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The role of cationic monomers in emulsion polymerization
European Polymer Journal 46 (2010) 1106–1110
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
The role of cationic monomers in emulsion polymerization Jose Ramos b, Jacqueline Forcada a,* a
Institute for Polymer Materials POLYMAT and Grupo de Ingeniería Química, Facultad de Ciencias Químicas, Universidad del País Vasco/EHU, Apdo. 1072, Donostia-San Sebastián 20080, Spain Grupo de Física de Fluidos y Biocoloides, Departamento de Física Aplicada, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain
b
a r t i c l e
i n f o
Article history: Received 21 October 2009 Received in revised form 7 January 2010 Accepted 16 January 2010 Available online 22 January 2010 Keywords: Cationic emulsion polymerization Kinetics (polym.) Cationic monomers Cationic latexes
a b s t r a c t [2-(methacryloyloxy)ethyl] trimethylammonium chloride (MATMAC), and vinylbenzyl trimethyl ammonium chloride (VBTMAC) were chosen to be used as ionic comonomers in the emulsion polymerization of styrene. The cationic nature of the two comonomers is the same (quaternary ammonium salts), however the styrene derivate (VBTMAC) is more hydrophobic than the methacrylic one (MATMAC). With the more hydrophobic cationic comonomer (VBTMAC) higher conversions were obtained due to the in situ creation of an amphiphilic copolymer with styrene and faster rates of polymerization were observed by increasing the cationic comonomer concentration. The same behavior was observed with the more hydrophilic cationic comonomer (MATMAC) at concentrations up to 0.012 M. At higher concentrations the ionic strength controls the colloidal stability of the system and coagulation occurs. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The mechanisms governing in cationic emulsion polymerization has been explored during the last decade. It was found that the knowledge of well-studied anionic systems could not be extrapolated to cationic ones. Van Berkel et al. [1] studied radical entry in the emulsion polymerization of styrene for four different systems, incorporating an anionic and a cationic initiator into both anionically and cationically charged latexes. They concluded that the initiator charge had a significant effect on the initiator efficiency: the anionic initiator had decreasing entry efficiency with increasing radical flux, whereas the cationic initiator had efficiencies that were relatively invariant with changing radical flux. In our previous works [2,3], the batch cationic emulsion polymerization of styrene was compared with the anionic one. The main difference found was that under the experimental conditions studied, kinetics of cationic systems were affected by particle size, while in the anionic system studied, * Corresponding author. Fax: +34 943 015270. E-mail address: [email protected] (J. Forcada). 0014-3057/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2010.01.012
due to the lower particle size and lower initiation rates, the rate of polymerization was not dependent on the volume of the latex particles. Furthermore, in the cationic systems a dependence on the particle size of the rate of polymerization per particle together with the average number of radicals per particle was found. These differences were explained taking into account the limited particle coagulation observed with cationic surfactants, and the high rate of radical formation of cationic initiators. Using hexadecyltrimethyl ammonium bromide (HDTAB) as cationic surfactant, lower dependences of the rate of polymerization and the total number of particles with surfactant concentration were found than by using dodecyltrimethyl ammonium bromide (DTAB), but a higher effect of the size of the particles on the rate of polymerization per particle and on the average number of particles was observed. On the other hand, different kinetic behaviors were observed with the two cationic initiators used, and they were due to the lower stabilizing effect of the cationic radicals provided by 2,20 -azobisisobutyramidine dihydrochloride (AIBA), which means that a lower amount of particles were nucleated. Using 2,20 -azobis (N,N0 -dimethyleneisobutyramidine) dihydrochloride (ADIBA), the same number of particles was obtained by increas-
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ing the initiator concentration, but a faster rate of polymerization was observed. This was due to the strong dependence of the average number of radicals per particle with the initiator concentration at the same particle size. Using AIBA, higher dependences of the total rate of polymerization and number of particles were obtained, and a double effect on the average number of particles was observed: the effect of the particle size and the effect of the amount of initiator added. Under the conditions studied the effect of the particle size was the dominant one. The addition of small amounts of ionic comonomers in an emulsion polymerization of styrene increases the rate of nucleation of newly formed particles giving them greater colloidal stability. Until now, the work reported involving emulsion copolymerization with ionic monomers was focused mainly on acid monomers. Acrylic and methacrylic acid are by far the most common ionic monomers cited in the literature. However, due to the anionic nature of the carboxylic group, they cannot be used in a cationic emulsion polymerization of styrene, if high colloidal stability is required, because of its amphoteric character [4]. In this sense, cationic monomers with quaternary ammonium groups are promising candidates to be used in a cationic emulsion polymerization because they are unaffected by changes in pH and the positive charge remains in acidic, neutral, and alkaline media, enhancing the stability of the polymer particles [5–9]. On the other hand, the hydrophobicity of an ionic monomer is an important parameter because hydrophobic monomers diffuse into the particle, polymerize and become part of the particle more easily than hydrophilic ones. Hydrophilic ionic monomers must be carried to the particle surface by oligomeric radicals that have polymerized in the aqueous phase, and usually very little becomes incorporated within the particle. The aim of this work was to study the role of cationic monomers in the emulsion polymerization of styrene. In this way, [2-(methacryloyloxy)ethyl] trimethylammonium chloride (MATMAC, M), and vinylbenzyl trimethyl ammonium chloride (VBTMAC, V) were chosen to be used as comonomers. The cationic nature of the two comonomers presented in this work is the same (quaternary ammonium salts), however the styrene derivate (V) is more hydrophobic than the methacrylic one (M).
