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Synthesis and nucleation mechanism of inverse emulsion polymerization of acrylamide by RAFT polymerization: A comparative study
Polymer 52 (2011) 63e67
Contents lists available at ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Synthesis and nucleation mechanism of inverse emulsion polymerization of acrylamide by RAFT polymerization: A comparative study Liu Ouyang a, Lianshi Wang a, F. Joseph Schork b, * a b
College of Materials Science and Engineering, South China University of Technology, Guangzhou, 510640, China School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332-0100, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 September 2010 Received in revised form 28 October 2010 Accepted 30 October 2010 Available online 5 November 2010
Well-defined poly (acrylamide) is synthesized by RAFT inverse emulsion polymerization using hydrophilic and lipophilic initiators. The kinetic behavior observed for RAFT inverse emulsion polymerization is similar to that for RAFT inverse miniemulsion polymerization. The nucleation mechanism of inverse emulsion polymerization of acrylamide is firstly investigated by RAFT polymerization and verified by GPC and SEM measurements. Droplet nucleation is found to be the primary mechanism in the inverse emulsion polymerization of acrylamide. However, polymerization occurring in the continuous phase is not negligible when lipophilic initiator is used. Ó 2010 Elsevier Ltd. All rights reserved.
This paper is dedicated to the late Professor John Vanderhoff who advocated some of the ideas in this paper over thirty years ago. Keywords: Inverse emulsion Reversible addition fragmentation chain transfer (RAFT) Acrylamide
1. Introduction Water-soluble polymers are an important class of materials because of their numerous applications. Polymers based on poly (acrylamide) (PAM) and its derivatives are widely used commercial polymers, particularly in wastewater treatment applications, as drag reduction agents and drilling fluids in enhanced oil recovery, as additives in paper making, and as drug-delivery agents [1e9]. Water-in-oil (inverse) emulsion polymerization is one of the ideal methods for obtaining such polymers with high molecular weight and low viscosity. An inverse emulsion, however, cannot be simply considered as an analogy to conventional emulsion in which the water is replaced by the oil and the hydrophobic monomer by the hydrophilic monomer. The kinetic mechanism of inverse emulsion polymerization can be affected by many factors such as initiators, surfactants, and oils [10]. Studies have been made to investigate the mechanism and kinetics of the inverse emulsion polymerization since 1960s. Vanderhoff et al. [11] suggested that the principal locus of particle nucleation was the monomer droplets in the inverse emulsion polymerization of p-sodium styrene sulfonate. Some later
* Corresponding author. Tel.: þ1 678 642 5366; fax: þ815 301 9729. E-mail address: [email protected] (F.J. Schork). 0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2010.10.063
studies [12e15] on inverse emulsion polymerization of acrylamide also showed the existence of droplet nucleation, although some of to the systems studied would now be called miniemulsions. Until now, a fully comprehensive description of the nucleation mechanism of inverse emulsion polymerization has not been made. As one of three major approaches to controlled (living) radical polymerization, reversible addition fragmentation chain transfer (RAFT) polymerization has been successfully implemented in heterogeneous systems [16e20]. Some studies have been made in RAFT inverse miniemulsion polymerization to obtain well-defined controlled water-soluble (co)polymers [21e23]. However, RAFT polymerization in inverse emulsion, has not been investigated. In this paper, well-defined PAM was synthesized by RAFT inverse emulsion polymerization, and the nucleation mechanism is discussed. 2. Experimental 2.1. Materials All chemicals were purchased from Aldrich except as otherwise stated. Acrylamide (AM, 99.5%) was recrystallized from chloroform. Azobisisobutyronitrile (AIBN, >98%), 2, 20 -Azobis [2-(2-imidazolin2-yl) propane] dihydrochloride (VA-044, Wako, >98%) were
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L. Ouyang et al. / Polymer 52 (2011) 63e67
purified by recrystallization from methanol. Span 80, B246SF (Uniqema) were used as received.
