Thihara Flavio

Eric Rodrigues

Sonia Marchioni

Florent Lordel-Madeleine

Anna Lucia Kuntz

Haasis Casañas

Diane Villavicencio

Julien-David

Flavio Thihara Rodrigues

Eric Marchioni

Sonia Lordel-Madeleine

Florent Kuntz

Anna Lucia Casañas

Haasis Villavicencio

Diane Julien-David
email: diane.julien-david@unistra.fr

Degradation of profenofos in aqueous solution and in vegetable sample by electron beam radiation

Keywords: Profenofos, electrons beam radiation, GC-MS/MS, decontamination HIGHLIGHTS

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

INTRODUCTION

Pesticides belong to a class of chemical compounds intended for preventing, destroying, repelling or mitigating any pest or used to control organisms that are considered a potential to damage crops. Organophosphate insecticides have been widely used by farmers to control fungal diseases and insect pests of vegetables, fruits, ornamental plants, turf, and other agricultural crops. An example of this pesticide class is profenofos.

Profenofos (O-(4-bromo-2-chlorophenyl phosphorothioate) O-ethyl S-

propyl) is a pesticide that remains in the environment and is widely used in Brazil, unlike in Europe where it is banned (EU pesticides database). According to the World Health Organization (WHO), profenofos has been classified as a moderately hazardous pesticide. In Brazil this pesticide is used for the cultivation of onions, corn, soybeans, coffee, tomato, cotton, beans, potatoes, peas and others. In peas, the maximum residue level (MRL) is 0.1 µg/g, established by the Ministry of Agriculture, Livestock and Food Supply (MAPA).

Several methods can be used for removing pollutants from contaminated environmental components [START_REF] Basfar | De-contamination of pesticide residues in food by ionizing radiation[END_REF]. In the field of chemical water treatment, several oxidative degradation processes, called advanced oxidation processes (AOPs), have been developed [START_REF] Picó | Evaluation of pesticide residue in grape juices and the effect of natural antioxidants on their degradation rate[END_REF]. These include UV photolysis, photocatalysis (hydrogen peroxide and ozone), Fenton reagent and water radiolysis ( (Angthararuk et al., 2017;[START_REF] Burrows | Reaction pathways and mechanisms of photodegradation of pesticides[END_REF][START_REF] Zamy | Phototransformation of selected organophosphorus pesticides in dilute aqueous solutions[END_REF]. Radiation is one of the most powerful AOPs, where the use of accelerated electrons or gamma radiation decomposes pollutants such as pesticide residues [START_REF] Basfar | De-contamination of pesticide residues in food by ionizing radiation[END_REF][START_REF] Hossain | Degradation of chlorpyrifos, an organophosphorus insecticide in aqueous solution with gamma irradiation and natural sunlight[END_REF][START_REF] Mohamed | Radiolytic degradation of malathion and lindane in aqueous solutions[END_REF]. Radiation is a process also used in the food industry. This consists of exposing food to ionizing radiation, such as high energy (E<10 MeV) electrons, X-rays produced by electron accelerators (E<5 MeV) or gamma rays (1.17 and 1.33 MeV) emitted from the radioisotope 60 Co. Depending on the absorbed dose, various effects can be achieved resulting in reduced storage losses, extended shelf life, and/or improved microbiological and parasitological safety of foods [START_REF] Farkas | Irradiation for better foods[END_REF]. The World Health Organization (WHO) has approved the irradiation of a variety of foods with doses greater than the level of 10.0 kGy recommended by the Codex Alimentarius Commission.

Most research studies working on the decomposition of pesticides by irradiation have used gamma rays [START_REF] Trojanowicz | Chapter 9 -Gamma-ray, X-ray and Electron Beam Based Processes[END_REF]. This choice could be explained by the fact that this technique is easier to use than other techniques such as electron beam radiation. Indeed, due to the high penetrative power of gamma rays, this technique does not require attention to the geometry of the sample to be irradiated. However, a major disadvantage is that it is much more polluting because it requires a radioactive source. Another technique, which is the most widely used for technical applications, is electron beam irradiation [START_REF] Trojanowicz | Chapter 9 -Gamma-ray, X-ray and Electron Beam Based Processes[END_REF]. One essential disadvantage is the limited penetration depth of the e-beam in the irradiated samples, which therefore requires a higher level of technical competency to use. Nevertheless, its main advantage is a much higher dose rate and instant on/off operation [START_REF] Trojanowicz | Chapter 9 -Gamma-ray, X-ray and Electron Beam Based Processes[END_REF]. Thus, in this study, the effect of accelerated electron beam radiation on the removal of pesticides was evaluated, and more particularly the degradation of profenofos in aqueous solution and in food samples (peas) was quantified. In addition, degradation products of profenofos formed in aqueous solution were also studied and detected by gas chromatography coupled with mass spectrometry (GC-MS).

Experimental procedure

Chemicals

Profenofos (C11H15BrClO3PS, 96.9%), atrazine (C8H14ClN5, 98.8%), anhydrous magnesium sulfate (MgSO4, ≥97%), acetonitrile (for pesticide residue analysis), ethyl acetate (for pesticide residue analysis), toluene (for pesticide residue analysis), carbon/NH2 dual layer SPE tube, QuEChERS salt kit containing 6 g of magnesium sulfate (MgSO4) and 1.5 g of sodium acetate (CH3COONa) and ethanol absolute (≥99.8%) were purchased from Sigma-Aldrich (Steinheim, Germany). The chemical structure of profenofos is as follows in Figure 1. The experiments were performed with water purified with a Synergy Milli-Q System (Millipore, Molsheim, France).

Pea samples were collected at random from a local supermarket in Strasbourg (France) and stored in refrigeration (4 ºC).

Sample preparation

Standard stock solutions of 1 mg/mL profenofos were prepared in acetonitrile.

Aqueous solution

The aqueous solution of profenofos was prepared as follows: 84 µL of a 1.0 mg/mL solution of profenofos were diluted in 3 mL of purified water to a final concentration of 28 µg/mL (corresponding to its limit of solubility). All samples were placed in 20 mL glass vials previously heated in an oven at 400 ºC for 8 hours, closed by teflon-based caps protected with aluminum foil. Each sample was assayed in triplicate.

Vegetable sample: pea

For the pea, a total of 10 µL of solution of 100 µg/mL of profenofos in ethanol were added to 10 g of pea. All samples were weighed, packed in polyamide/polyethylene plastic sachets, sealed and identified with their respective radiation doses. Each sample was assayed in triplicate.

Irradiation Source

Profenofos aqueous solutions and peas were irradiated with a Rhodotron E-beam accelerator (IBA, Louvain la Neuve, Belgique) facility installed at BGS GmbH & Co. KG, located in Bruchsal, Germany. The energy of the electron beam was 10 MeV with a 15 mA current and 160 cm scan width. Irradiation treatments were conducted at room temperature (25ºC) and atmospheric pressure (1 atm). For the aqueous solution of profenofos, the electron beam passed through the diameter of the tube (one millimeter of glass, one centimeter of water and one millimeter of glass).

The applied doses were 0 (control), 4.6, 11.7 and 31.6 kGy for the aqueous solution, and 0 (control), 4.5, 12.2 and 30.4 kGy for the peas. In both experiments, absorbed doses were monitored with FWT 60.00 radiachromic dosimeters (Far West Technology, Goleta, CA), previously calibrated with an alanine dosimeter (Aérial, Illkirch, France) [START_REF] Kuntz | Alanine-ESR in vivo dosimetry: a feasibility study and possible applications[END_REF]. Dosimetric readings confirmed that dose uniformity was about 10 % within the sample.

For the peas, the irradiated and control samples were frozen after electron-beam irradiation.

Extraction procedure

For aqueous solution of profenofos

After irradiation, the aqueous sample (3 mL) was transferred to a test tube to carry out the profenofos extraction process. In each tube, 84 µL of atrazine solution (1 mg/mL in acetonitrile) was added as an internal standard before extraction. Profenofos was then extracted with ethyl acetate (3 x 3 mL). To analyse the radiolytic breakdown of profenofos products, five irradiated test tubes were pooled into one and submitted to the same extraction procedure (3 x 3 mL) with ethyl acetate. The combined three organic extracts were evaporated to dryness under a gentle nitrogen flow.

To each dry extract 1 mL of ethyl acetate was added and carefully transferred using a pipette to a 2 mL injector vial and one microliter of each sample was injected into the GC-MS.

Vegetable sample: pea

After irradiation, 20 µL of internal standard solution (atrazine at 100 µg/mL in ethanol) was added to the pea samples. 10 g of peas were added to the 50 mL polycarbonate crushing vials with a stainless steel impinge. The pea samples were finely mixed in liquid nitrogen (Air Liquide, Paris, France) for 9 min with a cryogenic ground (6870 Freezer/Mill, Spex CertiPrep, Stanmore, UK) at a rate of 20 impacts/sec.

The QuEChERS procedure was employed in respect of each sample. Figure 2 illustrates the scheme of the entire analytical protocol used during the present study. Weigh 10 g of homogenized sample irradiated pea

GC-MS Analysis

GC-MS analysis was performed on a Varian STAR 3400 chromatographic system with an on-column SPI injector hyphenated with a

Varian SATURN 2000 mass sensitive detector (Varian, France). The instrumental control, data acquisition and processing were performed using Varian Saturn WS software 5.11.

Samples were separated with a VF-5ms capillary column (60 m, 0.25 mm internal diameter, Varian, France). The column temperature gradient was programmed from 80°C (held for 2 min) to 280°C at 5°C/min (held for 1 min).

The SPI injector operating conditions were as follows: injection volume 1 µL; initial injector temperature of 105ºC was increased to 280ºC at 100ºC/min held for 40 min. The carrier gas (helium, purity 99.9995%) flow was 1 mL/min.

Electronic impact (EI) ionization mode mass spectra were obtained at 70 eV. Quantitative data analysis was carried out using Selected Ion Monitoring (SIM) analysis of profenofos aqueous sample and using Multiple Reaction Monitoring (MRM) for profenofos contaminated pea samples.

Peak areas were normalized using atrazine as an internal standard. The analytes were quantified as follows (Equation 1): nC = (nis.Ac)/(RRFc.Ais)

Where: Ac and Ais are the areas of the chromatographic peak of profenofos and of atrazine, respectively; RRFc is the response factor of profenofos. The response factor of atrazine, by definition, is equal to 1. The RRFc was calculated using equation 2:

RRFC = (nis.Ac)/(nc.Ais) (2)
Where: nc and nis are the moles of profenofos and of atrazine, respectively present in the mixture injected in the GC-MS. Relative Response Factor (RRF) is an analytical parameter used in chromatographic procedures as a measure of the relative response of the detector to profenofos as compared to atrazine (internal standard). The RRFc was calculated within different concentrations of atrazine and profenofos.

