Investigation of the influence of sodium dodecyl sulfate concentration on the particle diameter and number particle density in the emulsion polymerization of styrene

The emulsion polymerization process is usually two-phase system in which the starting monomer and the resulting polymer are in the form of a fine dispersion in an immiscible liquid. The polymerization initiator is soluble in the liquid and may be present within the polymer particles during their formation. In addition to the monomer, polymerization medium and the initiator, one or more additives are added to the polymerization mixture to emulsify the monomer and to stabilize the monomer droplets and the resulting polymer particles. Various combinations are employed under empirically adjusted conditions to produce generally spherical polymer particles within range from 100 nm or smaller [1-3].

The initiator is, unlike in suspension polymerization, soluble in the medium, and not in the monomer. Under these conditions, the monomer is present in the mixture partly in the form of droplets (about 1-10~m or larger), and partly in the form of surfactant-coated micelles (ca. 50-100 A), depending on the nature and concentration of the surfactant. A small percentage of the monomer is also molecularly dissolved in the medium. For example, solubility of styrene in water at 70C is about 4 g/L [3].

The volume ratio of the monomer phase to the medium in emulsion polymerization is usually within about 0.1-0.5, and the polymerization is carried out at 40-80 C. Since the initiator is present only in the medium, the initial locus of polymerization is in the medium, which means outside the droplets and micelles. The oligoradicals formed in the polymerization medium are either surrounded by the dissolved monomer and surfactant molecules, or they are absorbed by the surfactant-coated micelles. In any case, the initially formed oligoradicals produce stabilized nuclei (primary particles) and subsequently, surfactant-stabilized polymer nuclei become the main loci of polymerization by absorbing further oligoradicals and monomer molecules from the medium, or in effect from the monomer droplets. In this way, the nuclei/particles grow gradually until the monomer is completely consumed. The size of the latex particles reported by emulsion polymerization is in the range of 50-300 nm [1-2]. It has been proposed that if the surfactant concentration is lower than the critical micelle concentration (CMC), the oligoradicals enter into the monomer swollen micelle (micellar nucleation). Otherwise, at higher surfactant concentrations, once the oligoradical has reached a critical chain length, the oligoradicals are forming primary particles (homogeneous nucleation) or the polymerization can be running in the monomer droplets (droplet nucleation) [4-7]. In surfactant-free emulsion polymerization, the polymerization is carried out in the same way as in classical emulsion polymerization, except that no emulsifier is used. Accordingly, nucleation takes place by precipitation of macroradicals (and macromolecules), as compared with micelle formation in normal emulsion polymerization. Since there is no emulsifier present in the medium, the nuclei thus formed are not stabilized by any emulsifier or stabilizer. As a result, the initially formed polymer nuclei collide and form larger and larger particles as the polymerization proceeds. However, latex particles produced in the absence of emulsifier are, to some extent, stabilized by the orientation of their own polymer chains, notably the chain ends originating from initiator molecules. For example, in the case of potassium persulfate initiator, the chain end groups are -OSO3- K+. As the particles grow, their surface charge increases, and at a certain size (usually bigger 100 nm), the particles become stabilized by their own electrostatic charge. It is important highlight that monomer to water ratio in emulsifier-free emulsion polymerization is much smaller than that in normal emulsion polymerization. In most cases, monomer concentration is less than 5%, and the size of the resulting particles is in the region of about 100-1000 nm [7].

The size of latex particles in emulsion polymerization has no direct relationship with the size of the initially formed monomer droplets or micelles. These do not contain any initiator and, hence, are not directly converted to the corresponding polymer particles. Instead, the fraction of the monomer molecularly dissolved in the polymerization medium plays a key role in determining the size of the final polymer particles. Therefore, the size of the latex particle obtained in emulsion polymerization is influenced by the surfactant concentration, polymerization temperature and number of particles nucleated.

The SmithEwart theory predicts that the number of latex particles nucleated per unit volume of water (p) is proportional to the surfactant concentration and initiator concentration to the 0.6 and 0.4 powers, respectively. This relationship shows that the most important parameter that controls the particle nucleation process is the surfactant concentration. Although the particle nucleation period is relatively short (up to about 1020% monomer conversion), it controls the particle size and particle size distribution of latex products. The application properties of emulsion polymers such as rheology and lm formation are strongly dependent on the particle size and particle size distribution [5,8].

