Benoit Delage

Benoit Briou

Thomas Brossier

Sylvain Catrouillet

Jean-Jacques J Robin

Vincent Lapinte
email: vincent.lapinte@umontpellier.fr

Polyoxazoline associated with cardanol for bio-based linear alkyl benzene surfactants

Keywords: Cardanol, surfactant, LAB, polyoxazoline, cationic polymerization

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Surfactants are largely used in applications such as detergency, paint formulations, phytosanitary products, wastewater treatment, metal extraction processes and so on. The demand in various chemical structures adapted to these specific uses remains high and the research is nowadays oriented towards the use of renewable raw materials such as lipids, polysaccharides, proteins... Some natural molecules such as acylsugars, glycolipids, sophorolipids exhibit surfactive properties [START_REF] Ghosh | Profiling, characterization, and analysis of natural and synthetic acylsugars (sugar esters)[END_REF][START_REF] Paulino | Current status in biotechnological production and applications of glycolipid biosurfactants[END_REF] but their extraction at industrial scale is economically not sustainable.

Cashew nut shell liquid (CNSL) is an agricultural by-product produced in significant tonnages in various areas of the world making it an attractive substitute in a broad range of applications. The chemical and thermal treatments by acids and distillation under reduced pressure lead to the decarboxylation of anacardic acid giving cardol, 2-methylcardol and cardanol [START_REF] Gedam | Cashew nut shell liquid -extraction, chemistry and applications[END_REF]. This last compound has been largely studied and functionalized following various routes to give polymers and additives as reviewed by Caillol and Wadgaonkar [START_REF] Voirin | Functionalization of cardanol: towards biobased polymers and additives[END_REF], [START_REF] Balachandran | Recent advances in cardanol chemistry in a nutshell: from a nut to nanomaterials[END_REF].

Concerning the surfactants, various modifications of cardanol have been reported. Epoxidized cardanol was modified by benzoxazine ring and polyether chains to give surfactants for the stabilization of epoxy emulsions [START_REF] Ambrozic | Epoxy emulsions stabilized with reactive bio-benzoxazine surfactant from epoxidized cardanol for coatings[END_REF]. The direct sulfonation of the aromatic ring has also been described by Peungjitton et al. giving surface tension close to that of dodecylbenzene sulfonate [START_REF] Peungjitton | Sodium Cardanol Sulfonate Surfactant from Cashew Nut Shell Liquid[END_REF]. Sulfonated cardanol-formaldehyde resins found applications in the area of coal water slurry as dispersants with a good surface activity [START_REF] Phulkerd | Coal water slurry using dispersant synthesized from cashew nut shell liquid (CNSL)[END_REF]. The indirect sulfonation of cardanol and hydrogenated cardanol where the sulfonate group is linked to the aromatic ring has been widely explored using for instance sulfones [START_REF] Anilkumar | Self-Assembled Cylindrical and Vesicular Molecular Templates for Polyaniline Nanofibers and Nanotapes[END_REF][START_REF] Castro Dantas | Micellization study and adsorption properties of an ionic surfactant synthesized from hydrogenated cardanol in air-water and in air-brine interfaces[END_REF][START_REF] Cheng | UV cured polymer based on a renewable cardanol derived RAFT agent[END_REF][START_REF] Kattimuttathu | Synthesis and Micellization Properties of New Anionic Reactive Surfactants Based on Hydrogenated Cardanol[END_REF]. Phosphorylated cardanol has been synthesized by direct phosphorylation of cardol for the coating of nanoparticles in the area of ferrofluid production [START_REF] Barreto | Novel ferrofluids coated with a renewable material obtained from cashew nut shell liquid[END_REF]. According to an easy synthetic route, surfactants based on carboxylated cardanol have been applied to the stabilization of microemulsions [START_REF] Scorzza | Synthesis and Physicochemical Characterization of Anionic Surfactants Derived from Cashew Nut Shell Oil[END_REF]. The synthesis of cardanol amine has been reported by Caillol et al. to lead to ionic and zwitterionic surfactants and epoxy hardeners [START_REF] Faye | Sustainable cardanol-based ionic surfactants[END_REF][START_REF] Darroman | New aromatic amine based on cardanol giving new biobased epoxy networks with cardanol[END_REF]. Otherwise, the synthesis of tertiary, secondary and primary amines have been reported [START_REF] Wang | Syntheses of cardanol-based cationic surfactants and their use in emulsion polymerisation[END_REF] and applied in the area of the organomodification of clays where they revealed a good efficiency in exfoliation and for the compatibilization with thermoset resins [START_REF] Puchot | Tailoring the reactivity of bio-surfactants as a tool for montmorillonite exfoliation in epoxy thermosets[END_REF][START_REF] Verge | Organomodification of sepiolite clay using bio-sourced surfactants: Compatibilization and dispersion into epoxy thermosets for properties enhancement[END_REF]. At last, some peculiar architectures based on cardanol such as gemini and betaïne surfactants should be reported [START_REF] Ahire | Novel Ester-linked Anionic Gemini Surfactant: Synthesis, Surface-Active Properties and Antimicrobial Study[END_REF][START_REF] Li | Interface Activity and Thermodynamic Properties of Cardanol Polyoxyethylene Ether Carboxylates[END_REF][START_REF] Luan | Study on Cardanol betaine Surfactants for Ultralow Interfacial Tension in a Low Range of Surfactant Concentration and Wide Range of Temperature Applied in Compound Flooding[END_REF][START_REF] Zhou | Synthesis and Characterization of Saturated Cardanol Sulfonate Salt Gemini Surfactant[END_REF][START_REF] Bhadani | Synthesis, characterization and comparative evaluation of phenoxy ring containing long chain gemini imidazolium and pyridinium amphiphiles[END_REF].

Nowadays, a crucial challenge is the replacement of toxic surfactants such as nonyl phenol ethoxylate (NPE) surfactants extensively employed in cleaning chemical formulations as wetting agents, dispersants or emulsifiers. The toxicity of NPE mainly affects the aquatic environment with risks of bioaccumulation. Substitution of NPE by cardanol ethoxylate (CE) has already been described and shows a better biodegradability even if the toxicity coming from the short PEO chains remains problematic [START_REF] Tyman | Synthesis and characterization of polyethoxylate surfactants derived from phenolic lipids[END_REF][START_REF] Tyman | Surfactant properties and biodegradation of polyethoxylates from phenolic lipids[END_REF] [27] (Figure 1). Consequently, this work focuses on the functionalization of the cardanol moiety by hydrophilic chains which would be neither PEO nor glycolipids as they have already been described [5, 25, 26] [27].

Polyoxazolines (POx)s substituted by methyl and ethyl groups, are promising water soluble polymers especially on account of their low toxicity. This class of polymers has been developed 50 years ago [START_REF] Tomalia | Homopolymerization of 2-alkyl-and 2-aryl-2oxazolines[END_REF] and renewed interest this last decade as it has been reviewed by Hoogenboom [START_REF] Hoogenboom | 50 years of poly(2-oxazoline)s[END_REF][START_REF] Verbraeken | The chemistry of poly(2-oxazoline)s[END_REF] and the special issue in European Polymer Journal (2017, Vol. 88).

