Abstract
We show the potential of miniemulsion photopolymerization for the continuous production of aqueous poly(acrylate) dispersions in a microreactor at room temperature. While the starting acrylate nanoemulsions are amenable to limit scattering, their polymerization within a microreactor provides additionally small microchannels and short diffusion path enabling an efficient mixing in order to alleviate the constraints associated with non-uniform through-cure in turbid medium. Two key features prove that this process design is highly eco-efficient: i) two types of energy-saving and compact UV sources (fluorescent or light-emitting diode) were employed; ii) high conversions were achieved using the fluorescent lamp with short residence times (10 min), low irradiance (3 mW cm-2) and without the need of solvent. The present study describes briefly the influence of various parameters – flow rate, photo-initiator type/concentration, droplet size, solid content, UV source – on the photopolymerization course (kinetics) and the properties of the nanolatex obtained (particle size and molecular weight).
1 Introduction
Sold in dry or colloidal form, butadiene-styrene, vinyl acetate or acrylate polymers prepared from aqueous dispersions represent a significant part of the worldwide production of polymers. These products of different emulsion polymerization processes, referred to as latex, are used in a wide range of applications including inks, adhesives, coatings, impact modifiers and tires. For starting and maintaining the polymerization, a free-radical initiator that decomposes to yield initiating free radicals at elevated temperature (60–100°C) is needed, for example sodium peroxodisulfate, organic peroxides, azo compounds, or redox systems active at ambient temperature (for example, the hydrogen peroxide/ascorbic acid couple). Although the latter reaction systems have led to many environmentally friendly materials, there is a continuous pressure to improve the synthesis conditions.
Today, considering the new environmental constraints and legislations, there is an urgent need to find alternative chemistry and processes. Photopolymerization may represent a breakthrough technology by introducing a safer, more compact and energy-efficient polymerization process provided that the problem of light attenuation within the monomer emulsion can be solved. Compared to traditional initiation means, ultra violet (UV) light may offer substantial benefits such as an efficient and fast initiation stage independent of temperature. Nevertheless, light penetration that governs reaction progress has been identified as the main bottleneck to achieve fast polymerization. Recently, several authors have studied the photopolymerization of acrylate [1, 2] or styrene [3] miniemulsions. The use of miniemulsions instead of conventional macroemulsions was found appropriate because of better kinetic stability and smaller droplet size, generally comprised between 40 and 500 nm [4]. Several studies indeed showed that light penetration could be improved by decreasing droplet size, thereby having a significant effect on UV light scattering and reaction kinetics [5–7]. Another distinctive feature of miniemulsion polymerization is a nucleation occurring predominantly inside the nanodroplets provided that any destabilization process is absent.
A different approach to improve light penetration is investigated here, and relies on improving reactor design and photochemical engineering. Our project was to polymerize similar low scattering nanodroplets of monomers (≈100 nm) in a photochemical microreactor providing small channels and efficient mixing. The microreactors may increase the portion of irradiated emulsions thanks to their small optical path length (100–1000 μm). Additionally, microreactors are known to intensify mass and heat transport due to higher surface area to volume ratio than conventional reactors, thus minimizing the problem of light penetration depth [8]. Recently, microreactors proved to be excellent vessels to conduct photochemical reactions including photoaddition [9], photocycloaddition [10], photorearrangement and photoisomerization reactions [11–13]. In organic preparative photochemistry, there is a general consensus that the combination of microscale channels (100–1000 μm) and flow conditions provides significant improvements with regard to efficiency (conversion, yield), safety and waste minimization. However, fewer studies describe the use of UV light to initiate polymerization in microreactor. While a minority of photopolymerization processes proceeds in solution [14], the main benefits of UV technology resides in the preparation of polymer microparticles from monomer microdroplets generated in situ within the microreactor [15, 16]. The tandem emulsification-photopolymerization approach enables to produce a continuous flow of cross-linked particles including core-shell or hybrid particles [17], microspheres [18], Janus or ternary particles exhibiting complex morphology [19].
