Diazonium salt-based photoiniferter as a new efficient pathway to clay–polymer nanocomposites

Abstract

Clay–polymer nanocomposites were prepared by living free radical photopolymerization initiated from a lamellar clay-anchored photoiniferter resulting from a cation exchange reaction of sodium by diethyldithiocarbamate benzyldiazonium tetra-fluoroborate salt. The success of the cation exchange process, followed by iniferter graft polymerization of glycidyl methacrylate (GMA) on clay nanoplatelets was confirmed by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA). Thereby, the original cation exchange method induced by diazonium salt produces intercalated nanocomposite materials with an important organic mass loading around 41 wt%. More importantly, the content of the organic component included in the final nanocomposite material could be controlled by the diazonium salt concentration used during the synthesis.

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1. Introduction

The design of clay–polymer nanocomposites^1–4 has progressed at a remarkable pace over the last two decades owing to the outstanding properties that clays impart to polymers such as flame retardancy,^5–7 barrier properties,^8–10 thermal stability,^4,8 resistance to heat distortion temperature^8,11 and outstanding mechanical properties.^1,8,12 This has led to the emergence of novel clay–polymer nanocomposites for a wide variety of timely applications such as drug release,¹³ adsorbents for pollutants,¹⁴ catalysts,¹⁵ nanocomposite hydrogels,¹⁶ and optical materials,¹⁷ to name but a few. Nevertheless, the preparation of clay–polymer nanocomposites is challenging due to the inherent incompatibility between the hydrophilic silicate lamellae and the organophilic polymer component. This may result in poor dispersion of the inorganic particles within the polymeric matrix, resulting in insignificant property enhancement of the final nanocomposite material.

Several methods were envisaged to enhance the dispersion of silicate nanoplatelets such as intercalation of polymer or pre-polymer from solution, in situ intercalative polymerization technique, melt intercalation and so on.⁸

In view of making polymer nanocomposites, one elegant route is to intercalate the clay with silane species or else onium salts bearing a polymerization initiator functional group. In this way, several polymerization strategies were explored, namely radical (photo)polymerization,^18–20 atom transfer radical polymerization (ATRP),^21,22 reversible addition–fragmentation chain transfer polymerization (RAFT),^23,24 nitroxide-mediated polymerization (NMP),^25,26 polymerization initiated by iniferter compounds,^27,28 oxidative polymerization of conjugated monomers (aniline, pyrrole, etc.),^29,30 cationic polymerization,³¹ etc. Despite such remarkable advances, only a few studies have been reported using the versatile surface chemistry of aryl diazonium salts as a building block providing initiator groups, whilst these onium salts have been widely employed as cationic photoinitiators.³²

Clay materials such as montmorillonites (MMT) used in this study have a significant ability to exchange ions. Actually, MMT structure consists of 1 nm thin layers composed of two tetrahedral silica sheets sandwiched with an octahedral alumina sheet. The aluminosilicate layers are negatively charged and this charge is counterbalanced by alkali cations such as Ca²⁺, Na⁺ or Li⁺ disposed in the interspace between the aluminosilicate platelets.³³

Diazonium salts are remarkable intercalant compounds as they proved to readily react with clay by cation exchange mechanism followed by reaction of the diazonium group with the aluminosilicate layer providing interfacial Si–O-aryl links.²⁰ By selecting a functional group in para position of the diazonium moiety, it becomes possible to impart to any kind of materials a specific functionality and reactivity.

In the domain of polymer science and engineering, diazonium salts were proposed by us³⁴ and others^35–37 as coupling agents for polymers, in the same manner as silanes and other coupling agents operate. Diazonium salts can be employed in this regard for either grafting onto or grafting from strategies³⁸ such as controlled radical polymerization,³⁹ radical photopolymerization,^40,41 polymerization induced by iniferter compounds,^42,43 oxidative polymerization of conjugated monomers,²⁹ anionic polymerization.⁴⁴