Peak areas were detected by MS operating in SIM mode for the aqueous solution and by MS-MS operating in MRM mode for the pea sample. The experiments were performed in triplicate.

Linearity and limit of detection

Limit of detection (LoD) was defined as the lowest analyte mass yielding a signal-to-noise (S/N) ratio of 3. The limit of quantification (LoQ) was defined as the lowest analyte mass yielding a signal-to-noise (S/N) ratio of 10. LoD and LoQ were obtained by successively injecting each pure compound solution in ethyl acetate with decreasing concentrations of analyte. Linearity was tested by triplicate analyses of profenofos at different concentrations ranging from 0.5 to 90 µg/mL in SIM mode and 0.002 to 0.5 µg/mL in MRM mode.

Linear regression analyses of the standard solutions were performed at four different concentrations levels, in triplicate. Linear plots of peak areas versus concentration were calculated.

Profenofos G-value

Radiation chemical yield of pesticide degradation (defined by the number of molecules formed or destroyed in solution absorbing 100 eV of radiation energy) was calculated at each absorbed dose using Equation 3 given by [START_REF] Kurucz | High Energy Electron Beam Irradiation of Water, Wastewater and Sludge[END_REF]:

G = ([R]. Na)/(D.(6.24 x10 16 )) (3)
Where: [R] is the change in profenofos concentration in mol.L -1 ; D is the dose absorbed in Gy; NA is Avogadro's number (6.02 x 10 23 ); 6.24 x 10 16 is the conversion factor from Gy to 100 eV.L -1 . To calculate the G-value in the SI unit (µmol/J), the value is multiplied by 0.1036.

Degradation of profenofos in aqueous solution

Electron beam irradiation is a process which involves accelerated electrons in a continuous exposure mode being able to penetrate different products to just beyond a few centimetres, depending on the thickness of the product, the energy of the e-beam and the electronic density of the processed material.

Degradation efficiency of profenofos is presented in Table 2, and Figure 3 indicates the relationship between the absorbed dose and profenofos concentration. The results indicate that degradation of profenofos fits with an exponential first order law. Removal percentages of 72.6 %, 96 % and 99.8 % can be achieved with absorbed doses of 4.6, 11.7 and 31.6 kGy respectively when irradiation is performed on aqueous solutions.

Results show that the degradation by E-beam of organophosphate compounds, like profenofos, in aqueous solution is possible. In fact, exposure of aqueous solutions to ionizing radiation causes radiolysis of water and as a result, the formation of primary radiolytic products (equation 6). These radical and molecular products are homogeneously distributed throughout the solution.

The simplified reaction scheme of water radiolysis could be summarized by: H2O HO•, e -aq, H•, H3O + , H2O2, H2, HO -(Le Caër, 2011). ( 6)

During this step, the radiolysis products can react with each other or with the molecules of solutes present in the solution. In this study, these species were leading to complete degradation of the investigated molecule.

The radiation chemical yield (G-value) and the dose constant (K) are two parameters used to quantitatively evaluate the elimination efficiency of profenofos after irradiation. From this value, D0.5 and D0.9 values were also calculated (table 2). The result showed that the G-value decreased with the augmentation of the dose. The radiation chemical yield trends reported here are similar to those reported for gamma ray irradiation of organophosphate and organochlored pesticides [START_REF] Mohamed | Radiolytic degradation of malathion and lindane in aqueous solutions[END_REF][START_REF] Ismail | Advanced oxidation for the treatment of chlorpyrifos in aqueous solution[END_REF]. The decrease of G-value could be explained by the formation of by-products and their competition with different reactive species [START_REF] Ismail | Advanced oxidation for the treatment of chlorpyrifos in aqueous solution[END_REF]. The D0.5 and D0.9 calculated values were equal to 3.57 kGy and 11.86 kGy respectively,

showing the influence of the absorbed dose on the degradation rate of profenofos. These values were notably higher than those observed in the degradation of an aqueous solution of organophosphate pesticides such as diazinon [START_REF] Basfar | Degradation of diazinon contaminated waters by ionizing radiation[END_REF] or malathion [START_REF] Mohamed | Radiolytic degradation of malathion and lindane in aqueous solutions[END_REF] after gamma ray treatment, but remain below those observed for an aqueous solution of an organochloride pesticide such as lindane [START_REF] Mohamed | Radiolytic degradation of malathion and lindane in aqueous solutions[END_REF] which reflects the role of pesticide chemical structure in degradation. The profenofos standards were not soluble in water and necessitated the use of ACN as a cosolvent. As the quantity of ACN and profenofos were the same, it can be theorised that ACN may act as a scavenger of the radialoytic compounds generated in water during radiation processing. The results presented in this work should be considered together with this scavenging capacity of ACN, decreasing the radiosensitivity of profenofos. Nevertheless, ACN is mandatory as profenofos is poorly soluble in water.

A radiolytic breakdown product was detected during this study. For this purpose, five irradiated samples in test tubes were pooled into one test tube, extracted, concentrated to dryness and finally dissolved in 1 mL of ethyl acetate. These steps were undertaken in order to increase the detection sensitivity of very low concentrations of profenofos radiolytic breakdown products, and then to facilitate their spectral identification. Figure 4 illustrates the chromatographic profiles obtained during the analysis of an 11.7 kGy irradiated solution of profenofos. One compound was detected and clearly separated from the peak of profenofos and atrazine (IS), indicating that at least one radiolytic breakdown product is formed. This product was detected only at absorbed doses of 4.6 and 11.7 kGy. This compound could not be identified by the NIST or other online libraries. Based mass spectrum a molecular mass of m/z = 259 amu (figure 4) was attributed to it, whose precursor is profenofos (374 amu) that has lost a bromide and a chloride [M-Br-Cl] + , the empirical formula of which can be presumed to be C11H15O3PS. This compound is formed after the cleavage of carbon-bromine and carbon-chlorine bonds on the benzene ring. The absence of characteristic peaks of bromine and chlorine atoms in the mass spectrum confirms the loss of these elements. To elucidate the structure of this compound, an analysis by RMN will be necessary. But due the low sensitivity of this technique it will be difficult to have the amount necessary knowing that it is formed in very few quantities. In fact to detect this compound by GC-MS, it was necessary to pool 5 tubes of irradiated aqueous solution of profenofos. This breakdown compound could not be quantified due to the lack of a standard, so the ratio of the peak area of this product to the peak area of the internal standard (atrazine) was used in order to evaluate the formation of this molecule (figure 5). This ratio is higher in the irradiated sample at 4.6 kGy, and the compound seems to be degraded by the electron beam, since its quantity decreases with higher doses. Some studies have investigated the photochemical transformation of profenofos in water using monochromatic light [START_REF] Angthararuk | Degradation products of profenofos as identified by high-field FTICR mass spectrometry: Isotopic fine structure approach[END_REF][START_REF] Zamy | Phototransformation of selected organophosphorus pesticides in dilute aqueous solutions[END_REF]. Several products were identified, but not the product of this study.

Degradation of profenofos in pea sample

QuEChERS method

The role of ionizing radiation in removal of pesticide residue in contaminated vegetables (in this study in peas) was investigated. Since the concentration of pesticides present in vegetable is extremely low, the extraction step, concentration and reduction or elimination of interference is a critical procedure.

Different techniques of sample preparation for pesticide analysis can be used, such as molecular imprinting technology [START_REF] Sanagi | Molecularly imprinted polymer solid-phase extraction for the analysis of organophosphorus pesticides in fruit samples[END_REF], solidphase extraction/microwave extraction [START_REF] Fang | Systematic optimisation of coupled microwave-assisted extraction-solid phase extraction for the determination of pesticides in infant milk formula via LC-MS/MS[END_REF], supercritical fluid extraction [START_REF] Rissato | Multiresidue Analysis of Pesticides in Soil by Supercritical Fluid Extraction/Gas Chromatography with Electron-Capture Detection and Confirmation by Gas Chromatography-Mass Spectrometry[END_REF] and many others. However, these methods are often time consuming and labour intensive. Compared to these techniques, QuEChERS is an alternative extraction method which enables covering a very wide analyte scope, including highly polar pesticides as well as highly acidic and basic ones, uses fewer organic solvents, produces lower detection limits and presents a simple operation protocol. Therefore, this technique was used in this study. According to information from the manufacturer, the carbon/NH2 cartridges have spherical, non-friable particles that improve flow characteristics and reduce susceptibility to the formation of interference. The particles with a carbon layer have the capacity to remove pigments and sterols, commonly present in many foods, and those with aminopropyl (NH2) layer removed organic acids, polar pigments and sugars.

Chromatographic determination using GC-MS/MS

Quantitation of profenofos degraded by e-beam in pea was calculated using equation 1. Removal efficiency of profenofos is illustrated in Table 3,

showing the influence of absorbed dose on the rate of degradation of profenofos in peas compared to the aqueous solution of profenofos. It was observed that profenofos samples were influenced by E-beam, and the highest degradation efficiency was achieved at 30.4 kGy absorbed dose with a removal percentage of around 47.9 %. However, compared to irradiated aqueous solution of profenofos, the percentage of removal in vegetable was much lower. In fact, for the same absorbed dose, the degradation of profenofos was twice as high when the pesticide is in water was when it was in the pea. This could be explained by the difference in concentration. Indeed, the concentration of profenofos in the experiment in aqueous solution was 28 µg/mL, whereas in the pea it was 1 µg/mL (corresponding to the MRL (0.1 µg/g) of profenofos in peas established at the MAPA in Brazil). Nevertheless, this difference can also be explained by the fact that plants are of more complex composition, in which a large number of organic substances can compete with pesticide residues for the various reactions produced with reactive species generated in peas after radiation.

These same observations were also noted in the study of [START_REF] Basfar | De-contamination of pesticide residues in food by ionizing radiation[END_REF] which decontamination of pesticide residues in an aqueous solution was more efficient than in contaminated fruits and vegetables after irradiation with gamma radiation.

CONCLUSIONS

This study has demonstrated that profenofos contained in aqueous solution or in vegetable could be effectively degraded and removed by E-beam irradiation. Increasing absorbed doses promoted a decrease in profenofos concentration. The removal of pesticide from an aqueous solution compared with vegetable was greater in irradiated aqueous solution than in vegetable.

Based on GC-MS analysis, one degradation product of profenofos in aqueous solution was detected but not quantified because of the lack of an authentic chromatographic reference. The QuEChERS method provided a fast and simple approach for extracting and analysing profenofos in pea samples.

The GC-MS/MS analysis allowed the analysis of trace of profenofos contained in vegetable.

Figure 1 .

