*4.2. PLA*/β*-Cyclodextrins*

The feeding ratio between β-CD and PLA, as well as the molecular weight of the latter, has a significant effect on the formation of IC prepared by the solution ultrasonic technique [118]. IC with PLA of Mn ≥ 500 kg/mol agglomerated in water, but if the Mn was <100 kg/mol then the IC was soluble in H2O. Both IC had lower Tg and were more hydrophilic than those of pure PLA. FTIR spectra showed a clear difference between the IC and the components β-CD and PLA. The ν (O-H) stretching modes of β-CD shifted to higher frequencies when the inclusion complexes were formed, indicating physical interactions between the components. The ν (C=O) stretching band of PLA (1750 cm<sup>−</sup>1) shifted to 1757 cm<sup>−</sup><sup>1</sup> and weakened in the spectrum of IC. Wide angle X-ray diffraction patterns revealed a specific crystal structure of IC that differed from those characteristic of neat β-CD and PLA. The Tg of the inclusion complex was lower than that of pure PLA, and the decrease was proportional to the amount of β-CD. The drop in the glass transition temperature was explained in terms of IC supramolecular structure formation with the molecular rings of β-CD hindering interactions between the PLA chains. The thermal stability of the IC was slightly lower than that of pure PLA.

The hydrogen bonds between polylactide and β-CD were confirmed by a shift of the ν (O-H) stretching IR mode of β-CD (3386 cm<sup>−</sup>1) towards a lower wavelength in the IC spectrum (3304 cm<sup>−</sup>1) [119]. Additionally, a shift in the ν (C-H) band of β-CD was noted. The association behaviour can be explained by the presence of an ordered phase with different chain conformation, as indicated by the ν (C=O) stretching band of PLLA. The second derivative of this band changes markedly in IC, with a decrease in the trans-gauche conformation component. Those changes may be related to the formation of hydrogen bonding between the blended molecules.

The absence of the hydrogen bonding hinders the propensity towards IC formation in polylactide/cyclodextrin systems. The addition of methyl-β-cyclodextrin (Me-β-CD) lowered the crystallinity of the PLLA and enhanced the mobility of the polyester chains [120]. The addition of Me-β-CD increased the amorphous component of the PLLA. The plasticizing of PLLA by Me-β-CD resulted in an improvement of the drawability of the composite (1000% at 60 ◦C in the presence of 17% of Me-β-CD). In situ measurements using combined differential scanning calorimetry (DSC) and Raman spectroscopy revealed a lowering of the glass transition temperature, cold crystallization temperature, and melting point. As no IC was formed and the amount of the amorphous phase increased, the thermal stability of the PLLA/Me-β-CD blend was lower than that of neat PLLA.

The mechanism of polylactide/β-cyclodextrin inclusion complex formation and the e ffects of its incorporation in the PLA matrix were investigated [121]. The changes in the thermal stability, surface morphology, and barrier and mechanical properties were studied for the composites at varying IC and β-CD concentrations. Samples admixed with neat β-CD featured agglomerates of the β-cyclodextrin molecules while PLA/IC composites had uniform structures. The reason was poor interfacial interactions between the polyester macromolecules and β-CD moieties. The structural variations were reflected in di fferent thermal stabilities. The Tg and Tc of the studied PLA composites were shifted to higher temperatures upon increasing the concentration of IC or β-CD. The composite films containing larger amounts of IC or β-CD exhibited higher oxygen and water vapour permeability but were less flexible and had lower tensile strength than neat PLA.

High molecular weight polymer chains of stronger steric hindrance cannot easily penetrate the cavities of cyclodextrins; thus, the macromolecules tend to form partial inclusion complexes with small stoichiometric ratios. Nevertheless, the partial inclusion complexes formed between β-CD and high molecular weight PLLA crystallized better than neat PLLA. An improved mechanical performance and thermal stability were also demonstrated [122].
