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Stretching a Polymer Brush by Making in Situ Cyclodextrin Inclusion Complexes Julie Joseph,†,§ Ce´cile A. Dreiss,*,‡ and Terence Cosgrove† School of Chemistry, UniVersity of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K., and Pharmaceutical Science DiVision, King’s College London, Franklin-Wilkins Building, 150 Stamford Street, London SE1 9NH, U.K. ReceiVed April 7, 2008. ReVised Manuscript ReceiVed July 3, 2008 The interaction between poly(ethylene oxide) (PEO) chains grafted onto polystyrene latex particles and R-, β-, and γ-cyclodextrins (CD) was studied by small-angle neutron scattering. The particles were contrast-matched to the solvent in order that only the scattering from the polymer layers was detected. The signal from the layers was fitted to a double-exponential volume fraction profile. The effects of adding cyclodextrin on the polymer profile are shown as a function of cyclodextrin concentration. The polymer layers are seen to extend on addition of CD, which is consistent with a complexation between the grafted PEO and the CD molecules. The effect is the strongest with R-CD.
Introduction Adsorbed polymers can contribute significantly to the stabilization of colloidal particles and thus a significant theoretical and experimental interest has been devoted toward understanding the structure of these layers.1 It is now well established that the volume fraction profiles of grafted and physically adsorbed polymers at the solid-liquid and liquid-liquid interfaces may be determined from SANS data.2-12 In this study, small-angle neutron scattering has been used to study the volume fraction profile of poly(ethylene oxide) (PEO) chains grafted onto colloidal polystyrene latex particles in the presence of cyclodextrin. The data have been analyzed and fitted by a nonlinear least-squares method.9-12 The scattering from the polymer layer is measured under conditions where the particle is contrast-matched in scattering length density to the solvent. The interfacial density profile φ(z) has been fitted to a doubleexponential profile and a term corresponding to local polymer concentration fluctuations.11,12 Grafting of the polymer was achieved through reaction of acrylate-terminated PEO polymer chains onto the surface of latex particles. The grafted chains produced are often referred to as brush polymers, and in the moderate-density grafting regime the chains can interact strongly with their neighbors. In a ‘good’ solvent the chains repel each other via excluded volume * To whom correspondence should be addressed. E-mail:
[email protected]. † University of Bristol. ‡ King’s College London. § Present address: National Starch & Chemical, National Adhesives, Wexham Road, Slough SL2 5DS, U.K. (1) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J.M.H.M.; Cosgrove, T.; Vincent, B., Polymers at Interfaces, Chapman and Hall, London, 1993. (2) Mir, Y.; Auroy, P.; Auvray, L. Phys. ReV. Lett. 1995, 75, 2863–2866. (3) Auroy, P.; Auvray, L. J. Phys. II 1993, 3, 227–243. (4) Auroy, P.; Mir, Y.; Auvray, L. Phys. ReV. Lett. 1992, 69, 93–95. (5) Auroy, P.; Auvray, L. J. Macromol. Sci.-Pure Appl. Chem. 1992, 29, 117– 124. (6) Auroy, P.; Auvray, L.; Leger, L. Physica A 1991, 172, 269–284. (7) Auroy, P.; Auvray, L.; Leger, L. J. Phys.-Condes. Matter 1990, 2, SA317– SA321. (8) Cosgrove, T. J. Macromol. Sci.-Pure Appl. Chem. 1992, 29, 125–130. (9) Cosgrove, T.; Heath, T. G.; Ryan, K.; Crowley, T. L. Macromolecules 1987, 20, 2879–2882. (10) Crowley, T. L. PhD Thesis University of Oxford, 1984. (11) Hone, J. H. E.; Cosgrove, T.; Saphiannikova, M.; Obey, T. M.; Marshall, J. C.; Crowley, T. L. Langmuir 2002, 18, 855–864. (12) Marshall, J. C.; Cosgrove, T.; Leermakers, F.; Obey, T. M.; Dreiss, C. A. Langmuir 2004, 20, 4480–4488.