They find applications in various areas such as hydrogels, surface modifications, health field, surfactants [START_REF] Tauhardt | Poly(2-oxazoline) functionalized surfaces: from modification to application[END_REF][START_REF] Hartlieb | Tailoring Cellular Uptake and Fluorescence of Poly(2-oxazoline)-Based Nanogels[END_REF][START_REF] Kempe | Preparation of non-spherical particles from amphiphilic block copolymers[END_REF][START_REF] Hartlieb | Covalently cross-linked poly(2-oxazoline) materials for biomedical applications -from hydrogels to self-assembled and templated structures[END_REF][START_REF] Tauhardt | Amine end-functionalized poly(2-ethyl-2-oxazoline) as promising coating material for antifouling applications[END_REF][START_REF] Farrugia | Poly(2-oxazoline) Hydrogels for Controlled Fibroblast Attachment[END_REF]. Their use as surfactant has been reported for the first time by Kobayashi by designing copolymers containing both hydrophilic and lipophilic POx blocks [START_REF] Kobayashi | Block copolymers from cyclic imino ethers -a new class of nonionic polymer surfactant[END_REF]. More recently, POx blocks have been linked to bio-based blocks such as glycopolymers, fatty derivatives or to petro-based blocks including polysiloxanes, aliphatic polyesters, fluorinated chains... giving various amphiphilic copolymers with surfactant or self-assembling properties [START_REF] Kempe | Poly(2-oxazoline) glycopolymers with tunable LCST behavior[END_REF][START_REF] Volet | Synthesis of monoalkyl end-capped poly(2-methyl-2-oxazoline) and its micelle formation in aqueous solution[END_REF][START_REF] Weberskirch | Synthesis and characterization of water-soluble block copolymers with an end-tagged naphthalene probe[END_REF][START_REF] Miyamoto | Preparation of block copolymer consisting of poly(2-methyl-2oxazoline) and poly(propylene oxide) blocks -a new nonionic surfactant[END_REF][START_REF] Rayeroux | One-pot synthesis of amphiphilic diblock copolymers of poly(styrene) and poly(2-methyl-2-oxazoline) by the direct combination of reverse iodine transfer polymerization (RITP) and cationic ringopening polymerization (CROP) processes[END_REF][START_REF] Chujo | Synthesis of polyoxazoline polysiloxane block copolymers[END_REF][START_REF] Kaberov | Novel triphilic block copolymers based on poly(2-methyl-2oxazoline)-block-poly(2-octyl-2-oxazoline) with different terminal perfluoroalkyl fragments: Synthesis and self-assembly behaviour[END_REF][START_REF] Mansfeld | Tuning the morphology of triblock terpoly(2-oxazoline)s containing a 2-phenyl-2-oxazoline block with varying fluorine content[END_REF][START_REF] El Asmar | Tuning the Thermo-Sensitivity of Micellar Systems through a Blending Approach[END_REF][START_REF] Guillerm | Well-defined poly(oxazoline)-b-poly(acrylate) amphiphilic copolymers: From synthesis by polymer-polymer coupling to self-organization in water[END_REF][START_REF] Rotta | Synthesis of Amphiphilic Polymers Based on Fatty Acids and Glycerol-Derived Monomers -A Study of Their Self-Assembly in Water[END_REF][START_REF] Stemmelen | Synthesis and self-assembly of amphiphilic polymers based on polyoxazoline and vegetable oil derivatives[END_REF][START_REF] Giardi | Nonionic polyoxazoline surfactants based on renewable source: Synthesis, surface and bulk properties[END_REF][START_REF] Travelet | Amphiphilic copolymers based on polyoxazoline and grape seed vegetable oil derivatives: self-assemblies and dynamic light scattering[END_REF][START_REF] Volet | Association of Hydrophobically alpha,omega-End-Capped Poly(2-methyl-2-oxazoline) in Water[END_REF][START_REF] Lee | Synthesis and micellar characterization of amphiphilic diblock copolymers based on poly(2-ethyl-2-oxazoline) and aliphatic polyesters[END_REF][START_REF] Korchia | Light-switchable nanoparticles based on amphiphilic diblock, triblock and heterograft polyoxazoline[END_REF]. Their use as surfactant for instance for emulsion polymerization or solubilization of dyes have recently been reported [START_REF] Kampmann | Synthesis of well-defined core-shell nanoparticles based on bifunctional poly(2-oxazoline) macromonomer surfactants and a microemulsion polymerization process[END_REF][START_REF] Kampmann | Formation of Well-Defined Polymer Particles in the Sub-100 nm Size Range by Using Amphiphilic Block Copolymer Surfactants and a Microemulsion Approach[END_REF].

Herein, the association of the peculiar phenolic lipid, cardanol, with an hydrophilic poly(2methyl-2-oxazoline) chain was explored. The synthesis of cardanol oxazolinate (COx n ) surfactants using cardanol as macroinitiator for cationic ring-opening polymerization (CROP) of 2-methyl-2-oxazoline (MOx) is described aiming at producing a bio-sourced linear alkyl benzene (LAB) alternative surfactants.

Analytical Techniques

Nuclear magnetic resonance (NMR) spectra were conducted on a NMR Bruker Avance I 300 MHz in CDCl 3 . The chemical shifts were in part per million (ppm), where (s) means singlet, (d) doublet, (t) triplet, (m) multiplet and (dd) doublet of doublet. For ( 1 H NMR) chemical shifts were referenced to the peak of residual CDCl 3 at 7.26 ppm. Size exclusion chromatography (SEC) analysis was performed in THF as solvent at a flow rate of 1 mL.min -1 at 30 °C. The apparatus was equipped with a guard column and PLGel 500, 10 3 , 10 4 columns (Polymer Laboratories). The molecular weight calibration curve was determined using poly(methyl methacrylate) (PMMA) standards from Polymer Laboratories. Differential scanning calorimetry (DSC) analysis was carried out on a Mettler Toledo apparatus with 5-10 mg of product. Samples were heated at 10 mL.min -1 to 150 °C and cooled at the same rate to ambient temperature. The samples were then heated at 10 °C/min from -150 to 100 °C. Mass spectrometry analyses were conducted with a Bruker Ultra-Flex MALDI-TOF mass spectrometer, equipped with a nitrogen laser (LSI, 337 nm, 10 ns pulse length) and one detector. Mixture of peptides was used for external calibration. The ions were accelerated by a potential of 25 kV and reflected with a 26.3 kV potential. All measurements were recorded in the reflection mode using R-cyano-4-hydroxycinamic acid (HCCA) as matrix with NaI. For each spectrum 300 transients were accumulated. The polymer was dissolved in acetonitrile at a concentration of 10 mg.mL -1 . Critical Aggregation Concentration (CAC) was determined by tensiometry using the du Noüy method with a tensiometer (Lauda TD2) fitted with a platinum ring with a total contact length of 0.08 m. Both the ring and the glass sample dish were cleaned thoroughly with acetone, 5 mol.L -1 nitric acid, and deionized water and dried overnight before use. The surface tension of water was checked to be 72 mN N -N . Stock COx solutions were prepared and aliquots were added to deionised water to give desired concentrations. The data were modeled using the Gibbs adsorption isotherm, which relates the surface excess (inversely proportional to the area per molecule) to the variation of the surface tension with concentration:

