The Stabilizer-Free Emulsion
Polymerization
The Stabilizer-Free Emulsion
Polymerization
Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben, voorzitter van het College voor Promoties,
in het openbaar te verdedigen op
dinsdag 24 september 2013 om 15.00 uur door
Marta Edyta DOBROWOLSKA
Master of Science in Chemistry, University of Warsaw geboren te Warschau, Polen.
Dit proefschrift is goedgekeurd door de promotoren: Prof. dr. J.H. van Esch
Copromotor: Dr. ing. G.J.M. Koper Samenstelling promotiecommissie: Rector Magnificus, voorzitter
Prof. dr. J.H. van Esch, Technische Universiteit Delft, promotor Dr. ing. G.J.M. Koper, Technische Universiteit Delft, copromotor Prof. dr. E.J.R. Sudholter, Technische Universiteit Delft
Prof. dr. K.U. Loos, Rijksuniversiteit Groningen Prof. dr. W.K. Kegel, Universiteit Utrecht Prof. dr. M.A. Cohen Stuart, Wageningen Universiteit
Dr. K. Tauer, Max-Planck-Gesellschaft
Prof. dr. S.J. Picken, Technische Universiteit Delft, reservelid The work described in this thesis was carried out in the Advanced Soft Matter group at the Delft University of Technology and was funded by the Dutch Polymer Institute (DPI).
© Marta Edyta Dobrowolska, 2013
Published by: Uitgeverij BOXPress, ‘s-Hertogenbosch ISBN 978-90-8891-688-5
All rights reserved. The author encourages the communication of scientific contents and explicitly allows reproduction for scientific purposes, provided the proper citation of the source. Parts of the thesis have been published in scientific journals and copyright is subject to different terms and conditions.
Dla Mamy i Taty.
“Nothing in life is to be feared, it is only to be understood. Now it is the time to understand more, so that we may fear less”
Contents Chapter 1
1. Introduction 1
1.1 General introduction to emulsion polymerization 2
1.2 Colloidal stability 3
1.3 Emulsification 4
1.4 Surfactant-free emulsion polymerization 4
1.5 Explanation of the thesis content 5
Chapter 2
2. Direct visualization of “coagulative nucleation” in surfactant-free emulsion polymerization 9
2.1 Introduction 10
2.2 Materials and methods 12
2.3 Results 13
2.4 Discussion 17
2.5 Conclusion 21
Appendix 26
Chapter 3
3. Optimal Ionic Strength for Nonionically Initiated Polymerization 31
3.1 Introduction 32
3.2 Materials and methods 34
3.3 Results 35
3.4 Discussion 37
3.5 Conclusions 40
Chapter 4
4. Bimodal molecular mass distribution in surfactant-free emulsion polymerization as a consequence of “coagulative nucleation” 45
4.1 Introduction 46
4.2 Materials and methods 48
4.3 Results and discussion 50
4.4 Conclusion 55
Appendix 58
Chapter 5
5. Natural charging on hydrophobic surfaces in water 63
5.1 Introduction 64
5.2 Materials and methods 67
5.3 Results 68
5.4 Discussion 71
5.5 Conclusion 73
Chapter 6
6. Stability of surfactant-free mini-emulsions 77
6.1 Introduction 78
6.2 Materials and methods 81
6.3 Results 82 6.4 Discussion 85 6.5 Conclusion 85 Summary 89 Samenvatting 93 Acknowledgements 97
Chapter 1
Introduction
ABSTRACT
The wide use of emulsion polymerization processes for the synthesis of polymer particles has resulted is an increased interest in the understanding and development of this method. From an environment point of view, emulsion polymerization poses the mildest process to synthesize polymer particles. However, the main disadvantage of the emulsion polymerization process is the contamination of the end product with surface-active species, either added as part of the formulation or produced as by-product of the polymerization reaction. The surface-active species are used for the stabilization of the polymer particles so that the resulting product remains a colloidal suspension.
The aim of the research described in this thesis is to design and develop a method for surfactant-free emulsion polymerization. In order to do so, three main issues had to be understood: colloidal stability, emulsification method and polymerization process.
1.1 . General introduction to emulsion polymerization
Emulsion polymerization is a widely used process for the production of polymers used for adhesives, paints, binders, additives etc.1 It is one of the most common ways of polymerization due to low viscosity of the product, easy heat removal as water is usually the continuous medium, good temperature control and high polymerization rates. Generally, there is no need to add any additional solvents and untreated monomers are easy to remove. High molecular weight polymers can be produced at high polymerization rates, which cannot be done in a bulk polymerization process. Nevertheless, the end product may consist of some impurities coming from the reaction such as surfactants and initiator molecules.
The term emulsion polymerization misleadingly seems to indicate that the polymerization reaction is taking place in the emulsion droplets. Yet, the polymerization always starts in the aqueous phase and the monomer droplets act as a reservoir for monomers. This also suggests that the monomer has to be slightly soluble in water in order to migrate to the growing particles. Polymerization starts upon the addition of an initiator, which can be water-soluble, but also oil-soluble and redox initiators are available for the synthesis. The rate of polymerization is dependent on the decomposition rate of the initiator, which is very strongly depending on temperature for the case of the commonly used thermal initiators. When micelles are present in the water phase, the oligomers enter the micelles, which are swollen with the monomer, shortly after polymerization has started when the polymer chains are no longer soluble in water. In this way the polymer particles gain stability and the particles can continue to grow. Other matter as radicals or even particles can also enter the micelles.
