Acrylate Nanolatex via Self-Initiated Photopolymerization

Florent Jasinski,1 Emeline Lobry,1 L�ena€ıg Lefevre,1 Abraham Chemtob,1

C�eline Croutxe-Barghorn,1 Xavier Allonas,1 Adrien Criqui2

1Laboratory of Photochemistry and Macromolecular Engineering, ENSCMu, University of Haute-Alsace, 3 bis rue,

Alfred Werner 68093 Mulhouse Cedex, France2M€ader Research - M€ADER GROUP, 130 rue de la Mer Rouge, 68200 Mulhouse, France

Correspondence to: A. Chemtob (E-mail: abraham.chemtob@uha.fr)

Received 3 February 2014; accepted 22 March 2014; published online 00 Month 2014

DOI: 10.1002/pola.27190

ABSTRACT: The use of UV light to initiate emulsion polymeriza-

tion processes is generally overlooked, whilst extensive litera-

ture exists on photocuring of monomer films. In this study, the

unique potential of UV light to produce at ambient temperature

polyacrylate latexes without initiator was exploited. Although

radical initiators are utilized at low concentration, their cost,

toxicity, and odor provide incentives for finding alternatives.

Starting with concentrated (30 wt %) and low scattering acry-

late miniemulsions (droplet diameter <100 nm), it was demon-

strated that acrylate self-initiation can promote an efficient and

fast photopolymerization in micrometer-scale reactor (spectro-

photometric cell) and lab-scale photoreactor. Herein, all kinetic,

colloidal, and mechanistic aspects involved in the self-initiation

of acrylate miniemulsion were extensively examined to provide

a complete picture. In particular, the effects of droplet size, ini-

tiating wavelength, optical path, and irradiance on the course

of the polymerization were thoroughly discussed. A diradical

self-initiation pathway is the most likely mechanism. VC 2014

Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem.

2014, 00, 000–000

KEYWORDS: photopolymerization; emulsion polymerization; ini-

tiators; colloids; self-initiation; miniemulsion

INTRODUCTION Over the last 40 years, miniemulsion poly-merization has resulted in the generation of many polymercolloidal structures, currently unattainable by other hetero-phase processes. The salient point of this technique is thevirtual lack of monomer transport across the aqueousphase.1 As a result, nucleation takes place preferentially inthe monomer droplets, making miniemulsion amenable tomany unconventional applications including water-sensitivereactions (ionic,2 catalytic,3 step-reaction polymerizations4),controlled radical polymerizations,5,6 encapsulation ofliquids,7 preformed polymers,8 or inorganic particles,9 andmany others.10–12 Recently, several examples of photoiniti-ated miniemulsion polymerizations driven by UV light havebeen very successful.13–22 The key feature here is the smallersize of the miniemulsion droplets (40–500 nm) comparedwith macroemulsion systems (500 nm–50 mm), leading to abetter light penetration and an optimized absorption/scatter-ing ratio. Additionally, photochemical initiation has providedunique capabilities as regards to temporal control of poly-merization20 and implementation of continuous operationmodes,17 not easily achievable with conventional thermalprocesses. Another distinguishing feature is ambient temper-ature polymerization, thus reducing energy consumption,

destabilization of the monomer miniemulsion, and risks ofrunaway in batch reactors. As outlined above, photopolyme-rization in miniemulsion suggests tremendous potentialitiesand innovative applications.

In this study, we explore the potential of this process to pro-duce latexes in the apparent absence of external initiator,through a spontaneous self-initiated photopolymerization.Except for styrene and alkyl acrylates at high temperature(>100 �C), most radical monomers when exhaustively puri-fied cannot undergo a purely thermal self-initiated polymer-ization.23,24 As a result, thermal initiators or redox systemsare indispensable ingredients to bring about a radical chainpolymerization in aqueous dispersed media where reactiontemperatures are generally limited.25 In contrast, a muchbroader range of monomers are able to self-initiate at ambienttemperature under UV exposure. Several thiol-ene26 and elec-tron donor–acceptor systems27 have demonstrated their self-photoinitiation ability as well as some specific vinyl28 or bro-minated acrylates.29 Recently, even simple alkyl acrylate andmethacrylate systems have proven to polymerize without pho-toinitiator through deep UV irradiation provided by excimerlamp30–33 (172 or 222 nm) or even medium-pressure mercury

Additional Supporting Information may be found in the online version of this article.

VC 2014 Wiley Periodicals, Inc.

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arc lamps (250–600 nm).34,35 In the continuity of these latterworks focused on UV curing; we show that miniemulsion pho-topolymerization can be used as an efficient method to makinginitiatorless water-based acrylate dispersions. In polymerindustry, acrylate latexes are currently employed at large scalein diversified customer markets ranging from coatings toadhesives.36 In addition to reducing cost and odor, the elimina-tion of initiator could open up new opportunities for medical,food, or microelectronic applications in which nontoxic poly-mer materials are in great demand.37 A final polymer filmwithout initiator residues may exhibit a decreased tendency toyellowing and sunlight degradation, therefore, providing thebenefits of a durable material suitable for outdoor applica-tions, which is rare in photopolymer materials.37 With regardto process, initiator-less monomer miniemulsions are likely tohave a prolonged colloidal stability. In addition, since light isattenuated only by droplet scattering and absorption of mono-mers rather than photoinitiator molecules, enhanced polymer-ization depth may be achieved when the extinctioncoefficients of monomers are not too high.

