Bridged-cyclodextrin supramolecular hydrogels: host–guest interaction between a cyclodextrin dimer and adamantyl substituted poly(acrylate)s

Abstract

The formation of a hydrogel constructed from a cyclodextrin dimer (βCD₂ur) and adamantyl substituted poly(acrylate) was investigated by 2D NOESY ¹H NMR and isothermal titration calorimetry (ITC). The two methods generally demonstrate enhancement of complexation as the length of the hydrophobic tether increases, which is likely to be due to the cooperative effect. An optimal tether length of 6 was found to maximize the complexation between βCD₂ur and the polymer. Increasing the tether length to twelve carbons slightly reduces the complexation affinity revealed by ITC experiments. Meanwhile, rheological analysis indicates the maximum degree of crosslinking was achieved by PAAADhn/βCD₂ur complexation since the rigid structure of PAAAD restricts the crosslinking based on the host–guest complexation between the cyclodextrin dimer and adamantyl substituent and the competitive binding from the long alkyl tether in PAAADddn reduces the degree of crosslinking.

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Introduction

Supramolecular polymeric hydrogels have attracted much attention due to their potential applications in tissue engineering, drug delivery, and bioadhesives.^1–3 Noncovalent bonds of ions,⁴ hydrophobic interactions,⁵ hydrogen bonds,^6,7 π–π stacking,^8,9 and metal-coordination bonds^10,11 to form supramolecular polymeric network are commonly reversible. In order to properly apply these materials for particular purposes, the fundamentals including the nature of the interactions and the structure of the hydrogels need to be more clearly understood. Recently, Harada, Wenz and other researchers employed host–guest complexation between cyclodextrin (CD) and a range of guests to prepare novel physical cross-linked polymeric networks.^12–22

In general, there are two basic approaches to generate supramolecular polymeric networks through host–guest complexation. One is to mixing the CD substituted polymer and the guest substituted polymer. The other interesting way is to using bridged-cyclodextrin to cross-link guest substituted polymer. In previous studies we have examined the formation of hydrogels formed by poly(acrylate)s bearing hydrophobic substituents which are complexed by αCD and βCD substituents on a second poly(acrylate),^23–25 and by linked βCD dimers and trimers to form hydrogels.^26–29 The optimal size match between βCD and adamantyl (AD) substituents on substituted polymers results in host–guest complexation that forms effective polymer crosslinking,^30–33 as do similar interactions between βCD and n-polyalkyl substituents in polymer systems.^23,34 These studies have substantially advanced the understanding of hydrogel formation.

In this study we seek to establish a relationship between interactions at molecular level and characteristics at the macroscopic level for bridged-cyclodextrin cross-linked polymer system. Three binary mixtures of N,N-bis(6^A-deoxy-6^A-β-cyclodextrin)urea (βCD₂ur), with poly(acrylate)s bearing 3% substituent of amidoadamantyl (PAAAD), 1-(6-aminohexyl)aminoadamantane (PAAADhn), and 1-(6-aminododecyl)aminoadamantane (PAAADddn) are designed to form polymeric networks (Scheme 1). ¹H NMR spectroscopy and isothermal titration calorimetry (ITC) are used to investigate the host–guest interactions at the molecular level, and steady and dynamic rheology is applied to study the characteristics of hydrogels at the macroscopic level. It is shown that steric interactions with the poly(acrylate) backbone and the length of the adamantyl tether greatly affect host–guest complexation and control the characteristics of the polymeric networks formed.

Scheme 1 The structures of the adamantyl substituted poly(acrylate)s (PAAAD, PAAADhn, PAAADddn) and bridged cyclodextrin (βCD₂ur).

