Victor H R Souza
email: souzavhr@gmail.com

Emmanuel Flahaut
email: flahaut@chimie.ups-tlse.fr

Aldo J G Zarbin
email: aldozarbin@ufpr.br

Conducting, transparent and flexible substrates obtained from interfacial thin films of double-walled carbon nanotubes

Keywords:

Introduction

The development of conducting and transparent substrates (CTS's) has received a huge attention from researchers since this material plays important role as a component on the manufacturing of optoelectronic devices, such as organic light emitting diodes, organic solar cells, touch panels, and so on [START_REF] Du | 25th anniversary article: carbon nanotube-and graphene-based transparent conductive films for optoelectronic devices[END_REF]. Low electrical resistance and high optical transmittance are desirable properties for materials intended to be applied in this field.

Tin-doped indium oxide (ITO) is with no doubt the most common substrate applied as CTS due to its outstanding optical (around 90 % of transparency) and electrical (sheet resistance ranging 10-25 X h À1 ) properties [START_REF] Du | 25th anniversary article: carbon nanotube-and graphene-based transparent conductive films for optoelectronic devices[END_REF][START_REF] Edwards | Basic materials physics of transparent conducting oxides[END_REF]. Moreover, the expertise in control of doping level and thickness of ITO make this substrate even more attractive and marketplace unanimously for CTS applications [START_REF] Kumar | The race to replace tin-doped indium oxide: which material will win?[END_REF]. However, the development of new technologies which employ CTS's claims for materials with properties beyond the high transparency and low electrical resistance like flexibility, weightlessness, low cost and easy processability. ITO presents several drawbacks to achieve these previous requirements: it easily cracks under bending, which prevents further application on flexible substrates; indium resources are becoming even scarcer and the market for CTS's is markedly expanding; high temperature and vacuum procedures are necessary for ITO deposition, which also increase the process cost and makes harder the deposition over plastic substrates [START_REF] Du | 25th anniversary article: carbon nanotube-and graphene-based transparent conductive films for optoelectronic devices[END_REF][START_REF] Minami | Present status of transparent conducting oxide thin-film development for Indium-Tin-Oxide (ITO) substitutes[END_REF][START_REF] Wang | Fabrication of highly conducting and transparent graphene films[END_REF][START_REF] Morag | Bottom-up" transparent electrodes[END_REF]. In order to overcome the drawbacks associated to ITO, several materials have been developed as an alternative for further application as CTS's, such as carbon nanomaterials (carbon nanotubes and graphene), metal gratings and random metallic nanowires networks [START_REF] Du | 25th anniversary article: carbon nanotube-and graphene-based transparent conductive films for optoelectronic devices[END_REF][START_REF] Kumar | The race to replace tin-doped indium oxide: which material will win?[END_REF].

In which concerns the use of carbon nanostructures as CTS's, carbon nanotubes are very promising materials as an alternative for ITO, due to the high electrical conductivity and high mechanical strength, which allows further applications of carbon nanotubes films in flexible devices [START_REF] Zhang | Transparent, conductive, and flexible carbon nanotube films and their application in organic light-emitting diodes[END_REF]. Conducting and transparent films of single-walled carbon nanotubes (SWCNT) have been developed with sheet resistance ranging on 30-500 X h À1 and transmittance at 550 nm achieving 90% [START_REF] Wu | Transparent, conductive carbon nanotube films[END_REF][START_REF] Dan | Continuous and scalable fabrication of transparent conducting carbon nanotube films[END_REF][START_REF] Geng | Effect of acid treatment on carbon nanotube-based flexible transparent conducting films[END_REF]. Double-walled carbon nanotubes (DWCNT) are composed of two concentric tubes and can be considered as a more promising material for CTS's compared to SWCNT due to the lower junction resistance as a result of the larger tube diameter while keeping the good conductivity [START_REF] Hou | Double-wall carbon nanotube transparent conductive films with excellent performance[END_REF]. Few works have been reported preparing CTS's from DWCNT, which achieved good values of sheet resistance and transparency [START_REF] Hou | Double-wall carbon nanotube transparent conductive films with excellent performance[END_REF][START_REF] Park | Fully organic ITO replacement through acid doping of double-walled carbon nanotube thin film assemblies[END_REF][START_REF] Green | Processing and properties of highly enriched doublewall carbon nanotubes[END_REF].

The processability of the active material as thin and transferable films is one of the most important steps for CTS's production. Several approaches have been reported for deposition of carbon nanotubes as thin films including filtration transfer, air-brushing, dip coating, drop casting, and so on [START_REF] Wu | Transparent, conductive carbon nanotube films[END_REF][START_REF] Dan | Continuous and scalable fabrication of transparent conducting carbon nanotube films[END_REF][START_REF] Hu | Carbon nanotube thin films: fabrication, properties, and applications[END_REF][START_REF] Béduer | Elucidation of the role of carbon nanotube patterns on the development of cultured neuronal cells[END_REF]. Nevertheless, several drawbacks associated to the carbon nanotubes deposition as non-homogeneity and post multi-steps are usually required, preventing the thin and transparent film to be deposit directly over the desired substrate.

The production of thin, transparent and freestanding films assembled at the interface of two immiscible liquids in a single step has been an outstanding method to prepare films with high homogeneity, controllable thickness and with a variety of applications [START_REF] Salvatierra | ITO-free and flexible organic photovoltaic device based on high transparent and conductive polyaniline/carbon nanotube thin films[END_REF][START_REF] De Souza | Thin and flexible all-solid supercapacitor prepared from novel single wall carbon nanotubes/polyaniline thin films obtained in liquid-liquid interfaces[END_REF][START_REF] Mehl | Thin and transparent films of graphene/ silver nanoparticles obtained at liquid-liquid interfaces: Preparation, characterization and application as SERS substrates[END_REF]. The interfacial route is a very simple way to prepare thin films in a single step, under environment conditions and easily transferable to different kind of ordinary substrates, including plastics [START_REF] Salvatierra | ITO-free and flexible organic photovoltaic device based on high transparent and conductive polyaniline/carbon nanotube thin films[END_REF][START_REF] De Souza | Thin and flexible all-solid supercapacitor prepared from novel single wall carbon nanotubes/polyaniline thin films obtained in liquid-liquid interfaces[END_REF]. CTS's from different carbon nanostructures assembled at the liquid-liquid interface have been prepared, where sheet resistance ranging 1.7-50 kX h À1 and transmittance up to 85% at 550 nm were achieved [START_REF] Souza | Flexible, transparent and thin films of carbon nanomaterials as electrodes for electrochemical applications[END_REF]. Films were also deposited over flexible substrates (PET) and high stability of sheet resistance was achieved even after several bending cycles.

In the presented study, thin and transparent films of DWCNT processed using different treatments have been prepared through the interfacial method for further application as CTS's. The values of sheet resistance and transparency have been correlated to the morphology of the films and the structure of the DWCNT. The relation between the purification treatments, the amount of DWCNT and the annealing process of the thin films has been studied. The interfacial thin films were also deposited over flexible substrates to enable further application on flexible optoelectronic devices.

Experimental

DWCNT's synthesis and all post-purifying procedures applied to carbon nanotubes were previously described elsewhere [START_REF] Flahaut | Gram-scale CCVD synthesis of double-walled carbon nanotubes[END_REF][START_REF] Bortolamiol | Double-walled carbon nanotubes: quantitative purification assessment, balance between purification and degradation and solution filling as an evidence of opening[END_REF]. Interfacial thin films of DWCNT's were produced following similar method already reported for SWCNT's and graphene [START_REF] Souza | Flexible, transparent and thin films of carbon nanomaterials as electrodes for electrochemical applications[END_REF][START_REF] Salvatierra | One-pot synthesis and processing of transparent, conducting, and freestanding carbon nanotubes/polyaniline composite films[END_REF]. It is noteworthy that the DWCNT's dispersions were prepared using two different solvents (deionized water or chloroform) depending on the purification treatment applied to the carbon nanotubes. Samples were named after the purifying treatment adopted.

DWCNT's purifying treatment

Briefly, all the samples investigated in this work were prepared from the same batch of raw DWCNT. Air/HCl treatment consisted in heating the sample for 30 min in air at 500 °C and washing it with aqueous solution of HCl 12 mol L À1 overnight at room temperature. HNO 3 oxidation was performed at 130 °C for 24 h in 3 mol L À1 of nitric acid followed by washing with either only water (H 2 O), or water followed by sodium hydroxide aqueous solution (4 mol L À1 ) at 130 °C for 24 h (NaOH). HNO 3 /H 2 SO 4 processing consisted in refluxing the DWCNT in a HNO 3 /H 2 SO 4 (1:3) aqueous solution at 130 °C for 5 h, followed by washing with either water (H 2 O), or water/sodium hydroxide aqueous solution as described before.

DWCNT's interfacial thin films

Table 1 summarizes the solvent and conditions used to prepare the DWCNT's dispersion.

The steps adopted to produce interfacial thin films are described as follow: a given amount of DWCNT was added into a beaker containing 32 mL of solvent (Table 1). The mixture was maintained under sonication using an ultrasonic probe system (Cole Palmer ultrasonic processor with 40% of amplitude and interval time of 59 00 on and 10 00 off), to obtain a stable dispersion. The DWCNT's dispersion with no visible flocculate was transferred into a round bottom flask containing 20 mL of the solvent to form the interfacial system (chloroform in the case of DWCNT dispersion in deionized water or deionized water in the case of DWCNT dispersion in chloroform). The interfacial system was maintained under magnetic stirring at 2500 rpm for 3 h. A thin and transparent film could be noticed, assembled at the liquid-liquid interface when the magnetic stirring was interrupted. Chloroform was removed and exchanged by toluene to keep the organic solvent on the top. The interfacial thin film was transferred with the aid of a pipette to a beaker containing the interfacial system (toluene/deionized water) and the substrates assembled on the bottom of the flask. The interfacial thin films were deposited on different substrates, removed from the bottom of the beaker with the aid of tweezers. Films were dried at 70 °C for 1 h in air. The schematic representation of all steps involved in the experimental procedure is shown in Fig. 1. The transparency and optical quality of the films obtained can be seen in the photograph image also presented in the middle of Fig. 1. DWCNT-Air/HCl and DWCNT-HNO 3 /H 2 SO 4 (H 2 O) films were also prepared using three different initial amounts of carbon nanotubes (0.05, 0.1 and 0.2 mg). For all other samples, films were prepared only using 0.2 mg of DWCNT.

