Mathieu Hugoud

Wilfried Uhring
email: wilfried.uhring@unistra.fr

Jean-Baptiste Kammerer

Jean-Bapstiste Schell

A High Dynamic Range High Speed Pixel Operating at 100 Million Frames Per Second

Keywords: High-speed imaging, CMOS, video sensor, current mirroring integration, burst imaging I

The burst imaging concept is a specific imaging method than allows achieving a pixel rate of about 1 Tera pixel per second. This huge data rate can be reached by storing the video frames within the sensor, thus circumventing the bottleneck of the input/output of the sensor. Consequently, a frame rate of several Mega frames per second (Mfps) can be obtained and the use of a fast pixel front end is mandatory. In this paper, a pixel front able to operate at a frame rate of 100 Mfps is proposed. Based on a current mirroring integration approach, it offers a high dynamic range feature compatible with the record of the very fast moving object without blurring effect. The feature is carried out by mirroring in parallel the photocurrent into several integration capacitances. The pixel consumption is as low as 12 µW for one single gain branch at 10 Mfps and 118 µW at a frame rate of 100 Mpfs.

INTRODUCTION

High-speed imaging is a key technology to study high-speed phenomena such as micromechanics, explosives, plasma formation mechanisms or laser ablation. The burst imaging concept introduced by Etoh allows to circumvolved the bottleneck of the data input/output of the sensor by storing the series of images within the sensor [1]. This way, a pixel rate of about 1 terapixel per second can be achieved while the conventional approach that extracts continuously the frames from the sensor is limited to a few tens of Gigapixel per second [2]. Thus, a frame rate of several Megaframe per second (Mfps) can be achieved with a spatial resolution of about 1 megapixel [START_REF] Suzuki | Over 100 Million Frames per Second 368 Frames Global Shutter Burst CMOS Image Sensor with Pixel-wise Trench Capacitor Memory Array[END_REF] and a frame rate of several Gigaframe per second has been demonstrated with a streak camera approach with a reduced spatial resolution [START_REF] Zlatanski | Sub-500 ps Temporal Resolution Streak-mode Optical Sensor[END_REF]. At such a frame rate, the exposure time is very short, thus the pixel analogue front end has to operate at a high speed while providing a high gain. In addition, the concept of storing the pixel within the sensor is relevant in the framework of a 3D stacked integrated circuit approach. Indeed, storing the images in an additional tier allows to save space on the first one that could be utterly dedicated to the sensitive part of the pixel in order to increase the fill factor. Additionally, the supplementary tier allows storing more frame in order to increase the movie length [START_REF] Suzuki | Over 100 Million Frames per Second 368 Frames Global Shutter Burst CMOS Image Sensor with Pixel-wise Trench Capacitor Memory Array[END_REF] . Moreover, pushing the 3D stacked up to 3 or more tiers makes it possible to implement more on chip functionality such as analogue to digital conversion [5][6].

Several high-speed pixel front-end architectures have been reported such as CCD [START_REF] Crooks | Kirana: a solid-state megapixel uCMOS image sensor for ultrahigh speed imaging[END_REF], pinned photodiodes [START_REF] Suzuki | Over 100 Million Frames per Second 368 Frames Global Shutter Burst CMOS Image Sensor with Pixel-wise Trench Capacitor Memory Array[END_REF], buffered and non buffered direct injection [START_REF] Bonnard | signal conditioning circuits for 3D -integrated burst image sensor with on-chip A/D conversion[END_REF], Classic 4T pixel [START_REF] Millet | A 5 Million Frames Per Second 3D Stacked Image Sensor With In-Pixel Digital Storage[END_REF] or capacitive feedback transimpedance amplifiers [START_REF] Uhring | A Scalable Architecture for Multi Millions Frames per Second CMOS Sensor With Digital Storage[END_REF], but none of these architectures offers a High Dynamic Range (HDR) feature. The HDR feature consists to capture an image with very different intensity levels without saturating. i.e., the brighter part of the image are not saturated to the maximum level and the darkest part of the image are not saturated to the minimal value. The classical operation of an HDR acquisition rely on the acquisition of several frames with varied integration time. Nevertheless, this mode of operation is incompatible with the acquisition of a very fast event as a blurring effect occurs while recording fast-moving objects. This paper presents a new architecture of high-speed pixel front end featuring a single shot HDR operating at up to 100 Mfps based on a Current Mirroring Integration (CMI) approach. Several gains are obtained in parallel with the integration of the photocurrent into some capacitances of different values with a single frame acquisition.

The architecture and operation of the pixel are presented in section 2. The pixel performances are assessed thanks to simulation results shown in section 3. The design constraints related to mismatch are then presented in section 4.

II. SYSTEM DESCRIPTION

The proposed architecture is based on a CMI approach that consists in copying the photocurrent I ph flowing through the photodiode into an independent capacitance C int as shown in Fig. 1. The benefits of this technique is twofold, 1) it allows to keep a constant bias on the photocathode, improving the photodiode transient response and 2) it allows separating the conversion gain from the photodiode capacitance, increasing the pixel sensitivity. Indeed, in the case of a high speed burst image sensor the pixel pitch is generally a few tens of micrometres, thus the photodiode area is relatively high. As a consequence, the capacitance of the junction Cdiode is generally high and integrating the photo-charge directly within the photodiode leads to a low conversion gain whereas mirroring the photocurrent in a smaller capacitance C int with C int << C diode offers a high conversion gain. Nevertheless, a high conversion gain can result in saturation of the pixel response in the case of a too strong light exposure. A HDR image sensor provides an image where the pixels with a higher illumination aren't saturated but still have an intensity related to its illumination. To provides this feature, we added some additional mirrored current integrating the photo-charge into different capacitances C intn . The schematic of the pixel is detailed in Fig. 2. The current mirror that copy the photo-current I ph from the photodiode to the integration capacitance C int is formed by M9-M10. In order to operate at a frame rate of 10 or 100 Mfps, the bandwidth of this current mirror has to be higher than 10 or 100 MHz. But this bandwidth can't be achieved without a DC biasing current flowing through the current mirror. Thus, an external source feeds the current I bias into the current mirror M1-M2 to provide an additional current I bias into the M9-M10 current mirror. Note that this biasing current can be externally adjusted to match the required bandwidth in regards to the acquisition frame rate.

The resulting current I ph +I bias is mirrored into the C int capacitance and a supplementary current mirror formed by M1-M6 withdraw the biasing current I bias from the C int capacitance. Providing that the current mirrors are perfectly matched, only the photocurrent I ph charges the integration capacitor C int resulting in a voltage increase ∆V out at the output node given by: ( ) In order to improve the current mirror matching, the cascode transistors M3, M4 and M5 ensure than the current mirrors formed by current M1-M2 and M6 are always identically biased. In the same way, the cascode transistors M7 and M8 ensure that the current mirror formed by M9 and M10 are biased similarly. This in particularly useful as the Out voltage is varying as the photocurrent is integrated while the photodiode voltage is almost constant.

