- Email: [email protected]
Preparation of polystyrene latex particles by miniemulsion polymerization using a predissolved fluorinated block copolymer as the sole co-stabilizer
Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 510–516
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Preparation of polystyrene latex particles by miniemulsion polymerization using a predissolved fluorinated block copolymer as the sole co-stabilizer Zhenqian Zhang ∗ , Xiang Ji, Pei Wang Department of Materials Science and Engineering, Changzhou University, Changzhou 213164, People’s Republic of China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• The fluorinated block copolymer is prepared by ATRP.
• The copolymers are used as sole co-stabilizer in the miniemulsion polymerization. • The fluorine content in copolymer has effect on the process of miniemulsion. • The final latex films show the better surface hydrophobicity. • The nucleation mechanism of miniemulsion polymerization is proposed.
a r t i c l e
i n f o
Article history: Received 20 July 2013 Received in revised form 10 September 2013 Accepted 3 October 2013 Available online 11 October 2013 Keywords: Miniemulsion polymerization Fluorinated block copolymer Atom transfer radical polymerization (ATRP) Co-stabilizer
a b s t r a c t The fluorinated block copolymer [P(St-b-DFM)] of styrene (St) and dodecafluoroheptyl methacrylate (DFM) was used as the sole co-stabilizer in St miniemulsion instead of the conventional co-stabilizers. The fluorinated block copolymer was prepared by atom transfer radical polymerization (ATRP). The fluorine content in P(St-b-DFM) was determined based on the relative number–average molecular weight of the macroinitiator and P(St-b-DFM). In St miniemulsion polymerization, the effects of the fluorine content in the block polymer on the size and number of the initial monomer droplets and latex particles were investigated. The ratio of the final number of latex particles to the initial number of monomer droplets was used to discuss the nucleation mechanism. The P(St-b-DFM) copolymers with 6% and 11% fluorine contents had a better effect on the stability of the monomer droplets, and monomer droplet nucleation almost dominated the process of St miniemulsion polymerization. The water contact angle on the surface of the polystyrene latex film demonstrated the surface hydrophobicity, and the surface tension was estimated according to Girifalco–Good–Fowkes–Young equation. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Miniemulsions are aqueous dispersions of relatively stable oil droplets having a droplet size in the range of 50–500 nm and consist of oil, water, a surfactant and a water-insoluble hydrophobe. The water-insoluble hydrophobe, called the co-stabilizer, is necessary
∗ Corresponding author. Tel.: +86 51986330096; fax: +86 51986330095. E-mail address: [email protected] (Z. Zhang). 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.10.019
to reduce the degradation of the monomer droplets from small droplets to large droplets (Ostwald ripening). Typical co-stabilizers normally have a low molecular weight, and examples include cetyl alcohol and hexadecane [1–3]. Recently, the use of polymers as co-stabilizers has been reported [1–6]. These water-insoluble hydrophobic polymers perform reasonably well and have the added advantage of being innocuous in the mixture [4,5]. The inclusion of a monomer–soluble polymer can slow the effects of Ostwald ripening. It has been found that polymer co-stabilizers can stabilize the droplets against diffusional
Z. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 510–516
511
O
degradation long enough to nucleate them into polymer particles [1–4]. Amalvy [6] applied the PVAc pre-polymer as a co-stabilizer in the miniemulsion polymerization of vinyl acetate and ultimately concluded that the particle size distribution of the coefficient d relations are as follows: d(HD+PVAc) > dHD > dPVAc . Jia [5] used liquid polybutadiene as the sole co-stabilizer in styrene miniemulsion polymerizations. Fluorinated polymers, which are regarded as high-value-added materials, are potential candidates due to their outstanding properties. The small size and high electronegativity of the fluorinated atom confer strong C F bond formation and a low polarizability. Such polymers show intermolecular interactions, which lead to low cohesive energy and therefore low surface energy. For the same reason, if the fluorinated block or graft copolymer is incorporated with other polymers, the surface of such a polymer blend will be enriched by the fluorinated segments to lower the surface energy, even if the concentration of the fluorinated block or graft copolymer in the blend system is low [7,8]. In the present study, the fluorinated block copolymer [P(Stb-DFM)] was prepared and used in place of the conventional co-stabilizer in St miniemulsion to improve the surface hydrophobicity of polystyrene (PS) latex film. P(St-b-DFM) was prepared by atom transfer radical polymerization (ATRP). Styrene (St) was the first monomer, and dodecafluoroheptyl methacrylate (DFM) was the second monomer. The influence of fluorine content in the P(Stb-DFM) on the size and number of the initial monomer droplets and latex particles was investigated. The form of the final latex particles was observed, and a mechanism of miniemulsion polymerization using a fluorinated block copolymer as the sole co-stabilizer was suggested. The effect of the fluorine content in the P(St-b-DFM) on the surface hydrophobicity of the PS latex film was discussed.
SDS (0.144 g) was dissolved in deionized water (90 mL) for the aqueous phase, and P(St-b-DFM) (0.30 g) was dissolved in St (10 g) for the oil phase. For pre-emulsification, the oil and aqueous solutions were mixed with a magnetic stirrer for 10 min. The resultant emulsion was then homogenized with a homogeneous dispersion machine (Misonix Sonicator 3000) for 15 min. The homogenized mix was immediately transferred into a 250-mL four-necked flask equipped with a mechanical stirrer, thermometer, nitrogen inlet and reflux condenser. The reactor was purged with nitrogen for 30 min before being placed into the thermostat at the reaction temperature. The initiator (KPS) (0.025 g) was then charged into the reactor to start the polymerization. The St miniemulsion polymerization temperature was kept constant at 70 ◦ C. The reaction was ended 180 min later after feeding KPS.
