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Probing the Surface of Polymer Colloids by Conductometric Surfactant Titration Jeffrey M. Stubbs,*,† Patrice Roose,‡ Patrick De Doncker,‡ and Donald C. Sundberg† Nanostructured Polymers Research Center, UniVersity of New Hampshire, Durham, New Hampshire 03824, and Research and DeVelopment, Cytec Surface Specialties, Drogenbos, Belgium ReceiVed October 7, 2005. In Final Form: December 1, 2005 This work revisits the use of surfactant titrations for the characterization of latex particle surfaces. Experiments were performed to study the effect of comonomer composition and the effect of acid comonomers, and the technique is applied to the characterization of particle morphology in composite latices for several different systems. It is confirmed that the packing density of surfactant on a polymer surface is a linear function of copolymer composition. Inclusion of acid comonomers has the expected effect of decreasing the amount of surfactant adsorbed on the polymer surfaces. The usefulness of the technique in the determination of particle morphology is demonstrated, in particular toward the detection of thin layers of either seed or second-stage polymer on the particle surface which are not easily detected by other techniques such as transmission electron microscopy (TEM). Finally, it is shown that the use of acid comonomers in composite particles greatly reduces the usefulness of the surfactant titration technique for morphology characterization. A possible explanation for this effect is proposed.
Introduction Protective and decorative coatings are subjected to increasingly stringent environmental regulations. In addition to a shift from solvent-based to water-based systems, there is a growing demand to avoid any use of volatile organic compounds in the final formulations. In particular, the simultaneous requirements with respect to film formation, hardness, and block resistance of aqueous dispersion binders for architectural and wood coatings are still often dealt with by using filming aids (coalescing agents) that evaporate upon film formation so as to develop the mechanical properties of the polymer. It has been shown that control of the morphology of the polymer particles, for example, prepared following a multiple-step emulsion polymerization process, offers an elegant alternative to achieve the seemingly contradicting coating properties without the use of volatile additives.1 For these composite particles, characterization of the particle structure, aside from the polymer structure and composition, is needed to establish the relationships with the coating properties. Morphological characterization of dispersed polymer particles is often addressed using advanced analytical techniques such as transmission electron microscopy, atomic force microscopy, or solid-state nuclear magnetic resonance.2,3 However, supporting data from thermal and mechanical analysis (e.g., differential scanning calorimetry, dynamic mechanical analysis) or particle size and particle surface analysis have proven to be of considerable value in resolving the ambiguities left by the former analyses. * Author to whom correspondence should be addressed. E-mail: jstubbs@ cisunix.unh.edu. † University of New Hampshire. ‡ Cytec Surface Specialties. (1) Schuler, B.; Baumstark, R.; Kirsch, S.; Pfau, A.; Sandor, M.; Zosel, A. Prog. Org. Coat. 2000, 40, 139. (2) Shen, S.; El-Aasser, M. S.; Dimonie, V. L.; Vanderhoff, J. W.; Sudol, E. D. J. Polym. Sci., Part A: Polym. Chem. 1991, 29, 857. Butt, H.-J.; Gerharz, B. Langmuir 1995, 11, 4735. Landfester, K.; Spiess, H. W. Acta Polym. 1998, 49, 451. Kirsch, S.; Landfester, K.; Shaffer, O.; El-Aasser, M. S. Acta Polym. 1999, 50, 347. Kirsch, S.; Pfau, A.; Stubbs, J.; Sundberg, D. Colloids Surf., A 2001, 183, 725. (3) Valtier, M.; Drujon, X.; Wilhelm, M.; Spiess, H. W. Macromol. Chem. Phys. 2001, 7, 202.
Although the examination of colloidal polymer surfaces using surfactant titration is well described in the literature,4-13 the application of this method in combination with conductivity monitoring is revisited thoroughly here. In essence, the amount of a probe surfactant (i.e., sodium dodecyl sulfate, SDS) required to saturate a latex particle surface, combined with the knowledge of its critical micelle concentration in water, the particle size, and the solid content, allows the determination of the crosssectional area occupied by a surfactant molecule at saturation (as) on the polymer particle, which in turn is related to the chemical composition at the surface. The paper is organized as follows. First, the good reproducibility of the method is illustrated for single-phase homopolymer and copolymer latices prepared by semibatch emulsion polymerization, and the dependence of the surfactant packing density on the surface of copolymer latex particles is addressed. Next, the usefulness of the technique is demonstrated in the characterization of structured styrene-acrylate polymer particles prepared by two-step emulsion polymerization. Part of this study was conducted within the framework of a large round-robin test aiming at the resolution of the morphology of model composite latex particles. Here, the good agreement between independent surfactant titration results is emphasized, and these results are confronted with data from other techniques (especially electron microscopy) to provide a consistent picture of the particle structure. Finally, the use of surfactant titration in studying composite latices containing carboxylic acid comonomers is investigated with the aim of evaluating its potential usefulness in such systems. As a technically important parameter, (4) Maron, S. H.; Elder, M. E.; Ulevitch, I. N. J. Colloid Interface Sci. 1954, 9, 89. (5) Paxton, T. R. J. Colloid Interface Sci. 1969, 31, 19. (6) Su¨tterlin, N.; Kurth, H. J.; Markert, G. Makromol. Chem. 1976, 177, 1549. (7) Piirma, I.; Chen, S.-R. J. Colloid Interface Sci. 1980, 74, 90. (8) Okubo, M.; Yamada, A.; Matsumoto, T. J. Polym. Sci., Polym. Chem. Ed. 1980, 16, 3219. (9) Vijayendran, B. R. J. Appl. Polym. Sci. 1979, 23, 733. (10) Kronberg, B.; Stenius, P. J. Colloid Interface Sci. 1984, 102, 410. (11) Maurice, A. M. J. Appl. Polym. Sci. 1985, 30, 473. (12) Kong, X. Z.; Pichot, C.; Guillot, J. Colloid Polym. Sci. 1987, 265, 791. (13) Stubbs, J. M.; Durant, Y. G.; Sundberg, D. C. Langmuir 1999, 15, 3250.
