Pulsed-Laser Polymerization in CompartmentalizedLiquids, 2a

Miniemulsions

Martin Jung, Eelco M. S. van Hamersveld, Thibault Julien, Alex M. van Herk*

Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The NetherlandsFax: 0031-402463966; E-mail: A.M.v.Herk@tue.nl

IntroductionIn order to understand the kinetics and mechanism of het-erogeneous polymerizations, the knowledge of the propa-gation rate coefficient (kp) and the local monomer con-centration is of paramount importance.[1]

Kinetic studies using pulsed initiation polymerization(PIP) have been performed in several compartmentalizedliquids like microemulsions,[2, 3] latex particles,[4, 5] andwithin vesicles.[6] The method of pulsed laser polymeriza-tion (PLP) is the method of choice for obtaining reliablevalues of kp. This method comprises the generation ofradicals through a photoinitiator, activated by a laserpulse.[7] The time between pulses determines the growthtime of a major part of the chains because biradical termi-nation occurs preferably just after the radical concentra-tion has been increased through a subsequent laser pulse.The molar mass distribution contains peaks of chains thathave grown an integer multiple of the time betweenpulses.

Other radical processes that can occur in the sameexperiment and contribute to the molar mass distributionare bimolecular termination between two pulses andchain transfer processes.

In heterogeneous polymerization systems like micro-emulsions and latices, the fate of the radicals is also influ-enced by the fact that the radicals are compartmentalized.Holdcroft and Guillet[2] introduced PLP for the first timein microemulsions in 1990. Different from homogeneouspolymerization systems, for PLP in microemulsions, con-ditions can be met where bimolecular terminationbetween two laser pulses is suppressed due to compart-mentalization of the radicals.[3] When these conditions inmicroemulsions are met, besides termination of growingchains by small radicals generated by the laser pulse, thetermination of growing chains by transfer to monomer isthe main chain-terminating mechanism and the molarmass distribution contains information on the transfer ratecoefficient.[4]

The same approach has also been applied to (non-trans-parent) latex particles.[4] Although the specific PLP peaks

Communication: The propagation rate coefficient (kp) ofmethyl methacrylate has been measured in miniemulsions.The molecular weight distributions show many higherorder peaks, which seems to be specific for pulsed initia-tion polymerization in compartmentalized liquids. The kp

values are very similar to bulk values although a fre-quency dependence is observed. This frequency depend-ence, which was overlooked in earlier research, is alsopresent in pulsed laser experiments in bulk.

Macromol. Rapid Commun. 2001, 22, No. 12 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2001 1022-1336/2001/1208–0978$17.50+.50/0

MWDs of PLP experiments in miniemulsion at 258C for dif-ferent frequencies.

978 Macromol. Rapid Commun. 2001, 22, 978–982

a For Part 1 of this series, cf. ref.[6]

Pulsed-Laser Polymerization in Compartmentalized Liquids, 2 979

appear in the gel permeation chromatography (GPC)traces, only a very small part of the reactor content can beirradiated. Stirring of the content leads to the total disap-pearance of the PLP peaks. This is caused by a large darkregion, where termination of the growing chains by newlyformed small radicals is less likely. Because mixing istherefore not desirable, temperature gradients in the poly-merization cell can occur. This has led to the investiga-tion of high energy electrons with a higher penetrationdepth in non-transparent systems as a source of initiatingradicals.[5, 8]

The chain length for the chains terminated with pulsesof small radicals is given by the simple equation

where: LO,i is the chain length (in monomeric units) ofthe polymer formed in the process of growth in the timebetween two laser pulses, kp is the propagation rate coef-ficient, [M] the monomer concentration at the site ofpolymerization, t0 is the time between two successivelaser pulses and i = 1, 2, 3,... for chains that were termi-nated after the subsequent pulse or survived one, two etc.subsequent laser pulses before termination, respectively.

Olaj[9] suggested that the inflection point at the low-molar mass side of the PLP peaks gives a good measurefor LO,i and, therefore, kp (provided of course that [M] isknown). However, in very small microemulsion droplets,indications were found that the best measure for kp shiftsfrom the inflection point to the peak maximum.[3, 10] Thechoice of the inflection point as a best measure of the truevalue of kp in heterogeneous systems can be debated,[3 – 5]

but has been chosen here because we believe this wouldonly result in a small systematic underestimation andwould not affect the trends. The size of the polymer parti-cles in the experiments described in this paper is largeenough to be able to use the inflection point.

