Solid inclusion complexes were prepared by solution precipitation of α-cyclodextrin (α-CD) and monodisperse poly(oxyethylene)s (POEs). The POE was either perdeuterated with molecular weights from 1.5 to 59.2 kg/mol or fully hydrogenated with molecular weights from 1.5 to 86.6 kg/mol. The number of cyclodextrins per chain increased linearly with molecular weight for both backbones. For a given chain length (X n), the perdeuterated POE backbone precipitated with a larger number of cyclodextrins than the hydrogenated POE. Assuming all of the cyclodextrins are threaded, the fractional coverage of the backbones decreases with increasing POE chain length but plateaus for chain lengths just below X n ≈ 500. At this chain length, the threading time becomes significantly longer (400 min) for perdeuterated POE as compared to the hydrogenated POE (28 min). Under the standard aqueous precipitation procedure, solid inclusion complexes could not be formed using perdeuterated POE with chain lengths greater than 1230 repeat units (59.2 kg/mol). Morphologies of the solid complexes were examined with wide-angle X-ray diffraction (WAXD) and small-angle neutron scattering (SANS). For molecular weights >3.2 kg/mol, POE backbones are partially covered with cyclodextrins that aggregate to form crystalline hydrogen-bonded stacks of α-CD-threaded POE separated by amorphous layers of unthreaded POE. As the POE molecular weight increases, the size of the crystalline α-CD stacks decreases. For molecular weights of 44 kg/mol (X n = 917), the stack size has decreased to the point that the materials exhibit very low crystallinity, even though 83% of the chain is covered with cyclodextrins. This low crystallinity is in contrast to the complex prepared from a hydrogenated chain of similar molecular weight (48.5 kg/mol), which exhibits higher crystallinity but is threaded by fewer cyclodextrins. These distinctive morphologies are related to the formation kinetics for these α-cyclodextrin inclusion complexes.
ASJC Scopus subject areas
- Materials Chemistry