10.1021/la052721n CCC: $33.50 © 2006 American Chemical Society Published on Web 02/11/2006
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Table 1. Composition and Polymerization Sequence of the P(BuA-co-MMA)-PS Composite Latices composite latex L1 L2 L3 L4
seed latex (I) P(BuA-co-MMA) (40 BuA:60 MMA) Tg ) 32 °C Tg ) 32 °C Tg ) 32 °C Tg ) 32 °C
second step (II) PS Tg ) 107 °C Tg ) 107 °C Tg ) 107 °C P(S-co-MAA) (93.4 S:6.6 MAA)
II:I weight ratio (%) 33
Table 2. Composition and Polymerization Sequence of the Model Non-acid Containing Composite Latices of the First Round-robin Test14 Composite latex
100 300 100
CL1
S1: PMMA Tg ) 123 °C
CL2
S2: P(MMA-co-MA)a (60 MMA:40 MA) Tg ) 60 °C S3: P(BuA-co-S) (27 BuA:73 S) Tg ) 58 °C
CL3
the minimum film formation temperature of the composite latices is also reported and discussed with respect to the titration results. a
Materials and Methods Single-Phase Copolymer Latices. Poly(n-butylacrylate-co-methyl methacrylate) (P(BuA-co-MMA)) latices of varying copolymer composition were prepared at Cytec Surface Specialties (CSS) according to the following recipe. A 2.5-L reactor equipped with an anchor stirrer was charged with 1459 g of demineralized water and 1.5 g of sodium dodecyl sulfate (SDS, Sigma, 99% purity). The reactor was heated to 80 °C and 1.5 g of ammonium persulfate, dissolved in 23.5 g of demineralized water, was added. The reactor was allowed to return to 80 °C over a period of 5 min after which the feed of (a mixture of) monomers was started at a rate of 2.8 g/min. After completion of the feed, the latex was left to stir at 80 °C for 60 min and was subsequently cooled to room temperature. The BuA:MMA ratios in the monomer feed were 0:100, 20:80, 40:60, 60:40, 80:20, and 100:0 and, hence, included the preparation of homopolymer latices of poly(n-butyl acrylate) (PBuA) and poly(methyl methacrylate) (PMMA). For reference, a polystyrene (PS) latex was produced under similar conditions. Multiple-Phase P(BuA-co-MMA)-PS Latices. A series of multiple-phase polymer latices were prepared at CSS by seeded emulsion polymerization using the P(BuA-co-MMA) latex with monomer ratio 40:60 BuA:MMA as seed material and forming polystyrene as the second-stage polymer. The composite latices were prepared according to the following recipe. A 2.5-L reactor equipped with an anchor stirrer was charged with the required amount of P(BuA-co-MMA) latex (20 wt % solids) which was used as the seed material. The reactor was heated to 80 °C. One gram of ammonium persulfate was dissolved in 25 mL of water and was added slowly to the seed latex. Neat styrene monomer or a mixture of styrene (S) and methacrylic acid (MAA) was added to the reactor over a period of 30-90 min depending on the quantity of monomer. During the same period, water and SDS were fed to the reactor using a separate feed stream at a quantity sufficient to adjust the end product solids content to 25 wt %. The amount of surfactant was adjusted to reflect the increase in surface area during the growth of the particles. The proportions of seed polymer and polystyrene were varied according to the weight ratios mentioned in Table 1. The conditions of the polymerization were such that, on the basis of much previous experience at CSS, starve fed conditions having high instantaneous monomer conversions were expected, although confirming measurements were not made in these particular experiments. Similarly, the amount of residual monomer was not measured, but given the expectations of starve fed behavior and the fact that the reactions were maintained at temperature for 1 h after the end of the monomer feed, residual monomer levels are certain to be very low. Composite Latices, First Round-Robin Study. For the comparative round-robin study,14 three model composite latices referred to as CL1-CL3 were prepared at the University of New Hampshire (UNH) by seeded emulsion polymerization. The compositions of seed and second-stage polymers were selected on the basis primarily of their respective polarities and glass-transition temperatures. The sequence of the multistep polymerizations is summarized in Table 2. The reactions were conducted in a jacketed glass reactor at 70 (14) Stubbs, J. M.; Sundberg, D. C. Polymer 2005, 24, 1125.
seed latex (I)
second step (II)
II:I weight ratio (%)
P(BuA-co-S) (30 BuA:70 S) Tg ) 52 °C PS Tg ) 107 °C
67
PMMA
82 100
MA: methyl acrylate.
