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Tailor-Made Fluorinated Copolymer/Clay Nanocomposite by Cationic RAFT Assisted Pickering Miniemulsion Polymerization Arindam Chakrabarty,† Longhe Zhang,§,⊥ Kevin A. Cavicchi,§ R. A. Weiss,§ and Nikhil K Singha*,†,‡ †

Rubber Technology Centre, Indian Institute of Technology Kharagpur, Kharagpur 721302, India School of Nano-Science and Technology, Indian Institute of Technology Kharagpur, Kharagpur-721302, India § Department of Polymer Engineering, University of Akron, Akron, Ohio 44325-0301, United States ‡

S Supporting Information *

ABSTRACT: Fluorinated polymers in emulsion find enormous applications in hydrophobic surface coating. Currently, lots of efforts are being made to develop specialty polymer emulsions which are free from surfactants. This investigation reports the preparation of a fluorinated copolymer via Pickering miniemulsion polymerization. In this case, 2,2,3,3,3-pentafluoropropyl acrylate (PFPA), methyl methacrylate (MMA), and n-butyl acrylate (nBA) were copolymerized in miniemulsion using LaponiteRDS as the stabilizer. The copolymerization was carried out via reversible addition−fragmentation chain transfer (RAFT) process. Here, a cationic RAFT agent, S-1-dodecyl-S′-(methylbenzyltriethylammonium bromide) trithiocarbonate (DMTTC), was used to promote polymer-Laponite interaction by means of ionic attraction. The polymerization was much faster when Laponite content was 30 wt % or above with 1.2 wt % RAFT agent. The stability of the miniemulsion in terms of zeta potential was found to be dependent on the amount of both Laponite and RAFT agent. The miniemulsion had particle sizes in the range of 200−300 nm. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) analyses showed the formation of Laponite armored spherical copolymer particles. The fluorinated copolymer films had improved surface properties because of polymer−Laponite interaction.



INTRODUCTION Fluorinated polymers in emulsion find wide applications in paints and coating because of their low surface energy. They also possess excellent resistance to oil, water, organic solvents, and flammability. In conventional emulsions, the properties of fluorinated polymers are somewhat restricted because of the presence of surfactants, which are low molecular weight organic molecules. In this regard, a surfactant-free emulsion of fluorinated polymer may achieve more importance. In 1907, Pickering identified inorganic nanoparticles as emulsion stabilizing agent.1 This kind of solid-stabilized emulsion is called a Pickering emulsion which does not need any conventional surfactants to be stable. Thus, it finds tremendous research interests because of its application in paints, coatings, cosmetics, and pharmaceutical industries.2 The approaches to prepare a Pickering emulsion were mainly based on the use of nanoclaylike montmorillonite (MMT)2−5 or Laponite6−11 as Pickering stabilizer. Among different kinds of nanoclays, Laponite is a disc-shaped clay mineral with a length of ∼30 and 1 nm thickness. Laponite has the proper © 2015 American Chemical Society

dimension to act as a Pickering stabilizer, and it is superior to other clay minerals in this respect.7,12,13 In this case, we are particularly interested in Laponite-RDS which is a synthetic layered silicate containing inorganic polyphosphate as peptiser. Addition of polyphosphate or pyrophosphates as peptiser together with nanoclay generally increases the emulsion stability by creating negative charge density on nanoclay edges.14,15 Laponite is hydrophilic as it swells in water. However, a Pickering emulsion polymerization with Laponite will be successful when it will remain at oil−water interface rather than in water phase only. In this case, a functional comonomer is generally used to promote polymer−Laponite interaction. Being a part of the polymer chain, the functional comonomers interact with Laponite via hydrophobic interaction or ionic attraction. Lami et al. used an acrylateterminated polyethylene glycol (PEGA) as a comonomer Received: May 15, 2015 Revised: September 17, 2015 Published: October 22, 2015 12472

