4-1 Introduction
Emulsion polymerizations have led to various kinds of colloidal structures and morphologies. For instance, core/shell1, hollow2, peanut3, and multilobbed4 latexes had been reported in the literatures. Miniemulsion polymerization, which is an alternative synthesis pathway for Polymer-encapsulated organic/inorganic core, polymer shell structures.5 In this case, monomer droplets play an important role during the particle nucleation. Another interesting class of composite polymer latexes is armored nature, which potentially have great performance applying in coatings or adhesives. This kind of complex structure is known to be synthesized via layer-by-layer (LBL) approach,6-8 or Pickering emulsion polymerization.
Disadvantages for the LBL approach are that these methods are time consuming and a dilute condition is required. Armes et al. proposed the synthesis of poly(methyl methacrylate)-silica nanocomposite particles in aqueous alcoholic media using silica nanoparticles as stabilizer, 9 recently extending this method to conduct in water using a glycerol-modified silica sol.10 Sacanna et al. showed that methacryloxypropyltrimethoxysilane, in the presence of nanosized silica led to spontaneous emulsification in water,11 a two-step polymerization procedure afforded armored particles with an outer shell of PMMA.12 Muller reported the use of Janus-type polymer particles as stabilizers in Pickering emulsion polymerization.13
In the early 1990s, pioneering work on in situ polymerization process for conductive polymer/silica composite particles via emulsifier free solid-stabilized emulsion was conducted by Armes et al.13 Over the last 20 years related work has extended the method to various types of vinyl monomers in the presence of
commercial silica sols, conducted in either water, alcohol/water mixtures or alcoholic medium. Taking advantage of hetero-coagulation between the organic core and the inorganic shell, these elegant bottom-up approaches have led to methods for the production of a great quantity of composite latexes in mild experimental conditions.14–18 In contrast with Pickering emulsion polymerization, this bottom up polymerization method does not require the emulsification process, a step undesirable for industrial scale-up. Up to now, very limited reports focus on this process and detailed reaction parameters on the resulting composite latexes has not been thoroughly studied.
Herein we conducted the emulsifier-free polymerization in the presence of commercial grade colloidal silica. Well-defined vinyl polymer core/silica shell with near monodisperse distributions could be readily obtained in a single step. Various syntheses parameters such as the pH of the solution, the kind of initiator employed into the polymerization, the amount of silica, and the ratio between the monomer and the silica were studied. In view of its nucleation mechanism, a “hetero-aggregation”
polymerization route was proposed, which accounted for its well-defined core/shell morphology.
4-2 Experimental Section 4-2.1 Materials
Methyl methacrylate (MMA; Acros), Styrene (ST; Acros), and butyl acrylate (BA; Acros) were distilled under a reduced pressure then passed through a basic aluminum oxide packed column and stored at 5 oC before use. Potassium persulfate (KPS; Aldrich), 2,2'-Azobis(2-methylpropionamide) dihydrochloride (AIBA; Acros, 98%), Ludox® TM40 colloidal silica (40 wt% suspension in water), Ludox® SM30 colloidal silica (30 wt% suspension in water), SYTON® HT-50 colloidal silica (50 wt% suspension in water) were purchased from Aldrich and used as received. Sodium hydroxide (NaOH; Shimakyu), hydrochloric acid aqueous solution (HCl; Shimakyu) were used without further purifications. All water used throughout the work was distilled and deionized to 18.2 MΩ.
4-2.2 Preparation of Polymer/Silica Core/Shell Nanocomposite Latexes by Emulsifier-Free Solid Stabilized Polymerization
A typical synthesis of polystyrene latex/ Ludox® TM40 silica core/shell nanocomposite latex is described as following. A 40 wt% aqueous sol of Ludox® TM40 silica nanoparticles (0.625g, equal to 0.25g dry silica powder) was added to deoxygenated water (37 mL). The pH of the sol was reduced to pH 3.3 by drop wise addition of HCl (aq) solution. The solution was then transferred to a 100 mL single necked glass reactor. To this 1.0 g of styrene was added. The mixture was then bubbled under a nitrogen gas inert atmosphere for 5 minutes whilst stirring. The mixture was heated to 60 °C and stirred at a rate that the vortex of monomer touched the paddles of the stirrer (500rpm). AIBA (0.015 g) dissolved in water (3.0 ml) was injected into the system to start the polymerization. The reaction was allowed over 24 hours at 60 °C.