°C under inert conditions with potassium persulfate (Alfa Aesar, 99.99% purity) as initiator and SDS (Sigma, 99% purity) as surfactant. Inhibitors were removed from the monomers prior to use. The seed latices were prepared by growing the particles from a preseed latex. All the polymerizations were performed under semibatch conditions, except for seed latex S1 that was produced under batch conditions. The process used to synthesize these latices is described in more details in ref 14. All second-stage polymerizations conducted at UNH (including those described in the following section) were conducted under starve fed conditions with high instantaneous conversion of monomer to polymer throughout the feed period. Samples were removed during the reaction and were analyzed gravimetrically and confirmed that the monomer concentration in the particles during the feed period was in the range of 0.5 M or less in all cases. As in the experiments at CSS, the reactor was maintained at reaction temperature for at least 1 h after the completion of the monomer feed so that the residual monomer levels were always very low. Composite Latices Containing Carboxylic Acid, Second Round-Robin Study. In a second round-robin study, another series of composite latices was synthesized to study latex morphology characterization methods in film-forming systems having one hard and one soft polymer phase with respect to the polymerization temperature. In addition, the latices contained an acid comonomer, methacrylic acid (MAA), copolymerized as a small percentage of the seed polymer. The synthesis conditions were nearly identical to those described for the latices in Table 2 and, hence, are not further described here. The major difference was that these polymerizations were buffered at a pH of approximately 3 because of the use of acid comonomers. The pH was controlled by using a buffer system composed of a combination of citric acid and monobasic sodium phosphate.15 Table 3 provides a description of the acid containing composite latices.
Latex Characterization Conductometric Titrations. Prior to titration, the latices were diluted to approximately 4-6 wt % solids and were cleaned by either serum replacement (method used at CSS) or passing through a mixed bed of ion-exchange resins (method used at UNH) to remove any surfactant and electrolyte remaining from their preparation. For cleaning by serum replacement, the latex was charged in a 400 cm3 ultrafiltration cell with a polycarbonate filtration membrane (pore size 100 nm, Osmotics). Distilled deionized water (conductivity ≈ 1 µS/cm) was fed to the cell, and the filtrate was collected. The cleaning process was controlled by online monitoring of the filtrate conductivity and was stopped when the latter reached the conductivity value of the incoming water (typically after 3 days). In some experiments, the extent of surfactant removal after different cleaning periods was inspected by surfactant titration.16 (15) Fukuhara, D.; Sundberg, D. Prog. Colloid Polym. Sci. 2003, 124, 18. (16) Roose; P.; De Doncker, P. J. Appl. Polym. Sci. 2004, 92 (5), 3226-3230.
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Table 3. Composition and Polymerization Sequence of the Model Acid Containing Composite Latices of the Second Round-Robin Test composite latex
seed latex (I)
second step (II)
II:I weight ratio (%)
ACL1
P(MMA-co-MAA)(98 MMA:2 MAA) Tg ) 126 °C P(MMA-co-MAA)(98 MMA:2 MAA) Tg ) 126 °C
P(BuA-co-S)(57 BuA:43 S) Tg ) 11 °C P(BuA-co-S)(30 BuA:70 S) Tg ) 55 °C
100
ACL2
For latices produced at CSS, the hydrodynamic particle size of the cleaned systems was measured by photon correlation spectroscopy using a Malvern Autosizer Lo-C with a 64 channel 7032 auto-correlator. The polydispersity index obtained from a quadratic cumulants analysis of the time auto-correlation function of the scattered light intensity was lower than 0.05, suggesting narrow particle size distributions for all the latices in this work.17 For latices produced at UNH, the particle size distribution (PSD) was measured using capillary hydrodynamic fractionation (CHDF, CHDF 2000, Matec Applied Sciences) and also exhibited narrow PSDs. The dry solid content of the latices was determined gravimetrically. The conductometric titrations were carried out at room temperature using an automatic titroprocessor (Metrohm, 716 DMS Titrino) and a conductometer (Metrohm, 712). A 0.1 M titrant solution of SDS (99% purity, Sigma) was added every 12 s in volume increments of 20 µL to ≈30 g of sample while stirring. A detailed description of the conditions of sample cleaning (mixed bed ion exchange) and conductometric surfactant titration at UNH can be found elsewhere.13 Glass-Transition Temperature. The reported polymer glasstransition temperatures, Tg, were measured by differential scanning calorimetry. At CSS, the measurements were conducted at a heating rate of 20 K/min using a Q10 DSC from TA Instruments. Measurements at UNH utilized a Q100 DSC from TA Instruments operated under temperature modulated conditions with an underlying heating rate of 3 K/min, an amplitude of 1.5 K, and a period of 60 s. All sample weights were about 10 mg. Minimum Film Formation Temperature (MFFT). MFFT values were determined on a Rhopoint temperature gradient bar ‘MFFT 90’. After stabilization of the temperature gradient (typically a span of 18-27 °C selected in the temperature range 0-90 °C) on the nickel-plated device substrate, the latex was applied by a brush and was left to dry while subjected to an air flow of 4 L/min. The MFFT was recorded as the lowest temperature at which a transparent crack-free film was formed after drying. Gel Permeation Chromatography (GPC). GPC was performed using a Waters system equipped with three Styragel columns (one HR3, one HR4, and one HR5) connected in series. The system is equipped with both refractive index (Waters 2414) and ultraviolet detectors (Waters 486) and was calibrated using EasiCal polystyrene standards obtained from Polymer Laboratories. Electron Microscopy. Both transmission and scanning electron microscopy (TEM and SEM) were used to characterize the latex particle morphologies. The details of these techniques, as applied to the latices discussed here, are reported separately.14
Results and Discussion Conductometric Surfactant Titration. In Figure 1, the conductometric SDS titrations of distilled deionized water, a cleaned PMMA, and a cleaned PBuA latex sample are shown. The SDS concentration where the slope of the titration curve decreases corresponds to the point where micelles start to form, (17) Finsy, R. AdV. Colloid Interface Sci. 1994, 52, 79.
