Nicolas Queyriaux

Emmanouil Giannoudis

Jean-François Lefebvre

Vincent Artero

Murielle Chavarot-Kerlidou
email: murielle.chavarot-kerlidou@cea.fr

Synthesis of ruthenium tris-diimine photosensitizers substituted by four methylphosphonate anchoring groups for dye-sensitized photoelectrochemical cell applications

Keywords: Ruthenium, photosensitizers, anchoring groups, photoelectrochemistry

The design and synthesis of ruthenium tris-diimine photosensitizers appropriately functionalized to be (i) anchored onto transparent conductive oxides (TCO) and (ii) covalently coupled with a water-splitting catalyst represents an important target for solar fuel production in dye-sensitized photoelectrochemical cells (DS-PECs). In this study, two different synthetic routes to prepare heteroleptic [Ru(4,4´-(CH2PO3Et2)2-bpy)2(N^N)](PF6)2 complexes are evaluated, the scope and limitations of the organometallic pathway involving half-sandwich  6 -arene ruthenium complexes as synthetic intermediates being especially studied. The spectroscopic and electrochemical characterizations of a series of novel structures varying by the nature of the third diimine N^N ligand are reported.

Introduction

One promising chemical approach to artificial photosynthesis relies on the development of tandem dye-sensitized photoelectrochemical cells (DS-PECs) for solar fuel production. [1][2][3][4] Tandem DS-PECs combine two photoelectrodes, (i) the photoanode where the light-driven oxidation of water occurs to supply (ii) the photocathode with the required amount of electrons and protons to drive the reduction of interest. A major requirement for this approach to be effective is a stable grafting of the molecular photosensitizer/catalyst assemblies onto suitable transparent conductive oxides (TCO). The most commonly employed TCOs are ntype TiO2 at the photoanode and p-type NiO at the photocathode; similar grafting strategies have nevertheless been applied at both electrodes, drawn on the experience gained from the dye-sensitized solar cell (DSSC) technology. [5] In that respect, the covalent grafting of carboxylic acid-substituted photosensitizers has been widely employed. Considering the specific aqueous conditions required for the DS-PECs, it is however relevant to focus on more stable anchoring systems, particularly with respect to hydrolysis. An improved stability of the grafting was observed for phosphonic acids [6][7][8] covalently anchored onto oxide films compared to carboxylic acids, especially at low pH (≤ 5), making them anchoring groups of choice for DS-PEC applications. [9,10] In a previous study, [11] we established that the number of phosphonate anchoring groups present on the Ru photosensitizer significantly improves the grafting efficiency on nickel oxide films, the surface concentration with four phosphonate anchoring groups being roughly twice higher than the one determined with only two anchoring groups. In addition, insertion of a methylene spacer between the bipyridine (bpy) ligand and the phosphonate group electronically insulates the anchors from the complex, which could help stabilizing the excess electron density on the third diimine ligand present in the Ru coordination sphere, thus away from the surface. [11,12] This warrants a better hole injection efficiency in the context of photocathode applications. Thus, bis-heteroleptic ruthenium tris-diimine photosensitizers bearing four methylphosphonate anchoring groups represent appealing targets for DS-PEC applications. Although synthetic routes were first reported in the 90's, an easy access to these complexes remains challenging due to low-yield procedures and difficult purification steps. The most straightforward route relies on the preparation of the cis-Ru(4,4´-(CH2PO3Et2)2-bpy)2Cl2 intermediate, [13][14][15][16][17] whereas a second approach exploits the properties of organometallic half-sandwich  6 -arene ruthenium (II) complexes. [18][19][20] The former procedure, called "traditional approach" below, is similar to the one previously developed for the synthesis of cis-Ru(bpy)2Cl2, however with a yield not exceeding 32% for isolated cis-Ru(4,4´-(CH2PO3Et2)2-bpy)2Cl2. [14] The organometallic route was first employed in 2010 by Bignozzi and coworkers, [18] and later reexamined by the group of T.J. Meyer, [19,20] in particular to study a novel photosensitizer-catalyst assembly anchored to TiO2. [19] In this study, we evaluate these two procedures for the synthesis of [Ru(4,4´-(CH2PO3Et2)2bpy)2(N^N)](PF6)2 complexes (RuP4 OEt -N^N), N^N being a third diimine ligand different from bpy, if possible functionalized for an ultimate coupling with a catalytic center. The scope and limitations of the organometallic pathway involving half-sandwich  6 -arene ruthenium complexes as synthetic intermediates is especially studied. The spectroscopic and electrochemical characterizations of all novel structures are also reported and compared to the reference [Ru(bpy)2(N^N)](PF6)2 complexes.

Results and discussion

Previous studies from the literature focused on the preparation of complexes bearing four methylphosphonic acid anchoring groups, the esters being most of the time hydrolyzed before the last purification step [13,14,18,20] to make it easier or the phosphonic acid ligand being employed at the beginning of the procedure. [19] In this study, we decided to isolate, purify and characterize the RuP4 OEt -N^N complexes under their phosphonate ester form, in order to warrant good solubility in organic media allowing further synthetic steps on the N^N ligand.

In addition to the reference 2,2'-bipyridine (bpy) ligand, we selected 1,10-phenantroline (phen) and phen-substituted derivatives, namely 5-amino-1,10-phenantroline (phenamine), dipyrido[3,2-a:2',3'-c]phenazine (dppz), 1,10-phenantroline-5,6-dione (phendione) and 2-(4ethynylphenyl)-1H-imidazo[4,5-f][1,10]phenanthroline (EPIP) (Figure 1). The latter allows further functionalization of the RuP4 OEt -N^N complex by copper-catalyzed azide-alkyne cycloaddition (CuAAC). [21] Figure 1. Schematic representation of the two synthetic routes to access the RuP4 OEt -N^N complexes and structure of the N^N ligands employed in this study.

The major difference between the two reported procedures lies in the sequential chelation order of the two 4,4´-(CH2PO3Et2)2-bpy and N^N to the Ru core, occurring at the last step for N^N in the traditional approach against the first step in the organometallic route (Figure 1).

We initially attempted to prepare our functionalized complexes by the former procedure in order to take advantage of the introduction of the functionalized ligand at the last step of the process. However, in our hands, the required cis-Ru(4,4´-(CH2PO3Et2)2-bpy)2Cl2 intermediate was isolated pure in very low yields (10-20 %, using either RuCl3 or Ru(DMSO)4Cl2 as precursors), which was a strong limitation, as previously mentioned by T.J. Meyer. [20] We thus decided to evaluate the scope of the organometallic route in order to extend it to substituted N^N ligands.

Preparation of the different organometallic [( 6 to the Ru center or formation of a Ru-alkynyl [22] complexes. Overall, the [( 6arene)Ru(N^N)Cl](PF6) complexes were isolated as PF6 salts after anion metathesis, with ≥ 90 % yields.

These complexes being relatively kinetically inert, substitution of the chloride ligand for a triflate (OTf) one was reported to facilitate thermal removal of the arene ligand and the coordination of the two 4,4´-(CH2PO3Et2)2-bpy. [19,20] The [( 6 -arene)Ru(N^N)(OTf)](OTf) derivatives were prepared by reacting [( 6 -arene)Ru(N^N)Cl](PF6) with triflic acid overnight at room temperature; complexes with bpy, phen or phenamine ligands were isolated with yields > 90 %. We however observed that alkyne hydration occurred for [( 6benzene)Ru(TMS-EPIP)Cl](PF6) and an alternative procedure relying on the use of AgOTf to displace the chloride ligand was unsuccessful. Isolation of the triflate complex also failed for [( 6 -arene)Ru(phendione)Cl](PF6) (either benzene or p-cymene). In the following, the TMS-EPIP and the phendione complexes were tested under their chloride form for the preparation of RuP4 OEt -N^N by the organometallic route. The [( 6 -arene)Ru(dppz)Cl](PF6) complex proved to be insoluble in most organic solvents (except for DMSO), preventing the preparation of [Ru(4,4´-(CH2PO3Et2)2-bpy)2(dppz)](PF6)2 by this approach.

Comparison of the thermal and microwave conditions on the RuP4 OEt -bpy model complex. In order to better understand and to optimize the synthesis of the different diethylmethylphosphonate RuP4 OEt -N^N complexes, we first studied various experimental conditions on the simplest case of the series, RuP4 OEt -bpy. The reaction in refluxing ethanol between two equivalents of 4,4´-(CH2PO3Et2)2-bpy and four different organometallic precursors varying by the nature of either the arene (benzene or p-cymene) or the anionic ligand (OTf or Cl) was monitored by UV-vis absorption spectroscopy (Figure 2). The wavelength of 459 nm was selected to detect the formation of RuP4 OEt -bpy (MLCT transition) [11] and to determine its conversion yield. As expected, the triflate [( 6arene)Ru(bpy)(OTf)](OTf) complexes are more reactive than their chloride counterparts with 30-35 % conversion for the former versus < 10 % for the latter. However, the conversion started to plateau after roughly 6 hours of reflux; this observation was confirmed by reacting the triflate precursors overnight at 90°C in a H2O/EtOH mixture: isolated yields in RuP4 OEtbpy didn't exceed 35 % (Table 1). The complex [( 6 -benzene)Ru(bpy)Cl](PF6) displays a very modest (6% conversion) but still slightly higher reactivity than [( 6 -p-cymene)Ru(bpy)Cl](PF6) (< 2% conversion) (Figure 2).

Taking into consideration that triflate complexes couldn't be prepared for two ligands of the series (TMS-EPIP and phendione), we decided to explore other reaction conditions in order to increase the reactivity of [( 6 -benzene)Ru(bpy)Cl](PF6). This complex being kinetically inert, higher temperatures are required for its thermal activation compared to its triflate counterpart.

We Microwave (MW) activation also represents an interesting alternative to thermal activation, as previously described by group of T. J. Meyer for the synthesis of RuP4 OH -bpy from [( 6benzene)Ru(bpy)(OTf)](OTf). [20] This procedure is appealing as it proceeds faster and with a lower energy consumption, comparatively to the thermal activation. [23,24] Using the previously reported experimental conditions, [20] RuP4 OEt -bpy was isolated in 58 and 60 % yield, using [( 6 -benzene)Ru(bpy)(OTf)](OTf) and [( 6 -p-cymene)Ru(bpy)(OTf)](OTf) respectively, which is very close to the 62 % reported yield. [20] We then examined whether such conditions could increase the reactivity of [( 6 -arene)Ru(N^N)Cl](PF6) precursors ( It is important to underline the formation of side-products bearing partially hydrolyzed phosphonate groups. The mono-hydrolyzed derivative could be isolated with yields up to 34 % (Table 2), by chromatography on silica gel (eluted after RuP4 OEt -bpy with 30% aqueous KNO3 (0.4 M) in CH3CN), whereas complexes with a higher degree of hydrolysis were irreversibly adsorbed on silica. Formation of these hydrolyzed derivatives was previously mentioned in the literature [25] and can account for the reported hydrolysis of the phosphonate ester groups before the purification. [13,14,18,20] It is however detrimental when preparation of the complexes under their phosphonate ester form is required.