In this study, the effect of surfactant concentration on particle nucleation during the emulsion polymerization of styrene on water has been studied using sodium dodecyl sulfate (SDS) and potassium persulfate (KPS) as initiator. Different formulations with a range of concentration ranging from concentrations lower than critical micellar concentration to concentrations higher than critical micellar were tested maintaining the styrene concentration at 20% with respect to the final mixture water:styrene. A sigmoidal dependence of the number of particles on [SDS] was found in the range investigated.

1. Experimental Part1.1 Polystyrene synthesisStyrene (stabilized, Merck, Darmstadt) stabilized with 15ppm 4-tert-Butylbenzcatechol was used as the monomer. The inhibitor was removed from the monomer by washing in a separation funnel with 5portions of 20mL of 1.0M sodium hydroxide solution (NaOH, Analytical Grade, Carl Roth) and then with 5portions of 50mL of water (Milli-Q, 18.2Mcm-1, Millipore, Darmstadt) to remove residual base. SDS (ultra pure, Carl Roth, Karlsruhe) was used as the surfactant, potassium persulfate (K2S2O8, ACS reagent, Carl Roth) purified by recrystallization from water at 50C and dried by vacuum suction was used as initiator, and sodium phosphate dodecahydrate (Na3PO412H2O, AG, Carl Roth) as the buffer.A 1.0L three-neck glass flask, surrounded by an oil bath to control the temperature, equipped at the top with a reflux condenser, a dropping funnel and nitrogen inlet, was used to carry out the reaction. A magnetic stirrer was used to mix the chemicals. 500mL of Milli-Q water were charged into the flask and the dropping funnel was charged with 30mL of 1.65M initiator solution. In order to deoxygenize the water, the reactor was evacuated and purged with nitrogen three times at a stirring rate of 800rpm. Afterwards, the temperature of the oil bath was raised to 130C to boil the water under reflux for 1h with a constant N2 flow of 1-2bubble per minute. The water was cooled down to 55C and then the N2 flow was stopped to charge the buffer, and the surfactant into the reactor (7 mL of water were used to transfer completely the chemicals). Once the buffer and the surfactant were completely dissolved, the styrene was added, the N2 flow was opened again and the reaction mixture was stirred until the reaction mix was homogeneous. The initiator solution was added quickly by opening the valve of the dropping funnel.The temperature of the mixture was maintained constant for 24h at 55C and raised to 85C for 3h. Afterwards the mixture was cooled down to ambient temperature and the product was collected and characterizated.

Twelve PS latexes were synthesized. Educts used for the synthesis are shown in Table 1.Table 1. Educts used for polystyrene synthesis.UP_PS_nStyreneWaterSDSKPSNa3PO412H2O

Mass m /g

9a131.0537.00.120171.337660.18082

10134.5537.00.248101.336000.18028

11134.5537.00.620551.338000.18000

12134.5537.01.242501.332500.18015

13134.5537.01.858981.335800.18493

14134.5537.00.031541.333530.18095

15134.5537.00.075811.333960.18077

16b201.0470.00.542302.002340.27629

18134.5537.06.195231.332320.18013

19134.5537.01.855252.166170.29295

20134.5537.00.318851.331240.18024

21134.5537.00.464191.330420.18014

a samples 9 to 15 and 19 to 21 have 20% styrene content, b sample 16 have 30% styrene content, c UP_PS_17 polystyrene emulsion was not synthesized.

1.2 Polystyrene analysisParticle diameter Dn:

Particle diameter of the obtained polystyrene (PS) latices was determined by dynamic light scattering (DLS) (Zetasizer Nano ZS Malvern Instruments GmbH, Kirschau). Measurements were performed at 173 (backscattering geometry). Additionally, the particle diameter was determined by scanning transmission electron microscopy (STEM, Quanta 250, FEI, Eindhoven, Netherlands).

Particle number density Np:

The particle number density Np was estimated based on the formula: (1)where Dn is the average particle diameter, Vp the total volume of polymer particles, Xm is the fractional conversion from monomer to polymer, Mo is the mass of the monomer per unit volume of water (or mixture?) in gmL-1 and is the polymer density. The fractional conversion Xm of the latexes was determined gravimetrically. A sample of 2.0mL of latex was dried at room temperature and then at 105C to determine the mass of polymer and the solid content of polymer. Xm was the calculated as follows: (2)Additionally, the conversion of styrene to polystyrene on samples UP_PS_11 and UP_PS_15 was determined at different reaction times, in the following manner: Samples were withdrawn from reaction mixture at different times, using a syringe equipped with a long needle and injected into vials, which contained 2drops of 2% aqueous hydroquinone solution (supplier, city) to inhibit the polymerization reaction. The samples were placed immediately in ice to avoid the evaporation of unreated monomer and weighed later. The latexes were poured into a petri dish, and the vial was rinsed with Milli-Q water. The water was also collected in the petri dish, evaporated at room temperature and then at 105C to determine the solid content of polymer.

