domain: chim phys

text:
Jakob Langenbach

Camille Bakkali-Hassani

Quentin-Arthur Poutrel

Antonia Georgopoulou

Frank Clemens

François Tournilhac

Sophie Norvez
email: sophie.norvez@espci.fr

Adhesion and stiffness matching in epoxyvitrimers/strain sensor fiber laminates

Keywords: soft robotics, elastomeric vitrimer, electrically conductive composites, matrixsensor fiber laminates, stiffness matching

come

Adhesion and stiffness matching in epoxyvitrimers/strain sensor fiber laminates

Introduction

Soft robotics is an emergent branch of robotics, which involves incorporation of elastomeric elements. Addition of soft materials into robotic devices allows better flexibility and adaptability to the environment than when robots are made of hard and rigid components. [START_REF] Hughes | Soft Manipulators and Grippers: A Review[END_REF] These features make soft robots a promising technology for biomedical, [START_REF] Cianchetti | Biomedical Applications of Soft Robotics[END_REF][START_REF] Atalay | Piezofilm Yarn Sensor-Integrated Knitted Fabric for Healthcare Applications[END_REF] food industry [START_REF] Zhang | State-of-the-Art Robotic Grippers, Grasping and Control Strategies, as well as their Applications in Agricultural Robots: A Review[END_REF] or space [START_REF] Pernigoni | Self-Healing Materials for Space Applications: Overview of Present Development and Major Limitations[END_REF] applications, insofar mechanical impedance of the robotic fingers could match those of the objects manipulated. However, transfer from academic research to industry still provides some challenges. [START_REF] Rus | Fabrication and Control of Soft Robots[END_REF] In particular, the loss of the robot functionality is a major concern after material damages, intrinsically favoured by the soft nature of the components. [START_REF] Terryn | A Review on Self-Healing Polymers for Soft Robotics[END_REF] To address this issue, the use of self-healing (SH) elastomers figures among available solutions. [START_REF] Terryn | Self-Healing Soft Pneumatic Robots[END_REF] SH ability has been demonstrated in diverse elastomers made of physical or chemical networks. SH physical networks may be obtained by association of macromolecules through small glassy or crystalline domains, ionic aggregates or hydrogen bonds. SH elastomers based solely on small molecules linked by hydrogen bonds have even been described. [START_REF] Cordier | Self-Healing and Thermoreversible Rubber from Supramolecular Assembly[END_REF] SH chemical networks may be built from dynamic covalent chemistry as well. In all cases, a trade-off between SH ability and mechanical properties is unavoidable. [START_REF] Hoogenboom | Hard Autonomous Self-Healing Supramolecular Materials-A Contradiction in Terms?[END_REF] The supramolecular rubbers exhibit creep under load or stress as the strength of the exchangeable bonds is low. Dynamic covalent networks with reversible links lose their integrity through dissociation of cross-linkers at high temperature. In dissociative systems, either non-covalent or covalent, the cross-link density depends on the equilibrium between the associated and dissociated bonds, as bonds are continuously broken and re-formed, and this equilibrium shifts to dissociation when temperature is elevated. To face the dilemma, a strategy combining two reversible cross-linkers working at different time scales, hydrogen bonds and Diels-Alder adducts, has been proposed. [START_REF] Zhang | Self-Healing, Malleable and Creep Limiting Materials Using Both Supramolecular and Reversible Covalent Linkages[END_REF] Slowly exchanging Diels-Alder cross-linkers limited creep at room temperature while faster exchange under heat allowed reshaping. Improvement of the mechanical performances of a covalent SH material using sacrificial hydrogen bonds grafts has also been considered. [START_REF] Neal | Enhancing Mechanical Performance of a Covalent Self-Healing Material by Sacrificial Noncovalent Bonds[END_REF] Still a dissociative mechanism cannot maintain shape at high temperature.

In contrast, systems using an associative mechanism may afford retention of the crosslink density, as in this case a bond cleavage occurs simultaneously with the formation of a new cross-link. This strategy was first worked out by Leibler and coworkers who named their materials vitrimers. [START_REF] Montarnal | Silica-Like Malleable Materials from Permanent Organic Networks[END_REF][START_REF] Capelot | Metal-Catalyzed Transesterification for Healing and Assembling of Thermosets[END_REF][START_REF] Capelot | Catalytic Control of the Vitrimer Glass Transition[END_REF] Prototypes of vitrimers were based on exchanges by transesterification in acid/epoxy networks; since then, numerous other inventive chemistries have been investigated. [START_REF] Pepels | Self-Healing Systems Based on Disulfide-Thiol Exchange Reactions[END_REF][START_REF] Rekondo | Catalyst-Free Room-Temperature Self-Healing Elastomers Based on Aromatic Disulfide Metathesis[END_REF][START_REF] Imbernon | Chemically Crosslinked Yet Reprocessable Epoxidized Natural Rubber via Thermo-Activated Disulfide Rearrangements[END_REF][START_REF] Obadia | Reprocessing and Recycling of Highly Cross-Linked Ion-Conducting Networks through Transalkylation Exchanges of C-N Bonds[END_REF][START_REF] Fortman | Mechanically Activated, Catalyst-Free Polyhydroxyurethane Vitrimers[END_REF][START_REF] Denissen | Permanent Organic Networks with Glass-Like Fluidity[END_REF][START_REF] Röttger | High-Performance Vitrimers from Commodity Thermoplastics through Dioxaborolane Metathesis[END_REF][START_REF] Denissen | Vinylogous Urea Vitrimers and Their Application in Fiber Reinforced Composites[END_REF][START_REF] Van Zee | Permanently Crosslinked Polymers with Dynamic Network Topology[END_REF] In vitrimer systems, the temperature increase accelerates the exchange rate of covalent bonds, while the total number of crosslinks remains constant.

Reprocessing and full recycling have been subsequently demonstrated in such systems.

Dissociative mechanism takes profit of local concentration of reactive species at the wound location, faciliting healing through bridging interfaces, but full reprocessing is not accessible without extensive loss of integrity. A strategy to provide integrity to reparable soft materials, deformable under load or heat, consists in introducing intentionally some permanent covalent links. [START_REF] Cash | Maximizing the Symbiosis of Static and Dynamic Bonds in Self-Healing Boronic Ester Networks[END_REF][START_REF] Poutrel | Dicarboxylic Acid-Epoxy Vitrimers: Influence of the Off-Stoichiometric Acid Content on Cure Reactions and Thermo-Mechanical Properties[END_REF][START_REF] Tangthana-Umrung | Epoxy Homopolymerization as a Tool to Tune the Thermo-Mechanical Properties and Fracture Toughness of Vitrimers, asap[END_REF] Poutrel et al. have recently shown that a slight off-stoichiometry in the ratio acid/epoxy greatly improved the mechanical properties of the resultant resin. [START_REF] Poutrel | Dicarboxylic Acid-Epoxy Vitrimers: Influence of the Off-Stoichiometric Acid Content on Cure Reactions and Thermo-Mechanical Properties[END_REF][START_REF] Tangthana-Umrung | Epoxy Homopolymerization as a Tool to Tune the Thermo-Mechanical Properties and Fracture Toughness of Vitrimers, asap[END_REF] From this point of view, even side reactions giving rise to permanent crosslinks may reveal as auxiliaries for self-healing behavior. The small amount of permanent network improves mechanical properties, yet allowing repeated repair or at least welding. Such materials have been referred to as being vitrimer-like. [START_REF] Poutrel | Dicarboxylic Acid-Epoxy Vitrimers: Influence of the Off-Stoichiometric Acid Content on Cure Reactions and Thermo-Mechanical Properties[END_REF][START_REF] Tangthana-Umrung | Epoxy Homopolymerization as a Tool to Tune the Thermo-Mechanical Properties and Fracture Toughness of Vitrimers, asap[END_REF][START_REF] Imbernon | Stress Relaxation and Self-Adhesion of Rubbers with Exchangeable Links[END_REF][START_REF] Kaiser | Stress Relaxation and Thermally Adaptable Properties in Vitrimer-Like Elastomers from HXNBR Rubber with Covalent Bonds[END_REF][START_REF] Kaiser | Design and Characterisation of Vitrimer-Like Elastomeric Composites from HXNBR Rubber[END_REF] To achieve automated operations and to detect a material damage (and possibly start a healing process), a soft robot must be equipped with a sensing system such as strain sensors. [START_REF] Liu | Electrically Conductive Polymer Composites for Smart Flexible Strain Sensors: a Critical Review[END_REF][START_REF] Georgopoulou | Piezoresistive Elastomer-Based Composite Strain Sensors and Their Applications[END_REF][START_REF] Lu | Recent Developments in Bio-Monitoring via Advanced Polymer Nanocomposite-Based Wearable Strain Sensors[END_REF] To produce stretchable electronics with self-healing properties, two strategies have been described: (i) combining self-healing elastomers with carbon black fillers, [START_REF] Latif | Self-Healing Materials for Electronic Applications: an Overview[END_REF] and (ii) embedding sensor fibers in an elastomer matrix. [START_REF] Georgopoulou | Sensorized Robotic Skin Based on Piezoresistive Sensor Fiber Composites Produced with Injection Molding of Liquid Silicone[END_REF] The first strategy yields to composites with significantly higher stiffness and shorter range of deformation than the second one. [START_REF] Georgopoulou | Effect of the Elastomer Matrix on Thermoplastic Elastomer-Based Strain Sensor Fiber Composites[END_REF][START_REF] Thuruthel | Using Redundant and Disjoint Time-Variant Soft Robotic Sensors for Accurate Static State Estimation[END_REF] The second strategy limits the undesired matrix stiffening but requires a good integration of the sensor fibers in the elastomer matrix. [START_REF] Georgopoulou | Effect of the Elastomer Matrix on Thermoplastic Elastomer-Based Strain Sensor Fiber Composites[END_REF][START_REF] Thuruthel | Using Redundant and Disjoint Time-Variant Soft Robotic Sensors for Accurate Static State Estimation[END_REF][START_REF] Georgopoulou | Piezoresistive Sensor Fiber Composites Based on Silicone Elastomers for the Monitoring of the Position of a Robot Arm[END_REF] In this work, epoxy-acid vitrimer or vitrimer-like networks with tunable mechanical properties are prepared as matrices for strain sensor integration. Four different vitrimer materials incorporating reversible hydrogen bonds, exchangeable ester bonds and small amount of permanent ether bonds are prepared in a straightforward two-step synthesis, and their mechanical properties are quantified. Their thermostimulated self-healing capability is exploited to devise a sandwich-welding procedure where a carbon/polymer composite fiber is embedded between two elastomer strips. The electrical and mechanical behaviors of the resulting sensor fiber composite (SFC) are investigated simultaneously in two different situations: (i) at large (> 100 %) deformation using quasi-static tensile testing conditions and