4-2.3 Characterization of Polymer/Silica Core/Shell Nanocomposite Latexes.
Field emission scanning electron microscopy analysis was performed using a Nova NanoSEM 230 (FEI Ultra-High Resolution FE-SEM with low vacuum mode).
Prior to analysis a drop of un-purified sample placing on an aluminum foil was sputter coated with AuPd for 90 seconds at 1.5 kV and 20 mA. The diameters of 50 latexes were used to calculate the average diameter (dn) and coefficient of variation (CV)
where N is the total number of counted latexes, and di is the diameter of ith latex.
The electrophoretic measurements were performed using dynamic light scattering instrument (Malvern Zeta Sizer 3000) and the Smoluchowski equation was used to calculate the latex zeta potential.
Dynamic light scattering (DLS) experiments were performed using a Malvern Zeta Sizer 3000 with a scattering angle of 173ﾟ. The Stokes-Einstein equation was used to convert the diffusion coefficients into hydrodynamic diameters. The measurements were repeated three times to verify the reproducibility of the experimental results.
TGA of the composite latexes were carried out using a Perkin-Elmer 7. The temperature was kept at 110 oC for 10 minutes and the sample was then heated to 800
oC at a rate of 10 oC /min under an nitrogen atmosphere. All measurements were taken under a constant flow of air of 30mL/min. The samples were dried in an oven at 110
oC prior to each run to remove the residual moisture. For determination of the exact silica content in the composite latexes, the colloidal dispersions were purified by three
centrifugation-redispersion cycles (5000 rpm, 3mins). The supernatant was carefully decanted in order to remove the free silica nanoparticles in the continuous phase.
Excessive centrifugation rates (>8000 rpm) and times (>1 h) were avoided, because these would otherwise result in sedimentation of the excess silica and make redispersion of the sedimented particles more difficult.
The morphologies and sizes of the latexes were also measured using a Hitachi H-7100 transmission electron microscope. The samples were prepared by evaporating dilute suspensions on a carbon-coated cooper grid.
4-3 Results and Discussions
4-3.1 Formation Mechanism of Emulsifier-Free Solid Stabilized Polymerization Figure 4-1 shows a schematic mechanism of Emulsifier-free solid stabilized emulsion polymerization, using commercial grade of silica sol as stabilizer. In this synthesis protocol, opposite charge between the solid particles and the initiators is a prerequisite for the successful formation of well-defined core/shell structure. As can be seen in Figure 4-1, the cationic initiator, AIBA, decomposes upon heating and adsorbed onto the negatively charge silica surfaces through electrostatic attraction.
The polymer nucleation takes place either in the aqueous phase or on the surface of silica nanoparticles. The large monomer droplets, acting as a reservoir was observed to be stabilized by the silica nanoparticles remaining in the aqueous phase with sizes ranging in tens to hundreds of micrometer, as determined by static laser light scattering measurements.
In conventional soap-free emulsion polymerization, the particle nucleation goes via coagulative homogeneous nucleation, depending on the solubility of the monomer in water. The primary nuclei are formed by association of single growing chains.
Growth of these nuclei through polymerization leads to colloidal instability which cause them to coalesce with the other, until the surface charge density as well as the radius of curvature have increased enough to impart colloidal stability. A constant number of mature latexes is achieved when new aqueous phase radicals have no other fate but to terminate or to enter the existing mature particles. When this nucleation is fast compared with the overall polymerization time, narrowly distributed even monodisperse size distribution could be obtained. In the present system with the presence of nanoparticles, they potentially can participate into the nucleation also polymerization process. It should be noted here that when a negatively charge initiator,
such as potassium persulfate (KPS) is used as an initiator, the formation of well-defined core/shell structures was unsuccessful. The as formed latexes with a silica-unattached surface and free silica dispersed in the aqueous phase coexisted after the reaction as shown in Figure 4-3.
As the reaction proceeds, the newly formed polymeric interfaces between latex particles and water increased. These need to be stabilized with sufficient surface charge or by other such as steric stabilization, to hinder coagulation. The nanoparticles present play a crucial role in emulsifier-free solid stabilized polymerization since it bears opposite charge to the initiator employed into the synthesis. We suggest that when a latex particle grows and thus increases its interfacial area, therefore reducing its surface charge density, it can heteroaggregate with nanoparticles. Upon collision the nanoparticle can adhere to the interface acting as a Pickering stabilizer and, will provide extra charge to secure sufficient electrostatic repulsion between growing polymer latex particles, the latter to avoid full coagulation of the system. The rate of heteroaggregation has been proved to be much faster than the rate of polymer growing to warrant its colloidal stability.18 After the polymerization reaches full conversion, colloidal stable core/shell nanocomposite particles could be obtained.