10 6 int M M bias bias ph ph out I I I dt I dt C V - + ⋅ = ⋅ = ⋅∆ ∫ ∫ (1)
Finally, the transistor M11 is used to reset the output voltage to an intermediate voltage ensuring the good operation of the current sources. The maximum output voltage is:

𝑂𝑂𝑂𝑂𝑂𝑂 𝑚𝑚𝑚𝑚𝑚𝑚 = 𝑉𝑉 𝐷𝐷𝐷𝐷 -𝑉𝑉 𝐷𝐷𝐷𝐷(𝑠𝑠𝑚𝑚𝑠𝑠) 𝑀𝑀8 -𝑉𝑉 𝐷𝐷𝐷𝐷(𝑠𝑠𝑚𝑚𝑠𝑠) 𝑀𝑀10
(2)

The minimum output voltage depends on the potential at the reset transistor node. Therefore, the theoretical voltage output range is given by:

∆𝑉𝑉 𝑂𝑂𝑂𝑂𝑂𝑂 = 𝑉𝑉 𝐷𝐷𝐷𝐷 -𝑉𝑉 𝐷𝐷𝐷𝐷(𝑠𝑠𝑚𝑚𝑠𝑠) 𝑀𝑀8 -𝑉𝑉 𝐷𝐷𝐷𝐷(𝑠𝑠𝑚𝑚𝑠𝑠) 𝑀𝑀10 + 𝑉𝑉 𝑟𝑟𝑟𝑟𝑠𝑠𝑟𝑟𝑠𝑠
Finally, the different conversion gains are obtained by adding n times the last branch of the circuit with some different integration capacitance C intn . Obviously, the gain can also be adjusted by varying the ratio of the current mirror. An additional system, not shown in Fig. 2, is mandatory to select the good output in regards of its value. The selection algorithm consists to select the higher gain output which is not saturated for the correspondent image value to be stored. An information coded in a analog or a digital format is also added to know at which gain the image value has been stored. As a consequence, an image is composed of a gain value and an analog value. The next section presents the simulation results of the front-end circuit.

I. CIRCUIT PERFORMANCE

The proposed circuit has been designed in a 180 nm CMOS image sensor technology and simulated using Spectre TM . The photocurrent is modeled using a homemade Verilog-A photodiode compact model, allowing a realistic representation At a frame rate of 10 Mfps the generated photocurrent by the pulse of light is set to 800 nA (black dashed line in Fig. 3). The rounded shape of the photocurrent is due to the internal capacitance of the photodiode. This pulse of light is sufficient to bring the higher gain branch close to the saturation (red line in Fig. 3) with a voltage swing of about 1.7 V. With a bias current Ibias of only 1.2 µA, the bandwidth of the pixel is adequate to integrate the light pulse with a total absence of lag on the next frame. Indeed, the output is rising as long as the photocurrent is generated and stop rising as the photocurrent return to zero. The reset width set to 6 ns is long enough to completely reset the output node. The lower gain branch responses are also depicted in Fig. 3 (green line for 50 fF and blue line for 100 fF). The simulated conversion gains at respectively 10 fF, 50 fF and 100 fF are equal to 6.8µV/e-, 2.6µV/e-and 1.4µV/e-corresponding to an equivalent integration capacitance of 23 fF, 62 fF and 114 fF. The parasitic capacitances of the output node are thus assessed to be 13 fF. The power consumption at this frame rate is of only 12 µW for one single gain branch and supplementary consumption of 4 µW is added for each additional gain branch.

The simulation results at a frame rate of 100 Mfps are depicted in Fig. 4. At this frame rate, the generated photocurrent by the pulse of light is set to 8 µA (black dashed line) to reach the saturation of the high-gain branch (red line) with a voltage swing of about 1.7 V. At a period of only 10 ns, the response of the photodiode is close to its limit as the photocurrent only have the time reaches 8 µA before returning to zero. Nevertheless, the total charge remains proportional to the light pulse energy and the response of the pixel at a bias current Ibias increased up to 12 µA is close to the 10 Mfps configuration as the final voltages reach almost the previous ones. This indicates that the bandwidth of the pixel is adequate and it is confirmed by the absence of lag on the next frame. In this simulation, the reset width is as short as 600 ps allowing an almost full aperture of the exposure time. The lower gain branch responses (green line for 50 fF and blue line for 100 fF) are also reaching the same final voltage. The simulated conversion gains at respectively 10 fF, 50 fF and 100 fF are equal to 6.6 µV/e - , 2.4 µV/e -and 1.2 µV/e -. This result confirms that the pixel is able to operate at 100 Mfps The power consumption at this frame rate is 118 µW for one single gain branch with a supplementary consumption of 49 µW per additional gain branch.

II. DESIGN CONSTRAINTS

The previous results showed that the current biasing technique of the current mirrors ensure a sufficient bandwidth event at 100 Mfps. Nevertheless, the current mirrors are particularly susceptible to device mismatch that can lead to a non-zero output current for a null photocurrent. Indeed, in this circuit, due to the effect of mismatches, the 10 M bias I and 6 M bias I current could not be exactly compensated and thus, the integration capacitor C int could be charged or discharged even with a null photocurrent I ph (see equation ( 2)).

In order to evaluate the effect of the mismatch on the pixel operation, a Monte Carlo analysis has been performed. The literature shows that a correlation coefficient over 0.9 is easily achievable with a common-centroid layout structure [START_REF] Lan | Matching performance of current mirrors with arbitrary parameter gradients through the active devices[END_REF][14] [START_REF] Lan | Matching performance of current mirrors with arbitrary parameter gradients through the active devices[END_REF]. Therefore, a correlation coefficient of 0.9 is introduced between each transistor of the pixel. Fig. 5 shows the Monte-Carlo simulation of the output voltage for a null photocurrent at a frame rate of 10 Mfps. The dispersion of the output due to the mismatches is higher for the high gain compared to the lower gain. Indeed, the compensation mismatch of the current at the output node is amplified by the conversion gain of this node. The peak to peak value of the dispersion are respectively 1 V, 340 mV and 190 mV for the 10 fF, 50 fF and 100 fF gain branch. These values are not negligible and they clearly highlight a drawback of the approach. However, this flaw can be compensated with a correlated double sampling feature and it can be seen as a simple Fixed Pattern Noise (FPN). Indeed, the subtraction of an image acquired in the dark can efficiently suppress this static dispersion. Moreover, the high-gain branches of the worst pixels still offer a large dynamic of 1.2 V and even if this branch of the pixel is saturated, the HDR feature ensure that the signal is still measurable on a lower gain branch. The Monte Carlo simulation of the output dispersion at a frame rate of 100 Mfps is also shown on Fig. 5. The peak to peak value of the dispersion is better to the previous one with respectively 450 mV, 200 mV and 120 mV for the 10 fF, 50 fF and 100 fF gain branch. This indicates that the pixels can still operate at the frame rate of 100 Mfps.

The pixel response non uniformity (PRNU), i.e. the dispersion of the pixel response when a spatially uniform light is applied on the sensor has also be assessed thanks to a Monte Carlo simulation. The response of a pixel to a given constant photocurrent is evaluated by subtracting the voltage of a pixel when the light is applied and the voltage of the same pixel when no light is applied. The relative dispersion of the pixel response, i.e. the ratio between the rms dispersion of the output and the mean value of the pixel has been simulated at a gain of 10, 50 and 100fF and a frame rate of 10 and 100 Mfps. The results obtained at 100 Mfps are depicted in Fig. 6. The assessed PRNU is as low as 1% and can be characterized and compensated by post processing. This results also shows that the relatively high FPN can be efficiently measured and withdrawn and thus is not a critical issue of the proposed architecture.

A transient noise analysis has also been performed in order to assess the signal-to-noise ratio (SNR) of the pixel. The output noise is in the range of a few mV (see table I and table II) and is obviously reduced on the lower gain branches due to the presence of a higher integration capacitance keeping the input referred SNR almost constant.

CONCLUSION

A high-speed pixel architecture well suited for high speed burst image sensor is proposed. It offers a multi-HDR feature carried out with a single frame exposure which is a mandatory functionality in the framework of ultrafast imaging. The pixel counts 11 transistors for a complete gain branch plus 5 transistors for each additional gain branches. The simulation results shows that the pixel is able to operate at a frame rate from 10 Mfps up to 100 Mfps. The biasing current of the pixel as to be adjusted in regards of the required frame rate. The pixel consumption is as low as 12 µW for one single gain branch at 10 Mfps and 118 µW at a frame rate of 100 Mpfs.