Overall, from this set of data, preparation of RuP4 OEt -bpy can be smoothly achieved from The synthetic limitations (poor orthogonality due to reactive organometallic intermediates, notably) observed at different stages of the organometallic route prompted us to reexamine the traditional approach. The group of S. Rau optimized it in order to prepare the phendione complex RuP4 OEt -phendione, key to access an immobilizable RuPd dyad. [16] Starting from previously reported by F. Odobel and coworkers to dramatically improved the first step of the procedure for related Ru complexes. [25] The synthesis of RuP4 OEt -phendione, reported with a 86% crude yield, [16] was reproduced with an overall 78% yield in our hands. Moreover, we succeeded to prepare RuP4 OEt -dppz in 55%. By contrast, RuP4 OEt -TMS-EPIP was isolated with a disappointing 10 % yield, in a first attempt. It could be improved to 20 % by increasing the amount of TMS-EPIP to 1.5 equivalent. Interestingly, protection of the terminal alkyne is not required anymore in this procedure: we succeeded to prepare RuP4 OEt -EPIP in 30 % yield by directly reacting 1.5 equivalent of EPIP ligand with [cis-Ru(4,4´-(CH2PO3Et2)2bpy)2Cl2-x(DMSO)x]Clx. This procedure thus proved to be more efficient and straightforward only two steps only from Ru(DMSO)4Cl2compared to the organometallic route and its overall 13 % yield (from RuCl3.3H2O) for RuP4 OEt -TMS-EPIP. 4. These spectra display intense ligand-centered - * absorption bands between 260 and 300 nm; RuP4 OEt -dppz displays an additional band at 358 nm attributed to a dppz-centered transition. [26] The visible part of the spectra is dominated by the classical metal-to-ligand charge-transfer (MLCT) transitions centered around 450-460 nm with a shoulder at 430 nm. In comparison to the parent complexes, introduction of four weak electron-donating methyl phosphonate substituents induces a slight bathochromic shift (5 to 7 nm) of the MLCT bands, due to stabilization of bpy-based * orbital. 4. The quasi-reversible one-electron process observed at a potential comprised between +0.85 and +0.95 V vs Fc + /Fc was unambiguously attributed to the metal-centered Ru III/II oxidation, according to previous studies on related ruthenium tris-diimine complexes, [9,27] For all complexes, the three quasi-reversible reductions on the diimine ligands are recorded on a range of potentials varying from -1.7 to -2.4 V vs Fc +/0 . The cyclic voltammograms of RuP4 OEt -dppz and of the parent [Ru(bpy)2(dppz)](PF6)2 are characterized by the presence of an additional process at -1.38 V vs Fc +/0 , assigned to the pyrazine-based reduction of the dppz ligand. [28,29] Overall, the introduction of four methylphosphonate anchoring groups in the coordination sphere of [Ru(bpy)2(N^N)](PF6)2 photosensitizers does not significantly modify their properties, consistent with the electronic decoupling provided by the methylene spacer. [11,12]

Spectroscopic and redox properties of the new

Conclusion

Novel ruthenium trisdiimine photosensitizers of general formula [Ru(4,4´-(CH2PO3Et2)2bpy)2(N^N)](PF6)2, bearing four methylphosphonate anchors, were prepared by two different synthetic procedures. The scope of the organometallic procedure was first studied; some synthetic limitations are highlighted, related to the introduction of the N^N ligand at the very beginning of the process. On the other hand, the traditional approach offers wider versatility, provided that the cis-Ru(4,4´-(CH2PO3Et2)2-bpy)2Cl2 intermediate is used as prepared.

Notably, synthesis of a novel alkyne-functionalized photosensitizer, [Ru(4,4´-(CH2PO3Et2)2bpy)2(EPIP)](PF6)2, is reported. Although increasing its isolated yield remains challenging, this complex offers interesting perspectives for the immobilization of dye-catalyst assemblies onto semi-conductor surfaces. Finally, the electronic properties of these light-harvesting units were studied and proved not to be significantly altered by the introduction of four phosphonate anchors thanks to the presence of a methylene spacer, which is an important parameter for future applications in the field of DS-PECs.

Experimental section

All reagents were purchased from Sigma Aldrich or Strem and used as obtained unless otherwise stated. Reagent-grade solvents were used without further purification. The 4,4´bis(diethylphosphonomethyl)-2,2´-bipyridine ligand (4,4´-(CH2PO3Et2)2-bpy) was customsynthesized by the company Oribase Pharma, according to a previously reported procedure. [9] The dipyrido[3,2-a:2',3'-c]phenazine (dppz), [29] phendione, [30] 2-(4trimethylsilylethynylphenyl)-1H-imidazo[4,5-f][1,10]phenanthroline (epip-TMS) [31] and 2-(4ethynylphenyl)-1H-imidazo[4,5-f][1,10]phenanthroline (epip) [21] ligands were prepared according to previously reported procedures. Spectroscopic and electrochemical characterizations of complexes RuP4 OEt -bpy and RuP4 OEt -dppz were previously reported. 1 H NMR experiments were carried out on a Bruker Avance 300 MHz and the resulting spectra are referenced to the residual solvent peak and reported in relative to tetramethylsilane reference (δ = 0 ppm). UV-vis absorption spectra were recorded either on a Shimadzu UV-1800 spectrometer or on an Agilent Cary 60 UV-Vis spectrometer. ESI-MS measurements were carried out on a Thermoquest Finnigan LCQ spectrometer.

[Ru( 6 -benzene)2Cl2]2. [32] A solution of RuCl3.3H2O (3 g, 11.5 mmol) and 1,4cyclohexadiene (11 mL, 115 mmol) in absolute ethanol (150 mL) was heated to reflux for 8 hours. The reaction mixture was then cooled down to -40°C overnight. The dark orange precipitate is finally filtrated, washed with diethyl ether and dried under vacuum to yield 2.02 g (71 %) of [Ru( 6 -benzene)2Cl2]2. 1 H NMR (300 MHz, CDCl3): δ (ppm) 5.71 (s, 6H).

[Ru( 6 -p-cymene)2Cl2]2. [32] A solution of RuCl3.3H2O (3 g, 11.5 mmol) and -phellandrene and N^N diimine ligand (0.6 mmol) in methanol (50 mL) was heated to reflux overnight.

Solvent was removed under reduced pressure and the resulting precipitate dissolved in water (5 mL). Dropwise addition of a saturated KPF6 aqueous solution gave a dark to bright yellow solid, which was collected, washed thoroughly with water (5 x 10 mL), diethyl ether (3 x 10 mL) and finally dried under vacuum.

[( 6 -benzene)Ru(bpy)Cl](PF6). Yield: 91 %. [Ru(4,4´-(CH2PO3Et2)2-bpy)2(bpy)](PF6)2 (RuP4 OEt -bpy). [11] Yield: 58% (method A: benzene); 58% (method B: benzene); 60% (method B: p-cymene).

[Ru(4,4´-(CH2PO3Et2)2-bpy)2(phen)](PF6)2 (RuP4 OEt -phen). Yield: 52% (method B: pcymene). General Procedure for the synthesis of [Ru(4,4´-(CH2PO3Et2)2-bpy)2(N^N)](PF6)2 by the traditional approach [16] [Ru(4,4´-(CH2PO3Et2)2-bpy)2(phendione)](PF6)2 (RuP4 OEt -phendione). [16,17] Yield: 78%.

[Ru(4,4´-(CH2PO3Et2)2-bpy)2(dppz)](PF6)2 (RuP4 OEt -dppz). [11] Yield: 55%. Supporting Information. 1 H NMR spectra, UV-vis absorption spectra and cyclic voltammograms for all new complexes.

Figure 2 .

 2 Figure 2. UV-vis monitoring (Abs @  = 459 nm) of the reaction between [( 6arene)Ru(bpy)L](PF6) (arene = benzene or p-cymene; L = Cl or OTf) and 2 equivalents of 4,4´-(CH2PO3Et2)2-bpy ligand in refluxing ethanol. Right: Conversions in RuP4 OEt -bpy calculated from the absorbance.

  Ru(DMSO)4Cl2 instead of RuCl3, the cis-Ru(4,4´-(CH2PO3Et2)2-bpy)2Cl2 intermediatebetter described as a mixture of [cis-Ru(4,4´-(CH2PO3Et2)2-bpy)2Cl2-x(DMSO)x]Clxwas isolated without any purification and directly used for the last step of the process, i.e. coordination of the functionalized N^N ligand. The use of Ru(DMSO)4Cl2 has been

  RuP4 OEt -N^N complexes. The UV/vis absorption spectra of RuP4 OEt -bpy, RuP4 OEt -phen, RuP4 OEt -phenNH2, RuP4 OEt -dppz and RuP4 OEt -EPIP were recorded in acetonitrile and compared to the parent [Ru(bpy)2(N^N)](PF6)2 compounds (Figures S7-11 in Supporting Information). Spectroscopic data are summarized in Table

(

  19 mL, 115 mmol) in absolute ethanol (150 mL) was heated to reflux for 8 hours. The reaction mixture was then cooled down to -40°C overnight. The dark red crystalline product is finally filtrated, washed with diethyl ether and dried under vacuum to yield 3.39 g (96 %) of [Ru( 6 -p-cymene)2Cl2]2. 1 H NMR (300 MHz, CDCl3): δ (ppm) 5.47 (d, 2H), 5.33 (d, 2H), 2.92 (hept., 1H), 2.15 (s, 3H), 1.27 (d, 6H). General procedure for the synthesis of [( 6 -benzene)Ru(N^N)Cl](PF6) (N^N = bpy, phen, phenamine, phendione, epip-TMS). A solution of [( 6 -benzene)RuCl2]2 (150 mg, 0.3 mmol)

and 2 -

 2 methoxyethanol (36 mL) was refluxed overnight. After cooling down to room temperature, 5 mL of a saturated KPF6 aqueous solution was added to the reaction mixture.Extraction with dichloromethane and removal of the organic solvent yield a reddish solid which was then subjected to chromatography on silica gel (MeCN / aqueous 0.4 M KNO3, 80:20). After removal of MeCN under reduced pressure and addition of a saturated KPF6 aqueous solution (5 mL), the aqueous phase was further extracted (3 x 50 mL) with dichloromethane and the organic phases washed with water. Evaporation of dichloromethane under reduced pressure gave a red-orange solid, which was collected and vacuum-dried overnight.Method B (microwave conditions):A solution of [( 6 -arene)Ru(N^N)(OTf)](OTf) (150 mg, 1 equivalent) and 4,4´-(CH2PO3Et2)2-bpy (2 equivalents) in ethanol (10 mL) was put in a sealed tube before being subjected to microwave irradiation (150°C) for 20 min. After cooling down to room temperature, 5 mL of a saturated KPF6 aqueous solution was added to the reaction mixture. Extraction with dichloromethane and removal of the organic solvent yield a reddish solid which was then subjected to chromatography on silica gel (MeCN / aqueous 0.4M KNO3, 80:20). After removal of MeCN under reduced pressure and addition of a saturated KPF6 aqueous solution (5 mL), the aqueous phase was further extracted (3 x 50 mL) with dichloromethane and the organic phases washed with water. Evaporation of dichloromethane under reduced pressure gave a red-orange solid, which was collected and vacuum-dried overnight.

