Prof Liliana M A P Joosten

Philippe Cassagnau

Eric Drockenmuller
email: eric.drockenmuller@univ-lyon1.fr

Damien Montarnal
email: damien.montarnal@univ-lyon1.fr

Prof E Cassagnau

Drockenmuller

Synthesis, Recycling and High-throughput Reprocessing of Phase-separated Vitrimer-Thermoplastic Blends

Keywords: vitrimers, thermoplastic vulcanizates, reactive processing, reprocessing, injection molding, thermoplastics, thermosets, viscoelastic properties, rheology

Phase-separated polymer blends including a transesterification vitrimer and polypropylene are synthesized from widely available precursors through reactive processing. We show that proper formulation during the process including a viscosity modifier of the vitrimer precursors and a reactive interface compatibilizer yields well-dispersed vitrimer phases with fractions up to 75 wt% in the polypropylene phase. These vitrimer blends can be easily reprocessed by multiple shredding-injection cycles and processed in high shear conditions through capillary rheometry. Rheological analyses reveal that a percolating dynamic network is formed across the vitrimer domains. It breaks apart during high shear reprocessing of the material and dynamically reforms after thermal annealing of the blends. It therefore ideally combines the mechanical and high-throughput (re)processing properties of the thermoplastic matrix minor phase with the chemical, thermomechanical and dynamic properties of the vitrimer major phase. This work illustrates the decoupling of processing at high shear rates enabled by the polypropylene processing aid from the kinetics of dynamic exchanges of the vitrimer phase and thermomechanical properties at service temperature of the blends.

Introduction

Thermosetting polymers are widely used in high-performance applications. [1] While polymer networks display outstanding performances such as enhanced chemical stability and dimensional stability at high temperatures or under loads, their recyclability is inherently limited to cost-and energy-intensive processes that are often undesirable in terms of sustainability. [START_REF] Fortman | [END_REF] Covalent adaptable networks (CANs) aim at overcoming these issues by integrating dynamic covalent cross-links with exchange reactions activated by triggers such as temperature or light. [3] Numerous developments have been recently reported on diverse covalent exchange chemistries involving either dissociative mechanisms (i.e. featuring sequential or simultaneous scission and reformation of cross-links) and associative mechanisms (also referred to as vitrimers, where dynamic exchanges occur while maintaining the network connectivity, i.e. constant cross-link density). [4][5][6][7][8][9] In both cases the bond reshuffling enables stress relaxation and allows the material to be reshaped under slow or limited deformations. In this regard, maintaining a low Weissenberg number (Wi is the product of the relaxation time of the material with the applied deformation rate) appears critical to avoid stress build up and fracture in the material. [10,11] In particular for vitrimers, the reprocessing is mostly carried-out by grinding and compression moulding that may involve transitory fracture, followed by multiple processes such as local relaxation, interfacial sintering and porosity decrease occurring at essentially zero shear rate. [12,13] Correspondingly, the reprocessing is directly governed by the exchange rate of the dynamic exchange reactions. [7] While the study of non-linear rheological properties of vitrimers has recently gained interest both from experimental and theoretical standpoints, flow at high shear rates is impossible for most highly cross-linked vitrimers and practically results in melt fracture or in the formation of crumbs. [14][15][16] Several groups have achieved regular extrusion of vitrimers by incorporating a small amount of dynamic cross-links into linear polymers such as polyolefins. [17][18][19] Another successful strategy is the combination of slow processing speeds with the design of vitrimers with very fast exchange dynamics. [20,21] For instance, Du Prez and coworkers designed vinylogous urethane-based vitrimers with extremely fast dynamics (relaxation time at 160 °C < 1 s), and managed to extrude at 150 °C smooth filaments using a double-twinscrew miniextruder at a speed of 1 rpm. [20] In all cases, the vitrimers had to be designed specifically to cope with the process: low cross-link density or low gel fraction, very fast exchange reactions induced by high amounts of catalysts or combination of dual dynamic bonds in series. Such workarounds unfortunately weaken the creep resistance of the vitrimers at service temperature. An alternative approach was recently proposed by Guo and co-workers. [22] They found that curing a styrene-butadiene rubber (SBR) in the presence of 10 phr of dithiol cross-linker containing dynamic dioxaborolane linkages leads to difficult processing: vane extrusion of this highly cross-linked dynamic rubber (100 rpm, 190 °C) essentially lead to fractured crumbs. However, after grinding the material into micron-sized powder, and blending it with an SBR containing 2 phr of dynamic cross-linker, the resulting material was processable by vane extrusion. This behavior is most likely related to the appropriate design of a two-phase blend with: i) comparable chemical nature of the polymer backbone and dynamic cross-linker affording ideal interfacial adhesion, and ii) highly cross-linked dispersed phase acting as reinforcement of the loosely cross-linked dynamic matrix enabling processability. Dispersion of highly cross-linked elastomers in thermoplastics by in-situ crosslinking during blending is in fact amongst the few successful strategies that have led to the industrial development of (re)processable permanent elastomers. These materials called thermoplastic vulcanizates (TPVs) have historically been obtained from the dispersion of a high volume fraction (>60 vol. %) of a cross-linked EPDM rubber in a polypropylene matrix, and subsequently extended to a larger variety of materials (e.g. polyamide matrixes, silicone rubbers). [START_REF] Coran | Rubber-Thermoplastic Compositions[END_REF] The desired objective is to combine the elasticity of the crosslinked rubber phase (e.g. low modulus, high creep resistance and elasticity recovery) with the melt processability in high-shear conventional devices (extruders, internal mixers, injection press) of the thermoplastic matrix. Blends such as TPVs are produced via a single reactive processing step, where the elastomer phase is selectively cross-linked during high-shear blending with a thermoplastic. In such blends, the mechanical performance is governed by the final morphology and the particle size. During processing, the blend morphology typically evolves from thermoplastic droplets dispersed in the uncross-linked elastomer matrix to cross-linked elastomer particles dispersed in the thermoplastic matrix following a phase-inversion process. [24][25][26][27][28][29] In the melt, attractive interactions across elastomer particles form a physical network, and the blend behaves as a yield stress fluid. [30] At service temperature, the crystallinity of the thermoplastic matrix dramatically increases the yield stress and the resulting materials demonstrate elastomer properties.

The present work applies the concept of TPVs to vitrimer-based materials and proposes a versatile strategy that enables high shear rate processing methods while mitigating the lower mechanical performance typical of highly cross-linked vitrimers (i.e. low strength and strain at break). It consists in a reactive blending strategy that converts commercially available low-cost vitrimer precursors and a molten thermoplastic into a fully cured phase-separated blend using a straightforward and up-scalable single-step process. It affords in relatively short reaction times a fine dispersion of cured vitrimer particles in a PP matrix. Extensive structural and rheological characterizations of blends involving 12-75 wt% vitrimer phase (S12-S75) are carried out and compared to the pure vitrimer (S100) and PP (S0) reference materials. For the first time in the field of CANs, capillary rheology has been implemented in order to measure viscosities at high shear rates and demonstrate in particular that the viscosity of the blends can be decoupled from the bond-exchange dynamics.

Results and Discussion

Reactive processing of PP/vitrimer blends

Up-scalable formulations involving inexpensive and largely available commercial precursors were selected to demonstrate the reactive blending of vitrimer precursors and a thermoplastic phase (Figure 1). A standard injection grade PP was chosen as the thermoplastic matrix. The vitrimer phase, relying on associative transesterification covalent exchanges, was obtained from the curing of di/trimerized fatty acids (Pripol 1040) and an aromatic diepoxy (DGEBA) in the presence of 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) to catalyze both network formation and subsequent transesterification exchanges. This classic vitrimer formulation is particularly robust and stable at high temperatures. [31] It has been widely studied, extended to various catalysts and epoxy/acid building blocks such as bio-based precursors and applied to different fields such as composites, 3D-printing of closed-loop recyclable materials or recyclable super-amphiphilic membranes for water filtration. [32][33][34][35][36][37][38][39] While this particular system is relatively brittle and displays rather slow exchanges kinetics, it is a very relevant candidate to decouple (re)processing timescales and network-relaxation timescales. TBD was chosen as catalyst, as it is soluble in Pripol at elevated temperature, thereby facilitating its homogeneous dispersion and catalytic activity during reactive processing. Curing kinetics at 200 °C of the pure vitrimer were studied by rheology: using 5 mol% of TBD with respect to COOH/epoxy groups, the gel point was reached within 80 s which is commensurate with high-throughput processing in an extruder and complete curing was obtained within 300 s (Figure S1, Supporting Information). The complete curing of the vitrimer was corroborated by , respectively), and the concomitant increase of the ester band (at 1735 cm -1 ) can also be observed within 400 s. Accordingly, the processing temperature was fixed at 200 °C and the processing time after adding all reagents in the melt mixer was maintained at 10 min throughout the study.

Reactive processing of a 12:88 (wt/wt) vitrimer/PP blend was initially carried out using a melt mixer by sequential addition of PP, Pripol/TBD mixture and DGEBA. The temperature and torque are recorded throughout the process, and the addition steps are visible in the profiles (Figure S3, Supporting Information). The sharp increase of the torque after 15 min corresponds to the increase in viscosity caused by the cross-linking reaction of the vitrimer phase after addition of DGEBA. To prevent thermal degradation of PP chains, a commercial antioxidant (0.5 wt% Irganox 1010) was added to the thermoplastic phase. HT-SEC confirmed that after melt blending PP with Irganox 1010 for 30 min at 200 °C, the decrease in number average molar mass of PP is negligible (Table S1, Supporting Information). Optical microscopy of the resulting blend demonstrates the presence of macroscopic vitrimer domains (median size of 0.6 mm) dispersed in the PP major phase (Figure S4a, Supporting Information). Two distinct strategies were then examined independently in order to improve this relatively poor mixing efficiency. First, PP grafted with maleic anhydride (PP-g-MA) was chosen as reactive interface compatibilizer. The grafted maleic anhydrides can indeed react with epoxides or alcohols in the vitrimer phase to form copolymers that reduce the surface tension between the vitrimer and PP phases. This strategy is well known to improve dispersion during reactive blending and eventually obtain nanophase structuration. [40] Herein, addition of PP-g-MA (10 wt% with respect to PP) during the processing results in a moderate decrease of particle sizes (median of 0.25 mm) compared to the blend without additives (Figure S4b, Supporting Information). In a second strategy, we aimed at reducing the viscosity mismatch between molten PP and vitrimer precursors. Droplet breakup when blending two immiscible liquids is indeed known to be primarily determined by the dimensionless viscosity ratio λ: [START_REF] Todd | Handbook of Industrial Mixing[END_REF] 𝜆 =