2. Experimental
2.4. Characterization
2.1. Materials
Fourier transform infrared (FTIR) analysis was used to obtain information on the polymer composition. FTIR spectra were recorded on an Avatar 370 Spectrum instrument. The powder sample was mixed with KBr and pressed. GPC analysis was carried out with a Waters 1515 instrument. Tetrahydrofuran was used as the eluent at a flow rate of 1.0 mL/min, and calibration was performed with PS standards. Data acquisition was performed with Waters software. The ratio of the relative mass–average molecular weight and relative number–average molecular weight (Mw /Mn ) was used to characterize the PDI. The droplet sizes (Z-average size, Dz , and volume–average size, Dv ) and distribution (intensity PSD) were measured by dynamic light scattering (DLS) with a Malvern Zetasizer Nano ZS instrument. Samples for the monomer droplet size measurements were prepared by diluting three drops of the homogenized mixture with approximately 25 g of St-saturated 0.16 wt% SDS solution. Approximately 2 mL of the diluted solution was placed in a quartz cuvette, and the droplet size was measured. The latex Z-average particle size and distribution (intensity PSD) were measured in a similar manner except that the dilution was performed with a 0.16 wt% SDS solution and a droplet of hydroquinone solution. The latex morphology was observed and imaged using transmission electron microscopy (TEM, JEM-1230 operated at 200 kV). The diluted sample solutions were applied to a 400-mesh carbon-coated copper grid and left to dry. From the TEM analyses, more than 1000 particles were counted for each sample. The average volume diameter Dv(TEM) was calculated using the following equation:
St was supplied by Shanghai Lingfeng Chemical Reagent Co. Ltd., distilled under reduced pressure and stored in a refrigerator until use. Potassium persulfate (KPS), sodium dodecyl sulfate (SDS) (Shanghai Lingfeng Chemical Reagent Co. Ltd., China) were of analytical grade and used as received. Chemical-grade DFM was supplied by the Harbin Xeogia Fluorine-Silicon Chemical Co. Ltd., China. Analytical-grade methylene iodide, tertrahydrofuran (THF), pentamethyldiethylenetriamine (PMDETA), ethyl-2-bromoisobutyrate (EBiB), CuBr, cyclohexanone and methanol were supplied by No. 2 Shanghai Reagent Co. Ltd., China. Deionized water was used in the experiments. 2.2. Preparation of the fluorinated block copolymer as a co-stabilizer A dry round-bottomed flask with a magnetic stir bar was charged with CuBr. The flask was sealed with a rubber septum, degassed and back-filled with nitrogen three times. Deoxygenated PMDETA, St and EBiB were introduced in turn using a syringe. The flask was then placed in an oil bath, heated at 110 ◦ C for 5 h and then cooled to room temperature. The homopolymer (PS-Br) was precipitated in methanol/H2 O (1:1, volume ratio) after passing through an alumina column to remove the copper complexes and dried under vacuum at 60 ◦ C for 24 h. The macroinitiator was successfully obtained. The reaction processes are shown in Fig. 1. Block copolymerization was performed in the same manner as homopolymerization except that a macroinitiator was used. PS-Br and CuBr were added to a dry round-bottomed flask with a magnetic stir bar and then degassed and charged with nitrogen three times. Deoxygenated DFM and PMDETA were introduced. Finally,
CuBr / PMDETA O
n
Br
cyclohexanone, 110 ºC
Br Fig. 1. Macroinitiator preparation (PS-Br).
the flask was placed in an oil bath and heated at 110 ◦ C for 5 h. The block copolymer P(St-b-DFM) was isolated by precipitation with methanol after passing through an alumina column to remove the copper complexes. It was then dried under vacuum at 60 ◦ C for 24 h. The reaction processes are shown in Fig. 2. 2.3. Miniemulsion polymerizations of styrene
1/3 ni Di 3 Dv(TEM) = ni
(1)
where ni is the number of polymer particles and Di is the diameter of the polymer particles.
512
Z. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 510–516
CH3
Br
n
C
CH2
C
O
+
(M2)
O
F 3C
F
F
H2C
C
C
C
CF3
H
F
CF3
CuBr/PMDETA M1M1M1M1
cyclohexanone, 110ºC
M1M1M2M2M2
M2M2M2
(M1:styrene unit;M2:DFM unit) Fig. 2. Preparation of the fluorinated block copolymer.
The conversion of the monomers during the polymerization was determined gravimetrically. The contact angle of water or methylene iodide on the surface of film was measured using a JC2000A contact angle measuring apparatus at 25 ◦ C. The latex films were prepared by spin-coating the diluted emulsions on newly cleaned glass plates, which were then dried in vacuum for 2 days at 50 ◦ C and for 2 h at 120 ◦ C, cooled to room temperature for measurement.
3. Results and discussion 3.1. FTIR of PS-Br and P(St-b-DFM) Fig. 3 shows the FTIR spectra of the macroinitiator and copolymers. In Fig. 3, the absorption bands at 3000 and 1600 cm−1 in the FTIR spectrum were characteristic of St. The band at 1400–1000 cm−1 in the FTIR spectrum of the copolymer was an important piece of evidence of C F bonds, and the band at 1700 cm−1 was attributed to the C O vibration of DFM, which proved that P(ST-b-DFM) was the copolymer of DFM and St.
P (St-b-DFM)
PS-Br
3.2. Mn and PDI of P(St-b-DFM) In the ATRP polymerization of DFM, PS-Br with different Mn and CuBr/PMDETA agent were used as the macroinitiator and catalytic systems, respectively. A P(St-b-DFM) copolymer with similar Mn could be achieved [9]. Mn and PDI (Mw /Mn ) of the PS-Br macroinitiator are shown in Table 1. The GPC results for the fluorine content in P(St-b-DFM) are plotted in Table 2. Tables 1 and 2 show that the PDI of P(St-b-DFM) was broader than that of PS-Br. In addition, a lower dispersion index (Mw /Mn < 1.5) was found. Using the relative number–average molecular weight of PS-Br and P(St-b-DFM), the fluorine content in P(St-b-DFM) was calculated (Table 2). P(St-b-DFM) with different fluorine contents were used as the sole co-stabilizer to stabilize the styrene miniemulsion. 3.3. Characterization of initial St droplets and final latex particles The initial St droplet size and distribution of miniemulsions co-stabilized by P(St-b-DFM) with different fluorine contents are shown in Fig. 4(a–d). The final latex particle size and distribution are given in Fig. 5(a–d). As shown in Figs. 4 and 5, the average volume diameter (Dv ) of the initial St droplets was measured by DLS, and the average volume diameter (Dv(TEM) ) of the final latex particles was measured by TEM. With the increasing fluorine content in P(St-b-DFM), the Dv of the initial St droplets increased from 94.0 nm to 112.8 nm, and the Dv(TEM) of the final latex particles increased from 84.7 nm to 93.5 nm. In the classical miniemulsion polymerization mechanism, an osmotic pressure in the monomer droplets counterbalances the Laplace pressure inside the original emulsion and retards the Ostwald ripening effect, which is characterized by diffusion of the monomer from small droplets to larger ones, thus providing a 1:1
Table 1 GPC characterization of the PS-Br macroinitiator.
3500
300 0
250 0
200 0
150 0
Wav enu mber / cm
100 0
500
-1
Fig. 3. FT-IR spectra of the macroinitiator and fluorinated block copolymer.
Sample
[EBiB]:[St]
PS-Br(1) PS-Br(2) PS-Br(3) PS-Br(4)
1:200 1:280 1:320 1:360
Mn 16,000 22,500 25,600 28,600
PDI 1.21 1.30 1.34 1.39
Z. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 510–516 25
B: F% 11
A: F% 6 20
Intensity / %
Intensity / %
20 15 10
15 10 5
5 0
40
60
80
100
120
0
140
40
60
80
100
120
140
Droplet size Dv / nm
Droplet size Dv / nm
(b)
(a)
D: F% 28
B: F% 11
20
15
20
Intensity / %
Intensity / %
513
10
5
15
10
5
0
40
60
80
100
120
0
140
40
60
80
100
120
140
Droplet size Dv / nm
Droplet size Dv / nm
(d)
(c)
Fig. 4. Size and distribution of initial St droplets in the system co-stabilized by P(St-b-DFM).
