Mohamed Sahnoune

Aurélie Taguet

Belkacem Otazaghine
email: belkacem.otazaghine@mines-ales.fr

Mustapha Kaci

José-Marie Lopez-Cuesta

Inner surface modification of halloysite nanotubes and its influence on morphology and thermal properties of polystyrene/polyamide-11 blends

Keywords: halloysite nanotubes, selective functionalization, blend morphology, blend compatibilization

The asymmetry of halloysite surface chemistry was used to perform a selective modification of its inner surface via grafting of a synthesized styrene/(methacryloyloxy)methyl phosphonic acid copolymer. Fourier transform infrared spectroscopy, thermogravimetric analysis (TGA) and pyrolysis gas chromatography/mass spectrometry were used to evidence and quantify the grafting. Then, raw and hybrid nanoparticles were incorporated in polystyrene (PS)/polyamide-11 (PA11) blends (80/20 and 60/40 wt%). Scanning electron micrographs showed differences in localization of the halloysite nanotubes (HNTs), since raw halloysite is concentrated in the PA11 phase while modified halloysite is also located at the PS/PA11 interface, leading to a better interfacial adhesion between PS and PA11. An inhibiting effect of modified halloysite on PA11 coalescence was evidenced by measuring the particle size distribution of the extracted nodules. Moreover, the presence of modified halloysite at the interface shows an improvement in terms of thermal stability as observed by TGA, but with no significant effects on PA11 crystallization behaviour as shown by differential scanning calorimetry results. Rheological measurements were carried out to study the influence of the surface modification of halloysite on the blend morphology. A gel-like behaviour was observed for the (60/40 wt%) HNTs reinforced composition that was enhanced in the case of 10% functionalized halloysite.

INTRODUCTION

Blending of polymers is a commonly used and well-known route to obtain new high-performance materials with various properties. 1,[START_REF] Utracki | Polymer Blends Handbook[END_REF] However, in the case of immiscible polymers, blends exhibit a narrow interface with high interfacial tension and weak chemical/physical interactions and thus poor properties. [START_REF] Park | [END_REF] Therefore, compatibilization is the main challenge in order to achieve polymer blends with improved properties. 4 To overcome this immiscibility, different compatibilizing agents can be used, such as block or grafted copolymers of appropriate chemical structure, 5,6 reactive compatibilizers 7,8 and more recently solid filler nanoparticles like fumed silica, 9,10 graphene, 11 titanium oxide 12 or organomodified clays. 13,14 Indeed, ternary systems of two polymers with incorporation of an inorganic filler have attracted a great deal of interest in the last few years, not only for their superior thermal and mechanical properties, 15 -17 but also for the stabilizing and compatibilizing effect of the incorporated nanoparticles. 18,19 The compatibilizing effect occurs when the filler is localized at the interface of the two polymers 20,21 and this final localization depends on several thermodynamic and kinetic parameters including incorporation sequence, 9,22 mixing time, 23,24 viscosity ratio of the two polymers 25 -27 and nanoparticle characteristics such as size, shape 28 and surface chemistry. 29,30 Polyamide-11 (PA11) and polystyrene (PS) are two widely and commonly used polymers with large field of applications from automotive to food packaging. So, a PS/PA11 blend combining the desirable properties of the two constituents will be welcomed. Many authors have worked on compatibility enhancement between PAs and PS. First, they focused on the use of PS bearing specific functions: acrylic acid, 31 manganese sulfonate 32 or maleic anhydride. 33 Then the focus was on direct in situ polymerization of the blend 34 and more recently on the utilization of mineral nanofillers such as silica and montmorillonite. 35,36 Halloysite nanotubes (HNTs) are a mineral of the kaolin group with the chemical composition Al 2 Si 2 O 5 (OH) 4 •nH 2 O. They have many advantages such as biocompatibility, thermal stability and reinforcing effect, 37 leading to a wide range of applications. Various polymer/halloysite nanocomposite systems have been reported, with, as direct 47 consequences, enhancements in mechanical properties like Young's modulus, toughness and tensile strength, improvement of thermal stability, nucleic effect and also flame-retardant behaviour. 38 -40 Added to these enhancing effects, halloysite possesses a unique tubular structure with aluminol groups at the inner surface and silanol ones at the outer. This asymmetry of composition leads to differences of reactivity, which allows selective modification of halloysite particles to be performed. Indeed, the reactivity between aluminol groups of HNTs and copolymers bearing phosphonic functions allows the inner surface of HNTs to be grafted. 41,42 There are few reported works in the literature on the use of halloysite as a compatibilizing agent in immiscible polymer blends. 43,44 HNTs modified with N-(𝛽-aminoethyl)-𝛾-aminopropyltrimethoxysilane were used to compatibilize a polyoxymethylene/polypropylene (PP) blend 43 and a syndiotactic PS/liquid crystalline polymer blend. 44 HNTs were also used with maleated styrene-ethylene/butylene-styrene to improve the performances of a PP/poly(ethylene terephthalate) blend. 45 However, there are no reports about the use of inner-surfacefunctionalized halloysite in immiscible polymer blends.

The main objective of the work reported here was to investigate the potential of halloysite, after its inner surface modification by grafting of copolymer chains, as a suitable filler and compatibilizing agent for a PS/PA11 blend. To this aim, halloysite inner surface was modified by grafting of a styrene/(methacryloyloxy)methyl phosphonic acid (P(S-co-MAPC 1 (OH) 2 )) copolymer, which was synthetized in our laboratory. Then, 80/20 and co-continuous 60/40 PS/PA11 blends filled with raw and modified HNTs were prepared by melt compounding.

EXPERIMENTAL Materials

PA11 (Rilsan ® LMNO) was kindly supplied by Arkema. This grade is free of any stabilizers or additives. Its mean molecular weight M n = 22 000 g mol -1 and average molar weight = 51 000 g mol -1 with a density of 1.02 g cm -3 . PS (CRISTAL 1340) was purchased from Total Petrochemicals. It is an amorphous PS with a high molecular weight, a melt flow index of 4 g (10 min) -1 (with 5 kg at 200 ∘ C) and a density of 1.05 g cm -3 . Raw halloysite (referenced as Hal) was derived from Djebel Debbagh deposit (eastern region of Algeria). Styrene was purchased from Sigma Aldrich, azobisisobutyronitrile (AIBN) from Fluka and MAPC 1 from Specific Polymers. Before use, HNTs and polymer pellets were dried under vacuum at 80 ∘ C for 24 h and overnight, respectively, to remove traces of moisture.

Synthesis of grafting agent

The first step consisted of a radical copolymerization of styrene and MAPC 1 under argon atmosphere. 46 Into a 100 mL flask fitted with a condenser, 15 g (1.4 × 10 -1 mol) of styrene, 3.3 g (1.6 × 10 -2 mol) of MAPC 1 , 0.26 g (1.6 × 10 -3 mol) of AIBN and 20 g of acetonitrile were introduced. Argon was bubbled through the mixture for 15 min. The mixture was then stirred and heated at 80 ∘ C for 15 h. After reaction, the polymer, P(S-co-MAPC 1 ), was purified by precipitation in methanol.

