Stereocomplex crystallite network in poly(d,l-lactide): formation, structure and the effect on shape memory behaviors and enzymatic hydrolysis of poly(d,l-lactide)

Abstract

Stereocomplex crystallization is a very interesting crystal modification formed between enantiomeric polymers, such as poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA). Herein, biodegradable poly(D,L-lactide) (PDLLA) and stereocomplex-poly(L- and D-lactide) (sc-PLA) blends were prepared by solution blending at various sc-PLA loadings ranging from 2.5 to 10 wt%. Wide-angle X-ray diffraction and differential scanning calorimetry results verified that complete stereocomplex crystallites without any evidence of the formation of homocrystallites in the PDLLA could be achieved. By a rheological approach, a transition from the liquid-like to solid-like viscoelastic behaviour was observed for the stereocomplex crystallites reserved PDLLA melt, and a frequency-independent loss tangent at low frequencies appeared at a sc-PLA concentration of 5 wt%, revealing the formation of stereocomplex crystallite network structure. By a delicately designed dissolution experiment, the structure of the formed network structure was explored. The results indicated that the network structure were not formed by stereocomplex crystallites connected directly with each other or by bridging molecules, but by the interparticle PDLLA chains which were significantly restrained by the crosslinking effect of sc-PLA. Accordingly, the mechanical properties of PDLLA were greatly enhanced after blending with sc-PLA. Moreover, the most intriguing result was that the shape memory behaviors of PDLLA had been improved obviously in the blends than in neat PDLLA, especially when a percolation network structure had formed, which may be of great use and importance for the wider practical application of PDLLA. Finally, it was found that the enzymatic hydrolytic degradation rates had been retarded in the blends than in neat PDLLA. The erosion mechanism of neat PDLLA and the blends was further discussed.