**2. Noncovalent interactions between polymers and cyclodextrins**

Rotaxanes are supramolecular structures consisting of at least one ring threaded through an axial molecule, with the particularity that the dissociation of this assembly is hindered by bulky groups at the ends of the axial molecule [17]. CDs may play the role of a ring that is lined up on a polymer chain. The general procedures for the synthesis of polyrotaxanes involve two pathways: the threading followed by rotaxination and the slippage of CD over a bulky group (**Figure 1**). These methods are used for the preparation of CD-based pseudorotaxanes and rotaxanes in aqueous media [18]. The formation of pseudorotaxanes is dependent on the relation between the sizes of the CD cavity and polymeric chain. Thus, α-CD will generate polyrotaxanes and pseudopolyrotaxanes with polyethylene glycols and polyamines [19, 20], while β-CD with polypropylene glycols [21] and γ-CD with polyvinyl alcohol [22], which often leads to gel systems. The endcapping of polyethylene glycols with bulky substituents, such as bis(3,5-dinitrobenzoyl) and bis(2,4-dinitrobenzoyl), will favor the formation of polyrotaxanes with γ-CD [23].

Many water-soluble polymers act as nonionic surfactants by self-association in micelles that are sensitive to temperature and the presence of other molecules. *Cyclodextrins as Bricks for Tuning Polymer Properties DOI: http://dx.doi.org/10.5772/intechopen.105688*

**Figure 1.** *Schematic representation of CDs and pseudopolyrotaxanes.*

CDs can form inclusion complexes with nonionic surfactants from the Triton X or pluronic classes. In the case of Triton X, α-CD can complex the polyethylene chain, while β-CD complexes the nonpolar head (the iso-octylphenyl and phenyl groups) [24]. CDs can also modify the critical micelle concentration or are able to disrupt the micelles due to the complexation of polymers or surfactants [25]. As an example, the Triton X-100 nonionic surfactant, consisting of a short chain of polyethylene glycol and an aromatic nonpolar head, is able to form micelles at concentrations higher than 2.2 mM [26]. CDs can form inclusion complexes with this molecule with different geometries and stoichiometries depending on the CD size. Using isothermal titration calorimetry (ITC) measurements, it was possible to evaluate the binding constants for β-CD and γ-CD by assuming a stoichiometry of 1:1 and 1:2 for β-CD and of 1:1 for γ-CD, both involving the complexation of the nonpolar surfactant head. Conversely, in the case of α-CD, the complexation was supposed to be through the formation of pseudorotaxane by the inclusion of the polyethylene chain, assuming 1:5 stoichiometry, although the ITC data were not conclusive [27]. The interaction of β-CD with Triton X-114 led to the formation of larger aggregates involving hydrogen bonds with β-CD. This has been studied as a function of β-CD concentration and temperature, and it was observed that, at higher β-CD concentration, a transition from micelle to vesicle occurred. This effect is different from the more common effect of CDs on surfactant aggregation [28].

The effects of various CDs on the micellization and gelation of pluronics have also been reported. For such systems, changes in micellar concentration, in gelation, as well as changes of the hydration layer around the polymer chains during phase transition, when CDs are placed among polymeric chains, can be the result of pseudorotaxane formation [29–33]. Being water soluble, CDs will be more probable to target the region of the micelles placed at the water interface. Two studies involving EPR and fluorescence spectroscopies along with rheological and tube inversion methods explore the effect of 2-hydroxypropyl-β-CD on the micelle-to-gel phase transition of pluronic F127 [34, 35]. It was revealed that the spectral parameters of

**Figure 2.**

*Representation of the micelle-to-gel transition in F127/HPB systems (A), variation of hyperfine splitting constant with temperature (B) and variation of EPR spectra of spin probe with temperature (C) in F127 system [35].*

molecular probes (commercially available spin probes, spin-labeled CDs, CDs labeled with fluorophores, or dual molecular probes) deviate from the linear dependence with temperature (**Figure 2**), thus indicating that the macroscopically observed phase transformation is related to changes at the nanoscale level. The results also led to the conclusion that the presence of CDs at the used concentrations does not induce micellar rupture but determines an increase in the micellar water content, which suggests an increase of the micellar size and excludes the formation of pseudopolyrotaxanes.

**Figure 3.** *Schematic representation of the supramolecular network of a sliding gel.* *Cyclodextrins as Bricks for Tuning Polymer Properties DOI: http://dx.doi.org/10.5772/intechopen.105688*

Rotaxanes and pseudorotaxanes generate networks that lead to the formation of hydrogels. A particular case refers to the formation of supramolecular networks named sliding gels that are characterized by mobile or sliding crossing points. A classical sliding network results from the intermolecular crosslinking of α-CD/polyethylene glycol pseudorotaxanes (**Figure 3**) [36–38]. The crossing points are, thus, mobile, and this will determine a mobility of the overall network. These gels are formed by linking CD units that belong to different chains.

The properties of these sliding gels can be described by topological parameters such as the complexation degree (number of CD units on a polymer chain), the crosslinker fraction (defined as the ratio of the mole number of crosslinker on the mole number of CDs), and the interactions between the swelling solvent and the constitutive parts of the network [39]. In many cases, the sliding motion depends on the swelling solvent, as well as on other factors such as the pH. For instance, in the case of pseudorotaxanes formed between a triblock copolymer consisting of polyethylene amine/ polyethylene glycol/polyethylene amine, and α-CD, with crossing points obtained in the presence of 1,10-carbonyldiimidazole, the gel properties are dependent on the pH value [40].