DOI: 10.1021/acs.langmuir.5b01799 Langmuir 2015, 31, 12472−12480

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sized as described previously.22 The fluorinated monomer, 2,2,3,3,3pentafluoropropyl acrylate (PFPA); a nonionic RAFT agent, 2-cyano2-propyl dodecyl trithiocarbonate (CPDTC); and the thermal initiator, 2,2′-azobis(2-amidinopropane) dihydrochloride (AAPH) were purchased from Sigma-Aldrich and were used as received. Polymerization Procedure. In a round-bottom flask, the required amount of Laponite-RDS was dispersed in borate buffer of pH 9.0 by ultrasonication for 30 min. The aqueous solution of the cationic RAFT agent was added dropwise to the Laponite dispersion for 10 min. The resulting solution was stirred at room temperature for 3 h to allow effective ionic interaction between the RAFT agent and the Laponite. Then, the mixture of BA, MMA, and PFPA monomers was added and stirred with continuous nitrogen purging for 15 min. The mixture was then ultrasonicated until the complete diffusion of monomer to aqueous phase. Finally, the aqueous solution of the initiator AAPH was injected through the sampling valve prior to the polymerization at 75 °C. Samples were taken at certain time intervals to measure the conversion by gravimetric method. The control experiments were also carried out with the nonpolar RAFT agent CPDTC. Polymer Recovery by Reverse Ion Exchange. The fluoropolymer/Laponite nanocomposite (0.2 g) was dissolved in 10 mL of 2 wt % LiBr solution in tetrahydrofuran (THF) under stirring. This solution was refluxed at 80 °C for 5 h under nitrogen atmosphere. The resulting solution was centrifuged at 6000 rpm for 15 min to separate the Li+ ion exchanged Laponite. The supernatant was filtered and dried in vacuum oven at 50 °C for 24 h. This Laponite-free polymer was subjected to NMR and gel permeation chromatography (GPC) analyses.

having hydrophobic interaction of the PEG unit with Laponite for the preparation of poly(styrene-co-butyl acrylate)/Laponite nanocomposite latex via Pickering emulsion polymerization.6 Some authors also used methacrylate-terminated polyethylene glycol (PEGMA) to achieve that goal.9,16 In 2011, Teixeira et al. reported the necessity of little amount of methacrylic acid as auxiliary comonomer for the successful Pickering emulsion copolymerization of styrene and butyl acrylate using LaponiteXLS as Pickering stabilizer.11 Currently, there are few approaches with a reactive cationic species responsible for polymer−Laponite interactions, but they should be possible as Laponite accumulates negative charge in aqueous solution. Recently, we reported the preparation of polyfluoroacrylate/ clay nanocomposite using a cationic comonomer.17 In the presence of a cationic species, Laponite undergoes a rapid cation exchange process in aqueous solution. This process induces sufficient hydrophobicity in Laponite to act as a Pickering stabilizer. In 2013, Kim et al. used hydrophobic Laponite obtained after the ionic modification of bare Laponite with cetyltrimethylammonium bromide (CTAB), a nonreactive cationic surfactant.10 Hence, there is a need to investigate the Pickering miniemulsion polymerization using other reactive cationic species which will take part in polymerization as well as interact with Laponite via ion exchange process. Reversible deactivation radical polymerization (RDRP) process is a revolutionary approach in polymer synthesis. It produces polymers with well-defined architecture, controlled molecular weights, and narrow dispersities. Among the different RDRP techniques, reversible addition−fragmentation chain transfer (RAFT)18 polymerization process has been very versatile in bulk, in solution, and in emulsion polymerization of fluoroacrylates.19−21 This process minimizes the gel content in the final polymer. To the best of our knowledge, there is no report on the Pickering emulsion polymerization of fluoroacrylate using a suitable RAFT agent. Herein, we report the preparation of fluorinated copolymer/ clay nanocomposite via RAFT mediated surfactant-free/ Pickering miniemulsion polymerization using Laponite-RDS as the sole stabilizer. In this case, a cationic RAFT agent was used, as it gets physically adsorbed onto the negatively charged Laponite surface via ionic interaction. This enhances the compatibility between fluorinated copolymer and Laponite. The 1H NMR spectrum of the fluorinated copolymer showed the presence of a cationic RAFT end group which is responsible for polymer−Laponite interaction. The kinetic observation clearly showed the effect of cationic RAFT agent on the Pickering miniemulsion polymerization. The polymers had controlled molecular weights and narrow dispersities. The fluorinated copolymer−Laponite nanocomposites were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and atomic force microscopy (AFM) analyses. Contact angle measurement showed the effect of cationic RAFT agent concentration, that is, polymer−Laponite interaction on the surface energy of the fluorinated copolymer films.