(

Where Γ is the surface excess, Γ CMC the surface excess at the critical micelle concentration (CMC), A CMC the area per molecule at the CMC, R the gas constant, T temperature, γ the surface tension, C the concentration and N A the Avogadro number. The prefactor m is dependent on the surfactant type and structure, as well as the presence of extra electrolyte in the aqueous phase [START_REF] Puchot | Tailoring the reactivity of bio-surfactants as a tool for montmorillonite exfoliation in epoxy thermosets[END_REF] [START_REF] Eastoe | Adsorption of ionic surfactants at the air-solution interface[END_REF]. For non-ionic and zwitterionic surfactants, a value of 1 for the pre-factor has been confirmed [START_REF] Verge | Organomodification of sepiolite clay using bio-sourced surfactants: Compatibilization and dispersion into epoxy thermosets for properties enhancement[END_REF], [START_REF] Ahire | Novel Ester-linked Anionic Gemini Surfactant: Synthesis, Surface-Active Properties and Antimicrobial Study[END_REF] [START_REF] Lu | Direct determination by neutron reflection of the structure of triethylene glycol monododecyl ether layers at the air-water-interface[END_REF][START_REF] Hines | Structure of an adsorbed layer of n-Dodecyl-N,N-dimethylamino acetate at the air/solution interface as determined by neutron reflection[END_REF]. The size of micelles in aqueous medium were measured by Zetasizer Nano-ZS (Malvern Instrument Ltd., UK) equipped with a He-Ne laser (λ = 632.8 nm). Samples were introduced into the cells (pathway: 10 mm) after filtration through 0.2 μm pore size Acrodisc filters to determine the hydrodynamic radius of polymer objects in aqueous solutions. The correlation function of DLS was analyzed via the general purposed method (NNLS) to obtain the distribution of diffusion coefficients (D) of the solutes. D is related to the apparent hydrodynamic radius, D h , of the solute by the Stokes-Einstein relation (equation 3):

(3) with k Boltzmann's constant, T the absolute temperature, and η the viscosity of the solvent.

Static Light Scattering (SLS) experiments were carried out on a Malvern Autosizer 4800 spectrogoniometer (Malvern Instruments, UK) equipped with a 50-mW laser operating at 532 nm. The static scattered intensities were corrected from pinhole size and normalized respective to toluene. The measurements were taken at 20 °C over a range of scattering wave vectors (q = 4πn.sin(θ/2)/λ, with θ being the angle of observation and n the refractive index of the solvent). The Rayleigh ratio, R θ , was determined using equation 4, [START_REF] Voirin | Functionalization of cardanol: towards biobased polymers and additives[END_REF] where I solution , I solvent and I toluene are the scattering intensities of the solution, solvent and reference (toluene) respectively, n is the refractive index (n water = 1.333, n toluene =1.496) and R toluene the Rayleigh ratio of toluene (R toluene = 2.54× 10 -5 cm -1 for λ = 532 nm). The optical constant, K, is defined by equation 5, where N a is the Avogadro number and dn/dC is the incremental refractive index. [START_REF] Balachandran | Recent advances in cardanol chemistry in a nutshell: from a nut to nanomaterials[END_REF] In all cases, it was verified that the apparent radius of gyration of the systems verified q × Rg < 1. The Zimm approximation can thus be used to obtain equation 6. Plotting KC/Rθ as a function of q 2 for each concentration yielded the apparent radius of gyration Rg of the scatterers as well as their apparent molecular weight extrapolated to zero angle, M a .

Representative plots are shown in

I I n R R I n               2 2 2 4 4 solvent a n n K N C            2 2 1 1 3 g a q R KC R M             
At a given concentration the Rayleigh ratio, R θ , is related to the apparent molecular weight of the sample, given by equation 4. It is only at infinite dilutions, where the interactions between scattering particles are negligible, that the apparent molecular weight is equal to the true molecular weight. [START_REF] Brown | Light Scattering: Principles and Development[END_REF] Multiple concentrations were measured and a plot of linear regression was used to determine the apparent molecular weight at a concentration of 0 mg.mL -1 .

Synthesis of chloroacetylated cardanol initiator (CIn)

The reaction between cardanol and chloroacetylchloride achieved in a tricol equipped with a condenser. Cardanol, triethylamine (1.1 eq) and anhydrous dichloromethane were mixed and cooled down in an ice bath and left under dry nitrogen stream. A solution of chloroacetyl chloride (1.5 eq) in dichloromethane was added dropwise. The reaction was completed after stirring at 0 °C during 30 min and the product was purified by washing the organic part with brine (EtOAc/brine) several times (yield: 98%). (H b , m), 0.9 (H a , m).

CROP of 2-methyl-2-oxazoline using CIn

NaI (1 eq) in dry acetonitrile was added dropwise to a 0.5 M solution of CIn in dry acetonitrile. The mixture was vigorously stirred 2 hours at room temperature before adding MOx. The monomer concentration in the final mixture was inferior to 4 M. The solution was stirred at 78 °C during 24 hours. At the end of the reaction, the mixture was quenched by addition of an adequate amount of piperidine (5 eq in respect to CIn) and stirred during 30 min at 40°C [START_REF] Nuyken | Systematic investigations on the reactivity of oxazolinium salts[END_REF]. After cooling, the polymer was isolated by 3 successive precipitations from cold diethyl ether.

Results and discussion

Oxazolines polymerized by cationic ring-opening polymerization (CROP). Efficient initiators for CROP must have good leaving group with appropriate nucleophilicity such as

