Nanoclay stabilized Pickering miniemulsion of fluorinated copolymer with improved hydrophobicity via RAFT polymerization

Abstract

This investigation reports the preparation of fluorinated copolymer/clay nanocomposite latex by Pickering miniemulsion polymerization using nanoclay sodium montmorillonite (NaMMT) as sole stabilizer. In this case the copolymerization of styrene (St), butyl acrylate (BA) and 2,2,3,3,4,4,4-heptafluorobutyl acrylate (HFBA) was carried out via Reversible Addition–Fragmentation chain Transfer (RAFT) process. Addition of 2-(acryloyloxy) ethyl trimethylammonium chloride (AETAC) increased the rate of polymerization. As nanoclays are negatively charged silicate layers, AETAC, a cationic monomer was introduced to increase the polymer–clay interaction via ion-exchange which has beneficial effect on the final properties of the copolymer. The copolymers had controlled molecular weights as well as narrow dispersity (Ð). Nanoclay armored morphology was observed by TEM and SEM analyses. The polymers had improved hydrophobicity and lower water uptake rate due to improved polymer–clay interaction.

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Introduction

Polymer/clay nanocomposites (PCN) have several advantages over pristine polymers, as they have several improved properties. An aqueous dispersion of PCN prepared via emulsion/miniemulsion polymerization finds application in paints and coatings. They can be prepared via conventional emulsion/miniemulsion polymerization using modified or unmodified nanoclay in presence of surfactant. In this case the water resistance of the polymer gets deteriorated due to the presence of conventional surfactants.¹

The pioneering studies by Ramsden in 1903 (ref. 2) and Pickering in 1907 (ref. 3) revealed an alternative way to stabilize an emulsion by solid particles instead of surfactants. Such emulsions are called as Pickering emulsion. In this case the solid particles should be wetted by both monomer and solvent.^4,5 There are reports to use nanosilica,^6,7 CaCO₃ nanoparticles,⁸ magnetic Fe₂O₃ and Fe₃O₄ nanoparticles,^9,10 graphene-oxide,^11,12 cellulose nanofibrils,¹³ starch nanocrystals¹⁴ as Pickering stabilizers. In this regard, nanoclays were widely used because of their availability, ease of modification, thermal and mechanical stability, precise dimension (length ∼ 25–150 nm, thickness ∼ 1 nm) etc.^15–29 Nanoclays are layered silicates which can be easily modified, intercalated or even exfoliated via ion exchange process in water. Pristine nanoclay has the compatibility with water, whereas modified nanoclay has the compatibility with organic substances. Researchers took this advantage to use nanoclay like LAPONITE®, montmorillonite (MMT) as the solid particle responsible to stabilize a surfactant-free emulsion residing at the monomer–water interface.^15–19 A modified nanoclay can stabilize the water-in-oil emulsion which is also called as inverse Pickering emulsion.^15,20 Similarly, unmodified nanoclay can be used to stabilize an oil-in-water emulsion i.e. Pickering emulsion.^21–23 However, the successful preparation of Pickering emulsion depends entirely on polymer–clay interaction. This interaction can be achieved by using a small amount of functional co-monomer in the polymerization recipe. In this case, the functional co-monomers are attached to nanoclay surfaces via ionic or covalent interaction and they simultaneously take part in polymerization.²⁴ Most of the authors took this advantage to introduce polymer–clay interaction in Pickering emulsion by using small quantity of a functional co-monomer. In 2007, Bourgeat-Lami et al. used methacrylate-terminated organosilane and poly(ethylene glycol) (PEG) as functional co-monomers to prepare PCN via Pickering emulsion polymerization.²⁵ In this case, methacrylate-terminated organosilane chemically binds nanoclay, whereas methacrylate-terminated PEG is adsorbed on nanoclay by hydrogen bonding with silanol (Si–OH) groups. In 2010, the same group reported the preparation of high solid content and film forming poly(styrene-co-butyl acrylate) latex stabilized by LAPONITE® RD, a disc shaped nanoclay.²⁶ In this case, they used methacrylate-terminated poly(ethylene glycol) as co-monomer to promote polymer-LAPONITE® association. Sheibat-Othman et al. also followed the same way in preparing polystyrene latex via surfactant-free emulsion polymerization using LAPONITE® RD.²⁷ Recently Bonnefond and colleagues used SipomerPAM100, a PEG based functional co-monomer to prepare poly(styrene-co-butyl acrylate) latex stabilized by sodium montmorillonite (NaMMT).¹⁷ In this case, the copolymer prepared by Pickering miniemulsion polymerization in presence of SipomerPAM100 showed improved hydrophobicity due to efficient polymer–clay interaction.

