CC BY
16
UDC 542.9:628.34
M. D. Goldfein, N. V. Kozhevnikov, N. I. Kozhevnikova, G. E. Zaikov
KINETICS AND MECHANISM OF POLYMER DISPERSION FORMATION ON BASED OF (METH)ACRYLATES. PART 1
Keywords: emulsion polymerization, metacrylic acid, ammonium persulfate, presence and absence of emulsifier, stabilization of
dispersions, kinetics, mechanism.
The present paper studies the emulsion polymerization of methylacrylate (MA) and methylmethacrylate (MMA). The authors' experimental study was conducted by dilatometry and the turbidity spectrum method. The possibility and conditions of applying the turbidity spectrum method in kinetic studies of emulsion polymerization are discussed. The kinetics and mechanism of the emulsion polymerization of (meth) acrylates in the presence of various emulsifiers (sodium lauryl sulfate, sulfated oxyethylated alkylphenols) were studied. It has been shown that the kinetics and mechanism of emulsion polymerization contradict to the classical concepts of this reaction due to a variety of nucleation mechanisms, the presence of several growing radicals in the polymer-monomer particles, the manifestation of gel effect, flocculation of the particles at different stages ofpolymerization, the partial solubility of some of the tested emulsifiers in the monomer, interactions between radicals in the aqueous phase, resulting in the formation of surfactant oligomers that act as a "self' emulsifier as well as chain termination.
Ключевые слова: эмульсионная полимеризация, метакриловая кислота, персульфат аммония, присутствии и отсутствие
эмульгатора, стабилизация дисперсий, кинетика, механизм.
В статье исследуется эмульсионная полимеризации метакрилата (MA) и метилметакрилата (ММА). Экспериментальное исследование авторов было проведено дилатометрическим и турбидиметрическим методами. Обсуждаются возможности и условия применения турбидиметрического метода в кинетических исследованиях эмульсионной полимеризации. Изучены кинетика и механизм эмульсионной полимеризации (мет) акрилатов в присутствии различных эмульгаторов (лаурилсульфата натрия, сульфированных оксиэтилированных алкилфенолов). Показано, что кинетика и механизм эмульсионной полимеризации противоречит классической концепции этой реакции вследствие различных механизмов нуклеации, наличия нескольких растущих радикалов в полимер-мономерных частицах, проявления гель-эффекта, флокуляции частиц на различных стадиях полимеризации, частичной растворимости некоторых из использованных эмульгаторов в мономере, взаимодействия между радикалами в водной фазе, в результате чего формируются поверхностные олигомеров, которые действуют как "само" эмульгаторы, также как агенты обрыва цепи.
1 Introduction
Emulsion polymerization remains one of the most problematic sections of radical polymerization as a whole.
Development of its general theory meets principal difficulties. The reasons are multi-phase structure of emulsion system, variety of parameters determining kinetics and mechanism of the process. These parameters depend not only upon reagents' reactivity, but also on their phase distribution, reaction topochemistry, mechanism of particles' nucleation and stabilization.
Emulsion polymerization goes through 3 main stages - PMP formation (when free emulsifier is available), stationary process (when monomer droplets in water phase are present, and monomer equilibrium concentration is established in PMP), and reaction completion (when monomer in PMP is depleted). For many monomer-polymer systems the parameters of equilibrium swelling of latex particles are well- known [1]. Such parameters indicate that in the case of polar monomers droplets' disappearance and completion of constant rate sections on kinetic curves must occur at relatively low conversions. For example, in the case of MA polymerization - at 16 % conversion, MMA - 34 % [1].
2 Experimental
The emulsion polymerization of the following monomers was studied: methyl acrylate (MA), methyl
methacrylate (MMA). These monomers were thoroughly purified (releasing from the stabilizer, drying, distilling under reduced pressure in an inert gas, and recondensation in vacuum).
The polymerization initiator, ammonium persulphate (APS), was purified by recrystallization from its water-alcohol solution.
The following surfactants were used as emulsifiers:
- sulfated oxyethylated alkylphenol S-10, which is a reaction product of the nonionic wetting agent OP-10 (monoalkylphenyl ether of polyethylene glycol CnH2n+rCaH4-(OC2H4)m-OH , where n = 8-10, m = 10-12) with concentrated sulfuric acid followed by neutralization with ammonia [2];
- Neonol AP9-12S, an analog of S-10, oxyethylated nonylphenol sulfate with an ethoxylation degree of 12;
- sodium lauryl sulfate (LS).