DWCNT's thermal annealing

Thermal annealing of DWCNT-Air/HCl and DWCNT-HNO 3 / H 2 SO 4 (H 2 O) samples at 200, 300 and 400 °C under oxidizing atmosphere was also performed. Films deposited over glass substrates were placed into the oven and heated to the desired temperature for 2 h using a heating rate of 10 °C min À1 in flowing air.

Characterizations measurements

DWCNT's interfacial thin films morphology on silicon substrates was evaluated using scanning electron microscopy (SEM-FEM/ Tescan). Raman spectra of the films on silicon substrates were recorded using a Renishaw Raman image spectrophotometer with excitation line of 1.96 eV (632.8 nm/He-Ne laser). The transmittance of the films over quartz substrates was acquired using a Shimadzu UV-2450 spectrophotometer. Sheet resistance (SR) and thickness of DWCNT interfacial thin films were recorded on glass substrates. SR was determined through the four-point method using a Jandel Universal probe equipment. The thickness of the films was measured using a Dektak 150 Veeco profilometer with a resolution and probe strength of 0.010 lm and 1 mg, respectively.

Results and discussion

The technique to assemble dispersible materials on the interface of two immiscible solvents as thin, transparent, and freestanding films has shown to be an outstanding method to prepare films capable to be deposited over different substrates with diverse applications [START_REF] Salvatierra | ITO-free and flexible organic photovoltaic device based on high transparent and conductive polyaniline/carbon nanotube thin films[END_REF][START_REF] De Souza | Thin and flexible all-solid supercapacitor prepared from novel single wall carbon nanotubes/polyaniline thin films obtained in liquid-liquid interfaces[END_REF][START_REF] Mehl | Thin and transparent films of graphene/ silver nanoparticles obtained at liquid-liquid interfaces: Preparation, characterization and application as SERS substrates[END_REF]. As can be noticed in the digital image presented in the middle of Fig. 1, transparent and macroscopically homogeneous films on glass substrates were produced from DWCNT samples. Fig. 2 shows the SEM images for the different samples of interfacial thin films of DWCNT (0.2 mg) directly deposited from the interface on silicon substrates. Through the images of raw DWCNT in Fig. 2 (a) and (b) the presence of large agglomerates of DWCNT as well as few carbon nanotubes randomly distributed over the substrate is clearly visible. The presence of these large agglomerates (also called ''diaphanous carbon") is resulting from CCVD synthesis of DWCNT and keeps the carbon nanotubes entangled [START_REF] Bortolamiol | Double-walled carbon nanotubes: quantitative purification assessment, balance between purification and degradation and solution filling as an evidence of opening[END_REF]. The morphology of DWCNT-Air/HCl in Fig. 2(c) and (d) is totally different from raw DWCNT sample and no diaphanous carbon could be noticed. The presence of many entangled nanotubes in DWCNT-Air/HCl comparing with images of raw DWCNT at the same magnification is notorious. The simple annealing of the DWCNT followed by washing with HCl is quite enough to remove oxide and residual catalyst without damaging the DWCNT surface [START_REF] Bortolamiol | Double-walled carbon nanotubes: quantitative purification assessment, balance between purification and degradation and solution filling as an evidence of opening[END_REF][START_REF] Osswald | Elimination of D-band in Raman spectra of double-wall carbon nanotubes by oxidation[END_REF][START_REF] Osswald | In situ Raman spectroscopy study of oxidation of double-and single-wall carbon nanotubes[END_REF]DWCNT For DWCNT-HNO 3 (H 2 O) sample in Fig. 2 (e) and (f), some entangled DWCNT agglomerates are still present, as also observed for DWCNT-HNO 3 (NaOH) sample (Fig. S1(a) and (b)). No agglomerates can be noticed for DWCNT-HNO 3 /H 2 SO 4 (H 2 O) sample in Fig. 2(g) and (h) as well as for DWCNT-HNO 3 / H 2 SO 4 (NaOH) in Fig. S1(c) and (d), indicating here a good condition to remove the diaphanous carbon from DWCNT, resulting in better homogeneity in the film morphology.

Raman spectra from different samples of DWCNT interfacial thin films are shown in Fig. 3(a). The presence of the main peaks concerning DWCNT can be noticed in Fig. 3(a), with the radial breathing modes (RBM) at low frequency region; D band around 1324 cm À1 , attributed to the presence of defects in the carbon nanotube structure; the G band split in G À and G + at 1560 and 1590 cm À1 , corresponding to circumferential and longitudinal carbon atoms vibration; and the 2D band at 2640 cm À1 , which is an overtone of the D band [START_REF] Osswald | Elimination of D-band in Raman spectra of double-wall carbon nanotubes by oxidation[END_REF][START_REF] Saito | Raman spectroscopy of graphene and carbon nanotubes[END_REF]. DWCNT-HNO 3 (NaOH) and DWCNT-HNO 3 /H 2 SO 4 (NaOH) exhibited very similar Raman spectra profiles for the respective samples washed with H 2 O (Fig. S2 (a)). As described previously, the I D /I G ratio is an important parameter to quantify the level of defects in the carbon nanostructure, and a large I D /I G ratio is normally related to a high level of defects on the surface of the carbon nanostructure [START_REF] Osswald | In situ Raman spectroscopy study of oxidation of double-and single-wall carbon nanotubes[END_REF][START_REF] Osswald | Monitoring oxidation of multiwalled carbon nanotubes by Raman spectroscopy[END_REF]. Fig. 3 Surprisingly, the DWCNT-HNO 3 (H 2 O) sample presented higher SR value even when compared to the raw DWCNT. This is in agreement with previous results which attributed a higher production of carboxylated carbonaceous fragments coating all outer DWCNT surface for samples purified with HNO 3 [START_REF] Bortolamiol | Double-walled carbon nanotubes: quantitative purification assessment, balance between purification and degradation and solution filling as an evidence of opening[END_REF].

(b) depicts

DWCNT-Air/HCl and HNO 3 /H 2 SO 4 (H 2 O) samples presented the best results of SR compared to the other samples prepared in this work. From this point of view, further experiments were developed to optimize the values of SR, besides the transmittance at 550 nm. Fig. 4(a) and (b) depict the SR values and transmittance (%) at 550 nm for interfacial thin films prepared from three different amounts of DWCNT Air/HCl and HNO 3 /H 2 SO 4 (H 2 O), respectively. As can be noticed in both graphics, the lowest SR values were obtained for DWCNT samples prepared from 0.2 mg of carbon nanotubes. On the other hand, the lower transmittance values were also achieved for these samples, with values of 59 and 80 % at 550 nm for DWCNT-Air/HCl and DWCNT-HNO 3 /H 2 SO 4 (H 2 O), respectively. High transmittance was obtained for samples with a low amount of DWCNT (0.05 mg), with values of 78 and 84 % at 550 nm for Air/HCl and HNO 3 /H 2 SO 4 (H 2 O) samples. The thickness of the DWCNT interfacial films also decreased when the amount of carbon nanotubes was lower, achieving 30.2 ± 7.9 and 14.8 ± 2.9 nm for DWCNT-Air/HCl and HNO 3 /H 2 SO 4 (H 2 O) samples, respectively, for films prepared from 0.05 mg of carbon nanotubes (Fig. S3(a) and(b)). Nevertheless, the SR values increased for these samples, although the transmittance and thickness were optimized. This increase in SR should be due to the decrease in connectivity arising from the lower amount of carbon nanotubes in the film [START_REF] Pereira | Upper bound for the conductivity of nanotube networks[END_REF][START_REF] Pereira | Electronic transport on carbon nanotube networks: a multiscale computational approach[END_REF]. DWCNT-Air/HCl and HNO 3 /H 2 SO 4 (H 2 O) films were annealed under oxidizing atmosphere at three different temperatures (200, 300 and 400 °C) to optimize the SR and transmittance values. Annealing process is known to improve SR and transmittance values of thin films prepared from different carbon nanostructures since this procedure removes carbon impurities, decreases the tube-tube junction resistance, and restores the sp 2 structure on the carbon nanostructures. [START_REF] Wang | Fabrication of highly conducting and transparent graphene films[END_REF][START_REF] Andrews | Multiwall carbon nanotubes: synthesis and application[END_REF][START_REF] Fujisawa | Enhanced electrical conductivities of N-doped carbon nanotubes by controlled heat treatment[END_REF] Fig. 5 presents SR and transmittance values obtained for each temperature, depending on the amount of carbon nanotube used to prepare the interfacial films. Table 2 summarizes all values obtained of SR and transmittance for each temperature. The improvement of the transmittance with the increase in temperature is very clear. The transmittance at 550 nm increased for all samples from 200 to 400 °C. The highest values of transmittance (Fig. 5 (a) and (c)) were achieved for samples prepared from 0.05 mg of DWCNT and annealed at 400 °C, being 86 and 90% for DWCNT-Air/HCl and HNO 3 /H 2 SO 4 (H 2 O), respectively. Even though some improvement was achieved on values of transmittance of the thin films, the changes in terms of SR were not so meaningful.

The SR slightly increased after annealing for DWCNT-Air/HCl sample prepared with 0.2 mg, achieving values twice higher at 400 °C (Fig. 5(b)). The thin films prepared with 0.1 and 0.05 mg presented significant decrease in SR until 300 °C, which become higher after annealing the DWCNT samples at 400 °C. Similar behavior could be noticed for all DWCNT-HNO 3 /H 2 SO 4 (H 2 O) samples (Fig. 5(d)), which exhibited low SR values for films annealed at 300 °C. The decrease in SR for almost all films from 70 °C to 300 °C can be associated to removing some impurities, as amorphous carbon for example, or even through restoring the CAC sp 2 bonds in the DWCNT from the decomposition of functional groups [START_REF] Souza | Flexible, transparent and thin films of carbon nanomaterials as electrodes for electrochemical applications[END_REF]. The SR value for all films increased when the annealing temperature was increased to 400 °C. This behavior can be attributed to the oxidation of carbon nanotubes, or at least some of them leading to both a lower number of remaining carbon nanotubes, and fewer interconnexions. In the case of the DWCNT-Air/HCl sample the explanation may be more complex as the nanotubes already experienced an oxidizing treatment at a temperature much higher than 400 °C (500 °C), but only for 30 min compared to the 2 h of the heat treatment discussed here. It is thus possible that some carbon nanotubes were burnt during this step.

As mentioned before, besides the capacity to develop conducting and transparent films, new technologies claim for materials capable to be assembled over flexible substrates standing the electrical and optical properties even under mechanical stress (as under stretching or bending, for example). To prove this capability, interfacial thin films of DWCNT-Air/HCl and DWCNT-HNO 3 /H 2 SO 4 (H 2 O) produced from 0.2 mg of carbon nanotubes were deposited over flexible substrates of PET. A transparent and homogeneous film of DWCNT-Air/HCl on the flexible substrate can be observed in Fig. 6 (a), which presented excellent adherence to the substrate and no macroscopic damage under bending (Fig. 6(b)). Reasonable values of SR and transmittance at 550 nm (Fig. 6(c)) were achieved for both DWCNT samples deposited on PET when compared to the same samples on glass substrates, indicating the maintenance of electronic and optical properties. Both samples on the flexible substrate were also applied as conducting substrate to close a circuit between a LED and a battery (video Supp. Inf), and it could be observed that both substrates worked well even under bending, indicating a promising viability for further applications in electronics devices.

The best results regarding SR and transmittance for conducting and transparent substrates in this work were achieved from the DWCNT-Air/HCl interfacial thin films. As described before, the lowest SR value was 0.5 ± 0.1 kX h À1 with transmittance at 550 nm of 59%, whereas the highest transmittance was 78%, followed by a SR of 1.7 ± 0.1 kX h À1 . The optoelectronic performance of these films is quite compatible with some results described elsewhere, like for DWCNT thin films obtained from a layer-by-layer assembly (0.413 kX h À1 and 87 % transmittance) [START_REF] Park | Fully organic ITO replacement through acid doping of double-walled carbon nanotube thin film assemblies[END_REF]; for DWCNT films prepared from vacuum filtration and transferred to glass substrates (0.146 kX h À1 and 75 % transmittance) [START_REF] Green | Processing and properties of highly enriched doublewall carbon nanotubes[END_REF]; for ureafunctionalized SWCNT rolled films (1.7 kX h À1 and 80 % transmittance) [START_REF] Azoz | Highly conductive single-walled carbon nanotube thin film preparation by direct alignment on substrates from water dispersions[END_REF]; and for DWCNT film obtained from filtration (0.083 kX h À1 and 79 % transmittance) [START_REF] Hou | Double-wall carbon nanotube transparent conductive films with excellent performance[END_REF]; The main breakthrough of this work is the production of conducting and transparent films of high structural quality DWCNT's assembled at the liquid-liquid interface, which are capable to be deposited over different substrates. Furthermore, the optoelectronic properties of these films can be easily tuned up only through the amount of DWCNT, and no complex post-step processing is required from the interfacial method for thin films production. So, the simplicity allied to the reproducibility of this method brings a contribution to the field of conducting and transparent substrates.

Conclusions

Conducting and transparent substrates obtained from thin films of double-walled carbon nanotubes assembled at a liquid-liquid interface were demonstrated here, which is an easy, cheap, and reproducible method to prepare thin films. Different approaches for DWCNT dispersions and further preparation of the interfacial thin films were strictly related to the purification treatment previously applied to the carbon nanotubes. Films were deposited on glass or PET substrates, and the morphology, the vibrational structure as well as the optical and electrical properties of these films were also evaluated. Samples prepared from DWCNT-Air/ HCl and DWCNT-HNO 3 /H 2 SO 4 (H 2 O) demonstrated the best electrical result. It was related to thin films with well-distributed carbon nanotubes on the substrates without the presence of entangled portions, combined to carbon nanotubes of very high structural quality (low I D /I G ratio). We demonstrated that the sheet resistance and the optical transparency of these two samples can be controlled through the amount of DWCNT and annealing process, which allows the modulation of the properties of the thin films depending on the future applications. The capability to prepare conducting and transparent films on plastic substrates allows these materials to be applied in fields where flexibility is required, which is impossible with ITO.

Fig. 1 .

 1 Fig. 1. Schematic representation of all steps involved in interfacial films production and photographic image (in the middle) of DWCNT's films deposited over glass substrate.

Fig. 2 .

 2 Fig. 2. SEM images of the films deposited over silicon substrates: raw DWCNT (a and b), DWCNT-Air/HCl (c and d), DWCNT-HNO 3 (H 2 O) (e and f) and DWCNT-HNO 3 /H 2 SO 4 (H 2 O) (g and h).

Fig. 3 .

 3 Fig. 3. Raman spectra of DWCNT thin films (a); detail from 1000 to 1750 cm À1 with the deconvolution of peaks (b); correlation between I D /I G ratio and sheet resistance of the different DWCNT films (c). Black full squares refer to I D /I G values and red full square refer to sheet resistance.

Fig. 4 .

 4 Fig. 4. Sheet resistance and transmittance at 550 nm vs DWCNT amount for Air/HCl (a) and HNO 3 /H 2 SO 4 (H 2 O), (b) films. Black full squares refer to sheet resistance and red full squares refer to transmittance.

Fig. 5 .

 5 Fig. 5. Transmittance at 550 nm vs the amount of DWCNT and the temperature of annealing of the Air/HCl (a) and HNO 3 /H 2 SO 4 (H 2 O) (c) films; sheet resistance vs the amount of DWCNT and the temperature of annealing of the Air/HCl (b) and HNO 3 /H 2 SO 4 (H 2 O), (d) films.

Table 1

 1 Summary of the setup used to prepare the interfacial thin films for each sample.

	Sample	Solvent	Amount of DWCNT	Sonication
			(mg)	time
	raw DWCNT	CHCl 3	0.2	20 min
	DWCNT-Air/HCl	CHCl 3	0.05/0.1/0.2	20 min
	DWCNT-HNO 3 (H 2 O)	H 2 O	0.2	30 min
	DWCNT-HNO 3 (NaOH)	H 2 O	0.2	30 min
	DWCNT-HNO 3 /H 2 SO 4 (H 2 O)	H 2 O	0.05/0.1/0.2	60 min
	DWCNT-HNO 3 /H 2 SO 4 (NaOH)	H 2 O	0.2

Table 2

 2 Sheet resistance (kX h À1 ) and transmittance (%) of the films vs both the amount of DWCNT and the annealing temperature.

	DWCNT-Air/HCl

Fig. 6. Photographic images of the DWCNT Air/HCl film deposit on PET at rest (a) and under bending (b); transmittance at 550 nm and sheet resistance of the Air/HCl and HNO 3 /H 2 SO 4 (H 2 O) films on PET (c). Black full squares refer to transmittance and red full squares refer to sheet resistance.

Acknowledgements

AJGZ and VHRS acknowledge CNPq, CAPES and INCT-Nanocarbon Materials for financial support. VHRS acknowledges CAPES for the fellowship. French National Research Agency is acknowledged for grant N°ANR-13-BS10-0014-03.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2017.04.084.


Khaled Bouaziz

Mohamed Ayadi

Noureddine Allouche

Abraham Chemtob
email: abraham.chemtob@uha.fr

Renewable Photopolymer Films Derived from Low-Grade Lampante and Pomace Olive Oils

Keywords: Ultra-violet, photopolymerization, auto-oxidation, thiol-ene, infrared spectroscopy

come   L'archive ouverte pluridisciplinaire

Introduction

Radiation curing is widely regarded as an emerging technology for the eco-efficient production of organic polymer films. Starting from a liquid film containing oligomers and multifunctional monomers still largely dependent on fossil resources (oil, gas, etc.), exposure to UV or visible light in the presence of a photoinitiator triggers a cross-linking reaction to create a solid network film, ideally both hard and chemically resistant [START_REF] Fouassier | Photoinitiators for Polymer Synthesis: Scope, Reactivy and Efficiency[END_REF]. The appeal of this curing method relies on a unique combination of solvent-free condition, reaction at room temperature, fast cycle times of less than one second, and small-scale production lines.

Applications span a large range of sectors, not only in industrial coatings, but also graphic arts and adhesives. Today, the economy of UV-curable coatings grows at a sustained high rate of 9 % per year [START_REF] Schwalm | UV Coatings Basics, Recent Developments and New Applications[END_REF]. An exceptional value when compared with that of waterborne coatings (3%) or powder coatings (2 %) produced by other energy-efficient technologies.

With the push for businesses to improve their green credentials, the radiation curing industry has embarked on a process aimed at incorporating an increasing part of bio-based monomers or oligomers into the photocurable formulation [START_REF] Sharma | Addition polymers from natural oils -A review[END_REF][START_REF] Meier | Plant oil renewable resources as green alternatives in polymer science[END_REF][START_REF] De Espinosa | Plant oils: The perfect renewable resource for polymer science?![END_REF][START_REF] Sharmin | Recent advances in vegetable oils based environment friendly coatings: A review[END_REF]. Foremost amongst the sustainable resources are vegetable oils which boast functionality, non-toxicity, straightforward extraction and wide source of supply [START_REF] Gandini | Progress of Polymers from Renewable Resources: Furans, Vegetable Oils, and Polysaccharides[END_REF][START_REF] Gandini | Polymers from Plant Oils[END_REF]. For reasons of reactivity, the main emphasis has been on structures based on unsaturated triglyceride esters of fatty acids [START_REF] Gan | CHAPTER 1 : Photo-cured Materials from Vegetable Oils[END_REF]. Because the internal carbon double bonds are generally not reactive enough towards most photopolymerization processes, the main solution has consisted in a preliminary chemical modification.

Epoxidation is the simplest way to make plant oils reactive via cationic photopolymerization [START_REF] Zou | UV-curable cycloaliphatic epoxide based on modified linseed oil: Synthesis, characterization and kinetics[END_REF], while (meth)acrylation is a multi-step reaction enabling their use in free radical photopolymerization [START_REF] Zovi | A solventless synthesis process of new UV-curable materials based on linseed oil[END_REF]. Additionally, oils with a high ratio of unsaturations can be photocured under daylight or UV irradiation through a photo-oxidation mechanism accelerated by metallic salt catalyst [START_REF] Fertier | The use of renewable feedstock in UV-curable materials -A new age for polymers and green chemistry[END_REF][START_REF] Ye | Photoassisted oxypolymerization of alkyd resins: Kinetics and mechanisms[END_REF]. Photoinduced thiol-ene reaction is the last strategy which has been extensively investigated over the last 10 years [START_REF] Klaasen | Fast drying cobalt-free high solids alkyd paints[END_REF]. Nevertheless, most studies use thiolene as a functionalization tool of unsaturated plant oils or fatty acid derivatives [START_REF] Samuelsson | Thiol-ene coupling reaction of fatty acid monomers[END_REF][START_REF] Palaskar | Original diols from sunflower and ricin oils: Synthesis, characterization, and use as polyurethane building blocks[END_REF][START_REF] Turunc | The thiol-ene (click) reaction for the synthesis of plant oil derived polymers[END_REF] rather than for cross-linking [START_REF] Bexell | Bonding of vegetable oils to mercapto silane treated metal surfaces: Surface engineering on the nano scale[END_REF][START_REF] Black | Thiol-ene UV-curable coatings using vegetable oil macromonomers[END_REF]. The thiol-ene polymerization reaction is a free radically initiated reaction of a thiol adding to an unsaturation of the triglyderide. It proceeds through a stepwise mechanism in which a photo generated thiyl radical reversibly adds to an unsaturation, producing a carbon radical able to abstract a new hydrogen from a thiol, and thus ensuring the polymerization propagation [START_REF] Hoyle | Thiol-Ene Click Chemistry[END_REF]. Thiol-ene photopolymerization using polyfunctional thiol cross-linkers was reported in a limited number of studies [START_REF] Bexell | Bonding of vegetable oils to mercapto silane treated metal surfaces: Surface engineering on the nano scale[END_REF][START_REF] Black | Thiol-ene UV-curable coatings using vegetable oil macromonomers[END_REF][START_REF] Luo | Thiol-ene" photo-cured hybrid materials based on POSS and renewable vegetable oil[END_REF][START_REF] Echeverri | Synthesis of elastomeric networks from maleated soybean-oil glycerides by thiol-ene coupling[END_REF].

All these studies involve modified oils (acrylate, maleate, fumarate, vinyl ether) to enhance reactivity because internal olefins of unmodified fatty acids are approximately 10 times less reactive than terminal ones toward the addition [START_REF] Denes | Thiyl Radicals in Organic Synthesis[END_REF]. Furthermore, most employed oils, including linseed or soybean oil, contain a high concentration of unsaturations, although their role is also crucial in human and animal consumption.

By contrast, we have focused our attention on thiol-ene photopolymerization of native (nonmodified) triglycerides with the emphasis on low-grade unsaturated plant oils. Lampante and pomace olive oils are two emblematic examples. Their structure and average composition in fatty acids are given in Scheme 1, this latter does not strongly differ from those of virgin olive oil. In green polymerization methods, olive oil has been generally ruled out because of its major role in human diet, and its high cost compared to other seed oils. Nonetheless, the situation is clearly different for these two systems:

-Lampante olive oil is defined as a low quality olive oil because of lack of care, parasites or diseases. It is declared unfit for human consumption because of acidity problems and high concentrations of polycyclic aromatic hydrocarbons [START_REF] Soucek | Review of autoxidation and driers[END_REF]. Locally, many small producers are producing lampante oil because of inadequate tools or improper installation. Despite the technical developments of the last 10 years, this category reaches between 10 and 20 %.

-The extraction of virgin olive oil also results in the production of solid by-products called olive oil cake, consisting of the dry matter of the olive (pulp, peel, stone...). When it is not used for livestock feeding or energy recovery [START_REF] Yucel | Optimization of biocatalytic biodiesel production from pomace oil using response surface methodology[END_REF], the olive cake is dried, grinded and treated with solvent, usually hexane, to produce a flavor-less pomace olive oil. About 20 % of olive oil is actually entrapped in the cake, meaning that it represents an under-exploited resource.

This study demonstrates the applicability of an eco-efficient thiol-ene photopolymerization to produce dry rubbery films with low cost olive oils resulting from non-edible grades (lampante) and waste production (pomace) without prior chemical modification. The resulting UV-curable films contain up to 80-85% of renewable resources and drying can proceed despite the limited unsaturations' concentration (the iodine value of olive oil is only 85 versus > 140 for typical drying oil such as linseed oil). However, the flexibility of the S-C bond together with the limited cross-linking density have resulted in poor network mechanical integrity. Photopolymerization was triggered with a conventional medium-pressure Hg lamp in presence of three different thiol cross-linkers (T1-3) and a radical photoinitiator (all structures are shown in Scheme 2). Cross-linking can proceed in principle by two concomitant mechanisms: photo-oxidation and thiol-ene photopolymerization, whose relative importance must be established. In this study, real-time Fourier transform infrared spectroscopy (RT-FTIR) has been mainly used to assess the reactivity of the olive oil/polythiol mixture using linseed oil as reference system.

Materials and Methods

Materials

2,2′-(ethylenedioxy)diethanethiol 95 % (T1), hexanedithiol 99.5 % (T2), trimethylolpropanetris(3-mercaptopropionate) 95 % (T3), 2-hydroxy 2-methylpropiophenone 97% (also called Darocure 1173, radical photoinitiator) were purchased from Sigma-Aldrich.

Byk 333 is a poly (ethylene oxide)-polydimethylsiloxane copolymer used as a wetting agent (BykChemie). The refined pomace olive oil and lampante olive oil were purchased from a local commercial refining oil factory located at Agareb in Sfax (Tunisia) in Sept. 2014.

Linseed oil was purchased from Sigma-Aldrich. All chemicals were used as received without further purification. Need comments on the different methods to characterize the native oils

Thiol-ene photopolymerization of olive oil/polythiol films

For a mixture containing a stoichiometric thiol-ene ratio, 1 g of pomace or lampante olive oil was mixed with 0.3 g of ethylene glycol dithiol (T1), 0.42 g of trimethylolpropanetris (3mercaptopropionate) (T2) and 0.24 g of hexanedithiol (T3) with 1% of radical photoinitiator D1173 and 0.1% of wetting agent. Under sub-stoichiometric conditions (thiol/ene: 1/1.45), 1 g of pomace or lampante olive-oil was mixed with 0.2 g of T1, 0.3 g of T2, and 0.16 g of T3 without photoinitiator and the same formulations was prepared again with 1 wt% of photoinitiator D1173. Additionally, quantities were adjusted to have the same thiol-ene molar ratio when using linseed oil exhibiting a higher content of unsaturations. In addition, oxophotopolymerization was performed without thiol derivatives, with the additional presence of photoinitiator in certain experiments.

All the irradiation experiments were performed at ambient temperature in the presence of air.

A liquid thiol-ene monomer film (6 μm thick) was applied onto a borosilicate glass plate by means of a calibrated wire-wound applicator without dewetting problems through the use of Byk-333. Two UV irradiation devices were implemented. First, the liquid films were exposed to the polychromatic light of a medium pressure mercury-xenon arc lamp (Hamamatsu L8252, equipped with a reflector at 365 nm) (λ: 250-600 nm, irradiance = I = 685 mW cm -2 ) focalized vertically on the sample through a flexible light guide placed at a distance of 3 cm from the film. The irradiation device was also equipped with an elliptical reflector (365 nm) that efficiently reflects the UV light, and lets heat rays pass through, preventing any significant increase of temperature. This cold reflector cuts off heat so that the local temperature increases of only ca. 3 °C during a typical photopolymerization experiment. With this lamp, the photopolymerization kinetics were monitored in situ by real-time Fourier transform infrared spectroscopy (see characterization section). Additionally, UV exposure was carried out through ten successive passes under a UV conveyor (Qurtech) equipped with an H-bulb lamp (Fusion). The belt speed of the conveyor was set to 10 m min -1 . Under these conditions, the emitted light dose for each pass is 1.35 J cm -2 UV A [320-390 nm]: 0.46 Jcm -2 , UV B [280-320 nm]: 0.31 J cm -2 , UV C [250-260 nm]: 0.08 J cm -2 and UV V [395-445 nm]: 0.50 J cm -2 ) and corresponds to an exposure time of 0.23 s.

Characterization

Real-time Fourier transform infrared (RT-FTIR) spectra were obtained with a Bruker Vertex 70 spectrophotometer equipped with a liquid nitrogen cooled mercury-cadmium-telluride (MCT) detector working in the rapid scan mode. The resolution of the spectra was 4 cm -1 with an average of 4 scans s -1 . The conversion kinetics were monitored online by measuring the decay of the IR absorption bands at 2860 cm -1 (SH stretching) and 3010 cm -1 (olefinic CH stretch) under simultaneous exposure to UV light. Differential scanning calorimetry (DSC) was carried out on a TA Q2000 DSC using a heat-cool-heat thermal cycle. 5 mg of poly(disulfide) film was placed in an hermetic aluminum pan. The first cycle started by heating the sample from -90 to 100 °C at 10 °C min -1 . The second cycle cooled the sample back to -90 °C. The third step heated the sample again from -90 to 100 °C at 10 °C min -1 .

Results and discussion

Table 1 shows a precise comparison of the composition of lampante and pomace olive oils with a conventional virgin olive oil in terms of acidity, peroxide value, polycyclic aromatic hydrocarbon composition and fatty acidy content. As expected, lampante olive oil has a much higher acidity, and peroxide value making it unsuitable to human consumption. Pomace olive oil is characterized by a much lower concentration in polyphenol and tocopherol because it has been refined. However, all olive oils show very slight differences in the proportion of fatty acids with a majority of oleic acid (C18:1, 62-64 %) and linoleic acid (C18:2, 18-20 %).

Investigation of UV-assisted oxopolymerization

Prior to incorporate thiols cross-linkers, the ability of pomace and lampante olive oils to selfpolymerize by a photo-assisted auto-oxidation process was explored. Without the addition of catalyst, several authors pointed out that UV irradiation can accelerate the oxidation rate. To provide consistent basis of comparison, linseed oil was also used. Due to its high concentration of unsaturations, this drying oil is known to be sensitive to auto-oxidation, and has been used for decades as painting and linoleum though its capacity to form a optically transparent film [START_REF] Claudino | Thiol-ene coupling of 1,2-disubstituted alkene monomers: The kinetic effect of cis/trans-isomer structures[END_REF].

Before irradiation, the high frequency transmission IR spectra of linseed oil and pomace olive oil, in Figure 1, are somehow similar. They show the presence of five distinct peaks. Four are straightforwardly assigned to CH stretching vibration from aliphatic groups at 2854, 2872, 2917 and 2954 cm -1 which are attributed to CH 2 symmetric, CH 3 symmetric, CH 2 antisymmetric and CH 3 antisymmetric vibrations, respectively. The singlet at 3007 cm -1 is indicative of olefinic carbon-hydrogen (=C-H) stretching vibration in hydrocarbon chain. The position of this CH mode peak supports the presence of internal olefin groups with a cis conformation. Its higher intensity for linseed oils indicates a higher concentration of unsaturation as expected. Note that the spectrum of lampante olive oil is similar to that of pomace olive oil, and has not been represented in Figure 1. After UV exposure (250 -600 nm, 685 mW cm -2 ) during 60 min, only this latter band has evolved, and has completely vanished below the noise level. This result is in agreement with a UV-assisted oxopolymerization mechanism involving, among other, various addition reactions of oxidation product radicals onto internal olefins as detailed below. The well-established photo-induced auto-oxidation mechanism has been fully commented on Figure 2 [START_REF] Soucek | Review of autoxidation and driers[END_REF]. Real-time FTIR spectroscopy enabled to monitor the conversion rates of C=C bonds during time. Consequently, the kinetic curve of Figure 3 reflects only one mode of cross-linking by radical addition onto olefins (eq 5 and 8 in Figure 3), while the other cross-linking reactions (eq 9) are difficult to assess by FTIR spectroscopy. Despite a higher number of reactive sites, the conversion-time curves of linseed oils, in Figure 3, shows a higher reactivity since a full conversion can be reached within 10 min whereas 60 min are necessary for pomace or lampante olive oils. Regardless of the oil, all films remain sticky after UV treatment. The tackiness is more pronounced with the two olive oils probably due to the lower concentration of unsaturations yielding a lower crosslinking density. The higher concentration in linoleate (15 %) and linolenate (57 %) segments in linseed oils compared to olive oil (19 % and <1%, respectively) accounts for its higher reactivity compared to the olive oils (see detailed composition in Table 1).

Figure 4 shows the conversion-time curves for the three oils in presence of conventional αcleavable radical photo initiator (2,2-dimethyl-2-hydroxyacetophenone, 1 wt%) conventionally used to trigger free radical photopolymerization. Under these conditions, oxidation initiation does no longer result from impurities (eq 1) but from benzoyl and alkyl radicals produced by the photolysis of the aromatic ketone species (Norrish type I reaction).

With a higher concentration of radicals in the medium, the conversion profiles become almost similar regardless of the type of oils. Full disappearance of C=C is accomplished in less than 10 min. Nevertheless, all the coatings remain sticky after irradiation although the quality of the film has been improved. This result can be related to the ability of both benzoyl and alkyl radical initiators to readily attack the π-bond of carbon-carbon double bond of unsaturated oils. However, as observed for most 1,2-disubstituted ethylene monomers, the approach of the resultant propagating radical to a new C=C is sterically hindered because of the βsubstituents, and therefore, propagation of polymerization is not possible (there is also a thermodynamic factor that superimposes to this kinetic factor) [START_REF] Claudino | Thiol-ene coupling of 1,2-disubstituted alkene monomers: The kinetic effect of cis/trans-isomer structures[END_REF]. This first study shows the applicability of a UV-driven photo-oxidation process for low-grade olive oils proceeding in a matter of minutes with a radical photoinitiator. To achieve subsequently dry films, we have attempted to combine photo-oxidation with thiol-ene photopolymerization

Investigation of UV-driven thiol-ene photopolymerization

Photopolymerization with dithiol T1.

In this new series of experiments, a 2,2′-(ethylenedioxy)diethanethiol(T1) / oil mixture was prepared to obtain a thiol:ene molar ratio of 1 for the three types of oil (linseed, pomace and lampante olive oils). Irradiation was performed in presence of 1 wt% radical photointiator. As the internal alkenes are less reactive, they will tend to cause the propagation reaction to be slow and rate limiting compared to chain transfer which involves a dithiol exhibiting readily abstractable hydrogen atoms. Scheme 3 depicts the idealized cross-linked thioether structure after thiol-ene photopolymerization.

In Figure 5, the high frequency region of the FTIR spectra shows five peaks assigned to CH stretching vibrations as seen previously, as well as a weakly intense single band at 2564 cm -1 attributed to the S-H stretching mode. Its distinctive presence establishes the nonvolatile character of the dithiol compound T1. Whatever the type of oil, there is a fast and concomitant decrease of this feature with maximum at 2564 cm -1 and the ν(=C-H) stretching vibration at 3007 cm -1 within only 30 s. All the three films are dry and rubbery after 1 min 30 s of irradiation.

The kinetic treatment by rapid scan FTIR analysis allows to distinguish the respective conversions of C=C and SH functions, as shown in Figure 6. The sample based on pomace olive oils copolymerizes faster than that based on linseed oil. Full conversion of C=C is achieved within 10 s while more than twice is necessary with the latter. This result may be related to the higher concentration in unsaturations of linseed oil. In both systems, there is a discrepancy between the thiols and carbon-carbon double bonds conversions which are fully converted at a higher rate. This result can be interpreted on the basis of a dual thiol-ene and photo-oxidation mechanism of photopolymerization for the C=C bonds. As a part of the olefins are consumed by oxopolymerization, they are not available for thiyl addition (present in stoichiometric amount). The larger gap in conversion rates in the case of linseed oil may translate a higher tendency for auto-oxidation because of the lability of the doubly allylic methylenes. The thiol-ene reactivity of triglycerides is ruled by two opposite effects: the steric hindrance of the reactive 1,2-disubstituted ethylene groups which tends to slowdown the addition; and conversely the electron-donating inductive effect of the two surrounded alkylene substituents which enhances the reactivity. Very important for linseed oil is that bis-allylic H abstraction (via auto-oxidation mechanism) can generate conjugated alkenes which are known to copolymerize very slowly with thiols, presumably because of the stability of the carboncentered allylic radical formed upon the addition of the thiyl radical to the carbon-carbon double bond. This can contribute to slower reactivity of linseed oil. In both cases the conversion rates remain high in the order of seconds versus minutes for auto-oxidation, suggesting that a majority of alkenes are actually involved into a thiol-ene coupling process.

A comment may be added about thiol consumption that has been pursued despite the absence of alkene. UV light (λ < 300 nm) can promote thiol photolysis yielding thiyl radicals which can dimerize into disulfide bonds by bimolecular recombination [START_REF] Feillee | Step-growth thiol-thiol photopolymerization as radiation curing technology[END_REF]. Finally, there is no difference in the kinetic behavior of pomace and lampante oils despite the fact that pomace contain a much higher concentration in polyaromatic species which may inhibit the radical polymerization.

Influence of various experimental parameters.

The influence of photoinitiator, nature of polythiol, thiol-ene molar ratio, film thickness and UV lamp are gathered in Table 2. Only the results of pomace olive oil (runs a) and linseed oil (runs b) are summarized since similar results were obtained with lampante olive oil.

One of the salient feature of thiol-ene photopolymerization is that for UV irradiation, no added photoinitiator molecules are required to initiate the polymerization [START_REF] Hoyle | Thiol-click chemistry: a multifaceted toolbox for small molecule and polymer synthesis[END_REF]. As the Hg medium pressure light is not filtered, a portion of light is emitted < 300 nm which is particular relevant for hydrogen abstraction of thiol. Compared to the reference experiment with photoinitiator (run a and b), the self-initiation is effective and a full conversion of SH and C=C is achieved despite slower polymerization rates for pomace olive oil (run a1) and linseed oil (run b1). The results show that films obtained by the formulations with photoinitiator exhibit better performance than those obtained without photoinitiator. Pomace olive oil was also copolymerized with two other polythiol of functionality 2 (T2: hexanedithiol, run a2) and 3 (T3: trimethylolpropanetris(3-mercaptopropionate), run a3). A higher reactivity was found with these novel thiols, but the resultant film was found to be sticky. This contradictory result was assigned to a partial miscibility of the thiol with the oil. In addition, increasing the thickness has a limited impact on reaction rates and final conversion (runs a4 and a5).

Production of films was also up-scaled using a UV-conveyor (whose technical characteristics are detailed in the method section) to perform thermos-mechaninal analysis. After 4 passes in the UV conveyor, dry films were produced based on pomace and lampante olive oils. DSC analysis of run a revealed a glass transition arising at -42 °C (Figure S1 of Supporting Information SI). Such value is consistent with that of conventional poly(thioether) network materials formed by thiol-ene step-growth polymerization [START_REF] Mcnair | Impact Properties of Thiol-Ene Networks[END_REF]. Though solid materials were achieved in all instances, the resultant films obtained from the two olive oils showed very poor mechanical properties that we have attributed to three factors: the inherent flexibility of thioether linkage, the long hydrocarbon groups of fatty acids, and the low cross-linking density due to the limited concentration of unsaturations in olive oils. Consequently, the determination of relevant mechanical quantities including hardness, elastic modulus, residual stress, or coating fracture toughnessnormally accessible by indentation testing or scratchinghas turned out to be impossible or inappropriate. This aspect is common to many low transition temperature poly(thiother) networks which are elastomeric and possess poor mechanical integrity at and above 25°C [START_REF] Kasprzak | Synthesis and thermomechanical behavior of (qua)ternary thiol-ene(/acrylate) copolymers[END_REF]. Works are underway to involve high glass transition resins, polystyrene or poly(methyl methacrylate for example, to increase modulus and improve the toughness and mechanical stress of the films [START_REF] Hearon | A High-Performance Recycling Solution for Polystyrene Achieved by the Synthesis of Renewable Poly(thioether) Networks Derived from d-Limonene[END_REF].

Conclusions

This study provides evidence that solid cross-linked transparent coatings can be produced from two low grade olive oils (pomace and lampante) by a tandem UV process combining essentially thiol-ene photopolymerization, and in a very minor extent auto-oxidation process.

2,2′-(ethylenedioxy)diethanethiol has proven to be an appropriate dithiol for reactivity and miscibility reasons. Using a Hg-Xe medium-pressure Hg arc, almost full conversion of thiol and C=C are achieved in less than 20 seconds. Under an industrial UV-conveyor large films of a few tenths cm 2 were also obtained after only 4 passes. While this study demonstrates conclusively the possibility to form poly(thioether) cross-linked films from low grade olive oils, improvements are required to enhance network mechanuical integrity. One means of improvement would to prepare a biphasic material based on a soft thiol-ene thermoset and a hard thermoplastic. Figure 3. Light absorbing impurities (RH) can produce radicals (R•) in the presence of air (oxygen) under UV-visible irradiation (eq 1), even in absence of any compound containing intra-molecular chromophoric groups [START_REF] Rabek | Polymer Photodegradation: Mechanisms and experimental methods[END_REF]. The second step is the formation of alkyl radicals (P • ) by hydrogen abstraction on unsaturated fatty acid chain of plant oils (PH). The key reaction in the propagation sequence is the formation of peroxy radicals (POO•) by the reaction of alkyl radicals (P•) with oxygen (eq 3). The two next propagation steps involving the peroxy radicals are hydrogen abstraction to generate a new alkyl radical (P•) (eq 4) and the addition onto carbon double bonds (eq 5). The efficiency of this couple of reaction (eq 4 and 5

) is strongly dependent on the photolysis of hydroperoxide species which exhibits a low extinction coefficient in the UV range (1 L mol -1 cm -1 at 300 nm but a quantum yield of 1) [START_REF] Verdu | Vieillissement oxydant des polymères aspects cinétiques[END_REF]. The decomposition of oxidation products generates alkoxy radicals (PO • ) and hydroxyl radicals (OH • ) (eq 6). These two oxygen reactive species are much more reactive than peroxy radicals and can be also involved in hydrogen abstraction (eq 7) on double bond addition (eq 8). In addition to reactions 5 and 8, cross-linking can proceed through a number of termination reactions via bimolecular recombination of various radicals (eq 9).

 R • + H 2 O • (under UV light and O 2 ) (1) R • + PH P • + RH (2)
Propagation sequence

P • + O 2  POO • (3) POO • + PH  P • + POOH (4) POO • + -CH=CH- POO-CH-CH • - (5)
Photolysis of hydroperoxide species

POOH  PO • + • OH (under UV light) (6)
Reactions involving peroxy radicals

PO • + PH  P • + POH (7) PO • + -CH=CH- PO-CH-CH • - (8)
Termination reactions via bimolecular recombination PO • / POO • / PO •  P-P / POP / POOP (9)

Figures captionFigure 1 .

 1 Figures caption

Figure 2 .

 2 Figure 2. Conversion profiles of C=C bonds of the three oil samples during UV irradiation (Hg-Xe, 60 min) as measured by RT-FTIR (integration of the olefinic =C-H stretching band at 3007 cm -1 ).

Figure 4 .

 4 Figure 4. Conversion-time curves during the photooxidation of three oils in presence of 1 wt% 2,2-dimethyl-2-hydroxyacetophenone (radical photoinitiator).

Figure 5 .

 5 Figure 5. Temporal evolution of FTIR spectra of T1/oil film during UV irradiation (Hg-Xe lamp) in presence of radical PI.

Figure 6 .

 6 Figure 6. Conversion profiles (C=C and SH) of two oil/T i films under UV irradiation (Hg-Xe lamp, 1 min 20 s).

Figure 2

 2 Figure 2

FF

  Figure 6

TableTable 1 . Virgin olive oil Lampante olive oil Pomace olive oil

 1

	Acidity (%)	0.45	4.75	0.12
	Peroxide value (meq O 2 \ kg)	6.35	24.5	14.03
	K232 (Absorbance at 232 nm)	2.28	3.18	3.105
	K270 (Absorbance at 270 nm)	0.22	0.385	0.853
	Polyphenol (ppm)	716.07	485.36	130.35
	Tocopherol (ppm)	850.35	547.32	350.24
	Carotene (ppm)	15.55	8.25	2.85
	Chlorophyll (ppm)	9.89	1.27	6.85
	C16:0	15.02	14.34	13.75
	C16:1	0.79	0.90	0.87
	C18:0	1.80	1.99	1.77
	C18:1	63.09	63.72	62.77
	C18:2	18.08	17.86	19.53
	C18:3	0.71	0.85	0.78
	C20:0	0.25	0.20	0.26
	C20:1	0.19	0.12	0.20

Table 2 .

 2 Comparison of conversion rate between olive pomace oil and linseed oil Example of triglyceride chain containing three fatty acid chains joined by a glycerol center found in pomace and lampante olive oil. The approximate fatty acid contents of these oils and that of linseed oil is given in the table below.

	Run	Oil	Thiol	PI	Thickness (µm) Conversion after 5s Conversion after 90s
						C=C	S-H	C=C	S-H
	A	pomace olive T1	Yes	6	100	70	100	100
	a1	pomace olive T1	No	6	25	10	100	100
	a2	pomace olive T2	Yes	6	60	40	100	100
	a3	pomace olive T3	Yes	6	100	65	100	100
	a4	pomace olive T1	Yes	10	100	70	100	100
	a5	pomace olive T1	Yes	4	100	70	100	100
	a6	pomace olive	T1	Yes	6	90	60	100	100
		(non-
		stoichiometric)
	B	linseed	T1	Yes	6	80	40	100	100
	b1	linseed	T1	No	6	75	-	100	100

Acknowledgements

This research was financed by Ministry of Higher Education and Scientific Research (Tunisia) under reference "UR11/ES74".

Conflict of interest

We declare no conflicts of interest.


Dr Emeline Lobry

Abdoul Salam

Bt T Bah

Loïc Vidal

Dr Esther Oliveros

Dr André M Braun

Adrien Criqui

Abraham Chemtob
email: abraham.chemtob@uha.fr

A S Bt Bah

Dr A Criqui Mäder

Dr Loïc A Chemtob

Vidal

Colloidal Tio

Colloidal and Supported TiO 2 : Toward Nonextractable and Recyclable Photocatalysts for Radical Polymerizations in Aqueous Dispersed Media

come

Introduction

Issues including initiator odor, toxicity, migration and contamination, which may affect the properties or uses of polymer products, are currently at the centre of public debate, especially in the context of consumer and worker protection. The problem is particularly acute in UVcuring technology, which is used for the large-scale production of industrial coatings, adhesives, inks and other photosensitive materials. [1,[START_REF] Green | Industrial Photoinitators, a Technical Guide[END_REF] Free radical UV-curing is characterized by sub-second cycle times and the need of high photoinitiator (PI) concentrations to overcome oxygen inhibition, especially in fields such as the printing industry. This leads to a significant amount of non-reacted PI remaining in cross-linked films. Due to their low-molecular-weight, PIs are able to migrate within the film network, and may extract from the surface. In the field of food packaging, this mechanism may result in contamination, and spawned the important food product scandals of recent years which have permanently affected the consciousness and driven more stringent regulations. [START_REF] Aparicio | [END_REF] At present, the main response of the industry concerns the development of polymer initiating systems based on oligomeric type II PIs, hence avoiding the release of low-molecular-weight photoproducts. These initiating systems typically involve a couple of non-migratory oligomeric photosensitizers (based on thioxantone or benzophenone [4] ) and a hydrogen donor (polymeric amine) generating initiating species of high-molecular-weight. [START_REF] Fouassier | Photoinitiators for Polymer Synthesis: Scope, Reactivity and Efficiency[END_REF] However, these "polymeric" PIs involve additional cost, lead to higher viscosities of the UV-curable formulations, and decrease polymerization reactivity when used at similar weight ratios as low-molecular-weight analogues.

To overcome these limitations, semiconductor nanoparticles have been recently presented as potential non-migrating radical photoinitiators. [START_REF] Schmitt | [END_REF] However, the capacity of semiconductors such as TiO 2 , ZnO or CdS to act as initiator of radical photopolymerization was already demonstrated several decades ago. [7][8][9][10][11][12] Today, nanoscale semiconductor particles, especially nanoTiO 2 , are best known as photocatalysts for advanced oxidation processes (AOP). [13,[START_REF] Oppenländer | Photochemical Purification of Water and Air: Advanced Oxidation Processes (AOPs): Principles, Reaction Mechanisms, Reactor Concepts[END_REF] TiO 2 -photocatalysis is induced by the generation of excitons (electron-hole pairs) upon absorption of UV-radiation by the semiconductor leading to redox reactions of compounds adsorbed at the particles surfaces, e.g. molecular oxygen (O 2 ) and/or water. The resulting reactive oxygen species (ROS), i.e., hydroxyl radical (HO . ) cause oxidation and subsequent mineralization of organic contaminants present in water or air. [START_REF] Ribeiro | [END_REF] Superoxide radical anion

(O 2 .-
), generated by the reduction of O 2 , might act as a reductant but is not known to induce subsequent radical reactions. Based on the premise that some ROS, such as HO . , might initiate radical polymerizations, Kraeutler et al. demonstrated as early as 1979 that fumed TiO 2 could replace a PI when dispersed in an organic solvent containing methyl methacrylate (MMA). [16] Mechanistically, it might be assumed that the holes of the electronically excited TiO 2 react by electron transfer with an alcohol (ROH) or a carboxylate (RCOO -) added as coinitiator, thus generating C-centered radicals (R . ) either by loss of a proton (primary or secondary ROH) or by loss of CO 2 from the intermediary RCOO . . Such initiating systems based on a semiconductor nanoparticle/coinitiator tandem have attracted considerable interest in radical photopolymerization. [17][18][19][20][21][22][23][24][25] Nevertheless, non-reacted co-initiators may be prone to migration in contrast to nanometer scale semiconductor particles that are not supposed to diffuse in the polymer network. Alternative pathways involving semiconductor nanoparticles proceeded by photoinduced electron transfer to transition metal ions to trigger atom-transfer radical polymerization (ATRP) [25,26] or bulk radical polymerization using surface Pt(II), Cu(II) or Fe(III) modified ZnO nanoparticles. [START_REF] Schmitt | [END_REF] Yet, the simplest method to avoid the formation of residual migrating species is probably to rely on hole oxidation of water to create initiating hydroxyl radicals (HO . ) that are known to be very effective for the oxidation of organic compounds in aqueous solution. The first TiO 2photocatalyzed polymerization studies were performed in aqueous solution using dispersed titania powder. This restricted the scope of this approach to a short list of water-soluble monomers (acrylamide, [8,24,27] acrylic acid [27] or methacrylic acid [28] ) or partially water-soluble monomers (MMA). [22,29,30] Given that most utility monomers are insoluble in water, we suggest that polymerization in aqueous dispersed media may be a more convenient and versatile system to perform nanoscale TiO 2 -photoinitiated polymerizations without generation of migrating photoproducts. Currently, polymerization in dispersed media accounts for nearly 20% of the worldwide polymer production, and encompasses a number of distinct processes. Mainly suspension and emulsion processes have been used industrially to produce some of the most important commodity thermoplastics, including poly(vinyl chloride) or styrene-acrylic polymers. Recently, Song et al. showed that solid TiO 2 nanoparticles can act both as photocatalyst and Pickering stabilizer (by adsorption on droplet surface) in emulsion polymerization of styrene. [31,32] Keeping this study in mind, we investigated in a first step a model MMA miniemulsion to confirm the feasibility of a TiO 2 -photoassisted polymerization in an aqueous dispersed medium. But in all likelihood, this approach may be applied to a range of aqueous monomer emulsions. By a careful control of the amount of added TiO 2 and the conditions of irradiation, we could show that the production of high-molecular-weight acrylic latex can be achieved without substantial chain degradation. In this work, two nanoscale TiO 2 forms have been investigated, as depicted in Figure 1: colloidal dispersion and nanosized layer coated on fused quartz fiber. Both systems have specific advantages, and their initiating efficiencies have been compared to that of a conventional water-soluble type I PI.

1. Colloidal TiO 2 offers significant process advantages: direct addition to any aqueous monomer dispersion without emulsification or dissolution stage, high surface-to-volume ratio to ensure optimal reactivity, and small average particle diameter (≈ 30 nm) to limit scattering and attenuation of UV radiation. However, the catalyst cannot be extracted after the latex synthesis stage. This raises concern on the UV photostability of the final product (waterborne coating or adhesive), in particular for outdoor applications.

2. TiO 2 -coated quartz fiber can act as a "supported" catalyst. [33] Quartz fibers offer an inert and UV transparent support, while the thin TiO 2 layer deposited on the micrometric fiber may provide a specific surface area for an optimal contact with the monomer emulsion and high reactivity. Supported TiO 2 may be seen as a promising avenue for recoverable and reusable PIs, lessening the environmental footprint of photopolymerization reactions. The main challenge is the risk of fouling of the fiber surface during photopolymerization, suppressing the photocatalytic activity.

Materials and methods

Materials

Technical grade methyl methacrylate (MMA) was purchased from Aldrich and used without further purification. Octadecyl acrylate (OA, Aldrich) was employed as co-stabilizer of monomer miniemulsions and sodium dodecyl sulfate (SDS, Aldrich) was used as a surfactant for preparing the MMA miniemulsions. Distilled water was used in the preparation of the miniemulsions. Used for comparison, the conventional type I water-soluble PI, Irgacure 2959

(2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone)
, was provided by BASF Specialty Chemicals. Nanoscale titanium dioxide was used in two different forms, (i) as a dispersion in water (33-37 wt%, specific surface area of 49 m 2 /g) produced by Buehler and sold by Aldrich. According to manufacturer information, this stable colloidal dispersion has an average diameter lower than 30 nm, and 90 % of the size distribution lies below the threshold value of 150 nm. (ii) as a TiO 2 -coated fused silica fiber provided by Saint Gobain (Quartzel® PCO). Currently used for air or water treatment, this product consists of a felt composed of entangled fiberglass with a nominal TiO 2 thickness of 15-20 nm, and a specific surface area of ca. 120 m 2 /g.

Miniemulsion photopolymerisation

The formulation of the 20 wt% solids content monomer miniemulsion is described in Table 1.

In a typical procedure, an organic phase was first prepared by dissolving the co-stabilizer (OA, 4 wt% / wt monomer ) into the monomer. The latter was then added to an aqueous SDS solution (2 wt% / wt monomer ). Both phases were then stirred during 10 min at 700 rpm. The resulting coarse macroemulsion was then sonicated during 5 min using a Branson Sonifier 450 (450 W/L) (9 output control, 80 % duty cycle) while maintaining the stirring. The PI was introduced differently depending on its structure. The water-soluble type I initiator or colloidal TiO 2 were simply added to the aqueous phase before the emulsification stage. As regards to the TiO 2 -coated fiber glass, the fabric was introduced in the reactor vessel containing the monomer miniemulsion just before irradiation. In the three cases, the weight concentration of the photochemically reactive substance (molecular PI or TiO 2 ) was kept constant with respect to the monomer content in order to enable comparison. Regarding c-TiO 2 , the concentration in TiO 2 can be easily deduced from the solids content value. For f-TiO 2 , the mass content was estimated based on the TiO 2 weight content compared to the total weight obtained from Energy-dispersive X-ray spectroscopy (EDX) data. Magnetic stirring of the miniemulsion is maintained throughout the photochemically initiated polymerization. Samples were drawn during polymerization to determine conversion.

Table 1. Miniemulsion formulation

Characterization

The average diameter of the monomer droplets (D d ), latex particles (D p ) and colloidal TiO 2 particles were measured by dynamic light scattering (DLS) using a Zetasizer nano ZS (Malvern Instruments). Conversion was assessed by gravimetric analysis. Molecular weights of the different latexes prepared were determined by size exclusion chromatography by using a PL-gel mixed D (5 µm, 300 mm  7.5 mm) Shimadzu column calibrated with polystyrene.

The molecular weights provided are then given in polystyrene equivalent. Both types of TiO 2

were characterized by X-ray diffraction (XRD, Philips X'Pert Pro PAnalytical). Transmission electron microscopy (TEM) was carried out on a Philips CM200 microscope operating at 30

kV. Scanning electron microscopy (SEM) was carried out on a Jeol JCM 6000 microscope operating at 15 kV. Quartzel® fibers were analyzed before and after photopolymerization and the TiO 2 content was measured by EDX.

3.

Results and discussion

Characterization of colloidal and supported nanoscale TiO 2

Colloidal TiO 2 (c-TiO 2 ) and TiO 2 -coated fiberglass (f-TiO 2 ) were characterized by a range of techniques prior to their use. All data are summarized in Table 2. Concerning the morphology of f-TiO 2 , the SEM analysis showed an entangled, curly continuous fiber structure displaying a mean fiber diameter of 10.5 µm. Fiber surface appeared smooth and default-free, i.e.

without any area non-covered by the titanium dioxide layer (15-20 nm thickness according to the manufacturer). EDX analysis revealed a TiO 2 concentration of 1.4 wt% (see Figure S1 in Supporting Information). TEM analysis of c-TiO 2 showed an ensemble of non-spherical particles that are roughly 24 nm in diameter. In contrast, DLS showed a much greater (hydrodynamic) average diameter (90 nm). TEM provides information about particles which are dehydrated and immobilized on a solid support. This can lead to structural distortions compared to the solvent swollen state of DLS analysis. Discrepancy between DLS and TEM size data is not necessarily contradictory; it may emphasize that inorganic nanoparticles like anatase structure that is known to have a higher photocatalytic activity than rutile but is thermodynamically less stable. [34,[START_REF] Luttrell | Intensively investigated as photocatalysts for the photodegradation of persistent organic contaminants, TiO 2 nanomaterials can be tamed to enable miniemulsion photopolymerization[END_REF] By contrast, the XRD pattern of f-TiO 2 revealed a coexistence of the two different crystallographic forms rutile/anatase in the ratio: ca. 80:20 %. To account for this result, we postulate that the immobilization of the catalyst may cause a less accessible surface area, and reduces the number of active catalyst sites compared to a finely dispersed c-TiO 2 system. A second most likely explanation is that the surface of f-TiO 2 absorbing the incident radiation may be smaller than the corresponding surface of c-TiO 2 dispersed over the whole reactor volume. The M n values of the precipitated polymers are of the same order or even larger in the case of the photocatalyzed polymerization. The highmolecular-weight values rule out that significant oxidative degradation may occur, and may reflect a segregation (compartmentalization) effect. As two radicals species located in two separate particles are unable to react, the molecular weight can increase due to a decreased termination rate. In addition, the formation of low-molecular-weight products may be limited by slower oxidation reactions compared to propagation reactions, or by a rather small concentration of ROS. The latter interpretation seems to be the most likely reason considering the much narrower polydispersity index (ca. 1.5) for the TiO 2 -initiated samples. In contrast, the reference sample has a PDI value of 2.8, more consistent with a conventional chain reaction (all the SEC traces are provided in Figure S3 in SI). Finally, the partially converted particles show size values very similar to those of the starting droplets, thus indicating a nucleation preferentially localized in the droplets. It must be emphasized that the number of monomer droplets in c-TiO 2 is about 290 times higher than the number of semiconductor nanoparticles. Consequently, the proportion of polymer/inorganic nanoparticles is assumed to remain much lower than pure polymer particles. With regard to this final point, a significantly lower concentration in TiO 2 has been used compared to other studies in aqueous solution. [22,28] One can expect that if photocatalyst particles were coated with polymer layer, the photocatalytic reactivity would be strongly reduced.

A tentative photoinitiation mechanism

While these first experiments illustrate the photoinitiation ability of nanoscale TiO 2 in dispersed media, a precise mechanism needs to be established. In principle, superoxide radicals (O 2 -) generated by a transfer of electrons from the conduction band to O 2 (reductive pathway), or hydroxyl radicals (OH  ) resulting from electron transfer from H 2 O to "holes" in the valence band (oxidative pathway ) may both initiate radical polymerization (Figure 2), provided that O 2 -and its protonated form would not dismutate to H 2 O 2 (Figure 3). A final possibility would imply the oxidation of MMA (oxidative pathway ) by "holes of the valence band and deprotonation to yield initiating C-centered radicals of the monomer substrate. ) in polymerization initiation. However, oxygen is not the only species able to be reduced by the electron in the conduction band. Among the other compounds present, it seems that only the carboxylate of MMA and the sulfate group of SDS are electron acceptors. The transfer of an electron to MMA would produce an anion radical that is protonated, the resulting allylic radical could be a weak initiator. The transfer of an electron to SDS would produce the dodecyl alcoholate and the radical of sulfurous acid anion that would instantly dimerize to dithionic acid, itself a weak oxidizing and reducing agent. Unless we would find sulfate by ion chromatography, we would have to assume that MMA is the electron acceptor. If this is the case, a slight acceleration of the polymerization could be only observed, meaning MMA reduction is not the main initiation reaction. The most likely explanation for the slight increase in conversion in the O 2 -free experiment is related to the suppression of the inhibition by oxygen. This means that the formation of less reactive ROO  radicals unable to add on new monomer molecules is prevented. In addition, a solution of MMA in anhydric acetonitrile failed to photopolymerize (0 % conversion) in presence of f-TiO 2 , revealing the minor contribution of oxidation of acrylate molecules by "holes" of the valence band to photopolymerization (pathway ). This set of experiments suggests that the presence of water is a prerequisite for the formation of initiating species, which are in all likelihood highly reactive hydroxyl radicals (pathway ). A tentative mechanism would therefore involve OH  primarily generated at the surface of the semiconductor. Despite their limited lifetime and diffusion ability, a rather large fraction of OH  may react with MMA at the aqueous/organic interface of the dispersed medium.

Subsequently, the C-centered MMA-radicals add to other MMA molecules present at high local concentration within the monomer droplets.

Primary process parameters affecting the TiO 2 photocatalyzed polymerization of

MMA

Irradiance. As summarized in Table 4, a decrease of irradiance from 430 to 115 mW/cm 2 reduces the final conversion from 82% to 58% in 10 minutes, and slightly affects the molecular weight values (ca. 500 kDa). For reaction systems implying c-TiO 2 , we observe a strong discrepancy between droplet size (50 nm) and particle size (85-155 nm), but the presence of TiO 2 particles may distort the final particle size value because of its propensity to form aggregates. The effect of irradiance is much more pronounced in the case of f-TiO 2 . At 115 mW/cm 2 , the conversion is only 20% versus 43% at 430 mW/cm 2 . This result translates a much lower concentration of initiating radicals causing a significant increase of M n . Given the fact that the efficiency of electronic excitation is much higher in nanoparticulate suspension that at immobile surfaces, the strong dependence of irradiance on the conversion of f-TiO 2 was expected. concentration has a marked effect on conversion and molecular weight (Table 5). A higher TiO 2 content enables the generation of a higher concentration of initiating species, yielding higher conversions and lower molecular weight values. However, it seems that c-TiO 2 conversion reached a threshold value. Above this value, the average molecular weight decreases. However, at the highest f-TiO 2 concentration (0.2 wt%), much shorter chains were obtained as well as a broader molecular weight distribution (PDI = 5.5), and these results may translate an increasing importance of degradation reactions. Particle sizes remain at a level close to those of droplets.

Conclusions

Due to their inherently large ratio of surface area to volume and the high efficiency of absorption of UV light, colloidal titania and titania deposited on fused silica fiber have been intensively used in the domain of photocatalysis. The idea behind this study was to prove that these beneficial properties can also be used in photopolymerizations of aqueous emulsions containing acrylate monomers. High molecular weight PMMA chains with a relatively narrow polydispersity index of about 1.5 were achieved using two different TiO 2 nanoscale forms (colloidal and supported titania). This new initiation mode combines the advantages of miniemulsion photopolymerization, such as the improvement of light absorption and fast kinetics, with the benefits of TiO 2 semiconductors: simple and low-cost process, no byproducts, and non-extractable PI residues in the polymer products. An emphasis on the f-TiO 2 system allowed to go beyond the proof of concept. First, the scale-up of the process was demonstrated and then, experiments were carried out to prove the recyclability. In the future, we foresee that such TiO 2 photoinitiating systems could be used for a broader range of waterinsoluble monomer dispersed into aqueous emulsions. In addition, new syntheses could also

Figure 1 .

 1 Figure 1. Supported and colloidal TiO 2 as photocatalysts for photoinitiation of radical polymerization of monomer aqueous emulsions

aFigure 2

 2 Figure 2 depicts the photopolymerization set-up. The reaction was performed in a 1 mm-thick

Figure 2 .

 2 Figure 2. Experimental set-up for miniemulsion photopolymerization 1. Medium pressure Xe-Hg arc lamp, 2. Long-wave elliptical reflector, 3. Light guide, 4. Glass filter, 5. Spectroscopic cuvette with an optical path of 1 mm containing the miniemulsion

TiO 2

 2 are prone to aggregation due to strong Van der Waals interactions. Additionally, TEM provides information about the high crystalline structure of the individual nanoparticles. The two main physico-chemically distinct polymorphs of TiO 2 are anatase and rutile. The precise crystalline phase was determined by XRD analysis (see Figure S2 of SI): c-TiO 2 has a pure

Table 2 .Table 3

 23 photopolymerizations were performed in a 1-mm thick spectroscopic cell to optimize the

Figure 3 .

 3 Figure 3. Photocatalytic activity of TiO 2

3. 5 Figure 4 )

 54 photopolymerization (Figure4) reveal that a minor part of the synthesized PMMA can adhere

Figure 4 .Figure 5

 45 Figure 4. SEM pictures of the f-TiO 2 before (left) and after (right) MMA miniemulsion photopolymerization

Figure 5 .Figure 6 .

 56 Figure 5. MMA conversion obtained during the photopolymerization of MMA miniemulsion when recycling f-TiO 2

Figure S3 .

 S3 Figure S3. SEC traces of PMMA obtained by miniemulsion photopolymerization catalyzed by c-TiO 2 , f-TiO 2 and a conventional type I PI (hydroxyacetophenone). Irradiation conditions: Hg-Xe lamp, 430 mW/cm 2 , 310-600 nm, 10 min UV exposure.

Table 3 .

 3 Comparison between different initiation modes for the miniemulsion photopolymerization of MMA. Irradiation conditions: Hg-Xe lamp, 430 mW/cm 2 , 310-600 nm, 10 min.

	PI	Conv. %	M n kDa	M n / M w PDI	Particle size a D p , nm
	Type I PI	63	176	2.8	63
	c-TiO 2	54	381	1.6	65
	f-TiO 2	43	181	1.4	56

a Determined by DLS Both nanoscale TiO 2 proved to be efficient PI, yielding ca. 50% conversions after 10 min of irradiation compared to 63% for a conventional PI. Control experiments without PI confirmed the absence of monomer consumption upon irradiation. Despite a greater surface area (0.84 m 2 vs 0.42 m 2 ), f-TiO 2 yields lower conversion (43 %) compared to c-TiO 2 (54 %).

Table 4 .

 4 Effect of irradiance on MMA miniemulsion photopolymerization initiated by nanoTiO 2 (D d = 50 nm) Regardless of the TiO 2 morphology, variation of the catalyst

	Nanoscale TiO 2 wt% / wt monomer	Irradiance mW cm -2	Conv %	M n kDa	PDI D p nm
		430	82	499	1.7 95
		335	76	548	1.6 85
	c-TiO 2	295	67	404	2.1 135
	3.88%	180	66	586	1.5 95
		135	62	594	1.5 115
		115	58	535	1.7 95
		430	43	181	1.4 56
	f-TiO 2	335	75	135	4.8 67
	0.17%	180	20	504	1.5 59
		115	20	548	1.5 52
	TiO 2 concentration.

Table 5 .

 5 Effect of the TiO 2 content on the MMA miniemulsion photopolymerization

		Nanoscale TiO 2	Conv	Mn	PDI	Dp
		wt% / wt monomer	%	kDa		nm
		0.07	15	641	1.4	60
	f-TiO 2	0.08 0.16	35 43	877 181	1.3 1.4	61 56
		0.20	49	108	5.5	60
		0.17	54	381	1.6	65
	c-TiO 2	0.39 0.79	66 77	600 615	1.5 1.5	66 68
		1.7	74	534	1.6	73
		3.88	82	499	1.7	95

Acknowledgement

The authors gratefully acknowledge the French Agence Nationale de Recherche (ANR), Programme Chimie Durable-Industries-Innovation (CDII, ANR-2012-CDII-006-02) for funding. The authors thank Emeric Gras from Saint Gobain who provide the f-TiO2 sample (Quartzel PCO®). Emeline Lobry acknowledges Dr Suqing Shi (College of Chemistry and Materials Science Northwest University, Xi'an, Shaanxi, China) for fruitful discussions. KEYWORDS: miniemulsion, photopolymerization, photoinitiator, photocatalysis, titanium dioxide

be envisaged such as polythiophene, poly (3,4-ethylenedioxythiophene), or polypyrrole via oxidative photopolymerization, proceeding through different mechanisms.

Acrylate miniemulsion