Langmuir 2006, 22, 9023-9029
9023
Characterization of Thermally Sensitive Interactions in Aqueous Mixtures of Hydrophobically Modified Hydroxyethylcellulose and Cyclodextrins Huaitian Bu,† Stine N. Naess,‡ Neda Beheshti,† Kaizheng Zhu,† Kenneth D. Knudsen,§ Anna-Lena Kjøniksen,† Arnljot Elgsaeter,‡ and Bo Nystro¨m*,† Department of Chemistry, UniVersity of Oslo, P.O. Box 1033, Blindern, N-0315 Oslo, Norway, Department of Physics, Norwegian UniVersity of Science and Technology (NTNU), Høgskoleringen 5, NO-7491 Trondheim, Norway, and Department of Physics, Institute for Energy Technology, P.O. Box 40, N-2027 Kjeller, Norway ReceiVed March 31, 2006. In Final Form: August 8, 2006 Effects of β-cyclodextrin (β-CD) or hydroxypropyl-β-cyclodextrin (HP-β-CD) addition and temperature on thermodynamic, rheological, and structural features of semidilute solutions of hydroxyethylcellulose (HEC) and its hydrophobically modified analogue (HM-HEC) are reported. Differential scanning calorimetric (DSC) measurements revealed a thermally induced crystal melting transition of β-CD at high concentrations in solutions of HEC and HM-HEC. No transition with HP-β-CD was observed in aqueous solution. Viscosity results indicated that at a cosolute concentration of 2 mm, the β-CD units are threaded onto hydrophobic tails of HM-HEC (C16 groups) to form columnar structures. This arrangement is more effective in the encapsulation of the hydrophobic chains than the monomer hydrophobic deactivation accomplished by the HP-β-CD units. At cosolute concentrations above 8 mm, no further decoupling of the hydrophobic interactions occurs for any of the cosolutes. Small-angle neutron scattering (SANS) experiments on HM-HEC/β-CD mixtures suggest that the large-scale association structures in HM-HEC/D2O solutions are reduced upon addition of β-CD, and an interesting temperature effect is observed at 2 mm β-CD addition. At high β-CD concentrations and low temperatures, the formation of large β-CD clusters or crystallites generates cross-links in the HEC and HM-HEC networks, resulting in a viscosity enhancement of several orders of magnitude. This strong temperature effect is not reflected in the structural features probed by SANS.
Introduction The most usual cyclodextrins (CDs) belong to a family of four cyclic oligosaccharides, R, β, γ, and δ, comprising six to nine R (1 f 4)-linked glucopyranose units, respectively.1,2 When a sufficiently high amount of cyclodextrin is dissolved in water, a temperature decrease can induce crystallization, where the CD molecules may be arranged in a herringbone pattern in such a way that the cavity of one molecule is blocked on both sides by adjacent, symmetry-related CD molecules.3 It has been reported4-11 that native cyclodextrins (CDs) develop inclusion complexes with various hydrophobically modified polymers, and it has also been found12,13 that CDs form crystalline complexes †
University of Oslo. Norwegian University of Technology and Science. § Institute for Energy Technology. ‡
(1) Harada, A. AdV. Polym. Sci. 1997, 133, 141. (2) Sa¨nger, W.; Jacob, J.; Gessler, K.; Steiner, T.; Hoffmann, D.; Sanbe, H.; Koizumi, K.; Smith, S. M.; Takaha, T. Chem. ReV. 1998, 98, 1787. (3) Saenger, W. Jerusalem Symposium of Quantum Chemical Biochemistry; Pullman, E. B., Ed.; Riedel, Co.: Dordrecht, The Netherlands, 1976. (4) Zhang, H.; Hogen-Esch, T. E.; Boschet, F.; Margaillan, A. Langmuir 1998, 14, 4972. (5) Amiel, C.; David, C.; Renard, E.; Se´bille, B. Polym. Prepr. 1999, 40, 207. (6) Karlson, L.; Thuresson, K.; Lindman, B. Carbohydr. Polym. 2002, 50, 219. (7) Liao, D.; Dai, S.; Tam, K. C. Polymer 2004, 45, 8339. (8) Guo, X.; Abdala, A. A.; May, B. L.; Lincoln, S. F.; Khan, S. A.; Prud’homme, R. K. Macromolecules 2005, 38, 3037. (9) Kjøniksen, A.-L.; Galant, C.; Knudsen, K. D.; Nguyen, G. T. M.; Nystro¨m, B. Biomacromolecules 2005, 6, 3129. (10) Galant, C.; Kjøniksen, A.-L.; Nguyen, G. T. M.; Knudsen, K. D.; Nystro¨m, B. J. Phys. Chem. B 2006, 110, 190. (11) Beheshti, N.; Bu, H.; Zhu, K.; Kjøniksen, A.-L.; Knudsen, K. D.; Pamies, R.; Herna´ndez Cifre, J. G.; Garcia de la Torre, J.; Nystro¨m, B. J. Phys. Chem. B 2006, 110, 6601. (12) Harada, A.; Kamachi, M. Macromolecules 1990, 23, 2821. (13) Harada, A.; Li, J.; Kamachi, M. Macromolecules 1993, 26, 5697.
