2346
Langmuir 1991, 7, 2346-2352
Polymer Displacement and the Role of Impurities in Homopolymer Adsorption: A Surface Force Balance Study David A. Guzonast and Michael L. Hair* Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario, Canada L5K 2L1 Received March 7,1991.In Final Form:May 20, 1991 The displacement of adsorbed polystyrene from mica surfaces in cyclohexane by acetone or pyridine has been studied by use of the surface forces apparatus. Both displacers alter the force-distance curves at parts per million concentrations by displacing polymer segments from the surface and, at high concentrations, can completely desorb the polymer. Acetone and pyridine are also capable of partially blocking polystyrene adsorption onto mica surfaces at concentrations of 100 ppm. These observations suggest that trace quantities of strongly adsorbing impurities can strongly influence the amount of polymer adsorbed and the quantitative details of the surface forces when the segment-surface adsorption energy is low.
Introduction The stabilization of colloidal dispersions by the adsorption of polymers is of great technological importance as well as being of scientific interest. The direct measurement of the force between adsorbed polymer layers on mica surfaces using a surface force apparatus such as that developed by Israelachvili' has been the subject of a number of studies in recent yeam2-'0 Pioneering experiments were due to Klein, and a comprehensive review of the data on polymer adsorption measurements obtained by using the surface forces apparatus has recently been published by Tirrell and Patel.2 One system that has been characterized and for which there is at least a good qualitative understanding of the surface forces data is that of polystyrene adsorbed on mica, where the force measurements have been carried out in several single solvents Marra and both below and above the 0 Hair7have published a study of the effect of solvent quality on the adsorption of polystyrene (using mixed solvents), as well as a study of the effect of moderate (1376 ) amounts of acetone on the desorption of polystyrene from mica in the presence of the poor solvent heptanea8 In that work it was shown that acetone could act as a displacer of polystyrene from the mica surface. Another recent work which suggests that small changes in solvent quality may have important effects on polymer adsorption is the work of Hu et alPI6where the adsorption on mica of polystyrene from cyclopentane (Te= 19 OC) was studied. Two types of cyclopentane were used, both of which contained linear pentanes as impurities. These linear pentanes are nonsolvents for polystyrene and the observed force-distance curves were different when measured in cyclopentane containing 5 % vs 1%of linear pentanes. A second related paper emphasizes the role of multilayer ads~rption.~ Current address: Virginia Polytechnic Institute & State Univmity,VirginiaCenter for Coal and MineralsProcessing,146 Holden Hall, Blacksburg, VA 24061-0268. (1) Israelachvili, J. N.; Adame, G. J. Chem. SOC., Faraday Trans. 1 1978, 79, 975. (2) Patel, S.; Tirrell, M. Annu. Rev. Phys. Chem. 1989, 40. (3) Klein, J. J. Chem. SOC., Faraday Trans. 1 1983, 79,99. (4) Israelachvili,J. N.;Tirrell,M.;Klein, J.;Almog, Y. Macromolecules 1984,17,204. ( 5 ) Hu, H.; Aleten, J. V.; Granick, S. Langmuir 1989,5, 270. (6) Hu, H.; Granick, S. Macromolecules 1990,23, 613. (7) Marra, J.; Hair, M. L. Macromolecules 1988,21, 2349. (8) Marra, J.; Hair, M. L. Macromolecules 1988,21, 2356. (9) Johnson,H. E.;Hu, H.;Granick, 5.Macromolecules 1991,24,1859. (10) Marra, J.; Christenson, H. K. J. Phys. Chem. 1989,93,7180.
