Elise Degrandi-Contraires

Ravindra Udagama

Elodie Bourgeat-Lami

Timothy Mckenna

Keltoum Ouzineb

Costantino Creton

Mechanical Properties of Adhesive Films Obtained from PU-Acrylic Hybrid Particles

relatively simple, the design of proper PSA is complex and relies heavily either on polymer chemistry (for acrylic and silicone polymers) or on formulation (for block copolymers and natural rubbers) in order to obtain sparsely crosslinked polymer networks with optimized properties [2][3][START_REF] Satas | Acrylic Adhesives[END_REF][START_REF] Sobieski | Silicone Pressure Sensitive Adhesives[END_REF][START_REF] Lindner | [END_REF][7] . This optimization means that the PSA must be able to dissipate energy during the peeling process (a property optimized for a highly viscous entangled polymeric liquid) and must be resistant to creep in shear (optimum when some crosslinking is present preventing flow).

Acrylic PSA [START_REF] Satas | Acrylic Adhesives[END_REF]8 are generally weakly crosslinked copolymers of low T g which have insoluble and soluble fractions, respectively called gel and sol. A very broad molar mass distribution (i.e. high polydispersity) of the sol fraction and a cohesive network formed by the gel is a good and relatively easy way to achieve a practical solution, but has limitations due to the impossibility of independently control the architecture of sol and gel during a simple synthesis. The fine control of the network structure of the gel becomes an essential aspect of advanced techniques to achieve a better balance of properties.

A possible design path to achieve a better control of the network structure is to incorporate another polymer (such as an alkyd or a polyurethane) to form hybrid materials. The preparation of these hybrid materials could potentially lead to a more favourable combination of properties compared with those of the initial purely acrylic PSA. Because of environmental concerns however, we have focused our study on emulsion-based acrylic PSA 8 . In emulsionbased systems, the independent control of the network structure of the gel and of the sol fraction is more difficult. Furthermore, the control of the interfaces between particles remaining after drying is problematic and can strongly affect the mechanical properties 9,10 .

However, the development of miniemulsion polymerization 11,12 has helped to overcome the main issue of the homogeneous incorporation of a hydrophobic polymer in a waterborne acrylic polymer. The advantage of miniemulsion polymerization is that it is possible to directly incorporate a hydrophobic polymer (e.g. PU) into the monomer droplets during the droplet formation step, and then directly polymerize the droplets to get a hybrid where the components are mixed on the scale of nanometers 12 . If these hydrophobic compounds contain reactive functional groups, such as unsaturated double bonds, then they can be chemically incorporated into the growing polymer chains during the free radical polymerization.

Many different components have been incorporated successfully into hybrid systems in this manner, and several reviews are available presenting the technique and the possibilities of hybridization [11][12][START_REF] El-Aasser | Miniemulsions: Overview of research and applications[END_REF][START_REF] Schork | Miniemulsion polymerization[END_REF] .

A number of studies have investigated the synthesis of acrylic-polyurethane (PU) systems.

Successful incorporations have been reported by the formation of interpenetrating networks 15 , by the formation of a core-shell structure 16 or by standard emulsion with a polyurethane dispersion as a seed 17 . In all cases, the transparency and homogeneity of the hybrid were better than those of the physical blends. However miniemulsion polymerization appears to be a versatile and powerful method to graft polyurethane to an acrylic polymer as exposed in the work of Udagama et al. 1 , of Wang et al. [18][19][20][21][22] or again of Li, El-Aasser and coworkers 23 . In the latter study, it is shown that the grafting prevents phase separation at the particle scale between polyurethane and acrylic copolymer. Most studies focused on applications as coatings, where advantages of combining acrylic with urethane for PSA applications are potentially numerous: better film-formation, better mechanical stability or temperature resistance [23][24][25] and where a precise control of the polymer architecture is not necessary since crosslinkers (dryers) are often added. To the best of our knowledge ours is the first study focusing on acrylic polymers for adhesive applications where the control of the polymer architecture is essential to obtain the desired properties and where a precise control of a low level of crosslinking 26 and of the entanglement structure is essential 27 .

In this study and in its companion paper 1 , we selected a model acrylic copolymer typical of those used for PSA, and investigated the incorporation of a minority fraction of a low molar mass reactive PU in the acrylic particle. Part I 1 focuses on the synthesis methods while part II, the current paper, focuses on the effect of changing synthesis parameters on the adhesive properties and relates such changes to the network structure, characterized by mechanical tests on dry films and by swelling experiments.

F o r R e v i e w . C o n f i d e n t i a l - A C S 5

Experimental

Synthesis

All samples have been synthesized in batch via miniemulsion polymerization. The details of the polymerisation procedure are described in a companion paper 1 and we only briefly recall here the main synthesis parameters which are necessary to understand the results.

The monomer composition of the acrylic matrix was: butyl acrylate (BA, 89.5 wt%), methyl methacrylate (MMA, 9.5 wt%) and acrylic acid (AA, 1 wt%). A NCO terminated polyurethane (PU) prepolymer (Incorez 701, Industrial Copolymers Limited) was incorporated in the acrylic polymer particles. This incorporation was performed in three steps:

1 ) Th e PU pre p ol y m er ch a ins w ere di s s olv ed i n t h e organic monomer phase, which also contained the hydrophobic costabilizer: octadecyl acrylate (ODA). Hydroxy ethyl methacrylate (HEMA) was added and a known amount of NCO functions was reacted with a known amount of HEMA. We thus define the percentage of HEMA to NCO (% HEMA to NCO). This step was conducted for 12 hours at 25°C using dibutyltin dilaurate as catalyst and results in the grafting of NCO to the HEMA.

2 ) A kn ow n am ou n t of Bi s ph en ol A (B PA ) w as adde d t o the organic phase containing PU and the dibutyltin dilaurate catalyst and reacted for 30 minutes at 25°C. The role of this BPA was to extend the PU chains and complete the reaction with the remaining NCO groups.

3 ) Th e o rg an i c ph as e c on t ain in g t he m on om e rs an d t he HEMA-functionalized BPAextended PU chains were added to the aqueous solution containing surfactant (Dowfax 2AI).

Nanodroplets were formed by ultrasonication. The radical polymerization was initiated with the addition of a redox initiator pair (Tertiary Butyl HydroPeroxide, TBHP, and Sodium Formaldehyde Sulfoxylate, SFS) and miniemulsion polymerization was carried out at 50°C. 1-D ode cy l m erca pt an w as us ed as a ch a in t rans f er a gent (CTA) to control the kinetic chain length and therefore gel formation. 0.2 wt% with respect to monomers was added unless mentioned otherwise. It is important to note that butyl acrylate is much more prone than methylmethacrylate to chain transfer during free radical polymerization and naturally leads to branched architectures of the polymer in the absence of CTA, ODA was used as hydrophobic component and was added to the particles to avoid Ostwald ripening. Three

F o r R e v i e w . C o n f i d e n t i a l - A C S 6
series of samples were prepared. In the first series, the molar ratio between OH and NCO functions was equal to 0.55 and the molar ratio of OH functions from the HEMA to OH functions from the BPA (the HEMA/BPA ratio) was kept equal to 0.22. The polyurethane weight fraction was changed from 5 wt% to 50 wt% based on monomers. In the second series, a fixed quantity of PU (25 wt%) was incorporated in the particles. The ratio OH/NCO was fixed at 0.55 and the ratio HEMA/BPA was changed from 0.11 to 0.50. In the last series, ODA was excluded from the synthesis and the ratio OH/NCO was increased to 1. Two samples with 25 wt% of PU were prepared with different HEMA/BPA (respectively 0.10 and 0.25) and different CTA contents (respectively 0.3 and 0.2). Finally, one blank latex with no PU, no HEMA and no BPA was prepared with a CTA content of 0.2%. Table 1 summarizes the important formulation parameters for the different samples.

Polymer characterization

The solid content of the latexes was measured gravimetrically and was ~ 50 wt% of polymer for all latexes. We measured the gel content and the swelling ratio, Q, by static measurements. A small piece of solid film was cut and weighed (W 1 ) and was then immersed in THF for a week. After a week, the sample which was completely swollen by the solvent was taken out and weighed again (W 2 ). It was then dried at 60 °C for 30 minutes to extract all the solvent and the dried sample was weighed (W 3 ).

F

The gel content was given by: %gel =

W 3 W 1 × 100 (1)
And the swelling ratio by:

3 2 W W Q = (2)
The glass transition temperature of the final polymer samples (Tg) was measured by Differential Scanning Calorimetry (DSC; TA Instrument DSC 2920). Two cycles were performed at cooling and heating rates of 10 °C.min -1 . The T g was obtained from the second cycle.

Mechanical characterization

Samples for the rheological and tensile experiments were prepared as follows: a given volume of latex was deposited in silicone moulds and dried at room temperature for one week. The moulds were covered with a bell-jar to protect them from dust. The volume of latex was calculated to give the required final film thickness depending on the latex solid content. After a week, the moulds were held for 5 minutes at 110°C. The samples were then removed, placed between low-release silicone papers and cut to the desired dimensions with a diecutter.

For the probe-tack experiments, the latexes were deposited on microscope glass-slides also protected from dust by a bell-jar, and dried 48 hours at room temperature. They were finally dried for 1 minute at 110°C and the resulting films, with an average thickness of 120µm, were kept in plastic boxes before use. All samples were used within a week of preparation.

The linear viscoelastic properties of the adhesives were characterized with a rheometer RDAII using parallel plate geometry. Frequency sweeps (0. To analyze the large-strain behaviour, we used a standard tensile INSTRON machine (model 5565) equipped with a videoextensometer (model SVE). The machine uses a 10 N load cell with a resolution of ± 1 mN. Rectangular strips of 5 mm wide (w 0 ) with an average thickness (e 0 ) of the order of 500µm were cut within the self-standing films. Individual values were controlled for each sample. The initial velocity of the crosshead was chosen to obtain an initial strain rate of 1 s -1 which is roughly equivalent to the initial strain rate applied during the adhesion tests and all tests were performed at room temperature. The force-displacement curve obtained is then converted to nominal stress σ N = F/A 0 vs. stretch λ = l/l 0 The adhesive properties were characterized with two types of tests: a laboratory probe test 28,29 and a series of standard industrial tests. In the probe tack test, a stainless steel probe comes in contact with the adhesive layer deposited on a glass slide. After a set contact time, the probe is withdrawn at a constant velocity. A mirror is installed upon the glass slide and allows the visualisation of the debonding mechanism and the measurement of the real contact area for each sample. We used standard parameters for the temperature (Room Temperature), approach velocity (V app = 30 µm.s -1 ), contact time (t c = 1 s) and contact force (F c = -70 N).