interactions, adopting a stretched configuration. The thickness of the brush is related to both the grafting density and the chain length. Auroy et al. 2 have demonstrated that (in a good solvent) the layer formed on colloidal particles by grafted polymer display concentration fluctuations whose correlation length ξ is equal to the distance between grafting sites. The shape of a polymer brush in a good solvent at intermediate coverage may be described by a parabolic decay, with an exponential tail.7 However, as the solvent quality decreases and the layer collapses,13 it can be described by an exponential profile (if the chain also physically adsorbs to the interface). Polymer layers grafted onto flat surfaces or colloidal particles have been widely studied, and their interactions with additives such as salts14 and surfactants15 have also been described. While the pseudopolyrotaxanes formed between unbound PEO and cyclodextrins have attracted much interest in the literature16 since the pioneering work of Harada,17 there are no published accounts of the interaction of grafted layers with cyclodextrins (CD). The aim of this paper is to discover the effect of adding three cyclodextrins analogues (R-, β-, and γ-CD) on the profile of grafted PEO layers.
Experimental Section Cyclodextrins. R-, methyl β-, and γ-cyclodextrins were commercial samples supplied by Reckitt Benckiser. They were used without further purification. Preparation of the Grafted Latex. 75% deuterated polystyrene latex particles were prepared using surfactant-free emulsion polymerization. 1.8 mL of H-styrene and 5.4 mL of D-styrene were distilled under vacuum and added to 2.5 L of degassed water (Millipore Milli) held at 95 °C under reflux. Styrene monomer was allowed to emulsify, then 0.8207 g of ammonium persulfate dissolved in 20 mL of Millipore water was added to the monomer and the polymerization was allowed to proceed. A portion of the dilute latex was removed (to construct the contrast-match plot for the SANS studies) and the remainder of the latex was grafted with polymer as follows. 2.8 mL of 50% solids PEOmethacrylate terminated polymer (2080 g/mol, used as received from (13) Cosgrove, T.; Heath, T. G.; Ryan, K.; Vanlent, B. Polym. Commun. 1987, 28, 64–65. (14) Wesley, R. D.; Cosgrove, T.; Thomson, L.; Armes, S. P.; Billingham, N. C.; Baines, F. L. Langmuir 2000, 16, 4467–4469. (15) Cosgrove, T.; Mears, S. J.; Obey, T.; Thomson, L.; Wesley, R. D. Coll. Surf. A. 1999, 149, 329–338. (16) Ceccato, M.; LoNostro, P.; Baglioni, P. Langmuir 1997, 13, 2436–2439. (17) Harada, A.; Li, J.; Kamachi, M. Macromolecules 1993, 28, 5698–5703.
10.1021/la801088q CCC: $40.75 2008 American Chemical Society Published on Web 08/20/2008
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Joseph et al. which controls the extension of the profile (or span). When fitting the SANS data with the double-exponential profile, the volume fraction of polymer at the interface φs and the decay length z0 of the first exponential were allowed to float, while the decay length of the second exponential was fixed to 1 Å. The contribution from local polymer concentration fluctuations was taken into account in the scattering equations by a Lorentzian term, characterized by a ‘fluctuation intensity’, linked to the rigidity of the layer. The fluctuations are also characterized by a correlation length, ξ, which is the distance between graft points. The correlation length was therefore calculated from the grafting density (obtained from fitting the SANS data from the layer) and fixed for all subsequent measurements. More precisely, ξ ) σ-0.5 where σ is the number of chains per unit area. Once the volume profile has been obtained, it is possible to derive a number of parameters. An important parameter is the adsorbed amount (Γ), or adsorbed mass of polymer per unit surface:
Figure 1. Log-Log plot representation of the scattering from 4% v/v latex, bare (empty circles) and with a PEO grafted layer (full circles) in contrast-match solvent. The fit to a double-exponential profile model is shown as a solid line.