 1 Figure 1. Molecular formula of profenofos

Figure 2 -

 2 Figure 2 -Schematic description of QuEChERS and cleanup procedures for irradiated peas

Figure 3 :

 3 Figure 3: Degradation of profenofos aqueous solution as a function of absorbed dose.

  Figure 4. GC-MS Chromatogram of profenofos solution irradiated at 11.7 kGy atrazine (peak 1), breakdown product (peak 2), profenofos (peak 3) and the mass spectrum of to the profenofos breakdown product (m/z 259.1).

  involves an initial extraction with acetonitrile used for sample cleanup. The addition of a sorbent (CH3COONa + MgSO4) improves the purification step, removes the water content in the extract of pea and consequently enhances the partition of the above-mentioned matrix interfering compounds. Pea sample extracts were percolated in an SPE tube to offer superior cleanup when conducting multi-residue pesticide analysis.

Table 2 :

 2 Degradation efficiencies and dose constants of profenofos in aqueous solution

	Absorbed doses (kGy)	Degradation (%)	G-value (µmol.J -1 )	Parameter
				K	D0.5	D0.9
	4.6	72.6	1.1 x 10 -2	0.19	3.57	11.86
	11.7	96.0	5.4 x 10 -3
	31.6	99.8	2.1 x 10 -3

Table 3 .

 3 Removal efficiency (%) of profenofos presents in peas and in aqueous solution after irradiation by E-beans at different doses.

	Estimated dose (kGy)	0	4.5	12	31
	Removal (%) in vegetable	0	38.2	41.8	47.9
	Removal (%) in aqueous solution	0	72.6	96.0	99.8

200 (quantification)

(identification) 1 Calibration curves were obtained by peak areas (y) vs. concentration (x)

ACKNOWLEDGEMENTS

The authors are grateful to CAPES and CNPq for their financial support.

Dose constant (K)

The dose constant, K, was obtained by plotting the logarithm (ln) of the profenofos concentration versus the absorbed dose (Gy). The absorbed dose required for degradation of 50% (D0.5) and 90 % (D0.9) were calculated by using the following formulas (Eq. 4, 5): D0.5 = ln 2/K D0.9= ln 10/K (4), ( 5)

RESULTS AND DISCUSSION

Quantification of profenofos

Chromatographic separation of atrazine, profenofos and its degradation product in different samples were well achieved on a VF-5ms capillary column and identified by their MS data. In the case of an aqueous solution of profenofos, quantification was performed using SIM mode. For the sample of contaminated peas, the use of selected tandem mass spectrometry (MS/MS) was used due to the very low amount of profenofos. Quantification of profenofos was done by using an internal standard, not isotope-labeled IS but another pesticide for economic purpose.

Mass spectra of profenofos gives characteristic isotopic clusters due to chlorine and bromine atoms with 35 Cl, 37 Cl and 79 Br, 81 Br significant natural isotopes and several characteristic ions produced due to the presence of the phosphorothioate group containing chlorine and bromine substituted on benzene ring. Most of the ions containing the aromatic ring have a significant intensity due to the stability of the aromatic system [START_REF] Hong | electron impact fragmentation of chlorinated organophosphorus pesticides[END_REF]. All data, retention time, RRF, the selected ions chosen for identification and quantitation along with their relative abundances are summarized in Table 1. In this table the linearity and limit of detection and quantification for profenofos in the analytical conditions (SIM mode and in MRM mode) described in the present study are also summarized.

The calibration of peak areas versus concentrations generated a linear function (R 2 = 0.998 in SIM mode, R 2 = 0.9958 in MRM mode).The LoD and LoQ were estimated to be 0.2 µg/mL and 0.5 µg/mL respectively in SIM mode and 0.02 μg/ml and 0.05 μg/ml in MRM mode. In this last case, the LoQ was lower than the maximum residue limit (MRL) for profenofos in peas established by the MAPA.


Maazouz

M Dkier

M Yousfi

K Lamnawar
email: khalid.lamnawar@insa-lyon.fr

A Maazouz

Chemo-rheological studies and monitoring of high-Tg reactive polyphtalamides towards a fast innovative RTM

Keywords: Hybrid TP-RTM, rheokinetics, composites, in-situ monitoring

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

Introduction

Nowadays, composites based on high-performance thermoplastic (TP) matrices have become particularly attractive in many industries, e.g., the aerospace and automotive sectors [START_REF] Henning | Fast processing and continuous simulation of automotive structural composite components[END_REF]. Because of their good mechanical resistance (intrinsic ductility and tenacity), their low environmental impact (ecofriendly production method, recycling), their potential to reduce the production cycle (thus a greater production capacity) and their welding properties, they strongly compete with composites based on a thermosetting matrix [START_REF] Bourban | Integrated processing of thermoplastic composites[END_REF][START_REF] Vaidya | Processing of fibre reinforced thermoplastic composites[END_REF]. However, the excessively high viscosity of the TP in the molten state makes it extremely difficult to produce continuous fiber-reinforced composites by injection at pressure lower than 15 bar and especially when the fiber content is high (≥30 vol%), as required for structure parts applications [START_REF] Faraj | Crystallization kinetics of new low viscosity polyamides 66 for thermoplastic composites processing[END_REF]. An alternative approach consists in using the TP matrices from monomers or prepolymers in a reactive processing route conventionally used for thermosetting matrices [START_REF] Cassagnau | Reactive processing of thermoplastic polymers: A review of the fundamental aspects[END_REF]. Among the methods available for the manufacturing of these materials, RTM (Resin Transfer Molding) makes it possible to obtain composite parts by injecting a lowviscosity resin onto pre-formed continuous fiber reinforcements in a mold. This process is undergoing rapid development because of its "closed mold" technology as well as the wide range of parts with complex shapes that become available. During the last decade, the idea of adapting the RTM process to make TP-based composites has awoken interest among scientists [START_REF] Hohne | Hexaphenoxycyclotriphosphazene as FR for CFR anionic PA6 via T-RTM: a study of mechanical and thermal properties[END_REF]. However, the choice of the thermoplastic matrix depends on the intended application. At present, the development of this new process has begun with specific polymers for which the chemistry is much more controlled. The constraints related to the manufacturing conditions, the polymerization and the concomitant crystallization therefore make it possible to consider only a relatively limited number of TP matrices. Despite the interesting literature dedicated to the composite field, it is a daunting task for both scientific and industrial communities to adapt RTM process to these new generation of TP fiber-reinforced composites. Indeed, many routes have been investigated involving bi or monoreactive component to form a TP matrix up to the impregnation. On one hand, structural reaction injection molding (SRIM) uses high pressure (8-10 bar) to impregnate a dry fiber preform, which is placed between two solid mold halves. The process is closely related to the classical RTM of thermoset composites. In order to obtain a good fiber impregnation, 1 Pa.s is generally regarded as the maximum viscosity limit of the reactive system. Upon completion of polymerization, the composite part can be demolded. On the other hand, vacuum infusion (VI) is also employed to compact and impregnate a dry and continuous fiber preform, which is placed between a solid and a flexible mold half. After mixing the contents of the two material tanks, the reactive system (viscosity: less than 1 Pa.s) is dispensed into a buffer vessel, which is required to separate the pressure required for dispensing and the vacuum necessary to promote infusion. The required time is generally longer than SRIM technology. Commingled yarns are one of the possible preforms used for continuous fiber reinforced thermoplastic composites in order to solve the problem of high melt viscosity during impregnation and consolidation steps. The tradename TWINTEX® is one commercial reference and it's a unidirectional thermoplastic/glass fiber composites prepared from an online combination of commingled E-glass and polypropylene filaments [START_REF] Wiegand | Commingled Yarn Spinning for Thermoplastic/Glass Fiber Composites[END_REF]. Due to the high melt viscosity of the thermoplastic polymer matrix, commingled yarn spinning process is considered one of the most promising impregnation techniques since a homogeneous distribution of continuous matrix filaments and reinforcement glass filaments took place during melt spinning. However, this technique should be improved for high Tg engineering polymers. Furthermore, the literature includes reports relating to the physicochemical study of TP formulations based mainly on c-PBT [START_REF] Archer | An investigation into thermoplastic matrix 3D woven carbon fibre composites[END_REF], PA6 [START_REF] Kim | Ultra-high-speed processing of nanomaterial-reinforced woven carbon fiber/polyamide 6 composites using reactive thermoplastic resin transfer molding[END_REF], PA12 [START_REF] Luisier | Initiation mechanisms of an anionic ring-opening polymerization of lactam-12[END_REF] and PMMA [START_REF] Gerard | Composite material via in-situ polymerization of thermoplastic (meth)acrylic resins and its use, ARKEMA[END_REF] for RTM applications or other processes such as pultrusion [START_REF] Ning | RIM-pultrusion of nylon-6 and rubber-toughened nylon-6 composites[END_REF], infusion [START_REF] Bhudolia | Optimizing Polymer Infusion Process for Thin Ply Textile Composites with Novel Matrix System[END_REF], RIM [START_REF] Williams | Resin infusion under flexible tooling (RIFT): A review[END_REF] and LCM [START_REF] Pillay | Liquid molding of carbon fabric-reinforced nylon matrix composite laminates[END_REF]. Pultrusion [START_REF] Luisier | Reaction injection pultrusion of PA12 composites: process and modelling[END_REF] and SRIM like processes [START_REF] Mairtin | Process investigation of a liquid PA-12/carbon fibre moulding system[END_REF][START_REF] Zingraff | Resin transfer moulding of anionically polymerised polyamide 12[END_REF][START_REF] Rosso | A novel polyamide 12 polymerization system and its use for a LCM-process to produce CFRP[END_REF] for TP based composites have intensively been developed in the past decade. The authors generally emphasize the interest of adapting the nature of the monomers/prepolymers (which influences the kinetics of polymerization) to the size of the parts to be manufactured (for thick parts one should favor a slower reaction to obtain good impregnation). Glass and/or carbon fiber-reinforced composite parts have been characterized by numerous authors in terms of fiber content, crystallinity, melting point of the matrix and for two different thermal cycles [START_REF] Williams | Resin infusion under flexible tooling (RIFT): A review[END_REF]. In particular, Mairtin et al. [START_REF] Mairtin | Process investigation of a liquid PA-12/carbon fibre moulding system[END_REF] found some sensitivity of PA12-based systems to humidity and the importance of having a good fiber-matrix interface and a better control of its impregnation. Luisier et al. [START_REF] Luisier | Time-temperature-transformation diagram for reactive processing of polyamide 12[END_REF] plotted a TTT (Time -Temperature -Transformation) diagram of PA12 in which a processing window was proposed. Zingraff et al. [START_REF] Zingraff | Resin transfer moulding of anionically polymerised polyamide 12[END_REF] studied the problem of forming voids in the composite (in and between wicks) in order to optimize the impregnation in the RTM process. Hakme et al. [START_REF] Hakme | In situ monitoring of cyclic butylene terephtalate polymerization by dielectric sensing[END_REF] reported on a competition between in-situ reaction of butylene terephthalate (CBT) oligomers and crystallization of the obtained c-PBT. The literature shows that the physicochemistry of some of these TP polymers has been widely studied. More specifically, Ageyeva et al. [START_REF] Ageyeva | Polymers and Related Composites via Anionic Ring-Opening Polymerization of Lactams: Recent Developments and Future Trends[END_REF] has recently summarized in a review article the literature data devoting to the use of TTT diagrams in the case of TP-RTM processing. Such diagrams have been developed for the PA12 reactive system [START_REF] Luisier | Time-temperature-transformation diagram for reactive processing of polyamide 12[END_REF] and for the PA6 reactive system [START_REF] Maazouz | Chemorheological study and in-situ monitoring of PA6 anionic-ring polymerization for RTM processing control[END_REF]. Despite the interesting kind of research dedicated to this subject, there are few papers which the subject is on the topic of the present work involving on high Tg polyamide based composites for fast RTM process. It is always challenging to adapt the usual technologies to intermediate viscous reactive monomers/prepolymers (close to 1 Pa.s) and especially with fast reactive systems. Indeed, the published reports in the last decade deal with systems that have a relatively known and simple chemistry and a service temperature in the range of 50 to 90 °C. Therefore, this class of materials cannot be used in most conventional automotive process chains or for other high performance applications. For example, the composite parts need to resist to the high temperature imposed during the cataphoresis treatment in which a maximum temperature of about 190 °C could be encountered [START_REF] Gratzl | Influence of the automotive paint shop on mechanical properties of continuous fibre-reinforced thermoplastics[END_REF]. Among the promising engineering polymers in these applications, we take note of the emergence of polyphtalamides (hereafter referred to as PPAs) which are considered as highperformance thermoplastic matrices with excellent mechanical and thermal properties. Compared to aliphatic and other classical polyamides, PPAs are typically distinguished by their higher melting points, higher glass transition temperatures and greater dimensional stability [START_REF] Novitsky | Eutectic melting behavior of polyamide 10,T-co-6,T and 12,T-co-6,T copolyterephthalamides[END_REF]. ARKEMA has developed and patented [START_REF] Capelot | Reactive compositions made from thermoplastic amino prepolymer and unsaturated extender for thermoplastic composite materials[END_REF][START_REF] Hochstetter | Process for manufacturing a thermoplastic composite part in a closed mould, with injection into a cold mould[END_REF] a number of polyphtalamide compositions making it possible to address the development of TP-RTM technology with high-performance polyphtalamides. The principle is to inject the prepolymer of low viscosity (~2.5 Pa.s) mixed with some reactive co-agent (~0.5 Pa.s) into a mold containing the reinforcement (glass or carbon). The temperature is then increased a few tens of degrees above the prepolymer melting temperature. Currently, the monitoring and modeling of a chemical reaction during this processing is a big challenge for scientists and industrial communities [START_REF] Konstantopoulos | Monitoring the production of FRP composites: A review of in-line sensing methods[END_REF]. Therefore, on-line process control with in-mold sensors is often used in industrial manufacturing of RTM parts. In-line systems such as dielectric sensors [START_REF] Hakme | In situ monitoring of cyclic butylene terephtalate polymerization by dielectric sensing[END_REF], ultrasound transducers [START_REF] Schmachtenberg | Application of ultrasonics for the process control of Resin Transfer Moulding (RTM)[END_REF] and point-voltage sensors [START_REF] Danisman | Monitoring of resin flow in the resin transfer molding (RTM) process using point-voltage sensors[END_REF] can be employed for this purpose. The advancement of a polymerization results in an increase in viscosity so the viscosity of the initial system must not exceed 1 Pa.s to ensure a good filling of the mold in which a porous and continuous reinforcement has been placed. The chemical reaction in the mold is accompanied by a viscosity increase of up to 1000 Pa.s in few minutes. Thus, the challenge lies in the need to characterize this rheological evolution during the first seconds to control and optimize the time required to inject the reactive system into the mold and thus also the impregnation of the reinforcement. Hence, the use of non-destructive and non-invasive methods such as ultrasound or dielectric spectroscopy should be a suitable route to attain this objective. The second scientific constraint that needs to be overcome relates to the adaptation of the devices to the high temperatures which these chemical reactions take place (molding temperatures ranging from 270 to 300 °C). These high processing temperatures implies to have a better choice of the fiber sizing and the interface/interphase properties with the formed matrix. The optimization and modeling of a manufacturing process require the design of an instrumented mold to monitor, in real time and in situ, the progress of the reaction and the resultant phase transformations. Indeed, the structural evolution of the reactive system is influenced by process parameters such as temperature, time and impregnation flow rate. The present work proposes the use of a so-called "chain-extender", which can react with the polyphtalamide prepolymer in the melt at a higher temperature. Consequently, this chain extender can potentially increase the molar masses of PPA within a short time. Firstly, the fast track of the viscosity increase and conversion will be also investigated by ex-situ fast scan FTIR/SAOS measurements. Secondly and to achieve the process control, in-situ monitoring by dielectric sensors will be performed [START_REF] Lin | A Kinetic study of the activated anionic polymerization of ε-caprolactam[END_REF][START_REF] Maazouz | Polyurethane and unsaturated polyester hybrid networks: Chemorheological and dielectric study for the resin transfer molding process (RTM)[END_REF]. The dielectric response will be combined with reference models. Then, the fast in-situ monitoring can then be compared to ex-situ measurements. The challenge is to convert the conductivity (from dielectric sensors) to the measured (even modelled) ex-situ resin viscosity (η) of the system towards monitoring of the reaction's advancement and the structure evolution of the matrix within the fabric. However, the difficulty lies in how to monitor the fast chain extension reaction inside the cavity at short times of polymerization (less than two minutes) in the presence of a high-temperature polymer such as PPA. Indeed, the difference in the melting temperature between the two components of the reaction (PPA prepolymer and the chain extender) leads to the choice of a confined geometry to ensure a good reliability of the monitoring results. The relevance of this approach is that data of the most important process milestones can be reliably generated from the dielectric sensors in real-time throughout the fabrication process. This data includes the injectability limit for the impregnation of the reinforcement and the maximum conversion rate reached by the system. In this study, titration analyses and chemo-rheology coupled to FTIR spectroscopy were also carried out for calibration. The obtained results will be discussed also in term of physico-chemical, morphological and mechanical properties of studied PPA matrix and its corresponding composites.