Following an initial feasibility study,19 we investigate exhaus-tively in this paper all the kinetic, colloidal and mechanisticaspects of initiator-free acrylate photopolymerizations usinghigh-solid content miniemulsions of 30 wt % required forcommercial applications. The dependence on initiating wave-length, droplet size, optical path, and irradiance has beenreviewed thoroughly as well as the critical issue of monomerminiemulsion stability. Photopolymerizations have been per-formed in small volume spectroscopic cell, and then scaled-up in an annular photoreactor. In a last part, the mechanismof initiation in spontaneous photoinduced polymerization ofacrylate and methacrylate is discussed. To date, there hasbeen a lack of evidence to draw any distinct conclusionabout the acrylate self-photoinitiation mechanism.

EXPERIMENTAL

MaterialsAll miniemulsions were prepared with distilled water. Technicalgrade monomers, butyl acrylate (BA), methyl methacrylate(MMA), and acrylic acid (AA), n-butyl methacrylate (BMA),methyl acrylate (MA) and ethyl acrylate (EA) were supplied byAldrich and used without further purification. Sodium dodecylsulfate (SDS, Aldrich) was used as received. Stearyl acrylate (SA,Aldrich) was added in the formulation as a reactive costabilizer.2-Methyl-1-[4-(methylthio)phenyl]-2-(4-morpholinyl)21-prop-anone (I 907) from BASF was used as an oil-soluble photoinitia-tor, benzophenone (BP, Aldrich), and thioxanthone (TX, Aldrich)as photosensitizers. Thermal polymerizations were performedwith potassium persulfate (KPS, Aldrich). In electron spin reso-nance experiments, tert-butylbenzene (TBB, Aldrich) was usedas inert solvent and 5,50-dimethyl-1-pyrroline-N-oxide (DMPO,Aldrich) as spin trap.38

Preparation of Monomer MiniemulsionsAn organic phase was first prepared by adding SA (4 wt %with respect to the monomer phase) to the monomer acry-

late mixture (BA/MMA/AA, 49.5/49.5/1 wt %). The aqueousphase was prepared separately by dissolving SDS in distilledwater (3.5 to 0.15 wt % with respect to the monomerphase). The weight concentration of the monomer phaselabeled as Cmonomer was kept constant at 30 wt % in allexperiments. Both phases were mixed for 10 min using amagnetic stirrer at 600 rpm. The coarse emulsion was thenhomogenized for 5 min with the aid of a Branson Sonifier450 (450 W/L) at 90% amplitude, while maintaining theagitation.

Initiatorless Miniemulsion Photopolymerizationin Spectroscopic Quartz CellIn a typical procedure, the photopolymerization of the mono-mer in the miniemulsion was carried out in a capped quartzrectangular cell (1 mm thick, 340 mL volume) without nitro-gen bubbling and stirring. Irradiation was applied with thepolychromatic light of a medium-pressure Hg–Xe arc lamp(Hamamatsu L8252, 200 W) coupled to a flexible light-guide.A picture of the illumination set-up is given in SupportingInformation Figure S1 in the Additional Supporting Informa-tion (ASI). The lamp is backed by a semi-elliptical mirror orreflector to focus radiation and minimize irradiance loss. Inthis study, a 254 or 365 nm reflector was used, each oneenhancing the reflection of the mentioned wavelength. Theend of the optical guide was placed at a distance of 4.2 cmfrom the sample and directed at an incident angle of 90� . Inthe spectral region below 300 nm in which acrylate mono-mers absorb, the light irradiance was respectively 150 mWcm22 (254 nm reflector) and 100 mW cm22 (365 nm reflec-tor). This irradiation set-up was used for the kinetic analysisof the polymerization by real-time Fourier transform near IRspectroscopy (RT-FTNIR) described in the characterizationsection. For comparison, thermally induced polymerizationswere also performed by heating the same spectroscopic cell(70 �C) containing the miniemulsion inside an environmentalchamber. After photopolymerization, the resulting latex wasprecipitated in methanol. After filtration and washing, thesolid polymer was then dissolved in filtered and distilled tet-rahydrofuran for molecular weight analysis.