  -3.82 (m, 16H), 3.55 -3.16 (m, 8H), 1.32 -0.99 (m, 24H). ESI-MS: m/z 604.3 [M-2PF6] 2+ . [Ru(4,4´-(CH2PO3Et2)2-bpy)2(TMS-EPIP)](PF6)2 (RuP4 OEt -TMS-EPIP). Yield: 20% (method A: benzene; 2.3 eq). 1 H NMR (300 MHz, CD3CN): δ (ppm) 9.04 -8.91 (m, 2H), 8.43 (d, J = 14.7 Hz, 4H), 8.26 (d, J = 8.1 Hz, 2H), 7.99 (d, J = 4.2 Hz, 2H), 7.79 -7.66 (m, 4H), 7.50 (d, J = 5.7 Hz, 2H), 7.40 (dd, J = 5.4 & 1.8 Hz, 4H), 7.15 (d, J = 5.7 Hz, 2H), 4.10 -3.89 (m, 16H), 3.43 (d, J = 22.5 Hz, 4H), 3.33 (d, J = 22.5 Hz, 4H), 1.23 -1.05 (m, 24H), 0.28 (s, 9H). ESI-MS: m/z 703.4 [M-2PF6] 2+ .

  (N^N: phendione, dppz, epip-TMS, epip): A solution of commercially available Ru(DMSO)4Cl2 (420 mg, 0.88 mmol) and 4,4´-(CH2PO3Et2)2-bpy (800 mg, 1.75 mmol) in methanol (100 mL) was refluxed overnight under argon. Removal of the solvent and drying under vacuum yielded the dark-red highly hygroscopic intermediate [Ru(4,4´-(CH2PO3Et2)2-bpy)2Clx(DMSO)2-x]Cl2-x. A solution of crude [Ru(4,4´-(CH2PO3Et2)2bpy)2Clx(DMSO)2-x]Cl2-x (1 equivalent, using an average molecular weight) and N^N (1 to 1.5 equivalent) in a water/ethanol (1:3) mixture was refluxed overnight. After cooling down to room temperature, 5 mL of a saturated KPF6 aqueous solution was added to the reaction mixture. Extraction with dichloromethane and removal of the organic solvent yield a crude reddish solid. After purification by flash chromatography on silica gel (MeCN/aqueous KNO3 (0.4 M), 80:20), MeCN was removed under reduced pressure and a saturated KPF6 aqueous solution (5 mL) added. The aqueous phase was further extracted (3 x 50 mL) with dichloromethane and the organic phases washed with water. Evaporation of dichloromethane under reduced pressure gave red-orange solids, which were collected and vacuum-dried overnight.

2 (

 2 RuP4 OEt -EPIP). Yield: 30% (1.5 eq of EPIP ligand).1 H NMR (300 MHz, CD3CN): δ (ppm) 9.09 (d, J = 8.5 Hz, 1H), 8.87 (d, J = 8.1 Hz, 1H), 8.42 (d, J = 15.4 Hz, 4H), 8.28 (d, J = 8.5, 2H), 8.00 (d, J = 5.1 Hz, 2H), 7.81 -7.74 (m, 6H), 7.49 (t, J = 6.0 Hz, 2H), 7.40 (d, J = 5.4 Hz, 2H), 7.15 (s broad, 2H), 4.10 -3.88 (m, 16H), 3.60 (s, 1H), 3.42 (d, J = 22.6, 4H), 3.32 (d, J = 22.6, 4H), 1.23 -1.05 (m, 24H). ESI-MS: m/z 667.3 [M-2PF6] 2+ .

Table 1 .

 1 Thermal

	Arene	L	Solvent, T

activation: Isolated yields (after purification by chromatography on silica gelsee experimental section) in RuP4 OEt -bpy for different reaction conditions.

, overnight 4,4´-(CH2PO3Et2)2-bpy RuP4 OEt -bpy

	benzene	Cl	2-MeOEtOH/H2O (4 :1), 120°C	2 eq 2.3 eq	40 % 58 %
		OTf	EtOH/H2O (4 :1), 90°C	2 eq	35 %
	p-cymene	Cl OTf	2-MeOEtOH/H2O (4 :1), 120°C EtOH/H2O (4 :1), 90°C	2 eq 2 eq	< 5 % 33 %

  thus switched to aqueous/organic solvent mixtures, and replaced EtOH by 2methoxyethanol (2-MeOEtOH). Results are summarized in Table 1. RuP4 OEt -bpy was isolated in 40 % and < 5 % yield, respectively, by refluxing overnight 2 equivalents of 4,4´-

	(CH2PO3Et2)2-bpy	with	either	[( 6 -benzene)Ru(bpy)Cl](PF6)	or	[( 6 -p-
	cymene)Ru(bpy)Cl](PF6) in a 2-MeOEtOH/H2O 4:1 (v:v) solvent mixture; this result is in
	agreement with the difference of reactivity observed for these two precursors during the
	course of the UV-vis monitored experiment in EtOH. Increasing the amount of 4,4´-
	(CH2PO3Et2)2-bpy from 2 to 2.3 equivalents allowed to prepare RuP4 OEt -bpy from [( 6 -
	benzene)Ru(bpy)Cl](PF6) with a satisfactory yield of 58 % after purification.

Table 2

 2

). Under the conditions applied for the triflate precursors (EtOH, 150°C, 20 min), RuP4 OEt -bpy was isolated in 42 % yield from [( 6 -benzene)Ru(bpy)Cl](PF6); this yield couldn't be improved by a longer reaction time (two 20 min cycles) or by adding an excess of 4,4´-(CH2PO3Et2)2-bpy ligand (2.3 instead of 2 eq). As previously observed, [( 6 -pcymene)Ru(bpy)Cl](PF6) is less reactive than [( 6 -benzene)Ru(bpy)Cl](PF6), with isolated yields in RuP4 OEt -bpy not exceeding 25%.

Table 2 .

 2 Microwave activation: Isolated yields in RuP4 OEt -bpy for different reaction conditions (EtOH, 150°C).

	Arene	L	Reaction time	4,4´-(CH2PO3Et2)2-bpy RuP4 OEt -bpy	Monohydrolyzed RuP4 OEt -bpy
		OTf	20 min	2 eq	58%	n.d.
			20 min	2 eq	42%	14%
	benzene		20 min	2.3 eq	42%	26%
		Cl
			2 x 20 min	2 eq	30%	n.d.
			2 x 20 min a	2.3 eq	30%	34%
	p-	OTf	20 min	2 eq	60%	n.d.
	cymene	Cl	20 min	2 eq	15%	13%

a 2-methoxyethanol instead of ethanol.

Table 3 .

 3 Synthesis of various RuP4 OEt -N^N complexes.

	this	way	obtained	from	[( 6 -arene)Ru(phen)(OTf)](OTf)	and	[( 6 -
	homoleptic [Ru(4,4´-(CH2PO3Et2)2-bpy)3](PF6)2 complex was instead observed under these
	conditions, the electron-deficient phendione probably being displaced by the more electron-
	donating 4,4´-(CH2PO3Et2)2-bpy. Finally, method A allowed isolating RuP4 OEt -TMS-EPIP in
	20 % yield.
		RuP4 OEt -phen	RuP4 OEt -phenamine RuP4 OEt -phendione	RuP4 OEt -dppz	RuP4 OEt -TMS-EPIP	RuP4 OEt -EPIP
	Method A	─		─	0 % a	─	20 % (13 % b )	─

[(

6

-arene)Ru(bpy)(OTf)](OTf) under microwave activation (Method B: EtOH, 150°C, 20 min; 60 % isolated yield); in addition, if the triflate precursor is not accessible, thermal conditions can be applied to [(benzene)Ru(N^N)Cl](PF6) (Method A: MeOEtOH/H2O, 120°C, overnight; 58 % isolated yield) using a 2.3 eq excess of 4,4´-(CH2PO3Et2)2-bpy.

Extension to various N^N ligands. The conditions defined above were tested for the synthesis of novel complexes, with different N^N ligands. Method B was employed each time the triflate precursor complex could be prepared. RuP4 OEt -phen and RuP4 OEt -phenamine were in arene)Ru(phenamine)(OTf)](OTf), in 52 % and 55 % yield, respectively (Table

3

). As already mentioned above, the lack of solubility of [( 6 -p-cymene)Ru(dppz)Cl](PF6) prevented us to test the preparation of RuP4 OEt -dppz by the organometallic route. Synthesis of RuP4 OEtphendione was attempted via method A, but proved to be unsuccessful; formation of the tris-

Table 4 .

 4 Spectroscopic a and redox properties b of complexes RuP4

OEt -bpy, RuP4 OEt -phen, RuP4 OEt -phenNH2, RuP4 OEt -dppz, and RuP4 OEt -EPIP

					together with the parent
	[Ru(bpy)3](PF6)2,	[Ru(bpy)2(phen)](PF6)2,		[Ru(bpy)2(phenNH2)](PF6)2,
	[Ru(bpy)2(dppz)](PF6)2 and [Ru(bpy)2(EPIP)](PF6)2 as reference complexes.
	Photosensitizer	Ref	abs () c	Eox	Ered1 Ered2 Ered3 Ered4
	RuP4 OEt -bpy	[11]	288 (85370), 456 (14830)	+0.87 -1.71 -1.90 -2.15	-
	[Ru(bpy)3](PF6)2	[11]	286 (94480), 451 (14710)	+0.89 -1.73 -1.92 -2.17	-
	RuP4 OEt -phen	this work	264 (57120), 455 (16580) 289 (67990),	+0.87 -1.72 -1.90 -2.16	-
	[Ru(bpy)2(phen)](PF6)2	this work	264 (56150), 449 (15900) 286 (63875),	+0.88 -1.75 -1.94 -2.20	-
	RuP4 OEt -phenNH2	this work	289 (60145), 464 (14025)	+0.95 -1.74 -1.94 -2.20	-
	[Ru(bpy)2(phenNH2)](PF6)2	this work	285 (59635), 458 (13540)	+0.93 -1.78 -1.98 -2.23	-
			285 (97975),
	RuP4 OEt -dppz	[11]	368 (13990),	+0.88 -1.38 -1.72 -1.93 -2.34
			457 (14331)
			283 (85090),
	[Ru(bpy)2(dppz)](PF6)2	[11]	367 (14780),	+0.88 -1.38 -1.84 -2.05 -2.31
			449 (14385)
	RuP4 OEt -EPIP	this work	288 (131360), 457 (20090)	+0.85 -1.74 -1.96 -2.38	-
	[Ru(bpy)2(EPIP)](PF6)2	this work	290 (106085), 463 (19250)	+0.88 -1.78 -2.02 -2.39	-
	a. Absorption spectra were recorded in acetonitrile.
	b. in V vs Fc + /Fc. Cyclic voltammograms were recorded at a complex concentration of 1 mM in a 0.1 M solution
	of n-Bu4NBF4 in degassed acetonitrile and at a scan rate of 100 mV.s -1 .
	c.absin nm; in L.mol -1 .cm -1 .