A different pathway is reported herein consisting in the continuous loading of acrylate miniemulsion inside a single lane microreactor under UV irradiation so that a polyacrylate nanolatex is obtained at the exit. The result is the possibility to implement a novel approach to continuous emulsion polymerization processes. Indeed, conventional routes relying on continuous stirred tank reactors (CSTR) [20–22] or continuous tubular reactors have shown many limitations. CSTR leads to conversion oscillations and broad droplet size distribution leading to unstable conditions [23], while tubular reactors are prone to fouling or plugging [23, 24]. In addition, the resultant latex could have a diameter of about 100 nm and film-forming ability, making it a suitable candidate for conventional latex applications [25]. Such characteristics are not easily achieved by in situ polymerization in a microreactor [26], but rather by emulsification or nanoprecipitation of preformed polymers in this vessel [27]. This study reports results obtained with two compact, flat and energy saving light sources to promote the radical chain photopolymerization: a polychromatic fluorescent UV lamp (280–380 nm) and a monochromatic light-emitting diode (LED) UV light source (λ=395 nm). The influence of different parameters – initial droplet size, monomer and photoinitiator concentration, residence time – on polymerization kinetics, colloidal properties and polymer characteristics are discussed in details.
2 Materials and methods
2.1 Chemicals
Technical grade monomers (Sigma-Aldrich, France) were used as received: methyl methacrylate (MMA), butyl acrylate (BuA), acrylic acid (AA) and octadecyl acrylate (OA). The aqueous phase for the preparation of the miniemulsion was composed of distilled water and sodium dodecyl sulfate (SDS) (Sigma-Aldrich). Two different photoinitiators were provided by BASF Specialty Chemicals (Germany): Irgacure 2959 (PI-1) (2-hydroxy-1-[4-(2-hydroxyethoxy) phenyl]-2-methyl-1-propanone) is water soluble whereas Darocur TPO (PI-2) (diphenyl (2, 4, 6- trimethylbenzoyl)phosphine oxide) is water insoluble.
2.2 Monomer miniemulsion preparation
In the classical recipe (see the structures of the compounds in Table 1), the solid content was 15 wt% and the organic and aqueous phases were prepared independently. The organic phase was composed of the monomer mixture (MMA/BuA/AA 49.5/49.5/1 wt%, respectively) in which the solid OA co-stabilizer (4 wt/wtmonomer%) was dissolved. The aqueous phase was a mixture of distilled water and SDS. The concentration of SDS (0.05–3.5 wt/wtmonomer%) was varied to obtain a range of droplet size comprised between 40 nm and 300 nm. Depending on the experiment, 1–5 wt/wtmonomer% of photoinitiator was added into the aqueous phase (PI-1) or into the organic phase (PI-2). PI-1 and PI-2 were used in combination with the fluorescent and the LED lamps, respectively, to ensure a good match between emission (lamp) and absorption (photoinitiator) spectra. Both phases were stirred during 10 min at 700 rpm. The resultant coarse emulsion was then sonified during 5 min using a Branson Sonifier 450 (450 W/L) while maintaining the stirring (9 output control and 80% duty cycle). The relative stability of the monomer miniemulsions was systematically checked using the back-scattering Turbiscan® technique from Formulaction.