2.2. RAFT inverse emulsion polymerization The inverse emulsion was prepared by mixing a solution of nonionic surfactant (i.e. Span 80 or B246SF) in cyclohexane with a water solution of AM. Two types of initiators were used: the hydrophilic initiator VA-044 was dissolved in the water solution or, the lipophilic initiator AIBN was dissolved in cyclohexane. The AM water solution was degassed for 5 min under vacuum prior to use. The reaction mixture prepared above was stirred under nitrogen at approx. 400 rpm for 5 min at room temperature. The resulting emulsion was charged into a 250 ml flask equipped with a nitrogen sweep and a magnetic stirrer, and then polymerized in an oil bath preheated to 60 C. All the experiments in this study are shown in Table 1. The ratio of monomer and RAFT agent was fixed at 100:1. For comparison purposes, RAFT inverse miniemulsion polymerization was also carried out. The preparation procedure for inverse miniemulsion was the same as for inverse emulsion except that the 5 min stirring process was replaced by sonicating with an OmniRuptor 250 ultrasonic homogenizer operated at 30% power output for 5 min. The emulsion was cooled in the ice bath during sonication. The CMC was measured by a Du Nouy Ring Surface Tensiometer (Fisher) equipped with a platinum-iridium ring. The surface tension was measured three times for each sample then the surfactant concentration in cyclohexane was increased. Measurements were made at room temperature and a constant solution volume of 50 ml. Molecular weights of the polymers were determined by aqueous size exclusion chromatography (ASEC) at 35 C. The ASEC system was comprised of a Waters 1525 Binary HPLC pump, a Waters 2414 refractive index detector (RI), a Waters 2487 dual l absorbance UV detector, an Ultrahydrogel Guard column, and Ultrahydrogel 2000A, 250A, 120A columns mounted in series. The mobile phase was 0.05 M Na2SO4 in water and the flow rate was maintained at 0.6 ml/min. PAM narrow standards were used to calibrate the ASEC by the universal calibration method. The conversion of the monomer was determined with ASEC using a known method by comparing the area of the RI signals that corresponded to the monomer and the polymer [24]. Morphology of polymer particles was investigated by scanning electron microscopy (SEM) with a JEOL JSPM-4500A (JEOL, Tokyo, Japan) instrument. For SEM analysis, a drop of latex obtained after 24 h of polymerization was placed on the glass and freeze-dried. This was then placed under vacuum, flushed with argon, and then sputter-coated with gold.
Fig. 1. Evolution of the conversion as a function of reaction time: (1) RAFT inverse emulsion polymerization with B246SF and VA-044 (Expt.1). (2) RAFT inverse miniemulsion polymerization with B246SF and VA-044 (Expt.2). (3) RAFT inverse emulsion polymerization with Span 80 and VA-044 (Expt.3). (4) RAFT inverse emulsion polymerization with B246SF and AIBN (Expt. 4). (5) RAFT inverse emulsion polymerization with B246SF and AIBN (Expt. 5). (6) RAFT inverse emulsion polymerization with B246SF and AIBN (Expt. 6).
3. Results and discussion The results of RAFT inverse emulsion polymerization of acrylamide with different initiators and surfactants are shown in Fig. 1. The kinetic data (curves 1, 3) are close to inverse miniemulsion polymerization (curve 2), and follow pseudo- first order kinetics. The induction time for inverse miniemulsion polymerization is a bit longer, most likely caused by the oxygen or other impurities which are introduced during sonication. Although different surfactants were used, curve 1 and curve 3 are almost identical. When the initiator was changed from water-soluble (VA-044) to oil-soluble (AIBN), as shown in curve 1 and curve 4, the polymerization rate decreased significantly and a much longer induction time was observed. This can be attributed to the long time interval that the oligoradicals take to enter droplets and initiate polymerization. This is discussed later in more detail. It is worth noting that for Expt. 4 and Expt. 5, the polymerization rate decreased with a higher surfactant concentration. This may be because of the steric stabilization employed in these inverse emulsions: the increased interfacial barrier caused by a higher surfactant concentration makes it more difficult for oligoradicals to enter droplets, resulting in a lower enter rate of the initiator. For the RAFT inverse emulsion system, the RAFT agent is watersoluble and mainly located in the droplets; hence, polymerization occurring in the continuous phase is uncontrolled due to the
Table 1 Experimental set up for the RAFT inverse emulsion polymerizations of acrylamide. Expt
Type
1 2 3 4 5 6 7
RAFT RAFT RAFT RAFT RAFT RAFT RAFT
Disperse phase/g
inverse inverse inverse inverse inverse inverse inverse
emulsion miniemulsion emulsion emulsion emulsion emulsion emulsion
Continuous phase/g
AM
CTA
VA-044
water
AIBN
Cyclohexane
Surfactant
2.5 2.5 2.5 2.5 2.5 2.5 2.5
0.089 0.089 0.089 0.089 0.089 0.089 0.089
0.028 0.028 0.028 e e e e
7.5 7.5 7.5 7.5 7.5 7.5 7.5
e e e 0.014 0.014 0.028 0.014
20 20 20 20 20 20 20
B246SF, 150% CMC B246SF, 150% CMC Span 80, 150% CMC B246SF, 150% CMC B246SF, 300% CMC B246SF, 150% CMC B246SF, 60% CMC
L. Ouyang et al. / Polymer 52 (2011) 63e67
65
Fig. 3. Mn and mole fraction of the PAMu as a function of conversion in Expt. 4 and Expt. 6
Fig. 2. RAFT polymerization of acrylamide: a) Evolution of Mn and PDI as a function of conversion. b) Evolution of Mn and PDI for controlled PAM as a function of conversion. c) ASEC chromatogram (RI and UV traces at 311 nm) evolution during the polymerization.
