Guillaume Bai
email: guillaume.bai@univ-lille.fr

Jean-Christophe Hornez
email: jean-christophe.hornez@uphf.fr

Ulrich Maschke
email: ulrich.maschke@univ-lille.fr

Phillipe Supiot
email: phillipe.supiot@univ-lille.fr

Etienne Brès
email: etienne.bres@univ-lille.fr

Modification of hydroxyapatite surface properties by electron irradiation

Keywords: Hydroxyapatite, Electron irradiation, surface modification, surface fonctionnalisation, surface wettability

In the present work, the modification of wetting properties of EB irradiated HA ceramics are studied under air and N2 gas atmospheres. We show a noticeable increase of the hydrophily of electron irradiated HA ceramics in an air atmosphere and a lesser increase when the samples are irradiated in an N2 atmosphere, such a phenomenon is correlated to a modification of grain morphology obtained under the two atmospheres. With a progressive water saturation of the ceramics, the wetting angles increase progressively until they reach a plateau value different from each set of samples (irradiated under air, irradiated under N2 atmosphere and pristine).

Profilometry, SEM/EDX, XRD and Raman spectrometry analysis do not show differences between irradiated and pristine samples which indicates that the EB irradiation modifies the surface chemical elements related to the surface wetting properties and not those analysed by the above techniques.

Introduction

Hydroxyapatite (HAP, Ca10(PO4)6(OH)2, P63/m) has a close similarity to the inorganic components of teeth and bones. Due to its biocompatibility to living tissues, HAP is currently used as a ceramic [START_REF] Kleebe | High-Resolution Electron Microscopy and Convergent Beam Electron Diffraction Studies of Sintered Undoped Hydroxyapatite[END_REF][START_REF] Dorozhkin | Calcium orthophosphate bioceramics[END_REF], a coating or as vector nanoparticles for biomedical applications [START_REF] Prakasam | Fabrication, Properties and Applications of Dense Hydroxyapatite: A Review[END_REF].

As an ionic material HAP show specific surface properties [START_REF] Brès | Observation of the surface structure of biological calcium phosphate apatite crystals from human tooth enamel[END_REF][START_REF] Tasker | The stability of ionic crystal surfaces[END_REF] that favours the accommodation of a large variety of molecules that can be used for inducing specific medical responses [START_REF] Damia | Functionalization of phosphocalcic bioceramics for bone repair applications[END_REF].

A further consequence of the HAP ionic nature is its specific behavior when subjected to electron irradiation in the electron microscope as a function of different beam characteristics: accelerating voltage, fluence and fluence rate, induced temperature rise…

Effect of electron beam irradiation on HAP 1.Radiation damage

Accelerating voltage: ballistic or ionization damage?

Recent observations by Eddisford et al. [START_REF] Eddisford | Identifying and quantifying the mechanism of electron beam induced damage and recovery in hydroxyapatite[END_REF] show that the damage is more important as the voltage decreases which indicates that the damage process is dominated by radiolysis, which would explain the breaking of the P-O covalent bonds and the displacement of the P 5+ and O 2-ions outside the apatite bulk. In addition to this, the formation of dislocation lines and dislocation loops was observed at very high voltages, 2 mV, during the observation of HAP crystals [START_REF] Senger | Ballistic damages induced by electrons in hydroxyapatite[END_REF].

Effect of the beam fluence and fluence rate on HAP

A significant dependence of the extent and type of damage of HA at a given fluences or fluence rates has been shown by Eddisford et al. and Reyes-Gasga et al. [START_REF] Eddisford | Identifying and quantifying the mechanism of electron beam induced damage and recovery in hydroxyapatite[END_REF][START_REF] Reyes-Gasga | Analysis of the electron-beam radiation damage of TEM samples in the acceleration energy range from 0.1 to 2 MeV using the standard theory for fast electrons[END_REF]. Nanovoids are formed on the faces of the crystals at fluence rates below 1.6 A cm -2 [START_REF] Nelson | High-resolution electron microscopy of electron irradiation damage in apatite[END_REF][START_REF] Meldrum | Electron-irradiation-induced phase segregation in crystalline and amorphous apatite: A TEM study[END_REF], these voids can be subsequently filled after prolonged irradiation [START_REF] Reyes-Garcia | Electron beam interaction, damage, and reconstruction of hydroxyapatite[END_REF]. This shows a structural destruction without material loss. Above 1.6 A cm -2 the bulk of the crystals become amorphous while CaO particles form at the surface of the crystals. Eddisford et al., Meldrum et al. and Brès et al. [7,11 and 12] have reported that a recovery process becomes appreciable above a fluence rate of about 80 Acm -2 . They also report that fluence seems to have no influence on damage below a value of about phase (space group: P63/m) into a monoclinic phase has been suggested [START_REF] Eddisford | Identifying and quantifying the mechanism of electron beam induced damage and recovery in hydroxyapatite[END_REF]. TEM-Electron energy loss spectrometry of HA at 150kV have shown no mass loss for fluence rates 160 A cm -2 and dose rate= 70 Acm -2 and no formation of CaO [START_REF] Brès | Probe size study of apatite irradiation in stem[END_REF].

Beam induced temperature rise at the sample

The temperature at a material irradiated by the electron beam inside an electron microscope is difficult to assess. It not only depends on the experimental parameters but also on the thermal diffusivity of the material itself [START_REF] Aoba | High-Voltage Electron Microscopic Study of Dislocations in Hydroxyapatite[END_REF]. Concerning calcium phosphate, indication on the temperature at the material level can be given by the existence or the absence of chemical reactions. In situ transformation of octacalcium phosphate (OCP) into hydroxyapatite does not show the presence of additional phase [START_REF] Reimer | Transmission Electron Microscopy[END_REF]. Such a transformation has been observed in situ has shown the existence of a transient phase in the transformation of calcium deficient apatite during TEM observation [START_REF] Nelson | High-resolution electron microscopy of octacalcium phosphate and its hydrolysis products[END_REF].

Electron beam induced properties modifications

Effects of electron irradiation on HA solubility

Recently, electron irradiation at very high energy 3-10 MeV (3.42, 10.27, 20.54 Gy, irradiation times: 5h, 15h, 30h) has been shown to increase HA solubility through the creation of crystal defects [START_REF] Zhu | Engineering microstructure of hydroxyapatite by electron beam irradiation to induce controllable in vitro degradation[END_REF].

Effect of electron irradiation on HA surface hydrophily

Electron and UV irradiation have been used for modifying the surface of HA solid surface [START_REF] Brès | Evidence of a transient phase during the hydrolysis of calcium-deficient hydroxyapatite[END_REF][START_REF] Aronov | Tunable hydroxyapatite wettability: Effect on adhesion of biological molecules[END_REF], a decrease of wettability induced by a low energy irradiation of the solids has been shown by Aronov et al. [START_REF] Aronov | Charge induced wettability modification[END_REF][START_REF] Aronov | Electron-induced wettability modification[END_REF]. The phenomenon being reversible when UV radiation is applied. A method for tuning the wettability properties of HA ceramics from 10° to 100° with an accuracy of +/-3° using very low energy and fluence has been described (electron energy = 100 eV, fluence rate = 100 nAcm -2 , exposure time = 0-3000 s, vacuum = 10 -7

Torr). This enabled the differential binding of biological materials with different surface properties, such as bovine serum albumin (BSA) and deoxyribonucleic acid (DNA) [START_REF] Aronov | Hydroxyapatite nanoceramics: Basic physical properties and biointerface modification[END_REF], controlling the bacterial adhesion on HAtitanium implants [START_REF] Aronov | Electron induced surface modification of hydroxyapatite-coated implant[END_REF]. Lately, we have shown a wettability modification of human tooth surfaces by water and UV and electron-beam radiation [START_REF] Tiznado-Orozco | Wettability modification of human tooth surface by water and UV and electron-beam radiation[END_REF].

Materials and methods

Processing of the pellets

Hydroxyapatite Calcium phosphate powders were precipitated from a Calcium Nitrate precursor Ca(NO3)2-4H2O (Brenntag, France) and an diammonium Phosphate precursor (NH4)2HPO4 (Carlo Erba, France) by aqueous precipitation 1-2 with the control of parameters such as: temperature, solution pH, stirring, For Hydroxyapatite (HAP) powders syntheses, temperatures were set at 50°C and pH at 8.0. The initial Ca/P molar ratios are 1.67 with an aging time of 20 hours. The precipitate was filtrated on a filtration set-up (i. e. Buchner funnel) and dried in an oven. After this, the solution was filtered, and the precipitate was dried at 70°C. Powders were then calcined in order to reduce the grain specific surface in order to improve slip casting. Then to make a slip, grains were calcined to become bigger and putted in solution to be crushed. This will lead to eliminate clusters induce by calcination. The powders whereas then again filtrated and dried. ground 48 hours to break up agglomerates formed during the thermal treatment and reduce the powder to its ultimate particle size. This grinding step was carried out by ball milling using a HDPE milling jar and yttrium stabilized zirconia grinding media. The slip was created by auditioning water and adjuvants to the powder. The slip was then shaped into pellets in plaster moulds. The HA slurries were prepared in water with a powder concentration of 65 wt. %. Slurry defloculation is obtained by adding polyacrylate ammonium (Darvan C, R. T. Vanderbilt Co.) with 1.5 wt % of HA powder. Slurry dispersion was ensured by a ball milling for 1 hour. Slurries were casted into a cylindrical plaster mould (Ф = 15 mm, h = 5 mm). And finally, pellets were sintered at 1250 °C in air atmosphere. The heating and cooling rates were 5°C.min -1 and dwell time was fixed to 3 hours.

Characterization of the pellets before irradiation

Samples were characterized by Fourier Transform Infrared spectrometry (FTIR), X-ray diffractometry (XRD), Scanning Electron Microscopy (SEM), energy dispersive X-ray spectrometry (EDX) and Time of flight Secondary Ions Mass Spectroscopy (Tof-SIMS) Profilometry.

Polishing the pellets

Pellets were polished using #800, #1200 and #4000 SiC Struers papers on a manual Struers Rotopol-22 polishing machine.

Conservation of the pellets

After polishing, all samples were washed in an ultrasonic bath (S30 H Elmasonic) in a solution of tetrahydrofuran (Sigma-Aldrich®) for 15 min at 30°C. The pellets were then placed in a vacuum desiccator until used.