(ii) at 20 % applied deformation, using dynamic cycling conditions. Dissimilar behavior observed at room temperature and at 50 °C is analysed in terms of difference in the Young's moduli of fiber and matrix.

Experimental Section

Materials

Pripol 1040 (vegetable fatty acid oil) with dicarboxylic (23 wt %) and tricarboxylic (77 wt %) acids were obtained from Croda. 2-Aminoethylimidazolidone (UDETA) was kindly provided by Arkema. A bifunctional diglycidyl ether of bisphenol A (DGEBA, DER332) and a tetrafunctional 4,4′-methylene-bis (N,N-diglycidylaniline) (TGMDA) were purchased from Sigma-Aldrich. 2-Methylimidazole (2-MI) supplied from Fluka was used as crosslinking catalyst. Pripol 1040, DGEBA, TGMDA, and 2-MI were used as received. UDETA with an initial purity of 83 wt % (95 mol %) was distilled under vacuum at 160 °C and recrystallized from a toluene/chloroform (mass ratio 80/20) mixture before use. Detailed formulae may be found in Table 1. The liquid precursor, SP50 with a target conversion of 50 mol % of acid into amide, was synthesized as already reported by reacting Pripol 1040 and UDETA as detailed in the Supporting Information. [START_REF] Sordo | Design of Self-Healing Supramolecular Rubbers with a Tunable Number of Chemical Cross-Links[END_REF]

Hybrid network synthesis

The hybrid networks are denoted as xD-(100-x)T_nepoxy, where x is the mol % epoxy (x = 0 or 75), n the excess mol % epoxy (n = 0, 5, 10 or 50), D and T represent DGEBA and TGMDA, respectively. Flat sheets of hybrid networks were obtained according to the procedure detailed in the Supporting Information. Table S1 summarizes the chemical composition of the five hybrid networks synthesized.

Sensor fiber synthesis

For the piezoresistive sensor composite, a styrene-based triblock copolymer by Kraiburg TPE (Waldkraiburg, Germany) was combined with carbon black obtained from TIMCAL (Bodio, Switzerland). The two components were mixed in a 1:1 mass ratio (50 wt % carbon black), using torque rheometer HAAKE Polylab Rheomix 600 from Thermofisher (Karlsruhe, Germany). Sensor fibers with a diameter of 0.3 mm were extruded using a capillary rheometer RH7 from NETZSCH (Selb, Germany). The temperature used for the mixing and the extrusion was 190 °C. The sensor fiber fabrication and properties have been reported

elsewhere. [START_REF] Melnykowycz | Comparison of Piezoresistive Monofilament Polymer Sensors[END_REF] The sensing mechanism is based on the rotation of the conductive particles, as well as on tunneling and hopping, and not on cracks formation as proposed by Liu and Shen. [START_REF] Liu | Stretchable Conductive Nonwoven Fabrics with Self-Cleaning Capability for Tunable Wearable Strain Sensor[END_REF][START_REF] Bu | Ultrasensitive Strain Sensor Based on Superhydrophobic Microcracked Conductive Ti3C2Tx MXene/Paper for Human-Motion Monitoring and E-Skin[END_REF] 1.5 Integration of sensor fibers into vitrimer matrices

The sensor fiber was pre-stretched to 50 % strain for one minute to ensure reproducible sensing properties. The ends of the fiber were connected to a thin metal wire with cable end sleeves and conductive glue. Dog-bone shaped specimen with a total length of 13 cm (gauge length 25 mm) were punched from the cured sheets. These samples were stored in an oven (22 °C, 30 % relative humidity) for several days to equilibrate the material and avoid dust attachment. For welding, the sensor fiber was placed between two dog-bone shaped samples, with no additional cleaning of the connected surfaces. Sensor fiber composites (SFCs) were sandwich-welded at 80 °C for 1 h with a 1 kg weight to induce flow of the assembly. After the heating period, the temperature is reduced slowly to improve the connection between the dog-bone sheets. For the sensor fiber composites, the sample was clamped with pneumatic holders, the wire cables were connected to a Keithley 2400 source-measure unit (sampling rate of 10 Hz) and the resistance was measured. The crosshead speed was 25 mm.min -1 for tensile until fracture and 12.5 mm.min -1 for cyclic loading. All tensile tests with sensor fiber composites were done at room temperature (22 °C) and the cyclic test was repeated at 50 °C. The cyclic test was performed between 0 and 5 mm displacement, which corresponds to a 20 % strain.

The relative resistance, which indicates the sensitivity of the strain sensor, is the ratio of the change in resistance (R-R0) divided by the resistance at rest (R0):

Relative resistance

0 0 R R R   (1)
1.9 Microscopic studies Images in polarized light were taken through crossed polarizers after fixing a weight of 100 g (1 N) at the bottom of the dog-bone composite in vertical position to exert a constant stress.

Polarizers were oriented at 45° or parallel to the stretching direction. The magnified images of the rupture spots were made using a camera installed on a Leica Wild MZ8 optical microscope.

Results and discussion

Our group previously designed tunable self-healing supramolecular rubbers, containing a density of chemical crosslinks, adjustable through a controlled ratio of bifunctional DGEBA and tetrafunctional TGMDA epoxy monomers. [START_REF] Sordo | Design of Self-Healing Supramolecular Rubbers with a Tunable Number of Chemical Cross-Links[END_REF] Self-healing ability was ensured by a fixed density of H-bonds provided by the UDETA moiety carried by the crosslinker SP50. With only 25% of tetra-epoxide for 75% di-epoxide in the formulation (75D-25T sample), the materials presented modest mechanical properties but complete recovery at room temperature after damage. With 100% tetra-epoxide (0D-100T sample), the material is much stiffer but the selfhealing at room temperature is poor. In another study, [START_REF] Poutrel | Dicarboxylic Acid-Epoxy Vitrimers: Influence of the Off-Stoichiometric Acid Content on Cure Reactions and Thermo-Mechanical Properties[END_REF] it was explored the effect of an excess of epoxy monomers on the thermomechanical properties of resins made of commercial diepoxide (Araldite) and sebacic acid. Homopolymerisation during curing of nonstoichiometric systems allowed formation of non exchangeable polyether bridges, improving mechanical properties of the resin. Off-stoichiometry [epoxy]/[COOH] thus appeared as a possible lever to improve stiffness without losing too much SH ability.