4-3.2 Effect of the pH
The intrinsic pH of the commerical grade silica Ludox® TM40, Ludox® SM30, and SYTON® HT-50 were around 10.3 to 11.0, which possess negatively charges on its surface. The zeta potential of Ludox® TM40 as a function of pH value is shown in Figure 4-4. When conducting the emulsifier-free solid stabilized polymerization at high pH conditions, no particles were formed and the monomer conversion was extremely low. This can be ascribed by the following two reasons. (1) As generally
known in Pickering (solid stabilized) emulsions, particles with intermediate hydrophilicities or moderate surface charges can act as an effective particulate emulsifier. Particles bear strong surface hydophilicity/ hydrophobicity or large surface charges would not able to reside at the oil/water interface. This assumption was also verified in our previous chapter indicating that stable Ludox® TM40 stabilized emulsion can be prepared only when the pH was lower than 5.5, in which Ludox® TM40 possessed zeta potentials less than -30mV. As a result, when newly formed polymeric interfaces between latex particles and water were increased, the silica particles were unable to attach on its surfaces. This led to the colloidal instability and therefore, rapid termination of the growing chains. (2) The second reason may be explained by the radical termination arising from the electrostatic attraction. We infer that this would not be the major reason since the well-defined core/shell structure can not be made as more AIBA were employed into the synthesis.
The recipe of polymerization is listed in Table 4-1. Upon decreasing the pH of the system, corresponding to decreasing the surface charges of silica nano particles, narrowly size distributed polystyrene core/ Ludox® TM40 shell nanocomposite latexes can be readily synthesized in a one step process. Also, the limiting monomer conversion increased as decreasing the pH of the system, as described in the last paragraph. The silica incoperation efficiency is therefore, increased as decreasing the pH of the system, which is manifested by the TEM images (Figure 4-5) as well as the TGA analysis.
4-3.3 Effect of the silica concentration
We performed emulsifier-free solid stabilized polymerization with various amounts of Ludox® TM40 silica with other conditions being fixed. Interestingly, limited control of particle sizes upon varying the concentrations of silica was
observed in our experimental results when the amount of silica was far more than the amount of AIBA presented in the system. More specifically, the net charges were predominately determined by the silica nanoparticles. Moreover, excess silica was observed when increasing the silica content by two fold (EFS-PS4) on the basis of EFS-PS3. The particle size was slightly decreased from 310nm to 300nm yet much lower silica incorporation revealing that not all of the silica nanoparticles were involved into the nucleation. However, as the amount of silica was decreased on the basis of EFS-PS3, the average size of the particles increased and its distribution has become broader in EFS-PS5 as shown in Figure 4-6(a), along with a decrease in limiting monomer conversion and the silica incoperation efficiency. When the amounts of silica employed into the polymerization are as low such as EFS-PS6, the reaction was seen to be retarded hence very low monomer conversion being achieved.
From the SEM image of EFS-PS6 in Figure 4-6(b), only small polystyrene latexes less than 100nm were formed, coexisted with the silica nanoparticles indicating that no adhesion nor core/shell structure formation had taken place. It is noteworthy that we also noticed a transition between the true emulsifier-free polymerization (EF-PS1, without addition of silica) and EFS-PS6 in which Ludox® TM40 was in a small amount of quantity (EFS-PS7). Judging from its TEM image as well as the rugged surfaces by SEM image as shown in Figure 4-7, most of the silica nanoparticle were encapsulated into the polystyrene latexes. The limiting monomer conversion was increased as decreasing the silica content within this interval.
The above results indicate that in emulsifier-free solid stabilized polymerization, the partcile sizes are predominately controlled by the decomposition of the initiator.
More accurately, the total interfacial area of polymer/water generated in the solution.
When the amount of silica in the medium is sufficient for the full coverage of all the polymer/water interface, well-defined colloidal stable core/shell structures are formed.