In regimes where the termination rate is very high, the-oretical calculations indeed show that the peak maximumis the best measure for the calculation of kp.[11] The reasonbehind using PLP and pulsed electron beam polymeriza-tion (PEBP) in heterogeneous systems is to obtain localmonomer concentrations in heterogeneous systems.[12]

When the value of kp (which has extensively beenreviewed[13, 14]) is known, Equation (1) can be used toobtain the local monomer concentration. In highly com-partmentalized systems, the radical concentration profilein the pseudo-stationary state in the pulsed initiationpolymerization experiments will consist of two regimesfollowing the rapid increase in radical concentration dur-ing the laser pulse. In the first part, just after the rapidincrease in radical concentration, the rate of terminationwill be high because the radical concentration will behigh. The second part comprises the stage where the num-ber of radicals per particle is less than two, and here the

termination rate will be lower than in a homogeneous sys-tem because termination involves the transferal of a radi-cal from one particle into another one followed by bimo-lecular termination. In this part, the main mechanism ofchain stoppage is chain transfer to monomer.[4, 10]

To our knowledge, the PLP method has not beenapplied to miniemulsion polymerization before. Minie-mulsion polymerization is sometimes used for preparinghybrid oil-acrylic latices.[15] Also in this system, the localmonomer concentration is important and difficult toobtain otherwise. The main difference between micro-and miniemulsion polymerization lies in the fact that thelatter system is thermodynamically not stable, and themonomer droplets are larger than in the case of micro-emulsions. In this paper we describe the application ofPLP to the miniemulsion polymerization of methylmethacrylate.

Experimental Part

Materials

Sodium dodecyl sulfate (SDS; Fluka BioChemica, verypure), hexadecane (HD; Merck) and photoinitiator 2,2-dimethoxy-2 phenylacetophenone (DMPA; Aldrich) wereused as received. Monomer methyl methacrylate (MMA;Merck), inhibited with 100–200 ppm hydroquinone, waspurified by passage over an inhibitor removal column(Aldrich). Super-Q water (Millipore) was used to prepare theemulsions.

Preparation of the Miniemulsion

The general recipe for the miniemulsion polymerizations isshown in Table 1. Initiator DMPA was mixed with monomerand hydrophobe. An emulsion was created by dispersing themonomer solution into an SDS/water solution. The emulsionwas sonicated (Branson Sonifier 250, 70% duty cycle) for 2min while cooling in a water bath and subsequently passedthrough a high-pressure homogenizer (NS 1001-Panda, Niro-Soavi) for five recycle passes. The homogenizer was oper-ated with the primary stage emulsification valve at 50 barand a second stage homogenization valve at 500 bar. Thenumber-average droplet diameter after homogenization was130 nm as determined by dynamic light scattering (Malvern4700; for details, see the literature[15]).

LO,i = i N kp N [M] N t0 (1)

Table 1. Typical recipe for miniemulsion polymerizations.

Amountg

Concentration10ÿ3

m

H2O (milli Q) 100methyl methacrylate (MMA) 25DMPA 0.64 20sodium docecylsulfate (SDS) 0.202 7Hexadecane (HD) 0.48 21

980 M. Jung, E. M. S. van Hamersveld, T. Julien, A. M. van Herk

Pulsed-Laser Polymerizations (PLP)

Photo-polymerizations were carried out in a 1-ml thermo-stated quartz cell (optical path length ca. 7 mm) in which theemulsion was injected and placed 10 cm from the laser.When experiments were performed at higher temperatures,the solutions were preheated to attain the temperature-dependent monomer equilibrium. The reaction mixture wasthen injected into the polymerization cell. The cell tempera-ture was controlled to l 0.18C and temperature was moni-tored by a small resistance element inside the cell.

The irradiation source was a pulsed excimer laser(Lambda Physics LPX110iMC) generating light at 351 nm(XeF line) and operating at 50 mJ per pulse, which corre-sponds to 1.5610 – 7 moles of photons per pulse. The pulsewidth was ca. 20 ns. Repetition rates varied between 2 and50 Hz and polymerization times were between 1.5 and 3min. Conversions were not higher than 8% as determined bygravimetry. Directly after laser irradiation, the sample wasquenched by the addition of a small amount of hydroqui-none. These samples were subsequently freeze-dried.