Cyclic and square wave voltammograms were recorded in degassed 0.1 M n-Bu4NBF4 solution in acetonitrile (Figures S12-16 in Supporting Information) and the redox properties of the different complexes are listed in Table

Acknowledgements

Colette Lebrun (CEA/DRF/INAC/SyMMES) is acknowledged for the ESI-MS measurements. This work was supported by the Labex program ARCANE (ANR-11-LABX-0003-01), the graduate school in Chemistry, Biology and Health of Univ. Grenoble Alpes (CBH-EUR-GS, ANR-17-EURE-0003), the European Research Council under the European Union's Seventh Framework Program FP/2007-2013 (ERC Grant Agreement n.306398).

Bibliography


Pierre Le Mouelli
email: pierre.le-mouellic1@etu.univ-nantes.fr

Sylvain Charlès
email: sylvain.charles@univ-rennes1.fr

Jean-Benoît Le Cam
email: jean-benoit.lecam@univ-rennes1.fr

Nicolas Boyard
email: nicolas.boyard@univ-nantes.fr

Jean-Luc Bailleul
email: jean-luc.bailleul@univ-nantes.fr

Thierry Gaudry
email: thierry.gaudry@cooperstandard.com

Jean-Marc Veillé

Thermomechanical Behaviour and Interface of Overmoulded Soft Thermoplastic Vulcanizate Elastomers

Keywords: overmoulding, vulcanizate thermoplastic elastomer, polymer interface, polymer junction

The influence of melt injection temperature on the thermomechanical behaviour of soft-soft overmoulded vulcanized thermoplastic elastomers (TPV) with different elastic properties was studied. Samples with two different overmoulding temperatures were tested under uniaxial loading conditions. The full deformation and temperature fields in each TPV were determined using digital image correlation technique and infrared thermography, respectively. The maximum interface strength was found to be equal to 70 N for a maximum injection temperature of 260 • C, which is consistent with the fact that high temperatures promote interdiffusion between the molten TPV and the TPV insert. The two TPV have different stiffness, leading to a significant change of the interface position along the specimens during stretching and to a significant necking in the softer material. The zone of influence of the interface in terms of stretch gradient is very different in size from one TPV to the other. In addition, thermal investigations have shown that the elasticity of the two TPV is due to both entropic and non-entropic effects, the former being the most significant at large strains.

Introduction

Injection moulding is a suitable technique for the production of numerous, highquality and complex geometry parts in short times. Overmoulding is an injection technique that consists of joining two polymeric parts without any adhesives. This injection technique is now used in several sectors such as automotive, medical or packaging. For the automotive market, more especially in the weather seals, overmoulding is used to joined extrudate profiles. Soft-soft combinations are often used to answer the specific needs of the customer such as insulation from noise, water and dust. The soft-soft combinations were classically made of ethylene-propylene-diene-monomer (EPDM) rubber, but recent trends show increasing use of thermoplastic elastomer vulcanizate (TPV). This type of thermoplastic elastomer (TPE) is produced by dynamic vulcanization [START_REF] Naskar | Thermoplastic elastomers based on PP/EPDM blends by dynamic vulcanization[END_REF], which leads to the elastomer crosslinking during the blending with a thermoplastic polymer [START_REF] Coran | Thermoplastic elastomers by blending and dynamic vulcanization[END_REF]. The resulting morphology is an elastomeric dispersed phase into a thermoplastic matrix. The elastomeric phase is usually composed of EPDM, and the thermoplastic phase is polypropylene (PP) [START_REF] Drobny | Handbook of Thermoplastic Elastomers[END_REF]. These materials have a great interest since they exhibit the features of elastomers with the processability of thermoplastics, allowing short cycle times and therefore a productivity increase. During the removal of the overmoulded part from the mould, the assembly on the vehicle or during the use, debonding at the interfaces can occur. It is then important to understand the adhesion mechanisms at TPV interfaces and to analyze the mechanical behaviour of these junctions.

Overmoulding fusion bonding between semicrystalline thermoplastics and TPE has been the subject of attention in various studies. Candal et al. [START_REF] Candal | Study of the adhesion strength on overmoulded plastic materials using the essential work of interfacial fracture (EWIF) concept[END_REF] studied the bonding strength of TPV on a polypropylene substrate by the Essential Work of Interfacial Fracture (EWIF) [START_REF] Lauke | Essential work of interfacial fracture: A method to characterise adhesion at polymer-polymer interfaces[END_REF]. They varied the overmoulding conditions and showed that increasing the melt and mould temperatures lead to higher adhesion since these parameters improve the wettability and interdiffusion of the TPV on the PP substrate. The authors used a 190 • C injection temperature and with the EWIF method the interfacial strength was found to be 2 kJ.m -2 , whereas, for an injection temperature of 260 • C they found an interface strength of 6 kJ.m -2 . They also showed that an increase in holding pressure decreased adhesion. The bonding between thermoplastic elastomer and Polyamide 12 with 0 to 50% glass fibre contents by a peeling test has been investigated by Persson et al. [START_REF] Persson | Adhesion between thermoplastic elastomers and polyamide-12 with different glass fiber fractions in two-component injection molding[END_REF]. The authors showed that adhesion increased with increasing melt temperature and injection rate for a certain range. Rosa-Sierra et al. [START_REF] Rossa-Sierra | Study of the interface behaviour between MABS/TPU bi-layer structures obtained through over moulding[END_REF] have studied the adhesion between methylmethacrylatebutadiene-styrene copolymer (MABS) and thermoplastic polyurethane (TPU) by peeling and tensile tests. The roughness of the insert was shown to be the most influential parameter on the adhesion strength. The authors used an insert roughness of 31.33 µm and with the peel test the interfacial strength was found to be 31.9 kJ.m -2 , whereas with a roughness of 3.24 µm the adhesion strength was found to be 5.2 kJ.m -2 . The authors also showed the increase of adhesion with the overmoulding temperature. Other studies [START_REF] Arzondo | Sequential injection overmolding of an elastomeric ethylene-octene copolymer on a polypropylene homopolymer core[END_REF][START_REF] Dondero | Adhesion control for injection overmolding of polypropylene with elastomeric ethylene copolymers[END_REF] focus on the influence of melt injection temperature and holding pressure on the adhesion strength between ethylene or octane copolymers on PP homopolymer substrate by peeling tests. It is demonstrated that the adhesion strongly depends on the interface temperature and thus on the development of entanglements between the two molten polymers. Weng et al. [START_REF] Weng | Fundamentals and material development for thermoplastic elastomer (TPE) overmolding[END_REF] have summarized the fundamental reasons for adhesion of an overmoulded TPE. They explain that the overmoulding process can be divided into three stages: wetting, diffusion of polymer chains and crystallization.

Wetting refers to the notion of intimate contact. The melted TPE is injected into a mould in which the TPE insert to be overmoulded has been previously positioned. The contact between both TPEs is not perfect due to the surface roughness of the insert. The notion of intimate contact is used to describe the evolution of physical contact between the two TPEs. The degree of intimate contact increases when the asperities of the TPE insert squeeze under the application of pressure and polymer flow of the injected TPE. Models to describe the evolution of intimate contact have been developed [START_REF] Dara | Thermoplastic Matrix Composite Processing Model[END_REF][START_REF] Lee | A model of the manufacturing process of thermoplastic matrix composites[END_REF][START_REF] Yang | A fractal Cantor set based description of interlaminar contact evolution during thermoplastic composites processing[END_REF], and generally show that the time required to achieve full intimate contact t ic is proportional to the viscosity η 0 and inversely proportional to the applied pressure P [START_REF] Lee | A model of the manufacturing process of thermoplastic matrix composites[END_REF]:

t ic ∝ η 0 P (1)
The intimate contact time refers to the time from the beginning of the process until the time for which polymer chains diffuse and create the junction strength. In other words, the intimate contact time precedes the healing time. The injection process involves melted polymer with low viscosities and high pressure. It is, therefore, possible to neglect the time necessary to achieve intimate contact (Equation ( 1)). This process is followed by healing which refers to the diffusion of polymer (PP) chains across the interface. These two processes are coupled because healing can occur only across the zones where intimate contact is established. The process of healing is described from the theory of De Gennes [START_REF] De Gennes | Dynamics of entangled polymer chains[END_REF] taken up by Doi and Edwards [START_REF] Doi | Dynamics of rod-like macromolecules in concentrated solution. Part 1[END_REF] and Wool [START_REF] Wool | A theory crack healing in polymers[END_REF]. The theory of De Gennes describes the movement of macromolecular chains in an entangled system constrained by the presence of neighbouring chains. A chain is then considered to be confined in a permanent and undeformable tube. The behaviour of the chain is anisotropic: lateral movements are not possible, and the polymer relaxes by monodirectional movements along its tube. The relaxation process of the polymer chain is associated with a characteristic time called reptation time depending on the temperature and molar mass. This characteristic time represents the time necessary for the chain to exit its initial tube and forget its initial configuration. Bastien and Gillespie [START_REF] Bastien | A non-isothermal healing model for strength and toughness of fusion bonded joints of amorphous thermoplastics[END_REF] introduce the degree of healing (Equation ( 2)) for non-isothermal temperature history at melted polymer interface as follows:

D h = σ σ ∞ = t p /∆t ∑ i=0 t 1/4 i+1 -t 1/4 i t * 1/4 rep ( 2
)
where σ is the Cauchy fracture stress, σ ∞ is the fracture stress of a fully healed interface, t p /∆t represents the division of the thermal history in time intervals characterized by an average temperature T * for which the reptation time t * rep is determined. The final performance of the overmoulded part is the combination of the contribution of wetting, diffusion and crystallization. If hard-soft combinations are studied in the literature, soft-soft combinations seem to be few to the author's knowledge.

The difference in elastic properties of the overmoulded TPV made their global mechanical behaviour complex. Indeed, contrarily to a soft-hard configuration, soft-soft configuration leading to large and very different deformation levels in the two materials, this induces a significant change in the position of the interface and in the loading applied locally in this zone. Even though mechanical models and experimental tests are reported in the literature for improving the prediction and analysis of the behaviour of materials at interfaces, such soft-soft configuration has never been considered to our best knowledge, which makes the innovative character of the present study. As a summary, this paper reports the first study on mechanical properties and interface of two TPV with very different stiffnesses. To do so, an original experiment is developed to characterize the material deformation processes at both the global and the local scales. The results reported in this study bring therefore new information on how characterizing the interface between two different TPV materials and data to better model interface effects in a continuum media. Therefore, the present work aims to study the thermomechanical behaviour of soft-soft combinations made of overmoulded TPV. In the present study, two TPV overmoulded at two different temperatures are considered. Two types of tests were performed at room temperature: a monotonous tensile test until failure and cyclic load-unload tests with a constant amplitude. The experimental setup and the theoretical framework for thermal and kinematic field measurements are detailed in Section 2. Section 3 gives the results obtained in terms of kinematic and thermal field. The mechanical strength of the assembly is discussed regarding the overmoulding temperature.

