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Langmuir 1995,11, 898-904
Adsorption Fractionation Studies by Size-Resolved Pulsed-Gradient Spin-Echo-NMR P. C. Griffiths and P. Stilbs” Physical Chemistry, Royal Institute of Technology Stockholm S 100 44 Received August 16,1994. I n Final Form: October 31, 1994@ In this paper the ability of pulsed-gradient spin-echo nuclear magnetic resonance (PGSE-NMR) to study adsorption fractionation is demonstrated. A n inverse Laplace transform applied to the attenuation functionyields a representation ofthe distribution of self-diffusioncoefficients. The self-diffusioncoefficient distributions of two model systems consisting of bimodal poly(ethy1eneoxide) (PEO) solutions have been examined in the absence and in the presence of silica particles. The results show that high molecular weight species are preferentiallyadsorbed over smaller ones as predicted by theory confirmingthe feasibility of the PGSE-NMR technique to study such distributions.
Introduction The role of polymers in today’s society is offundamental importance with applications ranging from sewage treatment to adhesion or 1ubrication.l The purpose of the polymer is to modify, in a controlled manner, some useful property of the dispersion such as viscosity, surface tension, or, most commonly, the stability. The ability of the polymer to perform such a variety of tasks arises through its propensity to aggregate at the interface between two immiscible phases like the solifliquid interface. The adsorption process depends on many parameters, notably the polymer molecular weight and concentration and the relative affinities of the polymer segment for the surface and solvent.2 Upon approach to such a n interface, the conformation of the polymer molecule is distorted, which entails a loss of entropy. This loss must be compensated for by a gain in enthalpy produced by (favorable) interactions between the polymer segments and the surface. The conformation of this “adsorbed polymer layer” is, thus, a balance of these competing effects. At low coverages, the polymer molecules adopt a flat conformation to maximize the number of segmentsurface contacts. At higher coverages, the polymer molecules rearrange to form a more extended layer, permitting more polymer molecules to adsorb onto the surface. The adsorption isotherm is used to describe the relationship between the amount of polymer bound to the surface per unit area, called the adsorbed amount, r,with units of mg/mz, and the concentration of polymer remaining in solution at equilibrium. Further parameters invoked to describe the conformation of the adsorbed polymer layer include measures of the layer thickness: the hydrodynamic thickness, d h , and the root-mean-square thickness, d,,,. The adsorbed amount and these various thicknesses have been shown to be dependent on molecular eight.^
For the case of homopolymer adsorption, the presence of a distribution in the polymer molecular weight often leads to anomalous behavior. In particular, the adsorption isotherm becomes much more rounded. The entropy of Abstract published in Advance ACS Abstracts, February 1, 1995. (1)See for instance: Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman and Hall: New York, 1993. (2) See for instance: Ploehn, H. J.; Russell, W. B. Macromolecules 1986, 22, 266. (3) Cohen Stuart, M. A.; Waajen, F. H. W. H.; Cosgrove, T.; Vincent, B.; Crowley, T. L. Macromolecules 1984,17, 1825. Lafuma, F.; Wong, K.; Cabane, B. J. Colloid Interface Sci. 1991, 143, 9. @
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mixing in solution is a strongly decreasing function of increasing chain length and thus, the smaller polymer molecules are preferentially held in solution. The adsorption process is, therefore, in favor of the larger molecule^,^-^ a process known as “adsorption fractionation”. In order to study adsorption fractionation, a technique is required that permits the molecular weight distribution to be probed with sufficient molecular weight sensitivity while not overemphasizing either the high or low molecular weight fractions. For example, light scattering has long be used to study molecular size, but since the scattered light depends on the square of the scattering volume, the larger molecules scatter disproportionately more light and, thus, are more heavily weighted in the final analysis.’ Ideally, a technique that measures a property that is linearly dependent on molecular weight is required. One such technique is gel permeation chromatography (GPC), which is routinely used for the measurement of such molecular weight distributions,8 but the technique is somewhat dependent on the nature of the polymer and the solvent. The biggest problem in adsorption studies of this nature is the need to “disassemble” the adsorption system: the adsorbed polymer must be separated from the polymer in solution by centrifugation of the particles plus adsorbed polymer. Clearly there is a n advantage to be gained by using a noninvasive, nondestructive, in-situ technique. Nuclear magnetic resonance techniques have these advantages, and perhaps most importantly, they are also chemically selective. A further bonus is that these sorts of experiments take only a few hours. The measurement of molecular weight dependent self-diffusion coefficients by the PGSE technique, therefore, seems to be a candidate to study this phenomena. The suggestion and experimental test of “size-resolved NMR”,gJObased on the PGSE-NMR technique for measuring self-diffusion have recently been extended by the work of Johnson et aZ.ll A 2D-NMR self-diffusion experi(4) Kolthoff, J. M.; Ray, L. N. J. Colloid Interface Sci. 1970,32,349. (5) Roe, R. J. Adhesion and Adsorption ofPolymers; Lee, L. H., Ed.;
Polymer Science and Technology, Part 12B; Plenum: New York, 1980. (6) Scheutjens, J. M. H. M.; Fleer, G. J. The Effect of Polymers on Dispersion Properties; Tadros, Th. F., Ed,; Academic Press: London, 1982. (7) Griffiths, P. C. Ph.D. Thesis; University of Bristol, 1991. ( 8 )Fleer, G. J.;Scheutjens, J. H. M. H. Coagulation andFlocculation: Theory and Applications; Dobias, B., Ed.; Surfactant Science Series, Vol. 47; Marcel Dekker: New York, 1993. (9) Stilbs, P. Prog. NMR Spectrosc. 1987, 87, 1. (10)Stilbs, P. Anal. Chem. 1981, 53, 2135. (11)Morris, K. F.; Johnson, C. S., Jr. J . Am. Chem. SOC. 1992,114, 3139.