absence of the RAFT agent. Considering the fact that the GPC RI trace accounts for the polymer species, and the UV trace at 311 nm reflects the presence of the C]S double bond in the RAFT agent [22], the fraction of polymer formed in the dispersed phase and continuous phase can be determined respectively, by comparing and calculating the areas of the RI and UV curves. In other words,
the kinetic mechanism of RAFT inverse emulsion polymerization can be analyzed by monitoring the RI and UV signals. The molecular weights of PAM are shown in Fig. 2a with the same number designation as in Fig. 1. Again, like the kinetic curves in Fig. 1, the molecular weights of Expt. 1, Expt. 2 and Expt. 3 are quite close and the GPC curves of Expt. 1 and Expt. 3, as shown in Fig. 2c, are unimodal and exhibit narrow molecular weight distribution. The RI curves have good overlay with the UV curves, indicating good control throughout the polymerization [10] and the same polymerization mechanism as inverse miniemulsion. Oil-soluble initiator AIBN (Expt. 3) was used to compare with the water-soluble initiator VA-044. From Fig. 2b, it is clear that the molecular weight evolutions of Expt. 4 and Expt. 6 show a downward deviation from Expt. 2 which was polymerized with the hydrophilic initiator VA-044. The decrease in molecular weight in Expt. 3 is most likely due to the lower concentration of living chains. The small wide peak that appears at shorter retention time in RI curve (3) (Fig. 2c), reflects the polymer fraction polymerized under uncontrolled conditions. When Qi [22] and coworkers used AIBN as the initiator in RAFT inverse miniemulsion polymerization, similarly bimodality in RI curves were observed, and the RI curves showed a poor overlay with the UV curves. Fig. 3 shows the molecular weight and mole fraction of the uncontrolled polymers (PAMu) in Expt. 4 and Expt. 6. The mole fraction of PAMu increased with monomer conversion, as did the molecular weight. For Expt. 4, the ratio [PAMu]/[PAM] remains low until high degree of conversion, indicating a dominant droplets nucleation. However, when the initiator concentration is increased by 200%, the fraction of PAMu grows extensively, especially in the late stage of polymerization, suggesting that polymerization in continuous phase is enhanced by the increased initiator concentration. Thus, the proposed nucleation mechanism (see Fig. 4) can be described as follows: For inverse emulsion polymerization with water-soluble initiators, the nucleation mechanism is similar to that of inverse miniemulsion polymerization. For inverse emulsion with oil-soluble initiators, the nucleation mechanism can be regarded as a mix of droplet nucleation and continuous phase (micellar or homogeneous) nucleation. Primary free radicals are formed by decomposition of initiator and grow by adding monomer units dissolved in the continuous phase. After the oligoradicals grow to a certain length, they become hydrophilic enough to enter droplets and initiate the RAFT polymerization. Meanwhile, a certain fraction of the oligoradicals are captured by monomer-swollen micelles, and form new particles by micellar nucleation. Alternatively, they may
66
L. Ouyang et al. / Polymer 52 (2011) 63e67
Fig. 4. Proposed nucleation mechanisms of RAFT inverse emulsion polymerization with a) water-soluble initiator b) oil-soluble initiator c) oil-soluble initiator, surfactant concentration below CMC.
