Langmuir 1991, 7, 43-45
43
Adlayer Thickness of Two Cationic Polyacrylamides Adsorbed onto Polystyrene Latices Rein Aksberg,l Maryann Einarson, John B e r g , * * 2 and Lars Odbergl Department of Chemical Engineering, BF-IO, University of Washington, Seattle, Washington 98195 Received March 27, 1990. I n Final Form: J u n e 21, 1990 The adsorption of two cationic, high molecular weight copolymers of acrylamide and [ (N,N,N-trimethylammonio)propyl]methacrylamide on two different, negatively charged polystyrene latices is investigated by photon correlation spectroscopy and conductometry. The results show that the reconformation of the investigated polymers on those latices is a more rapid process compared to the reconformation of polymers on cellulosic fibers. This result is explained by differences in surface characteristics of the materials.
Introduction Synthetic polyelectrolytes have long been used as flocculant, e.g., as a wet-end additive in the papermaking process. The interaction of these polymers with cellulosic fibers has been summarized by WBgberg and ddberg,3 while a more general overview of polymer adsorption has been published by Cohen-Stuart et al.4 The kinetics of polymer adsorption is of particular importance in the papermaking process because of the short contact times available between fibers and polymers, but relatively few studies have been made.+12 The existence of significant numbers of extended loops and tails is believed to be a prerequisite for bridging f l o c ~ u l a t i o n . Therefore ~~ the conformation of polymer chains on the surface as well as the extent of adsorption is important. Measurements of released counterions10J4J5 suggest that the conformation depends on the amount of polymer adsorbed, since the stoichiometry, i.e. the amount of released counterions per adsorbed charged group, depends on the adsorption level. An increase of stoichiometry with time has also been observed for charged high molecular weight polymers adsorbed onto cellulosic fibers,1°J2 suggesting a reconformation of the polymer chains with time. On a nonporous surface a reconformation would imply that an initially coiled polymer is spread out on the surface into a flatter conformation. On a porous surface the polymer chains might also penetrate into the pores. In both cases the adlayer thickness might be influenced. The purpose of this paper is to determine if a change of adlayer thickness with time occurs and how the adlayer thickness depends on polymer dosage. (1) Present address: Swedish Pulp and Paper Research Institute, Box 5604,5-114 86 Stockholm, Sweden. (2) To whom correspondence should be addressed. (3) Wigberg, L.; bdberg, L. Nord. Pulp Pap. Res. J. 1989,2, 135. (4) Cohen Stuart, G.; Cosgove, T.;Vincent, B. Adv. Colloid Interface Sci. 1986, 24, 143. (5) Lindetrbm,T.;Sbremark,C. J . Colloid Interface Sci. 1976,55,305. (6) Kindler. W. A,: Swanson. J. W. J . Polvm. Sci. 1971.. 9.. 853. (7) Goosens, J. W.'S.; Luner; P. Tappi 1976,59, 89. (8) Wigsten, A. L.; Stratton, R. A. In Polymer Adsorption and Dispersion Stability; ACS Symposium Series 240; Goddard, E. D., Vincent B., Ede.; American Chemical Society: Washington, DC, 1984, p 429. (9) Arsov, L.; Grcev, T.;Cvetkovska, M.; Petrov, G. Bull. SOC.Chim., Begrad 1983, 48, 417. (10) Wigberg, L.; bdberg, L.; LindstrBm, T.; Aksberg, R. J. Colloid Interface Sci. 1988, 123, 287. (11) Falk, M.; bdberg, L.; Wigberg, L.; Risinger, G. Colloids Surf. 1989,40, 115. (12) Aksberg, R.; bdberg, L. Submitted to Nord. Pulp Pap. Res. J . (13) Hunter, R. J. Foundations of Colloid Science; Oxford University Prees: Oxford, 1987; Vol 1, Chapter 8. (14) Winter, L.; Wigberg, L.; bdberg, L.; Lindstrbm, T. J. Colloid Interface Sci. 1986, 111, 537. (15) Wigberg, L.; Winter, L.; odberg, L.; LindstrBm, T. Colloids Surf. 1987, 27, 163.
