AbstractSemi‐interpenetrating networks have been synthesized from vernonia oil‐sebacic acid polyester network and poly(ethylene terephthalate) (PET). Bond interchange reactions during mixing of the two materials led to the formation of a miscible copolymer mixture, in which the vernonia oil was then cross‐linked with sebacic acid. The materials were phase‐separated, exhibiting two glass transitions, when the network was synthesized at 160°C, below the crystallization temperature of PET: however, a single stable glass transition (Tg) results after the material is heated to above the melting temperature of PET and cooled. When the vernonia polyester network was completely formed at 250°C, above the crystallization temperature of PET, noncrystalline, single‐Tgmaterial was created. The two‐phase semi‐IPNs were much tougher than were their constituent materials, with the 50 semi‐IPN over 15 times tougher than the PET from which it was made and over 50 times tougher than the neat vernonia oil elastomer, with tensile energy to break of 1780 kJ/m3. The single‐Tgmaterial was nearly 2.5 times as tough as the two‐phase material, with energy to break of 4400 kJ/m3. The microstructure of the two‐phase 50 semi‐IPN was investigated by transmission electron microscopy, which showed regularly shaped spherulites of 10–20 μm in diameter, as compared to irregularly shaped spherulites observed in a similar 50/50 castor oil urethane/PET semi‐IPN, in which the network formed simultaneously with PET crystallization. Scanning electron microscopy of the semi‐IPN fracture surfaces showed microscopic fibrils several hundred nanometers in diameter in both the two‐phase and single‐Tgmaterials, although only the two‐phase semi‐IPN had a macroscopically ro
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