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Manipulating the Morphologies of Cylindrical Polyelectrolyte Brushes by Forming Interpolyelectrolyte Complexes with Oppositely Charged Linear Polyelectrolytes: An AFM Study Youyong Xu,† Oleg V. Borisov,‡ Matthias Ballauff,§ and Axel H. E. M€uller*,† †
Makromolekulare Chemie II, and Bayreuther Zentrum f€ ur Kolloide und Grenzfl€ achen, Universit€ at Bayreuth, D-95440 Bayreuth, Germany, ‡Institut Pluridisciplinaire de Recherche sur l’Environnement et les Mat eriaux, UMR 5254 CNRS/UPPA, F-64053 Pau, France, and §Helmholtz-Zentrum Berlin f€ ur Materialien und Energie, D-14109 Berlin, Germany Received November 2, 2009. Revised Manuscript Received March 6, 2010 We present a study on water-soluble interpolyelectrolyte complexes (IPECs) formed by cationic cylindrical polyelectrolyte brushes (CPBs) and linear anionic poly(sodium styrenesulfonate) (PSSNa) using atomic force microscopy (AFM). The IPECs were prepared by dialysis of salt-containing solutions of the two polymeric components. The morphologies of the IPECs could be tuned by changing the charge ratio between the two polyelectrolytes, Z-/þ. Addition of increasing numbers of short PSSNa chains induced morphology changes of host CPBs from worms through intermediate pearl-necklace structures to fully collapsed spheres. Extremely long guest PSSNa caused the full collapse of the brushes to spheres even at very low charge ratios without intermediate states. In both cases we observe “disproportionation”, that is, inhomogeneous distribution of the PSS chains between the CPB for Z-/þ < 1. Unexpected micrometer-scale core-shell cylindrical objects were found by directly mixing CPBs with long PSSNa, which might be nonequilibrium structures caused by the kinetically controlled IPEC formation.
Introduction Responsive polymers have drawn attention due to their sensitiveness to various environmental stimuli, which may induce dramatic conformational transitions or changes of their macroscopic properties.1 Smart nanostructured polymers are of particular importance for the potential of building nanoscopic devices.2 Recently, coassembly processes in multicomponent polymer systems have attracted strong interest due to their versatility and multiresponsiveness of resulting nanostructures.3-5 Among these systems, interpolyelectrolyte complexes (IPECs), which form by the interaction of oppositely charged polyelectrolytes, have been of particular interest since they may be applied in different areas like gene transfer3-6 and layer-by-layer assembly,7 e.g., for capsules and nanotemplates.8-10 In general, electrostatically driven assembly enables formation of complexes of polyelectrolytes11 with surfactants, colloidal particles, and, in *To whom correspondence should be addressed. (1) Minko, S. Responsive Polymer Materials: Design and Applications; Blackwell Publishing Ltd.: Oxford, 2006. (2) Dai, L. Intelligent Macromolecules for Smart Devices: From Materials Synthesis to Device Applications; Springer-Verlag: London, 2003. (3) Kabanov, A. V.; Kabanov, V. A. Bioconjugate Chem. 1995, 6, 7–20. (4) Wolfert, M. A.; Dash, P. R.; Nazarova, O.; Oupicky, D.; Seymour, L. W.; Smart, S.; Strohalm, J.; Ulbrich, K. Bioconjugate Chem. 1999, 10, 993–1004. (5) Goffeney, N.; Bulte, J. W. M.; Duyn, J.; Bryant, L. H., Jr.; van Zijl, P. C. M. J. Am. Chem. Soc. 2001, 123, 8628–8629. (6) Bronich, T.; Kabanov, A. V.; Marky, L. A. J. Phys. Chem. B 2001, 105, 6042– 6050. (7) Decher, G. Science 1997, 277, 1232–1237. (8) Ariga, K.; Hill, J. P.; Ji, Q. Phys. Chem. Chem. Phys. 2007, 9, 2319–2340. (9) Quinn, J. F.; Johnston, A. P. R.; Such, G. K.; Zelikin, A. N.; Caruso, F. Chem. Soc. Rev. 2007, 36, 707–718. (10) Wang, Y.; Angelatos, A. S.; Caruso, F. Chem. Mater. 2008, 20, 848–858. (11) Mandel, M. Encycl. Polym. Sci. Eng. 1987, 11, 739–829. (12) Th€unemann, A. F. Prog. Polym. Sci. 2002, 27, 1473–1572. (13) Th€unemann, A. F.; M€uller, M.; Dautzenberg, H.; Joanny, J.-F.; L€owen, H. Adv. Polym. Sci. 2004, 166, 113–171. (14) Kabanov, V. A.; Kargina, O. V.; Ulyanova, M. V. J. Polym. Sci., Polym. Chem. Ed. 1976, 14, 2351–2356.