Lewis acids, strong protic acids and their corresponding esters, alkylhalides [START_REF] Aoi | Polymerization of oxazolines[END_REF] or molecular iodine [START_REF] Guillerm | Novel Investigations on Kinetics and Polymerization Mechanism of Oxazolines Initiated by Iodine[END_REF]. These aspects have been largely investigated by different authors who studied the covalent and ionic mechanisms of CROP of 2-R-2-oxazolines [START_REF] Hoogenboom | Parallel kinetic investigation of 2-oxazoline polymerizations with different initiators as basis for designed copolymer synthesis[END_REF][START_REF] Hoogenboom | The effect of temperature on the living cationic polymerization of 2-phenyl-2-oxazoline explored utilizing an automated synthesizer[END_REF][START_REF] Huber | New hyperbranched poly(ether amide)s via nucleophilic ring opening of 2-oxazoline-containing monomers[END_REF][START_REF] Kaya | Synthesis of oxazoline functionalized polypropene using metallocene catalysts[END_REF][START_REF] Kim | Organic-inorganic polymer hybrids using polyoxazoline initiated by functionalized silsesquioxane[END_REF][START_REF] Mcalvin | Polymerization of 2-ethyl-2-oxazoline using di-, tetra-, and hexafunctional ruthenium tris(bipyridine) metalloinitiators[END_REF][START_REF] Mcalvin | Synthesis of thermochromic iron(II) tris(bipyridine)-centered star-shaped polyoxazolines and their bipyridine-centered macroligand counterparts[END_REF][START_REF] Paulus | Acetyl halide initiator screening for the cationic ring-opening polymerization of 2-ethyl-2-oxazoline[END_REF][START_REF] Philippe | Handbook of Ring-Opening Polymerization[END_REF][START_REF] Uyama | Synthesis of poly(2-oxazoline) macromonomers having a vinyl ester group[END_REF]. The initiators were classified in term of polymerization rate depending on the nucleophilicity of the counterions MeOTf > MeOTs > MeI > benzyl bromide [START_REF] Hoogenboom | Parallel kinetic investigation of 2-oxazoline polymerizations with different initiators as basis for designed copolymer synthesis[END_REF][START_REF] Paulus | Acetyl Halide Initiator Screening for the Cationic Ring-Opening Polymerization of 2-Ethyl-2-Oxazoline[END_REF]. The initiation of the polymerization of oxazolines by chlorinated compounds is limited and only low monomer conversions are reached. To solve this problem, an in situ conversion of CH 2 -Cl into CH 2 -I is often considered. Indeed, the weaker C-I bond improves the initiation of the polymerization by an ionic mechanism via an oxazolinium species with I -as counter-anion.

Herein, the CROP of 2-methyl-2-oxazoline (MOx) occurred using bio-based initiator, and more specifically, a phenolic lipid named cardanol as illustrated in Figure 2. By 1 H NMR spectroscopy, the cardanol composition was calculated knowing that it is a singular lipidic derivative bearing an uneven number of carbon atoms on saturated (C15:0), mono-(C15:1), di-(C15:2) and tri-unsaturated alkyl chains (C15:3) (unsaturations under trans configuration) (Figure 1). From H h , H i and H g signals, [START_REF] Stemmelen | Synthesis and self-assembly of amphiphilic polymers based on polyoxazoline and vegetable oil derivatives[END_REF].7% of C15:3 were calculated while 12.4% of C15:2 and 37.3% of C15:1 were estimated from H g and aforementioned signals (Figure 3).

We noted only 0.6% of saturated chains of C15:0 considering H a signal. Finally, cardanol owns 2.1 unsaturations in average per chain.

First, cardanol was converted into chlorinated initiator (CIn) by esterification with chloroacetyl chloride. The trans-halogenation of CIn into the corresponding more reactive iodinated initiator was undertaken in an one step CROP process before adding MOx monomer without any isolation of the initiator. The optimized conditions of trans-halogenation have already been described [START_REF] Stemmelen | Synthesis and self-assembly of amphiphilic polymers based on polyoxazoline and vegetable oil derivatives[END_REF]. In 1 H NMR spectrum of COx n , the characteristic signals of cardanol aromatic ring (7.5-7.0 ppm) and alkyl chain (unsaturations in the 6.0-5.0 ppm region and terminal CH 3 group at 0.9 ppm) were detected as well as those of POx located at around 2.0 and 3.5 ppm (Figure 3). The integration of CH 2 POx signal (after deduction of H 3 ) related to H j (for instance) gave the degree of polymerization (DP n, NMR ). Thus, various amphiphilic POx oligomers were produced with a degree of polymerization ranging from 11 to 27 corresponding to a molar molecular weight of 1440-2720 g.mol -1 as summarized in Table 1 and illustrated in Figure SI-3 with the GPC traces. We noted the predominance of hydrophilic behavior of these nonionic surfactants with a hydrophilic/hydrophobic ratio between 4.1 to 8.7 using the molecular weight of each part of the amphiphilic molecule. and ΔM) as summarized in Table 2. The theoretical masses were expressed following equation 7: [START_REF] Peungjitton | Sodium Cardanol Sulfonate Surfactant from Cashew Nut Shell Liquid[END_REF] where M th is the calculated mass of the polymer of degree of polymerization nearest to the measured value, ΔM th is the mass of the monomer unit, and RM th represents the calculated residual mass. The M p values (mass peak corresponding to the maximum of the population) and the number average molecular weight (M n ) are also given in Table 2. The POx chemical structure (COx 11 ) was analyzed after the termination stage by piperidine (Figure 2). The interval between two consecutive peaks was 85. to 2723 Da (n = 27, COx 27 ). We noted that the number average molecular mass deduced by GPC was consistent with that of Maldi-Tof, 1360 and 1191 g/mol, respectively.

Self-organization of COx n

The Critical Aggregation Concentration (CAC) of the nonionic surfactants COx n was determined by tensiometry to estimate their surfactant properties in water (Figure SI-5). For a better understanding of the aromatic ring influence on COx n surfactant properties, a supplementary series of linear nonionic surfactants, C 12 -POx n and C 18 -POx n , were synthesized by using 1-iodododecane and 1-iodooctadecane initiators, respectively as previously reported [START_REF] Volet | Monoalkyl Poly(2-methyl-2-oxazoline) Micelles. A Small-Angle Neutron Scattering Study[END_REF]. The length of the POx hydrophilic block, the hydrophilic/hydrophobic ratio and the CAC values of the nonionic surfactants were gathered in Table 1. The CAC values of COx n are of the same magnitude order than C 12 -POx n and C 18 -POx n , ranging from 4 to 20 mol/L.

Moreover, for C 12 -POx n and COx n , the CAC values increase, as expected, with the length of the hydrophilic chain, represented in Table 1 by the DP n, NMR and shown in Figure 5 and The surface tensions of COx n in water were ranging from 41.0 to 53.0 mN.m -1 (Table 3).

From these data, the cross-sectional area of The influence of the size of the surfactant hydrophilic chain on the self-assembly in water was investigated on COx 11 , COx 14 , COx 24 and COx 28 , directly solubilized in water (ultra pure MilliQ). In a preliminary study, the auto-correlation functions were measured at 173° between 2.5 and 10 g/L and exhibited monomodal and quite narrow exponential decays (Figure SI-7).

