Crystallization and thermal characterization of biodegradable tri-block copolymers and poly(ester-urethane)s based on PCL and PLLA

In this paper the crystallization behavior of biodegradable linear tri-block copolymers PLLA-b-PCL-b-PLLA and their corresponding poly(ester-urethane)s was studied and related to their thermal stability. A series of tri-block copolymers was synthesized by ring opening polymerization of l-lactic acid thus using PCL block with constant molecular weight of about 8000 g/mol and varying the amount of PLLA. The poly(ester-urethane)s were synthesized by polycondensation of the tri-block copolymers with hexamehylene diisocyanate. The crystallinity nature of the polymers was investigated by wide angle X-ray diffraction (WAXD) measurements allowing the determination of the cell structure parameters. Furthermore, the kinetic of the crystallization process from the melt was studied with differential scanning calorimetry (DSC) measurements and by using the Avrami equation to describe the process in terms of crystallization growth and rate. Finally, the thermal degradation of the tri-block copolymers as well as of the poly(ester-urethane)s was investigated by dynamic thermogravimetric analysis (TGA). It was found that the differences on the molecular structure in terms of block copolymer composition, molecular weight, crystallinity and the presence of urethane bonds, affect the degradation behavior of each block.