The debonding rate was varied between 1 and 1000 µm/s but we report here only data for the debonding velocity (V deb = 100 µm.s -1 ) for the purpose of comparing materials. The force was measured by a load cell (250N, resolution 0.5 N). The displacement of the probe was measured with an LVDT extensometer (range 5mm, resolution 0.5µm). The force and displacement for each curve were normalized by the contact area of the probe (as visualized with the camera) and by the initial thickness of the film to obtain a stress vs. strain curve which will be used to compare adhesive films.

The characterization of the adhesive properties of the films with industrial standard tests (FINAT) was carried out as follows. The latexes were dried on a polyester (PE) backing film during 3 minutes at 110 °C to obtain dry adhesive film thicknesses of 23µm which are standard in industry. These adhesive + PE layers were then bonded to a stainless steel (SS) substrate.

The shear resistance was defined by measuring the holding time (in minutes) before failure of a 1 inch² adhesive layer to which a mass of 1 kg was applied (FINAT test method 8). The

Results and discussion

Before discussing the specific effects of the PU weight fraction and of the degree of grafting, it is useful to briefly summarize the DSC results. The PU has no apparent effect on the heat capacity scans and a single T g of -39 ± 3 °C is detected for all hybrid adhesive films. For comparison, the T g of the pure PU is estimated to be around -48 °C. Therefore, the large distribution of the T g of the copolymer and its proximity with the T g of the PU could mask a possible phase separation.

1.

Role of the PU weight fraction

Adhesive properties

In the first series of samples (sample T150 to T153 in table 1), the effect of the weight fraction of reactive PU incorporated in the polymer was investigated. Figure 1 presents probetack results for this series. The relevant parameters chosen to analyse these results were the plateau stress at intermediate and larger strain and the maximum strain. The plateau stress σ f is related to fibrillation observed during the debonding, while the maximum strain ε max corresponds to the final detachment from the probe 28 . Figure 1 shows that samples with a low amount of polyurethane (less than 25% based on the total amount of acrylic monomers) are characterized by a low fibrillation stress and a high maximum strain ε max a The residues a For 15wt% of PU, the stress does not go back to zero for technical reasons. The maximum scale of the extensometer measuring the probe displacement is 5 mm. For the 15% sample, when the probe reaches 5 mm, fibrils joining the probe and the adhesive layer remain. These fibrils neither fail for a high deformation nor after waiting a long time. They can break cohesively or adhesively when the test is stopped and the probe is removed manually. show almost no fibrillation plateau and the ε max is low. Despite nucleation of cavities at the onset of debonding, these cavities coalesce and interfacial propagation of cracks leads to an interfacial debonding 30,31 . Finally, the sample with 25% of PU forms a fibrillar structure upon debonding, and debonds at a maximum strain ε max of the order of 10 which is typical of many commercial PSA's. The final drop of the stress is characteristic of adhesive debonding, and results in the absence of residues on the probe. Thus, adhesive properties are best for this fraction of PU. with the pure acrylic matrix (T170) showing that both adhesion energy and resistance to shear have been improved by the incorporation of the PU in the material.

F

The interpretation of these results requires a finer characterization of the differences in the network structure.

This characterization was carried out with the help of three different methods which are complementary: Rheological measurements reveal the level of viscoelasticity, large strain tensile experiments are a sensitive probe of the finite extensibility of the network chains and of the dissipation of energy taking place at large strains and the swelling and gel fraction measurements inform on the architecture of the polymer.

Linear viscoelasticity

Since decades the linear viscoelastic properties of pressure-sensitive-adhesives have been considered the best predictor of the adhesive properties [32][33][34][35] . From fully empirical criteria, we have now moved to more mechanistically based semi-empirical criteria which are able to predict to a certain extent the performance of PSA 30 .

Figure 2 shows the variation of the storage modulus and tanδ with frequency, for three adhesive films with increasing PU content. At equivalent degree of grafting, the more PU is added to the hybrid, the higher is its elastic modulus and the lower is tanδ. It is clear that the dissipation becomes much lower and frequency-independent in the presence of high PU contents. This shows that the number of pendant chains and free chains decreases These linear viscoelastic properties can be used to predict the suitability of the soft network for PSA applications. The value of the storage modulus at 1 Hz should be below 100 kPa to allow an easy contact with a rough surface 36 . The value of tan δ/G' at the relevant debonding frequency can be used as a finer criterion to predict whether fibrillation will occur or not upon debonding 30,37,38 . For the sample with 50% of PU, the value of G' is 1.45.10 5 Pa, i.e. in the upper limit to conform to the surface. At the tack test frequency (1 Hz), the value of tanδ/G' is 0.21.10 -5 Pa -1 , which is inferior to the lower limit of 0.5.10 -5 Pa -1 reported to be required to have fibrillation debonding on steel 30,[START_REF] Deplace | Adhésifs Nanostructurés en Voie Emulsion[END_REF] . The fact that tanδ/G' is too low indicates a lack of dissipation coupled to a high value of the elastic modulus, completely consistent with the observed interfacial debonding.

F o r
Conversely, for 5% of PU, the storage modulus meets Dahlquist criterion (0.53.10 5 Pa) and the level of dissipation is very high. As a result the debonding criteria is very high (tanδ/G' ~ 1.15.10 -5 Pa -1 at 1 Hz). Dissipation dominates and we are in presence of a highly deformable material which readily forms fibrils upon debonding. This is again consistent with the cohesive failure observed in adhesion tests.

As observed with the tack test, the value of 25 wt% of PU in the latex represents an optimum value for the viscoelastic properties as well. Indeed, both the stiffness and the fibrillation Pa -1 at 1Hz, and fibrillation debonding is indeed observed.

The differences in viscoelastic behaviour reflect differences in the architecture of the polymer inside the particle. Table 2 gives the gel fraction in the adhesive film for the five PU contents.

For 5% PU, the gel fraction is too low to be measured. Then it increases to 70-80% for the high PU-content samples.

The characterization of the gel fraction is a good indicator of the viscoelastic behaviour. The partially crosslinked gel imparts elasticity while the pendant chains attached to the gel and the sol fraction control the time dependent stress relaxation. Based on the differences in linear viscoelastic properties and gel fraction, the incorporation of PU at fixed grafting ratio increases the crosslink density, the entanglement density (high frequency G') and decreases the density of pendant chains and the soluble fraction..

Large strain behaviour

While linear rheology experiments are useful to predict whether fibrils will form or not after the initial cavitation stage, the final detachment of these fibrils can only be predicted through the analysis of the large strain behaviour [START_REF] Glassmaker | [END_REF][41][42] . Figure 3 shows the results of tensile experiments for the five samples with different amounts of PU. Very significant differences due to the presence of PU are immediately apparent not only in the absolute values of the stress at equivalent strain but in the overall shape of the stress strain curve. The two samples with the lowest PU weight fractions (T150 and T151) do not present any hardening at large strain but simply flow at a nearly constant nominal stress. As it is obvious from looking at the curves of Figure 3, the tensile behaviour of these weakly crosslinked networks is highly non linear, and shows a pronounced relaxation or softening after about λ = 1.5. For almost all PSA's this softening is both strain and strain rate dependent and curves measured at a single strain rate can be fitted equally well by a viscoelastic model 26 or by a nonlinear elastic model 43 First the Young's modulus E y can be measured at small strains. It is defined as the slope of a line fitting the curve between λ=1 and λ=1.02. This quantity represents the unrelaxed modulus and should be directly comparable with G'(ω) at the appropriate strain rate.

F

Second, the deviation of the stress-strain curve relative to the prediction from Gaussian rubber elasticity can be analyzed from the Mooney representation. The reduced stress σ R , also named Mooney stress, is defined as:

      - = ² 1 N R λ λ σ σ
It corresponds to the nominal stress normalized by ( )

2 1 λ λ -
. For a Neo-Hookean rubber, the reduced stress is constant as a function of λ and is simply equal to the shear modulus G. For a PSA, this reduced stress is not constant and is typically represented as a function of the inverse of strain 1/λ 30 . Two characteristic parameters of the non-linear behaviour can be determined: the C soft to characterize the softening at intermediate strain and the C hard characterizing the onset of strain hardening at large strain 30 . C soft is defined as the slope of a line drawn between (σ R (0.8); 1/λ = 0.8) and (σ Rmin ; 1/ λmin ). C hard is defined as the reduced stress corresponding to the minimum in σ R . Another interesting parameter is λ hard, i.e. the strain level corresponding to the minimum reduced stress and hence to the onset of strain hardening. Strain hardening or more accurately strain stiffening 44 is correlated to the finite extensibility of the chains and thus to the network architecture. The ratio C soft /C hard characterizes, in large strain the balance between relaxing and non-relaxing crosslink points and should be optimized for a PSA [START_REF] Lindner | [END_REF]30 .

The Mooney representations as a function of PU content are shown in Figure 4 while Table 3 gives the values of the viscoelastic parameters C soft and C hard , the ratio C soft /C hard and the Young's modulus. The ratio C soft /C hard reflects the balance between the extent of relaxation at intermediate strain and hardening at large strain. It decreases significantly with increasing PU content, reflecting the less and less viscoelastic character of the films (see Figure 5). The Young's modulus is a small-strain high frequency modulus. It is affected by both chemical crosslinks and entanglements (that are all trapped at high frequencies). However in a fully acrylic matrix it is usually not much affected by a low level of crosslinking 26,43 . Its increase with PU content

Summary

These combined results indicate that the network becomes much more densely crosslinked when the PU weight fraction increases. However it is interesting to note that a small amount of PU actually reduces the gel fraction and the strain stiffening, relative to a purely acrylic matrix. This reduction in crosslinking is probably due to a reduction in the acrylic kinetic chain length since the bisphenol-A favors chain transfer 45,[START_REF] Lopez | Simultaneous Free Radical and Addition Miniemulsion Polymerization: Effect of the Diol on the Microstructure of Polyurethane-Acrylic Pressure Sensitive Adhesives[END_REF] .

The gel fraction and swelling experiments confirm that the NCO-terminated PU enhances crosslinking of the PU polymer chains to the acrylic network through the HEMA. In essence the PU would appear to act like any low molecular weight crosslinker. Yet this is misleading because, in addition, PU chains are also more densely entangled than the acrylic chains because of the PPG constituting their soft segment (M e ~ 3 -5 kg.mol -1 for the PU [START_REF] Florez | [END_REF] while M e ~ 20 -30 kg.mol -1 for the acrylics 48 ). Thus, the PU adds both chemical and physical crosslinks and this is the reason for the significant increase in the small-strain modulus. The very significant softening observed for all PU hybrids would be very difficult to obtain with a purely acrylic network.

F

The consequence on adhesive properties of this continuous variation in crosslinking and entanglement density with PU content is a clear transition from under-crosslinked to overcrosslinked materials. Only one of the adhesives gives a network architecture adapted to a PSA application and a reasonable balance between relaxation and hardening. This sample debonds adhesively in a fibrillation mode, with relatively high adhesion energy. An equivalent optimum in the PU weight fraction has been observed by Wang et al. 20 in the same type of PU/acrylic hybrid prepared for coating applications.