The second step consisted of hydrolysis of the phosphonate groups of the copolymer obtained in the first step. Into a 100 mL two-necked flask equipped with a Dean-Stark apparatus, 10 g of copolymer P(S-co-MAPC 1 ) (90/10 mol%) and 40 mL of dichloromethane were introduced under inert atmosphere. The mixture was degassed by bubbling argon for 15 min, stirred and heated at dichloromethane reflux for 1 h (water was eliminated by the Dean-Stark apparatus). The mixture was then cooled to room temperature and 2.7 g (1.8 × 10 -2 mol) of bromotrimethylsilane was added dropwise. After 15 h the solvent was partially eliminated under vacuum, before precipitation of the copolymer P(S-co-MAPC 1 (OH) 2 ) in methanol. A schematic representation of the grafting agent synthesis process is shown in Fig. 1.

Halloysite modification

Into a 100 mL flask fitted with a condenser were introduced 5 g of halloysite, 0.5 g of P(S-co-MAPC 1 (OH) 2 ) and 50 mL of toluene. The mixture was then stirred and heated at solvent reflux for 15 h. After the reaction, the mixture was centrifuged (5000 rpm) to eliminate the liquid phase and washed twice with tetrahydrofuran (THF) and twice with acetone. Finally, the obtained modified halloysite (referenced as Hal-M) was filtered and dried under vacuum before characterization and use. The reaction between halloysite and P(S-co-MAPC 1 (OH) 2 ) is schematically represented in Fig. 2.

Sample preparation

Raw and modified halloysite were incorporated at 3 and 10 wt% in PS/PA11 blends with two weight ratios (80/20 and 60/40), as summarized in Table 1. The samples were prepared by melt mixing under nitrogen atmosphere in a twin-screw mini-extruder DSM (Xplore) with a screw length of 135 mm and a chamber volume of 15 cm 3 . The temperature was set at 240 ∘ C and the screw speed was 60, 120 and 40 rpm during incorporation (1 min), mixing (4 min) and extrusion (almost 2 min), respectively. To avoid handling of volatile dry nanoparticles, a first solvent casting step was used to incorporate the HNTs into PS. To achieve that, 1 g of PS was dissolved in 15 mL of THF at room temperature before the introduction of the HNTs. The filler was then ultrasonically dispersed in the mixture. The solvent was then evaporated and the obtained film was dried under vacuum at 80 ∘ C overnight. The PS film containing HNTs was first pre-compounded with the rest of the PS pellets for 1 min. After pelletizing, the granules were dried under vacuum at 80 ∘ C overnight and then injection-moulded onto discs for rheological analyses using a mini injection-moulding machine (MERCATOR, Zamak). The barrel temperature was 240 ∘ C and the mould temperature was kept at 80 ∘ C. Holding pressure and injection time were 2 bar (200 kPa) and 10 s, respectively.

Annealing and selective extraction

Annealing was performed using an oven under nitrogen flow. Around 50 mg of blend was annealed at process temperature (240 ∘ C) for 10 min. Then the annealed materials were immersed into 20 mL of THF at room temperature without stirring overnight to remove PS. Finally, PA11 nodules were purified by three washing-centrifugation cycles using THF solvent and then collected for analysis.

Characterization 1 H NMR spectroscopy NMR spectra were recorded with Bruker AC 400 instruments, using deuterated chloroform as the solvent and tetramethylsilane as the references for 1 H nuclei. Chemical shifts are given in parts per million (ppm). The experimental conditions for recording 1 H NMR spectra were as follows: flip angle, 90 ∘ ; acquisition time, 4.5 s; pulse delay, 2 s; number of scans, 16.

Fourier transform infrared (FTIR) spectroscopy

The modifications to the chemical structure of halloysite after grafting were investigated using FTIR spectroscopy, performed using an IFS 66 spectrometer (Bruker). Samples were dispersed in KBr pellets (1:99 ratio) and spectra were obtained by collecting 64 scans between 400 and 4000 cm -1 with a resolution of 2 cm -1 .

Pyrolysis gas chromatography/mass spectrometry (Py-GC/MS)

A Pyroprobe 5000 pyrolyser (CDS analytical) was used to flash pyrolyse the samples in a helium environment. This pyrolyser was supplied with an electrically heated platinum filament. One coil probe enabled the pyrolysis of samples (<1 mg) placed in a quartz tube between two pieces of rockwool. The sample was heated at 900 ∘ C. The temperature was held for 15 s then the gases were drawn to the gas chromatograph for 5 min. The Pyroprobe 5000 was interfaced to a 450-GC chromatograph (Varian) by means of a chamber heated at 270 ∘ C. In the oven the initial temperature of 70 ∘ C was raised to 250 ∘ C at 10 ∘ C min -1 . The column was a Varian Vf-5 ms capillary column (30 m × 0.25 mm; thickness of 0.25 μm) and helium (1 L min -1 ) was used as the carrier gas. A split ratio was set to 1:50. The gases were introduced from the GC transfer line to the ion trap analyser of the 240-MS mass spectrometer (Varian) through the direct-coupled capillary column.

Thermogravimetric analysis

TGA was performed using a thermogravimetric analyser (Setsys Evolution, Setaram). In order to remove all the physisorbed water of the HNTs, an isothermal step (10 min, 110 ∘ C) was performed before starting the analysis and then the samples were heated to 900 ∘ C at a heating rate of 10 ∘ C min -1 . While the blend samples were heated to 600 ∘ C at 10 ∘ C min -1 . Measurements were carried out under nitrogen atmosphere with a flow rate of 20 mL min -1 for samples of approximately 20 mg.

Contact angle measurements

Contact angle measurements were carried out by depositing a liquid drop with controlled volume on the sample surface. The contact angle 𝜃 between the liquid and the substrate was measured using a KRÜSS Drop Shape Analyzer DSA30 goniometer apparatus. Thin flat discs of halloysite were prepared using a compression moulding laboratory press at 0 ∘ C and 30 bar (3 MPa). Then, contact angle measurements between sample surface and two liquids (water and diiodomethane) with different dispersive (𝛾 D L ) and polar (𝛾 P L ) contributions were performed three times for each surface-solvent pair. The dispersive (𝛾 D S ) and polar (𝛾 P S ) contributions to the surface energy of samples were calculated according to Eqn (1) using the Owens-Wendt model: 48

𝛾 L (1 + cos 𝜃) = 2 √ 𝛾 d S 𝛾 d L + 2 √ 𝛾 p S 𝛾 p L ( 1
)
where 𝛾 d L and 𝛾 p L are known for the two different liquids used.

Solvent dispersion test

A dispersion test in a water-toluene mixture was also completed on Hal and Hal-M. An amount of 5 mg of clay was introduced in 10 mL of water and ultrasonically dispersed for 5 min. Toluene (10 mL) was then added and the whole was mixed ultrasonically for another 5 min. The mixtures were left for a week and an image was obtained. Scanning electron microscopy SEM was used to evaluate the morphology of the polymer blends and the localization of halloysite, before and after modification. For that, an environmental SEM Quanta 200 FEG (FEI Company) was operated under high vacuum at 15 kV. The samples prepared by cryo-fracture were coated with a thin carbon layer.

Size distribution analysis

Size distribution measurements were made on the extracted PA11 nodules using a Coulter LS 13230 (Coulter Beckmann Co.) laser diffraction particle size analyser instrument. The PA11 nodules were collected after dissolving the PS matrix using THF solvent and the size measurements were performed using the micro liquid module (MLM, 15 mL) in THF with an obscuration of 10 ± 2%. At least two measurements were performed for each sample.