2. Experimental section Styrene (St) monomer was purified by washing with a 10% (wt.%) sodium hydroxide aqueous solution and stored at 18 °C until used. All the other materials were used as received, 2,20 -azobisisobutyramidine dihydrochloride (AIBA, A) (Wako Chemical Gmbh, Germany) was used as a cationic initiator. The surfactant used was hexadecyltrimethylammonium bromide (HDTAB, H) (Aldrich). [2-(methacryloyloxy)ethyl] trimethylammonium chloride (MATMAC, M) (Aldrich), 75 wt.% aqueous solution and vinylbenzyl trimethyl ammonium chloride (VBTMAC, V), (Fluka) were used as cationic comonomers. Double deionized (DDI) water was used throughout the work.
2.1. Synthesis of the polystyrene latex particles The batch emulsion polymerizations were carried out in a 1-L thermostated glass reactor fitted with a reflux condenser, stainless steel stirrer, sampling device, and nitrogen inlet tube. The stirring rate was 250 rpm, the reaction temperature was 70 °C, and the reaction time was 2 h. The recipes and the experimental conditions for the batch emulsion polymerization of styrene with different cationic comonomers are shown in Table 1. In all these polymerizations the amount of cationic surfactant (H) used is above its critical micelle concentration (cmc) [3]. In the first column the nomenclature used for the latexes is shown. The letter and decimal number of the latex name are the cationic comonomer type (V, or M), and the weight percent of cationic comonomer with respect to styrene, respectively. 2.2. Latex characterization To follow the evolution of some polymeric and colloidal characteristics of the synthesized latexes, samples were withdrawn during the reaction, and the polymerization was quenched with hydroquinone. The overall conversion was determined gravimetrically and by gas chromatography. The amount of coagulum can be calculated as the difference between the chromatographic and gravimetric conversions at the end of the reaction [3]. For the gravimetric conversion, samples were weighed, mixed with a drop of an aqueous solution of hydroquinone at 1 wt.%, and dried in the oven to constant weight. The weight of the dried sample is the combination of the polymer formed and the non-polymeric solids (surfactant, and initiator). For the chromatographic conversion, samples were diluted to a solids content lower than 2% by adding DDI water, and mixed with a known amount of cyclopentanone, which is the internal standard. Subsequently, samples were rotated end over end for 30 min at room temperature, and injected into a gas chromatograph (Shimadzu GC-14A) equipped with a flame ionization detector (FID) and an integrator (Shimadzu C-R6A). The column used was 25QC5/BP5 1.0 (Scientific Glass Engineering Inc.). The injector and detector temperature was
Table 1 Recipes used in the batch emulsion polymerizations of styrene varying the nature and the concentration of cationic comonomer. Latex
Cationic comonomer (g) MATMAC
VBTMAC
M0.0 or V0.0
–
–
M0.5 M1.0 M2.0
0.8 1.6 3.2
– – –
V0.5 V1.0 V2.0
– – –
0.8 1.6 3.2
Reaction conditions: T = 70 °C; rpm = 250; reaction time = 2–3 h. DDI water = 640 g; St = 160 g; HDTAB = 0.64 g; AIBA = 1.60 g. Variables: Nature and concentration of cationic comonomer.
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220 °C in both cases, and the temperature ramp was: 60 °C during 5 min, then it was heated at 10 °C/min up to reach 120 °C, and it was kept at this temperature during 1 min. The particle size distributions (PSDs) were determined by transmission electron microscopy (TEM, Hitachi H7000 FA) on representative samples of more than 500 particles in the case of monodisperse latexes, and more than 1000 particles for polydisperse latexes and analyzed using n ), weightthe software Bolero (AQ Systems). Number- (d v ) diameters as well as polyw ), and volume-average (d (d dispersity indices (PDI) were calculated from the PSD PCS ) [10]. The mean hydrodynamic particle diameters (d were obtained by photon correlation spectroscopy (PCS, Malvern Zetasizer Nano ZS). The rate of polymerization (Rp) and the total number of latex particles (Np) were calculated from the kinetic data obtained throughout the polymerization reaction, as in a previous work [2].
