Triple-shape memory effect of long-chain branched Poly(lactic acid)-b-poly(lactide-co-caprolactone) and its controllable shape recovery as self-fastening smart bone fixture

Abstract

Aiming to simultaneously realizing self-fastening and self-reinforcing function for PLA as smart bone fixation material, long-chain branched poly(lactic acid)-b-poly(lactide-co-caprolactone) (PLA-b-PLCL) block copolymer with controllable triple shape memory effect was fabricated. Under low strain, two given temporary shapes were well presented for PLA -b- PLCL at T high (>T g1 (glass transition temperature of PLA phase)) and T low (T g1 ∼ T g2 (glass transition temperature of PLCL phase)), and then two-stage shape recovery was triggered upon reheating, showing excellent triple shape memory effect. For highly oriented PLA -b- PLCL produced by two-stage drawing, under high strain, during the 1st step recovery at temperature range of T g1 ∼ T g2, recovery ratio reached as high as 92–99%, indicating excellent memory effect for the second temporary shape. Meanwhile, T g2 of PLCL phase, long period (Lac), amorphous layer thickness (La), orientation factor (F) decreased, while crystallinity (X c) , lamellae layer thickness (L c), lateral size of lamellae (Llateral) almost kept unchanged. During the 2nd stage recovery at temperature > T g1, samples showed low recovery ratio, indicating that after large deformation, oriented crystallization structure formed in PLA phase was stable. Accompanied by relaxation of oriented PLA chains in amorphous region and formation of new shish-kebab structure, X c/L Lateral increased, and F/L a/L ac decreased. Oriented PLA-b-PLCL bone screws were prepared, and after the 1st step recovery, cross-section area of screw increased resulting in an obvious increase in pullout force. Meanwhile, strength/modulus remained high values and the creep strain was low, exhibiting superior self-reinforcement and self-fastening effect for smart bone fixation.