Tailor-made polyfluoroacrylate and its block copolymer by RAFT polymerization in miniemulsion; improved hydrophobicity in the core–shell block copolymer

Journal of Colloid and Interface Science 408 (2013) 66–74

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Tailor-made polyfluoroacrylate and its block copolymer by RAFT polymerization in miniemulsion; improved hydrophobicity in the core–shell block copolymer Arindam Chakrabarty, Nikhil K. Singha ⇑ Rubber Technology Centre, Indian Institute of Technology Kharagpur, Kharagpur 721 302, India

a r t i c l e

i n f o

Article history: Received 6 May 2013 Accepted 17 July 2013 Available online 29 July 2013 Keywords: Fluoroacrylate Miniemulsion RAFT polymerization Core–shell morphology

a b s t r a c t Controlled/living radical polymerization (CRP) of a fluoroacrylate was successfully carried out in miniemulsion by Reversible Addition Fragmentation chain Transfer (RAFT) process. In this case, 2,2,3,3,4,4,4-heptafluorobutyl acrylate (HFBA) was polymerized using 2-cyanopropyl dodecyl trithiocarbonate (CPDTC) as RAFT agent, Triton X-405 and sodium dodecyl sulfonate (SDS) as surfactant, and potassium persulphate (KPS) or 2,20 -azobis isobutyronitrile (AIBN) as initiator. Being compatible with hydrophobic fluoroacrylate, this RAFT agent offered very high conversion and good control over the molecular weight of the polymer. The miniemulsion was stable without any costabilizer. The long chain dodecyl group (–C12H25) (Z-group in the RAFT agent) had beneficial effect in stabilizing the miniemulsion. When 2-cyano 2-propyl benzodithioate (CPBD) (Z = –C6H5) was used as RAFT agent, the conversion was less and particle size distribution was very broad. Block copolymerization with butyl acrylate (BA) using PHFBA as macro-RAFT agent showed core–shell morphology with the aggregation of PHFBA segment in the shell. GPC as well as DSC analysis confirmed the formation of block copolymer. The core–shell morphology was confirmed by TEM analysis. The block copolymers (PHFBA-b-PBA) showed significantly higher water contact angle (WCA) showing much better hydrophobicity compared to PHFBA alone. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction Fluorine containing polymers find great research interest due to their unique properties and wide range of applications. Small fluorine atoms having low polarizability are devoid of ‘‘London Dispersion Forces,’’ which causes low intermolecular force of attraction. Fluorinated polymers have excellent resistance to oil, water, and organic solvents as well as to flammability. Fluoropolymers have low surface energy and interesting surface properties. So, they find extensive applications in self-cleaning paints and coatings. Application of fluoropolymers in aqueous emulsion and miniemulsion drastically reduces the volatile organic content (VOC) in the environment. There have been several studies on the surface properties of different polyfluoroacrylates and polyfluoromethacrylates prepared by conventional free radical polymerization (FRP) [1–4]. However, FRP leads to uncontrolled molecular weights and broad polydispersity index in the polymers. Via FRP, it is also difficult to prepare polymers with controlled architecture and well-defined morphology like block, star, and brush like copolymers. Since the inception of controlled radical polymerization (CRP) in 1990s, they have been extensively used to prepare different tailor-made ⇑ Corresponding author. Fax: +91 3222 255303. E-mail address: [email protected] (N.K. Singha). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.07.031

functional polymers having controlled molecular weight and well-defined architectures [5]. Among the different types of CRPs, RAFT process [6] has been found to have good success in the emulsion system [7–9]. It is very difficult to polymerize very hydrophobic and high density fluoroacrylates via conventional emulsion polymerization, because of the poor transport of these monomers to the micelle through water phase [10,11]. Miniemulsion polymerizations have been successful in the polymerization of very hydrophobic monomers [12–19]. There are several reports on the preparation of fluorinated copolymers by RAFT polymerization in miniemulsion as well as in emulsion [20–23]. But these studies were mainly focused on the copolymerization of hydrophobic fluoromethacrylate (FMA) with protonated methacrylates. Zhou et al. [20] have reported that RAFT miniemulsion polymerization of only FMA was not successful in terms of conversion and latex stability. Addition of acetone can somewhat decrease the surface tension of water to achieve a stable dispersion of polyfluoroacrylate in water [24]. But this decreases the conversion of fluoromonomers and increases the VOC. Chen and Wu [25] reported decrease in conversion as well as in emulsion stability due to increased incorporation of fluoromonomers in its copolymer. Polymer lattices with core–shell morphology have several advantages over conventional polymer lattices [26–30]. They have better film formation properties. Core–shell polymers based on

