Langmuir 2000, 16, 4467-4469
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Hydrodynamic Layer Thickness of a Polybase Brush in the Presence of Salt Robin D. Wesley and Terence Cosgrove* School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, United Kingdom
Laurie Thompson Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, The Wirral L63 3JW, United Kingdom
Steven P. Armes, Norman C. Billingham, and Fiona L. Baines School of Chemistry, Physics and Environmental Science, University of Sussex, Falmer, Brighton, East Sussex BN1 9QJ, United Kingdom Received September 23, 1999. In Final Form: February 11, 2000 A grafted weak polyelectrolyte brush has been prepared by the incorporation of one block of a diblock copolymer inside a latex particle. The preformed brush shows a maximum in its hydrodynamic layer thickness radius as a function of added salt. Beyond 0.025 M NaCl, the data can be fitted to the scaling relationship for the salted-brush regime wherein the layer thickness varies as the inverse cubed root of the ionic strength.
Introduction Polymer brushes have been the focus of a large number of experimental and theoretical studies.1-6 An interesting category of these systems is that in which the brush can be charged. Recent theoretical work in this area1-3 has led to a phase diagram in which several distinct regimes can be identified. In this paper we focus on the transition between the so-called osmotic brush and salted brush regimes where the height of the polyelectrolyte layer passes through a maximum as a function of salt concentration. Beyond this transition with increasing salt the layer thickness will fall monotonically as the charges become progressively screened. A unique system for these studies is to prepare a polymer brush at the surface of a polymeric latex particle. This can be achieved by using a block copolymer that has one block that is compatible and can be incorporated into a latex particle and one block that is not. The system of choice in this study comprises a hydrophobic block of poly(methyl methacrylate) (PMMA) and a weak polyelectrolyte block, poly(2-(dimethylamino)ethyl methacrylate) (DMAEMA). These block copolymers have been studied in solution,7 at the liquidair interface8-10 and at the solid-liquid interface.11 In * To whom correspondence should be addressed. E-mail:
[email protected]. www.chm.bris.ac.uk/pt/ polymer/pig.htm. (1) Israels, R.; Leermakers, F. A. M.; Fleer, G. J. Macromolecules 1994, 27, 3087. (2) Lyatskaya, Y. V.; Leermakers, F. A. M.; Fleer, G. J.; Zhulina, E. B.; Birshtein, T. M. Macromolecules 1995, 28, 3562. (3) Zhulina, E. B.; Birshtein, T. M.; Borisov, O. V. Macromolecules 1995, 28, 1491. (4) Milner, S. T. Science 1991, 251, 905. (5) Szleifer, I.; Carignano, M. A. Adv. Chem. Phys. 1996, 94, 165. (6) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman and Hall: London, 1993. (7) .Wesley, R. D.; Cosgrove, T.; Thompson, L.; Armes, S. P.; Billingham, N. C.; Baines, F., submitted to Macromolecules. (8) An, S. W.; Thomas, R. K.; Baines, F. L.; Armes, S. P.; Billingham, N. C.; Penfold, J. J. Phys. Chem. B 1998, 102, 5120.
solution, the polymer forms micelles whose structures are sensitive to both ionic strength and added surfactant.7 At a hydrophobic solid-water interface, the PMMA is preferentially adsorbed and the DMAEMA block solvated.11 Using neutron reflection, the authors found that when they increased the pH, the DMAEMA block contracted whereas the MMA block expanded. At pH 9.5 the polyelectrolyte layer showed a slight increase with increasing salt concentration, but at pH 3 only a monotonic decrease was found. In this note we describe the variation in the hydrodynamic thickness at ∼pH 7 for a tethered DMAEMA chain. The pKa of DMAEMA is in the range from 6.612 to 7.311 and we compare the results with scaling predictions. Theory A weak polyelectrolyte does not have a fixed charge; the degree of dissociation of the ionizable groups is determined by the external pH. To allow direct comparison with the system we have studied experimentally, the theoretical situation for a polybase is discussed. An equilibrium exists between the neutral (B) and charged groups (BH+) of a polybase:
B + H2O h HO- + BH+ The degree of dissociation, R, which is the fraction of ionizable groups that carry a charge, is given by (9) An, S. W.; Thomas, R. K.; Baines, F. L.; Armes, S. P.; Billingham, N. C.; Penfold, J. Macromolecules 1998, 31, 7877. (10) An, S. W.; Su, T. J.; Thomas, R. K.; Baines, F. L.; Armes, S. P.; Billingham, N. C.; Penfold, J. J. Phys. Chem. B 1998, 102, 387. (11) An, S. W.; Thirtle, P. N.; Thomas, R. K.; Baines, F. L.; Armes, S. P.; Billingham, N. C.; Penfold, J. Macromolecules 1999, 32, 2731. (12) Lee, A. S.; Gast, A. P.; Butun V.; Armes, S. P. Macromolecules 1999, 32, 4310.