with different unmodified macromolecules. The crystal assemblies of CD inclusion complexes are usually classified into two main types, namely, cage and channel structures.1,3,14 The cage type is likely to be found when the guest molecule is small enough to be included within a single CD cavity, whereas the channel type crystalline structure is formed when long-chain molecules such as polymers are included as guests in the cavities of CD molecules. Several investigations6,9-11 on aqueous solutions of hydrophobically modified polymers have shown that CD may form inclusion complexes with pendant hydrophobic groups and thereby deactivate the hydrophobic interactions. In a recent study9 on semidilute aqueous solutions of alginate and a hydrophobically modified analogue (HM-alginate) in the presence of β-CD, we observed a strong viscosity enhancement for the alginate/β-CD sample at low temperatures. This was ascribed to the cross-linking of polymer chains through the formation of CD crystallites at low temperatures. To gain a deeper insight into this novel phenomenon where CD crystallization provides cross-links in the polymer network, we have conducted a thorough investigation of the interaction between CD and the uncharged hydrophilic polysaccharide hydroxyethylcellulose (HEC) and also its hydrophobically modified analogue (HM-HEC). The aim of this work is to characterize the interplay between the polymer and the CD, regarding both the cross-linking effect and the ability of CD to decouple hydrophobic associations. To accomplish this, aqueous semidilute solutions of HEC and HM-HEC in the presence of β-CD or hydroxypropyl-β-cyclodextrin (HP-β-CD) monomers are studied at different temperatures with the aid of calorimetry, rheology, and small-angle neutron scattering (SANS). These methods constitute important thermodynamic, structural, and (14) Harada, A.; Okada, M.; Kawaguchi, Y. Chemistry Lett. 2005, 34, 542.
10.1021/la0608664 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/12/2006
9024 Langmuir, Vol. 22, No. 21, 2006
Bu et al.
dynamical tools in the characterization of the synergistic interactions between polymer and cyclodextrin. It has recently been shown11 that the addition of HP-β-CD to semidilute aqueous solutions of HM-HEC at 25 °C can effectively decouple the hydrophobic interactions. A similar picture emerged from the viscosity and dynamic light scattering measurements of this work, namely, that HP-β-CD addition leads to deactivation of the hydrophobic interactions, and easier relaxation of the polymer chains was promoted. The cosolute β-CD crystallizes at low temperatures, whereas HP-β-CD does not form crystals in water, and is therefore employed as a reference in the evaluation of the impact of crystallization on the properties of the systems. Schematic illustrations of the chemical structures of HM-HEC, β-CD, and HP-β-CD are displayed in Figure 1. The results from this paper will demonstrate that cross-links, generated through β-CD clusters at low temperatures, can reinforce the polymer network. The strength of this effect can be modulated by the β-CD concentration and temperature. Furthermore, evidence will be presented for the formation of columnar β-CD structures in the encapsulation of hydrophobic tails of the HM-HEC polymer. The SANS experiments will show that the large hydrophobic association structures can be suppressed as the cosolute concentration increases. Experimental Procedures Materials and Solution Preparation. A hydroxyethylcellulose sample with the commercial name Natrosol 250 GR (lot no. A-0382), obtained from Hercules, Aqualon Division, was used as a reference and as the precursor for the synthesis of the hydrophobically modified analogue (HM-HEC). The degree of substitution of hydroxyethyl groups per repeating anhydroglucose unit is 2.5 (given by the manufacturer). The weight-average molecular weight (Mw ) 400 000) of this sample in dilute aqueous solution was determined by intensity light scattering15 at 25 °C. The cosolutes β-CD and HP-β-CD were supplied by Fluka and Aldrich, respectively, and were used without further purification. The HM-HEC polymer was synthesized according to a standard procedure,16 and the details and characterization of the present sample have been described elsewhere.11After completion of the hydrophobization reaction, acetic acid neutralized the liquid reaction mixture, and the product was collected by filtration. The product was washed thoroughly with acetone and dried at 70 °C for 24 h under reduced pressure to remove contents of acetone. The chemical structure (see Figure 1) and purity of HM-HEC were ascertained by 1H NMR, and the degree of hydrophobic modification (glycidyl hexadecyl ether groups) was determined from the peak ratios between the anisomeric protons and the methyl protons of the glycidyl hexadecyl chain. The degree of substitution of nC16H33 groups determined from NMR analysis was 1 mol %. To remove low molecular weight impurities, dilute HEC and HM-HEC solutions were thoroughly dialyzed against Millipore water (7 days) and isolated by freeze-drying. Regenerated cellulose with a molecular weight cutoff of about 8000 (Spectrum Medical Industries) was utilized as dialyzing membrane. All solutions were prepared in heavy water (D2O) by weighing the components, and the components were homogenized by stirring at room temperature for 1 day. All the measurements were conducted in the semidilute regime at a fixed polymer concentration (2 wt %) of HEC or HM-HEC in the presence of various amounts of β-CD or HP-β-CD. The experiments were carried out over a wide temperature domain (5-40 °C). Heavy water was employed as a solvent instead of light water to obtain good contrast and low background for the neutron-scattering measurements. Since the physical properties of the polymers and cosolutes may be slightly (15) Maleki, A., unpublished data. (16) Miyajima, T.; Kitsuki, T.; Kita, K.; Kamitani, H.; Yamaki, K. U.S. Patent 5,891,450, April 6, 1999.