20 15 10 5 0
25
A: F% 6
Particle number / %
Particle number / %
25
40
60
80
100
B: F% 11
20 15 10 5 0
120
Particle size Dv(TEM) / nm
40
60
(a) Particle number / %
Particle number / %
25
C: F% 18
20 15 10 5
40
60
80
100
Particle size Dv(TEM) / nm
(c)
100
120
(b)
25
0
80
Particle size Dv(TEM) / nm
120
D: F% 28
20 15 10 5 0
40
60
80
100
Particle size Dv(TEM) / nm
(d)
Fig. 5. Size and distribution of final latex particles in the system co-stabilized by P(St-b-DFM).
120
514
Z. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 510–516
Table 2 GPC characterization of P(St-b-DFM). Sample
Macroinitiator
[PS-Br]:[DFM]
P(St-b-DFM)a P(St-b-DFM)b P(St-b-DFM)c P(St-b-DFM)d
PS-Br(4) PS-Br(3) PS-Br(2) PS-Br(1)
1:50 1:60 1:70 1:80
Mn 32,400 32,000 32,600 32,100
copying of droplets to particles [10–12]. Based on the data for the initial number of monomer droplets (Nmi ) and the final number of latex particles (Npf ), the nucleation mechanism of the latex particles can be used to discuss the miniemulsion system. Under fairly ideal reaction conditions, where the particle nucleation process depends primarily on free radicals in monomer droplet situations, Npf /Nmi fluctuates around 1 theoretically. Thus, monomer droplet nucleation is almost the dominant process. The parameters Nmi and Npf can be calculated according to the following equation: Npf Nmi
3
=
(xf m dm ) 3
[p dp ]
(2)
where the parameters m and p are the monomer density (0.90 g/cm3 , 25 ◦ C) and polymer density (1.05 g/cm3 , 25 ◦ C), respectively. xf is the final monomer conversion. dm is the Dv of the initial St droplet. dp is the Dv(TEM) of the PS final latex particles. The calculated miniemulsion polymerization data are listed in Table 3. The PSD of the initial St droplets and final latex particles indicated that the dispersion was minimal. The Npf /Nmi of the final latex co-stabilized by P(St-b-DFM) with 6% and 11% fluorine content showed better adherence to the 1:1 copying of droplets to
PDI
nSt /nDFM
F (wt%)
1.45 1.49 1.39 1.45
1/0.03 1/0.06 1/0.12 1/0.26
6 11 18 28
particles. The TEM photographs of the final latex particles are shown in Fig. 6.
3.4. Evolution of the size and number of latex particles in the St miniemulsion polymerization The St conversions of the miniemulsion polymerization costabilized by P(St-b-DFM) with different fluorine contents (6%, 11%, 18%, 28%) are given in Fig. 7. Fig. 7 shows that fluorine content in P(St-b-DFM) had an effect on the polymerization conversion and rate. The final polymerization conversion was approximately 94%. Table 3 shows that the Dv of the initial St droplets in the homogenized mixtures were 94.0 nm, 97.2 nm, 105.7 nm and 112.8 nm for P(St-b-DFM) fluorine contents of 6%, 11%, 18% and 28%, respectively. In the above experiment, when the Dv of the initial St droplets decreased, the number of droplets increased. As a result, the number of droplets with radicals would increase accordingly. More particles in a unit volume should correspond to a higher polymerization rate. During the course of the St miniemulsion reaction, samples were analyzed with regard to particle size and number. The evolution of the size and number of latex particles (Np ) during polymerization is shown in Fig. 8. Np can be calculated according to the following
Fig. 6. TEM photographs of the final latex particles (co-stabilized by P(St-b-DFM) (a) F% 6; (b) F% 11; (c) F% 18; (d) F% 28).
Z. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 510–516
515
Table 3 Size and distribution of initial St droplets and final latex particles. Final particle size (nm) and distribution
Dv
Dz
PSD
Dv(TEM)
Dz
PSD
94.0 97.2 105.7 112.8
94.2 97.5 106.3 113.2
0.05 0.05 0.06 0.07
82.3 83.2 87.8 90.6
84.7 86.5 90.6 93.5
0.04 0.05 0.05 0.06
100
A
Conversion / %
80
B C
60
D
40
A:F% 6 B:F% 11 C:F% 18 D:F% 28
20 0 0
30
60
90
120
150
180
Time / min Fig. 7. St conversion of the miniemulsion polymerization.
equation:(3)Np =
(6Mm xf ) p Dv(TEM) 3
where Mm is the initial monomer-to-
water ratio in the reaction system. The parameter p is the polymer density (1.05 g/cm3 , 25 ◦ C). Dv(TEM) is the average volume diameter of the latex particles calculated by TEM.
3.2
60
3.0
50 A: F% 6
40 0
20
40
60
Conversion / %
80
Paticle size Dv(TEM) / nm
70
2.8 100
80
3.4
70
3.2 3.0
60
2.8 50
2.6
40
B: F% 11
0
20
40
2.8
70
2.6
60
2.4
50
2.2
40
C: F% 18 60
Conversion / %
80
2.0 100
Paticle size Dv(TEM) / nm
80
Np / 1014ml-1H2O
Paticle size Dv(TEM) / nm
3.0
(c)
1.21 1.29 1.40 1.55
80
2.4 100
(b)
90
40
60
Conversion / %
(a)
20
94.7 94.5 93.7 93.9
3.6
Np / 1014ml-1H2O
Paticle size Dv(TEM) / nm
3.4
Npf /Nmi
In St miniemulsion polymerization co-stabilized by P(St-b-DFM) with a 6% fluorine content, when the monomer conversion was 9.8%, the Dv(TEM) and Np of the latex particles were 42 nm and 2.89 × 1014 mL−1 H2 O, respectively. Dv(TEM) and Np reached 63.8 nm and 3.41 × 1014 mL−1 H2 O, respectively, at the 43.7% St conversion. In the later part of the polymerization, Dv(TEM) increased from 63.8 nm to 82.3 nm, while Np only changed slightly. The same trend lines were exhibited in Fig. 8(b–d), which was co-stabilized by P(Stb-DFM) with 11%, 18% and 28% fluorine content. The miniemulsion polymerization should afford a 1:1 copying of the droplets to particles in theory, while Table 3 shows that the Npf /Nmi was 1.55 in the St miniemulsion polymerization co-stabilized by P(St-b-DFM) with 28% fluorine content. Another form of nucleation may have played a role, in addition to monomer droplet nucleation, such as micellar nucleation or homogeneous nucleation. Under the conditions of the above experiment, homogeneous monomer nucleation probably occurred in the St miniemulsion system because of the critical micelle concentration of SDS (0.0081 mol/L). Below this concentration (in the above experiment, the SDS concentration was 0.0057 mol/L), micellar nucleation did not occur. In homogeneous nucleation, the water-soluble initiator decomposes, forming free radicals, which propagate in the aqueous phase until they become surface active and propagate further in the aqueous phase, precipitating to form particles [13]. The P(St-b-DFM) with 6% and 11%
3.6 80
xf (%)
Np / 1014ml-1H2O
F 6% F 11% F 18% F 28%
Initial droplet size (nm) and distribution
90
2.6
80
2.4
70
2.2
60
2.0
50
1.8
40
D: F% 28
0
20
40
60
80
Np / 1014ml-1H2O
Sample P(St-b-DFM)
1.6 100
Conversion / %
(d)
Fig. 8. Evolution of latex particle size and number during St miniemulsion polymerization (co-stabilized by P(St-b-DFM) (a) F% 6; (b) F% 11; (c) F% 18; (d) F% 28).