In the present work, we play on both levers to tune the thermo-mechanical properties of hybrid epoxy-acid networks. The workflow is presented in Figure 1. Firstly, the concentration of hydrogen bonding moieties is fixed by the initial feed ratio

[UDETA]0/[COOH]0 = 0.5 during the preparation of the liquid precursor SP50 (see section 1.2). The ratio of 50 mol % was selected to achieve high H-bonds concentration for healability at low temperature. In a second step, the remaining COOH functions of SP50 are reacted with epoxides to form the hybrid network. Following this two-step procedure, different Figure 1a illustrates the second step where the hybrid network is synthesized from the liquid precursor SP50. Depending on the stoichiometry [epoxy]/[COOH], only exchangeable ester bonds are formed in addition of existing H-bonds (Figure 1b), or a few supplemental non exchangeable ether bonds are produced by anionic ring-opening polymerisation (AROP), [START_REF] Morancho | Dual Curing of an Epoxy Resin with Dicarboxylic Acids[END_REF] when epoxy functionality is introduced in excess (Figure 1c). Such a dual-curing methodology involving epoxy excess have been proved as suitable to control cure parameters like gelation time in epoxy-acid thermosets. [START_REF] Morancho | Dual Curing of an Epoxy Resin with Dicarboxylic Acids[END_REF][START_REF] Morancho | Curing and Thermomechanical Properties of Off-Stoichiometric Anhydride-Epoxy Thermosets[END_REF] Here excess of epoxy was used to improve the mechanical properties of the rather soft 75D-25T material. Samples containing n mol % epoxy excess are denoted (75D-25T_nepoxy) in Table S1. The sample containing 50 mol % excess (75D-25T_50epoxy) was synthesized as a tool to assess the formation of ether bridges by spectroscopy.

Analysis of the cross-linking reaction

The network formation, which mainly consists in the addition of carboxylic acid functions onto epoxy rings, was monitored by ATR-IR to determine the conversion of both reacting groups.

Figure S1a shows the first and last spectrum of 75D-25T_10epoxy with a magnification of the significant peaks in Figure S1b. The carbonyl stretching band at 1710 cm -1 is attributed to the carboxylic acid function in the molten state, which shows an intensity decrease during the cross-linking reaction. The ester carbonyl stretching band at 1737 cm -1 increases over time, displaying the expected formation of ester bonds. The disappearance of the epoxy peak (low intensity) at 912 cm -1 shows the consumption of epoxy functions even if they are introduced in excess. The integration of these characteristic peaks during the cross-linking reaction is displayed in Figure S1c for the sample 75D-25T_10epoxy. The non-normalized values show the increase of the ester (red circles) and the decline of acid (black squares) and epoxy (blue triangles) peaks initially started with a similar slope. After one hour of cross-linking, the peak integrals stay quite stable which indicates the consumption of all reacting groups. The materials were left for 3 h at 130 °C to ensure the reaction is finished. The difficulty to identify a characteristic band of polyether and the small excess of epoxy functions used in this experiment, both render difficult the analysis of ether bonds formation by IR spectroscopy.

However, an increase of an alkyl-aryl ether peak at 1247 cm -1 suggests the formation of nonexchangeable ether bonds in the hybrid network. The integration method for this specific peak was confirmed using a test sample containing 50 mol % excess (75D-25T_50epoxy).

Peak integrations are shown in Figure S1d with all values normalized by the value of the first peak I0. The peak integrals of 75D-25T_10epoxy show a second increase after 50 minutes, which indicates the formation of ether bonds after all COOH groups were consumed. The final plateau is reached after one hour with higher values for an increasing epoxy excess.

The swelling behavior of hybrid networks is shown in Table S2 (images of swollen samples in Figure S2). As expected, the softest composition 75D-25T_0epoxy shows the highest solvent uptake (230 mass %) and soluble fraction (30 mass %), due to the lower number of chemical cross-links in the network. Small epoxy excess (75D-25T_5epoxy, 75D-25T_10epoxy) shows no influence on the solvent uptake (230 mass %) but gives rise to a slightly higher gel content (80 mass %). The few additional ether cross-links have only a small effect on the swelling behavior, as the cross-linking density is only slightly impacted.

The formulation containing only tetrafunctional epoxide (0D-100T) exhibits the lowest swelling uptake (130 mass %), in accordance with the high cross-link density of the network.

Mechanical and thermomechanical properties of the vitrimer materials

Thermomechanical properties of the hybrid network materials were investigated by DSC, DMA and creep analysis.

The glass transition temperature Tg for all compositions was measured by DSC (Table 2 and thermogram curves in Figure S3). For all materials, Tg stands below 20 °C, and slightly increases with the excess of epoxy groups, from 12 to 16 °C. As expected, the glass transition temperature rises with the cross-link density of the network.

DMA measurements in the film tension mode are presented in Figure S4. Below the glass transition temperature, apparent E' values coarsely match expectation (> 1 GPa).

Above 10 °C, the storage modulus drops in a same temperature range for all samples. At high temperatures the storage modulus reaches a plateau which increases with the number of cross-links. The values of E' at 150 °C are reported in Table 2. 75D-25T_0epoxy sample displays the lowest modulus (50 kPa), which rises slightly with excess of epoxy. The material with 100 % TGMDA (0D-100T) displays the highest modulus (0.9 MPa) in accordance to its highly cross-linked network. The dissipation factor, tan), shows a broad signal around 50 °C for all compositions, which is commonly associated to network defects such as dangling chains and sol molecules. The signal becomes narrower when the degree of crosslinking increases (see full width at half maximum values of tan) peaks in Table 2) which is attributable to a reduction of network defects. Thus, using an excess of epoxy, or changing the ratio of TGMDA are two possible levers to produce materials with tunable damping properties. On the one hand, a high dissipation capacity is useful for reducing contact bounce and improving grip. On the other hand, low dissipation is desirable to enable rapid and repeated tensile motions. Capability to manage the network defects is therefore of prime interest for applications in soft robotics. For application in soft robotics, sensor fibers will be integrated in the elastomers. Our sensor fibers being temperature-sensitive, temperature-dependent creep behavior is an interesting feature to test, for evaluation of brazing capability of our composite materials at moderate temperatures. Figure S5 shows the creep behavior of all formulations. The creep results were in turn used to determine  the viscosity of the materials, according to formula

(2):

  t        100 (2)
where  (in Pa) is the applied compressive stress and  (in % of the initial thickness) the linear strain; the time derivative is measured in the linear portion of the creep curves. The values of viscosity thus determined are plotted in Figure S6, using the Arrhenius representation (log10() as a function of 1/T). For each compound, the data correlate along a straight line, which is characteristic of vitrimers. The plot also enables to measure the activation energy, Ea = R d(ln)/d(1/T) where R is the ideal gas constant, and to evaluate the topology freezing transition Tv. Customarily, Tv is defined as the temperature below which the viscosity becomes greater than 10 12 Pa.s, where in practice, flowability is no longer perceptible. [START_REF] Montarnal | Silica-Like Malleable Materials from Permanent Organic Networks[END_REF] The 0D-100T sample exhibits negligible creep with a maximum deformation of 0.1% after 45 min at 160 °C. The corresponding viscosity approaches the limit values of 10 11 10 12 Pa.s. Even though the chemical structure is that of a vitrimer, in the absence of a transesterification catalyst, this formulation is physically arrested, showing poor ability to be reshaped in between 110160 °C. In contrast, 75D-25T_xepoxy samples, which show much lower viscosity (in the range of 10 8 10 9 Pa.s) are farther from the transition. 75D-25T_0epoxy (Figure S5b) sample deforms up to 20 % with increasing temperature, indicating good flow properties even at moderately high temperatures. As expected, the off-stoichiometric formulations 75D-25T_5epoxy (Figure S5c) and 75D-25T_10epoxy (Figure S5d) exhibit less creep. However, a linear increase of deformation is still present meaning that a low epoxy excess reduces the creep of the materials without quenching vitrimer properties. The activation energy for each composition was determined from the slope of the Arrhenius plot of viscosity (Table 2). From 75D-25T_0epoxy to 75D-25T_10epoxy, the activation energy decreases with the epoxy excess. Such trend was already observed in previous work [START_REF] Poutrel | Dicarboxylic Acid-Epoxy Vitrimers: Influence of the Off-Stoichiometric Acid Content on Cure Reactions and Thermo-Mechanical Properties[END_REF] and was interpreted as the onset of the transition from a vitrimer to a vitrimer-like (more permanent) network.