continue to propagate until they reach the limit of their solubility in the continuous phase and precipitate out (homogeneous nucleation). Both of these two possibilities will lead to the uncontrolled polymerization to high molecular weight. Based on the above, at the beginning of the polymerization, the polymerization rate in particles is mainly determined by the entry rate of oligoradicals; when most of the free oligoradicals have entered the droplets, they become the main polymerization locus. Therefore, the polymerization rate increases and the polymerization progresses by living polymerization kinetics. Increased initiator concentration will enhance the micelle nucleation due to the accelerated formation of new particles, and lead to a higher fraction of uncontrolled polymers. Inverse emulsion polymerization with low surfactant concentration (
disappearance of the uncontrolled polymer peak. The rest of the monomer in the dispersed phase polymerizes via a living mechanism. A schematic representation for this case is shown in Fig. 4c.The fitted line 7 shown in Fig. 2b is the molecular weight of controlled PAM in Expt. 7 as a function of conversion, which is parallel to the one in Expt. 2. That is because of the decreased ratio
Fig. 5. ASEC chromatogram (RI and UV traces at 311 nm) evolution of Expt. 4 and Expt. 7
L. Ouyang et al. / Polymer 52 (2011) 63e67
67
range particle sizes of RAFT inverse emulsion can be related to the absence of costabilizers, which are used to retard Ostwald ripening [25]. Fig. 6c is the SEM micrograph of the latex from inverse emulsion polymerization with AIBN as the initiator. Two populations of particles are observed. The submicron particles are thought to be derived from the uncontrolled PAM formed by continuous phase nucleation, while the larger particles come from droplet nucleation. The large particles, compared to Fig. 6a, exhibit a more uniform distribution and smaller particle size, which should result from the monomer diffusion from large droplets to newlyformed particles. 4. Conclusion RAFT inverse emulsion polymerization is a viable method to prepare controlled water-soluble polymers. It was found that when the water-soluble initiator VA-044 was used, the kinetics of inverse emulsion polymerization and inverse miniemulsion polymerization were similar and they have a same (droplet) nucleation mechanism. When the oil-soluble initiator AIBN was used to replace VA-044, a certain amount of uncontrolled polymer was observed and appeared as a wide peak in the GPC RI trace, which resulted from micellar or homogeneous nucleation. RAFT polymerization can be used as a universal technique in investigating the kinetic mechanism of inverse emulsion polymerizations of various monomers. Acknowledgements The authors would like to acknowledge the support of the US National Science Foundation (CTS-0553516) and China Scholarship Council (CSC) for this work. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] Fig. 6. SEM micrographs of the latex particles obtained from a) RAFT inverse emulsion polymerization, conversion: 95%, Expt. 1. b) RAFT inverse miniemulsion polymerization, conversion: 94%, Expt. 2. c) RAFT inverse emulsion polymerization with AIBN, 150% CMC, conversion: 80%, Expt. 6.
of monomer to RAFT agent due to the loss of uncontrolled polymer formed in continuous phase. To confirm the mechanism presented above, SEM was used to determine the size and morphology of final particles. The particle size of RAFT inverse emulsion polymerization (Expt. 1), as shown in Fig. 6a, is above 1 mm and has a wide distribution from 1 mm to 4 mm. Latex particles obtained from RAFT inverse miniemulsion polymerization (Expt. 2), shown in Fig. 6b for comparison, exhibit a much narrower distribution with a smaller average size. The wide
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
Wada T, Sekiya H, Machi S. J Appl Polym Sci 1976;20:3233. Virk PS. J Fluid Mech 1971;45:225. Enright DP, Perricone AC. Pet Eng Int 1988;60:55. Date JL, Shute JM. Tappi 1959;42:824. Buck S, Pennefather PS, Xue HY, Grant J, Cheng Y-L, Allen CJ. Biomacromolecules 2004;5:2230. Singh B, Chauhan GS, Kumar S, Chauhan N. Carbohydr Polym 2007;67:190. Chaw C-S, Chooi K-W, Liu X-M, Tan C-W, Wang L, Yang Y-Y. Biomaterials 2004;25:4297. Al-Karawi A, Ahemed Jasim M, AI-Daraji AHR. Carbohydr Polym 2010;79:769. Salehi R, Davaran S, Rashidi MR, Entezami AA. J Appl Polym Sci 2009;111:1905. Lovell PA, EL-Aasser MS. “Emulsion polymerization and emulsion polymers”. Wiley; 1997. Vanderhoff JW, Tarkowski HL, Shaffer JB, Brad-ford EB, Wiley RM. Adv Chem Ser 1962;34:32. Vanderhoff JW, Visioli DL, El-Aasser MS. Polym Mater Sci Eng 1986;54:375. Benda D, Snuparek J, Cermak V. Eur Polym J 1997;33:1345. Graillat C, Pichot C, Guyot A, El Aasser MS. J Polym Sci Part A: Polym Chem 1986;24:427. Hernandez-Barajas J, Hunkeler DJ. Polymer 1997;38:437. Jun A, Masamichi N, Kiyotaka S, Teruo O. J Polym Sci Part A: Polym Chem 2008;21:7127e37. PerB Z, Fawaz A, Masayoshi O. J Polym Sci Part A: Polym Chem 2009;47:3711e28. Oh JK. J Polym Sci Part A: Polym Chem 2008;46:6983e7001. An Z, Shi QH, Tang W, Tsung CK, Hawker CJ, Stucky GD. J Am Chem Soc 2007;129:14493e9. Lu FJ, Luo YW, Li BG, Schork FJ. Macromolecules 2010;43:568e71. Qi GG, Bennett E, Jones CW, Schork FJ. Macromolecules 2009;42:3906e16. Qi GG, Jones CW, Schork FJ. Macromol Rapid Commun 2007;28:1010. L. Ouyang, L.S. Wang, F.J. Schork, Macromol Chem Phys, in press. Albertin L, Stenzel M, Barner-Kowollik C, Foster LJR, Davis TP. Macromolecules 2004;37:7530e7. Ostwald W. Z Phys Chem 1901;37:495.
