Langmuir 2007, 23, 4389-4399
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Preparation and Characterization of Pure and Mixed Monolayers of Poly(ethylene Glycol) Brushes Chemically Adsorbed to Silica Surfaces Cathy E. McNamee,*,† Shinpei Yamamoto,‡ and Ko Higashitani† Department of Chemical Engineering, Kyoto UniVersitysKatsura, Nishikyo-ku, Kyoto, Kyoto 615-8510, Japan, and Institute for Chemical Research, Kyoto UniVersity, Gokasho, Uji, Kyoto 611-0011, Japan ReceiVed December 5, 2006. In Final Form: January 30, 2007 We prepared pure and mixed monolayers of methoxy-terminated poly(ethylene glycol)s (m-PEG’s) chemically attached to silica surfaces by using m-PEG silane coupling agents of three different molecular weights. These films were subsequently characterized in water by atomic force microscopy (AFM). Images of pure m-PEG monolayers showed the formation of polymer brushes on silica. Force curves between two modified surfaces suggested that an increase in the number of oxyethylene (OE) groups from 6 (PEG6 surface) to 43 (PEG43 surface) to 113 (PEG113 surface) decreased the flexibility of the m-PEG chains in the m-PEG brushes. Frictional force measurements also showed that the friction increased in the order PEG6 < PEG43 PEG43 > PEG113. Therefore, we think that the flexibility of the m-PEG brush decreases in the order of PEG6 > PEG43 > PEG113. This decreased flexibility for PEG113 compared to that of PEG6 may have been due to (1) PEG113 having a high density of m-PEG chains adsorbed to the silica surface than PEG43 or PEG6 and/or (2) the presence of entanglements between PEG113 chains or the hydrogen bonding of neighboring PEG113 chains through water clathrates.7 The density of adsorbed polymers could be estimated by fitting the Alexander-de Gennes theory for polymer brushes to the forces measured between a silica particle modified with m-PEG’s and a silica substrate modified with m-PEG’s in 100 mM NaCl. The Alexander-de Gennes theory for polymer brushes can be defined as47
F 16kTπL 2L 5/4 D 7/4 ) 7 + - 12 Nm-1 3 R D 2L 35s
[( ) ( )
]
(4)
where k, T, L, s, and D are the Boltzmann constant, the absolute temperature, the uncompressed brush layer thickness, the average distance between grafting points of the polymer on the surface, and the separation distance of the two surfaces. This expression is valid for D[dlt]R and D < 2L. The number of PEG6, PEG43, and PEG113 chains adsorbed per square nanometer on the silica substrate (σ) could subsequently be estimated using
σ)
1 chains nm-2 2 s
(5)
Figure 7A-C shows the best Alexander-de Gennes fits (solid line) to the reduced force (F/R)-distance curves (open circles) measured between two silica surfaces, which were modified with PEG6, PEG43, and PEG113, respectively. An increased deviation between the measured values and the fitted Alexander-de Gennes theory at smaller separations was noted as the m-PEG molecular weight increased. This difference in values may be due to the onset or presence of entanglements or interchain physical binding within the m-PEG brushes because this theory is valid for polymer brushes without interchain bonding. The determined values for L and s are included in Table 1. The values of L increased in the
order of the PEG chain length, as expected. The values of L obtained here were also within ∼5 nm of LS, which we estimated above for the thickness of an m-PEG film from the range of steric force. Although some deviation is seen in the values of L and LS, the relative increases in L and LS with the m-PEG chain length are the same. The differences in the L and LS values for PEG6, PEG43, and PEG113 are 0.5, 3.1, and 5.2 nm, respectively. The increased difference in the LS and L values with the m-PEG chain length may be a consequence of the Alexander-de Gennes theory fitting less to the experimental force data as the m-PEG chain length increased. The σ values for PEG6, PEG43, and PEG113 were estimated from L and s to be 4.0 × 10-2, 3.6 × 10-2, and 3.3 × 10-2 chains nm-2, respectively; these values are also included in Table 1. The decrease in σ with m-PEG chain length suggested that a longer m-PEG chain occupied more surface area on the silica substrate and that the m-PEG chain did not possess a vertical, straight conformation. The fact that PEG113 had the lowest σ suggested that the PEG113 chain must have been the least stretched chain. However, because we are comparing m-PEG’s of different molecular weights, it is difficult to judge the stretching of a chain using only σ. We may better judge the differences in the stretching of the m-PEG chain by calculating the stretching ratio (SR) of the m-PEG brushes. We may judge the differences in the stretching of the m-PEG chain by calculating the stretching ratio (SR) of the m-PEG brushes. We can determine the SR of the PEG6, PEG43, and PEG113 brushes using L or LS by
