G E Gomez
email: gegomez@unsl.edu.ar

R F D'vries

D Lionello

L M Aguirre-Díaz

M Spinosa

C S Costa

M C Fuert

R A Pizarro

A M Kaczmarek

J Ellena

L Rozes

M Iglesias

R Van Deun

C Sanchez

M A Monge

G J A A Soler-Illia
email: gsoler-illia@unsam.edu.ar

Exploring physical and chemical properties in new multifunctional indium-, bismuth-, and zinc-based 1D and 2D coordination polymers †

Keywords: Synthetic procedure details, experimental X-ray powder patterns, TGA and DSC

Main group element coordination polymers (MGE-CPs) are important compounds for the development of multifunctional materials. However, there has been a shortage of studies regarding their structural, optical, catalytic, mechanical, and antibacterial properties. This work presents an exhaustive study of a set of crystalline MGE-CPs obtained from bismuth and indium metals and iminodiacetate, 1,2,4,5-benzenetetracarboxylate, and 2,2'-bipyridine as building blocks. An in-depth topological analysis of the networks was carried out. Additionally, nanoindentation studies were performed on two representative low-dimensional compounds in order to find the relationships between their structural features and their intrinsic mechanical properties (hardness and elasticity). The solid-state photoluminescence (SSPL) properties were also studied in terms of excitation, emission, lifetimes values, and CIE chromaticites. Moreover, the heterogeneous catalytic activities of the compounds were evaluated with the cyanosilylation reaction using a set of carbonylic substrates under solvent-free conditions. Finally, the inhibitory effect of the Bi-CPs on the growth of microorganisms such as Escherichia coli, Salmonella enterica serovar Typhimurium, and Pseudomonas aeruginosa, which are associated with relevant infectious diseases, is reported.

Introduction

Inorganic-organic hybrid compounds as coordination polymers (CPs) or metal organic frameworks (MOFs) have become the focus of research due to their unique properties such as gas sorption, 1 optics, 2 and heterogeneous catalysis. 3 In comparison with transition metals from the d and f block, the main group elements (MGE) have been less explored for designing new coordination compounds. Most of the metal ions have been incorporated into these materials, but only recently have Bi 3+ -compounds with optical 4 or catalytic 5 properties been reported. For this reason, the synthesis and studies related to Bi-CPs are surprisingly scarce.

Particularly, our research has addressed the synthesis of new CPs and MOFs with optical, 6 mechanical, 7 catalytic, [8][9][10] thermo, 8,11 chemical, 6b and sensing properties 12 and explored structure-property relationships. Moreover, the catalytic properties of In-CPs have been widely explored by Monge's group, who found exceptional correlations between the structures and their catalytic performances. 13 Because of its geometrically flexible coordination environments, the trivalent Bi ion can be regarded as a novel node in the construction of CPs. Moreover, bismuth leads to unpredict-able and diverse structures because it presents a variety of coordination numbers. In this context, Inge et al. 14 recently reported an 8-coordinated Bi-MOF constructed from a tricarboxylate linker, revealing an unprecedented topological complexity.

From the point of view of the design of new CPs, during the last decade, iminodiacetic and 1,2,4,5-benzenetetracarboxylic acids have been employed as building blocks for the synthesis of CPs and MOFs as multifunctional platforms. 15 An increasingly urgent problem has been the antibiotic resistance observed in several pathogenic microorganisms, and diverse strategies have been proposed to overcome the rapid emergence of resistant organisms. 16 To counteract microbial pathogens, metallic or oxide nanoparticles (NPs) have been developed due to their novel physicochemical properties. 16 In the case of Bi, different compounds are being used against ulcers and in treatments for gastritis, diarrhoea, and dyspepsia. 17 Moreover, bismuth derivatives (for example, bismuth subsalicylate and bismuth citrate) have long been used as antimicrobial agents against several bacterial species, particularly Helicobacter pylori, a bacterium responsible for peptic ulcers and gastric cancer. 17e Their antimicrobial activity was observed at relatively high concentrations due to their limited water solubility. [START_REF] Cabral | Bismuto nanoparticles: antimicrobials of broad spectrum, low cost and safety[END_REF] In addition, it has been demonstrated that Bi-NPs reduce biofilm (growth of microorganisms on solid surfaces) formation by bacteria and fungi such as Streptococcus mutans and Candida albicans. [START_REF] Hernández-Delgadillo | Zerovalent bismuto nanoparticles inhibit Streptococcus mutans growth and formation of biofilm[END_REF]20 Biofilms represent a great problem for industry and human health because they form on a wide variety of surfaces, possess high resistance to antimicrobial agents, and are very difficult to eradicate. 21 To understand the processability and the mechanical properties of the new materials, nanoindentation studies have recently been adopted. In the pioneering work of Cheetham, 22 nanoindentation tests have been applied to demonstrate the intrinsic correlations between crystal packing and anisotropic mechanical properties (such as density, stability, and elasticity) in MOF materials.

The present work involves a complete study regarding the synthesis strategies, photophysical characterization, mechanical and catalytic analysis, and antimicrobial activity assay of a set of CPs based on main-group elements (MGE-CPs), or more specifically, two Bi-CPs and one In-CP.

Experimental section

Synthesis

[Bi (1,2,4,5-BTC) 0.5 (2,2′-bipyridine)

(NO 3 )(DMF)] (Bi-1), [Bi(IDA) (IDAH)] (Bi-2), [In(IDA)(Cl)] (In-1
) and [Zn (1,2,4,5-BTC) 0.5 (2,2′bipyridine)(H 2 O)] (Zn-1), were obtained as crystalline solids under solvothermal conditions using 43 mL Teflon-lined Parr reactors. The solid reactants and solvents were used as received without purification from Across Organics (1,2,4,5benzenetetracarboxylic acid) and Sigma-Aldrich (2,2′-bipyridine, Bi(NO 3 ) 3 •5H 2 O, N,N′-dimethylformamide, iminodiacetic acid, Zn(NO 3 ) 2 •6H 2 O and InCl 3 ) (see ESI section S1 † for synthesis details).

Fourier transform infrared spectroscopy (FTIR)

FTIR spectra were recorded with a Nicolet Protégé 460 spectrometer in the 4000-400 cm -1 range with 64 scans and a spectral resolution of 4 cm -1 by the KBr pellet technique. The FTIR spectra and band assignments are displayed in ESI section S2. †

Thermal analysis

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed with Shimadzu TGA-51 and DSC-60 apparatus, respectively, under flowing air at 50 mL min -1 , at a heating rate of 10 °C min -1 . TGA and DSC curves are displayed in ESI section S3. †

Powder X-ray diffraction (PXRD)

PXRD patterns were obtained with a Rigaku D-MAX-IIIC diffractometer using CuKα radiation (λ 1 = 1.54056 Å, λ 2 = 1.54439 Å) (see ESI section S4 †).

Scanning electron microscopy (SEM)

SEM micrographs were obtained and energy dispersive spectroscopy (EDS) (ESI section S5 †) was used with FEI Quanta 200 equipment. Samples were placed on an adhesive carbon tape coated with gold for the observations.

Single-crystal structure determination (SCXRD)

The SCXRD data for compounds Bi-1, Bi-2, and Zn-1 were collected at room temperature (293 K) on a Rigaku XTALAB-MINI diffractometer using MoKα radiation (0.71073 Å) monochromated by graphite. The In-1 compound was collected at room temperature (296 K) with a Bruker APEX-II CCD diffractometer using MoKα radiation (0.71073 Å) monochromated by graphite.