Electron irradiation (EB) 2.2.1. Description of the machine

The device use to conduct electron irradiation is a COMET-Beam Sources equipment (COMET Technology Model EBLab 180/210, Switzerland). It's composed of an electron beam that irradiates samples and a carriage that moves during exposure. Electrons are accelerated by an electric field in vacuum through a titanium window. The carriage speed, the accelerating voltage, the beam intensity and the distance between the titanium window and the surface of the sample are controlled.

Irradiation conditions

The EB radiation conditions were: accelerating voltage 150 kV, carriage speed 3 m/min, air gap 10 mm, beam current 12,5 mA, duration of irradiation 0.8 s for a surface dose around 460 kGy (~2.72x10 -2 mA.cm - 2 ). This correspond to an irradiation time of 0.8 s.

Gas atmospheres

EB irradiations were performed both in inert (N2, 2 SLPM (Standard Litre per Minute)) and ambient air gas atmospheres.

Contact angle measurements

Distilled water sessile 2 µL drops were deposited on the sample surface by a syringe and the contact angle were measured using a KRUSS Drop Shape Analyser 100. For this equipment, the standard deviation for the contact angle measurement is ±5°. Optical inspection of the drops was performed by a camera device and digital imaging techniques. The measurement of the contact angle was taken 10 s after the deposition of the drops, the estimated time of equilibrium to be reached. Two measurements were made for each drop, one from the left-hand side and one from the right-hand side of the drop. Average measurements were used for the graphs.

After each measurement, the water drop was wiped with absorbent paper. The homogeneity of the sample surface was evaluated by 5 measures, one at the center and 4 at each quadrant of the pellet as shown on Figure 1. This step permitted to concentrate our measurements at location 1 as explain below. In addition, measurements were carried out at regular time intervals at location 1. The results are present as a function of time. There is a 15s time interval between two measurements, corresponding to the necessary time of the stabilisation and the sweeping of the pellet surface with absorbent paper.

Figure 1. Locations of the drops on pellets for the contact angle measurements.

Profilometry

Profilometry data were recorded using an Alpha-Step IQ Surface Profiler. The roughness profile of all samples is the result of 3 measurements at different places on the surface. One on the centre and 2 others at different places. The scan length and the scan speed were 2 000 µm and 20 µm respectively. The average roughness Ra and the quadratic mean roughness Rq are the two main values mentioned.

XRD analysis

XRD (X ray diffractometry) data were recorded by using a Bruker AXS D8 Advance diffractometer at room temperature in a Bragg-Brentano geometry. Data were collected using the CuKα line in a 2θ range from 10 to 60° with a step size of 0.02° and 0.5 s counting time per step.

SEM/EDX analysis

The samples were examined in a JEOL JSM-7800F LV scanning electron microscope (SEM) at 10 kV equipped with an electron dispersion X ray spectrometry (EDX) detector (OXFORD INSTRUMENT X-Max N ).

The samples were metallized by a of Chromium coating prior to examination Samples were observed at different magnifications: x150, x1 200, x2 500, x5 000 and x15 000 on at least 3 different areas. In some specific cases, samples were examined at: x10 000, x20 000 and x30 000 with a diminution of the accelerated tension to 5 kV to improve the sharpness of the images. And for all those magnifications, backscattered electrons and secondary electrons images were taken.

Chemical composition variations were investigated using energy-dispersive X-ray spectrometry combined with SEM at an accelerating voltage of 10 kV and at a 2 500x magnification.

Raman spectrometry analysis

Raman analyses were performed with a Labram confocal microspectrometer (Horiba Gr, Jobin Yvon, Lille, France). A helium-neon laser with a wavelength of 632.82 nm was used. The Raman spectrometer was coupled with an Olympus microscope (BX40). The 100x (Numerical aperture 0.80) objective was used to focus the laser light on the sample and to collect backscattering light. Spectra were recorded between 200 and 2 400 cm -1 . The spectral resolution was 4 cm -1 . One each sample, 5 spectra were recorded from different locations (according to figure 1). Each spectrum was the result of the average from two accumulations of 5 s. In some samples, multiple spectra were realised along the pellet diameter.

A water immersion lens was used for observing possible spectral modifications of the water saturated HA.

XPS (X-ray photoelectron spectrometry)

XPS experiments were carried out with a Kratos AXIS Ultra DLD instrument using a monochromatic Al Kα X-ray source (10 mA, 12 kV). O 1s, C 1s, Ca 2p, and P 2p high resolution spectra were obtained. All spectra were charge corrected to give the adventitious C 1s components (i. e. the thin carbonate material usually found on surfaces exposed to air) a binding energy of 284.8 eV. Spectra were analysed using Casa XPS software (version 2.3.16, Casa Software Ltd.). The spectra were quantified after a Shirley background subtraction.

Results

The effect of EB irradiation on the sample surface has been analysed in terms of: 1) wetting property (contact angle), 2) surface roughness (profilometry),3) topography and surface composition (SEM EDX), 4) fine structure analyses of the pristine and irradiated pellets ( XRD, Raman and XP spectrometry).

Wetting property (contact angle)

The evolution contact angles vs time for repeated measurement/wipe processes is shown on Figure 2.

All three sets of samples show similar behaviours, first "starting" angles are measured on the dry samples, then the contact angles increase up to stable "plateau" contact angles values. The measurements obtained from the three curves are shown on Table 1. Those values came from the average of 3 different samples in each case.

The most extreme values both for the starting contact angles and for the time to reach the plateau angle are obtained for the sample irradiated in air gas atmosphere, the values for the sample irradiated in N2 gas atmosphere are in between the pristine and the sample irradiated in air.

This indicates that the modification of wetting property is obtained by a combination of EB irradiation and the chemical nature of the gas atmosphere surrounding the samples.

Surface roughness (Profilometry)

Roughness values are close between each sample with a low standard deviation. Non-significant differences were observed between the pristine and the irradiated samples, this indicates that the irradiation process does not influence the surface topography at the µm scale (Table 2

SEM/EDX

The surfaces of pristine and irradiated samples show a variety of holes as we can see on image A1, B1 and C1 Figure 3. These holes are probably produced during polishing. As controlled by EDX spectrometry (figure 4, Ca (Kα1) distribution), there appears to be a loss of Ca in these areas compared to the rest of the sample. Needle shape grains are observed on Figure 3, B6.

Out of "darker" area In the "darker" area Ca (%at) 21 19 P (%at) 13 14 Table 3. Ca and P % composition obtained by EDX for an irradiated sample under air. Each result is the average of 3 measurements.

EB irradiated sample in N2 atmosphere

Dark areas of smaller diameter (2 µm) similar to those observed on the air irradiated sample are seen (figure 3, C2 and C4). These areas are more uniformly distributed that those on the air irradiated sample.

XRD

No structure variation is observed between irradiated samples in gas atmosphere and pristine samples.

The presence of amorphous or CaO phases are not observed (Figure 5).

Raman spectrometry

No difference is observed between the pristine and the EB sample irradiated in air gas atmosphere (Figure 6).

Furthermore, spectra of pristine samples immersed from 1 minute to 3 hours (Figure 7) do not show any structure modification as no extra band is observed, only the intensities of existing bands are modified and this can't be allowed as a modification (due to water quantity modification during analysis).

XPS

XPS spectra O 1s, C 1s, Ca 2p and P 2p binding are identical for pristine, EB irradiated in air atmosphere and EB irradiated in N2 gas atmosphere (Figure 8). Ca/P ratio of pristine, EB irradiated in air atmosphere and EB irradiated in N2 gas atmosphere are also identical (table 4). Darker areas (Figure 3, B2 and B4) are too small to be detected by XPS analysis. No difference observed between pristine sample and both irradiated under air an N2 samples. Some carbon pollution was identified on samples by different carbonate groups like C=C, C=O and C-OH liaisons.

Discussion

The main results described in this paper are:

• EB induced time limited modification of the wetting property of dry HAP samples;

• Gas atmosphere dependence of EB irradiated grain morphology;

• Dependence of the wetting properties with water saturation for both pristine and EB irradiated samples.

For the first point, It must be noted that -at the dry state, for all characterization techniques usedexcept for the contact angle, no difference is observed between the pristine ad the irradiated samples which indicates that the irradiation process modifies solely modifies the surface elements related to the wetting properties and not those detected by the Tof SIMS and XPS spectrometry.

Important differences are observed with the previous work of Aronov et al. [START_REF] Aronov | Charge induced wettability modification[END_REF][START_REF] Aronov | Electron-induced wettability modification[END_REF] both in terms of behavior as we observe an increase of hydrophily while these authors describe opposed the opposite effect and of our previous work carried out on human tooth enamel and dentin [START_REF] Tiznado-Orozco | Wettability modification of human tooth surface by water and UV and electron-beam radiation[END_REF].

The most likely explanations for this difference is probably the large difference in irradiation conditions (voltage (accelerating voltage: 150keV vs 100eV, fluence: 2,34x10 -2 mC.cm -2 vs 100µC.cm -2 , fluence rate: 2.72x10 -2 mA.cm -2 vs 100nA.cm -2 ) and also the difference in atmospheric conditions as Aronov et al. [START_REF] Aronov | Charge induced wettability modification[END_REF][START_REF] Aronov | Electron-induced wettability modification[END_REF] have worked in 10 -7 Torr vacuum while the present work is carried out in N2 and in air atmospheres.

Different grain morphologies are observed when the EB irradiation is carried out in air and in N2 gas atmospheres. This indicates a coupling between the EB irradiation and the ambient gas atmosphere.

Although the voltage used in the present work (150kV) is similar to the one used in the electron microscope (150-400kV), the fluence and fluence rate of the two experimental setups are very different which might explain the absence of amorphization and of CaO grains in the present work.

It is also possible that the electric charge inbalance induced by the electron beam induced by the results in surface and subsurface reconstruction [START_REF] Brès | Observation of the surface structure of biological calcium phosphate apatite crystals from human tooth enamel[END_REF][START_REF] Damia | Functionalization of phosphocalcic bioceramics for bone repair applications[END_REF]. The surface being further modified by surface adhesion [START_REF] Leeuw | A Computer Modelling Study of the Adhesion of Apatite Thin Films on Silicate Surfaces[END_REF] which would explain the time limitation of the effect.

We confirm that contact angles reach a plateau with increased water saturation. This shows that the wetting properties of dry HA are not like the ones of water saturated HA. Hydroxyapatite (HA) surfaces exposed, at room temperature, to various aqueous media of different compositions (distilled water, saline solution, cell culture media, saturated HA solution) show an important decrease of the Ca/P atomic ratio of the surface layer was seen on all exposed surfaces, reaching 1, the Ca/P ratio of dicalcium phosphate dihydrate (DCPD) for the uppermost surface layer. Furthermore, HP0 2-ions have been observed by XPS and IR by Amrah-Bouali et al. [START_REF] Amrah-Bouali | Surface modifications of hydroxyapatite ceramics in aqueous media[END_REF]. Furthermore, the combined effect of water and HPO4 2--ion on HA lattice expansion has been clearly established by Elliott [27].

