5120
J. Phys. Chem. B 1998, 102, 5120-5126
Neutron Reflectivity of an Adsorbed Water-Soluble Block Copolymer at the Air/Water Interface: The Effects of pH and Ionic Strength S. W. An and R. K. Thomas* Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford, OX1 3QZ, U.K.
F. L. Baines, N. C. Billingham, and S. P. Armes School of Chemistry, Physics and EnVironmental Sciences, UniVersity of Sussex, Falmer, Brighton, BN1 9QJ, U.K.
J. Penfold ISIS, CCLRC, Chilton, Didcot, Oxon., OX11 0QX, U.K. ReceiVed: January 27, 1998; In Final Form: April 16, 1998
We describe the effects of pH and added electrolyte on the structure of layers of poly(2-(dimethylamino)ethyl methacrylate-block-methyl methacrylate) copolymer (poly(DMAEMA-b-MMA)) (70 mol % DMAEMA, Mn ) 10 000) adsorbed at the air/water interface. Previously, we had shown by means of neutron reflection measurements that at a pH of 7.5 there is a surface phase transition from a layer about 20 Å thick to one about 40 Å thick and that the adsorbed layer at the higher concentration has a cross sectional structure resembling that expected for a micelle. The present work shows that the “micellar” structure is promoted by any changes in the solution that enhance the surface coverage but is inhibited by an increase in the fractional charge on the polyelectrolyte part of the copolymer. Some surprising lack of correlation between surface tension and surface coverages is also observed, which may be related to this unusual surface structure.
Introduction In an earlier paper we described neutron reflectivity measurements on layers of the water soluble diblock copolymer poly(2-(dimethylamino)ethyl methacrylate-block-methyl methacrylate) copolymer (poly(DMAEMA-b-MMA)) (70 mol % DMAEMA, Mn ) 10 000) adsorbed at the air/water interface.1 At low concentrations the adsorbed layer was found to consist of an approximately uniform distribution of the two blocks of the copolymer along the direction normal to the interface with about half of the layer immersed in the underlying water. At high concentration the adsorbed layer was found to have a cross sectional structure resembling that expected for an adsorbed micelle, with the majority of the more hydrophobic MMA forming the core. This observation adds another type of adsorbed structure to the several identified experimentally, principally by Eisenberg and co-workers,2-5 although their work has focused on spread monolayers, i.e. species that are intrinsically insoluble. There appears to be a sharp phase change between the low and high concentration surface structures, which occurs close to the critical micelle concentration (cmc) in the bulk solution. Although it has been suggested that the surface and bulk phase changes need not necessarily coincide,6,7 the grid of points investigated was not fine enough to judge whether the surface and bulk phase transitions coincide. The DMAEMA block of this copolymer is a weak polyelectrolyte. Thus, by manipulation of the charge density on this block by variation of pH, or of the screening of the charge by variation of ionic strength, the solubility and surface activity of the copolymer can be controlled, which offers the possibility of observing the
effects of these variations on the surface structure, none of which has previously been accessible for a soluble polymer of this type. One of the more surprising features of both structures for the layer of poly(DMAEMA-b-MMA) is that some of the charged groups are apparently embedded in a region layer lying outside the water. This was also found for an insoluble diblock copolymer layer of poly(styrene-b-alkylvinylpyridinium iodide) by Li et al.4 However, it seems probable that as the polyelectrolyte block of a soluble material acquires a higher charge density, it should be more stable for it and its counterions to be fully hydrated and hence to remain in the water region of the layer. In this respect it should resemble more closely the typical models used in theoretical treatments where there is an insoluble “anchor” and a soluble “buoy”, the buoy here being the polyelectrolyte chain.8,9 The effect of added electrolyte should be to screen the charge on the polyelectrolyte block and hence effectively vary the interaction between solvent and buoy segments. This paper describes the effects of pH and ionic strength on the neutron reflectivity and surface tension of poly(DMAEMA-b-MMA) adsorbed at the air/solution interface. Experimental Details The preparation of poly(DMAEMA-b-MMA) and its partially deuteriated version (dMMA) has already been described.10 The DMAEMA monomer was polymerized first in these syntheses in order to ensure that any homopolymer contamination (typically less than 5%) was hydrophilic and therefore unlikely to affect the surface activity. The two isotopic species of the block copolymer have narrow unimodal distributions with Mn of 10 000 and 10 600 and Mw/Mn of 1.11 and 1.09, respectively,
S1089-5647(98)00903-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/05/1998
Neutron Reflectivity
J. Phys. Chem. B, Vol. 102, No. 26, 1998 5121
Figure 1. Surface tension of diblock copolymer as a function of polymer concentration at pH ) (4) 8.5, (b) 7.5, and (O) 6.5.