Glass transition temperature Tg:

The glass transition temperature Tg of the polystyrene latexes was determined by differential scanning calorimetry (DSC, DSC1, Mettler Toledo, Gieen). A sample of polystyrene latexes was dried at 40C, in an oven for 24 h. Approximately 5mg of the sample was placed in hermetic aluminum crucible of 40L. The samples were heated and cooled in three cycles from 25C to 150C, heating rate rh=20C min-1 and cooling rate rc=30Cmin-1. All measurements were performed under nitrogen atmosphere with a controlled flow of 30mLmin-1.

2. Results and Discussion2.1 Particle diameter determined by STEM and DLSIn Fig.1 to8, images of the samples studied are shown. The images show polymeric nanospheres with a narrow size distribution. Unfortunately, no enough images with properly resolution were registered. So, new images with resolution improved have to be registered in order to analyze the particle size and size distribution of the samples.

Fig.1: STEM Image of latex UP_PS_9

Fig.2: STEM Image of latex UP_PS_10

Fig.3: STEM image of latex UP_PS_11.

Fig.4: STEM image of latex UP_PS_12.

Fig.5: STEM image of latex UP_PS_14.

Fig.6: STEM image of latex UP_PS_15.

Fig7: STEM image of latex UP_PS_16.

Fig.8: STEM image of latex UP_PS_20

Table2: Particle diameter Dn determined by dynamic light scattering UP_PS_nDn (nm)

9646.8

10527.3

11128.0

1298.3

1392.1

14935.0

15754.9

16129.6

1874.6

20467.0

21365.3

1980.6

2.2 Effect of SDS on polymerization kineticsIn fig.9 the evolution of the particle diameter and in fig.10 the conversion vs. time are displayed for UP_PS_11 and UP_PS_15. In tab.2 the particle diameter and conversion are given for all synthesized latexes.

Fig.9: Evolution of the particle diameter during the synthesis of UP_PS_11 and UP_PS_15. The points correspond to experimental points. Continuous line is a guide to the eye.

Fig.10: Conversion of styrene during the synthesis of UP_PS_11 and UP_PS_15. The points correspond to experimental points. Continuous line is a guide to the eye.

Table.3: Latex formulations and calculated number particle density Np.UP_PS_nStyreneSDSaKPSDnNp

concentration based on water / mMnmcm-3

142.400.209.2935.05.39E+11

152.400.499.2754.91.00E+12

92.340.789.2646.81.58E+12

102.401.609.2527.32.94E+12

202.402.069.2467.04.27E+12

212.403.009.2365.38.99E+12

112.404.019.2128.02.13E+14

122.408.029.298.34.76E+14

132.4012.009.292.15.78E+14

182.4040.019.274.61.10E+15

164.114.0015.8129.63.47E+14

192.4011.9814.980.68.68E+14

a samples were organized by ascending SDS concentration and then by KPS concentration

Figs.9 and10 show a clear difference in the shape of the evolution of the particle diameter and the conversion of styrene during the reaction between a low surfactant concentration (0.49mM, UP_PS_15) and a high surfactant concentration (4.0mM, UP_PS_11). It can be seen that the higher surfactant concentration, the faster conversion rate of styrene. The conversion rate curve at 0.49mM of SDS is characteristic of polymerization reactions carried out below the CMC of the surfactant and the conversion rate curve at 4.0mM of SDS is typical for reactions performed above the CMC[7]. The particle number density for UP_PS_11 (4.0mM of SDS) reaches values with an order of magnitude of 1014cm-3, which is almost 100times higher than Np for UP_PS_15 (0.49mM of SDS). Therefore, an increase in the surfactant concentration yields a higher number of reaction loci and consequently the rate of polymerization is improved.

In fig.10 the effect of the reaction time on the conversion is shown. As is expected, increase the reaction temperature improves the conversion by acceleration of the rate of polymerization, which can be due to the higher formation radicals rate.2.3 Effect of SDS concentration on NpA sigmoidal dependence is obtained when Np is plotted against SDS concentration [SDS] in a logarithmic scale (fig.11). At low and high SDS concentration Np shows a low dependence on [SDS]. However, at SDS concentrations between 3.0mM and 4.0mM Np shows a strong dependence on [SDS], which suggest that the CMC of SDS in the reaction mixture is approximately 3.0mM.

