Mechanisms of Phase Separation in IPNs

Therefore, all the IPNs are thermodynamically unstable. The pecuharity of IPN formation determines their morphology, which is formed in the course of phase separation by curing. Below we discuss two possible mechanisms of phase separation in IPNs. 

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Generally, the following conclusion can be drawn from the analysis of the mechanisms of phase separation in IPNs. The structures arising at different... 

Because relatively few experimental SANS data are available for IPNs, presently it is difficult to draw any definite conclusions about the structure of IPNs. As is seen from the data considered, the mechanism of phase separation is not mentioned in any work cited above. Meanwhile, this mechanism should determine if the application of any theory is possible for a given system. One may suggest that the Porod and Hosemann models may be used only for the nucleation and growth mechanism of phase separation, most typical for sequential IPNs. For simultaneous IPNs, where spinodal decomposition, as a rule, is more probable, it seems to be more reliable to determine only the heterogeneity parameters, not the radii of particles, if any. It is also necessary to keep in mind the possible changes of the mechanism of phase separation in the course of reaction. 

Presently, a great deal of work is published concerning the structure of IPNs obtained using electron microscopy. From the preceding discussion it is evident that the morphology of IPNs should be determined by the following factors (1) the thermodynamic miscibility of two networks, (2) the kinetic conditions of the curing reactions, and (3) the mechanism of phase separation. In principle, three distinct features of IPN structure should be revealed. 

Various types of IPNs may also be classified by the mechanism of phase separation proceeding during IPN formation. These mechanisms are nucle-ation and growth, and spinodal decomposition. Differences in the conditions of phase separation predetermine the physical and morphological features of IPNs. As a rule, simultaneous IPNs are phase-separated by a spinodal mechanism, and sequential ones via a mechanism of nucleation and growth. Another approach to IPN nomenclature was proposed by Sperling [1], who used the differences in morphological features of IPNs. 

The comparison of the phase behavior of semi-IPNs based on almost miscible polymers—linear PS and poly-a-methylstyrene cross-linked by DVB and mixtures of the corresponding homopolymers—has shown that for homopolymer blends only one glass transition is observed, its position obeying the Fox equation. Simultaneously, for semi-IPNs there were two glass transitions, far from the glass transition temperatures of the components, for the same compositions where linear blends are miscible. A difference in phase behavior between blends and semi-IPNs seems to be evident. However, no phase diagrams allow one to determine the mechanism of phase separation. 

Structures typical for IPNs because of the mechanism of phase separation may appear only within a definite time interval, which is always less than the time of the gelation and of the formation of the final IPN structure. Subsequent (after At) cross-linking of both networks or one network occurs in the evolved microregions of phase separation up to when the final conversion degree is reached. 

A different approach was taken by Touhsaent et al. [2081. These authors synthesized two polymers, one of which formed a network, by simultaneous independent reactions in the same container. They have indicated that intercrosslinking reactions are eliminated by combining free radical (acrylate) and condensation (epoxy) polymerization. By this method, they modified an epoxy resin with poly(n-butyl acrylate) polymer. They have found that a two-phase morphology developed, consisting of co-continuous rubber domains (about 0.1—0.5 p-m) within the epoxy resin. The dimensions of the dispersed rubber phase domains and the extent of molecular mixing between the two components were found to depend on the relative reaction rates (or gel time) with respect to the rate of phase separation. Better mechanical properties resulted when the extent of molecular mixing was minimized and heterophase semi-IPNs were produced. 

The semi-IPNs filled with 6.5 to 10.5 vol% of -Fe203 are characterized by dual phase continuity. A very important characteristic of the phase separation in the IPNs is the degree of segregation, a, giving the fraction of material, which undergoes phase separation. When a = 1, the system is fully phase-separated at a = 0, it is fully miscible. The value of a can be estimated from the characteristic maxima of the mechanical loss curves according to the equation proposed by Rosovitsky and Lipatov in [59] ... 

As will be shown later, the phase separation begins at very low degrees of conversion and is enhanced by increasing MM and voliune fraction of copolymer. The kinetics of IPN formation determines the onset of phase separation and influences strongly this process as a whole, whereas phase separation does not influence the kinetic cmves. This fact may serve as an additional confirmation of the assumption that the phase separation proceeds according to a spinodal mechanism, because in this case the compositions of two evolving phases are very close. 

The data on the temperature dependence of the elastic modulus and of the mechanical loss tangent are given in Fig. 38. It is seen that semi-IPNs are typical two-phase polymeric systems, exhibiting two relaxation transitions (determined from the positions of loss tangent maximiun). The shape of the temperature dependence of elastic modulus is also typical of two-phase systems. At the same time, a substantial shift in Tg of the hnear polymer from 343 to 308 K (samples 8-3) and a less marked shift in Tg of the PU network indicate the formation in the system of two phases with dissimilar compositions. The concurrent processes of PU network formation and microphase separation caused by thermodynamic immiscibihty of components result in the formation of phases enriched in one of the components. 

For some systems a comparison of the properties of full and semi-IPNs has been made [227-229]. The main difference in dynamic mechanical properties consists in that the shift of temperature transitions in semi-IPNs is higher as compared with full IPNs. This effect was explained by the more complete phase separation in semi-IPNs as compared with full ones. 

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