Single crystals morphology of biodegradable double crystalline PLLA-b-PCL diblock copolymers

Abstract

The morphology of solution grown single crystals of a series of double crystalline diblock copolymers derived from l-lactide and ɛ-caprolactone has been investigated by means of transmission electron microscopy. The copolymers had a variable composition with a poly(l-lactide) weight percentage that ranged between 81 and 10%. All samples had a low polydispersity index (1.4–1.1) and a similar number average molecular weight (20,000–35,000g/mol).Bulk crystallization and melting behaviour of diblock copolymers were evaluated by DSC and the results demonstrated the double crystalline nature of the samples. Fractionated crystallization clearly occurred in copolymers having an intermediate composition.Isothermal crystallizations were performed in dilute n-hexanol solutions at temperatures that ranged between 80 and 50°C. Crystal morphologies were dependent on the crystallization temperature and even on the composition. Thus, the inability of poly(ɛ-caprolactone) (PCL) blocks to crystallize between 80 and 70°C rendered lozenge, truncated and spindle-shaped crystals associated to the poly(l-lactide) (PLLA) block. These usually had thicker edges due to PLLA overgrowths that mainly took place in their periphery. However, an overgrowth of irregular PCL crystals during subsequent cooling and crystallization at room temperature was also detected. Complex morphologies constituted by lamellar crystals of both PCL and PLLA blocks were developed at intermediate temperatures (70–65°C), whereas elongated hexagonal morphologies mainly associated to the PCL block were detected at the lowest crystallization temperature. In general, electron diffraction patterns showed for all samples’ reflections associated to both poly(ɛ-caprolactone) and poly(l-lactide) (α-form) crystals. The relative intensity between the two types of reflections varied according to the copolymer composition.