Contents lists available at ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Synthesis and nucleation mechanism of inverse emulsion polymerization of acrylamide by RAFT polymerization: A comparative study Liu Ouyang a, Lianshi Wang a, F. Joseph Schork b, * a b
College of Materials Science and Engineering, South China University of Technology, Guangzhou, 510640, China School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332-0100, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 September 2010 Received in revised form 28 October 2010 Accepted 30 October 2010 Available online 5 November 2010
Well-defined poly (acrylamide) is synthesized by RAFT inverse emulsion polymerization using hydrophilic and lipophilic initiators. The kinetic behavior observed for RAFT inverse emulsion polymerization is similar to that for RAFT inverse miniemulsion polymerization. The nucleation mechanism of inverse emulsion polymerization of acrylamide is firstly investigated by RAFT polymerization and verified by GPC and SEM measurements. Droplet nucleation is found to be the primary mechanism in the inverse emulsion polymerization of acrylamide. However, polymerization occurring in the continuous phase is not negligible when lipophilic initiator is used. Ó 2010 Elsevier Ltd. All rights reserved.
This paper is dedicated to the late Professor John Vanderhoff who advocated some of the ideas in this paper over thirty years ago. Keywords: Inverse emulsion Reversible addition fragmentation chain transfer (RAFT) Acrylamide
1. Introduction Water-soluble polymers are an important class of materials because of their numerous applications. Polymers based on poly (acrylamide) (PAM) and its derivatives are widely used commercial polymers, particularly in wastewater treatment applications, as drag reduction agents and drilling fluids in enhanced oil recovery, as additives in paper making, and as drug-delivery agents [1e9]. Water-in-oil (inverse) emulsion polymerization is one of the ideal methods for obtaining such polymers with high molecular weight and low viscosity. An inverse emulsion, however, cannot be simply considered as an analogy to conventional emulsion in which the water is replaced by the oil and the hydrophobic monomer by the hydrophilic monomer. The kinetic mechanism of inverse emulsion polymerization can be affected by many factors such as initiators, surfactants, and oils [10]. Studies have been made to investigate the mechanism and kinetics of the inverse emulsion polymerization since 1960s. Vanderhoff et al. [11] suggested that the principal locus of particle nucleation was the monomer droplets in the inverse emulsion polymerization of p-sodium styrene sulfonate. Some later
* Corresponding author. Tel.: þ1 678 642 5366; fax: þ815 301 9729. E-mail address: [email protected] (F.J. Schork). 0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2010.10.063
studies [12e15] on inverse emulsion polymerization of acrylamide also showed the existence of droplet nucleation, although some of to the systems studied would now be called miniemulsions. Until now, a fully comprehensive description of the nucleation mechanism of inverse emulsion polymerization has not been made. As one of three major approaches to controlled (living) radical polymerization, reversible addition fragmentation chain transfer (RAFT) polymerization has been successfully implemented in heterogeneous systems [16e20]. Some studies have been made in RAFT inverse miniemulsion polymerization to obtain well-defined controlled water-soluble (co)polymers [21e23]. However, RAFT polymerization in inverse emulsion, has not been investigated. In this paper, well-defined PAM was synthesized by RAFT inverse emulsion polymerization, and the nucleation mechanism is discussed. 2. Experimental 2.1. Materials All chemicals were purchased from Aldrich except as otherwise stated. Acrylamide (AM, 99.5%) was recrystallized from chloroform. Azobisisobutyronitrile (AIBN, >98%), 2, 20 -Azobis [2-(2-imidazolin2-yl) propane] dihydrochloride (VA-044, Wako, >98%) were
64
L. Ouyang et al. / Polymer 52 (2011) 63e67
purified by recrystallization from methanol. Span 80, B246SF (Uniqema) were used as received.
