Hydrophilic Polymers - American Chemical Society

21 Transient Polymeric-Bridging Dynamics of Colloids Downloaded by UNIV LAVAL on April 6, 2016 | http://pubs.acs.org Publication Date: January 15, 1996 | doi: 10.1021/ba-1996-0248.ch021

L. E. Dewalt, Z. Gao, and H . D. Ou-Yang Department of Physics and Polymer Interfaces Center, Lehigh University, Bethlehem, PA 18015

Telechelic poly(ethylene oxide) (PEO) polymers in aqueous solution can form bridges between colloidal surfaces by attaching one hydrophobe onto each surface. Colloidal clusters can form because of this polymeric-bridging-induced attraction between the particles. Using a suspension of 28-nm polystyrene colloidal particles and linear PEO polymers terminated on both ends by C H hydrophobes, we investigated the relaxation of transient aggregated colloidal clusters into smaller clusters or singlets. Polarized and depolarized static light scattering and dynamic light scattering were used to determine the sizes and molecular weights (in terms of the singlet mass) of the clusters as a function of time. After the transient clusters were made, a short, initial period of about 300 was dominated by multiple scattering, which indicates the presence of very large clusters. After that, two distinct relaxation regimes could be identified: one, the intermediate time regime, had a characteristic time of about 800 s, and the second, the long time regime, had a characteristic time of 10,000 s. The long time regime is believed to be the time when doublets relax to singlet particles. The temperature dependence of relaxation dynamics indicates that the breaking up of the low-order multiplets follows Arrhenius behavior with an activation energy of about 30 k T. 20

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^WATER-BASED ASSOCIATIVE POLYMERS are used in coating, adhesion, paper making, mining, water treatment, and various other material-

0065-2393/96/0248-0395$ 12.00/0 © 1996 American Chemical Society

Glass; Hydrophilic Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

Downloaded by UNIV LAVAL on April 6, 2016 | http://pubs.acs.org Publication Date: January 15, 1996 | doi: 10.1021/ba-1996-0248.ch021

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processing applications that involve colloidal suspensions as raw materials. In many of these applications, controlled polymeric-bridging flocculation from a marginally stabilized colloidal state is one of the primary concerns. Associative polymers used in commercial flocculation processes have very high molecular weights and functional groups that can strongly adsorb onto the colloidal surface (1-4). The phenomenon of incipient flocculation and a review of the related research were discussed extensively by Napper (5). Computer simulations of bridging flocculation by homopolymers were investigated recently (6). A n increasingly interesting area of flocculation for commercial applications and research is the use of polyelectrolytes (7-12). However, the regime near the onset of flocculation and, in particular, the kinetics of polymer-induced flocculation are still open fields for fundamental understanding, and they prompted this work (12-14). In this chapter we discuss studies of transient, finite clusters of polystyrene (PS) latex particles aggregated by water-soluble associative polymers. The model polymer we use is a telechelic polymer composed of a water-soluble, linear poly(ethylene oxide) (PEO) backbone with both ends terminated by a C20H41 hydrophobe. Because of its moderate association strength per chain and relatively low molecular weight, this model telechelic polymer is probably not the most favorable one for commercial flocculation applications. However, the relatively low association strength per chain allows the flocculation process to be reversible, and the simplicity of the polymer's linear, symmetric structure permits comparison of experimental observations with a simple model. For the model telechelic polymers we are studying, flocculation can be induced at a fixed particle concentration and polymer molecular weight by varying the polymer concentration. At low polymer and colloid concentrations, repulsion between individual polymer-coated particles renders the suspension stable. The suspension is also stable at very high polymer and colloid concentrations, at which an extended network of polymer forms throughout the system. In the intermediate concentration regime, when particle surfaces are partially covered by adsorbed polymers, finite clusters of particles can form because of polymer bridging. It is also relevant to introduce results from previous experiments on interactions of this type of telechelic P E O polymer with PS latex particles. At low particle concentration, such that the interparticle distance is much larger than the stretched polymer chain length, telechelic P E O chains in the adsorbed layer can form a dense brush with both C20H41 hydrophobic end groups attached on the PS surface and loops formed by the water-soluble P E O backbone extending into the