100
Figure 1. Conductometric SDS titrations of distilled deionized water (narrow line), a cleaned PMMA (medium line), and a cleaned PBuA latex (wide line). Roughly 240 aliquots were added during the titration, giving the appearance of a smooth line.
that is, the critical micelle concentration (cmc). Because of the partial adsorption of surfactant onto the polymer particles, more surfactant is required to saturate a latex in comparison to pure water. At saturation, the cmc is reached in the aqueous phase and the accessible polymer surface is covered to a maximum with surfactant. Throughout this study, the surfactant concentration at the saturation point was determined from the conductometric titration curves according to a procedure described by Stubbs et al.13 The cmc of SDS in deionized water was estimated from five replicate titrations. At 22 °C, a cmc value of 8.22 ( 0.04 mmol/L was found in very good agreement with other data cited in the literature.7,13,18 The random error was estimated at a 95% confidence level using Student’s t-distribution. The adsorption density of surfactant at a polymer/water interface for highest coverage (saturation) can be expressed as
σ)
nas nts - nws = Sp mpAsp
(1)
where nts denotes the total number of surfactant molecules added at the saturation point, nas the amount of surfactant adsorbed on the available polymer surface of area Sp, and nws the amount of surfactant residing in the aqueous phase. nws can be calculated from the cmc value of the surfactant in water.5 Sp was calculated from the polymer mass mp in the sample and the specific surface area Asp of the particles. In the assumption of spherical particles, Asp is reasonably estimated from the average hydrodynamic particle diameter d as
Asp )
6 Fd
(2)
where F is the particle density. Particle densities were approximated by a weighted average of the bulk homopolymer densities over the monomer composition. Finally, from the adsorption density, σ, the molecular adsorption area can be determined as as ) 1/σ. From the conductometric titrations of the PBuA and PMMA latices in Figure 1, as values of 66 ( 3 Å2 and 120 ( 5 Å2 (i.e., σBuA ) 1.52 nm-2 and σMMA ) 0.83 nm-2) were, respectively, (18) Jo¨hnsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solution; J. Wiley & Sons: Chichester, U.K., 1998.
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Figure 2. The dependence of the molecular adsorption area, as (open symbols) and packing density, σ (solid symbols), of SDS on the BuA-MMA copolymer composition. The 95% confidence intervals calculated from three independent measurements are shown as error bars. The straight line results from a linear regression to the packing data.
found for SDS. One can sometimes get disparate values for as if the surfactant has not been completely removed during the serum replacement (or ion exchange) step, but it is possible to verify the quality of cleaning from surfactant titrations in binary blends of latices, as described previously.16 It is also possible that sulfate end groups on the polymer chains may be present on the surface and may affect the measured as values. Our experience, for initiator concentrations typically employed in emulsion polymerization, shows that for more polar polymer surfaces such as PMMA, this may contribute up to a 10% change in as. For less polar surface such as PS, initiator end groups have very little effect on the as values. We have not explicitly considered this effect in this study but do not expect that it should have any effect on our conclusions. Application to Non-Acid-Containing Polymer Latices. Single-Phase Particles. It is known that the surfactant adsorption on homopolymer latex particle surfaces depends on the polarity of the polymer itself, with more dense packing on hydrophobic surfaces than on more hydrophilic surfaces. On the other hand, there are few data published which show the nature of the adsorption on well-controlled (i.e., no compositional drift) copolymer latex particle surfaces. The discussion presented below describes such an analysis for one of our own latices and also a reanalysis of some experimental results previously published by others. The adsorption of SDS within a series of butyl acrylate and methyl methacrylate homo- and copolymer latices with varying compositions was studied. As observed in Figure 2, the adsorption area, as, increases with MMA content in the polymer and is related to an enhancement in surface polarity.7,9,10,19 When the packing density (molecules per unit surface area, σ) of the surfactant on the surface is considered, a linear dependence on composition is expected for random copolymers. This is due to the random nature of the copolymer and the connectivity between the comonomer units, which prevents one of the comonomers in the chain from disproportionately covering the surface. A plot of σ as a function of composition in Figure 2 shows the expected linear behavior. There is some discontinuity between the points (19) Vijayendran, B. R. In Surfactants in Solution; Mittal, K., Ed.; Plenum Publishing Corporation: New York, 1989; Vol. 9, p. 435.