0 1995 American Chemical Society
Fractionation Studies by PGSE-NMR
Langmuir, Vol. 11, No. 3, 1995 899
ment is employed, called DOSY,the result of which is a conventional proton spectrum in one dimension, correlated with a distribution of self-diffusion coefficients in the second dimension. Initial studies on mixtures of surfactants, solvents, and micelles have shown the validity of the technique. However, in those studies and unlike the work described here, the spread of the self-diffusion coefficients is much wider. In this paper, the potential of this technique for studying molecular weight distributions is explored. The strategy employed is to measure the distribution of the self-diffusion coefficient of a model system before exposure to an adsorbing surface which will perturb the distribution in a controlled manner. The feasibility of this technique is then judged by its ability to correctly detect the perturbation in the self-diffusion coefficient distribution, as predicted by theory.
Experimental Section Samples. The low molecular weight poly(ethy1ene oxide) (PEO) samples (M,/M, < 1.1)and the poly(ethy1ene glycol) (PEG 600) (Mw/Mn= 2.0) sample were obtained from Fluka Chemicals and used with no further purification, while the PEOM, 400 000 sample (M,/Mn < 1.05)was prepared in-house. Deuterium oxide ('99.8% DzO) was obtained from Isotec, Inc. Ludox HS40, nominal radius 7 nm, was obtained from DuPont as a concentrated aqueous dispersion. The PEO solutions were prepared by dissolving the required masses of each polymer molecular weight into DzO in 5 mL vials. After allowing 48 h for dissolution and equilibration, the samples were then split into two: t o one half was added the concentrated silica dispersion, while to the other half was added the same amount of solvent. For the two highest silica volume fraction dispersions, the silica additions were consecutive. The adsorption samples were allowed to equilibrate for a further 24 h before measurement. NMRMeasurements. For isotropic Brownian motion,12the self-diffusion Coefficient, D,, is extracted by fitting to eq 1 the measured peak integral, A(d), as a function offield gradient pulse duration, 6, intensity, G, and separation, A:
A(6) = A(o) exp(-ZdT,) exp[-y2d2G2(A- d/3)D,] (1) where y is magnetogyric ratio and t is the rf pulse interval. The
A(o) term is determined by the number of protons in the sample and the first exponential term is the attenuation from spin-spin relaxation during the duration, 2t, of the experiment. When both the duration and intensity of the field gradients have been varied in a single experiment (at a constant value of t),eq 1 is often rewritten as
A(6) = A'(o) exp[-kD,]
(2)
where k = y2d2G2(A- 13/31. Hence, a plot of A(6) us k should be exponential with a time constant equal to the self-diffusion coefficient. The PGSE measurements were performed on a JEOL FXlOO employing a Hahn echo sequence and a Bruker MSL2OO employing the LED sequence.13 In the original Hahn echo sequence, the lower limit of the measurable self-diffusion coefficient is determined both by the magnitude of the gradient pulses and by the spin-spin relaxation time ofthe sample, since slow diffusion is usually accompanied by short spin-spin relaxation times. The LED sequence permits the measurement of slower diffusive motions by allowing A to be increased without the concomitant restrictions through spin-spin relaxation considerations i.e. t