Experimental section 2.1 Materials

The reaction medium consisted of 2 components: a semi-crystalline prepolymer kindly supplied by Arkema (France). This thermoplastic prepolymer [START_REF] Hochstetter | Process for manufacturing a thermoplastic composite part in a closed mould, with injection into a cold mould[END_REF] is a polyphtalamide (PPA) type PA 10T/11/6T with di-acid chain ends (a content of COOH terminal groups of 0.9 meq/g), obtained by polycondensation at 250°C of 1,6 hexanediamine, 1,10 decanediamine and terephthalic acid in the presence of undecanoic acid. Its chemical formula is shown in Figure 1. [START_REF] Hochstetter | Process for manufacturing a thermoplastic composite part in a closed mould, with injection into a cold mould[END_REF].

The chain extender agent used in this study was 1, 3 phenylene bis-2oxazoline (PBO), denoted "CA" throughout the paper, and was supplied from Evonik (Germany). The essential properties and characteristics of the materials used in this study are listed in Table 1. The prepolymer and chain extender (in solid form) were accurately dried under vacuum for 12h at 80°C and 50°C respectively.

It's important to remind than the main challenge is to develop a fast manufacturing of PPA based composites as patented [START_REF] Hochstetter | Process for manufacturing a thermoplastic composite part in a closed mould, with injection into a cold mould[END_REF]. There are few chain extenders dedicated to this target. With ARKEMA group, we have studied other chain extender as well di-epoxy molecules which the reaction kinetics was demonstrated to be equivalent but the mechanical performances of the final PPA matrix was less performant than the chosen PBO system. The reaction scheme between the prepolymer and chain extender is presented in Figure 2. The reaction mechanism involved chain lengthening between an acidic prepolymer and a bis-oxazoline. It occurred precisely between the acid/oxazoline functions and resulted in the formation of an esteramide bond at the heart of the formed chain. 1,3 phynelene bis-oxazoline (CA) groups are well known to afford quantitative conversion of carboxylic acids both in melt and solution processes [START_REF] Douhi | Study of bulk chain coupling reactions. III. Reaction between bisoxazolines and carboxy-terminated oligomers[END_REF]. Indeed, oxazoline groups, which react very quickly with the carboxylic end groups, have proven to be effective in achieving linear chain extension of the polymer by forming stable bisamide bridging segments (Figure 2a). This linear chain extension reaction yields esteramides quantitatively without formation of any byproducts [START_REF] Sano | Polymerization of bis(2-oxazoline) compounds with dicarboxylic acids[END_REF]. Moreover, it has been shown in the literature [START_REF] Vocke | Compatibilization of polyethylene polyamide 6 blends with oxazoline-functionalized polyethylene and styrene ethylene butylene styrene copolymer (SEBS)[END_REF] that the chain extender agent can also react with the amide section as described in Figure 2b. Thus, using a high concentration of 'CA' could lead to the formation of branched chains. In this study, an appropriate iso-stoichiometry ratio between chain extender and prepolymer was determined. For the clarity purpose of the present paper, it was unchanged throughout this work.

(a)

Reinforcing glass fibers

Woven glass fibers used in this study consisted of a taffeta weave fabric (reference TV 1184) supplied by Hexcel. The reinforcement has a specific weight area of 240 g/m 2 and a ply thickness of 200 µm. The choice of a plain fabric (also known as a taffeta) to reinforce the polyphtalamide matrix was made since this type of fabric is known for its good properties (good stability and low deformability during the manufacturing process). In addition, the plain weave structure gives the reinforcement substantially similar mechanical properties in both weft and warp directions giving rise to a relatively isotropic behavior. The weight fraction of Glass Fibers (GF) in the composite parts was verified by weighing 3 different specimens (5 cm x 5 cm) before and after incineration at 500 °C in air environment during at least 2 hours. The experimental volume and weight fractions of GF were thus measured to be around 30% and 60% respectively. It's useful to note that the glass fabric used in this study contained a special fiber sizing formulation based on PA and its copolymers (Hydrosize PA846) to make it resistant at high temperature (< 350 °C). This sizing is particularly suitable for high performance glass fiber applications [START_REF] Dkier | Etude rhéocinétique de polyamides HT: Application à la mise en forme de matériaux composites par des procédés réactifs[END_REF][START_REF] Gabet | Analysis and Optimization of fibrematrix Interface in Glass-PA66 based Laminate Composites[END_REF]. Finally, the GF fabrics were dried for 12 hours in a vacuum oven at 100 °C before being placed in the mold.