𝜂 𝑑 𝜂 𝑚 ( 1
)
where ηd is the viscosity of the dispersed phase and ηm the viscosity of the matrix (λ is initially about 10 -5

for the Pripol/PP mixture at 200 °C). In pure shear flow, which is the predominant mode in internal mixers, the breaking of droplets is most efficient for 0.1 < λ < 1 and decreases dramatically when λ <10 -2 . [START_REF] Utracki | [END_REF] Mixing efficiency may also be further decreased because we are mixing a Newtonian liquid (Pripol, then DGEBA) into a viscoelastic fluid. [43] While the viscosity of the vitrimer phase is expected to rise sharply during curing, the effective time-window for efficient mixing (λ >10 -2 ) before gelation occurs is extremely narrow (typically starting about 50 s after addition of DGEBA and lasting about 30 s). Our strategy aimed thus at adding inorganic fillers to increase the initial viscosity of the Pripol phase and benefit from an extended time-window for efficient mixing (typically 5 min of Pripol/PP mixing before addition of DGEBA). [44] A hydrophilic fumed silica (Aerosil 200 from Evonik) was dispersed in S4, Supporting Information. Significant curing of the vitrimer phase after the procedure steps was confirmed by FTIR (Figure S7, Supporting Information), demonstrating low amounts of residual carboxylic acids (typically <25%) and total disappearance of epoxy bands. The bulk morphology of blends S12-S75 (Figure 2) shows silica aggregates essentially localized in vitrimer domains (Figure S8, supplementary information), as anticipated from the higher polarity of the vitrimer phase than the PP phase and from the initial dispersion of silica in Pripol. The blends S12 to S75 show a good thermal stability, with a temperature at 5% weight loss in the 380-400 °C range independent of the vitrimer content (Figure S9, Supplementary Information) and ash residuals proportional to the amount of silica in each blend. DSC thermograms reveal that vitrimer domains barely impact the melting temperature of the PP phase (Table S2 and Figure S10, Supporting Information). The Tg of the vitrimer phase is noticeable for high vitrimer fractions around 15-20 °C. The crystallinity measured from the melting enthalpy and normalized by the PP fraction is constant about 45 %. Distinctions between samples can however be found during crystallization: whereas pure PP displays two separate crystallization exotherms at 113 °C and 123 °C, which are commonly attributed to a highly ordered and a less ordered α-phase, vitrimer blends show a single crystallization exotherm at 121-127 °C, characteristic of a nucleating effect resulting from the presence of small and well dispersed vitrimer domains. [46][47][48] This is interestingly corroborated by DSC characterization of S60 that does not contain silica and/or PP-g-MA additivestherefore comprised of very large vitrimer domains -and that does not exhibit such nucleating effect (Figure S11, Supporting Information). Further analysis of the crystalline structure by wide angle x-ray diffraction (WAXD, Figure S12, Supporting Information) reveals the predominance of the α-phase in the PP crystallites. However, S12 and to a lesser extent S24

Thermo-mechanical study

E'100°C a) [MPa] E'200°C a) [MPa] GN b) [kPa]  b) [s] Gel fraction c)
show an additional β-phase evidenced by a diffraction peak at 2θ = 16.2°. Similar β-nucleation effects have been previously reported for highly dispersed vulcanizate particles in PP/EPDM TPVs. [49,50] Small angle x-ray scattering (SAXS) at room temperature revealed for the pure PP distinctive scattering of a lamellar structure (q0 = 0.045 Å -1 after Lorentz correction; Figure S13a, Supporting Information). The correlation function analysis using the SasView software yields the typical profile expected for a semicrystalline arrangement, with a long period Lp = 13 nm and a narrow interface thickness of ca. 0.5 nm. Upon inclusion of 10% PP-g-MA and progressive increase of the vitrimer fraction, the scattering signal due to the lamellar structure of PP indicates first an increase of the long period and then becomes less and less visible in comparison to the strong scattering of silica aggregates in the vitrimer domains.

Kratky analysis (scattering signal around q = 0.01 Å -1 ) gives an estimation of silica aggregate sizes constant about 60 nm (Figure S13b, Supporting Information). The strong up-turn of the scattering intensities at low q-values for blends but not for S100 and S0 also indicates phase separation into domains with sizes well above 100 nm, consistently with the morphologies observed by SEM (Figure S4, Supplementary Information). S3, Supporting Information). PP shows the classical rigid and ductile behavior of a thermoplastic, including high modulus, yield point around ε = 20% and strain hardening at ε > 300%. The pure vitrimer presents a relatively high Young's modulus typical of a filled elastomer (E = 9 MPa) and a brittle elastic behavior (εmax = 80%). S12 and S24 evidence thermoplastic behaviors close to PP by large moduli and yield points, but highly reduced elongation at break. The behavior of the vitrimer phase is dominating for S48, S60 and S75, with an initial rigidity however dictated by the PP fraction. Interestingly, both stress and strain at break are significantly increased in the blends compared to the rather brittle S100 vitrimer reference. Pre-stretching the samples S60 and S75 at 50% strain (i.e. beyond the yield point of S0) is almost fully recoverable and results in softer materials comparable to highly filled elastomers, with moduli about 23 and 17 MPa, respectively (Figure S14a, Supporting Information). We confirmed the elastic recovery at 50% strain of sample S60 by performing cyclic tensile testing (Figure 3b and Figure S14b, Supporting Information). The corresponding elastic recovery above 90% after 10 stretch cycles indicates an excellent dimensional stability and supports the formation of a percolating network across vitrimer domains. The influence of phase dispersion on the tensile properties is illustrated in Figure S15b, Supporting Information. In absence of silica (following previously described recipes), the S60 blend shows poor properties (εmax = 70%), and almost immediate failure (εmax = 10%) in absence of compatibilizers. Other studies on vitrimer nanocomposites, reinforced with precipitated silica, graphene or carbon nanotubes, also found that the addition of nanofillers increases the moduli at the expense of decreasing the elongation at break. [51][52][53] The good mechanical performances in the sample containing both silica and compatibilizer in our case can be attributed to a finer morphology (See Figure S15c-f, Supporting Information), an effect already well-known in the case of conventional TPVs. [49] This result also emphasizes the importance of forming adequate interactions across vitrimer domains to control the mechanical properties of the blends, and the possible role of PP-g-MA in this regard. DMA analysis was performed from -80 to 220 °C to analyze the thermomechanical behavior of S0-S100

(Figure 4a,b). The α-transitions for vitrimer and PP phases are partially distinguishable (Table S2, Supporting Information). While Tα-PP remains constant at 20 °C, Tα-vit is gradually increasing with the vitrimer loading from 35 to 52 °C (for S12 and S100, respectively). Such evolution appears associated to the dispersion state of the vitrimer domains and might therefore originate either from a confinement effect or, more likely, from a better compatibilization and larger PP/vitrimer interphase as the ratio of PP-g-MA to vitrimer is effectively considerably higher for low fractions of vitrimer. Around 53 °C, a secondary Tα* transition is visible in PP-rich blends that corresponds to a crystal-crystal slip in the PP phase (Figure 4a). [START_REF] Mccrum | Anelastic and dielectric effects in polymeric solids[END_REF] Above the melting temperature of PP (near 170 °C), the sudden drop in modulus is concomitant with failure of samples S12 and S24. However, an elastic plateau visible in S48, S60 and S75

suggests the formation of a percolating vitrimer network. Complementary linear rheology measurements were thus performed in the melt (200 °C, Figure 4c). For S48, S60 and S75, the elastic plateaus at low frequencies on the storage moduli confirm the presence of a network across vitrimer domains. Stress relaxation enables to probe the dynamics of the blends beyond the relaxation of PP chains and define more accurately the modulus of the percolating network (Figure 4d). We found in this way that networks can be obtained with vitrimer fractions as low as 24%. While a similar formation of percolating network is often encountered in TPVs or rubber-modified thermoplastics, vitrimer blends display an additional relaxation at longer timescales. We attribute this relaxation to dynamic rearrangements within the percolating vitrimer network. [26,[START_REF] Ghahramani | [END_REF] The corresponding relaxation times (Table 1) are in the same order of magnitude (1000 to 2000 s at 200 °C), yet slight faster than that of pure vitrimer around 3400 s.

Discrepancies in these values might be related to variation in the effective formation of percolating vitrimer network that depends on the thermal and processing history of the samples as discussed further afterwards. The attribution of this relaxation event to dynamic network exchanges was further confirmed by preparing S60 with various concentrations of TBD catalyst at 2.5, 5.0 and 10 mol% with respect to COOH/epoxy groups (Figure S16, Supporting Information). The network relaxation of the blend is indeed accelerated for higher TBD contents (i.e.  = 2250, 1700 and 750 s for 2.5, 5.0 and 10 mol% TBD, respectively). Swelling in hot solvents, capable of dissolving the PP phase, is a particularly uncompromising test to determine co-continuity in blends, such as TPVs. [56,57] Herein, after immersion for 48 h in xylene at 110 °C, S0 dissolves completely while S100 shows a gel fraction of 93% and a swelling ratio of 1.7 (Table 1). Extraction of S12 leads to a dispersion of swollen vitrimer particles, while all other blends retain a monolithic shape and swell homogeneously (Figure 5). The gel content, normalized to the vitrimer fraction and assuming that all PP-g-MA remains tethered to the vitrimer network, remains constant at about 0.93 -0.99. This corroborates that mixing of vitrimer precursors and catalyst occurred homogenously in the dispersed vitrimer domains and produced fully cross-linked vitrimer domains, where the cross-linking density is independent of the vitrimer weight fraction in the blend. The swelling ratio increases significantly for samples having lower fractions of vitrimer phase (Table 1). We believe that this "sponge effect" is ascribed to free solvent trapped within open pores formed by the network of vitrimer particles. The linear rheology, structural and mechanical characterizations all point to the formation of a percolating vitrimer network with relatively slow relaxation dynamics (ca. 2000 s at 200 °C). In contrast, the reactive blending strategy chosen involves medium shear rates (estimated below 50 s -1 ) and yet leads to torques well within the working range of the melt mixer, as well as easy mold-pressing over short times. The presence of the PP phase and its role as processing aid appear thus essential to enable fast melt-processing independently from the slow exchange dynamics of the vitrimer network. This also suggests that the percolating network across vitrimer domains must be partially disrupted during high shear processing. To confirm this hypothesis, several creep steps with increasing stresses ranging from mm diameter) at high shear rates ranging from 100 to 2000 s -1 (Figure 6a). While the pure vitrimer cannot be processed through the die, all blends produce smooth extrudates and show shear-thinning behaviors with moderate viscosities (below 1000 Pa s for shear rates above 100 s -1 ). Compared to pure PP (S0), at 𝛾̇= 1000 s -1 the viscosities of the blends are increasing in line with the vitrimer fraction, up to 3-fold for S75, but remain well within the processing window of a standard extruder. A strong similarity can be found between these results and capillary rheology of conventional TPVs. [58] For all samples the viscosity dependence over shear rate is following a power law decay similarly to highly filled thermoplastics. [26,[START_REF] Ghahramani | [END_REF][59][60][61]

Strain [%] #1 #2 #3 #1 #2 #3
The easy (re)processability of vitrimer blends was further illustrated by injection moulding using sample S60. Samples undergoing multiple shredding-injection cycles were analyzed by tensile testing (Figure 7), confirming that the mechanical performance remains stable over the number of applied reprocessing cycles. Note that the initial sample (#1) was obtained by hot pressing and shows a modulus of about 2.5

MPa lower than reprocessed samples, in which the PP was oriented during injection. There is no noticeable change of mechanical performance between the second and third reprocessing cycle.

