Homo-IPNs as Model Networks

Most areas of modern research contain two concurrent factors a basic aspect, which somehow aims at an improvement in scientific understanding, and an applied aspect, which is directed toward a practical goal such as a new or improved material or process. While one or the other factor frequently predominates in a given piece of work, sometimes the two are inextricable. 

This chapter emphasizes the role of IPNs as model polymer compositions. It is concerned with aspects of rubber elasticity, physical and chemical crosslinks, and swelling behavior. While much of Chapters 7 and 8 to follow are much more practically oriented, a study of the molecular structure of IPNs will make more apparent the need for fundamental understanding in arriving at the best possible practical materials. 

In recent years, important advances in the theory of rubber elasticity have been made. These include the introduction of the so-called phantom networks by Flory and a two-network model for crosslinks and trapped entanglements by Ferry and co-workers, The latter builds on work by Flory and others on networks crosslinked twice, once in the relaxed state, and then again in the strained state. In other studies, Kramer and Graessley distinguished among the three kinds of physical entanglements as crosslink sites the Bueche-Mullins trap, the Ferry trap, and the Langley trap. 

These several papers discuss the contribution, or lack of contribution, of physical crosslinks to retractive stresses in the theory of rubber elastic-ity. In most sequential IPNs described in this monograph, a crosslinked polymer I is swollen with monomer II, plus crosslinking and activating agents, and monomer II is polymerized in Chapter 5 is devoted to a 

When both polymer networks are chemically identical, the naive viewpoint suggests that a mutual solution will result with few ways to distinguish network I from network II. As will be shown below, this is emphatically not the case. 

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