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Figure 3. The dependence of the molecular adsorption area, as (open symbols), and packing density, σ (solid symbols), of SDS on the MMA-S copolymer composition. Data obtained from ref 7 and replotted in the form of packing density as well as the adsorption area.
at 60 and 80%, but these points do not show a systematic deviation from the expected linear behavior and seem most likely because of experimental error. This result of linearity is of particular importance for the application of surfactant titration methods to the characterization of latex particle morphology, as described later in this paper. Some years ago, Piirma7 presented data for SDS adsorption on P(MMA-co-S) latex particles and reported a nonlinear relation between as and composition, as in Figure 3. A plot of their data in the form of σ versus composition shows a very linear plot, with almost no scatter in the data. In combination with the data presented in Figure 2, this provides an excellent confirmation of the expected linear relationship. Another useful set of data comes from Maurice11 for the adsorption of sodium dodecyl benzene sulfonate (SDBS) onto latex particle surfaces of P(MMA-co-S) and P(MMA-co-BuA). The data are plotted as as versus composition and as σ versus composition in Figure 4a and b. The latter curves are nearly linear, but the former are nearly linear as well. Thus, on the basis of the various data sources, we conclude that the surfactant packing (σ) on copolymer latex particle surfaces depends linearly upon polymer composition for random copolymers. Multiple-Phase Latex Particles. Here, the goal was to demonstrate the utility of surfactant adsorption toward the morphological characterization of several different latex systems which contain both homo- and copolymers as the two components. In this section, the discussion is restricted to systems that do not contain carboxylic acid monomers. The influence of such monomers is considered later. 1. Multiple-Phase P(BuA-co-MMA)-PS Latices. Figure 5 shows the saturation adsorption area of SDS on composite P(BuA-coMMA)-PS particles as a function of the weight fraction of the second-stage PS. The as values are also shown in Table 4. With a nearly constant as value in the range 85-92 Å2, the data essentially indicate that the surface composition of the composite particles is close to that of the soft, seed copolymer, even for polystyrene contents up to 75 wt % (25 wt % seed polymer). To understand the meaning of the surfactant adsorption data, we first consider the film formation temperatures of these latices reported in Table 4. The MFFT of 38 °C appears to be consistent with a particle containing 75% seed polymer (Tg ) 32 °C) and having a surface predominated with seed copolymer. The fact that the MFFT is some 12 °C above that of the pure seed copolymer
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Figure 6. UNHLATEX EQMORPH20 simulation results for (A) experiment L1, (B) L2, (C) L3, and (D) L4. (dark phase represents seed polymer, light phase represents second stage). Figure 4. (a) The dependence of the molecular adsorption area, as (open symbols) and packing density, σ (solid symbols), of SDBS on the MMA-S copolymer composition. Data reproduced from Maurice et al.11 with permission. (b) The dependence of the molecular adsorption area, as open symbols, and packing density, σ (solid symbols), of SDBS on the MMA-BuA copolymer composition. Data obtained from ref 11 and replotted in the form of packing density as well as the adsorption area.
Figure 5. Composition dependence of the molecular adsorption area of SDS for composite P(BuA-co-MMA)-PS latices. The filled symbols refer to emulsion latices produced with polystyrene in the second stage of the synthesis. The open symbol shows the result for an emulsion latex where MAA acid was added to the styrene in the second-stage reaction.
latex is consistent with the presence of a hard (PS) filler in the soft seed particle. The related particle morphology might be an inverted core-shell (ICS) or one with microdomains of PS dispersed within the seed copolymer. On the other hand, the MFFT data for the 50 and 75% PS composite particles show nonfilming latices, thus indicating nondeformable particles, despite the as values indicating the presence of soft, seed copolymer at the surface. This strongly suggests that for the
Table 4. Molecular Adsorption Area of SDS and Minimum Film Formation Temperature for the P(BuA-co-MMA), PS, and Composite P(BuA-co-MMA)-PS Latices latex
as (Å2)
MFFT (°C)
seed PS L1 (25% S) L2 (50% S) L3 (75% S) L4 (75% S/MAA)
90 47 92 85 87 101
26 >90 38 >90 >90 >90
styrene-rich latices the particle structure is not an inverted coreshell but that the outer particle region may contain a substantial fraction of polystyrene (possibly occluded) located below an outermost shell rich in soft seed polymer. Such structures would likely prevent film formation at temperatures significantly below the Tg of the PS. In a further attempt to reconcile the adsorption and MFFT data, the particle morphologies were predicted using the UNHLATEX software.20,21 Two simulation programs were utilized, EQMORPH20 which predicts the thermodynamically preferred, or equilibrium, morphology, and KMORPH21 which predicts how the morphology development will be impacted by kinetic considerations. The results of the EQMORPH simulations are summarized in Figure 6. Here, a plot is shown that describes the reduced interfacial free-energy surface for a range of possible morphologies (for these examples, it is only necessary to know that the left-hand side of each energy surface corresponds to a core-shell (CS) morphology and that the right-hand side corresponds to an inverted core-shell (ICS)). The equilibrium morphology is that which corresponds to the minimum energy on this surface and is shown for each experiment by the inset particle representations (light area corresponds to second-stage polymer and black is seed polymer). For experiments L1 (25% S), L2 (50% S), and L3 (75% S), the equilibrium morphology (20) Durant, Y. G.; Sundberg, D. C. J. Appl. Polym. Sci. 1995, 58 (9), 1607. (21) Karlsson, O. J.; Stubbs, J. M.; Carrier, R. H.; Sundberg, D. C. Polym. React. Eng. 2003, 11 (4), 589.
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Table 5. Molecular Adsorption Areas of SDS for the Reference and Composite Latices as Determined Independently at Two Laboratories Using Conductometric Titrationa latex b
S0 S1 S2 S3 CL1 CL2 CL3
as (Å2) CSS
as (Å2) UNH
MFFT (°C)
46 117 121 55 81 123 120
44 120 121 52 78 110 115
>90 >90 62 83 90 >90
a The adsorption areas are average values obtained from two (CSS) or three (UNH) titrations. Minimum film formation temperatures are also reported. b Refers to a PS latex.