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particular, oppositely charged polyelectrolytes (IPECs).12-22 Depending on the neutralization ratio, Z-/þ, of the two polyelectrolytes in solution, insoluble stoichiometric complexes or soluble nonstoichiometric complexes can be prepared. The driving force of the IPECs is mainly the entropy gain by liberating the condensed low-molecular-weight counterions into the solution. There are many factors that influence the structure and properties of IPECs:23 the type of the ionic groups (weak or strong electrolyte), the absolute and relative molecular weights, the stoichiometric charge ratio of the components, Z-/þ, solvent properties, such as pH, ionic strength, and valency of the counterions, and the preparation methods of the IPECs. Recently, the research efforts on polyelectrolytes have shifted from linear to nonlinear topologies, i.e., stars, cylindrical or spherical brushes, and hyperbranched polymers.24 Opposite to linear polyelectrolytes, most of counterions are strongly confined within polyelectrolyte brushes and branched polyelectrolytes.25,26 The corresponding high osmotic pressure leads to a stretching of the (15) Kabanov, V. A.; Kargina, O. V.; Mishustina, L. A.; Lubanov, S. Y.; Katuzynski, K.; Penczek, S. Makromol. Chem., Rapid Commun. 1981, 2, 343–346. (16) Izumrudov, V. A.; Savitskii, A. P.; Bakeev, K. N.; Zezin, B.; Kabanov, V. A. Makromol. Chem., Rapid Commun. 1984, 5, 709–714. (17) Kabanov, V. A.; Zezin, A. B. Macromol. Chem. Phys., Suppl. 1984, 6, 259– 276. (18) Kabanov, V. A.; Zezin, A. B. Pure Appl. Chem. 1984, 56, 343–354. (19) Izumrudov, V. A.; Bronich, T. K.; Zezin, A. B.; Kabanov, V. A. J. Polym. Sci., Polym. Lett. Ed. 1985, 23, 439–444. (20) Kabanov, V. A.; Zezin, A. B.; Rogacheva, V. B.; Prevish, V. A. Makromol. Chem. 1989, 190, 2211–2216. (21) Kabanov, V. A.; Zezin, A. B.; Izumrudov, V. A.; Bronitch, T. K.; Kabanov, N. M.; Listova, O. V. Makromol. Chem., Macromol. Symp. 1990, 39, 155–169. (22) Kabanov, A. V.; Bronich, T. K.; Kabanov, V. A.; Yu, K.; Eisenberg, A. Macromolecules 1996, 29, 6797–6802. (23) Philipp, B.; Dautzenberg, H.; Linow, K. J.; Koetz, J.; Dawydoff, W. Prog. Polym. Sci. 1989, 14, 91–172. (24) Mori, H.; M€uller, A. H. E. Prog. Polym. Sci. 2003, 28, 1403–1439. (25) Pincus, P. Macromolecules 1991, 24, 2912–2919. (26) Zhulina, E. B.; Borisov, O. V. Macromolecules 1996, 29, 2618–2626.
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arms in salt-free solutions.27-30 This osmotic pressure is relieved upon formation of an IPEC. Hence, the driving force for the complexation of branched or brushlike polyelectrolytes with oppositely charged groups should be much enhanced when compared to linear polyelectrolytes. Up to now, a few examples of IPECs of dendrimers,31-36 star polymers37,38 and micelles39-41 were reported. In general, research on IPECs of nonlinear polyelectrolytes is quite scarce. Evidently, this is due to the difficult synthesis of these structures which have only been overcome with the advent of controlled polymerization techniques. To the authors’ best knowledge, there are only a few studies of the IPECs of one-dimensional nanostructured polyelectrolytes, such as dendronized polymers42 and cylindrical polymer brushes.43 Cylindrical polyelectrolyte brushes (CPBs) consist of long backbones with densely grafted polycationic or anionic side chains.44,45 So far, grafting-from46 and grafting-through47,48 strategies have been adopted to prepare CPBs. We have recently reported the synthesis of cationic CPBs with poly([2-(methacryloyloxy)ethyl]trimethylammonium iodide) (PMETAI) side chains (Chart 1) by the grafting-from approach via atom transfer radical polymerization (ATRP) followed by quaternization.46 These brushes respond to the addition of mono-, di-, and trivalent counterions, which cause the transition of the (27) Ballauff, M. Prog. Polym. Sci. 2007, 32, 1135–1151. (28) Plamper, F. A.; Becker, H.; Lanzendoerfer, M.; Patel, M.; Wittemann, A.; Ballauff, M.; M€uller, A. H. E. Macromol. Chem. Phys. 