Three types of urethane-acrylate IPN's, two sequential and one simultaneous, were prepared in which the urethane was formed under thermal polymerization; and the acrylate, by photopolymerization. The IPN morphologies were studied by transmission electron microscopy (TEM) and dynamic mechanical analysis (DMA). When the urethane was reacted to relatively high conversion prior to photoinitiation of the acrylate, the IPN's obtained were characterized by very small domain sizes (<20 nm) and a single Tg between those of the two components. When the acrylate polymerization was induced following much smaller conversion of the urethane, or if the acrylate was polymerized first, domains of larger size (>100 nm) were obtained. In these two instances, if the Tg's of the individual components are well separated, dynamic mechanical analysis indicated separate Tg values close to the Tg's of the individual components. The degree of phase mixing in each IPN was estimated from the Tg values from the Fox equation, and these correlate with the various processing conditions.

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Two sequential urethane-acrylate interpenetrating network (IPN) systems were prepared in which the polyurethane (PU) phase is labeled with donor and acceptor dyes. Direct nonradiative energy transfer (DET) measurements on these systems indicate less efficient energy transfer in the IPN than in the corresponding pure PU matrix. This result is interpreted in terms of dilution of the dyes by mixing at the molecular level between the polyacrylate (PA) and PU components. Quantitative analysis of the changes in DET efficiency allows the extent of phase mixing to be calculated. The two SeqIPN's, PU g15 75% PA(EHA) 100% PU 100% and PU g180 100% PA(IBA) 100% , were labeled by incorporating phenanthrene and anthracene diols into the reaction mixture. Fluorescence decays of phenanthrene in these samples were measured by the single-photon-timing technique and analyzed in terms of both the Fo ¨rster model for DET and the Perrin model for static quenching. Both analyses gave similar extents of phase mixing, and these values are in good accord with the results of electron microscopy and dynamic mechanical experiments. Similar experiments were carried out during the polymerization reaction. These allowed features of the morphology evolution to be understood.

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