Table 6. Relative Surface Composition of the Composite Latices percentage of second-stage polymer at particle surface (%) latex
CSS
UNH
CL1 CL2 CL3
40 0 102
41 6 97
in all three cases is an ICS. The result for experiment L4 in Figure 6D will be discussed later. The results of KMORPH simulations are not shown, but in all cases it was predicted that the second-stage polymer radicals should easily be able to diffuse completely into the interior of the particles, which suggests that formation of the ICS structures may in fact be possible from a kinetic standpoint. Thus, it is not at all surprising that the surfactant adsorption data reflect the presence of the soft, seed copolymer at the extremities of the particles. The PS would be expected to be within the particles but may yet influence the filming characteristics of the latices, depending upon the total PS content in the particles. 2. Multiple-Phase Latices with Polar Seed Polymers and Nonpolar Second-Stage Polymers. Polar seed polymers (based on PMMA) and nonpolar second stage polymers (based on PS) were used for the experiments labeled CL1 and CL2 in Table 2. For CL1, the seed polymer was glassy at reaction temperature, and for CL2, the seed polymer was soft at reaction temperature. Thus, in the first case (CL1) we might have expected no penetration of the second-stage P(BuA-co-S) into the PMMA seed and should have seen this reflected in the surfactant adsorption data. The data are reported in Table 5 where one can see that the as values from two independent labs agree quite closely and average about 80 Å2. This value is midway between those of the pure second-stage polymer (S3) and the pure seed polymer (S1) in Table 5. From this set of conditions, we calculate (Table 6) that if both polymers are located on the surface of the particle, 40% of it is related to the second-stage P(BuA-co-S). This would suggest that the particle morphology is that of a hemispherical or a nonspherical lobed structure. Independent characterization of the particle morphology by TEM, SEM, and AFM14 clearly shows a lobed particle structure. The fact that this latex has an MFFT of 83 °C (the Tg’s of the first- and secondstage polymers are 123 °C and 52 °C, respectively) also attests that the particle surface is not uniformly composed of one polymer. Hence, the surfactant titration data offer reliable and complementary evidence that both polymers are present at the surface. Further, it seems possible to estimate the fraction of surface occupied by each of them. For experiment CL2, the relative Tg’s of the first- and secondstage polymers were switched without significantly altering their relative polarities. In this case, we would expect that the secondstage polymer radicals could penetrate the soft seed particles, thus yielding an ICS or occluded morphology, somewhat like
Figure 7. Microtomed TEM image of latex CL2. The second-stage polystyrene is the darker phase.
that shown in Figure 6B for sample L2 noted in Table 4. In CL2, the soft seed copolymer is very polar with an SDS adsorption area as ) 121 Å2 as shown in Table 5. The SDS adsorption area on the CL2 composite latex is about 117 Å2 (we note again the good agreement between results from different labs) clearly indicating that the external surface of these particles is composed of the seed copolymer (as detailed in Table 6). Thus, the PS second-stage material must be buried within the particle; the MFFT of 90 °C is consistent with this particle structure as it is intermediate to those for the seed and second-stage polymers. The morphology characterization by electron microscopy has been performed separately and does show occlusions of PS within the polar seed polymer represented by the dark areas in Figure 7. However, in general the TEM indicates a dark PS shell on the particles (albeit an occluded shell) but does not clearly reveal a light acrylic seed polymer layer covering the surface. This is an example of how surfactant titration is able to reveal information that is not revealed by microscopy techniques. In fact, surfactant titration is sensitive to even very thin layers covering the particle surface, while these features are easily overlooked by TEM. 3. Multiple-Phase Latex with Nonpolar Seed Polymer and Polar Second-Stage Polymer. This (CL3 in Tables 2, 5, and 6) is an interesting system in that one might expect that the equilibrium morphology will be core-shell (PMMA as shell) while also expecting that the PMMA radicals can easily penetrate the seed particle during polymerization. The MFFT (>90 °C) certainly suggests that the outside of the particle is quite glassy, and the as value of 118 Å2 (Table 5) clearly indicates that the surface is entirely PMMA, as shown by the calculated results listed in Table 6. In combination, both of these data sets point to a PMMA shell on the composite particles. However, neither method can offer a measurement of the thickness of the shell nor the fraction of the total PMMA that may be buried within the soft, seed polymer. Independent differential scanning calorimetry (DSC) measurements22 suggest that the vast majority of the PMMA in this sample is mixed with the seed polymer and is not well phase-separated from it but that a small portion of the total PMMA is well phase-separated. The surfactant adsorption results tell us that some or all of that PMMA is on the surface. Application to Acid-Containing Polymer Latices. Single-Phase Latices. The incorporation of acid comonomers is expected to increase the saturation adsorption area by increasing the polarity of the copolymer. Table 7 shows the saturation adsorption areas measured for a series of P(BuA-co-S-co-MAA) copolymers of varying MAA content. The conditions used to (22) Stubbs, J. M.; Sundberg, D. C. J. Polym. Sci. Part B: Polym Phys. 2005, 43, 2790.