To the best of our knowledge, it is the very first pixel dedicated to burst imaging sensors that provides an HDR feature.

Fig. 1 .

 1 Fig. 1. Principle of the Current Mirroring integration pixel with HDR feature

  current mirrored by the transistor M10 and M6.

Fig. 2 .

 2 Fig. 2. Schematic diagram of the pixel circuitof the current pulse which is integrated over C int . The photodiode model describes the behavior of a N Well over P sub photodiode with the assessment of the doping profiles for the used technology. For the higher conversion gain branch, the integration capacitor C int is set to 10 fF which is close to the parasitic capacitors and buffer input capacitor values. Two low gain additional branches have been added with an integration capacitance of respectively 50 and 100 fF. Fig.3 and 4show the simulation results at a frame rate of 10 Mfps and 100 Mfps. In order to match the required bandwidth, the I bias current has been set to respectively 1.2 µA and 12 µA for the frame rate of 10 and 100 Mfps. A new frame starts with the release of the reset which initialized the output voltage to 1.1 V. The V DD power supply is set to 3.3 V to achieve a high output voltage range close to 1.8 V. Two pulses of light are emulated on frame 2 and 4 thank to the photodiode model. The pulses start at the very beginning of the specified frames and last for half the period of a frame exposure.

Fig. 3 .Fig. 4 .

 34 Fig.3. Circuit simulation at 10Mfps, I bias = 1.2µA, generated photocurrent (dashed line), output voltage with a integration capacitance of 10 fF (red), 50 fF (green) and 100 fF (Blue)

Fig. 5 .

 5 Fig. 5. Monte-Carlo simulations of the output node at 10 Mfps (up) and 100 Mfps, without photonic current,i.e. dark image. The results represent the fixed patern noise (FPN).

Fig. 6

 6 Fig. 6 Monte-Carlo simulations of the output node at 10 Mfps (up) and 100 Mfps, with photonic current,i.e. bright image. The results represent the pixel response non uniformity (PRNU).

TABLE I .

 I SIMULATION RESULTS AT A FRAME RATE OF 10 MFPS

	At 10 Mfps	Cint = 10 fF	Cint = 50 fF	Cint = 100 fF
	Gain	6.8 µv/e -	2.6 µv/e -	1.4 µv/e -
	Temporal Noise rms	σ = 8.1 mV	σ = 2.8 mV	σ = 1.7 mV
	Output dynamic	ΔV = 1.7V	ΔV = 1.7V	ΔV = 1.7V
	SNR	209	607	1000
	FPN rms	σ = 145.2 mV	σ = 54.5 mV	σ = 30.3 mV
	PRNU	1.4%	1.3%	1.3%
	Total power consumption	12µW	+4µW	+4µW
	TABLE II.	SIMULATION RESULTS AT A FRAME RATE OF 100 MFPS

At 100 Mfps Cint = 10 fF Cint = 50 fF Cint = 100 fF

	Gain	6.6 µv/e -	2.4 µv/e -	1.2 µv/e -
	Temporal Noise rms	σ = 5.4 mV	σ = 5.1 mV	σ = 2.7 mV
	Output dynamic	ΔV = 1.7V	ΔV = 1.7V	ΔV = 1.7V
	SNR	314	333	629
	FPN rms	σ = 63.1 mV	σ = 28.1 mV	σ = 17 mV
	PRNU	0.9%	1%	1.6%
	Total power consumption	118µW	+40µW	+40µW

ACKNOWLEDGMENT

This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 777222 through the ATTRACT Digital Burst Image Sensor (DIBIS) Project.

III. REFERENCES


Paul Baptiste
email: baptiste.revol@gmail.com

Revol

Madeline Vauthier

Matthieu Thomassey

Michel Bouquey

Frédéric Ruch

Michel Nardin

Design of Experience to Evaluate the Interfacial Compatibility on High Tenacity Viscose Fibers Reinforced Polyamide-6 Composites

Keywords: A. Natural fiber composites, B. Fiber/matrix compatibilization, B. Surface treatments, C. Design of experience, E. Plasma treatment

In recent years, natural fibers reinforced composites have received much attention because of their lightweight, recyclable, nontoxic, low cost and biodegradable properties. However, compatibility between hydrophilic natural fiber and hydrophobic polymer is low. One way to increase the fiber/matrix compatibility is to treat the fiber surface, using hydroxyl groups present on the upper layer of the high tenacity viscose. However, hydroxyl groups are not always accessible on the fibers surface due to impurities. It is thus mandatory to study the various possibilities to determine a good cleaning treatment, which furthermore does not have a negative influence on the mechanical properties of the fibers. In this article, (3-aminopropyl)triethoxysilane (APTES) is used to functionalize the fiber surface after cleaning them, in order to prepare the fibers for reactive injection of polyamide-6. In order to determine the influence of each cleaning/treatment on the physical and chemical properties of the fibers and to determine optimal treatments to increase fibers/matrix compatibility, various characterization methods (XPS, FTIR-ATR, SEM, EDX) were combined to theoretical results obtained by a design of experience.