Evolution of the percolating vitrimer network over time and during various processes was monitored by repeated stress relaxations at 200 °C. [62] Figure 8a shows the relaxation moduli of S24 immediately after initial synthesis, and upon annealing at 200 °C for several hours in the rheometer. Initially (i.e. after reactive melt blending and pressing), the sample relaxes very fast and does not show any evidence of a percolating network (t1h in Figure 8a). The formation of a percolating dynamic network is shown after extended annealing at 200 °C (up to GN = 17 kPa with a relaxation time of 5400 s). After annealing, S24 was grinded and extruded at 2000 s -1 using the capillary rheometer, initially showing a very low modulus (GN = 0.6 kPa), demonstrating that high shear considerably alters the vitrimer percolating network. This phenomenon is however reversible, as significant network recovery (GN = 40 kPa) is obtained after 8h at 200 °C (Figure 8b). Similar build-up, break at high shear and recovery of percolating dynamic vitrimer networks are found for other blends. For instance, S60 immediately after synthesis shows a network modulus GN about 60 kPa with a relaxation time about 1700 s. The relaxation times quickly evolve to reach a constant value about 5000 s while the modulus continuously increases up to GN = 130 kPa after 8h at 200 °C (Figure S18a, Supporting Information).

Evidence of the formation of a percolating vitrimer network through annealing is also corroborated by swelling experiments: swelling S12 immediately after synthesis results in a dispersion of vitrimer particles whereas swelling after annealing for 8h at 200 °C leads to a monolithic piece (Figure S18b, Supporting Information). Dynamic percolation phenomena during static annealing have been observed in the case of polymer nanocomposites, such as PP nanocomposites containing organoclay or nanosilicates. [63][64][65][66] Time-and shear-dependent thixotropy or anti-thixotropy (e.g. decrease or increase of viscosity with time, respectively) has also been recently observed in silica/polymer nanocomposites featuring dynamic covalent links across interfaces. [67] In TPV, however, this phenomena appears to have been greatly overlooked. In our blends, the dynamics of the percolating vitrimer networks reformation appear to be commensurate with the bond-exchange transesterifications rates determined from stress relaxation experiments. It appears unlikely that this phenomenon would be related to direct coalescence and sintering of vitrimer particles as no evidence of morphological changes can be evidenced upon annealing (Figure S19, Supporting Information), and the phenomenon occurs even in blends containing fractions of vitrimers below the percolation threshold. We believe however that PP-g-MA chains grafted on and connecting vitrimer particles via ester bonds might also be involved in dynamic transesterification exchanges and might be the primary cause for the establishment of a dynamic percolating network with yield stress behavior.

Conclusion

A facile and up-scalable method for the synthesis of phase-separated vitrimer-thermoplastic blends through reactive mixing has been developed. This process was demonstrated using a well-established epoxy-acid vitrimer system dispersed into a polypropylene matrix. The corresponding blends demonstrate all characteristics of a percolating dynamic network connecting vitrimer domainswith solvent resistance, high-temperature dimensional stability, and stress relaxation. This system has the unique advantage of decoupling exchange dynamics in the vitrimer network from high-shear processing, that occurs through reversible rupture of the percolating vitrimer network and leads to low viscosities with easy processability. This concept addresses a major bottleneck in the current development of vitrimer materials and we believe it will open wide perspectives for high throughput syntheses and processing using conventional tools for thermoplastics such as extrusion, injection moulding, or 3D printing by FDM. Future optimizations of vitrimer-thermoplastic blends are currently ongoing in order to further decrease the size of vitrimer domains, either through processingusing for instance continuous reactive extrusionor through formulation, such as tuning the amount of compatibilizer. The model strategy system that we present might also be easily generalizable to other combinations of vitrimers (including for instance low Tg or high Tg vitrimer phases) and other thermoplastic phases.

Figure 1 .

 1 Figure 1. Reactive blending strategy between a thermoplastic phase (PP) and an epoxy-acid vitrimer phase.

  FTIR monitoring at 200 °C (Figure S2, Supporting Information). The disappearance of carboxylic acid and epoxy bands (at 1710 and 912 cm -1

Pripol ( 15

 15 wt% according to Pripol) using a planetary mixer until full transparency is reached. Curing kinetics at 200 °C of a pure vitrimer phase using Pripol/A20015%, DGEBA and TBD confirmed that adjunction of silica does not change the gelation time (FigureS1, Supporting Information). However, the final modulus and the Tg attained are slightly higher in presence of silica (1.63 vs 1.27 MPa, and 15 vs 12 °C with and without silica, respectively) and, more importantly, the similar values of G' and G'' before gelation indicate a more complex viscoelastic behavior. Additional characterization at 200 °C of the Pripol/A20015% mixture in absence of DGEBA indicates a viscoplastic behavior, with static and dynamic yield stresses about 140 Pa and 80 Pa, respectively (FigureS5, Supporting Information). When comparing to pure Pripol, the corresponding viscosities are increased by a factor up to 10 4 at 1 s -1 . Such improvement in the viscosity ratio of dispersed and matrix phases (λ ̴ 10 -1 at 200 °C) resulted indeed in a drastic improvement of the mixing efficiency between the vitrimer and PP immiscible phases, with median sizes of the vitrimer domains decreasing to 10 μm (FigureS4c, Supporting Information). Finally, the two strategies above were combined to carry out the reactive processing of all vitrimer/PP blends studied herein: i) addition of PP + 10 wt% PP-g-MA + 0.5 wt% Irganox 1010 in the melt mixer preheated at 200 °C, ii) after the torque has stabilized (about 5 min), addition of Pripol mixture in which 15wt% A200 silica and TBD catalyst were pre-dispersed, iii) after 5 min, addition of DGEBA, and iv) after 10 min of curing, recovery of the material and hot-pressing into 1.5 mm thick sheets. The fully formulated 12:88 vitrimer/PP blend comprised vitrimer domains with median sizes of 0.8 μm, and results in fully homogeneous sheets at the optical scale (FigureS4d, Supporting Information). The concept was further extended by varying the vitrimer:PP ratios from 12 to 75 wt.% vitrimer, referred to as S12 to S75 in the following. The S0 (PP:Irganox) and S100 (vitrimer:silica obtained by curing the liquid vitrimer precursors in the press) are added as references (FigureS6, supplementary information). The detailed compositions of all samples are given in Table

Figure 2 .

 2 Figure 2. SEM of vitrimer blends after surfacing by cryo-microtomy. a) S12; b) S24; c) S48; d) S60; e) S75 and f) S100. SEM-EDX analysis (Figure S8, supplementary information) enabled to readily assign the

  ; b) network moduli and relaxation time calculated from fitting the stress relaxation performed at 200 °C to a stretched exponential decay function (see Characterization Methods); c) determined after 48h in xylene at 110 °C, normalized to the weight fraction of the vitrimer phase and PP-g-MA.

Figure 3 .

 3 Figure 3. a) Tensile testing (10 mm min -1 ) of vitrimer/PP structured blends, PP and the pure vitrimer; b) Cyclic testing showing hysteresis curves up to 10 cycles for prestrained S60 sample. Experiments performed near 20 °C.

Figure 4 .

 4 Figure 4. Thermo-mechanical characterization of S0-S100. a,b) Temperature ramp measured by DMA (3 °C min -1 , 1 Hz), showing the traces of storage modulus (E'), and tan(δ). Characterization by rotational rheology: c) SAOS at 200 °C (1% strain, the loss moduli are omitted for clarity), and d) stress relaxation at 200 °C (1% strain). Data were fitted using a stretched exponential decay (Equation 2; black dashed lines).

Figure 5 .

 5 Figure 5. a) Gel fraction of PP/vitrimer blends swollen in xylene (24h at 110 °C), normalized to the initial content of vitrimer precursors and PP-g-MA. No measurable value was achieved for S12, as the sample disintegrates into particles; b) samples S12 -S100 in their swollen state, after 48h in xylene at 110 °C.

2. 3 .

 3 (re-)Processing and recyclability.

Figure 6 .

 6 Figure 6. a) Viscosity flow curve for S0-S75, obtained from capillary rheometry at 200 °C; and b) resulting extrudates from left to right: S12, S24, S48, S60 and S75.

  Pa were performed on sample S60, and the corresponding viscosities were determined from the compliance (Figure S17, Supporting Information). Up to 5000 Pa, S60 demonstrates relatively constant viscosity about 10 8 Pa s, i.e. in line with the result expected from linear relaxation data (η = G.τ = 1.2 × 10 8 Pa s) and thus characteristic of dynamic exchanges within the percolating vitrimer network. At 7000 Pa, a dramatic increase of shear rate indicates yielding into a viscous melt and breaking of the vitrimer network. Such rupture is however only partial as stopping and relaunching the creep steps experiment (2 nd run) gives the same results, albeit with a lower yield stress about 3000 Pa. Capillary rheometry was then used to investigate the melt flow of the vitrimer/PP blends through a narrow die (1

Figure 7 .

 7 Figure 7. Reprocessing of S60 by multiple shredding-injection moulding cycles. Tensile tests and corresponding tensile bars, showing first (green solid lines; #1), second (blue solid lines; #2) and third (orange solid lines; #3) reprocessing cycles.

Figure 8 .