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Table 7. Molecular Adsorption Area of SDS on P(BuA-Co-S-Co-MAA) Latex Particle Surfaces Containing Varying Amounts of Copolymerized MAA polymer composition (wt. %)
as (Å2)
30 BuA:70 S 35 BuA:60 S:5 MAA 40 BuA:50 S:10 MAA
59 87 93
synthesize these latices have been described elsewhere15 as have the conditions used to clean the latices and perform the surfactant titrations.13 The latices were produced under starve fed, semibatch conditions and resulted in a uniform statistical copolymer, as confirmed by NMR analysis.15 It is clear that the incorporation of MAA in the copolymer does in fact increase the adsorption area quite substantially. Composite Latices, Acid in the Second Stage. Experiment L4, described in Table 1 with the result given in Figure 5, shows that the addition of MAA during the second step of the synthesis results in a saturation adsorption area as > 100 Å2, which is higher than the values reported for polystyrene and seed polymer particles, that is, as ) 47 Å2 and 90 Å2, respectively. The likely explanation is that the incorporation of MAA into the secondstage polystyrene increases the as value from 47 to above 100 Å2 and that the P(S-co-MAA) forms the shell of the composite particles as was predicted from a thermodynamic standpoint in Figure 6D. Unfortunately, the adsorption area of neat P(S-coMAA) having the same composition was not measured nor was the morphology of the composite particles characterized, so this cannot be verified at the moment. The experiments and results described below further investigate the use of surfactant titration in acid-containing composite latices combined with more detailed morphological characterization. Composite Latices, Acid in the Seed, Second Round-Robin Study. For the latex systems introduced in Table 3, an acidic comonomer (MAA) was incorporated in the seed copolymer. The morphologies of these systems were well characterized by other methods and, hence, these experiments serve as an opportunity to evaluate the usefulness of the surfactant titration method toward determination of morphology in acid-containing systems. Predictions made using UNHLATEX EQMORPH20 show that the most stable morphology is an inverted core-shell (ICS) because of the much more polar nature of the seed polymer relative to the second-stage polymer. However, simulations using UNHLATEX KMORPH21 show that the second-stage radicals cannot diffuse fully (penetrate) into the seed particles, so that the second-stage polymer is thought to be formed entirely in the shell region. This means that formation of the ICS equilibrium morphology is prohibited by kinetic considerations. Other experiments14 have shown that such conditions are ideal for the formation of lobed particles such as the type described previously for system CL1 in Table 2. If the lobed particle structure is indeed formed, then the surfactant titration results should show that the surface is covered by a significant fraction of both the seed and the second-stage polymers. The results for system ACL1 obtained at UNH are shown in Table 8. The surfactant titration results suggest that only about 10% of the surface is covered by second-stage polymer. The MFFT values reported by CSS and UNH were 45 and 50 °C, respectively. These values are well above the MFFT for the pure second-stage polymer (12 °C) but are well below that for the pure seed polymer (120 °C). Clearly, the morphology cannot be inferred from these values alone. However, it is possible to eliminate the possibility of having an inverted core-shell or an occluded morphology in which the seed polymer comprises the continuous phase because in these cases the MFFT value would
Table 8. SDS Titration Results for the Systems ACL1 and ACL2a as of seed polymer as of second-stage polymer (P2) as of the composite particle fraction of the composite particle surface covered by P2
ACL1
ACL2
126 (125) 46 (57) 111 (117) 8% (12%)
126 52 95 23%
a The (average) values of the SDS adsorption area as are reported in Å2. The values determined at CSS for the sample ACL1 are shown in parentheses for comparison.
Figure 8. Microtomed TEM of the film derived from latex ACL1. The second-stage P(BuA-co-S) is the darker phase.
have to be close to the Tg of the seed polymer, which is well above 100 °C in this system. The clear determination of the particle morphology for this system was complicated by the film-forming nature of the latex, so that only the morphology of the film derived from the latex was observed in a microtomed TEM section (i.e., not the morphology of the individual latex particles). This electron micrograph, Figure 8, shows that the continuous, or film-forming, phase is the second stage P(BuA-co-S) which appears dark. This film morphology is consistent with either a lobed or a coreshell morphology but is not at all consistent with an inverted core-shell or an occluded morphology as suggested by the surfactant titration results. Both the MFFT and TEM results seem inconsistent with the surfactant titration results, but questions still remain because of the difficulty in characterizing the morphology of this film-forming latex. The second acid-containing system, ACL2 in Table 3, was designed such that a lobed structure would still be expected but with a Tg of the second-stage polymer above room temperature (but below reaction temperature) to facilitate morphology characterization of the single particles by other methods (not including surfactant titration). Then, this latex can be used to conclusively test the applicability of surfactant titrations in acidcontaining systems. Figure 9a and b shows SEM and microtomed TEM images for ACL2. From the SEM, it is clear that the particles are nonspherical as expected for a lobed structure. This is confirmed by the microtomed TEM which also confirms that the lobes consist of the second-stage P(BuA-co-S) copolymer (dark contrast in Figure 9b). Whole particle TEM (not shown) also confirmed the lobed structure. The results from the surfactant titrations shown in Table 8, suggesting that only 23% of the particle surface is covered by the second-stage polymer, do not seem to be in agreement with the observed lobed particle morphology. If all the second-stage polymer were present on the particle surface as lobes, and with
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Stubbs et al.