The cell determination and the final cell parameters of Bi-1, Bi-2, and Zn-1 were obtained on all reflections using the software CrystalClear. 23 Data integration and scaling were carried out using the software CrystalClear 23 and CrysAlisPro. 24 The structures were solved and refined with SHELXS-2013 25 software, included in WinGX 26 and Olex2. 27 Cell determination and final cell parameters of In-1 crystals were obtained on all reflections using the software Bruker SAINT included in the APEX2 software suite. 28 Data integration and scaling were carried out using the software Bruker SAINT. 27 In all cases, non-hydrogen atoms of the molecules were clearly resolved, and full-matrix least-squares refinements of these atoms with anisotropic thermal parameters were performed. Additionally, hydrogen atoms were stereochemically positioned and refined by the riding model. 25 ORTEP diagrams for all structures were prepared with the Diamond [START_REF] Brandenburg | Impact C., Crystal Impact, Kreuzherrenstr[END_REF] program. TOPOS [START_REF] Blatov | [END_REF] and Mercury 31 programs were used in the preparation of the artwork of the polyhedral and topological representations.

Mechanical characterization

Samples were prepared for mechanical characterizations including single crystals in acrylic resin (SUBITON®). Nanoindentation (NI) experiments were performed using an Agilent G200 Nano Indenter at room temperature. A Berkovich diamond tip with a final rounding of 20 nm was used in this study. The tip was calibrated using a standard fused silica sample. A group of 5 crystals was tested, and an array of 5 × 5 measurements up to a maximum penetration depth of 1200 nm was obtained for each selected crystal. The elastic modulus (E) and hardness (H) were calculated from the measurements by applying the Oliver & Pharr model, which was described in our previous work. 7

Solid-state luminescence measurements

The steady-state and time-resolved luminescence measurements were performed with an Edinburgh Instruments FLSP920 spectrometer setup, using a 450 W xenon lamp as the steady-state excitation source and an EPLED as the timeresolved excitation source with a fixed excitation wavelength of 331 nm. The emission was detected with a Hamamatsu R928P photomultiplier tube (PMT). Excitation spectra were corrected for the xenon lamp emission profile, whereas emission spectra were corrected for the detector response curve. All measurements were carried out at a step size of 1 nm.

Catalytic activity

Considering the nature of the new materials, among all possible organic transformations, only those that require a Lewis acid as catalyst were taken into account. Therefore, as a standard organic transformation, the cyanosilylation reaction (CSR) was chosen in order to evaluate the catalytic activity of Bi-1, Bi-2, and In-1 materials. In this order, catalytic amounts of the selected material (0.5 mol%, 1 mol%, and 2.5 mol%, respectively) were placed in a Schlenk tube under nitrogen atmosphere without solvents, together with the corresponding carbonyl compound (1 equivalent); trimethylsilyl cyanide (1.1 equivalent) was then added dropwise by syringe. The mixture was stirred until disappearance of the carbonyl compound; the kinetics of the reaction and its yield were confirmed by GC-MS.

Antibacterial assays

Stock solutions of bismuth compounds

Saturated aqueous solutions of the relatively insoluble compounds Bi-1, Bi-2, and bismuth citrate (Bi-cit) (Merck) were prepared in distilled and sterilized water by 0.22 µm-pore-size filters. Bi-cit was employed as a control because it is known as a bismuth derivative inhibitor of bacterial growth. 17b The concentrations of the bismuth compounds were expressed as elemental bismuth as determined by total reflection X-ray fluorescence spectrometry (Bruker PicoFox S2 TXRF). Bi-NPs and Bi-cit concentrations used for this study depended on their limited water solubility.

Bacterial strains and growth conditions

The employed set of bacteria consisted of Escherichia coli (E. coli, strain K12), Salmonella enterica serovar Typhimurium (S. Typhimurium, strain LT2), and Pseudomonas aeruginosa (P. aeruginosa, strain PAO1). For assays with planktonic (free) cells, E. coli, S. Typhimurium, and P. aeruginosa were cultivated in Nutrient Broth (Difco) at a temperature of 37 °C. For biofilm assays, P. aeruginosa was grown in LB broth (10 g tryptone, 5 g yeast extract, and 5 g NaCl in 1000 ml distilled water) at 37 °C.

Effect of Bi-NPs on planktonic cultures and biofilm formation

For assays with planktonic cultures, sterile Bi-compounds were serially suspended in tubes containing 1 milliliter of Nutrient Broth. Overnight cultures of the three bacterial strains were diluted 1/100 in saline solution, and 100 μl were added to each prepared tube as described. The tubes were incubated for 24 h at 37 °C. Cell mass was evaluated by optical density at 650 nm wavelength (OD 650 ). Quantification of viable microorganisms was determined by colony counts of bacteria plated on Nutrient Agar (Difco) at 37 °C and expressed as colony forming units per ml (CFU ml -1 ). Tubes without Bi-compounds were used as controls. For studies of biofilm formation, overnight cultures of P. aeruginosa were diluted 1/100 in LB in a final volume of 100 μl in the absence or presence of different concentrations of Bi-compounds in 96-well microplates. After 24 h incubation at 37 °C in a humidified chamber, planktonic cells were discarded and biofilm attached to the surface of the microplate wells was quantified by crystal violet staining. 32

Results

Synthesis

After the followed procedure detailed in Scheme 1, the phases were characterized by PXRD and EDS in order to confirm their purity (see ESI section S4 and S5 †). The morphological diversity was studied by SEM; growth habits were identified in Bi-1, Bi-2, and Zn-1; additionally, In-1 exhibited cubic crystals (Fig. 1). Good-quality crystals were carefully selected in order to perform SCXRD. The table in ESI section S6 † summarizes the crystallographic data for all the synthesized compounds.

Scheme 1 Synthesis of the reported CPs.

Crystal structure descriptions

Bi-1 compound crystallizes in the triclinic P1 ˉspace group. The primary building unit (PBU) consists of a nine-coordinated arrangement (BiO 7 N 2 ), where seven oxygen atoms belong to 1,2,4,5-BTC, DMF, and nitrate, and two nitrogen atoms from the 2,2′-bipyridine form a trigonal prism square-face tricapped polyhedron (TPRS-9) [START_REF] Connelly | Nomenclature of Inorganic Chemistry -IUPAC Recommendations[END_REF] (Fig. 2 left). As can be seen in Fig. 2 (right), the asymmetric unit of Bi-1 is composed of one trivalent Bi cation, one nitrate anion, one DMF, and one 2,2′bipyridine, in addition to a half of a crystallographically independent 1,2,4,5-BTC ligand.

In this compound, each 1,2,4,5-BTC ligand is coordinated to four Bi centers in chelate η 2 mode, giving rise to chains along the [1 0 0] direction (Fig. 3 left) with intermetallic distances of 6.3594(6) and 9.3852( 7) Å. The Bi-O bond lengths are in the range 2.326(5)-2.77(1) Å, which were seen in analogous Bi-MOFs. 14 The crystal structure of Bi-1 was analyzed with the TOPOS program in order to determine the underlying topology 34 of the associated simplified net. Through this simplification, it is possible to observe that the carboxylate ligand acts as a 4-connected node and the metal center acts as a bridge forming "ribbon-like" chains (Fig. 3 left). In addition, the compound can be classified as I 0 O 1 according to the proposed classification suggested by Cheetham et al. 35 describing "coordination polymeric chains".

The 3D supramolecular structure (Fig. 3) is based on weak hydrogen bonds that join the chains along the [0 1 0] and [0 0 1] directions, with distances C9-H9⋯O6 = 3.248( 1) and C18-H18A⋯Cg = 3.460(2) Å, respectively (Cg ring = C2, C3, C4, C2*, C3*, C4*).

For comparative purposes, the Zn-1 compound 36,37 was obtained. A deeper crystallographic description can be found in ESI section S7. † Similarly to Bi-1, here, the 1,2,4,5-BTC ligand is coordinated to four metallic centers giving rise to "ribbon-like" chains along the [1 0 0] direction, with intermetallic distances of 7.6958 (7) and8.0797( 8) Å (Fig. 4a). Despite presenting different coordination numbers, both compounds exhibit a similar arrangement of the ligand, giving rise to 1D coordination polymers.