Stabilization by means of surface-active molecules or nanoparticles is necessary to prevent phase separation of the emulsion into its constituting bulk phases before polymerization has terminated. Especially in the case of surface-active molecules, the stabilization mechanism does not prevent the exchange of material between the globules containing the dispersed monomers before or during the polymerization process. Small differences in
globule sizes are sufficient to drive the emulsion to phase separation due to Ostwald ripening because of the differences in Laplace pressure between the differently sized globules. This can be prevented by mini-emulsion polymerization2 where yet another additive - a higher than monomer molecular weight mass hydrophobe, which is completely insoluble in water – is added that by its large osmotic effect annihilates the effect of the Laplace pressure difference in the droplets.
1.2 . Colloidal stability
Droplets of a single phase dispersed in a continuous phase are not stable. They encounter a driving force to coalesce into larger droplets, which results in a phase separation of the emulsion with the result that the two phases separate out into two bulk layers. Addition of surface-active species such as surfactants, proteins, polymers or particles is needed to achieve stability of the system. Particles dispersed in continuous phase undergo Brownian motion; these are random movements resulting from collisions with molecules of the surrounding medium. If the particles have no protection mechanism that renders the interaction between them repulsive, these collisions will lead to coalescence of the particles and the dispersion will subsequently lose its stability. Colloidal stability requires repulsive forces between the particles otherwise they will naturally attract each other. Electric charge or an adsorbed layer of surface-active species can screen the attraction and moreover render the particles repulsive. Surfactants are molecules that have two sides; the one side of the molecule has more affinity for the one phase, while the other side of the molecule has more affinity for the other phase. That is why surfactants tend to adsorb at the interface. When surfactants are adsorbed at the interface, they usually form a repulsive barrier.
1.3 . Emulsification
Emulsions are dispersions of two immiscible fluids. They form the basis of many products and they need to be stabilized, which can be done with surfactants, proteins, polymers or particles. In general, there are two methods for the formation of emulsions3. First, by mechanical disintegration of the bulk material into fine emulsion droplets. Second, by condensation of the disperse phase into larger aggregates, which means changing the solvency of the continuous phase for the liquid forming the dispersed phase. The first class of methods is often used on the industrial scale, it relies on application of intense flow fields4. Most of the times, these kinds of systems contain an emulsifying or stabilizing agent to produce an interfacial layer between the two liquids. Any surface between two immiscible fluids has an interfacial tension, which gives rise to a force at the interface that tends to reduce the interfacial area.
1.4 . Surfactant-free emulsion polymerization
In recent literature5,6 claims are made that emulsion polymerization can be performed without stabilizing species. The potential economic impact of such claims is great because of the lower demand on reactants needed for the process and because of the simpler production pathway. In addition, it constitutes a significant step towards environmentally friendly production of synthetic polymers. Indeed, from a fundamental point of view hydrophobic globules in water should be colloidally stable due to the accumulation of hydroxide ions at the globule-water interface7. In practice, this stability is hardly ever attained and many reasons have been provided for this8. Exceptions have also been reported9,10,11 albeit that the employed method is not well understood. There are quite some discrepancies between theoretical predictions and experimental results on emulsion polymerization12, which might partly be due to the large number of components that plays a role in the process6. A surfactant-free process where also no ionic initiators or additives are used might significantly reduce this complexity.
1.5 . Explanation of the thesis content
The goal of this thesis is to develop and understand the mechanism of stabilizer-free emulsion polymerization. In chapter 2 we will deal with an important issue of “coagulative nucleation”, which has been recognized as an important step in the emulsion polymerization process, because of which it is possible to obtain dispersion of fine, relatively uniform polymer latexes. It will be argued that this step is crucial for the understanding of the process and possible improvement of the prediction for modeling of emulsion polymerization. In chapter 3 it will be shown that fine polymer particles can be synthesized by means of surfactant-free emulsion polymerization using nonionic initiator, not only with ionic as was appreciated up till now. The particles are being monodisperse and stable if specific conditions are met. The dependence on ionic strength is crucial, much less so the pH of the reaction solution. Again crucial step is the aggregation of small primary particles into much bigger secondary particles. It is then shown, in chapter 4, that the bimodal molecular weight distribution is a consequence of so-called “coagulative nucleation” and it is shown that in both cases of ionic and nonionic initiated systems it is visible. The initial stage of the polymerization, by which the primary particles are formed, is of the 01-kind which means that it can be assumed that the particles are so small that at any moment of time there is no more than one radical chain per particle. After the aggregation of primary particles into secondary particles, the so “coagulative nucleation” step, the polymerization kinetics in the subsequently coalesced secondary particles is of the pseudo-bulk kind which means that there are so many radicals in the particles that the polymerization process proceeds as in bulk. Chapter 5 discusses the ability of the water surface to donate or accept protons, which is critical to many chemical and biological processes. There is an ongoing debate about the nature of this charge. Many experimental and molecular-dynamics simulation results have been presented but still, it is not clear when and how this charge develops at the water/hydrophobe interface. In order to utilize this phenomenon, the charging behavior has been
that it is possible to form stable oil-in-water emulsions without the use of surfactants and that stabilization by charge is sufficient. Controlling the size of the emulsion droplets is hampered by the large fluctuations observed in these systems.
REFERENCES
1. Lovell, P. A.; El-Aasser, M. S., Emulsion Polymerization and Emulsion Polymers. Wiley: Chichester, England, 1997.
2. Schork, F. J.; Luo, Y. W.; Smulders, W.; Russum, J. P.; Butte, A.; Fontenot, K., Miniemulsion polymerization. In Polymer Particles, Okubo, M., Ed. Springer-Verlag Berlin: Berlin, 2005; Vol. 175, pp 129-255.
3. Sumner, C. G., On the Formation, Size and Stability of Emulsion Particles. I. A New Method of Emulsification. The Journal of Physical Chemistry 1932, 37 (3), 279-302.