Initiatorless Miniemulsion Photopolymerizationin an Annular UV ReactorThe annular photoreactor (UV-Consulting Peschl) shown inFigure 1 is composed of three parts: first, a standardmedium-pressure Hg arc lamp (Heraeus Noblelight TQ 150,arc length: 4.4 cm) emits a series of rays from 250 to 600nm (emission spectrum in Supporting Information Figure S2of ASI). This lamp is housed in a fused quartz sleeve offeringan excellence transmittance in the UV region down to ca.200 nm, which is essential to monomer excitation and self-initiation. Because of the heat liberation during lamp proc-essing, an external cooling jacket surrounds the sleeve vesselin order to hold the photoreactor contents at a temperaturebetween 20 and 25 �C throughout the polymerization reac-tion. Third, a borosilicate cylindrical section (outer annulus)is then installed around the sleeve to accommodate and irra-diate 300 mL of monomer miniemulsion when the reactor is

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full. The distance between the two annular sections (externalsleeve wall and inner reactor wall) is 9 mm and defines theoptical path length. Magnetic stirring of the miniemulsion ismaintained throughout the polymerization. Prior to the UVexposure, the monomer miniemulsion was degassed for 30min through nitrogen bubbling, which was also maintainedduring the irradiation process. Samples were drawn at differ-ent time intervals in order to determine the polymerizationkinetics (FTNIR spectroscopy) and the evolution of molecu-lar weights. Polymerization was stopped when a completeconversion was achieved or after a maximum exposure timeof 6 h.

MethodsColloidal CharacterizationThe miniemulsion stability was characterized using a Turbis-can instrument (Formulaction). In a typical measurement, 20mL of freshly prepared miniemulsion was placed in vial andplaced in the instrument. The temporal evolution of thebackscattered light irradiance normalized with respect to anon-absorbing standard reflector was assessed during 4 h.

The evolution of the backscattered light (at 45� from theincident beam) was determined by scanning the entire vol-ume of the miniemulsion from the bottom to the cap of thecell.39 Measurements were typically carried out at 25 �C atthe middle of the miniemulsion (�20 mm). Droplet and par-ticles diameters, respectively labeled as Dd and Dp, weredetermined through dynamic light scattering (DLS) with aZetasizer Nano ZS (Malvern Instrument). Typically, themonomer miniemulsion or the resultant latex wasdiluted 125 times in filtered and distilled water beforemeasurement.

Polymer Characterization and Reaction KineticsAcrylate conversion was followed in situ by RT-FTNIR. Inthese rapid scan experiments (temporal resolution: 0.5 s,spectral resolution: 4 cm21), a NIR probe beam and a UVexciting beam irradiated simultaneously the spectroscopiccell containing the miniemulsion (see Supporting InformationFigure S1 in ASI). The NIR region (k 5 970–1940 nm) is wellsuited to thick samples (1000 mm) and can accommodatethe high water concentrations of monomer miniemulsionswithout detector saturation. In this spectral range, the acry-late monomers exhibit a band at 6170 cm21 which isassumed to be a combination of two C–H stretching bands.40

The band is isolated enough from the other vibrational over-tones of water so that it can be used for quantitative pur-poses. Thus, polymerization kinetics was followed in situusing RT-FTIR by calculating the integrated absorbance ofthis band and monitoring its decrease during irradiation.The limited signal to noise ratio, especially at high conver-sion, provided motivation to fit the conversion–time plotsusing a sigmoidal function described in reference.41 Molecu-lar weights were determined by gel permeation chromatog-raphy (GPC). The GPC column was calibrated withpolystyrene standards, implying that all the molecular weightvalues (Mn) are considered as polystyrene equivalent.

Electron Spin Resonance (ESR) Spin Trapping ExperimentsESR spectra were measured with an X-band spectrometer(Miniscope 200 spectrometer, Magnettech). The acquisitionwas performed in 4 mm diameter capillary with 5 accumu-lations and a 400 gain. Sample solutions contained 9 3

1023 mol L21 of DMPO and 0.445 mol L21 of an acrylatemonomer. After 15 min bubbling with argon, the spectrumof the solutions were recorded following 100 s of UV irradi-ation with the same Hg–Xe lamp (I250–300 5 100 mW cm22)used for the photopolymerization in spectrophotometric cuv-ettes. The main issue arising from the acrylate and methac-rylate study in ESR is the very short lifetime of thegenerated radicals (<20 ms). In ESR spin trapping experi-ments, short-lived radicals were reacted with appropriatetrapping agents and converted to relatively stable nitroxideradicals (spin adducts). For this study, DMPO was used asspin trap to detect the propagating radicals. The structureof the trapped radicals were deduced or extracted from theanalysis of the ESR spectrum of the spin adduct through itshyperfine coupling constants involving nitrogen (aN) andhydrogen (aH).

FIGURE 1 Schematical view of the immersion-type photo-

chemical reactor. UV irradiation is provided by a medium-

pressure Hg arc lamp housed in a vertical arrangement of

immersion and cooling tubes.