CHARACTERIZATION GPC Analysis. The molecular weights (Mn) and dispersity (Đ) of the Laponite-free fluorinated copolymers 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 Refractive Index (RI) detector. THF was used as the eluent at a flow rate of 1 mL/min. Linear poly(methyl methacrylate) (PMMA) of narrow dispersity was used as the calibration standard. Data was analyzed by using OmniSEC 4.2 software. Samples were prepared in THF at a concentration of 2.0 mg/mL. NMR Spectroscopy. 1H NMR spectrum of the copolymer was recorded on a 400 MHz (Bruker) spectrometer using CDCl3 as a solvent. Dynamic Light Scattering (DLS) Analysis. The particle size, particle size distribution, and zeta potential of the fluoropolymer/Laponite nanocomposite latex were measured by DLS analysis using a Malvern Zetasizer ZS90 instrument with a 4 mW He−Ne laser (λ = 632.8 nm) operated at a scattering angle of 90°. Atomic Force Microscopy (AFM) Analysis. Surface morphology of the Pickering emulsion was studied by using an atomic force microscope (Agilent Technologies, USA model 5500) operating under tapping mode. Diluted nanocomposite latex was drop-cast onto small glass pieces and was dried in an oven at 50 °C for 48 h. Transmission Electron Microscopy (TEM) Analysis. The distribution of Laponite in the polymer matrix was studied by using transmission electron microscopy (TEM, TECHNAIG2 20 S-Twin) operated at an accelerated voltage of 120 kV. Thin films of nanocomposites were prepared by drop-casting the diluted latex on 300 mesh Cu grids and by drying in air for 3−4 days. X-ray Diffraction (XRD) Analysis. The d-spacing of Laponite and its nanocomposites was measured in an X-ray

EXPERIMENTAL SECTION

Materials. Unmodified commercial Laponite-RDS was purchased from Rockwood Additives Ltd. UK. The monomers methyl methacrylate (MMA) and n-butyl acrylate (n-BA) were purchased from Sigma-Aldrich, USA, and were used after purification by vacuum distillation. The cationic RAFT agent S-1-dodecyl-S′-(methylbenzyltriethylammonium bromide) trithiocarbonate (DMTTC) was synthe12473

DOI: 10.1021/acs.langmuir.5b01799 Langmuir 2015, 31, 12472−12480

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Langmuir Scheme 1. Chemical Structures of the Cationic and Nonionic RAFT Agents and Preparation Pathway of Tailor-Made Fluorinated Copolymer by Cationic RAFT Assisted Pickering Miniemulsion Polymerization Using Laponite-RDS

ization. Table S1 shows the amounts of different ingredients used for the present study. Here, the amounts of both Laponite-RDS and DMTTC were optimized by varying their concentrations within a certain range. The [RAFT]:[initiator] ratio was kept at 4:1. The effect of cationic RAFT agent toward the stabilization of the Pickering emulsion was established by performing a control experiment with CPDTC as the RAFT agent. Figure 1 shows the comparative kinetic study for the

diffractometer (WAXD, Rigaku, Ultima-III, Japan) with Cu target using an X-ray wavelength of 1.54 Å. The samples were scanned from 2θ = 2° to 10° at a scanning rate of 0.5°/min. For analysis, X’Pert PRO, PANalytical instrument, USA, software was used. Contact Angle Measurement. Contact angle was measured by CA Goniometer (Rame-Hart instrument co. Model no.- 260F4) using water and diiodomethane. Samples were prepared as films by casting the drops of emulsion on a glass slide and were dried in open atmosphere for 2−3 days.