Crystallization behaviour

Crystallization behaviour of the blend formulations was investigated by DSC with a Pyris Diamond DSC (PerkinElmer Instruments) under nitrogen environment (20 mL min -1 ). Specimens of about 10 mg were heated from 30 to 220 ∘ C at a rate of 10 ∘ C min -1 (first heating scan), equilibrated at 220 ∘ C for 5 min then cooled at 10 ∘ C min -1 to 30 ∘ C, equilibrated again at 30 ∘ C, afterwards heated again (second heating scan) always at 10 ∘ C min -1 . The measurements were performed from cooling and second heating scans, whereas the first heating scan was made to eliminate the sample thermal history. Crystallization temperature (T c ), melting temperature (T m ) and melting enthalpy (ΔH m ) of the PA11 phase were determined. The degree of crystallinity (X c ) of the PA11 phase was calculated using the following equation:

X c = ΔH m ΔH o m × w t (2)
where ΔH m is the melting enthalpy, ΔH o m the melting enthalpy of 100% crystalline polymer (=206 J g -1 ) 49 and w t the PA11 weight fraction in the composition.

Rheological measurements

Oscillatory shear measurements were performed using an advanced rheometric expansion system (ARES, TA Instruments). All the tests were carried out under nitrogen atmosphere on sample discs, which were dried under vacuum at 80 ∘ C for 24 h. The measurements were performed at 240 ∘ C using the plate-plate geometry. The plate diameter was 25 mm and the gap between the two plates was set to 1 mm. A shear strain of 5% was chosen for all subsequent frequency sweep tests. The frequency was varied from 0.07 to 100 rad s -1 . A minimum of three specimens were tested for each formulation.

RESULTS AND DISCUSSION

Grafting characterization

Before the characterization of the halloysite modified with the P(S-co-MAPC 1 (OH) 2 ) copolymer, a 1 H NMR analysis was performed in order to characterize the grafting agent.

Figure 3 shows the 1 H NMR spectra of the purified products P(S-co-MAPC 1 ) and P(S-co-MAPC 1 (OH) 2 ). The NMR spectrum of P(S-co-MAPC 1 ) (Fig. 3(a)) shows a large signal centred at 3.7 ppm corresponding to the protons of the methylphosphonate groups carried by MAPC 1 units. This signal also exhibits a peak corresponding to the protons of the methylene group in 𝛼-position of the phosphorus. A comparison between the intensity of this signal characteristic of MAPC 1 units and the intensity of the signal centred at 6.9 ppm characteristic of the aromatic protons of styrene units gives a molar percentage of about 5.4 for the phosphonated units. The spectrum of P(S-co-MAPC 1 (OH) 2 ) (Fig. 3(b)) shows the disappearance of the P-O-CH 3 protons and confirms the formation of phosphonic acid groups. region (Fig. 4(b)) and methylene stretching region (Fig. 4(c)) we note the presence of new peaks for Hal-M. Indeed, in the first region, Hal-M exhibits bands at 1454 and 1492 cm -1 assigned to the characteristic C-H bending of styrene, and in the methylene region the bands at 2852 and 2925 cm -1 are assigned to symmetric and asymmetric C-H stretching, respectively. The observation of these chemical groups indicates the successful grafting of P(S-co-MAPC 1 (OH) 2 ) on halloysite. The difficulty in observing these bands is due to the localization of the grafting at the inner surface. Indeed, the peak absorption shifts towards lower values due to the more constricted vibration of alkyl groups inside the confined spaces of halloysite lumen. 41 Py-GC/MS was also used to obtain evidence for the grafting, and the results are presented in Fig. 5. The first observation on halloysite modification is the disappearance of the main observed peak for Hal that is attributed to sulfur contaminants. The elimination of these sulfur compounds can be due to clay purification through the grafting process. The chromatogram shows the appearance of several peaks corresponding to benzene, toluene, ethylbenzene, styrene, methylbenzene and indene. These compounds result from the decomposition of the grafted P(S-co-MAPC 1 (OH) 2 ) chains. [START_REF] Kadi | Gener[END_REF]51 The continuous mass drop up to 450 ∘ C for Hal is attributed to the loss of intercalated water molecules in the interlayer space. The more pronounced loss in the same range for Hal-M is attributed to the beginning of the decomposition of the grafted P(S-co-MAPC 1 (OH) 2 ) chains, whereas the main part of this copolymer is decomposed at about 450 ∘ C. Then, Hal sustains a mass loss of 11% between 410 and 560 ∘ C caused by the structural dehydroxylation of the hydroxyl groups of the inner surface, while this decomposition of the inner hydroxyl occurs in the range 530-660 ∘ C for Hal-M. We assume that this shift towards higher values in the dehydroxylation temperature range is due to the reaction between the hydroxyl groups of halloysite inner surface and the phosphonic functions of P(S-co-MAPC 1 (OH) 2 ). TGA was also used to quantify the grafting rate. Indeed, in the case of pristine halloysite the residue at 900 ∘ C is 83.9% while it is only 77.2% for Hal-M. This additional mass loss of 6.7% is related to the grafting of P(S-co-MAPC 1 (OH) 2 ). Knowing that the P(S-co-MAPC 1 (OH) 2 ) rate during the grafting procedure was 10 wt%, the grafting success rate can be estimated to 67%.

The grafting leads to a decrease of the free surface energy of halloysite, from 76.9 to 46.4 mJ m -2 . To show the influence of this grafting on the surface energy of halloysite, a dispersion test in an immiscible water-toluene mixture was conducted. As shown in Fig. 7, Hal settles in the water bottom phase, whereas Hal-M segregates at the interface of water and toluene, and this suspension is still stable even after a week.

This change of affinity confirms the modification in the clay surface chemistry and consequently its polarity after grafting by P(S-co-MAPC 1 (OH) 2 ). Indeed, due to the grafting of long copolymer chains inside the lumen of halloysite, some of these chains can emerge outside the lumen (Fig. 8) changing the surface energy of halloysite edges thus forming an amphiphilic system, the edges being nonpolar, whereas the outer surface remaining polar.

Morphology

The morphology of the various systems was mainly studied using SEM. Figure 9 presents micrographs of the (80/20) and (60/40) unfilled blends. The micrographs show that the (80/20) blend presents a matrix/dispersed phase morphology with PS as the matrix and PA11 as nodules, whereas the (60/40) blend presents a co-continuous morphology, PA11 being more fluid than PS. These observations are in accordance with theoretical predictions of phase inversion. 52 When halloysite has been added, focus was made on the PS/PA11 interface in order to examine in detail the filler location. Figure 10 displays SEM observations of PA11 nodules in the (80/20) blends filled at 3 wt%. In 80/20 H3, PA11 nodules exhibit a smooth uniform surface due to the presence of all the HNTs inside the nodules, in contrast to 80/20 HM3, where the nodules show a rough surface. This roughness is due to the presence of Hal-M at the nodule surface, as evidenced by the observations under back-scattered electrons mode. Indeed, Hal is concentrated in the nodule core because halloysite is only compatible with hydrophilic PA11. In contrast, Hal-M is clearly visible on the nodule surface, hence at the PS/PA11 interface in the 80/20 HM3 sample, confirming thus the amphiphilic behaviour of Hal-M due to the presence of P(S-co-MAPC 1 (OH) 2 ) chains (Fig. 8). Indeed, bare halloysite totally migrates from PS matrix to dispersed PA11 phase, whereas modified halloysite segregates at the interface. It is well known that when nanoparticles are first dispersed in a polymer, the introduction of a second polymer with which the nanoparticles have better affinity will cause the migration of the nanoparticle into that second polymer. 26,53 In our study, thermodynamically, the raw HNTs have a higher affinity with PA11 than with PS. Hence as the mixing process is performed for a sufficiently long time, the thermodynamically favourable state should be reached. In addition, PA11 nodules seem to be well covered (no decohesion at the interface) by the PS matrix revealing a better adhesion between PS matrix and PA11 nodules. Figure 11 displays micrographs of the (60/40) blends filled with 10 wt% of halloysite. Unsurprisingly, in the case of 60/40 H10, all the HNTs are present in the PA11 phase. In addition, we observe a poor interfacial adhesion between PS and PA11. Nevertheless, in the presence of 10 wt% of Hal-M, the filler localization is changed. Indeed, in 60/40 HM10 we still observe HNTs in the PA11 phase, but with presence of some nanotubes in the PS phase (indicated with white circles in Fig. 11). This presence of Hal-M in the PS phase, despite its hydrophobicity, can be explained by the modification of the halloysite surface chemistry. The interface is also improved with addition of modified halloysite. These results are interesting especially because HNTs exhibits a high aspect ratio, leading to low interfacial stability with a high speed of transfer. Indeed, Göldel et al. 54 characterized the final morphology of blends containing carbon nanotubes and carbon black by investigating the transfer of nanoparticles at the blend interface. They proved that the transfer of a high-aspect-ratio particle through the blend interface is faster than that of a low-aspect-ratio one. Moreover, carbon nanotubes have a very low tendency to segregate at the interface. Despite that, if they localize at the interface, they are always parallel to this interface (not perpendicular). In their article, they classified HNTs in 'high aspect ratio nano objects with fast transfer and low interfacial stability'.