2005, 206, 1813–1825. (29) Plamper, F. A.; Schmalz, A.; Penott-Chang, E.; Drechsler, M.; Jusufi, A.; Ballauff, M.; M€uller, A. H. E. Macromolecules 2007, 40, 5689–5697. (30) Plamper, F. A.; Walther, A.; M€uller, A. H. E.; Ballauff, M. Nano Lett. 2007, 7, 167–171. (31) Miura, N.; Dubin, P. L.; Moorefield, C. N.; Newkome, G. R. Langmuir 1999, 15, 4245–4250. (32) Zhang, H.; Dubin, P. L.; Ray, J.; Manning, G. S.; Moorefield, C. N.; Newkome, G. R. J. Phys. Chem. B 1999, 103, 2347–2354. (33) Welch, P.; Muthukumar, M. Macromolecules 2000, 33, 6159–6167. (34) Leisner, D.; Imae, T. J. Phys. Chem. B 2003, 107, 13158–13167. (35) Leisner, D.; Imae, T. J. Phys. Chem. B 2004, 108, 1798–1804. (36) Kabanov, V. A.; Zezin, A. B.; Rogacheva, V. B.; Panova, T. V.; Bykova, E. V.; Joosten, J. G. H.; Brackman, J. Faraday Discuss. 2004, 128, 341–354. (37) Pergushov, D. V.; Babin, I. A.; Plamper, F. A.; Zezin, A. B.; M€uller, A. H. E. Langmuir 2008, 24, 6414–6419. (38) Larin, S. V.; Darinskii, A. A.; Zhulina, E. B.; Borisov, O. V. Langmuir 2009, 25, 1915–1918. (39) Pergushov, D. V.; Remizova, E. V.; Feldthusen, J.; Zezin, A. B.; M€uller, A. H. E.; Kabanov, V. A. J. Phys. Chem. B 2003, 8093–8096. (40) Pergushov, D. V.; Remizova, E. V.; Gradzielski, M.; Lindner, P.; Feldthusen, J.; Zezin, A. B.; M€uller, A. H. E.; Kabanov, V. A. Polymer 2004, 45, 367–378. (41) Burkhardt, M.; Ruppel, M.; Tea, S.; Drechsler, M.; Schweins, R.; Pergushov, D. V.; Gradzielski, M.; Zezin, A. B.; M€uller, A. H. E. Langmuir 2008, 24, 1769–1777. (42) G€ossl, I.; Shu, L.; Schl€uter, A. D.; Rabe, J. P. J. Am. Chem. Soc. 2002, 124, 6860–6865. (43) St€orkle, D.; Duschner, S.; Heimann, N.; Maskos, M.; Schmidt, M. Macromolecules 2007, 40, 7998–8006. (44) Zhang, M.; M€uller, A. H. E. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 3461–3481. (45) Sheiko, S. S.; Sumerlin, B. S.; Matyjaszewski, K. Prog. Polym. Sci. 2008, 33, 759–785. (46) Xu, Y.; Bolisetty, S.; Drechsler, M.; Fang, B.; Yuan, J.; Ballauff, M.; M€uller, A. H. E. Polymer 2008, 49, 3957–3964. (47) R€uhe, J.; Ballauff, M.; Biesalski, M.; Dziezok, P.; Gr€ohn, F.; Johannsmann, D.; Houbenov, N.; Hugenberg, N.; Konradi, R.; Minko, S.; Motornov, M.; Netz, R. R.; Schmidt, M.; Seidel, C.; Stamm, M.; Stephan, T.; Usov, D.; Zhang, H. Adv. Polym. Sci. 2004, 165, 79–150. (48) Hua, F.; Kita, R.; Wegner, G.; Meyer, W. ChemPhysChem 2005, 6, 336– 343.
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morphology from wormlike through helix to spherical, collapsed shapes.46,49 We also found that they can form complexes with the oppositely charged surfactant sodium dodecylsulfonate (SDS).50 In the present work we study the morphologies of the IPECs formed by the cationic CPB having PMETAI side chains with anionic linear polyelectrolyte PSSNa in a more systematic way, varying the ratio of anionic to cationic charges as well as the molecular weight of the linear polyanion. Pursuant to previous work51 on the formation of stable water-soluble IPECs between poly(acrylic acid) (PAA) CPBs and quaternized poly(4-vinylpyridine) (PVP 3 EtBr), we shall present a systematic study of the IPECs formed by this cationic CPB and poly(styrenesulfonic acid) (PSS). Our simulations predicted the existence of an interesting pearl-necklace structure of these IPECS.51 First atomic force microscopy (AFM) measurements on the IPECs formed between the host strong cationic PMETAI CPBs and linear poly(sodium sulfonate styrene) (PSSNa) corroborated these simulation results indeed.51 Since we are interested in the conformational changes of single brushes, most of the experiments were carried out in highly dilute solutions to avoid intermolecular aggregates. However, most characterization methods such as, e.g., dynamic light scattering (DLS) are not available or give rather insecure results in this concentration range. So in this work, AFM measurements52 were mainly employed to directly examine the IPECs formed by the cationic CPBs with linear polyelectrolytes.