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RESULTS AND DISCUSSION

Monomer Miniemulsion MetastabilityFour initiatorless acrylate miniemulsions (MMA/BA/AA/SA)with diameters ranging from 40 to 115 nm were preparedusing different SDS concentrations, but at a constant organicphase content (30 wt %) and costabilizer concentration (SA,4% wt/wtmonomer). As shown in Figure 2, the average drop-let diameter declines with higher surfactant concentrationsdue to the increase in surfactant interfacial area anddecrease in interfacial tension. The generation of nanosizeddroplets (Dd< 100 nm) is important to reduce UV lightattenuation caused by scattering, but very small droplet sizescan result also in stability problems.42 Although their poordeformability makes them relatively insensitive to coales-cence,43 the main source of instability in nanoemulsions isgenerally Ostwald ripening (diffusional degradation) arisingfrom the difference in Laplace pressure (chemical potentials)between droplets of different sizes.44,45

The stability of these monomer miniemulsions (40, 75, 90,and 115 nm) was examined by monitoring the temporal evo-lution of the diffuse reflectance R (fraction of incident lightreflected back at the sample interface). Figure 3 is a plotrepresenting R in the sample middle and at ambient temper-ature as a function of the ageing time (4 h). As expected, weclearly see that miniemulsions having the largest dropletsexhibit greater values of R due to a stronger scattering effi-ciency. But more importantly, all monomer miniemulsionshaving an average diameter larger than 75 nm were stablewithin the first 4 h of analysis, as suggested by a relativelysteady R during this period. As expected, such metastabilityis strongly dependent on the presence of costabilizer mole-cules in the monomer phase. As an evidence, the removal ofSA from the initially stable 90 miniemulsion led to a rapiddestabilization (see � in Fig. 3 and also Supporting Informa-

tion Figure S346). The case of the smallest miniemulsion(Dd 5 40 nm) prepared with the highest concentration inSDS (3.5% wt/wtmonomer) is particular since slight signs ofdestabilization were observed even in the presence of SA.Two factors are responsible for the enhanced Ostwald Ripen-ing. (i) The size distribution is broader in this case as dis-played in Figure 2, whereas other miniemulsions appearmore monodisperse (note that strictly monodisperse sampleswill not exhibit diffusional degradation). The increase in pol-ydispersity with decreasing miniemulsion size has beenalready reported in the literature47 and related to micellesformation or the technical limitations of a sonicator-inducedemulsification. (ii) Additionally, the high concentration inSDS may drive the migration of monomer molecules fromthe smaller droplets, across the aqueous phase, into thelarger droplets.48 Indeed, if too much surfactant is used, theexcess present in the aqueous continuous phase can enhancethe rate of monomer transfer between the droplets.

Proof of Acrylate Self-Photoinitiation in MiniemulsionFigure 4 compares the absorbance spectra of monomer mini-emulsions (Dd5 40, 75, and 115 nm) with that of an equiva-lent solution prepared in acetonitrile at similar monomerconcentration (30 wt %). As expected, monomer miniemul-sions had higher absorbance values which tend to increasewith droplet size owing to an enhanced scattering. The solu-tion system reveals that the acrylates absorption range startsbelow 280–300 nm, allowing them to match at least partiallythe emission spectrum of the Hg–Xe arc lamp plotted on thesame graphic. Although the overlap concerns only the

FIGURE 2 Droplet size distribution obtained by DLS for acry-

late monomer miniemulsions including different weight con-

centrations in SDS: 3.5% (Dd 5 40 nm, W), 0.75% (Dd 5 75 nm,

�), 0.5% (Dd 5 90 nm, �), 0.25% (Dd 5 115 nm, �).

Cmonomer 5 30 wt %, organic phase: MMA/BA/AA/SA.

FIGURE 3 Monomer miniemulsions stability assessed by the

reflectance measurements during ageing time in the middle of

the sample vial (Turbiscan data). The miniemulsions comprise

different weight concentrations in SDS: 3.5% (Dd 5 40 nm, W),

0.75% (Dd 5 75 nm, �), 0.5% (Dd 5 90 nm, �) and 0.25%

(Dd 5 115 nm, �). Full symbols are for miniemulsion containing

SA (4% wt/wtmonomer) and open symbols are costabilizer-free

systems. All droplet size data refer to “stable” miniemulsions

containing SA. Cmonomer 5 30 wt %, organic phase: MMA/BA/

AA/(SA).

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shortest wavelengths (250–300 nm), there are strong incen-tives to utilize mercury arc lamps, which are the workhorseof the photopolymerization in industry, and existing in avariety of size, geometry, and power at limited cost.

Figure 5 shows graphically the acrylate conversion of a 40nm miniemulsion without initiator as a function of irradia-tion time under different irradiation conditions. All kineticcurves were obtained by RT-FTNIR spectroscopy allowing

the real-time acquisition of NIR spectra during the irradia-tion process (see characterization section). A complete con-sumption of monomer was achieved in less than 6 min withthe Hg–Xe lamp fitted with a 365 nm reflector providing anirradiance of 100 mW cm22 in the 250–300 nm range. As aproof of concept demonstrating the self-initiation ability ofacrylates, the addition of a cutoff filter blocking the wave-lengths below 300 nm impedes the course of the polymer-ization. Because of differences in light absorption, thepolymerization rate is also predicted to be highly dependenton the wavelength of the irradiating light. As seen in Figure4, acrylates absorb light much more readily at lower wave-lengths. Accordingly, a 254 nm reflector emitting a higherirradiance of 150 mW cm22 (250–300 nm) induced a greaterradical generation rate, thereby increasing polymerizationrates. Hence, the same acrylate miniemulsion reached a fullconversion in 4 min instead of 6 min. Only faster kineticswas achieved when a water-insoluble photoinitiator (a-ami-noacetophenone radical) was added to the monomer organicphase. In this case, initiating radicals originate both fromacrylates excitation and initiator a-cleavage.