Bi-2 was obtained as a crystalline solid belonging to the monoclinic C2/c space group. The asymmetric unit is formed by one crystallographically independent trivalent Bi cation and two structurally different types of iminodiacetate ligand (IDA and protonated IDAH). The PBU can be described as an 8-coordinated metal center (BiO 7 N) formed by seven oxygens and one nitrogen atom from the ligands, forming a square anti- prism polyhedron (SAPR-8) arrangement. In this case, the metal centers are joined by the ligand-forming dimeric edgesharing secondary binding units (SBUs) (Bi 2 N 2 O 12 ), with an intermetallic distance of 3.9612( 9) Å. The formation of the dimeric SBUs is due to the µη 2 and µ 2 η 2 coordination modes of the carboxylate groups (see ESI section S8 †).

In the extended structure, the IDA acts as a tridentated ligand blocking the coordination sphere of the Bi 3+ , while the IDAH ligands join the SBUs along the [0 0 1] direction, giving rise to ribbon-like chains (Fig. 5 left and ESI section S8 †). Additionally, the IDAH ligands join two metallic centers forming dimeric SBUs, acting as 4-connected nodes. The 3D supramolecular crystal packing (Fig. 5 right) is given by hydrogen bonds along the [1 0 0] direction, with distances N1-H1⋯O4 = 2.932(6) and C2-H2C⋯O2 = 3.175(8) Å. Similar interactions are involved along the [0 1 0] direction, with distances N2-H2B⋯O2 = 2.908 (7) and N2-H2A⋯O4 = 3.091(6) Å.

The 3D supramolecular structure is formed by strong hydrogen bonds and π-π stacking interactions with distances O5-H5A⋯O1 = 2.657(3) and Cg⋯Cg = 3.758(2) Å, along the [0 0 1] and [0 1 0] directions, respectively.

In-1 crystallizes in the Pnma orthorhombic space group. The metal environment is formed by one Cl atom, one nitrogen, and four oxygen atoms, giving rise to a distorted octahedral geometry (Fig. 6 left). The asymmetric unit is formed by a half of the In 3+ cation, chloride anion, and IDA ligand. The ORTEP diagram of In-1 is displayed in Fig. 7 right. Each ligand is coordinated in µ 3 chelate fashion through the protonated nitrogen atom and two oxygen atoms of the carboxylate group. Moreover, the carboxylate group exhibits a µη 2 coordination mode, linking a pair of metallic centers (intermetallic distance = 5.645(2) Å) along the [0 1 1] and [0 -1 -1] directions (Fig. 8).

The junction of the metallic centers gives rise to layers in the (011) plane. As far as we know, this structure was topologically analyzed for the first time. In this case, each metal acts as a 4-connected node joined by the ligand, forming a sql/ Shubnikov topology net with point symbol (4 4 •6 2 ) (Fig. 8b). The 3D supramolecular structure is formed by the Cl⋯H-N interaction with a distance of 2.719(6) Å, which enables the stacking of the covalent layers along the [1 0 0] direction (Fig. 8c).

Nanomechanical properties

Dense CPs incorporate infinite inorganic connectivity, especially those based on metal-oxygen-metal motifs.

Recently, there has been an intense interest focused on CPs showing unique physical properties associated with their structures, which includes magnetic, electrical, optical, and multiferroic properties. Despite this, studies that have described the mechanical features of CPs are scarce. Additionally, good film stiffness and film-to-substrate adhesion strength is required for developing MOF/CP sensing devices. Moreover, elastic properties of the underlying framework are dominant in the performance of piezoelectric devices (such as actuators and sensors) and MOF coatings or membranes. 38 In this context, there is special interest in the mechanical characterization of low-dimensional 1D or 2D frameworks due to the possibility of delamination ("top-down" fabrication) and further deposition onto different substrates in order to build membranes and thin-film composites. 39,40 To elucidate structural-mechanical properties, NI experiments were performed on selected single crystals of In-1 and Bi-2 as representative models (Fig. 9a and10a). In both cases, a 5 × 5 field of NI was applied to the single crystals. To avoid ambiguities, NI experiments on an acrylic substrate were performed (see displacement (P-h) curves in ESI section S9 †) as a control experiment, obtaining H and E values of 0.11 ± 0.01 and 2.8 ± 0.1 GPa, respectively.

The crystallographic planes that probabilistically are exposed in the In-1 compound, which are also coincident to those that divide the nanolayers, are the family of h00 ones. In ESI section S1, † a preferential orientation into the [2 0 0] planes is clearly identified. These planes are exposed by deposition of the layers, which are located in the direction in which non-covalent interactions ensure the packing of the layers (see Fig. 9b). The same family of 200 planes in Bi-2 (Fig. 10b) exposes a higher concentration of metallic polyhedral, conferring structural stiffness. As can be seen in Fig. 9, 10c and d, the "Berkovich marks" were identified after NI measurements.

The average values of H and E measured for In-1 were 1.8 ± 0.1 GPa and 24.3 ± 0.8 GPa, respectively, and for Bi-2, were 1.1 ± 0.1 GPa and 12.4 ± 0.5 GPa. The obtained values from Bi-2 are considerably lower compared to In-1, possibly because of a decrease in stiffness due to being one-dimensional.

It is important to note the presence of "pop-in", or displacement events, in the performed NI experiments, which appears due to the breakages of intermolecular weak bonds. Similar displacements have been reported in low-dimensional CPs such as [Yb 2 (3-OHNDS) 2 (1,10-phen) 41 and [Mn (2,2-dms)]. 40 According to the classification reported by Cheetham and colleagues, 42 In-1 was located in the "dense hybrids" type materials and close to MOFs with similar mechanical performance (ESI section 10 †).

2 (H 2 O)]•3H 2 O, 7 [Cu(H 2 O) 2 (O 3 PCH 2 CO 2 ) 2 ]

Solid-state photoluminescence properties (SSPL)

According to a significant number of specialized articles 2 regarding photoluminescence (PL) properties, optical materials can be characterized by specific studies: (1) emission spectra (recorded as intensity vs. wavelength), ( 2) quantum yields (QYs), which refers to the efficiency of the luminescence process, and (3) the observed lifetime (τ obs ), which refers to the average time the molecule stays in its excited state before emitting a photon.

In contrast to the narrow and easily predictable emissions of 4f-4f transitions in rare earth elements, 43 the PL in transition metals and MGE commonly exhibits broad bands. The excited states in TM-and MGE-based compounds are different from the ground state, and considerable anti-bonding often produces broad emissions and large Stokes shifts. Additionally, many types of PL might occur in the same compound, as it is a challenge to identify the electronic transitions.

LMCT (ligand to metal charge transition) might occur in TM and MGE-CPs with metals in high oxidation states; however, the assignment of LMCT in these compounds is often difficult, especially in mixed ligand systems. The most likely candidates for metal-centered emission are CPs containing s 2 -ions such as Pb 2+ and Bi 3+ , where it is possible to observe s-p transitions. 44 The SSPL properties of Bi-1 and Bi-2 were studied by recording excitation-emission spectra, calculating lifetime values, and quantifying the color emission (QC) in comparison with the respective starting reactants (Bi(NO 3 ) 3 , H 2 IDA, and H 4 BTC). Usually, the spectroscopy of Bi 3+ with s 2 configuration in the ground state and sp configuration in the first excited state is discussed in Russell-Saunders-type electronic energy terms. Intense fluorescence emissions could be assigned to LMCT or the 1 P 1 ← 1 S 0 and 3 P 1 ← 1 S 0 transitions of the s 2 electron of trivalent Bi 3+ ions. 45 In Bi-compounds, the dominant excitation transition is commonly ascribed to the population of the excited state 3 P 1 from the ground state 1 S 0 (allowed by spin-orbit coupling), being that the 3 P 0 level is strongly forbidden and a metastable state. At high temperatures, the emission 1 P 1 → 1 S 0 transition is the most frequent in relation to the 3 P 1 → 1 S 0 transition, and is evidenced as broad bands accompanied by splitting components. 46 Therein, the emission peaks of Bi-1 are located at longer wavelengths (446, 486, and 511 nm) compared to Bi-2 (421, 435, and 464 nm) (Fig. 11). The signal located at 511 nm in Bi-1 could be attributed to π ← π* and/or n ← π* from 1,2,4,5-BTC and 2,2′-bipyridine ligands. For Bi-2, the contribution of the n ← π* transition from the IDA carboxylate linker cannot be discounted. Moreover, it is remarkable that although the band splitting between both spectra is different, the intensities are similar. For an in-depth analysis, Zn-1 was obtained and compared to Bi-1. Due to the fact that Zn-1 and Bi-1 have the same set of coordinated ligands, both compounds exhibit broad bands. The phosphorescence-emitting bands were further analyzed by recording intensity decay curves (see ESI section S11 †), and luminescence lifetimes were calculated by fitting mono-exponential functions, I = A exp(-t/τ obs ). The corresponding yielded τ obs were 16.7 μs and 0.52 μs for Bi-1 and Bi-2, respectively. The τ obs values of the Bi-CPs are higher compared to Bi(NO 3 ) 3 , H 2 IDA, and H 4 BTC (1.03, 1.32, and 1.02 ns, respectively).

