Dielectric studies of chain dynamics in homogeneous semi-interpenetrating polymer networks

Semi-interpenetrating polymer networks (semi-PPNs) were prepared from linea r polyurethane (PUR) and polycyanurate (PCN) networks. Wide-angle X-ray sca ttering measurements showed that the IPNs were amorphous, and differential scanning calorimetry and small-angle X-ray scattering measurements suggeste d that they were macroscopically homogeneous. Here we report the results of detailed studies of the molecular mobility in IPNs with PUR contents great er than or equal to 50% via broadband dielectric relaxation spectroscopy (1 0(-2)-10(9) Hz, 210-420 K) and thermally stimulated depolarization current techniques (77-320 K). Both techniques gave a single alpha relaxation in th e IPNs, shifting to higher temperatures in isochronal plots with increasing PCN content, and provided measures for the glass-transition temperature (T -g) close to and following the calorimetric T-g. The dielectric response in the IPNs was dominated by PUR. The segmental alpha relaxation, associated with the glass transition and, to a lesser extent, the local secondary beta and gamma relaxations were analyzed in detail with respect to the timescal e, the shape of the response, and the relaxation strength. The alpha relaxa tion became broader with increasing PCN content, the broadening being attri buted to concentration fluctuations. Fragility decreased in the IPNs in com parison with PUR, the kinetic free volume at T-g increased, and the relaxat ion strength of the alpha relaxation, normalized to the same PUR content, i ncreased. The results are discussed in terms of the formation of chemical b onds between the components, as confirmed by IR, and the reduced packing de nsity of PUR chains in the IPNs. (C) 2000 John Wiley & Sons, Inc.