Preparation of composites

The installation employed was chosen to match the specifications and constraints imposed by the selected chemistry. It consisted of a bi-component machine composed of a twin-screw extruder, connected to a closed mold. The prepolymer/CA systems with an optimal stoichiometric ratio were prepared thanks to the developed hybrid system (extrusion -injection) detailed below. The mixing

(b)
between the prepolymer and chain extender agent was carried out in a co-rotating twin screw extruder (Haake Polylab System, Rheocord RC400P, Thermo Scientific, Karlsruhe, Germany) with a screw diameter of 16 mm and an L/D ratio of 25:1. The parameters of extrusion were the following: screw speed of 800 rpm, temperature profile of 270, 275, 280, 280 and 280°C from the feed zone to the die. The screw profile was designed to be adapted for the extrusion of low-viscosity systems. The prepolymer powder was fed into the twin-screw extruder using a Brabender gravimetric feeder (DDW-P2-MT-1, Brabender, Duisburg, Germany). A high melting temperature of the prepolymer and an efficient mixing step is needed to unsure a total homogeneity between the chain-extender and the prepolymer before filling. Thus, a mixing head was specially designed and fitted to an area of the extruder in order to spray at high pressure the 'CA' onto the molten prepolymer. This mixing head was fed by a metering pump to control the amount of 'CA' to be sprayed inside the extruder (and ensure stoichiometric control), (Figure 3). The last zones of the extruder, the transfer station and the mold were maintained at a constant temperature of 280 °C, 290 °C or 300 °C. The set point temperature of the metering pump spraying the 'CA' into the extruder was set at 160 °C, i.e., just above the melting point of CA. Once the extruder and the mold were stabilized at the set point temperatures, the prepolymer and the chain extender were simultaneously introduced at a predetermined flow rate to ensure optimal stoichiometric conditions. Finally, a thermoregulated transfer station was conceived to connect the extruder to the mold with the homogeneous flow velocity profile through a sheet of fluid in the form of a fish tail. Thus, for a flow rate of 1.3 Kg/h, and in view of the geometry of the system (tube, then plate), the apparent shear rate was estimated to respectively 70s -1 and 10s -1 in the injection channel and at the entrance of the mold.

For the studied filling flow rate, the measured pressure at the injection part beyond the mold doesn't exceed 3 bar. It was measured by pressure transducers in the entrance zone of the mold. To help to avoid voids at the final parts, the filling could be assisted by vacuum through the event connected to a special pump. Since the viscosity of the medium was too low up to mixing zone (i.e. high speed), the time needed for the injection to produce the composite part was controlled to be as short as possible. With the chosen flow rate, the mold which it displays a small volume is completely filled beyond 20 s in which the viscosity is less than 10 Pa.s. After filling the cavity of the mold, and after a few seconds of evacuation through the vent, the extruder was stopped and the mold was isothermally maintained for 2 minutes. The cooling cycle was then quickly triggered by the thermoregulator to reach an indicated temperature of the heat transfer fluid of 100 °C. When adding the phases of demolding and cooling, the total cycle time to obtain the composite part did not exceed could reach 4-5 min in maximum.

Process installation used to monitor the hybrid T-ERTM process

A special instrumented mold was developed for this study with temperature and pressure transducers. It had a rectangular cavity with the following characteristics: H: 3 mm, L: 400 mm, W: 12 mm. No demolding agent was used before filling. Furthermore, no difficulty was observed to remove the composites parts during the demolding step (Figure 4). The mold was cooled by an independent oil circulation making it possible to vary the cooling rate.

The mold was pre-heated one hour before and the system was made inert by N2 gas. The computer acquisition system has been developed internally and allowed the collection of all the data from the pressure, temperature and dielectric sensors (Figure 5). These sensors are resistant to high temperatures and had a fast acquisition capacity (2 scans / sec). In addition to determining the pressure and temperature profiles throughout the mold cavity, the ultimate objective was to correlate the effective viscosity of the formulation with ionic conductivities obtained by in-situ metrology.

Figure 5. Thermoplastic extrusion resin transfer molding (hybrid HT-ERTM process) developed in this work.

Instrumented mold for process monitoring [START_REF] Maazouz | Chemorheological study and in-situ monitoring of PA6 anionic-ring polymerization for RTM processing control[END_REF].

Characterization of PPA and its composites

In-situ/online monitoring of the reaction in the mold

Dielectric spectroscopy (DRS) was applied for in-situ monitoring of the process and the measurements were conducted using a spectrometer (20 Hz to 200000 Hz) coupled to a Varicon sensor. A method for interfacing the instrument with the sample as well as with the procedures used for data analysis was developed. Data were collected especially at 20 Hz in order to capture the ionic conductivity and decouple it from dipolar relaxation. The system was programmed to store successive sets of data, rendering possible the real-time examination of the reaction process. The reaction process was depicted by a decrease in permittivity as well as by a drop in conductivity. The decrease of the specific conductivity resulted from a modification of the conductive paths and the increased viscosity. An experimental approach was proposed to follow the reaction process by dielectric measurements. The ultimate goal of this approach is to propose a correlation on the one hand between ionic conductivity of the system evaluated by in-situ measurements and on the other hand the viscosity of the medium determined by modeling at short times. This was in view of a better control of the process and in particular the impregnation step. DRS is extremely susceptible to small changes in material properties, such as the segmental motion, molecular relaxation, chemical reactions and dipole changes. Therefore, DRS can be a sensitive probe to detect local motions that involve chemical reactions, in terms of changes in ionic mobility. This technique is a good complement to rheological measurements by identifying the transitions from the electrical properties of the materials. The complex dielectric constant with relative permittivity and loss factor were determined directly. Their mathematical formalisms are given in the appendix (A).

Ex-situ monitoring of T-ERTM reactive processing

Small Amplitude Oscillatory Shear (SAOS) in coupling with FTIR

To validate the metrology used for in-situ measurements and in order to find a correlation between the temporal variations of the ionic conductivity in dielectrometry with those of viscosity, ex-situ measurements using chemo-rheology coupled to the fast scan FTIR spectroscopy (evolution of the area of peak absorbance of esteramide reactive groups during the reaction, located between 980-930 cm -1 ) have been performed. The change in viscoelastic properties with time and temperature were therefore recorded at the same time as an absorbance/transmittance spectrum in the confined heated chamber geometry. To achieve this goal, FTIR-ATR spectroscopy was carried out on molten samples at the same time as rheological tests using a Rheonaut ATR sampling module (Nicolet instruments) coupled to a thermo scientific HAAKE MARS 3 rheometer. Spectra were recorded at an average rate of 2 scans per second in the range 4000-700cm -1 with a resolution of 4 cm -1 , within a temperature range of 280°C to 300°C. The flow rate of nitrogen was set to 10 ml/min by a flow meter. Nitrogen purging (convection) was maintained throughout the experiments. It is worth mentioning that for accuracy of measurements, dynamic viscosity modulus given respectively by RheoMars (SAOS/FTIR coupling with cone plate geometry) and DHR rheometer (from SAOS with parallel-plate geometry) were compared. They showed to be very close and validate the specific confined geometry developed for SAOS decoupled measurements.

Small Amplitude Oscillatory Shear (SAOS)

SAOS measurements of PPA/CA formulations and their virgin constituents were determined in dynamic mode with a shear-stress controlled Discovery Hybrid Rheometer (DHR-2) from TA Instruments. The rheokinetic study was performed by monitoring the viscoelastic parameters in dynamic time sweep test (from 280 °C to 300 °C) and at an angular frequency of 1 rad/s. The studies were conducted in the linear viscoelastic regime previously established for each experiment. Before preparing the samples for the rheological measurements, the reactive systems and their virgin constituents were dried under vacuum for 12 hours at 80 °C in order to eliminate any trace of residual moisture. For only SAOS measurements, the sample in the form of compressed powder was placed in a cell contained in a confined controlled environment especially designed and adapted to the low viscosity of the starting systems. Experiments were performed in a plate-plate geometry using a plate diameter of 25 mm and a gap of ~ 1 mm. An axial force adjustment program was put in place to avoid any influence on the accuracy of measurements.

Measurements of torque using a micro-rheology compounder

5 g of PPA/CA blends were prepared using a twin-screw micro-compounder (Haake, Minilab II from Thermo-Scientific, Germany). For each formulation, the mixing torque (in N.m) was monitored as a function of time to follow and predict the end of the reaction. All the PPA/CA specimens were produced under an inert atmosphere thanks to a stream of nitrogen.

To mix the two constituents in a homogeneous manner, the chain extender was added vertically using a piston with a funnel to the top of the screws where the PPA prepolymer was in its molten state. The machine was purged and cleaned before each test. The reactive system could be circulated through a channel in order to pass through the compounder several times, making it possible to control the residence time of the reaction. The materials were blended in a temperature range from 280 to 300 °C for a variable duration of 1 to 20 min at a constant rotational speed of 50 RPM.

DSC measurements

Differential scanning calorimetry (DSC) measurements were performed on a Q20 (TA instruments, USA) calibrated with indium standards. A Rapid Cooling System (RCS) with a nitrogen flow of 50 mL min -1 was used down to -80°C. Samples of 5-10 mg were sealed in non-hermetic aluminum pans and were tested in the temperature range from ambient to 300 °C at a rate of 10 °C min -1 .

Size exclusion chromatography

Characterization by size exclusion chromatography (SEC) was used to measure the molar mass distribution of the reacted polyphtalamide samples at different experimental conditions (time, temperature). The analyses were performed with hexafluoroisopropanol (HFIP) as the eluent. The detection was carried out using an Agilent 1100 refractive index detector and the column was calibrated using a set of poly (methyl methacrylate) standards. The sample solutions in HFIP were prepared at 25 °C 1 day prior to the measurement in order to obtain good solubility. The PPA concentration was 1 g/L. Sample solutions were filtered through 10μm porous PTFE membranes prior to the measurements. SEC analyses made it possible to determine the molar mass characteristics (number and weight average data, and , respectively) and the polydispersity index = . The molar mass obtained are expressed as PMMA equivalents.

Size titration of carboxylic acid end groups

The reaction between the chain extender "CA" and the PPA prepolymer is a polyaddition resulting in the consumption of the terminal carboxylic acid groups 'COOH' by the oxazoline functions. The carboxylic acid end-group content was determined using the Waltz-Taylor method [START_REF] Pohl | Determination of Carboxyl End Groups in Polyester, Polyethylene Terephtalate[END_REF]. The sample (1 g) was dissolved in 50 mL of a trifluoroethanol/chloroform mixture. Titration of this solution was carried out using a Mettler Toledo automatic titrator with 0.05 N of tetrabutylammonium hydroxide in an acetic acid solution.