Experimental Section Materials. A cationic cylindrical polymer brush with side chains of poly([2-(methacryloyloxy)ethyl]trimethylammonium iodide) (PMETAI) was prepared by ATRP in our previous work.46 The backbone of the brushes has a degree of polymerization (DP) = 1500 and a polydispersity index of 1.04. The grafting density of the side chains is 50%, and their DP is 84. The number-average molecular weight of the brushes is Mn = 1.9 107 g/mol. Thus, in summary, each brush molecule carries 750 side chains, and each side chain carries 84 quaternary amine units. In total, each brush molecule consists of 6.3 104 elementary charged cationic groups. Linear poly(sodium styrenesulfonate)s (PSSNa) of two different molecular weights were purchased from Fluka and were used as received (sample 1: Mn = 1.32 104, DP = 64, PDI < 1.2; sample 2: Mn = 2.26 106, DP = 1.1 104, PDI < 1.2). The number of anionic groups carried by each PSSNa molecule is equal to its DP. NaCl was used as received. Millipore water was used for the preparation of samples and dialysis. Preparation of IPECs by Mixing in Salt Solutions and Further Dialysis. Stock solutions of PSSNa (0.001 M of anionic charges) were prepared by dissolving them in Millipore water. Then 10 mL of PMETAI brush dilute solutions (0.02 g/L) with 0.2 M NaCl was prepared separately. Different amounts of PSSNa were injected by a microsyringe according to the charge ratio between anions and cations, Z-/þ. Since the volume of the PSSNa added is minor comparing to the total volume, the ionic strength did not change significantly (for Z-/þ = 1, the volume of the added PSSNa is about 0.67 mL, about 6% of the total volume). Then the solutions were vigorously stirred for 5 min before they were subjected to dialysis against Millipore water for 4 days. (49) Xu, Y.; Bolisetty, S.; Drechsler, M.; Fang, B.; Yuan, J.; Harnau, L.; Ballauff, M.; M€uller, A. H. E. Soft Matter 2009, 5, 379–384. (50) Xu, Y.; Bolisetty, S.; Ballauff, M.; M€uller, A. H. E. J. Am. Chem. Soc. 2009, 131, 1640–1641. (51) Larin, S. V.; Pergushov, D. V.; Xu, Y.; Darinskii, A. A.; Zezin, A. B.; M€uller, A. H. E.; Borisov, O. V. Soft Matter 2009, 5, 4938–4943. (52) Sheiko, S. S.; M€oller, M. Chem. Rev. 2001, 101, 4099–4123.
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Figure 1. AFM height images of IPECs formed by CPBs and short PSSNa chains on mica. The AFM z range =10 nm. (a) Charge ratio Z-/þ = 0.1; (c) Z-/þ = 0.5. (b, d) Section analyses of the cursors displayed in (a, c), respectively.
The formation of IPECs results from the electrostatic interaction of oppositely charged polymer chains and subsequent release of counterions.53,54 Stopped-flow measurements have shown that the formation of IPECs is an extremely fast process, which takes place in less than 5 μs, close to the rate of the polyelectrolyte diffusion in solution.13 Evidently, there is a marked influence of the salt concentration in the solution since it tunes the strength of electrostatic interaction. The formation of the IPECs in water without added salt is fast and kinetically controlled. The final IPECs will be far from the thermodynamic equilibrium. However, adding even small amounts of salt to the system will partially screen the charges of the linear polyelectrolytes and weaken the driving force for the IPEC formation. Increasing the amount of
added salt facilitates equilibration of structure of the IPECs. At the same time the electrostatic repulsion between the IPEC particles in the solution is weakened. When a critical salt concentration is exceeded, this will lead to a rearrangement of the short guest molecules. They form completely complexed (coacervate) IPECs and precipitate out of the solution, leaving host polyelectrolytes in the solution. In the case of long linear host polyelectrolytes it was demonstrated that the supernatant phase contains virtually uncomplexed (“bare”) host polyelectrolyte chains.13,55 On the contrary, in the case of strongly branched host polyelectrolytes the coacervate precipitate phase coexists with partially complexed species in the supernatant.40,41 The critical salt concentration, according to some reports,55 is highly dependent on the type of the counterion. Further increase of the salt concentration will dissolve the IPECs by shifting the equilibrium to the side of the educts. For the PSSNa/poly(diallyldimethylammonium chloride) IPEC system, this salt concentration is as high as 4 M,13 whereas in the case of the IPECs between micelles of polyisobutylene-block-poly(methacrylic acid) and quaternized poly(4vinylpyridine), the disassociation starts from salt concentrations as low as 0.2 M, and 0.5 M NaCl is enough to cause the IPECs’ full dissociation.41 Thus, special attention must be paid to the salt concentration during the preparation of the IPECs. In this work, strong cationic PMETAI brushes and strong anionic linear PSSNa were employed for the formation of IPECs. Since both are strong polyelectrolytes, their ionization behavior is not dependent on pH. Nevertheless, all the experiments were performed at pH 7. Since ionic strength plays a vital role in the formation and properties of IPECs, we prepared the IPECs by different methods: (i) mixing in salt solutions with further dialysis
(53) Hofs, B.; Voets, I. K.; de Keizer, A.; Cohen Stuart, M. A. Phys. Chem. Chem. Phys. 2006, 8, 4242–4251. (54) Gummel, J.; Cousin, F.; Boue, F. J. Am. Chem. Soc. 2007, 129, 5806–5807.