Fig.11: Variation of number particles concentration on the final latex with the SDS concentration used in the reaction.

Evaluation of the slopes of curves log(Np) vs. log([SDS]) in the three different regions of surfactant concentration reveal it was obtained that at low concentration Np is proportional to the 0.88 power of SDS concentration, and at high surfactant concentration Np is proportional to the 0.52 power of SDS concentration, near to the 0.6 power predicted by the Smith-Ewart theory. A line of slope 10.93 can be drawn through the points corresponding to 3.00and 4.01mM SDS concentration, which shows the strong dependence of Np on SDS concentration close to the expected CMC.

Tab.3: Equation of linear regressions for dependences of Np on SDSSDS concentration range / mMEquation of linear regression

0.20 - 2.06log (Np) = 0.88 log([SDS]) + 12.3

3.00 - 4.01log (Np) = 10.93 log([SDS]) + 7.8

8.02 -40.0log (Np) = 0.52 log([SDS]) + 14.2

The CMC of SDS in pure water at 55C is 9.6mM[6] but in the reaction mixture the CMC changes due to the ionic strength of the solution, the water solubility of the monomer or the reaction temperature. According to the mixed micelle theory, when there are two or more surface active compounds in solution, mixed micelles should be formed [9]. Following this theory, once oligomers of styrene are produced, these can form mixed micelles with SDS molecules decreasing the CMC from the value in pure water, which is about 8.0 mM. This behavior agrees with the Np dependency found. Additional experiments are necessary to confirm this finding.Also it is necessary highlight that no conventional proportion of monomer:water:surfactant were used for the latexes marked as low SDS concentration, in consequence the mechanism of polymerization could be suspension polymerization. 2.4 Effect of SDS concentration on particle sizeIn fig.12 the effect of SDS concentration on the particle size is shown. There is a marked dependence of the particle diameter below 4.0mM of SDS. The size can be well controlled between 900nm and 130 nm by small variations of [SDS]. On the other hand, at SDS concentrations higher than 4.0 mM the CMC, big variations of [SDS] result in small changes of particle size. So, it is necessary to use SDS concentrations higher than approximately 8 mM when a smaller particle size than 100nm is looked for.The explanation to this size particle dependence on the surfactant concentration is simply because the shorter particle nucleation period when a low surfactant concentration is used, the narrower the resultant particle size distribution, and the higher concentration of surfactant, the higher the number of micelles and final number of particles nucleated. If the proportion of the monomer to the medium, in this case, styrene to water, is maintained constant, and the surfactant concentration is also constant, the latexes with a higher SDS concentration will have a higher number of particles nucleated, so, the particle size should be smaller, as the results obtained here.

Fig.12: Influence of SDS concentration on the particle diameter of latexes. The concentration of styrene (St) and potassium persulfate (KPS) are listed. Points correspond to experimental data and dotted line is a guide to the eye.

Two theories are accepted to explain the mechanism of particle nucleation in emulsion polymerization. According to the homogeneous nucleation theory, the free initiator radicals react with the dissolved monomer in the aqueous phase until the solubility of the oligomeric radical is negligible. The oligomers then precipitate to form a primary particle. This theory is complemented by the idea of coagulation between the primary particles as is seen to latexes with a SDS concentration lower than 4.0mM. According to the micellar nucleation theory a particle is nucleated when a free radical enters a monomer swollen micelle and reacts with the monomer. When SDS concentration is near to 30mM and an aggregation number of 160is consider, the number of micelles is around 1017 but the results show that Np is of the order of 1014, which implies that only a really small percentage of micelles are nucleated. Here, an increase of the SDS concentration has only a small effect on the particle size.2.5 Effect of KPS concentration on particle sizeIn order to synthesize particles with a diameter smaller than UP_PS_13 (Dn=92 nm) maintaining constant the SDS concentration, a new formulation,UP_PS_19, was prepared based by increasing the KPS concentration from 9.2mM to 14.9mM and maintaining the other educts constant. The particle diameter decreases from 92.1nm to 80.6nm (fig.12). This result suggests that the dependence of the particle diameter on the KPS concentration is stronger than on the SDS concentration for high surfactant concentrations.2.6 Increase of solid content to 30%Sample UP_PS_16 was synthesized based on the formulation of UP_PS_11 to obtain latex with a solid content of 30%. The molar ratios KPS/styrene and buffer/KPS and the SDS concentration were maintained constant. By this formulation a latex with almost the same particle size as UP_PS_11 was obtained. Nevertheless, a solid content of 29.1% was obtained. From this synthesis it can be concluded that if one aims to obtain latex with the same particle size but a higher solid content, the SDS concentration should not be upscaled.2.7 Glass transition temperature Tg of polystyrene nanoparticlesThe glass transition temperature can be used to determine small differences on cristalinity between polymers. In the range of 104-105gmol-1 for the molar mass of the nanoparticles, Tg increases with increasing molar mass, reaching a maximum value Tg = 101C in the case of the bulk material[10].