Aldrich) was added to the styrene emulsion polymer together with 0.1372 g of ammonium persulfate in water. Both the bare and the grafted latex were filtered and then purified by extensive dialysis against H2O (25 changes of the dialysate over one week). The latexes were concentrated initially by rotary evaporation and then further by centrifugation. Finally the latex particles were dispersed in D2O. Solids contents were determined for both the bare and grafted dispersions. The H2O content of both latexes was determined by high resolution NMR. The peak heights of the proton spectrum of several H2O/D2O mixtures were measured on a Jeol FX100 and used to produce a calibration curve. The H2O content of the dispersion could then be calculated by taking into account the volume occupied by latex particles. The size of the dried latex particles was measured by Transmission Electron Microscopy (TEM), yielding a radius of 455.2 Å, and by Photon Correlation Spectroscopy (PCS), giving a radius of 476.1Å. Preparation of the Pseudopolyrotaxanes. For all samples a final concentration of latex of 4% v/v was used. The cyclodextrins were added from stock solutions of D2O in the appropriate quantities (up to ca. 2 wt %). Samples were allowed to equilibrate for two weeks before measurement at the contrast matched solvent ratio. It was assumed that free CD did not make a significant impact at the scattering length density of the solvent. SANS Experiments. Measurements. The SANS measurements were performed on the LOQ instrument at ISIS spallation source (Rutherford Appleton Laboratory, Didcot, UK). All samples were measured in 2 mm path length quartz Hellma cells at 25.5 °C. The Q range measured was 0.007-0.249 Å-1, using neutrons with wavelengths between 2.2 and 10.0 Å collected simultaneously on a 128 cm × 128 cm position-sensitive detector 4.1 m away from the sample, and sorted by time-of-flight. Data were reduced using the Colette program at the ISIS facility. Data fitting. A detailed discussion of the equations used to fit the scattering from on-contrast layer scattering can be found in refs 11 and 12. In order to fit the scattering from an adsorbed or grafted layer, it is necessary to insert a model volume fraction profile into the scattering equation. In this study, the best fits were obtained by a double-exponential profile, where an exponential profile is cut off by another exponential profile to prevent a physically unrealistic extension of the profile to infinity. The exponential profile is described by
()
φ(z) ) φsexp
z z0
(1)
where φs is the volume fraction of the polymer at the solid interface, z is the distance normal to the interface and z0 is the decay length
Γ ) Fm
∫0s φ(z)dz
(2)
where Fm is the mass density of the polymer and the upper limit of the integral is the span. Another important parameter is the rootmean-square (rms) layer thickness, δrms defined by
δrms )
∫0s φ(z)z2dz ∫0s φ(z)zdz
(3)
Determination of the Contrast Match Point. The scattering length density of the latex was determined by a contrast-match plot, where the scattering from the bare latex in a range of H2O/D2O ratios was recorded and plotted as the square root of the intensity at Q ) 0, I0, against H2O concentration (experimentally the lowest Q value is used) is plotted. The contrast-match conditions obtained for I0 ) 0 were 20.4 v/v% H2O. All samples with the grafted PEO were therefore prepared in 20.4% H2O/79.6% D2O solvent. The corresponding scattering length density was calculated and fixed when fitting the SANS data.
Results and Discussion The scattering from the bare and grafted particles in contrastmatch solvent was measured and is reproduced in Figure 1. Under these conditions of contrast, the scattering from the latex particles is negligible and the signal originates mainly from the grafted PEO polymer layer and the invisible ‘hole’ of the colloidal particles. Oscillations characteristic of spherical particles are visible, but are smeared due to instrument resolution and polydispersity. The scattering data from the grafted latex in contrast-match solvent were fitted to a double exponential profile using the PLAYTIME program and the calculated function is displayed in Figure 1. The parameters resulting from the fit are displayed in Table 1. The instrument resolution width (10-3 Å-1) and the particle log-normal distribution width (0.042) were known and fixed for all the samples (they are indicated between brackets in Table 1 and are not reproduced in the following tables of results). The particle size was fitted and then fixed to the value obtained in subsequent fits (491.45 Å). The grafting density (or ‘adsorbed amount’) was determined to be 1.10 mg/m2 (Table 1); this gave a distance between grafting sites of 18 Å, which was taken as the correlation length and fixed for all samples. Important parameters resulting from the fits to the double-exponential layer profile are also given in Table 1: the adsorbed amount Γ, the rms layer thickness δrms and the span of the layer. Addition of r-CD to the Grafted PEO Layer. R-CD was subsequently added to the grafted colloidal latex. The amount
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Table 1. Parameters Resulting from the Double-Exponential Volume Fraction Profile Fit to the Grafted Layer Scattering in the Absence of Cyclodextrinsa
a
parameter
value from fit
decay length z0/Å resolution width /Å-1 log normal width average particle radius/Å correlation length ξ/Å fluctuation intensity volume fraction at interface φs adsorbed amount Γ/mg m-2 rms layer thickness δrms/Å span/Å
37.8 (1.0 × 10-3) (4.2 × 10-2) (491.45) (18) 0.100 0.250 1.10 52.7 124
Constrained parameters are shown in parentheses.