Figure 1. Schematic illustrations of the chemical structures of β-CD, HP-β-CD, and HM-HEC. different in light and heavy water, all experiments were carried out in heavy water. Differential Scanning Calorimetry (DSC). The calorimetric experiments were performed using a differential scanning calorimeter (Nano DSC series III from Calorimetry Sciences Corporation, Lindon, UT). The scans were carried out at a rate of 0.2 °C /min in a
Mixtures of Hydroxyethylcellulose and Cyclodextrins temperature range of 5-60 °C. All solutions were degassed prior to the measurements. The solutions were left in the instrument cells for an equilibration period of 15 min before the measurements were started. The instrument idle and equilibrium temperature was set to 5 °C. Three different sample preparation procedures were used for the calorimetric experiments: (i) the samples of HEC and HM-HEC containing β-CD or HP-β-CD were frozen to induce crystal formation. Then, the samples were slowly melted by keeping the temperature below 10 °C and loaded into the instrument. (ii) The samples of pure β-CD and HP-β-CD were heated to 60 °C to remove all the crystals and loaded into the instrument. (iii) For the pure HEC and HM-HEC solution, the samples were kept at room temperature and loaded into the instrument. These procedures were worked out to obtain reproducible and reliable results. Rheology. Shear viscosity measurements were conducted in a Paar-Physica MCR 300 rheometer using a cone-and-plate geometry, with a cone angle of 1° and a diameter of 75 mm. The samples were introduced onto the plate, and to prevent evaporation of the solvent, the free surface of the sample was always covered with a thin layer of low viscosity silicone oil (the viscoelastic response of the samples is not observed to be affected by this layer). The measuring device was equipped with a temperature unit (Peltier element) that provided a rapid alteration of the temperature and gave a good temperature control ((0.05 °C) over an extended time for all the temperatures investigated in this work. The shear viscosity measurements were conducted over an extended shear rate range at temperatures in the range of 5-40 °C. The samples were allowed to equilibrate for 60 min at each temperature of measurement. Small-Angle Neutron Scattering. Small-angle neutron scattering (SANS) experiments were carried out on 2 wt % solutions of HEC or HM-HEC in the presence of various amounts of β-CD or HPβ-CD and at different temperatures (temperature controlled to within (0.1 °C) at the SANS installation at IFE, Norway. The details of the spectrometer have been described elsewhere.17 The solutions were filled in 2 mm Hellma quartz cuvettes, which were placed onto a copper base for good thermal contact and mounted in the sample chamber. The scattering length densities calculated for the β-CD monomers (Fβ-CD ) 0.88 × 1010 cm-2) and the HP-β-CD derivative (FHP-β-CD ≈ 0.8 × 1010 cm-2) are quite close to that of HM-HEC (FHM-HEC ≈ 1.4 × 1010 cm-2). For this reason, contrast matching has not been used to observe the scattering from the individual components within the mixtures. However, it should be mentioned that the scattering from β-CD or HP-β-CD monomers in the samples is insignificant in comparison with that of HEC or HM-HEC. Each complete scattering curve is composed of three independent series of measurements, using three different wavelength-distance combinations (5.1 Å/1.0 m, 5.1 Å/3.4 m, and 10.2 Å/3.4 m). By using these combinations, scattering vectors q ) (4π/λ) sin(θ/2) (where θ is the scattering angle) in the range of 0.008-0.25 Å-1 were covered. Standard reductions of the scattering data, including transmission corrections, were conducted by incorporating data collected from empty cell, beam without cell, and blocked-beam backgrounds to obtain the scattered intensities in absolute scale (cm-1). Subtraction of the solvent scattering, as well as the estimated incoherent background originating from the solute molecules, was then done to obtain the coherent macroscopic scattering cross-section dΣ/dΩ(q) of the system.