Materials and Methods

Materials and Specimens Elaboration

Materials

The materials considered here are two polypropylene-ethylene-propylene-dienemonomer (PP-EPDM)-based thermoplastic elastomers (TPV) called TPV1 and TPV2. TPV1 is an injection moulding grade, TPV2 is an extrusion grade. Both TPVs are composed of several components, each of them providing different properties to the material:

1.

an elastomer (EPDM), for flexibility, deformation at break and impact resistance, 2.

a thermoplastic (PP), for the processing and the stiffness, 3.

fillers, for tunning specific mechanical and rheological properties [START_REF] Mishra | Structure-property relationship of a thermoplastic vulcanizate (TPV)/layered silicate nanocomposites prepared using maleic anhydride modified polypropylene as a compatibilizer[END_REF]. Generally, these fillers are carbon black [START_REF] Katbab | Carbon black-reinforced dynamically cured EPDM/PP thermoplastic elastomers. I. Morphology, rheology, and dynamic mechanical properties[END_REF] or silica [START_REF] Bazgir | Silica-reinforced dynamically vulcanized ethylene-propylene-diene monomer/polypropylene thermoplastic elastomers: Morphology, rheology, and dynamic mechanical properties[END_REF], 4.

a vulcanization agent, here phenolic resin, 5.

antioxidant agents.

It should be noted that each TPV of the present study is obtained by a dynamic vulcanization process such as described in [START_REF] Coran | Rubber-thermoplastic compositions. Part I. EPDM-polypropylene thermoplastic vulcanizates[END_REF], which consists of mixing the two melt phases (PP and EPDM) and then in vulcanizing the EPDM. In this process, a phase inversion is obtained in the sense that the PP phase is initially dispersed in the EPDM phase and after the vulcanization, EPDM is dispersed in the PP phase. The final morphology of the mixing is determined by the relative amount of the two phases, their viscosity, the crosslink density and the type and amount of the vulcanization [START_REF] Naderi | Microstructure-properties correlations in dynamically vulcanized nanocomposite thermoplastic elastomers based on PP/EPDM[END_REF][START_REF] Lee | Structure-property relationships in polymer blend nanocomposites[END_REF]. The main properties of TPV1 and TPV2 are given in Table 1. The morphology of the two TPV has been investigated thanks to a Bruker Multimode 8 NanoscopeV Atomic Force Microscope (AFM) in tapping mode. The scan sizes were 20 µm and 40 µm. The scan rate was 0.977 Hz. The cantilever parameters were a spring constant of 0.3 N/m and a tip radius of 10 nm. The overmoulded samples were previously cryo-microtomed at -80 • C. The images were processed using Gwyddion software.

The specific heat (C p ) of both TPV has been determined by differential scanning calorimetry (Q200, TA Instruments). The measurements were made at a heating rate of 10 K/min between 50 • C and 200 • C. The specific volume V sp , in m 3 /kg, was measured with a home-made PVT device [START_REF] Pignon | A new PvT device for high performance thermoplastics: Heat transfer analysis and crystallization kinetics identification[END_REF] for 10 MPa and 50 MPa at a cooling rate of 2 • C/min. Thermal conductivity k in W/(m.K) was measured for the temperature ranging between 23 • C and 260 • C thanks to a home-made guarded hot plate [START_REF] Pignon | Cristallisation des Polymères Semi-Cristallins en Condition Thermique extrême[END_REF].

The main characteristics of the PP constitutive of the TPVs matrices are listed in Table 2. Polypropylene 1,2 (PP1,2) is an ethylene-propylene copolymer and is representative of the matrix of TPV1 and PP1 is a homopolymer PP and is representative of the TPV2 matrix. These analyses were performed on a "Malvern viscotek" steric exclusion chromatography. The solvent was the 1,2,4-Trichlorobenzene. The elution speed of the solvent was fixed at 1 mL/min. The linear viscoelastic properties of the PP representative of TPV1 and TPV2 matrices were determined using a strain-controlled rheometer HAAKE MARS III (Thermo Scientific, Waltham, MA, USA) equipped with a 25 mm parallel plate geometry and a 1 mm gap. Dynamic frequency sweep measurements were performed for frequencies ranging between 0.01 rad/s and 100 rad/s, γ = 5 % for different temperatures between 200 • C and 230 • C.

Overmoulding Protocol

TPV2 were extruded with a FAIREX extruder (barrel diameter 60 mm). The extruded profiles were cut with a razor blade and stored at 23 • C for 24 h and 50% relative humidity. Their dimensions were 60 mm long, 20 mm wide and 2 mm thick. TPV1 were dried at 85 • C in a Moretto drier for 2 h. These profiles were then inserted in a specific mould, which is presented in Figure 1, and were overmoulded (LWB STEINL, 300 kN injection unit) with TPV1. The injection temperatures for TPV1 were set at 190 • C or 260 with an injection rate of 12 cm 3 /s, a hydraulic holding pressure of 0.7 MPa, a holding pressure time of 5 s and a mould temperature of 40 • C. The moulding cavity is instrumented with two pressure sensors Kistler© (type 6182CAG) positioned at 10 mm and 50 mm from the gate, respectively. Two pressure sensors are located at the inlet and at the outlet of the cavity. TPV1 is injected through a 3 mm diameter sprue.

Specimen Preparation

The cross-section of the parts is shown in Figure 2a. Once overmoulding was performed, rectangular bi-material specimens were cut from the overmoulded parts with a razor blade in the zone of interest centred on the interface (see Figure 2b). As shown in Figure 2c, their dimensions were 75 mm long, 15 mm wide and 2 mm thick. Fused Deposition Modelling (FDM) printed rolls of 6 mm in diameter were glued (Loctite 406, Henkel, Düsseldorf, Germany) to the ends of the specimen. Such rolls prevent the specimens from slipping into the jaws, which shape has been designed accordingly [START_REF] Trabelsi | Etude Statique et Dynamique de la Cristallisation des Élastomères sous Tension[END_REF][START_REF] Charlès | Inverse identification of constitutive parameters from heat source fields: A local approach applied to hyperelasticity[END_REF].

Mechanical Characterization

Full Kinematic Field Measurement

Images of the specimen surface were recorded at a frame rate equal to 10 Hz with an IDS camera equipped with a 55 mm telecentric objective. The charge-coupled device (CCD) of the camera has 1920 × 1200 joined pixels. The displacement field at the specimen surface was determined using the digital image correlation (DIC) technique. The software used for the correlation process was SeptD [START_REF] Vacher | Bidimensional strain measurement using digital images[END_REF]. Hardware and analysis parameters are summarized in Tables 3 and4, respectively.

Deformation Gradient Tensor Computation

As explained in [START_REF] Charlès | Inverse identification of constitutive parameters from heat source fields: A local approach applied to hyperelasticity[END_REF], the F ij components of the deformation gradient tensor are determined at the centre of the square elements, which are composed of four Zones Of Interest (ZOI). Each corner of the square element corresponds to a ZOI centre defined in the undeformed DIC grid. Within each square element i, the horizontal and vertical displacements, i U and i V respectively, are assumed to be bilinear functions of the coordinates (X, Y):

i U(X, Y) = a + bX + cY + dXY i V(X, Y) = e + f X + gY + hXY (3)
where a, b, c, d, e, f , g and h are constants obtained from the ZOI values. The displacement is interpolated at the centre of each square element, the calculation is carried out by considering that the element centre is the origin of the coordinate system. Finally, the deformation gradient tensor F components are calculated as follows:

F = ∂x ∂X ∂x ∂Y ∂y ∂X ∂y ∂Y = ∂X+ i U ∂X ∂X+ i U ∂Y ∂Y+ i V ∂X ∂Y+ i V ∂Y = 1 + b c f 1 + g (4)
The three principal stretches (λ 1 > λ 2 > λ 3 ) are defined as the square roots of the eigenvalues of the left Cauchy-Green tensor B (B = F F t ). λ 3 is deduced by assuming the material being is incompressible, i.e., J = detF = λ 1 λ 2 λ 3 = 1.

Full Temperature Field Measurements

Measuring the thermal response associated with the mechanical response of materials provides very complementary information on the origin of the elasticity, i.e., coupling between temperature and strain, as well as on dissipative effects, typically when selfheating occurs. Two types of thermoelastic coupling have been reported in the literature for elastomeric materials: isentropic [START_REF] Chrysochoos | Experimental analysis of thermomechanical coupling by infra-red thermography[END_REF] and entropic coupling [START_REF] Gough | Proceedings of the Literary and Philosophical Society of Manchester[END_REF][START_REF] Joule | On some thermodynamic properties of solids[END_REF]. They take place concomitantly and a competition is observed between both. Thus, the thermal response of vulcanized rubber is characterized by a slight cooling during stretching in the very low stretch range (typically for stretches lower than 1.1). Such thermo-sensitivity is explained by preponderant effects of internal energy changes at low stretches [START_REF] Ciferri | On the tension-temperature-coefficient of natural rubber[END_REF][START_REF] Roe | The contribution of internal energy to the elastic force of natural rubber[END_REF][START_REF] Allen | Thermodynamics of elasticity of natural rubber[END_REF][START_REF] Shen | Internal energy contribution to the elasticity of natural rubber[END_REF][START_REF] Boyce | The thermoelasticity of natural rubber in torsion[END_REF][START_REF] Allen | Thermodynamics of rubber elasticity at constant volume[END_REF][START_REF] Price | A thermodynamic analysis of the elastic properties of poly (dimethyl siloxane)[END_REF]. At higher stretches, the competition is in favour of the entropy changes, leading to a thermoelastic inversion and a strong heating [START_REF] Joule | On some thermodynamic properties of solids[END_REF][START_REF] Anthony | Equations of state for naturals and synthetic rubber like materials: Unaccelerated natural soft rubber[END_REF][START_REF] Pottier | Study on the use of motion compensation technique to determine heat sources. Application to large deformations on cracked rubber specimens[END_REF].