516
Z. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 510–516 50
45
40
90
35 80 30
70
5
10
15
20
25
P(St-b-DFM) were synthesized by atom transfer radical polymerization and used as the sole co-stabilizer in St miniemulsion polymerization. The fluorine contents in P(St-b-DFM) were calculated by the relative number–average molecular weight of the macroinitiator and block polymer. When the fluorine content in P(St-b-DFM) increased from 6% to 28%, the scattered monomer droplet size increased from 94.0 nm to 112.8 nm, and the size of final latex particles increased from 82.3 nm to 90.6 nm. In the process of St miniemulsion polymerization co-stabilized by P(St-b-DFM) with 6% fluorine content, the size of latex particles increased, while the number of latex particles changed only slightly after 43.7% St conversion. The P(St-b-DFM) with 6% and 11% fluorine contents in the St polymerization system had a better effect on the stability of the monomer droplets. The surface of the PS latex film with predissolved fluorinated block copolymer was hydrophobic.
Surface tension / mNm-1
Water contact angle / degree
100
4. Conclusions
25
F% in P(St-b-DFM) Fig. 9. Water contact angle and surface energy of the final latex film.
fluorine content in the St polymerization system exhibited a better effect on the stability of the monomer droplets, and the monomers diffused minimally into the aqueous phase, leading to smaller polymer latex particles. Monomer droplet nucleation almost dominated the process. 3.5. Water contact angle and surface tension of the PS final latex film Fig. 9 shows the water contact angle () on the surface of the final latex film. The surface energy ( S ) of the final latex film can be estimated from the Girifalco–Good–Fowkes–Young equation [14]: D (L1 )(1 + cos 1 ) = 2(SD L1 )
0.5
P + 2(SP L1 )
0.5
D (L2 )(1 + cos 2 ) = 2(SD L2 )
0.5
P + 2(SP L2 )
0.5
(4) (5)
where 1 is the water contact angle and 1 is the methylene iodide contact angle. S = SD + SP , where SD and SP are the dispersion force component and the polar force component of the solid surD + P , D (21.8 mN/m) and P face energy, respectively. L1 = L1 L1 L1 L1 (51.0 mN/m) are the dispersion force component and the polar force component of the surface tension for the water, respectively. D + P , D (49.5 mN/m) and P (1.3 mN/m) are the disperL2 = L2 L2 L2 L2 sion force component and the polar force component of the surface tension for the methylene iodide, respectively. With increasing F% in P(St-b-DFM), the film surface exhibited increased hydrophobicity. The water contact angle increased to 101◦ and the surface tension was 29 mN/m on the surface of this final PS latex film, which was approximately 3 wt% fluorinated block copolymer with 28% fluorine content. The surface of the film is enriched by the fluorinated segments, lowering its surface energy, despite the low concentration of the fluorinated block copolymer in the final latex film.
References [1] C.S. Chern, Emulsion Polymerization mechanisms and kinetics, Prog. Polym. Sci. 31 (2006) 443–486. [2] J.M. Asua, Miniemulsion polymerization, Prog. Polym. Sci. 27 (2002) 1283–1346. [3] P.J. Blythe, B.R. Morrison, K.A. Mathauer, E.D. Sudol, M.S. El-Aasser, Enhanced droplet nucleation in styrene miniemulsion polymerization: 1. Effect of polymer type in sodium lauryl sulfate/cetyl alcohol miniemulsions, Macromolecules 32 (1999) 6944–6951. [4] F.J. Schork, G.W. Poehlein, S. Wang, J. Reimers, J. Rodrigues, C. Samer, Miniemulsion polymerization, Colloids Surf. Phys. Chem. Eng. Aspects 166 (2000) 59–66. [5] G.W. Jia, N. Cai, Y.S. Xu, L. Chao, X.M. Tan, Miniemulsion polymerization of styrene with liquid polybutadiene as the sole co-stabilizer, Eur. Polym. J. 43 (2007) 4453–4459. [6] A. Imanol, J.I. Amalvy, M.J. Barandiaran, Influence of the polymeric hydrophobe on the kinetics of vinyl acetate miniemulsion polymerization, Colloids Surf. Phys. Chem. Eng. Aspects 153 (1999) 39–45. [7] V.C. Malshe, S. Elango, S.S. Bhagwat, S.S. Maghrabi, Fluorinated acrylic copolymers: part II. Polymeric surfactants, Prog. Org. Coat. 53 (2005) 212–216. [8] R. Souzy, B. Ameduri, Functional fluoro-copolymers for fuel cell membranes, Prog. Polym. Sci. 30 (2005) 644–687. [9] Z.C. Zhang, E. Chalkova, M. Fedkin, C.M. Wang, S.N. Lvov, S. Komarneni, T.C. Mike Chung, Synthesis and characterization of poly (vinylidene fluoride)g-sulfonated polystyrene graft copolymers for proton exchange membrane, Macromolecules 41 (2008) 9130–9139. [10] I. Capek, C.S. Chern, Radical polymerization in direct miniemulsion systems, Adv. Polym. Sci. 155 (2001) 101–165. [11] K. Landfester, N. Bechthold, F. Tiarks, M. Antonietti, Formulation and stability mechanisms of polymerizable miniemulsions, Macromolecules 32 (1999) 5222–5228. [12] C.M. Miller, E.D. Sudol, C.A. Silebi, M.S. El-Aasser, Miniemulsion polymerization of styrene: Evolution of the particle size distribution, J. Polym. Sci. Polym. Chem. Ed. 33 (1995) 1391–1408. [13] D.H. Li, E. David Sudol, M.S. El-Aasser, Miniemulsion and conventional emulsion copolymerization of styrene and butadiene: a comparative kinetic study, J. Appl. Polym. Sci. 101 (2005) 2304–2312. [14] F.Z. Hu, G.R. Chen, Y.J. Du, Material Surface and Interface, 2nd ed., East China University of Science and Technology Press, Shanghai, 2007, pp. 109–112 (chapter 6).