These differences might be related to their different coordination environment around Bi 3+ ions, 47 as Bi is nine-co-ordinated in Bi-1, while it is eight-coordinated in Bi-2. The SSPL and excitation spectra of both Bi-CPs suggest that two different emission metal centers are responsible for the resultant luminescence components. It is noticeable that the two maximum peaks in the excitation spectra are shifted by 30 nm, which is ascribed to the absorption of the bismuth-emitting center.

The QC for optical materials enables an accurate comparison between different materials based on their light-emitting performance. In this sense, the color coordinates are usually calculated employing the CIE (Commission International de L′ Éclairage) system (ESI section S12 †), proposed in 1931. [START_REF] Kickelbick | Hybrid Materials[END_REF] According to this approach, Bi-1 and Bi-2 were identified with bluish green and bluish purple emissions, respectively (Fig. 11 bottom). In comparison, similar colorimetric and spectroscopic features were observed in analogous Bi-HPyr (H 4 Pyr = 1,2,4,5-BTC), which exhibited pale green luminescence. 47 Based on this analysis, Bi-1 and Bi-2 can be used as precursor materials for blue-light OLED/LED devices.

Catalytic activity

Cyanosilylation reactions (CSRs) are important carbon-carbon bond-forming processes that are catalyzed by Lewis acids/ bases that can be used as homogeneous 49 or heterogeneous 50 catalysts. Among all the alternatives, MOFs and CPs are good choices as heterogeneous catalysts for Lewis acid reactions due to their unique reactivity and selectivity under mild reaction conditions. 8,10,13 Therefore, taking into account the excellent Lewis acid activity displayed by several In-MOFs reported in the literature, 10,11,13 we decided to test the catalytic activity of Bi-1, Bi-2, and In-1 in the cyanosilylation of acetophenone (Table 1).

It was found that the reactivity changes as a function of the metallic nature and the number of catalytic active sites pre- sented in each structure. The kinetic profile (Fig. 12) was studied to understand the catalytic performance of each material and to determine their turn over frequency (TOF) values.

The PXRD patterns of the recovered Bi-2 and In-1 after the catalytic reactions indicate that these materials do not undergo any structural change. However, Bi-1 does not maintain its structural integrity after the catalytic reaction (see ESI, section S13 †), and will not be considered as a heterogeneous catalyst for use in further experiments.

From the data presented in Table 1, it is evident that a structure/catalytic ability relationship exists. The comparison between Bi-2 and In-1, which have the same organic linker in their structure, allows the study based on the nature of the metal cation. It would be expected that Lewis acid behavior follows the order of In-1 > Bi-2, based on the cation size impact of the acid strength. Additionally, indium catalyst with octahedral metal centers forms a covalent framework with higher dimensionality (2D layers) than Bi-2 (1D chains) material. This higher covalent disposition of In-1 clearly seems to favor the interaction between the available active sites of the catalyst with the substrates, even for those that are the most sterically hindered, as can be appreciated in Table 2.

The scope of the reaction was studied for the cyanosilylation of ketones using In-1 as the catalyst (Table 2). In the case of non-aromatic ketones (Table 2, entries 4-8), higher yields were obtained compared to the aromatic ones (Table 2, entries 1-3). Reactions with linear aliphatic ketones (Table 2, entries 7 and8) proceeded efficiently giving the corresponding product at shorter times with yields over 85%. Higher yields of the cyclic ketones (Table 2, entries 4-6) were obtained (>97%). Finally, satisfactory yields of the less reactive aromatic ketones were also obtained (80-96%).

The results of the reactions carried out with acetophenone derivatives with electron-donating substituents, such as 2-methylacetophenone and 4-methylacetophenone, showed yields of 80% and 89%, respectively.

The recyclability tests show that the In-1 material maintains its crystallinity even after ten catalytic cycles, with only a small decrease of its catalytic activity, probably due to the losses during the recovery of the catalyst (see ESI, section S14 †). Hot filtration experiments revealed that In-1 is a truly heterogeneous catalyst.

Antibacterial assays

Firstly, the effects of Bi-NPs and Bi-cit on the cell growth of planktonic cultures of E. coli, S. Typhimurium, and P. aeruginosa were analyzed. Fig. 13 shows that 1.2 µg ml -1 of Bi-1 was required to decrease the amount of viable organisms by 6 logs in E. coli and P. aeruginosa and by 7 logs in S.

Typhimurium (viable counts compared to controls). In contrast, 295 µg ml -1 of Bi-2 were required to reduce by 2 logs the cell viability of E. coli and S. Typhimurium, and 4 logs in P. aeruginosa (Fig. 13b). Bi-cit decreased cell viability 1.5-2 logs at 6.8 µg ml -1 in all the studied microorganisms (Fig. 13).

Reduction of cell mass measured by OD 650 followed the same observed tendencies as those for the cell viability (Fig. 13).

According to these results, the greatest effect was produced by Bi-1, not only for the several orders of log reduction, but also for the lower concentration required to reduce bacterial growth (Table 3).

Then, the effect of Bi-NPs on the biofilm formation of P. aeruginosa was studied. P. aeruginosa is a relevant opportunistic pathogen of humans. Given its great capacity to form biofilms on diverse surfaces and cause severe damage to industry and human health, it is widely employed as a model in biofilm studies. Quantification of biofilms in control and test assays demonstrated that both Bi-NPs efficiently inhibited biofilm formation, although Bi-1 produced its antibiofilm effect at lower concentrations than Bi-2 (Fig. 14a). In contrast with Bi-NPs, Bi-cit did not inhibit biofilm formation at concentrations ≤3 μg ml -1 , but rather, it promoted its development. However, at higher concentrations, this effect was reversed and inhibition of biofilm formation was observed. Representative images of stained 24 h biofilms illustrate the inhibitory effect of Bi-NPs on biofilm formation (Fig. 14b). It is concluded that both Bi-NPs are effective antibiofilm agents in P. aeruginosa, with Bi-1 being more powerful than Bi-2.

Conclusions

A set of CPs based on bismuth, indium, and zinc were successfully obtained. Due to the lack of studies regarding the structural, optical, mechanical, and antibacterial properties of CPs, an exhaustive analysis covering these topics was carried out. These compounds were synthesized from bismuth and indium salts (IDA, 1,2,4,5-BTC, and 2,2′-bipyridine) as building blocks. The one-dimensional Bi-CPs, [Bi (1,2,4,5-BTC) 0.5 (2,2′-bipyridine)(NO 3 )(DMF)] and [Bi(IDA)(IDAH)], belonged to P1 ˉand C2/c space groups, and the layered In-CP, [In(IDA)Cl], belonged to the Pnma space group. Moreover, an in-depth analysis of the topology was carried out, where the Bi-CPs were simplified as "ribbon-like" chains, and a sql/Shubnikov tetragonal plane net with point symbol (4 4 •6 2 ).

To find relationships between the structural features and the intrinsic mechanical properties, nanoindentation studies were performed on a 1D and a layered compound (Bi-2 and In-1). The differences in the mechanical performances in both compounds were related to their crystal packing. According to H and E values, the CPs were located in the "dense hybrids" type of materials. These studies are important to evaluate the mechanical qualities of low-dimensional materials for device development.

