Stereocomplex formation in stereoblock copolymer networks composed of 4-armed star-shaped lactide oligomers and a 2-armed ε-caprolactone oligomer

Stereoblock copolymer networks were prepared by reactions of methylenediphenyl 4,4′-diisocyanate (MDI) with hydroxy-terminated 4-armed star-shaped L-lactide and D-lactide oligomers (H4LLAO and H4DLAO) with the degree of polymerization per arm n = 15 in the presence and absence of a hydroxy-terminated 2-armed e-caprolactone oligomer (H2CLO). Thermal and mechanical properties of the MDI-bridged stereoblock copolymer networks (MH-4scLAO/2CLOs 100/0, 75/25, 50/50, 25/75 and 0/100) were compared with those of a simple H4LLAO/H4DLAO blend (H4scLAO) and MDI-bridged homochiral (hc) networks of H4LLAO and H4DLAO (MH4LLAO and MH4DLAO). X-ray diffraction and differential scanning calorimetric analyses revealed that stereocomplex (sc) crystallites are formed without any hc crystallization for MH-4scLAO/2CLOs and H4scLAO, and that MH4LLAO and MH4DLAO are amorphous. The melting temperatures of sc crystallites for MH-4scLAO/2CLOs were much higher than those of hc crystallites of H4LLAO and H4DLAO, while the values were slightly lower than that of H4scLAO. Elongations at break and tensile toughnesses of MH-4scLAO/2CLOs (75/25, 50/50 and 0/100) were much higher than those of MH4LLAO and MH4DLAO.