Consequently, large amount of silica in excess would marginally change the final sizes of the latexes but remaining in the aqueous phase. Further decreasing the amount of silica employed into the polymerization firstly enlarged the average particle sizes also broaden its size distributions due to the less polymer/water interface that can be covered by the silica, and ultimately, gives no particle formation in the system. This is plausible since the net charges of the system are not as much for providing sufficient colloidal stability (within ±30mV), leading to massive coagulation of the system thereby low limiting monomer conversion in these cases. In EF-PS1, the reaction was carried out as conventional emulsifier-free polymerization using cationic AIBA as initiator.
Finally, we also performed this polymerization protocol for the synthesis of poly(methyl methacrylate)/ Ludox® TM40 and polystyrene/ Ludox® SM30 core/shell composite latexes (Figure 4-9). It shows that various kinds of vinyl polymer core/inorganic shell nanocomposite latexes can readily be prepared by this simple method.
4-4 Conclusions
In this chapter, we systematically studied the particle formation mechanism and morphology of polymer/silica core/shell nanocomposite latexes via emulsifier-free solid stabilized polymerization. Two criteria were met to prepare this well-define core/shell structure. (1) The nanoparticles possess opposite charge to the initiator employed into the polymerization. (2) The nanoparticles have intermediate hydrophilicities/surface charges. At the beginning of the reaction, the polymer nucleation mechanism is similar with the case in conventional emulsifier-free polymerization. However, the newly formed polymeric/aqueous interfaces were predominately “hetero-coagulate” by the opposite charge silica in order to maintain the colloidal stability in the system. As the amount of silica was reduced, the particle sizes had become larger with broader size distributions. The limiting monomer conversion gradually lowered owing to the insufficient charge density for the stabilization. On the other hand, the sizes of the composite latex were insensitive to the excess amount of silica presented in the system since it depends on the polymeric/aqueous interfaces generated during nucleation. We show that this one step emulsifier-free solid stabilized polymerization can apply to various kinds of vinyl monomer/ nano-sized particulate system.
Table 4-1.The recipe for emulsifier-free solid stabilized polymerization of polystyrene/silica composite latexes.a
Sample code
silica (g)
pH AIBA (g) Silica Incorperation Efficiency (%)
Conversion (%)d
dn(nm)b CV(%)b
EFS-PS1 0.25 10.3 0.015 /c / / /
EFS-PS2 0.25 4.5 0.015 60 72 240 3.8
EFS-PS3 0.25 3.3 0.015 89 91 310 3.9
EFS-PS4 0.5 3.3 0.015 58 88 300 5.3
EFS-PS5 0.1 3.3 0.015 81 69 580 27
EFS-PS6 0.025 3.3 0.015 / 18 / /
EFS-PS7 0.010 3.3 0.015 ~100 76 135 >30
ES-PS1 0 3.3 0.015 / 94 215 1.7
a Conditions: 1g styrene monomer. AIBA: 2,2'-Azobis(2-methylpropionamide) dihydrochloride. Stirring rate: 500rpm. Reaction temperature: 60 oC. b Average
diameter by counting 50 latexes. c No reaction. d Determined gravemetrically.
Figure 4-1. Schematic Representation of core/shell nanocomposite latexes synthesized from emulsifier-free solid stabilized polymerization using negatively charged silica and positively charged
initiator (EFS-PS2, 3, 4).
(a) (b)
(c) (d)
Figure 4-2. (a), (b) TEM, and (c), (d) SEM images of EFS-PS3.
(a) (b)
Figure 4-3. (a) TEM, (b) SEM image of emulsifier-free polymerization in the presence of Ludox® TM40 silica using KPS as initiator.
Figure 4-4. Zeta potentials as a function of pH values of Ludox® TM40 silica nanoparticles.
(a) (b)
Figure 4-5. (a) TEM image of EFS-PS4. (b) TEM image of EFS2.
Figure 4-6. (a) TEM image of EFS-PS5. (b) SEM image of EF-PS6.
Figure 4-7. (a) SEM, (b) TEM image of EFS-PS7.
Figure 4-8. TEM image of EF-PS1.
(a) (b)
(c)
Figure 4-9. TEM images of (a) polystyrene/ Ludox® SM30. (b) Poly(methyl methacrylate)/ Ludox® SM30. (c) Poly(methyl methacrylate)/ Ludox® TM40.
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Chapter 5 Novel Synthesis of Multi-Scaled, Surfactant-Free