On the other hand, no signal could be observed in chloroform (a good solvent of both blocks),

showing that the aggregation came only from the amphiphilic nature of the systems. At 5 g/L, the calculated values of D h were 11.7, 15.7, 21.0, and 24.4 nm respectively ( Complementary experiments of static light scattering were realized on these systems in order to determine the number of aggregation (N agg ) of the self-assemblies in aqueous media (concentrations ranging from 2.5 to 10 g/L). This parameter corresponds to the M w of the selfassembly, calculated by extrapolation of the apparent molar mass to zero concentration (Figure SI-9) divided by the molar mass of the block copolymer determined by SEC measurement as summarized in Table 4. Whatever the concentration, the scattering intensity did not depend on the scattering wave vector, indicating that the aggregates were small (R g < 20 nm Figure SI-1). The N agg values were between 32 and 57. Figure 6 put in evidence a crescent linear relationship between the length of POx chains and the D h of the selfassemblies while the N agg values were separated in two regions with the smaller values for the longer POx chains (DP 24 and DP 28 ). In general, the increase of N agg was explained by the need for the stabilization of hydrophobic block of the surfactant in water. Nevertheless, this magnitude of N agg has previously been described for other amphiphilic polyoxazolines but in general for smaller nanoparticles. [START_REF] Korchia | Photodimerization as an alternative to photocrosslinking of nanoparticles: proof of concept with amphiphilic linear polyoxazoline bearing coumarin unit[END_REF] Large nanoparticle with a low N agg can only be interpreted by the partial hydration of the core with a vague core-shell delimitation.

Figure 1 .Materials 2 -

 12 Figure 1. Chemical structure of COx n and nonyl phenol derivatives

  Figure SI-1 and SI-2.

Figure 2 .

 2 Figure 2. Synthetic route towards amphiphilic polymers COx n using CIn initiator.

Figure 3 . 1 H

 31 Figure 3. 1 H NMR spectra of CIn and COx n in CDCl 3 .

a : values determined by 1 H

 1 NMR spectroscopy. b : M philic = 85 x DP n + M piperine (84 g/mol) + M (O-CO-CH2) (58 g/mol). M phobic = 283 g/mol.c : values determined by tensiometry. MALDI-TOF mass spectroscopy confirmed the nature of the POx end-groups and the success of CROP process. The theoretical values were compared to the experimental ones (M, RM

  05 g/mol corresponding to the molecular weight of MOx monomeric unit as illustrating in the Maldi-Tof spectrum zoom of Figure SI-4. As expected, the detected family corresponding to POx structure initiated by CIn and terminated by piperidine associated to Na + counter-ion (MNa + type) predominates related to the minority MH + family (Figure 4). The main m/z population ranged from 511 (n = 1; COx 1 )

Figure 4 .

 4 Figure 4. Maldi-Tof spectrum of COx 11 .

Figure SI- 6 .

 6 Figure SI-6. Otherwise, for DP 25 , the CAC values were compared in function of the length of

Figure 5 .

 5 Figure 5. Evolution of CAC value of surfactants in function of DP n of the POx moiety of (■)

Figure 6 .

 6 Figure 6. Evolution of the number of aggregation (Nagg) (◊) and the hydrodynamic diameter (Dh) (▲) of various Cx n as a function of the degree of polymerization of the POx moiety.

  nonyl phenol ethoxylate (NPE) surfactants is described using cardanol as bio-based phenolic building block and polyoxazoline as PEO alternative. The control of the CROP process permits the synthesis of well-defined linear alkyl benzene (LAB) surfactants (COx n ) as evidenced by NMR and Maldi-tof spectroscopies. The surfactant properties of the nonionic surfactants evolved with the hydrophilic-hydrophobic balance between the length of the POx chain and the lipidic cardanol ring. The investigation of their behavior in water was monitored by dynamic and static light scattering experiments and the size and the composition of the self-assemblies were precisely estimated. In a future study, we will investigate the biodegradability and the toxicity in aquatic environment of these LAB surfactants knowing the toxicity in NPE decreases with the length of the PEO chain and that the meta position in benzene such as in cardanol ring, favors the degradation of the molecule.

  H NMR (300 MHz, CDCl 3 ): 7.3 (H j , dd), 7.1 (H k , d), 7.0-6.9 (H l , m), 5.8 (H i , m), 5.5-5.25

	(H h , m), 5.15-5.0 (H g , m), 4.5 (H y , s), 4-3.2 (H 4 and H 3 , m), 2.8 (H f , m), 2.6 (H e , t), 2.25 (H 5
	and H d , m), 1.9 (H 2 , s), 1.7 (H 1 , s), 1.6 (H c , m), 1.3 (H b , m), 0.9 (H a , m).

1

Table 1 .

 1 Characteristics of COx n and C m -POx nonionic surfactants.

		Alkyl chain	DP POx	a	M n (g/mol) a	M philic / M phobic b	CAC c (mg/L)	CAC c (mol/L)
	C 12 -POx 13	12 : 0	13		1300	6.6	13.1	10.2
	C 12 -POx 25	12 : 0	25		2300	12.7	29.1	12.6
	C 12 -POx 33	12 : 0	33		3075	16.7	61.5	20.0
	COx 11	15 : 2.1	11		1360	4.8	5.3	4.1
	COx 14	15 : 2.1	14		1615	5.7	23.5	12.0
	COx 24	15 : 2.1	24		2460	8.7	32.9	13.7
	COx 28	15 : 2.1	28		2800	10	45.8	17.0
	C 18 -POx 25	18 : 0	25		2400	8.5	19.1	7.95

Table 2 .

 2 Analytical Data Derived from COx 11 MALDI-TOF MS Spectroscopy.

	Assignemen
	RM formula	a	DM exp	a	RM th	a	RM exp	a	M p,th	a	M p, exp	a	Calculation
	t
	C 32 H 50 N 2												85.10n +
	COx n		85.05		425.28	425.37	1191.18 1191.72
	O 3												RM
	a Value expressed in g.mol -1 .

Table 3 .

 3 26, 84, and 95 Å 2 of COx 11 , COx 14 , and COx 28 respectively were calculated and gathered in Table 3. Similarly to previous work on triglyceride functionalized with POx, an increase of the surface of the head group is observed with increasing the DP n of the POx moiety.[50] Interestingly, COx 11 (26 Å 2 ) exhibits a much lower head group surface area than CO-POx 10 (88 Å 2 ), a triglyceride functionalized with a POx moiety of similar DP n . CAC values, maximum values of the surface excess concentration and surfactant head group surface area determined by tensiometry.