SR )
L LC
or
SR )
LS LC
(6)
where LC is the thickness of a fully stretched m-PEG chain, given by
LC ) ln nm
(7)
where l is the length of a monomer (OE) unit, reported as 0.365 nm,48 and n is the number of monomers (OE). Here we calculated SR using both the LS and L values, as L was determined using the Alexander-de Gennes theory, where the accuracy in determining L may be lower for the high-molecular-weight m-PEG’s that showed significant deviations in the fitted and actual force data. The LC values for PEG6, PEG43, and PEG113 can be calculated to be 2.2-3.3, 15.7, and 41.2 nm, respectively. This gave SR values of 1.4-0.94, 0.31, and 0.29 for PEG6, PEG43, and PEG113 when L was used and SR values of 1.8-1.2, 0.51, and 0.39 for PEG6, PEG43, and PEG113 when LS was used. Both of these data sets show that the PEG113 brush was the least stretched. Because high-molecular-weight chemisorbed polymers have been reported to be less stretched than short-chained polymers, this result may be due to the long chain length of PEG113. Alternatively, the low stretching of the PEG113 may be due to the presence of entanglements in the PEG113 chains or interchain hydrogen bonding. The entanglement behavior for PEG melts has been reported6 to occur for molecular weights of between 3200 and 4400 g mol-1, suggesting the presence of entanglements in the PEG113 sample. Physical binding between m-PEG chains has also been detected and explained7 by the water intercalates that form hydrogen bonds between neighboring m-PEG chains. This indicates the presence of interchain binding in long m-PEG chains such as PEG43 and PEG113, where the strength would increase with the OE chain length. Frictional force measurements between a silica particle and PEG6-, PEG43-, and PEG113-modified silica substrates were performed to determine whether the rheological properties of
PEG Brushes Chemically Adsorbed to Silica Surfaces
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Figure 8. (A and B) Models of the pure and the mixed m-PEG monolayers, respectively. Table 2. Friction Measurements of the m-PEG-Modified Silica Substrates in Distilled Water at pH 5.6 When a Load of 2 V (840 N) Was Applied and a 15-µm-Diameter Silica Probe Particle Was Used surface
frictional force (nN)
PEG6 PEG43 PEG113 35% PEG113 50% PEG113 75% PEG113
148 ( 74 307 ( 30 354 ( 18 143 ( 49 231 ( 12 349 ( 25
the surfaces were changing with an increase in the m-PEG chain length. The frictional force values calculated when a load of 2 V was applied to the surfaces (i.e., the same load applied in the normal force curve measurements) can be found in Table 2. Here we can note that the friction increased as the m-PEG chain length increased. This feature may have been due to the onset of entanglements in the adsorbed m-PEG chains and/or the interchain hydrogen bonding for the long-chained m-PEG’s. 3.2. Preparation and Characterization of Mixed PEG Monolayers. We can determine if interchain binding or chain entanglements are responsible for the change in the physical properties of the m-PEG brushes by making a mixed monolayer of m-PEG’s. Because the molecular weight of PEG6 is too low to allow entanglements and the number of OE groups is small enough to ensure only little interchain hydrogen binding ability, the introduction of PEG6 chains between PEG113 chains should decrease the number of entanglements and/or amount of interchain hydrogen bonding in the modified silica surface. We can therefore imagine that the number of entanglements or amount of interchain binding should decrease as the percentage of PEG113 decreases. Figure 8A,B shows a schematic representation of a pure