(55) Kabanov, V. Fundamentals of Polyelectrolyte Complexes in Solution and the Bulk. In Multilayer Thin Films; Decher, G., Schlenoff, J. B., Eds.; Wiley-VCH: Weinheim, 2002; pp 47-84.
Preparation of IPECs by Direct Mixing in Salt-Free Solutions. Stock solutions of PSSNa (0.001 M of anionic charges) were first prepared by dissolving them in Millipore water. Then 10 mL of dilute solutions (0.02 g/L) of PMETAI brush were prepared. Different amounts of PSSNa were injected by microsyringe according to the charge ratios between the polyanions and polycations. Characterization. Atomic force microscopy (AFM) measurements were performed on a Digital Instruments Dimension 3100 microscope operated in tapping mode. The microcantilevers used for the AFM measurements were from Olympus with resonant frequencies between 284.3 and 386.0 kHz and spring constants ranging from 35.9 to 92.0 N/m. The samples were prepared by spin-coating from very dilute (0.02 g/L) solutions onto freshly cleaved mica surfaces.
Results and Discussion
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Figure 2. (a) AFM height image of IPECs by CPBs and short PSSNa, charge ratio Z-/þ = 0.75, AFM z range = 20 nm. (b) AFM phase image, z range = 25°. (c) Magnified image of selected area in (a). (d) Section analysis of the cursor displayed in (a).
to pure water (since salt may cause some problems for the AFM measurements in the dry state) and (ii) direct mixing in pure water. The length of the linear polyelectrolyte may also have an influence on the IPECs. Therefore, short linear PSSNa (64 anionic units) and extremely long PSSNa (11 000 anionic units) were used for the comparison of the IPECs formation with PMETAI brushes, carrying 63 000 cationic units each. To fully compensate one brush’s cationic charges, there need to be either ca. 1000 short PSSNa chains or about six long ones. The DP of the brush’s side chain is 84, which is higher than that of the short PSSNa. The estimated contour length of the long PSSNa is ∼2800 nm, which is much larger than the length of the single brush (around 180 nm46). In order to avoid aggregation of the resulting IPECs, all AFM measurements were carried out by spin-coating solutions of only 0.02 g/L. Thus, the average distance between the brushes in solution is about 104 nm. IPECs Prepared in Salt Solution and Subsequent Dialysis. To obtain IPECs with well-defined structures, we first start by adding a relatively high concentration of salt (0.2 M) to the mixtures of cationic CPBs (0.02 g/L) with PSSNa. Subsequent dialysis for 4 days removed the excess of salt ions and counterions, resulting from the IPEC formation. Thus, dialysis is used to “freeze” the as-formed IPEC structures. IPECs with Short PSSNA Chains. For the IPECs of short PSSNa and CPBs at charge ratios Z-/þ e 0.5, the solutions were found to be clear, while at Z-/þ = 0.75, some precipitation was observed after the dialysis. At Z-/þ = 1, more precipitation occurred, and it was hard to find any objects in AFM, suggesting that most of the polymers had phase-segregated from the solution. When Z-/þ reached 2, the solution became transparent again. A similar behavior was found for the IPECs with the long 6922 DOI: 10.1021/la904167r
PSSNa. The only difference is that the precipitation happened at even lower charge ratio (full precipitation at Z-/þ = 0.75). This points clearly to an overcharging occurring in the process of IPEC formation. In the following, AFM measurements will provide the detailed information into the structure of the IPECs thus obtained. Figure 1 shows the height images and section analyses of IPECs of short PSSNa and CPBs with charge ratios Z-/þ = 0.1 and 0.5. For Z-/þ = 0.1, it is clearly seen in Figure 1a that some brushes are much thicker than others: Quantitative section analysis in Figure 1b revealed that most of the brushes are around 1.8 nm, which corresponds to the height of an uncomplexed brush.46 Some of the brushes have a height of around 5 nm, which is significantly higher than the original brushes, indicating the formation of the IPECs of the cationic CPBs with PSSNa. From Figure 1a, we also see that the original brushes adopt quite curvy morphologies (the bending angle sometimes even reaches 180°, probably due to the flattening of positively charged CPBs strongly adsorbed at the negatively charged mica surfaces),56 while the IPECs are straighter than the original brushes, but keeping the length of the original brushes (around 180 nm). This finding may be traced back to the cross-linking effect of the PSSNa inside the brush layer, which is expected to stiffen the brush. Also, the surface interaction between the IPECs and the mica substrate will be different as compared to the uncomplexed brushes. The above findings indicate that the formation of the IPECs with brushes is inhomogeneous, and there is an apparent (56) Khalatur, P. G.; Khokhlov, A. R.; Prokhorova, S. A.; Sheiko, S. S.; M€oller, M.; Reineker, P.; Shirvanyanz, D. G.; Starovoitova, N. Eur. Phys. J. E 2000, 1, 99– 103.
