SAXS analysis of the morphology of biocompatible and biodegradable poly(ε-caprolactone-b-glycolide) copolymers

Abstract

Poly(ε-caprolacrone-b-glycolide) diblock copolyesters have been synthesized by the sequential polymerization of ε-caprolactone and glycolide as initiated by aluminium alkoxides. Copolymerization is typically “living” and yields copolyesters of perfectly controlled molecular weight and composition. Diblock molecular weight (Mn PGA + Mn PCL) ranges from 5700 to 42000 and the ϱ = Mn PCL Mn PGA ratio varies from 1.5 to 13.1. Due to the inherent insolubility of the polyglycolide (PGA) segment in common organic solvent, the diblock copolyesters form stable non-aqueous colloidal dispersions e.g., in toluene, the stability of which results from the soluble poly(ε-caprolactone) (PCL) block. Combining all the experimental observations (PCS, TEM, WAXS, SAXS, AFM), a micelle model has been proposed which consists of a polyglycolide core surrounded with a corona of polycaprolactone (PCL). Both constituents are semi crystalline. From SAXS observations, the PGA core is better described by two concentric spheres. The internal sphere of a 5–6.7 nm diameter would essentially contain crystalline PGA. The diameter of the external sphere, D PGA, is in the range from 6.2 to 9.6 nm, at least for the investigated diblock copolymers. As a rule, this diameter increases as ϱ decreases at constant molecular weight and as the diblock molecular weight increases at constant ϱ. A scattering peak (weak) is observed in the range from 10.8 to 15.5 nm and the Bragg distance is close μD PGA, where μ is equal to (1+ 3 ϱ 2 ) 1 3 . From steric considerations, μ is the ratio between the diameter of the micelle and the diameter of the PGA core, so that this peak has been assigned to the characteristic intermicellar distance. At very small angles, several additional peaks are the signature of a hyperstructure which is possibly lamellar.