Full surfaces temperature field measurements were performed using a X6540SC FLIR IR camera, which detectors operating in wavelengths between 1.5 and 5.1 µm, equipped with a focal plane array of 640 × 512 px. The integration time was equal to 2700 µs and the acquisition frequency was equal to 10 Hz. The calibration of camera detectors was performed with a black body using a one-point Non-Uniformity Correction (NUC) procedure at this acquisition frequency. The thermal resolution or noise equivalent temperature difference (NETD) is equal to 20 mK for a temperature range between 5 and 40 • C and the spatial resolution of the thermal field was equal to 300 µm/px. The IR camera is switched on several hours before testing to ensure its internal temperature to be stabilised. The emissivity of the material was set at 0.94. The emissivity has been identified from the calibration procedure detailed in [START_REF] Charlès | Inverse identification of constitutive parameters from heat source fields: A local approach applied to hyperelasticity[END_REF]. The infrared camera was trigged at the same acquisition frequency as the optical camera one, i.e., 10 Hz. Due to large deformations undergone by the material, the material points observed by the IR camera move from pixel to pixel in the IR images and their motion has been compensated [START_REF] Pottier | Study on the use of motion compensation technique to determine heat sources. Application to large deformations on cracked rubber specimens[END_REF][START_REF] Samaca Martinez | Thermomechanical analysis of the crack tip zone in stretched crystallizable natural rubber by using infrared thermography and digital image correlation[END_REF]. This requires first describing the kinematic and the thermal fields in the same coordinate system. In the present paper, coupled full thermal and kinematic fields measurements have been carried out on both sides of the specimen. A calibration test pattern was placed in one of the grips. The reader can refer to [START_REF] Charlès | Inverse identification of constitutive parameters from heat source fields: A local approach applied to hyperelasticity[END_REF] for further information about the calibration procedure. This approach has been successfully applied for rubber [START_REF] Balandraud | Some specific features and consequences of the thermal response of rubber under cyclic mechanical loading[END_REF][START_REF] Le Cam | Energy storage due to strain-induced crystallization in natural rubber: The physical origin of the mechanical hysteresis[END_REF][START_REF] Samaca Martinez | Mechanisms of deformation in crystallizable natural rubber. Part 1: Thermal characterization[END_REF][START_REF] Samaca Martinez | Thermal and calorimetric effects accompanying the deformation of natural rubber. Part 2: Quantitative calorimetric analysis[END_REF][START_REF] Samaca Martinez | Filler effects on the thermomechanical response of stretched rubbers[END_REF][START_REF] Samaca Martinez | New elements concerning the Mullins effect: A thermomechanical analysis[END_REF][START_REF] Le Cam | Thermomechanical Analysis of the Singular Behavior of Rubber: Entropic Elasticity, Reinforcement by Fillers, Strain-Induced Crystallization and the Mullins Effect[END_REF], for Poly(methyl methacrylate) [START_REF] Jongchansitto | Comparison between 0D and 1D approaches for mechanical dissipation measurement during fatigue tests[END_REF] and for metallic materials [START_REF] Giancane | Deformation and dissipated energies for high cycle fatigue of 2024-T3 aluminium alloy[END_REF][START_REF] Wang | Energy-based approach for fatigue life prediction of pure copper[END_REF][START_REF] Chrysochoos | An infrared image processing to analyse the calorific effects accompanying strain localisation[END_REF][START_REF] Balandraud | Influence of the thermomechanical coupling on the propagation of a phase change front[END_REF][START_REF] Chrysochoos | Use of full-Field digital image correlation and infrared thermography measurements for the thermomechanical analysis of material behaviour[END_REF][START_REF] Maquin | Heat dissipation measurements in low stress cyclic loading of metallic materials: From internal friction to micro-plasticity[END_REF]. The analysis of the heat source release induced by the deformation remains delicate. Indeed, this analysis is based on a measurement of the effects (temperature variations) to go back to the causes (heat sources). 5 and6 present the images of the specimens overmoulded at 190 • C and 260 • C at different times of the test, respectively. These times correspond to letters a to h and a to j for 190 • C and 260 • C, respectively. The interface is highlighted with a white arrow. At the beginning of the test, the shape of the curves is close. The force-displacement relationship is almost linear. For higher displacement levels, the specimens overmoulded at 260 • C exhibits a higher stiffness. Moreover, the displacement at break is much lower for the specimens overmoulded at 190 • C (8 mm against 45 mm for the specimens overmoulded at 260 • C). For both overmoulding temperatures, specimen failure occurred at the interface between the two TPV. The failure occurs at a lower force in the case of specimens overmoulded at 190 • C than for specimens overmoulded at 260 • C. For specimens overmoulded at 260 • C, necking at the interface during the test is observed (Figure 6h). This may influence the strain state at the interface. Nevertheless, the spatial resolution of the full kinematic measurements did not enable us to further investigate this effect. This phenomenon is not observed for those overmoulded at 190 • C. High relative deformations are reached for TPV2, while TPV1 shows low deformations. These remarks will be discussed in more detail in the next section.

Results

Mechanical Analysis

Mechanical Cycle

Figures 7 and8 show the mechanical response in terms of force versus displacement during the mechanical cycle at a total displacement of 8 mm for the 190 • C overmoulded specimens and 25 mm for the 260 • C overmoulded specimens. For specimens overmoulded at 260 • C, necking can be seen for TPV1 (Figure 8e), but not for TPV2. This necking cannot be seen for specimens overmoulded at 190 • C. This has already been observed in the case of monotonous displacement test until failure (Figure 6h). This aspect will be more precisely detailed in the next section. The load levels obtained for the mechanical cycle are similar to the one obtained previously (Figure 4). The mechanical response exhibits a hysteresis for both specimens. This hysteresis loop can be associated with viscosity and/or damage. In the case of 190 • C overmoulded specimens, the residual strain is 1.67% against 5% for the 260 • C. The low residual strain obtained after relatively high strains applied indicates that a low level of damage, typically plasticity, takes place during the mechanical cycle in the two specimens. This is in good agreement with Babu et al. [START_REF] Babu | Recent developments on thermoplastic elastomers by dynamic vulcanization[END_REF], who used the TPV deformation model developed by Soliman et al. [START_REF] Soliman | Deformation mechanism of thermoplastic vulcanizates investigated by combined FTIRand stress-strain measurements[END_REF] to explain the behaviour of thermoplastic elastomers vulcanizate. This model explains that the deformation behaviour of TPV is the combination of matrix yielding and buckling because of the recovering tendency of the crosslinked EPDM particles (Figure 9). In our case, the mechanical response was the one of the specimens made of two different TPV. It could be useful to be able to distinguish their single responses during the test, i.e., to access mechanical information at a local scale. This is the reason the next sections focus on the response of each TPV using imaging.

Full Kinematic Field Measurements

The DIC technique enables the determination of the kinematic field for the two constitutive TPV. Figure 10 presents the stretch versus time for both injection temperatures and for monotonic tensile test until rupture. For an injection temperature of 190 • C, the stretch levels reached in each TPVs are lower than those observed for an injection temperature of 260 • C. Referring to Section 3.1.2, this behaviour is explained by the temperature dependence of the diffusion mechanisms of PP macromolecules at the interface. For both injection temperatures, it is observed that the elongation reached in TPV2 at the end of the test is higher than that in TPV1. For an injection temperature of 190 • C, λ TPV2 = 1.18 versus λ TPV1 = 1.06 (Figure 11), and for an injection temperature of 260 • C, λ TPV2 = 1.92 against λ TPV1 = 1.26 (Figure 12). Figure 13 presents the stretch versus time for both injection temperatures and for mechanical cycle tests. From these two figures, it should be noted that TPV2 is much softer than TPV1. Therefore, the whole specimen made with these two TPV is not submitted to a homogeneous loading and consequently, the interface failure cannot be seen theoretically as a crack propagating within a homogeneous medium, meaning that it is complicated to apply the existing models of fracture mechanics to solve interface failure problems [START_REF] Brown | A molecular interpretation of the toughness of glassy polymers[END_REF][START_REF] De Gennes | Toughness of glassy polymers: A tentative scheme[END_REF][START_REF] Dugdale | Yielding of steel sheets containing slits[END_REF].

The elastic properties of the two TPV are very different. This is partly due to their compositions, in particular, their respective proportions of PP and EPDM. Figure 14a,b present an AFM phase image of TPV1 and TPV2, respectively. TPV1 and TPV2 were previously cryo-microtomed from the 190 • C overmoulded specimens. The phase images can be used to visualize the distribution of the EPDM phase (dark areas) in the PP matrix (bright areas) [START_REF] Prut | Effects of blend composition and dynamic vulcanization on the morphology and dynamic viscoelastic properties of PP/EPDM blends[END_REF]. TPV1 has larger elastomer nodules and a higher proportion of PP than TPV2. This implies different mechanical responses as shown in Figures 10 and13. These observations are also in good agreement with the hardness ratio of both TPV presented in Table 1.

Full Temperature Field Measurement

Figure 15 shows the thermal response measured at the surface of the two TPV in specimen overmoulded at 190 • C for the tensile test at failure. It is observed that the temperature variation first decreases in the two materials. As the laboratory temperature is stabilised, the only explanation is that the isentropic coupling dominates the thermal response. Furthermore, given the very small temperature variations measured, the intrinsic dissipation caused by the viscosity is very low, even negligible. From a certain strain level, the curve slopes decrease and are inverted in the case of TPV2 for a stretch equal to 1.05. Such a temperature evolution is in a strong analogy with what is observed in elastomers and is generally referred to as the thermoelastic inversion [START_REF] Joule | On some thermodynamic properties of solids[END_REF][START_REF] Anthony | Equations of state for naturals and synthetic rubber like materials: Unaccelerated natural soft rubber[END_REF]. In addition, it is possible that the level of intrinsic dissipation due to viscosity also increases. Figure 16 shows the response obtained with the 260 • C specimen. Overall, we find the same trends as for the specimen 190 • C as shown in Figure 16. Indeed, when zooming in on the first part of the curves, i.e., at the lowest strains, the order of magnitude of temperature variation is the same. It should be noted that this increase in temperature can also be induced by a greater viscosity of the materials and therefore a greater intrinsic dissipation. As the interface is more resistant at 260 • C, the level of deformation reached in both materials is greater. This leads to one order of magnitude greater temperature variation in TPV1 between the 190 • C and 260 • C specimens. This issue will be addressed in more detail during the analysis of the cyclic tests.

