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Adsorption Kinetics of Amphiphilic Diblock Copolymers: From Kinetically Frozen Colloids to Macrosurfactants O. The´odoly,*,†,‡ M. Jacquin,† P. Muller,†,§ and S. Chhun† Complex Fluids Laboratory, CNRS FRE 3084, Rhodia Incorporated, 350 George Patterson BouleVard, Bristol, PennsylVania 19007, Laboratoire Adhe´sion et Inflammation, INSERM U600, CNRS UMR 6212, Case 937, 163 AVenue de Luminy, Marseille F-13009, France, Aix-Marseille UniVersite´, Faculte´ des Sciences/de Me´decine ou de Pharmacie, Marseille, F-13000, France, and UniVersite´ Strasbourg 1, Institut Charles Sadron, CNRS UPR 22, 23, rue du Loess F-67034 Strasbourg Cedex, France ReceiVed September 15, 2008. ReVised Manuscript ReceiVed October 21, 2008 We investigated the spontaneous adsorption properties of charged amphiphilic diblock copolymers on hydrophobic surfaces and explained the transition of behavior from depleting frozen colloids (that do not adsorb at all) to fast adsorbing macrosurfactants when the hydrophobicity of the nonsoluble block is reduced. Three copolymer families have been used with the same hydrophilic block poly(acrylic acid), a weak acid whose ionization R can be varied by changing the pH. The hydrophobic blocks polystyrene, PS, poly(n-butyl acrylate), PBA, and poly(diethylene glycol ethyl ether acrylate), PDEGA, have interfacial tensions with water γcore/solvent, respectively, of 32, 20, and 3 mN/m. We were mainly interested in the regime of high ionization R > 0.3, where PAA chains have no affinity for hydrophobic surfaces, and we verified experimentally that micelles do not adsorb directly. With the three copolymer families we show that the adsorption kinetics at an early stage is driven by the self-assembly properties in bulk solution: adsorption is hampered for PS-b-PAA (physically/kinetically frozen micelles in solution), controlled by unimer extraction for PBA-b-PAA (nonequilibrium micelles in solution with very low CMC < 10-4 wt %), and controlled by unimer diffusion and electrostatic repulsion for PDEGA-b-PAA (micelles at equilibrium in solution with high CMC ≈ 1-5 wt %). This explains the power law dependences of adsorption with concentration as C-1 for PBA-b-PAA and C-2 for PDEGA-b-PAA. It is finally the interfacial tension with water of the nonsoluble block and not its glass transition that is the main control of bulk solution self-assembly and consequently of the adsorption kinetics properties of amphiphilic diblocks. We also proved by preparative GPC that the fraction of non-self-assembling diblock chains, which exists in all highly hydrophobic amphiphilic diblock systems, plays a negligible role in the adsorption properties. Finally, we investigated the intrinsic thermodynamic affinity between amphiphilic diblocks and hydrophobic surfaces. We show quantitatively that this affinity depends dominantly on the interfacial energies between the hydrophobic block, the surface, and water: diblocks with strongly hydrophobic nonsoluble blocks (PS, PBA) have a low affinity for weakly hydrophobic surfaces, and oppositely, diblocks with weakly hydrophobic nonsoluble block (PDEGA) have a universal affinity for hydrophobic surfaces (like small-molecule surfactants but for different physical reasons). Finally, we showed via surface rheology that when adsorption occurs anchoring is strong and irreversible for very hydrophobic diblocks (PBA-b-PAA) and weaker and (partially) reversible for less hydrophobic diblocks (PDEGA-b-PAA).
1. Introduction There has been considerable interest in the adsorption of amphiphilic block copolymers at interfaces both theoretically1-5 and experimentally.6-21 From a practical point of view, water* To whom correspondence should be addressed. Phone: +33 (0)4 91 82 88 69. Fax: +33 (0)4 91 82 88 51. E-mail:
[email protected]. † Rhodia Inc. ‡ CNRS UMR 6212 and Aix-Marseille Universite´. § Universite´ Strasbourg 1.