3. Results and discussion As was previously reported, to enhance the colloidal stability of cationic latex particles, small amounts of cationic monomers should be added to the recipe [5–9]. All the reactions carried out in this work are based on a cationic system in which HDTAB was used as a cationic surfactant and AIBA was used as a cationic initiator and differ in the concentrations of the cationic comonomers (V or M) used, which were 0, 0.5, 1 and 2 wt.% with respect to styrene. As far as the different chemical structure of the cationic monomers is concerned, the molecular weights are very similar, so the concentration of cationic monomer used in the recipes is practically equal for both comonomers. Table 2 shows the final gravimetric conversion (XG), the percentage of coagulum (% coagulum), the volume-average v ), the PDI, and the mean particle diameter diameter (d PCS ) of the latexes prepared using the two cationic mono(d mers. As can be seen, the final gravimetric conversion increased and the particle diameters obtained by TEM and PCS decreased when the amount of cationic comonomer increased, except for reaction M2.0 in which some coagulum was formed and larger particle diameters were obtained. In the case of anionic systems, if carboxylic comonomers are used, the results show that increasing the amount of anionic comonomer smaller particle size and faster rates of polymerization are achieved. However, in the case of using MATMAC as cationic comonomer this behavior was not observed. Liu and Krieger [11] obtained the same unusual behavior by varying the content of 1-ethyl-2-methyl5-vinylpyridinium bromide used as cationic monomer. They found that increasing the cationic monomer content, particle diameters passed through a minimum. The authors suggested that two opposite factors produced this behavior. Initially, by increasing the cationic monomer concentration the rate of nucleation of newly formed particles is faster, and the stability is improved. Due to this, a higher particle number is achieved and smaller particles are produced. However, at higher cationic monomer concentrations the oligomeric radicals copolymerize with a
Table 2 Results of the batch cationic emulsion polymerizations of styrene using different type and concentration of cationic comonomer. Reaction
XG
V0.0 or M0.0
0.87
V0.5 V1.0 V2.0
0.99 Total Total
M0.5 M1.0 M2.0
0.99 0.99 0.82
v (nm) d
PDI
d PCS (nm)
8
90
1.051
118
0 0 0
73 61 59
1.043 1.045 1.035
87 79 73
0 0 14
82 68 95
1.041 1.053 1.006
94 88 118
Coagulum (%)
higher fraction of cationic monomer units (instead of styrene), which increase their solubility (requiring a longer chain length to precipitate). This higher solubility increases the time needed for precipitation and results in an increased chance of capture by existing particles. This would reduce the number of particles formed producing the increase in particle diameters. In order to corroborate this behavior and the differences between VBTMAC and MATMAC a kinetic study was performed. In Fig. 1, the evolutions of the conversions obtained for the different type and amounts of cationic comonomers are shown. In the case of using VBTMAC as a cationic comonomer, the rate of polymerization increased with increasing the cationic monomer concentration, due to the smaller size of the particles (see Table 2). This means that more particles were nucleated. However, in the case of using MATMAC, the kinetics were faster compared to M0.0 (reaction carried out without cationic comonomer), but with the highest amount of cationic monomer (reaction M2.0) the rate of polymerization was slower than when the amount of cationic monomer was 0.5 and 1 wt.% with respect to styrene (reactions M0.5 and M1.0). These results are justified by the larger particle diameter obtained in reaction M2.0, and it seems that less particles were nucleated as in the case of the results reported by Liu and Krieger [11]. The changes in the total number of particles with the conversion due to the type and amount of cationic monomer used are shown in Fig. 2. As can be seen, at low conversions the number of particles for the reactions in which cationic monomer was added was higher than in reactions carried out without it (M0.0 or V0.0). However, in reaction M2.0, with the highest amount of MATMAC, a significant increase in particle diameters implying a reduction in the number of particles is observed from conversion 0.2 to 0.4. This means that a coagulation process took place during the reaction. In Fig. 3, the changes in the rate of polymerization with conversion as a function of the type and amount of cationic monomer used are shown. This effect on the rate of polymerization was much stronger with VBTMAC than with MATMAC and much higher values were achieved due to the higher number of particles nucleated using VBTMAC (see Fig. 2). The differences in polymerization rates found between the two cationic monomers can be explained through their
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1
1
0.8
0.8
Conversion
Conversion
J. Ramos, J. Forcada / European Polymer Journal 46 (2010) 1106–1110
0.6 0.4
0.6 0.4
0.2
0.2
0
0 0
20
40
60
80
100
0
120
20
40
time (min)
60
80
100
120
time (min)
Fig. 1. Batch cationic emulsion polymerization of styrene. Effect of the concentration of VBTMAC (left) and MATMAC (right) on the evolution of the conversion. (j) V0.0; (d) V0.5; () V1.0; (N) V2.0; (h) M0.0; (s) M0.5; (}) M1.0; (4) M2.0. 18
18
10
Total Number of Particles
Total Number of Particles
10
17
10
0
0.2
0.4
0.6
0.8
17
10
1
0
0.2
0.4
Conversion
0.6
0.8
1
Conversion
Fig. 2. Batch cationic emulsion polymerization of styrene. Effect of the concentration of VBTMAC (left) and MATMAC (right) on the evolution of the total number of particles. (j) V0.0; (d) V0.5; () V1.0; (N) V2.0; (h) M0.0; (s) M0.5; (}) M1.0; (4) M2.0.
