Controlled Flocculation-Deflocculation Behavior of Adsorbed Block

It is well established that the flocculation-deflocculation behavior of adsorbed block copolymers in colloidal dispersions is controlled mainly by the...
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Langmuir 1994,10, 4031-4038

4031

Controlled Flocculation-Deflocculation Behavior of Adsorbed Block Copolymers in Colloidal Dispersions by Modifying SegmentlSurface Interactions: The Use of Small Displacer Molecules To Selectively Cleave Interparticle Bonds C.P. Tripp* and M.L. Hair Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario, L5K 2L1, Canada Received April 29, 1994. In Final Form: August 5,1994@ It is well established that the flocculation-deflocculation behavior of adsorbed block copolymers in colloidal dispersions is controlled mainly by the solvency of the extended polymer chain in solution. In this paper, we describe a colloidaldispersionin which it is possible to modifythe flocculation-deflocculation behavior by altering specificpolymer segmentisurfaceinteractions without affectingthe amount of adsorbed polymer. Infrared spectroscopy has been used to study the adsorption of three poly(ethy1ene oxidestyrene) block copolymers (PEO-PS) on silica particles suspended in CC4. The three block copolymers chosen for this study differ in the relative sizes of the PEO and PS blocks. Although cc14 is a good solvent for both PEO and PS, the PEO segments of the block copolymer preferentially adsorb on the surface hydroxyl groups (silanols) because the segmentisurface interaction of PEO is stronger than that of PS. With an excess of polymer in the solution, all three copolymers stabilize the silica dispersions. However, at these concentrations, it is found that the PEO segments may not adsorb exclusively because of the geometric constraints imposed by the relative block sizes. At polymer concentrations in solution that are not in excess, the relative number of PS segments adsorbed on the surface sites is higher due to the formation of interparticle bridges. Flocculationoccurs because the PS segments extendingfrom the surface from one particle form interparticle bonds by attaching to exposed silanol sites on other silica particles. We show that redispersioncan be induced by adding a small displacer molecule (triethylamine)to selectively cleave PS/silanol interparticle bonds.

Introduction Polymers exert various types of flocculation-deflocculation effects on colloidal dispersions.l Steric stabilization is the most important of these effects, and amphipathic block copolymers are good steric stabilizers. One block of the polymer is strongly adsorbed on the surface of the particle, and the second block is the stabilizing moiety that extends out from the surface into the solution. The solvency of the extended polymer chain is a key factor in defining the degree of dispersion of the colloidal system. When two particles approach each other, the extended polymer chains of the stabilizing block come into contact and, in a good solvent, give rise to a repulsive force that prevents the particles from aggregating. In a poor solvent, an attractive force between the polymer chains results in flocculation. Thus, flocculation can be induced in sterically stabilized dispersions by changing the solvency of the extended block. This can be done by changing temperature and/or pressure but most commonlyis achieved by adding a poor so1vent.l In the latter case, the effect is reversible with redispersion occurring upon dilution with a good solvent. Thus, the critical feature ofthe adsorbed polymer is the extended tail region, and as a result, much theoretical and experimental work has focused on understanding the parameters that control and define the polymer concentration profile.2 On the other hand, little attention is given to the detailed nature of segment'surface interaction of the adsorbed polymer layer in defining flocculation-deflocculation behavior. In sterically stabilized dispersions, the adsorpAbstract published inAdvanceACSAbstracts, October 1,1994. (1) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: New York, 1983. (2) Kawaguchi, M.; Takahashi, A. Adv. Colloid Interface Sci. 1992, 37, 219. @

tion criteria rarely probe the details of the surface interaction other than to specify that the polymer should adsorb at a high coverage and should be strongly attached to the surface of the particle. A high coverage is needed to avoid aggregation because the polymer may move laterally along the surface to escape the stress inside the particle contact region, which then allows for contact between the exposed surface area. Furthermore, a t low coverage, the adsorbed polymer may promote flocculation by linking to the free surface area of a second particle, forming an interparticle or bridged bond.3 A strong attachment of the polymer to the surface is desirable as a weakly attached polymer could be expelled from the particle contact region, resulting in displacement coagulation of the dispersion.' It is possible to achieve a strong attachment by two different ways. In one instance, a strong adsorption is dictated by the relative solubilities of each block. One block is in a good solvent and does not adsorb on the surface whereas the other block is in a poor solvent and prefers to escape the solvent by adsorbing on the surface. In a second way, the adsorption is driven by a specific segment'surface interaction. Preferential adsorption occurs because the anchor block is chemically bound to the surface; the stabilizing block either does not adsorb on the surface or is inhibited in doing so because the anchor block has a stronger interaction with the surface sites. In previous publications from our laboratory we have investigated the adsorption of PEO-PS block copolymers on mica from Both blocks of this polymer are (3) Kitahara, A.; Hasumuma, M. J . Colloid Interface Sci. 1972,41, 383. (4) Hair,M.L.;Guzonas, D. A.; Boils, D. Macromolecules 1991,24, 341. (5) Guzona8,D.A.;Boils, D.; Tripp, C. P.;Hair,M. L. Macromolecules 1992,25,2434.

