Kinetics and Mechanisms of Particle Formation and Growth in the

Chapter 5

Downloaded by STANFORD UNIV GREEN LIBR on August 6, 2012 | http://pubs.acs.org Publication Date: June 3, 1992 | doi: 10.1021/bk-1992-0492.ch005

Kinetics and Mechanisms of Particle Formation and Growth in the Emulsion Polymerization Initiated by the Oil-Soluble Initiator, 2,2'-Azobisisobutyronitrile M. Nomura, J. Ikoma, and K. Fujita Department of Materials Science and Engineering, Fukui University, Fukui, Japan This paper experimentally clarifies that in emulsion polymerizations initiated by oil-soluble initiator, 2,2'-azobisisobutyronitrile, polymer particles are generated from emulsifier micelles and that the polymerization proceeds mainly inside the polymer particles. Furthermore, it is demonstrated that the radicals which initiate the polymerization are those stemming from the initiator dissolved in the water phase. Based on these findings, a kinetic model for seeded emulsion polymerizations initiated by oil-soluble initiators is proposed and compared with experiment Oil-soluble initiators are known to initiate polymerization in emulsion and the kinetic behavior of this polymerization system is very similar to that of emulsion polymerizations initiated by water-soluble initiators despite the difference in the principal loci of radical production in both systems(7). We have reported that the kinetic behavior of the emulsion polymerization of styrene (ST) initiated by the oilsoluble initiator, 2,2'-azobisisobutyronitrile (AIBN) with sodium lauryl sulfate (NaLS) as emulsifier is quite similar to that initiated by the water-soluble initiator, potassium persulfate (K2S2O8) (2). The reasons for this similarity, however, have not been elucidated. Recently, Asua et al. (3) have presented a model which predicts the average number of radicals per particle in emulsion polymerization initiated by oilsoluble initiators, and arrived at a conclusion that the initiator radicals generated from the initiator distributed inside the polymer particles initiate the polymerization in this system. The aim of this paper is first to provide experimental evidence which supports the hypothesis that polymer particles are generated from emulsifier micelles and that the principal loci of the polymerization shifts at a comparatively early stage of polymerization from the monomer droplets to the resulting polymer particles, when the emulsion polymerization of styrene is initiated by the oil-soluble initiator, A I B N with NaLS as emulsifier. Secondly, the results in this paper demonstrate that the polymerization inside the polymer particles is initiated not by the radicals produced from the initiator distributed in the polymer particles and the monomer droplets, but mainly by the radicals generated from the initiator dissolved in the water phase. Based on these findings, a kinetic model for seeded emulsion polymerization which accounts for the rates of polymerization in both the monomer droplets and the polymer particles will be 0097-6156/92A)492-0055$06.00y0 © 1992 American Chemical Society In Polymer Latexes; Daniels, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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presented, introducing the theoretical approachreportedpreviously (4), and further, the validity of the proposed kinetic model will be demonstrated by comparing the model predictions with the experimental datareportedin the previous article (2). This study will, therefore, show why the kinetic behavior of emulsion polymerizations initiated by oil-soluble initiators are very similar to those initiated by water-soluble initiators.


Experimental Polymerization Apparatus and Procedure. Commercial ST monomer was purified by distillation after first washing with 15% KOH aqueous solution to remove hydroquinone inhibitor and then with deionized water until KOH was not detected in the effluent. AIBN and NaLS of extra-pure grade were used without further purification as initiator and emulsifier,respectively.All polymerization experiments were conducted at 50°C with the same experimental apparatus and procedure as previously described (5), except that high purity nitrogen gas (purity>99.995%) was used without further purification. In these experiments, the impeller speed was constant at 400 rpm. Monomer conversion was determined gravimetrically using methanol as a precipitant for the polymer. The number of polymer particles produced was determined by electron microscopy with the following expressions. 6 M

oX

M

KdpPp

0)

2>

(2) 3

where N j is the number of polymer particles per cm -water, Mo the amount of monomer initially charged per cm -water, X M the monomer conversion, p the density of polymer, and dp the volume average diameter of the polymer particles. 3

p

Water-Solubility of AIBN. The water-solubility of AIBN and its partition coefficient between the monomer and water phases were determined as follows. An appropriate amount of AIBN was dissolved in styrene monomer. A portion of this solution was allowed to contact distilled water for more than 30 min in a flask maintained at 50*C with the use of a thermostatted water bath. Moderate stirring was applied using a magnetic stirring bar to quicken the transport of AIBN molecules from the monomer droplets to the water phase. The mixture in the flask was then completely separated into a monomer layer and a water phase with a centrifuge and a small sample taken from each separated phase was subjected to measurement of the concentration of AIBN by high-performance liquid chromatography (HPLC). AIBN was separated on a TOSOH TSK-GEL ODS-80TM column by using a mixture of water-methanol (50:50 by volume) as the eluent. The concentration of AIBN was measured by a UV detector at a wavelength of 346 nm. Figure 1 is a plot of the measured equilibrium concentration of AIBN in the monomer phase, [I]d, versus that in the water phase, [I] , at 50*C. When [T^ is less than about 40 g/liter, the partition of AIBN between the monomer and water phases can be correlated by a linear relation. w