for the poly(DMAEMA-b-dMMA) and poly(DMAEMA-bhMMA), where d and h denote deuteriated or protonated PMMA blocks, respectively. All the glassware and PTFE troughs were cleaned by soaking them in alkaline detergent overnight and then rinsing several times with ultrapure water (Elgastat UHQ, Elga, U.K.). The solutions were made by dissolving first in methanol and then diluting with water to reach a final methanol concentration of 4 vol %. The pH of the solutions was adjusted using hydrochloric acid and maintained constant during the measurements by keeping the samples under a nitrogen or argon atmosphere. For the neutron measurements D2O and methanold4 were used as received from Aldrich. The surface tension of the aqueous copolymer solutions was determined on a Kru¨ss K10T digital-tensiometer using the du Nou¨y ring method with a Pt/Ir ring. The surfaces of solutions of this kind often show time effects, and all measurements were therefore made after the solution had stood for 10-30 min, the longer time being used only if necessary. Before each measurement, the ring was rinsed with pure water and flamed to remove contaminants. The temperature was maintained at 298 ( 0.2 K. The neutron reflection measurements were carried out on the reflectometers CRISP and SURF at Rutherford Appleton Laboratory (Didcot, U.K.). The instruments and the procedure for making the measurements have been fully described elsewhere.11,12 Results and Discussion (a) Surface Tension. Figure 1 shows the variation of surface tension with copolymer concentration of the diblock copolymer (poly(DMAEMA-b-hMMA)) in aqueous solution at 298 K and at three values of the pH, 8.5, 7.5, and 6.5. The pKa of the polymer was found by direct titration to be 7.3 ( 0.1 (here defined as the pH when the polyelectrolyte is half-ionized in the absence of electrolyte). We have shown in a previous publication that the surface pKa is slightly lower than this at about 6.7 and that the degree of ionization of the DMAEMA fragment at the surface is 0.12, 0.23, and 0.61 at the three pH values of 8.5, 7.5, and 6.5, respectively.13 A number of factors affect the surface behavior. At the highest pH of 8.5 the copolymer is only just soluble and both the MMA and most of the DMAEMA fragments are hydrophobic and will therefore be highly surface active. Adsorption at the surface then might be more characteristic of a very surface active homopolymer. At the lowest pH of 6.5 the copolymer will be more soluble and the hydrophobic (MMA) and hydrophilic (charged DMAE-
Figure 2. Surface tension of diblock copolymer with (b) and without (O) 0.1 M NaCl at pH (a) 7.5 and (b) 6.5.
MA) would be expected to be more clearly differentiated. This would be expected to make the copolymer less surface active but might facilitate the formation of micelles in the bulk solution because of the better segregation of hydrophilic and hydrophobic fragments. These expected features are all seen in Figure 1. Thus, the surface tension generally increases as the pH decreases, the critical micelle concentration (cmc) for the pH 6.5 solution is clearly seen and becomes less clear at pH 7.5, and there is no clear indication of a cmc at 8.5. Another important feature of the γ-ln c plot concerns its slope just below the cmc. We have shown elsewhere that an increasing degree of ionization of the polymer should lead directly to an increase in the magnitude of the slope. This is effectively because the number of adsorbed species (polyion and counterions) increases with degree of ionization.13 This feature is also clear in Figure 1. Parallel measurements on the partially deuteriated species, poly(DMAEMA-b-dMMA), gave surface tension curves very similar to those shown in Figure 1, establishing that isotope effects in this system are small, although this is not always the case with polymeric species. This is important for the analysis of the neutron reflectivity results. The addition of electrolyte may affect the DMAEMA block in two opposite ways. It should increase its degree of ionization because it lowers the activity of the counterion, and it should screen the electrostatic interactions between charged DMAEMA residues, conferring greater flexibility on the DMAEMA block. It is therefore not easy to predict the effect of added electrolyte on the surface behavior, and the results, shown for pH 7.5 and 6.5 in Figure 2, are indeed confusing. At pH 7.5 addition of electrolyte enhances the surface activity considerably at low polymer concentrations, but diminishes it slightly when the polymer concentration is above about 0.1 wt % (this is also seen at pH 8.5, for which the results have not been included in
5122 J. Phys. Chem. B, Vol. 102, No. 26, 1998
An et al. TABLE 1: Volumes and Scattering Lengths for Copolymer Fragments unit
volume/ Å3
scattering length × 105 Å
scattering length density × 106/Å-2
MMA-h9 MMA-d9 DMAEMA H2O D2O
140 140 225 29.9 30.2
14.92 98.24 17.98 -1.68 19.14
1.0 7.02 0.80 -0.56 6.34
TABLE 2: Coverages (mg m-2) and Thicknesses (Å) (in Parentheses) Obtained from the Best Fits of a Single Uniform Layer to Neutron Reflectivities from Poly(DMAEMA-b-dMMA) in Null Reflecting Water pH, cNaCl
Figure 3. Neutron reflectivity profiles of poly(DMAEMA-b-dMMA) in NRW at different concentrations at pH 8.5. The continuous lines are the best fits using a model whose parameters are given in Table 4. The bulk concentrations are (O) 0.2, (b) 0.1, (×) 0.04 and (O, lower set of data) 0.02 wt %.
