Langmuir 1996, 12, 3139-3142
3139
Studies of the Validity of the Dye Leaching Method for Investigating Particle Nucleation Mechanisms in Emulsion Polymerizations Dong Ryul Kim and Donald H. Napper* School of Chemistry, University of Sydney, Sydney, New South Wales 2006, Australia Received November 14, 1995. In Final Form: March 18, 1996X It has been proposed that the addition of a tracer dye to a styrene emulsion polymerization system prior to the commencement of polymerization can cast light on the mechanism of latex particle nucleation. It was argued that the dye would be able to be leached out of the resulting latex by dialysis if a micellar mechanism were operative whereas leaching would be precluded if the particles were nucleated by a coagulative mechanism. Experiments to test this proposed method have been reported previously by Pashley and co-workers: they showed that leaching occurred with an oil soluble initiator, thus suggesting a micellar nucleation mechanism, but was not observed if the water soluble persulfate anions were used as the initiator, implying a coagulative nucleation mechanism. It is shown here that the interpretation of such experiments, performed in unbuffered systems, was vitiated by large changes in pH, such as occurs with persulfate anions as the initiator. It was found that, irrespective of the chemical nature of the initiator, dye could be leached from high pH latexes (pH > 8.0) but could not be leached from low pH latexes. Identical results were obtained whether or not the dye was added to the emulsion system prior to the commencement of polymerization or postadded after the latex had been formed. Moreover, the leaching of the dye from a high pH latex could be halted by lowering the pH. The results suggest that considerable caution must be exercised in deducing nucleation mechanisms using the dye leaching method.
Introduction A detailed understanding of the mechanism by which polymer latex particles are nucleated in emulsion polymerization remains elusive. Several theories, however, have been postulated to explain latex particle generation. The first theory was that proposed by Harkins1,2 and subsequently rendered quantitative by Smith and Ewart.3,4 According to the Harkins theory, the primary free radicals generated by initiator decomposition in the aqueous phase enter the monomer swollen surfactant micelles and polymerize the monomer solubilized therein. The primary locus of particle nucleation is thus the interior of the micelles. Nowadays, it is recognized that the entry of free radicals into the micelles is likely to require the addition of one or more monomers to the primary free radicals in order to render them sufficiently amphiphilic to be able to enter into a micelle.5,6 Notwithstanding this minor modification, nucleation would still proceed primarily in conjunction with the micelles. It is known, however, that latex particles can be generated in the presence of submicellar concentrations of surfactants;7 indeed, stable latex particles can even be produced in the complete absence of added surfactant, provided an appropriate ionic initiator (e.g., persulfate anions) is used (the so-called “surfactant-free” emulsion polymerization).5,8-10 The nonmicellar mechanisms of X
Abstract published in Advance ACS Abstracts, May 15, 1996.
(1) Harkins, W. D. J. Chem. Phys. 1945, 13, 381. (2) Harkins, W. D. J. Am. Chem. Soc. 1947, 69, 1428. (3) Smith, W. V.; Ewart, R. H. J. Chem. Phys. 1948, 16, 592. (4) Smith, W. V.; Ewart, R. H. J. Am. Chem. Soc. 1948, 70, 3695. (5) Priest, W. J. J. Phys. Chem. 1952, 56, 1077. (6) Maxwell, I. A.; Morrison, B. R.; Napper, D. H.; Gilbert, R. G. Macromolecules 1991, 24, 1629. (7) Napper, D. H.; Parts, A. G. J. Polym. Sci. 1962, 61, 113. (8) Roe, C. P. Ind. Eng. Chem. 1968, 60, 20. (9) Kotera, A.; Furusawa, K.; Takeda, Y. Kolloid Z. Z. Polym. 1970, 239, 677; 240, 837. (10) Goodwin, J. W.; Hearn, J.; Ho, C. C.; Ottewill, R. H. Colloid Polym. Sci. 1974, 252, 464. (11) Hansen, F. K.; Ugelstad, J. J. Polym. Sci., Polym. Chem. Ed. 1978, 16, 1953. (12) Lichti, G.; Gilbert, R. G.; Napper, D. H. J. Polym. Sci., Polym. Chem. Ed. 1983, 21, 269.