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Figure 3. AFM height images of IPECs by CPBs and short PSSNa, charge ratio Z-/þ = 1: (a) from salt-free solution, AFM z range = 7 nm; (c) from 0.1 M NaCl solution, AFM z range = 30 nm; (b, d) section analyses of the cursors displayed in (a) and (c), respectively.
“disproportionation” of the PSSNa chains among the brushes, that is, a certain fraction of the brushes is complexed (loaded) with the guest PSSNa chains, whereas the others remain virtually “bare”. Large-scale measurements of the same sample indicate the formation of large aggregates as well. In Supporting Information Figure S1, several bright dots and larger aggregates are clearly seen, indicating that a very tiny amount of the brushes already formed fully charge-compensated complexes and aggregates. However, most of the CPBs are still in their wormlike nonaggregated form. A statistical analysis of Figure 1a shows that the complexed brushes are a fraction of 20% of the total number of flattened brushes. Assuming that all the PSSNa molecules bind to the 20% brushes, the charge ratio Z-/þ inside these complexes would be 0.5. The disproportionation is certainly not due to the deposition process on the surface. These salt-free systems are practically “frozen”, which should prevent a rearrangement of the guest molecules. Therefore, it is evident that two distinctly different species coexist in the solution.13,55 A possible explanation of the disproportionation effect may be sought in the fact that the initial salt concentration (0.2 M) may be above the critical concentration for phase separation in the mixture of CPBs with the oppositely charged linear polyelectrolyte. Moreover, because of the strong intrinsic hydrophobicity of PSS, the intramolecular complex coacervate domains possess high excess interfacial energy, which may also favor inhomogeneous loading of the CPBs by the guest PSS chains, particularly at high salt concentration, when electrostatic interactions are partially screened. Remarkably, this intermolecular disproportionation was not observed in mixtures of PAA CPBs with PVP 3 EtBr, which is less hydrophobic. Finally, one cannot rule out that the two populations of CPBs appear as a result of kinetically controlled association under low local ionic strength conditions Langmuir 2010, 26(10), 6919–6926
upon injection of the PSSNa solution into the solution of PMETAI CPBs. A similar finding is shown in Figure 1c for Z-/þ = 0.5. Again, species of different heights are observed in the AFM height image. The fraction of the thicker flattened brushes significantly increased to ca. 70%, while the remaining part of the brushes remained unchanged. The section analysis in Figure 1d demonstrates that the heights of some brushes kept their original values (around 1.8 nm), while the heights of those that formed IPECs were raised to 6 nm. Similarly, these IPECs adopted a straighter conformation than original brushes on the substrate. Again, assuming that all PSSNa molecules are inside these brushes, the composition of the complex particles corresponds to the charge ratio Z-/þ about 0.7. A closer look at these IPECs shows that some of the wormlike IPCs start to undulate. Figure 2 displays the AFM pictures of the complexes obtained for Z-/þ = 0.75. Here pearl-necklace structures are observed which were already reported in our previous work.51 Most of the flattened brushes have similar heights of around 8 nm, as evidenced by the section analysis in Figure 2d. The surface of the CPBs is uneven with some undulations, which might even hint to helical structures, similar to those observed in the interaction of PMETAI brushes with di- and trivalent counterions.46 We also observe a contraction of the formed IPECs to 140 nm, which is only ca. 80% of the length of the original CPBs. Considering that part of the complexes has precipitated from the solution, it is difficult to estimate the real charge ratio in the complexes left in the solution. However, Figure S2 (Supporting Information) clearly displays that there are already some spherical complexes and larger aggregates in the whole system. This again indicates that a certain degree of disproportionation of the PSSNa chains happened here, too. DOI: 10.1021/la904167r
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Figure 4. (a) AFM height image of IPECs by CPBs and long PSSNa, charge ratio Z-/þ = 0.2, AFM z range = 10 nm. (b) Section analysis of the cursor displayed in (a). Scheme 1. Schematic Morphology Changes of Cationic CPBs by Forming IPECs
