Photo-induced SI-ATRP for the synthesis of photoclickable intercalated clay nanofillers

Abstract

In situ photoinduced SI-ATRP is reported as a novel and efficient route for preparing intercalated nano-clay fillers bearing clickable functions. Poly(propargyl methacrylate) chains are grown inside the clay interlayer after silanisation and grafting of bromine ATRP initiator. The generic clickable character is demonstrated through grafting with azidomethyl benzene and mercaptosuccinic acid via photodriven 1,3-dipolar cycloaddition and thiol-yne click reactions, respectively.

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Clay nanoparticles due to their high aspect ratio, great surface area, impermeability, superior mechanical and thermal properties, low-cost and availability,^1–4 have attracted significant academic and industrial attention for potential applications in many areas such as sensing,^5,6 heterogeneous catalysis,⁷ pollutants removal,⁸ energy storage,⁹ drug delivery,^10,11 as well as reinforcement of nanocomposites.^12–17 Despite their attractive properties, clay nanoparticles suffer from incompatibility, hence weak interfacial interactions and poor dispersion with/in most organic materials because of their intrinsic hydrophilicity and strong interlayer interactions.^18,19 This is detrimental to the preparation of clay-based nanocomposites with exfoliated morphology but can be overcome by functionalizing the clay surface with organic modifiers, typically polymers, via either blending or in situ polymerization.²⁰ The latter approach is compatible with both aqueous and organic solvents, tolerates a wide range of monomers and functional groups and provides densely grafted polymer brushes as well as a uniform dispersion of clay layers within polymeric matrices.^21–24 The group of Singha prepared tailor-made poly(2-ethylhexyl acrylate) clay nanocomposites through thermally triggered living radical polymerisation using copper(I) halide, as catalyst, and N,N,N′,N′′,N′′-pentamethyl diethylenediamine, as ligand.²⁵ The initiator/ligand pair was simply added to the polymerization mixture containing organically modified clay providing high degree of intercalation.

Among the various in situ polymerization routes, the surface-initiated variant of controlled radical polymerization (SI-CRP) has been successfully applied as a versatile and promising approach in designing polymer brushes with narrow molecular weight distributions, predetermined molecular weights and controlled architecture and functionality. Growing polymer chains from nanoparticles via SI-CRP strategy permits tailoring both chemical and physical properties of a wide variety of nanoparticles.^26–31 SI-CRP enables notably the control polymer length via variation of monomer conversion affording, in the case of clay nanoparticles, tunable increase in the interlayer spacing over polymerization time. Using this approach for the fabrication of clay/polymer nanocomposites implies firstly the surface functionalization of silicate layers with polymerization initiator by either physical adsorption or chemical attachment, and then, in situ growing of the polymer chains from these macro-initiators.

To date, polymerization processes have been mainly initiated by using heating or chemical external stimulation. Montmorillonite clay layers have been effectively coated by homopolymers and block polymers via various thermal CRP methods, such as atom transfer radical polymerization,³² reversible addition fragmentation chain transfer polymerization, and nitroxide-mediated polymerization, using quaternary ammonium initiators.³³ In the cited examples, initiators have been incorporated inside the clay galleries by interlayer cations exchange. Thermal-assisted CRP initiated from silanised clay surface bearing initiating end-units has been demonstrated to be effective in increasing the basal plane spacing.³⁴ In other implementations, activated hydroxy groups on the clay surface were chemically modified with bromine-ATRP initiator, RAFT transfer agent, and NMP nitroxide terminating agent through esterification reaction and subsequent initiation of the polymerization process yielded intercalated or/and exfoliated layered silicates.³⁵ In other implementations, Singha et al. described the esterification of hydroxylated montmorillonite (Cloisite 30B) to graft ATRP initiator.^36,37 Herein, we opted for a three-step approach, involving acid activation of the clay prior to silanization with an aminosilane and subsequent grafting of the ARTP initiator via amidification reaction. Such acid treated clay exhibits increased density of surface silanol groups providing grafting of larger amount of organosilane as compared to native clay.³⁸ Silane grafting induces an increase in the basal spacing and a change in montmorillonite characteristics from hydrophilicity to lipophilicity.³⁹ Thus, the presence of an aliphatic segment arising from the 3-aminopropyl trimethoxysilane is expected to induce interaction with the propargyl methacrylate monomer favouring its incorporation within the interlayer galleries and thus the growth of the polymer chains. Only a few reports addressed the implementation of photochemically-driven strategies for such a purpose. Physically adsorbed quaternary ammonium salts based on dimethylaniline served as type II photo initiators in tandem with benzophenone (UV-light) or camphorquinone (visible light) for the in situ free radical polymerization of methyl methacrylate providing exfoliated nanocomposites as ascertained by the disappearance of the XRD peak.⁴⁰ As for thermal methods, photo-induced polymerization enables preparing polymers and copolymers with well-controlled molecular weight and topology from a large range of monomers and functional groups with the additional advantages of fast polymerization rates, easy process, and low temperature.