Chemical structures of the components of the acrylate miniemulsion.
| Monomers | MMA |  |
| BuA |  | |
| AA |  | |
| Co-stabilizer | OA |  |
| Surfactant | SDS |  |
| Water-soluble photoinitiator | PI-1 |  |
| Water-insoluble photoinitiator | PI-2 |  |
2.3 Experimental set-up and miniemulsion photopolymerization
The photopolymerization was carried out in a borosilicate glass microreactor (Mr Lab Series – LTF-V, 115×60×6 mm) provided by Little Things Factory GmbH (LTF). Note that borosilicate absorbs below 300 nm, preventing any risk of acrylate monomer self-initiation caused by short excitation wavelength. The LTF-V microreactor is single-lane with 22 straight channels of 1000 μm internal diameter of circular section connected with U-bonds, and a 1.7 ml volume. The total length of the channel is approximately 2 m. The experimental set-up is depicted in Figure 1. The prepared miniemulsion was loaded in a glass syringe (20 ml). The flow was controlled by a syringe pump (KD Scientific) fixing the residence time between 5 and 30 min. The connection tubes were made of ethylene tetrafluoroethylene of 1/8 inches. The syringe containing the monomer miniemulsion and the tubes were covered with aluminum foil to prevent any reaction before entry in the microreactor.
Experimental microreactor set-up.
The small space requirement and the geometry of the microreactor were suitable for an irradiation provided by energy-saving flat lamps. Two different UV lamps were used in this study. First, irradiation was provided by a compact system UV 236 (dimensions: 470×280×100 mm) purchased from Waldmann SA, and including two fluorescent tubes (UV6, 36 W, Philips) placed above the microreactor at about 8 cm. These lamps, used originally for phototherapy applications, cover a broad continuous spectrum, spanning essentially from 280 to 380 nm, with a total irradiance of approximately 3 mW cm-2 measured by radiometry (Sola check from Solatell). Compared to conventional mercury arc lamps, this UV source is distinguished by its very low heat development and low power consumption. A negligible heat release not exceeding 3°C was thus measured after 3 h of irradiation. This is very important to avoid any thermal contribution to the polymerization initiation. In this case, the water-soluble initiator PI-1 was implemented to trigger the photopolymerization. The emission (lamp) and absorption (photoinitiator) spectra are given in Figure 2A.
Emission spectra (solid line) of the two energy-saving light sources.
(A) fluorescent UV lamps and (B) LED UV lamps. On the same plot, the absorption spectrum (dotted line) of the corresponding photoinitiator is shown.
The second light source was a LED unit (dimensions: 94×125×103 mm) designed by Heraeus Noblelight and purchased from UV-consulting Peschl. The system includes three arrays of eight LED lamps emitting monochromatic radiation at 395 nm, and it was used in combination with the photoinitiator PI-2. Their respective emission and absorption spectra are shown in Figure 2B. The lamp was fitted with a cooling fan and connected to a power unit. The drawback of the LED unit was a non-homogeneous distribution of the photon flux reaching the microreactor: a maximum irradiance of 40 mW cm-2 was measured at the reactor center whereas a lower irradiance of 8 mW cm-2 was recorded on the edges. Moreover, there was a significant heat release.
2.4 Miniemulsion and latex characterization
Colloidal and molecular weight data were collected for samples withdrawn after four residences times. The mean droplet diameter (Dd) after sonication and the final nanoparticle diameter (Dp) were obtained by photon correlation spectroscopy using a Zetasizer nano ZS (Malvern Instrument). Samples were diluted to avoid multiple scattering. For droplet size analysis, all measurements were performed within 5 min following the miniemulsion preparation. The droplet size reported corresponds to the z-average diameter or hydrodynamic diameter. Based on these measurements and on the density of the products, the number of particles to droplets ratio, Np/Nd, was given by:
where X is the monomer conversion, ρd and ρp are the density of the monomer (droplet) and polymer (particle), respectively. This ratio provides insight into the nucleation mechanism operating during photopolymerization. The miniemulsion stability was characterized using a Turbiscan instrument (Formulaction). In a typical measurement, 20 ml of freshly prepared miniemulsion was placed in a vial and placed in the instrument. The temporal evolution of the backscattered light irradiance normalized with respect to a non-absorbing standard reflector was assessed for 4 h. Molecular weights and polymer dispersity index (PDI) were determined by size exclusion chromatography (SEC) after precipitation in methanol, filtration and dissolution of the purified polymer in tetrahydrofuran (THF). The SEC column was calibrated with polystyrene standards, implying that all the molecular weight values (Mn) are considered as polystyrene equivalent. Conversion rates were obtained by gravimetric measurements. A drop of a 1 mol% resorcinol solution was added to the sample to prevent further polymerization. Spectrophotometric measurements were performed using a UV-Visible Cary 4000 double beam spectrophotometer equipped with an integrating sphere (Internal DRA 900 Accessory, Agilent). In diffuse reflectance measurements, R, the sample was mounted in the reflectance port and a black polypropylene reference disk was pressed against to avoid reflection and re-entry of photons inside the sample. In addition, diffuse transmittance (Tdiff) was measured by changing the configuration. The forward scattered radiation transmitted by the sample was collected by the sphere using a polytetrafluoroethylene reference disk covering the reflectance port and closing the sphere. For Tdiff and R measurements, the samples were contained in quartz spectroscopic cuvettes of 100 μm optical path length to minimize light losses through the edges of the cell.