0 1994 American Chemical Society

4032 Langmuir, Vol. 10, No. 12, 1994 soluble in toluene, but strong adsorption occurs presumably through interaction of PEO segments with the mica surface; PS segments do not adsorb on mica from toluene. Theoretical arguments by Marques and Joannp predicted several trends for the surface density and length of the extended polymer chain as a function of the relative block size. Good agreement was found for the surface density measured by IR spectroscopy5and the extended PS length measured by the surface force apparatus' with these predicted theoretical values. In the experiments on mica, the block copolymers were adsorbed at fullsurface coverage,and this always produced a repulsive force when the two coated mica sheets were brought together in the surface force apparatus. In a recent surface force study, Dai and Toprakciogl~~ measured the force when PEO-PS diblock and PEO-PSPEO triblock copolymers were adsorbed on a single mica sheet and then brought into contact with a second bare mica sheet. A repulsive force was detected between the two approaching mica surfaces when coated with the PEO-PS block copolymer. In contrast, an attractive forve was found when the adsorbed PEO-PS-PEO triblock was used, and this was attributed to the bridging of the triblock between the two mica surfaces; the dangling PEO segments of the adsorbed triblock extend into solution and adsorb onto the opposing bare mica surface. These bridged bonds did not form when PEO-PS diblock copolymers were used because the extended PS segments did not adsorb onto the bare mica surface. Thisexperiment leads to the intriguing possibility of controlling the flocculation-deflocculation behavior by altering the adsorption behavior of the extended block. We examine this possibility in this study. Specifically,we report an in situ transmission infrared study of the adsorption of PEO-PS block copolymers on silica particles suspended in CC4. As with toluene, CCL is a good solvent for both PEO and PS so that the adsorption is driven by segment'surface interaction. However, silica is different from mica in that PS and PEO segments adsorb onto the surface by forming hydrogen bonds with the surface hydroxyl groups. PEO adsorption on silica is favored because the hydrogen bond formed with PEO segments is stronger than the bond formed with PS segments. We take advantage ofthe difference in PEO and PS interaction with the silanols and show that it is possible to change the flocculation-deflocculation behavior of the silica dispersion by modifying specific PEO and PS surface interactions. In the case of the silica surfaces, at a coverage below the maximum adsorbed amount, r, the PEO-PS forms a bridged bond because the PS segments that extend from the surface can bind to vacant silanol groups on a second silica particle. We show that it is possible to redisperse the silica suspension by using small molecules to selectively cleave the PS/silanol interparticle bonds. IR spectroscopy is used because it can detect and differentiate between PS and PEO interactions with the silanol groups. Furthermore, in a recent article,l0 we described an in situ colloidal infrared cell that can be used to measure segmenthilano1 interactions, the adsorbed amount, adsorption kinetics, and colloidal stability all under strict control of surface quality and particularly with respect to adsorbed water. Thus, infrared spectros(6)Boils, D.; Tripp, C. P.; Guzonas, D. A.; Hair, M. L. Langmuir 1992,8,2070. (7) Guzonas, D. A.; Boils, D.; Hair, M. L. Macromolecules 1991,24, 3383. (8)Marques, C.M.; Joanny, J. F. Macromolecules 1969,22,1454. (9)Dai, L.;Toprakcioglu, C. Macromolecules 1992,25,6000. (10)Tripp, C. P.; Hair, M. L. Langmuir 1993,9,3523.

Tripp and Hair Table 1. Characteristicsof the Polymer Used in This Study

sample PS 36311 334/19 87/29

PEO

Mm

(PSX,

300000 363000 334000 87000

2884 3490 3211 836

Mwm0

(PEOh)

1000 19000 29000 1900

23 431 659 43

Mw/Mn 1.06 1.14 1.37 1.27 1.08

copy combined with the in situ colloidal cell is well suited for studying the role of segmenthurface interactions in the behavior of colloidal silica particles.

