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Dynamics af Adsorbed Polymers. 2. Thickness Relaxation of Poly(ethy1ene oxide) on Glass as a Function of Segmental Binding Energy M. A. Cohen Stuart* and H. Tamaif Department of Physical and Colloid Chemistry, Wageningen Agricultural University, P.O. Box 8038, 6700 EK Wageningen, The Netherlands Received January 28, 1988. I n Final Form: April 14, 1988 We have used an electrokinetic method to study the thickness relaxation of adsorbed layers of water-soluble polymers (poly(ethy1eneoxide), poly(vinylpyrro1idone)) on glass. Very pronounced differences were found between pure (monodisperse)and mixed (polydisperse)polymer samples as the effective segmental binding strength was varied by means of pH variations. The results are indicative of strong blocking of the surface and very limited relaxation at low pH (strong binding) but of rapid exchange and relaxation at high pH (weak binding).
Introduction Static properties of adsorbed polymers have been amply investigated. Dynamic properties, however, have received relatively little attention. In particular, more information on the rate a t which the molecules change their conformation when going from the solution to the adsorbed phase would be most welcome. This rearrangement process is presumably very complex. Theoretical work on the dynamics of concentrated solutions and melts has already some history, but we are not aware of publications dealing with rearrangement dynamics a t interfaces. Experimental work on reconformation processes appears equally scarce. Thermal motions within adsorbed polymers were studied by means of spin resonance techniques.'2 However, these studies dealt with systems which were presumably largely relaxed. Some indirect information on relevant time scales is obtained from studies on polymer-induced flocculation of colloid^.^ Effects of short exposure times were also noted by Onoda et al.4 when glass slides were dipped into a solution of cationic polymer, quickly withdrawn, and subsequently "decorated" with silica particles. Interesting patterns were found, but no explanation was given. Recently, we proposed a method to investigate reconformation rates in a more direct way. The method is based on the well-known reduction of the streaming potential by adsorbed neutral polymers5i6and consists of measuring the streaming potential of a single glass capillary as a function of time after a short exposure of the capillary wall to polymer solution. The method is described in detail in a preceding paper,' where also results for the adsorption of poly(vinylpyrro1idone) (PVP) onto glass are given. The information obtained is essentially on the hydrodynamic thickness, 6H, as a function of time, for layers in the very initial stage of their formation. As long as free polymer is present (during an injection of polymer into the flowing electrolyte solution), 6H increases rapidly, but as soon as the concentration is switched to zero, a relaxation is observed, which we ascribe to changes in conformation (flattening) of the polymer molecules. An example is given in Figure 1,where the relaxation process was followed for eight consecutive injections (at two different polymer concentrations). Among the variables studied were the time during which the capillary was exposed to polymer solution (injection time), the polymer concentration, and 'On leave of absence from Faculty of Engineering, Hiroshima University, Japan.
Table I. Polymer Samples samde M, MJM, PEO PD (Aldrich) 600000 polydisperse PEO SE-8 (Toyo soda) 85000 1.06 PEO SE-70(Toyo soda) 570000 1.10 PVP K-30 (BASF) 43 000 polydisperse
the molecular weight. A central result of that study was that the relaxation rates observed decreased when the amount of polymer initially deposited increased, either by a longer injection time, a higher polymer concentration, or a lower molecular weight (faster diffusion). In this paper, we intend to focus on another parameter which is expected to be very important, namely, the segmental adsorption energy. Intuitively, one expects that polymers with tightly bound segments relax extremely slowly, whereas loosely bound segments lead to rapid and extensive reconformation. It would thus seem interesting to vary the (effective) segment/surface interaction energy. In a series of earlier we have shown that this can, in principle, be achieved by using binary solvents, where one of the components (the "displacer") adsorbs preferentially over a polymer segment. We have shown that a relation can be established between the theoretical interaction parameter (x,) and the concentration of the displacer and that experimental results agree quite well with prediction^.^ We therefore had to choose a suitable polymer/solvent/displacer combination. On the basis of literature data1"-12we decided that poly(ethy1ene oxide) (PEO)was a suitable water-soluble polymer and for that polymer dilute solutions of OH- could be used as displacer. It has been found that pure PEO as well as block copolymers of (1) Hommel, H.; Legrand, A. P.; Le Courtier, J.; Desbarres, J. Eur. Polym. J. 1983, 16, 631. (2) Facchini, L.; Legrand, A. P. Macromolecules 1984, 17, 2405. (3) Gregory, J. Colloids Surf. 1988, 31, 237. (4) Onoda, G.; Somasundaran, P. J. Colloid Interface Sci. 1987,118, 169. (5) Cohen Stuart, A.; Waajen, F. H. W. H.; Dukhin, S. S. Colloid Polym. Sci. 1984, 262, 423. (6) Cohen Stuart, M. A.; Mulder, J. W . Colloids Surf. 1985, 15, 49. (7) Cohen Stuart, M. A.; Tamai, H. Macromolecules 1988, 21, 1863.