Figure 3. Volume fraction profiles obtained from the fits to the SANS data for grafted PEO layers upon addition of R-CD: 0% R-CD (thick line), 1% R-CD (long dash), 5% R-CD (dot-dash), 10% R-CD (medium dash), 20% R-CD (dots), 33% R-CD (gray line), 50% R-CD (small dash) and 75% R-CD (thin line). Profiles close to the interface are shown in the inset.
Figure 2. Log-Log plot of the fitted experimental on-contrast scattering data for grafted PEO layers with added R-CD expressed in terms of complexation ratio with PEO: 1% R-CD (b), 5% R-CD (∆), 10% R-CD (×), 20% R-CD (2), 33% R-CD (0), 50% R-CD (9) and 75% R-CD (O).
of added CD is expressed in terms of ‘complexation’, where ‘full’ complexation is said to have occurred between PEO and R-CD when the whole length chain of PEO is complexed by R-CD, assuming a ratio of 2 EO units for 1 R-CD molecule.18 Obviously, it is unlikely that the total length of the grafted PEO chains will become threaded by cyclodextrins, being quite dense close to the surface, therefore a ‘100% complexation’ ratio does not correspond to an actual ‘full’ complexation but to the theoretical amount added to achieve 100% complexation (high complexation ratios would in fact lead to precipitation, which was not observed in any of the samples studied). The scattering after the addition of R-CD still arises mainly from the polymer layer (the scattering from R-CD in bulk solution is very weak, even at the highest concentrations studied) and the data were fitted with the same model used for the grafted polymer alone. The SANS data from the grafted polymer with R-CD complexation ratio between 0 and 75% is shown in Figure 2, together with the fits to the double-exponential profile model. The parameters used to obtain the best fits are shown in Table 2. The resulting volume fraction profiles are shown in Figure 3. Analysis of these profiles makes it possible to monitor changes in the polymer layer structure arising from the interaction with R-CD. Figure 3 shows that as increasing amounts of R-CD are added, the polymer layer extends further away from the surface, while (18) Harada, A. Coord. Chem. ReV. 1996, 148, 115–133.
Figure 4. Effect of adding R-CD on the rms thickness of the grafted PEO layer as a function of RCD/PEO complexation ratio.
its concentration at the interface decreases. This behavior agrees with previous studies19 performed on PEO layer adsorbed on silica particles and complexed with R-CD. Parameters derived from these profiles, displayed in Table 2, show that the maximum added amount of R-CD causes the layer to stretch to almost twice its original value: the rms thickness increases from 53 (no cyclodextrin added, c.f. Table 1) to 99 Å (maximum complexation ratio), while the span grows from 124 to 198 Å. Note that there is no additional layer extension from 50% to 75% R-CD complexation ratio (rms thickness and span are almost identical), suggesting that saturation has been reached, possibly due to steric effects. The rms thickness is plotted as a function of RCD/PEO complexation ratio in Figure 4. Addition of R-CD to the grafted polymer layer significantly alters the thickness of the layer (as already seen from the volume fraction profiles in Figure 3). The increase in layer thickness is more pronounced at low-tointermediate concentrations (10-30%). As the R-CD concentration increases further (above ca. 35%), the layer seems to reach (19) Dreiss, C. A.; Cosgrove, T.; Newby, F. N.; Sabadini, E. Langmuir 2004, 20, 9124–9129.