Noteworthy is that temperature does not increase more than10 �C above ambient temperature during irradiation, thusindicating that the polymerization is primarily photoinitiated.Such result is confirmed in Figure 6 by the absence of poly-merization upon heating the miniemulsion at 70 �C withoutirradiation. In contrast, the addition of KPS, a conventionalwater-soluble peroxide initiator, triggers a thermal initiatedpolymerization at 70 �C. However, the polymerization ismuch slower in this case compared to a photoinitiated path-way. It is assumed that faster initiation rates are promotedin acrylate self-initiated polymerization compared to KPSexhibiting a slow decomposition rate at 70 �C (half life of 10h at this T).

FIGURE 4 Emission spectra of the Hg–Xe arc lamp fitted with a

365 nm reflector (solid) or a 254 nm reflector (dot). In this latter

case, the light output irradiance below 300 nm is increased.

Absorption spectra of acrylate monomer miniemulsions with

four different droplet sizes: 40 nm (W), 75 nm (�), 90 nm (�),

and 115 nm (�). For comparison, the absorption spectrum of

an acrylate monomer solution in acetonitrile at similar concen-

tration is provided (*). Cmonomer 5 30 wt %, organic phase:

MMA/BA/AA/SA, 1 mm thick quartz cuvette.

FIGURE 5 Conversion–time curves of an initiatorless miniemul-

sion (Dd 5 40 nm) irradiated by the medium-pressure Hg–Xe

lamp fitted with a 365 nm reflector (1), a 254 nm reflector (3)

or a 300 nm cutoff filter (�). For comparison, the photopolyme-

rization kinetics (365 nm reflector) of the same miniemulsion

containing I907 as oil-soluble PI (2% wt/wtmonomer) is also dis-

played (*). Cmonomer: 30 wt %, 1 mm thick quartz cuvette.

FIGURE 6 Acrylate conversion evolution for acrylate miniemul-

sions heated at 70 �C without initiator (W) and with water-

soluble KPS initiator (�). The reference PI-free miniemulsions

irradiated using a Hg–Xe lamp (365 nm reflector) is also repro-

duced for comparison (3). Cmonomer 5 30 wt %, [Initiator] 5 2%

wt/wtmonomer, 1 mm thick quartz cuvette.

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Effect of Different Experimental Parameters onInitiatorless Acrylate PhotopolymerizationEffect of Droplet SizeFigure 7 illustrates the strong dependency between polymer-ization rates and miniemulsion droplet size. The reactionrates are fairly enhanced upon decreasing the average drop-let diameter by the following factors:

(i) Light penetration depth through the miniemulsion isimproved when decreasing droplet size (Fig. 4). A larger vol-ume of miniemulsion is thus illuminated, enabling a greaterfraction of monomer droplets to be initiated. Optically, bothmonomer absorption and droplet scattering impact the pene-tration of light inside the reaction medium. Recent spectro-photometric studies on acrylate miniemulsion have provedthat change in droplet diameter, in the range 40–300 nm,affects scattering, but not absorption.22

(ii) The decrease in droplet size is known to favor a radicalcompartmentalization effect.49 Very small droplet sizes(<100 nm) imply a high number of droplets, which favorsideally the confinement of a single radical in one droplet. Asthe termination reaction between macroradicals present indifferent entities becomes impossible, higher radical concen-tration, and higher polymerization rates are promoted whendecreasing droplet diameter. In our case, we cannot obvi-ously separate the two optical and chemical contributions.

Colloidal and molecular weight data were gathered in Table1. Regardless of the initial droplet size, all the self-initiatedsystems were characterized by a low droplet nucleation effi-ciency as reflected by a ratio of the number of particles tothe droplets number (Np/Nd) much lower than unity. Theuse of very small droplets implies some similarities with thenucleation mechanism operating in monomer microemulsion.Given the huge number of droplets initially formed, only afraction are effectively initiated and converted into particles,

while the rest serves as reservoirs and feed monomer, acrossthe aqueous phase, to polymerizing particles. Surprisingly,increasing droplet size results only in a slight increase of theNp/Nd ratio. This is because the light penetration is alsomore attenuated under these conditions. A stronger back-scattering spatially limits initiation to a small fraction ofdroplets in the close vicinity of the illuminated window. As aresult, the only way for the other monomer droplets to beconsumed is by monomer transfer to the illuminated areasor monomer diffusion through the aqueous phase.

