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Complexation between r-Cyclodextrin and Poly(ethylene oxide) Physically Adsorbed on the Surface of Colloidal Silica Edvaldo Sabadini,*,† Terence Cosgrove,‡ and Wirach Taweepreda‡ School of Chemistry, The University of Bristol, Cantock’s Close, Bristol BS8 1TS, United Kingdom, and Instituto de Quimica, Universidade Estadual de Campinas, Caixa Postal 6154, CEP 13084-862 Campinas, Brazil Received January 9, 2003. In Final Form: February 28, 2003 Aqueous solutions of R-cyclodextrin (R-CD), a cyclic oligomer formed by six glucose units, complex spontaneously with poly(ethylene oxide), PEO, and several molecules of R-CD thread onto the host polymer chain. A new nanocomposite has been synthesized by the complexation of PEO physically adsorbed on the surface of colloidal silica with R-CD. The PEO monomers adsorbed on the silica surface were displaced by changing the degree of complexation and the MW of the polymer. As the inclusion of R-CD can only start from the extremity of the PEO chain, the fraction and length of the tails can be studied by changing the number of R-CD molecules threaded. Adsorbed PEO of 1000 molecular weight can be completely included in the cavities of R-CD molecules, resulting in a full displacement of the adsorbed polymer. However, for PEO 6k and 20k, the complexation of the chain is partial, that is, some nonincluded EO units remain adsorbed on the silica surface. In these cases, a gel containing silica was formed in the hydrogel network. The effect of silica on the yield of the complexation was also studied. The solid-state complexes formed by [silica-PEO-R-CD] were characterized by FTIR, SEM, and X-ray diffraction.
Introduction A mixture of poly(ethylene oxide) (PEO) and R-cyclodextrin (R-CD) in aqueous solution spontaneously forms a complex known as a pseudopolyrotaxane. This supramolecular adduct can be used to generate other interesting structures such as molecular necklaces, molecular trains, and molecular tubes.1-3 In the formation of the complex, R-CD molecules thread along the polymer chain forming a necklace-like structure.4,5 The bonds involved in the formation of pseudopolyrotaxanes are not covalent, and therefore, the R-CD molecule has mobility along the polymer chain. This means that more than one molecule of R-CD can be threaded onto the polymeric host. The complex is thought to be promoted by hydrophobic interactions between the cavity of R-CD and -CH2CH2Ounits of PEO and also by hydrogen-bond formation between the hydroxyl groups situated along the rim of R-CD molecules threaded onto the PEO chain.6,7 The maximum ratio of PEO monomers to R-CD molecules in the final pseudopolyrotaxane formation is approximately 2:1.8 For PEO with low MW (in the range of 1k), the complex can be easily identified, as a solution obtained from a mixture of both components becomes turbid in a few minutes. There is no limit for the complexation between PEO and R-CD in terms of PEO MW; however, * To whom correspondence may be addressed. E-mail:
[email protected] or
[email protected]. † Universidade Estadual de Campinas. ‡ The University of Bristol. (1) Lo Nostro, P.; Lopes, J. R.; Ninham, B. W.; Baglioni, P. J. Phys. Chem. B 2002, 106, 2166. (2) Harada, A. Coord. Chem. Rev. 1996, 148, 115. (3) Fujita, H.; Ooya, T.; Yui, N. Macromolecules 1999, 32, 2534. (4) Harada, A.; Kamachi, M. Macromolecules 1990, 23, 2821. (5) Harada, A.; Li, J.; Kamachi, M. Nature 1992, 356, 325. (6) Ceccato, M.; Lo Nostro, P.; Baglioni, P. Langmuir 1997, 13, 2436. (7) Harada, A.; Okada, M.; Li, J.; Kamachi, M. Macromolecules 1995, 28, 8406. (8) Harada, A. Supramol. Sci. 1996, 3, 19.
