Structural features of inclusion complexes of γ-cyclodextrin with various polymers

The crystalline structures of inclusion complexes of γ-cyclodextrin (γ-CD) with poly(ethylene glycol), poly(ethylene adipate), poly(propylene glycol) and poly(isobutylene) were studied by electron microscopy, in combination with X-ray diffraction works and measurements of thermal properties by DSC and TGA. The crystalline structure of as-prepared complexes was tetragonal and its cell dimensions were a = b = 2.380 nm and c = 1.48 nm. When an as-prepared sample was dried in a vacuum at room temperature, the tetragonal modification was transformed into the monoclinic one with the projected cell dimensions of a = 1.75, b = 1.36 nm and γ = 110°. The transformation occurred by progressive 'shifting' of rows of polymer necklaces in the [110] direction along the (110) plane in an original tetragonal lamellar crystal. Complexes lost weight by 10-15% in the process of heating up to 140 °C. The tetragonal crystalline modification was transformed into the hexagonal one, and concurrently, the X-ray diffraction profiles of annealed complexes were broadened. When a sample was dried in a vacuum at room temperature or annealed at high temperatures, followed by exposure to water vapor, the original tetragonal crystalline structure was recovered, restoring the original degree of orientation of crystallites in the sample. When water molecules were removed, the lateral stacking order of γ-CD-polymer complexes was destroyed, but the basic necklace structure in which polymer chains threaded through the cavity of γ-CD rings' structure could be retained.