(1) Marques, C. M.; Joanny, J. F.; Leibler, L. Macromolecules 1988, 21, 1051. (2) Marques, C. M.; Joanny, J. F. Macromolecules 1989, 22, 1454. (3) Van Lent, B.; Scheutjens, J. M. H. M. Macromolecules 1989, 22, 1931. (4) Johner, A.; Joanny, J. F. Macromolecules 1990, 26, 5299. (5) Kopf, A.; Bashnagel, J.; Wittmer, J.; Binder, K. Macromolecules 1996, 29, 1433. (6) Amiel, C.; Sikka, M.; Schneider, J. W.; Tsao, Y. H.; Tirrell, M.; Mays, J. W. Macromolecules 1995, 28, 3125–3134. (7) Cosgrove, T.; Zarbakhsh, A.; Luckham, P. F.; Hair, M. L.; Webster, J. R. P. W. Faraday Discuss. 1994, 189. (8) Huguenard, C.; Varoqui, R.; Pefferkor, E. Macromolecules 1991, 24, 2226. (9) Munch, M. R.; Gast, A. P. Macromolecules 1990, 23, 2313. (10) Tripp, C. P.; Hair, M. L. Langmuir 1996, 12, 3952. (11) Tiberg, F.; Malmsten, M.; Linse, P.; Lindma, B. Langmuir 1991, 7, 2723. (12) 12An, S. W.; Su, T. J.; Thomas, R. K.; Baines, F. L.; Billingham, N. C.; Armes, S. P.; Penfold, J. J. Phys. Chem. B 1998, 102, 387–393. (13) An, S. W.; Su, T. J.; Thomas, R. K.; Baines, F. L.; Billingham, N. C.; Armes, S. P.; Penfold, J. Macromolecules 1998, 31, 7877–7885. (14) Matsuoka, H.; Maeda, S.; Kaewsaiha, P.; Matsumoto, K. Langmuir 2004, 20, 7412–7421.
soluble samples are of special interest due to their wide range of technical applications in emulsion polymerization, colloid stabilization, or wetting modification. Our main interest here is to develop charged diblock copolymers that may be more effective than traditional small-molecule surfactants. Given the large spectrum of applications of small-molecule surfactants in the industry (agriculture, cosmetology, oil recovery, detergency, wastewater purification) it is surprising that industrial applications of amphiphilic diblock copolymers have remained marginal. The main reason is that a large majority of existing charged diblock copolymers are poor surfactants.19-21 In the past decade, progress in macromolecular synthesis of block copolymers (atom transfer radical polymerization ATRP, stable-free radical polymerization SFRP, reversible addition (15) Toomey, R.; Mays, J.; Wade Holley, D. J.; Tirrell, M. Macromolecules 2005, 38, 5137–5143. (16) Toomey, R.; Mays, J.; Tirrell, M. Macromolecules 2006, 39, 697–702. (17) Abraham, T.; Giasson, S.; Gohy, J. F.; Jerome, R.; Muller, B.; Stamm, M. Macromolecules 2000, 33, 6051–6059. (18) Styrkas, D. A.; Butun, V.; Lu, J. R.; Keddie, J. L.; Armes, S. P. Langmuir 2000, 16, 5980–5986. (19) Matsuoka, H.; Matsutani, M.; Mouri, E.; Matsumoto, K. Macromolecules 2003, 36, 5321. (20) Kaewsaiha, P.; Matsumoto, K.; Matsuoka, H. Langmuir 2005, 21, 9938– 9945. (21) Bijsterbosch, H. D.; Cohen Stuart, M. A.; Fleer, G. J. Macromolecules 1998, 31, 9281–9294.
10.1021/la8030254 CCC: $40.75 2009 American Chemical Society Published on Web 12/15/2008
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fragmentation transfer RAFT, or macromolecular design via interexchange of xanthate MADIX) has increased the range of chemical compounds available for design of block copolymers both for laboratory use and for industrial-scale production.22 Imperfections of controlled radical polymerization products are largely compensated by the ability to precisely design samples with new properties, e.g., surface active properties.12,13 This permits exploring an endless range of chemistries and architectures to better understand the main factors that affect the interfacial behavior of amphiphilic block copolymers. In this paper, we are interested in the case of diblock copolymers with one hydrophobic (water-insoluble) block and one hydrophilic (water-soluble) block in water (selective solvent) and their interactions with a hydrophobic interface. It has long been recognized that an amphiphilic compound of large molar mass is not necessarily an efficient macromolecular surfactant or macrosurfactant.4 Widely studied amphiphilic diblock copolymers PS-b-PAA or PtBS-b-PSS form micelles in water, but they have been reported to be non-surface-active at low ionic strength.14,19,20,6 Also, amphiphilic diblocks take a long time to reach equilibrium (if they ever do) in adsorption experiments, whereas small-molecule surfactants form generally fast-adsorbing layers.21 The reason is that dense brushes form during initial diblock adsorption, which make it hard for additional molecules to adsorb. This effect is known to be even more important with charged species.23 The role of micelles and isolated unimers in diblock copolymer adsorption has also been under debate. An et al.12 proposed the existence of different regimes where layers build up by adsorption of micelles or unimers. Johner and Joanny,4 using scaling arguments, showed that the possibility for a micelle to bring its core into contact with the surface is hindered by a high barrier due to the large swollen and nonadsorbing corona. The surface layer is expected to proceed via attachment of free unimer chains. This was confirmed experimentally by Bijsterbosch et al.21 with diblock copolymers having very low CMC (