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Table 2. Parameters Resulting from the Double-Exponential Volume Fraction Profile Fit to the Scattering Data from the Grafted Layers in the Presence of r-CD % complexation
decay length z0/Å
fluctuation intensity
volume fraction at interface φs
adsorbed amount Γ/mg m-2
rms layer thickness/Å
span/Å
1% 5% 10% 20% 33% 50% 75%
34.51 36.65 38.56 61.70 68.51 76.90 75.96
0.088 0.103 0.095 0.102 0.087 0.103 0.103
0.261 0.256 0.247 0.198 0.226 0.211 0.216
1.05 1.09 1.10 1.40 1.77 1.84 1.86
48.1 51.1 53.7 83.5 91.1 99.6 98.7
116 122 126 172 184 198 196
Table 3. Parameters Resulting from the Double-Exponential Volume Fraction Profile Fit to the Scattering Data from the Grafted Layers in the Presence of mβ-CD % complexation
decay length z0/Å
fluctuation intensity
volume fraction at interface φs
adsorbed amount Γ/mg m-2
rms layer thickness/Å
span/Å
1% 5% 10% 20% 33% 50%
35.72 38.69 46.27 51.61 56.37 65.04
0.104 0.101 0.118 0.123 0.143 0.164
0.258 0.245 0.213 0.202 0.208 0.210
1.07 1.10 1.14 1.20 1.35 1.56
49.8 53.9 64.2 71.2 77.1 87.3
120 126 142 154 162 178
a plateau value, suggesting that complexation may have reached saturation, because of steric effects. It is well-known that R-CD can thread onto PEO and form a complex referred to as a ‘pseudopolyrotaxane’. The formation of such a complex is likely to be responsible for the changes observed in the polymer layer structure. As an increasing amount of R-CD threads onto the grafted PEO, the chains become more rigid and extend into the solution. The maximum extended length of the PEO chain can be estimated. Considering the relation20
R20 ) nl2C∞
(4)
where R20 is the mean square length of the polymer, n the number of bonds, l the bond length and C∞ the characteristic ratio (C∞ ) 4.1 for PEO), we can extract l, the length of a PEO unit by knowing that Rg is related to Ro by Ro ) Rg 61/2 and given (in nm) by21
〈R2g 〉 ) 1.87 × 10-2Mw0.598
(5)
This yields a total length of the PEO unit of 0.317 nm, and for 2080 g/mol PEO (47 units) a maximum extended chain length of 15.0 nm. Therefore, the addition of R-CD brings about quite a significant extension of the layer (Table 2). The value obtained
Figure 5. Adsorbed amount derived from the volume fraction profiles of the PEO grafted layer with added R-CD.
(higher than the estimated extended length) can be explained by polydispersity effects. The evolution of the adsorbed amount as a function of added R-CD (Figure 5) shows similar trends to the rms layer thickness: it increases sharply at low-to-intermediate concentrations of R-CD (10-30%) and then stabilizes with further addition of R-CD (above 33%). The adsorbed amount of polymer (expressed in mg/m2) is based on the molecular mass of the polymer. As CD molecules thread onto the polymer layer, the effective molecular mass of the layer increases (due to the presence of the CD) and hence the adsorbed amount rises. From the value of the adsorbed amount, it is possible to estimate how many CD molecules are threaded per polymer chain. At the highest concentration of R-CD measured (complexation ratio of 75%), we can calculate the adsorbed amount arising solely from R-CD by subtracting the value obtained for the grafted layer alone (1.10 mg/m). From this value, the number of R-CD molecules per unit surface area can be calculated (with MwR-CD ) 972.84 g/mol). Dividing by the number of PEO chain per unit area, we obtain a ratio of R-CD/PEO of ca. 1.6, which is the average number of R-CD molecules per polymer chain. This crude estimation gives quite a low number, and could be partially explained by the fact that the adsorption is underestimated (as the neutrons are not sensitive to the density of the long tails ends where some of the CDs could be located). Addition of Methyl β-CD (mβ-CD) to the Grafted Polymer Layer. Early studies by Harada et al.17 inferred that. unlike R-CD, β-CD could not form complexes with PEO. Ripmeester et al.22 however have reported a single-crystal structure analysis of solid polyrotaxanes of β-CD and PEO, in conditions of high concentration. It is therefore possible to envisage the occurrence of complexation between PEO and β-CD, although the interaction is likely to be ‘looser’ than in the case of R-CD. mβ-CD was added to the grafted colloidal suspensions up to a complexation ratio of 50% and the SANS data measured (the data are not shown as they are qualitatively very similar to the PEO/R-CD profiles shown in Figure 2). The data were fitted with the double exponential profile. The values of the parameters needed to obtain good fits are shown in Table 3 together with (20) Mark, J. E.; Flory, P. J. J. Am. Chem. Soc. 1965, 87, 1415–1423. (21) Sung, J. H.; Lee, D. C.; Park, H. J. Polymer 2007, 48, 4205–4212. (22) Udachin, K. A.; Wilson, L. D.; Ripmeester, J. A. J. Am. Chem. Soc. 2000, 122, 12375–12376.
