Thermodynamics and Phase Separation in IPNs

it is most probable that IPNs are two-phase microheterogeneous systems with some interpenetration between two constituent phases and with a molecular level of mixing in each phase, due to the impossibihty for these phases to be separated under conditions of IPN formation. 

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Cartoons that illustrate the two principal tjfpes of IPN are given in Figure 1.38. The properties of the IPN and the morphology that results are governed by the kinetics and thermodynamics of phase separation. 

In conclusion, one has to say that the general state of the investigation of the thermodynamics of IPN mixing and phase separation is far from the one we have for blends of linear polymers [52]. In particular, one very important factor is not accoimted for by studying the miscibihty of two networks, namely, the changes of volume by mixing. The last parameter is important not only for thermodynamics, but also for the structure and viscoelastic behavior of IPNs and is considered below. 

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. 

Exclusively mechanically interlocked linear polymer blends, typically, are not thermodynamically phase stable. Given sufficient thermal energy (Tuse>Tg), molecular motion will cause disentanglement of the chains and demixing to occur. To avoid phase separation, crosslinking of one or both components results in the formation of a semi-IPN or full-IPN, respectively. Crosslinking effectively slows or stops polymer molecular diffusion and halts the phase decomposition process. 

For example, increased crosslink density in polymer network I in an IPN clearly decreases the domain size of polymer II.This is illustrated by comparison of Figure 2.3, bottom left and bottom right. This effect appears reasonable because a tighter initial network must restrict the size of the regions in which polymer II can phase separate. However, the role of crosslinks in merely diminishing phase domain size, and in increasing compatibility in a thermodynamic sense, needs to be carefully distinguished. 

Phase structure and relaxation behavior. In previous publications [38,40,43-49], several methods were used for characterization of the microphase structure of the semi-IPNs studied. WAXS, SAXS, DSC [40], dielectric relaxation spectroscopy (DRS), and TSDC [48] measurements showed that pure PCN is characterized by a typical homogeneous structure, but in segmented TPUs microphase separation was observed on the level of the thermodynamically immiscible hard and soft domains. As for semi-IPNs, the destruction of the microphase-separated morphology of TPU was observed and microphase separation between the PCN and TPU phases, expected from the difference in their solubility parameters, was not found. Grigoryeva et al. [43] noted that... 

What is really interpenetrating in IPNs is the penetration of the problem connected with their formation. It is difficult to discuss kinetics without thermodynamics and vice versa. The thermodynamics of interaction between two network components and the reaction kinetics determine the onset of phase separation, the structure, and the viscoelasticity of IPNs. This is why we begin our consideration with thermodynamic analysis and the elucidation of the reaction kinetics and its interconnection with phase separation. Only after considering these problems does the possibility arise to analyze both the structure and the viscoelasticity of IPNs. 

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