Results and Discussion Differential Scanning Calorimetry. Calorimetry is a powerful method to evaluate morphological transitions of a sample. For example, temperature-induced crystallization or crystal melting of a system usually give rise to a peak in the heat flow curve. The aim of the present DSC results is not to establish a detailed description of crystallization or crystal melting but rather to disclose fundamental differences between the systems. No temperature-induced crystallization was observed for cosolutes with or without polymer during down-scans at a scan rate of 0.2 (17) Kjøniksen, A.-L.; Laukkanen, A.; Galant, C.; Knudsen, K. D.; Tenhu, H.; Nystro¨m, B. Macromolecules 2005, 38, 948.
Langmuir, Vol. 22, No. 21, 2006 9025
Figure 2. DSC scans for the systems indicated. The β-CD/D2O and HP-β-CD/D2O samples were heated to 60 °C before down-scans (0.2 °C/min) were performed (a). For the HEC/β-CD (c), HEC/ HP-β-CD (c), HM-HEC/β-CD (b), and HM-HEC/HP-β-CD (b) systems, the samples were loaded in the instrument at temperatures below 10 °C, and up-scans (0.2 °C/min) were carried out. More details are given in the text.
°C/min. The reason for this may be that the crystallization process is too slow or that too few crystals are formed. In up-scans (0.2 °C/min), transition peaks were detected. These transition peaks represent the crystal melting. In Figure 2, the DSC up-scans for various concentrations of β-CD or HP-β-CD in the absence and presence of HEC (2 wt %) or HM-HEC (2 wt %) are depicted. For the cosolutes without polymer (Figure 2a), an endothermic transition peak is only observed for the highest β-CD concentration. The fact that no peak is detected for the lower β-CD concentrations does not mean that no clusters or crystallites are melted, but they are probably too few to give off sufficient energy for DSC to sense a change. DSC can only register the massive onset of crystal melting. No sign of transition is detected for the HP-β-CD/D2O system. The scan for the 16 mm β-CD/D2O mixture divulges a temperature-induced crystallization transition, which is a phenomenon that has been reported18 for this system. The curves for the HM-HEC/β-CD and HM-HEC/HP-β-CD systems reveal no transition for the latter system, but marked transitions are observed for the former system at 12 and 16 mm β-CD concentrations, while only a very weak hump in the DSC curve can be discerned at 8 mm β-CD. A close inspection of the curves discloses that the melting of the crystals (this corresponds to the temperature where the peak flattens out at high temperatures) occurs at approximately 15, 25, and 35 °C for the β-CD concentrations of 8, 12, and 16 mm, respectively. A similar behavior is observed for the HEC/β-CD system, but in this case, the peaks are more pronounced, and the crystals have melted at about 18 and 38 °C for the β-CD concentrations of 8 and 16 mm, respectively. The trend of the transition temperatures toward higher values with increasing β-CD concentrations is reasonable since a higher level of β-CD addition gives rise to larger (18) Frank, J.; Holzwarth, J. F.; Saenger, W. Langmuir 2002, 18, 5974.
9026 Langmuir, Vol. 22, No. 21, 2006
Bu et al.
Figure 4. Shear rate dependencies of the viscosity (every second point is shown) for 2 wt % solutions of HM-HEC at the temperatures and β-CD concentrations indicated.
Figure 3. Comparison of the DSC curves for the systems β-CD/ D2O, HEC/β-CD/D2O, and HM-HEC/β-CD/D2O at two different β-CD concentrations. More details are given in the legend of Figure 2.
