Phase separation network formation

Phase separation during formation of three-dimensional networks. J. Polymer Sci. Pt. C-16, 1289 (1967). 

In this case of three-monomer polyurethane synthesis, there is no thermodynamic driving force for phase separation. The formation of clusters is fully controlled by the initial composition of the system, the reactivity of functional groups, and the network formation history (one or two stages, macrodiol or triol reacted with diisocyanate first, etc.). 

Crystallization is the most commonly cited mechanism for junction-zone formation in polymer gel networks. A distinction must be made between crystallization from solution resulting directly in network formation and crystallization which occurs subsequently to gelation. This distinction is not always apparent in the reports of crystallization and gelation. For example, crystallization may occur in an already phase-separated network or it may involve polymer molecules not included in the network structure. Another possibility is that a specific interaction or phase separation results in the formation of a weak network but that this network is consolidated by subsequent crystallization. Here the relative importance of the various mechanisms becomes unclear. These form the grounds for the current debate on the relative importance of crystallization and phase separation in gelation. In the examples which follow crystallization has been favoured as the primary mechanism. 

Explicit forms for the stress tensors d1 are deduced from the microscopic expressions for the component stress tensors and from the scheme of the total stress devision between the components [164]. Within this model almost all essential features of the viscoelastic phase separation observable experimentally can be reproduced [165] (see Fig. 20) existence of a frozen period after the quench nucleation of the less viscous phase in a droplet pattern the volume shrinking of the more viscous phase transient formation of the bicontinuous network structure phase inversion in the final stage. 

Depending on the conditions of synthesis, copolymerization of divinyl/vinyl-monomers in the presence of an inert solvent leads to the formation of expanded (preswollen) or heterogeneous (porous) structures [54,99,100]. If the solvent remains in the network (gel) phase throughout the copolymerization, expanded networks are formed. If the solvent separates from the network phase the network becomes heterogeneous. According to Dusek et al., heterogeneities may appear in poor solvents due to the polymer-solvent incompatibility (x-induced syneresis), while in good solvents due to an increase in crosslink density (v-induced syneresis) [99]. 

The previous discussion has shown that the CIPS technique allows one to produce macroporous epoxy networks with either a narrow or bimodal size distribution. However, no indication has been given on the type of phase separation mechanism to yield these morphologies. As discussed earlier, the formation of a closed cell morphology can result either from a nucleation and growth mechanism or from spinodal decomposition. 

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