The low molecular weight inflection points were used forfurther calculations in determining the kp values.[9]

Results were compared to the IUPAC benchmark valuesfor MMA[16] as given in Table 2 together with density andconcentration.

Size Exclusion Chromatography (SEC)

Prior to SEC analysis, the emulsions were freeze-dried andthe powder obtained was then dissolved in tetrahydrofuran(THF). The polymer solution was passed first over a cotton-wool pipette filter and second over a 0.2-m poly(tetrafluoro-ethylene) (PTFE) syringe filter (Alltech) to remove residualsurfactant crystals and other large contaminants. Theobtained polymer solution was then adjusted to a concentra-tion of ca. 1 mg/mL for SEC analysis. The SEC analyseswere carried out using four PLgel (Mixed-C) columns (Poly-mer Laboratories) at 408C. The injection volume was 100lL and THF was used as the eluent at a flow rate of 1 mL/min. A Waters 410 differential refractometer was applied fordetection. Narrow-distribution polystyrene (PS) standards(Polymer Laboratories) with molecular weights ranging from580 to 7.16106 g/mol were used to calibrate the SEC set-up. Mark-Houwink (M-H) correction has been carried outusing the M-H parameters for PS (a = 0.716; K = 1.14610–4

dL/g) and PMMA (a = 0.719; K = 0.994610 – 4 dL/g).

Results and DiscussionsIn Figure 1 the molecular weight distributions (MWDs)of the polymer obtained from PLP experiments in mini-emulsion at 258C and several frequencies are shown. TheMWDs show clear PLP features with several higher orderpeaks according to Equation (1). At a frequency of 2 Hz,these higher order peaks are almost baseline-separated.Very similar to PLP in microemulsion,[3] the value of i inEquation (1) visible in the MWDs is higher than normallyseen in bulk or solution PLP experiments. Apparently,there is a better chance for the radicals present in thecompartmentalized monomer/polymer droplets to escapefrom terminations several times. This might also be par-tially due to the inhomogeneous radical production withinthe sample as a consequence of light scattering andadsorption.

The same observations can be made at 608C, with thedifference that, especially for the low frequencies, somelow-molecular weight tailing can be seen (Figure 2). Thismight be associated with transfer to monomer occurring.In general, transfer to monomer is more pronounced athigher temperatures, which explains the somewhat differ-ent appearance of the MWDs obtained at 258C.

The MWDs show the expected direction of the fre-quency dependence. However, if we compare the experi-mental kp values with the IUPAC benchmark values itseems that there remains a frequency dependence of kp

that was not noticed in the original IUPAC paper.[16]

Therefore, we feel we are dealing with an apparent kp

value. In Figure 3, the ratio of the apparent kp over theIUPAC value is given as a function of frequency at threedifferent temperatures. And although the ratios are closeto one, there is a clear increase in this ratio with fre-quency (cf. Table 3). Most of the IUPAC values wereobtained at frequencies around 10 Hz.

The question now arises whether this is a specificeffect for miniemulsion polymerization or an intrinsic

Table 2. Parameters used for calculations.

T8C

kpaÞ

L=mol N sqbÞ

g=mLBulk

concentrationm

25 324099 0.9274695 9.263640 499227 0.910423584 9.093360 835943 0.887983256 8.8692

a) ln (kp) = 14.79 – (2687/T).[16]

b) q (T) = 0.9569–1.2129 N 10 – 3 N T + 1.6813 N 10 – 6 N T2 –1.0164 N 10 – 8 N T3 [g/mL]; T [ 8C].[16]

Figure 1. MWDs of PLP experiments in miniemulsion at 25 8Cfor different frequencies.

Pulsed-Laser Polymerization in Compartmentalized Liquids, 2 981

effect for the PLP of MMA. For that reason, we alsomeasured the MWDs of polymers obtained from PLPexperiments in bulk as a function of frequency. TheMWDs (Figure 4) are similar to those obtained fromminimemulsion polymerization at 608C although thereare less higher order peaks visible as expected. Also the

MWDs hint at some transfer-terminated material at thistemperature.