The dependency of molecular weight on droplet diameters(Table 1) supports such reasoning. A larger droplet sizemakes the miniemulsion medium more scattering; less drop-lets are thus nucleated resulting in larger molecular weights.Furthermore, at very small droplet sizes (40 nm), relativelylow molecular weights are produced (Mn 5 30,000 g mol21).There are three primary ways of understanding this result.(i) The high concentration in monomer behaving as an initia-tor contributes to the continuous formation of new initiatingradicals as long as acrylates have not been entirely con-sumed. (ii) In a photoinitiated reaction, the concentration ininitiating radicals is generally higher than in a thermal pro-cess in which radicals are produced more progressively.50 Asa result, the probability for primary radicals to terminate thepropagating polymer chain increases (primary radical termi-nation). (iii) Finally, in optically thick medium, whether scat-tering or not, there is a higher probability of a non-uniformrate of initiation leading to a continuous generation of radi-cals throughout the polymerization.

Effect of Optical PathGiven the strong dependence between penetration depth andreaction kinetics, optical path is predicted to be an importantparameter affecting the course of the reaction. For this rea-son, photopolymerization experiments were conducted withthree different quartz cuvettes exhibiting a thickness (e) of0.1, 0.5, and 1 mm. As shown in Figure 8, acrylate consump-tion was significantly faster when decreasing the thicknessof the cuvette. After 3 min irradiation, only 40% of acrylatewas converted at e5 1 mm, whereas 100% is polymerizedafter the same period at e5 0.1 mm. While the cuvette thick-ness had no major effect on particle size evolution forDd5 40 nm (Table 1), the variation of this parameteraffected substantially the molecular weights. Polymerizingvery thin cells enhanced the initiation efficiency, leading tothe generation of a high concentration in initiating radicalsyielding lower molecular weight products.

Effect of Light IrradianceAs shown in Figure 9, light irradiance affected mainly theinduction period resulting from inhibitor traces and oxygendissolved.51 At higher irradiance, the generation of more ini-tiating radicals can reduce the length of this retardationperiod. Thus, a full conversion was reached after only 6 minat 100 mW cm22 versus 12 min at 25 mW cm22. In contrastto nucleation, light irradiance had a marked effect on molec-ular weight distribution as revealed in Table 1. As expected,

FIGURE 7 Acrylate conversions as a function of irradiation

time for different initial sizes of initiator-free acrylate miniemul-

sions. Dd: 40 nm (W), 55 nm (�), 75 nm (�), 90 nm (�), 115

nm (*), 140 nm (1), 210 nm (3). Cmonomer 5 30 wt %, I250–300

(365 nm reflector) 5 100 mW cm22, 1 mm thick quartz cuvette.

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a decreased number of initiating radicals promoted at lowirradiance caused an increase in molecular weight.

Miniemulsion Photopolymerization withoutPhotoinitiator in a Labscale PhotoreactorSelf-initiated miniemulsion photopolymerization wasattempted in an annular photoreactor. Such immersion-typephotochemical reactor is one of the most common reactorconfiguration used at laboratory scale, and has been com-monly involved in many photolysis (water purification),52

photocatalysis,53 and photoinduced organic reactions.54 How-ever, there have been fewer examples of polymerizationsperformed in this vessel or even in other photochemicalreactors.55–59 As displayed in Figure 1, the lamp in this pho-toreactor is centered parallel to the axis of the reactor vessel

and separated from the miniemulsion by a cooling tube. Tomake monomer self-initiation effective, a medium-pressureHg arc lamp is employed (emission spectrum in SupportingInformation Fig. S2) and housed in a quartz cooling tubewith a transmittance extending down to 200 nm. Obviously,such scaling-up imposes different irradiation conditions incomparison with our initial model system based on the local-ized irradiation of miniemulsions contained in spectroscopiccells. The photoreactor set-up is notably characterized by alower irradiance measured at the cooling tube surface (5mW cm22 vs. 100 mW cm22 in the 250–300 nm spectralrange), a larger optical path (9000 mm vs. 1000 mm), and amuch greater irradiated volume (300 vs. 0.34 mL).

TABLE 1 Effect of Droplet Size (Dd), Irradiance (I250–300 nm), and Optical Path (e) on Colloidal Properties (Dp, Np/Nd), Reactions

Kinetics (Conv, Rpmax) and Number Average Molecular Weight (Mn)

SDS concentration

(% wt/wtmonomer)

I250–300 nm

(mW cm22) e (mm) Dd (nm) Dp (nm) Np/Nd

Conv after

16 min (%)

Rpmax

(mol L21 s21)

Mn (3103 g

mol21)

3.5 100 1 40 60 0.30 100 1.28 33.1

1.5 100 1 55 90 0.20 100 0.91 66.8

0.75 100 1 75 95 0.43 100 0.57 107

0.5 100 1 90 110 0.42 80 0.44 96.9

0.25 100 1 110 (150) – 64 0.34 51.8

0.15 100 1 140 (185) – 44 0.22 57.0

0.05 100 1 210 (265) – 16 0.14 45.1

3.5 100 0.5 40 55 0.33 100 2.29 24.4

3.5 100 0.1 40 55 0.32 100 4.34 16.1

3.5 75 1 40 55 0.33 100 0.51 47.9

3.5 50 1 40 60 0.21 100 0.41 58.9

3.5 25 1 40 55 0.36 100 0.30 69.1

FIGURE 8 Acrylate conversion as a function of irradiation time

at different cell thicknesses: 100 mm (W), 500 mm (�), and 1000

mm (�). Cmonomer 5 30 wt %, I250–300 (365 nm reflector) 5 100

mW cm22, Dd 5 40 nm.