0.1
Rate of Polymerization (mol/min)
Rate of Polymerization (mol/min)
0.1 0.08 0.06 0.04 0.02
0.08 0.06 0.04 0.02 0
0 0
0.2
0.4
0.6
0.8
Conversion
1
0
0.2
0.4
0.6
0.8
1
Conversion
Fig. 3. Batch cationic emulsion polymerization of styrene. Effect of the concentration of VBTMAC (left) and MATMAC (right) on the evolution of the rate of polymerization. (j) V0.0; (d) V0.5; () V1.0; (N) V2.0; (h) M0.0; (s) M0.5; (}) M1.0; (4) M2.0.
different hydrophobicity. The two cationic monomers are water soluble, but VBTMAC shows a higher affinity for the organic phase than MATMAC, so its copolymerization with styrene is more favored than in the case of using MATMAC. With MATMAC, homopolymerization in the water phase is favored and only a small amount of MATMAC is anchored to the particle surface. Due to this, more particles can be nucleated using VBTMAC than MATMAC as cationic comonomer, together with a faster rate of polymerization. Similar
results were obtained by Ganachaud et al. [12] in the emulsifier-free emulsion polymerization of styrene with vinyl benzyl amine hydrochloride, which is similar to VBTMAC, and aminoethylmethacrylate hydrochloride, which is similar to MATMAC. Furthermore, the higher compatibility of VBTMAC with styrene compared to MATMAC makes the stability of the particles higher, and so, no coagulation processes among particles were observed in reaction V2.0, while in reactions carried out with the highest amount of
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MATMAC (M2.0) coagulation took place due to the high ionic strength in the reaction medium and the lower colloidal stability of the particles. 4. Conclusions The cationic emulsion polymerization of styrene is affected by the addition of small amounts of cationic monomers. Polymerizations using the more hydrophobic cationic comonomer (VBTMAC) showed higher conversions due to the in situ creation of an amphiphilic copolymer with styrene improving particle stability, and faster rates of polymerization were observed by increasing the cationic comonomer concentration. With the more hydrophilic cationic comonomer (MATMAC) the same behavior was observed up to 0.012 M of MATMAC concentration. At higher concentrations the most of MATMAC homopolymerizes in water phase, and therefore, the ionic strength controls the colloidal stability of the system occurring coagulation. Acknowledgements This work has been supported by the Spanish Programa Nacional de Materiales(MAT 2009-13155-C04-01). J. Ramos
acknowledges the financial support by the Ministerio de Ciencia e Innovación: Subprograma Juan de la Cierva (JCI2008-2217).
References [1] van Berkel KY, Russell GT, Gilbert RG. Macromolecules 2003;36: 3921–31. [2] Ramos J, Costoyas A, Forcada J. J Polym Sci, Part A: Polym Chem 2006;44:4461–78. [3] Ramos J, Forcada J. Eur Polym J 2007;43:4647–61. [4] Miraballes-Martínez I, Forcada J. J Polym Sci, Part A: Polym Chem 2000;38:4230–7. [5] Miraballes-Martínez I, Martín-Molina A, Galisteo-González F, Forcada J. J Polym Sci, Part A: Polym Chem 2001;39:2929–36. [6] Ramos J, Martín-Molina A, Sanz-Izquierdo MP, Rus A, Borque L, Hidalgo-Álvarez R, et al. J Polym Sci, Part A: Polym Chem 2003;41:2404–11. [7] Ramos J, Forcada J. J Polym Sci, Part A: Polym Chem 2005;43:3878–86. [8] Dziomkina NV, Hempenius MA, Vancso GJ. Eur Polym J 2006;42:81–91. [9] Costoyas A, Ramos J, Forcada J. J Polym Sci, Part A: Polym Chem 2009;47:6201–13. [10] Unzueta E, Forcada J. Polymer 1995;36:1045–52. [11] Liu LJ, Krieger IM. J Polym Sci Polym Chem Ed 1981;19: 3013–26. [12] Ganachaud F, Sauzedde F, Ela A, Pichot C. J Appl Polym Sci 1997;65:2315–30.