For Z-/þ = 1 most of the polymers precipitate from the solution, and only very few objects could be observed in the AFM measurements (Figure 3a). In some areas, collapsed spherical objects (diameter of 80 nm) were detected, while no wormlike structure could be found any longer. Section analysis of these collapsed spherical objects (Figure 3b) indicated that they were a little higher (2.2 nm) than the original brushes. The precipitated IPECs may slightly swell in salt solutions. The IPEC precipitate with Z-/þ = 1 was added to a 0.1 M NaCl solution. After vigorous stirring overnight, a part of the precipitate was redispersed, and it was possible to prepare the AFM samples using this solution. More ellipsoidal and spherical objects (size around 90 nm) are shown in Figure 3c. Section analysis showed they have an average height of 9 nm, which is higher than those described above for Z-/þ = 0.1, 0.5, and 0.75. These results indicate that for Z-/þ = 1 the positive charges of the CPBs and the negative charges of PSSNa compensate each other and lead to the phase separation of the IPECs from the solution. Similar spherical morphologies were also observed in the complexes formed from the same brush with surfactant dodecylsulfonate sodium salt (SDS) when the charge ratio reached unity.50 When the CPBs were mixed with excess linear PSSNa (Z-/þ = 2), the coexistence of collapsed spheres and twisted worms was observed (see Supporting Information Figure S3). Again, this points to the uneven distribution of the guest PSSNa in the CPBs. Scheme 1 summarizes the conformational changes of the IPECs formed between the host CPBs and the short guest PSSNa chains. The results shown above clearly demonstrated that the morphologies of the brushes or the IPECs vary from flattened wormlike objects through intermediate pearl necklace to totally collapsed spheres. However, at most of the charge ratios, the IPECs formation is far from being homogeneous. Disproportionation of the guest PSSNa chains between the IPECs is a common phenomenon in this case. As we have discussed, the specific affinity of the hydrophobic PSSNa to the preformed IPECs or the critical phase separation salt concentration might be the possible reasons. 6924 DOI: 10.1021/la904167r
IPEC Formation with Long PSSNa Chains. We now turn to the discussion of complex formation of the CPBs with extremely long PSSNa chains having a DP = 11 000. Because of the high molecular weight of the PSSNa, there are much less long PSSNa chains in the solution at the same charge ratio. For instance, at Z-/þ = 0.2, ∼200 short PSSNa chains interact with each CPB molecule, whereas only a single long chain is needed for this charge ratio. Figure 4a shows the AFM image of the IPECs from CPBs and long PSSNa with charge ratio Z-/þ = 0.2. The CPBs are observed in coexistence with some collapsed globular objects. In opposite to the IPECs formed by CPBs and short PSSNa, no wormlike IPECs could be found here. The section analysis in Figure 4b indicates that the globular IPEC is 6.5 nm high, indicating the formation of IPECs between the long PSSNa and the CPBs. Since most of the brushes did not form IPECs, the real number of PSSNa chains in each IPEC spheres must be larger than unity. No intermediate wormlike or pearlnecklace states were detected. A good explanation for this is that PSSNa with extremely high molecular weight carries an enormous amount of negative charge, and several chains can induce the collapse of single brush molecules directly into spheres. In addition to the spherical IPECs, some much larger aggregates were also found (Supporting Information, Figure S4). These findings again indicate disproportionation of the PSSNa chains in the system. Upon the formation of IPECs with charge ratio Z-/þ = 0.5, a completely transparent solution was obtained. Figure 5a shows the AFM height image of the IPECs. Obviously, no wormlike structures can be observed anymore, and only some collapsed spherical objects with diameter of 90 nm are clearly seen. It is surprising that at Z-/þ = 0.5 most of the brushes were already collapsed into spheres. The section analysis displayed in Figure 5e shows that the height of the collapsed spheres is only around 3.5 nm, which is lower than that of the IPECs with short PSSNa at Z-/þ = 1. Again an uneven distribution of the PSSNa chains in the complexes is observed. Figure S5 in the Supporting Information Langmuir 2010, 26(10), 6919–6926
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Figure 5. (a) AFM height image of IPECs of CPBs and long PSSNa at Z-/þ = 0.5, z range 8 nm. (b) Phase image, z range 12°. (c) AFM height image of the same IPECs from 0.1 M NaCl solution, z range 35 nm. (d) Phase image, z range 20°. (e, f) Section analyses of the cursors displayed in (a, c), respectively.