Microscopic observations

To qualitatively investigate the fiber-matrix interface and to observe the quality of impregnation in the composite specimens, optical microscopy was carried out at different surface zones of composites using a stereomicroscope STEMI SV 6 from Zeiss at 3.2 magnifications with KL 2500 LCD lighting from Schott. For the investigation of the wetting between the fiber and matrix, Scanning electron microscopy (SEM) was performed. It was carried out in a HITACHI S-3500 N using an applied tension of 20 kV. To avoid damaging the morphological structure of composite parts before SEM observations, the composites samples were not cryogenically fractured. The observed layers have been delaminated from the composites specimens at the end of the mechanical tests, thus allowing to probe the quality of impregnation inter and intra-wicks.

Dynamic Mechanical Analysis (DMA)

DMA tests were performed using a Q800 DMA from TA instrument on samples with a rectangular shape (60 mm x 10 mm x 3 mm). The experiments were carried out according to the DIN EN ISO 6721 standard in flexural mode using a three-point bending clamp, with a constant heating rate of 3°C/min, a frequency of 1 Hz and a strain amplitude of 0.1%. Dynamic temperature weep tests were conducted in the temperature ranged from 25 to 250 °C. The rectangular samples were cut from the plates obtained from the forming parts (in the flow direction) and were dried for 12 hours at 100 °C before the measurements.

Flexural tests

Three-point flexural tests of all the composites and neat matrix were carried out on a Shimadzu AG-X tester (Japan) equipped with Trapezium® software with a 5-kN loading cell. The bending test speed was 2 mm min -1 and the span to sample thickness ratio was equal to 16. The flexural test was conducted according to NF EN ISO 14125 under the standard laboratory atmosphere with a temperature of 23 °C ± 1 °C and a relative humidity of 50% ± 5%. A minimum of five flexural specimens were tested along the flow direction for each reported value.

Results and discussion

Thermal properties of unreinforced PPA/CA samples

Figure 6 shows a typical DSC temperature scan of neat PPA, neat CA and a PPA/CA bi-component system carried out at a heating rate of 10 °C/min. It was clearly observed that the bi-component system before reaction (red curve) presented a glass transition temperature around 100°C corresponding to the Tg of the polyphtalamide (PPA) prepolymer, a melting temperature Tm of the 'CA' around 150°C and a melting temperature of PPA around 270 °C. After the reaction, a shift in the glass transition was noticed and Tg reached a value of 115 °C which signifies the presence of more cohesive PPA chains. This shift in the glass transition mirrors the extension chain reaction. It should be noted that the significant increase in Tg observed between the first and the second heating cycles confirmed that the reaction was very fast. Consequently, the use of DSC to probe polymerization kinetics at of reaction was very difficult.

During the first scan, a baseline difference was observed. In fact, the bit change of the baseline due to the chain extension reaction took place simultaneously with the endothermic peak corresponding to the melting of the of the polyphtalamide crystalline part. Indeed, it was difficult to separate these two phenomena and similar results were reported in the case of in-situ polymerized cyclic butylene terephthalate or caprolactam [START_REF] Hakme | In situ monitoring of cyclic butylene terephtalate polymerization by dielectric sensing[END_REF][START_REF] Maazouz | Chemorheological study and in-situ monitoring of PA6 anionic-ring polymerization for RTM processing control[END_REF]. However, the reaction is shown to not have a high exothermic energy. It presents a very low exothermal behavior as we have demonstrated by non anisothermal DSC investigations and kinetic modelling [START_REF] Dkier | Etude rhéocinétique de polyamides HT: Application à la mise en forme de matériaux composites par des procédés réactifs[END_REF]. For the clarity purpose, these results will not be shown in this paper. Hence, our chosen systems seems to not affect the interfacial bonding between polymer matrix and fibers in the contrary to other reactive thermoset systems [START_REF] Tominaga | Quantitative evaluation of interfacial adhesion between fiber and resin in carbon fiber/epoxy composite cured by semiconductor microwave device[END_REF].

Once completely reacted, a change in the melting peak was also observed. In fact, the chain extension reaction was found to lead to profound changes in the crystallization mechanism of PPA. This was manifested by the presence of a double melting peak, which may be explained by the presence of different crystal sizes. Chain extension reaction permits to form PPA long chains with different chain distribution as was demonstrated by SEC measurements (section 3.2.3). Thus, it allows them to crystallize differently. With a second and a third heating, the observed double peak is reproducible confirming two kinds of crystalline melting (i.e. thermodynamically different). Meanwhile, this double peak is shifted to high melt temperature with increasing reaction time. Furthermore, two melting peaks were observed which suggests the presence of different crystal sizes as reported in other literature data [START_REF] Ramesh | Studies on the crystallization and melting of nylon 66: 2. Crystallization behaviour and spherulitic morphology by optical microscopy[END_REF]. This hypothesis has not been validated during this work and it will be a subject of a featuring work including X-ray and Polarized Optical Microscopy investigations. The same reactive PPA/CA system was maintained isothermally at 280 °C at different reaction times (Figure 7). Subsequently, the evolution of Tg and Tm with the time of the reaction was measured by DSC and the results are summarized in Table 2. The glass transition temperature was found to increase gradually over the reaction time. Surprisingly, the increase in Tg was not significant at short times (2 and 4 minutes). As the reaction proceeded, the Tg continued to increase with increasing conversion, since a high conversion led to long-chain polymers and hence to a high molecular weight could be reached [START_REF] Fan | Synthesis and characterization of polyamide resins from soy-based dimer acids and different amides[END_REF]. The shift in the glass transition with increasing reaction time is assigned to the high conversion of the produced polyphtalamides, and hence to higher molecular weights.

Beyond the Tg zone, we observed the presence of a first endothermic peak located around 187 °C, believed to correspond to the melting of the polyundancanamide part (polycondensation of the undecanamide present in the initial formulation of the prepolymer). One could then observe the presence of two endothermic peaks corresponding to the melting of different-sized crystals of the formed polymer. Indeed, thicker and stable crystals exhibit a higher melting point than their finer counterparts. On the other hand, the melting temperature also increased with the evolution of reaction time. The increased conversion may lead to the formation of well-ordered crystals with strong inter-chain interactions. Since the melting point reflects the chain motion in the crystals, it was logical that the melting temperature was higher in the case of high molecular weight samples [START_REF] Meyer | Rigid-rod fully aromatic polyamides with controlled constitution: synthesis and some properties[END_REF]. A further discussion of this subject is given in section 3.2.3.

Chemo-rheological study

Torque measurements in a Minilab micro-rheology compounder

The occurrence of chemical interactions between the prepolymer and the chain extender in the molten state were qualitatively assessed by recording the evolution of the torque as a function of time using the micro-rheology compounder. In these experiments, the PPA prepolymer was introduced first after which the 'CA' was added at t=1min in an appropriate amount to achieve optimal stoichiometric conditions. The time required for the completion of the chain extension reaction was that when the torque reached its maximum value. Figure 8 shows the torque profile recorded at different studied temperatures. The introduction of the 'CA' into the initial prepolymer melt caused marked changes in the torque of the PPA/CA system. This rapid raise of the torque in the molten state was the result of the increase in melt viscosity of the formed PPA by the chain extension reaction between the oxazoline functions of the 'CA' agent and the carboxyl terminal groups of the PPA (see the proposed mechanism in the section 2 mentioned above). However, as previously mentioned, a secondary reaction of branching can take place. From Figure 8, it was clear that the shape of the torque profile depended on the temperature before reaching a plateau. An increase in reaction temperature reduced the time needed to achieve the end of the reaction and the maximum torque value. It was here demonstrated that this time decreased dramatically with increasing temperature. The chain extension reaction was completed after approx. 15 min at 280°C and approx. 7 min at 300°C. Nonetheless, a significant torque drop occurred at long times when the reaction temperatures were 290 °C and 300 °C, but not at 280 °C. PPA/CA samples taken at 2 and 15 minutes of reaction at 280 °C were immersed in HFIP for dissolution.

From Figure 9b, we can clearly observe some non-soluble parts of the polymer after 15 minutes of reaction, indicating the formation of a network (secondary reaction branching), similar to what we can obtain at high stoichiometry [START_REF] Hochstetter | Process for manufacturing a thermoplastic composite part in a closed mould, with injection into a cold mould[END_REF]. It is worth mentioning that the results shown in Figure 8 cannot determine with accuracy the reaction kinetics. In fact, the fairly rapid increase in torque following the addition of the CA agent into the PPA cannot be clearly probed from a kinetic point of view (the time needed to start and end the reaction). This result provides qualitative information indicating that the reaction that occurred in the PPA/CA systems was drastically influenced by changes in temperature and reaction time.

Influence of temperature and reaction times: chemo-rheological study

The polymerization process can also be followed by dynamic rheological measurements under isothermal conditions, since the chain extension reaction is directly related to the change in viscoelastic properties, such as storage modulus (G'), loss modulus (G'') and viscosity (η) of the materials. The influence of the polymerization temperature on the viscosity profile is presented in Figure 10 where viscosity spectra are plotted as a function of time comparing the PPA/CA systems with neat PPA prepolymer. It is worth mentioning that the time needed to stabilize the temperature for isothermal measurements was optimized to 1.5 min. For accuracy and confidence, the results were recorded after isotherms of two minutes. Making the situation more complex, it was found that the viscosity that was initially below 3 Pa.s rose very rapidly during the first two minutes required to reach and stabilize the temperature. Indeed, in 2 min, the viscosity reached 180 Pa.s and 73 Pa.s at 280 °C and 300 °C, respectively. The results show once again that the beginning of the reaction was very fast and that the reaction was almost instantaneous between the PPA and CA making rheokinetic studies at short times very difficult. Thus, rheometry showed its limit for the present system especially at first time of reaction. We also note a decrease in the viscosity when the reaction time reached 10 min at 300 °C probably due to the thermal degradation of the polymer at high temperatures.

Figure 10. The evolution of the dynamic viscosity modulus at an angular frequency of 1 rad/s for neat PPA and PPA/CA systems at 280°C, 290 °C and 300°C.

The effect of the reaction time on the molar mass properties

The molar mass properties of the obtained PPA were recorded at different reaction times (at 280 °C). They are illustrated in Figure 11. It can be seen that the values of and increased significantly at less than 5 minutes of reaction. Furthermore, the polydispersity indices continued to increase significantly indicating a high sensitivity of the weight average molecular weight ( ) to structural evolutions. The molar mass increased sharply to reach values around 260 Kg/mol. As a result, the distribution of macromolecular chains became broader at long times. In comparison to other polyamides systems, the PDI stabilized after 15 min of reaction at an index of 25. These results confirmed that some chain branching could influence those properties rather than chain extension. Hence, the first mechanism was the chain extension between the ends of COOH chains and oxazoline. The secondary mechanism consisted of branching of the macromolecular structure at temperatures above 280 °C and at long times manifested itself by a significant increase in polydispersity index.