Since polymerization is accompanied by volume effects, dilatometry fully conforms to the specified requirements to rate estimation [3]. There exists proportionality between the volumetric changes of the polymerizate during the reaction and the weight monomer-to-polymer conversion degree, determined by the densities of the monomer and polymer: AK _pn- pM AP , K Pn Po
where AV and AP are the changes in volume of the polymerizate and the weight of the monomer during polymerization, vo and Po the initial volume and weight
of the monomer, pm and pp the densities of the
monomer and polymer at the polymerization temperature.
Polymerization was carried out in special glass dilatometers of original design. The main elements of such a dilatometer are a measuring capillary, a reaction vessel, and a filling system providing the opportunity of releasing the monomer and dispersion medium from dissolved gases before polymerization. To avoid the influence of oxygen on the processes studied, the water-soluble and oil-soluble components of the reaction system were separately freed from dissolved air by multiple freezing (liquid nitrogen), high vacuum pumping, and thawing in vacuo with subsequent transfusions through the measuring capillary into the reaction vessel of the dilatometer. The tool filled with inert gas was then placed into a water bath over an electrical magnetic stirrer. The desired temperature was maintained via an ultrathermostat.
The estimation of the number and size of the resulting latex particles formed in the reaction was made by classical theory of emulsion polymerization [4] and turbidity spectrum method.
The classical theory establishes the nature of the influence of the concentration of initiator In and emulsifier E just on the number of particles in the emulsion N, which, starting from the second (fixed) stage of the reaction, was considered to be independent of the conversion degree and, for micellar nucleation, is described by: N = K[Inf 4[Ef 6. The emulsion polymerization rate is determined by the number of
particles, the average number n of radicals, and the monomer concentration [M]p therein:
W = kp n [M]pN/Na.
The turbidity spectrum method [5] based on the determination of the wavelength exponent in Angstrom's equation z = const 2~", describing the spectral dependence of the turbidity .zof colloidal solutions within a relatively narrow wavelength range. Turbidity is the coefficient in the exponent of Bouguer's
law I = Ioe
-T-l
where l is the scattering layer
thickness (the cuvette length) characterizing the ability of the disperse system to reduce the intensity of the incident light due to scattering. Turbidity depends on the number N of scattering centers, their sizes and optical properties: z = Nw2K(a,m),
where K(a,m) is the scattering efficiency factor (or scattering coefficient), a = 2nrn0/2 the relative size of a particle, m = n/no their relative refractive index (r the radius of a particle, 2the wavelength of light, n and no the refractive indices of a particle and the dispersion medium). The wavelength exponent u is a function of a and m and can be found by measuring the slope of the straight portion of the graphic dependence of turbidity
(or absorbance) D = lg (I0/I) = z ■ l/2.303 on 2
in the log-log coordinates.
It is possible to determine the parameters of the dispersion particles from the found wavelength exponent with the known dependences u(a,m) and K(a,m) on the relative size and the relative refractive index. These relationships for a discrete set of m according to the formulae of G. Mie's light scattering theory [6] were calculated [5]. However, these dependencies have an oscillating character for monodisperse systems. The allowance for the actual polydispersity of colloidal systems is typically provided by a graphical smoothing on the principle of symmetry of the oscillating curves plotted for specific values of the relative refractive index m. It is more convenient to use approximate analytic expressions for the characteristic functions of light scattering [7]. This approach allows computer processing of experimental data. Besides, due to the fact that the relative refractive index of the particles may continuously vary during the experiment (e.g., as the polymerization depth increases, the monomer-polymer ratio in the particles varies and, therefore, their spectral characteristics do as well), the table data calculated for a discrete set of m turn out to be inapplicable.
The mean radius and number concentration of particles in dispersed systems were calculated by the formulae [5]: r = aXrr/2mi0 , N =4n.z(2m}n02/2jK(a,m)a2, where 2m is the middle of the wavelength range in the
togartihrnc scale (Xm = ^¡2max2mm ).
The allowance for refractive index dispersion was made in accordance with the approaches to this problem developed in Ref. [8]. In this paper it is shown that the theoretical value of the wavelength exponent uo, which is calculated by the approximate equations with
r and N, and which corresponds to systems with negligible dispersion is related to the experimental parameter u by a relation like: uo = u + Au, where Au = kou + 2m(k - ko)/(m - 1) when u > 2 and Au = kou + um(k - ko)/(m - 1) when u < 2.
The quantities k and ko describe the refractive index dispersion of the dispersed phase n(2) and the dispersion medium no(2) and represent their logarithmic derivatives with respect to wavelength. The values of the parameter k are proposed [9] to be evaluated from the inverse relative dispersion S and the refractive index at the wavelength corresponding to the yellow line of sodium nD. Approximating n(2) by Cauchy's
binomial formula, we obtain:
k{I) = {nD -1)(V + A,?) i nDS{AF2 -Ac2) =
,
where 2 = [ )F~2 + )c~2 /2]~1/2 = 552.4 nm is the
middle of the spectral interval in the 1/) coordinates between the F and C lines.