FIGURE 9 Acrylate conversion as a function of irradiation time

for different irradiance I250–300 nm (irradiance is given in the

250–300 nm range where acrylate absorption is effective): 100

mW cm22 (W), 75 mW cm22 (�), 50 mW cm22 (�), and 25 mW

cm22 (�), Cmonomer 5 30% w/wmonomer, 1 mm thick quartz cuv-

ette, Dd 5 40 nm.

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Our main efforts have focused on investigating the impact ofdroplet size on polymerization kinetics as the first experi-ments have revealed the prominent role of this parameter.Despite major differences in experimental conditions, theconversion–time plots shown in Figure 10 revealed stronganalogies with the polymerization behavior found in irradi-ated cuvettes. Similarly, it is emphasized that small enoughminiemulsion droplets are essential to achieve high reactionrates and full conversion. In addition, Table 2 indicated againa lower number of particles in comparison with the initialnumber of droplets (Np/Nd< 1), translating a limited dropletnucleation efficiency. However, the most salient event occurswith the smallest monomer miniemulsion (Dd 5 40 nm)undergoing a very high increase of the miniemulsion viscos-ity during the polymerization. The high density of particlesin this case led to a short particle-particle distance andstrong particle interactions. This caused a substantial decel-eration of the polymerization at the end of the reaction aswell as filming problems on the cooling and immersion tubesindicative that diffusion was insufficient inside the reactor(see images in Supporting Information Fig. S4).

In our monomer miniemulsions, the ionic strength is lowbecause no ions other than those of the SDS are present inthe aqueous phase. Therefore, the electrical double layer sur-rounding the particles is relatively thick and the electrostaticinteractions are strong in a densely packed system compris-ing a high number of droplets. Consequently, the problemwas overcome by adding a small concentration of electrolyte([NaHCO3]5 0.03 mol L21) to the aqueous phase.60 Theaddition of salt compresses the double layer and lowers thezeta potential; thereby, reducing the electrostatic repulsion.However, it induces further coalescence after the sonicationprocess, and thus increases the average droplet size. In pres-ence of salt, the miniemulsion prepared with 3.5 wt % SDSshowed a diameter increasing from 40 to 60 nm, withoutdetrimental effects on its relative stability (Supporting Infor-mation Fig. S5). Remarkably, no filming was now reportedand a higher polymerization rate was even obtained becauseof the higher diffusion capacity in a low viscous medium. Asseen in Figure 10, a complete conversion was reached after1 h of irradiation, whereas the same salt-free miniemulsionrequired almost 2 h.

Self-Initiation Mechanism of Acrylate and MethacrylateMonomerInitiating RadicalsWhen photoinitiators are not utilized, the mechanism bywhich initiation of acrylate polymerization occurs has notbeen clearly identified. Attempts were made to identify thetransient species through laser flash photolysis supple-mented by computational analysis.30,61 Under excitation at222 nm, butyl acrylate was found to exhibit a transientabsorption decaying within 10 ms in acetonitrile solution.This transient was assigned to a triplet state. This was con-firmed by density functional theory (DFT) computationswhich further indicated that the spin density was highlylocalized on the double bond. Photolysis products were stud-ied using chromatography techniques showing that a-cleavage can occur from the singlet state and from unrelaxedtriplet state, leading to the formation of initiating radicals.61

Regarding the bimolecular reaction with a second acrylatemoiety, it was shown to occur preferentially through an addi-tion reaction rather than H-abstraction. The rate constant ofaddition being 7 3 108 mol L21 s21, this reaction wasthought to be predominant.30 According to the authors, a1,4-biradical in the triplet state was formed. Further infor-mation on the evolution of this triplet biradical can be found

FIGURE 10 Effect of the initial droplet size on polymerization

kinetics in a self-induced miniemulsion photopolymerization

carried out in a photoreactor. A range of droplet sizes was

obtained by changing the weight concentrations in SDS: 3.5%

(Dd 5 40 nm, W), 0.75% (Dd 5 75 nm, �), 0.5% (Dd 5 90 nm, �),

and 0.25% (Dd 5 115 nm, �). The open symbol is for miniemul-

sion prepared with an aqueous phase containing 3.5 wt % SDS

and 0.03 mol L21 of NaHCO3 (Dd 5 60 nm, w). Cmonomer 5 30 wt

%, organic phase: MMA/BA/AA/SA.