Tensile tests were conducted to evaluate the mechanical properties at high deformation. One tensile curve per composition is shown in Figure 2 (all curves are presented in Figure S7). The average and standard deviation values of maximum stress and strain are displayed in Table 2. 0D-100T has the highest and 75D-25T_0epoxy the lowest elastic modulus due to the different number of cross-links. The ultimate strength of 75D-25T_5epoxy and 75D-25T_10epoxy increases compared to 75D-25T_0epoxy due to the increased rigidity induced by the additional ether bonds. Interestingly, a slight epoxy excess thus can increase the elastic modulus, keeping suitable creep properties for sensor fiber integration. for all matrix compositions.

Preparation and adhesion of sensor fiber composite (SFC)

For the application in self-healing soft robots, a sensing system is essential to monitor the motions and detect possible damage of the structure. Herein, piezoresistive strain sensor fibers were integrated in the elastomers by a procedure referred as "sandwich-welding" (Figure 3). After the cross-linking step, the thin material sheets were used to punch dog-bone shaped specimens. The sensor was placed between two dog-bone samples, connected to cable wires, and the assembly was pressed for one hour at 80 °C (see section 1.5).

Composite samples will be identified using a C_prefix in their name. The quality of the strain sensor integration into the hybrid networks was measured by T-peel tests, as shown in Figure 4. For this experiment, rectangular samples have been prepared to insure a constant surface all along the specimen. As shown by Grande et al., [START_REF] Grande | On the Interfacial Healing of a Supramolecular Elastomer[END_REF] the initial slope of tensile curves is highly dependent on the stretching speed for these materials, due to the vicinity of Tg. Peeling experiments at 10 mm.min -1 (Figure S8) showed an overshot at small deformations and poor stability during the test. Consequently, the peeling speed was reduced to 2.5 mm.min -1 to ensure elastomeric behavior throughout the test. The C_0D-100T sample exhibits a low peeling strength (80 N.m -1 ), indicating a poor adhesion of both matrix sheets. Conversely, the C_75D-25T_0epoxy sample shows a high peeling strength (1100 N.m -1 ), largely deforming the stretched material and even breaking before delamination (see sample in Figure S9). C_75D-25T_5epoxy and C_75D-25T_10epoxy samples present a high peeling strength around 700 N.m -1 until delamination.

As mentioned above, the epoxy excess provides stiffer networks while keeping creep properties allowing good adhesion ability. Therefore, all C_75D-25T formulations seem suitable for sensor fiber integration, aiming at measuring the material deformation and thus possible damage.

Mechanical and electrical properties of SFCs

Mechanical and electrical properties of all sensor compositions were analysed simultaneously by tensile tests at large (quasi-static) and short (cyclic) deformations.

Figure 5a shows the stress-strain curve of the network in solid lines and the relative resistance in dotted lines. The ultimate strength is reduced for all materials containing sensor fibers compared to their corresponding matrice (Figure 2). The relative resistance of the integrated fiber increases almost linearly until rupture for all compositions, but the slope for the 0D-100T sample is higher than for the 75D-25T specimens due to a higher stress of the former. As shown by Thurutel et al., the change in relative resistance is determined by the applied strain and stress. [START_REF] Thuruthel | Using Redundant and Disjoint Time-Variant Soft Robotic Sensors for Accurate Static State Estimation[END_REF] The poor adhesion properties of 0D-100T composite led to delamination of the samples (Figure 5b). Conversely, the 75D-25T composites present a sharp local breakage (Figure 5c). The relative resistance of the well-integrated strain sensor shows a similar behavior for the three 75D-25T specimens, with rupture inside the material around 140 % strain, a value suitable for soft robotics applications. Repeated cyclic tensile displacements are key motions for soft robots. Figure 6 and Figure 7 show the mechanical (solid lines) and electrical (dotted lines) responses of the four SFCs during cycling tests at 020 % strain. The maximum stress of C_0D-100T decreases slightly during the cycling (Figure 6a). The plateau at low strain is due to buckling of the sample, when decreasing the crosshead distance just increases the sample bending, without appreciable elastic response. This effect is associated to the nearness of the glass transition temperature, just below room temperature. The piezoresistive strain sensor signal of C_0D-100T shows a positive linear increase of relative resistance with a high amplitude and almost no drift during the cycling. However, due to the poor adhesion of the two matrix moieties, delamination of the sandwich occurred during the cyclic testing (Figure 6b) and the experiment was not repeated after rest. For the three C_75D-25T composites at room temperature (Figure 7a-c), the amplitude of both stress and relative resistance decreases over time, in particular at the beginning of the experiment. These effects result from the viscoelastic behavior of the vitrimeric material within each cycle, as the material does not return to its initial length.

Consequently, the material undergoes less deformation each cycle, which results in a decreased amplitude of the relative resistance. None of the C_75D-25T samples showed delamination after cyclic loading (representative sample in Figure S10). After two days of rest at room temperature (colored curves in Figure 7a-c), a decrease of the stress amplitude could still be observed, as well as for the pristine samples. Nevertheless, the drift of the relative resistance is lowered, and the response of the strain sensor reaches a plateau after 10 cycles. In practice, several cycles thus could be performed to stabilise and calibrate the sensor before it reaches a stable response for cyclic tests with constant strain. The same cyclic tensile tests were performed at 50 °C, well above the glass transition temperature of the polymer matrices (Figure 7d-f). Figure 7g shows a representation of the last cycle at 20 °C and 50 °C for the C_75D-25T_10epoxy sample. All C_75D-25T samples exhibit a lower maximum stress and less buckling than at room temperature. Negative stress values originate from the clamping system and the softness of the material, which increases the sample length during cyclic testing. Buckling at room temperature was associated to a large hysteresis of stress, that is strongly reduced when far from the Tg. Conversely, the dependence of the relative resistance on stress [START_REF] Thuruthel | Using Redundant and Disjoint Time-Variant Soft Robotic Sensors for Accurate Static State Estimation[END_REF] seems slightly impacted by the temperature. A remarkable difference in the sensor behavior emerges between samples containing 0 % and 5 % epoxy excess (Figure 7d-e) and C_75D-25T_10epoxy composite (Figure 7f). While both first ones exhibit an early rupture of the fiber, after 3 cycles and 5 cycles, respectively, the stiffest sample C_75D-25T_10epoxy responds normally during the whole experiment. This difference in sensor rupture depending on the epoxy content was further investigated by measuring at 20 °C and 50 °C the Young's moduli of the separate components, the sensor fiber on one side, and the vitrimer matrices on the other side (Figure 8a). The Young's modulus of the matrices was determined by the slope of the first tensile cycle at each temperature. The modulus of the fiber was measured by the slope of a tensile experiment using DMA. At 20 °C, the fiber and the three matrices exhibit relatively comparable Young's moduli. S'2 = 48 mm 2 (Figure 9g). However, at 50 °C, more than 30 °C above their Tg, the stiffness of the polymer matrices drops, while the one of the sensor fiber barely decreases. At 20 °C, a rather homogeneous distribution of the tensile stress is expected in the composite because of comparable stiffnesses of fiber and matrix. Conversely, above the Tg of the matrices, the stress distribution may be highly impacted by two factors: not only the difference in Young's moduli of the composite components, but also the dog-bone shape of the composite sample. Indeed, in a tensile bar with varying cross-section, transverse stresses are generated under strain. As described by G'sell in the case of neck formation during bar stretching, [START_REF] G'sell | Effect of Stress Triaxiality on Neck Propagation During the Tensile Stretching of Solid Polymers[END_REF] the change of bar section when necking induces local effective stresses in addition to the applied stress. Those stresses may be tensile or compressive, on either side of the inflexion point in the shoulder part (Figure 9a). We investigated if such stress triaxiality in the vicinity of crosssectional gradients may be at the origine of the early rupture of our fibers, by observations of the dog-bone composite before and after break, under polarized or non polarized light (Figure 9b-f). Parameters for calculation of shear stress during strectching: E1, E2, Young's moduli of fiber, matrix, respectively ; S1, S2, cross-sections of fiber, matrix. The projection of fiber-matrix interface ST is underlined in red ; f 1, f 2 partial strengths applied to fiber, matrix. The (') symbol accounts for parameters in the large part of the specimen.

