Nonequilibrium States of IPNs

The microphase structure of IPNs may be described using the concept of the formation of various clusters physical clusters that are formed due to phase 

Both the chemical reactions and the phase separation proceed under nonequilibrium conditions simply because they proceed simultaneously. After some degree of chemical conversion and cross-linking is reached, microphase separation is impeded and the system freezes in a nonequilibrium structure characterized by incomplete phase separation. Thus, by the completion of IPN formation, reactions proceed in two evolved phases. The real structure of an IPN is a multiphase one, which is determined by the coexistence of at least three phases (not in a true thermodynamic sense). Two phases are formed by networks due to phase separation. Each phase may be considered as an independent IPN in which phase separation did not take place (the state of forced compatibility), and in which mixing on the molecular level is preserved. The composition of these two phases is determined by the reaction rate and the temperature. Each phase has an average composition that does not correspond to the network ratio in the entire IPN. The third phase is the nonequilibrium transition zone from one phase to another its size depends on the conditions of phase separation. This zone may be called mesophase and may be considered as a nonequilibrium IPN of some transition composition, since the molecular level of mixing should also be preserved. For spinodal decomposition there is no sharp border between coexisting phases. The transition zone may be arbitrarily chosen in such a way that its composition corresponds to the average composition of the IPN. 

In general, the microphase structure of IPNs may be described as a nonequilibrium one. Indeed, if the phase separation were realized under equilibrium, then, in accordance with the most general thermodynamic rules, the composition and the ratio of phases would be determined only by the phase diagram of the system, and not by the conditions of the separation or chemical kinetics. The situation typical of IPNs and of linear polymer blends may never be realized in polymer solutions or in alloys of low molecular mass substances. Thus, the first reason for the nonequiHbrium consists in the specific conditions of IPN formation. 

However, in such nonequilibrium IPNs one can still discern various microregions that can be described as in quasi-equiHbrium, that is, microregions with a near molecular level of mixing. Two thermodynamic states in the IPN can be distinguished. The IPN as a whole is a nonequilibrium system due to incomplete phase separation and thermodynamic immiscibility of the constituent networks. However, the two phases evolved may be considered as quasi-equilibrium phases because they are the result of microphase 

All the ideas connected with the nonequilibrium multiphase structure of IPNs are in good agreement with a comparatively low segregation degree in IPNs (see Chap. 4, this volume). In such a way the whole structure of IPNs may be presented as a mesophase matrix with embedded microphase regions, which represent two evolved phases. Such a structural model coincides with the spinodal mechanism of decomposition. 

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