RESULTS AND DISCUSSION Pickering Miniemulsion Polymerization. The surfactantfree/Pickering miniemulsion polymerization of n-butyl acrylate (n-BA), methyl methacrylate (MMA), and 2,2,3,3,3-pentafluoropropyl acrylate (PFPA) (n-BA/MMA/PFPA = 60:30:10) was carried out in the presence of Laponite-RDS at pH 9.0. In this case, copolymerization was carried out using two different types of RAFT agents: DMTTC, a cationic RAFT agent, and CPDTC, a nonionic RAFT agent (Scheme 1). Both of these RAFT agents contain a long alkyl chain, that is, a dodecyl group which can resist Ostwald Ripening, that is, coalescence between monomer droplets.19 As the RAFT agent itself can act as a cosurfactant, this miniemulsion copolymerization did not require any conventional cosurfactant like cetyl alcohol or hexadecane. Laponite is a negatively charged species because of the isomorphic substitution in its lattice structure. It is highly hydrophilic and forms a stable dispersion at neutral pH. However, addition of cationic species destroys the electrical double layer. Thus, the Laponite dispersion becomes unstable and thereby aggregates. Generally, Laponite shows fewer tendencies toward aggregation in the aqueous phase at basic pH because of the formation of more negative charges by the dissociation of −OH groups from Si−OH, Li−OH, and Mg− OH at the edges.23 In this case, all the Pickering polymerizations were carried out at pH 9.0 as the Laponite dispersion was quite stable at this pH even after the addition of the cationic RAFT agent. However, there is an optimum concentration of the RAFT agent up to which the aggregation can be prevented. With an increasing amount of RAFT agent used, it is more likely that the Laponite dispersion will become unstable. Thus, in our current study, the cationic RAFT agent, DMTTC, plays the key role in providing polymer−Laponite interaction for the successful Pickering miniemulsion polymer-

Figure 1. Kinetic plot [ln (1 − x)−1 vs time, x = conversion] for the Pickering miniemulsion polymerization using CPDTC as nonionic RAFT agent and DMTTC as cationic RAFT agent.

Pickering miniemulsion polymerization using the cationic RAFT agent DMTTC and the nonionic RAFT agent CPDTC. A conversion of 73% was achieved in 3 h using CPDTC, whereas the process with cationic RAFT agent offered about 99% conversion in 1 h only. Figure 1 indicates that about 10 times faster rate of polymerization was achieved with DMTTC compared to CPDTC as RAFT agent. This was due to the fact that being a cationic RAFT agent DMTTC was anchored onto the Laponite surface by ionic interaction, and thereby it formed stable droplets (Scheme 1). This kind of ionic interaction rather helps to form a stable Pickering emulsion at the early stage of polymerization. The addition of DMTTC turns the hydrophilic Laponite-RDS to amphiphilic so that it can reside at the monomer−water interface to act as a Pickering stabilizer. Figure 2 shows the 1H 12474

DOI: 10.1021/acs.langmuir.5b01799 Langmuir 2015, 31, 12472−12480

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Figure 2. 1H NMR spectrum of fluorinated copolymer prepared by RAFT mediated Pickering miniemulsion polymerization with Laponite-RDS.

Table 1. Results of Cationic RAFT Mediated Pickering Miniemulsion Polymerization Using Different Amounts of LaponiteRDSa run

laponite (wt %)

time (min)

conv. (%)

Mn,theo (g/mol)b

Mn,GPC (g/mol)

Đ

particle size (nm)c

size PDIc

zeta potential (mV)

1 2 3

20 30 40

120 60 60

54.2 99.6 99.5

23 300 42 500 42 500

17 600 45 200 41 600

1.22 1.21 1.25

534 210 197

0.907 0.257 0.180

−26.5 −33.9 −38.2

a Polymerization temp. = 75 °C. Conc. of RAFT agent = 1.2 wt % of the monomer. bTheoretical Mn is calculated by the equation Mn = MRAFT + x[M]0Mm/[RAFT]0 (x = conversion, [M]0 = initial conc. of monomer, Mm = mol. wt. of monomer, MRAFT = mol. wt. of RAFT agent). cDetermined by DLS analysis.