Of the relevance to the actual manuscript, Salmi et al.²⁰ have employed N,N-dimethylaminobenzyldiazonium tetrafluoroborate salt to intercalate Na⁺–montmorillonite and obtained a macro-photoinitiator for the radical polymerization of glycidyl methacrylate (GMA). The photopolymerization reaction provides an exfoliated clay–poly(glycidyl methacrylate) (PGMA) nanocomposite. Recently, in a similar approach, Jlassi et al.²⁹ intercalated bentonite clay with 4-diphenylamine diazonium tetrafluoroborate which served as nanoscale reactor for interfacial oxidative polymerization of aniline, allowing to the development of new nanostructured conducting polymer materials. This strategy permitted to obtain exfoliated clay–poly(aniline) (PANI) nanocomposites where the chains are bound to the clay nanosheets through aryl layers. In these systems the chains are covalently bound to the aryl groups from the diazonium precursors, and the latter are on their turn grafted to the sheets via covalent bonds after curing the diazonium-exchanged clay. We have thus reasoned that clay-anchored photoiniferter (initiator-transfer-termination) prepared from parent diazonium-intercalated clay could serve for in situ controlled free radical polymerization. In this technique, the polymerization process can be activated by either heat or light. The system utilizes a reversible chain transfer mechanism, with a negligible ratio of bimolecular termination reactions which provides the characteristics of a “quasi-living” nature of polymerization.^45–47 Photoiniferter compounds have been used to graft homopolymers and block copolymers to a range of substrates with some control over the number-average molecular weight (M_n), polydispersity (I_p) and thickness.⁴⁸ However, despite these challenging advances, this strategy has never been explored before using diazonium-intercalated clays, hence the motivation for this work.

The aim of this paper is to prepare polymer–clay nanocomposites by in situ living free radical photopolymerization. Cloisite and glycidyl methacrylate (GMA) were selected as model clay and monomer, respectively, for the demonstration of proof of concept. The nanocomposites were designed by surface-initiated photoiniferter of GMA in the presence of Cloisite modified by dithiocarbamate diazonium salt intercalated between the clay lamellas through cation exchange process. Several nanocomposites were synthesized using various diazonium salt concentrations expressed in cation exchange capacity (CEC) fractions. The clay–polymer nanocomposites and the reference materials were characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), transmission electron microscopies (TEM) and thermogravimetric analysis (TGA).

2. Experimental

2.1. Materials

Soda clay (Cloisite, CL) was supplied from BYK Company (ion-exchange capacity = 91 meq./100 g of clay). Glycidyl methacrylate (GMA, (97%)) was purchased from Aldrich. GMA was passed through a basic alumina column before using to remove the polymerization inhibitor. The organic solvents (acetone, methanol and tetrahydrofuran (THF)) were of analytical grade, de-ionized water was used for the pretreatment of the clay and any further washing steps. Phenylenediamine, tetrafluoroboric acid, terbutylnitrite, thionyl chloride, 2-(4-aminophenyl)ethanol, diethyldithiocarbamate, 4-nitrobenzyl chloride and RANEY® nickel used for the synthesis of diethyldithiocarbamate benzyldiazonium tetra-fluoroborate were obtained from Aldrich.

2.2. Synthesis of diazonium salt

Diethyldithiocarbamate benzyldiazonium tetra-fluoroborate (BF₄⁻, N₂⁺ C₆H₄CH₂SCSN(C₂H₅)₂) used as diazonium salt photoiniferter was prepared in three steps. Firstly, the 4-nitrobenzyl chloride reacts with the diethyldithiocarbamate to give the thiocarbamate intermediate a good yield. The latter was hydrogenated using RANEY® nickel and leads to the corresponding primary amine in quantitative yield. This amine was dissolved in acetone, cooled in an ice bath and three equivalents of tetrafluoroboric acid were added dropwise. Then terbutylnitrite was added in the same conditions to this solution. Twenty minutes later the reaction was stopped and diethyl ether was added leading to the precipitation of the diazonium salt which was filtered and washed with cold ether. NMR and IR characterization of the diazonium salt gave similar results to those previously reported in the literature.⁴²

IR, cm⁻¹: (N₂⁺, 2291), (C=C_Ar, 1598); ¹HNMR, δ ppm: 8.7 (d, 2H, j = 8.5 Hz), 4.3 (s, 2H), 3.7 (q, 2H, j = 6.5 Hz), 3.3 (q, 2H, j = 6.3 Hz), 1.2 (t, 6H, j = 7.5 Hz).