Experimental Section Details on the in situ mixing infrared cell and the IR spectrometer configuration or its operations with respect to polymer adsorption studies have been described in detail elsewhere.10 The fumed silica was Aerosil 90 obtained from Degussa A.G. and has a measured Nz(BET) surface area of 88 mVg. The silica (200mg) was pretreated by evacuation at 400 "C for 15 min, followed by cooling to room temperature before addition of about 75 mL of CC4. This pretreatment gave a surface containing only isolated/geminal hydroxyl groups as adsorption sites at a density of about 1.4 OW MI^.^' The CC4 was obtained from Aldrich and was dried over a molecular sieve and distilled before use. To prevent rehydroxylation and rehydration of the silica, the CC14 was added to the colloidalcell under vacuum. The suspension was stirred (usually overnight) to obtain maximum dispersion of the silica particles. Settling curves were used to monitor the degree of dispersion of the silica suspension, to measure the adsorbed amount of polymer, and to monitor changes in the relative stability of the silica particles. The procedure used to generate settling curves is detailed elsewhere.lOJ2 However, we provide a brief description because of the importance of the settling curves in supplying much of the data used in this study. A settling curve is generated by recording a spectrum while the suspension is stirred and then at equally spaced intervals after the stirring is stopped. When the stirring is stopped, the silica settles out from the beam area by gravitation, and this amount can be computed by monitoring the decrease in intensity of an infrared band of the silica. A plot of the intensity (normalized to the measured intensity recorded during stirring) versus time is referred to as a settling curve. The main use of the settling curves is in calculating the adsorbed amount of polymer. This is done by comparing the settling curve generated using a band due to the polymer with a settling curve produced using a band due to the silica. There are two extreme cases. In one where the polymer is completely adsorbed, the settling rate calculated using a polymer band is equal to the settling rate derived from a silica band. Secondly, when there is no polymer adsorbed, there is no settling of the polymer and therefore no change in intensity of the polymer band. The settling curve is also used to detect changes in the stability of the colloid. The first example of this use occurs when the silica is initially added to the CC4. Settling curves are generated during the initial stirring period (usually at 2-3 h intervals), and this is continued until the w e s are reproducible. Maximum dispersion is considered to be reached when the settling curves are independent ofthe stirring time. The settling curve obtained at this point is used as a reference point for comparison to other settling curves in order to monitor changes in the stability of the silica suspension after addition of various adsorbates. For comparative purposes, the adsorption ofthe homopolymers PEO and PS has been included. The homopolymers were obtained from Polyscience Inc., and the PEO-PS block copolymers are from Polymer Laboratories. The properties of the polymers are given in Table 1. The block copolymersare referred in the text by the molecular weight ratio; Le., PS-PEO block with Mwps= 334 000 and Mwm = 19 000 is denoted as 334/19. Triethylamine (TEA) was obtained from Aldrich and was added to the suspension from a solution containing about 0.2 mL of TEA in 25 mL of CCL. (11)Morrow, B. A.;McFarlan, A. J. Langmuir 1991,7 , 1695. (12)Tripp, C. P.;Hair, M. L. Langmuir 1992,8, 1961.

Controlled

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3000

20.00 c m - 1

Figure 1. Polymer adsorption on Aerosil90 using the SD protocol. Spectra are (a)Aerosil90 and after addition of (b)PS, (c) 363/1, (d) 334/19, (e) 87/29, and (f) PEO. Two different approacheswere used to add the polymer to the stirred silica suspension. In one case, the polymer was added in a single dose to the silica suspensionfrom a solutioncontaining about 45 mg of block copolymer (or about 20 mg of the homopolymers) in 10 mL of CCL and stirred for 48 h to ensure equilibrium. These polymer concentrations were about twice the maximum adsorbed amount and thus ensured full surface coverage. Equilibrium was checked by monitoring for changes in the adsorbed amount and in the spectra during the stirring period. In the second approach, a multiple-doseprocedure was used to enhance the formation of interparticle bonds. In this case, the same amount of polymer as was used in the single dose was added in about six equivalent doses. After each dose, the suspension was stirred for 48 h. Interparticle bonds are formed because the amount of polymer added in the first two or three doses was insufficient to cover all the surface sites. For brevity, we refer to the single-doseand multiple dose protocols as SD and MD, respectively.