4. Hall, C. W.; A.W., F.; A.L., R., Encyclopedia of Food Engineering. Avi PBL.
Company Inc.: Westport, Connecticut, 1986.
5. Ngai, T.; Wu, C., Double roles of stabilization and destabilization of initiator potassium persulfate in surfactant-free emulsion polymerization of styrene under microwave irradiation. Langmuir 2005, 21 (18), 8520-8525.
6. Tauer, K.; Deckwer, R.; Kühn, I.; Schellenberg, C., A comprehensive
experimental study of surfactant-free emulsion polymerization of styrene. Colloid &
Polymer Science 1999, 277 (7), 607-626.
7. Vanoss, C. J.; Absolom, D. R.; Neumann, A. W., The hydrophobic effect.
Essentially a van der Waals interaction. . Colloid Polym. Sci. 1980, 258 (4), 424-427.
8. Meagher, L.; Craig, V. S. J., Effect of dissolved gas and salt on the
hydrophobic force betwen polypropylene surfaces. Langmuir 1994, 10 (8), 2736-2742.
9. Karaman, M. E.; Ninham, B. W.; Pashley, R. M., Effects of dissolved gas on
emulsions, emulsion polymerization, and surfactant aggregation. J. Phys. Chem. 1996,
100 (38), 15503-15507.
10. Pashley, R. M., Effect of degassing on the formation and stability of
surfactant-free emulsions and fine teflon dispersions. J. Phys. Chem. B 2003, 107 (7), 1714-1720.
11. Hartmann, J.; Urbani, C.; Whittaker, M. R.; Monteiro, M. J., Effect of
degassing on surfactant-free emulsion polymerizations of styrene mediated with RAFT. Macromolecules 2006, 39 (3), 904-907.
12. Herrera-Ordonez, J.; Olayo, R.; Carrol, S., The kinetics of emulsion
polymerization: Some controversial aspects. J. Macromol. Sci.-Polym. Rev 2004, C44 (3), 207-229.
Chapter 2
Direct visualization of “coagulative
nucleation” in surfactant-free emulsion
polymerization
ABSTRACT
It is generally believed that surfactant-free emulsion polymerization involves four steps: initiation, nucleation into primary particles, coagulation into secondary particles, and growth. By high resolution SEM-imaging of the intermediate polymerization products, the evolution of the morphology of the polymer particles has been followed. This allowed us, to our best knowledge for the first time, to visualize “coagulative nucleation” which is the process where the primary nanoparticles aggregate into larger entities. The thus obtained visual information and data on particle size, number, and zeta potential, strongly suggest that coagulative termination is responsible for the coagulative nucleation phenomenon resulting in a dispersion of fine, relatively uniform polymer particles.
2.1. INTRODUCTION
After more than a century, emulsion polymerization1 has matured into the preferred production method for adhesives, paints, additives in paper, textiles, etc.2,3,4. It has gained its popularity as an easy production process because of the low viscosity of the final dispersion, its excellent heat removal as water is usually the continuous medium, good temperature control, and high polymerization rates5. In general, there is no need for additional solvents and also the unreacted monomers are removed without difficulty5. By developing surfactant-free processes, the disadvantage of contamination of the final product by emulsifier, as is the case in the classical synthesis route, is largely removed6.
Figure 1. Schematic representation (not to scale) of the polymerization process as a sequence of partially concurrent processes. See the discussion section for a description of the phenomena.
From a chemical engineering point of view, technological development is still hampered by the unsatisfactorily level of predictability due to the as yet incomplete understanding of the involved mechanisms7,8,9. The presently accepted description of surfactant-free emulsion polymerization, see Figure 1 for a schematic representation, dates back to the 1980s and involves an intricate interplay of partially overlapping processes, of which “coagulative nucleation” is the distinctive phenomenon that is generally attributed to Feeney et al.10, 11 to account for the initially increasing growth rate of particles with time. However, already in 1952 Priest suggested that “inter-particle combination”, the term then used for this phenomenon, could play a role12. Important contributions have been made by Fitch and coworkers, who noted that some coagulation is needed to explain particle
formation at low emulsifier concentrations13 and who studied coagulation dynamics by intentional destabilization14.
Coagulation processes in themselves are already notoriously difficult to model, largely because of the many unknown parameters involved so that approximation schemes are required. The lack of experimental evidence for such a process in emulsion polymerization renders even state-of-the-art modeling by means of population balances unsuccessful as discussed for instance by Hosseini et al.15 and recently reviewed by Vale and McKenna16. Presently, these models are verified by experimental particle growth data17 or molecular mass distributions18 only, whereas information on for instance critical primary particle size and concentration, particle morphology, and surface charge density is missing18. Nevertheless, it has recently again been verified that a coagulation process must be present in order to correctly predict experimental observations19.
By imaging of dispersion samples taken at successive time instants during the polymerization process, we will demonstrate how essential information on the formation process of polymer colloids can readily be obtained. For the present example we have followed the work of Tauer et al.6 and performed a surfactant-free emulsion polymerization using only styrene as monomer and potassium persulfate (KPS) as initiator. This procedure results in a relatively long pre-nucleation period followed by slow growth, which enables on-line monitoring of latex morphology as well as various particle parameters. Of four samples, which were taken at different time intervals after the initiation of the polymerization, micrographs were made by means of ultra-high-resolution scanning electron microscopy (UHR-SEM).
2.2. MATERIALS AND METHODS
Chemicals. Styrene (Sigma Aldrich, ≥99%) was purified by use of a pre-packed
column (Sigma Aldrich) for tert-butylcatechol removal. Potassium persulfate (KPS, Sigma Aldrich) and sodium nitrite (NaNO2, Acros Organics, 98.5% pure for analysis) was used as received. Water was purified by Millipore AFS 3 water purification system.