The SSPL properties of the Bi-CPs were studied, which involved excitation-emission, lifetime experiments, and quantification of colour emission. Regarding these results, the Bi-CPs were classified as blue-emitters in the visible region, with performances comparable with analogous Bi-compounds and TM-MOFs. These optical features highlight the importance of Bi-CPs for the design of photonic devices with blue light emissions.

From our catalytic studies on these new CPs under solventfree conditions, we can conclude that the factor that drives the reaction yield relies on the metal nature and framework dimensionality. Therefore, the catalytic activity decreases in the following order: In-1 > Bi-2. The In-1 material is capable of working as a real heterogeneous catalyst, using several hindered carbonyl derivates under mild reaction conditions and reaching high yields with a good recyclability rate.

Finally, the strong antibacterial effect of the Bi-CPs on E. coli, S. Typhimurium and P. aeruginosa was observed and compared to the well-known bismuth salt Bi-cit, especially in the case of Bi-1. The emergence and spread of antibiotic-resistant derivatives constitute a difficult problem that threatens the efficacy of treatment of many bacterial infections with antibiotics, and as a result, it is necessary to develop novel pathogen eradication methodologies.

Fig. 1

 1 Fig. 1 Micrographs of (a) Bi-1, (b) Bi-2, and In-1 obtained (c) without and (d) with growth modulator.

Fig. 2

 2 Fig. 2 Polyhedron representation of the PBU (left) and 50% probability thermal ellipsoid diagram showing Bi-1 (right). Hydrogen and disordered atoms were omitted for clarity.

Fig. 3

 3 Fig. 3 (a) Polymeric chain and decorated simplified chain along the [1 0 0] direction and (b) crystal packing view into the bc plane of Bi-1 compound.

Fig. 6

 6 Fig. 6 Polyhedron and simplified representation of the chains (left) and crystal packing view along the [0 0 1] direction of Bi-2 (right).

Fig. 7

 7 Fig. 7 Polyhedron representation of the In 3+ (left) and 50% probability thermal ellipsoid diagram for In-1 compound (right).

Fig. 8

 8 Fig. 8 (a) Representation of the 2D covalent network, (b) topological simplification, (c) 3D crystal packing, and (d) the coordination modes of the IDA ligand in In-1.

Fig. 5

 5 Fig. 5 PBU representation of the Bi 3+ cation (left) and 50% probability thermal ellipsoid diagram showing for Bi-2 (right). Hydrogen and disordered atoms were omitted for clarity.

Fig. 9

 9 Fig. 9 (a) P-h profiles of In-1 crystals. (b) Studied planes, in normal direction to the nanoindenter. (c and d) Indented cube crystals.

Fig. 10

 10 Fig. 10 (a) P-h profiles of Bi-2 crystals. (b) Studied planes, in normal direction to the nanoindenter. (c and d) Indented single crystals.

Fig. 11

 11 Fig. 11 Top: Excitation (left) and emission (right) spectra of Bi-CPs in comparison with Bi(NO 3 ) 3 , H 2 IDA, H 4 BTC, and Zn-1. Down: Chromaticities (CIE 1931) exhibited by Bi-1, Bi-2, and Zn-1.

Table 1 a

 1 Screening of materials as catalysts for the cyanosilylation reaction using acetophenone as a carbonyl source Solvent free reaction at 25 °C and 1 mol% of catalyst under N 2 atmosphere. b Yields (GC-MS). c TOF: (Δχ × [mol of reactant/mol catalyst]/ Δt ). d TON: (χ/mol catalyst). e Reaction employing 2.5 mol% of catalyst loading. f Reaction employing 0.5 mol% of catalyst loading. g Homogeneous catalyst [1 mol%] using CH 2 Cl 2 as solvent.

Fig. 12

 12 Fig. 12 Kinetic profiles of Bi-1, Bi-2, and In-1 catalysts in the cyanosilylation of acetophenone.

Fig. 13

 13 Fig.13Viable organisms per mL and OD 650 profiles of Bi-1, Bi-2, and Bi-cit.

Fig. 14 (

 14 Fig. 14 (a) Effect of bismuth compounds on biofilm formation in P. aeruginosa. P. aeruginosa was grown in 96-well microplates in the absence or presence of different concentrations of Bi-NPs and bismuth citrate. Biofilms that adhered to the wells were subsequently stained with crystal violet, and the absorbance of the solubilized stain was measured at 575 nm. Error bars represent the standard deviation of at least three independent experiments. (b) Visualization of biofilm formation in the absence or presence of different concentrations of Bi-NPs by crystal violet staining. Representative images are shown.

Table 2

 2 Scope of the In-1 cyanosilylation of ketones a

	Entry	Ketone	Time (h)	Yield b (%)	TON c
	1	Acetophenone	4	96	96
	2	4-Methylacetophenone	8	89	89
	3	2-Methylacetophenone	8	80	80
	4	Cyclopentanone	3.5	98	98
	5	Cyclohexanone	4	96	96
	6	4-Methylcyclohexanone	4	97	97
	7	2-Hexanone	3	>99	99
	8	3-Hexanone	3	85	85

a Solvent-free reaction at 25 °C and 1 mol% of catalyst under N 2 atmosphere. b Product yields (GC-MS), and c TON: (χ/mol catalyst).

Table 3

 3 Log 10 reductions of bacterial viability produced by Bi-compounds compared to controls, according to the maximum concentration of compound used

	Organism	Bi-1 (1.2 µg ml -1 )	Bi-2 (295 µg ml -1 )	Bi-cit (6.8 µg ml -1 )
	E. coli	6	2	2
	S. Typhimurium	7	2	2
	P. aeruginosa	6	4	1.5

Acknowledgements

This work was supported by the Consejo Nacional de Investigaciones Científicas y Técnicas, ANPCyT PICT 2012-2087 and CNEA. G. E. G. acknowledges the postdoctoral CONICET fellowship and HYMADE project. R. D. acknowledges Coordenação de Aperfeiçoamento de Pessoal de Nível Superior for the CAPES/PNPD scholarship from the Brazilian Ministry of Education and Universidad Santiago de Cali. D. F. L. acknowledges doctoral CONICET fellowship. Ramón Castillo Guerra, Ricardo Montero, Guillermo Arnaldo, Rodrigo Taboada, and Gonzalo Zbihlei, from Departamento de Materiales -(CNEA), are gratefully thanked for helping with sample preparation for NI testing. L. M. A.-D., M. I. and M. A. M. acknowledge the Spanish Ministry of Economy, Industry and Competitiveness (MINECO-Spain) projects MAT2013-45460-R, MAT2014-52085-C2-2-P, MAT2016-78465-R, and CTQ2014-61748-EXP, Fondo Social Europeo from the European Union. Comunidad Autónoma de Madrid S2013/ MIT-2740 (PHAMA 2.0). R. V. D. and A. M. K. acknowledge Ghent University's Special Research Fund (BOF) for a Posdoctoral Mandate ( project BOF15/PDO/091). J. E. is grateful to CNPq for the research fellowships. G. J. A. A. S. I. and M. C. F. are members of CIC-CONICET. Additionally, this project has received funding from the European Union's Horizon 2020 research and innovation program H2020-MSCA-RISE-2014, GA No. 645686.

Conflicts of interest

There are no conflicts of interest to declare.


