Semi-interpenetrating polymer networks (SIPNs) were prepared from PVC and 5-15 wt.% of di-(4,4-diisocyanatophenyl)methane (MDI) oligomers by directly mixing the liquid MDI with small (150 mu m dia.) porous (30% voids) unplasticized PVC particles at low temperatures followed by hot press curing. The tensile, flexural, and impact strengths increased significantly when these small amounts of isocyanate networks were created in PVC. These SIPN blends exhibited tan delta peak temperatures and single distinct loss modulus, E ", peaks at temperatures lower than those of PVC which had been exposed to the same processing temperatures. These observations rule out the presence of large PVC domains distinct from PVC/isocyanate SIPN domains and pure thermoset domains. A substantial fraction of the isocyanate appears to exist in SIPN type phases in these blends. Considerable amounts of unextracted residue (about 30-36%) remained after 48 h of continuous THF extraction of these SIPN blends. A crude correlation was noticed between the amounts of SIPN residue present and the mechanical strength improvements. Mathematical modeling of DMTA-derived T-g data by a T-g third power blend composition equation was employed to understand SIPN structures and the nature of PVC/isocyanate interactions. DMTA measurements of segmental mobility indicated that the isocyanate had a lower cross-link density when diluted in PVC than in the pure cured isocyanate. Fitting experimental T-g values gave parameters indicating that both the binary hetero-interactions (enthalpic effects) and the conformational redistributions (entropic effects) during the binary hetero-interactions contributed to SIPN formation. (C) 2000 Elsevier Science Ltd. All rights reserved.