Structure Evolution of Highly Oriented Poly(lactic acid)-b-poly(lactide-co-caprolactone) Block Copolymer during Two-Stage Solid Phase Hot Drawing

To realize a high degree of molecular orientation and highly efficient self-reinforcement for poly(lactic acid)-b-poly(lactide-co-caprolactone) (PLA-b-PLCL) block copolymer as a bone fixation material, two-stage solid phase hot drawing technology was established and the evolution of oriented crystalline/two-phase separated structure during drawing was studied. During the first stage drawing at a temperature higher than T-g1 (glass transition temperature of the PLA phase), crystallinity (X-c)/orientation factor (f)/content of mesophase increased with draw ratio, and a dense oriented crystalline structure together with the microfiber bundle structure formed. Meanwhile, the T-g1 moved toward high temperature, and T-g2 (glass transition temperature of PLCL phase) remained unchanged. Compared with PLA, motion ability and regularity of PLA-b-PLCL chains decreased, and thus the oriented mesophase was difficult to be transformed into a crystalline structure, showing lower X-c/f but higher mesophase content. During the second stage drawing at a temperature between T-g1 and T-g2, with increasing draw ratio, X-c changed slightly, while T-g2 moved to a high temperature. Meanwhile, PLCL phases were elongated and tended to coalesce, showing a continuous flat-banded structure. For samples with low draw ratio for the first stage drawing, L-c, L-a, L-lateral decreased, while f increased slightly during the second stage drawing. For samples with higher draw ratio for the first stage drawing, structural changes mainly focused on the ordered arrangement of PLCL chains and rupture of PLA lamellae. After two-stage drawing for PLA-b-PLCL, the modulus and tensile strength reached 4.3 GPa and 233 MPa, respectively, basically meeting the requirements for bone fixation.

Automated summary

In this study, a two-stage solid phase hot drawing technology was established to achieve a high degree of molecular orientation for poly(lactic acid)-b-poly(lactide-co-caprolactone) block copolymer. The evolution of the oriented crystalline/two-phase separated structure during drawing was studied. The results showed that by adjusting the draw ratio and temperature, the properties and structure of the material can be effectively controlled.