Results and Discussion

Background. In a recent paper, we showed the importance of the pretreatment conditions of the silica surface in polymer adsorption studies.1° The polymers adsorb on the surface hydroxyl groups, and the number and type of these groups on the silica surface vary with the activation temperature and with the amount of surface water that is present. In these experiments, we use silica that has been degassed at 400 "C. This pretreatment eliminates surface water and hydrogen-bonded hydroxyl groups, leaving only isolated geminal hydroxyl groups as sites for polymer adsorption. Although a silica degassed at 400 "C has surface siloxane groups (Si-0-Si), it is found that the polymers do not adsorb on these sites. In separate experiments, we found that PS or PEO homopolymers and the PEO-PS block copolymers used in this study do not adsorb on a silica that has been pretreated with a silanizing agent, hexamethyldisilazane. This silanizing agent replaces all isolatedlgeminal groups on the surface with Si(CH3)s groups.13 Similar findings for polymer adsorption on (13)Hertl, W.; Hair,M. L. J . Phys. Chem. 1971,75, 2181.

silanized silica surfaces have been reported by Eltekov and Ki~e1ev.l~ The spectrum of the fumed silica degassed a t 400 "C and dispersed in CCld is shownin Figure la. Assignments of the various bands are well documented.15-17 The most important feature is the band at 3690 cm-' which is assigned to isolatedgeminal hydroxyl groups (silanols). The bands below 2000 cm-' are various Si-0 bulk modes, and these bands are used to quantify the amount of silica in the infrared beam. For comparative purposes, the adsorption of the homopolymers is included in this study. The spectra of silica after addition of excess PS and PEO homopolymers are shown in Figure l b and lf, respectively. With PS, the hydroxyl band at 3690 cm-l shifts about 100 cm-l to 3590 cm-l due to a weak hydrogen-bonding interaction with the aromatic ring of the PS segments.l* With PEO, the band a t 3690 cm-l shifts about 400 cm-' to 3300 cm-' (Figure 10, which is consistent with much stronger hydrogen-bonding interaction between the oxygen of the PEO segments and surface silan~ls.'~ The three bands at 3690 (SiO-HI, 3590 (SiOH-PSI, and 3300 cm-' (SiOH-PEO) can be used to measure the fraction of surface sites occupied by polymer chains (&) and the fraction of segments attached to the surface, known as the bound fraction (e). The value for 6T is computed from the relative decrease in the integrated intensity of the band a t 3690 cm-'. In addition to a total 8T value, specific 8 values for PS (8ps) and PEO (8pEo) can be obtained from the bands at 3590 and 3300 cm-l. The integrated intensities of the bands at 3590 and 3300 cm-' were calibrated to the decrease in the integrated band at 3690 cm-l from the (14) Eltekov,Y. A.; Kiselev, A. V. J.Polym. Sci. Polym. Symp. 1977, 61, 431. (15) Kiselev, A. V.; Lygin, V. I. Infrared Spectra of Surface Compounds; John Wiley and Sons: New York, 1975. (16) Hair, M. L. Infrared Spectroscopy in Surface Chemistry; Marcel Dekker: New York, 1967. (17) Morrow, B. A. In Spectroscopic Analysis of Heterogeneous Catalysts, Part A: Methods of Surface Analysis; Fierro, J. L. G . , Ed.; Elsevier: Amsterdam, 1990. (18)Vander Linden, C.; Van Leemput, R. J . Colloid Interface Sci. 1978,67, 48. (19) Kawaguchi, M.; Hada, T.;Takahashi, A. Macromolecules 1989, 22, 4045.

4034 Langmuir, Vol. 10, No. 11, 1994

Tripp and Hair

Table 2. Measured Values for Adsorbed Polymers PS PEO q(mg/m2) 8 T segments/ segments/ sample f0.02 k0.02 eps emo nm2 eps n m 2 emo PS .58 .50 .50 3.0 .23 3634 SD 1.05 60 .57 .03 5.9 .13 .04 .93 MD 0.73 60 .57 .03 4.1 .19 .03 1.0 33419 SD 1.2 .70 .10 $0 6.3 .02 .88 .94 MD .93 6 8 .16 .48 4.5 .06 68 1.0 87/29SD 1.1 .88 .88 4.6 3.7 .33 MD 1.1 .86 .02 .84 4.6 .02 3.7 .32 PEO .36 .90 .90 4.9 .25 ~

spectra obtained for the adsorption of PS and PEO homopolymers. Values of e are calculated by ex VOH