A. Chakrabarty, N.K. Singha / Journal of Colloid and Interface Science 408 (2013) 66–74

fluoropolymers have high performance properties, and they are used in self-cleaning specialty coatings. These core–shell fluorinated copolymers are normally prepared by semi continuous or seeded emulsion polymerization [10,29,31–35]. In this case, fluorinated monomers are added after the formation of core of protonated acrylics. Fluorinated copolymers also sometimes show core–shell morphology upon migration of fluoropolymer to the surface [36,37]. Several authors have reported CRP of fluoroacrylate by Atom Transfer Radical Polymerization (ATRP) [38] and Reverse Iodine Transfer Polymerization (RITP) [39]. Recently, our group has reported RAFT polymerization of 2,2,3,3,4,4,4-heptafluorobutyl acrylate (HFBA) in solution [40]. But to the best of our knowledge, there is no report on the emulsion or miniemulsion polymerization of HFBA via any CRP method. There is also no report on the preparation of block copolymer based on PHFBA. This investigation reports preparation of tailor-made PHFBA in miniemulsion using 2-cyanopropyl dodecyl trithiocarbonate (CPDTC) as the RAFT agent, but without any costabilizer which is usually required in miniemulsion polymerization. In this case, long alkyl chain in the RAFT agent had beneficial effect in stabilizing the miniemulsion. Block copolymers of HFBA and BA were successfully prepared in miniemulsion by using PHFBA as macro-RAFT agent. TEM analysis showed the block copolymer had core–shell morphology. Formation of block copolymer was confirmed by GPC and DSC analysis. 2. Experimental part 2.1. Materials Butyl acrylate (BA) (Sigma–Aldrich) was purified by washing with 5% sodium hydroxide solution and followed by distillation under reduced pressure. The initiator, 2,20 -azobis isobutyronitrile (AIBN) was recrystallized from ethanol. The RAFT agents, 2-cyano-2-propyl dodecyl trithiocarbonate (CPDTC) and 2-cyano 2-propyl benzodithioate (CPBD) (Sigma–Aldrich), 2,2,3,3,4,4,4heptafluorobutyl acrylate (HFBA) (Sigma–Aldrich), Triton X-405 as nonionic surfactant (Dow chemicals), sodium dodecyl sulfate (SDS) (Merck) as anionic surfactant, and potassium persulfate (KPS) (Merck) were used as received.

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prepared by dissolving the PHFBA as macro-RAFT agent in requisite amount of BA, and then, it was added to the aqueous phase containing small amount of SDS as a surfactant. This mixture was ultrasonicated in ice-cold water for 2 min. This sonication produced a complete dispersion of monomer and macro-RAFT agent and monomer in water resulting a very stable white colored emulsion. The mixture was transferred to the glass reactor which was purged with nitrogen gas. Aqueous solution of KPS was added to the reactor, and the polymerization carried out at 75 °C. 3. Characterization 3.1. GPC analysis The polymer prepared by miniemulsion polymerization was washed several times with water and ethanol to remove the surfactant and dried in vacuum oven at 50 °C. The molecular weights (Mn) and polydispersity indices (PDI) of the polymers were determined by Gel Permeation Chromatography (GPC) analysis using a VISCOTEK, GPC instrument equipped with two ViscoGel mix bed columns (17360-GMHHRM) connected in series with a RI detector. Tetrahydrofuran (THF) was used as the eluent at a flow rate of 1 mL/min. PMMA of narrow PDI was used as calibration standard. Data analysis was carried out by using OmniSEC 4.2 software. GPC samples were prepared in THF at a concentration of 1.5 mg/mL. 3.2. NMR spectroscopy 1