10.1021/la991263d CCC: $19.00 © 2000 American Chemical Society Published on Web 04/14/2000
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R)
[BH+] [B] + [BH+]
Wesley et al.
(1)
At high pH, R approaches zero as the equilibrium shifts to favor B groups. At lower pH, R approaches unity as charged BH+ groups are favored. If one end of the polyelectrolyte chain is irreversibly attached to an interface, a brush is formed, provided the grafting density is sufficiently high. If the weak polyelectrolyte brush was dispersed in a (theoretical) zero ionic strength solvent, each charged base group will be balanced by an HO- counterion. Overall, the brush must remain electrically neutral so all the counterions must be contained in the vicinity of the brush as if trapped by an imaginary membrane. This trapping of the HO- counterions inside the brush causes the pH inside the layer to be higher than that in the bulk solution. This high local pH means that the equilibrium between uncharged and charged polybase groups will shift in favor of the uncharged groups, making the brush effectively neutral. The addition of salt to the system allows the passage of hydroxyl ions through the imaginary membrane into the bulk solution, the brush retaining its electrical neutrality by the diffusion of a salt anion back through the membrane for each hydroxyl ion that diffuses out. As HO- ions are released from the brush, the local pH can now begin to approach that of the bulk solution. The degree of dissociation will increase as the salt concentration increases, allowing more protons to diffuse into the bulk. As the degree of dissociation increases, the increased electrostatic repulsion leads to a stretching of the brush. Upon further addition of salt, a point is reached where the brush has reached its maximum degree of dissociation. Above this point, the screening of charges within the brush becomes important, and any further addition of salt causes the brush to collapse. The height, δ, of a brush, can be related to the chain length, N, and the grafting density, σ, using the scaling model of Alexander and de Gennes: 13,14 δ ∼ Nσ1/3. For a weak polyelectrolyte in the salt brush regime the effective surface density can be replaced by σνeff , where νeff is an effective excluded volume parameter which includes both electrostatic and nonelectrostatic contributions. In the salted brush regime the electrostatic contribution scales as 1/φs, where φs is the salt concentration. Combining these relations for a fixed brush length leads to the relation1 δ ∼ φs-1/3. Experimental Section A diblock copolymer based on poly(methyl methacrylate), PMMA, and poly(2-(dimethylamino)ethyl methacrylate), PDMAEMA, was prepared by sequential monomer addition [DMAEMA first] using group-transfer polymerization as described previously.15,16 The copolymer had a total number-average molecular weight (Mn), by 1H NMR, of 20 000 g mol-1 and a polydispersity (Mw/Mn) of 1.10 as determined by GPC using PMMA standards. 1H NMR also indicated that the copolymer had a DMAEMA content of 69.5 mol %. PDMAEMA can be considered to be a weak polybase. A positively charged PMMA latex was prepared using a surfactant-free emulsion polymerization technique. The PMMA latex was prepared at a temperature of 80 °C; higher temperatures were found to cause excess evaporation of the monomer, leading to a low yield of particles. 2,2′-Azobis(2-methyl-propionamidine)dihydrochloride (Acros organics, 98% purity) was (13) Alexander, S. J. Physique 1977, 38, 977. (14) de Gennes, P. G. Macromolecules 1980, 13, 1069. (15) Baines, F. L.; Billingham, N. C.; Armes, S. P. Macromolecules 1996, 29, 3416. (16) Baines, F. L.; Armes, S. P.; Billingham, N. C.; Tuzar, Z. Macromolecules 1996, 29, 8151.