the rate for the reaction and the proportion EOcomplexed/ EOfree reduce as the PEO MW increases. Depending on the PEO MW and the relative concentrations of the polymer and R-CD, a gel9,10 or a white crystalline phase can be obtained.11-13 The complexation of R-CD and PEO on the surface of colloidal particles is an interesting topic which has not been hitherto investigated. A range of properties can be envisaged due to the competition established between the processes of adsorption and desorption (due to the inclusion of EO monomers by R-CD) of the polymer chain onto the silica surface. A polymer adsorbed on a surface is commonly described by the looptrain-tail model. The trains are made up of segments in direct contact with the surface, whereas loops have no direct contact with the surface but may be in close proximity. Tails are nonadsorbed chain ends. Although tail segments may constitute a small proportion of all segments, they determine the hydrodynamic layer thickness of the adsorbed polymer.14 The complexation of R-CD by PEO grafted on the surface of a polystyrene latex has been studied using SANS. The complexation alters the layer thickness, which expands as the amount of R-CD threading onto the chains of the polymer increases.15 The complexation of PEO grafted onto dextrans has also already been reported.9 This paper describes the results of the complexation of PEO physically adsorbed onto the surface of colloidal silica, in which the effects of PEO MW and the degree of complexation were studied. (9) Huh, K. M.; Ooya, T.; Lee, W. K.; Sasaki, S.; Kwon, I. C.; Jeong, S.Y.; Yui, N. Macromolecules 2001, 34, 8657. (10) Li, J.; Harada, A.; Kamachi, M. Polym. J. 1994, 26, 1019. (11) Harada, A.; Okada, M.; Kawguchi, Y.; Kamachi, M. Polym. Adv. Technol. 1999, 10, 3. (12) Meschke, C.; Buschmann, H.-J.; Schollmeyer, E. Polymer 1999, 40, 945. (13) Gong, C.; Glass, T. E.; Gibson, H. W. Macromolecules 1998, 31, 308. (14) Jones, R. A. L.; Richards, R. W. Polymers at Surfaces and Interfaces; Cambridge University Press: Cambridge, U.K., 1999. (15) Joseph, J. Ph.D. Thesis, University of Bristol, 2001.
10.1021/la0340403 CCC: $25.00 © 2003 American Chemical Society Published on Web 04/05/2003
R-Cyclodextrin and PEO on Colloidal Silica
Figure 1. Photographs comparing the supramolecular gels produced from aqueous solutions of R-CD and free PEO 6k (identified by the subscript 0) and from PEO 6k adsorbed on the silica surface (identified by the subscript S). CR-CD/CPEO values for the three pairs of tubes are 6, 7, and 8.5, for the pairs A, B, and C, respectively. Csilica/CPEO for the samples containing silica is approximately 10.
Materials and Methods R-CD (Wacker) and PEO with MW 1k, 6k, and 20k g mol-1 (BDH) were used as received. The sample of silica Klebosol 30R 12, with low polydispersity, was supplied by Clariant. The dispersion has a specific surface area of 200 m2/g.16 Before use, the stock solution of silica was extensively dialyzed against Milli-Q plus water. The diameter of the particles was found to be close to 12 nm using TEM images. All the solutions were prepared using Milli-Q plus water. The mixtures of the components were prepared by weight (within (0.1 mg) from aqueous solutions of PEO (3%), silica (17.1%), and a saturated solution of R-CD (12.7%). Samples with adsorbed polymer were made by adding a known quantity of silica dispersion to a polymer solution with an equilibration time of 48 h, before the addition of R-CD, and then 7 days were left for the system to reach a final equilibrium (in this time no more changes in the amount of gel phase are observed). The samples were then centrifuged for 5 min (3200 rpm). To determine the yield for the reaction, the solid phase was separated from the supernatant and dried in an oven at 80 °C for 48 h. The yield was determined by the follow equation:
Y)
(
)
Wc × 100 WPEO + WR-CD
where Wc, WPEO, and WR-CD are the weights of the complex obtained, PEO, and R-CD, respectively. All the steps described were carried out in eppendorf tubes, allowing us to minimize losses of material. All the experiments were performed at room temperature. The experiments involving infrared spectra, X-ray diffraction, and SEM were performed using newly dried films of gels. The films were obtained from the centrifuged samples and dried for 2 days at room temperature and pressure. Infrared spectra between 530 and 4000 cm-1 with a resolution of 4 cm-1 (16 scans) were obtained with an FTIR Perkin-Elmer Spectrum One using a universal ATR sampling accessory. Micrographs were obtained using a JEOL model JSM5600 scanning electron microscope. The X-ray diffraction measurements were performed with a powder diffractometer (Bruker D8 Advanced; Cu KR radiation, λ ) 1.542 Å).
Results and Discussion The effect of colloidal silica on the pseudopolyrotaxane formation is clearly observed by comparing with complexation in the absence of silica. Three pairs of sample were prepared with and without silica, and Figure 1 shows (16) Catalogue of KlebosolsThe Colloidal SilicasClariant.