All the reported efforts to prepare Pickering emulsion by using nanoclay were based on hydrogen bonding interaction between polymer and nanoclay via PEG based functional co-monomers. Since in aqueous solution nanoclay has strong ionic attraction with cationic functionality, literature lacks the study with such ionic attraction between polymer and nanoclay in a Pickering emulsion. Recently, Kim et al. used LAPONITE® RDS as Pickering stabilizer after ionic modification with cetyltrimethylammonium bromide (CTAB), a non-reactive cationic surfactant.²⁸

Reversible Deactivation Radical Polymerization (RDRP) is a revolutionary approach to prepare polymers with controlled mol. wt and well-defined architectures. Among different types of RDRP processes reversible addition–fragmentation chain transfer (RAFT) process has been found suitable for emulsion/miniemulsion systems. Recently, we have reported the RAFT mediated Pickering miniemulsion polymerization using LAPONITE® as stabilizer.²⁹ However, there had been no report to use NaMMT as stabilizer in the Pickering miniemulsion polymerization of fluoroacrylate via RAFT process.

In this context, our present study deals with the preparation of fluoropolymer/clay nanocomposite via Pickering miniemulsion using NaMMT as stabilizer. In this case, a cationic co-monomer AETAC has been used to promote polymer–clay interaction via ion exchange. The effects of polymer–clay interaction on polymerization kinetics, particle size, miniemulsion stability, nanolatex morphology and hydrophobicity have been studied in a comparative manner with control experiments.

Experimental

Materials

Unmodified commercial nanoclay, sodium montmorillonite (NaMMT) was purchased from Southern Clay Products, USA. The monomers, styrene (St), n-butyl acrylate (BA), 2-(acryloyloxy) ethyl trimethylammonium chloride (AETAC) solution (80 wt% in H₂O) and 2,2,3,3,4,4,4-heptafluorobutyl acrylate (HFBA) were purchased from Sigma-Aldrich, USA and were used after the purification by passing through basic alumina column. The initiator, 2,2′-azobis isobutyronitrile (AIBN) (98%, ACROS Chemical) was recrystallized from ethanol. The RAFT reagent 2-cyano-2-propyl dodecyl trithiocarbonate (CPDTC) was purchased from Sigma-Aldrich and was used as received.

Polymerization procedure

The procedure adopted for entry no. 5 in Table 1 is described here. In a round bottom flask required amount of NaMMT (0.78 g, 15 wt% of monomer) was dispersed in pH 9.0 buffer solution (25 mL) by ultrasonication for 30 min. The aqueous solution of AETAC (0.7 mmol, 0.13 g) was added dropwise to the nanoclay dispersion for 10 min. The resulting solution was stirred at room temperature for 3 h to allow ion-exchange. Then the monomers St, BA and HFBA mixed (5.2 g) with the RAFT agent CPDTC (30 mg, 0.086 mmol) and free radical initiator AIBN (3.5 mg, 0.02 mmol) were introduced to that dispersion. The whole mixture was ultrasonicated until the diffusion of the oil phase (monomer) into the buffer solution. 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 minutes and then it was placed in the oil bath pre-heated at the reaction temperature (75 °C). Samples were taken at certain intervals to measure the conversion by gravimetric method. The control experiments were also carried out in absence of AETAC.