2.2. RAFT inverse emulsion polymerization The inverse emulsion was prepared by mixing a solution of nonionic surfactant (i.e. Span 80 or B246SF) in cyclohexane with a water solution of AM. Two types of initiators were used: the hydrophilic initiator VA-044 was dissolved in the water solution or, the lipophilic initiator AIBN was dissolved in cyclohexane. The AM water solution was degassed for 5 min under vacuum prior to use. The reaction mixture prepared above was stirred under nitrogen at approx. 400 rpm for 5 min at room temperature. The resulting emulsion was charged into a 250 ml flask equipped with a nitrogen sweep and a magnetic stirrer, and then polymerized in an oil bath preheated to 60 C. All the experiments in this study are shown in Table 1. The ratio of monomer and RAFT agent was fixed at 100:1. For comparison purposes, RAFT inverse miniemulsion polymerization was also carried out. The preparation procedure for inverse miniemulsion was the same as for inverse emulsion except that the 5 min stirring process was replaced by sonicating with an OmniRuptor 250 ultrasonic homogenizer operated at 30% power output for 5 min. The emulsion was cooled in the ice bath during sonication. The CMC was measured by a Du Nouy Ring Surface Tensiometer (Fisher) equipped with a platinum-iridium ring. The surface tension was measured three times for each sample then the surfactant concentration in cyclohexane was increased. Measurements were made at room temperature and a constant solution volume of 50 ml. Molecular weights of the polymers were determined by aqueous size exclusion chromatography (ASEC) at 35 C. The ASEC system was comprised of a Waters 1525 Binary HPLC pump, a Waters 2414 refractive index detector (RI), a Waters 2487 dual l absorbance UV detector, an Ultrahydrogel Guard column, and Ultrahydrogel 2000A, 250A, 120A columns mounted in series. The mobile phase was 0.05 M Na2SO4 in water and the flow rate was maintained at 0.6 ml/min. PAM narrow standards were used to calibrate the ASEC by the universal calibration method. The conversion of the monomer was determined with ASEC using a known method by comparing the area of the RI signals that corresponded to the monomer and the polymer [24]. Morphology of polymer particles was investigated by scanning electron microscopy (SEM) with a JEOL JSPM-4500A (JEOL, Tokyo, Japan) instrument. For SEM analysis, a drop of latex obtained after 24 h of polymerization was placed on the glass and freeze-dried. This was then placed under vacuum, flushed with argon, and then sputter-coated with gold.
Fig. 1. Evolution of the conversion as a function of reaction time: (1) RAFT inverse emulsion polymerization with B246SF and VA-044 (Expt.1). (2) RAFT inverse miniemulsion polymerization with B246SF and VA-044 (Expt.2). (3) RAFT inverse emulsion polymerization with Span 80 and VA-044 (Expt.3). (4) RAFT inverse emulsion polymerization with B246SF and AIBN (Expt. 4). (5) RAFT inverse emulsion polymerization with B246SF and AIBN (Expt. 5). (6) RAFT inverse emulsion polymerization with B246SF and AIBN (Expt. 6).