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Figure 2. Phase diagram for the complex PEO 6k and R-CD. The diagram is based on the proportion between the free amount of liquid (wFL) in equilibrium with the gel phase and the total weight of the sample (WT). The points corresponding to the samples AS, BS, and CS emphasize the silica effect. The standard deviation for the complexation is 3%.
their visual appearance. The relative concentrations of R-CD and PEO 6k (CR-CD/CPEO) for the three pairs of tubes were 6, 7, and 8.5 (A, B, and C). For the tubes on the left side (AS, BS, and CS), PEO was first adsorbed on the silica in the proportion Csilic/CPEO ≈ 10, and then R-CD was added. The subscript 0 (A0, B0, and C0) indicates the absence of silica. Apparently, the volume of gel formed with silica is lower than that without it. However, this result is associated with an increasing density of the gel due to the incorporation of silica particles. Despite this latter effect, for both samples the amount of gel increases with an increasing concentration of R-CD. The gel formation is highly dependent on the interactions between the complexed chains, which increase in volume with the threading of more R-CD molecules. The mechanism for the gel formation is complex. The chains of PEO containing threaded R-CD become hydrophobic due to the decreasing number of free hydroxyl groups of adjacent R-CD rings due to connectivity by hydrogen bonds. The interactions between the complexed chains produce a crystal or a gel phase, depending on the experimental conditions.10 In the case of gel formation, the network is made up of noncomplexed EO units linked by the aggregation of microcrystals of threaded R-CD, which act as physical crosslinking agents. For PEO 6k the inclusion of EO units is only partial, even if a saturated solution of R-CD is added to a solution of the polymer. We estimate from the work of Harada8 that for the PEO 6k sample approximately 58 EO segments form the inclusion complex but 79 EO remain free: part of the nonincluded EO segments remain adsorbed on the surface of silica when the complex is formed (loops and trains). As seen in Figure 1, the extent of the gel phase for the sample containing silica also increases as the degree of the complexation increases, implying that more of the polymer chain is displaced from the silica. Similar results (Figure 1) were observed for PEO 20k, in which the proportion of EO not complexed is higher than that for PEO 6k. The gel is only formed when the MW of PEO is higher than 2000. This is because for lower molecular weights, all of the EO units can be included by R-CD, forming an adduct in which the stoichiometry is 1R-CD/2EO.10 Figure 2 shows a phase diagram for the system PEO(6k)/R-CD, based on the free amount of liquid (WFL) in equilibrium with the gel phase, in which three regions can be observed.
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Figure 5. Fourier transform infrared spectrum of [silicaPEO (6k)-R-CD]. The main characteristic bands for the complex are indicated.
Figure 3. Yield for the complexation between R-CD and PEO 1k and PEO 6k in aqueous solution and PEO adsorbed on colloidal silica, as a function of CR-CD/CPEO. The proportion Csilica/ CPEO for the samples containing silica is approximately 10. The standard deviation for the complexation is 3%.
Figure 4. Yield for the complexation between PEO 6k and R-CD as a function of Csilic/CPEO. The complexation was carried out for two ratios: CR-CD/CPEO ) 4 (a) and 10 (b). For part a, due to the low degree of complexation, the gel is formed mainly by the nonadsorbed PEO chains; the yield decreases from 20% to 0 in the region of the monolayer (Csilic/CPEO ≈ 6-8). For part b, the high degree of complexation leads to the incorporation of silica particles in the network of the gel. The standard deviation for the complexation is 3%.
For small concentrations of R-CD, all the componentss PEO, free R-CD, and the complexsare soluble. The onset of the appearance of the gel phase (second region) is close to CR-CD/CPEO ) 5. Beyond this point, all of the liquid is held in the network of the hydrogel. The third region is characterized by the presence of free liquid in equilibrium with a crystalline phase. The points associated with the samples AS, BS, and CS were included in this diagram to emphasize the silica effect. The presence of silica shifts not only the equilibrium line for the sol-gel phases (to a region of higher R-CD concentration) but also the complexation yield. The yield for the reaction between R-CD and PEO 1k, 6k, and 20k adsorbed on the silica surface was found by changing CR-CD/CPEO from 0 to 10. The concentrations of silica and PEO were kept constant (Csilic/ CPEO ≈ 10). This proportion was chosen to be as close as possible to that to produce a monolayer of adsorbed PEO on the silica surface (estimated ) 0.6 mg m-2 (refs 1619)). The proportion Csilic/CPEO ≈ 10 corresponds to a situation in which 80% of the particle is covered by PEO.
Figure 6. SEM micrographs of the dried supramolecular gels formed from PEO (6k) and R-CD: free PEO (A) and PEO adsorbed on the silica surface (B). For both samples CR-CD/ CPEO ) 12, and for the sample containing silica Csilic/CPEO ) 9.