Introduction

Currently, composites using a thermosetting polymer matrix are mainly used compared to thermoplastic ones. [START_REF] Johnson | Thermoplastic vs. Thermoset Resins[END_REF][START_REF] Strong | Fundamentals of Composites Manufacturing: Materials, Methods, and Applications[END_REF] However, thermosetting polymers are hardly recyclable and may require the use of pollutant solvents. For environmental reasons and to have good mechanical properties, we decided to use thermoplastic polymer (polyamide-6) matrix reinforced by high tenacity viscose (HTV) fibers. [START_REF] Pickering | A review of recent developments in natural fibre composites and their mechanical performance[END_REF][START_REF] Revol | Single fibre model composite: Interfacial shear strength measurements between reactive polyamide-6 and cellulosic or glass fibres by microdroplet pullout test[END_REF][START_REF] Nikonovich | The supermolecular structure of viscose fibers and its dependence on spinning conditions[END_REF][START_REF] Caramaro | Fibres et Fils à Usage Technique[END_REF] Indeed, Pickering et al. described the recent interest for natural fiber composites due to their low environmental impact (renewable source, little energy), low cost and their wide range of applications (for instance door panels, railcars, engine, sun visors, seat backs or exterior panels) [START_REF] Pickering | A review of recent developments in natural fibre composites and their mechanical performance[END_REF] but, none of the presented studies used HTV fibers as reinforcement. For example, while polyamide-6 was used with cellulose reinforcement [START_REF] Fernandes | Polyamide-6 composites reinforced with cellulose fibers and fabricated by extrusion: Effect of fiber bleaching on mechanical properties and stability[END_REF][START_REF] Peng | Characterization of mechanical and morphological properties of cellulose reinforced polyamide 6 composites[END_REF] or with natural fibers (pineapple leaf fibers) [START_REF] Panyasart | Effect of Surface Treatment on the Properties of Pineapple Leaf Fibers Reinforced Polyamide 6 Composites[END_REF] it was never used with continuous HTV fibers implemented by reactive injection. Experimentally, fibers are wetted with a reactive mixture composed of ε-caprolactame, an initiator (sodium caprolactamate) and an activator (N,N-hexane-1,6-diylbis(hexahy-dro-2-oxo-1H-azepine-1-carboxamide)). This technique has two major advantages: (i) experimental conditions are very close to real composite reactive transfer molding (RTM) [START_REF] Van Rijswijk | Reactive processing of textile fiber-reinforced thermoplastic composites -An overview[END_REF][START_REF] Thomassey | Interest of a Rheokinetic Study for the Development of Thermoplastic Composites by T-RTM[END_REF] and (ii) the cellulosic fibers are not degraded during preparation. When optimized, the high tenacity viscose/polyamide-6 system could find applications in the automobile sector, [START_REF] Dahlke | Natural Fiber Reinforced Foams Based on Renewable Resources for Automotive Interior Applications[END_REF] with the production of recyclable composites presenting high mechanical properties. Moreover, it is well known that the fiber/matrix interface compatibility plays a crucial role in the transfer of mechanical properties from the polymer matrix to the fiber. [START_REF] George | A review on interface modification and characterization of natural fiber reinforced plastic composites[END_REF][START_REF] Le Moigne | Interfaces in Natural Fibre Reinforced Composites: Definitions and Roles[END_REF] It is then mandatory to investigate and improve this compatibility. In this purpose and in order to limit the influence of the cellulosic surface on the polymerization, fibers' cleaning and functionalization are fundamental steps. [START_REF] Kalia | Pretreatments of natural fibers and their application as reinforcing material in polymer composites -A review[END_REF][START_REF] Rozman | The effect of lignin as a compatibilizer on the physical properties of coconut fiber-polypropylene composites[END_REF][START_REF] Sain | Interface Modification and Mechanical Properties of Natural Fiber-Polyolefin Composite Products[END_REF][START_REF] Rizal | Interfacial Compatibility Evaluation on the Fiber Treatment in the Typha Fiber Reinforced Epoxy Composites and Their Effect on the Chemical and Mechanical Properties[END_REF] Indeed, the surface of cellulosic fibers can be naturally (triglycerides, waxes, hemicellulose, pectin, proteins or minerals) or artificially (sizing agents and oils used during the implementation) contaminated. [START_REF] Karmakar | Chemical Technology in the Pre-Treatment Processes of Textiles[END_REF] As a first step, the most common cleaning treatments (alkaline solution, [START_REF] George | A review on interface modification and characterization of natural fiber reinforced plastic composites[END_REF][START_REF] George | Improving the accessibility of hemp fibres using caustic to swell the macrostructure for enzymatic enhancement[END_REF] surfactant in a basic medium, [START_REF] Karmakar | Chemical Technology in the Pre-Treatment Processes of Textiles[END_REF] hot ethanol solution [START_REF] Narjès | A study of the surface properties of cotton fibers by inverse gas chromatography[END_REF][START_REF] Belgacem | Surface characterization of cellulose fibres by XPS and inverse gas chromatography[END_REF] and argon or oxygen plasma treatment [START_REF] Morent | Non-thermal plasma treatment of textiles[END_REF][START_REF] Vesel | Plasma modification of viscose textile[END_REF][START_REF] Kalia | Surface modification of plant fibers using environment friendly methods for their application in polymer composites, textile industry and antimicrobial activities: A review[END_REF]) are tested on the HTV fibers. Once the most effective cleaning treatment has been selected, it is applied in combination with a functionalization with (3aminopropyl)triethoxysilane. [START_REF] Abdelmouleh | Interaction of Silane Coupling Agents with Cellulose[END_REF][START_REF] Bledzki | Composites reinforced with cellulose based fibres[END_REF][START_REF] Brochier Salon | Hydrolysis-Condensation Kinetics of Different Silane Coupling Agents, Phosphorus, Sulfur, and Silicon and the Related Elements[END_REF] It is then necessary to determine parameters for verifying the effectiveness of the treatment of viscose. For instance, the probability of failure modeled according to a 2-parameter Weibull equation [START_REF] Revol | Influence of the sample number for the prediction of the tensile strength of high tenacity viscose fibres using a two parameters Weibull distribution[END_REF][START_REF] Weibull | A statistical theory of strength of materials[END_REF] was determined to ensure that treatments did not degrade the mechanical properties of the fibers. Finally, a design of experience is carried out to study the interactions between the surface properties (chemical and physical one) after cleaning and/or functionalization steps and the conversion rate of PA6 polymerization. The design of experience brings new information about the fibers/matrix interactions taking place during the preparation of HTV/PA6 composites.

Materials & methods

Materials

Non-treated high tenacity viscose fibers "Viscord Bohemia Super 2" were supplied by TextilCord (Luxembourg) in the form of a yarn of 2440 dtex containing 1320 filaments in the bundle. The density of viscose was 1.5 10 3 kg/m 3 and the theoretical fiber diameter was equal to 12.5 µm (value calculated from the data given by the supplier).

Concerning the polymeric resin of polyamide-6 (PA6), the reactive mixture consisted of three components: (i) a monomer, ε-caprolactam (CL) named AP-Nylon® Caprolactam solid (99%, Mw = 113.16 g/mol), (ii) an initiator, Brüggolen C10 (sodium caprolactamate, 1 mol/kg concentration in caprolactam) and (iii) an activator, Brüggolen C20P (N,N-hexane-1,6diylbis(hexahy-dro-2-oxo-1H-azepine-1-carboxamide), 2 mol/kg concentration in caprolactam).

CL, C10 and C20P were supplied in sealed polyethylene-lined aluminum drums (Brüggemann Chemical, Germany). After opening, sealed boxes were properly closed and stored in freezing bags filled with dry argon atmosphere.

Pluronic® F-127 (Sigma), a triblock copolymer of hydrophobic polypropylene glycol framed by two hydrophilic blocks of polyethylene glycol (PEG-PPG-PEG), was used as a non-ionic surfactant. The silane compound, (3-aminopropyl)triethoxysilane (APTES), was supplied by Sigma. All products were used as received without further processing or purification.

Methods

Surface cleaning and functionalization of high tenacity viscose (HTV) fabrics were carried out using a design of experience to determine the different interactions that can occur. The treatment conditions were summed up in Table 1 after selecting four factors: the plasma power, the plasma treatment time, the APTES amino-silane concentration and the APTES treatment time.

Liquid phase cleaning

Sodium solution (NaOH) treatments were carried out in beakers at room temperature for 30 minutes, with 1 g of fiber per 20 mL of solution. The concentration of NaOH was equal to 0.5w% or 8w%. Pluronic® F-127 concentration was equal to 0.1w%. It was combined with sodium carbonate (0.1w%) for the treatment to take place in basic medium.

Ethanol treatments (EtOH) were carried out at 50°C for 30 minutes or under Soxhlet for 6 hours.

Plasma treatment

Plasma treatments were carried out in a closed plasma-assisted chemical vapor deposition (PACVD) reactor from PLASSYS (MDS 100, pressure down to 2 x 10 -7 mbar). The reactor was equipped with a gas inlet and outlet. For the preliminary cleaning tests, plasma treatments on HTV fabrics were carried out for 5 minutes at a fixed power (50 W) at a constant pressure (1 mbar) in a closed chamber, saturated with argon or oxygen gas. Then, for the design of experience, plasma treatments were performed in the same chamber, with a power ranging from 10 to 60 W for a given time.

Surface functionalization

Functionalization treatments with (3-aminopropyl)triethoxysilane (APTES) were carried out after plasma cleaning, in ethanol/water (80/20) medium for a given time and with a given APTES concentration. Treatments were carried out at room temperature on fabric surfaces approximately equal to 5x7 cm². After treatment, fabrics were put in an oven at 120°C for 2 hours and then cleaned with ethanol for 30 minutes under gentle stirring.

All characterization techniques are described in Supporting Information (1. Experimental section).