Figure 1. A schematic representation of the grafted block copolymer. used as the initiator, which leads to the formation of -C(CH3)2C(NH2)2+ groups on the surface of the particles. The latex had an intensity averaged diameter of 112.4 ( 0.6 nm as determined by photon correlation spectroscopy. The technique of Dewalt17 was adapted to prepare a grafted polybase. The positively charged PMMA latex was swollen in a solvent containing 60:40 v/v water:tetrahydrofuran (THF). The PMMA-PDMAEMA diblock copolymer was added to the swollen latex dispersion. After being stirred for 72 h, the latex was deswollen by dilution with water. The deswelling of the latex is believed to trap the PMMA part of the copolymer in the PMMA latex particles, leading to a grafted layer of PDMAEMA chains exposed to the solution, as shown in Figure 1. Most of theTHF was removed by careful rotary evaporation under vacuum. The aqueous sample was then centrifuged at 10 000 rpm for 45 min. The grafted polyelectrolyte was then redispersed in a minimum amount of water by vigorous shaking. This centrifugation/redispersion process was repeated several times to remove any remaining THF and any other impurities such as ungrafted block copolymer. It is unlikely however that without dialysis that the core latex will revert exactly to its original unswollen diameter.18 For a series of photon correlation spectroscopy (PCS) measurements the grafted polybase was diluted with MilliQ Millipore water to give a final concentration of ≈100 ppm w/w at neutral pH. The size of the grafted particle was then measured as a function of added salt concentration.
Results and Discussion The thickness of the polybase layer can be estimated by determining the size of the bare particles before grafting and subtracting this from the total thickness of the particles and grafted layer. Such an approach assumes that the particle size is unaltered by the grafting process and as such is likely to give an overestimate of the polybase layer thickness. Small-angle neutron scattering from the same system can be used to find the radius of the core particle after the grafting procedure, by using contrast variation. If the DMAEMA layer is contrast-matched with an appropriate D2O/H2O ratio, then the observed scattering is dominated by the particle.19 The data gave a value of 66( 8 nm for the particle radius, which is (17) Dewalt, L. E.; Ou-Yang, H. D.; Dimonie, V. L. J. Appl. Polym. Sci. 1995, 58, 265. (18) Garreau, L. M. Sc. Thesis, University of Bristol, 1999. (19) Wesley, R. Ph.D. Thesis, University of Bristol, 1999.
Polybase Brush Layer Thickness near Salt
Figure 2. Hydrodynamic radius of the grafted particle as a function of added salt. The inset shows an expansion of the low-salt region.
considerably larger than the bare particle. This can be accounted for by the ingress of the MMA block of the copolymer and traces of residual THF. In Figure 2 we show the total hydrodynamic diameter of the core particle and the grafted brush obtained at a pH ≈7, at which the DMAEMA chain is ≈50% ionized.12 In the absence of added salt, the total radius was 74.2 nm. Subtracting the postgrafted core particle radius gives a hydrodynamic radius of 8.2 nm. The radius of gyration, RG, of the DMAEMA block can be estimated from lightscattering data20 to be ≈6.8 nm and the extended length 64 nm. The layer thickness as calculated is therefore of the order of RG. Figure 2 also shows the total hydrodynamic radius of the polyelectrolyte-grafted latex particles as a function of added salt. At very low salt concentrations a peak is observed in the thickness of the grafted polybase, but beyond 0.025 M NaCl, the layer thickness decreases. This region of the data has been expanded in the insert to the figure. This observation is in good qualitative agreement with the idea of a transition from an osmotic brush where the layer thickness is increased due to ionization and a salted brush where the charges become progressively screened. The effects seen here are quite dramatic and larger than the limited data observed at the solid-liquid interface.11 The hydrodynamic radius is known to depend (20) Armes, S. P., unpublished data.
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Figure 3. Hydrodynamic radius of the grafted particle as a function of added salt, showing the dependence on [NaCl]-1/3.
strongly on the presence of tails and in a charged brush the chain segments will be strongly stretched. In the neutron reflection studies referred to above, the thickness of the diffuse brush layer is not very sensitive to dilute tail segments and hence the observed effects are expected to be less pronounced. In Figure 3 we have replotted the data as a function of [NaCl]-1/3 and a clear linear region is found consistent within experimental error with the scaling prediction derived above. To obtain the linear region, a bare particle radius must be subtracted, and to give the correct exponent, we require a bare particle radius of 70 nm, which is consistent with the error bounds of the data above. Conclusions The influence of salt on the hydrodynamic thickness of a weak polyelectrolyte brush has been measured. A maximum has been found in the thickness at intermediate salt concentrations as has been predicted by theory. A good agreement with scaling theory in the salted-brush regime has also been found. Acknowledgment. R.W. and F.B. would like to thank EPSRC, Unilever and Courtaulds for CASE studentships. We also acknowledge NIST for the provision of neutron facilties and to Tania Slawecki for help in obtaining the experimental data. LA991263D