Table 1 GPC and DLS results of Pickering miniemulsion copolymerization using NaMMT^a

Entry	NaMMT content w.r.t. monomer	Conv. (%)	Mn,theo^b (g mol^−1)	Mn,GPC (g mol^−1)	Đ	Particle size^c (nm)	Size PDI^c	Zeta potential^c (mV)
a Polymerization time = 90 min, polymerization temperature = 75 °C, n-BA/St/HFBA = 60 : 30 : 10.b Theoretical Mn is calculated by 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).c Determined by DLS analysis.
1	5 wt%	48.7	34 100	38 400	1.41	329	0.301	−43.2
2	10 wt%	56.4	43 600	32 300	1.44	356	0.312	−45.3
3	15 wt%	55.6	33 700	30 400	1.32	267	0.214	−45.1
4	20 wt%	53.2	32 200	38 100	1.25	234	0.144	−46.3
5	15 wt% + AETAC (55 meq.)	78.8	46 900	57 200	1.31	332	0.135	−32.3

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Removal of nanoclay by reverse ion exchange

A solution of 2 wt% LiBr in THF was prepared. 25 mL of this solution was used to dissolve 0.5 g fluoropolymer/clay nanocomposite. The mixture was refluxed at 80 °C for 5 h under inert atmosphere. As a result, Li⁺ ion exchanged nanoclay was found to be precipitated. Nanoclay was nicely separated from the solution by centrifuging the mixture at 5000 rpm for 15 min. The supernatant was taken after filtration and evaporated in a vacuum oven at 50 °C for 48 h. Now the sample is ready for Gel Permeation Chromatography (GPC) analyses to determine molecular weight and dispersity (Ð).

Characterization

GPC analysis

The molecular weights and dispersity (Đ) of the nanoclay-free fluorinated copolymers were determined by GPC analysis in a VISCOTEK, GPC instrument equipped with two ViscoGel mix bed columns (17360-GMHHRM) connected in series with RI detector. THF was used as the eluent at a flow rate of 1 mL min⁻¹. Linear PMMA of narrow dispersity was used as calibration standard. OmniSEC 4.2 software was used to analyze the data. Samples were prepared in THF at a concentration of 2.0 mg mL⁻¹.

FTIR spectroscopy

FTIR spectra of unmodified and modified NaMMT were recorded on a Perkin-Elmer, Spectrum RX I FTIR spectrometer within a range of 400–4000 cm⁻¹. The samples were prepared in the form of KBr pellets.

Dynamic Light Scattering (DLS) analysis

The particle size, particle size distribution and zeta potential of the fluoropolymer/clay nanocomposite latex were measured by Dynamic Light Scattering (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°.

Transmission Electron Microscopy (TEM) analysis

The morphology of fluoropolymer/clay nanocomposite latex was studied by using Transmission Electron Microscopy (TEM, TECHNAI-G2 20 S-Twin) operated at an accelerated voltage of 120 kV. The samples were prepared by drop casting the diluted latex on 300 mesh Cu grids followed by drying in air for 2–3 days.

Scanning Electron Microscopy (SEM) analysis

Surface morphology of the nanocomposites was studied by using Scanning Electron Microscopy (SEM, JEOL JSM 5800, JEOL, Japan). Diluted nanocomposite latex was drop-cast onto small glass pieces (1 cm × 1 cm) and dried in air for 2–3 days. Gold coating on the samples was done prior to SEM analysis.

X-ray diffraction (XRD) analysis

The morphology of nanoclay in the nanocomposites was examined in a X-ray 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⁻¹. For analysis, X'Pert PRO, PANalytical instrument, USA software was used.

Thermal analysis

Thermogravimetric analysis (TGA) was carried out on a TGA instrument (model Q50 V6.1 Build 181, TA instrument). About 10 mg sample was heated from room temperature to 600 °C at a heating rate of 20 °C min⁻¹ under nitrogen atmosphere. 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 −60 °C to +60 °C. About 10 mg sample was taken for the measurement. The glass transition temperature (T_g) was determined from the inflexion point of the second heating curve in the plot of temperature vs. heat flow.

Water uptake

Water uptake measurements were carried out with the thick films prepared on glass slides by drying the latex in room temperature for one week. First the films were weighed out and immersed into a cup of water. At certain time intervals the films were taken out of water and soaked with paper. Then they were weighed out to determine the amount of water absorbed.

Contact angle measurement

Water contact angle was measured by CA Goniometer (Rame-Hart instrument co. Model no. 260F4) using deionized water. Samples were prepared as thin films by casting the latex on a glass slide and dried in open atmosphere for 2–3 days.