Results and discussion

Cleaning treatments

High tenacity viscose (HTV) was tested in two forms: in the form of filaments, and in the form of fabrics. Preliminary cleaning tests were carried out on HTV filaments. Some of the most common cleaning methods described in the literature (alkaline solution, [START_REF] George | A review on interface modification and characterization of natural fiber reinforced plastic composites[END_REF][START_REF] George | Improving the accessibility of hemp fibres using caustic to swell the macrostructure for enzymatic enhancement[END_REF] surfactant in a basic medium, [START_REF] Karmakar | Chemical Technology in the Pre-Treatment Processes of Textiles[END_REF] hot ethanol solution [START_REF] Narjès | A study of the surface properties of cotton fibers by inverse gas chromatography[END_REF][START_REF] Belgacem | Surface characterization of cellulose fibres by XPS and inverse gas chromatography[END_REF] and argon or oxygen plasma treatment [START_REF] Morent | Non-thermal plasma treatment of textiles[END_REF][START_REF] Vesel | Plasma modification of viscose textile[END_REF][START_REF] Kalia | Surface modification of plant fibers using environment friendly methods for their application in polymer composites, textile industry and antimicrobial activities: A review[END_REF]) were first tested on the filaments to determine the treatment of greatest interest for possible industrial applications.

Liquid phase cleanings were carried out with NaOH, sodium carbonate or ethanol solutions. NaOH (concentration equal to 0.5w% or 8w%) and sodium carbonate (concentration equal to 0.1w%) treatments occurred at room temperature, while ethanol cleaning was carried out at 50°C. X-ray photoelectron spectroscopy was known to be an effective way to check the surface condition before and after cleaning steps. [START_REF] Belgacem | Surface characterization of cellulose fibres by XPS and inverse gas chromatography[END_REF][START_REF] Rjiba | Comparison of surfaces properties of different types of cotton fibers by inverse gas chromatography[END_REF] It was important to point out that impurities (zinc, sulfur, nitrogen) were present in small amounts on the surface because of the cellulose treatments. Some traces of silicon and sodium were sometimes observed. However, these contaminations did not influence the results discussed (Table 2). The O/C ratio gave an indication of the amount of hydroxyl groups available on the surface. The amount of C-C groups gave a direct indication of the amount of impurities present at the surface because the theoretical cellulose did not have any. Finally, COOR groups were of interest when the functionalization of the surface was taken into consideration.

NaOH treatments were commonly used (mercerization) and can effectively clean the surface in many cases. [START_REF] George | A review on interface modification and characterization of natural fiber reinforced plastic composites[END_REF][START_REF] George | Improving the accessibility of hemp fibres using caustic to swell the macrostructure for enzymatic enhancement[END_REF] However, its influence was not significant here. Indeed, the amount of C-C groups did not change between uncleaned viscose and after 8w% NaOH treatment, and decreased from 15% to 12% in the case of treatment at 0.5w% NaOH. However, the O/C ratio varied from 53 to 60% for both sodium hydroxide concentrations. A decrease in free -OH groups after treatment with sodium hydroxide was sometimes observed. [START_REF] Ramamoorthy | Effect of alkali and silane surface treatments on regenerated cellulose fibre type (Lyocell) intended for composites[END_REF] Since the surface treated with sodium hydroxide was swollen and reactive, [START_REF] Roy | Étude de mélanges de cellulose dans des solutions aqueuses de soude[END_REF] it may react with impurities at the end of the treatment.

Sodium carbonate (Na2CO3) treatments were often used for basic cleaning in combination with a polar or an apolar surfactant. [START_REF] Karmakar | Chemical Technology in the Pre-Treatment Processes of Textiles[END_REF][START_REF] Peršin | Estimation of the surface energy of chemically and oxygen plasma-treated regenerated cellulosic fabrics using various calculation models[END_REF] In this study, the combination of Pluronic® F-127 (an apolar surfactant) with Na2CO3 gave interesting results since the O/C ratio increased from 53% to 61% without changing the amount of C-C groups. So, a larger proportion of the COOR groups for this cleaning may correspond to C-O-H groups but impurities remained present on the surface.

More interesting results were obtained by extraction in ethanol. Indeed, the O/C ratio approached the expected theoretical ratio (from 53% to 68%) and the amount of C-C groups decreased to 5%.

These results were consistent with those of Rjiba et al. for cotton cleaning by extraction under

Soxhlet in ethanol. [START_REF] Narjès | A study of the surface properties of cotton fibers by inverse gas chromatography[END_REF] Despite interesting results obtained with liquid phase treatments by extraction in ethanol, this treatment lacked industrial adaptability (mandatory heating, impurities brought to the surface, use of organic solvent, fibers swelling, etc.). In this purpose, further research was conducted by studying the efficiency of a plasma treatment.

Plasma treatment to efficiently clean high tenacity viscose filaments

Plasma technology offers many interesting possibilities for the production of high value-added materials. Here, plasma treatments with argon or oxygen gas were carried out for 5 minutes (power = 50 W) before analyzing the fibers by XPS. These results were compared to reference, untreated fibers (Table 3). Argon plasma treatment should have theoretically cleaned the surface without functionalizing it. However, the O/C ratio increased significantly (from 53 to 70%) after this treatment. This was also the case for the amount of C-C groups (from 14.7% to 17%) and COOR groups (from 0.7% to 8.2%). COOR groups corresponded to functionalization of the surface and C-C groups usually came from the presence of surface impurities. So, argon plasma treatment cleaned the surface while degrading it (C-C groups formed by chain breaks). Since the reactive, cleaned surfaces were exposed to air for a short time before XPS analysis, the surfaces may have partially been functionalized by the oxygen of the air. This method (functionalizing polymers through exposure to atmosphere after argon plasma treatment) had actually been used by Short's team. [START_REF] France | Plasma Treatment of Polymers: The Effects of Energy Transfer from an Argon Plasma on the Surface Chemistry of Polystyrene, and Polypropylene. A High-Energy Resolution X-ray Photoelectron Spectroscopy Study[END_REF] In the case of oxygen plasma, O/C ratio was equal to 83%, (similar to that of the theoretical cellulose). The surface was not degraded (amount of C-C groups equal to 2.4%) and functionalized (COOR groups present at 8.2%). Indeed, unlike argon plasma, oxygen plasma was known to lead to a combined effect of cleaning and functionalization. [START_REF] Vesel | Plasma modification of viscose textile[END_REF][START_REF] Peršin | Characterization of surface properties of chemical and plasma treated regenerated cellulose fabric[END_REF] For both types of plasma, a functionalization of the surface was observed. However, this was probably not caused by the same mechanism. In the case of argon treatment, the surface was cleaned and reactive after treatment, but also degraded. Subsequently, this surface reacted with the oxygen present in the air. In the case of treatment with oxygen plasma, the surface was better cleaned and not degraded (decrease in the amount of C-C groups), but also functionalized by reactions with oxygen during the plasma treatment. Similar observations and conclusions were made by Hua et al. when comparing oxygen and argon treatments on a cellulosic surface. [START_REF] Hua | Mechanisms of oxygen-and argon-RF-plasmainduced surface chemistry of cellulose[END_REF] So, oxygen plasma was an interesting choice to clean (and partially functionalize) cellulosic surfaces. It was however important to note that the effect of plasma treatment on the physical and chemical properties of the surface strongly depended on the process parameters. [START_REF] Morent | Non-thermal plasma treatment of textiles[END_REF][START_REF] Vauthier | Control of Interfacial Diels-Alder Reactivity by Tuning the Plasma Polymer Properties[END_REF][START_REF] Tsang | Comparative studies of physical and chemical properties of plasma-treated CVD low k SiOCH dielectrics[END_REF]

High tenacity viscose fabrics cleaning with various processes

The previous parts showed interesting results concerning plasma cleaning for HTV filaments. It was also mandatory to verify these results for a fabric, having more surface impurities. In Table 4, plasma treatment was proved to be effective for cleaning the fabrics' surface, despite a surface initially having a large amount of impurities. Indeed, O/C ratios were lower than in the case of the filaments, cleaning was at least as effective since this ratio increased by 118% between an untreated and a 5-minute treated sample. Moreover, the effectiveness of the treatment was dependent on the treatment time. Indeed, the O/C ratio was constant for a 5-minute treatment (O/C = 70% for 10 W and 60 W). The proportions of the different groups were similar in both cases. So, a maximum level of cleaning was achieved under these two conditions. A 10 W, 1 min plasma treatment was carried out and the amount of C-C groups did not change and remained about 48% but the O/C ratio increased from 32% to 36%, proving that this cleaning was not effective. Moreover, a 60 W, 1 min plasma treatment ended up with an O/C ratio increase up to 52% and a decreasing amount of C-C groups down to 36%, suggesting that the cleaning of the surface began but was not complete. The amount of COOR groups however reached a value of approximately 10%, equal to that reached after 5-minute treatments.