S0743-7463(95)01033-X CCC: $12.00
particle nucleation that have gained widespread acceptance are homogeneous nucleation,9,10 together with its subsequent extension to comprehend what has become known as “coagulative nucleation”.5,8-10 According to the homogeneous nucleation theory, the primary free radicals generated by initiator decomposition in the aqueous phase add monomer molecules until the resultant amphiphilic molecules are rendered insoluble. Precipitation of this insoluble species then ensues to form nascent latex particles, which can absorb monomer and grow. The coagulative nucleation theory postulates that the nascent particles, also termed “precursor” particles, are necessarily colloidally unstable and undergo coagulation until they form mature, and colloidally stable, latex particles. Although the various mechanisms postulated to describe latex particle nucleation are phenomenologically quite different, it has proved to be remarkably difficult to devise experiments that differentiate critically between them. Part of this difficulty may perhaps arise because neither micellar nucleation nor homogeneous nucleation occur exclusively in many systems; both mechanisms can in principle be operative simultaneously, although one may often be dominant. Karaman, Meagher, and Pashley13 have nonetheless published in this journal a dye tracer method which they claimed allows the micellar and homogeneous nucleation mechanisms to be differentiated. The idea underpinning this dye leaching method is to add the water soluble form of a dye (e.g., the disodium salt of fluorescein) to an emulsion polymerization of styrene prior to initiation. Pashley and co-workers argued that because the fluorescein dye was not surface active, it would not be incorporated into any of the micelles present prior to the commencement of polymerization but would remain dissolved in the aqueous phase. If micellar nucleation occurred, it would thus be expected that after polymerization, the dye could be readily removed from the aqueous phase of the resultant latex by dialysis. If, however, (13) Karaman, M. E.; Meagher, L.; Pashley, R. M. Langmuir 1993, 9, 1220. (14) Bleger, F.; Murthy, A. K.; Pla, F.; Kaler, E. W. Macromolecules 1994, 27, 2559.
© 1996 American Chemical Society
3140 Langmuir, Vol. 12, No. 13, 1996
homogeneous nucleation were operative, it would be expected that the adsorption of the dye molecules onto the hydrophobic surfaces of the precursor particles would result in the dye being entrapped permanently within the polystyrene latex particles after coagulation and the formation of mature particles. Their subsequent removal from the latex by dialysis would thus be precluded. Pashley and co-workers13 found that whether or not the tracer dye could be dialyzed from a polystyrene latex prepared by emulsion polymerization depended critically upon the chemical nature of the initiator studied. It was, however, independent on whether the surfactant was anionic or nonionic. With water soluble persulfate anions as the initiator, it was found that fluorescein could not be dialyzed from the latex; in contrast, with the oil soluble mixed initiator, 2,2′-azobis(2-methylproprionitrile) and lauroyl peroxide, the dye was readily removed by dialysis. These authors concluded that homogeneous nucleation was operative in styrene emulsion polymerizations initiated by the decomposition of persulfate anions whereas micellar nucleation generated the latex particles with the mixed oil soluble initiators. If its validity could be confirmed, the dye leaching method would represent a simple yet powerful method for discriminating between the various possible nucleation mechanisms in the emulsion and microemulsion polymerization of a wide variety of monomers. In what follows, we present new experimental results that explore the validity of this proposed methodology. Materials and Methods Styrene (Ajax) was vacuum distilled before use. Sodium dodecyl sulfate (SDS) Aldrich) was used as received. Cetyltrimethylammonium bromide (CTAB) (Aldrich) was recrystallized from acetone and methanol. 1-Hexanol (BDH) was vacuum distilled whereas 1-pentanol (BDH) was used as received. All water was double distilled. Potassium persulfate (KPS) (Ajax) was recrystallized from water and 2,2′-azobis(isobutyronitrile) (AIBN) (Fluka) was recrystallized from methanol. Potassium bicarbonate (KHCO3) (Merck) was used as received. A 60Co source with a dose rate of 0.44 kGy/h provided the γ-ray initiation. Visking dialysis tubing was boiled in water for 30 min prior to use. All polymerizations were performed in a round bottom flask (250 cm3), fitted with a mechanical stirrer (250 rpm). The flask was immersed in a thermostated water bath and could be flushed with nitrogen. Polymerization times were typically 3-5 h. The z-average latex particle sizes were measured by dynamic light scattering (DLS) using a Malvern 4700SM particle sizer at a fixed scattering angle of 90° and at an argon ion laser wavelength of 488 nm.