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
The role of cationic monomers in emulsion polymerization Jose Ramos b, Jacqueline Forcada a,* a
Institute for Polymer Materials POLYMAT and Grupo de Ingeniería Química, Facultad de Ciencias Químicas, Universidad del País Vasco/EHU, Apdo. 1072, Donostia-San Sebastián 20080, Spain Grupo de Física de Fluidos y Biocoloides, Departamento de Física Aplicada, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain
b
a r t i c l e
i n f o
Article history: Received 21 October 2009 Received in revised form 7 January 2010 Accepted 16 January 2010 Available online 22 January 2010 Keywords: Cationic emulsion polymerization Kinetics (polym.) Cationic monomers Cationic latexes
a b s t r a c t [2-(methacryloyloxy)ethyl] trimethylammonium chloride (MATMAC), and vinylbenzyl trimethyl ammonium chloride (VBTMAC) were chosen to be used as ionic comonomers in the emulsion polymerization of styrene. The cationic nature of the two comonomers is the same (quaternary ammonium salts), however the styrene derivate (VBTMAC) is more hydrophobic than the methacrylic one (MATMAC). With the more hydrophobic cationic comonomer (VBTMAC) higher conversions were obtained due to the in situ creation of an amphiphilic copolymer with styrene and faster rates of polymerization were observed by increasing the cationic comonomer concentration. The same behavior was observed with the more hydrophilic cationic comonomer (MATMAC) at concentrations up to 0.012 M. At higher concentrations the ionic strength controls the colloidal stability of the system and coagulation occurs. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The mechanisms governing in cationic emulsion polymerization has been explored during the last decade. It was found that the knowledge of well-studied anionic systems could not be extrapolated to cationic ones. Van Berkel et al. [1] studied radical entry in the emulsion polymerization of styrene for four different systems, incorporating an anionic and a cationic initiator into both anionically and cationically charged latexes. They concluded that the initiator charge had a significant effect on the initiator efficiency: the anionic initiator had decreasing entry efficiency with increasing radical flux, whereas the cationic initiator had efficiencies that were relatively invariant with changing radical flux. In our previous works [2,3], the batch cationic emulsion polymerization of styrene was compared with the anionic one. The main difference found was that under the experimental conditions studied, kinetics of cationic systems were affected by particle size, while in the anionic system studied, * Corresponding author. Fax: +34 943 015270. E-mail address: [email protected] (J. Forcada). 0014-3057/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2010.01.012
due to the lower particle size and lower initiation rates, the rate of polymerization was not dependent on the volume of the latex particles. Furthermore, in the cationic systems a dependence on the particle size of the rate of polymerization per particle together with the average number of radicals per particle was found. These differences were explained taking into account the limited particle coagulation observed with cationic surfactants, and the high rate of radical formation of cationic initiators. Using hexadecyltrimethyl ammonium bromide (HDTAB) as cationic surfactant, lower dependences of the rate of polymerization and the total number of particles with surfactant concentration were found than by using dodecyltrimethyl ammonium bromide (DTAB), but a higher effect of the size of the particles on the rate of polymerization per particle and on the average number of particles was observed. On the other hand, different kinetic behaviors were observed with the two cationic initiators used, and they were due to the lower stabilizing effect of the cationic radicals provided by 2,20 -azobisisobutyramidine dihydrochloride (AIBA), which means that a lower amount of particles were nucleated. Using 2,20 -azobis (N,N0 -dimethyleneisobutyramidine) dihydrochloride (ADIBA), the same number of particles was obtained by increas-
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ing the initiator concentration, but a faster rate of polymerization was observed. This was due to the strong dependence of the average number of radicals per particle with the initiator concentration at the same particle size. Using AIBA, higher dependences of the total rate of polymerization and number of particles were obtained, and a double effect on the average number of particles was observed: the effect of the particle size and the effect of the amount of initiator added. Under the conditions studied the effect of the particle size was the dominant one. The addition of small amounts of ionic comonomers in an emulsion polymerization of styrene increases the rate of nucleation of