To the best of our knowledge there is no report on combination of photochemical strategy with surface initiated atom transfer radical polymerization (SI-ATRP) method to design polymer grafts on clay platelets bearing surface-grafted initiator, hence the motivation for this study. Considering the previous report by Yagci's research group on preparation of well-defined bulk polymer chains through photochemical generation of copper(I) complex from copper(II) species without using any reducing agent,²² we expanded this attractive strategy to the area of the interfacial chemistry of clay.

Herein, we discuss the unprecedented room temperature preparation of azide- and thiol-sensitive clay/polymer intercalated nanofillers via photoinduced SI-ATRP. By combining this route with the surface grafting of ATRP initiator, we have successfully confined the growth of poly(propargyl methacrylate) chains from silicate layers for producing clickable and intercalated organoclay bearing alkynyl pendant groups. We also report here our preliminary results on click functionalization of the clay nanoparticles taking advantage of the alkyne-decorated polymer grafts as anchoring interface for azido- or thiol-containing agents via azide–alkyne cycloaddition and thiol-yne radical addition click reactions, respectively.^41–43 Both click reaction were performed at room temperature under photodriven irradiation conditions.

To establish the method, montmorillonite clay (MMT-Na) was firstly activated with diluted HCl 0.1 M solution in order to increase the surface density of hydroxyl groups and to improve the surface area as previously reported by He et al.⁴⁴ The activated clay (MMT-OH) was then functionalized by 3-aminopropyl trimethoxysilane by reaction of the trimethoxysilane moieties with OH groups on clay surface; the as-obtained MMT-NH₂ was then grafted with bromoisobutyryl bromide via amide bonds to prepare ATRP macroinitiator (MMT-Br). Finally, polymerization of propargyl methacrylate (PGM) from the bromine-clay was carried out at room temperature for 2 h under stirring using a UV irradiation at 365 nm in presence of CuBr/PMDETA complex as catalyst and 2,2-dimethoxy-2-phenylacetophenone (DMPA) as photoinitiator yielding clickable clay nanofillers (MMT-PPGM). The polymerization time was optimized on the basis of qualitative and macroscopic optical observations of the dispersion stability of polymer-grafted clays in common organic solvent. As a representative example, Fig. SI1† shows DMF solutions as observed for native MMT-Na (a) and MMT-PPGM (b and c) after standing for a period of 24 h. It is seen that native MMT is insoluble and sediment in DMF while increasing the polymerization time from 1 h (b) to 2 h (c) improves significantly the dispersion behaviour of the polymer-grafted clays. To fully evidence the reactivity of the alkynyl pendant groups, MMT-PPGM were allowed to react with mercaptosuccinic acid and azidomethyl benzene under UV-irradiation in presence of a catalytic amount of photoinitiator. The schematic procedures for making MMT-PPGM and their subsequent photoclick functionalization are is shown in Fig. 1 and SI2,† respectively. Experimental details are also available in the ESI.†