crystallization structures.19,20 Since the transitions peaks are found at much lower β-CD concentrations in the presence of polymer than without polymer, it is reasonable to assume that the polymer promotes the growth of crystals. Figure 3 shows a direct comparison of the calorimetric results for the HM-HEC/β-CD and HEC/β-CD systems and for the β-CD/ D2O samples at some given β-CD concentrations. We note that the transition peaks are more pronounced for HEC/β-CD than for the corresponding HM-HEC/β-CD sample, and the melting temperature for the crystals is shifted toward higher values for the HEC/β-CD system. These effects can be attributed to the fact that some β-CD molecules are consumed in the formation of inclusion complexes with the hydrophobic tails of the HM-HEC polymer, and thereby the concentration of β-CD in the bulk is lower for the HM-HEC/β-CD system. As a result, the peaks are more pronounced and shifted toward higher temperatures for HEC since more β-CD clusters are formed in the bulk for this polymer. Rheological Features. The effects of β-CD addition and temperature on the shear viscosity for the HM-HEC/β-CD system are depicted in Figure 4. The general trend (ignoring for the moment the viscosity behavior at low shear rates for the high β-CD concentrations at low temperatures) is the drop of the viscosity at low shear rates with increasing β-CD concentration, which suggests that the hydrophobic interactions are deactivated by the β-CD addition. At temperatures up to 25 °C, the viscosity curves collapse onto one another at β-CD concentrations above 2 mm, suggesting that this level is sufficient to decouple nearly all the hydrophobic interactions. At 40 °C, β-CD concentrations above 4 mm are required to deactivate the hydrophobic tails. This may indicate that the augmented thermal mobility of the (19) Georgalis, Y.; Schu¨ler, J.; Umbach, P.; Saenger, W. J. Am. Chem. Soc. 1995, 117, 9314. (20) Bonini, M.; Rossi, S.; Karlsson, G.; Almgren, M.; Lo Nostro, P.; Baglioni, P. Langmuir 2006, 22, 1478.
Figure 5. Effects of β-CD (a and c) or HP-β-CD (b and d) addition and temperature on the relative zero-shear viscosity for 2 wt % solutions of HEC and HM-HEC. The inset shows a magnification of the temperature effect.
chains at elevated temperature makes more hydrophobic moieties accessible for encapsulation. The strongest shear-thinning effect at higher shear rates is registered for the HM-HEC solution without β-CD (i.e., the system that exhibits the strongest association network). The progressive decrease in viscosity as the shear rate rises indicates that the intermolecular junctions are gradually disrupted. The most conspicuous feature in Figure 4 is the marked upturn of the viscosity at low shear rates for the lower two temperatures as the β-CD concentration is increased. The strong viscosity enhancement evident at low temperatures and high β-CD levels suggests the evolution of clusters or crystallites that act as crosslinkers of the polymer chains. These junction zones are broken up at fairly low shear rates, announcing that they are not strong. It is reasonable to assume that hydrogen bonds play an important role in the formation of crystal structures. Since these cross-links are formed at low temperatures and high β-CD concentrations, it is natural to assume that a large number of crystals of sufficient size are required to function as cross-linkers for the network. To remove the influence of trivial changes of the solvent viscosity with temperature, the viscosity results in Figure 5 are presented in terms of the relative zero-shear viscosity ηrel ≡
Mixtures of Hydroxyethylcellulose and Cyclodextrins
η0/ηheavy water, where η0 is the zero-shear viscosity and ηheavy water is the solvent viscosity. It should be noted that the viscosity of β-CD solutions without polymer is close to that of D2O (ca. 10-3 Pa‚s) at all conditions of temperature and β-CD concentrations. Figure 5 shows the influence of cosolute concentration (β-CD or HP-β-CD) and temperature on ηrel in 2 wt % solutions of HM-HEC and HEC. Let us first discuss the most prominent features of the polymers in the presence of β-CD (Figure 5a,c). For the HM-HEC/β-CD system at low cosolute concentrations, ηrel decreases as the amount of β-CD rises because of the deactivation of hydrophobic moieties. The value of ηrel is virtually independent of the level of β-CD addition for the HEC/β-CD system at higher temperatures because there are no attached hydrophobic groups on the polymer chains for β-CD to encapsulate. A detailed discussion about the temperature effect of ηrel for the HM-HEC sample at low β-CD concentrations is given next. The most drastic effect is the steep rise of ηrel at low temperatures and high levels of β-CD. The magnification of the effect in the inset plot divulges that a viscosity enhancement is even observed at 25 °C. A similar behavior is also found for the HEC/β-CD system (Figure 5c), but in this case, the onset of the raise of ηrel is located at lower β-CD concentration than at the corresponding condition for the HM-HEC/β-CD system. This can be ascribed to the fact that some β-CD is consumed in the formation of the inclusion complexes, and therefore, a higher concentration of β-CD is needed to establish the necessary crystallites for the evolution of cross-links for the HM-HEC network. In this context, it is interesting to note a recent rheological study9 on semidilute aqueous solutions of alginate and hydrophobically modified alginate (HM-alginate with 31 mol % of C8 groups) in the presence of β-CD. In this case, a strong viscosity enhancement was observed for the alginate solution at low temperatures and high β-CD concentrations, but no effect was detected for the HM-alginate sample. This behavior was attributed to steric hindrance due to the large amount of C8 groups. In the HM-HEC sample with the low amount of hydrophobic groups (1 mol %), steric hindrance was not expected to constitute a problem. The strong viscosity enhancement observed for the HEC/βCD and HM-HEC/β-CD mixtures at low temperatures and high cosolute concentrations can probably be rationalized in the following scenario. At low temperatures and high concentrations of β-CD, CDs crystal hydrates usually adopt a cage-type structure,14 in which β-CD molecules are arranged in such a way that each cavity is capped by other β-CD molecules. The organization of this structure is driven by hydrogen bonding. It has recently been reported20 from cryo-TEM and light scattering measurements on β-CD that the molecules self-assemble in sheetlike aggregates and fibers at high cosolute concentrations. Our conjecture is that in the presence of polymer chains, the clusters or crystallites undergo restructuring, and after rearrangements in solution, some of them form complexes with the polymer and establish cross-links in the network (see Figure 6a). In the polymer solutions with HP-β-CD (Figure 5b,d), the value of ηrel falls off with cosolute concentrations up to approximately 8 mm for the HM-HEC sample, whereas for the HEC/HP-β-CD system, no effect of HP-β-CD addition is visible. Thus, the hydrophobic tails are deactivated by the addition of HP-β-CD, but no drastic viscosification effect at low temperatures is observed because this cosolute is not capable of crystallizing. A closer inspection of the temperature effect on the viscosity reveals that the value of ηrel decreases with increasing temperature. This may be ascribed to enhanced mobility of the polymer chains at elevated temperature, leading to a loosening of the associative network.