Results and discussion

Pickering miniemulsion polymerization

Pickering miniemulsion copolymerization of n-BA/St/HFBA (60 : 30 : 10) was performed via RAFT polymerization by using NaMMT as stabilizer. In this case, CPDTC was used as RAFT agent and the RAFT to initiator ratio was maintained at 4 : 1. Different amount of NaMMT (5–20 wt% of the monomer) was used as stabilizer to study its effect on rate of conversion. Fig. 1a shows the comparative kinetic behavior for the Pickering miniemulsion polymerization with varying concentration of NaMMT. A significant increase in rate of conversion was observed when NaMMT content was increased. However, there was an insignificant change in final conversion. In this case, fastest rate of polymerization was observed with 15 wt% NaMMT. The solid particle responsible to stabilize a Pickering emulsion should have the affinity to both hydrophobic monomer and water. In this regard, AETAC, a cationic co-monomer was used to promote NaMMT at monomer–water interface. NaMMT is quite hydrophilic in nature and it can be easily dispersed in water. Fig. 1b shows the evolution of conversion for the polymerization in presence and absence of functional co-monomer AETAC. In presence of AETAC, a cationic co-monomer the polymerization proceeded at a faster rate and showed much higher conversion. The addition of cationic salt AETAC enabled cation exchange process in aqueous solution. The amount of AETAC was calculated w.r.t. the cation exchange capacity (CEC) of NaMMT (92.5 meq.). At this stage, AETAC enters into the clay galleries replacing Na⁺ ions in NaMMT.

Fig. 1 Evolution of conversion w.r.t. time for the RAFT mediated Pickering miniemulsion polymerization using (a) different amount of NaMMT and (b) in presence of AETAC as co-monomer.

The presence of AETAC molecules inside NaMMT was confirmed by the FTIR analysis. Fig. S1† shows the FTIR spectra of neat NaMMT and the same obtained after ion-exchange process prior to polymerization reaction. The evolution of a new absorption band at 1731 cm⁻¹ was due to the [double bond splayed left]C=O stretching vibration of AETAC. It proves the successful incorporation of AETAC units in the nanoclay to act as a compatibilizer between fluorinated copolymer and nanoclay. The presence of cationic co-monomer AETAC rather disturbed the electrical double layer on nanoclay surface making them aggregated and unstable in aqueous solution. It has been a major problem in undertaking ion exchange process as the technique to achieve polymer–clay interaction. The aggregation was prevented by carrying out the ion-exchange process at basic pH (pH 9.0). In this case, NaMMT shows fewer tendencies to aggregate during ion-exchange at basic pH due to the formation of more negative charge by the dissociation of –OH groups from Si–OH, Li–OH and Mg–OH at the edges.³⁰ After ion-exchange NaMMT becomes somewhat hydrophobic. This hydrophobicity took them to monomer–water interface to act as a Pickering stabilizer. Finally, the polymerization of AETAC present inside the clay galleries successfully produced a Pickering miniemulsion. Scheme 1 shows the chemical structure of the fluorinated copolymer prepared by Pickering miniemulsion RAFT polymerization using NaMMT as Pickering stabilizer and AETAC as cationic co-monomer. In this case, the amount of AETAC was varied according to the CEC of nanoclay.

Scheme 1 Chemical structure of the RAFT polymerized fluorinated random copolymer containing cationic functionality.

Table 1 shows the results of Pickering miniemulsion copolymerization using different amount of NaMMT and in presence of AETAC as functional cationic co-monomer. Unlike the polymerization kinetics, the final conversion was almost independent of the NaMMT content. However, the polymerization in absence of stabilizer NaMMT produced very little conversion of only 5.4% with complete phase separation. There was a slight increase in conversion with increase in NaMMT content from 5 wt% to 10 wt%. However, further increase in clay amount did not have any effect on final conversion. Interestingly, the conversion got a large increase from 55.6% (entry 3) to 78.8% (entry 5) in presence of AETAC, a cationic co-monomer. The molecular weight of the copolymers obtained from GPC analysis had the resemblance with the theoretical molecular weight. This showed the controlled nature of the copolymerization which had also been reflected in the lower dispersity (Đ) values (<1.5). However, the particle size and its distribution were dependent on the clay content in the Pickering miniemulsion. With the increase in clay content there was a gradual decrease in particle size with narrower size distribution (entry 1–4). Higher amount of nanoclay needed more surface area to reside at monomer–water interface. In order to increase the surface area the particle size decreased.