Impact of the degree of grafting of the PU

If the PU content is fixed, the second parameter that can be varied is the level of grafting of the PU chains to the acrylic matrix. The reaction between HEMA and NCO-terminated PU can be well controlled during the synthesis 1 . In the following series, the fraction of NCO actually consumed during HEMA grafting is varied from 5 to 20mol%, taking into account that 20% is the maximum level of grafting allowed by the reaction between HEMA and PU 1 .

For a fixed total amount of OH groups, the degree of grafting is directly connected to the HEMA/BPA molar ratio. In this section, we compare samples with a constant weight fraction of PU (25%) and HEMA/BPA ratios changing from 0.1 to 0.5 with a ratio between OH and NCO fixed at 0.55.

Adhesive properties

Figure 7 shows the probe tack curves obtained for different degrees of grafting at a PU content of 25% and a CTA amount of 0.2%. For these synthesis conditions, a fibrillation plateau is always observed but cohesive failure of fibrils is sometimes observed for the sample with HEMA/BPA=0. Pa.s -1 for the higher ratio (T149)). In conclusion changes of the grafting ratio at fixed PU content do not greatly affect the linear viscoelastic properties. This relative insensitivity of the linear viscoelastic properties with grafting ratio are confirmed by the gel fraction measurements which all show a gel fraction varying between 36 and 54 % and increasing slightly with increasing HEMA/BPA ratio.

Linear viscoelastic properties

Large strain experiments

The tensile experiments are more sensitive to the subtle changes in polymer architecture and they have been performed on the same samples at a nominal strain rate dε/dt=1s -1 . To emphasize these fine changes, we choose here the Mooney representation, as explained in 1.3

(Figure 9). The different curves have a similar shape for all the HEMA/BPA ratios. The main

(a) (b) F o r R e v i e w . C o n f i d e n t i a l - A C S 22
difference is in the onset of strain hardening, as can be seen on the magnified curve (fig. 9right). From the Mooney plot, one can observe that C soft and λ hard do not change much but C hard does. The increase of C hard shows that HEMA/BPA mostly affects the density of permanent crosslinks, i.e. the bridges created by the PU chains, but not the finite extensibility of the network presumably controlled by the longer chains of the acrylic network. Such an apparently conflicting result can only be obtained with a heterogeneously crosslinked network, i.e. some densely crosslinked zones (which swell less and provide stiffness) and some less crosslinked zones which percolate and can extend. The values of C soft (Table 5)

remain relatively constant except for the lowest grafting density where the relaxation is very fast and as a consequence the value of C soft /C hard decreases with grafting density. This decrease in viscoelastic character combined with the increased level of strain stiffening causes the differences in adhesion energy that are observed.

Measurements of the gel fraction (relatively constant for HEMA/BPA between 0.1 and 0.36) confirm this scenario: equilibrium swelling from Q = 85 at low grafting to Q = 55 at high grafting. Finally because the PU fraction is fixed the density of entanglements is also fixed and the small strain modulus slightly changes.

Summary

The crosslinked network architecture is modified by the HEMA/BPA ratio but differently than by the PU content. Decreasing the HEMA/BPA ratio is equivalent to decreasing the density of PU bridges incorporated in the network. However this time this change occurs mostly in the gel fraction and does not greatly affect the finite extensibility of the network probably because of a heterogeneous gel structure. As a result, the adhesion energy remains relatively constant but the material can be made a little less crosslinked (favourable for adhesion on low energy surfaces) or more crosslinked (favourable for high shear resistance).

One can conclude that at fixed 25% PU content, the change in grafting ratio affects the crosslinking density of more crosslinked domains in a heterogeneous gel structure and does not affect much the sol fraction. It affects the low frequency dissipation in the polymer network, improving the resistance to flow, and not the small strain and high frequency elasticity. It is then possible to use separate synthetic tools to adjust the storage modulus (PU fraction) and the finite extensibility (PU grafting).

Effect of the OH/NCO ratio

As described in a companion paper, during the miniemulsion synthesis, it is necessary to add a hydrophobic monomer to avoid Ostwald ripening of particles 12 . Here 5.15 % in weight (relative to the monomer content) of octadecylacrylate (ODA) was incorporated in the recipe.

In presence of this hydrophobic component, it was not possible to incorporate the total amount of BPA required to give a stoechiometric ratio OH/NCO = 1. For amounts higher than 50% of the theoretical amount, flocculation of the particles was observed during the synthesis 1 .

The consequence for the polymer architecture and mechanical properties is that the PU chains are expected to be less extended by BPA. Furthermore, as no isocyanate can be observed by [START_REF] El-Aasser | Miniemulsions: Overview of research and applications[END_REF] C NMR, it is reasonable to assume that reaction with water occurs. In other words, the reaction of polyurethane prepolymer chains is not completely controlled.

F o r
To promote a total reaction of the remaining isocyanate with BPA, some samples have been prepared without costabilizer ODA and with OH/NCO=1. No flocculation was observed and some solid films have been prepared from these new latexes to evaluate the impact of the OH/NCO ratio on the mechanical properties and the polymer architecture. In these samples, 25 wt% of PU is incorporated and two grafting ratios of the PU were chosen: 20% (high grafting) and 10% (low grafting). CTA content also varied from 0.2% for the high grafting to 0.3% for the low grafting (see Table 1). Probe-tack and tensile results of four samples are presented in figure 10. As seen in Figure 10, the effect of the change in process is different in the case of high grafting level and low grafting level. Nevertheless, in both cases, the increase in the amount Indeed, they indicate that the network is less crosslinked, but with only a small effect of the decrease in the gel fraction on the elasticity.

F o r
For low grafting, probe-tack tests show a longer plateau, due to a stabilized fibrillation process. In tensile experiments the softening takes place at a higher stress and hardening is more pronounced. These observations reflect that the material is less deformable with more BPA. In this case, it may be argued that the extension of PU chains by BPA can help to connect dangling chains together to create PU bridges and increase the gel content and thus the stiffness. Unfortunately, the extraction and swelling measurements could not be done due to the low level of crosslinking in the samples.

If the effect of the extension on the mechanical properties (see Figure 11) is clear in both cases, it is not easy to locate the transition from the first mechanism (reduction of acrylic chain length) to the second one (formation of new PU bridges) and the effect of reaction with water is also unclear.

On the point of view of the application as PSA, the best results in tack, peel and shear are obtained for hybrid latexes with 25% of polyurethane compared to the acrylic monomers, with OH/NCO=1, HEMA/BPA ~ 0. o BPA-extended chains which did not react with any HEMA are called free chains. They are part of the soluble fraction inside the particles.

These three types of chains coexist in the particles and their proportions and lengths obviously change according to the fraction of PU, the ratio OH/NCO and the ratio HEMA/BPA.

To have an appropriate representation of the architecture of the hybrid network inside particles, one has to take in account two other important parameters. The molar mass of the acrylic copolymers (300-4000 kg.mol -1 depending on the samples) is much higher than that of the PU, even extended. Then, assuming that the soft chain is mostly polyether (polypropylene glycol PPG), the PU chains are much more entangled than the acrylic ones. M e for this type of PU has been found to be around 3-5 kg.mol -1 47 when it is ~ 20 -30 kg.mol -1 for the main monomer, butyl acrylate 48,49 Taking in account the different connections of the PU chains, a schematic representation of the network is presented in Figure 11 (except that no reaction of NCO with water is shown). The presence of PU affects not only the viscoelastic balance but also the finite extensibility of the adhesive layer by connecting the acrylic backbones. As discussed above, even significantly extended PU grafted chains remain shorter than the acrylic chains. Moreover, most of the chains are bonded chains connected to the acrylic at both ends. In this context the onset of strain stiffening is controlled by the length of the acrylic chains between two grafting points. The change in mechanical properties as a function of grafting density suggest that the PU may not be distributed homogeneously in each particle, with some PU rich zones connected together by PU poor zones.

Two important caveats need to be mentioned here. First no reaction with water is considered in this description. In reality, some of the NCO functions will react with water during the miniemulsification process and the early stages of the synthesis reaction 1,23,50 . This fraction may modify the architecture of the polymer.

Second, in all the discussions above, we have ignored the particle morphology and assumed implicitly a homogeneous incorporation of the PU. This is consistent with the appearance of the films which are clear, transparent and crack-free at least at macroscopic level. This is also in good agreement with the unique T g measured by DSC. Nevertheless, the arguments above plead for the existence of heterogeneous structures and a phase separation occurring at the particle scale for 25 wt% of PU and a degree of grafting of 10 % cannot be ruled out. less crosslinked matrix polymer, seem to have little effect on the dissipation of energy during debonding but to greatly strengthen the resistance to shear of the material. This would also be consistent with a recently published study on nanostructured adhesives 51

Conclusion

The key findings of our study can be summarized as follows:

The amount of polyurethane (at fixed OH/NCO ratio) affected strongly the ratio between soluble and insoluble parts because of the addition of crosslinking points between the NCOterminated PU chains and the acrylic chains. Moreover, since the polyurethane is more entangled than the acrylic, it also affected the small strain elastic modulus with the incorporation of many entanglements, which behave like permanent crosslinks at high frequency but can relax at low frequency or high temperature.

The degree of grafting at fixed PU content (controlled by the ratio between the grafting agent HEMA and the chain extender BPA) plays an important role in the architecture of the insoluble part. The analysis of the tensile tests strongly suggest a heterogeneous particle structure with a minority of more crosslinked PU rich domains and a majority of less crosslinked and percolating PU-poor domains.

As a result of this heterogeneous structure, industrial standard tests on the adhesive properties of the PSA have shown a clear improvement in shear resistance while the adhesion energy remained acceptable for standard applications. This clearly showed that the detailed structure of the gel part of the PSA is as important as the gel fraction itself, a point which has been often neglected so far.

Clearly, the fine control of the polymer structure inside a particle with a bicomponent system remains very challenging. Several reactions compete kinetically and the final result is likely to depend on the details of the procedure and on the details of the composition such as amount of chain transfer agent used, nature of the initiator, and of course temperature of the reaction.

Furthermore the materials studied here have been produced with a batch process relatively far from the conditions used in industrial applications.

Nevertheless the general trends discovered should be broadly applicable to the design of hybrid networks containing both polycondensation reactions and free radical polymerization.

All previous publications on urethane/acrylic networks focused on high T g coatings which have much less propensity to chain transfer reactions and are more sensitive to the details of

F o r

  Properties of adhesive films obtained from PU-acrylic hybrid particlesElise Degrandi-Contraires, 1 Ravindra Udagama, 2 Elodie Bourgeat-Lami, 2 Timothy McKenna, 2 Keltoum Ouzineb,3 Costantino Creton 1 * adhesives (PSA) are soft polymeric materials displaying an instantaneous adhesion on most surfaces upon application of a light pressure. Although the function appears

  was 10000 min. After this limiting time, the test was stopped. If a cohesive failure occurred, CF was noted along with the value of the holding time at failure. It is worth noting that for a commercial PSA, cohesive failure is neither desirable in the case of the shear resistance test nor in the case of the probe test.