 8 Figure 8. a) Repeated stress relaxations at 200 °C and 1 % strain of S24 immediately after synthesis (solid lines) and after disrupting the percolating network of annealed S24 by extrusion at 2000 s -1 in the capillary rheometers (dashed lines). Annealing time is given at the end of the experiment, which is 1h (black lines), 4h30 (red lines), 6h15 (green lines) and 8h (purple lines).b) extrapolated G0 moduli, showing the increase

4 . 2 .

 42 Polypropylene PPH7060 (PP) was obtained from Total Energies. It features a melt flow index of 12 g 10 min -1 (at 230 °C, 2.16 kg) and a zero-shear viscosity of 1500 Pa s at 200 °C. HT-SEC in TCB at 150 °C gives Mn = 234 kg mol -1 and Ð = 5.5. Pripol 1040 (82 wt% trimer, 18 wt% dimer; with an acid value of 185 mg KOH g -1 , corresponding a COOH equivalent of 294 g molCOOH-1 ) with a viscosity of 13 mPa s at 200 °C was supplied by Croda International Plc. Polypropylene grafted maleic anhydride (PP-g-MA)Exxelor PO1020 (MA level 1.2 wt%; HT-SEC in TCB at 150 °C: Mn = 36 kg mol -1 , Ð = 2.53), with a melt flow index of 430 g 10 min -1 (at 230 °C, 2.16 kg) was purchased from ExxonMobil. Irganox1010 was supplied by BASF SE. Aerosil A200 fumed silica (SBET = 225 m² g -1 , Ø ca. 12 nm) was purchased from Evonik Industries. Bisphenol A diglycidyl ether (DGEBA, 98%, Merck), 1,5,7triazabicyclo[4.4.0]dec-5-ene (TBD, 98%, Merck) and xylene (99%, Acros Organics) were used as received. General procedure for the processing of PP/vitrimer blends. Synthesis of S60.

Table 1 .

 1 Physical and rheological properties.

	Sample	Vitrimer
	name	content [wt%]

Acknowledgements

We thank the staff of the Technological Centre of Microstructures, Claude Bernard University Lyon 1, especially Pierre Alcouffe, for the support and help conducting the microscopy analyses. We acknowledge SOLEIL for provision of synchrotron radiation facilities and we would like to thank Thomas Bizien for assistance in using beamline SWING. Additionally, we would like to thank François Tournilhac and Diego Cardi (ESPCI, Paris, France) for the fruitful scientific discussions and their support. This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant agreement No 860911 (www.vitrimat.eu).

While vitrimer domains appear well-dispersed into a "sea-island" morphology for low fractions (S12 and S24), a bimodal distribution appears for higher vitrimer loadings, with submicronic domains but also larger and more complex shaped domains due to droplet coalescence or less effective mixing during the synthesis. This results in elongated shapes as the shear-deformed vitrimer droplets are cross-linked by the fast curing reaction. For S48, S60 and S75 the delimitation between particles and matrix becomes ambiguous, typical of morphologies near co-continuity. [45]

was filled with the unreacted mixture, sandwiched between two steel plates and the pressure increased stepwise by 50 bar min -1 until 200 bars were reached. After 15 min, the mould was recovered, and the sample was cooled inside the mould at room temperature.

Reprocessing of PP/vitrimer blends.

The reprocessing of vitrimer blends was done by manually cutting the plates into ca. 1 mm pieces and feeding them into the injection barrel of a Xplore micro injection moulder preheated at 200 °C. After 5 min melting time, the material was compressed and injected into tensile specimen. After tensile testing, the bars were cut and injected again until the desired number of reprocessing steps was reached.

Characterization Methods

Size exclusion chromatography (SEC):

To determine the mollar mass of PP and PP-g-MA, SEC was performed using a Malvern Instruments Viscotek system equipped with three columns (Agilent Technologies, PLgel Olexis 300 mm × 7 mm I.D.). A solution (200 μL) was prepared with a concentration of 5 mg mL -1 for each sample, which was then eluted in 1,2,4-trichlorobenzene (150 °C, 1 mL min -1 ). 2,6-di-tert-butyl-4-methylphenol (butylated hydroxytoluene, BHT, 200 mg L -1 ) was used to stabilize the mobile phase. Detection was performed by using a differential refractive index detector.

Number average molar masses (Mn) and chain dispersities (Ð) were calculated using calibration curves obtained from poly(ethylene) standards using the OmniSEC 5.02 software.

Fourier-transform infrared spectroscopy (FTIR):

The samples were analyzed by a Thermo Scientific Nicolet iS50 FT-IR in transmission mode, with a resolution of 4 cm -1 and 32 scans between a wavenumber of 4000 and 600 cm -1 . For the analysis ca. 20 µm thick films were pressed with a hot press at 200 °C between Teflon sheets.

Thermogravimetric analysis (TGA): TGA was performed at a heating rate of 10 °C min -1 from room temperature to 600 °C, under nitrogen atmosphere using a Mettler Toledo TGA 2 instrument.

Differential scanning calorimetry (DSC):

A Mettler Toledo DSC 3 + device was used to study the samples.

Two heating and one cooling ramps at 10 °C min -1 were performed between -40 and 200 °C. Only the values of the second cycle were taken.

Dynamic mechanical analysis (DMA):

A DMA Q800 device from TA Instruments was used in the tension film mode under a nitrogen atmosphere. Rectangular samples of 1 mm thickness, 5 mm width and approximately 15 mm length were tested at a frequency of 1 Hz and an amplitude of 5 µm. Heating ramps of 3 °C min -1 were applied from -80 °C to 220 °C.

Rheological analysis: Rheology measurements were performed using a 25 mm plate-plate geometry with an Ares G2 rotational rheometer from TA Instruments equipped with a convection oven.

Flow curves for Pripol/TBD/silica mixtures were obtained from increasing steps of shear rates (0.001 to 2 s -1 ), with equilibration times between 30 and 60 s.

Gelation kinetics of the pure vitrimer were monitored by pouring the reactive mixture on the preheated geometries and immediately starting time-resolved small amplitude oscillatory shear (SAOS) experiments at 200 °C.

Samples of vitrimer blends were punched from rectangular plates with a thickness of ca. 1.0 mm, and a diameter of 25 mm, and introduced into the geometries preheated at 200 °C. Frequency sweeps and stress relaxations (SR) were performed with a strain of 1%, within the linear viscoelastic regime of the measured material. The average relaxation time τ* was calculated by fitting relaxation moduli G(t) using a stretched exponential decay function:

where τapp is the apparent relaxation time. The following correction gives τ * :

Successive stress relaxation experiments were performed to study the effect of prolonged annealing on the network properties, by repeating SRs for approximatively 8h.

A Mars 60 Rheometer from ThermoScientific, equipped with a Peltier oven and 8 mm parallel plate geometries was used to perform creep experiments. The stress imposed on the 1 mm thick samples was varied from 50 to 7000 Pa over 600s, at 200 °C.

Capillary rheometry: Experiments were performed in a Malvern Rosand RH10 at 200 °C, using a die with 1 mm diameter, 20 mm length and 180° entrance angle. A 10,000 Psi force captor was used to record the drop of pressure across the capillary. The amount of material required for single-bore measurements is ca. 20 g. We did not carry out Bagley corrections accounting for pressure drop at the entrance of the capillary as they required at least 40 g of materials (dual-bore mode) but specifically chose long capillaries (20 mm) and flat entrance to minimize this issue. Increasing steps of shear rates were applied and the apparent shear stresses 𝜏 𝑎𝑝𝑝 were determined from the pressure in front of the capillary after proper equilibration (about 30 s).

The Weißenberg-Rabinowitsch correction was performed automatically. Hence, the corrected shear rate (γ̇𝑡 𝑟𝑢𝑒 ) was given by: Tensile test: Dumbbell-shaped specimens were analyzed using a Shimadzu AGS-X tensile test machine at ambient temperature equipped with a force sensor of 10 kN and 10 mm min -1 crossbar speed. The samples were cut from a plate of 1 mm thickness, with a gauge width and length of 4 mm and 30 mm, respectively. The tensile strength, the elongation at break, and the Young's modulus were calculated for each sample from at least three specimens and the results were averaged. For cyclic tensile tests, injected dumbbell shaped specimen with a thickness of 2 mm, width of 4 mm and 30 mm gauge length were used on a MTS Criterion 42 device. Cyclic testing involves stretching up to ε = 50% and return to initial displacement at 2 mm min -1 . A 10 min recovery delay was performed before starting the next cycle.

Optical microscopy (OM):

Images were taken on a Leica stereo microscope with a magnification of ×11.7 and ×32.

Scanning Electron Microscopy (SEM):

Observations of the sample morphology was done on a FEI Quanta 250 FEG microscope with a voltage of 10 kV using a Secondary Electron detector (SED). For images with enhanced contrast or to identify the different phases, a Backscattered Electron detector (BSE) or an Energy Dispersive X-Ray detector (EDX) was used. The sample surfaces were prepared by cryo-ultramicrotomy using a UCL Leica at -110 °C with an oscillating diamond knife (Diatom), followed by a carbon coating to enhance conductivity.

X-ray analysis: Simultaneous SAXS and WAXS were recorded at the SWING Beamline of SOLEIL Synchrotron source, Saint-Aubin, France. The energy used for all experiments was 12 keV. Samples were melt-pressed into 1 mm thick metal washers, windows on both sides were obtained by sealing Kapton foil with epoxy glue. The samples were inserted in a custom-built heated block and analyzed at 25 and 160 °C. 2D SAXS and WAXS patterns were collected using an EigerX4M detector positioned at 6.5 m from the sample and a Merlin detector, respectively.

Gel Content and swelling ratio were determined by immersing samples (w0 of ca. 80 mg) in a closed vial in xylene at 110 °C for 48 h. Monolithic, swollen samples were recovered and quickly wiped and weighted (wS), then dried for 24 h at 40 °C under vacuum (final weight wd).

The gel content was calculated using the following equation:

The swelling ratio was calculated with the formula:

𝜌 𝑝 (7) where ρP and ρS are the density of polymer (approximated as 1 g cm -3 ) and for the solvent (xylene = 0.861 g cm -3 ), respectively.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Conflict of Interest

The authors declare no conflict of interest.