NMR spectrum of the fluorinated copolymer using DMTTC as the RAFT agent. The resonances at δ3.5, δ3.9, and δ4.5 are attributed to −O−CH3, −O−CH2−, and −O−CH2− protons of different comonomer units of MMA, BA, and PFPA, respectively, in the copolymer. Importantly, the resonances at δ7.3 and δ7.8 are due to − Ph protons showing the presence of cationic-RAFT end group in the copolymer. The molar composition of the different comonomers was calculated to be BA:MMA:PFPA = 65:28:7 which is comparable with the comonomers’ feed ratio of 60:30:10. Effect of Laponite-RDS Content. A series of Pickering miniemulsion polymerizations were performed with varying Laponite content. As Laponite is the sole stabilizer in our present study, its content will control the particle formation at the initial stage of polymerization. When Laponite content was 20 wt % of the monomer, the polymerization was slower with less conversion. About 55% conversion was achieved in 2 h, as this amount of Laponite was insufficient at monomer−water interface. With increase in Laponite content to 30 wt %, the Pickering miniemulsion polymerization showed a faster rate as well as a very high conversion of 99% in only 1 h. A further increase in Laponite content to 40 wt % showed a significant increase in rate of polymerization, especially at the initial stage of Pickering miniemulsion polymerization as evidenced from Figure S1a. Thus, a Laponite amount of 30 wt % or more proved to be sufficient for a successful Pickering miniemulsion polymerization. The effect of Laponite content on the rate of Pickering emulsion polymerization was also studied by Lami et al.16 with the free radical polymerization of styrene. An increase in Laponite amount increased the number of primary particles formed at the early stage of polymerization. Thus, an increase

in the number of polymerization sites stabilized by Laponite was responsible for faster rate of polymerization and higher conversion. In our present study, there was also a substantial effect of Laponite amount on the particle size of the Pickering miniemulsion. Figure S1b shows the particle size evolution with time for the Pickering miniemulsion polymerization using different amounts of Laponite-RDS. At 20 wt % Laponite content, it showed much larger particle size and its evolution in a broad range of 300−550 nm. Formation of larger particles is caused by lower surface coverage by a small amount of Laponite. This indicates instability in Pickering miniemulsion which was concomitant with the slower rate of polymerization and lower conversion. At 30 wt % or more Laponite content, the particle size became smaller with its evolution in a narrow range. This concentration of Laponite was found to be suitable to achieve higher conversion with smaller particle size. A much smaller particle size within a narrow range of 280− 200 nm was observed by using 30 wt % Laponite. This decrease in particle size of the Pickering miniemulsion with an increase in Laponite content could be attributed to the formation of more surface area to accommodate the higher amount of clay discs. Thus, there must be a substantial increase in particle number in order to increase the surface area. However, this decrease in particle size was not so much prominent when the Laponite amount was further increased to 40 wt %. At this concentration of Laponite, the particle size was much higher at the initial stage of polymerization. This may be due to the aggregation of Laponite clay discs at its higher concentration. Table 1 shows the results containing final conversion, GPC, particle size, and zeta potential analyses with the Pickering miniemulsion of fluorinated copolymer at different Laponite 12475

DOI: 10.1021/acs.langmuir.5b01799 Langmuir 2015, 31, 12472−12480

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Figure 3. Nature of particle size distribution with time for the Pickering miniemulsion polymerization with (a) 20 wt %, (b) 30 wt %, and (c) 40 wt % Laponite-RDS of the monomer.