Langmuir, Vol. 22, No. 21, 2006 9027
Figure 6. (a) Illustration of the formation of β-CD cross-links in a polymer network at high β-CD concentration and low temperatures. (b) Formation of columnar β-CD structures in semidilute solutions of HM-HEC and the break-up of these structures at elevated temperature.
Effects of temperature on ηrel in HM-HEC solutions with different concentrations of the cosolutes are scrutinized in Figure 7. In addition, the temperature dependence of ηrel for the HEC/ D2O system is displayed (Figure 7a). For both the HEC/D2O and the HM-HEC/D2O systems, ηrel decreases with increasing temperature. This behavior may be ascribed to augmented thermal mobility of the polymer chains as the intermolecular interactions break up. For the HM-HEC sample, the hydrophobic associations are weakened, whereas for the HEC/D2O system, the drop of ηrel may reflect the break-up of hydrogen bond interactions. At a level of 0.8 mm cosolute (Figure 7b), the relative viscosity for HM-HEC is lower than without cosolute, and ηrel assumes lower values in the presence of HP-β-CD than with β-CD. This suggests that while HP-β-CD is molecularly dispersed (no temperature-induced association), the β-CD solution contains minute amounts of clusters that are not effective in the decoupling of hydrophobic interactions, and therefore, at this low cosolute concentration, the number of encapsulated tails is less and the viscosity for the HM-HEC/β-CD system is higher. In the presence of 2 mm cosolute (Figure 7c), an intriguing temperature dependence of ηrel appears for the HM-HEC/β-CD system. In this case, the value of ηrel rises with increasing temperature, and at low temperatures, the values of ηrel are considerably lower than the corresponding ones in the presence of HP-β-CD. This may be explained in terms of a more efficient encapsulation of the hydrophobic tails with β-CD. It has recently been reported20 from cryo-TEM and light scattering experiments that β-CD self-assembles in water at a cosolute concentration between 2 and 3 mm in the form of columnar or channel-like arrangements. In our case, the hypothesis is that threading of β-CD molecules onto pendant hydrophobic chains occurs, and
9028 Langmuir, Vol. 22, No. 21, 2006
Bu et al.
Figure 8. SANS scattered intensity plotted vs scattering vector q (every third point is shown) at 25 °C for 2 wt % solutions of HEC and HM-HEC in the presence of various amounts of β-CD. The inset plot shows the effect of β-CD addition on the power law exponent γ (I(q) ∼ q-γ) in the small q range. Figure 7. Temperature dependencies of the relative zero-shear viscosity for 2 wt % solutions of HEC/D2O and HM-HEC/D2O (a) and for 2 wt % HM-HEC solutions (b-f) in the presence of β-CD or HP-β-CD at the cosolute concentrations indicated.