This bears some importance for the understanding of the properties of immersed HA as chemical functionalisation processes are performed in a wet state.

Conclusion

In the present work we have investigated the effect of EB irradiation on the surface properties of HA ceramics. Three main results are obtained for the irradiation conditions used: 1) time limited modification of the wetting properties of EB irradiated samples, 2) water saturation dependence of contact angles for pristine and EB irradiated samples, 3) gas atmosphere grain morphology dependence of EB irradiated samples. The results constitute the basic information for future developments of EB controlled functionalization of HAP. However, the dependence of HAP damage (or modification) on the irradiation conditions [START_REF] Eddisford | Identifying and quantifying the mechanism of electron beam induced damage and recovery in hydroxyapatite[END_REF][START_REF] Brès | Probe size study of apatite irradiation in stem[END_REF] must be considered. It cannot be excluded that different effects could be observed under different irradiation conditions. However, the results presented are important for the understanding of the effect of EB surface modification of HA surfaces and for the development EB induced functionalisation of HA bioceramic surfaces.

Table of figures

Figure 2 .

 2 Figure 2. Evolution of contact angles for pristine samples (a), EB irradiated samples under air (b) and EB irradiated samples under N2 atmospheres(c).

A B C 6 Figure 3 .

 63 Figure 3. SEM images of pristine sample (A), irradiated sample under air (B) and irradiated sample under N2 (C). Odd images were made with secondary electrons and even images with back scattered electrons.

3. 3 . 1 Figure 4 .

 314 Figure 4. EDX images of a pristine sample, « darker » area (Figure 3. B4) of an irradiated sample under air and of an irradiated sample under N2.

3. 3

 3 .1.1. EB irradiated sample in air atmosphere Darker areas are observed of roughly 20µm diameter are observed (Figure 3, B2 and B4).

Figure 5 .

 5 Figure 5. Diffractograms of the same sample at the pristine state (black) and at the irradiated under air state (red).

Figure 6 .

 6 Figure 6. Spectra of a pristine sample (black) and an irradiated sample under air (red).

Figure 7 .

 7 Figure 7. Raman spectra of the realized kinetic on a pristine sample (yellow spectrum at the back = initial time).

Figure 1 . 6 Figure 2 . 9 Figure 3 . 11 Figure 4 . 11 Figure 5 . 13 Figure 6 . 14 Figure 7 . 15 Figure 8 .

 16293114115136147158 Figure 1. Locations of the drops on pellets for the contact angle measurements. .................................... 6 Figure 2. Evolution of contact angles for pristine samples (a), EB irradiated samples under air (b) and EB irradiated samples under N2 atmospheres(c)..................................................................................................... 9 Figure 3. SEM images of pristine sample (A), irradiated sample under air (B) and irradiated sample under N2 (C). Odd images were made with secondary electrons and even images with back scattered electrons. .............................................................................................................................................................................. 11 Figure 4. EDX images of a pristine sample, « darker » area (Figure 3. B4) of an irradiated sample under air and of an irradiated sample under N2. ............................................................................................................. 11 Figure 5. Diffractograms of the same sample at the pristine state (black) and at the irradiated under air state (red). ............................................................................................................................................................. 13 Figure 6. Spectra of a pristine sample (black) and an irradiated sample under air (red)......................... 14 Figure 7. Raman spectra of the realized kinetic on a pristine sample (yellow spectrum at the back = initial time). .......................................................................................................................................................... 15 Figure 8. Superimposed XPS spectra of pristine, EB irradiated in air and EB irradiated in gas atmosphere samples. ............................................................................................................................................. 16

Table 1 .

 1 Starting contact angle value, plateau angle value, time for reaching the plateau angle value, amplitude between the starting contact and the plateau angle.

		Pristine sample	EB sample irradiated in	EB sample irradiated
			air atmosphere	under N2
	Starting contact angle	40±4°	19±4°	30±4°
	value
	Plateau angle value	85±5°	79±3°	77±3°
	Time for reaching the	550 s	260 s	470 s
	plateau angle value
	Amplitude between the	45±5°	60±4°	47±4°
	starting angle value and
	the plateau

Table 1 .

 1 ). Average roughness Ra and quadratic mean roughness Rq for two pristine samples, two EB irradiated under air samples and two EB irradiated under N2 samples.

		Pristine sample	EB irradiated	EB irradiated
			sample under air	sample under N2
	Average Ra	0,13 ± 0.03	0,12 ± 0.04	0,14 ± 0.06
	(µm)
	Average Rq	0,18 ± 0.04	0,18 ± 0.03	0,19 ± 0.04
	(µm)

Table 4 .

 4 Figure 8. Superimposed XPS spectra of pristine, EB irradiated in air and EB irradiated in gas atmosphere samples. Positions (eV), atomic percentage and Ca/P ratio of a pristine sample and both under air and N2 irradiated samples.

		Sample	Pristine	Irradiated under air	Irradiated under N2
	Position	O1s	531 / 48.01	531,2 / 49.62	531.1 / 51.74
	(eV) / %at	C 1s	284,8 / 27,03	284,8 / 24,15	284,8 / 21,84
	of groups	Ca 2p	347,1 /14,46	347,2 / 14,01	347,3 / 14,15
		P 2p	132,9 / 9,27	133,1 / 9,29	133,1 / 9,39
	Ca/P		1.6	1.5	1.5

Acknowledgments

We are grateful to Nicolas NUNS for Tof-SIMS analysis, Ahmed ADDAD and Alexandre FADEL for the help for SEM analysis, Guillaume PENEL and Guillaume FALGAYRAC for Raman analysis and also Pardis SIMON for XPS analysis.


Dalila Nesrine

Touaa

Chems Eddine Gherdaoui

Zohra Bouberka

Philippe Supiot

Pascal Roussel

Christel Pierlot

Ulrich Maschke
email: ulrich.maschke@univ-lille.fr

Titanium and iron modified delaminated muscovite as photocatalyst for enhanced degradation of Tetrabromobisphenol A by visible light

Keywords: Muscovite, persistent organic pollutant, Tetrabromobisphenol A, photolysis, photocatalysis

A new heterostructured titanium-iron-based material dispersed in alkaline muscovite (Ti-Fe-Mus) was synthesized by a typical impregnation process, and characterized by X-ray diffraction, adsorption/desorption of liquefied nitrogen and thermogravimetric/differential thermal analysis. This characterization suggests that the Ti-Fe-Mus system does not present sufficiently ordered silicate layers, but has a delaminated structure. The specific surface area and porosity have been significantly improved compared to the virgin muscovite, and the outer surface appears to be the dominant active surface. In particular, the Ti-Fe-Mus system was examined as a photocatalyst for the degradation of Tetrabromobisphenol A (TBBPA) selected as a model compound by visible light exposure. The Ti-Fe-Mus system showed excellent photocatalytic activity for TBBPA degradation and optimum catalyst concentration was found at 0.4 g L -1 . More than 60 % by weight of parent TBBPA with an initial concentration of 300 ppm were eliminated after 120 min at pH = 3. Acid conditions have resulted in a faster elimination compared to alkaline and neutral conditions. Based on these results, it has been shown that titanium and iron modified delaminated muscovite (Ti-Fe-Mus) has strong potential for application as a powerful photocatalyst for the elimination of persistent organic pollutants present in the environment.

Introduction

Titanium dioxide (TiO 2 ) photocatalysts are widely investigated for the degradation of organic pollutants, as they are readily available, photoactive, biologically and chemically inert, and relatively inexpensive. [1][2][3][4][5][6][7] Recently, TiO 2 clays generated significant interest; 8-10 therefore, these materials have been applied as photocatalysts for effective treatment of waste water containing toxic organic compounds. Some attempts have been made to develop heterogeneous catalysts prepared by loading iron oxide (III) on porous media such as clay, activated carbon and graphite using more or less ** corresponding author. simple techniques. 11 Clay materials are attractive to immobilize catalysts, and mica represents one of the most popular and stable mineral species. 12,13 Recently, photo-catalysts based on Fe-TiO 2 delaminated clays were designed as a set of inorganic structured materials. 14 Incorporation of a metal ion (Fe 3+ ) within a anatase structure is necessary to move the band-gap value to the visible light region, in order to obtain increased photocatalytic activity of the modified TiO 2 . 15 To the best of our knowledge, the application of Ti-Femuscovite to the decontamination of persistent organic pollutants has not yet been reported. Ti-Fe-muscovite was therefore applied to catalyze the photodegradation of Tetrabromobisphenol A (TBBPA) as a model compound.

TBBPA still represents one of the most commonly used brominated flame retardants in the world, although it may lead to biotoxicity, including immunotoxicity, neurotoxicity, endocrine disruption and cytotoxicity. 16 Elimination of this compound remains difficult because TBBPA has very limited water solubility. 17 The degradation of TBBPA under anaerobic and aerobic conditions was found to be ineffective. 18 Advanced oxidation processes, on the other hand, represent appropriate techniques to decontaminate TBBPA. 19,20 In addition, this study aims to valorize local low cost geological materials, without mineral interest. In this work, Ti-Fe-muscovite was synthesized, characterized and used to investigate photo-induced reduction of TBBPA.

Materials and methods

Starting Materials

Muscovite was obtained from a deposit at Timimoun (Algeria). Its structural formula is (K 1.46 Mn 0.01 Ca 0.29 Ti 0.11 ) (Si 6.93 Al 1.07 )(Al 1.95 Fe 2.35 Mg 0.80 )O 20 (OH) 4 , designated as "Mus". The pH of the suspension of Mus (pH = 7.52) appeared slightly alkaline, because muscovite, which has only siliceous faces, can trap protons in aqueous solution. The layer to layer cleavage of muscovite surface exposes K + , which can easily undergo hydrolysis and separate from surface. The surface of cleavage of muscovite being extremely hydrophilic and negatively charged, the cation exchange capacity (CEC) of Mus was found as 21.60 meq/100 g. 21 All reagents were of chemical grade (Sigma-Aldrich) and used as received.

