Pluronic and β-cyclodextrin in water: from swollen micelles to self-assembled crystalline platelets

Abstract

Cyclodextrin (CD) and copolymers of polyethylene oxide (PEO)-b-polypropylene oxide (PPO)-b-polyethylene oxide (PEO) (pluronics) form inclusion complexes (ICs) in water. A solution of native unmodified β-CD was mixed with a dilute solution of pluronic F-68 at 70 °C. The temperature was then quickly cooled down below the critical micellization temperature (CMT) of the triblock-copolymer. The mixture was left ageing at controlled temperature until turbidity, opalescence and finally sedimentation occurred. The self-organization behaviour of the system was investigated by small angle neutron scattering (SANS) in D₂O. At high temperature the formation of stable, β-CD-swollen micelles was observed. ICs were formed while the mixture was thermally quenched. Their subsequent aggregation was studied by SANS, and the resulting structures were observed by atomic force microscopy (AFM) after film deposition. With time, we evidenced the formation of thin platelets (17 nm) having large planar dimensions (about 1 µm at 20 °C to 4 µm at 40 °C) and a crystallographic angle of 105.4°. Fitting the SANS data, a model was proposed where the growth of a platelet occurred through association of IC bundles in a characteristic time of 190 min. Each bundle (2.4 nm in radius and 17 nm in length) of well-organized IC molecules is formed in a characteristic time of 17 min at 20 °C. The platelets are original because of their crystalline nature with well-defined angles, and their very flat and rigid shape. Moreover they form effortlessly through self-assembling in water.