this yields an efficient decoupling of hydrophobic interactions. In this channel-type structure, the dimer units formed by two neighboring cyclodextrin molecules (head-to-head or head-totail orientation) may be stacked to form longer columns.21-23 The inclusion complexes are stabilized by hydrogen bonds between the hydroxyl groups on neighboring β-CDs in columnar stacks and hydrophobic interactions established between hydrophobic tails and internal hydrophobic β-CD cavities. It has been argued21 that minimization of the mutual contact areas of the hydrophilic and hydrophobic parts leads to columnar aggregates of CDs and their inclusion complexes. The surmise is that the bulky hydrophobic groups (C16) attached to the HMHEC chains will be more effectively deactivated in the formation of inclusion complexes with β-CD dimers or longer columnar stacks than with monomers. In the latter situation, the ends of the hydrophobic tails may interact intermolecularly with adjacent chains with hydrophobic moieties. A dimer unit would be sufficient to encapsulate the whole C16 tail. As a result, the values of ηrel for HM-HEC/β-CD mixtures are lower than those for the HM-HEC/HP-β-CD system at low temperatures (Figure 7c). As the temperature increases, the portion of channel structures decreases (some are disintegrated into monomers) due to thermal disruption, and the viscosity increases. The difference in the value of ηrel for the systems at higher temperatures reflects the presence of some remaining β-CD aggregates in a similar way as for the solutions with the lower cosolute concentration (Figure 7b). The finding that weaker hydrophobic associations operate at 5 °C for the HM-HEC/β-CD (2 mm) system is supported by the SANS results presented next. (21) Polarz, S.; Smarsly, B.; Bronstein, L.; Antonietti, M. Angew. Chem., Int. Ed. 2001, 40, 4417. (22) Rusa, C. C.; Bullions, T. A.; Fox, J.; Porbeni, F. E.; Wang, X.; Tonelli, A. E. Langmuir 2002, 18, 10016. (23) Hunt, M. A.; Rusa, C. C.; Tonelli, A. E.; Balik, C. M. Carbohydr. Res. 2004, 339, 2805. Ibid, Carbohydr. Res. 2005, 340, 1631.
In the presence of 4 mm cosolute concentration (Figure 7d), a clear difference between the ηrel values of the systems is also in this case observed at low temperatures, and the crossover of the viscosity curves takes place at a higher temperatures (40 °C). Even at this cosolute concentration, the existence of columnar structures encapsulating hydrophobic tails is expected. The decrease of ηrel at higher temperatures reflects both enhanced thermal mobility of the polymer chains and a reduced fraction of columnar structures for encapsulation. The somewhat lower values of ηrel for the HM-HEC/β-CD mixture as compared with the HM-HEC/HP-β-CD system at 8 mm cosolute concentration (Figure 7e) can be related to the more effective decoupling in the presence of β-CD. At higher β-CD concentrations (Figure 7f), an intricate situation emerges with the growth of huge aggregates that coexist with smaller clusters with a broad size distribution. At this stage, it is difficult to predict how efficient the β-CD entities are to encapsulate hydrophobic tails. At 16 mm β-CD concentration, cage-like aggregates or crystallites are formed at low temperatures, and they serve as cross-links in the network structure. At sufficiently high temperatures, the β-CD clusters are disrupted, and the viscosity data for the samples with β-CD or HP-β-CD collapse onto each other. SANS Results. To gain insight into structural changes on a mesoscopic length scale, SANS experiments have been conducted on the HEC/β-CD and HM-HEC/β-CD systems at various β-CD concentrations and temperatures. Figure 8 shows the SANS intensity profiles for 2 wt % solutions of HEC and HM-HEC with different levels of β-CD addition at a temperature of 25 °C. No effect of β-CD addition is observed for the HEC solution, whereas for the HM-HEC sample, a weaker upturn of the scattered intensity at low q values is found upon the addition of β-CD. As expected, the weak interaction between HEC and β-CD does not generate any structural differences observable with SANS. To quantitatively depict the q dependence of the scattered intensity at low q values, we have employed a power law I(q) ∼ q-γ, which yields a good description of the SANS data in the low q region. The values of the power law exponent are given in the inset in Figure 8a. A strong upturn of the scattered intensity at
Mixtures of Hydroxyethylcellulose and Cyclodextrins
Langmuir, Vol. 22, No. 21, 2006 9029
of temperature on the scattered intensity can be discerned. To gain further insight into this system, we tried to conduct light scattering measurements on this system, but because of β-CD crystallization at low temperatures, the sample became turbid, and we encountered problems with multiple-scattering. This prevented us from performing a systematic light scattering study. Although the strength of the polymer network is enhanced through the establishment of cross-links at lower temperatures, our surmise is that the mesoscopic structure of the network is virtually intact (cf. Figure 6a).