Rémi Deleurence
email: remi.deleurence@saint-gobain.com

Tamar Saison
email: tamar.saison@saint-gobain.com

François Lequeux
email: francois.lequeux@espci.fr

Cécile Monteux
email: cecile.monteux@espci.fr

Foaming transient polymer hydrogels

Foams made with polymer hydrogels can be used in a variety of applications, such as scaffolds for biomedical applications or decontamination processes. However from a practical point of view it is difficult to introduce bubbles into viscous or viscoelastic fluids and to produce large volumes of hydrogel foams. In the present article we investigate the foaming process of PVA/Borax transient hydrogels, where PVA chains reversibly bind to Borax molecules. In a previous article we showed that foams obtained with PVA/Borax mixtures are highly stable due to both high interfacial and bulk viscosities and can be used to quickly absorb liquids, which makes them suitable for detergency or decontamination processes. To produce these foams we use a two-step foaming process which consists in first shearing a PVA solution to obtain a PVA foam and, second adding Borax to the PVA foam under continuous shearing. The obtained PVA/Borax foams are stable for weeks. In this study, we observe a shear-induced collapse of the foams for formulations containing a low Borax/PVA ratio while they remain stable under shear for high PVA/Borax ratios. Using scaling arguments we find that the shear induced collapse of the foams and bubbles is obtained below a critical ratio, N E /N B = 15, of the number of entanglements per chain, N E, and number of Borax per chain, N B . Rheology measurements show that the samples present a shear thickening behavior which increases with the Borax concentration. We suggest that during the foaming process when the shearing rate is of the order of 100 s -1 , the viscosity of these samples diverges leading to a viscous to fragile transition. To mimic the fast stretching of the PVA/Borax thin films during the foaming process we study the stretching of individual PVA/Borax catenoid-shaped thin films at high stretching rates. We observe that the films containing low PVA/Borax ratios do not minimize their surface area unlike what is theoretically expected for standard surfactant films. Moreover the films tend to be unstable and fracture because the PVA/Borax network does not have time to rearrange and relax stresses for high stretching rates.

Introduction

Foams, which are composed of air bubbles in a continuous matrix, are commonly used in multiple applications such as cosmetics, detergency and also as precursors for the production of lightweight materials. However, foams made with standard surfactants have a limited life time because of three destabilization mechanisms, ie the gravity-induced drainage flow of the continuous phase, Oswald ripening and bubble coalescence. Complex fluids such as polymer or peptide hydrogels [START_REF] Deleurence | Time scales for drainage and imbibition in gellified foams: application to decontamination processes[END_REF][START_REF] Testouri | Highly Structured Foams from Chitosan Gels[END_REF][START_REF] Andersen | Ionically Gelled Alginate Foams: Physical Properties Controlled by Operational and Macromolecular Parameters[END_REF][START_REF] Andersen | Ionically gelled alginate foams: Physical properties controlled by type, amount and source of gelling ions[END_REF][START_REF] Barbetta | Porous gelatin hydrogels by gas-inliquid foam templating[END_REF][START_REF] Colosi | Morphological Comparison of PVA Scaffolds Obtained by Gas Foaming and Microfluidic Foaming Techniques[END_REF][START_REF] Li | Long-Lived Foams Stabilized by a Hydrophobic Dipeptide Hydrogel[END_REF][START_REF] Malcolm | Foaming properties of a peptide designed to form stimuli-responsive interfacial films[END_REF] or colloidal suspensions [START_REF] Guillermic | Surfactant foams doped with laponite: unusual behaviors induced by aging and confinement[END_REF][START_REF] Liu | Aqueous foams stabilized by hexylamine-modified Laponite particles[END_REF][START_REF] Liu | Foams stabilized by Laponite nanoparticles and alkylammonium bromides with different alkyl chain lengths[END_REF][START_REF] Lesov | Factors controlling the formation and stability of foams used as precursors of porous materials[END_REF][START_REF] Lesov | Drying of particle-loaded foams for production of porous materials: mechanism and theoretical modeling[END_REF][START_REF] Deleurence | Mixtures of latex particles and the surfactant of opposite charge used as interface stabilizers -influence of particle contact angle, zeta potential, flocculation and shear energy[END_REF] can be used to produce highly stable foams as their high viscosity enables to slow down the drainage flow. Hydrogel foams were recently used for biological applications such as wound healing, tissue culture [START_REF] Barbetta | Porous gelatin hydrogels by gas-inliquid foam templating[END_REF][START_REF] Colosi | Morphological Comparison of PVA Scaffolds Obtained by Gas Foaming and Microfluidic Foaming Techniques[END_REF] . In a recent article, we showed that foams made of PVA-Borax physical hydrogels can be used to quickly absorb liquids, which makes them good candidates for a wide range of detergency applications, including decontamination during the dismantling of nuclear facilities. In the latter application, the high viscosity of the PVA/Borax mixtures enables to obtain foams that can remain highly stable during long decontamination processes. However foams made with hydrogels are delicate to produce and to handle. Indeed their high viscosity and elasticity prevents the incorporation of air bubbles during the foaming process. Moreover the handling and processing of foams made with covalent hydrogels is delicate because hydrogels are fragile and tend to fracture very easily.

The most current knowledge on foaming science is for Newtonian fluids [START_REF] Drenckhan | The science of foaming[END_REF] and there is very little knowledge concerning foams made with complex fluids although it would help producing new types of aerated materials with controlled structure. During the mechanical foaming of standard surfactant solutions, the foam volume and air fraction are controlled by the amount of air which is trapped into the fluid while the bubble size distribution is controlled by the balance between bubble break up and bubble coalescence which occurs as the bubbles and the fluid are sheared. The deformation and break up of a single bubble in a shear flow is controlled by the capillary number, Ca, which is a balance of capillary and viscous forces acting on the bubble. Grace [START_REF] Grace | PHENOMENA IN HIGH VISCOSITY IMMISCIBLE FLUID SYSTEMS AND APPLICATION OF STATIC MIXERS AS DISPERSION DEVICES IN SUCH SYSTEMS[END_REF] showed that the break-up of a single bubble in a Newtonian fluid in a shear flow occurs above a critical capillary number, Ca c , which value depends on the viscosity of the bulk phase. However it was recently shown that in foams, ie concentrated assemblies of bubbles, bubble breakup occurs for much lower capillary numbers [START_REF] Golemanov | Breakup of bubbles and drops in steadily sheared foams and concentrated emulsions[END_REF] . It is likely that the thin films between the bubbles play a role in this behaviour [START_REF] Drenckhan | The science of foaming[END_REF] .

Indeed in a sheared foam, the thin liquid films between the bubbles are strongly deformed and topological rearrangements of bubbles occur [START_REF] Dollet | Rheology of aqueous foams[END_REF][START_REF] Jiang | Hysteresis and avalanches in twodimensional foam rheology simulations[END_REF] which lead to bubble coalescence in the case of very thin films [START_REF] Biance | How Topological Rearrangements and Liquid Fraction Control Liquid Foam Stability[END_REF][START_REF] Carrier | Coalescence in Draining Foams[END_REF] .

In this article, we study the foaming of transient polymer networks made with Polyvinyl alcohol/Borax mixtures as well as the stability of the obtained foams under shear. PVA/Borax mixtures are non-Newtonian shear thickening fluids, whose viscosity increases with the shear rate and whose rheological properties can be tuned with the PVA/Borax composition. We showed in a recent article that PVA/Borax transient networks can be used to produce foams with a ultra-slow rate of drainage due to high bulk and interfacial viscosities 1 . To produce these foams we used a two-step foaming process, where a PVA solution is first foamed and then a Borax solution is added to the PVA foam under shear to obtain the PVA/Borax foams.

In the present article we show that the PVA/Borax foams with a low PVA/Borax ratios become highly unstable and collapse while they are sheared. Using scaling arguments we find that the shear induced collapse of the foams and bubbles is obtained below a critical ratio, N E /N B = 15, of the number of entanglements per chain, N E, and number of Borax per chain, N B, . Rheology measurements show that below this critical N E /N B ratio, the viscosity of the samples diverges. We suggest that the shear induced collapse of the foams is due to the fact that the PVA/Borax solutions are shear thickening and the thin films undergo a viscous to fragile transition break as they are stretched during the foaming process leading to a coalescence of the bubbles. A series of catenoid-shaped thin film stretching experiments show that the thin films break as they are stretched at high velocity below a critical PVA/Borax ratio, which is much higher than the critical ratio for foam collapse.