Tab.4: Glass transition temperature Tg of the synthesized polystyrene nanoparticles.UP_PS_nTg / C

9106.9 0.3

10107.2 0.2

11107.7 0.1

12106.3 0.1

13106.2 0.8

14107.8 0.1

15106.0 0.3

16108.1 0.4

18105.4 0.1

19108.4 0.2

20108.8 0.5

21107.2 0.2

In tab.4 the glass transition temperatures for the synthesized latexes are shown. The determined values lie in the range between 105.4C and 108.8C. As were expected, any relation between the glass transition temperature and SDS concentration or particle size were found, which means that all latex samples were composed by hard spheres fully polymerized. 3. Conclusions

The influence of sodium dodecyl sulfate concentration on the particle diameter and number particle density in the emulsion polymerization of styrene was studied. At low SDS concentration the Np is proportional to [SDS] to the 0.88 power, at intermediate surfactant concentration the order of dependence was 10.93. The order of dependence of Np in the final latex on the SDS concentration at higher SDS concentration than CMC is close to the Smith-Ewart prediction of 0.6. The curve S shaped of dependence of number particles with SDS concentration is simply explained for the broad range of CMC used, at lower SDS concentration than CMC, suspension polymerization was probably followed and a change to emulsion polymerization is followed when SDS concentration rise to the CMC. It is necessary follow further studies in order to know the CMC of SDS in the formulation tested.

4. References

[1] H. Zhenxing, Y. Xiaowei, L. Junliang, Y. Yuping, W. Ling, Z. Yanwei, An investigation of the effect of sodium dodecyl sulfate on quasi-emulsifier-free emulsion polymerization for highly monodisperse polystyrene nanospheres Europe. Polym. J. 47, 24 30 (2011).[2] M. Antonietti, K. Tauer, 90 Years of Polymer Latexes and Heterophase Polymerization: More vital than ever Macromol. Chem. Phys. 204, 207 219 (2003).[3] M. F. Kemmere, J. Meuldijk, A. A. H. Drinkenburg, A. L. German, Aspects of coagulation during emulsion polymerization of styrene and vinyl acetate J. Ap. poly. Sci. 69, 2409 2421 (1998).[4] W. V. Smith, R. H. Ewart, Kinetics of emulsion polymerization J. Chem. Phys. 16, 592 599 (1948).[5] C.S. Chern, Emulsion polymerization mechanism and kinetics, Prog. Polym. Sci. 31, 443 486 (2006).[6] S. Krishnan, A. Klein, M. S. El-Aasser, D. Sudol. Effect of surfactant concentration on Particle nucleation in emulsion polymerization of n-Butyl Methacrylate Macromol. 36, 3152 3159 (2003)[7] R. Arshady, Suspension, emulsion, and dispersion polymerization: A methodological survey Colloid Polym Sci 270: 717-732 (1992)[8] L. Varela de La Rosa, E. D. Sudol, M. S. El-Aasser, A. Klein Details of the Emulsion Polymerization of Styrene Using a Reaction Calorimeter J. Polymer Sci. Part A 34, 461 473 (1996)[9] R. Nagarajan,Molecular Theory for Mixed Micelles Langmuir 1, 331341 (1985).[10] M. Wagner, Thermal Analysis in Practice, Collected Applications METTLER TOLEDO 2009, 239.

Annex I.Samples studied by transmission electron microscopy were stored in a grid holder as is shown in tab.1. Tab.1: Samples studied by STEM. SamplePolystyrene concentration / %Grid Position

UP_PS_95.0A1

UP_PS_101.0A2

UP_PS_112.0A3

UP_PS_122.0A4

UP_PS_135.0A5

UP_PS_145.0B1

UP_PS_142.0B2

UP_PS_155.0B3

UP_PS_151.0B4

UP_PS_1610.0C1

UP_PS_165.0C2

UP_PS_161.0C3

UP_PS_192.0D1

UP_PS_202.0D2

UP_PS_212.0D3