To further confirm these changes in blend morphology, laser particle size analysis was performed to understand the effect of bare and modified halloysite on PA11 coalescence in the (80/20) blends. 55 Figure 12 shows the size distribution of extracted PA11 nodules before and after annealing. Regarding the unfilled (80/20) blend, PA11 nodules exhibit a broad size distribution with the presence of spherical particles in the 0-10 μm range and more elongated ones in the 10-40 μm range. So, two different phenomena are observed after annealing. First, a coalescence process that concerns the small spherical nodules is seen through a decrease of the volume fraction of the small spherical nodules (1-6 μm). Second, a relaxation process is seen of elongated nodules by an increase of the volume fraction of the largest PA11 nodules (6-18 μm) with a shift and a refinement of the broad peak (18-40 μm). With addition of 3 and 10 wt% of Hal, the system seems to be more disturbed, as the nodules exhibit an even broader size distribution with three peaks. The first one is still in the 0-10 μm range corresponding to the spherical PA11 nodules, and in the 10-80 μm range an 'M'-shaped curve, typical of particles with high aspect ratio, can be seen. The lower and higher peaks correspond to lower and higher diameters of particles with high aspect ratio, respectively. 56 After annealing, the same observations are made as for the unfilled blend with coalescence of the small nodules (especially for 80/20 H3) and relaxation of the elongated ones. Finally, with incorporation of Hal-M in the blends, both initial size distribution and evolution of the morphology during annealing are affected. Indeed, PA11 nodules show a more uniform size distribution with smaller spherical particles in the 0.3-15 μm range with fewer elongated nodules. After annealing, the volume fraction of the small nodules (0.3-15 μm) remains almost the same with still relaxation of the elongated nodules. This coalescence inhibition is the result of the stabilization of the microstructure against annealing by the presence of nanotubes of Hal-M at the PA11 nodules surface as seen in the SEM micrographs of Fig. 10.

Thermal stability

The TGA and DTG analysis for the (80/20) and (60/40) blends are shown in Figs 13 and14, respectively. Firstly, we remark from the DTG curves that the decomposition of the blends involves two steps, corresponding to the decomposition of PS and PA11, respectively. For the (80/20) blends (Fig. 13), the incorporation of both Hal and Hal-M does not improve the thermal stability of the PS phase because all the HNTs are in the PA11 phase. While the thermal stability of the PA11 phase is enhanced, but only with 3 wt% of Hal-M because of its localization at the interface. For 80/20 H10 and 80/20 HM10 the peak of the decomposition of the PA11 phase seems to be erased which can be due to the presence of a large amount of clay in the phase.

In the case of the (60/40) blends (Fig. 14), there is still an enhancement of the thermal stability of the PA11 phase upon incorporation of halloysite, but this time we also observe an effect on the PS phase. Indeed, in the 60/40 HM10 sample, the maximum degradation rate temperature increases from 389 to 400 ∘ C, while it is only 388 ∘ C for 60/40 H10. This is the confirmation of the presence of some Hal-M nanotubes in the PS phase as evidenced previously by the SEM observations (Fig. 11).

The enhancement of the thermal stability after halloysite inclusion is due to its intrinsic superior thermal stability which promotes polymer matrix stability via the formation of protecting chars during the decomposition process, and to its tubular structure with large lumen volume which allows entrapment of degradation products thus delaying mass transfer and increasing thermal stability of the polymer matrix. 57 12. Size distribution of PA11 nodules in the (80/20) blends before annealing (solid curves) and after annealing (dotted curves).

Temperature (°C)

Crystallization behaviour

Crystallization temperature (T c ), melting temperature (T m ) and calculated crystallinity (X c ) of the various formulations are reported in Table 2. The results show no difference between the unfilled (80/20) and (60/40) blends, which indicates that PA11 crystallization is not affected by its phase morphology in the blend. However, the presence of halloysite affects the crystallization of the PA11 phase. Indeed, a nucleating effect is observed through an increase in T c , particularly for blends filled at 10 wt% (+6.6 ∘ C for example in 80/20 H10). Nevertheless, no effects of halloysite modification are noted. This nucleating activity does not lead to increases in terms of crystallinity while the melting behaviour remains unaltered whatever the composition. 15(a)) exhibit a typical shear thinning behaviour with the appearance of a Newtonian plateau at low frequency (liquid-like behaviour) and no difference in this behaviour can be seen when 3% of raw or modified halloysites is added. Nevertheless, at 10 wt% of HNTs, we notice that the viscosity is even increased and the 80/20 HM10 sample has a higher viscosity than 80/20 H10 at medium and low frequencies (only in the 0.07-10 rad s -1 frequency range).

Temperature (°C)

However, as seen in Fig. 16(b), the (60/40) compositions filled with halloysite exhibit a gel-like behaviour at low frequencies (0.07-5 rad s -1 ), especially for the 60/40 HM10 sample, that is evidenced by the appearance of a plateau in the G ′ versus frequency plot. This gel-like behaviour is attributed to the formation a solid network of nanofillers due to the interfacial adhesion between the polymers and HNTs that restricts the polymer chains flow in the molten state. 10 The compositions also possess higher values of complex viscosity (Fig. 15(b)) than the corresponding unfilled blends, as filler content increases. While, at higher frequency (5-100 rad s -1 ), the effect of the modification does not appear. This increase of complex viscosity at low frequency is due to the particle-particle interaction or drop-drop hydrodynamic interaction and particle-induced clustering, 53 whereas the rheological behaviour is dominated by the polymer rather than the nanofillers at higher frequencies.