e, = 7 where x refers to PS or PEO, OH is the hydroxyl group density, and D is the number of polymer segments adsorbed per nm2. For our pretreatment conditions OH is 1.4OWnm2. The difference in the strength of the interaction of the surface with PEO and PS segments is important in displacement and competitive adsorption processes. Kawaguchi et a1.20 showed that PEO can completely displace preadsorbed PS from a silica surface and that PEO is preferentially adsorbed when PEO and PS are simultaneously added to silica. Such competitive adsorption processes are important factors in defining the conformation in the adsorption of the PEO-PS block copolymer. However, the adsorption ofthe PEO-PS block copolymer will differ from the homopolymers because of geometric constraints imposed by the relative size of each block. Thus, the number of PEO and PS segments adsorbed on the surfacewill reflect the differencein affinity ofthe PEO and PS for surface sites along with the relative size of each block.

Adsorption with the Single-Dose (SD) Protocol. The spectra of the silica after addition of the three PEOPS blocks using the SD protocol are shown in Figure 1, and values for 8 and e are given in Table 2. The adsorbed quantities (4)of the block copolymers are about twice the amount measured for the homopolymers. It is clear the PEO is not adsorbed exclusively as found with the homopolymer adsorption,2O and the number of segments of PEO and PS adsorbed is dependent on the relative size of each block. The number of attached PS segments is greatest with the highly asymmetric (36311)polymer and is less with the moderately asymmetric (334.49)polymer. No attached PS segments are observed when the symmetric (87129)block copolymer is adsorbed. The percentage of silanols covered by the polymers increases with the relative size of the PEO block and follows the order PS < 3634 < 334/19< 86/29< PEO. (Althoughthe molecular weight of PEO differs in these polymers, it is noted that both 8 T and e values for PEO homopolymers have been reported to be independent of molecular weight.19) For the adsorption of the 36311 polymer, a band at 3300 cm-' is not detected. However, the position of the 363/1 polymer in the above order along with the difference in adsorbed amounts between the block copolymers and the homopolymers suggests that adsorption of the 36311 does occur by PEO/silanol interaction. The number of PEO/ silanol interactions would be small, and this may explain the difficulty in detecting a broad band at 3300 cm-'. This explanation is supported by the 8ps and OPE0 values listed in Table 2. For the 36311 polymer, the decrease in silanol (20) Kawaguchi,M.; Sakai, A.; Takahashi,A. Macromolecules 1986, 19, 2952.

groups (8T) cannot totally be accounted for by the formation of PSlsilanol interactions. Subtracting the contribution 8ps from the total &, we find that about 5%of the total coverage is unaccounted for, and this discrepancy must be due to adsorbed PEO segments. This is not unreasonable, since a value of about 5%would be expected if all PEO segments of the 36311 adsorb on the surface. With this low number of PEOIsilanol bonds, the broad band at 3300 cm-' would be weak in intensity and would blend easily into the background. In contrast, the 334/19and the 87129both have a high proportion of silanols perturbed by PEO segments. Since the PEO/silanol interaction is stronger than the PSlsilanol, the bound fraction of PS segments decreases rapidlywith an increasein the relative size of the PEO block. Much of the data in Table 2can be rationalized in terms of excluded volume effects arising from polymer overlap. We recall that Kawaguchi et al.19have shown that PEO homopolymers can completely displace adsorbed PS from the silica surface. However, for the 36311 and 334/19block copolymers the exclusivity of PEO segments adsorbed on the surface is tempered because the PEO block must drag with it the much bulkier PS block in its quest for available surface sites. Free volume effects prevent the overlap of the bulkier PS segments near the surface, and this inhibits a full coverage of the surface sites by the PEO segments. Although the PEO units spread out flat on the surface (both 36311 and 334119 have high @PEO values), there are not enough PEO segments to cover all silanols in the area defined by the overlap of the PS blocks. This leaves exposed silanols as sites for PS adsorption. For 87/29, the surface coverage by PEO segments is not limited by the overlap of the PS tails. The PEO segments have access to all surface silanols and prevent any PS segments from adsorbing. In this case, there is an oversupply of PEO segmentsfor the available silanol sites, and this is reflected in a low value for @EO. When the SD protocol is used, the dispersion properties of the silica are greatly effected by the presence of the block copolymer. Figure 2 plots the settling curves for the silica particles before and after polymer addition. It is seen that all three blocks improved the stability to the same extent, and this can be compared with adsorbed PS that improved the stability slightly and PEO that destabilized the suspension. The drastic increase in stability when 363/1compared to PS is used adds support to the claim that the 3634 adsorbs at least to some extent via the PEO segments. Although the three block copolymers differ in their relative block size, it appears that all three block copolymers adsorbed using the SD protocol satisfy the conformationalcriteria for steric stabilization. In the next two sections of this paper, a comparison of the stability obtained using the SD protocol is drawn to those obtained with the MD protocol and to those obtained upon addition of a displacer molecule. For clarity, these changes in stability have been summarized and are listed in Table 3. Adsorption with Multiple-Dose(MD) Protocol. In our second set of experiments the PEO-PS was added in small doses and allowed to equilibrate afker each addition. The amount of polymer added in the first two or three additions was not enough to fully cover the surface, but the total amount of polymer eventually added was equivalent to the amount added in the SD experiments. The spectra obtained after additions of the block copolymers using the MD protocol are shown in Figure 3,and values for 8, e, and q are listed in Table 2. Although the total amount of polymer added was the same for both SD and MD samples, the two procedures produced different polymer conformations and stability curves.