These observations suggested that, in these conditions, plasma processing was power-and timedependent until a maximum level of cleaning was reached beyond which process time has no influence. For a higher power and/or a longer processing time, we guessed that the surface may have been degraded by the plasma treatment. [START_REF] Liston | Plasma surface modification of polymers for improved adhesion: a critical review[END_REF][START_REF] Ružbarský | Plasma Jet[END_REF][START_REF] Klemberg-Sapieha | Control and Modification of Surfaces and Interfaces by Corona and Low-Pressure Plasma, Interfacial Interact[END_REF] This solventless, vapor phase treatment seemed more interesting for industrial applications than liquid phase cleaning. However, it was mandatory to determine whether such a treatment had an influence on the mechanical properties of the HTV, and to what extent the surface quality influenced the functionalization mechanisms.

Oxygen plasma cleaning

In this part, the effect of oxygen plasma treatment was verified by comparing the properties of untreated HTV filament (CNRT) and oxygen plasma treated filament extracted from bundles at 50 W for 5 minutes (CRO2).

Mechanical properties

The diameter of the monofilaments was first measured from a longitudinal observation of the monofilaments (Figure 1) and no significant difference was observed whether the filament was cleaned with 50 W, 5 min oxygen plasma (13.9 ± 1 μm) or not (13.4 ± 1 μm). So, the same diameter value was used for mechanical properties calculus in both cases to allow the comparison of the mechanical properties. Indeed, in order to ensure that plasma treatment did not degrade the mechanical properties of the fibers, they had been tensile tested and compared with untreated fibers (Figure 2). In Figure 2.a, for an untreated filament (CRNT), the average value of rupture stress was 586 ± 64 MPa while it was equal to 636 ± 76 MPa for the plasma treated filament. As a first approximation, there was no significant difference in the rupture stress between these two samples.

The Weibull modulus [START_REF] Weibull | A statistical theory of strength of materials[END_REF] was then estimated to be 10 for the treated filament, compared with a nearly similar value of 11 for the untreated one, which meant that the distribution of defects in the monofilaments was almost identical. In Figure 2.b, the probability of failure modeled according to a 2-parameter Weibull equation was represented and proved that the O2-plasma treatment had no influence on the rupture stress. Finally, mechanical tests also revealed no significant difference of Young's modulus and elongation at break between treated and untreated filaments (Figure 3). Indeed, Young's moduli were respectively equal to 12 ± 2 GPa and 15 ± 3 GPa for treated and untreated viscose filaments and elongation at break was equal to 12 ± 3% for the treated filament and 13 ± 2% for the untreated one.

In conclusion, oxygen plasma treatment did not modify the mechanical properties of the HTV fabrics, proving that the treatment did not degrade the fabrics. However, the influence on surface properties was clearly spotted by XPS analyzes. Although plasma treatment was effective, it was difficult to deeply analyze its influence. This was one of the reasons why cleaning and surface treatment were simultaneously investigated for further analysis.

Cleaning and treatment of high tenacity viscose filaments

The objective here was to treat a surface cleaned by oxygen plasma with (3aminopropyl)triethoxysilane (APTES). However, plasma treatment conditions must be optimized to allow the best cleaning. In addition, whatever the plasma treatment used, the properties of the cleaned surface evolved in contact with air. It was therefore decided to analyze the synergy of cleaning and treatment by functionalizing the surface just after cleaning. In this way, by optimizing the conditions of cleaning and functionalization, the influence of air on the reactive surface was limited.

A design of experience (Table SI1) approach allowed us to determine the interactions between the different factors (cleaning and functionalization parameters). Thus, it was theoretically possible to quantify the effect of plasma cleaning on the functionalization treatment.

Determination of silane amount and polymerization conversion rate

Before considering the implementation of the design of experience, it was necessary to define a quantity to quantify the effectiveness of cleaning. This was why the amount of nitrogen on the surface of the samples was investigated by Fourier transform infrared spectrometry (FTIR-ATR) on the treated fabrics. Indeed, this parameter indicated the total amount of amino-silane APTES grafted and/or condensed on the surface of the sample. Moreover, the degree of conversion of polyamide-6 after wetting the treated fibers in the reaction mixture, an interesting parameter to verify that the treatment had no negative influence on the polymerization, was previously studied and published by our group. [START_REF] Revol | Single fibre model composite: Interfacial shear strength measurements between reactive polyamide-6 and cellulosic or glass fibres by microdroplet pullout test[END_REF]

Infrared spectroscopy

By infrared spectroscopy (FTIR-ATR), we observed a variation in peak intensity (absorbance) depending on the treatment conditions. In Figure 4, the peak at 1560 cm -1 on the treated fibers corresponded to the -NH2 groups present on the surface of the samples. [START_REF] Weibull | A statistical theory of strength of materials[END_REF][START_REF] Berger | Statistical analysis of the failure stresses of ceramic fibres: Dependence of the Weibull parameters on the gauge length, diameter variation and fluctuation of defect density[END_REF] This peak was absent for the untreated HTV, so it allowed to determine the total amount of APTES present on the surface, including the grafted or not grafted amino-silane in hydrolyzed or condensed form.

The FTIR curves were normalized with the peak at 1640 cm -1 which correspond to adsorbed water.

The correlation between the peak corresponding to the -NH2 groups and the amount of silane at the surface was verified in the following section. A not shown peak at 1740 cm -1 only for the untreated fabric corresponded to C=O bonds [START_REF] Abdelmouleh | Interaction of Silane Coupling Agents with Cellulose[END_REF] due to impurities. In some cases, a peak at 1480 cm -1 was observed, it could correspond to -NH3 + groups formed by protonation of -NH2 groups during exposure to air. [START_REF] Andersons | Modeling strength scatter of elementary flax fibers: The effect of mechanical damage and geometrical characteristics[END_REF] Other peaks could have been interesting to observe, especially the Si-OH groups (3030-3630 cm - 1 [START_REF] Weibull | A statistical theory of strength of materials[END_REF]) to determine the amount of hydrolyzed silane, or Si-O-C (1104 cm -1 ). However, the characteristic peaks of the HTV were very intense in these areas (-OH 3380 cm -1 , COC and CO between 1159 and 1000 cm -1 ), it is therefore difficult to analyze silane peaks at these wavenumbers.

X-ray Photoelectron Spectroscopy (XPS)

Analyzing XPS spectra allowed the determination of the amount of nitrogen present on the surface.

Since the untreated fabric did not possess (or in negligible quantity) nitrogen groups, it was assumed that the nitrogen amount visible by XPS analysis is totally due to the presence of silane (grafted, condensed or not grafted and uncondensed hydrolyzed) on the surface of the fabric. An XPS analysis was performed on four processed samples to verify the correlation between the amount of nitrogen quantified by XPS and the peak intensity of the -NH2 groups observed by FTIR-ATR at 1560 cm -1 (Figure 5). The analysis of the intensity of the peak at 1560 cm -1 by FTIR-ATR was therefore an effective means of evaluating the amount of nitrogen present on the surface of the HTV.