Figure 1 .

 1 Figure 1. Stress-strain tack curves for the 5 PU weight fractions with 0.2% CTA and HEMA/BPA=0.22; Vdeb=100 µm.s -1 (i.e. dε/dt=1 s -1 )

Figure 2 .

 2 Figure 2. Evolution of G' (a) and tanδ (b) as a function of frequency for different PU weight fractions: 5 wt% (T150), 25 wt% (T145) and 50 wt% (T153). T = 25°C

13 (

 13 tanδ/G') criteria are fulfilled with G'(1Hz) equal to 0.99 10 5 Pa and tanδ/G' equal to 0.6 10 -5

Figure 3 .

 3 Figure 3. Nominal stress versus lambda for the 5 different PU weight fractions; dε/dt=1s -1 ; the bottom figure is a magnification of the red square

Fig. 4 :

 4 Fig.4: Reduced stress versus inversed lambda for the 5 different PU weight fractions; dε/dt=1s -1 ; the bottom figure is a magnification of the region in the red square

Figure 5 .Figure 5 Figure 6 Figure 6 .

 5566 Figure 5. Evolution of the gel fraction (!) and of the swelling ratio (!) with the polyurethane content Figure 5 shows the change in gel fraction and equilibrium swelling of the gel as a function of PU content. The sharp increase in gel fraction with PU content is consistent with the viscoelastic measurements: the addition of PU reduces the uncrosslinked fraction and limits

Figure 7 .

 7 Figure 7. Stress-strain tack curves for 4 different HEMA/BPA ratios; Vdeb=100 µm.s -1 (i.e. dε/dt = 1 s -1 )

G

  ' and tanδ as a function of frequency for different HEMA/BPA ratios with 25 wt% of PU are shown in figure 8. Above 1 Hz, both change little with the ratio HEMA/BPA and are in the acceptable range for PSA applications. The resulting values of tanδ/G' are all predicting a fibrillation debonding (from 0.44.10 5 Pa.s -1 for the lower HEMA/BPA (T147) to 0.40.10 5

Figure 8 .

 8 Figure 8. Evolution of the G' (a) and of the ratio tanδ/G' (b) as a function of frequency for HEMA/BPA=0.10 (T147), for HEMA/BPA=0.22 (T145) and for HEMA/BPA=0.50 (T149); ε ~ 8 %.

Figure 10 .

 10 Figure 10. Probe tack (left) and tensile results (right) of samples with OH/NCO=0.55 and OH/NCO=1 for high grafting of PU (top; samples T149 and T161) and low one (down; samples T147 and T160); for probe-tack tests V deb = 100µm.s -1 (i.e. dε/dt = 1s -1 ); for tensile tests dε/dt=1s-1

27 Figure 11 :

 2711 Figure 11: Theoretical urethane/acrylic hybrid network

Table 1 .

 1 Description of the different series of latexes

	Lab code CTA a	PU b	HEMA (mol% to NCO)	OH/NCO HEMA/BPA b
	T150		5
	T151		15	10		0.22
	T152		35
	T153	0.2	50		0.55
	T147			5		0.11
	T145		25	10		0.22
	T148			15		0.36
	T149			20		0.50
	T160	0.3	25	10	1	0.10
	T161	0.2		20		0.25
	T170	0.2	0	/	/	/
	a Wt% relative to monomers. b Molar ratio of the OH groups from HEMA to the OH group from BPA.

  probe after the test indicate a cohesive debonding inside the fibrils. These tack curves are characteristic of samples with a low level of elasticity, due to a low level of crosslinking. On the other hand, for samples with 35 wt% or 50 wt% of PU, the tack curves

	o
	r
	R e
	v
	i e
	w .
	C
	o n
	f
	i
	d e
	n
	t
	i a
	l
	-
	A
	C S

10

remaining on the

Table 2 .

 2 Adhesion energy measured for V deb =100 µm.s -1 and industrial shear resistance results For V deb =100 µm.s -1 . b At 300mm.min -1 Table 2 presents results from probe-tack tests, gel measurements and industrial shear tests.The comparison between probe test results, which measure the energy dissipated during fast debonding, and shear resistance, which is a measure of the resistance to failure at long times illustrates the difficulty in finding an optimum. Clearly the samples with 5 and 15% of PU are still dissipative but shear resistance is not much improved relative to the standard acrylic. The sample with 50% PU has an exceptional resistance to shear but does not dissipate much energy upon debonding. In the end the best compromise in properties appears to be the sample with 25% PU which combines an improvement in tack and a 50 fold increase in shear resistance. The most interesting result here is the comparison of the best hybrid PSA (T145)

	Sample name	PU (wt%)	Gel fraction (%)	W adh (J.m -2 ) a	Shear resistance (min)
	T170	0	47.3	200.4 ± 21.2	181 ± 15 CF
	T150	5	Not measurable	161.4± 43.2 CF	46 ± 15 CF
	T151	15	25.7	379.3 ± 111.3 CF	222 ± 15 CF
	T145	25	37.4	284.9 ± 93.3	9463 ± 15 CF
	T152	35	57.9	143.62 ± 39.9	Not measured
	T153	50	73.8	49.6 ± 5.2	> 10000

a

  . However if tensile tests are carried out at two different strain rates with the same material, it is impossible to fit both curves with the same material parameters with either model. A quantitative modelling of the large strain behavior of a PSA would require a full fledged non-linear viscoelastic model including finite extensibility. Such not yet been developed. However it is possible to use a simpler and approximate methodology to characterize the PSA behaviour in large strain.

	a model has
	F
	o
	r
	R e
	v
	i e
	w .
	C
	o n
	f
	i
	d e
	n
	t
	i a
	l
	-
	A S C 15

Table 3 .

 3 Values of the Mooney-Rivlin viscoelastic parameters for different PU amounts;

	dε/dt=1s -1
	Sample Code	PU (wt%)	C soft (kPa)	C hard (kPa)	C soft /C hard	E (kPa)
	T150	5	84.5	No min in σ R	∞	1 4 1 . 4
	T151	15	115.9	No min in σ R	∞	2 3 9 . 7
	T145	25	109.4	11.8	9.3	275.6
	T152	35	147.3	44.8	3.2	329.6

  10. The maximum deformation ε max of the adhesive layer 20 decreases markedly when the ratio HEMA/BPA increases. A lower ε max means that fibrils are less stretched at detachment and this is due to a combination of more strain stiffening and probably less dissipation near the interface[START_REF] Glassmaker | [END_REF] . Conversely, the fibrillation stress σ f of the tack curves increases with HEMA/BPA.

	F
	o	1.0
	0.8 r						0.10 0.22
	R e 0.6 σ 0.4 N (MPa)				0.36 0.50
		0.2		v	increasing HEMA/BPA
		0.0	0	i e 6 4 2	8	10	12	14	16	18
				. w ε
					C
					o n
							f
							i
								d e
									n
										t
										i a
										l
										-
										A
										C S

Table 4 .

 4 Adhesion energy measured at V deb = 100 µm.s -1 and industrial shear and peel results for the blank sample and the 4 HEMA/BPA ratios; in all cases OH/NCO=0.55.

	Sample name	HEMA/BPA	W adh (J.m -2 )	Shear resistance (min)
	T170	0.0	200.4 ± 21.2	181 ± 15 CF
	T147	0.11	548.4 ± 128.2	1680 ± 15 CF

Table 5 .

 5 Values of the Mooney-Rivlin viscoelastic parameters for the 4 different HEMA/BPA ratios; dε/dt=1s-1

	σ R (MPa)	0.12 0.10 0.08 0.06 0.04	0.10 0.22 0.36 0.50				σ R (MPa)	0.06 0.05 0.02 0.04 0.03	0.10 0.22 0.36 0.50
		0.02						0.01
		0.00						0.00
		0.0	0.2	0.4	0.6	0.8	1.0	0.00		0.10	0.20	0.30
				1/λ						1/λ
	Figure 9. Reduced stress versus 1/λ for the 4 different HEMA/BPA ratios; dε/dt=1s -1 ; the
	bottom figure is a magnification of the red square.
		Sample name	HEMA/BPA C soft (kPa)	C hard (kPa)	C soft /C hard	E y (kPa)
		T147		0.11		67.9	8.9		7.7	191.5
		T145		0.22		109.4	11.8		9.3	275.6

  a positive effect. For high grafting the deformability of the sample is significantly improved. When a high degree of grafting of PU is used, the material synthesized with more BPA has a higher maximum extension before debonding while maintaining the level of plateau stress. Based on the previous conclusions, it is consistent with constant grafting and constant PU fraction. The tensile curves show a less pronounced softening for the sample with high BPA amount and a more progressive hardening at very large strains. The polymer network is more deformable with equivalent small-strain stiffness. Our hypothesis is that the presence of BPA may act there as a chain transfer and reduce the kinetic chain length of the acrylic, effectively making the hybrid network a bit more viscoelastic and extensible. This confirms the hypothesis of longer acrylic chains formed with the presence of a higher amount of BPA and thus higher deformability. The gel and swelling measurements partially confirm this interpretation. The gel content decreases from 54.5 % for OH/NCO=0.55 (less BPA) to 44.3 % for OH/NCO=1 (more BPA). On the other hand, the swelling increases from 57.0 (less BPA) to 74.6 (more BPA). Considering the extraction technique used to measure the gel content, values may appear closed but their trend is consistent with the other observations.

	of BPA has
	R e
	v
	i e
	w .
	C
	o n
	f
	i
	d e
	n
	t
	i a
	l
	-
	A S C 25

Network structure of the urethane/acrylic hybrids

  25 and 0.2% CTA. BPA-extended chains which have reacted on only one side with HEMA are dangling chains. They are terminated either by a BPA or by an amine (obtained after reaction with water). These chains are connected to the acrylic backbone at only one side.

	4. General discussion: Several molecular analysis have demonstrated three ideas 1 . First, all NCO groups have
	reacted with HEMA, BPA or water. Then, SEC analysis and inverse titration have shown that
	the molar mass of the PU prepolymer is ~ 3000 g.mol -1 , and finally it is known that only a F maximum of 20mol% of NCO groups can react with HEMA. Therefore, PU chains can be
	incorporated in three different ways: o r o BPA-extended chains which have reacted on both side with HEMA form
	bonded PU chains. After acrylic polymerization, these chains will be part of the
	e R gel fraction.
	v
	i e
	w .
	C
	o n
	f
	i
	d e
	n
	t
	i a
	l
	-
	26	A S C

o

  The possible existence of these nanodomains which are more crosslinked but are embedded in a

	acrylic backbone
		HEMA
		BPA
	bonding PU chain
	F	dangling PU chain
	o	free PU chain
	r
	R e	crosslinking point
	v
	i e
	w .
		C
		o n
		f
		i
		d e
		n
		t
		i a
		l
		-
	28	A S C

31 of 31 ACS Paragon Plus Environment Submitted to Macromolecules

  29the interfaces between particles19,20,23 . To our knowledge this is the first study on the design of such a network in soft adhesives.