Quasi-elastic light scattering or photon correlation spectroscopy (PCS)is a standard technique for measurement of the thickness of adsorbed polymer l a y e r ~ . ~It~ J ~ is, however, impossible to perform such measurements with polymers adsorbed to cellulosic fibers, since a monodisperse fraction of small fiber fragments, suitable for PCS measurements, is very difficult to prepare. Instead, two polystyrene latices were chosen as model substances. Since the surfaces of these latices lack the porosity of cellulosic fibers, the experiments should indicate whether reconformation of high molecular weight polyelectrolyte is a phenomenon general to charged surfaces and should permit identification of differences due to morphological factors, such as the porous structure of cellulosic fibers. The necessary comparison with stoichiometry measurements for fibers can be achieved if the amount of released counterions as a function of time is measured, e.g., by cond~ctometry.'~
Experimental Section Materials. The polymers used in these experiments were two cationic copolymers of acrylamide and [ (N,NJV-trimethylammonio)propyl]methacrylamide,kindly supplied by Allied Colloids, Bradford, U.K. Their charge densities, as determined by polyelectrolyte titration,'* were 1.4 mequiv/g for C-PAM4 and 2.6 mequiv/g for C-PAMG. The molecular weight of both polymers was determined to be 4 x 106 from intrinsic viscosity in 1 M NaCP as reported in ref 20. The polymers were received as powders, with chloride as counterion, and used without further purification. Since polyacrylamides are known to change conformation upon storage in salt-free solutions,2l the polymer solutions were stored 1 week before the experiments were performed. In the polyelectrolyte titration, an anionic polymer,potassium poly(vinylsu1fonate)(KPVS) from Wako Pure Chemicals, Ltd., Japan, was used together with a cationic indicator, orthotoluidine blue (OTB). Two negatively charged, monodisperse latices, purchased from Interfacial Dynamics Corp.,Portland, OR, were used. According to the supplier, Latex S, a sulfate latex, had a charge density of 4.03pC/cm2,corresponding to an area per charged group of 390 AZ,and a mean diameter of 0.486 pm f 1.0% as determined from transmission electron microscopy. Latex C, a carboxyl latex, had a charge density of 7.95 WC/cm2, corresponding to 202 A ' per charged group, and a mean diameter of 0.401M m 1.85 %. The
*
(16) Pecora, R., Ed. Dynamic Light Scattering; Plenum: New York, 1985. (17)Baker, J. A.; Berg, J. C. Langmuir 1988,4, 1055. (18) Horn, D. Prog. Colloid Polym. Sci. 1978,65, 251. (19) Mabire, F.; Audebert, R.; Quivoron, C. Polymer 1984,261317. (20) Einarson,M.; Aksberg, R.; bdberg, L.; Berg, J. C. To be submitted for publication. (21) Kulicke, W.-M.; Kniewske, R. Makromol. Chem. 1981,182,2277.
0743-7463/91/2407-0043$02.50/00 1991 American Chemical Society
Aksberg et al.
44 Langmuir, Vol. 7, No. I, 1991 Table I. Plateau Levels for Adsorption of C-PAM Polymers to Latices adsorption, mg/g polymer Latex S Latex C C-PAM4 3.8 10.8 C-PAM6 2.2 5.4 latices were received suspended in distilled water without any additives at concentrations of 9.1 and 4.1 wt %, respectively. The water used in the experiments was twice distilled after deionization and filtered through a 0.20-pm filter prior to use. Methods. The adsorption of polyelectrolyte on latex was determined from experimentsaccordingto a standard procedure22 where different amounts of polyelectrolyte are added to latex suspensions and the equilibrium concentration of polymer is measured by polyelectrolyte titration.18 In order to get measurable polymer concentrations,the concentration of latex was kept at 5 g/L in these measurements. After the addition of polymer, the suspensions were stirred gently for 24 h before the latex was separated from the polymer solution with a 0.30-pm filter. It was checked that the amount of polymer adsorbed in the filter membrane was negligible. Photon correlation spectroscopy (PCS) was used to measure the thickness of the adsorbed polymer layers on the latex particles as a function of polymer concentration and time after polymer addition. A photomultiplier, Model BI-DS,a 72-channel digital correlator, Model BI-2030, and computer software from Brookhaven Instruments Corp., Holtsville, NY, was used together with a SpectraPhysics Stabilite He-Ne laser, Model 124B. The equipment was held at a constant temperature of 24 O C , and the measuring angle fixed to 90°. The measurementswere performed at a wavelength of 632.8 nm. Equal volumesof latex suspensions and polymer solution were mixed in a test tube, and PCS measurements were carried out after different mixing times. The measurements were carried out at a latex concentration of 10 mg/L. The polymer concentration in the test solutions ranged from 1to 20 mg/L. Measured hydrodynamic radii were independent of latex particle concentration in the range