3 Results and discussion
3.1 Conditions for achieving stable performance in a microreactor
The microreactor operability (reproducibility and reliability) was first assessed for a miniemulsion based on an acrylate mixture, with 15 wt% monomer content and 90 nm diameter. The miniemulsion was irradiated 10 min (residence time) under a fluorescent UV lamp. The results of this experiment are shown in Figure 3. First, it was proved that conversion profiles and particle size evolution were similar for two runs performed under similar conditions (Figure 3A). Another critical point is that after 2–3 residence times only, the operation of the microreactor led to a steady state, with no fluctuation in monomer conversion (35%) and in particle size (105 nm). Ideally in this type of microreactor, each residence time is a different batch experiment and in the absence of plugging (not observed after at least seven residence times), monomer conversion and colloidal properties should be kept similar along the continuous flow process. Presumably, the kinetic stability of the miniemulsion (proved by back-scattering measurements showing a steady reflectance value during 4 h ageing, Figure 3B) is crucial to achieve stable performance in the microreactor. In conclusion, we have shown that a clogging-free operation could be carried out under these specific conditions. Consequently, these operating conditions [residence time (tR)=10 min] and miniemulsion characteristics (Dd=90 nm, Cmonomer=15 wt%) were chosen as the reference conditions for the rest of the study in which the effects of different parameters were studied (Table 2).
![Figure 3 (A) Evolution of conversion (open symbol) and particle size (full symbols) during the MMA/BA/AA (49.5:49.5:1 wt%) miniemulsion photopolymerization.The data of the first and second runs are presented as rectangles and circles respectively. [OA]=4 wt%, [SDS]=0.35 wt%, Cmonomer=15 wt%. Dd=90 nm, tR=10 min, Irradiance=3 mW cm-2.(B) The colloidal stability of the same monomer miniemulsion was assessed by measuring the reflectance measurements during ageing time in the middle of the sample vial (Turbiscan data).](/document/doi/10.1515/gps-2014-0051/asset/graphic/gps-2014-0051_fig4.jpg)
(A) Evolution of conversion (open symbol) and particle size (full symbols) during the MMA/BA/AA (49.5:49.5:1 wt%) miniemulsion photopolymerization.
The data of the first and second runs are presented as rectangles and circles respectively. [OA]=4 wt%, [SDS]=0.35 wt%, Cmonomer=15 wt%. Dd=90 nm, tR=10 min, Irradiance=3 mW cm-2.
(B) The colloidal stability of the same monomer miniemulsion was assessed by measuring the reflectance measurements during ageing time in the middle of the sample vial (Turbiscan data).