General procedure for polymerization. Reactions were performed in a three-neck
glass flask filled with deionized water (90 mL). To remove the oxygen and traces of other gases, water was bubbled with nitrogen and subsequently degassed under vacuum, which was repeated 3 times. Purified styrene (1.22 mL) was injected into the reaction flask and equilibrated for 2 h at 60 oC. Polymerization was started by injection of 10 ml of a 50mM aqueous potassium persulfate (KPS) solution. After 180 minutes, the reaction was stopped by the addition of sodium nitrite (NaNO2).
Scanning Electron Microscopy. During the polymerization a portion of the reaction
solution was taken out after 15, 45, 120 and 180 minutes under the flow of inert gas (N2) with gas tight syringe and freeze-dried under high vacuum. A part of the dried material was put on a specimen holder and subsequently a molecular layer of gold was deposited for analysis by Scanning Electron Microscope (FEI/Philips XL30 SFEG) operating at 5kV and using the Ultra-High Resolution mode (UHR-SEM).
Atomic Force Microscopy (AFM). The atomic force microscope (AFM), a NTMDT
Ntegra, was used to observe the molecular-scale images of polystyrene particles. A drop of the solution was taken from the reactor at a given time and then cast on the PDMS surface, such that the polymers were able to adsorb. The images were obtained using semi-contact mode in air with cantilevers with Si tip of spring constant of 11 N/m and tip radius 1-3 nm.
Particle size and zeta potential measurements of water-suspended samples were
analyzed by dynamic light scattering (DLS, Zetasizer Nano ZS, Malvern). The instrument is using 173o angle non-invasive back-scatter mode and M3-phase analysis light scattering mode, using a red 4.0 mW, 633 nm He-Ne laser. The multiple
peak high-resolution fitting procedure was used to obtain the particle size distribution from the auto-correlation function. For calculating the zeta potential, the Smoluchowski equation was assumed20, see supporting information.
Turbidity Measurements. Samples were taken out from the reactor, cooled down to
room temperature and transferred to a 1 cm quartz cuvette. Transmission was measured using a UV/Vis-spectrometer (Shimadzu UV 1800). Based on transmission data and average particle size, particle number was calculated as outlined by Tauer and Kühn21. The initial decay is extremely steep so that the first experimental data point, taken after 15 minutes, remains inaccurate despite being the average of three independent experimental determinations.
2.3. RESULTS
Figure 2. Raspberry structured polymer particles imaged by, from left to right, UHR-SEM
(with sputtering) and AFM (without sputtering), all scale bars are 200 nm.
In Figure 2 we present images made by UHR-SEM and AFM from a sample taken 45 minutes after the initiation of the polymerization experiments. A raspberry structure of very small particles aggregated into various medium sized spheres is visible on the UHR-SEM and on the AFM pictures. Clearly the surface structure is not due to the sputtering agent because the two images in Figure 2 are taken from sputtered and non-sputtered samples. This can also be seen from the UHR-SEM images in Figure 3 where exactly the same procedure is used for all pictures and where in particular the surface is smooth at later stages. Together with the UHR-SEM picture and
considering earlier results of Yamamoto et al, who also used atomic force microscopy22, 23, it is evident that the imaged surface structure is realistic.
Let us then further discuss the evolution of the surface structure following the time sequence presented in Figure 3. The first picture, Figure 3A, presents the UHR-SEM image of a sample taken 15 minutes after the initiation of polymerization and shows small particles of around 15 nm diameter. The particles are well separated and only show a weak tendency to coagulation. The particle diameter measured by dynamic light scattering (DLS, see appendix and Figure 4) is larger, about 35 nm, as is a known effect of both the swollen nature of the early stage polymer colloids and the fact that the particle size distribution is rather broad. In diluted solution the zeta potential has been measured to be -73.4 mV, which indicates good stability.
After 45 minutes of polymerization time, the particle diameter as obtained from the micrograph in Figure 3B has increased to 20 nm which is only slightly larger than in Figure 3A. From the same graph it is clear that there is a stronger tendency for coagulation, which is reflected by the larger mean particle diameter of 90 nm as obtained by DLS. Also, the particles are soft which is concluded from the deformation of the spheres upon aggregation. Some very large sphere-like structures are visible that apparently have formed from a large number of particles. The measured zeta potential is -67.5 mV, which is the value for the typical aggregates and not for the individual small particles.
The micrograph in Figure 3C is from a sample taken after 120 minutes of polymerization time. It shows aggregates with a much narrower size distribution and a typical diameter of around 190 nm. Although less clear, the primary particle structure is still visible and the primary particles appear not to be much larger than at 45 minutes. The particle aggregates have a zeta potential of -65.7 mV.
The final graph in Figure 3D, taken from a sample after 180 minutes of polymerization time shows that the particles have grown to a typical diameter of about 235 nm, whereas the morphology of the particles has becomes that of homogenous spheres. The zeta potential is determined at -65.2 mV at 120 minutes and remains constant.
Other techniques than UHR-SEM and AFM as used here do not yield clear images of the “coagulative nucleation” process as discussed here. For instance, TEM and cryo-TEM images of the same particles (see Supporting Information) show a homogeneous core which is probably due to the proceeding polymerization and the softness of the primary particles. The sequence of pictures in Figure 3 and the observed length exclude an interpretation in terms of heterogeneous nucleation as discussed by Tauer8.
Figure 3. SEM micrographs of freeze-dried samples taken during a surfactant free
polymerization process at successive time lapses after initiation. (A) 15, (B) 45, (C) 120, (D) 180 minutes (A and B scale bars are 200 nm, C and D scale bars are 500 nm).