H NMR spectra were recorded on a 400 MHz (Bruker) spectrometer using CDCl3 as a solvent containing a small amount of TMS as internal standard. 3.3. TEM analysis The particle size and distribution of latex were studied by using TEM (TEM; FEI™, Type 5022/22, Technai G2 20 S-Twin) operated at an accelerated voltage of 200 kV. A drop of diluted latex (diluted 20 times with deionized water) was placed on 300 mesh carbon coated copper grid. The grids were dried in a desiccator before analysis.

2.2. Homopolymerization of HFBA in miniemulsion HFBA (5 g, 22.5 mmol), RAFT agent CPDTC (0.08 g, 0.25 mmol), and AIBN (0.008 g, 0.062 mmol) were mixed together to make a homogeneous organic phase. The aqueous phase was prepared with deionized water (20 g) and surfactant. The organic phase was added dropwise to the aqueous phase with stirring to form a pre-emulsion. This pre-emulsion was then ultrasonicated in icecold water for 10 min. At this stage, the organic phase disperses completely in the aqueous phase forming a miniemulsion. This resultant miniemulsion was transferred to a four necked glass reactor equipped with a nitrogen inlet and outlet, sampling valve, and a magnetic stirrer. The reactor was deoxygenated by purging N2 for 30 min, and then, it was placed in the oil bath preheated at the reaction temperature (75 °C). At this stage, for KPS initiated system, the aqueous solution was injected through the sampling valve. Samples were taken at certain intervals to measure the conversion by gravimetric method. 2.3. Preparation of block copolymer, PHFBA-b-PBA using PHFBA as macro-RAFT agent The PHFBA-b-PBA was synthesized by the homopolymerization of BA using PHFBA as macro-RAFT agent. The organic phase was

3.4. Thermal analysis Differential scanning calorimetry (DSC) analysis was carried out in a DSC instrument (model Q100 V8.1 Build 251, TA instruments) at a temperature range of 100 °C to +100 °C. A sample weight of about 5 mg was taken for the measurement. The sample was quenched from room temperature to 100 °C and then scanned at a heating rate of 10 °C/min till 100 °C. The glass transition temperature (Tg) was determined from the inflexion point of the second heating curve in the plot of temperature vs heat flow. 3.5. DLS analysis The dynamic light scattering (DLS) analysis of the diluted latex was carried out by using a Malvern Nano ZS instrument employing a 4 mW He–Ne laser (k = 632.8 nm). 3.6. Contact angle measurement Contact angle was measured by the CA Goniometer (Rame-Hart instrument co. Model no. 190F2). Samples were prepared as film on a glass slide and annealed at 120 °C for 12 h.

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4. Results and discussion 4.1. Homopolymerization of HFBA RAFT polymerization of HFBA was carried out in miniemulsion by using AIBN as well as KPS as oil soluble and water soluble initiator, respectively. In this case, CPDTC and CPBD were used as the RAFT agent and Triton X-405 as the surfactant (Scheme 1). Fig. 1 shows the comparative kinetic plot of the RAFT polymerization of HFBA using CPDTC as RAFT agent and AIBN as well as KPS as thermal initiator. The kinetic plots were linear indicating that the polymerization reaction was well controlled. In both the cases, there was a short retardation time which is also reported by several authors in case of emulsion polymerization using RAFT process [41,42]. Fig. 1 indicates that polymerization initiated by KPS is faster than the same initiated by AIBN. Zhou et al. [43] studied RAFT polymerization of methyl methacrylate in miniemulsion, and they also reported that the KPS initiated system was much faster than the AIBN initiated one. This observation was attributed to the pattern of initiation. KPS dissociates in water and forms propagating radical in water phase. Ultrasonication produces monomer nanodroplets as the polymerization site. For further propagation, the KPS initiated propagating radicals enter the nanodroplet stabilized by surfactant. But AIBN dissociates inside the nanodroplets. This is a kind of bulk polymerization. A quick chain transfer to the RAFT agent is expected for AIBN initiated propagating radicals. So, the lifetime of AIBN initiated propagating radicals was much lower than KPS initiated system. Therefore, the AIBN initiated system had lower conversion as well as slower rate of polymerization than the KPS initiated system which followed zero–one condition [44]. According to Luo et al. [45], AIBN initiated polymerization causes super-swelling in the monomer droplet, and this led to lower conversion. Fig. 2a and b shows the plot of molecular weight of PHFBA vs conversion in the AIBN and KPS initiated system, respectively. There was a linear increase in molecular weight with conversion in both AIBN and KPS initiated system. This indicates a successful RAFT polymerization of HFBA in miniemulsion using both the initiators. But both the systems differ in PDI values during the course of polymerization. The PDI of the AIBN and KPS initiated RAFT polymerization of HFBA can be explained in terms of different initiation loci. In case of AIBN initiated system, the initiator decomposes in the droplets and forms several propagating radicals. At the early stage of polymerization, relatively short polymer

