Inclusion complexes between cyclodextrins and triblock copolymers in aqueous solution: A dynamic and static light-scattering study

Simultaneous static and dynamic light experiments have been made on various cyclodextrins and cyclodextrin derivatives, as well as the inclusion complexes formed between different polyethylene oxide/polypropylene oxide triblock copolymers (PEO-PPO-PEO) (pluronics) and dimethyl-beta-cyclodextrin (DIMEB). The inclusion complexes formed between DIMEB and pluronics are highly soluble, in contrast to the insoluble complexes formed between beta-cyclodextrin and the same substances. The static light-scattering (LS) data show that approximately 11 DIMEB molecules thread onto the copolymer chains and are located on the PPO block. With the inverse structure (PPO-PEO-PPO), about seven DIMEB molecules are present in the complex. NMR measurements are used to substantiate complex formation by means of characteristic changes in the proton signals. Hydrodynamic radii obtained from the dynamic LS data at infinite dilution for the cyclodextrins correspond well with dimensions determined using X-ray methods. Inverse Laplace transformation (ILT) allowed resolution of the relaxational modes from the cyclodextrin/pluronic complex and the excess cyclodextrin. The complexes formed with the DIMEB are shown to be significantly larger than the copolymer unimers, which may be due to accentuation of steric hindrance to flexing in the PPO block. With the inverse pluronic structure, on the other hand, the complex is smaller in radius than the unimer. At temperatures above which the copolymer forms micelles, addition of DIMEB inhibits both cluster formation and micellization of the pluronics and also prevents network formation with the inverse pluronic, whereas the trimethyl analogue (TRIMEB) does not have this effect.