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Preparation of polystyrene latex particles by miniemulsion polymerization using a predissolved fluorinated block copolymer as the sole co-stabilizer Zhenqian Zhang ∗ , Xiang Ji, Pei Wang Department of Materials Science and Engineering, Changzhou University, Changzhou 213164, People’s Republic of China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• The fluorinated block copolymer is prepared by ATRP.
• The copolymers are used as sole co-stabilizer in the miniemulsion polymerization. • The fluorine content in copolymer has effect on the process of miniemulsion. • The final latex films show the better surface hydrophobicity. • The nucleation mechanism of miniemulsion polymerization is proposed.
a r t i c l e
i n f o
Article history: Received 20 July 2013 Received in revised form 10 September 2013 Accepted 3 October 2013 Available online 11 October 2013 Keywords: Miniemulsion polymerization Fluorinated block copolymer Atom transfer radical polymerization (ATRP) Co-stabilizer
a b s t r a c t The fluorinated block copolymer [P(St-b-DFM)] of styrene (St) and dodecafluoroheptyl methacrylate (DFM) was used as the sole co-stabilizer in St miniemulsion instead of the conventional co-stabilizers. The fluorinated block copolymer was prepared by atom transfer radical polymerization (ATRP). The fluorine content in P(St-b-DFM) was determined based on the relative number–average molecular weight of the macroinitiator and P(St-b-DFM). In St miniemulsion polymerization, the effects of the fluorine content in the block polymer on the size and number of the initial monomer droplets and latex particles were investigated. The ratio of the final number of latex particles to the initial number of monomer droplets was used to discuss the nucleation mechanism. The P(St-b-DFM) copolymers with 6% and 11% fluorine contents had a better effect on the stability of the monomer droplets, and monomer droplet nucleation almost dominated the process of St miniemulsion polymerization. The water contact angle on the surface of the polystyrene latex film demonstrated the surface hydrophobicity, and the surface tension was estimated according to Girifalco–Good–Fowkes–Young equation. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Miniemulsions are aqueous dispersions of relatively stable oil droplets having a droplet size in the range of 50–500 nm and consist of oil, water, a surfactant and a water-insoluble hydrophobe. The water-insoluble hydrophobe, called the co-stabilizer, is necessary
∗ Corresponding author. Tel.: +86 51986330096; fax: +86 51986330095. E-mail address: [email protected] (Z. Zhang). 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.10.019
to reduce the degradation of the monomer droplets from small droplets to large droplets (Ostwald ripening). Typical co-stabilizers normally have a low molecular weight, and examples include cetyl alcohol and hexadecane [1–3]. Recently, the use of polymers as co-stabilizers has been reported [1–6]. These water-insoluble hydrophobic polymers perform reasonably well and have the added advantage of being innocuous in the mixture [4,5]. The inclusion of a monomer–soluble polymer can slow the effects of Ostwald ripening. It has been found that polymer co-stabilizers can stabilize the droplets against diffusional
Z. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 510–516
511
O
degradation long enough to nucleate them into polymer particles [1–4]. Amalvy [6] applied the PVAc pre-polymer as a co-stabilizer in the miniemulsion polymerization of vinyl acetate and ultimately concluded that the particle size distribution of the coefficient d relations are as follows: d(HD+PVAc) > dHD > dPVAc . Jia [5] used liquid polybutadiene as the sole co-stabilizer in styrene miniemulsion polymerizations. Fluorinated polymers, which are regarded as high-value-added materials, are potential candidates due to their outstanding properties. The small size and high electronegativity of the fluorinated atom confer strong C F bond formation and a low polarizability. Such polymers show intermolecular interactions, which lead to low cohesive energy and therefore low surface energy. For the same reason, if the fluorinated block or graft copolymer is incorporated with other polymers, the surface of such a polymer blend will be enriched by the fluorinated segments to lower the surface energy, even if the concentration of the fluorinated block or graft copolymer in the blend system is low [7,8]. In the present study, the fluorinated block copolymer [P(Stb-DFM)] was prepared and used in place of the conventional co-stabilizer in St miniemulsion to improve the surface hydrophobicity of polystyrene (PS) latex film. P(St-b-DFM) was prepared by atom transfer radical polymerization (ATRP). Styrene (St) was the first monomer, and dodecafluoroheptyl methacrylate (DFM) was the second monomer. The influence of fluorine content in the P(Stb-DFM) on the size and number of the initial monomer droplets and latex particles was investigated. The form of the final latex particles was observed, and a mechanism of miniemulsion polymerization using a fluorinated block copolymer as the sole co-stabilizer was suggested. The effect of the fluorine content in the P(St-b-DFM) on the surface hydrophobicity of the PS latex film was discussed.
SDS (0.144 g) was dissolved in deionized water (90 mL) for the aqueous phase, and P(St-b-DFM) (0.30 g) was dissolved in St (10 g) for the oil phase. For pre-emulsification, the oil and aqueous solutions were mixed with a magnetic stirrer for 10 min. The resultant emulsion was then homogenized with a homogeneous dispersion machine (Misonix Sonicator 3000) for 15 min. The homogenized mix was immediately transferred into a 250-mL four-necked flask equipped with a mechanical stirrer, thermometer, nitrogen inlet and reflux condenser. The reactor was purged with nitrogen for 30 min before being placed into the thermostat at the reaction temperature. The initiator (KPS) (0.025 g) was then charged into the reactor to start the polymerization. The St miniemulsion polymerization temperature was kept constant at 70 ◦ C. The reaction was ended 180 min later after feeding KPS.