Fig. 1. Kinetic plots of RAFT polymerization of HFBA in miniemulsion using KPS and AIBN as initiators and Triton X-405 as surfactant.

chains are formed [43]. So, it showed relatively broad PDI at initial stage. On the other hand, KPS dissociates in water and initiates the polymerization in water phase. The KPS initiated system followed predominantly zero–one condition. So, KPS initiated polymerization had relatively narrow PDI throughout the polymerization. However, toward the end of polymerization, both the systems have marginal difference in PDI. Fig. 3 shows the 1H NMR spectra of PHFBA prepared by KPS as an initiator and CPDTC as the RAFT agent. The different protons in the PHFBA are designated in Fig. 3. A resonance at 4.5 ppm is due to the –O–CH2–CF2– protons in PHFBA. The resonances at 3.3 ppm and 0.9 ppm are due to different protons of the RAFT end group as designated in Fig. 3. The resonance at 5.0 ppm is attributed to the proton of the last repeating unit connected to the RAFT end group. The molecular weight of this PHFBA was calculated by using the integral area of the resonances at 4.5 ppm (for –OCH2 protons in PHFBA) and at 5.0 ppm (for >CH– proton of the last repeating unit of PHFBA). The Mn,NMR (12,700 g mol1) was found to be comparable with the Mn,GPC (11,600 g mol1). Thus, the presence of well-defined RAFT end group also confirms the controlled RAFT polymerization of HFBA.

Scheme 1. RAFT polymerization of 2,2,3,3,4,4,4-heptafluorobutyl acrylate (HFBA) using different RAFT agents; with CPDTC and with CPBD.

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Fig. 2. Relationship of Mn and PDI with the conversion of HFBA for (a) AIBN initiated system (Expt. No. 2) and (b) KPS initiated system (Expt. No. 5).

Fig. 3. 1H NMR spectrum of PHFBA prepared via RAFT polymerization using CPDTC as RAFT reagent. (Expt. No. 9 in Table 2).

Table 1 Results of RAFT polymerization of HFBA in miniemulsion using Triton X-405e as surfactant and CPDTC as RAFT agent.

a b c d e

Expt. No.

Initiator

[M]:[RAFT]:[I]

Time (min)

Conva (%)

Mn(theo) (g/mol)b

Mn(GPC) (g/mol)

PDI

Particle size (nm)c

Zeta potential (mV)d

1 2 3 4 5 6

AIBN AIBN AIBN KPS KPS KPS

157:4:1 315:4:1 472:4:1 157:4:1 315:4:1 472:4:1

120 120 120 60 60 60

75.4 72.8 69.4 85.3 87.2 86.5

7800 14,800 21,100 8800 17,700 26,200

7200 16,000 19,700 9300 20,600 22,900

1.04 1.05 1.16 1.06 1.06 1.10

29.0 28.1 30.1 34.2 55.3 57.1

26.8 25.6 26.5 52.3 53.1 53.8

Determined gravimetrically. Theoretical Mn is calculated by equation Mn = MRAFT + x[HFBA]0MHFBA/[RAFT]0 (x = conversion of HFBA, MHFBA = molecular weight of HFBA). Determined by DLS analysis. Determined by DLS analysis. Triton X-405 used as 0.9% w.r.t. total mass.