Preparation of the catalyst

A muscovite dispersion (20 % in clay mass) was mixed with NaOH (5 M) under vigorous agitation, then 1M of solution of FeCl 3 6H 2 O (OH -/Fe 3+ = 5) was added in a ratio of 50 mmol of Fe 3+ per gram of clay, and maintained under agitation at 35°C. The end value of the pH was 3.5. Then, the solid was prepared by impregnation by wet process of Ti (IV) ions on solid Fe-Muscovite. After 5 min of agitation, 12 mL of concentrated TiCl 4 was added drop by drop under vigorous agitation (Fig. 1). The dissolution of muscovite is associated with a high consumption of acid and the formation of a very viscous gel structure. [START_REF] Macnaughton | Modelling the leaching behaviour of an uranium ore[END_REF] This gel was maintained at 50°C during 48 h before being washed until chloride free as indicated by the AgNO 3 test. The final product "Ti-Fe-Mus" was dried at 100°C for 4 months and was gently crushed in an agate mortar.

Characterization methods

The chemical composition was obtained by fluorescence spectrometry (Philips Magix-Pro). The results were reported in mass % of oxides for the major elements. X-ray diffraction measurements were carried out at room temperature using the diffractometer Siemens D-5000 with Cu K radiation. The X-ray tube was operated at 40 kV with a beam current of 30 mA in the range 2θ = 2-60° at a scanning rate of 2° 2θ min -1 . The database JCPDS (Joint Committee for Powder Diffraction Data-International Centre for Diffraction Data) was used to analyze the diffraction peaks. The surface of the solids was determined by adsorption of N 2 at 77 K using Micromeritics ASAP 2010, after outgassing the samples at 80°C during 12 h. The total surface area (SBET) was quantified by the BET method, while external or non-microporous surface area (SEXT) and the presence of micropores were estimated by the t-plot method. The distribution of the size of the micropores was analyzed according to the methods suggested by Horváth and Kawazoe. [START_REF] Horváth | [END_REF] The method of Barrett-Joyner-Halenda (BJH) was used to evaluate the average diameter of pores (APD).

Thermogravimetric analysis (TGA) combined with Differential Thermal Analysis (DTA) was carried out by using Perkin-Elmer Pyris 1 presenting a resolution in mass of 1 µg. The average mass of the samples was of 8mg (Platinium-HT-pans) and thermal analysis was performed under N 2 , while following a heating rate of 10°C min -1 from 20°C to 800°C. Zeta potential measurements were carried out at room temperature with a Zetasizer (Nano-ZS model, Malvern Instruments, England) by using samples with a solid/liquid ratio of 1 mg (3 mL) -1 . The infra-red absorption spectra (IR) were determined in transmission mode in the range from 4000 to 400 cm -1 using a spectrophotometer PHILIPS PU 9714. Mixtures of 99 mg pure KBr and 1 mg of sample were compressed under vacuum at room temperature.

Photodegradation experiment

Investigation on photocatalytic degradation was carried out by using TBBPA as model compound (CAS number: 79-94-7), purchased from Albermarle company (99 %). TBBPA is almost insoluble in water (20-30 ppm), with a partial solubility in acetone, chlorobenzene and o-xylene. [START_REF] Faust | Modern Chemical Techniques: An Essential Reference for Students and Teachers, Ultraviolet/visible spectroscopy[END_REF] Tetrahydrofuran (THF) was obtained from Sigma Aldrich in HPLC quality.

Aliquots of solutions containing 0.35 mL of TBBPA dissolved in THF/H 2 0 (80/20, V/V) and a photocatalyst were placed in Hellma quartz cells and were subjected to visible irradiation. At suitable exposure periods without interruption, i.e. no conduction of cumulated experiments; the vials were withdrawn from light exposure followed by immediate analysis. All experiments were repeated at least twice and average values are presented. The concentration of TBBPA was controlled by HPLC (Flexar, Perkin Elmer, USA), equipped with a UV detector and a symmetry column C18 (250 mm x 4.5 mm). The mobile phase consisted of acetonitrile (HPLC quality, Sigma-Aldrich) and of bidistilled purified water (90:10), flow rate (1 mL min -1 ), volume of injection 0.2 µL. Since the Flexar system was equipped with an array of photodiodes, the whole wavelength spectrum can be detected. The degradation of the TBBPA was determined by (2)

Photoreactor and light source

The irradiation processes were conducted at room temperature with 10 mm thick cells and the light was directed perpendicular to the cell. A source of Xenon light of 150 W (Hamamatsu LC8) associated with an optical fiber was applied, characterized by an emission spectrum covering a wide range of the UV-visible spectrum with a broad maximum in the visible region. The distance between fiber and the cell was fixed at 3 cm. The constant agitation of the solution was maintained by a vibration stirrer. The reactional solution was initially agitated during 30 min in the darkness to reach adsorption equilibrium.

Characterization of muscovite

The powder X-ray diffraction pattern of Ti-Fe-Mus compared to Mus is presented in Fig. Fe 2 O 3 PILC. [START_REF] Chen | [END_REF] The presence of a broad range of hydrolyzed Ti species of different sizes (such as monomeric TiO 2+ /Ti(OH) 2 2+ and polymeric species) 27 in the pillaring solution causes non-uniform pillaring of clay layers. 28 Tchoubar and Bergaya suggested that the hydrated iron species could aggregate together and become larger in size at higher OH -/Fe 3+ ratio, 29,30 resulting in delamination of certain host clay particles and the absence of long range face-to-face stacking of the clay layers. Indeed, the reflections are diffuse and of low intensity, which does not allow their observation on X-ray patterns. Several photocatalysts containing clay have been reported using layered montmorillonites and TiO 2 as an active phase. These reports are mainly centered on the improvement of porosity by delamination of structures and anchorage of other semiconductors, such as ZnO, Bi 2 O 3 , silver halides, and other ternary oxides, 31 necessary to study the relation between semiconductors and clay materials for the photocatalytic reaction. With to the XRD pattern of Fe 2 O 3 -pillared Montmorillonite, Mandalia et al. attributed the broad XRD peak in the area of very low 2 angles to certain disordered solids, 32 such as porous silica, 33 molecular sieves containing silica, 34 and biogenetic mesoporous silica, 35 which cannot be attributed to simple intercalation of iron hydroxyl cations, 29 This finding shows indirectly that clay presents important contents of collapsed layered-clay phases, because potassium is generally strongly placed in the ditrigonal holes of the tetrahedral layers, thus preventing any subsequent process of exchange of ions. [START_REF]Practical identification of clay minerals: A Handbook for Teachers and Students in Clay Mineralogy[END_REF][START_REF] Moore | X-ray Diffraction and the Identification and Analysis of Clay Minerals[END_REF] A high percentage of magnesium suggests that Mg belongs to the internal structure. [START_REF] Köster | [END_REF] The red colour is due to the iron oxide abundance (21.56 wt.%), indicating that this clay is a ferric aluminosilicate; Fe 2 O 3 can be present as a part of the structure, can serve as isomorphic substitution at the same time of Si and Al inside the layered structure, or as separate phases rich in Fe like phases of impurity on the surface of the sheet in form of extra-structural oxides. A noticeable increase of TiO 2 (5.5 wt.%) was observed for Ti-Fe-Mus, compared to 1.0 wt.% for Mus. A spectacular increase of Fe 2 O 3 was also observed, passing from 21.5 wt.% for Mus up to 67.6 wt.% for Ti-Fe-Mus (Table 1). The reduction of aluminium, potassium and magnesium of the clay structure after modification can be attributed to the conditions of pH. The pH treatment reduces CEC, 41 and can also release elements from the octahedral sheet. 42 The textural parameters are given in Table 2. According to IUPAC classification, both clay samples presented isotherms having a profile similar to that of the IVa type, exhibiting a welldefined H3 hysteresis loop. 43 The physisorption isotherms of type IV are associated with a capillary condensation V m

(cm 3 g -1 )
S ext

(m² g -1 ) S tot m² g -1 BJH V µP (cm 3 g -1 )
Horvath-Kawazoe APD occurring in the mesopores, limiting absorption on a wide range of P/P 0 , and monolayer-multilayer characteristics of adsorption at the initial isotherm part. A H3 hysteresis corresponds to a system formed by slit-shaped pores having a vertical adsorption branch at a relative pressure near to one, and a branch of desorption roughly at medium pressure. The curves of adsorption and desorption of Mus (Fig. 3(a)) and Ti-Fe-Mus (Fig. 3(b)) overlapped when P/P 0 was lower than 0.4, which revealed the existence of small micropores and monolayer adsorption. The quantity of adsorption of N 2 for Mus and Ti-Fe-Mus increased quickly when P/P 0 was higher than 0.4, which was caused by mesopore filling. The isothermal profiles and the loops of hysteresis indicated that the samples had a porous structure formed essentially of mesopores, associated with certain micropores, mainly with the slit-like format. Table 2 gathers the results obtained from BET analysis. Specific surface areas (S BET ) and the total volume of pores (V M ) increased from 3.61 m² g -1 to 44.6 m² g -1 and from 0.83 cm 3 g -1 to 10.25 cm 3 g -1 , while the average diameter of the pores decreased from 151.3 nm to 69.48 nm for Mus and Ti-Fe-Mus, respectively. The S BET value of Mus was found to be lower than that reported in literature, 44 due to the lower crystallinity in relationship with the presence of structural SiOH groups at crystal edge sites. During BET experiment, N 2 molecules occupy not only the external surface, but also penetrate in the capillaries. The porosity of initial muscovite results from an arrangement named "house of cards". [START_REF] Goldschmidt | Undersogelser over lersedimenter. Beretning om Nordiske Jordbrugsforskeres[END_REF] Moreover, the volume of the micropores (V µp ) accounted for 2 % of the total pore volume (TPV). In Ti-Fe-Mus, clay particles and some hydrated Fe and Ti cations outside the interlamellar space could overlap, forming a secondary "house-of-cards" structure. Consequently, mesoporosity would result from the intersections of the overlapping. These results thus suggest that the external surface was the predominant active surface. Figures 3(c The infrared spectra of Mus and Ti-Fe-Mus (Fig. 4) can be used for the identification of oxides and the distinction between the various forms of polymorphism. [START_REF] Farmer | The Infrared Spectra of Minerals[END_REF] Important peaks at 3622 cm -1 and near 3699 cm -1 can be assigned to OH groups on the internal surface. The broad OH-stretching band close to 3622 cm -1 coupled with the bands at 829 cm -1 and 750 cm -1 indicates the presence of muscovite. These bands come respectively from the Al-Mg-OH deformation [START_REF] Farmer | The Infrared Spectra of Minerals[END_REF] and the Al-O-Si in-plane vibration. The peaks at 1026 cm -1 were assigned to bands characteristic of silicates which were mainly related to the stretching vibrations of M-O (where M=Si, Al), and agreed well in the range of 1103-1036 cm - 1 . [START_REF] Vlasova | [END_REF] These stretching vibrations of the SiO 4 tetrahedrons produce a very broad band with two shoulders on the high frequency side and a shoulder on the low frequency side. The highest frequency band appears at 1108 cm -1 , which is very weak and can be easily missed. The peak at 916 cm -1 appears at a frequency appreciably lower, indicating a deformation of the Al-Al-OH group, 48,49 compared to the others, reflecting the influence of octahedral Fe and Mg on the spectrum. A vibration of the Al-Mg-OH band was observed at 829 cm -1 and the peak at 750 cm -1 was assigned to Mg-Fe-OH. Vibrations of Si-O and various modifications of silica (795 and 800 cm -1 ) were also observed by Akçay et al. 50 and Vlasova et al. [START_REF] Vlasova | [END_REF] The bands at 517 and 461 cm -1 were attributed to coupled Al-O and Si-O out-of-plane vibrations.