Q743-7463/91/2407-2346$02.50/0
Recently, Mama and Christensonlo have extended the earlier work of Marra and Hail.8 to study smaller concentrations of displacer molecules and have examined the effect of small amounts of water and acetone on the desorption of polystyrene (M, = 200 OOO) from mica in cyclohexane. They reported that 0.3% (3000 ppm) acetone had little effect on the amount of preadsorbedpolystyrene, whereas 3% acetone had a displacing effect, similar to that which had been observed when heptane was used as a solvent. The displacing action of the relatively large amounts of acetone reported by Marra and Christenson suggests that smaller amounts of a displacer may have a measurable effect on the force-distance curves. Water was also observed to have a displacing effect on the adsorbed polystyrene, but at the much lower concentration of about 50 ppm. The observation that 50 ppm water in the solvent could have a large effect on polymer adsorption as studied by the surface force balance is significant although not unanticipated in the literature. Israelachvili et al.' have attributed an observed difference in the amount of polystyrenes of two different molecular weights adsorbed on mica to the possible presence of water at the surface in one of the experiments. Recently, Watanabe et al." have attributed the failure of previous workers to obtain adsorption of polystyrene on mica from toluene to the presence of impurities. (But it should be pointed out that impurity arguments are double edged an "impurity" group in the polystyrene could also give rise to adsorption.) Clearly, the presence of any impurity which can compete with the weakly adsorbing polymer segments for surface sites will either reduce the adsorbed amount of the polymer or alter its configuration at the interface. Indeed, Watanabe et al." have shown that a strongly binding polymer can displace a more weakly interacting polymer from the surface of mica. Two possible roles of strongly adsorbing impurities in the solvent can be envisaged. First of all, small quantities of the impurity may preferentially adsorb on the surface and block surface sites which were previously available for the adsorption of the polymer segments. Experimentally, this situation can be realized by adding a controlled amount of a strongly adsorbing small molecule to the surface forces apparatus prior to the addition of the (11) Watanabe, H.; Patel, S.; Tirrell, M. In New Trends in Physics and Physical Chemistry of Polymers; Lee, Lieng-Huang, Ed: Plenum Press: New York, 1989.
0 1991 American Chemical Society
Langmuir, Vol. 7, No. 10,1991 2347
Polymer Displacement
polymer, thus effectively "precontaminating" the surfaces. Second, small polar molecules can act as displacers of previously adsorbed polymer by lowering the surfacesegment differential adsorption free energy,thus removing some of the polymer segments from the surface. A t the critical displacer concentration this causes total desorption of the polymer. Such polymer displacement has been discussed by Cohen-Stuart et a1.12who also experimentally measured the displacement of poly(vinylpyrro1idone)from silica by several molecules (water, l,ddioxane, N-methylpyrrolidine,N-ethylmorpholine, pyridine, dimethyl sulfoxide, and N-ethylpyrr~lidone).'~Displacement isotherms for five low molecular weight displacers (benzene, acetone, dioxane, triethylamine, and pyridine) have been reported by Kawaguchi et a1.,14who used UV spectroscopy to measure the adsorbed amount of polystyrene adsorbed on Aerosil 130 silica. To investigate these dual roles of impurities on homopolymer adsorption, we have chosen to work with the displacer molecules acetone and pyridine, which can also be considered to be models for strongly binding organic impurities. The homopolymer chosen was polystyrene, adsorbed on mica from the worse-than-8 solvent cyclohexane (Te= 34.5 "C). This polymer-solvent system was chosen because of the large body of data already in the literature. In this work, we report results of a study of the effect of small quantities of acetone and pyridine (concentrations of approximately 3 ppm up to 1%)on the force-distance curves of adsorbed polystyrene on smooth mica surfaces. In addition, the effect of "precontamination" of the mica surfaces by the addition of small amounts of acetone or pyridine on the subsequent