(8) Cohen Stuart, M. A.; Fleer, G. J.; Scheutjens, J. M. H. M. J. Colloid Interface Sci. 1984, 97, 515. (9) Cohen Stuart, M. A.; Fleer, G. J.; Scheutjens, J. M. H. M. J. Colloid Interface Sci. 1984, 97, 526. (10) Rubio, J.; Kitchener, J. A. J. Colloid Interface Sci. 1976,57,132. (11) Eremenko, B. V.; Sergienko, 2. A. Kolloidn. Zh. 1979, 41, 422. (12) Kokufuta, E.; Fujii, S.; Hirai, Y.; Nakamura, I. Polymer 1982,23, 452.
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!\
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Figure 1. Increase and relaxation of 8~ as observed for repeated injections. Polymer K-30 (fractionated); concentrations as indicated in the figure. Injection rate 0.05 mL s-l; injection time 10 s (from ref 7).
PEO with more hydrophic chains adsorb well on glass and on hydrophilic silica but that the adsorption decreases as soon as the pH becomes larger than 6. For pH 210.5 no more adsorption occurs.1&12The explanation of this result is that PEO segments adsorb through hydrogen bond formation with surface silanol groups, and these groups are gradually dissociated as the pH increases. Another question that remained to be answered was whether relaxation proceeds a t constant coverage or does some desorption take place. During the relaxation process, some molecules might drive others off the surface; especially small molecules would be desorbed relatively easily. We therefore suspected that the observed relaxations would be different for samples of different polydispersity. Experimental Section Several polydisperse polymer samples and two narrow PEO fractions were used; their properties and origin are listed in Table I. Analytical grade chemicals, as well as Millipore Super Q deionized water, were used throughout. A clean glass capillary of 21-cm length and 0.40-mm diameter was mounted between two containers, and NaCl solution of suitable pH and constant ionic strength M) was forced through the capillary under a constant pressure drop of 30 cmHg. Two Pt black electrodes were used to measure the streaming potential, which could be recorded as a function of time. On the high-pressure end of the capillary a Teflon injection piece was mounted through which polymer solution could be injected into the flowing electrolyte solution. A motor-driven buret was used to control the injection rate and duration. Under the conditions of the experiment the measured flow rate was 0.174 mL s-l. This implies an average liquid velocity of 1.4 m s-l and a Reynolds’ number of 560. The flow can thus be considered laminar, with an approximate entrance length (according to the Boussinesq expression) of about 15 mm. In agreement with these conclusionsis the finding that the streaming potential was always proportional to the pressure drop. The relation between electrokinetic data and hydrodynamicthickness has been studied by several a u t h o r ~ . ~These * ~ J references ~~~ show that given appropriate conditions 6~ can be obtained simply from the streaming potentials V,,o and V, of the bare and covered surface, respectively, by
(13)Cohen Stuart, M.A.; Cosgrove, T.; Vincent, B. Ado. Colloid Interface Sci. 1986, 24, 143. (14)Varoqui, R.Nouu. J. Chim. 1982, 6, 187. (15)De Gennes, P.-G. C. R. Acad. Sci. Paris 1983,197, 883. (16)Koopal, L. K.;Hlady, V.; Lyklema, J. J. Colloid Interface Sci. 1988, 121, 49.
Figure 2. Streaming potential V,,oof a bare glass capillary, as
a function of pH.
where K is the Debye screening length. The relation holds well provided K ~ 5.5 are larger for the polydisperse PEO than for the monodisperse PEO. Presumably, the exchange process leads to a surface population with a larger than average molecular weight. At the high pH end, only the largest molecules have a tendency to adsorb, since the energy per segment is now very small. However, these molecules experience large forces from the flow field and desorb eventually. The monodisperse sample does not show all these complications but a rather regular decrease in hH(0)(reflecting a decreasing “capture efficiency” of the surface) as well as a gradual decrease in BH as the molecules spread out. Both 6H(O) and & are zero a t pH 10.7, indicating the absence of a fraction of very large molecules, cf. Figure 4.