2. Experimental
2.4. Characterization
2.1. Materials
Fourier transform infrared (FTIR) analysis was used to obtain information on the polymer composition. FTIR spectra were recorded on an Avatar 370 Spectrum instrument. The powder sample was mixed with KBr and pressed. GPC analysis was carried out with a Waters 1515 instrument. Tetrahydrofuran was used as the eluent at a flow rate of 1.0 mL/min, and calibration was performed with PS standards. Data acquisition was performed with Waters software. The ratio of the relative mass–average molecular weight and relative number–average molecular weight (Mw /Mn ) was used to characterize the PDI. The droplet sizes (Z-average size, Dz , and volume–average size, Dv ) and distribution (intensity PSD) were measured by dynamic light scattering (DLS) with a Malvern Zetasizer Nano ZS instrument. Samples for the monomer droplet size measurements were prepared by diluting three drops of the homogenized mixture with approximately 25 g of St-saturated 0.16 wt% SDS solution. Approximately 2 mL of the diluted solution was placed in a quartz cuvette, and the droplet size was measured. The latex Z-average particle size and distribution (intensity PSD) were measured in a similar manner except that the dilution was performed with a 0.16 wt% SDS solution and a droplet of hydroquinone solution. The latex morphology was observed and imaged using transmission electron microscopy (TEM, JEM-1230 operated at 200 kV). The diluted sample solutions were applied to a 400-mesh carbon-coated copper grid and left to dry. From the TEM analyses, more than 1000 particles were counted for each sample. The average volume diameter Dv(TEM) was calculated using the following equation:
St was supplied by Shanghai Lingfeng Chemical Reagent Co. Ltd., distilled under reduced pressure and stored in a refrigerator until use. Potassium persulfate (KPS), sodium dodecyl sulfate (SDS) (Shanghai Lingfeng Chemical Reagent Co. Ltd., China) were of analytical grade and used as received. Chemical-grade DFM was supplied by the Harbin Xeogia Fluorine-Silicon Chemical Co. Ltd., China. Analytical-grade methylene iodide, tertrahydrofuran (THF), pentamethyldiethylenetriamine (PMDETA), ethyl-2-bromoisobutyrate (EBiB), CuBr, cyclohexanone and methanol were supplied by No. 2 Shanghai Reagent Co. Ltd., China. Deionized water was used in the experiments. 2.2. Preparation of the fluorinated block copolymer as a co-stabilizer A dry round-bottomed flask with a magnetic stir bar was charged with CuBr. The flask was sealed with a rubber septum, degassed and back-filled with nitrogen three times. Deoxygenated PMDETA, St and EBiB were introduced in turn using a syringe. The flask was then placed in an oil bath, heated at 110 ◦ C for 5 h and then cooled to room temperature. The homopolymer (PS-Br) was precipitated in methanol/H2 O (1:1, volume ratio) after passing through an alumina column to remove the copper complexes and dried under vacuum at 60 ◦ C for 24 h. The macroinitiator was successfully obtained. The reaction processes are shown in Fig. 1. Block copolymerization was performed in the same manner as homopolymerization except that a macroinitiator was used. PS-Br and CuBr were added to a dry round-bottomed flask with a magnetic stir bar and then degassed and charged with nitrogen three times. Deoxygenated DFM and PMDETA were introduced. Finally,
CuBr / PMDETA O
n
Br
cyclohexanone, 110 ºC
Br Fig. 1. Macroinitiator preparation (PS-Br).
the flask was placed in an oil bath and heated at 110 ◦ C for 5 h. The block copolymer P(St-b-DFM) was isolated by precipitation with methanol after passing through an alumina column to remove the copper complexes. It was then dried under vacuum at 60 ◦ C for 24 h. The reaction processes are shown in Fig. 2. 2.3. Miniemulsion polymerizations of styrene
1/3 ni Di 3 Dv(TEM) = ni
(1)
where ni is the number of polymer particles and Di is the diameter of the polymer particles.
512
Z. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 510–516
CH3
Br
n
C
CH2
C
O
+
(M2)
O
F 3C
F
F
H2C
C
C
C
CF3
H
F
CF3
CuBr/PMDETA M1M1M1M1
cyclohexanone, 110ºC
M1M1M2M2M2
M2M2M2
(M1:styrene unit;M2:DFM unit) Fig. 2. Preparation of the fluorinated block copolymer.
The conversion of the monomers during the polymerization was determined gravimetrically. The contact angle of water or methylene iodide on the surface of film was measured using a JC2000A contact angle measuring apparatus at 25 ◦ C. The latex films were prepared by spin-coating the diluted emulsions on newly cleaned glass plates, which were then dried in vacuum for 2 days at 50 ◦ C and for 2 h at 120 ◦ C, cooled to room temperature for measurement.
3. Results and discussion 3.1. FTIR of PS-Br and P(St-b-DFM) Fig. 3 shows the FTIR spectra of the macroinitiator and copolymers. In Fig. 3, the absorption bands at 3000 and 1600 cm−1 in the FTIR spectrum were characteristic of St. The band at 1400–1000 cm−1 in the FTIR spectrum of the copolymer was an important piece of evidence of C F bonds, and the band at 1700 cm−1 was attributed to the C O vibration of DFM, which proved that P(ST-b-DFM) was the copolymer of DFM and St.
P (St-b-DFM)
PS-Br
3.2. Mn and PDI of P(St-b-DFM) In the ATRP polymerization of DFM, PS-Br with different Mn and CuBr/PMDETA agent were used as the macroinitiator and catalytic systems, respectively. A P(St-b-DFM) copolymer with similar Mn could be achieved [9]. Mn and PDI (Mw /Mn ) of the PS-Br macroinitiator are shown in Table 1. The GPC results for the fluorine content in P(St-b-DFM) are plotted in Table 2. Tables 1 and 2 show that the PDI of P(St-b-DFM) was broader than that of PS-Br. In addition, a lower dispersion index (Mw /Mn < 1.5) was found. Using the relative number–average molecular weight of PS-Br and P(St-b-DFM), the fluorine content in P(St-b-DFM) was calculated (Table 2). P(St-b-DFM) with different fluorine contents were used as the sole co-stabilizer to stabilize the styrene miniemulsion. 3.3. Characterization of initial St droplets and final latex particles The initial St droplet size and distribution of miniemulsions co-stabilized by P(St-b-DFM) with different fluorine contents are shown in Fig. 4(a–d). The final latex particle size and distribution are given in Fig. 5(a–d). As shown in Figs. 4 and 5, the average volume diameter (Dv ) of the initial St droplets was measured by DLS, and the average volume diameter (Dv(TEM) ) of the final latex particles was measured by TEM. With the increasing fluorine content in P(St-b-DFM), the Dv of the initial St droplets increased from 94.0 nm to 112.8 nm, and the Dv(TEM) of the final latex particles increased from 84.7 nm to 93.5 nm. In the classical miniemulsion polymerization mechanism, an osmotic pressure in the monomer droplets counterbalances the Laplace pressure inside the original emulsion and retards the Ostwald ripening effect, which is characterized by diffusion of the monomer from small droplets to larger ones, thus providing a 1:1
Table 1 GPC characterization of the PS-Br macroinitiator.
3500
300 0
250 0
200 0
150 0
Wav enu mber / cm
100 0
500
-1
Fig. 3. FT-IR spectra of the macroinitiator and fluorinated block copolymer.
Sample
[EBiB]:[St]
PS-Br(1) PS-Br(2) PS-Br(3) PS-Br(4)
1:200 1:280 1:320 1:360
Mn 16,000 22,500 25,600 28,600
PDI 1.21 1.30 1.34 1.39
Z. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 510–516 25
B: F% 11
A: F% 6 20
Intensity / %
Intensity / %
20 15 10
15 10 5
5 0
40
60
80
100
120
0
140
40
60
80
100
120
140
Droplet size Dv / nm
Droplet size Dv / nm
(b)
(a)
D: F% 28
B: F% 11
20
15
20
Intensity / %
Intensity / %
513
10
5
15
10
5
0
40
60
80
100
120
0
140
40
60
80
100
120
140
Droplet size Dv / nm
Droplet size Dv / nm
(d)
(c)
Fig. 4. Size and distribution of initial St droplets in the system co-stabilized by P(St-b-DFM).
