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NMR Solvent Relaxation in Studies of Multicomponent Polymer Adsorption Andrew Nelson, Kevin S. Jack, Terence Cosgrove,* and Darby Kozak School of Chemistry, The University of Bristol, Cantock’s Close, Bristol BS8 1TS, United Kingdom Received November 19, 2001. In Final Form: January 8, 2002 NMR spin-spin solvent relaxation measurements have been performed on a series of aqueous dispersions of silica with physisorbed poly(ethylene oxide) (PEO) and poly(vinyl pyrrolidone) (PVP). Enhancements in the specific relaxation rate constant as a result of polymer adsorption were observed. Measurements of the relaxation rate constant as a function of the concentrations of silica and polymer in the system have been used to determine the amount of polymer trains adsorbed at the particle surface, at saturation. Studies of PEO-PVP-silica dispersions have shown that preadsorbed PEO is completely displaced when the silica concentration is small enough to allow the added PVP to saturate the surface. When the concentration of silica is increased beyond this point, then PEO and PVP are able to coexist on the particle surface.
Introduction The applications of polymers in controlling the dispersion properties of colloidal systems are diverse, ranging from industrial and pharmacological formulations to wastewater treatment. Moreover, it is well-known that the extent of steric stability (or flocculation) of these dispersions can be tuned by varying the type of polymer used, its molecular weight, and/or adsorbed amount.1 In all these applications, a detailed understanding of how interfacial polymer modulates interparticle interactions is needed. Many of these systems, however, contain multiple components that can compete for adsorption sites on the surface or can displace each other if added sequentially. Studies of these multicomponent systems have been less extensive than those where a single polymer adsorbs. It is these competitive and consecutive adsorption processes that are studied in this paper. The most important factor in determining the adsorption/desorption behavior of a polymer at an interface is the net segmental adsorption energy, χs.2 If the segment/ surface interaction energies are stronger than the solvent/ surface and the segment/solvent interaction energies, then the polymer will adsorb (χs is positive). However, χs has to be greater than a certain critical value, χsc, to compensate for loss in conformational entropy that a polymer experiences on adsorption. If a second component is added to the system, then it may be able to displace the adsorbed polymer if it has a higher affinity for the surface and the adsorption of the first component is reversible. Cohen Stuart et al. determined the segmental adsorption energy of poly(vinyl pyrrolidone) (PVP) onto silica, from water, by using different solvents that acted as low molecular weight displacers.3,4 This method has been used to study the strength of adsorption of various polymers * To whom correspondence may be addressed:
[email protected].
Terence.
(1) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces, 2 ed.; Chapman and Hall: London, 1993. (2) Silberberg, A. J. Chem. Phys. 1968, 48, 2835. (3) Cohen Stuart, M. A.; Fleer, G. J.; Scheutjens, J. J. Colloid Interface Sci. 1984, 97, 515. (4) Cohen Stuart, M. A.; Fleer, G. J.; Scheutjens, J. J. Colloid Interface Sci. 1984, 97, 526.