Results and Discussion Styrene Emulsion Polymerizations. Unfortunately, Pashley and co-workers13 did not publish any details of the recipes that were used in their styrene emulsion polymerization studies. It was therefore not possible to reproduce exactly the conditions under which their experiments were performed. Nevertheless, we were able to reproduce their general results for unbuffered styrene polymerizations, using the recipes shown in Table 1. The fluorescein tracer was released on dialysis of the latex generated using the oil soluble initiator 2,2′-azobis(isobutyronitrile) (see experiment 3) but not when persulfate anions were the initiator (see experiment 2). Note that in a separate experiment, the average size of the latex particles was found to be identical whether or not the dye was present, suggesting that its presence did not perturb the nucleation process perceptibly. Surfactant-free Styrene Polymerizations. Surfactant-free styrene polymerizations almost certainly
Kim and Napper Table 1. Recipes and Leaching Results for the Unbuffered Styrene Emulsion Polymerizations experiment designation 2 ingredient/g styrene SDS KPS AIBN dye water temperature/°C average diameter/nm dye leaching
7.94 0.81 0.434 0.0038 104.3 70 37 no
5
6a
9.05
9.05
1.5 0.18
0.10
0.10
0.0018 100.0 70 523 no
nil 100.0 70 445 b
3
4
7.94 0.81 0.264 0.0038 104.3 70 56 yes
0.002 13.5 25 yes
a
Initiated by γ-rays; 0.15% (w/v) of KHCO3 added prior to polymerization. b Not applicable.
proceed by a homogeneous/coagulative nucleation mechanism since surfactant micelles are clearly absent. If the concepts underlying the dye leaching methodology are correct, then it would be expected that latexes generated in this way, using persulfate as the initiator, would not release the fluorescein tracer or dialysis. The result of experiment 4 in Table 1 confirms the validity of this prediction. Note, however, that in this instance (see experiment 5 in Table 1), the size of the latex particles was increased somewhat by the presence of the dye, suggesting that the fluorescein may have promoted coagulation in this system. Styrene Emulsion Polymerizations Initiated by γ-Rays. A styrene emulsion polymerization was initiated by γ-rays at 25 °C. Unexpectedly, the dye was found to be released on dialysis of the resulting latex (see experiment 6 in Table 1). This observation might be interpreted to suggest that when γ-rays induced initiation, particle nucleation in styrene emulsion polymerizations proceeded via the micellar mechanism, in marked contrast to the homogeneous/coagulative mechanism indicated by the same method to be operative when persulfate anions were used as the initiator. Yet in both cases, the species that caused initiation (SO4•- for persulfate and predominantly •OH for γ-rays) were generated in the aqueous phase. Note that in this latter experiment, potassium bicarbonate (0.15% (w/v)) was added to the initial recipe to provide an initial pH of 8.8; this value was reduced only marginally (to 8.7) during the course of the polymerization. Application to Styrene Microemulsion Polymerization. There seems to be general agreement in the literature13 that in styrene microemulsion polymerizations, particle nucleation occurs by the entry of free radicals into the microemulsion droplets, irrespective of the chemical nature of the initiator or surfactant. As such microdroplets can crudely be considered to be “hyperswollen” micelles, it would be expected that any added dye would be readily removed by dialysis from the microlatexes generated by microemulsion polymerization. The results for the microemulsion polymerizations of styrene presented in Table 2 do not bear out this prediction entirely. The microemulsions prepared from the cationic surfactant CTAB with 1-hexanol as the cosurfactant (experiment 13-1) gave results that unexpectedly paralleled those for conventional emulsion polymerizations. With the oil soluble initiator, azobis(isobutyronitrile), the dye was released on dialysis but no dye release was observed when persulfate anions constituted the initiator. Note that some ambiguity could attend the interpretation of these data because the cationic surfactant could interact strongly with the anionic dye, confounding the observations which were difficult to make in this instance. However, the microlatex generated by polymerizing a
Latex Particle Nucleation by Dye Leaching