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Table 4. Parameters Resulting from the Double-Exponential Volume Fraction Profile Fit to the Scattering Data from the Grafted Layers in the Presence of γ-CD % complexation
decay length z0/Å
fluctuation intensity
volume fraction at interface φs
adsorbed amount Γ/mg m-2
rms layer thickness/Å
span/Å
1% 5% 10% 20% 33% 50%
37.2 37.7 37.8 43.2 48.9 48.6
0.098 0.102 0.104 0.107 0.119 0.116
0.259 0.248 0.257 0.238 0.222 0.222
1.12 1.09 1.13 1.19 1.25 1.24
51.8 52.5 52.7 60.1 67.7 67.2
122 124 124 136 148 146
the parameters derived from the grafted layer profile (adsorbed amount Γ, rms layer thickness δrms and span of the layer). As in the case of R-CD, the volume fraction profiles show that as the concentration of mβ-CD increases, the grafted layers become increasingly expanded while the polymer concentration close to the surface decreases. The rms thickness increases by 66% (against 89% with R-CD). Stretching of the layer however is not as extensive as in the case of complexation with R-CD. This suggests that interaction with mβ-CD is weaker. As with R-CD, we can estimate the number of cyclodextrins per PEO chain. Considering an average molecular weight of 1331 g/mol for mβ-CD and a maximum adsorbed amount Γ ) 1.56 mg/m, this gives a mβCD/PEO ratio of 0.7, i.e., less than half the number of R-CD per PEO chain. Again, this confirms that the complexation of the PEO layer with R-CD is much stronger than with mβ-CD. Addition of γ-CD to the Grafted Polymer Layer. Finally, the effect of adding γ-CD to the grafted polymer layer was investigated. The formation of pseudopolyrotaxanes between γ-CD and PEO was first reported by Harada23 who observed double-stranded inclusion complexes (two PEO chains are threaded through the CDs). More recently, Goh et al.24 reported the crystalline inclusion complex formed by low molecular weight multiarm PEGs (4 and 6 arms) with γ-CD and Sabadini and Cosgrove25 studied the kinetics of complexation between γ-CD and high molecular weight star-polymer (13 and 15 arms). The scattering from the polymer layer in the presence of γ-CD was fitted to a double exponential profile. The parameters of the volume fraction profiles are shown in Table 4. Addition of γ-CD, as observed with both R- and β-CD, also results in an extended profile of the layer, with less affinity for the surface of the particle. The extent of the changes is much weaker than those observed with β-CD, with a modest increase in rms layer thickness of 27% (respectively 66% and 89% with β- and R-CD). The increase in adsorbed amount yields a number of γ-CD molecules of 0.2 per grafted chain (with Mwγ-CD ) 1297 g/mol). If one molecule of γ-CD can effectively thread onto two PEO chains, this would raise this value to 0.4 γ-CD/ chain, which is still lower than with added β-CD (0.7), and significantly lower than with R-CD (1.6). The probability that two chains become threaded by γ-CD is dependent on their spatial separation (or grafting distance) and on the dynamics of the chains. If the chains are highly mobile, the process of threading onto two chains is indeed quite difficult, which could partly explain the low degree of complexation obtained with γ-CD. Comparison of the Effect of r-, mβ-, and γ-CD on the PEO Grafted Layer. On addition of all analogues of CD to the grafted PEO layer, the changes observed in the polymer profiles display similar trends, namely, an extension of the layer into the bulk as complexation proceeds. These effects become less pronounced as the cavity size of the CD increases (from R-CD to γ-CD). For (23) Harada, A.; Li, J.; Kamachi, M. Nature 1994, 370, 126–128. (24) Jiao, H.; Goh, S. H.; Valiyaveettil, S. Macromolecules 2002, 35, 1980– 1983. (25) Sabadini, E.; Cosgrove, T. Langmuir 2003, 19, 9680–9683.