Titration analysis

Reaction kinetics were also studied using the titration method. The acid content of the PPA prepolymer before and after processing in the micro-rheology compounder was determined by titration of acid end groups. The apparent conversion of the polymerization was calculated using equation 6:

= (Eq. 6)
Where ( ) is the conversion rate, the initial concentration and the concentration at time t.

Figure 12 portrays the reaction conversion as a function of time at 280 °C. The results show that the beginning of the reaction was very fast and that the conversion reached 80% after two minutes without any significant changes in residual acid detected after 15min of polymerization. Since the reaction between PPA and CA was so fast, it was extremely difficult to track the reaction kinetics at short times by the titration method. Conversion rate (%)

Coupling Fourier transform infrared spectroscopy to rheology

Time (min)

The monitoring of the polymerization reaction at short times by conventional techniques (rheometry, DSC and titrations) was inconclusive. To overcome these constraints, an Infrared Rheology Coupling System was chosen. This system had the advantage of coupling the kinetic study of the reaction via infrared monitoring to the evolution of the dynamic viscosity modulus. To attain this objective, both theory and experiments were performed to establish a rheo-kinetic model allowing us to quantify the viscosity, especially at short times, and also in order to predict the processing window.

The peak area at (980-930 cm -1 ) was used for the quantitative analysis of the reaction rate. The abovementioned peak area was assigned to esteramide [START_REF] Rotter | FTIR separation on nylon-6 chain conformations: Clarification of the mesomorphous and γcrystalline phases[END_REF]. At 0 min (neat prepolymer), the band at 980 cm -1 , characteristic of the esteramide group, was completely absent, then started to appear as the reaction progressed. In fact, an esteramide is the result of a reaction between oxazoline and a carboxyl group as mentioned when discussing the mechanism of the chain extension reaction cited above. These bonds were monitored independently during the polymerization to follow the reaction kinetics. The absorption intensities were normalized by the hydrogen bonds at 2900 cm -1 for the neat prepolymer and were used as a reference. Figure 13 shows a plot of the amide region (980-930 cm -1 ) as a function of time. The extent of reaction with time was determined from the evolution of the amide area (peak area 980 cm -1 ). The normalized fractional conversion from the FTIR absorption data was calculated based on equation ( 7):

) = ) (Eq. 7)
Here, p is the fractional conversion, A(t) is the area of the peak during time, and A(tf) is the area of the peak at the end of the reaction. For each system, the area of the relevant band was measured taking into consideration slight variations in the baseline. Figure 15(a) highlights that the reaction seemed to be very fast and that a maximum was reached in less than 120 s.

To establish a rheokinetic model, we studied the kinetics of the reaction by in-situ FTIR as well as the increase in viscosity during the reaction. The adequate equation for the variations of the viscosity was derived from the molecular weight dependence. It follows a simple law expressed in the following equation.

) = ! " # $ %& ' )
Eq. ( 8) where Ea is the activation energy, T the temperature in Kelvin and n an exponent that is adapted depending on whether is lower than (or equal to) 1 or higher than critical entanglement molecular weight Mc (i.e., equal to 3.4). The is expressed as a function of the reaction conversion p(t) [START_REF] Pichaud | Chemorheological and dielectric study of epoxy-amine for processing control[END_REF]. This apparent reaction extent could be evaluated according to equation 7.

) = ( ) )*+ ) , * -. /0.
)

) 1 2 3) ,
Eq. ( 9) Subsequently, this viscosity evolution can be expressed as a function of time and conversion p according to:

M 5 t ( ) =
η t) = A exp " D E FG ' H" 7 8 3 9 ) :; *7 8 3 9 ) :<= > ' I )*1 2 3) , * -J /0J 2 3) ) 1 2 3) , KL M Eq. (11)
At short times (t<90 sec), the logarithm of the zero shear viscosity was plotted as a function of log

). It was found that the viscosity of the medium was proportional to the molar mass of formed macromolecules and a slope of 3.1 was obtained (Figure 14). This value of the slope indicated that we were in the presence of an entangled polymer even for very short reaction times (more than 10 s). Theoretical values from equation 11 were compared with the experimental measurements at reaction times over 2 min in the linear viscoelastic regime. Figure 15(b) shows the evolution of the complex viscosity as a function of time. The evolution of the apparent conversion followed that of the viscosity especially at long times. Figure 15(b) also displays how the experimental viscosity corresponds to that calculated from equation 11. The results show that the beginning of the reaction was very fast and that the conversion reached 80% and 70% at 280 °C and 300 °C, respectively, in 2 min. It was thus clearly demonstrated that the conversion as well as the viscosity increased drastically during the first 2 minutes. range. Moreover, the viscosity profile showed a signature of both chain extension and branching reactions especially at longer times. Indeed, a slight deviation from the experimental data was observed, especially after 10 minutes of reaction. However, the fit was less effective at 300 °C due to thermal degradation as we have demonstrated by molar masses measurements. These results also highlight a processing window for HT-ERTM processes including time for impregnation and to a maximum of reaction conversion. Thus, with these reactive system, the time frame for injection was consequently very short and making more complicate it's RTM adaptation. In sum, 2 minutes is a maximum holding time in the mold in order to reach 85% of conversion. Experimentally and with our chosen flow rate, the mold which it displays a small volume is completely filled beyond 20 s in which the viscosity is less than 10 Pa.s. The total cycle time before demolding could reach 4-5 min in maximum depending on the desired PPA molar masses.

In-situ monitoring of the reaction in the mold by dielectric measurements

The values of conductivity as a function of the reaction time were calculated from equation ( 5) and are represented in Figure 16(a). The dielectric properties of the system can be related to the material processing: (i) the increase in the PPA conductivity corresponds to the melting of the sample and stabilization of the temperature, (ii) the decrease in ionic conductivity after the peak maximum signified the start of the reaction, (iii) the conductivity plateau correlated well with the end of the reaction. Based on the validation of the procedure previously discussed in the experimental part of this paper, Figure 16(b) portrays the evolution of the reaction conversion with time for PPA/CA systems at 280 °C. It compares the conversion determined by FTIR spectra with that calculated by equation 5 from the in-situ dielectric measurement. Interestingly, the obtained evolution of apparent conversion recorded by the in-situ dielectric measurement during the process corroborated the exsitu counterpart obtained by FTIR coupled to rheology and by titration as given in Figure 12.

Correlation of dielectric in-situ monitoring with the evolution of viscosity during polymerization in the mold

Figure 17 shows the variations of the ionic conductivity measured in the mold by dielectrometry as a function of the modulus of the complex viscosity determined experimentally or by modeling at short times. The mold temperatures were either 280 °C, 290 °C or 300 °C. The obtained results demonstrated similarities between the variations of the ionic conductivity from dieletricometry with those of the viscosity as a function of time and temperature. According to the literature, Kranbuehl et al. [START_REF] Kranbuehl | Dynamic dielectric analysis: Nondestructive material evaluation and cure cycle monitoring[END_REF] noticed that there was an inversely proportional relationship between viscosity and conductivity. Thus, the ionic conductivity decreases with increasing viscosity according to the socalled Stokes-Einstein law. In contrast, a deviation from the Stokes law is generally observed for non-homogeneous reactive media. Consequently, the so-called Walden's correlation can be given as follows [START_REF] Szwajczakaf | On the relation Between Mobility of Ions and Viscosity. the Walden's Rule[END_REF]: σ t) η t) O = Const with m < 1.

Here, (σ) is the ionic conductivity and (η) is the viscosity. Thus, our original contribution consists in taking into accounts the evolution of viscosity from in-situ/modeling approach. Figure 17 shows that the higher the reaction temperature, the lower the amplitude of the ionic conductivity, thus resulting in a high reactivity of the PPA/CA system. Moreover, the presence of side reactions at long times leads to a loss of validity of the Walden equation since the fit is divergent in this zone [START_REF] Southall | Ionic conductivity and viscosity correlations in liquid electrolytes for incorporation into PVDF gel electrolytes[END_REF]. 3 and4 summarize all the measured glass transition temperatures and melting enthalpies. It's important to note a certain reproducibility of measured Tg and melting enthalpies, regardless of where the sample was taken. One again, two melting peaks were observed which may have their origin in a double distribution of the crystalline phase. At a temperature close to 189 °C, the melting of polyundecanamide is clearly identified. These values of Tg, melting points and melting enthalpies corroborate those measured with unreinforced matrices as discussed earlier. This reflected some isotropy in properties in the different areas of the composite parts. To the best of our knowledge, the melting enthalpy of 100% crystalline PPA is not available in the literature, so the absolute degree of crystallinity for the PPA samples cannot be determined directly. Thus, it could be concluded that the chain extension reaction had indeed taken place homogeneously within the mold. In addition, the high values of Tg validated the efficiency of the used process and in particular the injection head and the screw profile. Composite part 58

Microscopic observations of reactive GFRTC

The morphological observations at the micro scale on composites surfaces were conducted at different areas of the composite parts obtained at 280 °C as illustrated in Figure 19. It was found that a homogeneous wetting was obtained within and between the wicks. The glass fiber reinforcements were completely coated by the PPA matrix which seemed to be free from air bubbles. In addition, no lack of material was noticed indicating a good matrix/reinforcement impregnation. Therefore, the quality of the impregnation seemed to be the same regardless of the cutting area in the composite parts (a), (b) or (c). To analyze the impregnation inter and intra-wicks, delaminated layers of PPA/CA composites have been analyzed using Scanning Electron Microscopy (SEM). As can be seen in Figure 20(a), a good wetting between fibers and matrix was observed. At high magnification, Figure 20(b) shows that a thorough wetting of the individual strands was achieved. Nevertheless, the micrographs indicate the presence of some holes and bubbles.

Thermo-mechanical properties

The evolution of dynamic mechanical properties versus temperature of unreinforced and woven glass fabric reinforced composites was performed to investigate the glass transition temperature and the viscoelastic behavior at the solid state of the samples. Figure 20(a) depicts the storage modulus (E') and the damping factor (tan δ) as a function of temperature for PPA/CA specimens prepared at the optimal processing temperature, 280 °C. The peak of tan δ curve around 120 °C reflects the dynamic α-relaxation (glass transition). The storage modulus (E') in the glassy state was around 1.6 GPa. As soon as the Tα transition was approached, a decline in storage modulus occurred due to the enhanced chain mobility. However, as the melting of crystals approached, the elastic modulus (E') fell even more steeply. It should be mentioned that the storage modulus above the Tg (E'rubbery) presented a non-flat rubbery plateau probably due to the alpha star transition (Tα * ) associated with the slippage between crystallites [START_REF] Guerrero | CO2 Separation in Nanocomposite Membranes by the Addition of Amidine and Lactamide Functionalized POSS (R) Nanoparticles into a PVA Layer[END_REF] and it was characterized by the appearance of a second peak in the region above the Tg located at about 183 °C. Moreover, some cold crystallization could appear.