Figure 4. Record of the surfactant-free emulsion polymerization process with determination
of particle diameter from DLS (squares) and particle number (circles) as obtained from turbidity (see text).
2.4. DISCUSSION
On the basis of the above experimental findings, let us now rationalize the observed primary and secondary particle diameters. The primary particle is initially built up from small insoluble oligomers although some partially soluble inactive oligomers may contribute. The radical oligomers subsequently grow by polymerization involving chain growth of the oligomers inside the particle using the available monomers in the solution and by adsorption of still water soluble oligomers. This view is supported by an estimate of the zeta potential24,25 on the basis of this particle diameter and expected average molar mass for the oligomers of about 2500 (taken from ref.6), which yields the value of about -80 mV guaranteeing a stable dispersion (see Supporting information).
From experiments performed under very similar conditions to ours, Tauer et al. concluded from the inflection point in the conductivity versus time curve, that particles are being formed after a pre-nucleation period of about 7 minutes6. In this pre-nucleation period, the oligomers polymerize to about 6 monomeric units6 at which degree of polymerization they reach such a low solubility that they aggregate and
correlation spectroscopy, a relatively monodisperse particle size distribution (see supporting information) emerges. Similar information can be deduced from the UHR-SEM pictures taken after 15 minutes (see Figure 3A). This strongly suggests a
burst nucleation process as described by Privman26 and by Sugimoto27 albeit that the original literature dates back to LaMer28,29. The idea is that the concentration of insoluble oligomers increases until it reaches a critical level after which nucleation sets in and subsequently lowers the concentration of oligomers never to exceed the critical level again9 do to the low monomer solubility and the slow transport from the bulk monomer phase.
After this initial burst, the average particle size increases and the particle number decreases as can be seen from Figure 4. Data for similar experimental conditions were reported by Tauer et al.6, 21 and much earlier by Goodall et al30. From our UHR-SEM pictures taken from samples after longer polymerization times, it is clear that after the initial busts the small, so-called primary particles have aggregated into larger clusters. These data suggest that the nucleated primary particles are colloidally unstable. This is in contradiction with the fact that the zeta potential of the primary particles was experimentally determined to be about -70 mV, which is normally considered to hold for colloidally very stable dispersions even at more elevated ionic strengths31.
Nevertheless, it is generally accepted that the primary particles undergo limited aggregation32. In a canonical aggregation process, dispersed particles irreversibly aggregate into clusters that subsequently grow by aggregating, again irreversibly, with other clusters or with remaining free particles. The term coarsening is often used for this process, as the size distribution of the dispersion gets broader with a median value going to larger and larger values with time. Important to note is that the initial part of the aggregation process, which largely involves the individual particles, is much faster than the final part of the process, which involves the large particle clusters. In a dispersion of primary particles there are two ways to achieve a stationary particle size distribution with a given mean value. One way involves reversible aggregation, where the size distribution is the consequence of the balance between the aggregation rate and the break-up rate. The other way is by means of a
limiting mechanism such as a protective surfactant coating around the clusters to inhibit further aggregation from the moment that the coating layer is complete. In the following we shall argue that in the present situation the aggregation process is reversible. In actual fact, the secondary particles are involved in at least one growth process, which is due to the continued polymerization. Another growth process involving aggregation may be indiscernible from this first one. The fact that the size distribution remains stationary and does not broaden significantly with time leads us to the conclusion that the aggregation process is reversible.
The question that remains to be answered concerns the nature of the aggregation process. Following Feeney et al.33 one could invoke the classical Smoluchowsky theory complemented with the Fuchs stability factor34 to describe aggregation. The particle interaction then is subsequently described by the classical DLVO-potential34. This method does not appear to yield satisfactory results, see the recent review by Vale and McKenna35, and often a simpler approach is chosen35,36 where the interaction strength is determined by the colliding particle sizes only37. Recall that the stability factor itself hardly depends on particle sizes34,38. The discrepancy of modelling predictions and experimental results as mentioned above may be due to the fact, that the outlined scenario does not involve a limit to aggregation. As discussed above, in order to achieve a limiting effect, for instance a build-up of repulsion between particles is needed in such a way that the resulting size distribution becomes relatively mono-dispersed26. Feeney et al. suggested that incompletely polymerized oligomeric chains become surface active by charged initiator groups remaining after radical generation and subsequently adsorb on particles to render these charged33. Once the particles have aggregated to the extent that the available surface area has been reduced to accommodate almost all surfactant molecules, further particle aggregation stops39. Many experimental facts plead against this suggestion, for instance the fact that both ionic and non-ionic initiators lead to relatively mono-dispersed particles whereas the non-ionic initiators do not render surface-active species40.
Figure 5. Schematic representation of reversible aggregation of two particles with active
chains.
But more importantly, a stationary particle size distribution will not emerge if the aggregation is not reversible, as for instance discussed recently by Privman39. One feasible mechanism would be that when two particles collide (Figure 5.), active chains from the one and the other particle combine and terminate their polymerization. Break-up of the particles is still possible for small primary particles where the chains are relatively short so that they may detach from one of the particles again. A similar mechanism has been worked out in detail for systems of colloids with controlled ligand-receptor stoichiometry41. The available cross-linking agent and the available surface-active ligands then control the resulting particle size. In actual fact, this mechanism of coagulative termination has also been proposed by Herrera-Ordonez et al to explain the development of molar mass and polymerization reaction rate in emulsion polymerization42,43. The mechanism has already been discussed before in the context of the polymerization of vinyl chloride44, although in their approach the more appropriate term for the mechanism would be radical entry driven particle coalescence. The relatively monodisperse secondary particles that are visible in Figure 3 could indeed result from such a process of limited, reversible aggregation.