Langmuir, Vol. 10, No. 11, 1994 4035

Controlled Flocculation-DeflocculationBehavior 0 A

0

i¶

334119 31/29 363/1

PBO

PS A90

1.00

., R

E

x

0.00

x

x

I

0

SO0 T h e (mc)

Figure 2. Settling curves for the addition of polymers using the SD protocol. Table 3. Effects of Adsorbed Polymer on the Stability of Silica Suspension8 polymer protocol effect on stability PS SD slight improvement 334/19 SD stable +TEA stable-no change flocculation MD +TEA stable 36311 SD stable +TEA stability reduced MD stable +TEA stability reduced 87/29 SD stable +TEA stable-nochange flocculation MD +TEA stable PEO SD flocculation

For all three block copolymers, the same fraction of surface silanols was involved in segmentisurface interactions in both SD and MD samples. The differences appeared in the relative number of PS and PEO segments adsorbed on the surface. For 87/29, the amount adsorbed is equal in both SD and MD samples, but we now see that there are a few PS segments adsorbed in the MD sample. The spectrum of the adsorbed 87/29 on silica in Figure 3a also shows a band at 3590 cm-l (SiOH-PSI, and this band is not present in the spectrum of the SD sample shown in Figure le. The small number of attached PS segments coupled with an equal value for the adsorbed amount suggest that the conformation of the adsorbed polymer is similar in both SD and MD samples. However, the presence of the few PSIsilanol bonds has a tremendous effect on the stability of the dispersion. In Figure 4 we have plotted the settling curve of silica before and aRer 87/29 addition for the MD sample. Clearly, the addition of 87/29 using the MD protocol leads to flocculation, and this is opposite to the effect of the SD protocol shown in

Figure 2. The formation of interparticle bonds would explain the differences in the settling curves for SD versus MD samples even though both samples have the same total amount of adsorbed polymer. In the first two to three additions of polymer in the MD samples there is insufficient polymer to fully cover the surface sites. A higher number of PS segments adsorb because the PS block either lies in a flatter configuration on the surface or forms interparticle bonds through the adsorption of PS segments with exposed silanols on a second silica particle. As the amount of polymer is increased, the PEO segments displace most of the PS segments, leaving few attached to the surface. In support of this model, it is noted that the spectrum recorded after the first and second dose had a higher number of adsorbed PS segments than in the final spectrum. The presence of PS/silanol interparticle bonds also explains the incomplete displacement of the PS by the PEO segments: The PS segments are not completely displaced because the 87/29 would have difficulty in reaching the confined regions between the flocculated particles. A similar trend was found in the adsorption of the 334/ 19 polymer. However, for the 334/19 MD sample, the changes were more pronounced; there was a 3-fold increase in the number of PS/silanol bonds that were formed and a marked decrease in the adsorbed amount. Nevertheless, as with 87/29MD, the settling curves of 334/19 MD showed similar behavior (see Figure 5). The 334/19 MD sample had flocculated, and further support for this comes from the "spikes" that are observed in the settling curves. These are due to large flocs entering the beam area.1° For 363/1, the values of f3T, Ops, and &EO in both SD and MD samples were equal. This is not too surprising since the adsorption of this polymer in the SD sample is already dominated by PWsilanol interactions. However, the adsorbed amount is reduced for the MD sample, and this gives rise to a higher eps value. Unlike the 87/29 or 334/ 19, this higher eps value did not translate into a reduced stability of the dispersion. In contrast, the silica particles were stable in the sense that the settling curves for both the SD and MD samples are similar (Figures 2 and 6). We defer further discussion of the 3634 MD sample to the next section. Effect of Displacer Molecule. It is well known that small molecules can displace polymer segments from the surface if the strength of the interaction of the small molecule with the surface is stronger than the polymer segmentisurface interaction. Cohen Stuart et a1.21,22 used small molecules to estimate the adsorption energy parameter (xs)by measuring the critical volume of the small molecule needed to displace the polymer from the surface. This work was extended by Kawaguchi et al. for PEO19 and PSZ3adsorbed on silica from CC14. Kawaguchi et al. found that the volume fraction of the displacer required to remove the polymer from the surface was smaller for PS than PEO, and this was rationalized in terms of PEO being more strongly bound to the surface. Rather than completely desorbing the polymer, our aim is to exploit this difference in strength of the segmentisilanol interaction between PEO and PS and use a small displacer molecule to selectively cleave the PS/silanol interaction from the surface. In this manner, those adsorbed PS units that form interparticle bridged bonds in the MD samples can be removed. (21)Cohen Stuart, M. A.; Fleer, G. J.; Scheutjens, J. M. H. M. J. Colloid Interface Sei. 1984,97, 515. (22) Cohen Stuart, M. A.; Fleer, G. J.; Scheutjens, J. M. H. M. J. Colloid Interface Sei. 1984,97, 526. (23) Kawaguchi, M.; Chikazawa, M.; Takahashi, A. Macromolecules 1989,22, 2195.