Miniemulsion photopolymerization experiments performed in a microreactor irradiated with the fluorescent UV lamp.
| Entry | Monomer content (wt%) | [PI-1] (wt%) | tR (min) | Dd (nm) | Dp (nm) | X (%) | Np/Nd | Mn (103 g mol-1) | PDI |
|---|---|---|---|---|---|---|---|---|---|
| Ref | 15 | 2 | 10.0 | 90 | 107 | 35 | 0.3 | 28 | 5.2 |
| S1 | 15 | 2 | 10.0 | 40 | 73 | 74 | 114 | 3.0 | |
| S2 | 60 | 78 | 63 | 0.3 | 55 | 2.7 | |||
| S3 | 80 | 84 | 53 | 0.4 | 24 | 5.0 | |||
| S4 | 100 | 111 | 33 | 0.2 | 22 | 5.5 | |||
| M1 | 5 | 2 | 10.0 | 60 | 66 | 83 | 0.5 | 116 | 2.9 |
| M2 | 30 | 60 | 80 | 35 | 0.2 | 22 | 5.5 | ||
| I1 | 15 | 1 | 10.0 | 90 | 97 | 32 | 0.2 | 52 | 3.5 |
| I2 | 3 | 85 | 99 | 37 | 0.2 | 28 | 5.5 | ||
| I3 | 4 | 90 | 103 | 35 | 0.3 | 26 | 5.4 | ||
| I4 | 5 | 95 | 97 | 35 | 0.3 | ||||
| τ1 | 15 | 2 | 5.4 | 94 | 106 | 25 | 0.1 | 23 | 4.0 |
| τ2 | 14.0 | 94 | 115 | 68 | 0.3 | 26 | 4.5 | ||
| τ3 | 20.0 (2 reactors in series) | 90 | 98 | 69 | 0.3 | – | – |
Ref, reference system; S runs, study of the effect of the initial droplet size; M runs, study of the monomer concentration effect; I runs, study of the initiator concentration effect; and τ runs, study of the residence time effect.
3.2 Influence of miniemulsion droplet size
Light penetration inside the microreactor is assumed to be strongly influenced by the initial droplet size of the monomer miniemulsion. This hypothesis was investigated by measuring the diffuse R and Tdiff spectra of acrylate monomer miniemulsions (Figure 4A, B) having different droplet sizes (40–360 nm), and contained in a spectroscopic cell of the same thickness (1 mm) as the channel of the microreactor. Tdiff and R, representing respectively the forward scattered and back scattered radiation, were evaluated through the use of an integrating sphere (see experimental part). In the 250–300 nm wavelength range where acrylate monomers and photoinitiator strongly absorb, both Tdiff and R values are close to 0 regardless of the average droplet size. There was no scattering because all the photons were absorbed. The dependence on droplet size is significant in the 300–800 nm range where PI-1 absorption is moderate (Figure 2A). For droplets comprised between 40 and 150 nm, any diameter increase resulted in an enhancement of back-scattering R and consequently in a decrease of Tdiff. However, above a threshold size of 150 nm, there was no significant effect of droplet size on R or Tdiff. With these larger miniemulsions, multiple scattering dominated the two spectra, and it was impossible to distinguish the miniemulsions of different droplet sizes. As droplet size decreased below 150 nm, more light could penetrate through the miniemulsion, suggesting that this size range is the most suitable for photopolymerization.
Diffuse reflectance (A) and diffuse transmittance (B) spectra of a series of acrylate miniemulsions containing PI-1, and displaying different average droplet diameters.
● 50 nm, ∆ 70 nm,
Five monomer miniemulsions differing in their initial droplet size (40–100 nm diameter) were then prepared (Ref and S1–S4). Photopolymerization experiments were performed in the microreactor with a residence time of 10 min. All colloidal properties, conversions and molecular weight data are listed in Table 2. Decreasing droplet size led to higher conversions. The improved light penetration can account at least partially for such acceleration. However, it is impossible to consider the enhanced UV transparency as the single reason for improvement, as compartmentalization can also contribute to promote faster polymerization kinetics. After only 10 min of irradiation, a conversion of 74% was achieved in the miniemulsion with the smallest droplet size [average diameter of 40 nm (S1)], but this conversion dropped to 35% in the reference experiment having droplets of 90 nm (Ref). With regard to colloidal properties, the particle size turned out to be higher than the initial droplet size regardless of the initial size. The result was a small Np/Nd indicative of a low efficiency of droplet nucleation, because of the high initial number of droplets implying that the nucleation of all these droplets was not favored. Finally, average molecular weight increased consistently with conversion. The molecular weight values were relatively low, suggesting the generation of relatively short polymer chains.