Within the secondary particles, polymerization can take place in two ways: by intra-particle polymerization, i.e. chain propagation inside the primary intra-particles, and by inter-particle polymerization where chains form between the particles. The fact that the primary particle structure keeps the same length scale while slowly fading away indicates that the growth of the inter-particle network is the dominant process. Just like the primary particle size, the secondary particle size will be controlled by the accumulated charge due to radical remnants from the initiator24,45. As polymerization progresses, the internal particle structure is cured as is obvious from the
disappearing internal structure, which leads to a much more uniform size distribution. Simultaneously more polymer is also created on the surface of the particles, which results in a slow but gradual growth of the particle diameter with time. In our example this amounts to about 50 nm per hour for the monomer concentration used, see Figure 4.
2.5. CONCLUSION
In conclusion, we have found evidence for the aggregation process of primary particles into secondary particles, the so-called “coagulative nucleation”, which lies at the basis of surfactant-free emulsion polymerization growth mechanism. An important condition that allowed us to find this evidence, is the relatively low polymerization rate achieved by our experimental conditions. For higher rates, the initial part of the formation process where the “coagulative nucleation” occurs may be too short for experimental assessment.
The here presented evidence of a “coagulative nucleation” stage in emulsion polymerization will make predictive modelling much more realistic. However, it also involves a serious complication, as not only information on the molecular scale46 but also on the meso-scale of the primary and in particular the secondary particles is required. It is expected, that the interplay of these processes on the various levels and time scales remains a challenge for the unravelling of emulsion polymerization in the near future.
ACKNOWLEDGMENT
Rienk Eelkema and Emanuela Negro are acknowledged for the discussions. Marcel Bus for obtaining AFM micrographs. Job Boekhoven for obtaining cryo-TEM micrographs. Louw Florusse for obtaining TEM micrographs.
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Appendix
Conductivity study
The average conductivity and zeta potential measured for samples at different polymerization time (measured some time after polymerization)
Sample 15min 30min 45min 60min 75min 90min 120min Conductivity [mS/cm] 1.47 1.43 1.46 1.49 1.54 1.47 1.56 Zeta potential [mV] -73.4 -71.4 -67.5 -70.8 -65.7 -66.9 -65.2
Estimation of the surface charge
Following Tauer 24, we estimate the surface charge for a 15 nm particle consisting of chains with an average degree of polymerization of 2,500. We begin by calculating the volume of the particle as
3 4 3
V= πa
from which the mass follows with the styrene mass density ρ=0.9 10 kg/m⋅ 3 3as 3
4 3
m=ρ πa
The number of chains with an average degree of polymerization Pis calculated using the molar monomer mass M =104.15 g/mol
3 4 3 chains A A a N mN N PM PM ρ π = =
Assuming that on average each chain has f ≈1.5 charged end-groups finally yields the surface charge to be
4
Where −F N/ Ais the charge of the end-group with the Faraday constant
4 9.6 10 C/mol
F = ⋅ . The above result, as Tauer24 experimentally prove, is proportional to the particle size and inversely proportional to the average degree of polymerization. The zeta potential is calculated using the Smoluchowsky equation20.
0 0 8 3 F a M P σ ρ ζ ε εκ ε εκ = = − in which ε0 8.854 10 F/m12 −
= × is the dielectric permittivity of vacuo, ε =78the relative
dielectric permittivity of the surrounding medium water, and κ the inverse Debye length that we depends on ionic strength I through
2 0 2F I RT κ εε =
With R the gas constant and T =300 K temperature. We estimate the ionic strength,
using the conductivity data measured by the same instrument and assuming contributions largely from protons at 3 mM. With the given values, the estimated zeta potential becomes 80 mV, quite close to the measured value.
Cryo Transmission Electron Microscopy. Pictures were obtained on a Philips
CM120 electron microscope operating at 120 KV. Samples were prepared by depositing a few microliters of suspension on a holey carbon coated grid (Quantifoil 3.5/1, Quantifoil micro tools GmbH, Jena, Germany), after blotting away the excess of liquid the grids were plunged quickly in liquid ethane. Frozen hydrated specimens were mounted in a cryo specimen holder (Gatan, model 626). Micrographs were recorded under low dose conditions on a slow scan CCD camera (Gatan, model 794).
Transmission Electron Microscopy. Pictures were taken using FEI Monochromated
Tecnai 200STEM-FEG microscope operated at 200 kV, in which images were acquired with Gatan CCD camera. Samples were prepared by depositing small portion of the reaction solution on a Quantifoil R copper microgrid and left overnight for drying.
Figure S2. TEM (left) and cryo-TEM (right) images of early stage polymerized polystyrene
Chapter 3
Optimal Ionic Strength for
Nonionically Initiated
Polymerization
ABSTRACT
Surfactant-free emulsion polymerization involving a nonionic, and hence uncharged, initiator presents a new approach towards environmentally friendly procedures to synthesize latex particles. Under optimal solvent conditions, notably pH and ionic strength, the latex particles are stabilized by the natural development of ionic charge at the surface of the particles. We emphasize, that the present process does not at all involve the addition of stabilizers such as surfactants or the creation of surface-active species from ionic initiators. The width of the size distribution is found to vary strongly with the experimental conditions, notably the ionic strength and to a much lesser extent pH. The phenomenon is explained by a critical ionic strength dependence of the aggregation of the just nucleated primary particles into larger secondary particles, the so-called “coagulative nucleation” step.