Distinct changes were observed on the FTIR spectrum of Ti-Fe-Mus compared to Mus. The presence of a broad band centered at 3438 cm -1 of Ti-Fe-Mus is attributed to a silicate species, [START_REF] Moore | X-ray Diffraction and the Identification and Analysis of Clay Minerals[END_REF] indicating a new substance, in agreement with X ray diffraction results. The bands at 3622 and 3699 cm -1 of Ti-Fe-Mus, reduced in intensity compared to Mus, can be attributed to the release of octahedral cations from the structure, and were confirmed by chemical analysis (Table 1). The band at approximately 1026 cm -1 , due to the Si-O stretching in the case of Mus, moved to 1451 cm -1 for 0,2 0,4 0,6 0,8 1,0 0 Ti-Fe-Mus, which implies structural changes in the tetrahedral sheets. The absorption bands at 461 and 517 cm -1 can be attributed to bending vibrations of the Si-O band, which decrease for Ti-Fe-Mus. The presence of anatase in Ti-Fe-Mus could be detected around 698 cm -1 . Comparatively, the TGA curve of Ti-Fe-Mus (Fig. 5(b)) show a loss of 4.4 mass-% up to 200°C, which can be attributed to adsorbed water loss. It is interesting to note that the weight loss in the case of Ti-Fe-Mus was quite higher than that for Mus, which indicates that Ti-Fe-Mus has a much stronger hydrophilic character. Moreover, several other thermal phenomena occurred in Ti-Fe-Mus, in particular that around 633°C, which corresponds to a loss of approximately 8.9 mass-%, ascribable to partial oxidation of Ti and Fe species impregnated in Mus. The high weight loss of 12.7 mass-% around 900°C can be attributed to the dehydration of polycations belonging to clay layers, which the structure against a structural rearrangement of the pillars.

SEM micrographs (Fig. 6) revealed modifications of texture of Mus and Ti-Fe-Mus: the morphology of Ti-Fe-Mus is compact because of delamination, in agreement with the results from XRD, while muscovite presents flaky and large particles characteristic of muscovite. SEM images of Ti-Fe-Mus show many small well dispersed aggregates, which are probably broken platelets and agglomerates. Figure 7 illustrates the effect of pH on the zeta potential of Mus and Ti-Fe-Mus. An increase of pH involved an increase of the negative charge of muscovite. The intersection of the curve with the x-axis at zeta potential equal zero gives pHiep (iep = isoelectric point). This material did not show a iep because isomorphic substitution remains independent of the pH in the acidic range. In the basic range, the edge surface of the muscovite, where the octahedral sheet is broken, can be compared with the surface of alumina or silica particles. The hydroxyl-ion acts like an ion determining the potential and causes a negative charge of the surface of the edge. 21

Photocatalytic behavior

No degradation of TBBPA without photocatalyst was observed in dark control samples. Moreover, tests of Figure 8 shows clearly that the photo-induced degradation of TBBPA by adding Ti-Fe-Mus as catalyst was faster and more efficient than photolysis. Moreover, pure Mus was applied as catalyst yielding nearly the same results as those obtained for photolysis. After 120 min of reaction time, more than 60 wt.% of TBBPA were removed by Ti-Fe-Mus, but only 40 wt.% were eliminated by adding pure Mus, or by direct photolysis in the absence of catalyst. Figure 9 presents the effect of Ti-Fe-Mus mass on TBBPA degradation to determine the optimal catalyst loading needed for maximal photodegradation. The initial slope of the curve increases considerably by increasing the catalyst mass from 5 to 10 mg, and then the degradation rate remains almost constant. With the increase of the catalyst mass, the total active surface increases and thus more active sites are available on the catalyst surface, 52 then, by increasing again the catalyst mass, the degradation rate starts to decrease. This can be explained by the aggregation of Ti-Fe-Mus particles with raised concentrations, involving a reduction of active sites on the surface, and a decrease of the penetration of visible light. 53 The photocatalytic abatement of other organic pollutants also showed the same dependence with respect to the catalyst concentration. 54 In the present work, it can be concluded from Fig. 9 that the optimal catalyst loading was 10 mg of Ti-Fe-Mus in 25 mL TBBPA solution. The photocatalytic reactions were carried out in a range of pH values between 3 and 11, since hydroxyl and hydrogen ions affect considerably the dissociation of TBBPA and the catalyst surface during the process of oxidation. Figure 10 shows the percentage of degradation of TBBPA related to pH and exposure time, initially revealing a strong increase in the elimination of TBBPA, which becomes then constant during an exposure time from 60 to 120 min. Moreover, the percentage of degraded TBBPA increased with the reduction of the pH value; the highest degradation occurred at 120 min exposure time with pH = 3 (60 wt.%), and the lowest degradation with pH = 11 (16 wt.%). An optimal degradation was thus achieved under acid conditions (pH ≈ 3.0), and this pH value was maintained for all subsequent degradation experiments.

The interpretation of the effects of the pH on the efficiency of the process of photocatalytic degradation remains a difficult task because the photodegradation rates of TBBPA depend on the distribution of the various species of TBBPA and the other concomitants in the solvent (hydrogen donor), as well as on the Ti-Fe-Mus catalyst. Several authors reported that the efficiency of elimination of TBBPA increased with the reduction of the pH values, [55][56][57] which corresponds to the results presented here. It was explained that the low pH values were beneficial for the debromination and the value of the highest debromination was obtained with pH = 4. On the other hand, a better removal efficiency of TBBPA in alkaline conditions was also reported, 58 which is explained by the fact that the higher pH can provide more hydroxide ions (OH -) to react with the holes and to form hydroxyl radicals, improving thereafter the degradation rate of TBBPA. As the effect of the pH cannot be generalized, Gogate and Pandit recommended to realize laboratory studies to establish the optimum pH conditions. 59 It was shown that pzc for Ti-Fe-Mus was equal to 5.5 (Fig. 7), above this value (TiFe-Mus) - is mainly formed and with pH values lower than pzc, the catalyst surface is protonated to form (Ti-Fe-Mus) + . Consequently, Ti-Fe-Mus was positively charged at pH = 3 and 5 while negatively charged at pH = 7, 9 and 11. In addition, TBBPA has two sites associated with the hydroxyl groups presenting different pKa values of 7.5 and 8.5, respectively. 60 Ogunbayo and Michelangeli 61 determined a global value of pKa of TBBPA in aqueous solution (9.8 +/-0.3) thus a value higher than the above mentioned values. In the explored pH range, TBBPA exists in neutral molecular conformation (pH = 3, 5, 7), and as a dissociated charged species (anion) with pH = 9, 11.

Since the surface of Ti-Fe-Mus was negatively charged with a pH higher than pzc, electrostatic repulsion between TBBPA and the surface of catalyst becomes dominating with a pH more raised, involving a reduction in the elimination of TBBPA. The effect of various initial concentrations of TBBPA on the efficiency of photocatalytic degradation is shown in Fig. 11. Table 3 accounts for the data from Fig. 11 in terms of removal efficiency (%), rate constant k and half-life t 1/2 for TBBPA degradation. The degradation efficiency decreases significantly with the increase of the initial concentration of TBBPA. The maximum efficiency (90 wt.%) was reached when the concentration of TBBPA was 90ppm together with the highest value of k (0.0350 min -1 ). When the TBBPA concentration was increased to 300 ppm, only 60 wt.% of TBBPA were degraded. The rate constant k decreased with the increase of the initial concentration of TBBPA. This finding could be attributed to the high quantity of TBBPA, occupying active sites of the surface of the catalyst and, consequently, decreasing the path length of the photons entering the solution, thus generating a reduced number of oxidizing species. Consequently, the generation of hydroxyl radicals will be reduced and light irradiation will become less effective. Figure 11 also shows that the photocatalytic degradation of TBBPA follows pseudo-firstorder kinetics in agreement with other reports. 57,58 The photocatalytic process implies many contradictory reactions. Figure 12 proposes a mechanism of the increased photocatalytic activity for the heterostructured Ti-Fe-Mus catalyst. The muscovite clay is known to be a very good electrical insulator. Consequently, the oxidation of TBBPA is initiated by photoexcitation of the semiconductor Ti-Fe-Mus, followed by the formation of electron-hole pairs on the catalyst surface.

(3) These holes react with water by producing highly reactive hydroxyl radicals ( • OH), and the electrons can be trapped by O 2 available on the surface, to initiate the presence of reactive species ( and then H 2 O 2 , 62,63 involving oxidation and further removal of TBBPA. The photoinduced electrons will form active oxygen species, in particular superoxide radical ion and hydroxyl radical • OH, which react with the TBBPA molecules and degrade them effectively. The co-solvent THF also plays a significant role in the photocatalytic reaction because this solvent reacts not only as VB hole scavenger, but also as a donor of hydrogen atoms during the process of debromination.

Stability of photocatalyst

The stability and thus the reusability of the Ti-Fe-Mus photocatalyst is a parameter of vital importance for practical applications. The ageing of the photocatalyst was therefore examined and efforts were made to optimize the regeneration process. Repeated photodegradation cycles were performed to examine the recyclability of Ti-Fe-Mus and to determine the degradation efficiency of TBBPA. After each photocatalytic cycle the catalyst was recovered from the reaction mixture by centrifugation, washed several times with THF/H 2 0 (80/20, V/V) and dried at 100°C in an oven. Measurements of the catalyst weight were performed before and after regeneration. No significant loss of catalyst was observed during this process. The reusability of Ti-Fe-Mus was investigated up to four cycles of TBBPA degradation. The results are shown Fig. 13, revealing that the photocatalytic activity of Ti-Fe-Mus was practically maintained after four successive degradation reactions with a slight decrease of the percentage of TBBPA degradation.