3.3 Effect of photoinitiator concentration
Other parameters can play a role on the light penetration, such as the photoinitiator concentration that was varied between 1 and 5 wt% in the experiment series I1–I4, keeping tR=10 min and the droplet size unchanged (90 nm). Photoinitiator can induce a light screening effect because of its strong UV absorption. However, the results summarized in Table 2 show that the PI concentration had only a small effect on conversion that remains close to 35%, while it leads to a drastic decrease of the molecular weights. A higher concentration in initiating radicals may promote primary radical termination.
3.4 Effect of monomer concentration
Monomer concentration is another parameter that may have a significant effect on light penetration. Three different monomers contents (5, 15 and 30 wt%) were assessed (entry S2 and M1–M2, Table 2). In each case, a similar droplet diameter of 60 nm was chosen, while the other parameters were kept unchanged. Decreasing monomer concentration for a similar droplet size causes the decrease of scattering. As the density of droplets decreased, the absorbed photon flux was increased. At a constant irradiation time of 10 min (tR), the conversion was almost complete at 5 wt% of monomer content, whereas it was limited to 35% at a concentration of 30%. Additionally, much higher molecular weights were obtained upon decreasing monomer concentration.
3.5 Influence of residence time
In a continuous-flow reactor, the irradiation can be easily adjusted by varying tR. In a set of experiments (Ref and τ1–τ3), tR was changed by adjusting the flow rate of the pump or with two microreactors connected in series. As expected, higher conversions were achieved upon decreasing the flow rate. Indeed, increasing tR led to an increased absorbed photon flux and therefore to an increased radical concentration. Despite greater conversions, the molecular weights remained unchanged. Nevertheless, when tR was too long (30 min, not reported in Table 2), clogging occurred systematically because particle-wall interactions were enhanced as the flow rate was too low [26].
3.6 Influence of the UV light source
A second flat irradiation source was used in this study, instead of the fluorescent lamp. The same acrylate mixture miniemulsions were irradiated under a LED unit (λ=395 nm). For these experiments the water-soluble photoinitiator (PI-1) was also replaced by a water-insoluble photoinitiator (PI-2) dissolved in the organic phase during the miniemulsion preparation. The change of initiator was necessary to ensure a good match between the lamp emission spectrum and the absorption spectrum of the photosensitive compound (Figure 2). The results and operating conditions are given in Table 3 (entry L1–L2). At a tR of 30 min and using an acrylate miniemulsion of 50 nm diameter, clogging took place inevitably after 3–4 residence times. The instability of this particular miniemulsion containing the PI-2 may account for such result (Figure 5). Unfortunately, decreasing the concentration of the photoinitiator from 4 to 0.5 wt% did not bring up any improvement (entry L2, Table 3).
Back-scattering measurements of the MMA/BA/AA (49.5:49.5:1 wt%) miniemulsion containing PI-2, reflecting its physical stability.