CONCLUSIONS

Halloysite selective modification was successfully achieved via grafting of P(S-co-MAPC 1 (OH) 2 ) copolymer chains at the inner surface. FTIR, TGA, Py-GC/MS and solubility test results evidenced this grafting with a change in surface chemistry, the halloysite becoming amphiphilic after its functionalization. The influence of halloysite modification on the morphology of two PS/PA11 blends (80/20 and 60/40) was examined. SEM micrographs revealed a change in halloysite location after modification as the HNTs migrate from the core of the PA11 phase to the PS/PA11 interface. This inhibits the coalescence process as evidenced by laser particle size analysis of extracted PA11 nodules before and after annealing. Since in the 60/40 blends, Hal-M is present at the interface with some nanotubes in the PS phase as confirmed by TGA, the thermal stability of the PS phase is enhanced due to the presence of some Hal-M nanoparticles. DSC results showed that both Hal and Hal-M act as nucleating agents. Finally, rheological measurements showed the limited effect of the modification on the rheology of the (80/20) compositions, while Hal-M enhances the gel-like behaviour observed in the filled (60/40) blends.

Figure 1 .

 1 Figure1. Schematic representation of P(S-co-MAPC 1 (OH) 2 ) synthesis.47

Figure 2 .

 2 Figure 2. Schematic representation of reaction between halloysite and P(S-co-MAPC 1 (OH) 2 ).

Figure 3 . 1 H

 31 Figure 3. 1 H NMR spectra of purified products: (a) P(S-co-MAPC 1 ); (b) P(S-co-MAPC 1 (OH) 2 ).

Figure 4 .

 4 Figure 4. (a) FTIR spectra of Hal, Hal-M and P(S-co-MAPC 1 (OH) 2 ), with (b) the 1400-1550 cm -1 region and (c) the methylene region of Hal and Hal-M.

Figure 5 .

 5 Figure 5. Py-GC/MS chromatograms of Hal and Hal-M.

Figure 4 (

 4 Figure 4(a) shows the FTIR spectra of pristine halloysite, modified halloysite and P(S-co-MAPC 1 (OH) 2 ) copolymer. At first view no differences between the chemical structures of Hal and Hal-M are observed, but with a detailed view of the 1400-1550 cm -1

6 .

 6 (a) TGA and (b) DTG curves of Hal and Hal-M. (c) TGA and DTG curves of P(S-co-MAPC 1 (OH) 2 ) copolymer.

Figure 7 .

 7 Figure 7. Dispersion stability of the HNTs in water-toluene mixture: (a) Hal; (b) Hal-M; (c) reference blend.

Figure 6

 6 Figure6displays the TGA and differential thermogravimetric (DTG) curves of Hal, Hal-M and P(S-co-MAPC 1 (OH) 2 ) copolymer. The mass loss in the range 40-220 ∘ C for both HNTs is due to the release of water adsorbed on the surface of the particles.[START_REF] Kadi | Gener[END_REF]51 The continuous mass drop up to 450 ∘ C for Hal is attributed to the loss of intercalated water molecules in the interlayer space. The more pronounced loss in the same range for Hal-M is attributed to the beginning of the decomposition of the grafted P(S-co-MAPC 1 (OH) 2 ) chains, whereas the main part of this copolymer is decomposed at about 450 ∘ C. Then, Hal sustains a mass loss of 11% between 410 and 560 ∘ C caused by the structural dehydroxylation of the hydroxyl groups of the inner surface, while this decomposition of the inner hydroxyl occurs in the range 530-660 ∘ C for Hal-M. We assume that this shift towards higher

Figure 8 .

 8 Figure 8. Schematic representation of halloysite lumen before and after P(S-co-MAPC 1 (OH) 2 ) grafting.

Figure 9 .

 9 Figure 9. SEM topography micrographs of unfilled (a) (80/20) and (b) (60/40) PS/PA11 blends.

10 .

 10 SEM observations of PA11 nodules in 80/20 H3 (left) and 80/20 HM3 (right) blends, under secondary electrons (top) and back-scattered electrons (bottom).

Figure 11 .

 11 Figure 11. SEM observations of 60/40 (left) and 60/40 HM10 (right) blends, under secondary electrons (top) and back-scattered electrons (bottom).

Figure 13 .

 13 Figure 13. (a) TGA and (b) DTG curves of the (80/20) blends.

Figure 14 .FrequencyFigure 15 .

 1415 Figure 14. (a) TGA and (b) DTG curves of the (60/40) blends.

FrequencyFigure 16 .

 16 Figure 16. Shear storage modulus versus frequency of (a) (80/20) blends and (b) (60/40) blends.

Table 1 .

 1 Composition of samples

	Sample	PS/PA11	Hal (wt%)	Hal-M (wt%)
	80/20	80/20	0	0
	80/20 H3	80/20	3	0
	80/20 HM3	80/20	0	3
	80/20 H10	80/20	10	0
	80/20 HM10	80/20	0	10
	60/40	60/40	0	0
	60/40 H3	60/40	3	0
	60/40 HM3	60/40	0	3
	60/40 H10	60/40	10	0
	60/40 HM10	60/40	0	10

Table 2 .

 2 T m , T c and X c values of the various PS/PA11 composites

	Sample	T m ( ∘ C)	T c ( ∘ C)	X c (%)
	80/20	191.1 ± 0.9	165.9 ± 0.8	20.1 ± 0.2
	80/20 H3	189.9 ± 1.1	167.6 ± 0.3	19.5 ± 1
	80/20 HM3	191.9 ± 0.7	166.8 ± 0.7	19.7 ± 0.9
	80/20 H10	189.6 ± 0.1	172.5 ± 0.3	19 ± 1
	80/20 HM10	191.9 ± 0.7	169.1 ± 0.3	19.3 ± 0.1
	60/40	193.8 ± 1.5	165.2 ± 0.7	19.3 ± 0.54
	60/40 H3	193.2 ± 0.2	168.4 ± 0.2	20.3 ± 0.2
	60/40 HM3	191.5 ± 0.7	166.7 ± 0.3	18.9 ± 0.2
	60/40 H10	191.5 ± 0.6	171.3 ± 0.2	19.9 ± 0.4
	60/40 HM10	193.8 ± 1.2	170.2 ± 0.7	19.8 ± 0.6
	Rheological behaviour
	Complex viscosity (𝜂*) and shear storage modulus of all samples
	are shown in Figs 15 and 16, respectively. One can note that
	the (80/20) blends exhibit higher values of 𝜂* than the (60/40)
	ones because PS is more viscous than PA11. The (80/20) blends
	(Fig.

ACKNOWLEDGEMENTS

The authors are grateful to EGIDE for its financial support through the Tassili program 13MDU891, to Jean-Jacques Flat from Arkema and to the Algerian company Soalka for supplying polyamide-11 and raw Algerian halloysite, respectively. MS is also grateful to Loic Dumazert for his help in the experimental work.


Quentin Benito

Nicolas Desboeufs

Alexandre Fargues

Alain Garcia

Florian Massuyeau

Charlotte Martineau-Corcos

Thomas Devic

Sandrine Perruchas
email: sandrine.perruchas@cnrs-imn.fr

A photoactive copper iodide phosphine-based coordination polymer

Keywords: Copper iodide cluster, diphosphine ligand, coordination polymer, luminescence thermochromism

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Introduction.