3. Results and discussion The results of RAFT inverse emulsion polymerization of acrylamide with different initiators and surfactants are shown in Fig. 1. The kinetic data (curves 1, 3) are close to inverse miniemulsion polymerization (curve 2), and follow pseudo- first order kinetics. The induction time for inverse miniemulsion polymerization is a bit longer, most likely caused by the oxygen or other impurities which are introduced during sonication. Although different surfactants were used, curve 1 and curve 3 are almost identical. When the initiator was changed from water-soluble (VA-044) to oil-soluble (AIBN), as shown in curve 1 and curve 4, the polymerization rate decreased significantly and a much longer induction time was observed. This can be attributed to the long time interval that the oligoradicals take to enter droplets and initiate polymerization. This is discussed later in more detail. It is worth noting that for Expt. 4 and Expt. 5, the polymerization rate decreased with a higher surfactant concentration. This may be because of the steric stabilization employed in these inverse emulsions: the increased interfacial barrier caused by a higher surfactant concentration makes it more difficult for oligoradicals to enter droplets, resulting in a lower enter rate of the initiator. For the RAFT inverse emulsion system, the RAFT agent is watersoluble and mainly located in the droplets; hence, polymerization occurring in the continuous phase is uncontrolled due to the
Table 1 Experimental set up for the RAFT inverse emulsion polymerizations of acrylamide. Expt
Type
1 2 3 4 5 6 7
RAFT RAFT RAFT RAFT RAFT RAFT RAFT
Disperse phase/g
inverse inverse inverse inverse inverse inverse inverse
emulsion miniemulsion emulsion emulsion emulsion emulsion emulsion
Continuous phase/g
AM
CTA
VA-044
water
AIBN
Cyclohexane
Surfactant
2.5 2.5 2.5 2.5 2.5 2.5 2.5
0.089 0.089 0.089 0.089 0.089 0.089 0.089
0.028 0.028 0.028 e e e e
7.5 7.5 7.5 7.5 7.5 7.5 7.5
e e e 0.014 0.014 0.028 0.014
20 20 20 20 20 20 20
B246SF, 150% CMC B246SF, 150% CMC Span 80, 150% CMC B246SF, 150% CMC B246SF, 300% CMC B246SF, 150% CMC B246SF, 60% CMC
L. Ouyang et al. / Polymer 52 (2011) 63e67
65
Fig. 3. Mn and mole fraction of the PAMu as a function of conversion in Expt. 4 and Expt. 6
Fig. 2. RAFT polymerization of acrylamide: a) Evolution of Mn and PDI as a function of conversion. b) Evolution of Mn and PDI for controlled PAM as a function of conversion. c) ASEC chromatogram (RI and UV traces at 311 nm) evolution during the polymerization.
absence of the RAFT agent. Considering the fact that the GPC RI trace accounts for the polymer species, and the UV trace at 311 nm reflects the presence of the C]S double bond in the RAFT agent [22], the fraction of polymer formed in the dispersed phase and continuous phase can be determined respectively, by comparing and calculating the areas of the RI and UV curves. In other words,
the kinetic mechanism of RAFT inverse emulsion polymerization can be analyzed by monitoring the RI and UV signals. The molecular weights of PAM are shown in Fig. 2a with the same number designation as in Fig. 1. Again, like the kinetic curves in Fig. 1, the molecular weights of Expt. 1, Expt. 2 and Expt. 3 are quite close and the GPC curves of Expt. 1 and Expt. 3, as shown in Fig. 2c, are unimodal and exhibit narrow molecular weight distribution. The RI curves have good overlay with the UV curves, indicating good control throughout the polymerization [10] and the same polymerization mechanism as inverse miniemulsion. Oil-soluble initiator AIBN (Expt. 3) was used to compare with the water-soluble initiator VA-044. From Fig. 2b, it is clear that the molecular weight evolutions of Expt. 4 and Expt. 6 show a downward deviation from Expt. 2 which was polymerized with the hydrophilic initiator VA-044. The decrease in molecular weight in Expt. 3 is most likely due to the lower concentration of living chains. The small wide peak that appears at shorter retention time in RI curve (3) (Fig. 2c), reflects the polymer fraction polymerized under uncontrolled conditions. When Qi [22] and coworkers used AIBN as the initiator in RAFT inverse miniemulsion polymerization, similarly bimodality in RI curves were observed, and the RI curves showed a poor overlay with the UV curves. Fig. 3 shows the molecular weight and mole fraction of the uncontrolled polymers (PAMu) in Expt. 4 and Expt. 6. The mole fraction of PAMu increased with monomer conversion, as did the molecular weight. For Expt. 4, the ratio [PAMu]/[PAM] remains low until high degree of conversion, indicating a dominant droplets nucleation. However, when the initiator concentration is increased by 200%, the fraction of PAMu grows extensively, especially in the late stage of polymerization, suggesting that polymerization in continuous phase is enhanced by the increased initiator concentration. Thus, the proposed nucleation mechanism (see Fig. 4) can be described as follows: For inverse emulsion polymerization with water-soluble initiators, the nucleation mechanism is similar to that of inverse miniemulsion polymerization. For inverse emulsion with oil-soluble initiators, the nucleation mechanism can be regarded as a mix of droplet nucleation and continuous phase (micellar or homogeneous) nucleation. Primary free radicals are formed by decomposition of initiator and grow by adding monomer units dissolved in the continuous phase. After the oligoradicals grow to a certain length, they become hydrophilic enough to enter droplets and initiate the RAFT polymerization. Meanwhile, a certain fraction of the oligoradicals are captured by monomer-swollen micelles, and form new particles by micellar nucleation. Alternatively, they may
66
L. Ouyang et al. / Polymer 52 (2011) 63e67
Fig. 4. Proposed nucleation mechanisms of RAFT inverse emulsion polymerization with a) water-soluble initiator b) oil-soluble initiator c) oil-soluble initiator, surfactant concentration below CMC.