Materials and methods

Sodium tetraborate decahydrate Na 2 B 4 O 7 .10H 2 O, or Borax is purchased from Aldrich and used without further purification. In water, it dissociates into Na + and B(OH)4 -. PVA of various molar mass and degree of hydrolysis were purchased from either Sigma-Aldrich or Polyscience. The entanglement concentration C* was obtained using rheology measurements as explained below. The PVA solutions with concentrations ranging from 2.5 to 15wt% are heated to 85°C under vigorous stirring during 3 hours to reach total polymer dissolution, and then cooled to room temperature before use. For all PVA/Borax mixtures, Borax solutions are added to a tenfold greater volume of PVA solution, so that final Borax concentrations are approximately ranging from 0.1 to 0.5wt%.

Table 1. PVA molecules used

Foam preparation

200g of PVA, poly(vinyl alcohol) solution is placed into the bowl of a kitchen mixer (Kenwood® KWY90) and sheared (Figure 1a) at a maximal speed (100 s -1 ) during 3 minutes. PVA, which is a surface active polymersurface tension around 45 mN/mis indeed a very good foaming agent and we obtain 3 to 4 L of low density fine foam (bubble size 400 µm, volume fraction of air from 93 to 95%). After the PVA foam is obtained, an appropriate volume of borax solution at a concentration 10 times the final one is added to the PVA foam under continuous mixing for time varying between 10 and 180 seconds. To ensure good homogeneity of the PVA/Borax gel, we add rhodamine to the Borax solution and find that 10 seconds is enough to homogenize the foam. The air fraction is measured by measuring the weight of a given volume of foam. Reproduced from Deleurence et al., [1] with the permission of the Royal Society of Chemistry.

Product

Measurement of flow curves and storage and loss modulii

Rheological measurements of PVA/Borax solutions are carried out with a AR-G2 Rheometer (TA Instruments) using a cone-plate geometry (cone angle 2°, diameter 40mm, truncation 52µm). All measurements are made at 25°C, and with a solvent trap to avoid evaporation. The

Catenoid stretching experiment

To mimic the stretching of foam bubbles during the foaming process where the foams are sheared, we performed an experiment where a catenoid shaped thin film is stretched. In this set up (Figure 2), two rings made of stainless steel (internal diameter, 2R = 15 mm, thickness In Figure 3a and 3b we present the rheological measurements obtained for solutions of PVA 5% with increasing Borax concentrations. The PVA 5% solution with no Borax, behaves like a purely Newtonian viscous fluid. The solution which contains 0.1% Borax, has a weak shear elastic modulus but its flow curve shows a constant viscosity with the shear rate. For 0.2% and 0.4% Borax the samples are viscoelastic, with G' and G'' crossing at a frequency  c of the order of 5 to 10 rad.s -1 . The corresponding samples present a sharp increase of the viscosity at high shear rate. The shear thickening behavior appears at a strain rate of about  c evidencing that if the system does not have the time to relax its links it undergoes a strong shear thickening. The shear thickening behavior of PVA/Borax solutions was reported previously [START_REF] Inoue | Rheological properties of poly(vinyl alcohol)/sodium borate aqueous solutions[END_REF] . Above 20 s -1 , it becomes impossible to measure the viscosity as the samples get ejected from the rheometer gap.

In Figure 3c we present flow curves obtained for two PVA concentrations, 5% and 10%, and two Borax concentration of 0.2 and 0.4%. For a given Borax concentration the shear thickening is less pronounced for the highest PVA concentration. Similarly to us, Liu et al. [START_REF] Xu | Divergent Shear Thinning and Shear Thickening Behavior of Supramolecular Polymer Networks in Semidilute Entangled Polymer Solutions[END_REF] observed that the shear thickening behaviour of another type of polymer transient networks can be reduced by increasing the polymer concentration with a fixed cross linker concentration. c. Flow curves of PVA 5 and 10% with 0,2 and 0,4% Borax. At high shear rates, the samples get sometimes ejected from the rheometer, preventing any measurement. The dotted lines represent the evolution of the viscosity that we expect with respect to the collapse of the foams.

Shear instability of PVA/Borax foams

The PVA/Borax foams are produced in a two-step process consisting in shearing a PVA solution using a kitchen mixer and then adding the Borax under continuous shearing. For a given range of PVA/Borax compositions, the sheared PVA/Borax foams become highly unstable during the shearing process. Figure 4 provides an example of such a shear-induced collapse of the foams. For a foam containing 7.5% wt PVA (Mowiol 8-88) and 0.4% of Borax, the air fraction drops from 92% to 25% after 60 seconds of shear and the foam volume falls quickly. However for a foam containing the same PVA concentration with only 0,1% wt of Borax, the air fraction and foam volume remains stable while the foam is sheared over 3 minutes. weeks for the highest PVA/Borax concentrations in zone III, as reported in our previous article [START_REF] Deleurence | Time scales for drainage and imbibition in gellified foams: application to decontamination processes[END_REF] .

We note that when the mixer speed is reduced to its minimum, corresponding to a shear rate of approximately 20s -1 , the frontier moves toward higher Borax concentrations. These results

show that the collapse of the foams during mixing depends on the composition and on the mixing velocity.

Combining the rheology measurements and the foam collapse observations we suggest that the foam collapse during the mixing process is caused by a strong increase of the viscosity during the mixing which leads to a ductile to fragile transition, similarly to what was observed in other polymer transient networks [START_REF] Ligoure | Fractures in complex fluids: the case of transient networks[END_REF] . Below we discuss in more details the influence of the composition on the shear thickening behavior and foam collapse.

Frontiers between zones I, II, III

The gelation of PVA/Borax mixtures was studied in References [START_REF] Kurokawa | Phase behaviour and sol-gel transition of poly(vinyl alcohol)-borate complex in aqueous solution[END_REF][START_REF] Pezron | Reversible gel formation induced by ion complexation. 2. Phase diagrams[END_REF][START_REF] Leibler | Viscosity behaviour of polymer solutions in the presence of complexing ions[END_REF][28] . Keita and Ricard 28 The dashed frontier between zones II and III shown in Figure 5c is obtained using a critical ratio N E /N B = 15. Hence the shear induced coalescence of the foams is obtained when

N E /N B <15.
We note that when the mixer speed is reduced to its minimum, corresponding to a shear rate of approximately 20s -1 , the critical ratio decreases down to N E /N B =7. This means that when the mixing velocity is lower, the frontier moves toward higher Borax concentrations.

In Table 2, we indicate the foam collapse observations made for two mixing velocities equal to 20 and 100 s -1 for the four PVA/Borax solutions which flow curves are given in Figure 3c, PVA 5 and 10% with Borax 0,2 or 0,4%. The solutions containing 5% of PVA with 0,2 or 0,4% Borax collapse at mixing velocities of both 20 and 100 s -1 , which can be correlated to the strong increase of the viscosity starting before 20 s -1 . The ejection of the sample at high shear rate makes the viscosity measurement impossible but the dotted lines in Figure 3c are a suggestion for the viscosity evolution that we expect for these two samples at high shear rate.

The foam containing PVA 10% and Borax 0,2% is stable at 20 and 100 s -1 , which can be attributed to the slow increase of the viscosity with the shear rate. Finally, the foam containing 10% of PVA and 0,4% of Borax is stable for a mixing velocity of 20s -1 but collapses if the mixing velocity is increased to 100 s -1 . As suggested by the dotted line in Figure 3c, a weak increase of the viscosity up to 20s -1 followed by a sharp increase of the viscosity above 20 s -1 could explain such behavior.

We note that the foams containing 5% PVA and 0,1% Borax which are not shear thickening (Figure 3b) are noted as unstable in Figure 5c. In fact, the foam observations were not reproducible for this specific sample, which lies close to the frontiers between zones I, II and III, where the homogeneization of the samples is probably critical.

Table 2. Foam collapse observations for the same concentrations as in Figure 3c.

Stretching catenoids of PVA/Borax mixtures

When a foam is sheared, the thin liquid films between the bubbles are either stretched or compressed. To mimic the stretching of thin liquid films inside the foams during the shearing process we performed a series of experiments which consist in a fast stretching of catenoidshaped films made with PVA/Borax mixtures. 