	F F
	o o
	r r
	R e R e R e
	v v v
	i e i e i e
	w . w . w .
	C C C
	o n o n o n
	f f f
	i i i
	d e d e d e
	n n n
	t t t
	i a i a i a
	l l l
	---
	A A A
	C S C S C S

31

Page

Acknowledgments

The EU Framework 6 Integrated Project NAPOLEON NMP3-CT-2005-01184 is acknowledged for the financial support provided to this project.


G Billuart

E Bourgeat-Lami

M Lansalot

V Monteil

Free radical emulsion polymerization of ethylene

Keywords: Ethylene, free radical, emulsion polymerization, nanoparticles, supercooling, film formation

Free radical polymerization of ethylene in aqueous media was performed under mild conditions (T < 90 °C and P ethylene < 250 bar) using an anionic initiating (ammonium persulfate, APS) and stabilizing (sodium dodecyl sulfate, SDS) system, yielding polyethylene latexes. pH regulation was a key to the feasibility of this process. Particle sizes and morphologies as well as polymerization yields were affected by SDS concentration. In any case, yields increased with polymerization time, especially in presence of surfactant. When increasing the polymerization pressure, the increase on yields was much stronger in presence of surfactant and particle sizes increased more when no surfactant was used. Stable polyethylene latexes with relatively high Page 1 of 36

ACS Paragon Plus Environment

Submitted to Macromolecules 2 solids content (30 %) were synthesized. This article also reports a supercooling effect on the crystallization of the polyethylene particles and evidences promising water-repellency properties of paper coated with the PE latexes synthesized in our laboratory.

Introduction

Polyethylene (PE), the top manufactured polymer, can be synthesized by two different polymerization routes. The free radical way, discovered in the 1930s by Imperial Chemical Industries (ICI), produces Low-Density PolyEthylene (LDPE). This synthesis requires very harsh polymerization conditions: high temperatures (T ≈ 300 °C) and high pressures (P Ethylene > 2000 bar). [START_REF] Soares | Coordination Polymerization In Handbook of Polymer Reaction Engineering[END_REF][START_REF] Kalyon | [END_REF][3] The obtained polymer is highly branched, thus leading to a low density and low crystallinity material. The other route, a catalytic process discovered by Ziegler and Phillips Petroleum in the 1950s, yields High-Density PolyEthylene (HDPE): a very linear PE with very high crystallinity. By copolymerizing ethylene with α-olefins such as 1-butene or 1-hexene, a slightly branched PE is obtained, named LLDPE for Linear Low-Density PolyEthylene. [START_REF] Soares | Coordination Polymerization In Handbook of Polymer Reaction Engineering[END_REF]4 LLDPE exhibits an intermediate crystallinity between LDPE and HDPE.

Despite its hydrophobicity, PE is used by certain industries (e.g. inks, concrete, wood…) under the form of dispersions in water. To obtain such products, the aforementioned LDPE is poured into hot water, in presence of surfactants, and dispersed under the effect of a strong mechanical stirring, leading to 10 µm wax particles. [START_REF] Engels | Process for the production of storage stable wax dispersions[END_REF][START_REF] Gravatt | Novel wax dispersion formulations, method of producing same, and uses[END_REF] The dispersion is then submitted to a high pressure homogenization process to reach particle sizes around 2 µm. As one can imagine, this process involving several steps to obtain a PE dispersion is long, energy consuming and thus expensive.

In our laboratory, it has been shown lately that the production of PE via a free radical pathway can be efficient under much milder temperature and pressure conditions than the ones described Page 2 of 36 ACS Paragon Plus Environment Submitted to Macromolecules 3 above. Slurry polymerization was, at first, carried out in organic solvents using 2,2'azobisisobutyronitrile (AIBN) as organosoluble initiator. This polymerization process benefited from an important activation effect from the solvent. Indeed, polar organic solvents such as THF or dialkyl-carbonates greatly improved the polymerization yields. 7 However, despite interesting yields, chain transfer reactions of the growing radical to the solvent produced low molar mass PEs, usually lower than 7000 g mol -1 . 8 To lower the extent of transfer reactions, ethylene polymerization was then studied in water, a polar, non-transferring solvent. 9 A cationic, hydrosoluble equivalent of AIBN, 2,2'-azobis(2-methylpropionamide) dihydrochloride (AIBA), was used to initiate the reaction. Stable latexes of low solids content (1 to 4 %), containing spherical PE nanoparticles (average hydrodynamic particle diameters: 60-100 nm), were obtained yielding dispersions of PE nanoparticles in a one-step synthesis. The PE particles were assumed to be stabilized by positively charged initiator fragments located at chain ends. In presence of a cationic surfactant, cetyl trimethyl ammonium bromide (CTAB), the obtained latexes were still stable and produced with much higher yields (up to 30 % of solids content) and high molar masses (typically larger than 50 000 g mol -1 ). In this case, the PE nanoparticles were smaller (20-50 nm) and presented disc-like shapes, as evidenced by TEM analysis. These unusual particle morphologies were attributed to the crystallinity of the polymer which seems to prevent the formation of spherical objects in the presence of surfactant. Aqueous dispersions of semi-crystalline PE nanoparticles, obtained from aqueous catalytic processes, exhibiting such particle morphologies have already been reported. [10][11][12][13][14][15] It is also important to keep in mind that free radical polymerization of ethylene in water was reported as early as in the 1940s but in these pioneering works no precise information on colloidal properties was given. [16][17][18][19][20] On top of making the use of emulsion polymerization techniques for ethylene possible, the polymerization process in aqueous dispersed media also tackles the question of the locus of polymerization. Indeed, during a slurry free radical polymerization of ethylene (FRPE) in an organic solvent under medium pressure conditions, the polymerization medium can be monophasic (solvent and ethylene form a single phase) or biphasic, depending on both the experimental conditions and the solvent's supercritical properties. 21 Emulsion polymerization benefits from the thermodynamic properties of water, i.e. a very high critical point (347 °C, 221 bar) compared to organic solvents, preventing the formation of a supercritical monophasic medium: a liquid aqueous phase coexists with the supercritical ethylene phase. Consequently, as this polymerization process uses a hydrosoluble initiator, the polymerization will be initiated in the aqueous phase.

In line with our previous works, 9 we wanted to explore the synthesis of negatively charged PE latexes through emulsion polymerization. Indeed, controlling the surface charge of the particles is important to expand the versatility of this process. Herein, we will therefore study the feasibility of the free radical emulsion polymerization of ethylene using a very conventional, anionic initiating and stabilizing system, commonly used in emulsion polymerization. The thermal properties of the resulting latexes will then be discussed. Finally, the water-repellency properties of paper coated with PE latexes will be evaluated.

Experimental.

Materials:

Ethylene (99.95 %, Air Liquide), 2,2'-azobis(2-methylpropionamide) (AIBA, 98% Acros Organics), 4,4'-azobis(4-cyanopentanoic acid) (ACPA, >98% Fluka), ammonium persulfate (APS, 98% Acros Organics), cetyl trimethyl ammonium bromide (CTAB, 99% Sigma-Aldrich), sodium dodecyl sulfate (SDS, 99% Acros Organics), sodium bicarbonate (99,7%, Sigma-Aldrich), potassium carbonate (99%, Sigma-Aldrich), sodium hydroxide (>98%, Sigma-Aldrich), LDPE (Acros Organics) were used as received. Water was purified by an ELGA Labwater Classic: impurities were removed by ultrafiltration and UV irradiation.

Characterizations:

Latex colloidal properties:

Hydrodynamic particle diameters (Z av, nm) were measured by a Malvern Zetasizer Nano ZS. A 633 nm wavelength laser beam was sent to an infinitely diluted sample and the scattered signal intensity analyzed at a 173° angle, at 25 °C. Dynamic Light Scattering (DLS) allowed access to the particle sizes and the broadness of the size distribution (indicated by the poly value -the higher this value, the broader the size distribution) by computation using the cumulant analysis method. From DLS data, particle numbers were calculated using equation (1). N p = 6τ / (πZ av 10 -21 dp) (1) With N p , the number of particles (mL -1 ), τ the polymer concentration (g mL -1 ), Z av the average particle size (nm) and d p the polymer density (g cm -3 ). Here, d p was set to 0.86 g cm -3 , the density of a very low density polyethylene (VLDPE). [START_REF] Peacock | Handbook of Polyethylene: Structures: Properties, and Applications[END_REF] of the Université Claude Bernard Lyon 1, Villeurbanne, France) on a Philips CM120 transmission electron microscope. The sample was diluted until reaching a 1% solids content. A drop of the solution was placed on a Quantifoil R2/1 copper grid with 100 holey Carbon support film. The grid was then plunged into liquid ethane, leading to the formation of amorphous ice that will imprison the particles, preserving their morphology, as opposed to regular TEM analysis.

Initial and final pH values (pH i and pH f , respectively) of the aqueous phase were probed by a pH-meter (Seven Easy, Mettler Toledo) using a combined glass electrode (Mettler Toledo).

To remove the free surfactant from the latexes, dialyses were performed. A fraction of the latex was introduced in a dialysis tubing (molar mass cutoff 6000 -8000 g mol -1 ), itself immerged in ultrapure water. The water was renewed every day for one week.

Polymer characterizations:

Polymer molar masses were measured by Size Exclusion Chromatography (SEC) using a Viscotek Malvern HT-GPC Module 350A equipped with three mixed bed columns (300×7.8 mm from Malvern Instrument) and a guard column (75×7.5 mm). Dry polymer samples were dissolved at 150 °C in 1,2,4-trichlorobenzene (stabilized with 2,6-di(tert-butyl)-4-methylphenol), at a 3 mg mL -1 concentration. The determination of the molar masses was based on a triple detector set: viscometer, refractometer, light scattering at a low angle (LALS) and a right angle (RALS) under a 1 mL min -1 flow. This triple detection allows an absolute measurement of the polymer molar masses (number-average molar mass, M n and weight-average molar mass, M w ) and molar-mass dispersity (Ð =M w /M n ), without need for calibration standards.

Branching degrees of the polymers were measured by 13 C NMR on a Bruker Avance II 400 Ascend in a tetrachloroethylene (TCE) / deuterated benzene mixture (2/1 v/v) in 10 mm tubes at 90 °C with a 10 mm SEX- 13 C/ 1 H probe at the NMR Polymer Center of Institut de Chimie de Lyon (FR5223) (Villeurbanne, France). Branching density calculations were performed according to the work of Galland et al. [START_REF] Galland | [END_REF] Differential Scanning Calorimetry (DSC) measurements were performed on a Mettler-Toledo DSC-1. The dried samples (bulk PE) were submitted to two successive heating (10 to 160 °C at 5 °C min -1 ) and cooling (160 to 10 °C at -20 °C min -1 ) cycles in a standard 40 µL aluminum crucible, with an empty reference crucible. The analyzed data (melting temperature, T m and crystallinity, X C , %) are extracted from the second heating. Crystallinity was calculated with the following equation where ∆H f,∞ is 293 J g -1 .

X c = (∆H f,measured / ∆H f,∞ ) x 100 DSC was also performed directly on the PE particles dispersion. To this purpose the latexes were submitted to an analog cycle as for bulk PE, this time in a medium pressure 120 µL stainless steel crucible sealed with a Viton O-ring, with temperatures ranging from 0 to 130 °C and heating or cooling rates of 5 °C min -1 or -5 °C min -1 . For each sample, the reference crucible was filled with an aqueous solution of surfactant, whose concentration was close to the one used to synthesize the sample. Crystallization temperature (T c ) was extracted from the cooling segment, whereas T m was obtained from the second heating. A DLS analysis was performed on the latexes in order to check their colloidal stability after the DSC cycles.

Permeability measurements:

Cobb test measurements were performed by submerging a known surface of coated paper in 1 cm of water (coated face up) during 1 minute. Surplus water was then removed by pressing a sheet of blotting paper on the wetted surface. By weighing the sample before and after wetting, the amount of absorbed water could be calculated in g m -2 .

Moisture Vapor Transmission Rate (MVTR) was determined by sealing a cup containing a dessicant salt (anhydrous CaCl) with a piece of coated paper (coated face up) in a controlled atmosphere storage room. Two types of conditions were used: standard conditions (T = 25 °C, humidity = 50 %) and tropical conditions (T = 38 °C, humidity = 90 %). The MVTR (in g m -2 day -1 ) was determined by weighing the amount of water absorbed in 24 hours by the dessicant salt.

Standard ethylene polymerization:

Ethylene polymerizations were carried out in a 160 mL stainless steel autoclave purchased from Parr Instrument Co. Previous to the polymerization, the reactor had been set under an inert argon atmosphere and preheated to the desired temperature. In parallel, a 50 mL initiator aqueous solution with or without surfactant and pH regulator was degassed by argon bubbling for 15 minutes. This solution was loaded into the reactor through a cannula, under argon flow. Ethylene was introduced immediately after in the reactor, which marked the beginning of the polymerization. Ethylene pressure was maintained during the polymerization by small ethylene additions, making this type of polymerization a semi-batch process. After the desired polymerization duration, the reactor's jacket was cooled down by water. The reactor was then slowly degassed until reaching atmospheric pressure to collect the latex. A fraction of the Page 8 of 36

ACS Paragon Plus Environment

Submitted to Macromolecules obtained latex was dried for solids content measurement, which after subtraction of nonpolymeric species gave the polymer content (PC, %). When the latex was stable, the PC was used to calculate the yield of the polymerization. The dried polymer was then used for polymer characterization (DSC, NMR, SEC).

Results and discussion

Development of a negatively charged initiating and stabilizing system

As interesting as they are, positively charged latexes currently find fewer applications. Free radical emulsion polymerization of ethylene with negatively charged initiator and surfactant was thus investigated. A first series of experiments was carried out using 4,4'-azobis(4cyanopentanoic acid) (ACPA, 1 g L -1 ) as an anionic equivalent of AIBA. Since ACPA has two carboxylic acid functions, it needs a slightly basic solution to dissolve. Thus, the polymerizations took place in presence of NaHCO 3 (1 g L -1 ), with or without surfactant (Sodium Dodecyl Sulfate, SDS). Small amounts of PE polymer in a stable latex were obtained (PC ≈ 0.5 %). Once the latex had been dried, a waxy polymer of very low molar mass was obtained. To increase the yields, different experimental conditions (ethylene pressure and surfactant concentration, pH) were investigated. However, despite producing stable latexes, the amounts of polymer obtained remained low (PC ≤ 0.5 %). The feasibility of free radical emulsion polymerization of ethylene was then studied using a more common initiator, ammonium persulfate (APS).

a t = 4 h, T = 80 °C, [APS] = 0.84 g L -1 , [SDS] = 3 g L -1
, 50 mL H 2 O, P ethylene = 100 bar, stirring rate = 250 rpm. b pH regulator: [NaHCO 3 ] = 1 g L -1 . c pH regulator: K 2 CO 3 (1.6 g L -1 ). d pH regulator: NaOH (1 g L -1 ).

Polymerizations of ethylene in presence of APS (0.84 g L -1 ) with surfactant (SDS, 3 g L -1 ) yielded a stable latex with 95 nm PE particles containing a negligible polymer amount (0.59 g, run 1 in Table 1). A pH drop from 5.4 (pH i in Table 1) to 2.8 (pH f in Table 1) before and after polymerization could be measured. When regulating the pH with salts such as NaHCO 3 , K 2 CO 3 or NaOH (runs 2 to 4 in Table 1) stable latexes exhibiting substantial amounts of polymer (PC ≥ 4 %) were obtained. No significant pH variation was observed before and after polymerization in these three cases.

These results concur with the well-known sensitivity of persulfate initiators to pH. [24][25][26] The need for a pH regulator for the emulsion polymerization of monomers such as ethylene-vinyl acetate mixtures 27 or butadiene 28 using persulfate initiators has been indeed reported in the literature. By comparison, the pH regulator is not necessary for emulsion polymerization of styrene. 24 The emulsion polymerization of monomers such as butadiene or ethylene with persulfate initiators would thus require a steady, basic pH value to obtain significant amounts of polymer.

This trend has been verified in our case for the emulsion polymerization of ethylene. Using different pH regulators, initial pH values between 8.2 and 12.4 were investigated (runs 2 to 4 in Table 1). Polymerization yields increased with increasing pH. Higher decomposition rate of APS in alkaline conditions 25,29 could partially explain these findings. However it is known that the polymerization rate is not strongly influenced by the initiator decomposition rate in conventional emulsion polymerization. It thus seems clear that the nature of the monomer plays a determinant role in our case as styrene does not lead to similar trends.

Particle sizes remained around 25 nm and high poly values were obtained by DLS (poly > 0.1), indicating non-spherical, disc-like particle morphologies. 9 Indeed, it has been evidenced in previous works that in the case of PE latexes a high poly value (≥ 0.10) did not imply a poor stabilization or an heterogeneous particle size distribution but non-spherical particle shapes.

Thermal properties of polethylenes remained the same, independently from the pH value (T m ≈ 90 °C, X c ≈ 20 %), indicating that the polymer microstructure was not affected by this parameter.

For the next experiments, NaHCO 3 (1 g L -1 ) was chosen as the pH regulator.

Influence of the surfactant concentration

Having established the determinant role of pH on the synthesis of PE particles using APS as initiator, we next investigated the effect of SDS concentration for a fixed polymerization pH of 8.2. Polymerizations of ethylene were performed in aqueous solutions with SDS concentrations ranging from 0 to 5 g L -1 in presence of NaHCO 3 . Experimental conditions and results are reported in Table 2 (runs 2 and 5 to 9 in Table 2) and Figure 1. Without surfactant (run 5, Table 2), a stable latex was obtained, but with a low polymer content. The addition of SDS provided much more significant yields with an optimum reached for 5 g L -1 of SDS (run 9, Table 2 and Page 11 of 36

ACS Paragon Plus Environment

Submitted to Macromolecules

Figure 1). Particle sizes decreased with surfactant concentration, until 5 g L -1 . Above this concentration, the latexes were not stable anymore. 2).

Above 2 g L -1 of SDS, the poly value measured by DLS was high (≥ 0.10), indicating that the PE particles probably exhibited non-spherical morphologies. 9 The non-spherical particle shape was indeed confirmed by cryo-TEM for run 2 (Figure 2). Even if it is difficult to obtain accurate particle dimensions from cryo-TEM, the particle size shows a good accordance with DLS, and confirms that the high poly value is likely due to the non-spherical shape of the particles rather to a high particle size heterogeneity.

Figure 2.

Cryo-TEM picture of a PE latex synthesized with 3 g L -1 SDS (run 2, Table 1).

Despite the fact that average diameters measured by DLS assume a spherical shape for particles (and thus correspond to the equivalent spherical diameters of disk-like particles), the particle number N p was estimated according to equation (1). Figure 3 shows the dependence of N p on SDS concentration. It points out that the higher the surfactant concentration, the greater the number of particles. This figure illustrates the very strong particle nucleation taking place during emulsion polymerization of ethylene. 2).

Figure 3 also indicates that the molar mass first increased with increasing surfactant concentration from 0 to 2 g L -1 (runs 5 to 7, Table 2) and then significantly decreased. This is once again in contradiction with standard emulsion polymerization theory which usually predicts that the molar mass increases when the number of latex particles increases. The likeliest scenario is that a transfer reaction to the surfactant occurs, as already observed in early works of Mantell on free radical polymerization of ethylene in water. 16 Indeed, the similarity between the alkane moiety of SDS and a polyethylene chain makes transfer reactions of growing PE macroradical to SDS possible, as it could be transferred to another polyethylene chain by in an intermolecular reaction, usually responsible of branches formations. There would thus be a competition between compartmentalization of radicals, responsible for higher molar masses, and transfer to the surfactant, which would shorten the chains. This competition favors at first the compartmentalization and higher molar masses, but as the surfactant concentration increases, molar masses end up decreasing. NMR studies failed to evidence this transfer to surfactant as it is not possible to find a common NMR solvent for both PE and sulfonate moieties.

Kinetic studies of APS initiated FRPE, without or with SDS (3 g L -1 )

To better understand the particle nucleation in absence or presence of SDS, kinetic studies were undertaken for both systems. From a practical viewpoint, this implied to carry out a new experiment for each investigated time as no withdrawal of sample was technically possible during the reaction. FRPE was thus carried out under 100 bar of ethylene, without SDS (runs 5 and 10 to 14, Table 2) and with 3 g L -1 of SDS (runs 2 and 15 to 19, Table 2). The polymerization durations spread from 0.5 to 6 h. As observed in our earlier work on the synthesis of cationic PE latexes, yields increased with time in both cases (Figure 4). 9 Whatever the polymerization time, higher yields were reached when SDS was present. Particle sizes increase was very strong without surfactant (Figure 5). The number of particles is almost constant (Figure 5), implying that particles are created at the beginning of the polymerization and grow bigger during the polymerization. In contrast, in presence of surfactant, particle sizes are constant with a rather narrow particle size distribution (vide supra) when yields increase (Figure 5, Table 2). Here, particles are created all along the polymerization. This seems to indicate that particles, continuously nucleated during polymerization, failed to grow bigger. These trends are very similar to those previously observed during the synthesis of cationic PE latexes with or without surfactant. 9 Analogous results have been also evidenced in latexes of semi-crystalline polymers 2).

Molar masses increased with time in both cases (Figure 4). However, the obtained molar masses were much higher in the case of FRPE in presence of surfactant (up to M n ≈ 7.5 10 4 g mol - 1 ), as already mentioned. Surprisingly, especially in the presence of surfactant, the molar masses increased with time, and thus with the polymer yield. We currently have no rational explanation for this phenomenon which deserves deeper investigation. particles by FRPE in aqueous dispersed media using APS as initiator and SDS as surfactant (runs 2, 5 and 10 to 19, Table 2).

Towards higher solids contents

Influence of ethylene pressure

In the next series of experiments, the effect of ethylene pressure has been investigated for a constant stirring rate of 250 rpm. Without (runs 5 and 20 to 22, Table 3) or with surfactant (runs 2 and 23 to 25, Table 3), yields, particle sizes and molar masses reached higher values when ethylene pressure was increased (Figure 6). In all cases, a stable latex was obtained. It can be noticed that in the absence of surfactant, particle sizes were multiplied by a factor of 3 (63 to 173 nm) with increasing pressure from 50 to 200 bar, while yield increase was moderate (0.08 to 2.41 g). In contrast, in presence of 3 g L -1 SDS, yields increase was drastic (0.21 to 6.46 g) while particle sizes were 3 times larger as well (16 to 42 nm). Similar results were observed in our Page 18 of 36

ACS Paragon Plus Environment

Submitted to Macromolecules previous work on the synthesis of cationic PE latexes. 9 As expected, very high molar masses have been reached with surfactant (M n = 2.5 10 6 g mol -1 under 200 bar), but these were much lower without surfactant: M n = 8.5 10 5 g mol -1 under 200 bar. 3).

13

C NMR analysis was used to determine the branching degree of samples obtained for ethylene pressures of 50, 100, 150 and 200 bar, respectively (runs 23, 2, 24 and 25 in Table 3). [START_REF] Galland | [END_REF] The branching density decreases with increasing ethylene pressure, in agreement with DSC measurements (the crystallinity increases, Table 3). Indeed, the formation of polymer crystals is hindered by branches. This decrease in branching degree is probably due to higher monomer concentration when its pressure increases, thus favoring a linear growth of the chains (i.e.

propagation) rather than intermolecular transfer reactions. When compared to a LDPE produced by an industrial process provided by Alfa-Aesar, the PE produced using emulsion polymerization under mild conditions exhibits a much lower crystallinity and a much higher branching density.

Indeed, the industrial sample crystallinity is around 40 %, which is much higher than the 10 to 20 % of the PEs obtained by FRPE in water. The branching density range of the latter is of 24 to 50 branches / 1000 C when the industrial sample possesses only 13 branches / 1000 C. Actually, with a more branched structure than an industrial LDPE, the PE produced by free radical emulsion polymerization relates more to very low density polyethylene (VLDPE). [START_REF] Peacock | Handbook of Polyethylene: Structures: Properties, and Applications[END_REF] This is confirmed by the melting temperatures and crystallinities measured on our samples.

Effect of the stirring rate

In a heterogeneous polymerization process, the stirring rate is a decisive parameter which will influence the colloidal properties of the obtained latex. [START_REF] Odian | Principles of Polymerization[END_REF] Accordingly, the effects of stirring rate on FRPE in aqueous media using an anionic initiating and stabilizing system were investigated at an ethylene pressure of 100 bar (runs 2 and 26 to 28, Table 3). Yields were multiplied by a factor 3 when increasing the stirring rate from 250 to 950 rpm, without latex destabilization. Particle sizes remained unchanged, around 25 nm, with high poly values indicating non-spherical morphologies, whatever the stirring rate. The increase in yields can be explained by a better renewing of the ethylene-water interface.

The fact that particle sizes were not affected by stirring rate could originate from the crystallinity of PE inside latex particles. One can imagine that crystallinity protects the particles from coalescence phenomena, in a way similar to what can be observed for the synthesis of polystyrene particles in presence of a relatively high amount of cross-linker, which would favor the nucleation of new particles. 32 Consequently, as yields increased and particle sizes remained constant, the number of particles increased with stirring rate, as shown in Figure 7. This effect was very important between 250 and 500 rpm and then became less pronounced. By comparison Page 21 of 36

ACS Paragon Plus Environment

Submitted to Macromolecules 22 in a conventional emulsion polymerization process where the polymer would be totally amorphous, a higher shear rate would increase coalescence phenomena and reduce the number of particles, as it is the case for butadiene emulsion polymerization. 33 Nevertheless, the influence of crystallinity on particle formation should be considered with care because crystallinity values of bulk PE are very low (< 30%), and latex particles are mostly composed of amorphous PE.

Investigations on thermal properties of PE latex itself will confirm that these hypotheses are subject to caution, as shown below. 3).

Page 22 of 36 ACS Paragon Plus Environment

Submitted to Macromolecules

In order to further assess the influence of stirring in our process, we next investigated the effect of stirring rate on the melting temperature and crystallinity of PE isolated from the latex. Figure 8 shows an increase of both series of data. According to the aforementioned study on the effects of pressure in FRPE in aqueous media, this change in thermal properties is the reflection of a change in the structure of the PE chains. Indeed, a higher crystallinity has been identified as the result of a less branched polymer. As explained with the effect of ethylene pressure on the structure of the chains, a better monomer provision by higher stirring may help create more linear chains (intermolecular reactions are disfavored) and thus a more crystalline polymer, although still less crystalline than an industrial LDPE. 3).

Increase of the solids content

To make this FRPE in aqueous media system interesting for further applications, we next investigated the possibility to increase the solids content. To do so, the range of ethylene pressures has been narrowed to 100-250 bar and the stirring rate increased to 750 rpm instead of the usual 250 rpm to favor a fast renewing of the ethylene/water interface and thus a better monomer supply to the particles (runs 27 and 30 to 31, Table 3). Yields were indeed increased, causing the latexes to be destabilized by the high amounts of PE produced under 200 and 250 bar of ethylene, where 24 and 32 g of polymer were produced, respectively, in 50 mL of aqueous dispersion (runs 30 and 31 in Table 3). In these two cases, the yield was obtained by drying the whole content of the reactor. Nevertheless, under 150 bar of ethylene, a stable latex was obtained with a 30% solids content and an average particle diameter of 63 nm (run 29, Table 3). These high polymerization activities often go along with high molar masses as it was also the case here (M n = 2.1 10 6 g mol -1 ). Higher molar masses seem to be obtained for the higher ethylene pressures (200 and 250 bar), as the corresponding polyethylenes flowed directly through the columns without retention. Our SEC apparatus was consequently not able to give us valid molar mass values.

Thermal properties of PE latexes

While polyethylenes isolated from the latex can be analyzed by DSC giving access to the thermal properties of the bulk material, the measurement of the thermal properties directly on the Page 24 of 36

ACS Paragon Plus Environment

Submitted to Macromolecules 25 latex is much more instructive. Indeed DSC analysis shows that the crystallization of PE occurs at a much lower temperature when polymer is located into the confined environment of a latex nanoparticle than under its bulk form. 34,35 On the other hand, no significant differences between melting temperature of bulk PE and confined PE is observed. These phenomena have also been evidenced on PE dispersions made by catalytic processes [10][11][12][13][14] and more generally on semicrystalline polymers under situations of confined environment (nanoparticles, ultrathin films, block copolymers mesophases…). 36 This phenomenon, usually referred as supercooling, evidences a change of nucleation mechanism from heterogeneous nucleation in the bulk to surface or homogeneous nucleation in the confined situation. The intensity of this phenomenon can be quantified by ∆T = T c Bulk -T c Latex (with T c Bulk the crystallization temperature in the bulk polymer and T c Latex the crystallization temperature of the particles in the latex).

This supercooling effect has been investigated for a range of latexes synthesized using different SDS concentrations (runs 2 and 5 to 9 in Table 2) and the expected trends were effectively observed (exemplified for run 7 in Figure 9). Table 2) compared to a bulk PE obtained by drying the same latex.

In matter of supercooling, the critical parameter to consider is the size of the confined domains, that is in our case the size of latex nanoparticles. Figure 10 shows that the depression of crystallization temperature of PE in latex, that is the intensity of supercooling, increases with decreasing nanoparticle size. Surfactant-induced supercooling can be ruled out by the fact that supercooling in a latex synthesized without surfactant but under a higher ethylene pressure, thus with bigger particles, gives a different supercooling value (data not shown).

The stability of the latex during DSC analysis is also very appealing. Indeed the latexes were submitted to temperatures up to 130 °C two times and were still stable afterwards as evidenced by DLS analyses of the resulting latex, which indicated no change of the particle size after thermal treatments. This indicates that PE can melt and crystallize inside nanoparticles of the latex without affecting the colloidal stability. Use of these PE latexes for phase change materials (PCM) applications such as heat transfer fluids (HTF) could be then envisaged. 2).

Another important conclusion can be drawn from the study of thermal properties of PE latex. Indeed, low T c values for PE inside the latex particles indicate that during polymerization at 80 °C, PE could be amorphous and would then crystallize during the cooling process of the latex at the end of the polymerization. This could strongly impact some of our interpretations on the influence of crystallinity on particle formation. Indeed, the process of PE crystallization under the polymerization conditions (i.e. under high pressures of ethylene) is difficult to assess. More investigations are necessary to precisely evaluate if, during the polymerization process, PE will crystallize as it is formed or will remain amorphous.

Water-repellency properties of films obtained from PE latex

Page 27 of 36

ACS Paragon Plus Environment

Submitted to Macromolecules

Coatings applications are usually targeted for polymer latexes. In the case of polyethylene the apolar character of the polymer could lead to attractive barrier properties to water. Commercial aqueous wax dispersions, obtained by redispersion of bulk PE in water as described in the introduction, are indeed used for coating applications in particular for paper industry.

We evaluated our PE latexes for coatings application on paper substrate and compared them with commercial wax dispersions. To this purpose, PE latexes produced by free radical emulsion polymerization in our laboratory were coated on paper (vegetable parchment, 75 g m -2 ), to evaluate their effectiveness as protective coating against water. Aiming for a good interaction with the negatively charged cellulose, cationic latexes were used. They were synthesized using cationic initiator (AIBA, 1 g L -1 ) and surfactant (CTAB, 1 g L -1 ) under 250 bar of ethylene at 70 °C for 4 hours with a stirring rate of 250 rpm according to our previous work. 9 The solids content of the produced latex was increased to 30% by evaporating water for the coating needs, without destabilization or major average particle size alteration (Z av ≈ 50 nm). The coating was performed at 120 °C instead of the usual 160 °C used in industry. Two factors can account for this decrease of the coating temperature. Firstly, the lower melting temperatures of the PE contained in the latexes: the PE isolated from the latexes produced in our laboratory melts around 90 °C instead of 98-115 °C for a usual LDPE. 17 Secondly, the PE nanoparticles obtained from emulsion polymerization are much smaller than the particles present in a wax dispersion, which are a few microns large. [START_REF] Gravatt | Novel wax dispersion formulations, method of producing same, and uses[END_REF] Indeed, smaller particles have been reported to decrease the minimum filmforming temperature (MFFT). 38 The obtained film was homogeneously shiny with no visible cracks. Furthermore, water-repellent properties of the coated paper were investigated and compared to papers coated with a commercial wax dispersion. As observed in Figure 11, the commercial wax dispersion yields films with a good resistance to water vapors under tropical 11a) and to liquid water (Figure 11c), but the PE latex from our laboratory gave better results against water vapors under standard conditions (Figure 11b). These results are quite promising, given that the commercial wax dispersion is formulated and specially made for this type of application.

Other substrates can be envisaged (fabrics, concrete, wood…) and depending on their surface charges, anionic or cationic PE latexes will be preferred, thus giving a very wide application range.

Conclusions

Free radical emulsion polymerization of ethylene has been successfully performed using an anionic stabilizing (SDS) and initiating system (APS). The use of a pH regulator was decisive to obtain significant yields. In absence of surfactant, stable PE latexes of low polymer contents, consisting of small sphere-like PE nanoparticles (Z av ≈ 100 m), have been synthesized. With the addition of SDS, polymer contents are higher and the PE particles much smaller (Z av ≈ 20 nm), presenting non-spherical morphologies. High molar masses were attained (M n = 2.3 10 5 g mol -1 ), despite possible transfer reactions to the surfactant. Different nucleation mechanisms have been proposed, depending on the presence of surfactant or not, but some interpretations, in particular the moment of crystallization of PE in the particles (during polymerization or during the cooling down of the latexes inside polymerization reactor) are still subject to debate.

To make FRPE in aqueous media viable for applications, the synthesis of high solids content latexes was investigated. To do so, ethylene pressure and stirring rate were varied. An increase in ethylene pressure led to higher solids contents, larger particles and higher molar masses, up to M n = 2.5 10 6 g mol -1 . When investigating the effects of stirring rate in presence of surfactant, an

Page 30 of 36

ACS Paragon Plus Environment

Submitted to Macromolecules important process parameter for emulsion polymerization, no influence on particle sizes was evidenced but the polymer content drastically increased resulting in an increase of the particle number. By combining higher pressures and stirring rates, stable latexes with polymer contents up to 30 % could be successfully obtained. Both parameters appeared to improve the monomer feed to the growing chains, leading to slightly less branched and thus more crystalline PEs.

A supercooling phenomenon, whose governing parameter seems to be particle size, has been evidenced when studying the thermal properties of the latexes. Latexes stability was not affected after being submitted to extreme temperatures during DSC analysis (0 to 130 °C). To estimate the water repellency properties of the PE films, our cationic latexes were coated on vegetable parchment. The low melting point of the PE and the small particle sizes allowed a coating temperature of 120 °C instead of the usual 160 °C used for commercial wax dispersions.

Permeability to water was measured (Cobb test and MVTR) on the coated paper and showed promising results without even formulating the latex, especially against water vapor.

aFigure 1 .

 1 Figure1. Influence of SDS concentration on polymer yields (□) and particle sizes (Z av , ■) during the synthesis of PE particles via FRPE in aqueous dispersed media using APS as initiator and NaHCO 3 as pH regulator (runs 2 and 5 to 9, Table2).

Page 14 of 36 ACS 1 )Figure 3 .

 3613 Figure 3. Influence of SDS concentration on the polymer molar mass (M n , ■) and on the number of particles (N p , •) during the synthesis of PE particles by FRPE in aqueous dispersed media (runs 2 and 5 to 9, Table2).

Page 16 of 36 ACS 1 )Figure 4 .

 3614 Figure 4. Polymer yields (• without surfactant and ■ with surfactant, 3 g L -1 ) and molar masses (M n , ○ without surfactant and □ with surfactant) vs time curves during the synthesis of PE particles by FRPE in aqueous dispersed media using APS as initiator and SDS as surfactant (runs 2, 5 and 10 to 19, Table2).

Page 17 of 36 ACS 1 )Figure 5 .

 3615 Figure 5. Particle sizes (Z av , • without surfactant and ■ with surfactant) and particle numbers (N p , ○ without surfactant and □ with surfactant) vs yield curves during the synthesis of PE

a t = 4 Figure 6 .

 46 Figure 6. Effect of ethylene pressure on polymer yields (• without surfactant and ■ without surfactant) and on particle sizes (Z av , ○ without surfactant and □ with surfactant) during the synthesis of PE particles via FRPE in aqueous dispersed media (runs 2, 5 and 20 to 25, Table3).

1 )Figure 7 .

 17 Figure 7. Evolution of the particle sizes (Z av , ■) and the number of particles (N p , •) with the stirring rate during the synthesis of PE particles via FRPE in aqueous dispersed media (runs 2 and 26 to 28, Table3).

Figure 8 .

 8 Figure 8. Effect of stirring rate on the melting temperature (T m , •) and crystallinity (■) of PE particles synthesized by FRPE in aqueous dispersed media (runs 2 and 26 to 28, Table3).

Page 25 of 36 ACSFigure 9 .

 369 Figure 9. DSC analysis of a PE latex obtained by FRPE in aqueous dispersed media (run 5 in

37

 37

Figure 10 .

 10 Figure 10. Particle sizes (Z av , •) and quantification of the supercooling effect (∆T, ■) vs surfactant concentration (runs 2 and 5 to 9, Table2).

Figure 11 .

 11 Figure 11. a) Moisture vapor transmission rate (MVTR) under tropical conditions (T = 38 °C, Humidity = 90 %), b) MVTR under standard conditions (T = 25 °C, Humidity = 50 %) and c)

Table 1 .

 1 Experimental conditions for FRPE in aqueous medium for the development of a negatively charged initiating and stabilizing system.

	Run	pH i	pH f	Yield (g) (PC) (%)	Z av (nm) (poly)	T m (°C) X c (%)	M n (g mol -1 ) (Ð)
	1 a	5.4	2.8	0.59 (1.2)	95 (0.26)	88	17	700 (6.3)
	2 a,b	8.2	8.9	2.40 (4.8)	23 (0.30)	84	13	49 400 (2.9)
	3 a,c	9.9	9.4	2.04 (4.1)	22 (0.32)	85	22	55 000 (2.7)
	4 a,d	12.4	12.2	3.58 (7.2)	27 (0.18)	86	18	85 000 (7.0)

Table 2 .

 2 Experimental conditions for FRPE in aqueous media for the investigation of the effect of surfactant concentration and kinetic study a

	Run Duration (h)	[SDS] (g L -1 )	Yield (g) (PC) (%)	Z av (nm) (poly)	T m (°C) X c (%)	M n (g mol -1 ) (Ð)
	5	4	0	0.77 (1.5)	91 (0.03)	89	24	3 400 (5.3)
	6	4	1	1.27 (2.5)	58 (0.01)	88	22	57 300 (2.0)
	7	4	2	1.79 (3.6)	37 (0.10)	87	26	230 000 (2.1)
	(2)	4	3	2.40 (4.8)	23 (0.30)	84	13	49 400 (2.9)
	8	4	4	2.76 (5.5)	20 (0.42)	83	22	30 000 (4.2)
	9	4	5	2.89 (5.8)	18 (0.15)	83	23	16 600 (4.7)
	10	0.5	0	0.00 (0.0)	-	-	-	-
	11	1	0	0.00 (0.0)	-	-	-	-
	12	2	0	0.05 (0.1)	40 (0.01)	nd	nd	-
	13	3	0	0.40 (0.8)	79 (0.02)	88	18	-
	(5)	4	0	0.74 (1.5)	91 (0.03)	89	24	3 400 (5.3)
	14	6	0	0.96 (1.9)	108 (0.01)	90	19	7 600 (3.4)
	15	0.5	3	0.20 (0.4)	11 (0.43)	85	11	-
	16	1	3	0.34 (0.7)	19 (0.45)	81	11	5 200 (5.2)
	17	2	3	1.27 (2.5)	20 (0.37)	82	19	20 000 (4.7)
	18	3	3	1.62 (3.2)	19 (0.32)	83	21	21 500 (3.7)
	(2)	4	3	2.40 (4.8)	23 (0.30)	84	13	49 400 (2.9)
	19	6	3	2.53 (5.1)	22 (0.31)	84	17	73 300 (2.2)

Table 3 .

 3 Experimental conditions for FRPE in aqueous media for the investigation of the effect of ethylene pressure, stirring rate and feasibility of high solids content a

	Run	Stirring rate (rpm)	P ethylene (bar)	Yield (g) (PC) (%)	Z av (nm) (poly)	T m (°C)	X c (%)	M n (g mol -1 ) (Ð)
	20 b	250	50	0.08 (0.2)	63 (0.08)	80	2	-
	(5) b	250	100	0.74 (1.5)	91 (0.03)	89	24	3 400 (5.3)
	21 b	250	150	1.16 (1.3)	120 (0.02)	95	24	14 700 (3.4)
	22 b	250	200	2.41 (4.8)	173 (0.04)	91	32	84 700 (4.1)
	23	250	50	0.21 (0.4)	16 (0.32)	51	3	4 900 (1.4)
	(2)	250	100	2.40 (4.8)	23 (0.30)	84	13	49 400 (2.9)
	24	250	150	4.61 (9.2)	30 (0.25)	91	23	220 000 (2.4)
	25	250	200	6.46 (12.9)	42 (0.20)	91	26	585 000 (2.3)
	(2)	250	100	2.40 (4.8)	23 (0.30)	84	13	49 400 (2.9)
	26	500	100	4.90 (9.8)	23 (0.24)	89	22	-
	27	750	100	6.70 (13.4)	26 (0.20)	91	25	-
	28	950	100	5.93 (11.9)	24 (0.20)	91	25	-
	(27)	750	100	6.70 (13.4)	26 (0.20)	91	25	260 000 (3.0)
	29	750	150	15.17 (30.3) 63 (0.11)	84	18	2.1 10 6 (2.4)
	30	750	200	24.00 (-)	Unstable	96	33	-d
	31	750	250	32.50 (-)	Unstable	102	44	-d

Page 28 of 36 ACS Paragon Plus Environment Submitted to Macromolecules 29

conditions (Figure

ACS Paragon Plus EnvironmentSubmitted to Macromolecules

Acknowledgments

ANR (Agence Nationale de la Recherche) and competitiveness clusters AXELERA and PLASTIPOLIS are acknowledged for financial support (project FRaPE; ANR 2011 JS08 008 01).

The authors thank Pierre-Yves Dugas for his help with cryo-TEM and DLS analyses, Olivier Boyron for SEC analyses and Jean-Pierre Broyer and Sébastien Norsic for technical support. We

Author Information

*E-mail: vincent.monteil@univ-lyon1.fr

Supporting information available

13

C NMR spectra of PE with branching assignments.

For Table of Contents use only

Free radical emulsion polymerization of ethylene