A force f = 1 N was applied to a C_75D-25T_5epoxy sample placed between crossed polarizers. When crossed polarizers are oriented at 45° with respect to the stretching direction, a strong birefringence reflects the stress gradient in both shoulders of the dog-bone (Figure 9b). The darkest line that initiates at both points of inflexion on external sides of the shoulders probably follows the line of strongest stress in the sample. When crossed polarizers are oriented along the stretching direction (Figure 9c), orange quarters appear in both shoulders. The concentration of stress in this region, as pointed out by G'sell et al. [START_REF] G'sell | Effect of Stress Triaxiality on Neck Propagation During the Tensile Stretching of Solid Polymers[END_REF] in necking cases, is clearly visualized here. In the case of our composite samples, the transverse efforts (extensive and compressive) lead to a stress concentration on the fiber in this region. At the root of the shoulder, in the concave part, stresses are all tensile, causing a phenomenon of cavitation that possibly initiates rupture. The position of the damage in the broken C_75D-25T_5epoxy sample of Figure 7e was observed post-mortem under re-strech of the dog-bone sample. As observed in Figures 9d and9e, the fiber was ruptured at two spots, indeed in the root of one shoulder, and also in the central gauge region, where the fiber was not well-centered. After relaxation of the broken sample (Figure 9f), the fiber shows buckling in the area of the narrow cross-section due to the increased stress described above.

In these experiments, rupture and buckling of the fiber are induced by a change of shape or a lack of parallelism in the tensile bar, a situation which may be quite representative of robotic parts, whose contours are seldom rectangular.

To describe quantitatively the situation of rupture in our samples, the difference of stiffness between fiber and matrix must be taken into account, in addition to triaxiality effects appearing in the shoulder region. Indeed, as the dog-bone is made of components having different stifnesses, a shear stress  is generated all along the interface ST between fiber and matrix in the shoulder region.  may be written as the difference of forces f applied on the fiber in the narrow (f 1) and large (f' 1) parts divided by the interfacial surface ST in the shoulder region (Figure 9g):

T S f    (3)
The overall force f applied to the dog-bone bar may be decomposed into partial forces f 1 and f 2, f' 1 and f' 2, applied to the fiber and to the matrix, in both extremities of the shoulder:

' ' f f f f f     (4)
and the variation f expressed as:

2 2 1 1 ' ' f f f f f      (5)
Expressing the partial strengths in narrow and large parts of the bar versus their respective strain, surface, modulus, it follows that:

               2 2 1 1 2 2 1 1 1 1 ' 1 1 E S E S E S E S E S f f (6)
where  describes the relative variation of the force when the composite sensor is under strain (see calculation details in Supporting Information). Plotting  against E2 clearly shows that this ratio passes through a maximum value for a Young's modulus of the matrix around 0.025 MPa, and drops rapidly with softer or stiffer matrices (Figure 8b). This extremum corresponds also to a maximum of the shear stress . The E2 values measured for the three matrices at 50 °C were 0.07 MPa, 0.085 MPa and 0.13 MPa for 0 %, 5 %, and 10 % epoxy, respectively. Those matrix moduli may seem quite similar, as compared with the fiber's one, 9 at 50 °C. However, the curve of Figure 8b, where the corresponding  values are indicated, may explain why samples C_75D-25T_0epoxy and _5epoxy broke at 50 °C while C_10epoxy sample did not. Both first ones suffered much more shear stress during cyclic experiments.

Taken the value of interfacial surface ST = 9.4 mm 2 in the shoulder area, it comes that the maximum shear stress  exerted for f = 1 N would be around 26 kPa. This relatively low value must be considered in regard of the small strain applied on the dog-bone bar using a 100 g weight. It is also worth to point out that this simple calculation gives rise to an average value on all the interface, without taking into account the localized stress concentrations.

The dog-bone shape of the specimen, useful to apply strong elongation while avoiding break in the clamps, but rather unusual for continuous fiber composites, has underlined the importance of matching the Young's moduli of matrix and fiber. This problem is important to integrate sensors in soft robotics grippers whose shapes are rarely if ever rectangular. This also raises the question of matrix glass temperature. A glass temperature well below room temperature is desirable for improving the mechanical properties of the matrix (faster return, less drift, in particular), but meanwhile possibly worsens the Young's moduli mismatch with a fiber of high modulus. In addition, the analysis of triaxiality in dogbone shaped specimen may be of interest in experiments involving samples with internal structuration, as for example 3D-printed objects described by Shen et al. [START_REF] Shen | Macromolecular Engineering and Additive Manufacturing of Poly(styrene-b-isobutylene-b-styrene)[END_REF] These authors showed that the printing angle, i.e. the angle between the traveling direction of the nozzle and the long axis of the dog-bone, strongly affects the mechanical properties, especially the modulus. Triaxiality effects in their dog-bone shaped samples could in addition affect the inlayer and interlayer failure modes.

At least, the stiffest sample C_75D-25T_10epoxy showed fast elastic return at 50 °C and did not break during the 15 first min of cyclic testing, meaning that the contrast of elastic modulus between fiber and matrix may be acceptable for working at a moderately high temperature. Such a composite would thus be suitable for robots operating in warm environments, for instance manipulating in ovens.

Conclusions

In this work, a hybrid network based on the epoxy-acid reaction is synthesized and used to integrate a strain sensor fiber for soft robotics finalities. A sandwich-welding procedure is worked out to encapsulate a sensor fiber between two elastomer strips, merged upon application of heat and pressure. The hybrid network displays advantages such as facile synthesis, tunability of thermo-mechanical properties, vitrimer or vitrimer-like behavior and easy processing for sensor integration. Network topology is adjusted by varying the ratio diepoxy/tetraepoxy monomers, and by introduction of a small excess of epoxy groups. Hence, the thermo-mechanical properties of the matrix may be managed, allowing for welding at moderate temperature (80 °C) the temperature-sensitive sensor fiber. Moreover, the subsequent good adhesion fiber-matrix avoids premature rupture of the fiber and interfacial delamination of the sandwich composite.

The electrical resistance of the sensor increases almost linearly with deformation, and the device breaks at relatively high deformation (140 % strain). The tendency of delamination of the two sandwich moieties strongly depends on matrix stiffness and can be avoided by chosing appropriate composition. Cyclic mechanical and electrical tests at room temperature reveal buckling and drift of the sensor response associated to the vicinity of the glass transition temperature of the matrix (1216 °C). However, the drift can be reduced with precycling or at larger distance from Tg (50 °C).

The current study shows the interest of using a weldable hybrid network coupled with resistive sensing technology for soft robotics applications, as well as the importance of the Young's moduli matching between sensor fiber and matrix. Current work is in progress to improve the dynamic properties using SH materials based on backbones showing lower Tg values.

  Prepolymer liquid precursor (SP50) preparation

  networks have been synthesized by (i) modifying the ratio of functional epoxy groups ([DGEBA]/[TGMDA]) and (ii) using an off-stoichiometry ratio [epoxy]/[COOH] > 1, where [epoxy] = [DGEBA]+[TGMDA], targeting different compromises between static and dynamic properties.

Figure 1 .

 1 Figure 1. a) Workflow of sample preparation with adjustable quantities of liquid precursor SP50 and epoxy cross-linkers. Illustration of underlying chemical reactions: b) through acidepoxy addition at 1:1 stoichiometry ([epoxy]/[COOH] = 1) or c) with involvement of anionic ROP in the presence of an excess of epoxy groups. Red spots represent ether bridges.

Figure 2 .

 2 Figure 2. Tensile curves recorded at room temperature at a deformation rate of 25 mm.min -1

Figure 3 .

 3 Figure 3. Scheme for the integration of piezoresistive sensor fiber in vitrimer matrix (sandwich-welding). After cross-linking, dog-bone shaped specimens are stamped and sensor fibers are connected to cable wires. Fibers are between two dog-bones and sample is welded at 80 °C for 1 h.

Figure 4 .

 4 Figure 4. Adhesion measurements: a) T-peel test of a rectangular composite sample (here C_75D-25T_0epoxy) with integrated sensor fiber. b) Peeling strength depending on displacement for the four SFCs (crosshead speed of 2.5 mm.min -1 ).

Figure 5 .

 5 Figure 5. a) Tensile tests of sensor fiber composite at room temperature at a deformation rate of 25 mm.min -1 . b) C_0D-100T composite delaminated due to low adhesion matrix-fibermatrix. c) Sample ruptured locally without delamination (C_75D-25T_0epoxy).

Figure 6 .

 6 Figure 6. a) Cyclic test (20 cycles of 020 % strain) of C_0D-100T composite at room temperature at a deformation rate of 50 %.min -1 . b) Delaminated sample after cyclic loading.

Figure 7 .

 7 Figure 7. Cyclic tests (20 cycles at 020 % strain) for the three C_75D-25T SFCs at a deformation rate of 50 %.min -1 . Solid lines represent the stress values and dotted lines the relative resistance. Black curves correspond to the pristine samples and colored curves to

Figure 8 .