Luminescent coordination polymers are attracting attention due to their potential applications for chemical sensing and detection. 1,2,3,4,5 The most commonly studied metallic ions are Zn(II) and Cd(II) and in those cases, the luminescence properties arise from the ligands having in general conjugated organic backbone. 6,7 Lanthanide-based coordination polymers are also widely studied but the intrinsic photophysical properties of the Ln(III) ions, limit their stimuli-responsive properties. 8 Copper(I) iodide complexes exhibit a wide range of optical properties and constitute attractive components for designing original emissive materials. 9,10,11,12,13,14,15,16 In particular, molecular copper iodide clusters present extremely rich photoluminescent stimuli-responsive properties. 17,18,19,20,21 Amongst those clusters, the [Cu4I4L4] (L = organic ligand) one of cubane geometry stands out. This cluster is built on a Cu4 tetrahedron embedded into a larger I4 one, and the four organic ligands are coordinated to the Cu atoms (Figure 1). 22 [Cu4I4L4] clusters present a large panel of photoluminescent properties with stimuli-responsive features such as luminescence thermochromism, 23,24 mechanochromism, 25 rigidochromism, 26 or vapo/solvatochromism. 27 Combining multistimuli-responsive properties is particularly appealing to access original multifunctional materials toward sensory applications. Promising applicative perspectives have been already reported with the successful use of [Cu4I4L4] clusters as ink 28 and as phosphors 29 or emitters 30 in lighting devices.

The study of [Cu4I4L4] clusters for the synthesis of coordination polymers is thus especially appealing to obtain original photoactive materials. Moreover, the particular geometry of this polymetallic node is expected to lead to coordination network of high dimensionality. Indeed, coordination to the Cu atoms of the Cu4 tetrahedron should lead to a layout of the ligand in a tetrahedral fashion. The [Cu4I4] metallic center can be thus viewed as an expanded version of a four-fold coordinated single metallic center. These interesting structural and photophysical characteristics motivated investigations and several coordination polymers incorporating the [Cu4I4] metallic center have been reported. Most of the reported coordination polymers were synthesized with nitrogen 29,31 and sulfur-based 32 ligands having flexible structure while the study of rigid ligands is less documented. 33,34,35,36,37 The use of rigid ligands is well known to lead to network of high dimensionality with potential porous character. 38 Bidentate rigid phosphine ligands are commonly employed in coordination chemistry but their use for designing coordination polymers is rarely encountered 39 and to our knowledge, not reported with copper iodide derivatives. Note however that cyclodiphosphazane derivatives were successfully employed to generate [Cu4I4]-based coordination polymers but their optical properties were not investigated. Here, we report on the synthesis, structural and optical characterizations of a coordination polymer based on the [Cu4I4] cluster core connected by rigid diphosphine ligands (Figure 1), formulated [Cu4I4(Ph2P(C6H4)2PPh2)2]n (1). Although straightforward structural characterization by SCXRD analysis was not possible, combination of PXRD and solid-state NMR analysis permit to get insights into the nature of the polymeric structure. The photophysical characterizations reveal a contrasted luminescence thermochromism with dual emission interplay ascribed to the photoactive [Cu4I4(PPh2-)4] cluster unit.

Experimental Section.

Synthesis.

The solvents, tetrabutylammonium iodide (TBAI) and copper(I) iodide (CuI) were purchased from Aldrich and used as received. The 4,4'-bis(diphenylphosphino)biphenyl phosphine ligand (Ph2P(C6H4)2PPh2) was synthesized according to literature procedure 41 by lithiation of 4,4'-dibromodiphenyl followed by reaction with chlorodiphenylphosphine.

[Cu4I4(Ph2P(C6H4)2PPh2)2]n (1). To a suspension of CuI (270 mg, 1.4 mmol) in 5 mL of acetonitrile was added TBAI (530 mg, 1.4 mmol) and Ph2P(C6H4)2PPh2 (250 mg, 0.48 mmol). The reaction mixture was placed in a Parr autoclave and heated at 120 °C for 63 h. The resulting yellowish powder was filtrated and rinsed with acetonitrile. Yield = 40 % based on CuI (1.01 g, 0.56 mmol, for [Cu4I4(Ph2P(C6H4)2PPh2)2] M = 1806 g.mol -1 ). Anal. Calcd (% wt.) for C72H56P4Cu4I4: C, 47.86; H, 3.12; Found: C, 47.83; H, 3.27.

Characterizations.

Elemental analyses (C, H, N) were performed by the Service de microanalyse de l'ICSN -CNRS Gif-sur-Yvette, France.

The solid state static 65 Cu NMR spectra of 1 and 2 were recorded on a Bruker Avance 500 MHz spectrometer (B0 = 11.7 T) using a quadrupolar echo in static condition, with an inter-pulse delay of 100 µs. The recycle delay was 1 s. Total acquisition time is 3 days per sample. The spectra are referenced to CuI at 0 ppm. The samples were packed in 4 mm outer diameter rotor. The 1 H MAS, 1 H-31 P cross-polarization (CP) magic-angle spinning (MAS) and 1 H- 13 C CPMAS NMR spectra of 1, 2 and the ligand were recorded at MAS frequency of 10-12.5 kHz. For both CP experiments, the contact time was set to 3 ms and the recycling delay to 10 s. 1 H SPINAL-64 decoupling is applied during the signal acquisition. The 1 H and 13 C chemical shifts were referenced to TMS at 0 ppm, the 31 P chemical shifts were referenced to a solution of H3PO4 at 0 ppm. All spectra were analyzed using the Dmfit software. 42 Luminescence spectra were recorded on a SPEX Fluorolog FL 212 spectrofluorimeter (Horiba Jobin Yvon). The excitation source is a 450 Watt xenon lamp, excitation spectra were corrected for the variation of the incident lamp flux, as well as emission spectra for the transmission of the monochromator and the response of the photomultiplier (Peltier cooled Hamamatsu R928P photomultiplier). Low temperature measurements were recorded with a liquid helium circulation cryostat SMC TBT Air Liquid model C102084. The absolute internal quantum yields () were measured by using the Jobin Yvon integrating sphere. The error of the measured values has been evaluated to 10 %. Emission lifetimes () were recorded with a time-resolved photoluminescence setup consisting of a frequency-tripled regenerative amplified femtosecond Ti:sapphire laser system from Spectra Physics to obtain λex = 266 nm and a C7700 streak camera from Hamamatsu coupled to an SP2300 imaging Acton spectrograph for the luminescence detection. Data were analyzed by exponential curve fitting using Origin software.

The high resolution powder X-ray diffraction (PXRD) diagram was measured at room temperature on a Bruker D8 Advance diffractometer with a Debye-Scherrer geometry, in the 2θ range 7-100°. The D8 system is equipped with a Ge(111) monochromator producing Cu Kα1 radiation (λ = 1.540598 Å) and a LynxEye detector. The XRD pattern was indexed by means of the software DICVOL, 43 with an absolute error in peak positions of 0.03 °(2θ). The subsequent Le Bail structure-independent fit was performed using FullProf software 44 integrated in the WinPlotr suite. [START_REF] Roisnel | Proceedings of the Seventh European Powder Diffraction Conference (EPDIC 7)[END_REF] FTIR (Fourier Transform Infra-Red) spectra was recorded on powder with a Bruker alpha spectrometer in ATR mode.

TGA (Thermogravimetric Analysis) was performed on a Mettler Toledo TGA/DSC 1, STAR®System apparatus under pure O2, at a heating rate of 5°C.min -1 up to 600°C. N2 sorption measurement was performed at 77 K on BEL Japan Belsorp Mini apparatus. Prior to measurement, the solid was heated at 373 K under primary vacuum for one day. CO2 sorption measurement was carried out at 298 K up to 20 bars using a gravimetric IGA Hiden Isochema apparatus. The sample was outgassed under a secondary vacuum (10 -6 Pa) at 373 K for one day prior to the measurement.

Results and Discussions.