Loïc Leclercq

Pierre Bauduin

Véronique Nardello-Rataj

Supramolecular "Big-Bang" in single ionic surfactant/water system driven by electrostatic repulsions: From vesicles to micelles

Keywords: Double-tailed ammonium, Dynamic and Static light scattering, Self-diffusion NMR, Freeze-fracture TEM, Micelles to vesicles transition

In aqueous solution, dimethyl-di-n-octylammonium chloride, [DiC 8 ][Cl], forms spontaneously dimers at low concentrations (1-10 mM) in order to decrease the hydrophobic/water contact. Dimers represent ideal building blocks for the abrupt edification of vesicles at 10 mM. These vesicles are fully characterized by dynamic and static light scattering, self-diffusion NMR and freeze-fracture transmission electron microscopy. An increase in concentration leads to electrostatic repulsions between vesicles which explode into small micelles at 30 mM. These transitions are detected by means of surface tension, conductivity and solubility of hydrophobic solutes as well as by isothermal titration microcalorimetry. These unusual supramolecular transitions emerge from the surfactant chemical structure that combines two contradictory features: (i) the double chain structure tending to form low planar aggregates with low water solubility, and (ii) the relatively short chains giving high hydrophilicity. The well balanced hydrophilic-hydrophobic character of [DiC 8 ][Cl] is then believed to be at the origin of the unusual supramolecular sequence offering new opportunities for drug delivery systems.

Introduction.

Molecular assemblies allow the formation of complex and fascinating structures with new and interesting properties. [START_REF] Schneider | Principles and Methods in Supramolecular Chemistry[END_REF][START_REF] Steed | Supramolecular Chemistry[END_REF][START_REF] Lehn | Supramolecular Chemistry. Concepts and Perspectives[END_REF] The supramolecular self-assembling and their resulting properties can be externally controlled with numerous stimuli (e.g., temperature, concentration, salt, pH, etc.). [START_REF] Borovkov | The origin of solvent-controlled supramolecular chirality switching in a bis(zinc porphyrin) system[END_REF][START_REF] Xu | A pH switch in supramolecular polymeric capsules[END_REF] Surfactants are typical examples of molecules which give supramolecular self-assemblies. For instance, the aggregation of ionic surfactants can be easily tuned by varying the concentration, the charge of the aggregates (salt effect), the temperature as well as their spontaneous curvature (or the molecular packing parameter). [START_REF] Israelachvili | Theory of self-assembly of hydrocarbon amphiphiles into micelles and bilayers[END_REF][START_REF] Nagarajan | Molecular Packing Parameter and Surfactant Self-Assembly. The neglected role of the surfactant tail[END_REF] In respect to this, aggregates as diverse as micelles (e.g., spheres, disks, ribbons, and elongated micelles), vesicles, bilayers, and hollow regular icosahedra can be obtained. [START_REF] Kaler | Spontaneous vesicle formation in aqueous mixtures of single-tailed surfactants[END_REF][START_REF] Zemb | Self-assembly of flat nanodiscs in saltfree catanionic surfactant solutions[END_REF][START_REF] Khan | Synergism and polymorphism in mixed surfactant systems[END_REF] Sometimes, transitions of one structure into another offer interesting opportunities for drug delivery systems and for membrane protein reconstitution. [START_REF] Malmsten | Surfactants and polymers in drug delivery[END_REF] However, transitions from vesicles to micelles by increasing concentration are usually observed for surfactant mixtures systems such as carboxylic/carboxylate, cationic/anionic, phospholipid/surfactant, and surfactant/cosurfactant mixtures. [START_REF] Renoncourt | Spontaneous vesicle formation of an industrial single-chain surfactant at acidic pH and at room-temperature[END_REF][START_REF] Stuart | Two distinct mechanisms of vesicle-to-micelle and micelleto-vesicle transition are mediated by the packing parameter of phospholipid-detergent systems[END_REF][START_REF] Yin | Temperature-induced micelle to vesicle transition in the sodium dodecylsulfate/dodecyltriethylammonium bromide system[END_REF][START_REF] Oberdisse | Experimental study of the micelle-to-vesicle transition[END_REF][START_REF] Viseu | spontaneous vesicles formed in aqueous mixtures of two cationic amphiphiles[END_REF] For instance, catanionic surfactants (obtained from equimolar mixing of cationic and anionic surfactants) form spontaneously stable vesicles which present reversible transitions between vesicles and micelles upon temperature or composition changes. [START_REF] Jiang | Bile salt-induced vesicle-to-micelle transition in catanionic surfactant systems. Steric and electrostatic interactions[END_REF] These transitions were clearly attributed to the balance between electrostatic, van der Waals, hydrophobic and steric forces. [START_REF] Jiang | Bile salt-induced vesicle-to-micelle transition in catanionic surfactant systems. Steric and electrostatic interactions[END_REF][START_REF] Philp | Self-assembly in natural and unnatural systems[END_REF][START_REF] Tian | Spontaneous aggregate transition in mixtures of a cationic gemini surfactant with a double-chain cationic surfactant[END_REF] However, the transition of stable vesicles to micelles remains unknown with a single surfactant/water system.

On the other hand, di-n-alkyldimethylammonium surfactants, [DiC n ][X] (where X is the counterion), are widely used in the field of phase transfer catalysis, biocides, immunosuppressing drugs, wetting and antistatic agents, etc. [START_REF] Leclercq | Supramolecular assistance between cyclodextrins and didecyldimethylammonium chloride against enveloped viruses. Toward eco-biocidal formulations[END_REF][START_REF] Kuniega | Solution behavior of dialkyldimethylammonium chloride in water. Basic properties of antistatic fabric softeners[END_REF] In respect with their packing parameter, double-tail ammonium surfactants are known to readily form vesicles or lamellae. [START_REF] Warr | Microemulsion formation and phase behavior of dialkyidimethylammonium bromide surfactants[END_REF] For instance, in 1977, Kunitake and Okahata reported the formation of [DiC 12 ][Br] vesicles after dispersion and sonication in water. [START_REF] Kunitake | A totally synthetic bilayer membrane[END_REF] However, it is noteworthy that energy input (sonication or heating) is not required to obtain vesicles: their formation is totally spontaneous. [START_REF] Fontana | Kinetics of breakdown of vesicles from didodecyldimethylammonium bromide induced by single chain surfactants and by osmotic stress in aqueous solution[END_REF] Interestingly, the existence of transition between small unilamellar to large multilamellar vesicles (SUVs → LMVs) was clearly identified for [DiC 12 ][Br] aqueous solution. [START_REF] Soltero | Rheological behavior in the didodecyldimethylammonium bromide/water system[END_REF] As the vesicles are highly charged (≈ 50% near the critical vesicle concentration, CVC) [START_REF] Soltero | Rheological behavior in the didodecyldimethylammonium bromide/water system[END_REF] , we can suggest a tentative mechanism to explain this transition. Indeed, as more [DiC 12 ][Br] is added, there is an increase in the concentration of SUVs, as well as an increase in the electrostatic repulsions between them. Above a given [DiC 12 ][Br] concentration, the electrostatic repulsions between SUVs becomes strong enough to allow the formation of LMVs (i.e., onion-like vesicles). However, it is noteworthy that the disruption of the stable SUVs cannot occur because the electrostatic repulsions cannot overcome the van der Waals chain-chain interactions in the bilayers. As the strength of van der Waals interactions decreases with the alkyl chain length, we can assume that transition from vesicles to smaller structures (e.g., vesicles → micelles) can be expected. At this point, it is worth mentioning that [DiC 8 ] cation constitutes a borderline case between hydrotropes, ≤ [DiC 6 ], and surfactants, ≥ [DiC 10 ], leading to a contradiction between two features (i) the double chain structure tending to form planar aggregates with low water solubility, and (ii) the relatively short chains giving high hydrophilicity. [START_REF] Hong | Binary and ternary phase behaviors of short double-chain quaternary ammonium amphiphiles. surface tension, polarized optical microscopy, and SAXS investigations[END_REF] This balanced hydrophilichydrophobic character of [DiC 8 ][Cl] leads to a particular aggregation behavior with the formation of dimer at low concentrations, a transition to large aggregates at intermediate concentrations and finally to the formation of micelles at high concentrations. Vesicles are observed between 10 and 30 mM whereas micelles are observed for higher concentrations (> 30 mM, see Figure 1). In 2011, we reported Small-Angle Neutron Scattering (SANS) experiments which proved the formation of ellipsoid micelles above 30 mM. [START_REF] Collinet-Fressancourt | Counter anion effect on the self-aggregation of dimethyl-di-n-octylammonium cation. A dual behavior between hydrotropes and surfactants[END_REF] However, the data obtained for aqueous solutions between 10 and 30 mM suggested discoid micelles (radius of 62.9 nm) or vesicles (radius of 102.5 nm). In consequence, we concluded that "the precise structure of the large aggregate [observed between 10 and 30 mM] cannot be resolved from the SANS results". [START_REF] Collinet-Fressancourt | Counter anion effect on the self-aggregation of dimethyl-di-n-octylammonium cation. A dual behavior between hydrotropes and surfactants[END_REF] To our knowledge, this result is rather uncommon as vesicle → micelle (or discoid micelle → ellipsoid micelle) transitions are usually exclusively observed with surfactant mixtures. [START_REF] Renoncourt | Spontaneous vesicle formation of an industrial single-chain surfactant at acidic pH and at room-temperature[END_REF][START_REF] Stuart | Two distinct mechanisms of vesicle-to-micelle and micelleto-vesicle transition are mediated by the packing parameter of phospholipid-detergent systems[END_REF][START_REF] Yin | Temperature-induced micelle to vesicle transition in the sodium dodecylsulfate/dodecyltriethylammonium bromide system[END_REF][START_REF] Oberdisse | Experimental study of the micelle-to-vesicle transition[END_REF][START_REF] Viseu | spontaneous vesicles formed in aqueous mixtures of two cationic amphiphiles[END_REF][START_REF] Jiang | Bile salt-induced vesicle-to-micelle transition in catanionic surfactant systems. Steric and electrostatic interactions[END_REF] To go deeper into the structure of the large aggregates formed by [DiC 8 ][Cl] and to understand the origin of this unusual transition, we have combined in the present work solubilization of hydrophobic solutes, isothermal titration microcalorimetry, dynamic and static light scattering, self-diffusion NMR and freeze-fracture electron microscopy. Comparison of the data obtained by all these techniques has enabled to determine the exact nature of the aggregates formed in the 10-30 mM concentration range and the previous SANS results were further discussed in the light of the results achieved here. On the basis of the present results, a mechanism for this unusual transition could thus finally be proposed.

Results and discussion.