contents. It shows a sharp increase in final conversion when the Laponite content was increased from 20 wt % to 30 or 40 wt %. However, the fluorinated copolymers had controlled molecular weights and narrow dispersities (Đ) irrespective of the Laponite content in the Pickering miniemulsion. It also shows a parallel relationship between Laponite content and particle size with distribution. As the Laponite content increases, the particle size as well as the size distribution of the final fluoropolymer latex decreases. However, the change was not so prominent when Laponite content was 30 wt % or above. Zeta potential values showed that the least stable miniemulsion was with 20 wt % Laponite. The stability increased as the Laponite amount was increased beyond 20 wt % showing more negative zeta potential values. Figure 3 shows the particle size distribution (PSD) profile at different time intervals for the Pickering miniemulsion polymerization with different amounts of Laponite-RDS. At 20 wt % Laponite content, the Pickering miniemulsion was not stable showing bimodal PSD at the early stage of polymerization (Figure 3a). At this stage, both the initiation and the propagation were hindered because of the instability in the miniemulsion. However, at the final stage, a unimodal and broad PSD was observed. Thus, this concentration of Laponite was not sufficient to form a stable emulsion as reflected in conversion, particle size, and its distribution. Our earlier observation showed tremendous effect on polymerization kinetics and

conversion and particle size when the Laponite content was beyond 20 wt %. This effect was also reflected in the nature of PSD with polymerization time (Figure 3b and c). A unimodal and sharp PSD was observed during the polymerization at 30 wt % Laponite content (Figure 3b). However, a broad and bimodal distribution was observed after ultrasonication when the Laponite content was 40 wt % (Figure 3c). This may be due to the aggregation of excess Laponite in water phase rather than at monomer−water interface before the polymerization. Eventually, the PSD sharpened and displayed a unimodal distribution during the polymerization. Here, the excess Laponite was accommodated at the monomer−water interface because of the increase in surface area by a quick decrease in particle size at the early stage of polymerization. Effect of RAFT Agent Content. A series of Pickering miniemulsion polymerizations with Laponite-RDS were carried out using different amounts of cationic RAFT agent. In our earlier discussion, it was shown that the Pickering miniemulsion polymerization with Laponite-RDS was successful by virtue of the cationic RAFT agent DMTTC. Generally, a stable Pickering emulsion is formed because of the presence of solid particles preferentially at the oil−water interface. When hydrophilic solid particles are used as Pickering stabilizer, they remain suspended in the water phase. In this case, they are not adsorbed by the hydrophobic monomer droplets. In the same way, very hydrophobic solid particles are totally adsorbed 12476

DOI: 10.1021/acs.langmuir.5b01799 Langmuir 2015, 31, 12472−12480

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Table 2. Results of Cationic RAFT Assisted Pickering Miniemulsion Polymerization Using Laponite-RDS with Varying Amounts of RAFT Agenta

a

run

time (min)

RAFT (wt %)

conv. (%)

Mn,theo (g/mol)

Mn,GPC (g/mol)

Đ

particle size (nm)

size PDI

zeta potential (mV)

4 2 5 6

60 60 60 120

0.8 1.2 1.6 2.0

98.8 99.6 84.6 52.3

63 000 42 500 27 200 13 700

52 300 45 200 24 100 16 200

1.28 1.21 1.18 1.20

261 210 251 288

0.395 0.257 0.172 0.928

−39.7 −33.9 −21.5 −16.1

Polymerization temp. = 75 °C. Laponite-RDS used = 30 wt % of the monomer.

Scheme 2. Orientation of Laponite Clay Discs as Evidenced from DLS Profiles for the Pickering Miniemulsion Polymerization Using Nonionic and Cationic RAFT Agents

polymerization was carried out using RAFT concentration of 0.8−1.6 wt % of the monomer. However, the particle size remained almost unchanged in the course of polymerization when the RAFT concentration was 2.0 wt %. In this case, highly hydrophobic Laponite had no effect on the polymerization as well as the particle size during polymerization. This fact also supported the conversion behavior at highest RAFT concentration. At this stage, Laponite clay discs are too hydrophobic to act as a Pickering stabilizer. In our present study, the hydrophobicity of Laponite-RDS was successfully tuned by varying the cationic RAFT concentration to construct an ideal condition for Pickering miniemulsion polymerization. The kinetic behavior in terms of conversion and particle size clearly depicted the effect of hydrophobicity in a Pickering stabilizer. Table 2 shows the GPC and DLS results of the Pickering miniemulsion polymerization with increasing amount of RAFT agent. The fluorinated copolymers had narrow dispersity (