[Cu4I4(Ph2P(C6H4)2PPh2)2]n (1) was synthesized by reacting CuI and the diphosphine ligand in acetonitrile under solvothermal conditions. The addition of TBAI to the reaction medium appeared to enhance the reaction yield probably due to an increase of CuI solubility in the reacting medium. The compound was obtained as a yellowish powder. Different synthesis conditions were explored by varying the reactants ratios and concentrations, solvents, temperature, but all attempts to obtain single crystal were unsuccessful. Instead, no or lower reaction yields were obtained. TGA indicated no mass loss up to 330 °C that is in agreement with the absence of any entrapped solvent (Figure S1). The Ph2P(C6H4)2PPh2 ligand can be viewed as two connected PPh3 units (Figure 1). If we consider that each bidentate ligand is coordinated to [Cu4I4] centers, then the formed coordination polymer is the result of the fusion of [Cu4I4(PPh3)4] units. The molecular cluster [Cu4I4(PPh3)4] (2) is the prototypical member in the family of cubane copper iodides coordinated by phosphine ligands and has been well characterized. 24,[START_REF] Kitagawa | [END_REF]47,48 Comparing data of 1 and 2 appears thus relevant to get insights into the structure and to rationalize the photophysical properties of the coordination polymer 1.

In order to analyze the local structure of 1, solid-state NMR experiments were conducted. The different nuclei, 1 H, 13 C, 31 P and 65 Cu were analyzed. The last two are very informative regarding the geometrical characteristics of copper iodide complexes. The 31 P CPMAS NMR spectrum of 1 presents a broad signal centered at -25 ppm, as shown in Figure 2. The spectrum of the molecular cluster [Cu4I4(PPh3)4] (2) is also reported for comparison. 49 The latter displays well resolved peaks but with a similar pattern (envelop) and a close chemical shift value compared with 1. The multiple peaks observed in 2 result from the one-bond J-couplings between the 31 P and the two copper 65 Cu and 63 Cu isotopes (30.8 and 69.2 % of natural abundance, respectively) both having a nuclear spin value of 3/2. In that case, each independent phosphorus atom (two in 2) gives rise to two quartets which overlap in 2. The broad feature of the spectrum of 1 indicates static or dynamical disorder in the structure. This latter could be attributed to the flexibility of the phenyl groups (PPh2) having free rotation around the P-C bonds. Comparable chemical shifts in 1 and 2, confirms that the phosphorus atoms are all coordinated to copper atoms. This is further validated by the lack of signal of uncoordinated phosphorus as that observed for the free ligand with a single peak at -10 ppm. Note that little phosphine oxide is detected around +25 ppm. This small amount can be attributed to oxidation of the compound's surface during the measurement, since it is not detected in other analysis (elemental analysis). The 65 Cu static NMR spectrum of 1 is shown in Figure 2 and presents a broad feature extending over more than 400 kHz due to the large quadrupolar moment of the 65 Cu nucleus. The individual components are strongly overlapped, even at high magnetic field (11 T), preventing deconvolution. Similarity with the spectra of [Cu4I4(PPh3)4] (2) is striking, the span and shape of the patterns are almost identical. This signature is typical of copper atoms in PCuI3 four-fold coordination geometry 50 that is clearly distinct from the three-coordinated PCuI2 51 and the four-coordinate P2CuI2 and CuI4, copper environments. 49 Therefore, this result confirms without doubt the presence of [Cu4I4] cluster core of cubane geometry in 1 for which the Cu atoms are coordinated by the phosphine ligand. The resonance in the 65 Cu NMR spectrum of 1 is slightly broadened compared with those of 2 indicating small local disorder as already detected in the 31 P NMR analysis. Such disorder has been already observed in coordination polymers presenting some flexibility of the inorganic or organic part. 52 The 13 C and 1 H NMR spectra of 1 are reported in Figure 3. The 13 C NMR spectrum is typical of phenyl-based derivatives with signal of aromatic carbons around -130 ppm. The 1 H NMR spectrum presents a broad signal centered at -9 ppm in accordance with phenyl and phenylene groups. As expected, both 13 C and 1 H NMR spectra are comparable to those of the free ligand and of 2. No signal of other carbon-and hydrogen-based compounds is detected in agreement with no entrapped solvent in the structure of 1.

From the NMR analysis, it is deduced that each phosphorus atom of the ligands is coordinated to one copper atom of [Cu4I4] centers so the expected coordination polymer can be represented as illustrated in Figure 4 and is formulated [Cu4I4(Ph2P(C6H4)2PPh2)2]n. As highlighted in Figure 4, 1 can be viewed as connected [Cu4I4(PPh3)4] units through one phenyl group of the ligands (H atoms in para position suppressed), as already mentioned. The [Cu4I4(Ph2P(C6H4)2PPh2)2]n formula is confirmed by elemental analysis pointing out the 1:2 ligand:Cu stoichiometry. This analysis also confirms the absence of solvent in the structure. Only one other example of coordination complex of copper iodide with the studied ligand has been reported which is formulated [(CuI)2(Ph2P(C6H4)2PPh2)2]. 53 This complex consists in a molecular mesocate involving mononuclear CuI units with a 1:1 ligand:Cu stoichiometry. The FTIR spectra were recorded for 1, 2 and the ligand in the two 45-650 and 400-4000 cm -1 ranges (Figure 5 and SI). The low frequencies region was analyzed because the vibrational bands of the [Cu4I4] cluster core lie in this range. 54,55,56 Below 400 cm -1 (Figure S2), all vibrations of 1 correspond to the PPh2(C6H4)2PPh2 ligand. In the 45-650 cm -1 range, 1 presents two bands at 95 cm -1 and 132 cm -1 . These two bands also displayed by 2 are characteristic of molecular [Cu4I4L4] clusters of cubane structure and correspond to Cu-I bond vibrations. 54,55,56 The band observed at 434 cm -1 only presents in the spectra of 1 and the ligand can be attributed to C-H vibration of the phosphine ligand. The broad band centered at 511 cm -1 can be attributed to vibrations of the phenyl groups. The band at 618 cm -1 in the spectra of 1 and 2 may be attributed to Cu-P bond vibration. The IR analysis agrees with the 31 P solid-state NMR study and confirms the [Cu4I4] cluster core coordinated by PPh2(C6H4)2PPh2 ligands in the structure of 1. The PXRD diagram of 1 presents well defined diffraction peaks attesting the crystallinity of the coordination polymer (Figure 6). Indexation of the diagram (DicVol FullProf) gave as the best result a cubic cell of parameter a = 14.9894( 7) Å (V = 3367.9(3) Å 3 ) while extinctions suggest I23, I213, Im-3, I432, I-43m, Im-3m as possible space groups. As mentioned above, the NMR study and chemical analyses indicate a stoichiometry of two ligands per [Cu4I4] unit. Nitrogen and carbon dioxide sorption experiments performed at 77 and 298 K respectively, revealed the non-porous character of 1 (ESI, Figures S3 andS4). One can thus expect an average volume par atom of ca. 20 Å 3 in this structure, as found in dense compounds of similar composition (20.9 and 18.9 Å 3 per atom in [Cu4I4(PPh3)4] [START_REF] Kitagawa | [END_REF] and [(CuI)2(Ph2P(C6H4)2PPh2)2], 53 respectively). Considering the determined cell volume, this corresponds to the formula [Cu4I4(Ph2P(C6H4)2PPh2)2] in the unit-cell. Preliminary attempts to solve the structure permits to localize the [Cu4I4] cluster core but the organic part was not resolved because of a symmetry conflict. Indeed, the 3-fold axis presents in the cubic cell passing through the long diagonal of the [Cu4I4] center is not compatible with the phosphine ligand (Ph2P(C6H4)2PPh2), as the ligand itself cannot sustain a 3-fold axis.