The comparative yields for PEO 1k and 6k are shown in Figure 3. What is noticeable from this figure is that the curves for PEO 1k with or without silica are rather close. This means that the majority of the adsorbed polymer was displaced from the surface of silica due to the complexation and no silica is retained in the gel. In the case of PEO 6k, the yield for the samples containing silica is higher than that for those without silica. This is attributed to the incorporation of the silica particles in the mass of the complex. The mass of the particles was (17) Van der Beek, G. P.; Cohen Stuart, M. A.; Cosgrove, T. Langmuir 1991, 7, 327. (18) Lafuna, F.; Wong, K.; Cabane, B. J. Colloid Interface Sci. 1991, 143, 9. (19) Killmann, E.; Maier, H.; Kaniut, P.; Gutling, N. Colloids Surf. 1985, 15, 261.
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Figure 7. X-ray diffraction patterns for PEO 6k (a), R-CD (b), dried gel formed from free PEO and R-CD (c), and dried gel formed from PEO adsorbed on the silica and R-CD (d). For these samples CR-CD/CPEO ) 12, and for the sample (d) Csilic/CPEO ) 9.
not used in the calculation, resulting in yields higher than 100%. The incorporation of silica was also observed for the gel formed with PEO 20k (the results are not shown). The displacement of PEO from colloidal silica is interesting from a thermodynamic point of view. The most important factor in determining the adsorption/desorption behavior of a polymer at an interface is the net segmental adsorption energy, χs.20 χs has to compensate for the loss in conformational entropy that a polymer experiences on adsorption. Adsorption only occurs when χs is greater than a critical value χsc. PEO can be completely displaced from the silica surface when poly(vinylpyrrolidone) is added to the system, due to the higher value of χs for the adsorption of this polymer on the silica surface.21 However, in the present study the displacement of PEO is not due to the adsorption of R-CD on the silica surface. The main driving force for the PEO displacement is the free energy involved in the supramolecular adduct formation. Despite the fact that this process is entropically unfavorable, as the linear polymer chain must fit into several host units, it is an overriding enthalpic effect associated mainly with the strong interactions established between the polymer units and the hydrophobic cavity of R-CD which leads to complex formation.2,22,23 To verify the effects of silica coverage, the complexation yield was also determined by fixing the concentration of R-CD at two ratios (CR-CD/CPEO ) 4 and 10) and varying the proportion Csilic/CPEO from 0 to 15. The results are shown in Figure 4. For the lowest ratio of CR-CD/CPEO ) 4, the yield is close to 20% with no silica (Csilic/CPEO ) 0). This value decreases when the Csilic/CPEO ratio is in the range 6-8. This is the region in which a monolayer of PEO is formed. Beyond this point, the yield is approximately zero. This does not mean that no further (20) Silberberg, A. J. Chem. Phys. 1968, 48, 2835. (21) Nelson, A.; Jack, K. S.; Cosgrove, T.; Kozak, D. Langmuir 2002, 18, 2750. (22) Lo Nostro, P.; Lopes, J. R.; Cardelli, C. Langmuir 2001, 17, 4610. (23) Pozuelo, J.; Mendicuti, F.; Mattice, W. L. Macromolecules 1997, 30, 3685.
complex is formed, as the yield is based only on the mass of complex that is not soluble. Probably a colloidal complex of [R-CD-PEO-silica] remains dispersed, in equilibrium with free R-CD. In this range of concentration, the presence of silica inhibits the physical cross-linking between the threaded R-CD: the tails of the complex are not long enough to allow the formation of the microcrystals. The behavior is opposite when the proportion between R-CD and PEO is 10; the yield increases progressively as the proportion of silica increases. The displacement of PEO and the increasing number of threaded R-CD’s lead to cross-linking among the complexed chains. The particles of silica therefore remain joined to the part of nonincluded EO units in the gel structure. The infrared spectrum of the dried gel formed from [silica-PEO 6k-R-CD] is shown in Figure 5. The spectrum shows the characteristics bands of the complex at 1297, 1057, 950, and 699 cm-1.24 The structure of the spectrum in the region of 1000 cm-1 is broadened due to the superposition of the band associated with the stretching of SiO2. This result qualitatively confirms the incorporation of silica particles into the matrix. A film of dried silica-containing gel is more transparent and more homogeneous than the film of dried gel without silica. Electron microscopy images were obtained for both newly dried gels (Figure 6). The proportion of CR-CD/CPEO for both samples is 12, and for the sample with silica Csilic/ CPEO ) 9. The images show layered crystalline regions. However, there are some important differences in the morphologies. The silica-containing gel shows an isotropic and well-pronounced homogeneous network of twodimensional platelets of similar dimensions. The sample without silica contains, for the most part, massive threedimensional aggregates. Similar structures were observed by Panova et al.25 for gels formed with PEO and low and high concentrations of R-CD, respectively. Even though the same concentration of R-CD was used in both samples, (24) Rusa, C.; Tonelli, A. E. Macromolecules 2000, 33, 1813. (25) Panova, I. G.; Gerasimov, V. I.; Topchieva, I. N. Polym. Sci. 1998, 40, 336.