In Figure 6 we present the shape variation of catenoids made from PVA 8-88 10% with Borax concentrations between 0 and 0,4%. Along with images of the catenoids, we present the variation of a/R as a function of h/R, where is a the neck radius, R the radius of the catenoid at the rings and h the height of the catenoid. The dashed line corresponds to the theoretical shape of surfactant catenoid in quasi static conditions, which corresponds to a minimal surface area 29 . Theoretically, a surfactant catenoid becomes unstable for h/R above 1,33. The behavior of the catenoids containing only PVA is very close to the theoretical dashed line for h/R up to 1,3. However we note that the rupture of the catenoids is delayed and occurs for h/R = 1,8 probably because of the fast stretching rate and to the relatively high viscosity of the PVA solution in comparison to standard surfactant films. The destabilization of the films is delayed. For increasing Borax concentrations, to 0,4%, the shape of the catenoids deviates from the theoretical prediction more and more significantly. Indeed a/R is higher than expected for a given h/R meaning that the surface area is not minimal. Moreover the catenoids break for lower values of h/R as the Borax concentration is increased. These simple experiments shows that the thin liquid films containing PVA/Borax mixtures undergo a viscous to brittle transition as the Borax concentration increases. We suggest that for PVA/Borax mixtures for which the viscosity diverges strongly at high deformation rates, the PVA/Borax network cannot flow to minimize their surface energy and become brittle. We therefore suggest that the shear induced collapse of the foams that we observe during the foaming process is due to the rupture of thin liquid films as they are strongly stretched. We note that the critical Borax concentration for which the fracture of the catenoids occurs for a critical ratio N E /N B = 150, well above the ratio of 15 or 7 found in the shearing foam experiments for 100 or 20 s -1 showing that the thin films undergo a viscous to fragile transition for much lower Borax concentrations than in the foams. Moreover, for 0,1%

Borax, the catenoid films undergo a fragile break up although the foams are stable during the mixing process and the solutions do not present any shear thickening behavior (see Supporting Information S1). The catenoid experiment is a purely extensional flow, in contrary to the shear rheology experiments and the elongational viscosity of these PVA/Borax systems may be much larger than their shear viscosity.

Conclusion

In this article, we investigate the foaming process of foams made with PVA/Borax transient networks.

To produce these foams we used a two-step foaming process, where a PVA solution is first foamed and then a Borax solution is added to the PVA foam under shear to obtain the PVA/Borax foams. For formulations with a low PVA/Borax ratio, we observe a shear-induced collapse of the foams. For high PVA/Borax ratios, the foams remain stable under shear (mixing velocity of 100 s -1 ) and the produced foams are subsequently highly stable at rest. Using scaling arguments we find that the shear induced collapse of the foams is obtained below a critical ratio N E /N B <15 of the number of entanglements per chain, N E and number of Borax per chain, N B . For a mixing velocity of 20s -1 , the critical ratio falls down to 7. Rheological measurements of the PVA/Borax solutions show that the solutions are shear thickening. The rate at which the viscosity diverges with the shear rate increases when the PVA/Borax ratio decreases.

Combining the foam observations and the rheological measurements, we suggest that the foam collapse is due to the strong increase of the viscosity during the mixing process, which induces a viscous to fragile transition of the samples. Experiments consisting in a fast stretching of PVA/Borax catenoids show that the thin films undergo a viscous to brittle transition below a critical PVA/Borax ratio : the films do not minimize their surface energy and become brittle upon stretching. However we note that the critical ratio for film fracture is much higher than the one measured for the foam collapse, of the order of 150 : the thin films break at very low Borax concentrations. We suggest that this discrepancy between films and foams measurements is due to the fact that in the film experiment the thin film is larger than the ones is the foams increasing the probability of rupture. Moreover in the thin film stretching experiment the flow is purely elongational, possibly leading to higher viscosities than in a shear experiment. To summarize we suggest that the shear induced collapse of the PVA/Borax foams observed during the foaming process is due to the rupture of the thin liquid films and subsequent coalescence of the bubbles and is related to a transition from a viscous to a fragile behavior, which depends on the PVA/Borax ratio and shear rate.

Figure 1

 1 Figure 1. a) 2-step foaming process: a PVA solution is foamed using a mixer and Borax is

  value of C*, the entanglement concentration, was obtained by measuring the steady shear viscosity as a function of the bulk concentration. The change of exponent of the viscosity variation with respect to concentration indicates C*. The PVA/Borax gel are prepared by mixing PVA and Borax and gently stirring the sample.

  mm) are dipped into the PVA solution and some of the solution is collected between the two rings. The rings are then withdrawn from the solution and pulled apart by 1 cm to form a catenoid thin film. The PVA amount is measured by weighing the rings after dipping them in the PVA solution. The Borax is then added dropwise onto the film (homogeneity of the film is checked by adding a dye, rhodamine, to the Borax solution). Then the catenoid is stretched by letting the bottom ring fall. The resulting stretching velocity increases to approximately 450 mm/s corresponding to a stretching rate of the order of 40 s -1 . Using a high speed camera we then extract the neck radius, a and h the distance between the rings.

Figure 2 .

 2 Figure 2. Experimental set up used to stretch catenoid thin films containing PVA/Borax

Figure 3 .

 3 Figure 3. Rheological properties of PVA/Borax mixtures obtained for PVA 8-88.

Figure 4 Figure

 4 Figure 4 Evolution of the air fraction of the PVA/Borax foams as a function of the mixing time for a shear rate of 100 s -1 . The two photos show the quantity of foam obtained in the

  investigated experimentally and theoretically the sol/gel transition of PVA/Borax mixtures. Above the gel point, the PVA chains form a percolated network. Their experimental gel point curve, which represents the critical Borax concentration for gelation as a function of C/C*, is very close to the frontier that we measure between zone I and zones II/III in Figure 5c, suggesting that in zone I the PVA/Borax mixtures are below the percolation point. Keita and Ricard predicted the shape of the gel point curve is obtained as follows : the gel point is reached if all PVA chains contain at least one cross-link, either a PVA/Borax/PVA dicomplex or an entanglement between two PVA chains. The latter condition writes theoretically [B -]DP~(C/C*) -2 Equation1with [B -] the concentration in Borate anions, DP the degree of polymerization of the PVA chains, C the PVA concentration and C* the overlap concentration above which PVA chains start overlapping. In our case, we find the frontier of zone I is well described by the following Equation 2 enables us to obtain a divergence of the Borax needed to obtain gelation for C=C*, a condition which is observed experimentally both in our case and in Keita's case.For PVA and Borax concentrations above the gel point such that stable (zone III) or unstable (zone II) during the mixing process depending on the PVA/Borax composition.Interestingly the frontier between zones II and III corresponds to a critical ratio of the number of entanglements and Borax links per chain. The number of Borax per polymer chain, N B , and number of entanglements, N E , per polymer chains write respectively[START_REF] Kurokawa | Phase behaviour and sol-gel transition of poly(vinyl alcohol)-borate complex in aqueous solution[END_REF][START_REF] Pezron | Reversible gel formation induced by ion complexation. 2. Phase diagrams[END_REF][START_REF] Leibler | Viscosity behaviour of polymer solutions in the presence of complexing ions[END_REF]

Figure 6 .

 6 Figure 6. Stretching of the catenoids and values of a/R as a function of h/R for catenoid films

  Supporting information. S1. Shear rheology measurements of a PVA10% Borax 0.1% solution 28. Keita, G. & Ricard, A. Phase diagram of borate-PVA system: Sol-gel transition and demixtion. Polym. Bull. 1990, 24, 633-640. 29. Salkin, L., Schmit, A., Panizza, P. & Courbin, L. Influence of boundary conditions on the existence and stability of minimal surfaces of revolution made of soap films. Am. J. Phys. 2014, 82,

Acknowledgements

We thank Saint-Gobain as well as ANRT for financial support of RD's PhD thesis. We acknowledge Guylaine Ducouret for her help and fruitful discussions concerning the rheology measurements.