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solvent (J5). These extended polymeric brushes give rise to an effective repulsive interaction between the particles that renders PS stable. However, at lower polymer surface coverage, exchange of polymer chains between colloidal surfaces is substantial. Strong evidence suggests that the exchange of polymer chains between colloidal surfaces occurs by direct transfer from one surface to another rather than by desorption of chains into the solution first and then reabsorption onto another surface (16,17). This evidence implies that a transient bridged state of two (or more) colloidal particles exists during the chain exchange process. The bridged pairs or clusters must be transient, because the overall particle concentration was kept low enough that thermodynamics dictate that the suspension remain dominated by singlet particles. However, if the adsorption interaction is stronger or the particle concentration is higher, the system will remain flocculated. A general strategy for investigating the flocculation phenomenon is to approach the critical flocculation point (CFPT) from the stable phase. Our intention here is to investigate the cluster and the transient lifetimes of the aggregated doublets. The present study focuses on the regime of low particle concentration and low polymer concentration so that the system is below the C F P T . The challenge is to create transient flocculated colloidal clusters whose lifetimes are long enough for observation. By injecting a drop of high-concentration polymer solution into a dilute, uniform PS latex particle solution, we are able to locally quench the system above the C F P T , thus creating flocculated clusters. This process of making transient flocculated clusters also works when we reverse the mixing procedure, that is, when we inject a small amount of concentrated PS latex particle solution into a dilute, uniform polymer solution. Since the overall colloidal suspension is thermodynamically stable, flocculated colloidal clusters are only transient. Using this approach, we were able to observe the transient behavior of low-order colloidal clusters (multiplets) during relaxation of the system to the stable suspension state (singlet-dominated state). Light-scattering measurements are used in this study to monitor the forming and breaking of the multiplets. Since the flocculated clusters are typically anisotropic, they give rise to depolarized (VH) scattering. We measure both the polarized ( W ) and the V H scattering intensities as a function of time. We also obtain W dynamic-lightscattering data to provide additional information on the size of the multiplets. From the decay of low-order multiplets, we are able to obtain the lifetimes of transient doublets. Modeling the breaking of the bridged pairs as an activation process, we can obtain a measure of the activation energy from temperature-dependent data.