Tripp and Hair

4036 Langmuir, Vol. 10, No. 11, 1994

1

.5

0 3500

4000

cm-1

3000

Figure 3. Block copolymers (a) 87/29, (b)33419, and (c) 363/1 adsorbed on Aerosil90 using the MD protocol and after addition of TEA to (d) 87/29, (e) 334/19, and (0 36311.

0

-

1 .oo

1.00

0.50

0.50

2

I \

H

0

500

1000

Time ( ~ e c )

0

500

TI-.

1000 (1.c)

Figure 4. Settling curves for the addition of 87/29 using the MD protocol.

Figure 5. Settling curves for the addition of 334/19 using the MD protocol.

In a series of experiments we have tested this hypothesis by adding triethylamine (TEA) to the adsorbed PEO-PS block copolymer on silica. TEA was selected because it forms a strong hydrogen bond with the surface silanols that gives a shift of about 900 cm-1?4 This shift in frequency is larger than those obtained for either adsorbed PS or PEO segments, and therefore TEA can displace both PS and PEO segments from the surface. Because the strength of the PS/silanol is much weaker than the PEO/ silanol, the addition of small quantities of TEAwill displace

the more weakly adsorbed PS segments in preference to the PEO segments. Furthermore, the adsorbed TEA “poisons”the silanol sites for further polymer adsorption. Thus, the number of PEO segments attached to the surface will not increase when the adsorbed PS segments are removed by TEA because the PEO segments cannot displace the more strongly adsorbed TEA molecules. For comparative purposes, TEA was added to the PEOPS silica suspensions produced by the MD and SD protocols. In both SD and MD samples, the TEA was added in small incremental doses until there was no evidence of adsorbed PS segments on the surface (i.e.,

(24)Tripp, C.P.;Hair, M.L. J. Phys. Chem. 1993,97,5693.

Controlled Flocculation-Deflocculation Behavior

Langmuir, Vol. 10, No. 11, 1994 4037

1 1.00

I\

~

0.00

0.00 0

500

1000

Tlme (see)

0

SO0 Time (sec)

Figure 6. Settling curves for the addition of 36311 using the

Figure 7. Settling curves of A90 and A90 treated with TEA.

