Long-Chain Branched Poly(lactic acid)-b-poly(lactide-co-caprolactone): Structure, Viscoelastic Behavior, and Triple-Shape Memory Effect as Smart Bone Fixation Material

A novel fully biosbased poly-(lactic acid)-b-poly-(lactide-co-caprolactone) (PLA-b-PLCL) with a two-phase structure and long-chain branches was specifically designed and prepared through reactive melt processing. The results showed that PLCL segments were introduced onto PLA chains successfully. With the increase of PLCL content, the blockier distribution of LA/CL chain sequences of the sample was exhibited. PLA-b-PLCL showed two distinct thermal transitions, corresponding to the glass transition of PLA and PLCL domains, respectively, whereas the phase morphology changed from a seaisland to a co-continuous structure with increasing PLCL content. Because of the long-chain branched structure, PLA-b-PLCL samples showed a much higher viscoelasticity, strong molecular entanglement, and obvious strain-hardening behavior, resulting in a high draw ratio of the sample during orientation process, whereas the tensile strength and the modulus of the oriented sample reached up to 173 MPa and 5.4 GPa, respectively, which basically met the requirements of bone screws. Moreover, PLA-b-PLCL showed a triple-shape memory effect at 55 and 120 degrees C, respectively. For PLA-b-30 wt % PLCL, the recovery ratio can reach up to 98.1% under 55 degrees C, while high mechanical properties can be maintained, realizing self-reinforcement and self-fastening effect simultaneously as a smart bone fixation material.