Design of experience

The first step was to define the factors of interest for the study as well as the experimental field explored. Factors identified as having an influence on the results were: plasma power, plasma treatment time, APTES concentration and APTES treatment time (Table 1). The treated samples always had the same surface of 5x7 cm². The levels of plasma power and plasma time factors were selected after preliminary experiments on cotton and high tenacity viscose (HTV). The APTES concentration parameters and APTES treatment time were determined from the literature. [START_REF] Abdelmouleh | Interaction of Silane Coupling Agents with Cellulose[END_REF][START_REF] Brochier Salon | Silane adsorption onto cellulose fibers: Hydrolysis and condensation reactions[END_REF] Once the factors and their levels of variation had been determined, it was possible to draw up an design of experience (Table SI3).

The conversion rate of polyamide-6 determined by 1 H-NMR (Y2) varied from 57% to 72%, except in one case. It was interesting to note that the influence of the treatment conditions on the conversion rate was relatively limited, except in the case of experiment 13 (Y2 = 16%) (Table SI3).

In this particular case, the tissue was poorly cleaned (10 W, 1 min) and the treatment was carried out at high concentration for a long time (5w%, 2 h). Maybe APTES was present in excessive amounts on the surface of the sample in grafted and condensed form, but also in hydrolyzed form which would then disrupt the polymerization.

In the studied experimental domain, the four best conversion rates were obtained for experiments number 6, 10, 11 and 16 (low amount of APTES) with values of 69, 69, 72 and 72% respectively.

The conversion rate after wetting a reference sample of untreated HTV by the reactive mixture (and polymerization) was 61%. The plasma/APTES treatment therefore allowed a slight improvement in the polymerization of the reaction mixture. From an experimental point of view, it would then be possible to choose one of these conditions to allow the fabrication of composites whose polymerization would theoretically not be disturbed.

The normalized infrared intensity (i.e., the amount of nitrogen at the surface of the samples) varied from 0.5 to 4 a.u. However, at this stage of the analysis, it was impossible to conclude on the optimum amount of APTES to add. Indeed, a greater amount of silane on the surface of the HTV did not necessarily imply a better fiber/matrix interface. For example, the largest amount of APTES corresponded to the lowest conversion rate (experiment 13). So, it probably existed a maximum amount of silane beyond which the polymerization was disturbed.

In order to deepen the analysis, the interactions between the factors were investigated in order to determine which parameters had the greatest influence on the responses studied (namely the amount of nitrogen groups and the conversion rate).

Coefficients study: amount of silane on the treated fibers

Incertitude on the coefficients was evaluated in order to only consider the significative coefficients (Table 5). The study of the coefficients was first carried out on the amount of nitrogen present on the surface, determined by infrared spectroscopy. The details of the coefficient calculations were explained in Supporting Information.

The significant coefficients for the amount of silane on the fibers are presented in Figure 6. For example, when the concentration of (3-aminopropyl)triethoxysilane (coefficient C) increased from 0.5w% to 5w%, the amount of nitrogen present at the surface of the sample increased by an average of 0.43*2 = 0.86 (around 60%). With the same analyze, we noted that two single interactions (C and D) and two multiple interactions (ABD and ABCD) had a positive effect on the amount of nitrogen at the surface. The only interaction having a negative impact was the one combining plasma time, APTES concentration and APTES time (BCD).

Single interactions

The design of experience proved that the APTES concentration (C) had a positive effect on the amount of silane on the surface (Figure 6). In 2002, Gandini's team demonstrated that the adsorption of APTES-type silanes on the surface of cellulose was dependent on the initial concentration [START_REF] Abdelmouleh | Interaction of Silane Coupling Agents with Cellulose[END_REF] and, according to the work of Brochier-Salon et al., the percentage of hydrolyzed silane was constant during a fixed time. [START_REF] Brochier Salon | Hydrolysis-Condensation Kinetics of Different Silane Coupling Agents, Phosphorus, Sulfur, and Silicon and the Related Elements[END_REF] So, it was consistent to observe an effect of the APTES concentration independently of the other parameters because APTES could react with a -OH group of the cellulosic surface and with a -OH group available on a silane molecule already grafted on the surface.

When APTES treatment time (D) rised from mins to 2 hrs, the amount of APTES on the surface increased for around 0.86 a.u., i.e. around 60%. The dependence of the amount of silane present at the surface as a function of time had also been observed for the functionalization of porous silicone surfaces by Aissaoui et al and by Majoul et al. They reached a plateau value after respectively 12 hours [START_REF] Aissaoui | Silane Layers on Silicon Surfaces: Mechanism of Interaction, Stability, and Influence on Protein Adsorption[END_REF] and 24 hours [START_REF] Majoul | Progress of porous silicon APTES-functionalization by FTIR investigations[END_REF]. This increase was attributed to the adsorption of APTES by the surface to an upper limit and the grafting mechanisms were represented by a model of nucleation and growth on a heterogeneous surface.

Multiple interactions

Although interactions ranked higher than two were difficult to interpret from a physical point of view, it was intriguing to note that plasma processing parameters (A and B) had an influence as multiple interactions (ABD, BCD and ABCD) but were not significative as a double interaction. These interactions were still interesting to determine that the silane grafting was influenced by the surface cleaning and to verify that the cleaning of the surface depended on the plasma conditions.

Coefficients study: polyamide-6 conversion rate

Incertitude on the coefficient was evaluated in order to only consider the significative coefficients (Table 5). As presented in Figure 7, almost all factors were significant for the study of the conversion rate.

Single interactions

Interestingly, plasma treatment time (D) had an influence on the amount of silane but none on the conversion rate. Since better cleaning previously resulted in a better conversion rate, it was normal to observe that plasma power (A) and plasma treatment time (B) had a positive influence on the conversion rate (Figure 7). However, increasing APTES concentration (C) had a negative effect on the conversion rate while it had a positive effect on the amount of silane at the surface (Figure 6). This observation was consistent with the experiment 13 previously discussed (Table SI3) for which the high amount of silane at the surface (4 a.u.) was accompanied by a low conversion rate (16%). So, it was necessary to graft a sufficient quantity of APTES to improve the fiber/matrix interface, but low enough not to disturb the polymerization reaction.

Multiple interactions

By comparing Figure 6 and Figure 7, it was interesting to observe that double interactions (AB, AC, AD, BD, CD) had an influence on the PA6 conversion rate but not on the amount of silane at the surface. First, the AB interaction had a negative influence on the conversion rate whereas plasma power (A) and plasma treatment time (B) had a positive influence when considered individually. Thus, a combination of high power/high time lead to a degradation of the surface [START_REF] Yasuda | The competitive ablation and polymerization (CAP) principle and the plasma sensitivity of elements in plasma polymerization and treatment[END_REF][START_REF] Siffer | A chemometric investigation of the effect of the process parameters during maleic anhydride pulsed plasma polymerization[END_REF] and/or the creation of new functional groups (Table 4), disrupting the polymerization reaction. Then, correlations between one cleaning-parameter (A or B) and one functionalizationparameter (C or D) demonstrated that the combined effect of cleaning and functionalization had a positive effect on the conversion rate (see AC, AD and BD interactions). Finally, CD interaction (APTES concentration and APTES treatment time) negatively affected the PA6 conversion rate, confirming that too much silane on the surface interfered with the PA6 polymerization.

Concerning triple interactions, we interestingly observed that ABD interaction had a negative effect on the conversion rate (Figure 7) but a positive one on the amount of silane (Figure 6) while BCD interaction had a positive effect on the conversion rate (Figure 7) but a negative one on the amount of silane (Figure 6).