Figure 2). The break in the slope of the γ-ln c plot, which probably corresponds to the cmc, is shifted to about one-tenth of its value in the absence of salt. This is similar to the effect of electrolyte on the cmc’s of small molecule surfactants. At a pH of 6.5, the addition of electrolyte decreases the surface activity (i.e., increases the surface tension) at concentrations above about 0.05 wt %, which is similar to the effect at pH 7.5. We defer discussion of these effects until after the neutron reflection results have been presented. (b) Specular Reflection. The most direct way of examining both composition and structure of the monolayer at the air/water interface is to measure the reflectivity of the partially deuteriated copolymer in null reflecting water (NRW) because the reflected signal is then entirely from the adsorbed polymer. Figure 3 shows a set of reflectivity profiles at different concentrations of poly(DMAEMA-b-dMMA) in NRW (the 4% methanol in the solution was also adjusted to give zero reflectivity) at a pH of 8.5. There is a steady increase in the reflectivity with increasing concentration at low values of the momentum transfer κ, which corresponds to an increase in the amount adsorbed (the magnitude of the reflectivity is approximately proportional to the square of the surface excess at low values of κ ). Although the surface tension at pH 8.5 is always significantly lower than for the corresponding copolymer concentration at pH 7.5, we found almost no difference in the reflectivities at comparable concentrations; that is, there is negligible difference in the amount adsorbed. Also, just as was found at pH 7.5, there is a marked increase in the thickness of the layer above a concentration of about 0.1 wt %; this can be seen from the change in slope of the reflectivity, a greater thickness causing a steeper decay of the reflectivity with momentum transfer. The most direct method of determining the coverage and mean thickness of the adsorbed layer is to assume that, for the partially deuterated polymer in NRW, the distribution of polymer along the normal direction z can be approximately represented by a Gaussian, giving the distribution of the scattering length density as
( )
F ) F0 exp -
4z2 σ2
(1)
where σ is the full width at 1/e of the maximum. For this
2.5
6.5
6.5, 0.1
conc./ wt % 0.02 0.6(11) 0.04 0.5(14) 0.05 0.06 0.07 0.8(16) 0.1 0.5(10) 1.2(18) 0.2 1.3(20) 0.5 0.8(17) 1.0 1.0(25)
7.5
7.5, 0.1
8.5
1.8(11) 0.8(20) 2.5(25a) 0.8(20) 1.5(15) 1.7(20) 3.1(35a) 1.6(20) 1.9(21) 2.3(26) 1.6(16) 2.0(20) 3.1(35a) 1.5(15) 2.4(25) 3.1(35a) 2.5(28) 1.7(20) 3.0(37) 3.1(35a) 2.7(35a)
8.5, 0.1
2.8(30a) 3.2(45a) 3.2(45a) 4.0(50a) 4.5(54a)
a
Approximate values because the single uniform layer does not fit the data well.
distribution the reflectivity R is given approximately by
κ2R =
16π2bp2Na2Γp2 1040
( )
exp
-κ2σ2 8
(2)
where bp is the known scattering length in angstroms of the polymer (the scattering lengths of the different fragments are given in Table 1), κ is the momentum transfer ()(4π sin θ)/λ) (Å-1), and the surface excess Γp is in mol mol-2. Hence
(
ln(κ2R) ) 2 ln
)
4πbpNaΓp 20
10
-
κ2σ2 8
(3)
Thus the extrapolation of the linear plot of ln(κ2R) against gives the surface excess, and although the polymer is not necessarily well represented by a Gaussian, the slope of the plot gives σ2/8 and hence the relative thickness of the polymer layer. We have shown elsewhere that the value of the surface excess is accurately determined by this procedure even when the polymer distribution is not well represented by a Gaussian.14 The surface excesses obtained by this analysis are given in Table 2 for pH values of 2.5, 6.5, 7.5, and 8.5 and plotted for the last three values of the pH in Figure 4a. The accuracies of the thicknesses given in Table 2 are generally about 10-15% but become less good when either the coverage is low (