continue to propagate until they reach the limit of their solubility in the continuous phase and precipitate out (homogeneous nucleation). Both of these two possibilities will lead to the uncontrolled polymerization to high molecular weight. Based on the above, at the beginning of the polymerization, the polymerization rate in particles is mainly determined by the entry rate of oligoradicals; when most of the free oligoradicals have entered the droplets, they become the main polymerization locus. Therefore, the polymerization rate increases and the polymerization progresses by living polymerization kinetics. Increased initiator concentration will enhance the micelle nucleation due to the accelerated formation of new particles, and lead to a higher fraction of uncontrolled polymers. Inverse emulsion polymerization with low surfactant concentration (
disappearance of the uncontrolled polymer peak. The rest of the monomer in the dispersed phase polymerizes via a living mechanism. A schematic representation for this case is shown in Fig. 4c.The fitted line 7 shown in Fig. 2b is the molecular weight of controlled PAM in Expt. 7 as a function of conversion, which is parallel to the one in Expt. 2. That is because of the decreased ratio
Fig. 5. ASEC chromatogram (RI and UV traces at 311 nm) evolution of Expt. 4 and Expt. 7
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range particle sizes of RAFT inverse emulsion can be related to the absence of costabilizers, which are used to retard Ostwald ripening [25]. Fig. 6c is the SEM micrograph of the latex from inverse emulsion polymerization with AIBN as the initiator. Two populations of particles are observed. The submicron particles are thought to be derived from the uncontrolled PAM formed by continuous phase nucleation, while the larger particles come from droplet nucleation. The large particles, compared to Fig. 6a, exhibit a more uniform distribution and smaller particle size, which should result from the monomer diffusion from large droplets to newlyformed particles. 4. Conclusion RAFT inverse emulsion polymerization is a viable method to prepare controlled water-soluble polymers. It was found that when the water-soluble initiator VA-044 was used, the kinetics of inverse emulsion polymerization and inverse miniemulsion polymerization were similar and they have a same (droplet) nucleation mechanism. When the oil-soluble initiator AIBN was used to replace VA-044, a certain amount of uncontrolled polymer was observed and appeared as a wide peak in the GPC RI trace, which resulted from micellar or homogeneous nucleation. RAFT polymerization can be used as a universal technique in investigating the kinetic mechanism of inverse emulsion polymerizations of various monomers. Acknowledgements The authors would like to acknowledge the support of the US National Science Foundation (CTS-0553516) and China Scholarship Council (CSC) for this work. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] Fig. 6. SEM micrographs of the latex particles obtained from a) RAFT inverse emulsion polymerization, conversion: 95%, Expt. 1. b) RAFT inverse miniemulsion polymerization, conversion: 94%, Expt. 2. c) RAFT inverse emulsion polymerization with AIBN, 150% CMC, conversion: 80%, Expt. 6.
of monomer to RAFT agent due to the loss of uncontrolled polymer formed in continuous phase. To confirm the mechanism presented above, SEM was used to determine the size and morphology of final particles. The particle size of RAFT inverse emulsion polymerization (Expt. 1), as shown in Fig. 6a, is above 1 mm and has a wide distribution from 1 mm to 4 mm. Latex particles obtained from RAFT inverse miniemulsion polymerization (Expt. 2), shown in Fig. 6b for comparison, exhibit a much narrower distribution with a smaller average size. The wide
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