2.3. Diazonium modification of clay

To an aqueous suspension of clay (0.100 g dissolved in 100 ml of de-ionized water), a solution of diazonium salt (INIF): 0.125, 0.050, 0.025 and 0.0175 g dissolved in 5 ml of acetone, which corresponds to 1, 2, 5 and 9 times the clay cation exchange capacity (CEC) was added. As described by Salmi et al.,²⁰ addition of the diazonium salt solution leads immediately to the swelling of the clay, visible with the naked eye, an indication of the success of the cation exchange reaction. The diazonium-modified clays were washed several times with water/acetone solvent (90%/10%) until the washings were clear. Finally, the modified clay samples were dried in the oven at 60 °C during 2 hours. In the following sections, the modified clay by diazonium salt will be noted CL-INIFx (x stands for the CEC fraction).

It is to note that the procedure is scalable, and for laboratory scale one can start with 500 mg of pristine clay and use proportional amounts of diazonium salt.

2.4. Synthesis of clay–PGMA nanocomposites

In this study, the weight ratio of glycidyl methacrylate (GMA)/CL-INIFx was set at 2.5 for all nanocomposite syntheses. In a typical procedure, CL-INIFx sample (20 mg) was dispersed in 5 ml of THF with GMA as monomer (50 mg). The photosensitive formulation was left in dark under magnetic stirring for 1 h. Then, the formulation was sealed and purged with nitrogen, and exposed to UV light in the chamber of the Dr Honle apparatus (Hg/Xe lamp, 70 mW cm⁻², München, Germany) for 2 h. The solid precipitate was recovered by centrifugation and thoroughly washed with THF and cold methanol. Finally, the product was dried in oven at 60 °C overnight. It is to note that THF is a good solvent for either GMA or PGMA but does not facilitate dispersion of the nanocomposites which precipitate in this solvent. It follows that any polymer that formed spontaneously by polymerization initiated by the monomer itself would have been removed in the washing procedure. In the following, the synthesized nanocomposite samples will be recorded CL-INIFx–PGMA, where x stands for the fraction of CEC (1, 2, 5 and 9).

NB: as for Section 2.3., it is possible to prepare larger amounts of clay–polymer nanocomposites starting with 500 mg of pristine clay, a fact that stresses the scalability of the process.

2.5. Characterization

The infrared absorption spectra were recorded by an IR spectrophotometer from Bruker in ATR mode in the range between 400 and 4000 cm⁻¹ with a resolution of 4 cm⁻¹.

Thermogravimetric analyses (TGA) were conducted using a Setaram instrument (Setsys Evolution 16 model). The samples were heated up from 20 to 800 °C at a linear heating rate of 10 °C min⁻¹ under argon.

XPS spectra were recorded using a K Alpha (Thermo) fitted with a monochromatic Al K X-ray source (spot size: 400 μm). The pass energy was set to 200 and 50 eV for the survey and the narrow regions, respectively. The spectra were calibrated against the C–C/C–H C1s component set at 285 eV. The composition was determined using the manufacturer sensitivity factors.

XRD patterns were recorded on an X'pert Pro diffractometer (Panalytical Company) operating at 40 kV and 40 mA, with an anode using Co Kα as the radiation source (λ = 1.7902 Å).

Transmission electron micrographs were obtained on a JEOL 2010 microscope equipped with a CCD camera. For the purpose of TEM images, clay samples were impregnated in a resin called AR Spurr. The kit used for the preparation of the resin was purchased from EMS (Electron Microscopy Sciences, USA) under the reference 14300 RT. The ImageJ free software from National Institutes of Health (NIH) in America was used for TEM image analysis.

3. Results and discussion

3.1. Strategy of making Cloisite/PGMA nanocomposites

Fig. 1 displays the procedure employed to prepare clay–PGMA nanocomposites using diazonium salt-based photoiniferter. The first step consists in modifying the soda clay by diazonium cation exchange mechanism (step 1, Fig. 1). The modified clay is further heated in a dry oven at 60 °C to achieve dediazonation reaction. The thermal decomposition of the diazonium salt leads to a covalent attachment of benzyl diethyldithiocarbamate moieties on the aluminosilicate clay platelets (step 2, Fig. 1). In the step 3, the modified clay can be used as macro-photoinitiator to initiate the GMA monomer photopolymerization. Actually, when the modified clay grafted with the photosensitive iniferter is irradiated with a UV light, the reaction provides free radicals resulting from a photochemical cleavage of the C–S bond.⁴⁹ The photolysis reaction leads to the generation of a radical pair: a benzyl radical (Fig. 1) and a dithiocarbamyl chain terminator radical (Fig. 1). Benzyl radicals linked to the clay platelets constitute highly reactive compounds able to initiate the polymerization of vinyl monomers such as GMA (Fig. 1, step 3). Due to the extremely high reactivity between the different radicals produced, a spontaneous recombination reaction occurs, allowing to the formation of “dormant species”.^45,48–50 However, under continuous irradiation a new pair of active radicals is regenerated; this process gives a living character to the polymerization using photoiniferter. Finally, due to a controlled polymerization process, crosslinking reactions are limited, involving the formation of a clay–polymer nanocomposites including well-defined polymer part (Fig. 1, step 4). For instance, it can be noticed that the filtrate has been recovered which confirms the absence of any eventual uncontrolled polymerization process within the solution. Thereby, the filtrate was evaporated and washed thoroughly with cold methanol. No precipitate or turbid solution assignable to the presence of PGMA polymer in solution has been observed, confirming a polymerization mechanism confined to the interlayer spacings of the modified clay.