until the point where the band at 3590 cm-' (SiOH-PS) completely disappeared). The spectra obtained after addition of TEA to the 87/29 MD sample are shown in Figure 3d. No PEO segments are removed (band at 3300 cm-l did not change), and there was no change in the adsorbed amount. The ability ofthe TEA to adsorb on the residual silanols in SD and MD samples shows that TEA has no problem in navigating through the polymer layer t o reach the surface sites. Thus, the TEA can easily reach the PS/silanol bond in the confined interparticle region of the flocculated suspension. The removal of the PS/silanol bonds in the MD sample produced a dramatic improvement in the settling properties of the silica. This is shown in Figure 4. We recall that the original PS/silanol bond in this sample was due to interparticle adsorption that gives rise to the flocculated dispersion. ARer all PS/silanol bands are broken, the silica is stabilized to about the same level as obtained with the SD sample (cf. Figure 2). Breaking the interparticle bonds results in the flocculated suspension being redispersed. In the SD sample, there was no band at 3590 cm-l to remove and the addition of TEA resulted only in a small amount of TEA adsorbed on the remaining few silanol groups; the TEA treatment did not change the settling rate. As a final comment, it is noted that the breaking of the PS/silanol bonds is paralleled by the adsorption of TEA on these sites. Therefore, it is possible that the adsorbed TEA in itself affects the stability of the silica particles. This possibility was tested in a separate control experiment. TEA was added to a silica sample degassed at 400 "C and dispersed in CCld. The TEA adsorbed on all the isolatedgeminal groups and the settling curves recorded before and after TEA treatment are shown in Figure 7. The adsorbed TEA increases the settling rate, and this is opposite to the effect of the adsorbed TEA on breaking of interparticle PS/silanol bonds. This shows that the adsorbed TEA in itself does not account for the improved stability of the silica particles.

The effect of TEA adsorption on the 334/19 MD sample was similar to that observed with the 87/29 MD sample. We recall that the 334/19 SD sample differed from the 87/29 SD sample in that the former had PS segments adsorbed on the surface whereas the latter did not. When a 334/19 SD sample was treated with TEA to remove all the PS/silanol bands from the surface,there was no change in the adsorbed amount and no change in the settling curve. This shows that the PS/silanol bonds in the 334/19 SD sample are not due to interparticle linkages. For the 334/19 MD sample, however, each intermediate dose of TEA produced a gradual improvement in the settling curve. This continued until all PS/silanol bonds were eliminated. Again there was no change in the adsorbed amount, and the settling curve obtained after removal of all PS/silanol bonds is shown in Figure 5. As expected, the total amount of TEA (750 pmollg silica) used in the removal of all PS/silanol bonds for the 334/19 MD sample was much higher than the 20 pmollg silica required for the 87/29 sample. In contrast to the 87/29 settling curves, the curve obtained for the 334/19 MD after TEA addition did not match the same level of stability of the 33409 SD curve as shown in Figure 2. This lower stability is likely due to a smaller adsorbed amount in the 334/19 MD sample with respect to the SD sample (see Table 2). It is noted that both 87/29 SD and MD sampleshad identical adsorbed amounts of polymer. The addition of TEA to both the 363/1 MD and SD samples had an opposite effect to that observed with either the 334/19 or 87/29 treated silicas. We recall that unlike the 334/19 and 87/29 samples, both the 36311 SD and MD samples gave rise to stable suspensions. With TEA addition to the 363/1 SD and MD samples, the PS/silanol bonds were reduced by 85% (Figure 3f) without causing a reduction in the adsorbed amount, and this was accompanied by a reduction in the stability of the suspension (Figure 6). For 363/1, the adsorption is dominated by PS/silanol bonds, and the settling curve data indicate the absence of any bridged PS/silanol bonds. When TEA is introduced, the PS is displaced from the surface, leaving large areas on the surface devoid of

MD protocol.

4038 Langmuir, Vol. 10,No. 11, 1994 adsorbed polymer. These sites are now occupied by adsorbed TEA, and as shown in Figure 7 this itself may produce an increase in the settling rate. Since the bulkier PS block is no longer held in place by PS/silanol bonds, it can more easily undergo a lateral displacement upon approach of a second particle with subsequent flocculation.

Conclusion Studies of the adsorption ofblock copolymers on surfaces most often involve systems where one block adsorbs and the other does not. Here, the conformationof the adsorbed polymer is dictated by the relative size of each block of the copolymer. In the system discussed here the PEO-PS block copolymers adsorbed on silica from CCl4 are unique in that both the PS and PEO segments adsorb on the surface. Thus, the polymer conformation is dictated not only by the relative block size but also by the relative

Tripp and Hair

strength of interaction of the PS and PEO segments for the surface sites. We have taken advantage of this additional criteria and shown that it is possible to selectivelybreak segmentfsurfacebonds without desorbing any polymer, and in doing so we can control the dispersion properties of the colloidal suspension. In particular, we used a small displacer molecule, triethylamine, to selectively cleave PS/silanolinterparticle bonds, and this causes the deflocculation of the suspension. This idea may be useful in several areas because it lends itself to the possibility of controlling or modifying the behavior of the colloidal suspension through a change in the surface chemistry. For example, the number and type of surface sites could be modified before polymer adsorption, or as we have shown, the segmentfsurface interactions could be disrupted by chemical means using probe molecules.