 8 Figure 8. a) Compared Young's moduli of the fiber sensor and the three 75D-25T matrices, at 20 °C and 50 °C. b) Dependence of the ratio = f /f at the sensor-matrix interface on E2,

Figure 9 .

 9 Figure 9. a) Scheme of stress profile (dark arrows) in shoulder region of a dog-bone sample. Bi-axial stress profile in convex (extensive and compressive) and in concave (all tensile) areas. b, c) A SFC sample bearing with 1 N weight observed in polarized light between crossed polarizers oriented at 45° or parallel to the stretching direction. d) Image of stretched C_75D-25T_5epoxy sample showing two spots of fracture of the sensor fiber. e) Zoom-in on the points of rupture observed with a binocular magnifier. f) Broken sample C_75D-25T_5epoxy after relaxation, showing the fiber buckled in the narrow cross-section area. g)

Table 1 .

 1 Molecular structure, scheme, and name/role of all used reactants.

	Component	Scheme	Name/Role

Mixture (trimer 77 wt % and dimer 23 wt %)

Table 2 .

 2 Glass transition temperature Tg, as measured by DSC. Storage modulus E' as measured by DMA at 150 °C. Full width at half maximum (FWHM) of the tan() peak, as measured by DMA. Activation energy of viscosity, tensile stress, and strain at break for the four matrix compositions (standard deviations of three samples minimum from different sheets per composition).

		Tg	E'	FWHM	Activation	Maximum	Maximum
			(150 °C)	of tan	energy	stress	strain
		[°C]	[MPa]	[°C]	[kJ/mol]	[MPa]	[%]
	0D-100T	16	0.90	45	67	5.6 ± 0.4	143 ± 7
	75D-25T_0epoxy	12	0.05	76	55	2.9 ± 0.2	451 ± 26
	75D-25T_5epoxy	13	0.06	66	50	3.9 ± 0.1	401 ± 8
	75D-25T_10epoxy	16	0.13	57	38	4.3 ± 0.2	333 ± 5

Acknowledgements. This work was funded by the European Union's Horizon 2020 FET Open Project Self-Healing Soft Robotics [grant agreement No. 828818]. The authors thank ESPCI-PSL, CNRS, EMPA, for supportive financial environment, as well as Arkema for kind gift of UDETA.

Supporting Information.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/.... Supramolecular Prepolymer liquid precursor (SP50) preparation (1.2); Hybrid network synthesis (1.3); Quantities of reactants for the preparation of hybrid network sheets (Table S1); Chemical analysis (1.6); Thermomechanical analyses (1.7); ATR-IR spectra (Fig. S1); Solvent uptake and gel content values after immersion in trichlorobenzene for 24 h at 150 °C (Fig. S2 and Table S2); Glass transition temperature Tg of all compositions (Fig. S3); Storage modulus E' and tan() from -50 to 200 °C for the four matrix compositions (Fig. S4); Creep measurements for the four matrix compositions (Fig. S5); Arrhenius plots of the viscosities for the four matrix compositions (Fig. S6); Tensile test until rupture for three samples per composition (Fig. S7); Peeling strength depending on displacement for the four SFCs, at a crosshead speed of 10 mm.min -1 (Fig. S8); 75D-25T_0epoxy rectangular sensor fiber sample after rupture of one sheet before delamination (Fig. S9); Representative image of 75D-25T_0epoxy sample after cyclic loading at room temperature with no delamination of matrix sheets (Fig. S10); Calculation of  variation with matrix modulus E2.


Muhammad Mumtaz

Emmanuel Ibarboure

Christine Labrugère

Synthesis of PEDOT Nano-objects Using Poly(vinyl alcohol)-Based Reactive Stabilizers in Aqueous Dispersion

come

The nano-objects formed show electrical conductivity up to 1.6 × 10 -2 S cm -1 . The size of these nano-objects ranges from 30 to 200 nm (excepting in the case of doughnuts) depending upon the experimental conditions.

3,4-Ethylenedioxythiophene (EDOT), N-methyl-2-pyrrolecarboxylic acid, and poly(vinyl alcohol) (PVA) were purchased from Aldrich and used without further purification. Dimethyl sulfoxide (DMSO) was distilled over CaH 2 . Ammonium persulfate, iron(III) p-toluenesulfonate hexahydrate [Fe (III) (OTs) 3 • 6(H 2 O)] of technical grade, and p-toluenesulfonic acid (98.5%) were purchased from Aldrich and used as received. 4-(Dimethylamino)pyridinium 4-toluenosulfonate (DPTS) was prepared as described in the literature. 30 In a typical procedure, PEDOT particles of 180-200 nm diameter were prepared by dispersion polymerization as following: (PVA-g-Py) (270 mg, M j w ) 18 000 g mol -1 ) (see its synthesis in Supporting Information (SI)) was introduced in a 250 mL flask equipped with a mechanical stirrer. A mixture (1:4, v/v) of methanol and water (40 mL) was then added, and the temperature of the system was raised up to 60 °C in order to dissolve the stabilizer. The temperature was then lowered to 40 °C, and EDOT (0.5 g, 3.5 mmol) was charged in the flask in one shot. A solution of ammonium persulfate (1.30 g dissolved in 10 mL of 1:4 methanol-water mixture) was then introduced in a single dose. The reaction mixture was stirred for 72 h at room temperature. The resulting blue dispersion was centrifuged at 10 000 rpm at 5 °C for 30 min. The supernatant was carefully decanted and the dark blue sediment was redispersed in methanol/water mixture. This redispersioncentrifugation cycle was repeated three times in order to ensure the complete removal of inorganic material such as ammonium sulfate and eventual unattached reactive stabilizer.

1 H NMR spectra were recorded using a Bruker AC-400 NMR spectrometer. Conductivity measurements of the PEDOT samples (pressed pellets) were performed using a Keithley 2400 Source Meter four-probe instrument. SEM images of the PEDOT samples were taken using a JEOL JSM-5200 scanning microscope. Atomic force microscopy (AFM) images were recorded in air with a Nanoscope IIIa microscope operating in tapping mode (TM). The probes were commercially available as silicon tips with a spring constant of 42 N m -1 , resonance frequency of 285 kHz, and a typical radius of curvature in the 10-12 nm range. Both the topography and the phase signal images were recorded with a resolution of 512 × 512 data points. Samples for AFM were prepared by solvent casting at room temperature from water/ methanol solutions. Typically, 100 μL of a dilute solution (0.1 wt %) was cast on a 1 × 1 cm 2 freshly cleaved mica. Dynamic light scattering (DLS) measurements were performed at 25 °C using an ALV laser goniometer. TGA measurements were taken using Perkin-Elmer thermogravimetric analyzer (TGA 7). XPS measurements were made using an ESCALAB 220-iXL spectrometer (Thermo-Electron, VG Co.). Photoemission was stimulated by a monochromatized Al KR radiation (1486.6 eV). An area of about 250 μm diameter was analyzed for each sample. Surveys and highresolution spectra were recorded and then fitted with an Avantage processing program provided by ThermoFisher Scientific. UV-vis spectra were recorded with a Spectramax-M2 spectrometer. Infrared measurements (FTIR) spectra were performed on a Bruker Tensor 27 spectrometer.

In order to prepare PEDOT core-shell particles, we derivatized a series of PVA-g-Py (that are used as steric reactive Conducting polymers are an attractive class of materials that combine the general properties of traditional organic polymers with the electrical conductivity of metals or semiconductors. They have potential applications in electrochromics, 1 supercapacitors, 2 antistatic and electrostatic coatings, 3 light-emitting devices, 4,5 photovoltaics, 6 sensors, 7 etc. However, their insolubility in some solvents limits their wider use. One method to improve the processability of these polymeric materials is by preparing them in water-borne or organic dispersions. This process may have considerable environmental and economic advantages for large scale concerns. Aqueous dispersions of polypyrrole and polyaniline have been already prepared by chemical oxidative polymerization of the corresponding monomers in the presence of suitable water-soluble steric stabilizers. [8][9][10][11][12][13][14][15][16] Among conducting polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) has been well investigated due to its good environmental stability, low band gap, low redox potential, and high optical transparency in its electrically conductive state, all properties that make it an excellent candidate for use in optoelectronics. [17][18][19] Despite the fact that the PEDOT-poly-(styrenesulfonate) (PEDOT-PSS) mixture is largely used for photovoltaics, there is still a need to optimize the synthesis and the morphology of PEDOT in order to tune the optoelectronic properties.