Fig. 1 Sequential steps for the preparation of Cloisite–PGMA nanocomposites.

3.2. XRD characterization

Fig. 2 shows the XRD patterns of crude Cloisite (CL), Cloisite sample after diazonium cation exchange reaction (CL-INIF), and the associated nanocomposite CL-INIF–PGMA at a CEC fraction of 9. As it can be visualized, CL-INIF sample exhibits a typical intercalated structure characterized by the increase of d-spacing from 1.17 nm, for the unmodified Cloisite to 1.50 nm. In the case of the CL-INIF–PGMA nanocomposite, the d₀₀₁ diffraction peak observed on the XRD patterns is shifted to lower angle, meaning the d-spacing increases up to 1.69 nm after in situ photopolymerization process as a result of the intercalation of polymeric chains between the clay interlayer spaces.

Fig. 2 XRD patterns of Cloisite, CL-INIF and CL-INIF–PGMA nanocomposite at a CEC fraction of 9.

3.3. TEM analysis

TEM micrographs presented in the Fig. 3 show basal spacings between the clay platelets for the unmodified Cloisite sample (Fig. 3a), CL-INIF material (Fig. 3b) and CL-INIF–PGMA nanocomposite (Fig. 3c) prepared at a CEC fraction of 9. In line with the previous XRD measurements, the interlamellar distance increases from an average value of 1.1 nm measured using ImageJ software for the Cloisite to 1.7 nm observed after cation exchange reaction by diazonium salt based photoiniferter, then sharply increases to 2.0 nm upon photopolymerization process. These results indicate the intercalation of diazonium moieties according to an efficient cation exchange mechanism ensuring, in a second step, the intercalation of a polymer component. It is noteworthy that the interlayer distance value determined after nanocomposite preparation remains less than 5 nm indicating a non-exfoliated clay structure.

Fig. 3 TEM micrographs of (a) Cloisite, (b) CL-INIF, and (c) CL-INIF–PGMA at a CEC fraction of 9 (arrows indicate the interlamellar distance between the clay layers).

3.4. TGA characterization

Fig. 4 displays the TGA thermograms of Cloisite, CL-INIF and CL-INIF–PGMA nanocomposite at a CEC fraction of 9. Aryl grafting on the clay nanoplatelets leads to a relatively important mass loss about 20 wt% compared to Cloisite sample (CL). This result indicates an efficient sodium exchange reaction induced by diazonium salt based photoiniferter. After in situ photopolymerization, CL-INIF–PGMA nanocomposite is found to have an organic mass loading of 41 wt%, indicating an efficient photopolymerization process.

Fig. 4 TGA thermograms of Cloisite (CL), CL-INIF and CL-INIF–PGMA nanocomposite at a CEC fraction of 9.

3.5. Infrared spectroscopy characterization

FTIR spectra are shown in Fig. 5 for Cloisite sample, diazonium modified clay materials determined before and after thermal curing at 60 °C, respectively noted CL-INIF before curing, CL-INIF after curing, and nanocomposite formation, CL-INIF–PGMA at a CEC fraction of 9.

Fig. 5 FTIR spectra of Cloisite, CL-INIF at room temperature (RT) before thermal treatment, CL-INIF after thermal treatment at 60 °C, and CL-INIF–PGMA nanocomposite at a CEC fraction of 9.