adsorption of polystyrene on the mica surfaces will also be reported. Experimental Section Polystyrene (M,= 300000,M,/M, = 1.06) was purchased from Pressure Chemical Co. (Pittsburgh, PA) and purified by repeated precipitation from THF into methanol. All solvents used were obtained HPLC grade, dried over molecular sieves, and distilled prior to use. Solutions of polystyrene in cyclohexane were made up by heating the solvent slightly, since cyclohexane is worse than 8 solvent for polystyrene a t 25 "C. The surface forces apparatus used is that previously described in detail in the literature.7~8except for the addition of an imageintensified CCD video camera at the exit slit of the spectrometer. The camera output is fed to a video monitor and to one channel of a MATROX IMAGER AT video processing board in an IBM compatible AT computer. A mouse-driven, C language software package grabs the video input for signal averaging and provides instant output of the surface separation. The maximum distance resolution obtainable with this system is f0.2 nm. The force-distance curves are measured between two freshly cleaved, molecularly smooth mica surfaces which are silvered on one side with a -460 A thick silver layer and then glued silver side down on cylindrically curved glass disks. These disks are mounted in the surface forces apparatus in the crossed cylinder configuration. The forces measured are then normalized by the measured local radius of curvature R, to obtain the interaction energy E ( D ) per unit area between two flat parallel surfaces according to the Derjaguin approximation, F ( D ) / R= 2 ?r E(D).l5 The separation between the surfaces is controlled by a threestage mechanism consisting of a stepping motor for coarse (1 pm) positioning, a dc synchronousmotor for positioning to within 2 nm, and a piezoelectriccrystal which allows positioning to within 0.2 nm. The outputs of these two devices are also fed into an (12) Cohen-Stuart, M.; Fleer, G. J.; Scheutjens, J. M. H. M.J. Colloid Interfacial Sci. 1984, 97, 515. (13) Cohen-Stuart, M.; Fleer, G. J.; Scheutjens, J. M. H. M.J. Colloid Interfacial Sci. 1984, 97, 526. (14) Kawaguchi, M.; Chikazawa, M.; Takahashi, A. Macromolecules 1989,22,2195. (15) Deryaguin, B. V. Kolloid Zh. 1934,69, 155.
6
4 h
E
\
2
E
Y
2
K
\ U.
Inward Jump
0
-2
-
-
Outward J U I P n ,
a
,
a
,
I
,
I
A/D converter installed in the computer. The force is measured by the deflection of a double leaf spring of calibrated spring constant. The separation between the surfaces is measured interferometrically by the method known as fringes of equal chromatic order (FECO), and the distances can be measured to f0.2 nm. At the start of each experiment, the surfaces are first brought together in an atmosphere of dry nitrogen and this contact position is defined to be D = 0. The apparatus is then filled with dry cyclohexane and the forcedistance profile measured. The presence of an oscillatory profile as reported in the literature is taken as a sign of solvent and surface cleanliness. Once the condition of the surfaces has been checked in this manner, the polymer is added to the apparatusto give a workingconcentration of 10pg/mL and adsorption is allowed to proceed for 12 h with the surfaces held far (about 3 mm) apart, as in ref 8. The addition of the displacer to the apparatus for the impurity experiments was made as follows: A solution of the displacer in cyclohexanewas prepared with a concentration of approximately 1000 ppm, and small aliquots of this were then injected into the apparatus to give the desired final concentration. The apparatus was allowed to sit for several hours after the addition of the smallest aliquots to ensure sufficient time for diffusion. Unfortunately, one very interesting parameter was not experimentally accessable: the actual amount of the displacer adsorbed on the mica surface. Also, the probability of adsorption of the displacer onto the walls and internal parts of the surface forces apparatus should be noted. This would lower the actual concentration of displacer in the apparatus. Thus the displacer concentrations reported here should be considered as upper bounds. All experiments were carried out at a temperatue of 25 "C. Measurements were reproduced a t different contact points on the same surface as well as in experiments carried out on fresh mica sheets.