Table 1 summarizes the results of RAFT polymerization of HFBA in miniemulsion using Triton X-405 as surfactant and CPDTC as RAFT agent. Much higher conversion was achieved in case of KPS initiated system than the AIBN initiated one. KPS initiation is associated with sulfate ion (SO2 4 ) in propagating radical which creates negative charge on the particle surface. This caused higher negative zeta potential in KPS initiated system than in the AIBN initiated system. Higher the negative value of zeta potential, higher

is the stability of the latex, which is explained in terms of electrostatic repulsion. Thus, KPS initiated polymerization produced more stable latex and led to higher conversion of HFBA. 4.2. Effect of surfactant type and concentration The effect of surfactant on the polymerization kinetics of HFBA was studied by using both Triton X-405 and SDS. Triton X-405 is a

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TritonX-405 SDS

2.0

Expt. 5

2.0

-2

k app = 4.2 x 10 min-1

1.5

k app = 4.29 x 10

Expt. 8

R = 0.9667 -2

k app = 3.55 x 10

-1

ln (1-X)

1.0

-2

2

1.0 k app = 2.02 x 10

-1

k app = 1.0 x 10 min

0.0

-1

min

2

R = 0.9402

0.5

2

R = 0.9747

0.5

-1

min

R = 0.9586

-2

-2

-1

min

2

1.5

2

R = 0.9667

-1

ln (1-X)

Expt. 7

0.0 0

20

40

60

80

100

10

120

20

30

40

50

60

Time (min)

Time (min) Fig. 4. Kinetics of KPS initiated RAFT miniemulsion polymerization of HFBA with different surfactants (0.9% to total mass).

nonionic surfactant based on hydrophilic polyethylene oxide (PEO) and has hydrophilic–lipophilic balance (HLB) value of 17.9 and critical micelle concentration (CMC) of 0.8 mM. SDS is an anionic surfactant and has HLB value of 40 and CMC of 7–10 mM. Surfactants were taken in three different concentrations. Fig. 4 shows the comparative kinetic plots of KPS initiated RAFT polymerization of HFBA with two different surfactants. Rate of polymerization was much faster with Triton X-405 than with the SDS. So, steric stabilization by nonionic surfactant is efficient with our system. Table 2 summarizes the results of RAFT miniemulsion polymerization of HFBA using two different surfactants as well as varying their concentrations also. About 87% conversion was achieved in 1 h using 0.9% (wt% to total mass) Triton X-405 as surfactant. But with SDS, it shows 67% conversion in 2 h. This result could be attributed to the poor affinity and compatibility of fluoroacrylate with anionic surfactant like SDS. Moreover, Triton X-405 has much lower CMC compared to SDS. Triton X-405 being based on PEO has better compatibility with the environment. Advantages of polymeric surfactant over low molecular weight surfactants are to improve film formation due to their polymeric component and to prepare micro-latex (particle size <50 nm) [45,46]. Generally, increase in surfactant concentration increases emulsion stability [47] and the rate of both emulsion and miniemulsion polymerizations [43] due to the formation of more polymerization sites i.e. micelle. But RAFT miniemulsion polymerization of HFBA exhibited a reverse trend. Fig. 5 shows the comparative kinetic plots of RAFT miniemulsion polymerization of HFBA at different concentrations of Triton X-405. The conversion as well as rate of polymerization decreased as the surfactant concentration was increased. Usually, in the miceller nucleation, the rate of polymerization increases as the concentration of surfactant increases. In our case, we observed that the rate of polymerization as well as conversion decreased with increase in surfactant concentration (Fig. 5). Capek [48] reported that surfactant with high enough

Fig. 5. Kinetics of KPS initiated RAFT miniemulsion polymerization of HFBA with different concentrations of Triton X-405.