Many groups have reported different techniques to synthesize PEDOT nano-objects with the aim of improving its processability. [20][21][22][23][24][25][26] Recently, Sun et al. reported the synthesis of PEDOT nanowires in the presence of poly(acrylic acid). 27 We also described the synthesis of PEDOT nanoparticles and vesicles by dispersion polymerization of EDOT in alcoholic media in the presence of reactive poly(ethylene oxide) endcapped with EDOT, fluorene, pyrrole (Py), and thiophene functions. 28,29 In this Communication, we discuss the synthesis in aqueous dispersion of well-defined PEDOT nano-objects with various morphologies, including particles and donuts, in the presence of Py-grafted poly(vinyl alcohol) (PVA-g-Py) used as a reactive stabilizer. Ammonium persulfate and iron(III) p-toluenesulfonate hexahydrate [Fe (III) (OTs) 3 • 6(H 2 O)] were tested as oxidants. stabilizers) with different degrees of substitution and molar mass by partial esterification of PVA with N-methyl-2-pyrrolecarboxylic acid in the presence of 4-(dimethylamino)pyridinium 4-toluenesulfonate (DPTS). 30 The general synthetic procedure is shown in Scheme 1.

1 H NMR of the PVA-g-Py in DMSO is shown in Figure 1. The appearance of the signals at 7.05 (h), 6.80 (f), and 6.07 ppm (g) due to pyrrole ring resonance confirmed the grafting of pyrrole units along the PVA backbone. 1 H NMR analysis was used to evaluate the density of grafting of pyrrole units per PVA chain in particular for degree of substitution higher than 5%.

In order to improve the solubility of EDOT in water, the dispersion polymerization was performed in a mixture of methanol:water (1:4). Two types of oxidants, i.e., ammonium persulfate and Fe (III) (OTs) 3 • 6(H 2 O), were used for the oxidative polymerization of EDOT. The effect of the concentration, degree of substitution, and the molar mass of the PVA-g-Py stabilizers on the morphology and size of the nano-objects was investigated and analyzed by dynamic light scattering (DLS). The data of EDOT dispersion polymerization in the presence of PVA-g-Py are summarized in Table 1.

Unlike polymerizations of EDOT performed in the presence of nonmodified PVA (run 1), the grafting of PVA hydroxyl moieties with pyrrole was shown to be essential in order to prepare PEDOT nano-objects having various morphologies (runs 2-15) (see Figure SI1).

Above 5 mol %, the degree of PVA substitution ([Py]) has a little effect on the average PEDOT particle size which remains in the range 100-200 nm (runs 5-15). However, at a very low degree of substitution (runs 3 and 4), there is an amazing formation of rings, or so-called "donuts" (for more images see Figure SI7). Although we do not have a full explanation on the formation of donuts, there is an evident relationship between the density of pyrrole grafting onto the PVA reactive stabilizers and the morphology of PEDOT objects. A lack of thermody- namic stability is probably the cause of the formation of hollow spheres. It is noteworthy that ring structure have been already reported by Dı `ez et al. in the case of synthesis of polypyrrole in aqueous media. 31 However, in this specific work the authors use templates (β-naphthalene and fluorosurfactants) for pyrrole polymerization. In our case, we have checked by DLS measurements that no particular self-organization of the PVA-g-Py reactive stabilizer was formed prior polymerization. The change in PEDOT morphologies from donuts to dense particles was noticed while increasing the concentration of pyrrole units onto PVA backbone (see runs 3 and 5). Spherical particles were only obtained above a critical value of pyrrole units as it is illustrated in Figures 2and 3.

In accordance with literature data, 9,32 the average size of the PEDOT particles decreases from 170 to 100 nm as the concentration in stabilizer increases from 10 to 35 wt % (see runs 6-8 and 10, 11). This is logically explained by a higher surface coverage in the presence of a higher amount of steric reactive stabilizer (see Figure SI3c,d).

As expected, the molar mass of the stabilizer also affects the size of the PEDOT particles. The latter decreases as the molar mass of the PVA-g-Py increases from 18 000 to 88 000 g mol -1 , even at low stabilizer concentrations (see runs 4, 5, 12, and 13). This is again explained by the higher surface coverage afforded by the high molar mass stabilizer molecules, which leads to the formation of a larger number of stable primary particles. Similar results were obtained and reported by Mandal and colleagues for the dispersion polymerization of pyrrole. 11 AFM images (Figure 3) (for more images see Figure SI2) clearly show that each PEDOT nano-object is formed by the aggregation of the very small nanoparticles having a size between 20 and 30 nm which were also called particulates or metastable particles by Mandal and colleagues. 33 These results thus further strengthen the mechanism of the particle formation given by Paine that each large stable particle is formed by the combination of small metastable elementary particles. [34][35][36] Similar results were observed and reported later by Armes et al. with the help of scanning tunneling microscopy. 32 As already noted, 28,29 PEDOT particles were obtained in high yield when using Fe (III) (OTs) 3 • 6(H 2 O) as an oxidant at 85 °C (see runs 14 and 15). This is explained by a higher efficiency of Fe (III) (OTs) 3 .6(H 2 O) in the given experimental conditions as compared to ammonium persulfate. In addition, the PEDOT particles formed in these conditions are smaller in size as compared to the ones obtained using ammonium persulfate as an oxidant for a same degree of substitution and concentration of stabilizer (see runs 5 and 14). The PEDOT samples synthesized in the presence of Fe (III) (OTs) 3 • 6(H 2 O) are insoluble in any common organic solvent, preventing their characterization by NMR and SEC.

The conductivity of the PEDOT particles was measured using the conventional four-probe technique on dried and compressed PEDOT powders under the form of disk pellets. PEDOT samples prepared using ammonium persulfate as an oxidant show low conductivities (up to 9.0 × 10 -6 S cm -1 ) while those prepared using Fe (III) (OTs) 3 • 6(H 2 O) exhibit higher conductivities (up to 1.6 × 10 -2 S cm -1 ). This is explained by the higher content and molar mass of PEDOT within the latter samples and also because in case of ammonium persulfate PEDOT samples are overoxidized.

In order to further analyze the size, size distribution, and morphology of PEDOT nano-objects, dynamic light scattering (DLS) experiments of PEDOT samples were performed in methanol/water (1:4) mixture. DLS results for a PEDOT sample (run 5) are shown in Figure SI4.

The autocorrelation functions C(q,t) (Figure SI4a) and the corresponding relaxation time distributions G(t) (Figure SI4b) of a PEDOT sample measured at different angles show a single population with very narrow relaxation time distributions in each case. This confirms that PEDOT nano-objects have very narrow size distribution which was already illustrated by AFM images. The evolution of the relaxation frequency as a function of the square of the wave vector permits us to calculate the apparent diffusion coefficient of PEDOT particles. The linear relation between relaxation frequency and square of the wave vector (Figure SI4c) confirms the homogeneous size and spherical nature of these PEDOT nano-objects. The size of these latter particles was found to be 200 nm, which is in agreement with the results obtained by AFM and SEM. These DLS results further prove that these particles are monodispersed and are well separated from each other; i.e., no aggregation occurs among final PEDOT particles.

The core-shell nature of the formed particles, i.e. shell of PVA onto PEDOT core, was proved by X-ray photoelectron microscopy (XPS) analyses. XPS survey spectra of PEDOT core-shell particles and PEDOT bulk powder are presented in Figure 4. Both samples show four peak signals originating from C 1s, O 1s, S 2s, and S 2p. The relative intensity of the signals due to S 2s and S 2p in PEDOT core-shell particles is lower than PEDOT bulk powder. In addition, XPS analyses allowed us to estimate the surface coverage of the PEDOT particles by the PVA stabilizer as revealed for instance by the presence of the N 1s signal. As anticipated, data gathered in Figure 4c reveal an increase of the N% together with the expected mol % of pyrrole groups per chain. For runs 3 and 5, the binding energy of the maximum at 399.9 eV is consistent with the Nmethylpyrrole groups on the PVA stabilizer chains (see also Figure SI5a,b). For run 8, the N 1s maximum is located at 401.4 eV, suggesting some remaining ammonium ions from (NH 4 ) 2 S 2 O 8 onto the particles (Figure SI5c).