The particle size distribution was quite narrow with AETAC as an additive (entry 5 in Table 1) compared to the same in the other experiments carried out in absence of AETAC (entry 1–4). This observation holds well in favor of AETAC, a cationic co-monomer having ion-exchange interaction with nanoclay. AETAC attracted the nanoclay to reside at monomer–water interfaces which made the particle size distribution quite narrower. The highly negative zeta potential values indicated that the fluorinated copolymer emulsions were quite stable. But, addition of AETAC disturbed the emulsion stability to some extent as observed from the large decrease in zeta potential value from −45.1 (entry 3) to −32.3 (entry 5). In this case, addition of AETAC, a cationic co-monomer partially neutralized the negatively charged electrical double layer present on the particle surfaces.³¹ In the present study, the Pickering miniemulsion polymerization was successfully carried out with higher yield and faster rate of polymerization in presence of AETAC by sacrificing the miniemulsion stability to some extent. Fig. S2† shows the GPC profiles of the fluorinated copolymers prepared by Pickering miniemulsion polymerization. The molecular weight distribution was quite broad at lower nanoclay content (10 wt%). However, it became narrower when clay content had been increased.

Effect of AETAC content

In the present study, the nanoclay stabilized Pickering miniemulsion polymerizations were carried out with different amount of AETAC. The amount of AETAC was varied according to its cation exchange capacity (CEC) with nanoclay. NaMMT has the cation exchange capacity of 92.5 meq./100 g clay. In this case, the AETAC solution was used in equivalence to below CEC, at CEC and above CEC. Fig. 2 shows the evolution of conversion w.r.t. time for the Pickering miniemulsion polymerization carried out in presence of AETAC with different concentrations according to its CEC. In this case, the polymerization was quite faster when AETAC concentration was 55 meq./100 g clay (AETAC-55) i.e. partial ion-exchanges took place at below CEC. There was a regular decrease in rate of polymerization as well as conversion with increase in degree of ion-exchange by increasing AETAC concentration. The Pickering miniemulsion polymerization with fully ion-exchanged nanoclay (AETAC-92) showed about 65% conversion in 5 h. Accordingly, the polymerization achieved much lower conversion of about 30% when AETAC concentration was much higher (167 meq.) (AETAC-167) than CEC of nanoclay (92.5 meq.).

Fig. 2 Evolution of conversion with time for the Pickering miniemulsion polymerization using AETAC at different CEC.

Table 2 shows the results of Pickering miniemulsion polymerization using different concentrations of AETAC w.r.t. the CEC of nanoclay. Highest conversion was achieved with the AETAC concentration of 55 meq. i.e. below the CEC of nanoclay. With increase in the concentration of AETAC the conversion got a rapid decrease due to the increase in hydrophobicity of nanoclay. Generally, addition of a cationic substance to the aqueous dispersion of nanoclay makes the dispersion unstable due to the rapid cation exchange. In our present study, the increase in the AETAC amount increased the hydrophobicity of nanoclay causing the colloidal instability due to aggregation. The particle size analysis showed the aggregation phenomena of nanoclay by the considerable increase in particle size with increase in AETAC content. Similarly, the colloidal instability was observed from the rapid decrease in zeta potential value with increase in AETAC amount.