Experimental

Materials

βCD from Nihon Shokuhin Kako Co. LTD, 1-methyl-2-pyrrolidone (NMP) (99.5%), dicyclohexylcarbodiimide (99%) and methanol (99.5%) from Aldrich, sodium hydroxide (97%) from EM Science, and 1,2-diaminoethane (Ajax), and N,N-dimethylformamide (DMF) (Ajax) were used as supplied. Poly(acrylic acid) (PAA) (M_w = 250 000, M_w/M_n ≈ 2) was purchased from Aldrich as a 35% wt solution in water and freeze-dried to a constant weight. The βCD dimer (βCD₂ur) and the 3.0% AD substituted poly(acrylate)s (PAAAD, PAAADhn and PAAADddn) were prepared as previously described.^28,29

Characterization

The 2D ¹H NOESY NMR spectra were recorded at 298.2 K with a mixing time of 300 ms for solutions of the adamantyl substituted poly(acrylate)s and βCD₂ur with an equimolar ratio of βCD groups to AD substituents in D₂O at pD 7. Solutions were equilibrated at the thermostated probe temperature of 298.2 K for 30 min in 5 mm NMR tubes prior to the spectra collection.

The isothermal titration calorimetry (ITC) measurements were performed with a MicroCal VP-ITC equipment. Solutions were prepared in aqueous phosphate buffer at pH 7.0 and I = 0.10 M. All solutions were degassed at 298.2 ± 0.1 K immediately prior to titration. The titrations were carried out under concentration conditions where the product of the total adamantyl substituent concentration, the complexation constant, K, and the number of βCD₂ur complexing each adamantyl substituent, N, yielded a sigmoidal variation of heat released against titrant added for each system. The concentrations pertaining to two of the titrations appear in the figure captions. 2 mL of each of the three adamantyl substituted poly(acrylate)s whose concentrations ranged from 0.2 to 0.4 wt% were titrated by adding 10 μL aliquots of βCD₂ur solution, whose concentrations also appear in the figure captions, from a computer-controlled micro-syringe at intervals of 210 s. The contributions from heats of dilution were determined by titrating the buffer solution into buffered PAAAD, PAAADhn and PAAADddn solutions at appropriate concentrations, and by titrating βCD₂ur at the appropriate concentrations into buffer solution. The heat change observed from the above blank titrations was less than that 1% of those observed for the host–guest complexation titrations. An algorithm for complexation by βCD₂ur and the adamantyl substituents of PAAAD, PAAADhn and PAAADddn to the experimental data points provided the best fit using the Origin 7.0 MicroCal protocol to yield K, ΔH and TΔS.

Both the steady and dynamic rheological measurements were performed on a Physica MCR 501 (Anton Paar GmbH) stress-controlled rheometer with a 25 mm cone and plate geometry. The temperature was controlled to within ± 0.1 °C by a Peltier plate. Samples for the rheological measurements were prepared by dissolving the substituted poly(acrylate) in 0.10 M aqueous NaCl in order to screen the electrostatic interactions between the poly(acrylate) carboxylate groups, and the solution pH was adjusted to 7 using 0.10 M aqueous NaOH.

Results and discussion

Host–guest complexation identification

The structural information of the polymeric networks formed by substituted poly(acrylate)s (PAAAD, PAAADhn and PAAADddn) and βCD₂ur with βCD groups and adamantyl substituents equimolar were gained from 2D NOESY ¹H NMR spectroscopy (Fig. 1, S1 and S2†). The cross-peaks of the adamantyl protons in PAAAD/βCD₂ur system is smaller than that in PAAADhn/βCD₂ur system, which indicates that steric effect from the PAA backbone restricts the host–guest complexation between βCD₂ur and adamantyl substituents. Increasing the length of tether provides more flexibility to the adamantyl group. Furthermore, NMR signal from dodecyl protons in PAAADddn/βCD₂ur pair is much stronger than the case of hexyl protons in the PAAADhn/βCD₂ur system probably due to the existence of stronger complexation arising from the complexation between longer alkyl tether and βCD group (Fig. 2).

Fig. 1 2D NOESY ¹H NMR spectrum of PAAADddn and βCD₂ur with equimolar βCD groups and adamantyl substituents. The cross-peaks enclosed in rectangles A and B arise from interactions of the AD substituent H²⁻⁴ and dodecyl tether protons with the annular H^3,5,6 of βCD₂ur, respectively.