As a conclusion from all the above mentioned studies, we hypothesized about silane grafting mechanisms in the presented case: the hydrolyzed silane was grafted onto the surface-available -OH groups, possibly according to a Langmuir model. [START_REF] Abdelmouleh | Interaction of Silane Coupling Agents with Cellulose[END_REF] In parallel, it condensed both in the solution and on the surface (eventually according to a nucleation and growth model [START_REF] Vollhardt | Nucleation in insoluble monolayers. 1. Nucleation and growth model for relaxation of metastable monolayers[END_REF]). This approach was consistent with the mechanisms reported in Schwartz's review on the formation of monolayers. [START_REF] Schwartz | Mechanisms and Kinetics of Self-Assembled Monolayer Formation[END_REF] It would be interesting to further correlate the amount of silane condensed on the surface with the amount of APTES present in solution and the conversion rate.

Complementary characterizations

For model surfaces such as silicon wafers, the high-resolution X-ray photoelectron spectroscopy (XPS) spectrums were useful to observe the Si-O-Si groups resulting from condensation or grafting of silanes. However, for HTV, this distinction was not possible. So, XPS wide spectrum analysis was performed on the four most interesting samples selected using the design of experience (Table 6). These results corresponded to those used to plot the correlation between the amount of nitrogen observed by XPS and by FTIR-ATR (Figure 5).

For treated samples, the presence of silane on the surface as a function of the amount of nitrogen and silicon groups was identified. As silicon was a relatively common contaminant, the amount of silane was assumed to be equal to the amount of nitrogen present at the surface of the samples. As indicated in Table 6, the amount of silane on the surface thus varied between 4.5% and 11% depending on the treatment carried out. Moreover, it was proved in the literature that a correlation existed between the amount of nitrogen present at the surface and the thickness of the layer formed on silicon wafers. [START_REF] Metwalli | Surface characterizations of mono-, di-, and tri-aminosilane treated glass substrates[END_REF] So, a quantity of nitrogen equal to 2% would then correspond to a layer of about 10 Å and a layer of about 50 Å for 4.5% of nitrogen. Curiously, percentages of nitrogen and silicon on the samples' surface were rarely equivalent although they were similar, maybe due to impurities present on the HTV.

Scanning electronic microscopy (SEM) analyses were also used to investigate the surface covering of three functionalized samples. The analysis was completed by an Energy Dispersive X-ray spectroscopy (EDX) to know the local chemical composition of the surface. The results are presented in Supporting Information (Figure SI1, Figure SI2).

As a conclusion, complementary analyzes carried out after surfaces-functionalization to complete FTIR-ATR analyzes, confirmed the presence of silane groups on the surface of treated samples.

The amount of silane at the surface was dependent on the treatment conditions and was not homogeneously distributed over the entire treated surface.

Conclusion

The study of various cleaning treatments allowed identifying an efficient and non-polluting cleaning treatment: oxygen plasma cleaning. However, the reactivity of the surface after treatment led to subsequent reactions with the air. So, a combined study of cleaning and functionalization was an interesting alternative.

Investigating the amount of silane on the fibers/fabrics surface was a useful parameter to validate the interest of the cleaning and to be aware of the interactions involved in silane grafting. After proving that the amount of silane present on the surface could easily be determined by FTIR-ATR spectra, the conversion rate was an interesting tool to verify that the polymerization was not disturbed. The complex study of interactions showed that a large amount of silane at the surface was not necessarily beneficial to the conversion rate. Indeed, a study of surface treatments with a design of experience was complex because many parameters needed to be considered. Such an approach nevertheless provided answers to implementation issues regardless of the complexity of the mechanisms that occurred. This study by design of experience revealed at least 3 treatment conditions to allow the fabrication of composites. It was also possible to control the amount of grafted silane on high tenacity viscose fibers in order to compatibilize them with a polymer (polyamide-6) matrix.

Finally, the examination of these treatments on polyamide-6 / high tenacity viscose composites properties will provide answers to processing questions in view of their use in industry. Since the fabrication of the composite materials related to this paper would induce a lot of additional experiments and results, it deserves a standalone study.

Figure Captions

Table 1. Experimental conditions used for the 4-factors, 2-levels design of experience. SI1) as well as the untreated fabric as a reference (no peak, zero intensity). Table 5. Normalized infrared intensity values and conversion ratio after treatment.

Tables and Figures

Figure 1 .

 1 Figure 1. Diameter distribution for untreated (CRNT) and O2-plasma treated (CRO2) filaments.

Figure 2 .

 2 Figure 2. a) Rupture stresses of cellulose filaments treated with oxygen plasma (CRO2) and untreated cellulose filaments (CRNT) and b) representation of the breaking probability of CRO2 and CRNT according to a 2-parameters Weibull model.

Figure 3 .

 3 Figure 3. a) Young's modulus and b) elongation at break for treated (CRO2) or untreated (CRNT) filament, with a gauge length of 34 mm.

Figure 4 .

 4 Figure 4. FTIR-ATR spectra realized with Ge crystal of an untreated fabric (blue) or after various treatments (red: 10 W 1 min, 0.5% 2 h; green: 60W 1 min, 5% 30 min).

Figure 5 .

 5 Figure 5. Relationship between the quantity of nitrogen groups determined by XPS and by FTIR-ATR. The experiments analyzed were 1, 11, 13 and 16 (see TableSI1) as well as the untreated

Figure 6 .

 6 Figure 6. Values of significant coefficients impacting the amount of nitrogen groups present at the surface of the fibers. A: plasma power; B: plasma treatment time; C: APTES concentration; D: APTES treatment time.

Figure 7 .

 7 Figure 7. Values of significant coefficients impacting the conversion rate of polyamide-6 on treated fibers. A: plasma power; B: plasma treatment time; C: APTES concentration; D: APTES treatment time. Table 6. Wide-spectrum XPS analysis of various cleaned and treated samples. *First line: plasma treatment, second line: APTES treatment. **Impurities (Zn, Cl) in a small amount (<0.5%).

Figure 2 Frequency

 2 Figure 2

Figure 4 Figure 5

 45 Figure 4

Table 2 .

 2 C1s XPS analyzes of high tenacity viscose filaments after various types of chemical cleaning treatments.

Table 3 .

 3 C1s XPS analyzes of high tenacity viscose filaments after different plasma cleaning processes.

Table 4 .

 4 C1s XPS analyzes of high tenacity viscose fabrics after oxygen plasma cleaning.

Table 2 Treatments

 2

	Table 1
	Factors		Codes	Level -	Variations Level 0	Level +
	Plasma power (W)		A	10	35	60
	Plasma time (min)		B	1	3	5
	APTES concentration (w%)	C	0.5	2.75	5
	APTES time (min)		D	30	75	120
	Uncleaned	53		14.7	73.3	11.3	0.7
	NaOH 8w%	59		15.5	71.8	12.7	0
	NaOH 0.5w%	60		11.8	73.2	14.8	0.2
	Na2CO3	61		15.6	73.5	10.6	0
	EtOH 50°C	63		7.3	76.5	15.7	0.5
	EtOH Soxhlet	68		4.9	83.4	11.7	0
	Theory (cellulose)	83		0	83	17	0
	Table 3
	Treatments	O/C ratio (%)	C-C (%)	C-O-C, C-OH (%)	O-C-O, C=O (%)	C-OOR (%)
	Uncleaned		53	14.7	73.3	11.3	0.7
	Ar plasma 50 W, 5 min		70	17.0	53.9	17.6	11.4
	O2 plasma 50 W, 5 min		83	2.4	68.7	20.7	8.2
	Theory (cellulose)		83	0	83	17	0

O/C ratio (%) C-C (%) C-O-C, C-OH (%) O-C-O, C=O (%) C-OOR (%)

Acknowledgements

The authors thank Philippe Fioux, Philippe Kunemann and Aissam Airoudj for their help with surface characterizations and access to the IS2M characterization platforms. Dr. B.P. Revol thanks gratefully Cetim Grand Est for the machines access and fundings.