Table 3 Results of RAFT polymerization of HFBA in miniemulsion with different RAFT agents. Expt. RAFT No. agent 7 12

Time (min)

CPDTC 60 CPBD 240

Conv Mn(theo) (%) (g/mol)

Mn(GPC) (g/ PDI Particle size Size mol) (nm) PDI

81.3 49.0

17,800 6300

16,500 10,000

1.10 34.2 1.15 168.2

0.067 0.514

concentration forms thick interfacial layer which acted as a barrier for the entry of propagating radicals, and thus, the rate of polymerization decreased. It is reported that at higher surfactant concentration, the particle size decreases to accommodate the excess surfactant [49–51]. In the present case, we observed that as the concentration of surfactant increased, the particle size also decreased. At higher surfactant concentration, the excess surfactants are adsorbed on the monomer droplet which decreases the zeta potential [52]. In our case, we also observed the decrease in zeta potential (Table 2) with increase in surfactant concentration. So, all the above observations showed the adsorption of excess surfactant on the monomer droplets indicating predominantly droplet nucleation process. Again, formation of coagulum was observed at much lower surfactant concentration [53]. Increase in concentration of Triton X-405 reduced the emulsion stability in terms of electrostatic repulsion. But, excess SDS increased emulsion stability showing consistent zeta potential values. This observation was also in good agreement as Ramos et al. [54] reported for the preparation of polyurethane nanoparticles in miniemulsion by using SDS as anionic surfactant and Tween 80 as nonionic surfactant. 4.3. Effect of RAFT agent Miniemulsion polymerization is associated with droplet polymerization where the monomer droplets are stabilized by using

Table 2 Results of RAFT polymerization of HFBA in miniemulsion with varying surfactant and its concentration. Expt. No.

Surfactant

Surf. conc. (wt% to total mass)

Time (min)

Conv (%)

Mn(theo) (g/mol)

Mn(GPC) (g/mol)

PDI

Size (nm)

Zeta potential (mV)

5 7 8 9 10 11

TX-405 TX-405 TX-405 SDS SDS SDS

0.9 1.9 2.9 0.9 1.9 2.9

60 60 60 120 120 180

87.2 81.3 61.1 67.3 56.2 45.3

17,700 16,500 12,500 13,700 11,500 9300

20,600 17,800 18,700 11,600 8700 6700

1.06 1.10 1.16 1.13 1.15 1.18

55.3 34.2 22.3 49.9 38.5 28.5

53.1 47.1 35.8 51.2 52.3 52.4

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Fig. 6. DLS profiles of PHFBA lattices prepared by using different RAFT agents: (a) with CPBD and (b) with CPDTC. Fig. 7. GPC traces of homopolymer (PHFBA) and diblock copolymer (PHFBA-b-PBA) of Expt. 13.

long chain hydrophobes as costabilizers like cetyl alcohol, hexadecane, etc. They are usually dissolved in the monomer prior to ultrasonication. In this miniemulsion polymerization, we have not used any low molecular weight long chain hydrophobes as costabilizer which usually leads to high VOC in the environment. So, in this case, we carried out successful miniemulsion polymerization of HFBA by using a RAFT agent which had a long alkyl chain as hydrophobe. Thus, the RAFT agent could serve dual purpose. It was effectively acting as a chain transfer agent (CTA) to control the molecular weight, and its long alkyl chain was helpful in stabilizing the latex. Miniemulsion polymerization of styrene and methyl methacrylate was found to be dependent on the type of RAFT agent i.e. the structure of the chain transfer agent [13,15,55]. In this case, polymerization of HFBA was carried out with two types of RAFT agent like CPDTC (having a long alkyl chain as the end group) and CPBD (having phenyl group as the end group). Faster rate of reaction and higher conversion were achieved with CPDTC rather than CPBD (Table 3). Particle size distribution in DLS profiles (Fig. 6) shows the effect of chemical structure of the RAFT agent toward miniemulsion stability. Fig. 6 shows that CPBD mediated polymerization produced broad size distribution compared to CPDTC mediated polymerization. CPDTC is such a RAFT agent which contains a dodecyl group as the Z-group, whereas CPBD has a phenyl ring as the Z-group. In this case, CPDTC was found to be successful to act as a costabilizer as well as a chain transfer agent. Complete dispersion of HFBA in water was achieved in 5 min of ultrasonication. No coagulum was found after the reaction. Stable nanoparticles of PHFBA in water were formed which was evident from DLS and TEM study. The assumption with the RAFT agent like CPDTC to act as a costabilizer was proved correct by using a RAFT agent like CPBD which is having a phenyl group