-
Results (a) Force-Distance Curves in a "Pure" System. Although surface force data for polystyrene adsorbed on mica in cyclohexane have previously been reported, it is worthwhile to first present our data for this system in order to set the stage for the discussion to follow. A composite force-distance curve from a number of experiments performed using polystyrene (M,= 300 000)adsorbed on mica from cyclohexane is plotted in Figure 1for high (saturation) surface coverage. The actual data points are omitted, but all of the experimentally measured force distance curves lie inside the shaded area. Ten independent experiments involving mica sheets cleaved at different times (and thus at potentially different ambient humid-
2348 Langmuir, Vol. 7,No. 10,1991 ities) were performed over a period of about 2 years, with each experiment being carried out under what were believed to be identical conditions in the surface forces lab. In every one of these experiments, the force-distance curves were reproducible over a number of cycles, in or out, and at a number of contact points. In each and every experiment, the qualitative shape of the force curve was the same, but variations in the quantitative details (e.g. distance at which surfaces jumped into contact, depth of the adhesive minimum) of the force-distance curve were observed. In each experiment the adsorbed amount was estimated by measuring the refractive index of thematerial between the mica surfaces as the surfaces were brought to the distance at which a hard wall repulsion was noted (-8mN/m). At thisdistance the polymer volume fraction could be calculated from the measured refractive index and this, coupled with the bulk density of polystyrene, enables an evaluation of the amount of adsorbed polymer per surface. The refractive index measurements gave a value for the polymer volume fraction of 0.8-0.85, and the adsorbed amount was found to range between 4 and 5 mg/m2 in all of the experiments. This value compares well with measurementsof the amount of the samepolymer adsorbed on mica from cyclohexane (4.7 mg/m2) recently obtained by using an infrared spectroscopic method.leThe value is similar to those reported by Israelachvili et aL4 (5.5 mg/m2 for M, = 6 X lo6 and 3.6 mg/m2 for M, = 9 X lo5) and Klein (6 mg/m2 for M, = 6 X 105) but is somewhat larger than the 1 mg/m2 reported for 200 OOO M, PS by Marra and Christenson.lo The latter authors attributed their smaller adsorbed amount to the lower molecular weight of their polystyrene, since the adsorbed amount has been shown to increase with polystyrene molecular weight for Mw up to 500 OOO. It is perhaps worth repeating at this point that all of these experiments refer to adsorption below 8 conditions, i.e., approaching conditions in which the polymer is “insoluble”. In Figure 1,an attractive force first becomes measurable at a distance of 30 f 5 nm. A little further in, at a distance of about 25 f 5 nm (within the shaded area labeled inward jump), the gradient of the attractive force becomes greater than the spring constant and the surfaces jump inward to a distance of 12 f 2 nm. This attractive force has been attributed by earlier authors to a combintionof an osmotic force due to the negative free energy of mixing of the polymer segments and a bridging contribution which is caused when the separation is small enough that polymer chains become adsorbed on both surfaces. The inward jump took up to 5 min to complete. This behavior has also been noted by previous workers and attributed to the hindered drainage of solvents from between the surfaces by the presence of the polymer. Further inward movement of the surfaces is possible when pressure is applied and a minimum distance of about 9.5 nm is achieved at a force of -8 mN/m. At this point the force curve rises very steeply and little further compression is possible under small loads. Upon separation of the surfaces, an adhesive minimum occurs at a distance of about 15 f 3 nm (illustrated by the area labeled ‘outward jump”). The location of this minimum corresponds to D = R,, which for 300 OOO M, polystyrene is 15.9 nm.17 The depth of the adhesive minimum was found to range from 0.6 to 1.6 mN/m, with values at both ends of the range being reproducible. An average value for the well depth is therefore 1.1 mN/m. Previously reported values for the depth of the adhesive minima for polystyrene in cyclo(16) Tripp, C. P.;Hair, M . L. Appl. Spectrose., in press. (17)Schmidt, M.;Burchard, W.Macromolecules 1981, 14, 210.
Cuzonas and Hair (A) No acetone (6) 3ppm acetone (c) 3oppm acatona (D) 1OOppm acetone
-2
t 0
\’
c-r 20
10
30
40
50
80
70
80
SEPARATION (nm)
Figure 2. Effect of acetone at 3,30, and 100 ppm on the force distance curves of polystyrene adsorbed on mica from cyclohexane.
hexane range from 0.5 to 1.2 mN/m and were seemingly independent of the molecular weight of the polymer used. (b)Addition of a Displacer to a System Containing Preadsorbed Polystyrene. The effects of the addition of trace (