on different substrates,5-7 including that of poly(ethylene oxide) (PEO) onto silica from carbon tetrachloride.8 Polymer displacement may also be achieved using a chemically different polymer that has a higher affinity for the surface, as described by Fleer.9 Only a small difference in χs is needed for one polymer to be completely displaced by another due to the cooperative effects of many segments.10 Several polymer/displacer/substrate systems have been studied7,11-13 using a variety of experimental methods. This paper builds on an earlier study,14 using NMR, of the displacement of PEO and PVP from silica using a low molecular weight displacer. In this paper, we extend the applications of this NMR technique to study both individual adsorption of PEO and PVP on silica as well as the polymeric displacement of one polymer (PEO) by another (PVP). The NMR technique used, that of solvent relaxation measurement, is an efficient, noninvasive method, which allows an indirect study of the polymer layer by measuring the overall dynamics of the solvent in the system. Theory NMR solvent relaxation measurements rely on the fact that mobile (liquid-like) and immobile (solid-like) protons have very different relaxation times.15 For small solvent molecules moving isotropically, the proton spin-spin (5) van der Beek, G. P.; Cohen Stuart, M. A.; Fleer, G. J.; Hofman, J. E. Langmuir 1989, 5, 1180. (6) van der Beek, G. P.; Cohen Stuart, M. A.; Fleer, G. J.; Hofman, J. E. Macromolecules 1991, 24, 6600. (7) van der Beek, G. P.; Cohen Stuart, M. A.; Fleer, G. J. Macromolecules 1991, 24, 3553. (8) Kawaguchi, M.; Hada, T.; Takahashi, A. Macromolecules 1989, 22, 4045. (9) Fleer, G. J. Colloid Surf., A 1995, 104, 271. (10) Frantz, P.; Leonhardt, D. C.; Granick, S. Macromolecules 1991, 24, 1868. (11) Kobayashi, K.; Dochi, A.; Yajima, H.; Endo, R. Bull. Chem. Soc. Jpn. 1993, 66, 1938. (12) Kawaguchi, M.; Sakai, A.; Takahashi, A. Macromolecules 1986, 19, 2952. (13) Botham, R. A.; Thies, C. J. Polym. Sci., Part C: Polym. Symp. 1970, No. 30, 369. (14) van der Beek, G. P.; Cohen Stuart, M. A.; Cosgrove, T. Langmuir 1991, 7, 327. (15) Halle, B.; Piculell, L. J. Chem. Soc., Faraday Trans. 1 1986, 82, 415.
10.1021/la0156863 CCC: $22.00 © 2002 American Chemical Society Published on Web 02/21/2002
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relaxation time (T2) is proportional to the inverse of the correlation time (τc), which describes the lifetime of a particular dynamic process (e.g., rotation of the molecule). Free, or bulk, solvent molecules are highly mobile and hence have relatively short values of τc. These relatively high mobilities lead to 1H spin-spin relaxation times that are typically of the order of several seconds. In contrast, when a solvent molecule is bound at an interface, its molecular motions become anisotropic and more restricted (i.e., τc becomes longer). This reduction in mobility leads to more efficient spin-spin relaxation processes in the sample, resulting in a shorter 1H T2 value, typically 0.06. This second transition point may be measurable at lower polymer concentrations, as the silica concentration at the transition point would be much smaller and flocculation would be less likely to occur. Conclusions Spin-spin solvent relaxation measurements have been shown to be an effective, yet simple, method for probing the adsorption of poly(ethylene oxide) and poly(vinyl pyrrolidone) on silica. An enhancement in the specific relaxation rate constant was seen when either polymer was adsorbed onto the silica surface and can be used as a simple technique to qualitatively determine the interfacial activity of macromolecules at a particle surface. In measurements of specific relaxation rate data as a function of silica concentration (for a given polymer concentration) a transition from a region of constant polymer train density (and excess polymer chains in solution) to a region of decreasing polymer train density (and excess silica surface area) was observed. Moreover, from measurements of this transition point as a function of the initial polymer concentration, the total amount of polymer adsorbed at the plateau on the train adsorption isotherm was determined. Such a determination could be made by studying only a single initial polymer concentration; however by increasing the number of polymer concentrations studied, the uncertainty in the determination can be reduced. Relaxation measurements of PEO-PVP-silica solutions have proven to be a valuable technique for investigating competitive and consecutive adsorption of polymer chains in these systems. In a displacement-adsorption experiment, we found that PVP totally displaced preadsorbed PEO from the silica surface, due to its higher adsorption energy. Only in the presence of excess silica, however, where the PVP had adsorbed to its maximum extent, could PEO chains absorb on the surface of the particles unoccupied by PVP and hence coexist at the particle surface. The generality of the solvent relaxation technique suggests that it should be equally valuable in measuring competitive displacement/adsorption and consecutive adsorption in a diverse range of mixed component systems. Furthermore, it can be seen that this method could also be used to study kinetics of these processes providing the
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rates of adsorption or exchange are slower than the time taken to make a measurement. From measurements performed on other dispersed systems, it is evident that the measurement time can be reduced to 1-2 min. It may even be possible to reduce this further using some form of in situ (within the magnet) mixing of the system.
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Acknowledgment. K.S.J. wishes to thank Imperial Chemical Industries (ICI) for funding of this work via a Senior Research Fellowship. A.N. acknowledges the EPSRC for part funding of this work. LA0156863