Table 2 Results of RAFT mediated Pickering miniemulsion polymerization using 15 wt% nanoclay and different concentrations of AETAC w.r.t. the CEC of nanoclay

Entry	Samples	Conc. of AETAC (meq./100 g clay)	Conv. (%)	Particle size (nm)	Size PDI	Zeta potential (mV)
5	AETAC-55	55	78.8	332	0.135	−32.3
6	AETAC-92	92	62.5	501	0.140	−14.1
7	AETAC-167	167	32.5	736	0.733	−2.2

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Latex morphology

Morphology of the fluorinated copolymer latex particles was observed by microscopic analysis. Fig. 3 shows the TEM images of the fluorinated copolymer particles prepared by using bare NaMMT as Pickering stabilizer. In this case, the spherical fluorinated copolymers are well distributed as observed in the Fig. 3a–c taken at different sections with different magnifications. Here, Fig. 3b and c show the nanoclay layers that are well separated from the spherical polymer particles. In this case, NaMMT was fully dispersed in aqueous phase as bare NaMMT is hydrophilic in nature. When the Pickering miniemulsion polymerization was carried out with only NaMMT (entry 1–4), there was no interaction between polymer and nanoclay. So, the clay layers were not on the polymer surface rather they are aggregated in aqueous phase as observed in the TEM images. Some authors used graphene oxide (GO) as a Pickering stabilizer for the surfactant-free emulsion polymerization of styrene, aniline etc.^32,33 GO remains at the monomer–water interface because of its amphiphilic nature.³⁴ As a result, the polymer particles prepared by Pickering emulsion polymerization are covered with GO sheets. However, the hydrophilic nature of NaMMT makes them reside at the aqueous phase. In this study, we have introduced electrostatic interaction between negatively charged NaMMT and a cationic comonomer AETAC to keep NaMMT at the monomer–water interface.

Fig. 3 TEM images of the fluorinated copolymer latex particles prepared by Pickering miniemulsion polymerization using only NaMMT (a–c).

Being a cationic monomer, AETAC undergoes a cation exchange process with nanoclay and simultaneously polymerizes with other monomers. Thus, the addition of AETAC made significant changes in the morphology of nanoclay in the fluorinated copolymer latex. Fig. 4 shows the TEM images of different sections in the Pickering miniemulsion of fluorinated copolymer prepared by using NaMMT in combination with AETAC. Due to the presence of AETAC, the polymer particles interacted with nanoclay to form nanoclay armored morphology. In this case, all the polymer spheres were observed to be covered with nanoclay layers due to the electrostatic interaction between polymer and nanoclay as observed in the Fig. 4a–c taken at different sections. Thus, the effect of ionic attraction exerted by AETAC could be proved by comparing the nanolatex morphology obtained in presence and absence of AETAC. A similar observation was also reported by Sorbier and co-workers where nanosilica coated polymer spheres were obtained because of the ionic interaction between positively charged nanosilica and negatively charged initiator.³⁵

Fig. 4 TEM images of the fluorinated copolymer copolymer latex particles prepared by Pickering miniemulsion polymerization using NaMMT and AETAC (a–c).

There had been a very similar observation on the effect of AETAC in FESEM analysis. Fig. 5 shows the FESEM images of the fluorinated copolymer latex particles. When only NaMMT was used as Pickering stabilizer, the polymer particles showed quite smooth surface morphology (Fig. 5a–c).

Fig. 5 FESEM images of the fluorinated copolymer latex particles prepared by Pickering miniemulsion polymerization with only NaMMT (a–c) and NaMMT together with AETAC (d–f).

This ensured the absence of nanoclay at the polymer surface as the system lacks any sort of polymer–clay interaction. However, the presence of cationic functionality in AETAC initiated the polymer–clay interaction via ion exchange. So, the copolymer latex particles were covered with nanoclay causing a rough surface morphology as observed from the FESEM analysis (Fig. 5d–f). A quaternary ammonium cation was incorporated to the fluorinated copolymer with the addition of AETAC during the polymerization. Nanoclay is a kind of clay mineral which is prone to ion-exchange in its aqueous solution in presence of cationic species. So, the fluorinated copolymer containing the quaternary cation entered the clay galleries via ion-exchange. In this case, an exfoliation of clay layers is expected to happen due to the incorporation of fluorinated copolymer inside the clay galleries. Fig. 6 shows the XRD profiles of pristine nanoclay and polymer–clay nanocomposites prepared in presence and in absence of AETAC. The fluorinated copolymer prepared in absence of AETAC showed peaks at 2θ values of about 7.0 and 6.0 because of the agglomeration and intercalation of nanoclay respectively. In this case, absence of any sort of strong interaction between polymer and nanoclay made the clay layers agglomerated and intercalated.