First of all, after sodium exchange reaction, it is important to note the decrease in the intensity of the –OH bands at 3410 and 1637 cm⁻¹ associated with water molecules within the clay layers. This modification endorses that clay behavior becomes organophilic and no longer hydrophilic. Thus, this result reflects an efficient intercalation of diazonium salt based photoiniferter between the clay nanosheets. In addition, CL-INIF before and after curing FTIR spectra exhibit the presence of two intense absorption bands at 2935 and 2890 cm⁻¹ corresponding respectively to the antisymmetric and symmetric –CH₃, –CH₂ and –CH stretching modes of diazonium salt, which confirms the intercalation of organic moieties. Similarly, CL-INIF FTIR spectra determined before and after thermal curing reveal the apparition of two IR bands at 1550 and 1275 cm⁻¹ assigned respectively to –CN and –C=S vibrational stretching modes, which is in line with the intercalation of dithiocarbamate diazonium species. Finally, CL-INIF FTIR spectrum observed before thermal curing indisputably brings evidence of the diazonium intercalation. Indeed, it is visualized by an IR band at 2200 cm⁻¹ characteristic of –N≡N⁺ functions as for pure diazonium salt powders.⁵¹ It is interesting to notice that this IR band disappears after 2 hours of curing in the oven at 60 °C, resulting from an efficient dediazonation process²⁰ (Fig. 5, CL-INIF after curing) which is likely to yield the grafting of benzyl diethyldithiocarbamate species on the surface of the clay nanosheets.

FTIR spectrum associated to CL-INIF–PGMA sample confirms the strategy used in this study to prepare nanocomposites. In fact, it is observed a sharp absorption at 1715 cm⁻¹ reflecting the C=O stretching vibration of PGMA. In addition, it can be remarked the presence of characteristic alkyl groups from the polymer backbone at 2935 and 2890 cm⁻¹ confirming the success of the polymer grafting on the surface of the modified clay.

3.6. Characterization of the surface chemical composition by XPS technique

X-ray photoelectron spectroscopy (XPS) was used to provide information on chemical bonds/atomic bonding, based on the chemical shifts in their core levels in a certain chemical environment, and on the surface chemical composition of the material. Fig. 6 displays the XPS survey spectra for Cloisite, CL-INIF (after thermal curing) and CL-INIF–PGMA nanocomposite synthesized with a CEC fraction of 9.

Fig. 6 XPS survey spectra of Cloisite, CL-INIF and CL-INIF–PGMA nanocomposite at a CEC fraction of 9.

Fig. 7 displays high resolution C1s peaks from the pristine and the modified clays. The main peaks attributed to the unmodified Cloisite are Si2p, Al2p, O1s, and Na1s centered respectively at 103, 74, 533, and 1072 eV (Fig. 6). Surprisingly, from the Fig. 6, it is observed a small C1s peak centered at 285 eV assigned to an adventitious hydrocarbon contamination. In fact, a noisy C1s signal with a low intensity is observed in the case of pristine Cloisite indicating a surface describing a small hydrocarbon concentration deposit as shown in the Fig. 7a.

Fig. 7 High resolution C1s spectra of (a) Cloisite, (b) CL-INIF and (c) CL-INIF–PGMA at a CEC fraction of 9.

CL-INIF sample obtained after diazonium cation exchange reaction exhibits several additional peaks in comparison with the unmodified clay XPS survey. Indeed, CL-INIF spectrum displays an additional peak at 400 eV which is ascribed to N1s atomic bonding. Similarly, S2p and S2s peaks centered respectively at 165 eV and 229 eV are also detected. Moreover, CL-INIF sample displays a higher relative intensity of the C1s peak centered at 285 eV (Fig. 7b) in comparison with the unmodified Cloisite C1s signal (Fig. 7a). In view of the C1s spectra associated to CL-INIF material, XPS signal can be fitted with four components centered at 285, 286, 288 and 290 eV assigned respectively to C=C/C–C/C–H,⁵² C–N/C–S,^51–53 C=N⁵³ and π–π* shape-up satellite characteristic of aromatic species.^53,54 These modifications can be ascribed to an efficient intercalation of benzyl diethyldithiocarbamate moieties on the aluminosilicate nanolayers, following the diazonium salt dediazonation process. Furthermore, as seen in the CL-INIF XPS survey diazonium cation exchange reaction induces the disappearance of Na1s signal (1072 eV) and its corresponding Auger peak (Na KLL) at 497 eV (Fig. 6). These results account for a complete cation exchange reaction leading to a total replacement of Na⁺ species from the soda clay by diazonium cations.