newly formed particles giving them greater colloidal stability. Until now, the work reported involving emulsion copolymerization with ionic monomers was focused mainly on acid monomers. Acrylic and methacrylic acid are by far the most common ionic monomers cited in the literature. However, due to the anionic nature of the carboxylic group, they cannot be used in a cationic emulsion polymerization of styrene, if high colloidal stability is required, because of its amphoteric character [4]. In this sense, cationic monomers with quaternary ammonium groups are promising candidates to be used in a cationic emulsion polymerization because they are unaffected by changes in pH and the positive charge remains in acidic, neutral, and alkaline media, enhancing the stability of the polymer particles [5–9]. On the other hand, the hydrophobicity of an ionic monomer is an important parameter because hydrophobic monomers diffuse into the particle, polymerize and become part of the particle more easily than hydrophilic ones. Hydrophilic ionic monomers must be carried to the particle surface by oligomeric radicals that have polymerized in the aqueous phase, and usually very little becomes incorporated within the particle. The aim of this work was to study the role of cationic monomers in the emulsion polymerization of styrene. In this way, [2-(methacryloyloxy)ethyl] trimethylammonium chloride (MATMAC, M), and vinylbenzyl trimethyl ammonium chloride (VBTMAC, V) were chosen to be used as comonomers. The cationic nature of the two comonomers presented in this work is the same (quaternary ammonium salts), however the styrene derivate (V) is more hydrophobic than the methacrylic one (M).
2. Experimental section Styrene (St) monomer was purified by washing with a 10% (wt.%) sodium hydroxide aqueous solution and stored at 18 °C until used. All the other materials were used as received, 2,20 -azobisisobutyramidine dihydrochloride (AIBA, A) (Wako Chemical Gmbh, Germany) was used as a cationic initiator. The surfactant used was hexadecyltrimethylammonium bromide (HDTAB, H) (Aldrich). [2-(methacryloyloxy)ethyl] trimethylammonium chloride (MATMAC, M) (Aldrich), 75 wt.% aqueous solution and vinylbenzyl trimethyl ammonium chloride (VBTMAC, V), (Fluka) were used as cationic comonomers. Double deionized (DDI) water was used throughout the work.
2.1. Synthesis of the polystyrene latex particles The batch emulsion polymerizations were carried out in a 1-L thermostated glass reactor fitted with a reflux condenser, stainless steel stirrer, sampling device, and nitrogen inlet tube. The stirring rate was 250 rpm, the reaction temperature was 70 °C, and the reaction time was 2 h. The recipes and the experimental conditions for the batch emulsion polymerization of styrene with different cationic comonomers are shown in Table 1. In all these polymerizations the amount of cationic surfactant (H) used is above its critical micelle concentration (cmc) [3]. In the first column the nomenclature used for the latexes is shown. The letter and decimal number of the latex name are the cationic comonomer type (V, or M), and the weight percent of cationic comonomer with respect to styrene, respectively. 2.2. Latex characterization To follow the evolution of some polymeric and colloidal characteristics of the synthesized latexes, samples were withdrawn during the reaction, and the polymerization was quenched with hydroquinone. The overall conversion was determined gravimetrically and by gas chromatography. The amount of coagulum can be calculated as the difference between the chromatographic and gravimetric conversions at the end of the reaction [3]. For the gravimetric conversion, samples were weighed, mixed with a drop of an aqueous solution of hydroquinone at 1 wt.%, and dried in the oven to constant weight. The weight of the dried sample is the combination of the polymer formed and the non-polymeric solids (surfactant, and initiator). For the chromatographic conversion, samples were diluted to a solids content lower than 2% by adding DDI water, and mixed with a known amount of cyclopentanone, which is the internal standard. Subsequently, samples were rotated end over end for 30 min at room temperature, and injected into a gas chromatograph (Shimadzu GC-14A) equipped with a flame ionization detector (FID) and an integrator (Shimadzu C-R6A). The column used was 25QC5/BP5 1.0 (Scientific Glass Engineering Inc.). The injector and detector temperature was
Table 1 Recipes used in the batch emulsion polymerizations of styrene varying the nature and the concentration of cationic comonomer. Latex
Cationic comonomer (g) MATMAC
VBTMAC
M0.0 or V0.0
–
–
M0.5 M1.0 M2.0
0.8 1.6 3.2
– – –
V0.5 V1.0 V2.0
– – –
0.8 1.6 3.2
Reaction conditions: T = 70 °C; rpm = 250; reaction time = 2–3 h. DDI water = 640 g; St = 160 g; HDTAB = 0.64 g; AIBA = 1.60 g. Variables: Nature and concentration of cationic comonomer.