PH
Figure 6. Initial thickness 6,(0) and relaxed thickness &, as a function of pH, for the results with monodisperse PEO SE-70,
1
given in Figure 5. Experimental conditions: see Figure 3. 4-
6 ~ nm /
32-
01
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8
L
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PH Figure 7. Initial thickness 6,(0) and relaxed thickness 6, as a function of pH for a 1:l mixture of low molecular weight (PEO SE-8) and high molecular weight (PEO PD) polymer. Experimental conditions: see Figure 3. PEO we carried out a similar set of experiments with PVP. Since PVP adsorbs through essentially the same hydrogen-bonding mechanism, it is expected to respond in a qualitatively similar way to pH changes. The results are indeed very similar to those for polydisperse PEO: little or no relaxation and small values of
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Langmuir 1988,4, 1188-1193
In order to check whether the above picture makes sense, we carried out a similar set of experiments with a mixture of two samples of PEO (SE-8 and PD, respectively) in a 1:l weight ratio. All experimental conditions were exactly the same as those for Figures 4 and 6. If our explanation is correct, we should again expect a maximum in both aH(0)and as a function of pH and substantial relaxation in the neighborhood of the maximum. The result, presented in Figure 7, shows exactly that pattern; the maximum is quite large, and for pH >9 both curves drop very rapidly. There is still some doubt as to the importance of the flow field in the relaxation process.18 We therefore checked for a particular experiment (polydisperse, PVP, pH 8.3) whether the relaxation would proceed differently in the absence of flow. The result is shown in Figure 8 where we plot V,(t) while switching the pressure off and on several times. It seems from this result that relaxation continues even in the absence of flow, which supports the idea that the flow field is of little importance even when the polymer is relatively weakly bound. (18)Lee, J. J.; Fuller, G. G. J . Colloid Interface Sci. 1985, 103, 569.
Conclusions Thickness relaxations observed in the streaming potential experiment developed by us show very strong dependence on the polydispersity of the polymer sample, especially when the segments are strongly bound to the surface. This could be concluded from data sets obtained with polydisperse (PEO, PVP) and monodisperse (PEO) samples a t various pH values. The results are consistent with an essentially diffusion-controlled enrichment of the surface with small molecules a t low pH and a thermodynamically driven preference for the large molecules to adsorb at neutral and high pH. The role of the flow field seems to be minor, except a t pH >10 where the adsorption becomes extremely weak. Acknowledgment. We thank Prof. J. Lyklema for his encouraging discussions. We are indebted to A. J. van der Linde for his help with the streaming potential measurements. H. Tamai also wishes to thank the Wageningen Agricultural University for the financial support that enabled him to stay in the Netherlands. Registry No. PVP, 9003-39-8; PEO, 25322-68-3.
Interactions of Poly(dimethylsi1oxane) with Lewis Bases Sydney Ross* and Nguyet Nguyen Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York 12180-3590 Received March 14, 1988 The surface activity of poly(dimethylsi1oxane) in a synthetic ester lubricant is traced to an acid-base interaction between solute and solvent, which is enough to confer a degree of solubility on the poly(dimethylsiloxane). The acid character of poly(dimethylsi1oxane) is revealed by its interaction with a basic second solute, namely, N-phenyl-1-naphthylamine; this interaction further increases the solubility of the poly(dimethylsi1oxane) in the lubricant and so reduces its surface activity. The relative measures of the surface activity of the polymer are provided by the concentration required to reach the 50% point of transition between the regimes of Rybczynski-Hadamardand of Stokes in the rate of rise of a bubble in the solution. Shifts of the NMR spectra when poly(dimethylsi1oxane) interacts with a “soft”base (triethylamine) and with a ”hard” base (ethyl acetate) confirm that poly(dimethylsi1oxane) is a Lewis acid and a hard acid at that. The silicon atom is the source of its hard acid character because of ita small size, slight polarizability, and empty d-orbitals, which can accept electrons from a base. Introduction The problem of the foaming of aircraft lubricants was a serious one for the defenders during the Battle of Britain in the second world war. Before takeoff, the pilots of intercepter planes (Spitfires) would pump gasoline into the cold oil so as to reduce its viscosity in order to make the motors start without delay; only, on rising to a high altitude, to suffer a loss of lubricant through the breather holes, because of the expansion of the volume of the oil by entrapment of vaporized gasoline. The technical problem thus posed was how to separate the entrained vapor from the liquid phase by immediate rupture of the foam lamellae. Various foam inhibitors were tried, but none proved as effective as either fluorinated hydrocarbons or poly(dimethylsi1oxane). The properties that make these compounds so effective are low volatility, low surface tension, and insolubility in hydrocarbon oil. These properties give the foam inhibitor a positive surface-entering coefficient’ with respect to the oil medium. A positive 0743-7463/88/2404-1188$01.50/0
surface-entering coefficient corresponds exactly to a negative spreading coefficient of the medium with respect to the insoluble droplet. In practice that results in a mechanical action, namely, the retraction of the oil in the foam lamella from the insoluble droplet situated on its surface. The process is called dewetting. The force of its mechanical action or, as some conceive it, the bridging of the liquid lamella by a droplet whose adhesion to the medium is too weak to hold the composite structure together, causes it to collapse. The problem arose anew when synthetic esters, such as trimethylolpropane heptanoate, were used as lubricants in the place of hydrocarbon oils. The presence of poly(dimethylsiloxane), instead of removing the difficulty, seemed to augment it. Work done in this laboratory under contract with the U.S. Air Force found that poly(di(1) Robinson, J. V.; Woods, W. W. J . SOC.Chem. Ind., London 1948, 67, 361.
0 1988 American Chemical Society