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Experimental Details The model associative polymers used in this study are telechelic copoly mers with a molecular structure of R-0-(DI-PEO) -DI-0-R, as obtained from Union Carbide. Here P E O is a PEO segment with a nominal molecu lar weight of 8,200 g/mol, DI is an isophorone diisocyanate group that links the P E O segments with a polymerization index m of 12, and R is a terminating hydrophobic end group (C20H41). For simplicity, the sample is labeled according to the number of carbons in the end group as well as the overall molecular weight of the chain (in thousands of grams per mole). Therefore, C20-116 is a sample with C20H41 end groups and a total molecular weight of 116,000 g/mol. From previous studies (18) we learned that both the P E O backbone and the DI linkers adsorb to PS particles in water. However, the adsorption strength of the end groups dominates, so we can assume that the model polymer effectively has the A-R—A tele chelic architecture. The adsorption substrate for these experiments is 28-nm-diameter PS spheres (Duke Scientific) in aqueous suspension. In all of the experi ments, we compared the sizes of the particles with adsorbed polymer to the sizes of bare particles as obtained by dynamic light scattering; the sizes of the bare particles agreed with the manufacturer's stated size of 28 nm. Our purposes here were to create transient polymer-bridged colloidal clusters and to study the relaxation of the cluster decay through lightscattering observations. A transient cluster can by produced by one of two means: addition of a small amount of high-concentration polymer solution to a uniform, dilute suspension of colloidal particles or addition of a small amount of high-concentration particle suspension to a uniform, dilute polymer solution. These two processes yield different initial clusters sizes, but the long-time relaxation is very similar. Throughout this study we followed the first procedure. When 40 μϊ of 0.5% (by weight stock) solution of C20-116 polymer chains was mixed with a 28-nm PS particle suspension at a PS volume fraction of 6 x 10 ~ (this mixture is a solution containing about 10 chains per sphere, on average), a slightly opaque sample was initially produced. The opacity of the sample decayed within a few minutes. We simultaneously monitored, at symmetric scattering angles of 45°, the W and V H scattering intensities. The W and V H intensity data were accumulated by two Brookhaven Instruments BI-2030 autocorrelators and averaged over periods of 30 to 60 s. This time was chosen to provide adequate time resolution for the kinetics of relaxation. For a fixed particle concentration, the W scattering intensity in the low-concentration limit is linearly proportional to the square of the mass of clusters in solution and is therefore very sensitive to clustering of the particles. V H scattering comes from two effects: multiple scattering and rotations of anisotropic particles (19). Figures 1 and 2 show the normalized W scattered intensity versus time for two samples prepared as described above. Time t = 0 is the point at which the collection of data began; this time was, at most, several seconds after addition and subsequent mixing of the polymer with the uniform colloidal suspension. The normalized intensity data from Figure 2 are replotted in Figure 3 on a semilog scale to reveal the relaxation times. The V H scattering of the event shown in Figure 2 is recorded by


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Figure I. Normalized W scattering intensity l(t)/l() = n(t) for a mix ture of 10 C20-116 chains per 28-nm PS particle at 25 °C. The solid line represents a single exponential in the range of η = 2 —• I.

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Figure 2. Normalized W scattering intensity for a mixture of 10 C20116 chains per 28-nm PS particle at 25 °C.




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Figure 3. Semilog plot of the data in Figure 2, showing two characteris tic decay times. The fast decay time is 800 s, and the slow decay time is 10,000 s.

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Figure 4. Semilog plot ofVH scattering for the same sample as in Figure 2, also showing two characteristic decay times. The fast decay time is 350 s, and the slow decay time is 10,000 s.


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Figure 5. Particle size versus time plotted for the same sample as in Figures 2-4. a second photomultiplier tube and shown in Figure 4 on a semilog plot. The W and V H data are synchronized within an error of about 5 s, a short time compared to the relaxation time scale. The W scattering data were also used to calculate the intensity-intensity autocorrelation function av eraged over the collection period of about 60 s. The autocorrelation func tions were then used to determine the average hydrodynamic radius of the scattering clusters by cumulative expansion of the correlation function. Figure 5 shows the cluster size obtained from dynamic-light-scattering analysis of the W scattering data. Each point in Figure 5 was calculated from an individual correlation function.

Data Analysis and Discussion The initial observation that the sample increases in opacity when the polymer is introduced suggests that significant flocculation is occur ring. However, this flocculation is transient, since the sample becomes transparent again within several minutes. From the facts that at long times the suspension remained transparent and that light-scattering intensity carefully measured at long times confirmed that individual singlet particles dominate the suspension, we conclude that the free energy favors colloidal particles in the singlet form under the mixing conditions just described. Thus, by mixing polymer solution with the 28-nm-diameter PS particles in solution, we create a transient nonequilibrium state in which large flocculated clusters first form and then relax to an equilibrium state composed of single particles with polymer adsorbed to the surface. By analyzing the decay in the light scattering, we can follow the


