EFFECT OF SOLIDS CONCENTRATION ON POLYMER ADSORPTION AND CONFORMATION

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1 2 EFFECT OF SOLIDS CONCENTRATION ON POLYMER ADSORPTION AND CONFORMATION Tsung-yuan Chen,. Chidambaram Maltesh,2 and Ponisseril Somasundaranl IHerny Krumb School of Mines Columbia University New York, New York alco Chemical Co. Naperville, Illinois ABSTRACT The effect of solids concentration on the adsorption and conformation of polymers at solid/liquid interface was investigated using depletion adsorption and fluorescence techniques. The fluorescence of pyrene labeled polyethylene oxide at silica/water interface as a function of solids concentration revealed a surprisingly measurable change in polymer conformation as the solids content was increased. Adsorption density as well as coiling of the polymers was found to decrease with increase in solids concentration. These changes are correlated with the rheological behavior of the suspensions. 2. INTRODUCTION Concentrated particulate suspensions are widely used for coatings, paints, electronic pastes, clay and ceramic slurries in many industrial processes. To maintain the stability of these concentrated suspensions and to facilitate the processing, polymers are frequently used for controlling the suspension rheology.' Although polymers are currently used for dispersing concentrated suspensions, the role of polymer properties, particularly their confonnation and orientation, in stabilization of colloidal particles in concentrated slurries remains unclear. Recently, we have used fluorescence spectroscopy to estimate the confonnation and interaction of polymers at surfaces!--4 Infonnation obtained for polymer confonnation at solid/liquid interface has been correlated with the flocculation properties of particulate suspensions. In this paper, effects of solids concentration on polymer adsorption and confonnation on selected model systems are presented. Water Soluble Polymers, edited by Arnjad Plenum Press, New YOrlc:,

2 14 Tsung-yuan Chen et at 3. EXPERIMENTAL 3.1. Materials AKP50 alumina powder (Sumitomo Chemical, New York) of 0.2 mm diameter and 10.9 m2/g specific surface area was used for the adsorption study. Particle size distribution measured with Photon Correlation Spectroscopy (Brookhaven Instruments, BI-8000AT), shows a log normal distribution with an average particle size of 0.21 mm (Figure I). The alumina particles are nonporous as indicated by the scanning electron microscopic examination. Purity of the powders is specified by the manufacturer as 99.9%. For silica, Geltech powder (Geltech Tech., Florida) of 1.0 mm diameter and 4.2 m2/g specific surface area was used. This silica powder is manufactured using a sol-gel process and SEM examination showed nonporous spherical shape. Particle size distribution measured using the PCS method indicates the silica to be relatively monodisperse with an average diameter of 0.92 mm (Figure I). The density of the silica was measured to be 2.1 g/cmj and their purity is claimed to be 99.9% by the manufacturer. Polyacrylic acid polymer of molecular weight 90,000 and polyethylene oxide polymer of molecular weight were used for the adsorption study. Both polymers were purchased from Polysciences, Inc. and used as received. Polyethylene oxide polymer with pyrene probes attached on both ends was synthesized by Kumar and Aguilar of University of Florida. The labeling ratio of pyrene molecule on polyethylene oxide polymer was 2 to I. The attachment of pyrene probes to the polymer was confirmed by NMR measurements. Polyvinyl pyrrolidone polymer of molecular weight 40,000 supplied by Fluka Chemical Company was also used in the adsorption study. Sodium nitrate (Fisher Scientific) of ACS grade was used for ionic strength control. Water of resistivity 17 MW after distillation and deionization (Bamstead Nanopure System) processes was used for making all solutions os Diameter, om Figure 1. Particle size distribution of AKP50 alumina and Geltech silica powders measured by PhotOn Comlation Spectroscopy.