3.1. INTRODUCTION
Emulsion polymerization is a widely employed process for the production of an extensive range of polymers used in many applications1,2, 3. The control over size and polydispersity in any of these applications is highly required and in order to achieve this, surfactants are used. Surfactants are adsorbing on the surface of the monomer droplets providing protection against coagulation and subsequent coalescence. Nonionic surfactants are providing steric stabilization and ionic surfactants are providing electrostatic stabilization1. Nevertheless, surfactants are the most significant contamination of the final product and there have been many attempts to reduce their use and provide rules for surfactant-free processes4. In such systems only monomer, initiator and water are employed where the initiator can be both oil-soluble and water-soluble. Oil-soluble initiators are less frequently used in surfactant-free systems because these do not provide additional electrostatic stability as usually is provided by the water-soluble, ionic initiators. The absence of a stabilizing mechanism in the case of oil-soluble initiator usually leads to a higher polydispersity of the final latex particles due to coagulation during and after the polymerization process.
Already in the 19th century it has been observed that air bubbles in water are negatively charged so that in an electrophoretic experiment these migrate towards the positive electrode5. More recently6, studies investigating the zeta potential of air bubbles under a wide range of pH values show that these are negatively charged and display an isoelectric point at a pH value of about 4. Below that pH value the bubbles become positively charged. The interface of neutral water with hydrophobic surfaces, such as hydrocarbon oils, is similar to the interface between air and water, and it has long been known that such interfaces also acquire a negative charge when the water is of neutral pH7-8. Similar to the situation of interfaces between air and water, various authors9-10 have proven that the point of zero charge for hydrophobic surfaces in contact with water is between pH 3 and 4. As pointed out by Beattie et al.9, hexadecane in water emulsions prepared at pH 7 are stable for several hours.
In order to maintain the pH while refining the emulsion, additional hydroxide needed to be added. Based on Grahame’s equation and Stern’s isotherm, Marinova et al.11 modeled the behavior of the zeta potential as measured for oil droplets in water as a function of pH and ionic strength of the aqueous solution. By fitting the model to experimental data, they obtained reasonable model parameter values such as equilibrium constant for hydroxyl adsorption. Recently, Roger et al.12, suggested that the negative surface charge is originating from unreacted traces of fatty acids dissolved in the oil. However, the authors did not convince the scientific community and in actual fact presented another piece of evidence for the presence of hydroxide ions at the interfaces13.
In a preliminary experiment, we verified that styrene-in-water emulsions behaved according to the predictions of Marinova et al11. An important result is, that the observations are independent of particle size at least up to micrometers. Moreover, the ionic strength dependence of the emulsion stability followed classical DLVO theory predictions14,15. Recently, Yamamoto demonstrated the synthesis of polystyrene, micron-sized particles using a non-ionic initiator in a soap-free environment16. This author attributed the origin of the stabilizing negative charge to the polarization of the electron-active functional groups decomposed from the initiators and the pi-electron cloud of the benzene ring in styrene. However, this is not very likely to be the case because the concentrations of monomer and initiator used should lead to stabilization at much smaller particle sizes as follows from a simple geometrical analysis17. Here, we study in more detail the conditions under which surfactant-free emulsion polymerization of styrene with non-ionic initiator can be performed.
3.2. MATERIALS AND METHODS
Polystyrene particles were synthesized by surfactant-free emulsion polymerization. Given amount of sodium chloride (Sigma Aldrich, ≥99.5%) was added to 100 mL of MilliQ water in three-neck glass flask. The solution was bubbled with N2 and degassed with vacuum, the procedure was repeated 3 times, while stirring (150rpm). Subsequently, 0.68 g of styrene (Sigma Aldrich, ≥99%) was injected into the flask. Prior to that step, protective inhibitor was removed from the styrene using a pre-packed column (Sigma Aldrich, for removal tert-butylcatechol). The solution was equilibrated in a closed flask for 2 h at 70 oC. Polymerization was started by injection of initiator, 0.019 g azobisisobutyronitrile (AIBN) dissolved in 0.23 g styrene. The solution was then allowed to polymerize under nitrogen conditions for 5 h. During the polymerization a portion of the reaction solution was taken out after 90 and 180 minutes under the flow of inert gas (N2) with gas tight syringe and freeze-dried under high vacuum. A part of the dried material was put on a specimen holder and subsequently a molecular layer of gold was deposited for analysis by Scanning Electron Microscope (FEI/Philips XL30 SFEG) operating at 5 kV and using the Ultra-High Resolution mode (UHR-SEM). Water-suspended samples were analyzed by dynamic light scattering (DLS, Zetasizer Nano ZS, Malvern).
3.3. RESULTS
Figure 1. UHR-SEM micrographs of samples taken at two instants after initiation of
the polymerization process. The left micrograph was taken after 90 min of polymerization time clearly shows that particles are small aggregates of primary particles. The right micrograph was taken after 180 min polymerization time and shows the relatively homogeneous final particles. The scale bars are 200nm.
Before starting the polymerization, the aqueous phase is a dilute emulsion with rather large monomer droplets of about 400 nm as can be seen from the Figure S1 in the appendix. The size distribution of this emulsion is quite broad and the aqueous phase is saturated with monomer. After addition of initiator dissolved in an aliquot of monomer, polymerization starts and the observed particle size steeply decreases and subsequently slowly grows with polymerization time. From the micrographs in Figure 1 we learn, that the emulsion polymerization process conducted here clearly involves a coagulation step where primary particles aggregate into larger secondary particles18,19 . Coagulation rate, growth rate, and final particle size are determined by solvent conditions as will be discussed below. The zeta potential of the aggregates has been measured to be around -50 mV and hardly varies with time, ionic strength, or initiator concentration.