Clay–polymer nanocomposite XPS spectrum prepared following photopolymerization process shows sharp C1s feature and almost disappearance of both Si2p and Al2p peaks from the aluminosilicate nanosheets as visualized in the Fig. 7c. This result brings strong supporting evidence for the intercalation and wrapping of the clay by PGMA grafts.

The high resolution C1s region from CL-INIF–PGMA nanocomposite can be fitted with three components centered at 285, 286.5 and 289 eV, assigned to C–H/C–C, C–O and O–C=O contributions respectively. It is interesting to note that the decomposed peak areas of these three components C–H/C–C//C–O//O–C=O are in the ratio of 3.5/3.2/1 matching 3/3/1 expected for pure PGMA polymer.⁵⁵

3.7. Effect of diazonium salt concentration on bulk and surface chemical compositions of the modified clay

The effect of diazonium salt concentration, expressed in CEC fraction, on the extent of the cation exchange reaction was monitored by XPS and TGA experiments.

(a) XPS analysis. Fig. 8 compares the C/Si and O=C–O/Si ratios determined by XPS analysis, through cation exchange reaction induced by diazonium salt based photoiniferter and photopolymerization process involving different CEC fractions (i.e. 1, 2, 5 and 9).

Fig. 8 Histogram relating C/Si (blue diagonal line pattern) and O–C=O/Si (black squared pattern) atomic ratios to diazonium salt CEC fractions.

As it can be visualized in the Fig. 8, that increasing the CEC part allows to a gradual increase in the C/Si atomic ratio (blue diagonal, line pattern, Fig. 8). Actually, the modified clay treated with a CEC fraction of 9 induces a 4 fold diazonium loading compared to clay cationically exchanged at an initial CEC fraction of 1. For CEC fraction of 9, we estimated an average degree of polymerization (DP) following the approach reported by Matrab et al.³⁹ poly(n-butyl methacrylate) chains grafted to iron by ATRP. Instead of bromine as an elemental marker for ATRP initiator, here we use sulfur as the elemental marker for the iniferter: DP = C_O–C=O/(S/2) atomic ratio where C_O–C=O is the carbon atom in the ester repeat units of PGMA. We divided the sulfur content by 2 because there are 2 sulfur atoms per iniferter. The DP was estimated to be 20 for CEC fraction of 9.

After having considered the influence of the CEC fraction on the performance of cation exchange process, the impact of the diazonium salt concentration on the nanocomposite composition is investigated.

Thereby, Fig. 8 presents the O–C=O (ester groups from the PGMA backbone)/Si fractions associated to the synthesized nanocomposites prepared with different CEC ratios of diazonium salt. As it can be observed, the CEC parameter induces an enhancement of the O–C=O/Si ratio (Fig. 8). A gradual enhancement of the O–C=O/Si fraction is visualized in consequence of the CEC increasing from 1 to 9. This result can be ascribed to an improvement of the photoiniferter grafting on the clay surface during the diazonium cation exchange process upon increase of the diazonium initial concentration (expressed in CEC fraction). Accordingly, raising the CEC fraction leads to highly grafted clay with photoiniferter moieties capable to initiate efficiently the photopolymerization of GMA monomer.

(b) TGA analysis. TGA analyses performed on the clay samples modified by diazonium cation exchange process, namely CL-INIF and the associated nanocomposites, CL-INIF–PGMA synthesized at different CEC ratios confirm the previous results obtained by XPS experiments. Fig. 9 depicts the weight loss determined at 800 °C attributed to CL-INIF and CL-INIF–PGMA materials prepared at several CEC fractions. It is observed that increasing the CEC ratio in the case of CL-INIF samples leads to a weight loss enhancement. The modified clay prepared at a CEC ratio of 1 exhibits a weight loss of 15 wt%, while in the same conditions, sample prepared at a CEC of 9 provides a weight loss of 20 wt%. This result is interpreted by an improvement of the aryl grafting between the clay nanoplatelets, after the diazonium cation exchange reaction, progressing with the increasing of the CEC.

Fig. 9 Final weight loss observed at 800 °C determined from the TGA thermograms of CL-INIF (black squares) and CL-INIF–PGMA materials (red rings) synthesized at different CEC ratios. PGMA component included in the nanocomposite material developed with the different INIF concentrations is shown in the inset.

