Relaxation Spectra of IPNs

It is known that the most fundamental characteristic of relaxation behavior is the relaxation spectriun, which may be calculated from the following equation  

From the temperature dependence of tan 8 it follows that for the 65 35 IPN the continuous phase is the PU-enriched phase, whereas for the 50 50 IPN the continuous phase is PBMA-enriched phase. From the temperature dependence of tan 8 at various frequencies the activation energies of the relaxation processes in IPNs have been calculated [192] using the Arrhenius equation  

Here r was found from the condition cot = I for the maximum tan 8 co is the frequency). Because IPNs have two glass transition temperatures, it is pos- 

It can be seen that for each IPN the values of Ea are practically the same for both phases. For the 65 35 IPN, Ea for both phases is very close to Ea for pure PU, whereas for the 50 50 IPN, Ea is different for the two phases and only for the PBMA-enriched phase is Ea the same as for pure PBMA. These results seem to be rather puzzling and may be connected to the dual-phase morph- 

network ratio PU-enriched phase PBMA-enriched phase 

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In such a way, IPNs have a much broader set of relaxation times than the pure constituent networks. The relaxation spectra of IPNs cannot be obtained by simple superposition of the spectra of the constituent networks. The broader spectra of the IPNs may be explained by the existence of a two-phase structure where each phase is enriched in one of the components. Simultaneously, the existence of two phases is reflected in the relaxation spectra by their broadening and shift along the time axis. It is evident that in spite of the incompatibility of the two networks, there exists a strong physical interaction between macromolecular chains of dissimilar chemical nature, which may be described in terms of entanglements, and strong polar interactions. The latter may be the reason why 50 50 IPN has its relaxation spectrum shifted to higher relaxation times than that for 65 35 IPN. 

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