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Figure 6. Relaxation model of the transient bridged state from the inter mediate to the long time regime. relaxation process of the cluster breakup. From relaxation data shown in Figures 3 and 4, we define three time regimes: the short time covers the first 300 s, intermediate times are from 300 to about 3,000 s, and long times are 3,000 to 30,000 s. The data presented thus far suggest the model indicated in Figure 6. Just after the concentrated polymer solution is mixed into the particle suspension, a population of transient polymer-bridged colloidal clusters is formed. In the very short time regime, the very large clusters start to break up quickly, in about 300 s. The finite, smaller clusters begin to break up during the interme diate time regime. Finally, in the long time regime, the relaxation is dominated by the breaking of doublets into single particles. We will analyze the data according to this model. Quantitatively interpreting the data from the short time regime is difficult because reproducing the initial conditions from run to run is difficult (discussed later). Also, the sample appears milky because of multiple scattering and is difficult to analyze by conventional lightscattering techniques. However, the sample is transparent in the inter mediate and long time regimes. Using conventional light-scattering theory, we can analyze the relaxation process by the following way. The small-angle W light scattering at a fixed colloid concentration can be expressed as I cluster

=

/singlet

(1)

where η is the number of singlet particles per aggregated cluster ( 19). Normalizing the W scattering intensity data to the scattering intensity at long times, the vertical axes of Figures 1 and 2 can be considered to be the number of particles in the scattering clusters. As stated ear lier, in the short time regime of Figure 1, we see very large clusters, but in the same regime of Figure 2, we do not. Thus the nonreproduceability of the mixing process is shown. Looking at the intensity relaxation data in Figure 3, we clearly see two relaxation times, one for the intermediate and one for the long time



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regimes; hence the definition of the time regimes. The intermediate characteristic decay time is 800 s, a time that is similar to the characteristic time found in Figure 1 (solid line). The long decay time has a characteristic time of 10,000 s. In the intermediate regime, in which the average number of particles in a cluster is on the order of 2 to 5, several relaxation channels in which the clusters can break up are present. For example, a cluster of 5 particles can relax by breaking up into clusters of 1, 2, 3, or 4 particles. In contrast, the long time relaxation occurs in the time domain in which the average cluster size is relaxing from about 1.2 to 1 particles. If we assume that the population of clusters in this time range is dominated by doublets and singlets, then only one decay channel exists, that is, from doublets to singlets. However, at this point we cannot exclude the possibility of a recombination channel yielding a finite distribution of doublets and singlets because the colloidal singlets are not completely sterically stabilized at 10 chains per particle. According to the size and intensity analyses for long times, the doublet population, if it exists, is very small. The dynamic-light-scattering data in Figure 5 confirm the existence of clusters in the intermediate time regime. After about 2000 s, singlets begin to dominate the scattering. At very long times, the average particle size approaches 32 nm. This size implies that the 10 polymer chains form an adsorbed layer 2 nm thick, a value that is not an unreasonable according to our experience of polymer adsorption in a similar system. The polydispersity of the diffusion constant analyzed at long times shows a value of about 13%, which is comparable to that obtained from bare particles. This value also supports the point that at long times, singlet particles dominate. In addition, the particle diameter at each point is taken from a time-averaged correlation function over 60 s, which is somewhat shorter than the ideal length of 200 s or longer. However, the lack of scatter in the data at long times suggests that a clear trend was obtained in this measurement. V H scattering, shown in Figure 4, supports the relaxation picture of our model. At very short times, we anticipate very large clusters, which are indicated by the milky appearance and multiple scattering. The first decay at a very short time in the multiple scattering regime has a relaxation time of 350 s. At the very long time the relaxation is due to the decay of doublets into singlets. As a matter of fact, the V H scattering from singlets should be very small, because the singlets are isotropic. This relaxation time is the same as that determined from W scattering. The intermediate time scale has a relaxation time between 350 and 10,000 s but has not been analyzed in detail. For the intermediate regime we estimated the lifetimes of the bound particles by assuming that particles leave multiplets by an activation process. The temperature dependence of the characteristic


