Solid-State Complexation of Poly(Ethylene Glycol) with α−Cyclodextrin

Low-molecular-weight liquid poly(ethylene glycol) (PEG) spontaneously forms an inclusion compound (IC) when combined with α-cyclodextrin (α-CD) powder at room temperature. This process can be followed with wide-angle X-ray diffraction (WAXD). The WAXD data shows that the α-CD crystals undergo a solid-state crystal−crystal transformation from the cage to the channel crystal structure upon IC formation over a period of about 8 h. The time dependence of the 2θ = 20° α-CD channel structure X-ray peak can be described by a simple first-order kinetic model. The effects of changing the temperature, PEG:α-CD molar ratio, PEG molecular weight, and vacuum-drying the CD have been studied. The barrier opposing the PEG inclusion-induced solid-state transformation of α-CD from the cage to the channel crystal structure appears to be dominated by changes in the packing/interactions of α-CDs, rather than the loss in the conformational entropy experienced by the PEG chains during the inclusion process.