[I]d = ^[I]w

#

(k=115 at 50 C)

In Polymer Latexes; Daniels, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

(3)

5. NOMURA ET AL.

57

Particle Formation in Emulsion Polymerization

The partition coefficient of AIBN between the polymer particle and water phases, X\ is usually different from X. However, it is reasonable to consider in this case that #


h=X\ so that we regard, as a first approximation, also that X= X =115 at 50 C. Since the thermal decomposition rate constant for AIBN is comparatively small and also the volume change of die total organic phase due to polymerization is not so large in this system, we can safely regard that the concentration of AIBN in the monomer droplets is approximately equal to the initial value, [I]o, which is given by dividing Io, the initially charged weight of AIBN, by the total volume of the monomer and seed polymer particles initially charged. [Hd = [I]p = [I]o

(4)

Considering the monomer partition in the case of emulsion copolymerization (6), the following relationships appear to hold and hence, to be applicable in obtaining more precise values for [I]p, [I]d, and [T|w. n 3 f = rSn" • [MJd [M]

V

*TO*

+

V

P™P

+ V w [ I ] w

= Iexp(-kdt)

*

p

(5)

where [M]d is the monomer concentration in the monomer droplets, Vd is the total volume of the monomer droplets contained per cm -water, kd the thermal decomposition rate constant for AIBN, and t the reactiontime.The suffixes, d, p, and w denote the properties associated with the monomer droplet, the polymer particle, and the water phase, respectively. Furthermore, the total volume of organic phase in the reactor, (Vd+Vp), is approximately constant and equal to the total volume of the monomer and seed polymer particles initially charged, because the densities of the monomer and the resulting polymer are not very different. 3

Thermal Decomposition Rate Constant for AIBN. AIBN initiator thermally decomposes to produce a pair of radicals in the water, monomer droplet and polymer particle phases, respectively. As mentioned above, however, only radical production in the water phase and, in some cases, also in die polymer particles is important and their rates are expressed, respectively, as r

iw

= 2k f [I] dw

w

(6)

w

r = 2kdpf[I]pVpNT ip

(7)

p

where rj and f are the rate of radical production and the initiator efficiency in each phase and v is the average volume of a polymer particle. The equality of these parameters among the phases is also a reasonable assumption. p

f

f

kdw=kdp=kd, d= p= f

(8)

In order to determine the thermal decomposition rate constant for AIBN, bulk polymerizations of styrene were carried out at 50*C with varying initial concentrations of AIBN, [T)o. Tte value of kd was determined from the measured values of the rate of polymerization, Rp, calculated from the slope of the conversion versustimecurves and the number average molecular weight of the resulting polymers, P , measured by gel permeation chromatography (GPC) with the expression: n


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POLYMER LATEXES

(9)

According to Equation 9, the reciprocal of the value of P observed experimentally was plotted against the corresponding value of [I]o/R in Figure 2. All the experimental points fall on a straight line whose slope is k^f and the intercept with the ordinate is kmVkp, the chain transfer constant to monomer. Thus, the following values were determined: kdf=8.4xl0- 1/sec, k /k =9.2 x 1(H (10) n

p

7

mf

P


Experimental Results and Discussion Polymerization Loci. Figure 3 shows the effect of the initial emulsifier concentration, So, on the progress of the unseeded emulsion polymerization of styrene carried out with the initial initiator and monomer concentrations fixed at 10=36.1 g/dm -monomer and Mo=0.2 g/cm -water, respectively (2). The number of polymer particles, Nj, reached a constant value very early and is plotted against So in Figure 4, along with the rate of polymerization, R , calculated from the slope of the linear portion (30 to 50% X M ) of the monomer conversion versustimecurves shown in Figure 3. The number of polymer particles formed increases abruptly in the vicinity of So=2.0 g/dm -water. This particle nucleation behavior is quite similar to that observed in the case of the emulsion polymerization of styrene initiated by the watersoluble initiator, K2S2O8 with NaLS as emulsifier. These data are also shown in Figure 4. Considering that the CMC of NaLS in the absence of electrolyte is about 8xlO- mole/dm -water (2.3 g/dm -water), these experimentalresultsappear to imply that the polymer particles are generated from the emulsifier micelles independently of the kind of initiator used. 3