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1/Γ(Κ) Figure 7. Semilog plot of relaxation time versus 1ΓΤ for results from conventional W static scattering. The solid line is a fit to the Arrhenius function with an activation energy of 30 keT. decay time obtained from W scattering is plotted versus 1/Γ in Figure 7. The Arrhenius plot indicates an activation energy of 30 k T. The error bars shown in Figure 7 are an indication of the reproduceability of the data. We are in the process of determining the temperature dependence of the relaxation in the long time regime. This calculation should pro vide the effective binding energy for a pair of bridged particles, and this energy can be compared to the binding energies of the individual hydrophobes. Work is also being done on the effects of particle and polymer concentrations. These results will be reported elsewhere. B

Conclusions In this chapter we show the relaxation behavior of low-order colloidal multiplets formed by transient bridging flocculation. By using W and V H light-scattering techniques, we found three relaxation regimes. The fast regime is a fast relaxation from large clusters in the multiple scattering regime. In the intermediate regime the large clusters have broken up into small clusters, probably with 1 to 5 particles per cluster. Here, the clusters still have several decay channels, and these chan-


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nels allow a faster relaxation to equilibrium. In the slow regime, only one decay channel is left (doublet to singlet), and the relaxation time becomes significantly longer. The Arrhenius behavior in the interme diate time regime indicates an activation energy of 30 k T, a value 3 to 4 times that for the interaction between a C o H i hydrophobe and a PS particle. B

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Acknowledgments We acknowledge the Donors of the Petroleum Research Fund, admin istered by the American Chemical Society, for partial support of this research. We are also thankful for support from the N S F - I U C R C for Polymer Interfaces at Lehigh University. L . E . Dewalt is supported by a graduate fellowship from the U.S. Department of Education.

References 1. Russel, W. B. In Colloid-Polymer Interactions; Dubin, P.; Tong, P., Eds., ACS Symposium Series 532; American Chemical Society: Washington DC, 1992. 2. Scheutjens, J. M. H. M.; Fleer, G. J.; Cohen Stuart, M. A. Colloids Surf. 1986, 21, 285. 3. Gregory, J. Colloids Surf. 1987, 31, 231. 4. Dickinson, E.; Eriksson, L. Adv. Colloid Interface Sci. 1991, 34, 1. 5. Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Aca demic: London, 1983. 6. Dickinson, E.; Euston, S. R. J. Chem. Soc. Faraday Trans. 1991, 87, 2193. 7. Mabrire, F.; Audebert, R.; Quivoron, C. J. Colloid Interface Sci. 1984, 97, 120. 8. Wang, T. K.; Audebert, R. J. Colloid Interface Sci. 1987, 119, 459. 9. Durand-Piana, G.; Lafma, F.; Audebert, R. J. Colloid Interface Sci. 1987, 119, 474. 10. Muthukumar, M. J. Chem. Phys. 1987, 86, 7230. 11. Wang, T. K.; Audebert, R. J. Colloid Interface Sci. 1988, 121, 32. 12. Pelssers, E. G. M.; Cohen Stuart, Μ. Α.; Fleer, G. J. J. Chem. Soc. Faraday Trans. 1990, 86, 1355. 13. Pelssers, E.; Cohen Stuart, Μ. Α.; Fleer, G. J. Colloids Surf. 1989, 38, 15. 14. Virden, J. W.; Berg, J. C. J. Colloid Interface Sci. 1992, 149, 528. 15. Ou-Yang, H. D.; Gao, Z. J. Phys. II 1991, 1, 1375. 16. Gao, Z. Ou-Yang, H. D. In Colloid-Polymer Interactions; Dubin, P.; Tong, P., Eds.; ACS Symposium Series 532; American Chemical Society: Washington, DC, 1992. 17. Gao, Z.; Dewalt, L.; Ou-Yang, H. D. Colloids Surf. A 1994, 86, 255. 18. Gao, Z. Ph.D. Dissertation, Lehigh University, August 1993. 19. Berne, B.; Pecora, R. Dynamic Light Scattering; John Wiley: New York, 1976. RECEIVED for review April 16, 1994. A C C E P T E D revised manuscript May 16, 1995.


Hydrophilic Polymers - American Chemical Society

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