TABLE 2 Effect of Droplet Size on Colloidal Properties (Dp, Np/Nd) and Reactions Kinetics (Rpmax) for a Self-Initiated Polymerization

Performed in an Annular Photoreactor

[SDS] concentration (% wt/wtmonomer) Dd (nm) Dp (nm) Np/Nd Conv after 2 h (%) Rpmax (mol L21 s21)

3.5 40 70 0.21 94 0.10

3.5 [0.03 mol L21 of NaHCO3(aq)] 60 70 0.51 100 0.25

0.75 75 100 0.32 98 0.10

0.5 90 120 0.35 66 0.04

0.25 115 150 0.30 33 0.03

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from works devoted to thermal self-initiation of alkyl acryl-ates and methacrylates.23,24,62 DFT conclusively proved theoccurrence of a Flory mechanism during the thermal self-initiation of acrylate or methacrylate by evaluation of thechemical reaction transition state energies. The initiationmechanism was thought to proceed from a 1,4-biradical inthe triplet state which subsequently reacts with a thirdmonomer molecule to create two monoradical initiating spe-cies via hydrogen abstraction (favored with methacrylate) orhydrogen transfer (favored with acrylates). The proposed ini-tiation mechanism was depicted in Scheme 1.

A qualitative insight into whether triplet states can react withacrylates to initiate the polymerization was provided by a sim-ple experiment. A monomer miniemulsion including a sensi-tizer (2% wt/wtmonomer) such as benzophenone (BP) orthioxanthone (TX) was irradiated through a longpass filter(k>300 nm) to hinder acrylate excitation. The kinetic analysisin Figure 11 showed a slow but complete conversion. Thedeactivation of aromatic ketones by acrylates is known tooccur with relatively low rate constants (5.4 3 107 and 1.5 3

107 mol L21 s21 for the quenching of the triplet state of ben-zophenone and thioxanthone by methyl methacrylate, respec-tively).63 The direct hydrogen transfer from MMA to

SCHEME 1 Proposed mechanism for photochemical self-

initiation of alkyl acrylates and methacrylates.

FIGURE 11 (a) Proposed initiation mechanism of (meth)acry-

late monomers in the presence of an aromatic ketone.

Although the initiation pathway shows a limited efficiency, the

kinetic study indicates qualitatively that excited BP or TX can

also react with the monomer, leading to initiating monoradical

generation (X@H for acrylates and X@CH3 for methacrylates).

(b) Acrylate conversion as function of irradiation time for differ-

ent acrylate miniemulsion containing a photosensitizer: BP (W)

and TX (�).

FIGURE 12 Experimental ESR spectrum of a MMA solution

containing DMPO spin trap (a). Simulated spectrum of the spin

adducts arising from MMA reaction only (b).

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benzophenone triplet state was suspected in,63 although thisreaction was ruled out in a more recent paper.64 Besides thispossible hydrogen transfer reaction, the possible formation ofa 1,4-biradical between the ketone triplet state and the mono-mer was pointed out as the main reaction pathway.63 Thisassumption was further supported by semi-empirical calcula-tions showing that (i) the thermodynamics of the processdepends on the ketone triplet state energy and (ii) a nice rela-tionship can be drawn between the quenching rate constantand the ketone triplet state energy.64 Hence, the 1,4-biradicalis expected to be involved in the polymerization reaction, in aquite similar way as the acrylate triplet state (Fig. 11).

Propagating RadicalsESR experiments were performed by irradiating monomerorganic solutions in the presence of a spin trap (DMPO) toidentify the propagating radical. Figure 12 shows the resultsin the case of MMA (but comparable results were obtainedwith other acrylates or methacrylates). The spectrum b ofthe main radical was simulated from the experimental spec-trum a after eliminating peaks identified as the degradationproducts of DMPO. The hyperfine coupling constants found(aN 5 1.4 mT, aH 5 2.1 mT) were fully consistent with thoseof carbon centered radical species.65,66 Similar results wereobtained during the self-initiation of other. In all likelihood,this radical is thought to be the conventional methacrylatepropagating radical (Scheme 2). As additional evidences, sim-ilar constants were obtained when adding a photoinitiator tothe MMA.

CONCLUSIONS

High-solid content acrylic latexes (30 wt %) were producedwithout initiator via the UV-driven self-initiation of alkylacrylate/methacrylate miniemulsions. A high reactivity wasfound for photopolymerizations performed both in spectro-photometric cells (a few minutes) and photoreactor (�1 h)when using small enough monomer droplets (40 nm) tominimize the light attenuation. Industrially, the strength isthat such high polymerization rates could be achieved with aconventional medium-pressure Hg arc lamp displaying only alimited match with the absorption spectrum of the mono-mers. However, the necessity of handling concentrated nano-sized emulsions implies that issues of colloidal stability andviscosity are addressed. Other key parameters affectingmolecular weight and reaction kinetics included the opticalpath and the irradiance. Mechanistically, the generation ofinitiating radicals by hydrogen abstraction or transfer from adiradical intermediate was proposed as a likely explanationfor experimentally observed spontaneous initiation.

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