The analysis of the average temperature variation during the test in each of the two materials far from the interface, i.e., without thermal gradients induced by the stress and strain concentrations of the interface, enabled us to show that the elasticity of the TPVs constituting the specimen is both entropic and isentropic. A competition between the two types of elastic couplings occurs at the low strains. The isentropic coupling is first preponderant, then the entropic coupling takes over. Nevertheless, this analysis does not allow the study of the mechanical behaviour and the effects of the interface in the different materials to be further investigated. This is the reason Figures 17 and18 show the temperature variation fields and the temperature variation profiles along the specimens at 190 • C and 260 • C, respectively. Please note that these profiles were determined by averaging the temperature variations across the width. The temperature profiles along the specimen length show a different thermal response for the two TPV as detailed previously. For the smallest and moderate strains (t = 2, 4, 6 and 7 s for specimen at 190 • C, and t = 2.5, 15, 30 and 37 s for specimens at 260 • C), the thermal response is different from one TPV to another, but it is homogeneous in each TPV. The interfaces do not have a specific thermal signature. Nevertheless, for the highest strains, i.e., close to those at failure, the thermal field exhibits a strong singularity in the interface zone. Indeed, a strong increase in temperature occurs in this zone and can be considered to be a precursor of the interface failure. Such temperature increase is observed in many materials when damage takes place [START_REF] Samaca Martinez | Heat and strain measurements at the crack tip of filled rubber under cyclic loadings using full-field techniques[END_REF]. Please note that the increase in temperature is mainly observed in TPV2. The zone where the temperature is observed is larger (approximately 15 mm) in TPV2 in specimen at 260 • C than in specimen at 190 • C (approximately 10 mm). It should be noted that the temperature rise is 0.16 • C and 1.2 • C for the overmoulded samples at 190 • C and 260 • C, respectively. The fact that an increase in temperature is mainly observed in TPV2 before failure can be interpreted as the effect of an increase of intrinsic dissipation due to internal friction accompanying PP chain disentanglements. Nevertheless, additional experiments must be carried out to further investigate the physical origin of this increase in temperature rise.

Prediction of Adhesion

The failure observed at a lower strength level in the case of the 190 • C specimens in Figure 4 may be explained regarding the overmoulding process. Assuming diffusion of the molten TPV1 into TPV2 insert, the strength of the overmoulded specimen is increasing with injection temperature because of the decreasing value of reptation time with temperature (Equation ( 2)) [START_REF] Giusti | Modeling the Adhesion Bonding Strength in Injection Overmolding of Polypropylene Parts[END_REF]. In the following considerations, we assume an instantaneous intimate contact. We will focus on the calculation of the degree of healing based on a rheological determination of relaxation times of PP representative of the matrix of both TPVs. Indeed, PP can be considered to be the main material in the composition of the TPV contributing to adhesion, as observed by AFM as presented in Figure 21. TPV2 is located on the left side and TPV1 on the right one. The interface between the two overmoulded TPVs is represented by dotted lines and is made of PP. The mechanical strength of the assembly of the two overmoulded TPVs is thus linked to the entanglements formed by the PP chains that will renew their configuration and cross the interface.

Rheological Characterization

The mastercurve was determined at 200 • C from an Arrhenius plot of the shift factors a T versus the inverse of the temperature. The activation energies were E a,PP1,2 = 41.94 kJ/mol and E a,PP1 = 39.76 kJ/mol for PP1,2 and PP1, respectively. and are in good agreement with the literature [START_REF] Koscher | Influence of shear on polypropylene crystallization: Morphology development and kinetics[END_REF][START_REF] Galgali | A rheological study on the kinetics of hybrid formation in polypropylene nanocomposites[END_REF]. The mastercurve was fitted with a N = 5 mode Maxwell model (Equations ( 5) and ( 6)).

G (ω) = N ∑ i=1 G i (ωτ i ) 2 1 + (ωτ i ) 2 (5)
G (ω) = N ∑ i=1 G i ωτ i 1 + (ωτ i ) 2 (6)
The relaxation moduli G i and the associated relaxation time τ i were calculated at 200 • C. The average relaxation time given by τ w =

∑ G i τ 2 i ∑ G i τ i
. At 200 • C, τ w = 1.37 s for PP1 and τ w = 0.80 s for PP1,2. The values obtained were compared to the Cole-Cole plot. From the plot of imaginary η versus real η components of the complex viscosity one can determine a weight-average relaxation time from the frequency at the maximum of η . The average relaxation time τ w obtained by this technique for PP1,2 is approximatively 0.63 s and 1.23 s for PP1. The differences in the calculated values can be explained by the polydispersity of both polypropylenes, i.e., they have a distribution of relaxation times reflecting the whole chains dynamics. The relaxation times obtained are then extrapolated with an Arrhenius relation using the activation energies.

Thermophysical Properties

The calculation of the relaxation times needs to be coupled with the simulation of the cooling of the overmoulded part to predict the degree of healing. The modelling of heat transfer during cooling in semicrystalline polymer requires the knowledge of thermophysical properties as a function of temperature. Figure 22 presents the apparent heat capacity of the two TPV versus temperature. Between 125 • C and 175 • C, the two peaks correspond to the melting of the PP matrices. Thanks to these measurements, C p has been evaluated in J/(kg.K) for the two TPV as a function of temperature T in • C. The results are presented in Table 5. The PVT diagram of TPV1 and TPV2 is presented in Figure 23. TPV1 has a lower density than TPV2 for both isobars. The data are fitted following a linear regression in the molten and in the solid states. The results are presented in Table 6.

TPV1 TPV2

V

sp [m 3 /kg] R 2 V sp [m 3 /kg] R 2
Solid state 7.25 × 10 -7 T + 1.08 × 10 -3 0.99 6.01 × 10 -7 T + 1.01.10 -3 0.99 Amorphous state 8.20 × 10 -7 T + 1.07 × 10 -3 0.99 7.27 × 10 -7 T + 1.01 × 10 -3 0.99 Figure 24 presents the results obtained for the two TPV. The data are fitted following a linear regression, as presented in Table 7. The relaxation time thus obtained make it possible to calculate the degree of healing as a function of the temperature field during the cooling of the overmoulded part. In the following, we assume 1D heat transfer through the overmoulded part. TPV2 is located between x = 0 and x = e and TPV1 between x = e and x = 2e (Figure 25). The cooling of the TPV1 was modelled by the 1D heat transfer equation (Equation ( 7)) through the part (e < x < 2e). At the initial time, the temperature field was assumed to be uniform in TPV1 and equal to the injection temperature T inj (Equation ( 11)). The TPV1 crystallization rate was modelled by the differential form of Nakamura (Equation ( 8)) [START_REF] Nakamura | Some aspects of nonisothermal crystallization of polymers. I. Relationship between crystallization temperature, crystallinity, and cooling conditions[END_REF][START_REF] Patel | Crystallization kinetics during polymer processing-Analysis of available approaches for process modeling[END_REF], and a heat source term was added in the heat transfer equation (Equation ( 7)). A non-linear solving system was used to take into account the thermodependency of thermal properties and crystallization.

∂ ∂x k TPV1 ∂T TPV1 ∂x + ρ TPV1 ∆H ∂α ∂t = ρ TPV1 C p,TPV1 ∂T TPV1 ∂t f or e < x < 2e and ∀t > 0 ( 7
)
where ∆H is the crystallization enthalpy.

∂α ∂t = n.K Nak .(1 -α).(-ln(1 -α)) (n-1) n (8) + k TPV1 ∂T TPV1 (2e, t) ∂x = φ mould (t) at x = 2e (9)
k TPV1 ∂T TPV1 (e+, t) ∂x = φ 1 (t) at x = e+ (10)

T TPV1 (x, t = 0) = T inj f or e ≤ x ≤ 2e (11)
The cooling of the TPV2 was modelled by the heat transfer (Equation ( 12)) through the part (0 < x < e). At the initial time, the temperature field was assumed to be uniform and equal to the mould temperature T ex (Equation ( 15)). It is assumed that the TPV2 does not undergo a phase change.

∂ ∂x k TPV2 ∂T TPV2 ∂x = ρ TPV2 C p,TPV2 ∂T TPV2 ∂t f or 0 < x < e (12)
-k TPV2 ∂T TPV2 (0, t) ∂x = φ mould (t) at x = 0 (13) -k TPV2 ∂T TPV2 (e-, t) ∂x = φ 1 (t) at x = e- (14)
T TPV2 (x, t = 0) = T ex f or 0 ≤ x ≤ e (15)
The continuity of heat flux densities at the interface is expressed as (Equation ( 16)):

-k TPV1 ∂T TPV1 (e+, t) ∂x = -k TPV2 ∂T TPV2 (e-, t) ∂x = T TPV2 (e-, t) -T TPV1 (e+, t) TCR (16)
We considered for the simulation a constant thermal contact resistance (TCR) value at the TPVs interface (4 × 10 -3 K • m 2 /W). This value was taken as equal to the TCR determined by Somé et al. [START_REF] Somé | Modeling of the thermal contact resistance time evolution at polymer-mold interface during injection molding: Effect of polymers' solidification[END_REF] at PP/mould interface since TCR at polymer interfaces during the overmoulding process appears to be non-existent in the literature to the author's knowledge. Before overmoulding, the surface roughness of TPV2 was measured (Mitutoyo SJ-301) at R a = 0.43 µm. The cooling profiles of TPV1 are then calculated at x = e + 50 µm from the interface, i.e., outside the thermally disturbed zone.

Figure 25 presents the cooling profiles calculated at a position located at 50 µm from interface in TPV1 for injection temperatures of 190 • C and 260 • C. The degree of healing was only calculated for the case of a 260 • C overmoulding since in this configuration the TPV1 and TPV2 are melted at the interface. The average of the Maxwell model relaxation times of PP1 and PP1,2 was considered for the calculations. In the case of an overmoulding at 190 • C, only the TPV1 is melted at the interface. The degree of healing cannot be calculated since TPV2 remains in the solid state. Referring to the work of Nguyen et al. [START_REF] Nguyen | Adhesion control for injection overmolding of elastomeric propylene copolymers on polypropylene. Effects of block and random microstructures[END_REF], the adhesion of TPV1 to TPV2 is carried out by Rouse-type chain movements of PP1,2 in the amorphous regions of PP1 which is in the semicrystalline state. The adhesion levels observed in this case are lower than those observed for partial TPV1/TPV2 interface remelting, i.e., in the case of a 260 • C injection melt temperature, where the degree of healing reaches a maximum value for t ≈ 0.5 s.

Conclusions

This paper presents the first thermomechanical study of an assembly of two soft TPV obtained by overmoulding process and their interface. The rheological and thermophysical properties of the two TPV have first been fully characterized. Then, the mechanical tests carried out with the assemblies have highlighted that under monotonous tensile loading, the failure occurs at the interface. The DIC measurements have shown that the TPVs have different stiffnesses, which leads to a significant change in the interface position along the specimens during stretching. Furthermore, these measurements have shown that a significant necking is observed in the softer material and that the zone of influence of the interface in terms of stretch gradient is very different in size from one material to the other. Therefore, the interface resistance can be addressed neither as a crack propagating in a homogeneous medium nor a crack propagating at the interface between a soft and a rigid body. These questions undoubtedly on how to apply fracture mechanics on overmoulded soft-soft materials with a significant difference in stiffness. In addition, thermal investigations have shown that the elasticity of the two TPV is due to both entropic and non-entropic effects, the former being the most significant at large strains. Moreover, for high strain levels, the interface exhibits a singular behaviour characterized by a significant rise in temperature. The thermally influenced zone is larger for specimens moulded at higher temperatures. The heat production is even more important as the resistance of the interface is important. Analysis of average temperature profiles close to the interface reveals a strong heat production when the rupture occurs. In addition, the influence of the overmoulding temperature has been studied. It was shown that the increase in injection temperature induced an increase in the strength of the overmoulded part. Assuming a diffusion of the polypropylene macromolecules present at the interfaces, an acceleration of the adhesion kinetics was shown with the increase of the injection temperature.

