Highly toughened poly(ʟ-lactide) by poly(ᴅ-lactide)-containing crosslinked polyurethane shows excellent malleability, flexibility and shape memory property

Abstract

The brittle nature of poly(ʟ-lactide) (PLLA) limits its extensive application. In the present work, PLLA was toughened by crosslinked polyurethane (CPU), which was formed by reactive blending of poly(ε-caprolactone) (PCL), poly(d-lactide) (PDLA), hexamethylene diisocyanate (HDI) and glycerol. PCL segments in the network acted as soft segments while PDLA segments served as hard segments to form stereocomplex (SC) crystallites as physical crosslinking points. The disappearing peak at 2270 cm−1 and the emerging absorption peak appeared at 1530 cm−1 in the FTIR spectrum proved the formation of the network in the modified PLLA. The melt-quenched specimens containing the network showed significant increase in elongation at break, and the CPU80/18/2 blend showed a maximum value of 428.6%. Tensile-fractured surfaces of all the blends exhibited fibril-like morphologies because of the matrix shear yielding caused by large deformation. And the crystallization rate could be accelerated because of the PDLA chains in the sample comparing to the CPU80/20 without PDLA. Interestingly, when SC crystallites could be formed after annealing of the melt-quenched specimens at 110 °C for 1h, CPU80/18/2 still owned a maximum value of 152.8% of elongation at break. It was found that the CPU phase was presented as continuous phase in the material. Due to the compatibility, the added PDLA chains were distributed in the interface of CPU and PLLA phase, which provided additional interfacial adhesion, improved the stress transmission and endowed the material with enhanced toughness. The CPU80/18/2 sample also showed the malleability, flexibility and shape memory capacity, implying great application potential as biodegradable green composites.