The chemical composition of the Ti-Fe-Mus recovered after the four cycles of photodegradation (REC) was analyzed and the data inserted into the Table 1. Only minor changes in weight were observed for REC compared to pristine catalyst. In particular no significant difference in weight of TiO 2 and Fe 2 O 3 was found in both cases, indicating high stability of Ti-Fe-Mus under the chemical environment of photodegradation even after four application cycles.

Conclusions

In this work, a new photocatalyst made up of muscovite modified with titanium and iron was prepared by the method of impregnation and characterized by several analytical methods such as X-ray diffraction and nitrogen adsorption-desorption analysis. A high surface area and high photocatalytic performance could thus be obtained.

The synthesized catalyst effectively eliminated the persistent organic pollutant TBBPA from solution (THF/H 2 O), definitively better than by photolysis. The effect of the pH revealed that the removal efficiency of TBBPA increased when the value of the pH decreased from 11 to 3, where TBBPA was in molecular form. The highest photocatalytic efficiency was obtained with an amount of catalyst of 0.4 g L -1 and an initial concentration of TBBPA of 90 ppm. Under these conditions, an elimination of 90 wt.% of TBBPA was obtained after an exposure period of 120 min. The kinetics of degradation of TBBPA followed the pseudo-first-order model. The mechanism proposed of TBBPA removal suggests multiple pathways of action on TBBPA by hydroxyl radicals.

Fig. 1 .

 1 Fig. 1. Synthesis of Ti-Fe-muscovite by impregnation method.

  concentrations before (t = 0) and after an exposure time period (t), respectively. The pseudo first-order rate k (min -1 ) was estimated by non-linear least square regression analysis

Fig. 2 .

 2 Fig. 2. XRD patterns of Mus (lower diffractogram in black color) and Ti-Fe-Mus (upper diffractogram in red color). The Ti and Fe introduction into layered clay resulted in significantly different XRD patterns; in particular the intensity decreased compared to Mus. The disappearance of the d 001 reflection together with the loss of other (001) reflections, characteristic of muscovite and kaolinite, indicates disordering of the clay sheets in the c-direction, which can be associated to their delamination: The diffuse XRD pattern observed for TiO 2 -PILC indicates a highly disordered structure, similar to that previously observed for

  but to the existence of a porous structure. XRD patterns of Ti-Fe-Mus show peaks with 2 values of 26.66°, 37.95°, 48.02°, 53.78° and 56.25° corresponding to the planes (101), (103), (200), (105) and (211) of anatase type (JCPDS No. 21-1272). The peaks with 2 values of 24.62°, 30.04°, 35.63°, 40.63° can be attributed to (012), (104), (110) and (113) crystalline structures, corresponding to hematite (JCPDS file Card, No. 33-0664).

  ) and micropore volume (V µP ) of Mus and Ti-Fe-Mus catalyst. V m : monolayer capacity derived from BET treatment. S tot : Total surface area derived from the slope of the straight line passing through the origin of the t-plot. V µp : Liquid micropore volume derived from the ordinate at the origin in the second straight line of the t-plot. S ext micro: Surface area out of micropores derived from the slope of the second straight line of the t-plot. V tot : Total pore volume (TPV), derived from the amount of nitrogen adsorbed at P/P 0 of 0.995.

Fig. 3 .

 3 Fig. 3. N 2 adsorption-desorption isotherms of (a) Mus and (b) Ti-Fe-Mus. The corresponding pore size distributions are shown as well for (c) Mus, and (d) Ti-Fe-Mus.

  ) and3(d) show the evolution of the pore size distribution of Mus and Ti-Fe-Mus, calculated from desorption experiments using the method of Barrett-Joyner-Halenda (BJH). Both figures show one peak dominating in the mesoporous range. The peak maximum of Mus at 8.63 nm was shifted to 5.84 nm for Ti-Fe-Mus, indicating a narrow pore size distribution of the photocatalyst.

Fig. 4 .

 4 Fig. 4. FTIR spectra of (a) Mus, and (b) Ti-Fe-Mus.

Fig. 5 .

 5 Fig. 5. TGA curves of (a) Mus and (b) Ti-Fe-Mus.

Figures 5 (

 5 Figures 5(a)-5(b) show the results from TGA analysis of Mus and Ti-Fe-Mus, respectively. Figure5(a) exhibits a relatively important loss of approximately 4 mass-% between room temperature and 100°C, corresponding to water loss retained on the surface of the sample, mainly due to the high percentage of the fine fraction of this clay. The loss of mass in the range from 340 to 670°C (4.7 mass-%) is due to the dehydroxylation of the clay. According to Gridi-Bennadji et al.,51 kaolinite is dehydroxylated between 450 and 550°C and muscovite between 475 and 950°C. The loss of hydroxyl groups breaks the bonds between tetrahedral silicon and octahedral aluminium, destroying the laminar structure and transforming clay into metakaolinite.Comparatively, the TGA curve of Ti-Fe-Mus (Fig.5(b)) show a loss of 4.4 mass-% up to 200°C, which can be attributed to adsorbed water loss. It is interesting to note that the weight loss in the case of Ti-Fe-Mus was quite higher than that for Mus, which indicates that Ti-Fe-Mus has a much stronger hydrophilic character. Moreover, several other thermal phenomena occurred in Ti-Fe-Mus, in particular that around 633°C, which corresponds to a loss of approximately 8.9 mass-%, ascribable to partial oxidation of Ti and Fe species impregnated in Mus. The high weight loss of 12.7 mass-% around 900°C can be attributed to the dehydration of polycations belonging to clay layers, which the structure against a structural rearrangement of the pillars.SEM micrographs (Fig.6) revealed modifications of texture of Mus and Ti-Fe-Mus: the morphology of Ti-Fe-Mus is compact because of delamination, in agreement with the results from XRD, while muscovite presents flaky and large particles characteristic of muscovite. SEM images of Ti-Fe-Mus show many small well dispersed aggregates, which are probably broken platelets and agglomerates.

Fig. 6 .

 6 Fig. 6. SEM images for Mus (on the left) and Ti-Fe-Mus (on the right).

Figure 7

 7 Figure7illustrates the effect of pH on the zeta potential of Mus and Ti-Fe-Mus. An increase of pH involved an increase of the negative charge of muscovite. The intersection of the curve with the x-axis at zeta potential equal zero gives pHiep (iep = isoelectric point). This material did not show a iep because isomorphic substitution remains independent of the pH in the acidic range. In the basic range, the edge surface of the muscovite, where the octahedral sheet is broken, can be compared with the surface of alumina or silica particles. The hydroxyl-ion acts like an ion determining the potential and causes a negative charge of the surface of the edge.21 Figure 7 also shows that Ti-Fe-Mus introduced a significant positive charge under acidic conditions, probably related to the formation of larger polyhydroxo complexes on the external surface of muscovite because of flocculation with high percentages of titanium and iron, involving a reduced electrophoretic mobility. The point of zero charge (pzc) of Ti-Fe-Mus occurs at pH = 5.5.

Fig. 7 .

 7 Fig. 7. Zeta potentials of Mus and Ti-Fe-Mus.

  modified delaminated muscovite as photocatalyst for enhanced degradation of Tetrabromobisphenol A by visible light 7 adsorption were carried out in the darkness to elucidate the role of the Ti-Fe-Mus surface on the adsorbed quantities of TBBPA. The elimination produced by Mus was zero after 120 min of reaction time and only 2 wt.% Ti-Fe-Mus was removed from the initial concentration after the same time.

Fig. 8 .

 8 Fig. 8. Degradation of TBBPA under visible light (a) without catalyst, (b) with pure Mus as catalyst and (c) with Ti-Fe-Mus catalyst (concentration 30 mg/25 mL). Other experimental conditions: TBBPA concentration 300 ppm; pH = 3 and T = 25°C.Consequently, one can conclude that visible irradiation and presence of Ti-Fe-Mus particles are essential to obtain efficient photodegradation of TBBPA. Indeed, the Ti-Fe-Mus catalyst presents high-performance photocatalytic efficiency by using luminous energy to create e-/h+ pairs on the surface. The e-/h+ pairs are then available for the processes of degradation, which generally imply the formation of reactive oxygenated species, such as HO • and O 2•-, which contribute then to the of the

Fig. 10 .

 10 Fig. 10. Effect of pH on the photocatalytic degradation of TBBPA. Experimental conditions: Ti-Fe-Mus concentration 30 mg/25 mL; TBBPA concentration 300 ppm and T = 25°C.

Fig. 11 .

 11 Fig. 11. Effect of initial concentration of TBBPA towards removal efficiency. Experimental conditions: Ti-Fe-Mus concentration 10 mg / 25mL; pH = 3 and T = 25°C. Table 3. Kinetic parameters of TBBPA degradation, applying various initial TBBPA concentrations. Experimental conditions: Ti-Fe-Mus concentration 10 mg / 25 mL; pH = 3 and T = 25°C. C 0 (ppm)

9 Fig. 12 .

 912 Fig. 12. Proposed photocatalytic process of TBBPA by Ti-Fe-Mus.

Fig. 13 .

 13 Fig. 13. Results from repeated application cycles of Ti-Fe-Mus catalyst for the photodegradation of TBBPA. Experimental conditions for each cycle : TBBPA concentration 300 ppm; Ti-Fe-Mus concentration 10 mg / 25mL; exposure time t = 120 min; pH = 3 and T = 25°C.

Table 1 .

 1 Chemical

		SiO 2	Al 2 O 3	Fe 2 O 3	CaO	MgO	MnO	K 2 O	TiO 2
	Sample	(mass%)	(mass%)	(mass%)	(mass%)	(mass%)	(mass%)	(mass%)	(mass%)
	Mus	47.72	17.66	21.56	1.90	3.70	0.13	7.92	1.01
	TFM	15.32	8.36	67.68	0.41	----	----	1.18	5.54
	REC	15.43	8.32	67.92	0.36	----	----	1.12	5.34

composition of Mus; Ti-Fe-Mus (TFM); Ti-Fe-Mus (REC) (recovered after four photodegradation cycles).

Table 1

 1

summarizes the data of chemical composition of Mus and Ti-Fe-Mus determined by XRF. Mus is characterized by high percentages of SiO 2 and Al 2 O 3 , representing principal oxides. The elements Si-and Al-are generally associated with structures of clay, quartz and feldspars. The ratio SiO 2 /Al 2 O 3 = 2.7 is thus greater than 1, and is in the interval most frequently published from 2.3 to 5. 36,37 A high quantity of K 2 O was found, indicating the presence of muscovite/illite mica, also confirmed by XRD.