as Z-group. After the polymerization with CPBD, a red coagulum was found to be sticking to the stirring magnetic needle.

4.4. Preparation of PHFBA-b-PBA diblock copolymer Block copolymers of HFBA and BA were prepared in miniemulsion using PHFBA as macro-RAFT agent and KPS as initiator. In this case, PHFBA of relatively low molecular weight (Mn = 7200, PDI = 1.04) was used as macro-RAFT agent, so that it was easily soluble in BA. Table 4 shows the results of preparation of block copolymer using PHFBA as macro-RAFT agent. The chain extension reaction was quite slower due to the restricted movement of macro-RAFT agent. Higher stability of the block copolymer miniemulsion was attributed to the steric stabilization rather than similar charge repulsion which was evident from the zeta potential value. Zeta potential of the block copolymer was comparable with the homopolymer. But block copolymer emulsion was stable for above 6 months without any coagulation, whereas coagulation of PHFBA homopolymer was observed after 2 months. GPC analysis of PHFBA showed negative refractive index in differential refractive index detector (Fig. 7). Matsumoto et al. [56] also observed very weak or even negative signal peaks for fluorinated polymers due to less refractive index of fluorinated polymers than THF, which was used as eluent. But after block copolymerization ve GPC traces of PHFBA shifted toward lower elution volume with positive refractive index, as PBA segment has greater refractive index than THF. Particle size measurement was also in good agreement with the formation of block copolymer. This observation was further confirmed by TEM analysis.

Table 4 Preparation of block copolymer of HFBA and BA by using KPS as initiator and PHFBA as macro-RAFT agent (Mn = 7200, PDI = 1.04).

a

Expt. No.

[M]:[macro-RAFT]:[I]

Time (h)

Conv (%)

% PHFBAa

Particle size (nm)

Zeta potential (mV)

Water contact angle (°)

13 14 15 16

625:4:1 1250:4:1 1875:4:1 2500:4:1

5 5 5 5

87.0 70.3 86.3 78.4

40.0 19.0 11.0 7.0

127.2 170.0 179.7 232.4

46.6 54.1 41.6 42.5

112.5 106.0 98.3 93.5

Obtained from 1H NMR.

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Fig. 8. 1H NMR Spectrum of PHFBA-b-PBA of Expt. 13.

Fig. 9. TEM images of (a) PHFBA and (b) core–shell structure of poly (HFBA-b-BA) and EDX analysis on the shell.

Molar composition of HFBA and BA in the block copolymer was determined by 1H NMR analysis (Fig. 8). The 1H NMR spectrum shows the resonances at 4.5 ppm for –O–CH2–CF2 protons of

PHFBA segment and 4.0 ppm for –O–CH2 protons of PBA segment. The ratio of the integral area of these resonances shows that the block copolymer has almost 40% PHFBA. The molecular weight

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Fig. 10. Photographs of water droplet on thin films of homopolymer (a) and diblock copolymer (b).