Results of photopolymerizations experiments using the LED UV lamp.
| Entry | Monomer content (wt%) | [PI-2] (wt%) | tR (min) | Dd (nm) | Dp (nm) | X (%) | Np/Nd |
|---|---|---|---|---|---|---|---|
| L1 | 15 | 4.0 | 30 | 50 | 56.4 | 40 | 0.4 |
| L2 | 15 | 0.5 | 30 | 50 | 54.0 | 48 | 0.3 |
4 Conclusion and outlook
Microreactors have already proved to be excellent vessels to conduct a number of photochemical organic reactions. This study extends their use to polymer nanoparticle synthesis via miniemulsion photopolymerization. Such waterborne UV-mediated polymerization performed at ambient temperature is in line with the main green process principles [28]. The resultant combination of low scattering nanodroplets and microreactor attributes – narrow microchannels and short diffusion path – offer an efficient way to photopolymerize turbid monomer emulsions. In this study, the conditions to achieve a steady state of microreactor operation with an acrylate mixture miniemulsion were established, and the influence of various parameters on the polymerization course were then screened. The role of droplet size and monomer concentration on polymerization rate was emphasized, reflecting the significant effect of UV light penetration. In addition, the versatility of microreactors enabled an easy variation of different processing parameters (residence time, flow rate). However, in most cases, the problems of limited conversions and fouling were found. Productivity limitation prevents microreactor from appearing as a viable alternative to common semi-batch polymerizations in stirred tank reactor. By contrast, it may offer a credible alternative for the polymerization of specific monomer emulsions whose polymerization cannot be carried under conventional conditions. The second issue of reactor fouling during the polymerization is an obstacle limiting the general use of microreactor for all emulsion polymerization processes. It seems that the colloidal stability of the monomer miniemulsion fed into the reactor is essential to avoid the clogging and fouling issue. The surface properties of miniemulsion systems and reactor walls are also very important in the clogging issue [29]. We foresee that the value of photopolymerization in microreactor could be raised in the future by focusing on highly exothermic monomers in the form of stable miniemulsions.
About the authors
Emeline Lobry obtained her PhD degree in 2012 at Toulouse University (Chemical Engineering Laboratory, LGC), France. Her PhD was centered on the “batch to continuous transposition of vinyl chloride suspension polymerization”. She joined the Laboratory of Photochemistry and Macromolecular Engineering (University of Haute Alsace, France) as a post-doctoral fellow. Her research is focused on monomer miniemulsion photopolymerization.
Florent Jasinski studied photopolymerization at the Laboratory of Photochemistry and Macromolecular Engineering in Mulhouse (France) and at the Institute of Physics and Chemistry of Materials in Strasbourg (France). He graduated in polymer chemistry at the University of Haute Alsace (France) in 2011. He is currently finishing his PhD thesis at the University of Haute Alsace on the synthesis of nanolatexes via photopolymerization in miniemulsion.
Marta Penconi graduated in 2008 and obtained her PhD in chemical sciences from the University of Perugia (Italy) in 2012. During her PhD, her main research interests included the development of novel photocatalysts for water splitting and the study of the up-conversion process based on triplet-triplet annihilation. She joined the IMRCP Laboratory at the University Paul Sabatier in Toulouse (France) as a post-doctoral fellow and her research has focused on the polymerization in miniemulsions under UV irradiation for the production of synthetic latexes.
Abraham Chemtob has authored 45 articles in refereed international journals and three book chapters. He obtained his PhD in macromolecular chemistry in 2003 at the University of Bordeaux (France) on the development of ring-opening metathesis polymerization in dispersed medium. After two postdoctoral internships in world-class institutions (University of Sydney and University of Basque Country), he became in 2006 an assistant professor at the Laboratory of Photochemistry and Macromolecular Engineering (University of Haute Alsace, France). His research spans a number of areas including self-assembled hybrid materials, colloidal polymer science and photoinitiated polymerizations.
Céline Croutxé-Barghorn graduated in chemistry from the University of Bordeaux (France) and the Technische Hochschule Darmstadt (Germany). She obtained her PhD in physical chemistry in 1996 at the University of Haute-Alsace (France) on the use of photopolymers for the generation of optical elements. She is currently a professor at the University of Haute-Alsace and the head of a research group in the Laboratory of Photochemistry and Macromolecular Engineering. Her present research interest is the study of photopolymerization processes in hybrid sol-gel films and nanocomposites, and their characterization for specific applications (coatings or bulk materials).