Conclusion
Figure 9. SANS scattering profiles (every second point is shown) for 2 wt % HM-HEC solutions at different temperatures and β-CD concentrations.
low q values announces the formation of large-scale heterogeneities24 in the solution or multichain domains.25 This type of behavior has previously been reported11,26,27 from SANS measurements on aqueous solutions of hydrophobically modified polymers. In light of this, the results in Figure 8 suggest that the hydrophobic interactions give rise to large association structures, and the intensity of the hydrophobic interactions is reduced as the β-CD concentration increases because of the formation of inclusion complexes. The inset plot indicates that the evolution of large-scale aggregates in the HM-HEC/β-CD system is suppressed at β-CD concentrations above 8 mm. This is consistent with the viscosity results, which suggest that the hydrophobic interactions are decoupled at this level of β-CD addition. Figure 9 illustrates the effect of temperature on the scattered intensity for 2 wt % HM-HEC solutions with different concentrations of β-CD. In general, a very little effect of temperature is seen, but an interesting feature is depicted in Figure 9a for the solution containing 2 mm β-CD. The slightly weaker upturn of the scattered intensity at low q values at 5 °C may signalize less pronounced association structures, which is compatible with the lower viscosity reported at this condition. This was attributed to the efficient encapsulation of the hydrophobic tails of the polymer chains through the formation of the channel-like β-CD structures. No temperature effect on the scattered intensity at low q values can be traced for the HM-HEC sample with 8 mm β-CD (Figure 9b), which is consistent with the interpretation of the viscosity results at this condition, namely, that virtually all hydrophobic moieties are deactivated at this β-CD concentration. For the 2 wt % HM-HEC solution with a β-CD concentration of 16 mm, a marked viscosity enhancement was observed at low temperatures. To examine whether this effect has an impact on the local structure of the network, SANS experiments were conducted on this sample at different temperatures (see Figure 9c). Despite the strong viscosity effect, practically no influence (24) Horkay, F.; Basser, P. J.; Hecht, A.-M.; Geissler, E. Polym. Prepr. (Am. Chem. Soc., DiV. Polym. Chem.) 2002, 43, 369. (25) Ermi, B. D.; Amis, E. J. Macromolecules 1997, 30, 6937. (26) Esquenet, C.; Terech, P.; Boue´, F.; Buhler, E. Langmuir 2004, 20, 3583. (27) Bu, H.; Kjøniksen, A.-L.; Knudsen, K. D.; Nystro¨m, B. Langmuir 2005, 21, 10923.
In this investigation, we have examined the effects of β-cyclodextrin addition and temperature on the thermodynamic, structural, and viscosity properties of semidilute solutions of HEC and a hydrophobically modified analogue (HM-HEC). In addition, measurements have also been performed in the presence of a reference cosolute (HP-β-CD) that has no tendency to crystallize at low temperatures to evaluate the impact of the crystallization process on the behavior of polymer solutions with β-CD. The DSC experiments show that crystal formation takes place in the HEC/β-CD and HM-HEC/β-CD systems at low temperatures and at sufficiently high β-CD concentrations. It is found that the crystallization transition is induced at lower levels of β-CD addition in the polymer solutions than in the corresponding β-CD/D2O mixtures. The viscosity results reveal that decoupling of hydrophobic interactions starts at very low concentrations of β-CD or HPβ-CD, and at 2 mm, the β-CDs are threaded onto the hydrophobic tails to form columnar structures, which are suppressed at elevated temperatures. The less pronounced upturn in scattered intensity from SANS at low q values for the HM-HEC/β-CD (2 mm) system at the lowest temperature suggests that the deactivation of hydrophobic moieties is more efficient because of a large portion of columnar structures. These channel-type arrangements are more effective to encapsulate hydrophobic tails than β-CD monomers. At higher cosolute concentrations, the values of the relative zero-shear viscosity are lower in the presence of β-CD, which indicates that β-CD is a more effective inhibitor of hydrophobic interactions than HP-β-CD. At levels of cosolute addition above 8 mm, no differences between HM-HEC/β-CD and HM-HEC/HP-β-CD systems are detected, and no further decoupling of hydrophobic sites is visible. The viscosities of the HEC/D2O system and solutions of HM-HEC with various levels of β-CD or HP-β-CD decrease with increasing temperature, and this effect is ascribed to enhanced thermal mobility of the polymer chains at elevated temperatures, leading to a loosening of the network and easier relaxation of the chains. At high β-CD concentrations and low temperatures, both HEC and HM-HEC chains are cross-linked through the formation of large β-CD clusters or crystallites, and a dramatic viscosity enhancement occurs. The β-CD units are probably assembled in cage-like structures. Despite the strong viscosity rise, no changes of the structure on a mesoscopic length scale are disclosed. The conjecture is that no large-scale structures are formed as in the case of hydrophobic associations but rather that the network is strengthened through the cross-links. Acknowledgment. B.N, H.B., K.Z., N.B., A.E., and K.D.K. gratefully acknowledge financial support provided by the Norwegian Research Council through a NANOMAT Project (158550/431). K.D.K. thanks the Marie Curie Industry Host Project (Contract G5TR-CT-2002-00089) for support. LA0608664