20 15 10 5 0
25
A: F% 6
Particle number / %
Particle number / %
25
40
60
80
100
B: F% 11
20 15 10 5 0
120
Particle size Dv(TEM) / nm
40
60
(a) Particle number / %
Particle number / %
25
C: F% 18
20 15 10 5
40
60
80
100
Particle size Dv(TEM) / nm
(c)
100
120
(b)
25
0
80
Particle size Dv(TEM) / nm
120
D: F% 28
20 15 10 5 0
40
60
80
100
Particle size Dv(TEM) / nm
(d)
Fig. 5. Size and distribution of final latex particles in the system co-stabilized by P(St-b-DFM).
120
514
Z. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 510–516
Table 2 GPC characterization of P(St-b-DFM). Sample
Macroinitiator
[PS-Br]:[DFM]
P(St-b-DFM)a P(St-b-DFM)b P(St-b-DFM)c P(St-b-DFM)d
PS-Br(4) PS-Br(3) PS-Br(2) PS-Br(1)
1:50 1:60 1:70 1:80
Mn 32,400 32,000 32,600 32,100
copying of droplets to particles [10–12]. Based on the data for the initial number of monomer droplets (Nmi ) and the final number of latex particles (Npf ), the nucleation mechanism of the latex particles can be used to discuss the miniemulsion system. Under fairly ideal reaction conditions, where the particle nucleation process depends primarily on free radicals in monomer droplet situations, Npf /Nmi fluctuates around 1 theoretically. Thus, monomer droplet nucleation is almost the dominant process. The parameters Nmi and Npf can be calculated according to the following equation: Npf Nmi
3
=
(xf m dm ) 3
[p dp ]
(2)
where the parameters m and p are the monomer density (0.90 g/cm3 , 25 ◦ C) and polymer density (1.05 g/cm3 , 25 ◦ C), respectively. xf is the final monomer conversion. dm is the Dv of the initial St droplet. dp is the Dv(TEM) of the PS final latex particles. The calculated miniemulsion polymerization data are listed in Table 3. The PSD of the initial St droplets and final latex particles indicated that the dispersion was minimal. The Npf /Nmi of the final latex co-stabilized by P(St-b-DFM) with 6% and 11% fluorine content showed better adherence to the 1:1 copying of droplets to
PDI
nSt /nDFM
F (wt%)
1.45 1.49 1.39 1.45
1/0.03 1/0.06 1/0.12 1/0.26
6 11 18 28
particles. The TEM photographs of the final latex particles are shown in Fig. 6.
3.4. Evolution of the size and number of latex particles in the St miniemulsion polymerization The St conversions of the miniemulsion polymerization costabilized by P(St-b-DFM) with different fluorine contents (6%, 11%, 18%, 28%) are given in Fig. 7. Fig. 7 shows that fluorine content in P(St-b-DFM) had an effect on the polymerization conversion and rate. The final polymerization conversion was approximately 94%. Table 3 shows that the Dv of the initial St droplets in the homogenized mixtures were 94.0 nm, 97.2 nm, 105.7 nm and 112.8 nm for P(St-b-DFM) fluorine contents of 6%, 11%, 18% and 28%, respectively. In the above experiment, when the Dv of the initial St droplets decreased, the number of droplets increased. As a result, the number of droplets with radicals would increase accordingly. More particles in a unit volume should correspond to a higher polymerization rate. During the course of the St miniemulsion reaction, samples were analyzed with regard to particle size and number. The evolution of the size and number of latex particles (Np ) during polymerization is shown in Fig. 8. Np can be calculated according to the following
Fig. 6. TEM photographs of the final latex particles (co-stabilized by P(St-b-DFM) (a) F% 6; (b) F% 11; (c) F% 18; (d) F% 28).
Z. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 510–516
515
Table 3 Size and distribution of initial St droplets and final latex particles. Final particle size (nm) and distribution
Dv
Dz
PSD
Dv(TEM)
Dz
PSD
94.0 97.2 105.7 112.8
94.2 97.5 106.3 113.2
0.05 0.05 0.06 0.07
82.3 83.2 87.8 90.6
84.7 86.5 90.6 93.5
0.04 0.05 0.05 0.06
100
A
Conversion / %
80
B C
60
D
40
A:F% 6 B:F% 11 C:F% 18 D:F% 28
20 0 0
30
60
90
120
150
180
Time / min Fig. 7. St conversion of the miniemulsion polymerization.
equation:(3)Np =
(6Mm xf ) p Dv(TEM) 3
where Mm is the initial monomer-to-
water ratio in the reaction system. The parameter p is the polymer density (1.05 g/cm3 , 25 ◦ C). Dv(TEM) is the average volume diameter of the latex particles calculated by TEM.
3.2
60
3.0
50 A: F% 6
40 0
20
40
60
Conversion / %
80
Paticle size Dv(TEM) / nm
70
2.8 100
80
3.4
70
3.2 3.0
60
2.8 50
2.6
40
B: F% 11
0
20
40
2.8
70
2.6
60
2.4
50
2.2
40
C: F% 18 60
Conversion / %
80
2.0 100
Paticle size Dv(TEM) / nm
80
Np / 1014ml-1H2O
Paticle size Dv(TEM) / nm
3.0
(c)
1.21 1.29 1.40 1.55
80
2.4 100
(b)
90
40
60
Conversion / %
(a)
20
94.7 94.5 93.7 93.9
3.6
Np / 1014ml-1H2O
Paticle size Dv(TEM) / nm
3.4
Npf /Nmi
In St miniemulsion polymerization co-stabilized by P(St-b-DFM) with a 6% fluorine content, when the monomer conversion was 9.8%, the Dv(TEM) and Np of the latex particles were 42 nm and 2.89 × 1014 mL−1 H2 O, respectively. Dv(TEM) and Np reached 63.8 nm and 3.41 × 1014 mL−1 H2 O, respectively, at the 43.7% St conversion. In the later part of the polymerization, Dv(TEM) increased from 63.8 nm to 82.3 nm, while Np only changed slightly. The same trend lines were exhibited in Fig. 8(b–d), which was co-stabilized by P(Stb-DFM) with 11%, 18% and 28% fluorine content. The miniemulsion polymerization should afford a 1:1 copying of the droplets to particles in theory, while Table 3 shows that the Npf /Nmi was 1.55 in the St miniemulsion polymerization co-stabilized by P(St-b-DFM) with 28% fluorine content. Another form of nucleation may have played a role, in addition to monomer droplet nucleation, such as micellar nucleation or homogeneous nucleation. Under the conditions of the above experiment, homogeneous monomer nucleation probably occurred in the St miniemulsion system because of the critical micelle concentration of SDS (0.0081 mol/L). Below this concentration (in the above experiment, the SDS concentration was 0.0057 mol/L), micellar nucleation did not occur. In homogeneous nucleation, the water-soluble initiator decomposes, forming free radicals, which propagate in the aqueous phase until they become surface active and propagate further in the aqueous phase, precipitating to form particles [13]. The P(St-b-DFM) with 6% and 11%
3.6 80
xf (%)
Np / 1014ml-1H2O
F 6% F 11% F 18% F 28%
Initial droplet size (nm) and distribution
90
2.6
80
2.4
70
2.2
60
2.0
50
1.8
40
D: F% 28
0
20
40
60
80
Np / 1014ml-1H2O
Sample P(St-b-DFM)
1.6 100
Conversion / %
(d)
Fig. 8. Evolution of latex particle size and number during St miniemulsion polymerization (co-stabilized by P(St-b-DFM) (a) F% 6; (b) F% 11; (c) F% 18; (d) F% 28).
