Structure and shape memory effects of poly(L-lactide) and its copolymers

A series of biodegradable poly(L-lactide) (PLLA) and its copolymers with poly(ε-caprolactone) (PCL) is synthesized and characterized. The mechanical properties and shape memory effects are studied by means of dynamic mechanical analysis (DMA) and tensile tests. It is found that the mechanical properties are significantly affected by the polymer compositions. With the ε-caprolactone (ε-CL) content increasing, the maximum stress decreases linearly and the strain at break increases gradually. By adjusting the compositions, the polymers exhibit excellent shape memory effects and a maximum recovery stress of about 6.2 MPa can be obtained for the PCLA70 copolymer. Structure of the polymers during the shape memory process is investigated using differential scanning calorimetry (DSC) and x-ray diffraction (XRD). The polymers are observed to consist of the PLLA crystal and amorphous phase which serve as physical cross-links and reversible phase, respectively. Shape memory effect is attributed to be the deorientation of the oriented amorphous chains due to thermodynamic entropy effect.