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J. Ramos, J. Forcada / European Polymer Journal 46 (2010) 1106–1110
220 °C in both cases, and the temperature ramp was: 60 °C during 5 min, then it was heated at 10 °C/min up to reach 120 °C, and it was kept at this temperature during 1 min. The particle size distributions (PSDs) were determined by transmission electron microscopy (TEM, Hitachi H7000 FA) on representative samples of more than 500 particles in the case of monodisperse latexes, and more than 1000 particles for polydisperse latexes and analyzed using n ), weightthe software Bolero (AQ Systems). Number- (d v ) diameters as well as polyw ), and volume-average (d (d dispersity indices (PDI) were calculated from the PSD PCS ) [10]. The mean hydrodynamic particle diameters (d were obtained by photon correlation spectroscopy (PCS, Malvern Zetasizer Nano ZS). The rate of polymerization (Rp) and the total number of latex particles (Np) were calculated from the kinetic data obtained throughout the polymerization reaction, as in a previous work [2].
3. Results and discussion As was previously reported, to enhance the colloidal stability of cationic latex particles, small amounts of cationic monomers should be added to the recipe [5–9]. All the reactions carried out in this work are based on a cationic system in which HDTAB was used as a cationic surfactant and AIBA was used as a cationic initiator and differ in the concentrations of the cationic comonomers (V or M) used, which were 0, 0.5, 1 and 2 wt.% with respect to styrene. As far as the different chemical structure of the cationic monomers is concerned, the molecular weights are very similar, so the concentration of cationic monomer used in the recipes is practically equal for both comonomers. Table 2 shows the final gravimetric conversion (XG), the percentage of coagulum (% coagulum), the volume-average v ), the PDI, and the mean particle diameter diameter (d PCS ) of the latexes prepared using the two cationic mono(d mers. As can be seen, the final gravimetric conversion increased and the particle diameters obtained by TEM and PCS decreased when the amount of cationic comonomer increased, except for reaction M2.0 in which some coagulum was formed and larger particle diameters were obtained. In the case of anionic systems, if carboxylic comonomers are used, the results show that increasing the amount of anionic comonomer smaller particle size and faster rates of polymerization are achieved. However, in the case of using MATMAC as cationic comonomer this behavior was not observed. Liu and Krieger [11] obtained the same unusual behavior by varying the content of 1-ethyl-2-methyl5-vinylpyridinium bromide used as cationic monomer. They found that increasing the cationic monomer content, particle diameters passed through a minimum. The authors suggested that two opposite factors produced this behavior. Initially, by increasing the cationic monomer concentration the rate of nucleation of newly formed particles is faster, and the stability is improved. Due to this, a higher particle number is achieved and smaller particles are produced. However, at higher cationic monomer concentrations the oligomeric radicals copolymerize with a
Table 2 Results of the batch cationic emulsion polymerizations of styrene using different type and concentration of cationic comonomer. Reaction
XG
V0.0 or M0.0
0.87
V0.5 V1.0 V2.0
0.99 Total Total
M0.5 M1.0 M2.0
0.99 0.99 0.82
v (nm) d
PDI
d PCS (nm)
8
90
1.051
118
0 0 0
73 61 59
1.043 1.045 1.035
87 79 73
0 0 14
82 68 95
1.041 1.053 1.006
94 88 118
Coagulum (%)
higher fraction of cationic monomer units (instead of styrene), which increase their solubility (requiring a longer chain length to precipitate). This higher solubility increases the time needed for precipitation and results in an increased chance of capture by existing particles. This would reduce the number of particles formed producing the increase in particle diameters. In order to corroborate this behavior and the differences between VBTMAC and MATMAC a kinetic study was performed. In Fig. 1, the evolutions of the conversions obtained for the different type and amounts of cationic comonomers are shown. In the case of using VBTMAC as a cationic comonomer, the rate of polymerization increased with increasing the cationic monomer concentration, due to the smaller size of the particles (see Table 2). This means that more particles were nucleated. However, in the case of using MATMAC, the kinetics were faster compared to M0.0 (reaction carried out without cationic comonomer), but with the highest amount of cationic monomer (reaction M2.0) the rate of polymerization was slower than when the amount of cationic monomer was 0.5 and 1 wt.