Preliminary investigations. 2a andb. [START_REF] Leclercq | Stepwise aggregation of dimethyl-di-n-octylammonium chloride in aqueous solutions. From dimers to vesicles[END_REF] These two techniques can give information on the transition between unimers (or dimers) and aggregates. The results were not in agreement with each other as two different critical concentrations were obtained at 12.0 ± 0.5 and 32.0 ± 0.5 mM. For the following discussion, the CAC obtained by surface tension and by conductivity measurements are represented, respectively, by CAC 1 (≈ 10 mM) and CAC 2 (≈ 30 mM). It is noteworthy that the specific conductivity also reveals that the aggregates formed between CAC 1 and CAC 2 is totally ionized whereas the aggregates above CAC 2 are partially ionized: the binding degree of the counterion are 0 and 0.46, respectively.

To get a better insight into the discrepancy between the CAC values, we performed solubility measurements of an hydrophobic dye (Sudan II) in the presence of increasing concentrations of Beyond CAC 2 , a second regime is observed in which the dye solubilization is more efficient, i.e higher slope. This abrupt solubility increase above CAC 2 can be attributed the solubilization of hydrophobic dye molecules in the inner core of the micelle. In contrast, it is likely that the solubilization between CAC 1 and CAC 2 is related to the presence of larges aggregates. As the solubility enhancement is very weak, we can assume that the hydrophobic dye is solubilized in the aggregates formed in this concentration regime. The hydrophobic dye may be for example dissolved into the palisade layer of the large planar aggregates (i.e., discoidal micelles or vesicles) through hydrophobic interactions. It is noteworthy that only ~1% of the [DiC 8 ][Cl] total concentration participates in the formation of the large aggregates between CAC 1 and CAC 2 . [START_REF] Collinet-Fressancourt | Counter anion effect on the self-aggregation of dimethyl-di-n-octylammonium cation. A dual behavior between hydrotropes and surfactants[END_REF] Therefore the dye solubility remains very weak in this region due to the low concentration of surfactant participating to the formation of the large aggregates. The presence of NaCl (0.1 M) has also a clear effect on the values of CAC 1 and CAC 2 (15 and 72 mM with NaCl vs. 10 and 32 mM without NaCl, compare Figures 2c andd). This seems to indicate that the large aggregates are stabilized by addition of salt.

[
In contrast at 45 °C, only one transition is observed due to the disappearance of these large aggregates (see Figure 2e and the following discussion).

Next, isothermal titration microcalorimetry (ITC) experiments by dilution of a concentrated micellar solution show a complex evolution with a non-monotonic behavior (peaks) that is likely to be related to the complex mechanism of aggregation (Figure 3).

) 2 exp( 1 ) ( 2        g (1)
where  is the intercept of the correlation function and  is the average characteristic line width. For spherical particles,  is related to the translational diffusion coefficient D provided that the internal motions are negligible. In the cumulant approach, the z-averaged translational diffusion coefficient was obtained from the average line width, , based on the following equation:

2 Dq   with           2 4 sin n q (2)
where q is the magnitude of the scattering vector, n is the refractive index of dispersant,  is the wavelength of the laser and  is the scattering angle. DLS measurements were carried out at different concentration of 20 mM. It is noteworthy that, each experiment was performed in triplicate and that, after preparation, the [DiC 8 ][Cl] aqueous solution was stored overnight at room temperature before performing DLS measurements. In each case, the auto-correlation functions can be fitted with a single exponential decay (eq. 1) for all investigated detection angles indicating a single diffusion mode observed with spherical aggregates (see the typical auto-correlation function presented in Figure 4a). In order to confirm the spherical or the discoid structure of the aggregates formed between 10 and 30 mM, the relationship between the scaled average characteristic line width (/q 2 ) and the magnitude of the scattering vector (q 2 ) have been plotted in Figure 4b. The /q 2 values are constant and independent of the scattering angle, indicating a one mode diffusion typical of spherical shapes aggregates. The diffusion coefficient (D) is then estimated by D/q 2 . The hydrodynamic radius (R h ) of the aggregates can be calculated from D by using the Stokes-Einstein equation.

D kT R h   6 ( 3
)
where k is the Boltzmann's constant, T is the absolute temperature and  is the viscosity. R h obtained by fitting the auto-correlograms is 85.9 ± 9 nm for all the investigated angles each in triplicate. It is

noteworthy

Static light scattering (SLS).

Static Light Scattering (SLS) was used to measure the scattering curve I s (q) which contains information about the scattering particle's size and its shape (see Figure 5a). The Guinier approximation was used to determine the radius of gyration (R g ) by fitting the scattering spectra:

          3 exp ) ( 2 0 é g s q R I q I (4)
Here, the best fit is obtained for R g = 86 ± 5 nm (see the red dashed line in Figure 5). Further information on the aggregate structure can be obtained by the R g /R h , ratios. Indeed, hollow spheres (vesicles), cylinders, and rods give typical R g /R h values of 0.775, 1.0, 1.7, and >2. [START_REF] Burchard | Dynamic light scattering from polymer solutions[END_REF] For 20mM, the R g /R h ratio is indeed 1.0 which suggesting that the scattering objects are vesicles. All the presented results support the formation of spherical vesicles between 10 and 30 mM. Moreover, as the scattered intensity decreases very abruptly for concentrations above CAC 2 (see Figure 6a) it can be assumed that the vesicles (i.e., the main contributor to the scattered intensity compared to dimer and micelles) disappear in a very narrow range of concentrations Increasing the temperature also leads to disrupt the vesicles (20 mM of [DiC 8 ][Cl]) above around 20°C (see Figure 6b). These results support the solubility of Sudan II at 25 and 45 °C which showed that the intermediate concentration zone, where vesicles are formed, vanished at high temperatures (see previous discussion). The presence of NaCl (0.1 M) produces a significant increase in the vesicle size. Indeed, the best fit of the SLS spectrum is obtained for R g = 130 ± 16 nm (see the red dashed line in Figure 5b and eq. 4). As the R h obtained by fitting the auto-correlograms is 134 ± 16 nm for all the investigated angles with 0.1 M of NaCl, the value of the R g over R h ratio is 0.97, which is again close to unity the value expected for hollow spheres (vesicles). As the R g /R h values are close with or without NaCl (1.0 and 0.97, respectively), the evolution of the 2 nd virial coefficient by adding salt can be neglected compared to the effective change in the vesicle size. Therefore, the fit with a Guinier model in this region is sufficient. In our case, the increase in the size of the vesicles by adding salt is apparently totally related to a screening effect of the electrostatic repulsions acting between the charged polar head of the surfactant. This effect leads to decrease the average area per polar head in the vesicles which promotes a stronger charge compensation by the counterions. The decrease in the electrostatic repulsions leads to decrease the free energy difference between the vesicles and the micelles formed at higher concentrations as suggested by the extension of the realm of existence of the vesicles observed experimentally (see Figure 2e and the discussion below in the supramolecular speciation section).

Water self-diffusion coefficient. In order to support SLS and DLS results, water self-diffusion measurements were performed for three [DiC 8 ][Cl] concentrations 18, 20 and 22 mM. From a theoretical point of view, in the case of vesicles, self-diffusion coefficients of water can be linked to the behavior of two types of water molecules: inside (entrapped water) and outside of the vesicles (bulk water). Depending on the relation between the NMR time scale and vesicle permeability (the vesicles are not always completely impermeable), two water signals can be observed or be averaged to a unique signal. The NMR time scale is intrinsically related to the distance traveled by molecules.

In the case of monodisperse vesicles, if the NMR time scale is small, the water molecules will have a diffusivity similar to that of bulk water because the water molecules do not collide the vesicle wall.

In this case only one signal will be detected. In contrast, if the NMR time scale is large, the water molecules diffuse more slowly than bulk water because at this time scale water molecules can be collided a few times with the vesicle wall. Then, two cases can then occur: (i) if the vesicles are impermeable (i.e., multilamellar vesicles), the water will not escape the vesicle and will have the same diffusivity as the vesicle or (ii) if the vesicles are permeable (i.e., unilamellar vesicles), the water molecules will escape after some time, within the experimental time. [START_REF] Silva | Unusual vesicle-micelle transitions in a salt-free catanionic surfactant. Temperature and concentration effects[END_REF] In the situation of fast exchange, if it is assumed that the monodisperse unilamellar vesicles which behave as hard spheres, an averaged self-diffusion coefficient of water, D w , will be observed according to the two populations of water molecules, corresponding to inside and outside of the vesicles, with volume fractions P and (1 -P), respectively. where D ves and 0 w D are the self-diffusion coefficients of entrapped and bulk water (i.e., the neat water self-diffusion coefficient), respectively. It is noteworthy that A is the excluded volume which in the case of hard spherical monodisperse vesicles is given by:

1 2 1           ves A (6)
where  ves is the volume fraction of vesicles. For large vesicles (i.e., larger than the bilayer thickness), the fraction, P, corresponds to  ves and when the vesicles are so large that their diffusion can be neglected, we can obtain from the water reduced self-diffusion coefficient (D w / 0 For large monodisperse single bilayer vesicles, the vesicle radius (R ves ) can be approximated from equation 8 as the surfactant length, l s , is very small compared to R ves . where  s is the surfactant volume fraction which can be related to the total surfactant molar concentration, C T , by  s = N A × C T × v s , where N A is the Avogadro number. It is noteworthy that l s is defined as l s = v s / a s , where v s is the surfactant molecular volume and a s is the area occupied by the surfactant molecules at the interface (88 Å 2 ). [START_REF] Leclercq | Stepwise aggregation of dimethyl-di-n-octylammonium chloride in aqueous solutions. From dimers to vesicles[END_REF] An estimation of the vesicle volume fraction and radius is given in Table 1. As depicted in Table 1, the averaged vesicle radius, R ves , is around 106 ± 8 nm and it is close to the value obtained by SLS (R g = 86 nm from Guinier analysis) and by DLS (R h = 85.9 nm). This result support the formation of monodisperse unilamellar vesicles which behave as hard spheres. Freeze-fracture transmission electron microscopy. In order to prove unambiguously the presence of vesicles in the 10-30 mM concentration range, freeze fracture-TEM imaging in this region have been performed. As depicted in Figure 7, freeze-fracture of [DiC 8 ][Cl] vesicles vary in appearance, depending on the fracture plane and on the structural heterogeneity of vesicles. Indeed, the vesicles may be convexly (Figures 7A andB), concavely (Figure 7D) or cross-fractured (Figure 7C). For instance, in Figure 7A, the external surface (ES) of a convexly fractured vesicle is seen. A similar vesicle in which the fracture has followed the external surface (ES) and then partially penetrated inside the bilayer of the vesicle is shown in Figure 7B leading to view innermost membrane (IS). Figure 7C illustrates examples in which the vesicles have been cross-fractured: the vesicle core (VC) appears relatively featureless. Despite these advantages, freeze-fracture technique has a serious drawback for estimating the vesicle size, the fracture plane passes through the sample but the vesicles are randomly positioned in the frozen section resulting in non mid plane fracture (see Figure 7). Consequently, the observed radius depends on the distance of the vesicle center from the plane of the fracture, while the probability that a vesicle will be in the fracture plane depends on the vesicle radius. In any event, the vesicles look like spherical and the cross-fractured vesicles (where the plane of the fracture passes through the mid plane of vesicles) reveals a size compatible with the radii, R, determined from SLS, DLS, SANS and Self-diffusion NMR experiments (80 < R < 100 nm). It is noteworthy that additional micrographs are available in Supporting Information (Figures S1-S7 Supramolecular speciation from conductivity and SANS data treatment. In the following the amount of surfactant participating to the aggregates formed, dimer, vesicle and micelles will be estimated from the result obtained by the SANS spectra [START_REF] Collinet-Fressancourt | Counter anion effect on the self-aggregation of dimethyl-di-n-octylammonium cation. A dual behavior between hydrotropes and surfactants[END_REF] and discussed together with the conductivity.