This information could be used in future work to implement predictive models of the thermomechanical behaviour of soft-soft overmoulded junctions.

Figure 1 .

 1 Figure 1. Overmoulding mould. The green and red profiles represent TPV2 and TPV1, respectively. Two pressure sensors are located at the inlet and at the outlet of the cavity. TPV1 is injected through a 3 mm diameter sprue.

Figure 2 .

 2 Figure 2. (a) Cross-section of the profile. (b) Zone of interest centred on the interface. (c) Dimensions of the specimens. FDM printed rolls of 6 mm diameter were glued to the ends of the specimens.

Figure 3

 3 Figure 3 gives an overview of the experimental setup. It is composed of optical and infrared cameras, one on each side of a home-made biaxial testing machine. The tensile machine used is composed of four independent RCP4-RA6C-I-56P-4-300-P3-M (IAI) electrical actuators. They were driven by a PCON-CA-56P-I-PLP-2-0 controller and four PCON-CA (IAI) position controllers. The actuators were piloted by an in-house LabVIEW program. Two load cells measure the force in the perpendicular direction. Only one loading direction of the biaxial tensile machine was used for the present study and the specimens were mounted in the vertical direction. Two types of loading were applied. The first one corresponds to a monotonous tensile test until specimen failure at a rate of 36 mm/min

Figure 3 .

 3 Figure 3. Photography of the experimental setup for mechanical characterization.

3 . 1 . 1 .

 311 Figure 4 presents the mechanical behaviour during a monotonic tensile test until failure of the specimens overmoulded at 190 • C and 260 • C. Figures 5 and 6 present the images of the specimens overmoulded at 190 • C and 260• C at different times of the test, respectively. These times correspond to letters a to h and a to j for 190 • C and 260 • C, respectively. The interface is highlighted with a white arrow. At the beginning of the test, the shape of the curves is close. The force-displacement relationship is almost linear. For higher displacement levels, the specimens overmoulded at 260 • C exhibits a higher stiffness. Moreover, the displacement at break is much lower for the specimens overmoulded at 190 • C (8 mm against 45 mm for the specimens overmoulded at 260 • C). For both overmoulding temperatures, specimen failure occurred at the interface between the two TPV. The failure occurs at a lower force in the case of specimens overmoulded at 190 • C than for specimens overmoulded at 260 • C. For specimens overmoulded at 260 • C, necking at the interface during the test is observed (Figure6h). This may influence the strain state at the interface. Nevertheless, the spatial resolution of the full kinematic measurements did not enable us to further investigate this effect. This phenomenon is not observed for those overmoulded at 190 • C. High relative deformations are reached for TPV2, while TPV1 shows low deformations. These remarks will be discussed in more detail in the next section.

Figure 4 .

 4 Figure 4. Tensile mechanical response up to failure of 190 • C and 260 • C overmoulded specimens. Letters (a) to (i) and (a) to (h) refer to the letters of images taken at different displacements and given in Figures 5 and 6, respectively.

Figure 5 .

 5 Figure 5. Pictures related to Figure 4, specimens overmoulded at 190 • C. TPV1 is at the bottom and TPV2 at the top.

Figure 6 .

 6 Figure 6. Pictures related to Figure 4, specimens overmoulded at 260 • C. TPV1 is at the bottom and TPV2 at the top.

Figure 7 .

 7 Figure 7. Cyclic loading 8 mm of specimens overmoulded at 190 • C. Images (a-g) show a close-up view of the interface at different displacements during the test. TPV1 is at the bottom and TPV2 at the top.

Figure 8 .

 8 Figure 8. Cyclic loading 25 mm of specimens overmoulded at 260 • C. Images (a-g) show a close-up view of the interface at different displacements during the test. TPV1 is at the bottom and TPV2 at the top. Necking can be seen for TPV1 in image (e).

Figure 9 .

 9 Figure 9. Deformation mechanisms of TPV [57].

Figure 10 .

 10 Figure 10. Stretch-time evolution for each TPV overmoulded at 190 • C and 260 • C, in the case of monotonic tensile test until rupture.

Figure 11 .

 11 Figure 11. Stretch evolution for each TPV overmoulded at 190 • C, in the case of monotonic tensile test until rupture.

Figure 12 .

 12 Figure 12. Stretch evolution for each TPV overmoulded at 260 • C, in the case of monotonic tensile test until rupture.

Figure 13 .

 13 Figure 13. Stretch-time evolution for each TPV overmoulded at 190 • C and 260 • C, in the case of mechanical cycle.

Figure 14 .

 14 Figure 14. AFM phase image of TPV under study: (a) TPV1, injection grade. (b) TPV2, extrusion grade. The bright areas correspond to the thermoplastic phase (PP) and the dark areas correspond to the elastomer phase (EPDM). (images size (20 × 20) µm 2 ).

Figure 15 .

 15 Figure 15. Temperature change with stretch for specimen overmoulded at 190 • C, in the case of monotonic tensile test until rupture.

Figure 16 .

 16 Figure 16. Temperature change with stretch for specimen overmoulded at 260 • C, in the case of monotonic tensile test until rupture.

Figure 17 .

 17 Figure 17. Temperature variation fields and the temperature variation profiles along the specimens overmoulded at 190 • C. (a) t 1 = 2 s, (b) t 2 = 4 s, (c) t 3 = 6 s and (d) t 4 = 7 s.

Figure 18 .

 18 Figure 18. Temperature variation fields and the temperature variation profiles along the specimens overmoulded at 260 • C. (a) t 1 = 2.5 s, (b) t 2 = 15 s, (c) t 3 = 30 s and (d) t 4 = 37 s. Figures 19 and 20 show the thermal response of the overmoulded specimens at 190 • C and 260 • C respectively for the mechanical cycle. The thermoelastic inversion for TPV2 is observed for both overmoulding temperatures during the loading phase. For the 190 • C overmoulded specimens, the TPVs temperature is increasing during the unloading phase reflecting intrinsic dissipations due to viscosity and/or damage. For the 260 • C

Figure 19 .

 19 Figure 19. Temperature change with time for specimen overmoulded at 190 • C, in the case of cyclic loading.

Figure 20 .

 20 Figure 20. Temperature change with time for specimen overmoulded at 260 • C, in the case of cyclic loading.

Figure 21 .

 21 Figure 21. AFM phase image: TPV1-TPV2 interface observation by AFM.

Figure 22 .

 22 Figure 22. Apparent specific heat versus temperature for our two grades of TPV.

Figure 23 .

 23 Figure 23. PVT diagram for TPV1 and TPV2 at two pressure levels.

Figure 24 .Table 7 . 3 .

 2473 Figure 24. Thermal conductivity versus temperature for TPV1 and TPV2. Table 7. Thermal conductivity of TPVs under study. TPV1 TPV2 k [W/(m.K)] R 2 k [W/(m.K)] R 2 -7.3 × 10 -5 T + 0.197 0.90 -1.8 × 10 -4 T + 0.225 0.99

Figure 25 .

 25 Figure 25. Temperature evolution for a node located at x = e + 50 µm from interface in TPV1.

Author

  Contributions: Conceptualization, J.-B.L.C., N.B., J.-L.B.; methodology, J.-B.L.C., S.C., P.L.M.; software, S.C., P.L.M.; validation, J.-B.L.C., S.C. and P.L.M..; formal analysis, J.-B.L.C., N.B., J.-L.B., S.C., P.L.M.; investigation, J.-B.L.C., S.C., P.L.M.; resources, J.-B.L.C., S.C., P.L.M.; data curation, S.C., P.L.M.; writing-original draft preparation, J.-B.L.C., P.L.M.; writing-review and editing, J.-B.L.C., N.B., J.-L.B., P.L.M.; visualization, J.-B.L.C., S.C., P.L.M.; supervision, J.-B.L.C.; project administration, J.-B.L.C.; funding acquisition, T.G., J.-M.V. All authors have read and agreed to the published version of the manuscript.

Table 1 .

 1 Main properties of the studied materials.

	Properties	TPV1	TPV2
	Hardness [Shore A]	81	72
	Elongation at break, 23 • C [%]	610	450
	Young modulus, 23 • C [MPa]	25.5	19.6
	Tensile stress at 100%, 23 • C [MPa]	4.1	3.0

Table 2 .

 2 Molar mass, polydispersity index and activation energy of PPs under study.

	Material M PP1,2	80,528	167,612	1.8
	PP1	96,800	197,600	2.0

n [g/mol] M w [g/mol]

I p

[-]

Table 3 .

 3 DIC Hardware parameters.

	DIC Hardware Parameters	Detail
	Camera	IDS UI-3160CP Rev. 2
	Image Resolution	( 1920 × 1200 ) px
	Lens	55 mm C-mount partially telecentric.
		Constant magnification over a range
		of working distances ±12.5 mm
		of object movement before 1% error
		image scale occurs
	Field-of-View	(186.8 × 116.8) mm 2
	Image Scale	10 px/mm
	Stand-off Distance	1500 mm
	Image Acquisition Rate	10 Hz
	Patterning Technique	White spray on black specimen
	Pattern Feature
	Size (approximation)	6 px

Table 4 .

 4 DIC Software parameters.

	DIC Analysis Parameters	Detail
	DIC Software	7D©
	Image Filtering	None
	Subset Size	8 px/0.78 mm
	Step Size	4 px/0.39 mm
	Subset Shape Function	Affine
	Matching Criterion	Zero-Mean Normalized Cross Correlation
	Interpolant	Bi-cubic
	Strain Window	5 data points
	Virtual Strain Gauge Size	24 px/2.34 mm
	Strain Formulation	Principal Maximal
	Post-Filtering of Strains	None
	Displacement Noise-Floor	0.011 px/10.7 µm
	Strain Noise-Floor	0.0023

Table 5 .

 5 Apparent specific heat of TPVs under study.

		TPV1		TPV2
		C p [J/(kg.K)]	R 2	C p [J/(kg.K)]	R 2
	Solid state	5.22T + 2026	0.99	4.53T + 1815	0.99
	Amorphous state	4.19T + 2011	0.99	2.79T + 2023	0.99

Table 6 .

 6 Specific volume of TPVs under study.

Acknowledgments:

The authors thank Cooper Standard Automotive for financial support. We also thank Erwan Verron for the fructuous discussions, M Yvon Tirel for the mould design, M Eric Josso for the FDM printed rolls, Mathieu Miroir, Eric Robin, M Vincent Burgaud and M Mickael Le Fur for the design of the biaxial tensile machine.

Funding: This research was funded by the National Center for Scientific Research (MRCT-CNRS and MI-CNRS), Rennes Metropole, and Région Bretagne.

Data Availability Statement: Data available on request due to restrictions eg privacy or ethical.

Conflicts of Interest:

The authors declare no conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript: