Ali Hamieh

Carole Coufort-Saudejaud

Aline Couffin

Alain Liné

Christine Frances
email: christine.frances@toulouse-inp.fr

Temperature influence on MBS latex aggregate morphology

Keywords:

Introduction

The synthesis of grafted copolymers in the form of particle suspensions is often achieved by emulsion polymerization. To be used as an impact modifier for improving material properties, the latex suspension is for specific applications subjected to an aggregation process to recover the product as a dry powder with specific properties: regular shape, narrow particle diameter distribution, high bulk density for storage and transportation, and low fines content for safety and environmental reasons [START_REF] Li | Slow Pelleting Coagulation of MBS Latex[END_REF]. In particular, MBS (Methacrylate Butadiene Styrene) aggregates are used as modifiers in the PVC manufacturing process to improve impact resistance and processability. Their morphological properties have an important effect on the functional properties of the final product [START_REF] Zhou | Toughening of Polyvinylchloride by Methyl Methacrylate-Butadiene-Styrene Core-Shell Rubber Particles: Influence of Rubber Particle Size[END_REF][START_REF] Chen | Effect of the Shell Thickness of Methacrylate-Butadiene-Styrene Core-Shell Impact Modifier on Toughening Polyvinyl Chloride[END_REF][START_REF] Zhou | Effects of Core-Shell Particle Growth Manners on Morphologies and Properties of Poly(Vinyl Chloride)/(Methyl Methacrylate-Butadiene-Styrene) Blends[END_REF]. They can be controlled by monitoring the physical mechanisms during the aggregation process.

In practice, the destabilization of a stable suspension of primary particles is most often achieved adding a coagulant or changing the pH of the suspension which will act on the charged surface of the particles and lead to their collision [START_REF] Duan | Coagulation by Hydrolysing Metal Salts[END_REF]. Aggregation studies of latex colloidal systems are usually performed under turbulent conditions in a stirred tank. Spicer and al. [START_REF] Spicer | Shear-Induced Flocculation: The Evolution of Floc Structure and the Shape of the Size Distribution at Steady State[END_REF] were among the firsts to investigate the influence of the shear rate and coagulant concentration on latex aggregation in a stirred tank by studying the evolution of the size and structure of polystyrene aggregates by image analysis. The study of Kuster et al. [START_REF] Kusters | Aggregation Kinetics of Small Particles in Agitated Vessels[END_REF] also focused on the effects of the impeller speed and the concentration on latex aggregation kinetics, final size distribution, and structure of aggregates. Selomulya et al. [START_REF] Selomulya | Evidence of Shear Rate Dependence on Restructuring and Breakup of Latex Aggregates[END_REF] highlighted the shear rate dependence on the restructuring and breakup of latex aggregates. In their study, the evolution of the size and structure of latex particle aggregation induced by shear was observed using small angle light scattering. This influence of the shear rate has led some authors to analyze the impact of breakage and regrowth steps on the size and morphology of latex aggregates in stirred tanks under industrially relevant conditions [START_REF] Vaccaro | Aggregation of Concentrated Polymer Latex in Stirred Vessels[END_REF][START_REF] Soos | Effect of Shear Rate on Aggregate Size and Morphology Investigated under Turbulent Conditions in Stirred Tank[END_REF]. Other studies have examined the performance of sequenced hydrodynamic flocculation experiments inside a Taylor-Couette reactor, as the hydrodynamics and flow regimes inside this type of reactor are well characterized and controlled [START_REF] Frappier | Floc Cohesive Force in Reversible Aggregation: A Couette Laminar Flow Investigation[END_REF][START_REF] Guérin | Dynamics of Aggregate Size and Shape Properties under Sequenced Flocculation in a Turbulent Taylor-Couette Reactor[END_REF][START_REF] Guérin | Fractal Dimensions and Morphological Characteristics of Aggregates Formed in Different Physico-Chemical and Mechanical Flocculation Environments[END_REF]. Moreover, Ehrl et al. [START_REF] Ehrl | Dependence of Aggregate Strength, Structure, and Light Scattering Properties on Primary Particle Size under Turbulent Conditions in Stirred Tank[END_REF] investigated the effect of primary particle size on the latex aggregate size and structure for different values of shear rate and solid volume fraction. Furthermore, besides the effect of mixing, Kostansek [START_REF] Kostansek | Controlled Coagulation of Emulsion Polymers[END_REF] was interested in the effect of coagulation temperature on the morphology of latex aggregates. He found that the size of the aggregates increases significantly as the coagulation temperature increases above the glass transition temperature of the copolymer.

To cover most of the relevant parameters affecting more specifically the aggregation of MBS latex by charge neutralization in a stirred tank, Li et al. [START_REF] Li | Slow Pelleting Coagulation of MBS Latex[END_REF] investigated the effects of pH, temperature, feeding mode, agitation, polymer concentration, and residence time on the properties of the aggregates by image analysis. According to this study, to produce spherical aggregates with uniform size and high bulk density, the amount of coagulant must be strictly controlled to achieve a well-specified pH range. Moreover, larger aggregates appeared with increasing temperature: a linear correlation is found between temperature and average particle diameter. Whereas the increase in the agitation speed induces a decrease in the mean particle diameter and a narrower size distribution.

The objectives of the present work are to better understand the aggregation mechanisms of MBS latex and more specifically to analyze the effect of temperature on the morphological properties of aggregates, defined in terms of size, shape and fractal dimensions. The coupled influence of the physicochemistry of the medium and the hydrodynamics of the reactor on the morphological properties of MBS latex aggregates is investigated. For this purpose, aggregation experiments of a latex nanoparticle suspension in presence of a sulfuric acid solution were performed in a laboratory-scale stirred reactor under standard industrial conditions. During the different phases of the process, samples were collected and analyzed using a particle size analyzer and a microscope coupled to an image analysis software to analyze the change of size and shape distributions as well as fractal dimension over time. The experimental data were analyzed in terms of number, surface, and volume distributions to obtain information on the entire population, from primary particles to agglomerates several orders of magnitude larger. The mechanical properties of the material were also characterized via DMA (Dynamic Mechanical Analysis) analysis in order to highlight the relationship between the process conditions and the morphological properties of the agglomerates produced.

Materials and methods

Materials

The experimental study was carried out using a suspension of MBS latex nanoparticles supplied by the industrial group ARKEMA France. The mass fraction of solids in the initial suspension is 43%. The nanoparticles are made of a spherical core-shell copolymer based on polybutadiene and polymethyl methacrylate synthesized by emulsion polymerization and stabilized by the potassium salt of fatty acid (anionic surfactant) [START_REF] Navarro | Process for Core-Shell Impact Modifiers and Impact Modified Thermoplastic Composition with Enhanced Hydrolytic Resistance[END_REF]. An aqueous sulfuric acid solution having an initial concentration of 1 mol/l and a density of 1.06 Kg/l from Lch Chimie was chosen as coagulant and pellets of sodium hydroxide with a purity >98% from Sigma-Aldrich were used to neutralize the pH of the suspension after destabilization.

Characterization of the MBS latex and slurries

Morphological properties of the particles

The morphological properties of particles refer to their properties of size, shape and fractal dimension.

Laser diffraction

The particle size distributions of the initial and agglomerated suspensions were determined by laser diffraction using a Mastersizer 3000 equipped with the liquid sample dispersion unit Hydro MV (Malvern Panalytical ® ). The refractive index and the absorption index of the latex are 1.528 and 0.1, respectively. The results will be presented as volume, surface or number distributions according to particle size classes. Laser diffraction allows obtaining an equivalent optical spherical diameter. The number and surface distributions were then deduced from the volumetric distribution by calculating the number and surface corresponding to the volumetric fraction for each size class. From these distributions, the values of characteristic diameters can be obtained, among which the mode, which refers to the most probable size, and the median diameter, which is widely used in particulate processes and refers to the value where half of the population resides above this point, and half resides below this point.

The volume, surface, and number distributions of the initial MBS latex suspension are shown on the same graph in Figure 1. The median diameters are 410, 380, and 340 nm in volume, surface, and number respectively. All three types of distributions are unimodal with sizes ranging from 0.2 to 0.8 μm. The mode of the volume distribution is 426 nm while the mode of the number distribution is shifted towards the smallest sizes (340 nm) but all three distributions are consistent and characteristic of relatively monodisperse populations. As shown in the SEM photo of the initial MBS latex suspension, inserted in Figure 1, the latex particles have a relatively spherical shape and are fairly similar in size in agreement with the particle size distributions.

Moreover, according to the data provided by laser diffraction, the global fractal dimension [START_REF] Gregory | Monitoring Particle Aggregation Processes[END_REF][START_REF] Lachin | Aggregation and Breakup of Acrylic Latex Particles inside Millimetric Scale Reactors[END_REF] D 3 can be deduced directly from the negative slope of a log-log plot of the relative scattering intensity (I) versus the modulus of the scattering wave vector (k) given by the following equation where λ is the wavelength and θ the scattering angle: Indeed, in the fractal regime when R g -1 <<k<< r 0 -1 where r 0 is the radius of the primary particles and R g the radius of gyration of the aggregates, the intensity decays according to a power law which can be defined as the opposite of a scaling factor or fractal dimension D 3 for mass fractal objects [START_REF] Soos | Aggregate Breakup in a Contracting Nozzle[END_REF][START_REF] Sorensen | Light Scattering by Fractal Aggregates: A Review[END_REF]:

Image Analysis

An off-line analysis of samples was also performed using Morphologi G3 (Malvern Panalytical ® ), which is a characterization tool consisting of an optical microscope (coupled to a CDD camera) associated with an image analysis software [START_REF] Guérin | Dynamics of Aggregate Size and Shape Properties under Sequenced Flocculation in a Turbulent Taylor-Couette Reactor[END_REF]. For each analysis, a sample is injected between two glass plates separated by a joint in order to reduce the risk of deforming the aggregates and thus biasing the results during image analysis. More than 10000 isolated and digitized aggregates were collected for each sample. From the data provided by the image analysis, the software is able to calculate characteristic shape parameters, such as circle equivalent diameter (CED), circularity and other parameters. In this study, circularity (C) was chosen as a characteristic parameter of the overall shape and surface roughness of the aggregates [START_REF] Guérin | Dynamics of Aggregate Size and Shape Properties under Sequenced Flocculation in a Turbulent Taylor-Couette Reactor[END_REF][START_REF] Vlieghe | Morphological Properties of Flocs under Turbulent Break-up and Restructuring Processes[END_REF]. It corresponds to the ratio between the perimeter of a circle having the same surface (A) as the projected image of the aggregate and the perimeter (P) of the aggregate:

Its value varies between 0 for an elongated object to 1 for a disk. Furthermore, the 2D-fractal dimension (D 2 ) determined from the results obtained by the image analysis treatment is used to characterize the aggregates [START_REF] Florio | The Use of the Perimeter-Area Method to Calculate the Fractal Dimension of Aggregates[END_REF]. It is determined by the exponent of a relation between the intrinsic area (A s ) and a characteristic length (l c ) of the aggregate:

The maximum distance, which is the farthest distance between two points of the perimeter, also defined as the maximum Feret diameter, was chosen in this work as the characteristic length of the aggregates. D 2 can take values from 1 to 2 that corresponds to a disk since the area is proportional to the square of the length.

MBS suspension stability analysis

The MBS latex suspension is stable due to electrostatic repulsive forces between the negatively charged particles by the carboxylate group of the potassium salt present on their surfaces. To confirm this, the variation of the zeta potential of the latex suspension was measured as a function of pH by applying the Smoluchowski equation [START_REF] Sze | Zeta-Potential Measurement Using the Smoluchowski Equation and the Slope of the Current-Time Relationship in Electroosmotic Flow[END_REF] using a Nanosizer ZS (Malvern Panalytical ® ) (Figure 2). The measurement was performed with a diluted suspension of nanoparticles (0.0215% w) and the pH was adjusted by carefully adding drops of sulfuric acid (15 mmol/l) or sodium hydroxide (2.5 10 -3 mmol/l) to the diluted suspension. For each pH value, the error bars presented in the graph correspond to three different samples, each measured three times to check the stability of the initial product. According to the DLVO theory, the higher the potential, the better the stability. The zeta potential of the diluted suspension (pH  6) is -35mV which confirms its good stability (|zeta| being higher than 30 mV) [START_REF] Hunter | Zeta potential in colloid science[END_REF]. In basic medium, the zeta potential increases to reach constant values of -48 mV beyond pH=8.5. In acidic medium, the absolute value of the zeta potential decreases to zero around pH=2. This observation reflects the fact that particles can aggregate under acidic conditions.

MBS suspension density

Measurement of the change in density of the MBS latex suspension with temperature was performed using an oscillating tube density meter (Anton PAAR DMA 4100M). The values measured for a 21.5%w suspension are presented in the supplementary material (see Figure S1). The evolution of density with temperature is similar to that of water although lower values than water were measured over the entire temperature range considered. The density of the suspension as a function of temperature (T) can be described by the following equation:

where ρ is the density and ρ 0 =0.9974 g/cm 3 the density at T=0°C and T 0 =273.15K.

MBS suspension rheological behaviour

The rheological behavior of an MBS latex suspension was measured at room temperature using a rotational rheometer (Kinexus pro+ Malvern Panalytical ® ) with a double-gap geometry that consists of filling the sample into a double cylinder when a second cylinder is lowered into it to create different shears on the fluid. The suspension behaves as a shear thinning fluid as shown in Figure S2. The rheological data from 1 to 500 s -1 were fitted to the Cross model (see Figure S2) relating apparent viscosity (µ) to shear rate according to the following equation:

Where assuming the zero-shear viscosity µ 0 =0.012 Pa.s and the infinite viscosity at very high shear rates µ ∞ =0.0011 Pa.s, we obtain c=1.41 the consistency coefficient and m=0.84 the rate constant reflecting the degree of dependence of the viscosity on the shear rate.

Thermomechanical properties

The thermomechanical properties of the core-shell MBS copolymer aggregates were evaluated by Dynamic Mechanical Analysis (DMA). This technique was chosen because it is widely used to characterize thermomaterial properties as a function of temperature, time, frequency, stress, atmosphere or a combination of these parameters [START_REF] Saba | A Review on Dynamic Mechanical Properties of Natural Fibre Reinforced Polymer Composites[END_REF]. DMA applies an oscillatory force at a specified frequency to the sample and reports changes in stiffness and damping that can be expressed in terms of (tan δ) defined as the ratio of loss and storage modulus. This type of analysis cannot be performed directly on a solid-liquid suspension. For this purpose, the suspensions were filtered and then dried in a fluidized bed at a temperature of 50°C. The powder obtained was pressed to form a disk of 1.71mm thickness and 5mm diameter. Two disks were mounted in the shear clamp and analyzed with a Metler Toledo DMA 1. The heating was from -100°C to 180°C at 3K/min based on results in literature [START_REF] Zhou | Toughening of Polyvinylchloride by Methyl Methacrylate-Butadiene-Styrene Core-Shell Rubber Particles: Influence of Rubber Particle Size[END_REF][START_REF] Sun | Agglomeration of the Poly(Butadiene-Styrene) Latex Triggered by CO2 Bubbling and the Preparation of Poly(Methyl Methacrylate-Butadiene-Styrene) Core/Shell Particles with a Wide Size Distribution[END_REF]. Measurements were performed in a frequency series at 10 and 1 Hz to determine the glass transition of MBS aggregates which corresponds to the temperature range where a polymer changes to a softer and more compliant state. It can be determined when the maximum of (tan δ) occurs.

Experimental Setup and protocol

Aggregation experiments were performed in a 1-liter jacketed and cylindrical batch reactor equipped with four long and flat baffles. A scheme is presented in Figure 3. Two superposed propellers (BOHLC443-08), commercialized by BOLA Rührwellen, were used in all experiments presented in this article. They are fixed on a 10 mm shaft in the center of the tank and separated by a distance of 60 mm. Their diameter is 60 mm and the distance between the lower propeller and the bottom of the tank is 18 mm. Each propeller consists of four blades made entirely of PTFE and inclined at 45° to achieve good axial flow with low shear force. A thermostatically controlled silicone oil bath controls the temperature of the suspension by circulating the cooling or heating fluid through the jacket around the vessel. Moreover, a bulb condenser consisting of a long glass tube with a water jacket allows the condensation of the vapors produced during the process. The evolution of the process at different steps can be followed by taking samples through an outlet located at the bottom of the reactor. Latex aggregation runs were performed according to a specific protocol (Figure 4) derived from the coagulation process of the core shell polymer cited in a patent specification deposed by ARKEMA [START_REF] Navarro | Process for Core-Shell Impact Modifiers and Impact Modified Thermoplastic Composition with Enhanced Hydrolytic Resistance[END_REF]. First, a 500 ml latex suspension, diluted to 2.15%w with demineralized water, is introduced into the reactor. Stirring is set at N=200 rpm or 450 rpm and the temperature of the suspension is raised up to T 1 (20, 30 or 42°C). A 215 ml solution of a sulfuric acid prepared at a specific concentration (7.5, 15 or 30 mmol/l) is injected into the reactor by a peristaltic pump at a flow rate of 0.23 ml/s (duration : 16 minutes). Then 60 ml of a sodium hydroxide solution is added at the same flow rate to obtain a neutral pH (around 6 or 7). Hence, the total volume in the reactor is equal to 775ml and the concentration of the acid corresponds to 2.08, 4.16 (denoted C a ) or 8.32 mmol/l. The reactor temperature is then raised to T 2 (42, 60, 80 or 95°C) and maintained for 30 min. At the end of the run, the mixture is cooled to ambient temperature.

In this paper, the influence of the coagulant concentration, the stirring rate and the temperatures of the two steps of the protocol (T 1 ) and (T 2 ) on the size and shape distributions of the agglomerates was studied.

Results and discussion

Morphological changes during the aggregation process

Before analyzing the effect of the different parameters on the evolution of the size and shape distributions, the results of Exp #1 are first presented and discussed. Table 2 reports the modes (most probable sizes) and median diameters of the initial suspension and at the end of the acidic destabilization and heating steps during which aggregation occurs, Figure 5 shows the corresponding volume, surface and number size distributions.

Destabilization Step

Under these reference conditions (Exp #1), the distribution starts to evolve from the first drops of acid addition until it reaches a constant distribution after the addition of 100 ml (get at 7 min) of sulfuric acid which leads to a pH close to 2. Indeed, it is the pH obtained during that step that controls the aggregation phenomenon. When an acidic medium (pH=2) is obtained by adding sulfuric acid, the zeta potential becomes almost zero (see Figure 2). As a result, the repulsive electrostatic forces are reduced compared to the attractive Van der Waals forces, which leads to the aggregation of the latex nanoparticles according to the DLVO theory.

During the rest of the destabilization step, the size distributions do not evolve anymore. Hence, in relation to Figure 2, it can be stated that the amount of coagulant is more than sufficient to completely destabilize the suspension. The main aggregation phenomenon is the charge neutralization, which is quite rapid. These "intermediate aggregates" have a wider and larger size range than the primary particles. It can be stated that the size distributions are broad. The volume distribution of aggregates ranges from 2 to 200 μm represented by a distribution with a streak on the left emphasizing the presence of some small flocs. While for the number representation, the range is more shifted to the left between 1.8 and 30 µm indicating the presence of most small flocs. Furthermore, the surface distribution can be considered as an intermediate between these two representations. As a consequence, the aggregation population is quite large and heterogeneous as it is mainly composed of numerous small aggregates of about 2µm and larger aggregates with sizes up to 200µm; the span of the sizes being spread over 2 decades.

Neutralization step

During the neutralization step, the distributions (not shown here) remain stable. Thus, after neutralization, the pH of the suspension has no more effect on the aggregation phenomenon.

Heating step

The second phase of aggregation takes place during the temperature rise. The size distribution gradually evolves with temperature until a steady-state distribution is reached when the temperature is equal to T 2 =95°C. After that, the aggregate size distributions do not evolve, even if the suspension remains at T 2 for a long time. The size distributions are not affected by the cooling phase. The volume size distribution determined by laser diffraction is unimodal with sizes ranging from 130 to 1000 μm. The heating step therefore leads to the formation of very large aggregates. Similarly, the surface distribution shows the same range with a small left shift of the peak. While the number representation shows a size range between 110 and 900 μm with a more left-shifted peak. It can be noticed in Figure 5 that the overall population at the end of the heating step is much more homogeneous in terms of sizes than that at the end of the destabilization step. In Table 2, the mode and median diameter are close to each other.

The evolution of the fractal dimension D 3 of latex aggregates is plotted in Figure 6 as a function of temperature. For the initial suspension, the fractal dimension has no physical significance because the first aggregates formed cannot be considered as fractal objects [START_REF] Soos | Effect of Shear Rate on Aggregate Size and Morphology Investigated under Turbulent Conditions in Stirred Tank[END_REF][START_REF] Kostansek | Controlled Coagulation of Emulsion Polymers[END_REF][START_REF] Takayasu | Determination of the Equivalent Radii and Fractal Dimension of Polystyrene Latex Aggregates from Sedimentation Coefficients[END_REF]. Thus D 3 was measured only a few minutes after the start of the acid injection. During the sulfuric acid injection (which lasts 16 minutes), the D 3 increases slightly with the size of the aggregates. The further increase of the fractal dimension until the end of the destabilization step may be related to a kind of compaction of the aggregates that has already been underlined in the literature. At the end of the destabilization step, we obtain aggregates with a fractal dimension of about 2.2; value consistent with those found in the literature [START_REF] Selomulya | Evidence of Shear Rate Dependence on Restructuring and Breakup of Latex Aggregates[END_REF][START_REF] Guérin | Fractal Dimensions and Morphological Characteristics of Aggregates Formed in Different Physico-Chemical and Mechanical Flocculation Environments[END_REF][START_REF] Vlieghe | QMOM-Based Population Balance Model Involving a Fractal Dimension for the Flocculation of Latex Particles[END_REF][START_REF] Spicer | Effect of Shear Schedule on Particle Size, Density, and Structure during Flocculation in Stirred Tanks[END_REF][START_REF] Oles | Shear-Induced Aggregation and Breakup of Polystyrene Latex Particles[END_REF]. This value continued to evolve during the heating step to reach a final value of 2.9 at 95°C, reflecting that the aggregates become denser.

From the analysis of the images provided by the Morphologi G3, it is possible to determine the distribution of basic morphological characteristics of more than 10000 aggregates collected for each sample. To represent the morphology of the whole population, a 3D graph is proposed [START_REF] Guérin | Dynamics of Aggregate Size and Shape Properties under Sequenced Flocculation in a Turbulent Taylor-Couette Reactor[END_REF]; on the same graph is presented the distribution according to the aggregate size and the circularity. This representation allows highlighting the relationship between the size and the shape of the aggregates. An example is shown in Figure 7 for the number and volume distributions of size and circularity at the end of the destabilization and heating steps for Exp #1.

After the addition of acid, the number distribution (Figure 7.a) shows that two different populations appear: (i) a population of primary aggregates with a size between 1 and 10 µm with a mean circularity of 0.68 and (ii) a population of larger aggregates with a size between 10 and 300 µm with a mean circularity of 0.76 while the volume distribution (Figure 7.b) shows aggregates between 40 and 500 µm with a mean circularity of 0.73. It should be remembered that, in contrast to laser diffraction measurements, the data from image analysis are natively in numbers and then transformed to obtain the volume distributions. We observe here that the population, highlighted by the volume distribution corresponds in fact to very few large aggregates. Overall, these graphs indicate that during the destabilization step, the particles form primary aggregates with low circularity values before these coalesce to form intermediate and more circular aggregates. After the heating step, the number distribution (Figure 7c) shows a first sub-population between 1 and 200 µm with an average circularity around 0.8. This sub-population is a part of the aggregates formed during the destablization step that did not aggregate during the second heating step. But the main population is constituted by much larger aggregates between approximately 200 and 900 µm with a mean circularity of 0.83. This second population of quite large and circular aggregates is well put in evidence in the volume distribution (Figure 7d). Moreover, this observation is also in agreement with the volume size distribution determined by laser diffraction and reported previously in Figure 5, suggesting that a monodisperse population is finally recovered at the end of the heating step. As for the number size distribution, also shown in Figure 5, as already mentioned in the introduction, it is calculated from the volume distribution and may therefore not highlight the presence of a few small aggregates detected by image analysis. Both analysis methods (laser diffraction and image analysis) provide complementary data, and analysis of the results interpreted on the basis of number and volume distribution sheds new light.

The 2D fractal dimension, deduced from the image analysis, evolves little, from 1.86 after the destabilization step to 1.99 after the heating step, but it confirms the formation of very circular aggregates.

Thus, it can be stated that the final aggregates formed after heating are larger and more circular than those obtained after the destabilization step. Indeed, the small aggregates obtained spontaneously by the addition of coagulant tend to aggregate over time, under the effect of heating, to form more circular aggregates. These observations are also supported by images of the aggregated suspensions recovered at the end of the destabilization step and at the end of the heating step and reported in Figures 7e and7f respectively. In addition, a selection of images of aggregates is shown as supplementary material in Figure S3.

c) Number and d) Volume distributions at the end of the heating step (e) Image of the aggregated suspension recovered at the end of the destabilization step (f) Image of the aggregated suspension recovered at the end of the heating step

This evolution of the latex according to the different steps of the protocol was observed in all the experiments performed. Indeed, whatever the physicochemical and hydrodynamic conditions applied during the experiment, we can distinguish two main aggregation phases : the coagulation phase which corresponds to the destabilization by the addition of sulfuric acid 400µm 400µm and a subsequent aggregation phase during the heating step. Moreover, once the aggregation is achieved, the aggregates do not change with time, which implies that the two aggregation phenomena mentioned above are fast, permanent and irreversible.

Factors influencing aggregation

Stirring speed

The effect of the stirring speed on the aggregate size can be analyzed by considering Exp #1 and Exp #9. These experiments were performed with the same concentration of acid, T 1 and T 2 , only the rotation speed (N) of the stirrer was different: 200 rpm for Exp #1 and 450 rpm for Exp #9. By comparing the distributions of Exp #1 and Exp #9, it can be noted that the mode of the distribution depends on N (it is equal to 352 µm for N=200 rpm and 211 µm for N=450 rpm). A number of studies [START_REF] Bouyer | Experimental Analysis of Floc Size Distributions in a 1-L Jar under Different Hydrodynamics and Physicochemical Conditions[END_REF][START_REF] Li | The Strength and Fractal Dimension Characteristics of Alum-Kaolin Flocs[END_REF][START_REF] Xiao | PIV Characterisation of Flocculation Dynamics and Floc Structure in Water Treatment[END_REF][START_REF] Wang | Characterization of Floc Structure and Strength: Role of Changing Shear Rates under Various Coagulation Mechanisms[END_REF][START_REF] Coufort | Flocculation Related to Local Hydrodynamics in a Taylor-Couette Reactor and in a Jar[END_REF][START_REF] Coufort | Analysis of Floc Size Distributions in a Mixing Tank[END_REF] show that there is a relationship between the aggregate size and the turbulence scales. The ratio of the Kolmogorov microscales for Exp #1 and Exp #9 can be estimated by the following equations : [START_REF] Kusters | Aggregation Kinetics of Small Particles in Agitated Vessels[END_REF] and ( 8) giving [START_REF] Vaccaro | Aggregation of Concentrated Polymer Latex in Stirred Vessels[END_REF] Where P w is the supplied and dissipated power, N p the power number of the impeller, ρ the suspension density, N the stirring speed, D the impeller diameter, ε the volume average of the viscous dissipation of the kinetic energy, V the volume of liquid in the reactor, η the average Kolmogorov microscale, and υ the kinematic viscosity of the suspension. This ratio can be compared to the ratio of the modes of the volume distributions  1.7. Both ratios are quite similar, indicating that even in the case of a heating step, the agglomeration phenomena seem to be closely related to hydrodynamics.

Coagulant concentration

The effect of coagulant concentration was examined while keeping the other experimental conditions constant and can be analyzed by referring to Exp #1 Exp #2 and Exp#3. To avoid variation in the total reactor volume, 215ml sulfuric acid solutions at different concentrations were used to destabilize the latex suspension. Figure 8 shows the size distribution for three experiments with an acid concentration of 2.08, 4.16 and 8.32 mmol/l (denoted C a /2, C a , 2.C a ) at 2 minutes, 7 minutes and 16 minutes after the start of sulfuric acid injection (during the destabilization step), respectively.

The kinetics of the destabilization step is affected for low coagulant concentration. Indeed, the size distribution of the aggregates shows a similarity between the three experiments after 7 min (Figure 8.b) and at the end of the destabilization step (Figure 8.c). While after 2 min of addition (Figure 8.a) the suspension is not yet coagulated for C a /2 which is particularly shown by the peak of number distribution between 0.2 and 0.9 µm. In volume representation, this can be noticed by the presence of a minor peak at this size range. This implies that the amount of coagulant added is not yet sufficient. In agreement with this observation, the number median diameter for C a /2 evolves from 0.34 µm for the initial suspension to 0.45 µm after 2 min, then to 3.5 µm after 7 min which remains almost fixed until the end of the destabilization step. While for C a or 2.C a , the number median diameter is already 3.5 µm after 2 min, but remains around 3.9 µm at the end of the step. The variation of the acid concentration has no persistent influence on the aggregates formed after heating (not shown here). Similarly, it was observed that it does not affect the global fractal dimension determined by laser diffraction. Its value changes from 2.1 after the destabilization step to 2.7 after the heating step, as shown in Figure 6. These experiments prove that the acid concentration does not affect the size or morphology of the intermediate and final aggregates as long as the rate of acid injection is moderate and the amount of acid is sufficient to achieve a low pH corresponding to negligible zeta potential values.

Destabilization temperature T 1

The influence of temperature during the destabilization step can be discussed by referring to Exp #1, Exp #4 and Exp #5.

In the graphs in Figure 9, the volume and number size distributions at different times of the destabilization step are given for the aggregates of three different experiments performed with destabilization temperatures T 1 of 20°C, 30°C, and 42°C, respectively. Note that regardless of the temperature, approximately the same size distributions are obtained at each time point during the destabilization step. The number distributions at the end of the destabilization step vary between 1.8 and 30 μm represented by a monomodal distribution with a peak at 2 μm and a streak on the right emphasizing the presence of fewer aggregates between 10 and 30 μm. The volume representations show the presence of aggregates between 2 and 200 µm from the beginning of the injection and regardless of the temperature of the destabilization step. Nevertheless, it can be noticed that the kinetics of the onset of aggregation seems to be somewhat affected by temperature. Indeed, in the Figure 9.a, the higher the temperature, the greater the distribution mode. At the end of the destabilization step (Figure 9.c), the mode of the volume distribution is slightly shifted to the right for a temperature of 42°C.

Similarly to the sulfuric acid concentration, no effect of the destabilization step temperature on the fractal dimension was observed. The temperature of the first aggregation phase has no significant effect on the size and morphology of the aggregates.

Heating temperature T 2

The effect of the temperature of the heating step can be analyzed by referring to Exp #6 to Exp #9. Note that for these 4 experiments, the destabilization step was performed at a temperature of 42°C ; with a concentration of C a sulfuric acid and a rotation speed of 450 rpm. The volume and number size distributions are presented in Figure 10 for the four experiments performed at T 2 of 42°C, 60°C, 80°C and 95°C respectively. When the temperature is maintained at 42°C during the heating step, the volume representation (Figure 10.a) shows a monomodal distribution between 20 and 120 µm with a small fraction of smaller aggregates below 10 µm. While for the number representation (Figure 10.b), the peak is even more shifted to the left revealing that the majority of the aggregates are smaller than 10 µm in size. These distributions are similar to those obtained at the end of the destabilization step but they are narrower, i.e. fewer large aggregates (>10µm) are observed in the number representation and fewer small aggregates (<10µm) in the volume representation. When heated to 60°C, the volume distribution starts to shift to the right as the mode is close to 100µm, but a significant portion of small aggregates between 10 and 50µm is noticeable. Looking at the number distribution, it is obvious that there is still a large portion of the population with sizes between 2 and 10 µm. It can also be noticed that increasing the heating temperature from 42 to 60°C leads to a slight change in the size of the smallest aggregates.

Looking at the results for T 2 = 80°C, the volume distribution shows a range close to that of heating at 60°C but with the disappearance of the smallest flocs between 10 and 50µm, while the number distribution shows that the aggregates are significantly larger (>30µm), the mode of the distribution being close to 60µm. For T 2 =95°C, the aggregates are even larger either in number or in volume. Indeed, the size range is between 90 and 700 μm in volume representation and between 80 and 600 μm in number representation. In summary, it is clear that the higher the temperature, the larger the aggregates. Moreover, the size distribution becomes narrower with increasing temperature and the number and volume distributions become closer together.

Let us now consider the fractal dimensions of the latex aggregates. The 2D fractal dimension was determined from image analysis according to (Eq. 4) and the 3D global fractal dimension was deduced from laser diffraction measurements (Eq. 2). The determination of the 3D global fractal dimension in the fractal regime from laser diffraction measurements for the different runs varying the temperature of the heating step is reported in supplementary material (see Figure S4). Although correlations have been developed in the literature [START_REF] Lee | Prediction of three-dimensional fractal dimensions using the twodimensional properties of fractal aggregates[END_REF][START_REF] Wang | Investigation of the relationship between the 2D and 3D box-counting fractal properties and power law fractal properties of aggregates[END_REF] to link 2D and 3D fractal dimensions, and thus predict for example the 3D fractal dimension from 2D images, it seems difficult here to directly compare volume fractal dimensions, obtained by laser diffraction of one part and calculated from 2D analyzes of another part, knowing that the values come from experimental data based on very different physical principles. However the evolution of 2D and 3D fractal dimensions follow similar trends. The correspondence between temperature changes and 2D or 3D fractal dimensions of the flocs can be seen in Figure 11. Remember that the range of variation of 2D and 3D fractal dimensions is not the same, depending on their respective definitions. The 2D fractal dimension varies between 1 (for a line) and 2 (for a disk), while the 3D fractal dimension varies between 1 (for a needle) and 3 (for a sphere). Despite this, the same evolutionary trends as a function of temperature were observed. After the destabilization step, performed at 42°C for Exp #6 to Exp #9, the 2D fractal dimension is on average equal to 1.9 and the D 3 fractal dimension to 2.2. After the heating step, the 2D fractal dimension becomes almost equal to 2 (maximum value) and D 3 reaches 2.9 for the highest heating temperatures (80°C and 95°C). It is slightly lower (2.7) if the heating step is performed at 42°C or 60°C. Thus, the fractal dimension increases slightly if the temperature of the second step is the same as that of the destabilization step (case of Exp #6). This increase, as indicated above, is accompanied by a slight narrowing of the distribution. We can think that the largest aggregates maintained under agitation at constant temperature undergo a slight erosion. Finally, from these results, it can be assumed that the larger aggregates have higher fractal dimension values, or that increasing the temperature leads to the formation of larger, but denser aggregates.

In summary, the heating step, when performed at high temperature, leads to an efficient agglomeration phase since it allows the formation of a population of large, dense and relatively circular aggregates. To our better knowledge, there are only a few articles in the literature [START_REF] Li | Slow Pelleting Coagulation of MBS Latex[END_REF][START_REF] Ehrl | Dependence of Aggregate Strength, Structure, and Light Scattering Properties on Primary Particle Size under Turbulent Conditions in Stirred Tank[END_REF] mentioning the influence of temperature on the agglomeration efficiency. In order to better understand the underlying mechanisms, DMA analyses were performed on the agglomerates after the destabilization and heating steps.

The DMA results for the aggregates obtained in experiments with different heating temperatures (42°C and 95°C) are presented in Figure 12. The heating cycles were repeated several times: the first and second heating are represented by the red and blue line for both samples (42°C and 95°C), respectively, and the third heating is represented by the green line performed only for the sample obtained at 42°C. In dotted lines, are reported the curves realized at 10 Hz and in solid lines, the curves realized at 1Hz. The determination of the different transitions of MBS copolymer is based on the results of the literature [START_REF] Zhou | Toughening of Polyvinylchloride by Methyl Methacrylate-Butadiene-Styrene Core-Shell Rubber Particles: Influence of Rubber Particle Size[END_REF][START_REF] Sun | Agglomeration of the Poly(Butadiene-Styrene) Latex Triggered by CO2 Bubbling and the Preparation of Poly(Methyl Methacrylate-Butadiene-Styrene) Core/Shell Particles with a Wide Size Distribution[END_REF].

From Figure 12, a first phenomenon, weakly dependent on the frequency of solicitation, is detectable around -75°C for both samples at different heating cycles and may correspond to the alpha transition of the polybutadiene. At about 20-40°C, the frequency curves overlap, probably indicating crystallization of the polybutadiene. Around 95°C, a second phenomenon, corresponding to the alpha transition of methyl methacrylate, is evident for both samples. It is frequency dependent and is particularly noticeable during the second heating cycle (blue line) due to a separation of the butadiene/metacrylate zones that occurred during the first cycle. If we compare the two samples, we notice that the height of the peak is greater for the aggregates obtained at 42°C than for those at 95°C in the first pass, whereas in the second pass, the values are approximately the same. This implies that after the heating step of the aggregation process, the aggregates are more constrained and therefore less damped. In summary, the DMA results indicate that the core-shell MBS particle consists of a nuclear layer, i.e., a core with a glass transition temperature of -75 °C, and a shell with a T g of approximately 95 °C -100 °C. This is consistent with previous reports [START_REF] Zhou | Toughening of Polyvinylchloride by Methyl Methacrylate-Butadiene-Styrene Core-Shell Rubber Particles: Influence of Rubber Particle Size[END_REF][START_REF] Sun | Agglomeration of the Poly(Butadiene-Styrene) Latex Triggered by CO2 Bubbling and the Preparation of Poly(Methyl Methacrylate-Butadiene-Styrene) Core/Shell Particles with a Wide Size Distribution[END_REF]. These transitions indicate subtle changes in the material. The glass transition T g being the temperature range where the material softens, we can assume that agglomeration phenomena are promoted as soon as the temperature approaches T g allowing the formation of large and dense aggregates. Therefore, the higher the temperature of the heating step, the larger the aggregates formed. This coincides with what is found in the literature [START_REF] Ehrl | Dependence of Aggregate Strength, Structure, and Light Scattering Properties on Primary Particle Size under Turbulent Conditions in Stirred Tank[END_REF]; when T g is exceeded, the polymer is more "sticky", which facilitates penetration between the chains of colliding particles. Glass transition agglomeration is a common process in fluidized bed spray agglomeration processes [START_REF] Palzer | The effect of glass transition on the desired and undesired agglomeration of amorphous food powders[END_REF][START_REF] Avilés-Avilés | Fluidised bed agglomeration of particles with different glass transition temperatures[END_REF][START_REF] Rieck | Monte Carlo modeling of binder-less spray agglomeration in fluidized beds[END_REF] where particles, being wetted by a liquid, collide at wet spots; the material becoming sticky if the process temperature is close to the glass transition temperature. Solid bridges can finally be created, leading to the formation of agglomerates during drying contrary to the agglomeration process in liquid suspensions. A few papers [START_REF] Qiao | Effect of Particle Agglomeration and Interphase on the Glass Transition Temperature of Polymer Nanocomposites[END_REF][START_REF] Zhang | Study of Glass Transition and Reinforcement Mechanism in Polymer/Layered Silicate Nanocomposites[END_REF] address the effect of particle agglomeration and interphase on the glass transition temperature of polymer nanocomposites but they are mostly devoted to thin polymer films and not, as in our case, for suspended MBS nanocomposites, i.e. closer to industrial aggregation conditions.

Conclusions

Aggregation experiments of MBS latex particles were performed in a laboratory-scale stirred reactor according to a specific two-step protocol (destabilization and heating steps) derived from the industrial core-shell polymer coagulation process. The results have shown that two aggregation mechanisms occurred successively: particle destabilization by charge neutralization due to sulfuric acid addition and aggregate consolidation during the heating process. During destabilization step, sulfuric acid acts on the negatively charged surface of the latex particles and leads to the reduction of the potential barrier, which leads to a preliminary aggregation phase. During the heating step, the evolution of the aggregate size distribution and the fractal dimension D 3 derived from the laser diffraction analysis shows that the aggregates formed are larger and larger and their circularity increases. This suggests that small aggregates are built by a particle-cluster mechanism, whereas large agglomerates are formed by the aggregation of small aggregates with each other, leading to large agglomerates structured hierarchically. Furthermore, image analysis shows that the morphology of the aggregates, is also sensitive to temperature. The 3D distribution (size and circularity) and the fractal dimension D 2 also highlight that the aggregates formed after heating are larger and more circular. The ability to form consolidated aggregate during the heating step seems to be related to the intrinsic characteristics of the material. Indeed, the DMA (Dynamic Mechanical Analysis) analysis showed that the glass transition temperature of the MBS shell (polymethyl methacrylate) is around 90-100°C. In this interval, the mechanical properties of the latex evolve towards a greater deformability. This is reflected by the influence of the variation in heating temperature on the size and shape distributions and on the fractal dimensions D 2 and D 3 . Consequently, the higher the temperature, the larger and more circular the aggregates. In other words, the closer the heating temperature is to the glass transition temperature, the more aggregation is favored. A clear dependence was shown between the temperature (during the heating step) and the fact that the agglomeration was favored. The closer the heating temperature is to the glass transition, the larger and circular agglomerates are formed. The glass transition temperature of PMMA (shell material of the MBS latex) is around 95-100°C. All experiments were carried out at lower temperatures (42, 60, 80 and 95 °C), so sintering, strictly speaking, does not occur, except possibly for the test carried out at 95°C. So, the major mechanism and observed effect of temperature are probably due to the fact that as the temperature approaches the glass transition temperature, the aggregates become stickier and more deformable, which favors agglomeration efficiency and the formation of more circularshaped aggregates.

The impact of hydrodynamics on the morphological properties of the aggregate population obtained in a stirred pilot reactor is now under further investigation. This experimental database will then be used in a perspective of development generic tools allowing an advance understanding of the suspension aggregation processes in order to control the morphological properties of the solid product formed and to propose innovations on the design of the reactors. Table 1: Experimental conditions for aggregation runs Table 2: Mode and median diameters of the initial suspension and at the end of the destabilization and heating steps

List of Figures

List of Tables

Figure 1 :

 1 Figure 1 : Volume, surface and number size distributions of the initial MBS nanoparticle suspension and SEM image

Figure 2 :

 2 Figure 2: Zeta potential of MBS latex suspension versus pH

Figure 3 :

 3 Figure 3: Scheme of the experimental set-up for MBS aggregation experiments

Figure 4 :

 4 Figure 4 : Experimental protocol

Figure 5 :

 5 Figure 5 : Number, surface and volume size distributions of the initial suspension I (dotted lines), at the end of the destabilization step D (dashed lines), and at the end of the heating step H (continuous lines) -Exp #1

Figure 6 :

 6 Figure 6 : Fractal dimension D 3 and Temperature versus time -Exp #1

Figure 7 :

 7 Figure 7 : 3D distribution of size and circularity -Exp # 1. a) Number and b) Volume distributions at the end of the destabilization step. c) Number and d) Volume distributions at the end of the heating step (e) Image of the aggregated suspension recovered at the end of the destabilization step (f) Image of the aggregated suspension recovered at the end of the heating step

Figure 8 :

 8 Figure 8: Number and volume size distributions during the destabilization step for different acid concentrations (Exp #1, Exp #2, Exp #3). (a) t= 2min, (b) t=7min, (c) t=16min.

Figure 9 :

 9 Figure 9: Number and volume distributions during the destabilization step for different T 1 (Exp #1, Exp #4 and Exp #5). (a) t= 2min, (b) t=7min, (c) t=16min.

Figure 10 :

 10 Figure 10: Volume (a) and number (b) size distributions at the end of the heating step for different T 2

Figure 11 :

 11 Figure 11: D 2 and D 3 fractal dimensions versus Temperature (Exp. # 6 to Exp #9)

Figure 12 :

 12 Figure 12: DMA results -Tangent Delta versus Temperature (°C) for 2 experiments with different heating temperatures: (a) T 2 =42°C and (b) T 2 =95°C

GlossaryA

  : surface (m²) A s : intrinsic surface (m²) c: consistency coefficient (-) C: circularity (-) C a : acid concentration (mol/L) CED: circle equivalent diameter (m) D: impeller diameter (m) DMA: Dynamic Mechanical Analysis D 3 : global fractal dimension (-) D 2 : 2D-fractal dimension (-) I: scattering intensity (s -1 ) k: modulus of the scattering wave vector (m -1 ) l c : characteristic length (m) MBS: Methacrylate Butadiene Styrene m: rate constant (-) N: stirring speed (rev.s -1 ) N p : power number (-) P w : power (W) P: perimeter (m) tan δ: dissipation factor (-) T: temperature (K) T 1 : temperature of destabilization step (°C) T 2 : temperature of heating step (°C) V: volume (m 3 ) ε: viscous dissipation of the kinetic energy (m².s -3 ): shear rate (s -1 ) λ: wavelength (m) µ: apparent viscosity (Pa.s) υ: kinematic viscosity (m 2 .s -1 ) η: Kolmogorov microscale (m) θ: scattering angle (rad) ρ: density (kg. m -3 )

Figure 1 : 2 : 1 Figure 6 : 1 Figure 7 :

 121617 Figure 1: Volume, surface and number size distributions of the initial MBS nanoparticle suspension Figure 2: Zeta potential of MBS latex suspension versus pH Figure 3: Scheme of the experimental set-up for MBS aggregation experiments Figure 4: Experimental protocol Figure 5: umber, surface and volume size distributions of the initial suspension I (dotted lines), at the end of the destabilization step D (dashed lines), and at the end of the heating step H (continuous lines) -Exp #1 Figure 6: Fractal dimension D 3 and Temperature versus time -Exp #1 Figure 7: 3D distribution of size and circularity -Exp # 1. (a) Number and (b) Volume distributions at the end of the destabilization step. (c) Number and (d) Volume distributions at the end of the heating step (e) Image of the aggregated suspension recovered at the end of the destabilization step (f) Image of the aggregated suspension recovered at the end of the heating step Figure 8: umber and volume size distributions during the destabilization step for different acid concentrations (Exp #1, Exp #2, Exp #3). (a) t= 2min, (b) t=7min, (c) t=16min.Figure 9: Number and volume distributions during the destabilization step for different T 1 (Exp #1, Exp #4 and Exp #5). (a) t= 2min, (b) t=7min, (c) t=16min.Figure 10: Volume (a) and number (b) size distributions at the end of the heating step for different T 2 Figure 11: D 2 and D 3 fractal dimensions versus Temperature (Exp. # 6 to Exp #9) Figure 12: DMA results -Tangent Delta versus Temperature (°C) for 2 experiments with different heating temperatures: (a) T 2 =42°C and (b) T 2 =95°C

Figure 9 :

 9 Figure 1: Volume, surface and number size distributions of the initial MBS nanoparticle suspension Figure 2: Zeta potential of MBS latex suspension versus pH Figure 3: Scheme of the experimental set-up for MBS aggregation experiments Figure 4: Experimental protocol Figure 5: umber, surface and volume size distributions of the initial suspension I (dotted lines), at the end of the destabilization step D (dashed lines), and at the end of the heating step H (continuous lines) -Exp #1 Figure 6: Fractal dimension D 3 and Temperature versus time -Exp #1 Figure 7: 3D distribution of size and circularity -Exp # 1. (a) Number and (b) Volume distributions at the end of the destabilization step. (c) Number and (d) Volume distributions at the end of the heating step (e) Image of the aggregated suspension recovered at the end of the destabilization step (f) Image of the aggregated suspension recovered at the end of the heating step Figure 8: umber and volume size distributions during the destabilization step for different acid concentrations (Exp #1, Exp #2, Exp #3). (a) t= 2min, (b) t=7min, (c) t=16min.Figure 9: Number and volume distributions during the destabilization step for different T 1 (Exp #1, Exp #4 and Exp #5). (a) t= 2min, (b) t=7min, (c) t=16min.Figure 10: Volume (a) and number (b) size distributions at the end of the heating step for different T 2 Figure 11: D 2 and D 3 fractal dimensions versus Temperature (Exp. # 6 to Exp #9) Figure 12: DMA results -Tangent Delta versus Temperature (°C) for 2 experiments with different heating temperatures: (a) T 2 =42°C and (b) T 2 =95°C

Figure 10 : 2 Figure 11 :

 10211 Figure 1: Volume, surface and number size distributions of the initial MBS nanoparticle suspension Figure 2: Zeta potential of MBS latex suspension versus pH Figure 3: Scheme of the experimental set-up for MBS aggregation experiments Figure 4: Experimental protocol Figure 5: umber, surface and volume size distributions of the initial suspension I (dotted lines), at the end of the destabilization step D (dashed lines), and at the end of the heating step H (continuous lines) -Exp #1 Figure 6: Fractal dimension D 3 and Temperature versus time -Exp #1 Figure 7: 3D distribution of size and circularity -Exp # 1. (a) Number and (b) Volume distributions at the end of the destabilization step. (c) Number and (d) Volume distributions at the end of the heating step (e) Image of the aggregated suspension recovered at the end of the destabilization step (f) Image of the aggregated suspension recovered at the end of the heating step Figure 8: umber and volume size distributions during the destabilization step for different acid concentrations (Exp #1, Exp #2, Exp #3). (a) t= 2min, (b) t=7min, (c) t=16min.Figure 9: Number and volume distributions during the destabilization step for different T 1 (Exp #1, Exp #4 and Exp #5). (a) t= 2min, (b) t=7min, (c) t=16min.Figure 10: Volume (a) and number (b) size distributions at the end of the heating step for different T 2 Figure 11: D 2 and D 3 fractal dimensions versus Temperature (Exp. # 6 to Exp #9) Figure 12: DMA results -Tangent Delta versus Temperature (°C) for 2 experiments with different heating temperatures: (a) T 2 =42°C and (b) T 2 =95°C

Figure S3 : 8 9

 S389 Figure S3: Examples of some aggregates obtained at the end of the destabilization step and at the end of the heating step

Table 1

 1

	summarizes the operating conditions of each

Table 1 :

 1 Experimental conditions for aggregation runs

Table 2 :

 2 Mode and median diameters of the initial suspension and at the end of the destabilization and heating steps

Table 3 :

 3 Mode and median diameters at the end of the heating step for different T 2

		Volume	Surface	Number
	Heating Temperature	Mode (µm)	Median (µm)	Mode (µm)	Median (µm)	Mode (µm)	Median (µm)
	42°C	62.9	58.9	48.7	41.1	1.77	2.1
	60°C	119	95.3	92.25	35.9	2.28	3.1
	80°C	104.55	112	92.25	93.7	62.9	70.7
	95°C	225.5	244	198.5	211	153.5	165

Table 3 :

 3 Mode and median diameters at the end of the heating step for different T 2

	Destabilization step		Heating step
	Aggregate image	CED	C	Aggregate image	CED	C
		206.14 0.712		352.44 0.956
		170.82 0.797		297.78 0.954
		140.41 0.861		242.21 0.95

CED : Circle Equivalent Diameter C : Circularity

Acknowledgments

The authors thank the Agence Nationale de la Recherche (ANR) in France which founded this research, project ANR-20-CE07-0025-01.

Supplementary materials


Why engage with a canonical playwright? Isn't there enough work to do trying and recovering the works of playwrights who have all but been erased from the canon of Irish theatre history and whose plays have not made it past the stage of the premiere production? Do the plays of J. M. Synge still speak to twenty-first-century Ireland? What about the feminist in me? Shouldn't she concern herself with the work of female playwrights? Surely there are more pressing issues than performance as the world faces into an unprecedented ecological catastrophe, and as, after protest movements such as Rhodes Must Fall (2015) for example, the West is just about beginning to realise, the urgency of addressing the unbearable, continuing legacies of Europe's colonial past? Those are some of the many questions that crossed my mind as I was writing Performance, Modernity and the Plays of J. M. Synge.

Well, thinking about performance, about the possibilities that it opens up and the sense of retrieving agency that it confirms might be precisely the thing to do. The more I inquired into Synge's engagement with early twentieth-century Western modernity, the more relevant to contemporary Ireland Synge's plays became. The plays compel the audience, the critics and most importantly the theatre practitioners that engage with them to think about time: its acceleration, the deadliness of teleological time, disrupted linearity and temporal alternatives to modern time. In Synge's work, these are issues begging to be explored. The temporal revolution of the late-nineteenth and early twentieth-century is of a different order than the one forced on us by new technologies but the questions are similar.

My book is concerned primarily in the transformative possibilities of performance. Specifically, I am fascinated by the capacity of performance to disrupt narratives of modernity and progress, and so to challenge representation and offer alternatives. To this end, the book pays particular attention to Synge's interest in keening, which, I argue, can be thought about as a ritual of female empowerment. Rather than apprehend keening as a nostalgic tribute to a lost Ireland, or as the vestigial remains of a supposedly archaic culture, my concern is to highlight the transformative and utopian possibilities of this collective performance practice rooted in a woman's protest tradition. And this is where the works of J. M. Synge and that of internationally renowned, Irish visual artist Alice Maher, who was kind and generous enough to authorize one of her 2018 pieces, Vox Hybrida I, be reproduced on my book's front cover, intersect.

When it came to look for a visual for the cover, Jack B. Yeats was my first port of call. An obvious choice, undoubtedly. A powerful one, nonetheless. "The Barrel Man" (1912) was the painting I had in mind. The fierce defiance in the Barrel Man's eyes that Yeats's painting captures marvelously exemplifies the vitality, indomitability, violence, but most of all the spirit of resistance of a marginalized Irish culture that fired Synge's imagination as it did Yeats's. Copyright matters meant that I had to think again. In hindsight, it was a blessing in disguise.

Alice Maher's exploration of the aesthetic and political potential of recalcitrant objects and hybrid forms in her 2018 installation Vox Materia resonates very much with the argument that I articulate in Performance, Modernity and the Plays of J. M. Synge. I was particularly sensitive to the way in which one piece, Vox Hybrida 1, a large-scale wood relief watercoulour print recalling a silhouetted figure, problematizes the body and refuses to see it as a self-contained, fixed entity. Vox Hybrida 1 affirms the transformative possibilities held by moving bodies, not just for the individual but, most importantly for the collective. This is an area that is especially congruent with the argument of my book, which is also deeply interested in the promises of social reconfigurations that performance holds.

Another aspect of Alice Maher's work which I find thought provoking and challenging has to do with its engagement with the past. If the source of inspiration for Vox Hybrida 1 lies in times immemorial -a twelfth-century carved stone mermaid from Kilcooley Abbey, Co. Tipperary (Ireland) -I see the piece as an invitation to apprehend the past not as a frozen memento or as disjoined from the present, but, on the contrary as retrieving the agency of the past in the present, as recognizing the past as a force within the present and therefore as an invitation to think anew about agency.

Maher's Vox Hybrida I, not unlike performance, is fundamentally about hope and invites thinking of (female) collective identities as not fixed entities, but as forever fluctuating and therefore open to change. Her work talks fruitfully to Synge's and vice versa. Synge's plays are definitely contemporary.


Olha Sysova

Paule Durin

Corinne Gablin

Didier Léonard

Alexandre Téolis

Stéphane Trombotto

Thierry Delair

Dominique Berling

Isabelle Servin

Raluca Tiron

Arnaud Bazin

Jean-Louis Leclercq

Yann Chevolot

Olivier Soppera
email: olivier.soppera@uha.fr

Green Deep-UV photoresist based on Chitosan for microelectronics

Keywords: Photolithography, chitosan, photoresist, Deep-UV, biosourced materials, photoacid generator

à la diffusion 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

The industry of integrated circuits is one of the most competitive and demanding industries in the modern age. It is constantly developing to increase the performance and efficiency of the manufacturing while decreasing the cost and the size of the devices.

Photolithography is a technique that uses light to create patterns over a thin film to protect the silicon wafers during the etching or deposition steps. Despite the considerable amount of research and undeniable progress in nanolithography [START_REF] Sebastian | Nanolithography and Its Current Advancements[END_REF] , including Extreme UV lithography [START_REF] Manouras | High Sensitivity Resists for EUV Lithography: A Review of Material Design Strategies and Performance Results[END_REF][START_REF] Wu | The Recent Progress of Lithography Machine and the State-of-Art Facilities[END_REF][START_REF] Fu | EUV Lithography: State-of-the-Art Review[END_REF] and e-beam lithography [START_REF] Winkler | 3D Nanoprinting via Focused Electron Beams[END_REF][START_REF] Randall | Next Generation of Extreme-Resolution Electron Beam Lithography[END_REF] , the Deep UV lithography remains for now the most used technique in the microelectronics industry. The potential hazards and long-term consequences of the exposure to chemical substances during this process, such as polymers, solvents, developing agents, sensitizers and additives, have however raised awareness among world health institutions. [START_REF] Huang | Health Risk Assessment of Photoresists Used in an Optoelectronic Semiconductor Factory[END_REF][START_REF] Rim | Prevention of Occupational Diseases from Chemicals Due to Development of the Semiconductor Industry[END_REF] Most of the chemicals are classified as hazardous or cancerogenic while some resist formulation ingredients are listed as a trade secret, so there is no disclosed information about their toxicity. [START_REF] Jang | Evaluation of Hazardous Chemicals with Material Safety Data Sheet and By-Products of a Photoresist Used in the Semiconductor-Manufacturing Industry[END_REF] These substances are also toxic for the environment and are not sustainable as they are derived from oil. [START_REF] Weissermel | Industrial Organic Chemistry[END_REF][START_REF]Ullmann's Encyclopedia of Industrial Chemistry[END_REF] Therefore, it is crucial to find an eco-friendly replacement issued from biosourced materials [START_REF] Grebenko | Gentle Patterning Approaches toward Compatibility with Bio-Organic Materials and Their Environmental Aspects[END_REF][START_REF] Herbert | Recent Advances in Printing Technologies of Nanomaterials for Implantable Wireless Systems in Health Monitoring and Diagnosis[END_REF][START_REF] Yu | Processes, and Facile Manufacturing for Bioresorbable Electronics: A Review[END_REF] . In the last years, there has been a considerable number of studies on "green" alternatives for the lithography. For example, silk fibroin was claimed to be a good candidate for e-beam and DUV lithography, [START_REF] Kim | All-Water-Based Electron-Beam Lithography Using Silk as a Resist[END_REF][START_REF] Kurland | Precise Patterning of Silk Microstructures Using Photolithography[END_REF][START_REF] Park | Eco-Friendly Photolithography Using Water-Developable Pure Silk Fibroin[END_REF] however, a complex preparation protocol was required.

Egg albumin was used to obtain sub-100 nm patterns and as a positive and negative resist capable of submicron resolution in e-beam and UV lithography. [START_REF] Huang | Health Risk Assessment of Photoresists Used in an Optoelectronic Semiconductor Factory[END_REF][START_REF] Rim | Prevention of Occupational Diseases from Chemicals Due to Development of the Semiconductor Industry[END_REF] Polysaccharides have received a lot of interest as well. Dextrin [START_REF] Takei | Ecofriendly Ethanol-Developable Processes for Electron Beam Lithography Using Positive-Tone Dextrin Resist Material[END_REF][START_REF] Amano | Evaluation of Nano-Patterning Performance of Water-Soluble Material for Photoresist Using Sugar Chain[END_REF] , cellulose [START_REF] Dore | A Water-Processable Cellulose-Based Resist for Advanced Nanofabrication[END_REF][START_REF] Takei | EUV Lithography Using Inedible Cellulose-Based Biomass Resist Material[END_REF] and chitosan [START_REF] Caillau | Fifty Nanometer Lines Patterned into Silica Using Water Developable Chitosan Bioresist and Electron Beam Lithography[END_REF][START_REF] Caillau | Sub-Micron Lines Patterning into Silica Using Water Developable Chitosan Bioresist Films for Eco-Friendly Positive Tone e-Beam and UV Lithography[END_REF][START_REF] Ray | Quadratic Nonlinear Optics to Assess the Morphology of Riboflavin Doped Chitosan for Eco-Friendly Lithography[END_REF][START_REF] Grebenko | Lithography for Delicate Materials[END_REF] may become a good alternative to the industrial resists.

In this context, a previous paper presented works to use chitosan as a water-developable resist for DUV (193 nm) lithography. [START_REF] Sysova | Chitosan as a Water-Developable 193 Nm Photoresist for Green Photolithography[END_REF] No chemical modification was required since chitosan showed some photosensitivity to 193 nm irradiation and deionized water could be used for development. However, the irradiation doses for the photopatterning were typically 1000 mJ/cm 2 . This is significantly higher than that for the commercial photoresists, which is limiting for industrial applications. In commercial photoresists, photosensitive compounds are usually added. They can be either photoinitiators or photoacid generators (PAGs), depending on the kind of photoresist. Here, since the chitosan polymer solubility is highly modified by the pH of the solution, we propose to evaluate the impact of the addition of a PAG on the photosensitivity of a chitosan-based resist. Many PAGs have been developed. They usually rely on diaryliodonium, triarylsulfonium, dialkylphenacylsulfonium and dialkyl-4hydroxyphenylsulfonium salts molecular structures. Under irradiation, these molecules are able to generate Brønsted acids and can thus locally modify the pH. 29,30 Since water is used as a solvent for the chitosan photoresist preparation, a water-soluble PAG is needed. However, the vast majority of industrial photoresists are using organic solvents, which strongly limits the choice of suitable PAG active in the DUV region around 200 nm. 30 Some works are describing the crosslinking of water-soluble Poly(vinyl alcohol) photoresists with PAG at 254 nm and 248 nm 31,32 or a water-developable chemically amplified negativetone resist for 248 nm, 33 but no reports have been identified for 193 nm applications.

In this work, we used Dimethyl-2,4-dihydroxyphenylsulfonium triflate, a water-soluble ionic PAG that absorbs at 193 nm and does not present acute toxicity hazard 34 (Fig. 1). The interaction between the new formulation and DUV was studied. We demonstrate a significant improvement of photopatterning performance at 193 nm and a mechanism of the PAG action is proposed. Etching tests are also performed to verify if the new formulation is suitable for pattern transfer into silica.

Experimental section

Materials

Chitosan was purchased from Mahtani Chitosan PVT, Ltd (India) and purified according to the Rinaudo procedure. 35 For this, chitin extracted from squid pens was partially hydrolyzed in alkaline medium. 36,37 In this study, the degree of acetylation (DA) of the chitosan was increased from 1% to 35%. 38 The DA value was confirmed with 1 H NMR according to the Hirai procedure 39 using an AVANCE III 400 MHz spectrometer. The average weight molar mass (Mw) and average molar mass (Mn) were 705 kg.mol -1 and 334 kg.mol -1 respectively (Dispersity Đ = 2.1), which was confirmed by size exclusion chromatography (SEC).

Dimethyl-2,4-dihydroxyphenylsulfonium triflate (purity 99,9 %) was purchased from Fluorochem Ltd. Acetic acid (glacial acetic acid 99 %) and deionized water for the solution preparation were purchased from Merck (Sigma-Aldrich).

Chitosan solution with (Dimethyl-2,4-dihydroxyphenylsulfonium triflate)

An aqueous solution of dimethyl-2,4-dihydroxyphenylsulfonium triflate was prepared by dissolving the powder (0.1 % w/w) in deionized water. The solution was left to stir magnetically for at least 2h so that the product could be completely dissolved.

Chitosan DA35 % at 0.8 % (w/w) was dissolved in the aqueous solution of PAG in the presence of acetic acid (Merck, glacial acetic acid, concentration 0.2 % (v/v)). The solution was left to stir magnetically overnight so that the chitosan could be completely dissolved.

Formulations without PAG, used for comparison, were prepared following the same procedure.

Thin film preparation and characterization

Silicon wafers were cleaned using acetone and ethanol followed by at least 15min of treatment in a UV-ozone cleaner. Chitosan and chitosan/PAG films were prepared by spincoating at 3000 rpm and 600 r/s 2 for 60 s. After this, the film was heated on a hotplate at 150°C for 5 min resulting in 100±2 nm thin films. An UVISEL ellipsometer from Horiba-Jobin-Yvon (spectral range 190 -830 nm) was used to measure the thickness of the film with the New Amorphous model used to fit the experimental data.

UV-VIS spectroscopy

Absorption spectra were recorded on a Lambda 950 UV/Vis spectrometer from Perkin Elmer (spectral range 200 -800 nm). Standard rectangular quartz cuvettes with pathlength of 10mm were used for the measurements in solutions. Films were spin-coated on fused silica substrates followed by a soft-bake at 150°C for 5 min.

Photopatterning

An Excistar (Coherent) nanosecond ArF Excimer laser (193 nm) was used to irradiate the samples. The power density of the beam was measured at the sample level before each experiment and used to calculate the exposure time. The mechanical shutter enables remote control of the irradiation dose, typically between 10 and 1000 mJ/cm 2 . The sample was placed on the x,y,z-motorized stage connected to the shutter. The beam was expanded and homogenized with a beam expander. Fused Silica Ronchi Rulings with chromium line patterns (100, 200 and 600 LPMM -line-pairs per millimeter) were used as a mask for the photopatterning. After the exposure, the sample was developed in deionized water for 30 s.

Additional tests on a commercial 193 nm stepper were carried out. For this, a 300 mm coat & develop track SOKUDO DUO (SCREEN) was used. The exposure tool was an ArF dry scanner Nikon NSR307E at NA 0.85.

Fourier transform infrared (FTIR) spectroscopy ATR

The FTIR spectra of chitosan and chitosan/PAG film were recorded on a Thermo Fisher Scientific iS50 spectrometer with an ATR Germanium grazing angle θ = 64° (Harrick -VarigATR) accessory. Chitosan/PAG films were deposited on silicon wafers. Spectra were recorded with a spectral resolution of 4 cm -1 and were averaged over 64 scans in the range 4000 -700 cm -1 .

Atomic Force Microscopy (AFM)

AFM scans of the patterns were obtained using a PicoPlus 5500 System model from Agilent. Scanning was performed in a tapping mode at 1 line/sec resulting in a 50µm x 50µm

scan with an image resolution of 512 x 512 pixels.

Profilometry

A DektakXT stylus profilometer was used. The range for the acquisition was from 1Å to 6.5 µm with the Stylus Tracking Force that equals to 3 mg. The radius of the stylus was 2 µm.

The scan range was from 200 to 300 µm.

Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS)

ToF-SIMS measurements of the chitosan/PAG films were obtained using a TRIFT III ToF-SIMS instrument (Physical Electronics, USA) operated with a pulsed 22 keV Au + ion gun (2 nA ion current) rastered over a 300 × 300 µm 2 area. The electron gun was operated in pulsed mode at low electron energy for charge compensation. The ion dose was kept below the static conditions limit. Data were analyzed using the WinCadence software. Mass calibration was performed on hydrocarbon secondary ions.

Plasma etching

Silicon wafers (thickness of 500 µm) with a 200 nm thick CVD SiO2 layer were used.

Patterns written in the chitosan/PAG layer were transferred into the underlying silica using a reactive ion etcher (RIE Corial 200S) operating under the follows conditions. The etching process was composed of three etching steps. The first step was a 5 s O2 descum plasma with a flow gas of 40 sccm (standard cubic centimeter per minute at STP), 40W RF source power and a pressure of 50 mT. The purpose of this step was to remove the remaining chitosan at the bottom of the features after development. Then the silica was etched with CHF3 plasma during 6min (flow gas of 100 sccm, 140 W RF source power and a pressure of 50 mT). Finally, a stripping step was performed to remove the chitosan mask layer with the following conditions:

O2 flow 100sccm, 100W RF source power and a pressure of 100 mT during 2 min.

Results and discussion

Influence of PAG on the chitosan resist properties

The photolysis mechanism of the PAG used in this study, namely dimethyl-2,4dihydroxyphenylsulfonium salts, was extensively studied by Crivello et al. 29,30 Based on the NMR data and final products analysis, they proposed a photolysis mechanism (Fig. 2a): the irradiation leads to the dissociation of the sulfonium salt and the formation of resonancestabilized ylide and solvated protonic acid. This acid is able to induce chemical reactions in the resist. In order to qualitatively show the impact of the PAG with irradiation at 193 nm, an aqueous solution with PAG (0,1% wt.) was irradiated with DUV light (l = 193 nm) with a dose of 4 J/cm 2 . This dose is chosen to be much higher than the doses used later in photopatterning to ensure the most complete photolysis of the PAG. The pH decreased from 4.6 to 3.3 as a result of formation of Trifluoromethanesulfonic acid. The appearance a pale orange color confirmed the photoinduced reaction (Fig. 2b).

The evolution of the UV absorption spectra before and after the irradiation confirmed this trend (Fig. 2c). Before the irradiation, the PAG exhibits three well defined absorption bands: at The UV spectra were also recorded in thin films with thickness of 100 nm (Fig. 3a). The absorbance spectra of chitosan and chitosan/PAG films are similar to the spectra of the solutions, confirming that the photolysis of the PAG is efficient also in the chitosan/PAG film.

We noted a slight shift of the absorption bands of the PAG before irradiation which may be assigned to a partial aggregation of the PAG in the thin film, after water evaporation. Resulting intermolecular interactions affect the energy states and as a consequence, absorption maxima. 40 The extinction coefficient of PAG e(200) was calculated as 20300 l . mol -1. cm -1 from the linear regression of the absorbance versus concentration, using the Beer-Lambert law (see inset in Fig

3.a)

. This value is high, which denotes a strong absorbance of the PAG at this wavelength, which is obviously needed to significantly increase the photosensitivity of the photoresist.

Fig. 3b shows the variation of the UV-spectrum in the chitosan/PAG thin film upon DUV irradiation. The tendency is similar to the UV-spectra for the chitosan/PAG solution. We observe a total bleaching of the PAG after irradiation with 1 J/cm 2 . Note that this dose was chosen lower than for solutions since the thickness is much lower (100 nm). The photobleaching of the PAG is thus also efficient in thin chitosan/PAG films. Thus, the release of H + in the irradiated zones is expected from these results.

These data are confirmed by ToF-SIMS measurements performed on chitosan/PAG thin films before and after irradiation. PAG characteristic signatures are detected at m/z 156.02 (C7H8O2S + ), 169.03 (C8H9O2S + ) and 171.05 (C8H11O2S + ) in positive ion mode (Fig. S1a and1b) and m/z 148.96 (CF3SO3 -) in negative ion mode (Fig. S2a andS2b). In agreement with the photobleaching mechanism of PAG (Fig. 2a), the relative intensity of the positive ion mode signatures decreases after irradiation (Fig. S1c andS1d) while that of the negative ion signature (CF3SO3 -) does not significantly change (Fig. S2c andS2d).

In our previous study, FTIR in transmission mode was used to investigate the modification of the chitosan photoresist upon DUV irradiation. [START_REF] Sysova | Chitosan as a Water-Developable 193 Nm Photoresist for Green Photolithography[END_REF] A remarkable stability of the chitosan chains and functional groups upon DUV irradiation was noted, with very limited chemical modification. The impact of the PAG is investigated here, using FTIR -ATR. In Fig. 4a, the FTIR-ATR spectra of chitosan with and without PAG are first compared. As expected, the characteristic peaks of chitosan can be found. In the region between 3500 cm -1 and 2500 cm -1 , there is a broad asymmetric band that corresponds to C -H stretching bands at 2882 cm - b) FTIR -ATR spectra of the chitosan + PAG film before and after DUV irradiation (1 J/cm 2 ).

In the "fingerprints" region, the strong peaks at 1072 cm -1 and 1035 cm There is a new peak in a region between 1700 cm -1 and 1200 cm -1 which is related to the presence of PAG. It is a band at 1241 cm -1 that corresponds to C -O stretching in hydroxyphenyl. The other peaks are related to the chitosan structure, such as the C = O stretching (amide I) at 1649 cm -1 , N -H bending (amide II) at 1564 cm -1 and C -N stretching (amide III) at 1316 cm -1 . 42 The last two sharp peaks at 1378 cm -1 and 1423 cm -1 can be assigned to the CH3 symmetrical deformation mode. 43 In order to investigate the mechanism of the PAG action, FTIR-ATR analysis of the irradiated films was performed. It can be seen from the spectrum (Fig. 4b) that most of peaks do not change after 1 J/cm 2 . This result is in line with our previous results, without PAG, that showed a limited modification of chitosan under DUV irradiation. It is interesting to note that the bands related to PAG are not modified by irradiation, although the UV-visible and ToF-SIMS measurements showed an efficient photolysis of PAG. This result can be explained by attributing these FTIR bands to functions preserved during irradiation. However, thanks to the use of ATR instead of transmission mode, a close look reveals that one new peak appears after the irradiation. It is a weak signal at 1718 cm -1 (Fig. 5a). This wavenumber corresponds to C = O stretching. Such vibration mode usually has a strong absorption, which denotes a very low concentration of C=O appearing upon irradiation. This peak is observed with and without PAG added in the chitosan film. The area of this peak was evaluated for DUV doses ranging between 0 and 5 J/cm 2 . The results with and without PAG are plotted in Fig. 5b. It clearly shows that the appearance of C=O during irradiation is increased with addition of PAG in the chitosan thin film. This denotes a higher reaction yield in the presence of the PAG. According to other studies on the modifications of chitosan under irradiation, 44-47 chain scission of chitosan can occur after the H • abstraction reaction of a radical on one of the saccharide cycles. As depicted in Fig. 6, the chain scission mechanism creates a new C=O function that could explain the appearance of the new peak in the FTIR-ATR spectrum. It is however surprising to observe an increase of the rate of creation of this new C=O band when PAG is added to chitosan as the PAG is essentially expected to decrease the pH. However, it is known that radical species are produced during the photolysis of this PAG. [START_REF] Fouassier | Photoinitiators for polymer synthesis[END_REF] Some of these radicals could have participated in the chain scission mechanism, as it seems to be highlighted by this ATR measurement.

Photopatterning at 193 nm

The motivation of the addition of PAG was initially to decrease the pH in the irradiated zones and thus increase the solubility of chitosan in these zones. It is indeed known that in acidic environment, the amine functions of chitosan are protonated, which decreases the probability of formation of intermolecular bonds between the chitosan chains (Fig. 7). By increasing the scission rate, made possible by the addition of PAG, a significant effect on the photolithography rate of chitosan in the presence of PAG is expected, which is tested in the following. This improvement of the chitosan photosensitivity by adding PAG was also confirmed in a micropatterning configuration, to check that a contrast in solubility can be obtained after a spatially controlled irradiation. A binary chrome mask with 100 line-pairs per mm (LPMM) was used. The irradiation doses ranged from 50 mJ/cm 2 to 600 mJ/cm 2 . After being exposed, the samples were developed in DI water for 30s. The height of the patterns was then measured using the profilometer and the results are plotted in the Fig. 8b. The sensitive curve for the pure chitosan is added for the comparison. As it can be seen on the figure, the pattern height increases linearly first, reaching a plateau after 450 mJ/cm 2 . Compared to the pure chitosan film, the doseto-print decreased almost two times becoming 500 mJ/cm 2 compared to the initial 900 mJ/cm 2 .

The maximum pattern height remains 80±5 nm. This proves that the effect of the PAG can be spatially localized in the irradiated area. The effect of the concentration of PAG on the photolithography properties was also investigated. Three different concentrations of PAG were then tested (0.01, 0.05 and 0.1 wt %). Note that 0.1 wt % corresponds to the limit of solubility of the PAG in the photoresist. The results (Fig. 9) show that the dose needed to obtain 80 nm-patterns and to remove the residual layer in the exposed zones increases proportionally to the decrease in the PAG concentration.

This justifies the concentration of PAG as 0,1 wt % used in the tested formulation and confirms the crucial role of the PAG in the increase of the photosensitivity of the photoresist.

It was also confirmed that smaller lateral dimensions could be obtained (Fig. 10) by applying the photopatterning procedure with masks with smaller features (5 µm, 2.5µm and 0.375 µm line widths). Well defined patterns were obtained in all cases. A full patterning down to the substrate could be obtained for 5 and 2.5 µm line widths, with a slight decrease of maximum patterns heights in the case of the 2.5 µm line widths. Submicron patterns can be obtained but, in this case, the maximum height is smaller. This was attributed not only to the resist limitations, but also because of the optical set up limits (diffraction limits). Note that the best patterns (with maximum height patterns) for the lower line width were obtained for a slightly smaller dose (340 vs 530 mJ/cm 2 ). Finally, the last step was to make sure that the new formulation is compatible with a commercial 193 nm stepper and the etching step since these aspects are vital for its further implementation at the industrial production level. Fig. 11a shows a photograph of a silicon wafer covered with a chitosan film deposited by spincoating. This image shows the good quality of the deposition on the whole wafer, except for defects on the edges which can be corrected with optimization work. The thickness was measured by ellipsometry along the wafer diameter and evaluated at 100+/-11 nm. The thickness deviation will also need to be optimized to meet the requirements for industrial use but these initial results are very encouraging. Fig. 11b shows SEM images of patterns obtained with the Nikon NSR307E scanner at NA 0.85. The contrast curve is identical to the one shown in Figure 8.a. A dose of 400 mJ/cm 2 was used. Well defined patterns are observed for widths of 5 µm and 3 µm. A decrease in the quality of the patterns is obtained for line widths of 2 µm and 1.5 µm but these results are also consistent with those shown previously [START_REF] Amano | Evaluation of Nano-Patterning Performance of Water-Soluble Material for Photoresist Using Sugar Chain[END_REF] and show that the chitosan-based resin is well compatible with industrial tools, even if optimization work is still necessary to allow results usable in an industrial context.

Features were transferred into a 200 nm thick SiO2 layer on Si substrate using CHF3 etching 13,56 MHz RIE plasma. The initial pattern in the chitosan film was produced using a chromium mask consisted of 5 µm lines separated by 5 µm. Etching tests showed that the selectivity of the chitosan/PAG resist layer was similar to the one of pure chitosan. The resist was etched as long as possible to reach patterns of almost 200nm high as displayed in Fig. 11c. The width of the patterns (taken from the bottom) was 5.2µm ± 0.1µm and deviated by 4% from the expected value, with a slope of about 10°. The low slope value is easily explained by the high ratio width over depth of the patterns which is just over 25. Finally, the Root Mean Square (RMS) surface roughness at the top and bottom of the features were 0.2nm and 8.5nm, respectively. Therefore, an increase of roughness as measured by the RMS at the bottom of the features was observed compared to our results using pure chitosan as a resist. The increase in roughness is not fully explained but may be due to a not complete descum (there is still some resist on the bottom before etching which accentuates the roughness), or to the presence of PAG or the formation of non-volatile compounds that can lead to micromasking. In conclusion, we demonstrate that the presence of the PAG in the chitosan photoresist does not degrade the etching properties of the photoresist. Work is in progress to optimize the roughness criteria.

Conclusions

In this paper, we demonstrate that the photosensitivity of the chitosan can be significantly improved by adding a photoacid generator to the formulation. We studied the mechanism of interaction of DUV laser beam and PAG as well as its influence on the chitosan resist. It was shown that the PAG favors the chain scission mechanism under DUV irradiation, which facilitates the dissolution of the chitosan during the development step. A nearly two-fold lower dose is necessary to obtain patterns. Moreover, we showed that the presence of PAG did not degrade the resistance of the photoresist layer to etching. Micrometric features were transferred into the silica layer with excellent fidelity. The addition of PAG does not seem to modify the etching selectivity of the resist under CHF3 plasma and thus allows the etching of patterns in silica. However, a slight increase of the roughness at the bottom of the features was observed compared to our previous results. With this work, it is shown that photoresist chitosan has the potential to reach industrial standards and its use in this context will reduce the negative impact of the microelectronics industry on the environment.

Figure 1 .

 1 Figure 1. Simplified scheme of the photolithography procedure. 1. The substrate is first cleaned

Figure 2 .

 2 Figure 2. a) Mechanism of PAG degradation under UV irradiation (adapted from[START_REF] Dore | A Water-Processable Cellulose-Based Resist for Advanced Nanofabrication[END_REF][START_REF] Takei | EUV Lithography Using Inedible Cellulose-Based Biomass Resist Material[END_REF] ). b)

  285 nm, 244 nm and at 200 nm. After the irradiation, those bands disappeared, confirming the efficient photolysis of the PAG upon 193 nm irradiation. In the inset of Fig.2c, the spectra of the solution containing only chitosan (before and after DUV irradiation) are shown. These spectra show the absorbance of the chitosan solution and also the absence of modification of this absorbance spectrum during irradiation.

Figure 3 .

 3 Figure 3. a) UV absorption spectra of chitosan with and without PAG in thin films. The inset

1

  and axial O -H and N -H stretching at 3363 cm -1 .

Figure 4 .

 4 Figure 4. a) FTIR-ATR spectra of non-irradiated chitosan and chitosan/PAG (0.1% w/w) films.

  -1 are related to skeletal signals (glycosidic bonds' vibration, C -O -C and C -O single bond vibrations),which is a characteristic of the chitosan polysaccharide structure.41

Figure 5 .

 5 Figure 5. a) FTIR -ATR spectra of the chitosan/PAG film before and after DUV irradiation

Figure 6 .

 6 Figure 6. The proposed chain scission mechanism of the chitosan, initiated by the radical

Figure 7 .

 7 Figure 7. The solubility switch of the chitosan depending on the pH.

Figure 8 .

 8 Figure 8. a) Contrast curve plotted after measuring the thickness of the residual film on

Figure 9 .

 9 Figure 9. Evolution of patterns (5µm width) height with the increase of the dose for three

Figure 10 .

 10 Figure 10. AFM images of patterning using chitosan/PAG resist. Patterns with 100

Figure 11 .

 11 Figure 11. a) Photography of a 300 mm Si wafer covered by a 100 nm chitosan film

Acknowledgments

The authors thank the Agence Nationale pour le Recherche (ANR) for funding this project (Lithogreen project, Grant ANR-19-CE43-0009). Simon Grée and Philippe Kunemann are gratefully acknowledged for their technical assistance in the FTIR-ATR and profilometry measurements.

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