Fig. 1 Schematic representation of the preparation of poly(propargyl methacrylate)-grafted clay (MMT-PPGM) via (i) acidic activation and (ii) silanisation of clay surface, (iii) surface-grafting of ATRP initiator and (iv) photoinduced surface initiated atom transfer radical polymerization. The clay modification is illustrated on one clay layer for clarity purpose. Although the scheme is not to scale, the increase in the interlayer distance is shown following successive chemical modification steps.

Chemical structures of the neat and modified clays were analyzed by infra-red spectroscopy (FT-IR) as shown in Fig. 2. For comparison purposes, all spectra were normalized to the area under the Si–O vibration band (at about 1000 cm⁻¹). It can be observed for all samples, a weak peak at 3620 cm⁻¹ corresponding to the inner OH stretching vibration of Si–OH and Al–OH in the smectite type clays, a strong peak centred at 1000 cm⁻¹ assigned to the Si–O band, and a deformation band at 1647 cm⁻¹ indicating the presence of adsorbed water. Expanded spectrum between 3500 and 3900 cm⁻¹ (noted A in Fig. 2) is shown in order to better highlight the effect of the mild acid treatment on the clay surface. The intensity increase of the band at 3620 cm⁻¹ corresponding to hydroxyl groups suggests increase in the OH density, demonstrating thus success of the clay surface activation. After silanisation reaction, two weak bands appeared at 2930 and 2867 cm⁻¹ corresponding to the asymmetric and symmetric stretching of CH₂ groups, respectively.^45–47 The two large bands centred at 3300 and 3367 cm⁻¹ (expanded spectra noted B) can be assigned to the vibrations of NH₂ from the organosilane. It is worth noting that the silanisation step is accompanied by a slight intensity decrease in OH stretching vibrations at 3620 cm⁻¹, thus suggesting the consumption of the silanol groups via grafting of silane molecules.^45–47

Fig. 2 FTIR spectra of (a) pristine clay (MMT-Na), (b) activate clay (MMT-OH), (c) silanised clay (5MMT-NH₂) (d) ATRP initiator-grafted clay (MMT-Br) and (e) poly(propargyl methacrylate)/clay nanocomposite (MMT-PPGM).

Initiator grafting is evidenced by the apparition of characteristic signals of C=O stretching vibration at 1630, 1680 and 1712 cm⁻¹, assigned to the amide groups. In addition, appearance of new peak at 1530 cm⁻¹ typical for the NH of O=C–NH clearly indicates the successful reaction between the amine groups and bromoisobutyryl bromide. After the polymerization reaction, one can observe the increase of the intensity of the carbonyl stretching vibration due to the presence of C=O ester groups of poly(propargyl methacrylate) accompanied with a shift from 1712 to 1732 cm⁻¹. Success of the chain growth in the clay intergalleries is further confirmed by the apparition of a new peak at 3293 cm⁻¹ corresponding to the stretching vibrations of terminal alkyne (≡C–H). From these results, one can state that the poly(propargyl methacrylate) chains are well grown from the clay surface leading to clickable nanofillers.

Thermogravimetric analysis (TGA) has been further used to gather quantitative information about surface functionalization. Clearly, degradation of organic content was observed in the temperature range from 100 to 800 °C, indicating the successful grafting of organic molecules (Fig. 3). The decrease in mass loss at ≈100 °C attributed to the amount of physically adsorbed water as observed upon successive functionalization steps reveals conversion of clay surface from hydrophilic character to hydrophobic one.

Fig. 3 TGA curves of (a) pristine clay (MMT-Na), (b) activate clay (MMT-OH), (c) silanised clay (MMT-NH₂) (d) ATRP initiator-grafted clay (MMT-Br) and (e) poly(propargyl methacrylate)/clay nanocomposite (MMT-PPGM).

In addition, one can note that the onset temperature of neat clay is remarkably decreased after surface clay modification from approximately 600 °C for MMT-Na to approximately 350 °C for MMT-NH₂ and 300 °C for both MMT-Br and MMT-PPGM. This finding indicates reduced thermal stability providing further evidence for the grafting of organic moieties at the inorganic clay surface. Weight losses of 12.5, 11.9, 19.3, 27.5 and 37.3% are observed for MMT-Na, MMT-OH, MMT-NH₂, MMT-Br and MMT-PPGM, respectively. Such a continuous increase in the weight loss confirms the grafting of organic moieties at each step.⁴⁸

As well known, the interlayer spacing plays a decisive role in the compatibility and dispersion ability of organo-modified clays with organic materials. To access this parameter, X-ray diffraction (XRD) measurements were conducted on all clay samples and the interlayer d-spacing was calculated according to Bragg equation (Fig. 4). Pristine montmorillonite (MMT-Na) exhibits a strong diffraction peak at 2θ = 7.58°, corresponding to the (001) basal reflection of the silicate layers and giving a d-spacing of 1.16 nm which is in well concordance with the value given by Southern Clay Products (1.17 nm). The d₀₀₁ peak was shifted to about 4.93° after silanisation and 4.12° after bromination. This suggests a clear increase in the interlayer spacing from 1.16 nm for the MMT-Na clay to 1.79 nm for the MMT-NH₂ and 2.13 nm for the MMT-Br.

Fig. 4 (a) XRD patterns of (a) pristine clay (MMT-Na), (b) silanised clay (MMT-NH₂), (c) macroinitiator clay with surface grafted ATRP initiator (MMT-Br) and (d) poly(propargyl methacrylate)/clay nanofiller (MMT-PPGM). (b) Plot of the variation of the interlayer spacing as a function of the mass loss variation. The linear dependence suggests preservation of the layered structure. Pristine clay was considered as reference, and the plot is representative of MMT-NH₂, MMT-Br and MMT-PPGM.

It should be underlined that the d-spacing value obtained after silanisation step is comparable to the one given by Piscitelli et al.⁴⁵ fitting with a bilayer organization of aminosilanes within the clay interlayer. Moreover, the absence of the d₀₀₁ peak after polymerization in the studied angle region (2θ from 4 to 10°), may be explained by a shift towards a lower diffraction angle (2θ < 4° for MMT-PPGM). In addition, one can note the apparition of new diffraction peak for all modified clays, which can be attributed to the (002) plane. A shift is also observed for the 002 peaks, from 2θ = 9.66° for MMT-NH₂ to 2θ = 8.01° for MMT-Br. This behavior is to be correlated to the shift in the d₀₀₁ peak as discussed here above and corresponds to the incorporation of ATRP initiator between aluminosilicate layers.

As expected for layered structures, all d₀₀₂ diffraction peaks appeared at positions corresponding to about twice the 2θ values of the corresponding d₀₀₁ peaks, so intuitively position of the d₀₀₁ peak of MMT-PMMG may be estimated to appear at 2θ = 2.96° corresponding to a d-spacing of 3 nm. This finding suggests growing of the polymer chains from clay surface with conservation of the parallel layers organization which is in full line with our aim to preserve the intercalated structure for the polymer-grafted nanofillers. This XRD-calculated d-spacing agrees well with morphological investigations performed by electron microscopy.