By varying the NaCl concentration between 10-7 M to 10-2 M, the effect of the aqueous phase ionic strength on the size distribution was studied. It was observed that as the ionic strength increased, the size distribution became narrower up until a concentration of 10-3 M. At higher concentrations the synthesized particles were becoming more polydisperse and in addition were
suggesting that the quality of the particle dispersion strongly varies with ionic strength. However, as was discussed before, it has been seen that the zeta potential, which is a measure of the colloidal stability of the samples, hardly varies with ionic strength and in all situations a moderate surface charge has been measured. Other types of monovalent salts resulted in very similar behavior compared to what is reported here. To quantify the results, we have defined a quality factor as the inverse of the polydispersity index (PDI) calculated from the size distribution of the samples as obtained from the emulsion polymerization experiments at various values of ionic strengths. We stress the quality factor as defined here is based on the idea that a shallow size distribution is optimal. There are of course situations where a broad size distribution is optimal in which case a better definition of a quality factor would be the PDI itself. The results of our analysis have been plotted as a function of ionic strength in Figure 2.
Figure 2. Optimal conditions as determined by the polydispersity of polystyrene
latex particles as a function of ionic strength. Scanning electron micrographs of polystyrene particles polymerized with no additional salt (A), with 1mM NaCl (B) and 5mM NaCl (C).
3.4. DISCUSSION
As discussed in the Introduction, the commonly held view is that hydrophobic surfaces become negatively charged in aqueous environments, which is assumed to be due to the adsorption of hydroxide ions that are naturally present in water. To ensure reproducible conditions, the solvent is adjusted to slightly basic with a pH of at least 8. Hydrophobic particles thus obtain a zeta potential that does not get above -30 mV at ionic strengths lower than 10 mM11. Even if the mechanism in itself is not known in detail, the experimental facts clearly indicate that the charge development both can be deduced from electrophoretic mobility measurements11 as well as from stability experiments9.
Let us therefore review the effect of ionic strength on the quality of the latex dispersions in some more detail. As is known from electrolyte theory, see e.g. the treatment in the monograph by J. Israelachvili20, the ionic strength mainly affects the thickness of the double layer, which is the region within which the surface potential decays to vanish beyond. For larger separations than the double layer thickness, particles do not interact with one another and hence only when the double layers of two particles are overlapping there is a significant, initially repulsive interaction. Upon closer approach, the particles are attracted by Van der Waals forces and subsequently attach irreversibly to each other. The latter process is called flocculation and when the particles are soft they subsequently merge or coalesce. The thickness of this double layer varies inversely with the square root of the ionic strength and for a monovalent electrolyte its characteristic Debye length scale amounts to 3 nm at 10 mM and becomes about 30 nm for 0.1 mM. The typical distance between particles depends on their concentration as well as on their size. As a consequence, these are then also parameters in the flocculation rate of the particles21 in addition to the barrier in the repulsive interaction.
In a surfactant free emulsion polymerization process there are two steps where aggregation can take place. The first is where the primary particles, that nucleated from the insoluble oligomeric chains, coagulate into secondary particles, the so called “coagulative nucleation” step22. The second step is the, usually undesired, aggregation of secondary particles into even larger structures. After nucleation, the primary particles continue to grow by polymerization and under favorable conditions aggregate to form secondary particles. The final size distribution of the secondary particles is, again under favorable conditions, relatively sharp. The conditions for this coagulation step are quite similar to what has been found for KPS-initiated emulsion polymerization. Also in the present case of nonionic initiator, the particles are relatively highly charged which would normally guarantee good colloidal stability. Despite being highly charged, these particles do aggregate. It has recently been argued by Herrera-Ordonez et al.23 that attachment occurs through termination of active chains on either surface of colliding particles.
Similar arguments have been already been put forward by Tauer et al in the early nineties24. Such attachment is partially reversible due to the fact that the chains are still relatively short and can hence easily desorb from one of the particles. As argued in chapter 2, a partially reversible process accounts for the experimentally observed stationary particle size distribution with increasing median value as for instance discussed by Privman25. The observed effect of ionic strength can now be rationalized as follows. Too high an ionic strength results in a thin double layer which layer, which always leads to flocculation. In addition, it reduces the repulsive barrier. As a consequence, the primary particles have no chance to form reversible bonds; they immediately are irreversibly flocculated by means of the Van der Waals forces. Also clusters of primary particles can participate in this coagulation process, which leads to a broad particle size distribution. At optimal ionic strength the particles can approach close enough to have a reasonable chance to bind through termination of active chains on either particle. However, at too low ionic strength this process is inhibited and the reversible aggregation into secondary particles does not take place. Instead, the primary particles will continue to grow. As recently discussed by Privman25, this prolonged growth always leads to broad size distributions. In addition, due to fluctuations in surface charge and active chain length, some particles still aggregate albeit most often irreversibly which leads to coalescence and hence even broader size distributions.
Similar experiments as discussed here, were performed by Yamamoto16 who focussed more on size than on dispersion quality. The ionic strength variation in this case was brought about changing the valence of the counter-ion in chloride compounds, which leads to the exploration of a much smaller range of ionic strength values than done here.
3.5. CONCLUSION
In conclusion, we have shown that surfactant-free emulsion polymerization can be achieved with non-ionic initiators while maintaining good dispersion quality of the final result. This allows one to circumvent the use of ionic initiators that lead to hydrophilic surface-active species that deteriorate the final product. In order to obtain the optimal result, the solvent quality has to be adapted. In the present case this largely affects the ionic strength and hardly the pH. This is the case, because the phenomenon uses the apparent accumulation of hydroxyl ions at the solid water interface, which is relatively insensitive to acidity. Interestingly, the required ionic strength is very comparable to what is employed for the same emulsion polymerization experiment performed with KPS26.
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