Considering the content of the unit-cell, the heteroatoms (Cu, I, P) should lie on a crystallographic site of multiplicity lower or equal to 8. Nevertheless, in the possible space groups, these sites are either located on a 3fold axis or incompatible with the geometry of the [Cu4I4P4] unit, which was unambiguously identified by solidsate NMR and luminescence studies (see below). Hence, the unit cell parameters and/or the symmetry are wrong, and either additional diffraction peaks or peaks splitting must be present. Nevertheless, none of these features were detected on PXRD data. This probably arises from the moderate crystallinity of 1, which gives rise to peak broadening preventing the detection of peak splitting, and hence the definitive indexation of the PXRD pattern. Regarding the photophysical properties, the yellowish powder of [Cu4I4(Ph2P(C6H4)2PPh2)2]n (1) emits in the yellow range under UV illumination, at room temperature (Figure 7). When cooled to 77 K, the emission becomes bright green and more intense. With this contrasted emission color change, which is completely reversible, 1 displays thermochromic luminescence properties. Temperature-dependent solid-state emission and excitation spectra were recorded from 20 to 290 K and are shown in Figure 8. At 290 K (λex = 300 nm), the emission spectrum displays a single unstructured broad emission band centered at 573 nm (LE for Low Energy), in agreement with the yellow light observed (Figure 8a). The absolute internal quantum yield determined at room temperature is 2 % (λex = 390 nm). Emission lifetime recorded at room temperature is  = 2.95 s. The Stokes shift calculated between excitation and emission maxima (LE) is of 8190 cm -1 . By lowering the temperature, another emission band at higher energy appears (HE for High Energy). This HE band is centered at 510 nm at 75 K and corresponds to the green emission observed in liquid nitrogen (Figure 7). This band presents a structured feature with three resolved peaks at 475, 510 and 545 nm at 20 K (Figure 8a). Similar excitation spectra recorded for each of these peaks confirms their same origin and their regular spacing (~ 1330 cm -1 ) indicates their vibronic character (Figure SI). The intensity of the HE band progressively increases upon cooling with concomitant disappearance of the LE band (Figure 8b). Note that the LE band can be selectively excited. For excitation at 415 nm, the LE band is largely predominant over the whole temperature range (Figure S5).

The broad emission band observed at room temperature can be attributed to the LE band classically observed for molecular [Cu4I4L4] cubane clusters. The value of λmax = 573 nm lies in the same range than that reported for [Cu4I4(PPh3)4] (2) of 545 nm (HE at 415 nm) with a close emission lifetime value (4.30 s) but the related quantum yield value is much higher for 2 (64 %), as reported in Table S1. 24 From DFT calculations, this LE band is assigned to a transition of mixed XMCT and MM character (X = I and M = Cu). 24,57 The corresponding triplet excited stated is commonly labelled 3 CC for Cluster Centered. The nature of the HE band is different and implies in that case the ligands. The corresponding transition is of 3 (X,M)LCT character. The luminescence thermochromism displayed by 1 results from intensity variation of the two emission bands (HE and LE) with the temperature. This thermochromism is thus similar to that usually exhibited by molecular [Cu4I4L4] cubane clusters. The thermal equilibrium between the two corresponding excited states is at the origin of the intensity variation of the emission bands. At low temperature, the excited state of lowest energy is populated and is responsible for the HE emission. Upon increasing the temperature, thermal energy permits to populate the second excited state leading to the LE emission. The main difference with the [Cu4I4L4] cubane clusters, is the well resolved vibronic structuration of the HE band of 1. The involvement of the ligand in this transition ( 3 (X,M)LCT) is in accordance with the vibronic structure which has been indeed already observed for such aromatic ligands. 58 This vibrational effect can be boosted by the rigid nature of the coordination network of 1. Amongst the reported coordination polymers incorporating [Cu4I4] units, only two examples exhibit luminescence thermochromism with interplay of two emission bands 59,60 but in those cases, the emissions originate from two different copper species. Otherwise, only a shift of the single emission band is observed with temperature variation but in any case no vibronic structuration was reported. 31,61,62,63,64

Conclusion.

A coordination polymer based on the polymetallic [Cu4I4] center was synthesized by coordination with phenyl based-phosphine ligands. The combination of different characterization techniques permits to get insights into the polymeric structure of this compound formulated [Cu4I4(Ph2P(C6H4)2PPh2)2]n. This coordination polymer is built on [Cu4I4] centers linked by bidentate phosphine ligands forming a high dimensional structure with possible interpenetrated networks. This coordination polymer exhibits contrasted luminescence thermochromic properties ascribed to the photoactive copper iodide unit with vibronic structuration of the emission band. This study demonstrates that original photoactive coordination polymers based on copper iodide derivatives can be obtained. Such materials with additional porosity properties promises attractive potentialities for chemical sensing and detection. The rich photophysical properties and economic advantages of copper-based materials surely offer great possibilities to design readily accessible and functional light-emitting materials.

40

 40

Figure 1 .

 1 Figure 1. Molecular structure of the [Cu4I4] cubane cluster core and the studied diphosphine Ph2P(C6H4)2PPh2 ligand.

Figure 2 .

 2 Figure 2. Solid-state 31 P CPMAS NMR spectra of 1, 2 and the ligand Ph2P(C6H4)2PPh2 with * denoting a small amount of phosphine oxide, and 65 Cu static WURST-QCPMG NMR spectra of 1 and 2.

Figure 3 .

 3 Figure 3. Solid-state NMR 13 C and 1 H MAS spectra of 1 and 2, * denotes spinning side bands.

Figure 4 .

 4 Figure 4. Schematic drawing of the polymeric structure of 1. Structural analogy with connected [Cu4I4(PPh3)4] clusters is highlighted by the circle in dotted line.

Figure 5 .

 5 Figure 5. FTIR spectra of 1, 2 and the ligand.

Figure 6 .

 6 Figure 6. PXRD diagram and pattern matching of 1.

  The 'primary' structure of the studied coordination polymer [Cu4I4(Ph2P(C6H4)2PPh2)2]n (1) consisting of [Cu4I4] centers coordinated by the rigid diphosphine ligand (Ph2P(C6H4)2PPh2) is ascertained by the different characterizations results. However, the global spatial layout of this polymeric structure could not be elucidated by X-Ray diffraction analysis. Formation of low dimensionally compounds (1-D) is unlikely due to the steric hindrance of the eight phenyl groups that should favor the isotropic repartition of the ligands around the [Cu4I4] cluster core. Instead, the tetrahedral coordination geometry of the polymetallic center preserved by the rigid ligand, should promote the formation of either 2-D or 3-D networks. The non-porous character of 1 could be related to the bulkiness of the ligand and/or networks interpenetration.

Figure 7 .

 7 Figure 7. Photographs of powder of 1 under ambient light and UV irradiation (365 nm UV lamp) at room temperature (298 K) and in liquid nitrogen (77 K).

Figure 8 .

 8 Figure 8. Temperature dependent solid-state luminescence spectra of 1. (a) Normalized emission (plain lines) and excitation spectra (dotted lines) at 20 and 290 K. (b) Emission spectra from 20 to 290 K at lex = 300 nm.

Acknowledgments.

The CNRS is thanked for funding. C. M. C. thanks the Institut Universitaire de France (IUF) for funding.

Electronic supplementary information (ESI) available:

Other characterization data.