Fig. 5 shows the TEM micrographs together with SEM images of (a) MMT-Na and (b) MMT-PPGM. The hydrophilic pristine clay forms agglomerates, with stacks of multiple parallel layers. The basal spacing of MMT-Na is around 1.1 nm, which corresponds well with the value measured by XRD. For the polymer-grafted clay, the flakes became smaller and their surface is much smoother due to wrapping with PPGM. The average interlayer space increased from 1.1 ± 0.1 nm for MMT-Na to 3 ± 0.2 nm for MMT-PPGM. In addition, the TEM image of MMT-PPGM shows clearly the individualisation of the layers through an increase in the interlayer distance while maintaining their parallel organisation. This behaviour is in good agreement with the tendency shown in Fig. 4b. In this figure, change in d-spacing is plotted versus variation in weight loss, considering the values characteristic for pristine clay as references. A direct proportionality is nearly observed confirming preservation of the intercalated morphological regime. Moreover, the increase in the d-spacing is nearly strictly governed by the amount of organics incorporated between the clay layers. Due to clickable character and intercalated morphology of the as-prepared clay–poly(propargyl methacrylate) nanofillers, the interlayer spacing may be further increased by coupling of azide- or thiol-functionalized (macro)molecules and thus favouring random dispersion of silicate layers.^49,50

Fig. 5 TEM images of (a) pristine clay and (b) poly(propargyl methacrylate)/clay nanocomposite. The insets show SEM micrographs for the corresponding samples.

Surface reactivity of the as-synthesised clickable clay nanohybrids was evaluated towards copper-catalysed Huisgen cycloaddition (CuAAC) and thiol-yne radical addition (Fig. SI2†). Both reactions were performed under UV irradiation conditions with model molecules and the resulting functional clay nanofillers, named MMT-COOH and MMT-Phe in Fig. SI2,† were analysed by infrared spectroscopy (Fig. SI3†). Success of both the thiol-yne (Fig. SI3a†) and CuAAC cycloaddition (Fig. SI3b†) click reactions is firstly demonstrated by the complete disappearance of the terminal alkyne characteristic absorption peaks detected at 3293 cm⁻¹ in the MMT-PPGM spectrum (Fig. 2c). Moreover, the apparition of a broad band centered at 3250 cm⁻¹ together with the intensity increase of the peak at 1720 cm⁻¹ assigned to the C–OH and C=O stretching vibrations in carboxylic acid functions are clearly seen after thiol-yne coupling of mercaptosuccinic acid molecules. The CuAA cycloaddition between MMT-PPGM and azidomethyl benzene was further revealed by the appearance of C=C stretching typical for aromatic ring at 1227 and 1497 cm⁻¹, N=N vibration band at 1455 cm⁻¹, as well as the triazole breathing mode at 1129 and 1142 cm⁻¹.⁵¹ In addition, one can notice the absence of peaks at 2555 and 2089 cm⁻¹ attributed to thiol and azide antisymmetric stretch from mercaptosuccinic acid and azidomethyl benzene respectively, indicating that the clay nanocomposite functionalization is achieved via covalent click coupling.⁴² The so-designed functional nanoclays, with robust and hydrolytically stable anchoring of the polymer grafts on the clay surface may be used as adsorbents for the removal of metal ions and aromatic pollutants from wastewater.⁵²

Conclusion

In summary, photoinduced SI-ATRP approach has proved effective for producing intercalated clickable clay nanofillers. Through this strategy, clay surface was firstly functionalized with amino silane coupling agent, followed by grafting of bromine ATRP-initiator, and finally growth of poly(propargyl methacrylate) chains has been UV-initiated from the silicate layers in presence of free photoinitiator. The as-obtained clickable alkynyl nanohybrids were readily coupled with azidomethyl benzene and mercaptosuccinic acid small molecules through photo-initiated Huisgen cycloaddition and thiol-yne click reactions, respectively. These results revealed that the strategy of photoinduced SI-ATRP easily provides generic clay-based nanoplatforms for the preparation of a variety of new multifunctional nanomaterials.

Acknowledgements

This work has benefited from a French government grant managed by ANR within the frame of the national program of Investments for the Future ANR-11-LABX-022-01 (LabEx MMCD project).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details section and pristine clay XRD pattern. See DOI: 10.1039/c6ra14724k

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