monolayer of PEG113 and a mixed monolayer of PEG6 and PEG113, respectively. The mixed monolayers of PEG6 and PEG113 were initially prepared in an ethanol solvent, which is the same solvent used to prepare the pure monolayers. However, imaging of the surface (images not shown here) and analysis of the film thickness indicated that mostly only PEG113 adsorbed to the silica surface, even when the content of PEG113 was only 5%. This may be explained by the greater number of OE groups in the PEG113 chain making PEG113 more hydrophilic than PEG6 and the silica surface being more hydrophilic than the ethanol solution as a result of the small layer of water situated in the gel SiOH layer at the silica surface.49 Because the PEG6 and PEG113 are competing for the free SiOH sites to which they can chemically bind, the m-PEG that is less energetically stable in the solvent (i.e., PEG113) would be preferentially adsorbed. If, however,
Figure 9. Height and phase images (4 × 4 µm2) of mixed m-PEG monolayers, imaged by tapping mode AFM with a k ) 0.06 cantilever in water. (A-C) Height images for the silica wafers modified with 35% PEG113-65% PEG6 (35% PEG113), 50% PEG113-50% PEG6 (50% PEG113), and 75% PEG113-25% PEG6 (75% PEG113) mixed monolayers, respectively, when the z scale bar was 10 nm. (D-F) Phase images for the silica wafers modified with 35% PEG113, 50% PEG113, and 75% PEG113 mixed monolayers, respectively, when the z scale bar was 10°. The height profiles of H1-H3 shown between the height and phase images correspond to the solid lines drawn in A-C, respectively, and range from -5 to 5 nm. The dotted circles in the images show the size of the PEG113 clusters in each case. The FFTs of the height images of 35% PEG113, 50% PEG113, and 75% PEG113 are shown in the upper right-hand corners of A-C, respectively.
we choose a reaction solvent that is energetically stable for both PEG6 and PEG113, we can expect to obtain a mixed monolayer of PEG6 and PEG113. Here, we chose to use methanol because it is more hydrophilic than ethanol but less hydrophilic than water, thereby seeming to be suitable for the PEG6 and PEG113 polymers. Figure 9 shows the AFM tapping images of mixed PEG6 and PEG113 monolayers of different PEG113 fractions when the reaction solvent was methanol. The height images of 65 wt % (bulk concentration) PEG6 and 35 wt % (bulk concentration) PEG113 (35% PEG113), 50 wt % (bulk concentration) PEG6 and 50 wt % (bulk concentration) PEG113 (50% PEG113), and 25 wt % (bulk concentration) PEG6 and 75 wt % (bulk concentration) PEG113 (75% PEG113) are shown in Figure 9AC. The phase images of 35% PEG113, 50% PEG113, and 75% PEG113 are shown in Figure 9D-F, respectively. A comparison of the height and the phase images of each m-PEG-modified surface also showed that a similar image quality was obtained for the 35% PEG113, 50% PEG113, and 75% PEG113 surfaces. Here, clusters were observed on the surface of the mixed monolayers. Dotted circles on the images indicate examples of some of these clusters that are observable on the images. A height profile analysis of these clusters (shown between the height and phase images and labeled as Figure 9H1-H3 for 35% PEG113, 50% PEG113, and 75% PEG113, respectively), determined for the clusters along the solid line shown in the images, indicated that these clusters were approximately 5 nm taller than the other regions of the mixed m-PEG monolayer. Additionally, the size (width) of these clusters increased as the percentage of PEG113 increased. These data suggest that the clusters were composed of PEG113 chains and the other areas of the mixed m-PEG monolayer were composed of PEG6 chains. An increase in the PEG113 percentage must have increased the number of PEG113 chains adsorbing in each cluster. The FFTs
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Figure 10. Forces resulting between two surfaces modified with mixed m-PEG monolayers, when measured in 100 mM NaCl. (AC) 35% PEG113, 50% PEG113, and 75% PEG113 cases, respectively. The arrows labeled CC indicate the constant compliance regions of these forces. The solid arrows show the forces due to the compression of the m-PEG brushes, where PEG113 and PEG6 indicate the PEG113 and the PEG6 chains, respectively.