The conductivity curve was fitted by taking into account the following considerations and assumptions. The conductivity of [DiC 8 ][Cl] solutions can be expressed as the sum of contributions of dimers, micelles and vesicles as: For C T < CAC 1 , only dimers contribute to the conductivity, and  dim can therefore be obtained from the conductivity curve ( dim = 0.095 mS.cm -1 .mM -1 ). For CAC 1 < C T < CAC 2 , we have

mic mic ves ves dim dim ] Cl ][ DiC [ ] Cl ][ DiC [ ] Cl ][ DiC [       
C T = [DiC 8 ][Cl] dim + [DiC 8 ]
[Cl] ves . It can be observed that the slope in the conductivity curve (Figure 2b) does not change in the regions C T < CAC 1 and CAC 1 < C T < CAC 2 . This may indicate that (i) the vesicles concentration is low compared to the dimer concentration because the vesicles do not contribute to the conductivity due to their large size, and (ii) the dimer concentration continuously increases in the regions C T < CAC 1 and CAC 1 < C T < CAC 2 .

In order to check these hypothesis, information on the concentrations of dimers and vesicles can be obtained from the fit results of the SANS spectra at C T = 10 and 20 mM. [START_REF] Collinet-Fressancourt | Counter anion effect on the self-aggregation of dimethyl-di-n-octylammonium cation. A dual behavior between hydrotropes and surfactants[END_REF] It was shown that the two SANS spectra at 10 and 20 mM, obtained from plotting the absolute scattered intensity normalized by the surfactant concentration versus q, completely overlap over the whole q-range.

This indicates that in the concentration

dim ves ves ] Cl ][ DiC [ ] Cl ][ DiC [ ] Cl ][ DiC [ 8 8 8    (10)
where

C T = [DiC 8 ][Cl] dim + [DiC 8 ][Cl] ves in the range CAC 1 < C T < CAC 2 .
The dimer concentrations for 10 and 20 mM surfactant solutions were previously determined: 2.35×10 -3 and 4.7×10 -3 dimer/nm 3 which corresponds to [DiC 8 ][Cl] dim = 7.8 and 15.61 mM. [START_REF] Collinet-Fressancourt | Counter anion effect on the self-aggregation of dimethyl-di-n-octylammonium cation. A dual behavior between hydrotropes and surfactants[END_REF] The molar fraction, , can therefore be calculated  = 0.22 (7.8/10 and 15.61/20).

By taking all these considerations into account, a model for fitting the conductivity curve can be expressed by the following set of equations:

                                           mic T dim ves di dim ves T dim T dim CAC C CAC CAC CAC CAC C CAC C 2 1 2 1 1 1 1 1 for           2 2 1 1 CAC C CAC C CAC CAC C CDC T T T (11)
where C T is the surfactant concentration and dim, mic and ves stand for dimer, micelle and vesicle. In equation 11, the fitting procedure of the experimental data only considers then one fitting parameter,  mic , as the other parameters could be determined separately. The conductivity curve can be well fitted by using this model (see Figure 8). Therefore the conductivity and SANS measurement are in good agreement confirming that only a small fraction of the surfactant participate in the formation of the vesicles, i.e., 22% of the surfactant in the range CAC 1 < C T < CAC 2 . Consequently, the other fraction of surfactant participates in the formation of dimers observed in the vesicle core (see above). Proposed mechanism. As previously mentioned, transitions from vesicles to micelles by increasing concentration are usually observed for surfactant mixtures systems (see above).

Moreover, simple considerations using the packing parameter concept enables to predict such a phase transition by changing parameters such as pH, salt concentrations, etc. For water/pure surfactant binary mixtures vesicle-to-micelle transition has never been reported. In the last years a family of non-classical amphiphilic boron-based macro-anions (~1 nm in size), the metallacarboranes (MCs), has attracted much attention. [START_REF] Bauduin | A Theta-shaped amphiphilic cobaltabisdicarbollide anion. Transition from monolayer vesicles to micelles[END_REF][START_REF] Brusselle | lyotropic lamellar phase formed from monolayered theta-shaped carborane-cage amphiphiles[END_REF][START_REF] Vinas | Surfactant behavior of metallacarboranes. A study based on the electrolysis of water[END_REF][START_REF] Gassin | Surface activity and molecular organization of metallacarboranes at the air-water interface revealed by nonlinear optics[END_REF] These anions, although lacking the classical hydrophilic-hydrophobic sequence of surfactants, have shown surfactant behaviors: micelle/vesicle formation, [START_REF] Bauduin | A Theta-shaped amphiphilic cobaltabisdicarbollide anion. Transition from monolayer vesicles to micelles[END_REF] lyotropic lamellar phase formation, [START_REF] Brusselle | lyotropic lamellar phase formed from monolayered theta-shaped carborane-cage amphiphiles[END_REF] foaming, [START_REF] Vinas | Surfactant behavior of metallacarboranes. A study based on the electrolysis of water[END_REF] surface activity. [START_REF] Gassin | Surface activity and molecular organization of metallacarboranes at the air-water interface revealed by nonlinear optics[END_REF] It was shown that cobalt bis(diarbollide), the archetype molecule of MCs, presents a vesicle-to-micelle transition in binary mixtures with water by increasing concentration. [START_REF] Bauduin | A Theta-shaped amphiphilic cobaltabisdicarbollide anion. Transition from monolayer vesicles to micelles[END_REF] A mechanism based on Coulombic explosion was proposed recently to explain this counterintuitive transition. [START_REF] Brusselle | lyotropic lamellar phase formed from monolayered theta-shaped carborane-cage amphiphiles[END_REF][START_REF] Vinas | Surfactant behavior of metallacarboranes. A study based on the electrolysis of water[END_REF] For the present surfactant system, a similar mechanism can be proposed to explain the origin of the vesicles to micelles transition. Indeed vesicles are also formed here at lower concentrations than micelles, indicating that vesicles are of lower free energy than micelles. Therefore the spontaneous packing parameter of [DiC 8 ][Cl] is around one and leads to the formation of vesicles. The increase in the vesicle concentration, observed by SANS and confirmed by conductivity for CAC 1 < C T < CAC 2 , leads then to electrostatic repulsions between the vesicles because of their charge. Increase in the vesicle-vesicle repulsions, i.e., when Debye length starts to overlap, i.e., when C ~ CAC2, leads to increase tremendously the energy of the system which is then released by formation of (small) micelles. In order to check this hypothesis, the CAC 1 /CAC 2 was determined (Figure 2d) in the presence of background salt ([NaCl] = 0.1 M) which aims at screening the vesicle-vesicle electrostatic repulsions. It was found that the addition of salt leads to increase the domain of existence of vesicles, as a consequence of a screening effect, which is in agreement with the proposed mechanism.

Another surprising behavior is the formation of dimers at low concentrations. The low dimer concentration indicates that monomers tend to decrease their free energy by forming dimers. This may be a way for the surfactants to decrease the hydrophobic/water contact between the two bulky alkyl chains in water. Therefore, dimers, formed by a tail-tail self-assembly process, can be seen as the building block of the more complex aggregates: unimers → dimers → vesicles.

Conclusion.

We reported here convergent evidences for the abrupt unprompted edification of [DiC 8 ][Cl] vesicles at low concentrations after the surface saturation (CAC 1 = 10 mM), followed by an unexpected transition into micelles (CAC 2 = 30 mM). These vesicles were fully characterized by dynamic and static light scattering, self-diffusion NMR and freeze-fracture transmission electron microscopy whereas the micelles have been previously described from small-angle neutron scattering. [START_REF] Collinet-Fressancourt | Counter anion effect on the self-aggregation of dimethyl-di-n-octylammonium cation. A dual behavior between hydrotropes and surfactants[END_REF] As and the addition of small volumes to ultrapure water ( = 72.0 mN.m -1 at 25 °C) was used to increase the solution concentration. After each addition, the solution was gently stirred for 30s.

[
Equilibrium surface tension was measured for each concentration. All surface tension values were mean quantities of at least three measurements. The standard deviation of the mean never deviated ±1.5 % of the mean. The precision of the force transducer of the surface tension apparatus was 0.1 mN.m -1 and before each experiment, the platinum plate was cleaned in red/orange color flame. The temperature stabilization was estimated as better than ±0.05 °C with a thermoregulated bath.

Conductivity measurements. Conductance measurements were taken with a CDM210 conductivity meter (Radiometer). All measurements were taken in a thermostated water bath. The temperature stabilization was estimated as better than ±0.05 °C with a thermoregulated bath. The conductance was measured for each surfactant concentration after a sufficient time to reach a constant value. The maximum error limit of conductance measurements was ± 0.5%. The sample were filtered through 0.2 µm filter before its transfer into a cylindrical scattering cell.

Solubility

Each experiment was performed in triplicate to ensure reproducibility.

DOSY measurements. The self-diffusion coefficient of the nonionic surfactants were determined by the pulsed-field gradient NMR (PGSE-NMR) technique, by monitoring the 1 H signal on a Bruker Avance 500 spectrometer equipped with a field gradient probe unit. This method was first introduced by Stejskal and Tanner, [START_REF] Stejskal | Spin diffusion measurements. Spin echoes in the presence of a time-dependent field gradient[END_REF] and in our experiments the BPP-STE-LED sequence, [START_REF] Wu | An improved diffusion-ordered spectroscopy experiment incorporating bipolar-gradient pulses[END_REF] combining constant time, bipolar pulse, stimulated echo, and the longitudinal eddy current delay method, was used.