Figure 9. Microscopy results for system ACL2. (a) SEM; (b) microtomed TEM.
a stage ratio giving an equal volume of seed and second-stage polymer, it is hard to believe that the second-stage polymer would only cover 23% of the surface. Of all the systems discussed here which suggested a lobed morphology [CL1, ACL1 (expected but not fully confirmed), and ACL2], only the non-acid-containing system (CL1) produced a surfactant titration result that was consistent with a lobed morphology. It seems clear that surfactant titrations for the acid-containing systems overestimate the fraction of the surface that is covered by the acid-containing polymer. However, these results merely indicate that the presence of the acid comonomers is the cause for the apparent contradiction between surfactant titration and morphology, but they do not reveal any reason behind it. Hence, further characterization experiments were designed in an attempt to elucidate this anomaly. If it assumed that a small fraction of low molecular weight and MAA enriched oligomers are formed in the MAA containing seed latices, one can reasonably assume that they adsorb onto the particle surface with very little partitioning to the water phase and, hence, are not removed easily during the ion exchange or dialysis steps prior to the surfactant titrations. Such oligomers might be bound fairly strongly to the surface but might be short enough so that they have some mobility to move around on the surface. If this were the case, then when lobes are formed on the seed particle surface during the second-stage polymerization, these oligomers can migrate and cover the surface of the lobes. In one sense, they would be acting like surfactant molecules, preferentially covering the less polar surface to decrease the interfacial tension between that surface and the aqueous phase. On the other hand, they are behaving quite differently than a classical surfactant, which distributes over a surface by a desorption/readsorption mechanism. Since these hypothetical oligomers are not easily desorbed from the surface, they must diffuse along the surface to redistribute over the lobes forming from the second-stage polymer. Eventually, this would result in a larger adsorption area for surfactant on the lobe surfaces, giving rise to larger as values measured for the composite particles, which is of course the result that has been observed. A convenient method to detect the presence of low molecular weight (MW) oligomers is gel permeation chromatography (GPC). However, since we are interested in a minor fraction of the seed polymer, preparative separation is required prior to GPC analysis. The following procedure was followed. Both the acidcontaining P(MMA-co-MAA) seed latex (used in both ACL1 and ACL2) and the pure PMMA seed latex (S1 used in CL1) were cleaned with ion-exchange resins in the normal manner. Both latices were then dried at room temperature, and the resulting dry polymers were dissolved in acetone at a concentration of about 3 wt %. These acetone solutions were then titrated slowly with deionized water. As water was added to the solutions, the solvency power of the acetone/water mixture decreased and the
Figure 10. GPC chromatograms of oligomers remaining soluble in acetone/water solution. Black lines represent P(MMA-co-MAA) sample, gray lines represent PMMA sample. Thick and thin lines represent first injection and repeat injection, respectively.
polymer started to precipitate. The high molecular weight chains became insoluble and precipitated first, with the lower MW chains remaining soluble to higher concentrations of water. After precipitation of most of the polymer (g99%), the resulting suspensions were centrifuged and the completely transparent supernatant was removed. The supernatant was then dried and the small amount of residual polymer (estimated to ≈0.5 to 1 wt % of the total polymer, although with little accuracy given the small masses of polymer remaining soluble) was dissolved in THF and was analyzed by GPC. The results of the GPC analysis are presented in Figure 10. The GPC data are very reproducible with essentially no difference between repeat runs of each sample. The most important point is that a peak is observed at slightly above 30 min for the P(MMA-co-MAA) sample but not for the straight PMMA sample. This peak elutes at essentially the same time as a polystyrene standard having a molecular weight of 5400 g mol-1. We conclude that there is a small percentage of oligomers in the P(MMAco-MAA) sample having an MW of approximately 5000 g mol-1, which is not present in the PMMA sample. This would correspond to oligomers of about 50 repeating units, which is in the range that might be expected to be surface active but having very little solubility in water (this of course depends on the MAA content in the oligomers). This result reasonably agrees with our hypothesis that the surfactant titrations are affected when using latices containing acid comonomers because of the presence of a small fraction of surface-active oligomers that are not removed during ion exchange or dialysis cleaning steps prior to the surfactant titrations. This appears to be the reason for the contradiction between the surfactant titration results and the observed particle morphology. Of course, these experiments have not proven this explanation. We have only confirmed the presence of these oligomers and not that they reside on the surface or are able to redistribute on that surface by a diffusive mechanism. However, it remains that this explanation is one that can explain
Probing the Surface of Polymer Colloids
the experimental results entirely. A full conclusion on this topic is left to future study. We finish by summarizing the main message of this section which is that, unfortunately, the value of surfactant titration data for characterization of structured latex particles is seriously impaired by the inclusion of acid comonomers in the particles.
Conclusions The technique of surfactant titration for the characterization of latex particles, in particular for structured particles, has been revisited. It has been shown that surfactant titration is a reliable and reproducible method to probe the specific polymer composition at the surface of polymer particles. Furthermore, for random copolymers, we have obtained further confirmation that surfactant-packing density is a linear function of copolymer composition. As a method for characterizing the morphology of composite latex particles, surfactant titration is perhaps unique in its ability to specify the polymer composition at the particle
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surface when there is only a very thin layer of that polymer on the surface. Finally, it has been shown that the use of acid comonomers degrades the usefulness of the titration technique, probably because of the existence of low MW acid-containing oligomers. A possible explanation for this effect is that such oligomers can migrate to all regions of the latex particle surface through a surface diffusion mechanism. Acknowledgment. J.M.S. and D.C.S. would like to thank the members of the UNH Latex Morphology Industrial Consortium (Arkema, Cytec Surface Specialties, DSM/NeoResins, Rohm and Haas) for their financial support. We also thank Daisuke Fukuhara of Asahi Chemical Co. for performing some of the experimental work. P.R. and P.D.D. extend thanks to Hans De Brouwer and Steven Van Es for their practical support of this work. LA052721N