Fig. 6 XRD profiles of pristine nanoclay and polymer–clay nanocomposites.

The agglomerated clay layers were observed in the TEM images as described earlier. But, the presence of AETAC made the clay layers to be exfoliated as evidenced by the broadening of XRD profile. In this case, AETAC induced the compatibility between fluorinated copolymer and nanoclay.

Thermal analysis

Thermal stability of the nanocomposites prepared with and without AETAC was measured by TGA. Fig. 7a shows the TGA thermogram for fluorinated copolymers containing 15 wt% nanoclay. In this case, the copolymer prepared in absence of AETAC showed T_onset (10% degradation temperature) at 344.8 °C whereas the same prepared in presence of AETAC showed higher T_onset at 355.3 °C. This substantial increase in thermal stability refers to the improved polymer–clay interaction offered by AETAC. It was further established by measuring T_g in DSC analysis. Fig. 7b shows the DSC curves of the fluoropolymer–clay nanocomposites prepared in presence and absence of AETAC. In this case also, a considerable increase in T_g from 28.9 °C to 36.8 °C was observed because of the improved polymer–clay interaction via ionic attraction in presence of AETAC co-monomer.

Fig. 7 (a) TGA and (b) DSC curves of fluoropolymer–clay nanocomposites.

Film properties

The polymer–clay nanocomposite can have improved water resistance property due to the polymer–clay compatibility aroused by a cationic functionality. The water absorption behavior of the fluoropolymer–clay nanocomposite films was measured for the samples prepared with and without AETAC. Fig. 8 shows the water uptake behavior of different nanocomposite films. There was a regular increase in rate of water absorption with increase in clay content in the fluorinated copolymer. However, the copolymer with AETAC showed improved hydrophobicity i.e. lower water uptake compared to the other copolymers. Higher polymer–clay interaction made the clay layers hydrophobic enough to show lower water uptake.

Fig. 8 Water uptake behavior of different copolymer films.

The water contact angle (WCA) analysis of the fluorinated copolymer–clay nanocomposites (Fig. 9) showed similar observation to that of water absorption. The nanocomposite prepared by using only NaMMT showed WCA value of 57.8° which indicated the hydrophilic character of the copolymer. But, there had been a remarkable increase in WCA values from 57.8° to 96.1° in the nanocomposite prepared in presence of AETAC. This increment in hydrophobicity supported the act of AETAC as compatibilizer between polymer and nanoclay.

Fig. 9 Nature of water drops on different copolymer films.

Conclusions

Fluorinated copolymer/clay nanocomposite with controlled mol. wt was successfully prepared by RAFT mediated Pickering miniemulsion polymerization using nanoclay NaMMT as sole stabilizer. The polymerizations were carried out via RAFT process to obtain the nanocomposites with controlled molecular weights and narrow dispersities. The polymer–clay compatibility was achieved via ionic interaction offered by a cationic co-monomer, AETAC. Due to the presence of quaternary ammonium cation, AETAC entered the clay galleries via ion-exchange and polymerized with other monomers leading to better dispersion of nanoclay in the polymer. In this case, the Pickering miniemulsion polymerization was much faster in presence of AETAC with a concentration below the CEC of nanoclay. Due to the presence of such polymer–clay interaction, the fluoropolymer/clay nanocomposite showed nanoclay armored morphology with higher surface roughness as observed from TEM and SEM analyses. In this case, nanoclay was exfoliated in presence of AETAC, whereas its absence showed a mixture of agglomeration and intercalation as observed from XRD analysis. There had been a significant increase in thermal stability, T_g and hydrophobicity in the fluorinated copolymer/clay nanocomposite where nanoclay was armored and exfoliated in presence of AETAC. This polymer–clay nanocomposite emulsion may find potential application in hydrophobic paints and coatings.

Acknowledgements

AC thanks to Council of Science and Industrial Research (CSIR), New Delhi (Grant no. 09/081 (1110)/2010-EMR-I) for research fellowship. SP and NKS are thankful to CSIR, New Delhi for financial assistance.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra25808a

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