Esther Oliveros received her degree as a chemical engineer from the Ecole Nationale Supérieure de Chimie de Toulouse (France) in 1970. In 1972, she joined the Centre National de la Recherche Scientifique (CNRS) and started her research work in organic and mechanistic photochemistry in the Laboratoire des Interactions Moléculaires et Réactivité Chimique et Photochimique of the University Paul Sabatier in Toulouse. She received her doctoral degree (Doctorat ès Siences Physiques) in 1977 and her habilitation in 1986. From 1988 to 1992, she was a research scientist at the Ecole Polytechnique Fédérale de Lausanne in Switzerland, then she moved to Germany and worked as a lecturer, research adviser and professor (2003) at the Lehrstuhl für Umweltmesstechnik at the University of Karlsruhe (now Karslruhe Institute of Technology, KIT). She was an Associate Editor of the Journal of Photochemistry and Photobiology (B: Biology) from 1993 to 2001 and of Photochemical and Photobiological Sciences during its first year of publication (2002). In January 2007, she moved to her former Laboratory in Toulouse to head the group “Photobiology, Environment and Photochemistry”. Her main research interests include photooxidation reactions and singlet oxygen sensitization by molecules of biological interest and mechanistic studies of advanced oxidation processes applied to the treatment of water and air. She is a co-author of two books on photochemical technology and published 180 articles and book chapters, including several reviews on water treatment by photochemical methods.
André M. Braun studied chemistry at the Universität Basel, Switzerland (1961–66) and obtained a PhD degree (Physical Organic Chemistry, C.A. Grob, 1966). After several postdoctoral positions (Universität Basel, California Institute of Technology and Yale University), he became a project leader (J.R. Geigy AG and Ciba-Geigy AG, Basel 1969–77) and privat-docent (Ecole Polytechnique Féderale de Lausanne, Switzerland, 1977–91). He held the Hewlett-Packard Chair (Techniques of Environmental and Process Analysis, 1992–2006), and was then a research chemist at Universität Karlsruhe, Germany, and a member (1993–2001) and President (1995–2001) of the DAAD-PROCOPE Binational (Germany-France) Commission of Project Evaluation. At present, he is the head of Quantapplic, Mainz, Germany (Scientific and technical consulting in the domain of photochemical technology). His research interests are mechanistic photochemistry, photochemical technology: industrial projects for synthesis, product engineering and environmental protection. He has authored two books and some 260 publications. He has supervised or co-supervised 35 PhD theses, as well as 60 Diploma theses.
Adrien Criqui obtained his Engineering degree from the Engineering School of Chemistry of Mulhouse (ENSCMu) and a Master degree in the field of surfaces and interfaces for polymers and colloids (UHA). His Master thesis was on “colloidal stabilization of hydrophobic polymer particles in a photopolymerisable monomer”, under the supervision of Professors G. Riess and C. Delaite. In 2010, he received his PhD at the Department of Photochemistry (LPIM, Mulhouse) under the supervision of Professors X. Allonas, J. Lalevée and J.-P. Fouassier. His PhD was centered on “new radical photoinitiators systems for UV curing of coatings under air”, with the collaboration of MÄDER (CIFRE contract). He joined MÄDER in 2010 and worked on the creation of MÄDER RESEARCH (2011), the long term research center of MÄDER based in Mulhouse. As the responsible person for coating innovation, one of his main research topics is the elaboration of innovative (photo)polymerization systems for varnishes, paints or composites.
Acknowledgments
The authors gratefully acknowledge Little Things Factory (Germany) for the microreactor. Emeline Lobry and Marta Penconi thank the French National Research Agency (ANR, Programme Chimie Durable-Industries-Innovation) for their post-doctoral grants under contract number ANR-2012-CDII-006-02. This work has been funded by the Agence Nationale de la Recherche, (Grant/Award Number: ‘ANR-CD2I’).
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