3

p

3

3

3

3

As an example, the polymerization was started under thereactionconditions shown in Figure 5. 15 min after the start of the polymerization, about 20 g sample of thereactionmixture was withdrawn from the sampling cock attached at the bottom of thereactionvessel and a portion of this sample was poured into excess methanol to precipitate the polymer. The precipitated polymer was separated byfiltrationwith a glass crucible and dried in a vacuum oven. Then, the total monomer conversion was gravimetrically determined to be 4.3%. The solid line A shown in Figure 5 indicates the molecular weight distribution (MWD) determined by GPC of the total polymer thus collected. The rest of the sample was, on the other hand, subjected to separation of the mixture with a centrifuge into a monomer layer and a serum containing polymer particles. The polymers contained in each separated phase were precipitated and collected by the same procedure as mentioned above. The broken line B shows the GPC chromatogram of the polymer collected from the monomer layer, the dashed line C indicating that of the polymer collected from the serum. The MWD shown by line B agreed perfectly with that of the polymer obtained in the very beginning of the bulk polymerization of styrene carried out under the same temperature and the same concentration of AIBN initiator. Therefore, we believe that in this emulsion polymerization system, polymerization takes place simultaneously in both the monomer droplets and the polymer particles. Suspension polymerization which follows homogeneous bulk kinetics occurs in the monomer droplets. On the other hand, Figure 6 shows how the shape of the MWD changes with the progress of polymerization. The polymerization in the monomer droplets is important in the beginning, but the polymerization in the polymer particles becomes dominant with the progress of the polymerization.


NOMURA ET AL.

Particle Formation in Emulsion Polymerization x10"^


81

1

1

[I]

w

1

1

r

[g/cc-water]

Figure 1. Determination of the partition coefficient of AIBN, X, between the monomer droplets and the water phase.

Figure 2. Determination of the thermal decomposition rate constant of AIBN.



POLYMER LATEXES

Reaction tire t Cnrin]

Figure 3. Effect of initial emulsifier concentration on the progress of polymerization (IQ=36.1 g/dm -water, MQ=0.2 g/cm -water, So= 0 25.0, E20.0, (112.5, 36.25, € 3 . 1 3 , B2.50, • 2.0, • 1.25, • 0.8, A 0.5, A 0 g/dm -water), (Reproduced with permission from ref. 2. Copyright 1991 Wiley.) 3

3

3

Figure 4. Effect of initial emulsifier concentration on the number of polymer particles produced, Nj, and the rate of polymerization, R . p



5. NOMURA ET AL.


* / 1 I !

M »0.2

61

-water

o

3

I =36. lg/dra -monomer Q

3

S -6.25g/dm -water Q

I I

X - 4.3 % M

II

J 10

3

I I I I 10* 10 10 10 Polystyrene Molecular Weight 5

6

7

Figure 5. The molecular weight distributions (MWD's) of the polymer produced in the monomer droplets and in the polymer particles (line A: the MWD of the total polymer, line B: the MWD of the polymer produced in the monomer droplets, line C: the MWD of the polymer produced in the polymer particles).



62

POLYMER LATEXES

J

I 3

JO

I 4

10

10?

I

ioP

Polystyrene Molecular

L_ 7

10

Weight

Figure 6. The variation of the MWD of the polymer with the monomer conversion.


5. NOMURA E T AL.


The amount of polymer produced in each reaction locus can be determined first by separating, for example, the chromatogram A with two peaks shown in Figure 5 into two individual chromatograms and then, by calculating the area bounded by each chromatogram and the base line. Let P and Pd be die amounts of polymer produced in the polymer particles and in the monomer droplets,respectively.The experimental value of Pd thus determined was plotted against the monomer conversion, X M , in Figure 7. The solid lines indicate the values calculated by the kinetic model for the seeded emulsion polymerization proposed later. The data in Figure 7 show that the polymerization in the monomer droplets is dominant only in the very beginning and becomes negligible above ca. 30% conversion. This is because most of the monomer in the monomer droplets, which also contains a small amount of polymer, has already been transferred into the polymer particles at ca. 30% conversion. Thus, the rate of polymerization calculated from the slope between 30 and 50% conversion of the conversion versus time curve, R , can be safely regarded to show the rate of polymerization in the polymer particles, r , which is expressed by Equation 11 given later. Hence, the average number of radicals per particle, n, can be calculated by applying the observed values of r and [M] to Equation 11 presented later.


p

p

p

p

p

The Average Number of Radicals per Particle. The average number of radicals per particle is one of the important parameters which determine the average volumetric growth rate per particle. The solid lines in Figure 8 show the theoretical values of n which were computed by applying the method previously proposed for the case of K=a /a =ri /ri =0.05 (4). The experimental values of ii (closed circles) obtained by applying the procedure mentioned above to the experimental data shown in Figure 3 are also plotted against ^ ( ^ w V p / k ^ x ) in Figure 8. The value of n was calculated by Equation 6. For the concentration of AIBN in the monomer droplets, [I]d, which is necessary for estimating the value of [I]w using the correlation shown in Figure 1, the initial value [IJo, defined by Equation 4, was employed, because the change in the total volume of the organic phase in thereactionsystem would not be so large, as already presumed, even with the density change in the polymer particles and the monomer droplets occurring during the polymerization. The experimental data shown by open circles, on the other hand, indicate those found in the emulsion polymerization of styrene initiated by thewater-soluble initiator, K2S2O8 (7). The dotted lines show the theoretical values of n corresponding to the case for water-soluble initiators(K=oo). From this figure, it is clear that the solid lines are w

p

w

p

w

perfecdy superimposed upon the dotted lines in the range where the values of a and m are both less than about 10~ . This means that in this range, any radical production inside the polymer particles does not contribute to an increase in the average number of radicals per particle. This can also be taken as a theoretical evidence that the polymerization in the polymer particles proceeds mainly by the radicals stemming from the initiator dissolved in the water phase, not by the radicals producedfromthe initiator distributed in the polymer particles as long as the rate of radical desorption from the polymer particles is not so high (that is, in the range where the values of m and a are both less than about 10" when K=5xl0" ) (4). The fact that the experimental values of ii obtained with AIBN (closed circles) are in complete agreement with those obtained with K2S2O8 (open circles) can beregardedas a decisive experimental evidence for supporting the validity of the conclusions mentioned above. Furthermore, this result also supports the validity of the assumption that the radicals produced in the monomer droplets do not affect the radical balance in the water phase because only a negligible amount of them can desorb into the water phase owing to a small surface area per unit volume of the monomer droplets, so that almost all of them are terminated inside the monomer w

2

2

2

w



64

POLYMER LATEXES

Monomer c o n v e r s i o n X

[-]

Figure 7. The effect of emulsifier concentration initially charged on the amount of polymer produced in the monomer droplets, Pa versus monomer conversions (solid line: model predictions by Equation 13).

Figure 8. A plot of ii versus oc (Experimental:#=AIBN, 0=K2S208; Theoretical value: dotted line=n for water-soluble initiators with K=°°; solid line =n for oil-soluble initiators with K=0.05). w


5. NOMURA E T AL.

65


droplets. The radicals produced inside the polymer particles will not contribute to the polymerization inside die polymer particles because the volume of a polymer particle is so small that instantaneous radical termination takes place as soon as a radical pair is born inside the polymer particles. Based on the analysis given above, we propose a kinetic model for seeded emulsion polymerization which can predict the rates of polymerization in both the monomer droplets and the polymer particles.


A Seeded Emulsion Polymerization Model. (1) The polymerization rate in the polymer particles is given by:

rp = ^

= (MM]pnN >(^] T

(11) 3

where P is the weight of polymer produced inside the polymer particles per cm water, k the propagation rate constant, M the molecular weight of styrene, N A Avogadro's number. The average number of radicals per particle, n can be calculated by the method previously proposed (4), or by the following approximate expression (6): p

p

G

2

(12)

(2) The polymerization rate in the monomer droplets is:

r =f d

= (MM]a[RVaN }(^)

^

d

3

where Pd is the weight of polymer produced inside the monomer droplets per cm water. Vd is the average volume of a monomer droplet and Nd the number of monomer droplets per cm -water. The product VdNd is expressed by Equation 19 shown later. [R*]d is the concentration of radicals in the monomer droplets, which is determined exclusively by the events occurring inside the monomer droplets, that is, the production and mutual termination of radicals, if the exchange of radicals between the monomer droplets and the water phase can be neglected as mentioned above. This means that polymerization in the monomer droplets follows suspension polymerization kinetics. Therefore, [R*]d is given by 3

1

[R*] = (k f[I] ^ ) / d

d

d

2

td

14)

(

where k d is the mutual termination rate constant for the radicals in the monomer droplets. The total monomer conversion, X M , is defined by t

X

M

= (Pp + Pd)/Mo

(15)

(3) Monomer concentrations in the polymer particles and the monomer droplets are treated as follows. The chemical potential of monomer in a given phase i in the form of spheres with diameter di is well known to be expressed by


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POLYMER LATEXES

_-lnd + X

Kinetics and Mechanisms of Particle Formation and Growth in the

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