shows the coexistence of worms, spheres, and larger aggregates in some area. This again points to disproportionation. When NaCl (0.1 M) was added to the soluble IPECs, the size of the collapsed spheres became smaller (around 60 nm), and core-shell structures were observed in both the height and phase images, displayed in Figure 5c,d. Section analysis shows that the height of the core reaches 12 nm. Adding salt to the system weakens the complexes, and the chains have more freedom to rearrange to a thermodynamically more stable state. The cores in Figure 5c,d are the coacervate stoichiometric complexes, while the shell should be the free PMETAI chains, which solubilized them in the solution. For IPECs between CPBs and long PSSNa at Z-/þ > 0.5 (0.75 and 1), most of the polymers precipitated from the solution, and no objects were observed in the AFM measurements. Langmuir 2010, 26(10), 6919–6926
IPECs with excess long PSSNa (Z-/þ = 2) were prepared, too. Very few objects were found on the substrate, probably due to the repulsion of the negatively charged IPECs by the negatively charged mica surface. Ellipsoidal structures could be seen in Figure 6a. Quantitative section analysis shows that the IPECs are as high as 17 nm, which is higher than all the IPECs described above, probably due to the high molecular weight of the complexed long PSSNa. Scheme 2 summarizes the experimental observations for complexation of the CPB with long PSS chains. In contrast to the IPECs formed from CPBs with the short PSSNa chains, the IPECs formed with long PSSNa chains do not form intermediate cylindrical or pearlnecklace structures before collapsing to spherical complexes (see Scheme 2). This could be due to the greater chain length and charge numbers of the long PSSNa chains. However, the disproportionation DOI: 10.1021/la904167r
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Figure 6. (a) AFM height image of IPECs by CPBs and long PSSNa, charge ratio Z-/þ = 2, AFM z range 30 nm. (b) Section analysis of the cursor displayed in (a).
Figure 7. (a) AFM height image of IPECs by CPBs and long PSSNa via direct mixing, charge ratio Z-/þ = 2, AFM z range 3 nm. (b) Section analysis of the cursor displayed in (a). Scheme 2. Morphology Transition Induced by the IPECs Formation between PMETAI Brushes and Extremely Long Linear PSSNa
PSSNa was calculated as 2.7 μm (1.1 104 0.25 nm), which is longer than the cylinders’ diameters. Thus, we assume that the shell could be formed by PSSNa.
Conclusions
of the PSSNa chains between the brushes is still present. Thus, it seems to be a general phenomenon for the present systems. IPECs Prepared by Direct Mixing. As already discussed, the formation of the IPECs in pure water is kinetically controlled. Hence, the resulting complexes are expected to be far from the thermodynamic equilibrium. For the IPECs prepared by directly mixing in salt-free solutions, it is found that intermolecular aggregation dominates. Large aggregates are observed of the IPECs formed by CPBs with short PSSNa via direct mixing in salt-free solution (Z-/þ = 0.5) (see Figure S6 in the Supporting Information); no single brushes were detected. However, when long PSSNa was directly mixed with CPBs at charge ratio 2, micrometer-scale cylindrical objects together with some aggregates were recorded by AFM. Figure 7a shows a typical example of such aggregates. The diameter of the cylinders is around 900 nm, and the length varies from 1 to 5 μm. From Figure 7a, we can see the flattened cylinders have noncontinuous narrow cores and relatively long shells. The section analysis in Figure 7b shows that the height of the core is around 1.8 nm, which is close to that of the uncomplexed brushes. The contour length of the fully stretched long 6926 DOI: 10.1021/la904167r
We have studied the IPECs formation between host cationic CPBs and guest anionic PSSNa. AFM was mainly employed to measure the morphology change of the CPBs and their complexes. The results may be summarized as follows: 1. The morphologies of the IPECs formed by CPBs with PSSNa can be tuned by changing their charge ratios. With increasing charge ratio there is a transition from extended worms through pearl-necklace intermediates to collapsed spheres in case of short PSS chains. Extremely long PSSNa induces only the transition to spheres without any intertwining state. 2. Disproportionation, that is, the inhomogeneous distribution of the PSS chains between the CPB, is a general feature of the present system. This is in contrast to previous findings on the PAA/PVP 3 EtBr system. A possible reason for this finding may be sought in the salt concentration for the preparation of IPECs which may be higher than the critical phase separation concentration. Future work must pursue this problem in more detail. Acknowledgment. Financial support from Deutsche Forschungsgemeinschaft within SFB 481 is gratefully acknowledged. Helpful suggestions by Dr. Dmitry V. Pergushov (Moscow State University) are appreciated. Supporting Information Available: AFM height images of IPECs at further charge ratios, Z-/þ. This material is available free of charge via the Internet at http://pubs.acs.org. Langmuir 2010, 26(10), 6919–6926