Fig. 2 Schematic structures of the host–guest complexation between βCD₂ur and either of PAAAD, PAAADhn or PAAADddn in equimolar solutions.

Isothermal titration calorimetry (ITC)

ITC was performed to understand the binding events of such βCD₂ur/AD substituted poly(acrylate) systems. Fig. 3, S3 and S4† present the raw data of titrations of the βCD₂ur into the AD substituted poly(acrylate) solution. The exothermic process was observed for all the AD substituted poly(acrylate) titrations. Even though it is still controversial regarding to the factors influencing the sign of enthalpy change, the results apparently indicate that the hydrophobic interactions involved in such system are exothermic. Integration of the negative peaks from raw data gives the binding isotherm of the βCD₂ur/AD. Since only 3% AD was grafted on the polymer chain, each AD binding site is assumed to be independent from other adamantyl binding sites. Therefore, the binding isotherm was fit by a one site independent binding model described in eqn (1) which has been discussed in details elsewhere in our previous work.²⁹The binding constant (K), enthalpy (ΔH), entropy (ΔS), and number of βCD₂ur complexing to each adamantyl substitute (N) are listed in Table 1. PAAAD, PAAADhn, and PAAADddn were also titrated with native βCD as control experiments in which the derived binding constants and negative enthalpies are smaller than the cases for βCD dimers. However, the entropies show opposite trend. This is likely because of the cooperative complexation between βCD moieties. In order words, complexation of the first βCD group makes the complexation of the second βCD much easier when βCD₂ur complexes with adamantyl functionalized PAA chains. However the binding constants for βCD₂ur are generally smaller than for βCD₃bz (1,3,5-N,N,N-tris-(6^A-deoxy-6^A-β-cyclodextrin)-benzene) except for the case of PAAADddn as investigated before probably due to less cooperative effect.²⁹ As the tether between polymer backbone and adamantyl group becomes longer and hence more hydrophobic arising from the intra-strand aggregation, the binding constant increases as shown in Table 1. The hydrophobic intra-strand facilitates the complexation between βCD₂ur and polymer probably due to better orientation is favored for host–guest complexation as the tether becomes longer. However, in the case of 12 carbon tether, the binding constant becomes smaller than that of PAAADhn. This might suggest that an optimum length of the tether exists. The contribution from entropy change (TΔS) becomes less significant for βCD dimers and generally the complexations are gradually more enthalpically controlled with stronger binding constants seen. Meanwhile, the number of βCD₂ur per adamantyl substituent (N) increases along with the binding constants, but still smaller than the optimal case (N = 0.5). A consequence of more regular or constrained structure is also reflected by smaller entropy values shifting from positive to negative values. Such results are intuitively reasonable since more complexation crosslinks the structure more tightly.

Fig. 3 (Top) ITC data for the βCD₂ur/PAAADhn system where 3.27 × 10⁻³ mol dm⁻³ βCD₂ur was titrated into 0.2 wt% PAAADhn for which the [adamantyl substituent] = 0.59 × 10⁻³ mol dm⁻³ in aqueous phosphate buffer at pH 7.0 and I = 0.10 mol dm⁻³ at 298.2 K. (Bottom) The solid curve shows the best fit to the experimental data points of an algorithm for equilibrium analogous for the interaction of the βCD with the adamantyl substituent.

Table 1 Thermodynamic parameters derived from ITC data^a

	K^b (dm^3 mol^−1)	ΔH (kJ mol^−1)	TΔS (kJ mol^−1)	N^c
a In aqueous phosphate buffer at pH 7.0 and I = 0.10 mol dm^−3 at 298.2 K.b K is defined through eqn (1) and their equivalents. The errors shown for K and the associated parameters are the data fitting error. When experimental error is also included it is estimated that the overall error in K is ≤±5%.c N is the number of either βCD or βCD2ur complexing each adamantyl substituent in the dominant complexation interaction.d Data from ref. 29.
PAAAD/βCD^d	3.35 × 10^3	−3.00	17.12	0.77
PAAADhn/βCD^d	1.44 × 10^4	−15.45	8.29	0.85
PAAADddn/βCD^d	5.77 × 10^3	−16.58	4.89	0.83
PAAAD/βCD2ur	1.56 × 10^4	−22.86	1.07	0.14
PAAADhn/βCD2ur	1.05 × 10^5	−39.82	−11.13	0.31
PAAADddn/βCD2ur	7.81 × 10^4	−35.48	−12.75	0.29