of the height images are shown in the top right corner of Figure 9A-C, respectively. The 35% PEG113 and 75% PEG113 surfaces displayed a similar pattern that suggested that the m-PEG chains did not adsorb to the silica surface to give a periodic pattern. The FFTs for the 50% PEG113 surface, however, showed an increase in definite structure compared to those for the 35% PEG113 and 75% PEG113 cases. This result suggests that the pattern being obtained is being controlled, perhaps entropically. A study by Preechatiwong and Schultz50 has also suggested that the structures formed by mixed monolayers of PEGs in the presence of salts may be explained in terms of the molecular disorder in the PEGsalt phase, the configurational entropy, and/or the degree of dissociation of the salt. Chou and Chu15 have also shown that the entropy of a surfactant and PEG mixed monolayer increased with the temperature or molar percentage of PEG. The (F/R)-D profiles obtained between two surfaces modified by identical mixed monolayers of PEG6 and PEG113 in the presence of 100 mM NaCl when the fraction of PEG113 was varied are shown in Figure 10. The force profiles for 35% PEG113, 50% PEG113, and 75% PEG113 are shown in Figure 10A-C, respectively. For each case, we could see a repulsive force beginning between ∼20-24 nm for the 35% PEG113, 50% PEG113, and 75% PEG113 cases, indicated by the arrow labeled PEG113 in Figure 10. This distance was approximately the same as the distance of onset of the steric force obtained for the pure PEG113 case. This repulsion was therefore presumably a steric repulsion due to the compression of the m-PEG chains. Because the system was symmetric, we could conclude that the thickness of one film was between 10 and 12 nm. Because the thickness of a pure PEG113 monolayer was determined above from the Alexander-de Gennes equation (i.e., L) to be 10.8 nm and the thickness determined from the steric force measurements (i.e., LS) was 16 nm, we could conclude that this repulsion was due to the compression of the PEG113 portions of the mixed monolayers. The repulsion was seen to increase as the separation distance between the two surfaces decreased until a separation distance of approximately 15 nm was reached. The slope of the repulsion force then decreased in the order of 35% PEG113 > 50% PEG113 > 75% PEG113 in the region between 15 and 6 nm. The magnitude of the slope of FR(steric force)/D was suggested above as indicating the ability of a chain in a polymer brush to be compressed, where the chains in a polymer brush with a large FR/D slope would be more compressible than those in a polymer brush with a low FR/D slope. Therefore, the fact that the slope in this region varied in the order of 35% PEG113 > 50% PEG113 > 75% PEG113 suggested that the PEG113 chains could be compressed the most in the 35% PEG113 mixed
McNamee et al.
monolayer and the least in the 75% PEG113 mixed monolayer. In the case of 75% PEG113, we could also observe that the repulsive force gave an approximately constant value with decreasing surface separations in this region. Because similar behavior was observed for the pure PEG113 monolayer, we could presume that the rigidity of the 75% PEG113 film was similar to that of the pure PEG113 monolayer. This suggested that the PEG113 islands were significantly rigid in the 75% PEG113 monolayer. An indent was subsequently seen in the force profiles for 35% PEG113, 50% PEG113, and 75% PEG113 at a separation distance of between ∼5 and 6 nm; see the arrow labeled PEG6. This distance corresponded to a film thickness of ∼3 nm. Because the thickness of a pure PEG6 monolayer was determined above from the Alexander-de Gennes equation (i.e., L) to be 3.5 nm and from the steric force measurements (i.e., LS) to be 4 nm, we think that this repulsion was presumably due to the commencement of the compression of the PEG6 chains. The existence of indents for both the PEG6 and PEG113 chains also showed that we indeed did have a mixed monolayer of PEG6 and PEG113. The constant compliance regions for these forces are indicated by the arrow labeled CC. Again, the data spacing was not constant overall in these force plots. This can again be explained by the facts that the spacing between the data points was controlled in our experiment by the time interval and that an attraction may also have been present in this system. The above data suggest that the flexibility of the PEG113 brush portions