Freeze

27

 27

Figure 1 .

 1 Figure 1. Structuration of dimethyl-di-n-octylammonium chloride, [DiC 8 ][Cl], aqueous solution as a

Figure 2 .

 2 Figure 2. Surface tension,  (a), specific conductivity,  (b), solubility of Sudan II with NaCl = 0 M

Figure 3 .

 3 Figure 3. Variation of the enthalpy, DH, as a function of [DiC 8 ][Cl] concentration at 25.0 °C (see

  scattering angles 30, 40, 50, 55, 60, 70, 80, 90, 95, 100, and 105° at 25 °C and at a fixed [DiC 8 ][Cl]

Figure 4 .

 4 Figure 4. (a) Typical DLS auto-correlation functions for a 20 mM [DiC 8 ][Cl] aqueous solution at 25

  that the R h values are very similar for [DiC 8 ][Cl] aqueous solution between 10 and 30 mM: 82.4, 89.2 , 106.7, and 63.4 nm (S.D.: ± 18%) for 10, 15, 25 and 30 mM, respectively. Moreover, the presence of NaCl (0.1 M) for a 20 mM [DiC 8 ][Cl] aqueous solution at 25 °C has a clear effect on the R h values (134 ± 16 nm with NaCl vs. 85.9 ± 9 nm without NaCl).

Figure 5 .

 5 Figure 5. Experimental () and fitted (dashed line, see eq. 4) SLS spectra for a 20 mM [DiC 8 ][Cl]

Figure 6 .

 6 Figure 6. SLS scattered intensity, I s , as a function of [DiC 8 ][Cl] concentration at 25 °C (a) and as a function of temperature for 20 mM of [DiC 8 ][Cl] (b) (detection angle: 90° with equal attenuation for

Figure 7 .

 7 Figure 7. Freeze-fracture transmission electron micrographs of [DiC 8 ][Cl] aqueous solution (20

  8 ][Cl] x and  x are the surfactant concentration involved in the aggregate x and the specific conductivity of the aggregate x (with x = dimer, vesicle or micelle). Moreover, C T = [DiC 8 ][Cl] dim + [DiC 8 ][Cl] ves + [DiC 8 ][Cl] mic . The contribution of the unimer to the conductivity is assumed to be negligible in the range of surfactant concentration used in the conductivity experiment. The critical dimer concentration (CDC) is indeed much lower than CAC 1 and CAC 2 , CDC (0.1 mM) << CAC 1 < CAC 2 , therefore for [DiC 8 ][Cl] > CDC then [DiC 8 ][Cl] unimer << [DiC 8 ][Cl]. The specific conductivity of the aggregates is related to their charge and size. As the vesicles are much larger than the dimers and vesicles, it can be assumed that  ves <<  mic <  dim .

  range CAC 1 < C T < CAC 2 , [DiC 8 ][Cl] dim and [DiC 8 ][Cl] ves both increase in the same way, i.e., by keeping the [DiC 8 ][Cl] dim /[DiC 8 ][Cl] ves ratio constant. The molar fraction of surfactant involved in the formation of vesicles can be defined as:

Figure 8 .

 8 Figure 8. Experimental () and calculated (dashed line) specific conductivity, , as a function of

  of hydrophobic dye. Solubility measurements were performed at 25.0 or 45.0 °C. For all [DiC 8 ][Cl] solutions (with or without NaCl, 0.1 M), Sudan II was added in excess. These solutions were mixed in a temperature-controlled water batch. After equilibration (72 h), these solutions were filtered and the Sudan II solubility was determined by measuring the absorbance of diluted solutions at 543 nm with Cary 50 UV-visible spectrophotometer (Varian). Solubility (in arbitrary units, a.u.) was obtained by multiplying the absorbance of diluted solutions by the dilution factor. Isothermal titration calorimetry. ITC experiments were performed using a nano-ITC titration calorimeter from TA Instruments at 25 °C. A 250 μL injection syringe was used with stirring at 400 rpm. [DiC 8 ][Cl] was dissolved in deionized water and the solutions were degassed gently under vacuum before use. Each titration comprised 25 × 10 μL injections of [DiC 8 ][Cl] (265 mM) into deionized water (950 µL). Control experiment with deionized water into deionized water was used to correct titration data.Dynamic and static light scattering. The measurements used a 3D spectrometer (LS Instruments, Fribourg, Switzerland) equipped with a 25 mW HeNe laser (JDS uniphase) operating at 632.8 nm, a two channel multiple tau correlator, a variable-angle detection system, and a temperature-controlled index matching vat. The scattering spectrum was measured using two single mode fiber detections and two high sensitivity APD detectors (SPCM-AQR-13-FC, Perkin Elmer).

  -fracture transmission electron microscopy. Freeze-fracture samples were prepared by first depositing the sample (aqueous solution of [DiC 8 ][Cl] = 20 mM) on a gold tablet containing a

DiC 8 ][Cl] with

	values are in good agreement with the CAC 1 and CAC 2 values obtained from surface tension (12.0
	mM) and from conductivity measurements (32.0 mM), respectively.
	or without NaCl (Figures 2c and 2d). As can be seen, the amount of dye solubilized
	at a [DiC 8 ][Cl] concentration below 10 mM is negligible and close to zero, proving the absence of
	aggregates. The first slight onset of solubilization is observed between 10.0 and 32.0 mM. These

Table 1 .

 1 Estimated vesicle volume fraction,  ves , and vesicle radius, R ves , obtained from the reduced

	water self-diffusion coefficients, D w / 0 w D , as a function of [DiC 8 ][Cl] concentrations at 25 °C.
	C T (mM) D w / 0 w D	a	 ves	R ves (nm) b
	18	0.748		0.184	115
	20	0.743		0.187	106
	22	0.743		0.187	96

a The self-diffusion coefficient of neat water, 0 w D , is 2.30×10 -9 m 2 /s at 25 °C. b Calculated with a s = 88 Å 2 and eqs. 7 and 8 (see earlier discussion).

  The synthesis of dimethyl-di-n-octylammonium chloride, [DiC 8 ][Cl], was prepared as previously described.[START_REF] Leclercq | Stepwise aggregation of dimethyl-di-n-octylammonium chloride in aqueous solutions. From dimers to vesicles[END_REF] The purity of [DiC 8 ][Cl] obtained by this method was ≥ 99.9% only moisture trapped in the sample are observed due to hygroscopic nature of this salts ( 1 H NMR and elemental analysis).27 1 H NMR (300 MHz, CDCl 3 , 20 °C, TMS): (ppm) = 0.86-0.90 (m, 6H; CH 3 ), 1.27-1.36

	(m, 20H; CH 2 ), 1.69 (m, 4H; N-CH 2 -CH 2 ), 3.43 (s, 6H; NCH 3 ), 3.50 (m, 4H; NCH 2 ). 13 C NMR (75
	MHz, CDCl 3 , 20 °C): (ppm) = 14.0, 22.5, 22.7, 26.2, 29.0, 29.1, 31.6, 51.3, 63.6. Anal. Calc. for
	(C 18 H 40 NCl); 1/3H 2 O: C, 69.30%; H, 13.14%; N, 4.49%; O, 1.71%; Cl, 11.36%. Found: C, 68.96%;
	H, 13.57%; N, 4.46%; O, 1.79%; Cl, 11.18%. mp 75 °C. (Yield 96%). It is noteworthy that
	[DiC 8 ][Cl] was dried and store under dry nitrogen.
	Experimental section.
	Materials. All the reagents were purchased from Sigma-Aldrich chemicals and used without

DiC 8 ][Cl] is known to form spontaneously dimers at very low concentrations (1-10 mM) in order to minimize the area of contact between the water molecules and the nonpolar alkyl tails of the surfactant (i.e., under the well-known hydrophobic effect), we can reasonably supposed that dimers represent ideal building blocks for the spontaneous formation of vesicles at CAC 1 .

[START_REF] Leclercq | Stepwise aggregation of dimethyl-di-n-octylammonium chloride in aqueous solutions. From dimers to vesicles[END_REF]

However, electrostatic repulsions between vesicles increase with concentration resulting in a mechanism close to Coulombic explosion into micelles at CAC 2 . This unusual behavior is totally ascribed to the hydrophilic-hydrophobic balance of [DiC 8 ] cation which constitutes a borderline case between hydrotropes, ≤ [DiC 6 ], and surfactants, ≥ [DiC 10 ]. This particular character leads to a contradiction between two features (i) the double chain structure tending to form planar aggregates with low water solubility, and (ii) the relatively short chains giving high hydrophilicity. Due to the current interest in the development of switchable drug delivery systems, fruitful developments are expected for pharmaceutical formulations with unusual supramolecular sequences. further purification. All products were dried in a Freeze Dryer (Alpha 1-2 LD plus). In all experiments, the water used was Millipore (18.2 M/cm; Simplicity 185).

Synthesis.

[START_REF] Schneider | Principles and Methods in Supramolecular Chemistry[END_REF]

H Surface tension measurements. The tensiometer K100MK2 (Krüss) and the Wilhelmy plate method have been used for the surface tension measurements. A concentrated solution was prepared

Acknowledgements.

Chevreul Institute (FR 2638), Ministère de l'Enseignement Supérieur et de la Recherche, Région Hauts-de-France and Fonds Européen de Développement Régional (FEDER) are acknowledged for supporting and funding partially this work.

small well. The samples were frozen by plunging the sample into a liquid propane (-200 °C). The frozen sample was transferred to the sample block of a freeze-fracture device (Leica BAF 060).

After temperature and pressure equilibration, the sample was fractured with knife and the resulting surfaces were replicated with platinum and carbon (≈ 30 nm). The replicas were washed in water and ethanol mixture in order to dissolve any remaining surfactant. The replicas were collected on copper TEM grids and observed using a Hitachi H600 transmission electron microscope at 75 KeV. The experiment was performed twice to ensure reproducibility.

Associated content.

Supporting Information

Copies of 1 H and 13 C NMR spectra. Seven additional freeze-fracture transmission electron micrographs. This material is available free of charge via the Internet at http://pubs.acs.org.

Author information.

Corresponding authors *E-mail: loic.leclercq@univ-lille1.fr (L.L.) *E-mail: veronique.rataj@univ-lille1.fr (V.N.-R.)

Author Contributions

All authors contributed equally to this work and approved the final version of the manuscript.

Notes

The authors declare no competing financial interest.