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Figure 8. Schematic illustration of the structure of a supramolecular gel formed by PEO physically adsorbed on a colloidal silica surface and R-CD. The cyclic molecules thread onto the tails of PEO. The length of the complexed tails increases as the number of threaded R-CD’s increases. The aggregation of R-CD acts as physical cross-linking, producing a network. The small particles of silica are bonded by part of the EO units of PEO not include by R-CD molecules.
when the polymer is adsorbed on the surface of the silica, the number of nuclei and the growth of crystallites are reduced. This can be associated with the fact that a tail of complexed PEO adsorbed on the silica surface has a lower mobility than the extremity of a free complexed polymer in solution. Despite the similarity between the structures of our matrixes and the matrix obtained by Panova et al., the silica-containing gel becomes opaque in 2 weeks and SEM images reveal a collapse of the platelets. The new structure is similar to that of the matrix without silica (Figure 6a). To characterize the crystalline structure of the dried gels, we obtained the X-ray diffraction patterns for the matrixes of dried gels with and without silica (Figure 7). There are no peaks of PEO in the gel matrixes, indicating that the chains of PEO not included by R-CD are in the amorphous phase. Both matrixes show a peak at 2θ ) 20° (interplanar spacing, d0 ) 4.44 Å), due to the 210 reflection from the hexagonal lattice of the complex. This peak is associated with the channel-type crystalline structure of the long-chain nature of the guest molecules.9,24,26-29 The peak is less intense for the sample containing silica. This (26) Harada, A.; Li, J.; Kamachi, M. Macromolecules 1993, 26, 5698. (27) Harada, A.; Okada, M.; Kamachi, M. Macromolecules 1995, 28, 8406. (28) Huang, L.; Allen, E.; Tonelli, A. Polymer 1999, 40, 3211. (29) Huang, L.; Allen, E.; Tonelli, A. Polymer 1998, 39, 4857.
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result is in qualitative agreement with the SEM pictures. The cross-linking of the R-CD-PEO chains is less effective when the polymer is adsorbed on the silica surface. The strong peak at 2θ ) 11° exhibited for both samples is due to nonequatorial reflections and is associated with the formation of R-CD aggregates in which water molecules are bonded in the sequence of the R-CD molecules. Such crystalline aggregates are induced by inclusion complex formation9 and are destroyed if the gel is dried by heating. The peak is not observed in the X-ray pattern of pure R-CD (Figure 7). From these results and on the basis of the model for the R-CD-PEO gel structure proposed by Harada,10 we suggest a possible configuration for the gel formed in the presence of silica (Figure 8). The PEO chains adsorbed on the silica surface exist as trains, loops, and tails. With the addition of R-CD, the tails of PEO penetrate into the cavities of R-CD. The polymers are displaced from the silica surface as more R-CD molecules thread along the chain. The threaded R-CD molecules interact together by hydrogen bonds, and the complexed chains become hydrophobic and can self-aggregate. A network is produced by the microcrystals of threaded R-CD. The majority of the EO units remain free, and they are distributed as trains, loops, and tails. Therefore, tiny particles of silica and aqueous solution are held in the network, producing a dense gel. Conclusions A supramolecular complex was made by threading R-CD onto PEO adsorbed on the surface of colloidal silica. The inclusion process can only start from the tails of the adsorbed PEO and leads to a displacement of the polymer from the silica surface. For low-MW PEO (1k), all of the chain is displaced. However, for PEO in which the inclusion of the EO units is only partial, the polymer still remains bonded to the silica surface. As the degree of complexation increases, more polymer is displaced from the silica surface. The presence of silica in the network of the supramolecular gel can decrease the physical cross-linking between the threaded chains. The formation of the complex on the silica surface was characterized by FTIR and X-ray diffraction. Acknowledgment. The authors thank Clariant (Division Functional Chemicals) for the sample of silica, Wacker-Chemical AG for the sample of R-CD, David Jones for the photographs of the samples, Jonathan Jones for the electronic micrographs, and Dr Suzanna D. Kean for useful comments. E.S. would like to thank CNPq-Brazil for funding of this work via a Senior Research Fellowship. W.T. would like to thank the Thai government for the Ph.D. studentship. LA0340403