		CAC	γ CAC	Γ CAC	Head group
		(mol/L)	(mN.m -1 )	(μmol.m -2 )	surface area (Å 2 )
	COx 11	4.1	41.2	6.46	26
	COx 14	12.0	52.8	1.98	84
	COx 28	17.0	46.7	1.76	95

Table 4 )

 4 . A clear increase of D h with the degree of polymerization of the POx block was observed. For the longest POx block (COx 24 and COx 28 ,), a high D h was measured related to C 12 -POx 25(8.4 nm), CoumPOx 25 (9.9 nm) and C 18 -POx 25 (10.8 nm) as previously described.[START_REF] Korchia | Photodimerization as an alternative to photocrosslinking of nanoparticles: proof of concept with amphiphilic linear polyoxazoline bearing coumarin unit[END_REF] This discrepancy of D h cannot be justified by a difference of POx length knowing that the persistence length is around 2.0 nm as previously calculated by Volet et al. considering semiflexible chain for worm-like micelle model.[START_REF] Volet | Monoalkyl Poly(2-methyl-2-oxazoline) Micelles. A Small-Angle Neutron Scattering Study[END_REF] Nevertheless, a similar value (24.0 nm) has already been reported for a POx bearing a crowded triglyceride hydrophobic structure.[START_REF] Travelet | Amphiphilic copolymers based on polyoxazoline and grape seed vegetable oil derivatives: self-assemblies and dynamic light scattering[END_REF] We checked by differential scanning calorimetry the absence of a melting point corresponding to - stacking of aromatic rings in the hydrophobic nanoparticle core. A glass transition at only 18 °C for DP 11 and around 30 °C for DP 14 , DP 24 and DP 28 was detected. In our case, the fatty chain of cardanol acts as a plasticizer regarding the Tg value of poly(2-methyl-2oxazoline) at 80 °C[START_REF] Hoogenboom | Microwave-assisted synthesis and properties of a series of poly(2-alkyl-2-oxazoline)s[END_REF] and that of raw cardanol at -97 °C (FigureSI-8).

Table 4 .

 4 Characteristics of COx n self-assembly in water.

	DP POx	M w self-assembly a	M w unimer b (g/mol)	N agg	D h	a
		(g/mol)			(nm)
	11	77 700	1 443	54	11.7
	14	83 000	1 953	58	15.7
	24	87 600	2 378	32	21.0
	28	100 700	2 718	37	24.4

a Determined by SLS measurement. b Determined by SEC in DMF using PMMA standards.

Acknowledgments

We thank Corine Gerardin for SLS experiments, and Gilles Valette for Maldi-Tof experiments.


Zhening Yang

Alexandru Takacs

Samuel Charlot

Daniela Dragomirescu
email: daniela.dragomirescu@laas.com

Flexible Substrate Technology for Millimeter Wave Wireless Power Transmission

In this paper, a technology based on thin flexible polyimide substrate (Kapton) to develop antennas for millimeter wave wireless power transmission is presented. Firstly, we characterize the Kapton polyimide (relative permittivity and loss tangent) by using a ring resonator method up to V band. A 60 GHz patch antenna is designed, fabricated, and measured to validate our technology. Crossed dipoles array antennas at Ku band and K band for energy harvesting are also designed, fabricated and measured. Then a prototype of crossed slot dipole antenna at V band is proposed. Finally, a resistivity characterization of Au bump used in flip chip packaging is done, which leads us one step further towards a heterogeneous integration on flexible substrate of different components for Wireless Sensor Network nodes.

INTRODUCTION

There is an increasing demand of high efficiency antennas and passives for applications where high frequency operation, lightweight, and conforming to a curved surface are required. Flexible electronics have spanned impressively over the past years, and flexible substrate became a strong competitor compared to his rigid counterparts because they are typically lighter, more rugged and portable, and less expensive to manufacture [START_REF] Wong | Flexible Electronics: Materials and Applications[END_REF]. Table 1. Comparative properties of state-of-art flexible substrates [START_REF] Vyas | Liquid Crystal Polymer (LCP): The ultimate solution for low-cost RF flexible electronics and antennas[END_REF].

Material

Flexible substrate choice

The choice of flexible substrate for the applications at microwave frequencies is a challenge, because the substrate must exhibits good electrical (e.g. low losses), mechanical and chemical properties. Moreover, the substrate should keep these electrical properties over a wide range of frequencies and under external conditions such as temperature and pressure etc. Finally, substrates used in RF devices should be able to withstand the rigors (e.g. humidity, gasing) of their operating environment and of the fabrication processes used in their packaging [START_REF]Printing Technology for Flexible Substrates[END_REF].

Several flexible substrates used for printed circuit packaging are listed in Table 1, one can see the characterizations of polyethylene teraphthalate (PET) [START_REF] Hettak | Flexible plastic-based inkjet printed CPW fed dipole antenna for 60 GHz ISM applications[END_REF], polyethylene naphthalate (PEN) [START_REF] Bisognin | Inkjet Coplanar Square Monopole on Flexible Substrate for 60-GHz Applications[END_REF], Kapton [6], liquid crystal polymers (LCP) [START_REF]Printing Technology for Flexible Substrates[END_REF] and paper-based substrate [START_REF] Shaker | Inkjet Printing of Ultrawideband (UWB) Antennas on Paper-Based Substrates[END_REF]. LCP has a nearly constant relative permittivity of 3.1 and low loss (tan δ = 0.002-0.005) up to millimeter wave frequency range, along with a near-hermetic nature (water absorption < 0.04%) which make it suitable for high frequency designs. The paper based substrate is a good candidate thanks to its low cost, but it has limitations issues related to high frequency, absorption, and humidity. Finally, we selected Kapton as substrate for the development of our passive circuits due to its good RF and thermal properties, very good flexibility over a wide temperature range (-73°C to +400°C) and its resistance to many chemical solvents.

Fig. 1. Comparison of the processing steps involved in subtractive microfabrication, screen printing and inkjet printing [START_REF] Chauraya | Inkjet printed dipole antennas on textiles for wearable communications[END_REF].

B) Process technology choice

The most widespread technologies used in microfabrication are photolithography, screen printing, and inkjet printing (Fig. 1). Inkjet printing is a direct write technology which presents a low cost advantage because the design pattern is transferred directly to the substrate and does not require masks. However, the liquid metal nanoparticles used for inkjet printing have much lower conductivity compared to the bulk conductor used for lithography [START_REF] Swaisaenyakorn | Characterization of ink-jet printed CPW on Kapton substrates at 60 GHz[END_REF][10], which can degrade the printed circuit's performances at RF and millimeter wave frequency. In addition, millimeter wave applications require high accuracy, and both screen printing and inkjet printing can only offer a highest resolution around 20 µm [START_REF] Singh | Inkjet printing-process and its applications[END_REF]. Antennas fabricated on Kapton at lower frequency have been reported in [START_REF] Ahmed | A Compact Kapton-based Inkjet Printed Multiband Antenna for Flexible Wireless Devices, Antennas and Wireless Propagation Letters[END_REF] [13] by using screen printing or inkjet printing, but the traditional photolithography remains the most suitable method for microfabrication at millimeter frequency range.

This paper is organized as follows. The second chapter presents a description of the fabrication process we have developed, followed in the third chapter by a study of Kapton dielectric properties using a ring resonator method. Then, a design of a 60GHz grounded coplanar waveguide feeding patch antenna and two crossed dipoles array antennas at Ku band and K band are described. A prototype of crossed slot dipole antenna at V band is also analysed here. Finally, experimental results concerning the resistivity of Au bump used in flip chip packaging is presented.

II. TECHNOLOGICAL PROCESS FOR FLEXIBLE SUBSTRATE INTEGRATION

To manufacture our circuits onto a flexible substrate, we have chosen the traditional photolithography, the main difficulty during the fabrication process lies on the flexible film handling and its use in micro-technology equipment. In order to overcome this difficulty, a 4inch silicon wafer is used as a host carrier. One of the critical obstacles consists in finding a way to adhere the polyimide film on the Si support. This adhesion has to be compatible with the various technological stages (vacuum, solvent and temperature) and allows after manufacture a peeling without any physical or chemical constraint.