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Entropy–enthalpy linear relationship

A linear relationship between TΔS and ΔH has been observed for complexation process between small guest molecules and native, mono-modified or bridged β-cyclodextrins after linear least squares fits with eqn (2).^35,36

TΔS = αΔH + TΔS₀ (2)

In contrast to small guest molecule system (α = 0.80 and TΔS₀ = 11 kJ mol⁻¹ for native βCD,³⁵ α = 0.85 and TΔS₀ = 23.5 kJ mol⁻¹ for βCD dimers³⁶), the thermodynamic data of the adamantyl substituted poly(acrylate) systems yields α = 0.89 and TΔS₀ = 20.5 kJ mol⁻¹ regardless of the type of the hosts including native βCD, βCD dimer (βCD₂ur), or βCD trimer (βCD₃bz and βCDen₃bz) (Fig. 4). This suggests that complexation in the βCD annulus substantially controls the thermodynamics of host–guest complexation within the substituted poly(acrylate) systems studied. The larger slope (α = 0.89) and intermediate intercept (TΔS₀ = 20.5 kJ mol⁻¹) suggests a more intense conformation change and a medium desovlation process respectively upon complexation.

Fig. 4 A plot of TΔS against ΔH for the 1 : 1 complexes formed by the native βCD,²⁸ βCD dimer and βCD trimmers²⁸ with adamantyl substituted poly(acrylate)s.

Effects of stoichiometry of βCD₂ur/adamantyl substituent on complexation

The stoichiometry of the host–guest complexations between βCD₂ur and the adamantyl substituents in poly(acrylate)s was studied by rheology. The zero-shear viscosities of the adamantyl substituted poly(acrylate)s alone increase with the length of adamantyl-backbone tether in poly(acrylate)s (Fig. 5), suggesting the occurrence of stronger substituent aggregation. As βCD₂ur/AD stoichiometry is increased, all zero-shear viscosities of its mixtures with adamantyl substituted poly(acrylate)s increase monotonically and reach a plateau. Due to stronger steric effects from the PAAAD backbone and the competitive complexation between the ddn tether and the adamantyl substituents in PAAADddn (Fig. 2), the zero-shear viscosities for βCD₂ur + PAAAD and βCD₂ur + PAAADddn systems are much less than βCD₂ur + PAAADhn system.

Fig. 5 Effect of variation of [βCD₂ur]/[adamantyl substituent] on the zero-shear viscosity of 5 wt% solutions of adamantyl substituted poly(acrylate)s with various tether lengths.

Concentration effect on zero-shear viscosity

The zero-shear viscosities of solutions of adamantyl substituted poly(acrylate)s and their mixtures with βCD₂ur as a function of polymer concentration are shown in Fig. 6. The increase in zero-shear viscosity upon increasing polymer concentration is much more significant for βCD₂ur cross-linking system compared to poly(acrylate) alone. The non-specific hydrophobic interactions among substituents in polymer allow the formation of both intra and inter molecular associations.³⁷ When the concentration reaches the overlap concentration c*,^38–42 the probability of inter molecular associations increases which results in the enhancement of solution viscosity. For the adamantyl substituted poly(acrylate)s alone (Fig. 6A), c* decreases in the sequence of PAAAD (2.6 wt%) > PAAADhn (1.7 wt%) > PAAADddn (1.0 wt%) which is consistent to the fact that a longer tether length causes a shorter AD substituent distance, hence increasing the probability of forming inter molecular associations. In comparison, c* for the βCD₂ur cross-linking system (Fig. 6B) decreases in the order of βCD₂ur + PAAAD (1.5 wt%) > βCD₂ur + PAAADddn (0.8 wt%) > βCD₂ur + PAAADhn (0.7 wt%). This is because of the existence of the competitive complexation (K₁ and K₂) arising from the interaction between adamantyl substituents and βCD₂ur and hydrophobic aggregates from the long tether and βCD₂ur.

