Yielding and Fracture of Polymer Networks

There are two main sets of materials properties involved in any problem of 

For a given material, there are generally several possible mechanisms of yielding and fracture, each characterized by the influence of temperature, loading rate, hydrostatic pressure, time (physical aging). A vast literature deals with the influence of network structure on yielding or on fracture properties, but we have to be very careful with the results obtained because of the different types of networks used in these experiments. 

We can distinguish first between homogeneous and inhomogeneous networks (Chapter 7). In homogeneous networks a distinction between ideal and nonideal structures may be performed. This concept is presented in Chapters 10 and 11. 

A nonideal network may be obtained as in the previous case but using different nonstoichiometric molar ratios or arresting the polymerization at different conversions, to modify the structure. In these cases, the presence of a sol fraction and dangling chains will introduce an additional plasticization effect, surimposed on the new architecture (Vallo et al., 1993). 

The last case concerns inhomogeneous networks often produced by chain polymerization (acrylate networks, unsaturated polyesters), where a gradient of crosslink densities is the result of the reaction mechanism and, in some cases, thermodynamic effects (Chapter 7). 

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The effect (or lack of effect) of crosslinks on basic physical properties of thermosetting polymers is discussed in Chapter 10, while the effect on elastic and viscoelastic properties is analyzed in Chapter 11. Yielding and fracture of neat and modified thermosetting polymers are discussed in Chapters 12 and 13. Finally, the very important problem of the durability of polymer networks is presented in Chapter 14. 

A polymer is more likely to fail by brittle fracture under uniaxial tension than under uniaxial compression. Lesser and Kody [164] showed that the yielding of epoxy-amine networks subjected to multiaxial stress states can be described with the modified van Mises criterion. It was found to be possible to measure a compressive yield stress (Gcy) for all of their networks, while the networks with the smallest Mc values failed by brittle fracture and did not provide measured values for the tensile yield stress (Gty) [23,164-166]. Crawford and Lesser [165] showed that Gcy and Gty at a given temperature and strain rate were related by Equation 11.43. 

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. 

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