Polymer networks prepared from 4-arm star-shaped L-lactide oligomers with different arm lengths and their semi-interpenetrating polymer networks containing poly (L-lactide)

Polymer networks and semi-interpenetrating polymer networks (semi-IPNs) based on poly(L-lactide) (PLA) were prepared by reactions of 2,4-tolylene diisocyanate (TDI) and hydroxy-terminated 4-arm star-shaped L-lactide oligomers (H4LAO(n)'s) with the degrees of polymerization of lactate unit per one arm, n = 3, 5 and 10 in the absence and presence of PLA, respectively. Thermal, mechanical properties and morphologies of the TDI-bridged H4LAO(n)'s (TH4LAO(n)'s) and TH4LAO(n)/PLA semi-IPNs were evaluated by comparing with those of PLA. Scanning electron microscopic observation revealed that compatibility between the two components of TH4LAO(n)/PLA semi-IPNs becomes better with increasing PLA content and n value. Especially, no clear phase-separation was observed for TH4LAO(10)/PLA 25/75. All the TH4LAO(n) networks were substantially amorphous and their tan delta peak or glass transition temperatures increased with decreasing n value. The tan delta peak temperatures of TH4LAO(3) (86.2 degrees C) and TH4LAO(5) (79.8 degrees C) were much higher than that of PLA (55.1 degrees C). All the semi-IPNs except for TH4LAO(10)/PLA 25/75 displayed two tan delta peaks related to TH4LAO(n) and PLA. TH4LAO(3)/PLA 75/25 exhibited a much higher E' at around 50-90 degrees C than PLA. TH4LAO(5) had the highest tensile strength (90 MPa), modulus (2.5 GPa) and tensile toughness (2.1 MJm(-3)) among the TH4LAO(n)'s, which were higher than those of PLA (51 MPa, 2.0 GPa and 0.5 MJ m(-3)), while the semi-IPNs did not exhibit much better tensile properties than PLA did.