investigated. The viscosities of the different polymer solutions, necessary for calculation of hydrodynamic radii from diffusion coefficients obtained by PCS, were measured with an Ostwald capillary viscometer. Conductivity measurements14 were performed to investigate the kinetics of the release of counterions. Equalvolumes of latex suspensions and polymer solution were mixed while conductivity was measured as a function of time. The concentrations of latex and polymer after mixing were 2.0 g/L and 100 mg/L, respectively. These higher concentration levelswere necessary to obtain a measurable amount of counterions. A conductivity meter, Model CDM 83 Radiometer A/S, Copenhagen, Denmark, was used. The suspensions were continuously stirred in a nitrogen atmosphere to avoid absorption of carbon dioxide. No pH adjustments were made except in the experiments with Latex C, where a pH adjustment to 7 was necessary to ensure total ionization of the carboxyl groups. The pH of the distilled water was 5.2. Results and Discussion Thickness of Adsorbed Polymer Layers. It has been shown that the adsorption of charged polymers on latex surfaces is of the high affinity type. The ratio of adsorbed charges to surface charges is close to unity a t low salt concentration^.^^ The adsorption of both C-PAM polymers t o the latices was measured, and the plateau levels determined as given in Table I. The charge ratios are approximately 1. Hydrodynamic radii, Rh,m, from PCS measurements are presented in Figures 1 and 2. At low concentrations of C-PAM6, 0-2.5 mg/L, the adlayer (22) Wigberg, L.; Winter, L.; Lindatriim, T. In Papermaking Raw Materiakr, Transactions oftheEight FundamentalResearch Symposium held at Oxford;Punton, V., Ed.; Mech. Eng. Publ. Ltd.: London, U.K., 1985 D 917.
(23j Tanaka, H.; Odberg, L.; Wigberg, L.; Lindetr6m, T. J. Colloid Interface Scr. 1990, 134, 219.
,
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Y
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-;
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. C-PAM4 C-DAM6
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Figure 1. Hydrodynamic radii, Rh-, for Latex C with adsorbed polymers. Rh,,,, is shown as a function of dosage of C-PAM4 and C-PAMG.
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Figure 2. Hydrodynamic radii, Rh,,,,, for Latex S with adsorbed polymers. Rh,,,, is shown as a function of dosage of C-PAM4 and C-PAMG. thickness increased with concentration for both latices. In the high polymer concentration regime, 2.5-20 mg/L, however, the adlayer thickness decreased with increasing concentration. This behavior is observed because in the plateau region, an increase in polymer concentration does not significantly increase the amount of adsorbed polymer, but the increased ionic strength of the solution results in a decrease of the adlayer thickness (“polyelectrolyte effect”). For C-PAM4 adsorbed on Latex C, the adlayer thicknesses reached a maximum at concentrations between 2.5 and 5.0 mg/L. An increase was observed throughout the investigated concentration range on Latex S, leveling off a t concentrations above 10 mg/L. The differences in behavior between C-PAM6 and C-PAM4 can be explained by the lower charge density of the latter polymer, which gives a conformation less dependent on the ionic strength of the suspension. For both latices, C-PAM4 gave thicker adlayers than C-PAM6. Since the amount of adsorbed polyelectrolyte depends on the number of available charged groups on the polymer,23the thicker adlayer is a result of the higher adsorption level of the low charge density polymer, C-PAM4. A comparison of the adlayers on different latices shows that very little difference in adlayer thickness is observed. The maximum adlayer thicknesses measured were approximately 50 and 140 nm for C-PAM6 and C-PAM4, respectively. The surface charge densities of the investigated latices are of the same order of magnitude as the number of charged groups divided by the projected area of a polymer coil, where coil diameters are assumed t o be 0.3 pm, i.e. the number obtained from classical light scattering.20 These observations show that the confirmation of the polymers on the surfaces are rather flat and that the polymers do not have t o spread out over a large area to reach the charged groups on the latex surface. Kinetic Effects. Two time effects may occur. First, the adlayer thickness can decrease due to reconformation
Polyacrylamides Adsorbed onto Polystyrene Latices
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Langmuir, Vol. 7,No. 1, 1991 45
1-
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Time after polymer addition (s) Figure 3. Hydrodynamic radii, &a, as function of time after polymer addition for C-PAM4 and C-PAM6 on Latex S. The dosages of polymer were 20 mg/L and the concentration of latex was 10 mg/L. The decrease of indicates a reconformation of the polyelectrolyte.