determination via GPC analysis showed that the block copolymer has 36% PHFBA, which was comparable with the value obtained in 1H NMR analysis. DSC analysis (Fig. S1 in Supplementary Section) of the homopolymer PHFBA showed an inflexion point at 17 °C indicating the glass transition (Tg) of PHFBA. But the block copolymer, PHFBA-b-PBA, showed two Tg s, 54 °C for PBA segment and 16 °C for PHFBA segment, respectively (Fig. S2 in Supplementary Section). 4.5. TEM analysis TEM images of PHFBA and the block copolymer poly (HFBA-bBA) are shown in Fig. 9. Fig. 9a shows that PHFBA particles were finely dispersed in the miniemulsion, and there was no aggregation. The average particle size of PHFBA is 60 nm which increases to 135 nm on block copolymerization. The important feature of the TEM image of the diblock copolymer is that it has core–shell morphology. The energy dispersive X-ray (EDX) analysis of the shell (Fig. 10) shows the presence of fluorine indicating the presence of PHFBA segment predominantly in the shell. Fluorinated polymers have very low surface energy. So, they always tend to reside on the surface. Misra et al. [57] reported core–shell structure of fluorinated copolymer where the fluorinated segment occupied the shell. The DLS analysis showed that there was a large increase in particle size from homopolymer to diblock copolymer indicating successful block copolymerization. 4.6. Contact angle measurement The contact angles of water droplet on the polymer films were measured to know about the hydrophobicity of both homopolymer and diblock copolymers. A polymer film was prepared by casting the emulsion on a glass slide and dried in air for 2–3 days. Then, it was annealed at 120 °C for 12 h before contact angle measurement. Fig. 10 shows the nature of water drop on the thin film of PHFBA and PHFBA-b-PBA. Table 4 shows a significant increase in contact angle with increase in PHFBA content in the block copolymers. This increase in hydrophobicity from homopolymer to diblock copolymer could be attributed to the proper aggregation of fluorinated polymer chains. According to Honda et al. [58], polyfluoroacrylates with short pendent fluorinated group [(CF2)n where n 6 6] show poor hydrophobicity due to disordered arrangement arising from molecular mobility. In our case, PHFBA is a fluorinated polymer having very short side chain. But after chain extension, molecular mobility was

somewhat restricted showing greater contact angle values. Nanophase-separated morphology of the block copolymer plays an important role on surface properties of the polymer [59]. In this present case, core–shell morphology of PHFBA-b-PBA showed nanophase separation as observed in TEM. EDX analysis of the shell indicated the presence of fluorine atom. The measurement of water contact angle (WCA) indicated that the block copolymer had much higher WCA than the PBA homopolymer (45.3°) [60] and as the PHFBA content increased the WCA of the block copolymer also increased (Table 4). Thus, it indicated that with the increase in PHFBA content, hydrophobicity increased indicating most of the PHFBA chains reside on the surface. Several authors also reported core–shell fluoroacrylate latex with fluoropolymer in the shell [31–37,57] and fluorinated block copolymer with aggregation of fluorinated segment on the surface [60,61]. 5. Conclusions RAFT miniemulsion polymerization of HFBA was successfully carried out without using any costabilizer. The RAFT agent CPDTC containing dodecyl group (Z-group) was able to prepare PHFBA particles with high conversion as well as with good colloidal stability. KPS initiated polymerization was more successful in terms of rate of reaction, colloidal stability than AIBN initiated polymerization. The RAFT agent offered very good control over the molecular weight with narrow PDI < 1.2. It was also observed that the rate of polymerization was much faster with nonionic surfactant Triton X405 compared to SDS, an anionic surfactant. 1H NMR showed the presence of well-defined RAFT end group in the PHFBA which was successfully used as macro-RAFT agent to prepare AB diblock copolymer, poly (HFBA-b-BA). TEM and EDX analysis confirmed the core–shell morphology of the block copolymer in which PBA formed the core and PHFBA formed the shell. Block copolymerization with BA using PHFBA as macro-RAFT agent remarkably improved the hydrophobicity due to the formation of core–shell structure with the aggregation of PHFBA in the surface. Acknowledgments AC thanks to Council of Science and Industrial Research, New Delhi (Grant No. 09/081(1110)/2010-EMR-I) for Junior Research Fellowship. Thanks are due to Asian Paints, Mumbai for financial support. The authors are also thankful to Prof. Nilmoni Sarkar, Dept. of Chemistry, IIT Kharagpur and Mrs. Sangita Singh, senior research fellow, Rubber Technology Centre, IIT Kharagpur for

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