Complexation behavior of α-, β-, and γ-cyclodextrin in modulating and constructing polymer networks

A systematic study of the host-guest complexation by α-, β-, and γ-cyclodextrin (CD) in either the free state or as substituents of poly (acrylic acid) (PAA) with the hydrophobic n-octadecyl groups, C18, substituted onto PAA (HMPAA) and its effect on polymer aggregation and network formation is reported. Free α-CD, β-CD, and γ-CD mask hydrophobic associations between the Cl8 substituent of HMPAA in aqueous solution and form host-guest complexes with a 1:1 or CD:C18 substituent stoichiometry at 0.5 wt % polymer concentration. For α-CD this host-guest stoichiometry changes to 2:1 or 2α-CD:C18 at ≥1 wt % polymer concentrations but not for β-CD and γ-CD. Shear-thickening occurs when γ-CD complexes C18 HMPAA substituents. Upon addition of sodium dodecyl sulfate, SDS (SDS:CD = 1:1), the hydrophobic associations between C18 diminished by α-CD masking were fully restored, were only partly restored in the case of β-CD, and not restored for γ-CD. When α- and β-CD substituted PAA (α-CDPAA and β-CDPAA) were mixed with HMPAA polymer, networks formed. As for free β-CD, the β-CD substituents of β-CDPAA also formed 1:1 or β-CD:C18 stoichiometry host-guest complexes with the C18 substituents of HMPAA. The α-CD substituents of α-CDPAA also formed 1:1 or α-CD:C18 stoichiometry host-guest complexes with some indication of the formation of 2:1 or 2α-CD:C18 stoichiometry host-guest complexes at polymer concentrations ≥ 1 wt %. The polymer networks formed by β-CDPAA with HMPAA are less viscous than those formed by α-CDPAA, for which shear-thickening occurs at polymer concentrations >2 wt %. It is evident that the difference in CD annular size and its match with the C18 of HMPAA control the diversity of the interactions of α-CD, β-CD, γ-CD, α-CDPAA, and β-CDPAA with HMPAA.