of the adlayer, resulting in a decrease in Rh,m. Second, if flocculation occurs, as well as the polydispersity of the suspension, will increase. The two effects should thus be distinguishable. Flocculation was observed in experiments with Latex S for both polymers when the suspensions were mixed vigorously after polymer addition. For C-PAM4, Rh,m, increased to 3 pm within 30 min. The latex flocculated much slower in the experiment with C-PAMG, where Rh,m, increased to 1.7 pm within 24 h. The polydispersities of the samples also notably increased. Since vigorous mixing resulted in flocculation, further experiments were carried out gently mixing the suspensions in test tubes as described above. On Latex S, no change of Rh,m was observed with time after polymer addition at dosages of 10 mg/L and less for both polymers. This indicates that the equilibrium conformation is reached rapidly. At the highest dosage, 20 mg/L, on the other hand, a decrease OfRh,mWaSobserved a t short times as shown in Figure 3. The slower rate of reconformation a t high dosages can be expiained by intersegmental crowding restraining polymer reconformation. On the more highly charged surface of Latex C, no decrease of Rh,m with time was observed at any of the investigated dosages. The polymers appear to have reached their final conformation in a very short time after contact with the latex. There are more charged groups on the surface of Latex C than on Latex S. Thus, polymer chains have to spread to a lesser extent to reach their final conformation on the surface of Latex C than on Latex S. If reconformation of a polymer can no longer occur on the surface, a porous structure can provide further reconformation by the slower process of r e p t a t i ~ nwhere , ~ ~ the polymer chains penetrate into the porous structure, e.g., those of cellulosic fibers. A slow increase in adsorption stoichiometry has been observed when a high molecular mass polyelectrolyte is adsorbed to the surface of cellulosic fibers.10120 Release of Counterions. A direct comparison between results from PCS measurements and conductivity measurements is difficult due to the different experimental conditions. The effect of the release of counterions, observed as an increase in conductivity, is counteracted by the flocculation of the latices. In the experiments with Latex C only a rapid decrease of conductivity with time was observed for both polymers. For Latex S, however, Figure 4 shows an increase in conductivity between 0 and 30 s for C-PAM4 and no effect for C-PAMG. The increase of conductivity is an indication of increasing adsorption (24) de Gennes, P.4. Scaling Concepts in Polymer Physics; Cornel1 University Press: Ithaca and London, 1979; p 229.
01
0
10
20
Time after polymer addition (SI Figure 4. Conductivity as function of time after polymer addition for C-PAM4 and C-PAM6 on Latex S. The dosages of polymer were 2.0 g/L and the concentration of latex was 100 mg/L. stoichiometry, since the initial adsorption of polyelectrolyte in these systems is a very rapid process.'O The decrease of conductivity with time observed in the experiments with Latex C can be explained by the higher charge density of this latex, contributing to the conductivity to a larger extent than Latex S. Consequently, the decrease of conductivity due to flocculation dominates the increase of conductivity due to the release of counterions. Conductivity measurements20on polystyrene latex, bentonite, and carboxymethylated cellulosic fibers show that for fibers the release of counterions is much slower. This indicates again that the structure of a surface on which a polyelectrolyte adsorbs influences the rate of reconformation of the polymer chains. Conclusions The measurements of adsorbed polymer layer thicknesses presented in this paper show that charged C-PAM type polymers adsorbed onto polystyrene latices reach their final conformation after a short time, i.e., a few seconds in most cases. A decrease in adlayer thicknesses as a function of time was observed only a t high polymer dosages. These observations, together with earlier investigations of polyelectrolyte adsorption to cellulosic fibers, indicate that the reconformation of charged polymers is affected not only by surface charge density and polymer characteristics but also by the morphology, mainly porosity, of the surface. The following model is suggested. Initially a high molecular weight polyelectrolyte is adsorbed in a coiled conformation on a charged surface. The polymer will quickly reconform to a flatter conformation occupying in some cases a larger area than that of the coil initially. The rate of this initial reconformation is affected by the degree of surface saturation and the surface charge density. A high surface saturation will decrease the rate of reconformation. An increase in surface charge density will increase the rate of reconformation also by reducing the necessity for the polymer to spread over the surface. After the quick initial reconformation a slower reconformation of the polymer can occur if the surface is porous, as for, e.g., cellulosic fibers. This process is due to polymer chains penetrating into the porous structure. This reptation process will furthermore increase the adsorption stoichiometry.
Acknowledgment. The authors wish to thank Swedish Board for Technical Development (STU) and the IBM Corporation for financial support. Registry No. (Amylamide)([ (trimethy1amino)propyllmethacrylamidecl) (copolymer), 58627-30-8.