the purpose of comparison, the rms thickness derived from the volume fraction profiles for all the CD analogues have been reproduced in Figure 6. These data show similar trends for all CDs, namely, a steep increase in layer thickness at low-tointermediate CD concentrations, followed by a plateau at higher concentrations (although not as clear for β-CD). The larger increase in rms thickness is observed with the addition of R-CD. However, at low concentrations, the changes are comparable. The more marked effect of R-CD on the polymer layer profile is expected since the physical changes in solution properties on mixing PEO and R-CD are significant. Addition of mβ and γ-CD to the grafted layer also causes conformational changes, but the layer is stretched to a lesser extent. The internal cavity of R-CD
Figure 6. rms thickness of the PEO grafted layer on addition of R-CD (2), mβ-CD (b), and γ-CD (0) as a function of PEO/CD complexation ratio.
Figure 7. rms thickness of the grafted layer on addition of R-CD (2), mβ-CD (b), and γ-CD (0) plotted as a function of Γ1/3 (values derived from the fits).
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CDs, as pseudopolyrotaxanes are formed and the effective molecular weight rises. Addition of R-CD has a more visible effect on the conformation of the polymer chain, forming the highest yield of pseudopolyrotaxanes, and hence the adsorbed amount increases most notably with R-CD. mβ-CD and γ-CD also cause the adsorbed amount to increase, but less significantly.
Conclusions
Figure 8. Adsorbed amount derived from the volume fraction profiles of the grafted layers with addition of R-CD (2), mβ-CD (b), and γ-CD (0).
presents a better geometrical fit with PEO and therefore is likely to form a more rigid structure, as CDs line up along the polymer chain. The polymer chain instead is less restricted by mβ and γ-CD, hence the association is less rigid and the layer is not stretched to the same extent. It is expected that the rms thickness should be correlated to the adsorbed amount.1 For a brush, the thickness is related to the number of chains per unit area by δ ) Nσ1/3. Figure 7 shows the data of Figure 6 plotted this time as the rms thickness as a function of Γ1/3: as expected, the three plots for the different cyclodextrins roughly fall on the same master curve, which is linear. The effect on the adsorbed amount upon addition of the three CD analogues follows a similar trend, as summarized in Figure 8. The adsorbed amount (linked to the weight of polymer and cyclodextrin at the surface) increases upon addition of all three
The effect of adding cyclodextrins on the volume fraction profile of PEO layers grafted on colloidal particles was investigated. Using SANS measurements and a detailed modeling of the data using a double-exponential profile, we found that R-, mβ- and γ-CDs all affected the structure of the grafted layer, causing an extension of the PEO chains into the bulk. Parameters derived from the fits such as the rms layer thickness, the span of the layer and the adsorbed amount, all increased sharply at low-to-intermediate cyclodextrin concentrations before reaching a plateau at higher concentrations. The effects observed are attributed to the formation of a complex, or pseudopolyrotaxane, between the CD molecules and the PEO chains. The most significant changes in the layer profile were observed with R-CD, which gave rise to a 90% increase in rms layer thickness and 77% in adsorbed amount. A crude estimation from the values of the adsorbed amounts showed that the average number of CD molecules threaded per PEO chains were respectively 1.6, 0.7 and 0.2 for R-, mβ- and γ-CD. The more marked changes observed with R-CD can be attributed to the stronger complexation of PEO with R-CD, which has been widely documented in the literature. Acknowledgment. J.J. acknowledges Malcolm McKechnie and Gay Cornelieus at Reckitt Benkiser for the provision of a CASE award. The authors thank ISIS for the provision of beam time and Steve King for help with the experiments. LA801088Q