Langmuir, Vol. 12, No. 13, 1996 3141
Table 2. Recipes and Leaching Results for the Styrene Microemulsions experiment designation 13-1 ingredients/g water CTAB SDS dye styrene 1-hexanol 1-pentanol AIBN KPS temperature/°C particle diameter/nm dye leaching
13-2
95.3 1.9
95.3 1.9
0.0025 1.9 0.95
0.0033 1.9 0.95
13-3 82.3 9.1 0.005 4.85 3.85
0.0095 0.16 60 25 no
60 24 yes
0.0254 70 24 yes
Table 3. Recipes and Leaching Results for Buffered Systems experiment designation ingredients/g AIBN water NaOH KHCO3 SDS dye styrene water KPS temperature/°C particle diameter/nm initial pH final pH dye leaching a
65
66
67
68
69
18-6
79.6 nil nil nil nil 7.5 8.3 0.13 70 760 6.6 2.2 a
80.0 nil nil 1.2 nil 10.0 10.0 0.9 70 39 7.6 2.9 a
79.4 0.05 0.113 1.2 0.015 10.0 10.0 0.13 70 35 11.4 10.7 yes
79.4 nil 0.113 1.2 0.015 10.0 10.0 0.13 70 35 8.8 8.7 yes
79.9 nil nil 1.2 0.015 10.0 10.0 0.13 70 40 7.6 2.9 no
0.03 90.0 nil nil 1.2 0.005 10.0 nil nil 70 77 3.0 3.0 no
Not applicable.
microemulsion prepared using SDS as the surfactant and 1-pentanol as the cosurfactant, coupled with a relatively low concentration of persulfate anions as the initiator, was found to release its dye on dialysis, in conformity with the prediction detailed above. The Effects of Buffering of the Emulsion Polymerization Systems. Some of the results presented above appear to support the validity of the dye leaching method but there remained disturbing exceptions to this agreement. Especially difficult to explain is the result observed for the latex prepared using the γ-radiation induced emulsion polymerization of styrene. A key feature of this experiment was the fact that the pH of the system remained essentially constant at ca. 8.8 throughout the polymerization. This high and constant value contrasts markedly with the dramatic drop in pH that occurred in unbuffered styrene emulsion polymerizations with or without surfactant (see experiments 65 and 66 of Table 3) when persulfate anions were used as the initiator. Typically, if the pH of the system was between 6 and 8 prior to the commencement of polymerization, the final pH of the latex produced was between 2 and 3. The cause of this severe reduction in pH was the hydrolysis of the persulfate anions to form bisulfate anions, which are acidic in character:
S2O82- + H2O f 2HSO4- + 1/2O2 HSO4- + H2O h H3O+ + SO42One conclusion to be drawn from the foregoing results is that two variables (initiator type and pH) were varied in
the experiments of Pashley and co-workers. The observations presented here imply that the large decrease in pH that accompanied the use of persulfate anions as the initiator may have been implicated in some way with the failure of the fluorescein dye to leach from the resultant latexes. To test the validity of this inference, two emulsion polymerizations initiated by persulfate anions were performed, each containing potassium bicarbonate as a buffer (see experiments 67 and 68 of Table 3). Sodium hydroxide was added to the emulsion system denoted as experiment 67 to ensure that its initial pH (11.4) was significantly higher than that in experiment 68 (pH ) 8.8). In both cases the final pH of the latex generated was greater than 8.0. Moreover, in both cases the fluorescein dye was rapidly leached from the resultant latexes on dialysis. To exclude the possibility that in both experiments 67 and 68 more dye was present than could be adsorbed by the latex particle surfaces, experiment 69 was undertaken. It was identical in its ingredients (see Table 3) with experiments 67 and 68, except for the absence of potassium bicarbonate and sodium hydroxide. Its initial pH was 7.6, but the pH declined as polymerization proceeded to its final final value of 2.9. As observed previously in the absence of buffering, dye leaching could not be detected from the resulting latex. Since the average size of the particles generated in experiment 69 was similar to those generated in experiments 67 and 68, the surface areas available for dye adsorption were comparable in all three cases. If the area available in experiment 69 was sufficient to adsorb all the dye, then presumably it was also sufficient in experiments 67 and 68. Hence the leaching observed in experiments 67 and 68 did not arise from insufficient latex surface area for dye adsorption. It seems reasonable to infer that the observation of leaching in