The storage modulus (E') and damping factor (tan δ) of the glass fiber/polymer composites are presented in Figure 20(b). As can be seen, the composites showed significantly higher E' values than their unreinforced PPA/CA counterparts. Indeed, an increase in the elastic modulus of about 160% was observed (E'composites ~ 4.2 GPa). This augmentation of E' over the analyzed temperature range was attributed to the usual reinforcement effect caused by the fibers allowing a high degree of stress transfer at the interface. In contrast, the α relaxation temperature (Tα ~ 122 °C) remained almost unchanged. Nevertheless, the DMA results revealed a broadening of the tan δ peak with the incorporation of GF layers. This phenomenon should be assigned to the broadening of the spectrum of relaxation times due to the reduction of mobility by the anchoring of the chains in contact with the GF folds [START_REF] Ishak | Mechanical properties of woven glass fabric reinforced in situ polymerized poly(butylene terephthalate) composites[END_REF]. Indeed, the properties of the interphase usually differed from that of the bulk owing to different relaxation behaviors. Another effect of the reinforcement was the decrease in the height of the tan δ peak which could be assigned to a reduction of the mobility of the macromolecular chains in the vicinity of the glass fibers due to a good interfacial interaction between PPA matrix and GF folds which led to lower damping at the interface.

Flexural properties

The average values of the flexural modulus and the yield strength defined as the maximum bending stress, are given in Table 5. It can be seen that the virgin polymer had a moderate yield stress of 33 MPa and featured a flexural modulus of 2.2 GPa. Furthermore, the composite parts presented a flexural modulus of the order of 16 GPA and a yield stress of the order of 120 MPa. Thus, the results obtained for the composite showed some adhesion and corroborated the adhesion properties revealed by the morphological studies. A Takayanagi model was used to predict the modulus values of the composite. The model considers the material as composed of two elements working in parallel. The first element consists of the entire fiber and part of the matrix, acting in series. The second element consists of the rest of the matrix [START_REF] Alvarez | Dynamic mechanical properties and interphase fiber/matrix evaluation of unidirectional glass fiber/epoxy composites[END_REF]. The expression of the composite modulus using the Takayanagi model is:

T U = T V "1 - Y Ф ' + \ Y # ] #
# ] Ф , * ) Ф)Ф# ^ Eq. [START_REF] Ning | RIM-pultrusion of nylon-6 and rubber-toughened nylon-6 composites[END_REF] where Em and Ef are the modulus of the matrix and the fiber, Ec is the storage modulus of the composite, Ф is an adjustable parameter. The parameter Ф could be related to the morphology of the composite. Comparison between the experimental results and the theoretical predictions gives a value of composite modulus of 16.4 GPa. The obtained value corroborates the experimental one. The parameter Ф determined from the data fit using the above equation, using the values of the neat matrix (2.2 GPa) and GF fiber modulus (70 GPa). The calculated parameter Ф was 0.986 indicating a higher efficiency of stress transfer between glass fibers and PPA matrix. Hence, the present manufacturing approach, which avoids some of the issues encountered with in-situ polymerization of reactive monomers [START_REF] Van Rijswijk | Reactive processing of textile fiber-reinforced thermoplastic composites -An overview[END_REF], allow to preserving a relatively low viscosity compatible with liquid composite molding processes and to reach satisfactory mechanical properties.

Conclusions

This article presents a new manufacturing method to prepare high Tg and melting temperature polyphtalamide thermoplastic polymer composites, using reactive extrusion of a low viscosity prepolymer with a chain extender, injected into the mold containing the GF reinforcement, similar to RTM process. Firstly, the chain-extending reaction is demonstrated to be very rapid. Indeed, a series of adapted experimental studies have been performed to characterize the process and predict the processing window. Hereto, conventional ex-situ monitoring experiments (DSC, titration, rheometry and torque measurements) have shown that the reaction between PPA and CA agent occur at less than 2 min for which reason these techniques were unable to track the reaction at short times. Secondly, the reaction kinetics was therefore monitored using high temperature FTIR spectroscopy coupled to rheology. Subsequently, a rheo-kinetic model was established allowing to quantify the viscosity, especially at short times and therefore to predict the processing window for this kind of reactive PPA systems. Moreover, the modelled viscosity of this bi-component system was sufficient for filling and 30 seconds are a limit before rapidly rising. Interestingly, it was found that the evolution of the apparent conversion recorded by in-situ dielectric measurement during the process accurately matched the ex-situ counterpart obtained by FTIR coupled to rheology or by titration. Hence, it possible to achieve 85% of conversion at less than 2 minutes at the optimal reaction time of 280°C. The process could be under pressure (3-4 bar) and impregnation is helped with vacuum. Finally, high Tg PPA based composites were obtained with good impregnation properties. Overall, this study highlighted the potential of the High Temperature hybrid process (HT-ERTM) developed for this engineering polymer matrix. The obtained results demonstrated that the mechanical and thermomechanical properties of the resulting composites could be tuned through the control of processing parameters and especially time and reaction temperature. Hopefully, the present findings will provide a new guidelines for manufacturing PPA based composites for high service temperature applications in automotive and aerospace industries.

Figure 1 .

 1 Figure 1. Chemical structure of the prepolymer used in this study[START_REF] Hochstetter | Process for manufacturing a thermoplastic composite part in a closed mould, with injection into a cold mould[END_REF].

Figure 2 .

 2 Figure 2. Reaction mechanism for 1, 3 phenylene bis-oxazoline and carboxylic acid: polymerization via chain extension reaction (scheme a) and possible secondary branching reaction especially at high CA content (scheme b).

Figure 3 .

 3 Figure 3. Photo of the used mixing head coupled to the metering pump for the injection of the chain extender agent onto the molten prepolymer in the extruder.

Figure 4 .

 4 Figure 4. Design (left) and realization (right) of the instrumented and thermoregulated RTM mold.

Figure 6 :

 6 Figure 6: DSC curve at a heating rate at 10°C/min for neat PPA, CA and the PPA/CA system. Comparison of 1 st , 2 nd and 3 rd heating scans.

14 Figure 8 .

 148 Figure 8. The evolution of the normalized melt torque [C(t)/C( t=0)] as a function of the reaction time at different reaction temperatures.

Figure 9 .

 9 Figure 9. Example of the solubility of the PPA/CA bi-component system (a) after 2 minutes, and (b) after 15 minutes of reaction at T=280°C.

Figure 11 .

 11 Figure 11. Evolution of the weight average molecular weight ( ), the number average molecular weight ( ) and the polydispersity index (PDI) as a function of time for PPA/CA systems at 280 °C.

Figure 12 .

 12 Figure 12. Conversion rate as a function of the reaction time of PPA/CA system at 280°C.

Figure 13 .

 13 Figure 13. In-situ FTIR spectra during the reaction of PPA with CA at 280°C in the zone of (980-930 cm -1 ).

M 5

 5 from equation 10 can be introduced in equation 9 in order to model the viscosity evolution.

Figure 14 .

 14 Figure 14. Complex viscosity at 1 rad.s -1 as a function of the weight average molecular weight at 280 °C (logarithmic scale).

  Figure 15(b) highlights a good agreement between the experimental data of the measured viscosity and its theoretical counterpart at 280 °C. The viscosity evolution as calculated by equation 11 fit the experimental data quite well in the first conversion

2 Figure 15 .

 215 Figure 15. (a) Evolution of the apparent normalized conversion for PPA/CA systems at 280 °C and 300 °C. (b) Comparison of theoretical viscosities predicted by equation 11 with the experimental values of the dynamic viscosity modulus versus time.

Figure 16 .

 16 Figure 16. (a) Evolution of ionic conductivity as a function of time at T=280°C at a f = 20 Hz. (b) Evolution of the apparent conversion obtained from FTIR and dielectric spectroscopies as a function of time at 280°C.

Figure 17 : 3 . 8

 1738 Figure 17: Logarithmic evolution of the ionic conductivity (measured at 20 Hz by in-situ dielectrometry in the mold) as a function of the complex viscosity modulus.

Figure 18 :

 18 Figure 18: DSC scans (2st heating) of continuous glass fiber reinforced parts (PPA/CA bi-component systems injected at 280 ° C +60% by weight of glass fibers). Sampling at (A) the beginning of the flow, (B) inthe middle area and (C) at the end of the flow.

Figure 19 .

 19 Figure 19. Optical microscopy observations of PPA/CA based composites. Sampling at (a) the beginning of the flow, (b) in the middle area and (c) at the end of the flow.

Figure 20 .

 20 Figure 20. (a). Scanning electron microscopy (SEM) image of a cross section of PPA/CA composites. (b) SEM micrograph at high magnification.

Figure 20 .

 20 Figure 20. Evolution of storage modulus and tan (δ) as a function of temperature for (a) a neat PPA matrix, and (b) glass fiber-reinforced PPA composites obtained at a reaction temperature of 280 °C.

Table 1 :

 1 Main characteristics of the prepolymer and the chain extender agent.

	: number average

Table 2 :

 2 Values

		of the glass transition temperature (Tg), melting point Tm,1 (first peak), melting
	point Tm,2 (second peak), at different reaction times. The reactive bi-component system
			corresponded to a temperature of 280 ° C.
	Reaction time	0	2	4	6	10	20
	Tg (°C)	100	102	104	107	110	115
	Tm,1 (°C) Tm,2 (°C)	270	239 253	242 256	242 254	247 260	247 260

Figure 7: Comparison of DSC thermograms performed at 10°C.min -1 on PPA/CA systems at 280 ° C following different reaction times (2,4,6,10 and 20 minutes).

Table 3 :

 3 Glass transition temperatures and melting points in the three areas of the composite parts.

	Sample n° Tg (°C)	Tm,1 (°C) Tm,2 (°C)
	A	110	244	258
	B	113	247	261
	C	109	248	262

Table 4 :

 4 The average melting enthalpy obtained using three different areas of the composite parts in comparison with pure matrix.

	Sample n°	Melting Enthalpy (J/g)
	Pure PPA matrix	59

Table 5 :

 5 Yield strength and flexural modulus of glass fiber-reinforced composites obtained at a reaction temperature of 280 °C. SD stands for standard deviation.

		Yield strength (MPa)	Flexural modulus (GPa)
	Sample	σ (MPa)	SD (MPa)	E (GPa)	SD (GPa)
	Neat PPA/CA	33.0	0.7	2.2	0.3
	PPA/CA +60% GF	120.0	8.0	16.4	1.5

Acknowledgements

The authors acknowledge ARKEMA for providing the prepolymer and chain extender. The PhD thesis was performed in COMPOFAST project which was a framework of academic labs and many companies.