Table 2 .

 2

Specific surface area (S BET ), external surface area (S ext

Acknowledgments

The authors gratefully acknowledge the support of the Algerian Ministry of Higher Education and Scientific Research, University of Sciences and Technology (Oran/Algeria), CNRS, CAMPUSFRANCE, and University of Lille (France).


Amina Bouriche

Lamia Bedjaoui Alachaher

Ulrich Maschke
email: ulrich.maschke@univ-lille1.fr

Phase behaviour and electro-optical response of systems composed of nematic liquid crystals and poly (2-ethylhexylacrylate)

Keywords: eutectic liquid crystal, poly(2-ethylhexylacrylate), phase behavior, morphology, photopolymerization, electro-optical response

Phase behaviour and electro-optical response of systems composed of nematic liquid crystals and poly (2-ethylhexylacrylate)

Amina Bouriche, Lamia Bedjaoui Alachaher, Ulrich

Introduction:

Polymer Dispersed Liquid Crystal (PDLC) film consists generally of micron-sized nematic liquid crystalline (LC) domain dispersed in a polymer matrix [START_REF] Doane | Polymer dispersed liquid crystals[END_REF][START_REF] Drzaic | Liquid crystal dispersions[END_REF][START_REF] Crawford | Liquid crystals in complex geometries[END_REF][START_REF] Jeon | Application and new developments in polymer dispersed liquid crystal simulation studies[END_REF][START_REF] Javadian | A new approach to study interaction parameters in cyanobiphenyl liquid crystal binary systems[END_REF]. They have been subject of intensive studies [START_REF] Pauluth | Advanced liquid crystals for television[END_REF][START_REF] Silva | Improvement of permanent memory effect in PDLC films using TX-100 as an additive[END_REF][START_REF] Marinov | Electro-optical response of polymerdispersed liquid crystal single layers of large nematic droplets oriented by rubbed teflon nanolayers[END_REF], because of both fundamental interest and potential applications in the field of smart windows, optical shutters, active display devices, sensors, microlenses and smart food packaging. Their unique electro-optical properties make them suitable for a large range of opto-electronical to biomedical devices [START_REF] Liu | Surface acoustic wave driven light shutters using polymerdispersed liquid crystals[END_REF][START_REF] Lee | Properties of thiol-vinyl PDLC films without additional photoinitiator[END_REF][START_REF] Su | Characterization of optically switchable holographic polymer-dispersed liquid crystal transmission gratings[END_REF][START_REF] Lai | A novel gas sensor using polymer-dispersed liquid crystal doped with carbon nanotubes[END_REF][START_REF] Xiong | Phototunable microlens array based on polymer dispersed liquid crystals[END_REF][START_REF] Bezrodna | Low-temperature photoluminescence of 5CB liquid crystal[END_REF][START_REF] Perju | Thermotropic and optical behaviour of new PDLC systems based on a polysulfone matrix and a cyanoazomethine liquid crystal[END_REF][START_REF] Li | Electrically switchable high-fold-helix spiral phase plate based on polymer dispersed liquid crystals[END_REF][START_REF] Lin | An electrically switchable surface free energy on a liquid crystal and polymer composite film[END_REF][START_REF] Hsu | Concentric polymer-dispersed liquid crystal rings for light intensity modulation[END_REF]. In the field off-state, the PDLC film scatters light due to mismatch between the effective refractive index (n eff ) of the LC and that of the polymer (n p ). In the field on-state, LCs of positive anisotropy tend to align with their nematic director parallel to the field direction. In such a state, refractive index matching occurs and the PDLC film becomes transparent.

Generally, PDLC materials can be obtained by different methods [START_REF] Mucha | Polymer as an important component of blends and composites with liquid crystals[END_REF]: thermally induced phase separation (TIPS), solvent evaporation induced phase separation (SIPS), polymerization/crosslinking induced phase separation (PIPS) and emulsion or encapsulation [START_REF] Bronnikov | Polymer-dispersed liquid crystals: progress in preparation, investigation, and application[END_REF]. The choice of one of these methods can lead to specific features of PDLC films [START_REF] Perju | Polymer-dispersed liquid crystal composites for bioapplications: thermotropic, surface and optical properties[END_REF]. In many applications, commercial multicomponent LC mixtures like the well-known eutectic nematic E7 are used, since they possess high anisotropy over a large temperature range. As an example, E7 exhibits a single nematic to isotropic transition temperature at T NI =61°C and a glass transition temperature at T g =-62°C, providing a large nematic range of approximately 120°C. In most cases these transition temperatures seems to remain identical in the presence of a polymer, however some authors report about exceptions which were explained by preferential solubility effects of the constituents of these eutectic LC blends towards the polymer. A short overview of the literature shows that this phenomenon was obtained with several polymer/LC systems like UV curable resin NOA65/E7 [START_REF] Nolan | Liquid crystal microdroplet composition in a UV cured PDLC film[END_REF] and NOA65/E44 [START_REF] Nwabunma | Phase behavior, photopolymerization, and morphology development in mixtures of eutectic nematic liquid crystal and photocurable monomer[END_REF], for which the phase behavior of the LC-rich domains deviate remarkably from those of pure eutectic LC mixtures. For the first system [START_REF] Nolan | Liquid crystal microdroplet composition in a UV cured PDLC film[END_REF], gas chromatographic (GC) analysis showed an important deviation compared to the original composition of E7. In the second system NOA65/E44, the heterogeneity of LC domains formed after photopolymerization, compared with results involving a pure LC (7CB), was attributed to different miscibilities of the E44 compounds towards NOA65.

Other systems were concerned by large shifts of phase transition temperatures in polymer/LC films; for example thermo-optical studies of poly(methylmethacrylate)/E7 [START_REF] Deshmukh | Effect of temperature on the optical and electro-optical properties of poly(methylmethacrylate)/E7 polymer-dispersed liquid crystal composites[END_REF] showed an increase of T NI , indicating preferential solubility during the phase-separation process. The same effect was observed on poly(methylphenylsiloxane) (PMPS)/E7 phase diagrams [START_REF] Gogibus | Phase diagrams of poly(siloxane)/liquid crystal blends[END_REF][START_REF] Bedjaoui | Preferential solvation of the eutectic mixture of liquid crystals E7 in a polysiloxane[END_REF], by varying the molecular weight of the polymer. It has been noticed in the case of the molecular weight of 60000g/mol that the nematic-isotropic transition temperature increased by nearly 20°C when adding 80weight% of polymer to E7. In a study of the phase behavior and morphology of poly(n-butylacrylate) (polyABu)/E7 blends, High Performance Liquid Chromatography (HPLC) analysis of the separated LC domains revealed relatively high solubility of 5CB with polyAbu, compared with PMPS [START_REF] Bouchaour | Phase equilibrium of poly(n-butyl acrylate) and E7[END_REF][START_REF] Bedjaoui | Solubility effects of multicomponent liquid crystal blends towards poly(n-butyl-acrylate)[END_REF].

In the present work, the range of investigated polymeric systems was extended to poly (2ethylhexylacrylate) (poly-2EHA), and analysis of the preferential solubility phenomenon was conducted by several techniques, such as Polarizing Optical Microscopy (POM) and Differential Scanning Calorimetry (DSC). In addition, GC coupled with Mass Spectrometry (GC-MS) was employed as promising method for the determination of the content of segregated LC domains from the poly-2EHA/E7 system.

To the best of our knowledge, no data are currently available in literature describing the consequences of the phenomenon of preferential solubility on electro-optical properties of corresponding PDLC films. Generally, electro-optical characteristics of PDLC films were examined by varying various parameters such as nature, architecture and concentration of LC and polymer, film thickness, and morphology [START_REF] Ellahi | Effects of di and tetra functional epoxy monomers structure on the morphology and the electro-optical properties of polymer-dispersed liquid crystal films[END_REF], [START_REF] Ellahi | Characterization and morphology of polymer-dispersed liquid crystal films[END_REF]. In this report the electro-optical performance of photochemically crosslinked poly-2EHA (polycross-2EHA)/LC films will be analyzed, using model LC blends prepared according to the compositional results obtained from GC-MS results.

Experimental

Materials

The monofunctional monomer 2-Ethyl Hexyl Acrylate (2EHA) (Figure 1a) was obtained from Sigma-Aldrich, the difunctional isotropic crosslinking agent hexanedioldiacrylate (HDDA) (Figure 1b

Sample preparation

The poly-2EHA/E7 blends were prepared by the TIPS process. The DSC apparatus was calibrated using indium and zinc as standards at a heating rate of 10°C/ min.

GC-MS analysis

GC-MS analysis was carried out on a Perkin Elmer Turbo Mass Spectrophotometer (Norwalk, USA) which includes a Perkin Elmer Auto sampler XLGC. The column used was a Perkin Elmer Elite-5 capillary column measuring 30m×0.25mm with a film thickness of 0.25mm composed of 95% Dimethylpolysiloxane. The carrier gas used was Helium at a flow rate of 2.5mL/min. Injection volumes of 0.4μL were used per sample. The oven temperature program was as follows: temperature was kept at 100°C for 15min and then increased to 150°C with a rate of 10°C/min followed by an isotherm period of 10min.; then temperature was increased to 240°C (10°C/min) followed by another isothermal period of 10min. Injection port and transfer line temperatures were set at 300°C and 180°C, respectively. GC-MS analysis was carried out using electron impact ionization at 70eV and data were evaluated using total ion count (TIC) for compound identification and quantification. The spectra of the components were compared with known components stored in the GC-MS database. Measurement of peak areas and data processing were carried out by Turbo Mass-OCPTVS-Demo SPL software. To establish the calibration curves, the internal calibration method was chosen by adding 4CB as standard. This standard was added to the LC sample to be analyzed with a constant concentration of 1mg/mL.

Electro-optical measurements

Results and discussion

Thermograms of the poly-2EHA/E7 system obtained from DSC measurements are displayed in Figure 2.