Surprisingly, these data also show a relatively strongfrequency dependence when plotting kp

app/kpIUPAC (Fig-

ure 5). Within a frequency range from 2 to 50 Hz, theminiemulsion kp values increase by approximately 30%,the bulk kp values by about 20%. As the effect does notseem to depend on temperature (see Figure 3) transfer tomonomer or polymer is probably not the cause of thiseffect. All the MWDs show higher order peaks, indicatingthat we are not in the high termination limit. The higherorder peaks qualitatively show the same frequencydependence, therefore, a chain-length-dependent kp can-

Figure 2. MWDs of PLP experiments in miniemulsion at 60 8Cfor different frequencies.

Table 3. Experimental PLP results.

T8C

FrequencyHz

Timemin

log(L1) kpapp/kp

IUPAC

25 2 1.5 5.0734 0.79025 2 3 5.0727 0.78925 5 1.5 4.7073 0.85125 5 3 4.7090 0.85425 10 1.5 4.4340 0.90725 10 3 4.4227 0.88325 20 1.5 4.1437 0.92925 20 3 4.1284 0.89725 20 1.5 4.1652 0.97725 20 3 4.1622 0.97025 30 1.5 4.0014 1.00525 30 3 3.9980 0.99725 50 1.5 3.7869 1.02225 50 3 3.7905 1.030

40 2 1.5 5.2481 0.78240 2 3 5.2692 0.82140 5 1.5 4.8809 0.83940 5 3 4.8867 0.85140 10 1.5 4.6145 0.90940 10 3 4.6178 0.91640 20 1.5 4.3251 0.93440 20 3 4.3312 0.94740 20 1.5 4.3560 1.00240 20 3 4.3489 0.98640 30 1.5 4.1757 0.99340 30 3 4.1771 0.99640 50 1.5 3.9809 1.05740 50 3 3.9724 1.036

60 2 1.5 5.4920 0.84060 2 3 5.5077 0.87160 5 1.5 5.1033 0.85860 5 3 5.1029 0.85760 10 1.5 4.8210 0.89660 10 3 4.8260 0.90660 20 1.5 4.5528 0.96660 20 3 4.5359 0.92960 20 1.5 4.5595 0.98160 20 3 4.5634 0.99060 30 1.5 4.3996 1.01860 30 3 4.4039 1.02860 50 1.5 4.1910 1.05060 50 3 4.1844 1.034

Figure 3. Ratio kpapp/kp

IUPAC versus the frequency for three dif-ferent temperatures obtained from miniemulsion polymeriza-tion.

Figure 4. MWDs of PLP experiments in bulk at 60 8C for dif-ferent frequencies.

982 M. Jung, E. M. S. van Hamersveld, T. Julien, A. M. van Herk

not be the explanation either because the higher orderpeaks of course occur at higher chain length.

ConclusionsThe propagation rate coefficient (kp) of methyl methacry-late were measured in miniemulsions and found to bevery similar to bulk values although a frequency depend-ence could be observed. The explanation for this fre-quency dependence does not seem to be transfer to mono-mer or polymer because the effect seems to be tempera-ture-independent. Because of the presence of higher orderpeaks, both in miniemulsion and in bulk, we are not oper-ating in the high termination limit. So most of the IUPACcriteria for proper PLP experiments are met, except forthe frequency dependence of kp, which was not noticed inthe original IUPAC paper on the kp of MMA.[16]

For acrylates, a frequency dependence of kp is verycommon with good explanations at hand[17] The main rea-son for the failure of the experiment in the case of acry-lates seems to be transfer to polymer.

Recently, Olaj[18] described the same effect for bothmethyl methacrylate and styrene. Our results are comple-tely in line with his. The explanation for this effectremains unclear as stated in the Conclusions paragraph,[18]

although a chain-length dependence of kp seems to be themost logical explanation.

Received: March 26, 2001Revised: May 28, 2001

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[17] A.M. van Herk, Macromol. Rapid Commun. 2001, 22, 687.[18] O.F. Olaj, P. Vana, M. Zoder, A. Kornherr, G. Zifferer,

Macromol. Rapid Commun. 2000, 21, 913.

Figure 5. Ratio kpapp/kp

IUPAC versus the frequency at 60 8C inbulk.