516
Z. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 510–516 50
45
40
90
35 80 30
70
5
10
15
20
25
P(St-b-DFM) were synthesized by atom transfer radical polymerization and used as the sole co-stabilizer in St miniemulsion polymerization. The fluorine contents in P(St-b-DFM) were calculated by the relative number–average molecular weight of the macroinitiator and block polymer. When the fluorine content in P(St-b-DFM) increased from 6% to 28%, the scattered monomer droplet size increased from 94.0 nm to 112.8 nm, and the size of final latex particles increased from 82.3 nm to 90.6 nm. In the process of St miniemulsion polymerization co-stabilized by P(St-b-DFM) with 6% fluorine content, the size of latex particles increased, while the number of latex particles changed only slightly after 43.7% St conversion. The P(St-b-DFM) with 6% and 11% fluorine contents in the St polymerization system had a better effect on the stability of the monomer droplets. The surface of the PS latex film with predissolved fluorinated block copolymer was hydrophobic.
Surface tension / mNm-1
Water contact angle / degree
100
4. Conclusions
25
F% in P(St-b-DFM) Fig. 9. Water contact angle and surface energy of the final latex film.
fluorine content in the St polymerization system exhibited a better effect on the stability of the monomer droplets, and the monomers diffused minimally into the aqueous phase, leading to smaller polymer latex particles. Monomer droplet nucleation almost dominated the process. 3.5. Water contact angle and surface tension of the PS final latex film Fig. 9 shows the water contact angle () on the surface of the final latex film. The surface energy ( S ) of the final latex film can be estimated from the Girifalco–Good–Fowkes–Young equation [14]: D (L1 )(1 + cos 1 ) = 2(SD L1 )
0.5
P + 2(SP L1 )
0.5
D (L2 )(1 + cos 2 ) = 2(SD L2 )
0.5
P + 2(SP L2 )
0.5
(4) (5)
where 1 is the water contact angle and 1 is the methylene iodide contact angle. S = SD + SP , where SD and SP are the dispersion force component and the polar force component of the solid surD + P , D (21.8 mN/m) and P face energy, respectively. L1 = L1 L1 L1 L1 (51.0 mN/m) are the dispersion force component and the polar force component of the surface tension for the water, respectively. D + P , D (49.5 mN/m) and P (1.3 mN/m) are the disperL2 = L2 L2 L2 L2 sion force component and the polar force component of the surface tension for the methylene iodide, respectively. With increasing F% in P(St-b-DFM), the film surface exhibited increased hydrophobicity. The water contact angle increased to 101◦ and the surface tension was 29 mN/m on the surface of this final PS latex film, which was approximately 3 wt% fluorinated block copolymer with 28% fluorine content. The surface of the film is enriched by the fluorinated segments, lowering its surface energy, despite the low concentration of the fluorinated block copolymer in the final latex film.
References [1] C.S. Chern, Emulsion Polymerization mechanisms and kinetics, Prog. Polym. Sci. 31 (2006) 443–486. [2] J.M. Asua, Miniemulsion polymerization, Prog. Polym. Sci. 27 (2002) 1283–1346. [3] P.J. Blythe, B.R. Morrison, K.A. Mathauer, E.D. Sudol, M.S. El-Aasser, Enhanced droplet nucleation in styrene miniemulsion polymerization: 1. Effect of polymer type in sodium lauryl sulfate/cetyl alcohol miniemulsions, Macromolecules 32 (1999) 6944–6951. [4] F.J. Schork, G.W. Poehlein, S. Wang, J. Reimers, J. Rodrigues, C. Samer, Miniemulsion polymerization, Colloids Surf. Phys. Chem. Eng. Aspects 166 (2000) 59–66. [5] G.W. Jia, N. Cai, Y.S. Xu, L. Chao, X.M. Tan, Miniemulsion polymerization of styrene with liquid polybutadiene as the sole co-stabilizer, Eur. Polym. J. 43 (2007) 4453–4459. [6] A. Imanol, J.I. Amalvy, M.J. Barandiaran, Influence of the polymeric hydrophobe on the kinetics of vinyl acetate miniemulsion polymerization, Colloids Surf. Phys. Chem. Eng. Aspects 153 (1999) 39–45. [7] V.C. Malshe, S. Elango, S.S. Bhagwat, S.S. Maghrabi, Fluorinated acrylic copolymers: part II. Polymeric surfactants, Prog. Org. Coat. 53 (2005) 212–216. [8] R. Souzy, B. Ameduri, Functional fluoro-copolymers for fuel cell membranes, Prog. Polym. Sci. 30 (2005) 644–687. [9] Z.C. Zhang, E. Chalkova, M. Fedkin, C.M. Wang, S.N. Lvov, S. Komarneni, T.C. Mike Chung, Synthesis and characterization of poly (vinylidene fluoride)g-sulfonated polystyrene graft copolymers for proton exchange membrane, Macromolecules 41 (2008) 9130–9139. [10] I. Capek, C.S. Chern, Radical polymerization in direct miniemulsion systems, Adv. Polym. Sci. 155 (2001) 101–165. [11] K. Landfester, N. Bechthold, F. Tiarks, M. Antonietti, Formulation and stability mechanisms of polymerizable miniemulsions, Macromolecules 32 (1999) 5222–5228. [12] C.M. Miller, E.D. Sudol, C.A. Silebi, M.S. El-Aasser, Miniemulsion polymerization of styrene: Evolution of the particle size distribution, J. Polym. Sci. Polym. Chem. Ed. 33 (1995) 1391–1408. [13] D.H. Li, E. David Sudol, M.S. El-Aasser, Miniemulsion and conventional emulsion copolymerization of styrene and butadiene: a comparative kinetic study, J. Appl. Polym. Sci. 101 (2005) 2304–2312. [14] F.Z. Hu, G.R. Chen, Y.J. Du, Material Surface and Interface, 2nd ed., East China University of Science and Technology Press, Shanghai, 2007, pp. 109–112 (chapter 6).