A matured fabrication process is used where first a PDMS (PolyDiMethylSiloxane) spin coating is performed for the adhesion of the polyimide on the holding wafer. Then the Kapton polyimide is patterned on the PDMS-Si support using a Shipley 360N laminator. A resin spin coating is then realized in a fully automated resist processing tool EVG120, followed by a photolithography process [START_REF] Jatlaoui | 3D heterogeneous integration of wireless communicating nano-sensors on flexible substrate[END_REF]. During the different fabrication processes, Kapton polyimide is metallized with Ti/Au layers (50 nm/ 200 nm) or Ti/Cu layers (50 nm/500 nm) using the Electron Beam Physical Vapor Deposition (EBPVD). In case of Cu coating, the fabrication process will be terminated with a surface finishing (gold immersion deposits) to prevent Cu from oxidizing. Both wet-etching and lift-off methods were carried out during different tests, each of them is able to obtain a sufficient resolution for the metallization tracks.

The skin depths of gold and copper at microwave frequencies are given in Fig. 3. One can see the skin depth decrease with higher frequency. So with the metal thickness mentioned above, selected mainly for V-band applications, high losses at lower frequencies should be expected.

III. CHARACTERIZATION OF KAPTON

A commercially 127-µm Kapton (type 500HN) was chosen as our flexible substrate, which is the thickest product available in the HN series. In fact there is a minimum substrate thickness that must be respected for designing an effective patch antenna in V-band. This antenna is composed by a rectangular patch on the top side of the substrate and a ground plane on the back side. In order to avoid undesired capacitive effect and to ensure a highly-efficient radiation mechanism of a patch antenna, a quick parametric study is done. It shows the influence of substrate thickness versus maximum antenna gain in HFSS simulation [15] (Fig. 4). One can see that the thicker the substrate, the better the antenna gain, regardless its metallization layer: Perfect Electric Conductor (PEC), copper or gold. To design properly any high-frequency structure on this 127-µm Kapton by using numerical simulation, the knowledge of dielectric properties of substrate becomes necessary. The dielectric properties of Kapton: the relative permittivity ε r and the loss tangent tan δ were extracted from S-parameters measurement of a ring resonator [START_REF] Thompson | Characterization of liquid crystal polymer (LCP) material and transmission lines on LCP substrates from 30 to 110 GHz, Microwave Theory and Techniques[END_REF]. This resonator is composed by a ring with mean radius of 2.95 mm. The width of the microstrip on the Kapton surface is 310 µm to give us a characteristic impedance in the range of 50 Ω. Additionally, a grounded coplanar waveguide (GCPW) to microstrip transition was optimized with the help of HFSS electromagnetic (EM) software to minimize the impedance mismatch. There are two 70 µm gaps at the edges of the ring to couple the resonator with the measurement system, which provides us sufficient coupling to measure the resonator without overload the test equipment (see Fig. 5).

The same structures were also modeled using ADS Momentum software [17]. In order to fit measured and simulated data for the entire frequency band, we tuned the dielectric parameters. It is clear that the full 3D Finit Element Method (FEM) used in HFSS software is more accurate than the Method of Moments (MoM) used in ADS Momentum, but in our case, a simulation from 10 to 65 GHz with ADS Momentum is much faster. The correlation between experimental and simulation results is depicted in Fig. 6. Simulated and measured resonant frequencies and quality factors are given in Table 2. The experimental results were obtained by using an Agilent PNA network analyzer and Cascade Microtech GSG probe with a 150 µm pitch. A 2-port on-wafer SOLT calibration method was used. The dielectric properties found for this Kapton polyimide are ε r = 3.2 ± 0.03 and tan δ = 0.012 ± 0.004 within the entire frequency band.

IV. DESIGN AND MEASEREMENT OF KAPTON BASED PASSIVE CIRCUITS A) Patch antenna

A 60 GHz grounded coplanar waveguide (GCPW) feeding rectangular patch antenna (see Fig. The simulated and the measured return loss of the patch antenna versus frequency are presented in Fig. 8. The agreement between experiments and simulations is very good: a relative frequency shift of only 0.7% is observed between the simulated (HFSS) and measured results. The difference is due to the uncertainties of the substrate permittivity/permeability value and under/over etching of the conductive patterns. The measured impedance bandwidth, defined by return loss less than -10 dB, is from 60.05 GHz to 61.8 GHz (3% of relative bandwidth).

The far-field radiation pattern and gain measurement were performed with a probe based antenna measurement setup at LAAS (Fig. 7b). The antenna is fed through a 150-µm Ground-Signal-Ground (GSG) probe, and this probe is directly connected to a 65 GHz VNA (Anritsu 37397D) with a flexible V-cable. Dielectric foam from Rohacell was used below the antenna to prevent reflections from the metallic part of the setup. The Antenna Under Test (AUT) was illumined by the field generated by a calibrated VT-15-25-C horn antenna from Vector Telecom.

Under these conditions, a maximum gain of 5 dBi was measured in the perpendicular direction of the antenna at 60.3 GHz. The simulated and measured radiation patterns in the Hand E-plane are given in Fig. 9 and Fig. 10. The difference of gain between simulation and measurement is about 1.7 dB, which may due to insertion loss introduced by the CPW probe and the metallization quality of conductive tracks. The E-plane radiation pattern has a restricted range due to architecture of the measurement setup because the probe was placed in the E-plane. Ripples observed in the E-plane are due to reflection and diffraction effects on the metallic micro-positioner and the probe. The 3dB beamwidth is 56° for the H-plane.

The results demonstrate the quality of fabrication on flexible polyimide substrate and the accuracy of measurement setup for millimeter-wave antenna.

B) Cross dipoles array antenna

For the interest of energy harvesting for satellite health monitoring, the cross dipoles array antenna (CDAA) on Kapton polyimide is proposed, which consists of four printed half-wave dipoles arrays on the top side of substrate. As shown in Fig. 11, CDAA is a coplanar stripline (CPS) structure. A T-junction CPS-to-microstrip was designed and optimized by using intensive electromagnetic simulations to allow the excitation of the CDAA by a microstrip and the proper connection with a K "end launch" connector from Southwest for measurement purposes [START_REF] Takacs | Design and Characterization of Effective Antennas for K-band Rectennas[END_REF]. Two CDAAs operating in K u and K band are fabricated and measured respectively, their dimensions are summarized in Table 3. S-parameter measurement and simulation results are reported in Fig. 12 and Fig. 13. One can see a large impedance bandwidth is obtained (measured at S11 = -10 dB) for two proposed antennas, 15.45 GHz to 19.35GHz for the K u band CDAA (22% of relative bandwidth), and 21.7GHz to 25.7GHz for the K band CDAA (17% of relative bandwidth).