E7

The poly-2EHA/E7 blends were analyzed using increments of 10wt.-% starting from pure poly-2EHA to pure E7. In Figure 2 The combination between GC and MS represents a powerful method to identify and quantify LC compounds included in LC mixtures like E7. Calibration curves were established for each component relative to an added internal standard which was 4CB. Figure 4a shows the result of the separation of pure E7 by GC-MS, and Figures 4b and4c display the outcome of the analysis of phase separated LC domains in poly-2EHA/E7 in terms of the variation of the concentrations of 5CB, 7CB, 8OCB, and 5CT. The x-axis of Figure 4b and4c correspond to the poly-2EHA concentration of the examined poly-2EHA/E7 blends. Similar results as for PMPS/E7 and PABu/E7 were found, i.e. 5CB and 7CB concentrations decrease with increasing poly-2EHA concentration, whereas 8OCB and 5CT display opposite behavior. The dependences presented in Figures 4b and4c show that 5CB and 7CB are miscible to some extent with poly-2EHA while 8OCB and 5CT show poor affinity with the polymer. The variation of the 5CT concentration reveals to be quite important, and can lead to a strong increase of T NI of the segregated LC domains due to its high nematic-isotropic transition temperature which corresponds to 240°C. The results are consistent with the hypothesis proposed that the LC composition from separate domains varies depending on the polymer concentration, due to preferential miscibility phenomenon. It can be concluded that GC-MS data were helpful to qualify and quantify this effect observed by and DSC.

To the obtained GC-MS results, DSC were performed on samples made from four individual LC mixtures that correspond to the composition of the single LC compounds from segregated domains of poly-2EHA/E7 blends, which contain 10-40 wt.-% poly-2EHA. The concentration data of these LC mixtures are reported in Table 1, and these LC-compositions were analyzed by DSC and POM. below room temperature, related to crystallization and melting effects, meaning that these mixtures cannot be furthermore considered as eutectic blends. These results clearly confirm the presence of a preferential miscibility effect of the poly-2EHA/E7 system and show an important impact for practical applications. Figure 7a shows the superposition of electro-optical responses obtained for PDLC films based on photochemically crosslinked poly-2EHA (polycross-2EHA)/LC films exhibiting both a thickness of 19μm. An important improvement of the electro-optical response of the film prepared with E7 (4) was observed with respect to that of the commercial LC E7. In particular, the E7 (4) based film required much lower voltages to achieve the transparent state.

Note also that the hysteresis effect was lower, and a slightly higher transmittance T ON was found compared to the film containing E7.

A certain number of parameters can be applied to evaluate more in detail the electro-optical properties of PDLC films, such as the threshold voltage (V 10 ) [START_REF] Kim | Optical properties and optimized conditions for polymer dispersed liquid crystal containing UV curable polymer and nematic liquid crystal[END_REF]. V 10 is defined as the electric voltage required to obtain a transmittance of 10% of the PDLC film, and was found to be determined by the balance between surface anchoring and electric forces [START_REF] Ellahi | Study on the effects of isotropic cross-linked pristine morphology and electro-optical properties of PDLC films[END_REF]. As shown in Figure 7b, V 10 increases with increasing PDLC film thickness. Moreover, the films prepared with the modified LC present significantly lower V 10 values compared to those based on the pure LC E7. The presence of E7 (4) leads thus to a significant enhancement of the electrooptical properties.

Conclusions

The phase behavior of a poly-2EHA/E7 system was examined by two different techniques POM and DSC, exhibiting a strong increase of the nematic-isotropic transition temperature when increasing the poly-2EHA concentration, which can be explained by preferential miscibility effects and phase-separation phenomena involving the polymer, as well as the different LC constituents. The GC-MS analysis allowed to obtain qualitative and quantitative data of the LC content confined in phase separated domains, and revealed thus the change of LC composition within these regions, compared to pure E7. Interestingly, the electro-optical response obtained for PDLC films elaborated with a model LC blend, E7 (4), chosen as a

  ) was supplied by Cray Valley and the photo-initiator 2-hydroxy-2-methyl-1phenyl-1-propanone (Darocur 1173) was obtained from Ciba-Geigy. The poly (2-Ethyl Hexyl Acrylate) (poly-2EHA) had an average molecular weight M w =92.000g/mol and was supplied from Sigma Aldrich. The polydispersity of this polymer was determined by gel permeation chromatography (GPC) performed in tetrahydrofuran (THF) at room temperature yielding M w /M n =3. E7 represents a commercially available eutectic LC mixture and was purchased from Synthon GmbH, Germany. It contains 51wt.-% of 4-cyano-4'-n-pentyl-biphenyl (5CB), 25wt.-% of 4-cyano-4'-n-heptyl-biphenyl (7CB), 16wt.-% of 4-cyano-4'-n-oxyoctyl-biphenyl (8OCB) and 8wt.-% of 4-cyano-4''-n-pentyl-p-biphenyl (5CT) [31]. E7 exhibits a single T NI at 61°C and a glass transition temperature T g at -62°C. The refractive indices of E7 at T= 20°C are given as n o =1.5183; n e =1.7378 (λ= 632.8 nm), leading to a birefringence of Δn=n en o =0.2195 [32]. The details of the chemical structures of LC molecules involved in E7 were presented in Figure 1c. The LC 4-cyano-4'-n-butyl-biphenyl (4CB) supplied by Merck was used as a standard to establish the calibration curves needed for GC-MS analysis; its chemical structure is illustrated in Figure 1d.

Figure 1 )

 1 Figure 1) Chemical structures of (a) 2-Ethylhexylacrylate (2EHA) monomer, (b) Hexanedioldiacrylate (HDDA), (c) nematic LC E7, and (d) 4-cyano-4'-n-butyl-biphenyl (4CB).

  x wt.-% of E7 (x=10; 20; . . . ; 90) and (100-x) wt.-% of poly-2EHA were mixed together at room temperature for several hours. The phase behavior of poly-2EHA/E7 mixtures was already established experimentally by POM[START_REF] Kara-Slimane | Thermophysical behaviour of monofunctional acrylate and liquid crystal systems[END_REF][START_REF] Kara-Slimane | Phase properties of poly(2ethylhexylacrylate)/E7 systems[END_REF] and will thus be analyzed by DSC. To investigate the electro-optical performance of polymer/LC systems, PDLC films were prepared by the PIPS technique.Blends of (2EHA/HDDA/Darocur 1173) (96/2/2wt.-%) together with a LC mixture, either E7 or a model blend, were polymerized/crosslinked under UV-visible irradiation for 30min. The LC concentration of the precursor blends was kept at 70wt.-% to achieve a homogeneous thermodynamic state. The photopolymerization/crosslinking process was carried out by irradiating the sample cells under a Philips TL08 UV light source, possessing a maximum output at a wavelength of =365nm, and a dose rate of 0.5mJ/cm 2 . The cells were assembled using two 1mm thick plane parallel glass plates, which were coated inside with thin transparent conductive layers of indium tin oxide (ITO). The size of the cells was 1cmx1cm.2.2.1 Polarized optical microscopyFor polarized optical microscopy (POM) experiments, an Olympus BX41 apparatus, equipped with a Linkam heating/cooling stage LTS350 together with a temperature-controlling unit TMS 94, was used. The samples were placed between two micro glass slides, then heated up until they became optically clear, and then cooled down slowly to ambient temperature. The heating and cooling rates were 0.5°C/min. The same heating/cooling procedures were repeated three times for each sample to minimize experimental uncertainties.2.2.2 Thermal characterizationThermal properties like determination of phase transition temperatures and enthalpies of polymer/LC blends were investigated by DSC (Perkin-Elmer model 8000). Samples were prepared by introducing about 8-10mg of the initial mixture into aluminum DSC pans, to avoid evaporation effects during the temperature treatment. A rate of 10°C/min (heatingcooling cycle) was used in the temperature range from 72°C to +100°C. The program consisted first in cooling the sample, followed by three heating and cooling cycles to take into account eventual thermal events related to the sample preparation history. A Pyris computer software from Perkin-Elmer was used to determine the onset temperature of the various phase transitions. Only thermograms from the second heating scan were considered for the analysis.

For

  electro-optical measurements, the collimated beam from a He/Ne laser (=632.8nm) passed through the sample cell at an angle of 90° compared to the cell surface, and the transmitted light was collected by a photodiode. Using a standard frequency and function generator together with a voltage amplifier, an electric field was applied across the samples (1kHz-sinusoidal). Optical transmission curves were thus obtained as function of the applied voltage, allowing to extract characteristic parameters such as V 10 , representing the voltage corresponding to 10% of transmitted light. T OFF and T ON correspond to the light transmitted in the OFF and ON states, respectively.

Figure 2 )

 2 Figure 2) Thermograms obtained from DSC measurements (heating rate 10°C/min) for a series of poly-2EHA/E7 mixtures for LC concentrations ranging from 0 to 100 wt.-%. T g represents the glass transition temperature and T NI stands for the nematic to isotropic transition temperature.

Figure 3 )

 3 Figure 3) (a) Superposition of T NI data obtained by POM and DSC of the poly-2EHA/E7 system. T b-NI and T e-NI correspond to beginning and end of appearance of the T NI transition observed by POM, respectively. (b) Variation of T NI-m /T NI-p of polymer/E7 systems versus polymer concentration obtained from POM. T NI-m corresponds to the nematic-isotropic transition temperatures of LC of polymer/E7 mixtures (PMPS/E7 [25], PABu/E7 [35]). T NI-p represents T NI of pure LC E7.

Figure 4 )

 4 Figure 4) (a) Chromatogram of LC E7 obtained by GC, (b) and (c): Variation of the concentration of E7 components (5CB, 7CB, 8OCB and 5CT) depending on poly-2EHA content of the initial poly-2EHA/E7 mixtures, obtained from GC-MS data.

Table 1 :

 1 Composition of individual LC mixtures denoted E7 (1), E7 (2), E7 (3), and E7 (4), corresponding to the results from GC-MS measurements of segregated domains of the poly-2EHA/E7 system.

Figure 5

 5 Figure 5 gathers the thermograms of these four LC mixtures, showing a single glass transition and a single nematic-isotropic transition together with a large nematic region for E7 (1) and E7 (2). On the other hand, E7 (3) and E7 (4) present additional transitions at temperatures

Figure 5 )Figure 6 )Figure 7 )

 567 Figure 5) DSC thermograms of model LC mixtures composed of 5CB, 7CB, 8OCB and 5CT, which correspond to the compositions of the LCs from the segregated domains of the poly-2EHA/E7 system, obtained from GC-MS investigations. The temperatures given on the thermograms correspond to the peak maxima of the T NI transition.

Acknowledgements

This work has been accomplished in the framework of an international research program. The authors gratefully acknowledge the support of the Algerian Ministry of Higher Education and Scientific Research (MESRS), the University of Tlemcen/Algeria, the French Ministry of Higher Education and Research (MENESR), the CNRS, the University Lille 1 -Sciences and Technologies/France, and the CROUS of Lille/France.