Similarly, due to the introduction of an organic polymeric component between the clay interlayer spaces, nanocomposite samples CL-INIF–PGMA yields higher weight loss than the corresponding modified clay CL-INIF. It can be also noticed that the mass loss progresses with the increase in the CEC fraction. Indeed, the lowest weight loss value (20 wt%, Fig. 9) is observed for the nanocomposite prepared with a CEC ratio of 1, while the highest mass loss (41 wt%, Fig. 9) is visualized for the material synthesized with a CEC ratio of 9. This result is interpreted in terms of enhancement of the photopolymerization performances induced by higher density of photoiniferter grafting on the clay lamellae. This result is illustrated in the inset of Fig. 9, which displays the PGMA component included in the nanocomposite material elaborated with several diazonium salt based iniferter concentrations (INIF).

Elsewhere, Kongkaew et al.⁵⁶ have investigated the effects of reaction conditions on methyl methacrylate (MMA) monomer polymerized by living free radical polymerization using iniferter species. They concluded that to achieve low-polydispersity PMMA from living free radical polymerization through the use of the benzyl diethyl dithiocarbamate (BDC) iniferter, the polymerization should be carried out under the following conditions: high BDC/MMA molar ratio (typically, 0.02–0.03). Herein, the ratio of grafted INIF/GMA is in the 0.009–0.037 range overlapping that employed by Kongkaew et al.⁵⁶ Nevertheless, XPS analysis permits to determine the sulfur content (in atomic%) from the iniferter (C₁₂S₂NH₁₆) used to modify the clay through cation exchange reaction. Under the same conditions as those defined in the work of Kongkaew et al.,⁵⁶ we have determined that a ratio of Cloisite-INIF/GMA around 2.5 is necessary to provide an INIF/GMA molar ratio in the range between 0.009 to 0.037 (see Table 1).

Table 1 Determination of INIF/GMA molar ratio by XPS analysis

CEC fraction	Atomic% of sulfur	Atomic% of iniferter C12S2NH16	INIF/GMA molar ratio
1	1.06	4	0.009
2	2.11	8	0.019
5	3.63	14	0.031
9	4.40	16	0.037

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From the above, it is thus clear that iniferter participates to the reactions and the aryl groups grafted to the clay initiate indeed the polymerization process.

So far, we have considered XPS and TGA results; both can actually be correlated as surface composition versus bulk composition of the nanocomposites. Fig. 10 depicts the simplest XPS-vs.-TGA plot for the nanocomposites in the sense it correlates surface C/Si atomic ratio (determined by XPS) and the total mass loss (determined by TGA). The choice of C/Si for XPS measurement lies in the fact that C is an elemental marker for organic materials while silicon is representative of clay. Fig. 10 shows that the surface and bulk compositions correlate very well.

Fig. 10 Surface C/Si atomic ratio (determined by XPS) versus total organic mass loading (determined by TGA experiments) for CL-INIF–PGMA materials at indicated CEC fraction F1, F2, F5 and F9.

4. Conclusion

Clay–PGMA nanocomposites were successfully synthesized by living free radical photopolymerization initiated by silicate-anchored photoiniferter. Clay was modified by cation exchange reaction of sodium by diethyldithiocarbamate benzyldiazonium tetra-fluoroborate salt which intercalated the silicate layers and reacted on the surface of the silica platelets within the interlayer spacings. This modified clay can act as an efficient macro-photoinitiator applied to the polymerization of glycidyl methacrylate as confirmed by the different characterization techniques employed (XPS, FTIR, XRD, TEM and TGA).

It has been shown that the concentration of diazonium salt used through the cation exchange process plays a paramount role during the nanocomposite preparation. Indeed, a nanocomposite material with a high organic content (∼41 wt%, as determined by TGA analysis) was obtained from the synthesis using the highest CEC fraction due to the formation of a high density of photoiniferter grafting surface.

As stated above, on the one hand, living free radical photopolymerization in the presence of clay-bearing photoiniferter groups grafted by diazonium cation exchange mechanism constitutes an unusual, but very efficient route for making clay–polymer nanocomposites. On the other hand, it adds up a new and interesting brick in the surface and interface chemistry of aryl diazonium salts of relevance to polymer science and engineering.

Acknowledgements

The authors would like to thank the Labex MMCD (Multi-Scale Modelling & Experimentation of Materials for Sustainable Construction) from Paris-Est University (France) for financial support (ANR-11-LABX-022-01).

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Footnote

† Actual address: Univ Paris-Sud, ICMMO, UMR 8182, Bât. 420, 15 rue Georges Clemenceau, 91405 Orsay, France.

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