3 Effect of Solids Concentration on Polymer Adsorption and Conformation Methods Polymer Adsorption. Solutions of polymers at desired concentration were prepared in 0.03 M NaNO) solution and the solution ph adjusted. 10 ml of each polymer solution was then placed in a 20 ml glass vial and alumina or silica powder was added gradually into above glass vial while the polymer solution was being stirred. The suspensions were then stirred for 30 minutes, ph adjusted, and further stirred for 18 hours for equilibrium adsorption. After adsorption, samples were centrifuged at 2400 rpm for one hour and the supernatants were removed for determination of polymer concentration. Polymer concentration was determined using a DC-90 Total Organic Carbon Analyzer (Dohrmann Instrument). Adsorption density of polymers on the solid surface was calculated from the difference between the initial and residual polymer concentrations. For the study of polymer conformation at solid/liquid interface, mixtures of pyrene labeled polymer with unlabeled polymer were used Polymer Conformation. The conformation of pyrene labeled polyethylene oxide polymers in solution and at solid/liquid interface was monitored using the fluorescence spectrum of the pyrene probe using a LS-I fluorescence spectrometer (Photon Technology International Inc.). Coiling index, defined as the ratio of intensity of the excimer peak (480 om) to that of the monomer peak (373 nm) in the pyrene fluorescence spectrum, was used as a measure of the polymer conformation (Figure 2). When the polymer is stretched, the possibility of pyrene excimer formation is low and this yields a low excimer peak, and therefore the coiling index is low. In contrast, when the polymer is coiled, the possibility of excimer formation is high and the coiling index is higher. Thus, the coiling index of pyrene labeled polymers serves as an in-situ measure of the polymer conformation in the solution or at the solid/liquid interface. In all the experiments, suspensions with nearly no residual polymer are used for confonnation study to minimize the influence of fluorescence spectra of free polymers in the supernatant. 4. RESULTS AND DISCUSSION The effect of solids concentration as well as suspension ph on the adsorption of polyacrylic acid on alumina was first studied and the results obtained are shown in Fig- PolYIrB" Stret hed LAlwIe/lm Coiling Index = IeJIm PoIynMJr Coiled High k/iid FIgure 2. Schematic presentation of coiling index and polymer confomlation. Ie and 1m represent the fluorescence intensities of the excited monomers and excimers, respectively. The coiling index is low when the polymer is stretched and high when polymer is coiled.

4 Tsung-yuan Chen et al Me } om 8. {/I Figure 3. Effect of solids concentration on the adsorption of polyacrylic acid on alumina. ure 3. Interestingly, the solids content was found to markedly decrease the adsorption when it was increased from 0.5 volume percent to 27 volume percent both at ph 4 and ph 9. Surprisingly, the decrease in adsorption density occurred even in the low solids concentration region when interactions between particles are less than at high concentrations. The effect of suspension ph on the adsorption of polyacrylic acid on alumina existed in the entire concentration range tested with the adsorption higher at lower ph. Since the adsorption of polyacrylic acid on the alumina surface depends on the population and the ionization status of hydroxyl groups presented on the alumina surface, this suggests the electrostatic interaction between the polymer and alumina surface to playa dominant role in determining the adsorption behavior of polyacrylic acid polymers even when the solids content of system is high. The marked effect of solids concentration on polymer adsorption was also observed in the case of polyvinyl pyrrolidone-alumina system. When the solids concentration in the suspension increased from I volume percent to 20 volume percent, the adsorption density of polyvinyl pyrrolidone on alumina surface dropped a couple of orders of magnitude (Figure 4). Such huge difference in the polymer adsorption due to solids concentration change has not been reported previously to our knowledge. The effect of solids concentration on the adsorption of polyacrylic acid and polyvinyl pyrrolidone on alumina can best be illustrated by plotting log-log the polymer adsorption density against the available surface area per suspension volume (which will be termed as surface area loading later). Figure 5 shows an inverse correlation between polymer adsorption density and the surface area loading. The slope of this plot between the polymer adsorption density and surface solids loading is an indicator of the effect of solids concentration on polymer adsorption. For the polyvinyl pyrrolidone system the slope is , while for the polyacrylic acid system the slopes are and at ph 4 and ph 9, respectively with the higher slope suggesting the stronger dependence of adsorption on solids concentration. Similar effects of solids concentration on adsorption of polyethylene oxide on silica are illustrated in Figure 6. ph was found to have a measurable effect on polyethylene ox-

5 Effect of Solids Concentration on Polymer Adsorption and Conformation 27 Residual conc.. ppm Figure 4. Adsorption of polyvinyl pyrrolidone on alumina surface at low (1 vol. %) and high (20 vol. %) solids loadings. ide adsorption even though polyethylene oxide is a nonionic polymer. This is attributed to ph dependence of possible hydrogen bonding as a result of the ph dependent ionization of hydroxyl groups on silica. Solids concentration affects the adsorption of polyethylene oxide on silica at ph 4 at solids loading below 10 volume percent and at ph 9 at solids loading above 25 volume percent. The change in polymer adsorption behavior at high solids content is particularly 2 Swface area per volwne, m /mi Figure S. Correlation between the polymer adsorption density and surface area per suspension volume for polyacrylic acid and polyvinyl pyrrolidone/alurnina systems.

6 28 Tsung-yuan Chen et al Figure 6. Effects of solids concentration and suspension ph on polyethylene adsorption at silica/water interface. interesting since most industrial processes use such highly concentrated suspensions. To understand the reasons for the behavior, fluorescence experiments were done with mixtures of pyrene labeled polymers in unlabeled polymers along with their adsorption isotherms. In Figure 7, the coiling index of the adsorbed polymer is plotted against solids loading along with polymer adsorption density. There is a clear correlation between the polymer conformation change and the decrease in polymer adsorption density with the increase in solids concentration. The coiling index of adsorbed polymers drops from 0.75 at 5 volume percent of solids to 0.3 at 35 volume percent solids of concentration. This sug Solids loading, vol. % Figure 7. Clation between polymer confonnation and the polymer adsorption density.