Fig. 6 Zero-shear viscosity as a function of concentration for (A) adamantyl substituted poly(acrylate)s alone and (B) adamantyl substituted poly(acrylate)s in the presence of βCD₂ur with βCD groups and adamantyl substituents equimolar.

Temperature effect on host–guest complexation

The steady shear viscosities of the gel formed with a mixture of 5 wt% PAAADhn and βCD₂ur solutions at a variety of temperatures were tested as a function of shear rate as shown in Fig. 7. Overall, the viscosities decrease monotonically, while the onset of shear thinning occurs at higher shear rate with increasing temperature from 10 to 40 °C and the transitions becomes more ambiguous as well. Such observations should be caused by a decrease of relaxation time and a reduction of cross-linking.⁴³

Fig. 7 Viscosity of a 5 wt% PAAADhn and βCD₂ur mixture with βCD groups and adamantyl substituents equimolar as a function of shear rate at various temperatures.

The relaxation modes of the βCD₂ur/PAAADhn system vary systematically with temperature in the range of 15–35 °C, hence “master curves” were obtained by time–temperature superposition^44–46 of the modulus at different temperatures (Fig. 8). The storage and loss moduli (G′ and G′′) at any temperature can be deduced by eqn (3) where a_T and b_T are the horizontal and vertical shift factors, respectively, and T_ref denotes the reference temperature (25 °C).

Fig. 8 Master curves for (A) the storage modulus G′ and (B) the loss modulus G′′ of a 5 wt% PAAADhn and βCD₂ur mixture with βCD groups and adamantyl substituents equimolar as a function of frequency after time–temperature superposition.

Assuming that the temperature effect on the host–guest complexation follows the Arrhenius equation, the activation energies were calculated by fitting of the shift factors through eqn (4) and (5):

where E_a and E_b are the activation energies. The average values of E_a and E_b are 75.8 ± 0.8 kJ mol⁻¹ and −4.4 ± 0.1 kJ mol⁻¹, respectively.

Conclusions

The host–guest interactions between adamantyl modified polyacrylates and βCD₂ur were fully studied. The complexation between adamantyl substituents and βCD group was clearly observed from 2D NOESY ¹H NMR spectra. Increasing of tether length to six or twelve accounts for the complexation between alkyl tether and βCD group. Such competitive complexation of the tether and the adamantyl substituents was further understood by thermodynamic analysis (ITC) and the results were compared with our previous work. The general binding constants follows a trend: βCD trimer > βCD dimer > βCD monomer. Such observations can be explained by the stronger cooperative binding for dimer or trimers i.e. complexation of βCD to polymer facilitates the complexation of the second βCD and so force. In consistent to our previous findings, the longer tether makes the complexation stronger. However, the crosslinking degree for PAAADhn was shown by rheological experiments including stoichiometry and concentration studies to be higher than PAAAD and PAAADddn probably due to the combination of steric and competitive complexation effects. Finally, the storage and loss modulus of the mixture solution of βCD₂ur and PAAADhn obey time–temperature superposition which results in the activation energies of E_a = 75.8 ± 0.8 kJ mol⁻¹ and E_b = −4.4 ± 0.1 kJ mol⁻¹, respectively.

Acknowledgements

We gratefully acknowledge NSFC Grants 51403062, 51273063, 21306049 and 20774030, the Fundamental Research Funds for the Central Universities, the higher school specialized research fund for the doctoral program (222201313005 and 222201314029), the China Postdoctoral Science Foundation (2013M541485), the Open Project of State Key Laboratory of Chemical Engineering (SKL-ChE-14C01), the Australian Research Council Grant DP110103177 and 111 Project Grant (B08021) for support of this work.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra06163f

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