experiments 67 and 68 was associated with their high pH. Postaddition of Dye. To confirm further the pivotal role played by pH in determining whether or not dye leaching could be observed, dye was postadded to a sample of the latex generated in experiment 69 at its final pH of 2.9. No dye leaching could be observed on dialysis of this latex. Yet when the pH of this latex was raised to greater than 8.0, dye leaching could be readily observed. Similarly, lowering the pH of a sample of the latex generated in experiment 68 to ca. 3.0 brought about an immediate cessation of leaching. The experiments described above strongly suggest that the conclusions reached by Pashley and co-workers with regard to nucleation mechanisms in styrene may have been vitiated by large changes in pH in their unbuffered systems. Curiously, these authors did specifically consider this possibility, even conducting an emulsion polymerization experiments at a pH that was controlled around 7-8, observing no effect of pH between 3 and 8. Unfortunately, in light of the present results, the upper limit of the pH range studied was not quite high enough; had these authors investigated a somewhat higher pH say, above pH ) 8.0, they are likely to have reached the diametrically opposed conclusion. Oil Soluble Initiator at Low pH. Finally, an additional check on the validity of the foregoing interpretation of the effects of pH on leaching was performed as follows: a styrene emulsion polymerization was carried out using the oil soluble initiator azobis(isobutyronitrile) at a low pH of 3.0 (see experiment 18-6 in Table 3). This low pH was achieved by the addition of hydrochloric acid. Under these acidic conditions, it was found that the fluorescein was not leached out by dialysis, contrary to the observations at higher pH reported by Pashley and co-workers and which was confirmed in these studies. It
3142 Langmuir, Vol. 12, No. 13, 1996
seems most unlikely that the mechanism of nucleation in these systems would be strongly pH dependent; the most plausible explanation for the different leaching behavior observed lies in a strong pH dependence of leaching. The structure of the fluorescein molecule shows why a strong pH dependence of leaching might be expected. It contains both a carboxylic acid group (pKa ≈ 4) and a phenolic hydroxyl group (pKa ≈ 8). At the low pH observed at the end of a persulfate-initiated emulsion polymerization, the fluorescein is likely to exist primarily in its undissociated nonionic form. This would limit its solubility in water and explain its strong and apparently irreversible adsorption onto the polystyrene latex particles. Above pH ≈ 8, it would exist primarily in its doubly ionized form, which would explain its high water solubility and ease of leaching on dialysis. Conclusions The interpretation of the observation by Pashley and co-workers that the tracer dye fluorescein could be leached from polystyrene latexes generated by emulsion polymerizations that were initiated by an oil soluble initiator but not from latexes initiated by persulfate anions appears to have been confounded by the large drop in pH that the
Kim and Napper
latter system underwent in the absence of buffering. Dye leaching was not observed in the current studies from any low pH polystyrene latex, irrespectively of whether the initiator was the water soluble persulfate or an oil soluble azonitrile initiator. Moreover, dye leaching could be observed from buffered polystyrene latexes at high pH (pH > 8.0) that were prepared using persulfate initiators. Dye leaching from unbuffered latexes at high pH could be halted by lowering their pH. These observations were reproduced whether or not the fluorescein was added to the initial emulsion prior to the addition of initiator or postadded after the completion of polymerization. It follows that the inference by Pashley and co-workers that the differences observed in the leaching behavior of polystyrene latexes according to the chemical nature of the initiator used can be ascribed to different mechanisms of nucleation must be treated with considerable caution. While it is not yet known whether their general conclusion associating different chemical initiators with different mechanisms of nucleation is correct, it seems unlikely from the present studies that it was justified by the experimental results that they reported. LA9510339