7 Effect of Solids Concentration on Polymer Adsorption and Conformation 29 gests that polymers become stretched when the solids content is increased above 25 volume percent. Confonnational changes correlate well with this change in polymer adsorption density since a stretched polymer will occupy more area on the surface than a coiled polymer. This suggests that the change in polymer adsorption density results primarily from the stretching of polymers at silica surfaces rather than the change in the nature of polymer-surface interaction. The effect of polymer coverage on confonnation of adsorbed polymers at two different solids concentrations is shown in Figure 8. Interestingly, in the complete range of adsorption density, the coiling index of the adsorbed polymer at high solids loading is always lower than that at lower solids loading. The adsorbed polymer is thus more stretched at high solids loading even when the particle surface is not completely covered. The rheology data for silica suspension sheds some lights on the possible reasons for this effect. Zamrnan et. al.5 have reported the relative viscosity of silica suspensions of the same composition but without polymers to show an increase from the value predicted by Batchelor model6 when the solids concentration was above 25 volume percent. This suggests a stronger particle-particle interaction than that dictated by the Batchelor model at solids concentration above 25 volume percent. Such interaction can indeed cause the polymer to stretch on the surface. The observed increase in polymer stretching can be accounted for by the enhanced interaction between particles that are closer to each other at high solids loading. 5. SUMMARY Solids concentration exhibits a marked effect on the adsorption of polyacrylic acid, polyvinyl pyrrolidone, and polyethylene oxide polymers on solids. Adsorption of polyacrylic acid and polyvinyl pyrrolidone on alumina was found to decrease as the solids concentration was increased from 0.5 to 27 volume percent with a logarithmic correlation u j co 0.4 '0U Adsorption density, mg/m2 0.4 Figure 8. Effects of solids concentration and polymer adsorption density on conformation of adsorbed polyethylene oxide polymers.

8 30 Tsung-yuan Chen et ol of the adsorption with solids surface area loading. The effect of solid5 c,oncentration on adsorption was measurably higher for polyvinyl pyrrolidone than for polyacrylic acid. In the case of adsorption of polyethylene oxide on silica at high ph the adsorption density decreased at solids concentration above 25 volume percent. Investigation of the polymer conformation at silica/water interface using fluorescence spectroscopy indicates polymer to become more stretched as the solids loading was increased. The decrease in polymer adsorption density with solids can be accounted for by considering the stretching at high solids concentration. With more stretched polymers on the surface, the area occupied per polymer is larger; and hence the total number of polymer molecules on the surface and hence adsorption density is lower. It was also found that the conformation of the adsorbed polymer at low solids loading is different from that at high solids concentration even at low adsorption densities. This suggests strong influence of particle interaction on adsorbed polymer conformation even when the surface coverage is low. Correlation between the suspension rheology and the polymer adsorption density and conformation change suggests that increased interaction between particles at high solids to be a possible reason for the observed effect of solids loading on polymer conformation and adsorption density. ACKNOWLEDGMENTS The authors acknowledge the financial support of the Engineering Research Center (ERC) for Particle Science and Technology at the University of Florida, the National Science Foundation (NSF) grant #EEC , and the Industrial Partners of the ERC. REFERENCES I. Cesarano III J, and Aksayl A, "Processing of Highly Concentrated Aqueous a-alumina Suspensions Stabilized with Polyelectrolytesfl, J. Am. Ceram. Soc., 1988;71(12): Chanar P, Somasundaran P, Turro NJ, and Watemlan KC, "Excimer Fluorescence Determination of Solidliquid Interfacial Pyrene-Labeled Poly(acrylic acid) Confonnations, Langmuir, 1987;3: Somasundaran P and Krishnakumar S, "In-situ spectroscopic investigations of adsorbed surfactant and polymer layers in aqueous and nonaqueousystems", Colloids and Surfaces, 1994;93: Xiang Yu and Somasundaran P, "Role of Polymer Conformation in Interparticle-Bridging Dominated Flocculation", Journal of Colloid and Interface Science, 1996; 177: Zaman AA, Moudgil BM, Fricke Al, and EI-Shali H, "Rheological Behavior of Highly Concentrated Aqueous Silica Suspensions in the Presence of Sodium Nitrate and Polyethylene Oxide", J. Rheology, 1996;40(6): Batchelor OK, "The Effect of Brownian Motion on the Bulk Stress in a Suspension of Spherical Particles", J. Fluid Mech ;83: