Local monomer concentrations in emulsion polymerization
Citation for published version (APA):
German, A. L., Manders, L. G., Zirkzee, H. F., Klumperman, B., & Herk, van, A. M. (1996). Local monomer
concentrations in emulsion polymerization. Macromolecular Symposia, 111, 107-120.
https://doi.org/10.1002/masy.19961110112
DOI:
10.1002/masy.19961110112
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Published: 01/01/1996
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LOCAL MONOMER CONCENTRATIONS
IN EMULSION POLYMERIZATION
Anton L. German', Bart G. Manders, Hennie F. Zirkzee, Bert Klumperman, and Alex M. van Herk
Eindhoven University of Technology, Laboratory of Polymer Chemistry P.O. Box 513, 5600 MB Eindhoven, The Netherlands
Abstract: Knowledge of local monomer concentrations in the case of heterogeneous polymerizations is extremely important, since these local concentrations determine the kinetics of the reactions. In this paper two techniques to determine the local concentrations are reported. The first technique makes use of the well known pulsed initiation method, which is fiequently used nowadays Íirr the experimental determination of the propagation rate constant (k"). The second technique is applied Íbr the determination of monomer concentrations in vesicle bilayer structures.
INTRODUCTION
Over the last decade there has been a continuous increase in the use of emulsion polymerization as a means to minimize negative eÍfects of polymer production on the environment. Especially in the coating industry there are continuing efforts to replace solvent-based paints by waterborne coatings. It is very clear that the simple substitution of water and surfactant for solvent hardly ever leads to a useful product. The polymer microstructure, i.e. copolymer composition distribution (CCD) and
molar mass distribution (MMD) are strongly affected by the change from
homogeneous to heterogeneous polymerization techniques. Whereas in bulk and solution copolymerization, instantaneous CCD is determined by the reactivity ratios, in emulsion polymerization. monomer partitioning among the various phases appears to have a significant effect on the CCD.
1 0 8
In order to understand the importance of monomer partitioning a brief overview of some work that has been carried out in our group will be given. Subsequently, some advanced techniques will be discussed to measure monomer concentrations at the site of polymerization in heterogeneous systems. One of the techniques is carried out on the actual polymerizing systems, which results in measurements with fewer assumptions than e.g. the various dialysis andior monomer swelling-based techniques. Another technique is presently used in the determination of the swelling behavior of surfactant vesicles. Some experimental results of this work will be presented. 1 . 0 -o"-t a- o---Í'
-..í
f, M AFigure l. Copolymer composition F"n vs monomer feed composition fro of the bulk copolymerization of methyl acrylate (MA) and vinyl pivalate. Copolymerizations were carried out at 50'C to conversions below 3%.
0 . 8
Fn,to
0 . 2 0 . 0 1 . 0 0 . 8MONOMER PARTITIONING IN EMULSION COPOLYMERIZATION
The rules-of-thumb Íbr the occurrence of the composition driÍt in a regular solution copolymerization are relatively simple. No composition drift occurs at the trivial extremes of homopolymerization, and furthermore, there may be a so-called azeotropic composition where also copolymer composition equals monomer Íèed composition. In the simple case of Mayo-Lewis or terminal model kinetics, an azeotropic point occurs when either both reactivity ratios are greater than unity or, more common, both are smaller than unity. Further, under non-azeotropic conditions the general rule is that the composition driÍt is stronger with increasing deviation fiom the azeotrope. In Figure I it is shown how the copolymer composition varies with instantancous comonomer Íèed. This graph is applicahle to a homogeneous polymerization of methyl acrylate and vinyl pivalate in bulk Íbr w h i c h t h e r e a c t i v i t y r a t i o s h a v e b e e n d e t e r m i n e d a s r * : 6 . 1 , a n d r r = 8 . 7 * 1 0 r . l n this case it is Íbund that monomer concentration has no significant eÍï'ect on copolymer composition, neither at low conversion, nor as a Íunction of conversion. The simple reason tbr this low dependence of copolymer composition on elution is the homogeneous charactcr ofthe solution. Although there have been several reports o n t h e e Í ï è c t o f s o l v e n t o n c o p o l y m e r c o m p o s i t i o n r , t h i s h a r d l y e v e r s h o w e c l u p i n a c o n c e n t r a t i o n - d e p e n d e n t w a y . I n o t h e r w o r d s , i Í ' o n e n e e d s to c a l c u l a t e or predict copolymer composition, it is necessary to usc reactivity ratios determined in the particular solvent of choice. There is no need to make adlustments Íor monomer concentration.
I f w e n o w c o n s i d e r e m u l s i o n c o p o l y m e r i z a t i o n , t h e s i t u a t i o n is q u i t e d i Í Í e r e n t . T h e majority ol'monomer conversion takes place in latex particles. On the other hand there is a large aqueous phase which makes up 50 ro 90% of the latex volume. If we now Íbcus on two comonomers with relatively large diÍÍerences in wateÍ solubility, it is easy to envisage that the more water-soluble monomer will have a relative preference to reside in the aqueous phase, whereas the less water-soluble monomer has a relative pref'erence for the monomer droplets and latex particles. Then taking into account the fact mentioned above that polymerization mainly takes place in the
' 1 1 0
latex particles, it is easily understood that the comonomer ratio in these particles, i.e. aï Íhe site of propagation, may difÍèr significantly from the overall (charged) comonomer ratio. One step further is the effect of overall monomer concentration. Imagine we have two extreme situations in terms of monomer/water ratio. One where the volume of the aqueous phase is much larger than the volume of the monomer phase (l), and the other where the volumes of aqueous phase and monomer phase are approximately equal (2). In situation (l) a large amount of the more water-soluble monomer will be in the aqueous phase, and therefore not directly available to the polymerization, which mainly takes place in the polymer particles, as indicated before. On the other hand in situation (2) the aqueous phase is smaller in volumc. and can thereÍbre take less of the water-soluble monomer. Now if we comparc the comonomer ratio in the latex particles it is easy to imagíne that the relative amount of the more water-soluble monomer in the latex particles is s m a l l e r i n s i t u a t i o n ( 1 ) t h a n i t i s i n s i t u a t i o n ( 2 ) . I f a c o p o l y m e r i z a t i o n i s s t a r t e d in the latex particles, the actual comonomer ratio to use will be the one inside the latex particles. ThereÍirre, if we consult an ordinary graph of copolymer composition as a Íunction of overall comonomer Íèed, both copolymerizations will yield different products. This is clearly shown in Figure 2. This specific example was taken Íiom previous work in our group on the copolymerization of methyl acrylate with vinyl p i v a l a t c r .
In Figure 2 thc eÍÍect of conversion on the residual monomer composition is shown. 'fhis
is reÍlected in the direction of the copolymer composition driÍi as observed tn the cmulsion copolymerization shown in Figure 2. Only by adjusting the monomer/water ratio, the cornposition driÍt can be shiÍted Íiom driÍting towards homopolymerization of vinyl pivalate via pseudo-azeotropic conditions to drifïing towards homopolymerization of methyl acrylate. This clearly illustrates the importance of local comonomer ratios in the case of emulsion copolymerization.
ln order to ensure knowledge on monomer partitioning in emulsion
copolymerization, quite a Íèw studies have been conducted to theoretically predict this phenomenon, or to experimentally measure it. Theoretical pre<.lictions in general are only considered useful if practical evidence supports the predictions ln the
o-remainder of this paper we want to discuss two relatively new techniques for experimental determination of latex particle swelling.
The first technique is based on the well known pulsed laser polymerization The other technique is based on a swelling experiment which was devekrped in our laboratorv for the swellins of twin-tailed surÍactant vesicles.
r.oo
0 . 8 0
0 . 6 0
0 . 4 0
0 . 2 0
0.oo
o.o0
o . 2 0
0 . 4 0
0 . 6 0
o . E 0
converslon
Figure 2. Residual Íronomer composition (Í*n,,) as a Íunction of conversion firr the emulsion copolymerization of methyl acrylate (MA) and vinyl pivalate at various monomer/water (M/W) ratios.
THE USE OF PULSED INITIATION METHODS TO DETERMINE LOCAL MONOMER CONCENTRATIONS
For bulk polymerization a method, pulsed laser polymerization (PLP), has been developedr5)to study the kinetics of free radical polymerization. With this method it is possible to determine the propagation rate constant (k") in bulk or solution systems. The method comprises the repetitive generation of radicals from a
1 1 2
initiator during very short laser pulses at constant time intervals which will lead to a large instantaneous increase of the radical concentration. This sharp increase in radical concentration will lead to a much higher termination rate. Therefore, growing chains that started during a previous laser pulse will have a higher probability of termination with a small radical species. A definite part of the polymer formed will effectively have had a growth time that equals the time between two consecutive laser pulses. In a molecular weight distribution (MWD) of polymer formed during a PLP experiment, apart iiom background polymer Íbrmed because of termination during the dark period, typical PLP peaks appear as a result of this. The molecular weight of this polymer can easily be related to the time between two p u l s e s a c c o r d i n g to t h e l ' o l k r w i n g e q u a t i o n ' s '
M n = i k n ' [ M l t u M , ,
where Mn is the molecular weight of polymer derived Íiom the position of the PI-P material, i is an integer that arises liom the Íact that overtones may be observed due
to termination of chains aÍier more than one pulse, IMl is the monomer
concentration, q) is the time between two pulses and M,, is the molecular weight of a monomer unit. Simulationst) have shown that the inflection point at the low-molecular-weight side of the PLP peaks in the MWD is the best measure of Mn as given in Eq. I . As kn is determined by the position of the PLP peaks in the MWD' it is vital to have correct MWD measurements. This is possible to within 5 % when
using size-exclusion chromatography (SEC) in combination with primary
calibration, l.e. using narrow polydispersity standards with knclwn molecular weight made from the same polymer as is being analysed.
Contrary to bulk systems where with a known monomer concentration a \ value can be cletermined Íiom the propagation frequency k"[MJ, in emulsion systems, wÍth the use of kn as obtainecl Íiom bulk experiments, the monomer concentration can be determined.ó,t) In this case the heterogeneity of the system will ínÍluence the pseudo-stationary kinetics as observed for bulk systems to some extent, but the same principles remain: when growing chains are being terminated by small radicals
directly after a laser pulse, the MWD will contain the typical pl-p peaks that can be evaluated.E)
PLP has been applied to polystyrene (pS) emulsion systems that have been swollen to maximum (saturation) swelling with styrene, although the non-transparent character of the latex prohibits a homogeneous irradiation of the sample. This will lead to delicate experiments,') as optimal oonditions (e.g. pulse energy, initiator concentration, time of irradiation) Íbr PLP have to be Íbund by trial and error. This makes the method not less time consuming than other methods of determining monomer concentrations. C)n the other hand, with PLP no separation steps are needed and the method will yield the monomer concentration at the locus of reaction. An example of a MWD obtained with PLP on latex systems is given in Figure 3. Several experiments done at different temperatures indicate the absence of a temperature dependence of the equilibrium degree of swelling,') at least within exDerimental err0r. l ^ 6 rc n 7 rc @ o 0 . 0 2- 5 1 . 0 3 . - 5 4 . 0 4 . . s 5 . 0 5 . 5 I o g M _- n l 6 . 5 7 . 0 '7 .5
Figure 3. A typical molecular weight distribution (MWD) obtained from a pulsed laser polymerization experiment with styrene-swollen polystyrene (pS) latex particles (unswollen particle size dp : 92 nm) at 60 'C, time between two pulses tr : 0.1 s. Drawn curve is the MWD, dotted curve is the first derivative.e)
1 1 4
Results for styrene yield an average value of [Mln'"' : 6.4 mol L'for pS particles with an unswollen diameter of 92 nm and a value of 5.3 mol L' for pS particles with an unswollen diameter of 34 nm. Both values coincide with values Íbund with other methods. "')
Although the results are very promising, in order to make the method a real success a better initiation system is needed. Electrons offer the possibility to homogeneously irradiate layers of water of up to 1 cm, irrespective of optical transparency. A pulsed-electron-beam polymerization (PEBP) setup míght thereÍbre prove valuable Íbr the type of experiments described above. This has been tested with a pulsed 3 M e V V a n t l c C r a a l l' a c e e l e r l t t o r . I r ' 0 . 8 0 fÍq
z
r o l o rÍq" 7
? o . o
dl 3 0 . 4 0 . 1 14
l o e M
' p iFigure 4. A typical molecular
weight distribution
(MWD) obtained
Íiom a
pulsed-electron-beam
polymerization
experiment with styrene-swollen
polystyrene (pS)
latex particles (unswollen
particle size d, : 46 nm) at 23 "c, time between
two
pulses
!, : 0.2 s, close
per pulse
was 1.49 Gy. Drawn curve is the MWD, dotted
curve is the first derivative.'"
Radicals may be generated inside the latex particles or in the water phase (e-"0, H., OH ). Optimum doses are between 1 and 4 Gy per pulse. Calculations with the experimentally used doses show that direct initiation in latex particles is negligible. The conclusion therefore is that the main source of initiation is the radicals generated in the water phase. This means that there is a small time delay after a pulse before radicals reach a particle. As this is the case with every pulse, the only result is that the initiation period is somewhat broadened. This will not endanger the PLP/PEBP principles as put Íbrward above. A typical MWD for styrene is given in Figure 4. In this figure a large amount of oligomeric material is present with a length of 2-3 styrene units. This can be attributed to ionic polymerizations which are suppressed by the large amount of water present in a latex system.'''t)An average v a l u e o f [M]p'"' : 5.8 mol l, I f r r r t h e p S p a r t i c l e s w i t h a n u n s w o l l e n d i a m e t e r o f 4 6 n m i s Í b u n d . " )
'Ihe
PLP method and the PEBP mcthod represent alternative methods to determine monomer concentrations in saturated latex particles which conÍirm the conventional measurcments involving separation steps. The good agreenrent between monomer concentrations deterrnined Íiom the PLP/PEBP method and Íiom traditional methods confirms the corrsctness ol' the assumption that the propagation rate constant as determined in bulk can bc used Íbr latex systems. The PEBP method is more Í'lexible than the PLP method as the Íbrmer method does not require optical transparency of the samplc.
EXPERIMENTAL DETERMINATION OF PHASE EQUILIBRIA IN
VESICULAR SYSTEMS
Vesicles are structures that can be Íirnned Íiom twintailed surfactant rnolecules. Vesicles are normally spherical structures with water inside and outside. The bilayer itself is a double layer of surÍactant molecules, that are oriented with therr hydrophilic head towards the vesicle phase and with their hydrophobic tails towards the inside of the bilayerra. When Dimethyldioctadecylammonium bromide (DODAB) is used, the vesicle structure may be somewhat labile. This means that upon addition of a large amount of monomer initially, the vesicles may break up, and the
1 1 6
surfactant molecules will stabilize the monomer droplets, similar to the sinration in an ordinary emulsion polymerization. In order to overcome this practical problem, a special vessel was designed in our laboratory to 'swell' vesicles with monomer without direct contact between the vesicle solution and the monomer phaser'. The experimental setup which was used Íbr the swelling of the vesicle bilayer is presented in Figure 5.
The swelling vessel is equipped with a glass tube in which a dialysis tube was placed, containing the vesicle solution. Typical DODAB concentrations were l-6 mM. The vessel was Íilled with distilled water and Íinally a monomer layer was placed on top of the system. The vessel could be thermostatted within the temperature range of 20-60'C. With this experimental setup direct contact of vesicles with an excess of monomer was prevented. The monomer will diffuse through the aqueous phase to the vesicle bilayer. Samples could be taken separately fiom the aqueous phase as well as Íiom the vesicle solutkrn, and the monomer concentrations can bc determined using UV-spectroscopy and IIigh PerÍirrmance I-iquid Chromatography (IIPLC).
Sample point
Dialysis
tube
Vesicle solution
In Figure 6 a characteristic 'swelling 20 "C, is shown.
experiment of DODAB vesicles with styrene at
Figure 6. Styrene concentration in the aqueous phase (o) and in the vesicle phase (À) as Íunction of time.
The average diameter of the vesicles usod in the swelling experiments was typically 30 nm. The styrene concentration in the aqueous phase and vesicle solution were monitored as a Íunction oÍ' time. ln the beginning of the swelling experiment the aqueous phase was saturated with styrene. As soon as the dialysis tube, that contained the vesicle solution, was inserted into the vessel the styrene concentration in the aqueous phase decreased strongly. 'Ihis is attributed to the Íact that the initial styrene concentration in the vesicle bilayer is zero, i.e. a high concentration gradient between the vesicle bilayer and the aqueous phase is present which causes a high initial diffusion rate. However, aÍter approximately two hours the aqueous phase was saturated with styrene again. The styrene concentration in the vesicle bilayer gradually increased over a relatively large period of time. aÍter which styrene reached its sanlration value of approximately 1.5 M in the vesicle bilayer.
1 1 8
The swelling experiments were also perÍbrmed as a function of temperature, in order to investigate the inÍluence of the phase transition temperature of the vesicle bilayer upon the swelling behaviour of the vesicle bilayers with styrene.
1 . 4
Figure 7. Styrene concentration at saturation swclling in the vesicle bilayer as a Íunction of tcmperature.
As is clearly demonstrated in Figure 7 temperature has a prtlnounced eÍÍect on the styrene saturation concontration in the vesicle bilayer. This is attributed k) the fact that below the phase transition temperature the alkane part ol'the surlactant is in a semt-crystalline state. A sirnilar eÍ1èct of temperature on swelling was observed by Simon et alr6 in the case of phosph<tlipid vesicles swollen with hexane and benzene. Above the phase transition temperature the styrene concentration in the vesicle bilayer is approximately a Íactor of 2 higher than below the phase transition temperature'
2 . 8 o 2 . 4 a . z o 'E 2.O . g > r ó U) t o
Temperature
('C)
DISCUSSION AND CONCLUSIONS
From the examples given above it will be clear that, especially in the case of heterogeneous polymerizations, it is very important to know local monomer concentrations. The methods described in this paper have clear advantages over the traditional methods Íbr the determination of local monomer concentrations. The putsed initiation method allows the determination of monomer concentrations at the site of propagation in a rcacting system. The major improvement achieved by the use of pulsed electron beam over the use of pulsed laser consists of the more homogeneous initiation of the entire sample, l.d. PEBP, in contrast to PLP, does not require optical transparency Íirr a hornogeneous irradiation. The swelling method which is described Íirr the swelling of vesicles is a convenient method. The use of'the dialysis membrane in the described vessel, to separate the monomer phase fiom the vesicle solution appears to bc vcry useÍul. The experirnental setup prevents the vesicles to break up. Furthermore it greatly simpliÍies the deterrnination of monomer concentrations in the diÍÍerent phases. It is lbr this latter rcason that the experimental sctup will be very suitable Íbr the dctermination oï latex particle swelling as well.
Knowledge of local ÍronoÍler concentrations in heterogeneous polymerizations has proven to be of great value in the kinctic rnodelling of these systcms. With the experimental techniques described in this paper wc are able to validate predictions b a s e d o n v a r i o u s s w e l l i n g m o d e l s .
R E F E R E N C E S
( 1 ) H . J . H a r w o o d , M u k r o m o l . C h e m . , M a c r o m o l S , v m p l0 / l l , 3 3 1 ( 1 9 8 7 ) (2) E.F.J. Verdurrnen NoëI, PH.D. thesis - University of Eindhoven (1994) (3) V. N. Genkin, V. V. Sokolov, Dokl. Akad. Nauk. SS.SR 234,94 (1917) ( 4 ) A . P . A l e k s a n d r o v , V . N . G e n k i n , M . S . K i t a i , I . M . S m i r n o v a , V . V . Sokolov, Kvantovaya Elektron. (Mosr:ow) 4, 976 (1977); Transl: Sor,. J. Quantum Electron. 7. 547 0977\
( 5 ) O . F . O l a j , I . B i t a i , F . H i n k e l m a n n , M a k r o m o l . C h e m . 1 8 8 , 1 6 8 9 ( 1 9 8 7 ) (6) J. Schweer, R. J. Pijpers, Appl. Patent P4331296.9,1994