% with respect to styrene (reactions M0.5 and M1.0). These results are justified by the larger particle diameter obtained in reaction M2.0, and it seems that less particles were nucleated as in the case of the results reported by Liu and Krieger [11]. The changes in the total number of particles with the conversion due to the type and amount of cationic monomer used are shown in Fig. 2. As can be seen, at low conversions the number of particles for the reactions in which cationic monomer was added was higher than in reactions carried out without it (M0.0 or V0.0). However, in reaction M2.0, with the highest amount of MATMAC, a significant increase in particle diameters implying a reduction in the number of particles is observed from conversion 0.2 to 0.4. This means that a coagulation process took place during the reaction. In Fig. 3, the changes in the rate of polymerization with conversion as a function of the type and amount of cationic monomer used are shown. This effect on the rate of polymerization was much stronger with VBTMAC than with MATMAC and much higher values were achieved due to the higher number of particles nucleated using VBTMAC (see Fig. 2). The differences in polymerization rates found between the two cationic monomers can be explained through their
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1
1
0.8
0.8
Conversion
Conversion
J. Ramos, J. Forcada / European Polymer Journal 46 (2010) 1106–1110
0.6 0.4
0.6 0.4
0.2
0.2
0
0 0
20
40
60
80
100
0
120
20
40
time (min)
60
80
100
120
time (min)
Fig. 1. Batch cationic emulsion polymerization of styrene. Effect of the concentration of VBTMAC (left) and MATMAC (right) on the evolution of the conversion. (j) V0.0; (d) V0.5; () V1.0; (N) V2.0; (h) M0.0; (s) M0.5; (}) M1.0; (4) M2.0. 18
18
10
Total Number of Particles
Total Number of Particles
10
17
10
0
0.2
0.4
0.6
0.8
17
10
1
0
0.2
0.4
Conversion
0.6
0.8
1
Conversion
Fig. 2. Batch cationic emulsion polymerization of styrene. Effect of the concentration of VBTMAC (left) and MATMAC (right) on the evolution of the total number of particles. (j) V0.0; (d) V0.5; () V1.0; (N) V2.0; (h) M0.0; (s) M0.5; (}) M1.0; (4) M2.0.
0.1
Rate of Polymerization (mol/min)
Rate of Polymerization (mol/min)
0.1 0.08 0.06 0.04 0.02
0.08 0.06 0.04 0.02 0
0 0
0.2
0.4
0.6
0.8
Conversion
1
0
0.2
0.4
0.6
0.8
1
Conversion
Fig. 3. Batch cationic emulsion polymerization of styrene. Effect of the concentration of VBTMAC (left) and MATMAC (right) on the evolution of the rate of polymerization. (j) V0.0; (d) V0.5; () V1.0; (N) V2.0; (h) M0.0; (s) M0.5; (}) M1.0; (4) M2.0.
different hydrophobicity. The two cationic monomers are water soluble, but VBTMAC shows a higher affinity for the organic phase than MATMAC, so its copolymerization with styrene is more favored than in the case of using MATMAC. With MATMAC, homopolymerization in the water phase is favored and only a small amount of MATMAC is anchored to the particle surface. Due to this, more particles can be nucleated using VBTMAC than MATMAC as cationic comonomer, together with a faster rate of polymerization. Similar
results were obtained by Ganachaud et al. [12] in the emulsifier-free emulsion polymerization of styrene with vinyl benzyl amine hydrochloride, which is similar to VBTMAC, and aminoethylmethacrylate hydrochloride, which is similar to MATMAC. Furthermore, the higher compatibility of VBTMAC with styrene compared to MATMAC makes the stability of the particles higher, and so, no coagulation processes among particles were observed in reaction V2.0, while in reactions carried out with the highest amount of
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J. Ramos, J. Forcada / European Polymer Journal 46 (2010) 1106–1110
MATMAC (M2.0) coagulation took place due to the high ionic strength in the reaction medium and the lower colloidal stability of the particles. 4. Conclusions The cationic emulsion polymerization of styrene is affected by the addition of small amounts of cationic monomers. Polymerizations using the more hydrophobic cationic comonomer (VBTMAC) showed higher conversions due to the in situ creation of an amphiphilic copolymer with styrene improving particle stability, and faster rates of polymerization were observed by increasing the cationic comonomer concentration. With the more hydrophilic cationic comonomer (MATMAC) the same behavior was observed up to 0.012 M of MATMAC concentration. At higher concentrations the most of MATMAC homopolymerizes in water phase, and therefore, the ionic strength controls the colloidal stability of the system occurring coagulation. Acknowledgements This work has been supported by the Spanish Programa Nacional de Materiales(MAT 2009-13155-C04-01). J. Ramos
acknowledges the financial support by the Ministerio de Ciencia e Innovación: Subprograma Juan de la Cierva (JCI2008-2217).
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