In addition, Figure 5a,b shows the C 1s core-line spectra of PEDOT core-shell particles and PEDOT bulk powder. The peak at 286.4 eV position in C 1s core-line spectra of PEDOT core-shell particles corresponds to C-O-H moieties in PVA. [37][START_REF] Beamon | High Resolution XPS of Organic Polymers[END_REF][START_REF] Shakesheff | [END_REF] In addition, the presence of peak at 532.6 eV (Figure 5c) originating from O 1s is due to C-O-H and is in agreement with the data already available in the literature. 37 Such a peak was not observed in O 1s core-line spectrum of PEDOT bulk powder (Figure 5d), which confirms the presence of PVA on the PEDOT surface of PEDOT core-shell particles. The S 2p core-line spectra of the samples are shown in Figure 5e,f. The peaks at 164-165.2 eV correspond to neutral S of PEDOT backbone while the peaks at 167.6-168.9 eV are due to sulfate anions. 40 The signals at 165.8-167.2 eV due to S + of PEDOT were difficult to observe in PEDOT core-shell particles probably due to the presence of the PVA shell. Finally, the amount of sulfur in the PEDOT-PVA sample was 8.5% against 12% in PEDOT bulk powder, which is again in accord with the presence of PVA on the PEDOT surface.

The formation of PEDOT latexes was also confirmed by FTIR analyses (see in Figure 6 FTIR spectra of the PEDOT bulk powder and PVA stabilized PEDOT particles using different oxidants). 41,42 The occurrence of peaks at 3424 cm -1 (O-H stretching vibration) and 2950 cm -1 (C-H stretching vibration of CH 2 group of PVA) in PEDOT particles (Figure 6b,c) confirms the presence of PVA since such peaks are not seen in PEDOT bulk powder (Figure 6a).

The composition of the PEDOT core-shell particles was also determined by thermogravimetric analysis (TGA). The TGA curves for PEDOT bulk powder, PVA, and PEDOT core-shell particles are shown in Figure SI6. The TGA curves for PEDOT-PVA core-shell particles fall in between the ones of PEDOT bulk powder and PVA, which corroborates the presence of PEDOT and PVA in the PEDOT core-shell particles samples. As expected, the TGA curve for PEDOT run 12 is more close to PEDOT bulk powder as compared to PEDOT run 13 due to greater amount of PEDOT in the former.

In conclusion, we have shown that narrow distributed PEDOT nano-objects can be easily prepared in the presence of PVA-based reactive stabilizer. No stable PEDOT dispersions were formed using unmodified PVA while stable dispersions are formed using pyrrole-modified PVA, proving the key role played by the reactive stabilizer toward the formation of stable PEDOT dispersions. The morphology of these nano-objects can be tuned by changing the concentration and degree of pyrrole function of the PVA stabilizer. These results are also a clear proof of the Paine's theory for the mechanism of particles' formation in dispersion polymerization. Because of PVA nature, latexes are very stable with respect to time and exhibit interesting filmforming property that is currently under investigation. Such a methodology is currently investigated to prepare polyaniline and polypyrrole latexes with original morphologies. Results will be published in a forthcoming paper.

Scheme 1 .Figure 1 . 1 H

 111 Scheme 1. (a) Synthesis of PVA-g-Py and (b) Synthesis of PEDOT Particles and Donuts

Figure 2 .

 2 Figure 2. SEM images of PEDOT particles and donuts prepared using different PVA-g-Py stabilizers with various degrees of pyrrole grafting and concentrations: (a) 2.5% of Py grafting, 20 wt % (run 2); (b) 5% of Py grafting, 20 wt % (run 4); (c) 10% of Py grafting, 20 wt % (run 9); (d) 5% of Py grafting, 35 wt % (run 5).

Figure 3 .

 3 Figure 3. AFM images of PEDOT particles and donuts prepared using PVA-g-Py stabilizer having 5 mol % of pyrrole grafting, at different concentrations: (a, b) 20 wt % (rn 4); (c, d) 35 wt % (run 5).

Figure 4 .

 4 Figure 4. XPS survey spectra of (a) PEDOT core-shell particles and (b) PEDOT bulk powder. (c) Table for the estimation of chemical composition on surface of PEDOT-PVA samples.

Synthesis of PEDOT Nano-objects Using Poly(vinyl alcohol)-Based Reactive Stabilizers in Aqueous Dispersion Muhammad Mumtaz, †, ‡ Emmanuel Ibarboure, †, ‡ Christine Labruge `re, § Eric Cloutet,* , †, ‡ and Henri Cramail* , †, ‡

	Laboratoire de Chimie des Polyme `res Organiques, CNRS,
	16 aVenue Pey Berland, Pessac Cedex F33607, France;
	Laboratoire de Chimie des Polyme `res Organiques,
	UniVersite ´ de Bordeaux, ENSCPB, Pessac Cedex F33607,
	France; and Institut de Chimie de la Matie `re Condense ´e de
	Bordeaux, CNRS, 87 aVenue du Docteur Schweitzer,
	Pessac Cedex F33608, France

Table 1 . Synthesis of PEDOT Particles Using PVA-g-Py as a Reactive Stabilizer in Water/Methanol (1:4) at 40 °C

 1 Measured by the four probe method. b Measured from SEM images. In all cases averages on 100 objects were made. c Reaction mixture kept at 85 °C for 48 h.

			PVA-g-Py Mw	mol % of Py	PVA-g-Py		conductivity a	particle
	run no.	oxidant type	(g mol -1 )	per chain	introduced (wt %)	yield (%)	(S cm -1 )	size b (nm)	remarks
	1	(NH4)2S2O8	18 000	0	20	50	nd	-	aggregates
	2	(NH 4)2S2O8	18 000	2.5	20	55	nd	-	aggregates
	3	(NH 4)2S2O8	18 000	2.5	35	60	3.5 × 10 -6	300-400	donuts
	4	(NH 4)2S2O8	18 000	5.0	20	70	3.2 × 10 -6	400-800	donuts
	5	(NH 4)2S2O8	18 000	5.0	35	60	7.5 × 10 -5	180-200	particles
	6	(NH 4)2S2O8	18 000	7.5	10	55	9.0 × 10 -6	150-180	particles
	7	(NH 4)2S2O8	18 000	7.5	20	60	3.6 × 10 -6	100-140	particles
	8	(NH 4)2S2O8	18 000	7.5	35	50	1.6 × 10 -6	80-120	particles
	9	(NH 4)2S2O8	18 000	10.0	20	60	7.2 × 10 -6	150-200	particles
	10	(NH 4)2S2O8	88 000	1.0	10	60	4.4 × 10 -6	150-190	particles
	11	(NH 4)2S2O8	88 000	1.0	20	50	4.0 × 10 -6	100-170	particles
	12	(NH 4)2S2O8	88 000	5.0	10	60	nd	80-100	particles
	13	(NH 4)2S2O8	88 000	5.0	20	60	nd	50-100	particles
	14 c	Fe(OTs)3	88 000	5.0	35	80	1.6 × 10 -2	100-140	particles
	15 c	Fe(OTs)3	18 000	7.5	50	80	1.8 × 10 -3	50-60	particles
	a

Acknowledgment. The authors are thankful to the Higher Education Commission, Government of Pakistan, and French Ministry of Education for financial support.

Supporting Information Available: Synthesis of poly(vinyl alcohol)-based reactive stabilizers; PEDOT latexes as obtained using (A) unmodified PVA and (B) modified PVA (PVA-g-Py) (Figure SI1); AFM images of PEDOT particles, donuts prepared using PVA-g-Py stabilizer with 5 mol % of pyrrole grafting, at different concentrations, (a) 20 wt % (run 4) and (b) 35 wt % (run 5) (Figure SI2); SEM images of PEDOT samples prepared using PVA-based stabilizers with various degrees of pyrrole substitution, concentrations, and molar mass, (a) 0% of Py grafting, 20 wt %, M j w ) 18 000 g mol -1 (run 1), (b) 2.5% of Py grafting, 35 wt %, M j w ) 18 000 g mol -1 (run 3), (c) 1% of Py grafting, 20 wt %, M j w ) 88 000 g mol -1 (run 10), and (d) 1% of Py grafting, 35 wt %, M j w ) 88 000 g mol -1 (run 11) (Figure SI3); (a) autocorrelation function C(q,t), (b) relaxation time distribution G(t) at different angles, and (c) relaxation frequency in the function of q 2 , of PEDOT sample (run 5) prepared using PVA-g-Py (M j w ) 18 000 g mol -1 , 5% pyrrole grafting) as a reactive stabilizer in water/methanol mixture (1:4) at 25 °C (Figure SI4); XPS N 1s core-line spectra for the PEDOT particles prepared using PVA-g-Py stabilizers having different degree of pyrrole grafting (a) 2.5 mol %, (b) 5.0 mol %, and (c) 7.5 mol % (run 3, 5, and 8, respectively) at high resolution (Ep ) 1) and (NH4)2S2O8 (run 5, Table 1) as oxidants, respectively.