Inclusion complexes containing poly(ε-caprolactone)diol and cyclodextrins. Experimental and theoretical studies

Inclusion complexes (ICs) between poly(ε-caprolactone)diol (PEC) with α-cyclodextrin (α-CD) (α-CD-PEC) and γ-cyclodextrin (γ-CD) (γ-CD-PEC) were prepared and characterized by FT-IR, ¹H NMR, thermogravimetry, surface activity and wettability measurements. The thermal stabilities of the inclusion complexes are very similar. The thermal stability of PEC is better than ICs and CDs. Stable monolayers of PEC and α-CD-PEC and γ-CD-PEC complexes have been obtained at the air-water interface using the Langmuir Technique. The surface pressure-area isotherms (π-A) were found to be of different types, depending on the CD utilized. From the surface free energy values of PEC and ICs it was possible to conclude that ICs are more hydrophobic than cyclodextrins. PEC is the most hydrophobic. The surface parameters the minimum area A₀, the critical surface pressure π_c, and static elasticity ε₀ were also estimated for ICs and PEC. In order to describe the experimental results, molecular dynamic simulation (MDS) was performed. In addition, the physical properties that stabilize CD-CD, CD-polymer and CD-solvent interactions were elucidated by MDS. Theoretical results have demonstrated that complexes are stabilized by hydrophobic interactions between the cavity of CDs and the -(CH₂)₅-units of PEC, and also by hydrogen-bond formation between the hydroxyl groups situated along the rim of CD molecules threaded onto the PEC chain. CD-CD hydrogen-bond formation is maximized in 1:2 γ-CD-PEC complex and 1:1 α-CD-PEC complexes.