Tough, optically clear simultaneous interpenetrating polymer networks (SINs) of polyurethane (PU) and poly(allyl diglycol carbonate) (ADC) at different compositions were synthesized. The effects of the molecular weight of PU soft segment on the morphology, mechanical properties, and thermal transition behavior of the SINs at two levels of crosslinking agent were studied. The miscibility of PU/ADC SINs, studied by TEM and DMA, was greatly influenced by the SIN composition and the molecular weight of poly(caprolactone) diol (PCL) as the PU soft segment. A single-phase morphology at a PU concentration of 10% changed to a very fine microheterogeneous morphology as the molecular weight of PCL changed from 530 to 1250. The two-phase morphology of the PU10/ADC90 SIN based on higher PCL molecular weight (PCL 1250) was also confirmed by DMA, which displayed a sharp peak for the ADC-rich phase and a small shoulder for the PU-rich phase transition in the tan 8 plot. The SINs at 20-30% PU composition exhibited co-continuous phase morphology in the transmission electron micrographs, in which the phase regions grew larger as the PCL molecular weight increased from 530 to 1250. All the SIN samples possessed excellent optical transparency except two samples with 30% PU composition based on PCL 1250, which showed a hazy appearance. The tensile strength, modulus, and toughness of the SINs decreased by increasing the molecular weight of PCL from 530 to 1250, whereas the elongations at break remained nearly unchanged. (C) 2003 Wiley Periodicals, Inc.