfaces together, raising the temperature initiated a self-crosslinking of polymer III to form the IPN. 

Years earlier, Staudinger and Hutchinson employed a homo-IPN of acrylic composition to make optically smooth surfaces. As amplified in Chapter 1, network I was swollen with more monomer of the same type to smooth out surface wrinkles by the stretching incurred on swelling. Polymerization of the new monomer yielded a homo-IPN. 

For a homo-IPN, the free energy change, AG, of mixing during swelling may be written as a sum of three contributions, a term for the sum of the contributions of polymer and solvent mixing forces, AGm, and a term for each network s elastic retractive forces ... 

Figures 4.2,4.3, and 4.4 show the modulus predicted by equation (4.8) vs. Young s modulus, E (experiment). As with the swelling data, the network imperfections and the contributions of the physical crosslinks, if any, were minimized by determining the two crosslink levels required for E (theory) on the separate homopolymer networks. Unfortunately, Millar did not report modulus data for his polystyrene/polystyrene homo-IPNs.

Figure 4.2. Young s modulus behavior of PS/PS homo-IPNs. Shibayama and Suzuki s data plotted according to equation (4.8) for E (theory) vs. experiment.

Figure 4.5. Morphology of 50/50—0.4/4% DVB + 1 % isoprene homo-IPN. Polymer network II, darker regions stained with OSO4, appear as domains near 75 A in diameter. " ...

The PS/PS homo-IPNs, discussed in Chapter 4, also show evidence of domain formation even though both polymers are identical. In this case, y is certainly zero and the predicted 60-100 A domain sizes are indeed found. 

Because electron micrographs and glass transition studies as such cannot prove interpenetration, some modeling of the data is necessary to arrive at a coherent picture. While some of the quantitative aspects of the interpenetration and phase continuity were discussed in Chapter 4 on homo-IPNs, a qualitative review of the evidence is nevertheless instructive. 

Copolymers of Styrene and Divinj benzene, Rubber Chem. Tech. 40, 476 (1967). Homo-IPNs of polystyrene and polystyrene. Swelling and mechanical behavior. 

D. L. Siegfried, J. A. Manson, and L. H. Sperling, Viscoelastic Behavior and Phase Domain Formation in Millar Interpenetrating Polymer Networks of Polystyrene, J. Polym. Sci. Polym. Phys. Ed. 16(40), 583 (1978). PS/PS homo-IPNs. Visoelastic and morphological behavior. 

J. J. P. Staudinger and H. M. Hutchinson, Process for the Production of Strain-Free Masses from Crosslinked Styrene-Type Polymers, U.S. Pat. 2,539, 377 (1951). Homo-IPNs and semi-I IPNs based on polystyrene. 

J. L. Thiel and R. E. Cohen, Synthesis, Characterization, and Viscoelastic Behavior of Single-Phase Interpenetrating Styrene Networks, Polym. Eng. Sci. 19, 284 (1979). Polystyrene/polystyrene homo-IPNs. Swelling equation for single-phase IPNs. Equilibrium swelling studies as a function of crosslink level. 

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