The radiation pattern characterizations were performed in an anechoic chamber. Fig. 14 and Fig. 15 report the measured and simulated radiation patterns in two orthogonal planes at 17 GHz for the K u band CDAA. Fig. 16 and Fig. 17 report the measured and simulated radiation patterns at 22 GHz for the K band CDAA. We note that the simulation results were obtained in HFSS and the "end-launch" connector was not included in the model. The difference of gain between measurement and simulation is about 2.7 dB may due to (i) the losses introduced by the "end-launch" connector (estimated to be in the range of 1 dB); (ii) the assembling mechanical process for connecting the Kapton structure to the connector that can lead to an imperfect contact; (iii) the skin effect. Metallization layers are composed by: (a) adherent layer of Ti about 50 nm; (b) cooper layer (thickness: 500 nm); (c) gold layer (thickness: a few nanometers). As mentioned in section II, the skin depth of copper at 20 GHz is 460 nm which is comparable with the total metal thickness of 600 nm, thus unexpected RF losses may occur. The ripples observed in the YOZ plane (phi = 90°) are due to reflection and diffraction effects on the connector and the coaxial cable. The 3dB beamwidth is 52° in the XOZ plane and 65° in the YOZ plane for the K u band CDAA at 17 GHz. For the K band CDAA, The 3dB beamwidth is 58° in the XOZ plane and 62° in the YOZ plane at 22 GHz. As depicted from Fig. 12 to Fig. 17, the proposed CDAAs are wideband antennas (22% of relative bandwidth in Ku band and 17% of relative bandwidth in K band). The radiation pattern do not change significantly in the relative bandwidth and the measured maximum gain is about 4.3 dBi (included the losses added by the end lunch connector and the associated mounting process and by CPS-to-microstrip T junction). We note that the gain can be increased by 3dB if a metallic plate acting as reflector is properly putted below the CDAA. A parametric study for the angle between slots is shown in Table 4. It is seen that when the angle α increase, the peak antenna gain decrease along with a larger half power beamwidth (HPBW) on H-plane. This phenomenon can be explained by the increase of the coupling effect when the slots approach each other.

As demonstrated by the simulation, CSDA has interesting performances in 60 GHz band. This antenna has no ground plane and in V band its performances (input matching and radiation pattern) can be impacted by the surrounding environment placed behind the antenna (the metallic chuck of the probe station used for S-parameters measurement or the dielectric

Gain in dBi

Angle in degree

H-Plane E-Plane
supporting plate of the antenna setup shown in Fig. 7 b). In order to avoid such issues and obtain a good correlation between simulation and measurements two solutions can be envisaged: (i) redesign this antenna by using a reflector/ground plane properly positioned behind the antenna and (ii) re-simulate the antenna taking into account a representative 3D model of the surrounding environment. Both solutions are time consuming and were not implemented before the submission of this paper.

V. INTEGRATION ASSEMBLY TEST

For the purpose of integration of different components on Kapton polyimide, different dummy circuits are realised and mounted by using a well matured flip chip technique established in LAAS-CNRS as shown in Fig. 21 [START_REF] Jatlaoui | Wireless communicating nodes at 60 GHz integrated on flexible substrate for short-distance instrumentation in aeronautics and space[END_REF]. To measure the quality of the contacts, four probe method was used to characterize the bumps resistivity as shown in Fig. 22. A resistance about 10 mΩ was measured for the Au bump, which proves a very good DC contact has been achieved by using this technique. The presented Kapton supported technology can be used to develop millimeter wave circuits by heterogeneous integration for the implementation of wireless sensors network (including autonomous wireless sensors by using energy harvesting or wireless power transfer techniques). The RF power used for that application is generally very low and is subject to regulation. For example in V-band, the EIRP limitations for operation in the US limits [20] the EIRP at 40 dBm (conducted power up to 27 dBm). In Europe [21], stipulates a maximum conducted power of less than +10 dBm. To the author best knowledge, these (maximum) authorized power levels are not critical for Kapton technology that has a very good thermal behavior.

VI. Conclusion

In this work, we demonstrate the feasibility of manufacturing microwave passive circuits at high frequency on a commercially available polyimide Kapton substrate through a traditional photolithography process. A 60 GHz patch antenna has been designed, fabricated, and measured to validate our technology. Crossed dipoles array antenna at Ku band and K band for energy harvesting have also been designed, fabricated and measured. Simulation results agree well with the experimental results, only 0.7% of frequency shift is observed for return loss of 60 GHz patch antenna, 6.2% and 3.1% for K u and K band CDAA respectively. Future measurement for the cross slot dipole antenna will be carry on. The results of flip chip test are one step further towards a heterogeneous integration on flexible substrate of different components for autonomous wireless sensor nodes using wireless power transfer.

Fig. 2 .

 2 Fig. 2. a) Shipley 360N Laminator; b) EVG120 resist processing system

Fig. 3 .

 3 Fig. 3. Skin depth versus frequency, the horizontal lines show the selected metallization thickness

Fig. 4 .

 4 Fig. 4. Maximum simulated antenna gain versus substrate thickness at 60 GHz.

Fig. 5 .

 5 Fig. 5. Ring resonator design

Fig. 7 .Fig. 8 .

 78 Fig. 7. (a) 60GHz GCPW feeding rectangular patch antenna; (b) Measurement setup; (c) Manufactured patch antenna on Kapton

Fig. 9 .

 9 Fig. 9. H-plane radiation patterns of the patch antenna at 60.3 GHz

Fig. 11

 11 Fig. 11. a) Cross dipoles array antenna: top view of the layout (the bottom layout is represented in filled with green trellis); b) Simulation model; c) manufactured antenna

Fig. 12 .Fig. 13 .Fig. 14 .Fig. 18 Fig. 19 .

 1213141819 Fig. 12. Simulated and measured return loss of the K u band CDAA

Fig. 21

 21 Fig. 21 Description of flip chip process

Table 2 .

 2 Resonant frequencies & quality factors

		0
		-10
		-20
		-30
	Transmission	-70 -60 -50 -40
		-80
		19	24	29	34	39	44	49	54	59	64
	Simulation					Measurement
	Resonant Frequency Q factor Resonant Frequency		Q factor
	20.27		72.4			20.345			36.7
	30.36		89.3			30.265			39.1
	40.37		98.5			40.34				42
	50.28		104.7			50.415			44.4
	60.17		114.6			60.18				50.2

Loss in dB Frequency in GHz Measurement Simulation

Table 3 .

 3 Cross dipoles array antenna dimensions

	Band	lax (mm) lay (mm) d (mm) Ld (mm) Ldy (mm)
	Ku	8.5	12.42	8	93.18	4.42
	K	5.7	7.5	4.5	6.26	3

Table 4 .

 4 Study of angle α effect on radiation

	α (°) fc (GHz) Return Loss (dB) Maximum Gain HPBW (°)
	40	59.7	-21.09	4.14	82
	45	60.2	-22.76	4.02	86
	50	61.15	-27.48	40.8	90