appeared to increase as the percentage of PEG113 in the mixed PEG6-PEG113 monolayer decreased. The size of the PEG113 islands was also seen from the images to decrease as the percentage of PEG113 decreased. We can therefore conclude that the flexibility of the PEG113 brush increased as the size of the PEG113 islands decreased. A decrease in the packing between PEG chains from a close to a more sparse packing has been shown8 to change the m-PEG rheology from an elastic-gel-forming material to a fluid viscouslike material. Decreasing the packing density of m-PEG chains in a 2D case (brush) may give the same effect as lowering the molecular weight of m-PEG chains in a 3D case (bulk).51 A decrease in the percentage of the high-molecular-weight m-PEG may have the effect of reducing or eliminating the entanglements/physical interchain bonds between the high-molecular-weight m-PEG chains in a mixed monolayer of a low- and a high-molecularweight m-PEG film. Frictional force measurements were performed between a silica particle and substrates modified with 35% PEG113, 50% PEG113, and 75% PEG113 to verify that the surface rheology was indeed changing as the PEG6-PEG113 fraction was varied. Again, the frictional forces were measured in water and at the same load that was applied in the normal forces. As can be seen in Table 2, the frictional force of the modified silica substrates decreased in the order of 75% PEG113 > 50% PEG113 > 35% PEG113. These facts suggest that the properties of the film may be changing from an elastic-gel-forming material to a fluid viscouslike material as the fraction of PEG6 is increased. Additionally, the value of 35% PEG113 was the same as that of PEG6 when the standard deviation was taken into account. This suggests that the 35% PEG113 film possessed the same elasticity as a PEG6 film (i.e., the 35% PEG113 film may have been a fluid viscouslike material). The above therefore shows that we may be able to control the flexibility of a long-chained m-PEG polymer by changing the fraction of PEG113 in a mixed monolayer of PEG6 and PEG113. This result may be important in the design of a drug delivery system carrier, as we showed in a separate study that a flexible
PEG Brushes Chemically Adsorbed to Silica Surfaces
PEG113 chain can bind more strongly to a living cell than can a rigid PEG113 chain.52
4. Conclusions We have shown that we can create pure and mixed monolayers of m-PEG brushes that are chemically adsorbed to silica by using m-PEG silane coupling agents and the appropriate reaction solvent and ammonia. The pure m-PEG monolayers were seen to adsorb on the silica surfaces to give polymer brushes. The effect of increasing the chain length of OE groups from 6, 43, to 113 was seen from the force curves to decrease the flexibility of the m-PEG chains in the m-PEG brushes. Because PEG113 had the smallest σ, a PEG113 chain was determined to occupy the largest surface area and therefore to be the least stretched. Frictional force measurements also showed that the friction increased in the order of PEG6 < PEG43 < PEG113. A pure PEG113 monolayer was therefore thought to contain entanglements or interchain hydrogen bonding. Images of the mixed PEG6 and PEG113 monolayers showed that the size (area) of the islands of PEG113 decreased as the fraction of PEG113 decreased. The force curves between two
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identically modified surfaces appeared to show that the flexibility of the mixed monolayers decreased and the film became more rigid as the fraction of PEG113 increased. Frictional force measurements also showed that the friction decreased as the fraction of PEG6 in the PEG6-PEG113 mixed film increased. The entanglements in the PEG113 islands were therefore thought to decrease as the fraction of PEG113 in the mixed monolayer decreased. The above therefore shows that the flexibility of a long-chained m-PEG polymer could be controlled by manipulating the fraction of PEG113 in a mixed monolayer of PEG6 and PEG113. Acknowledgment. C.M. thanks the Japanese Government for the financial support provided through the JSPS Postdoctorial Fellowship For Foreign Researchers. Supporting Information Available: SEM images of the colloid probes used. Force between the bare cantilever tip and m-PEG-modified silica surfaces in various aqueous solutions. This material is available free of charge via the Internet at http://pubs.acs.org. LA063512L