The relation for k(2) was used for the final polymer dispersion when the conversion q approached
100%, and the latex particles were composed of polymer entirely. However, this approach is inapplicable in analysis of the properties of latex particles depending on the polymerization depth, as the particles consist of the polymer and monomer with their varying ratio depending on q. In the absence of monomer droplets, when almost all the monomer and polymer are in the latex particles, the refractive index of the scattering centers can be estimated through the corresponding values for the monomer nm and polymer np, in view of the volume ratio 9 of the monomer and polymer in a particle, calculated, in turn, from the conversion degree: n = (pnm + np)/(1 + 9).
But the values of S for such particles are unknown.
It has been shown [8] that the parameter k( X)
can also be estimated based only on nD, since for more than 100 samples of polymers it is described by a general dependence approximated as: - k( X) = Bo +
B1(nD - 1.4) + B2(nD - 1.4)2, where Bo = 0.01675, B-i = -0.026858, and B2 = 0.780829. The use of this approximation requiring the knowledge of nD, within the range nm -r- np (in the case of MA polymerization it is 1.4040-1.4725) to assess the value of k, apparently gives wrong results, since when nD = 1.4 - Bo/2B2 = 1.4172, this function has a minimum, which contradicts our experimental data. The dependence of k on the refractive index in this range of its values can be more accurately represented by linear interpolation between the values calculated for the monomer and polymer.
The parameter ko for the medium in the case of aqueous dispersions can be obtained from direct spectral measurements of no(X). Approximation of the data from Ref. [10] for no(X) of water at 20°C by Cauchy's tripartite formula gives ko = -0.0155 (X = 552.4 nm).
Turbidity spectra were recorded on a SF-26
spectrophotometer. The value Xcp=X = 552.4 nm was selected as the wavelength mid-range, and measurements were performed with a constant logarithmic step AlgX= 0.02.
The polymer dispersion formed by the emulsion polymerization of acrylates (the monomer concentration is 10-20%, the initiator is ammonium persulfate, APS) have relatively high optical density; therefore, they were diluted with water before measuring turbidity. However, it turned out that the results of measurements, and, in particular, the wavelength exponential in Angstrom's equation and the reduced turbidity (the turbidity multiplied by the dilution R) depended on the dilution (Fig. 1), especially at its relatively small values.
These dependencies are not observed at high dilutions. These data indicate that in sufficiently concentrated dispersions there is multiple secondary scattering due to which more (as compared with single scattering) light passes through the cuvette and reaches the receiver as if to reduce the turbidity of the system. The theory underlying the turbidity spectrum method does not account for multiple light scattering. Therefore, to estimate the parameters of the dispersion it is
necessary to dilute it to such an extent that the reduced turbidity becomes independent of R and remains constant at any dilution, which serves as the criterion of no multiple scattering [9].
The obtained data have allowed us to estimate the contribution of multiple light scattering at different dilution degrees, since the proportion of multiply scattered light at the exit of the cell T2 can be expressed [9] through the reduced turbidity at the given dilution (tR) and infinite dilution (tR)w = lim (tR) :
R—
T2 = I2/I = 1 - 10D-A, where D - D- = l (tR) - (tR)a
2.303 R
(I) being the intensity of light transmitted through the dispersion, which consists of the intensity / weakened due to single scattering, and the additional intensity I2 arising due to multiple scattering of radiation in the direction of the receiver).
u t R
4,0
3,6
3,4
1
100 80 60 40 20
0
20
40
60
80
R
Fig. 1 - Dependence of the wavelength exponent (24) and the reduced turbidity of the dispersions (1, 5, 6) obtained by MA emulsion polymerization (50°C) with the monomer concentration of 10 (2, 4-6) and 20% (1, 3) on the dilution prior to measuring turbidity. The emulsifier (1%) is LS (1-3, 6) and Neonol (4, 5); [APS] x 103 = 2 (2, 4-6) and 36 mol/l (1, 3)
The effect of multiple light scattering quickly decreases with increasing dilution (Fig. 2).
The necessary dilution degree depends on the dispersion properties, in particular, it is higher when sulfated oxyethylated alkylphenols (C-10, nonoxynol-9-12) are used as an emulsifier in comparison with sodium lauryl sulfate (LS), and it also depends on the monomer content in the initial emulsion.
Dilution simultaneously facilitates washing of the latex particle surface from the emulsifier molecules stabilizing them, whose refractive index differs from the corresponding quantities of the monomer or polymer, which, in the absence of dilution, introduces an additional uncertainty into the results. Therefore, polymer dispersions are pre-diluted with water by 100200 times and measurements are usually performed in 0.3-cm cuvettes with the value of transmittance maintaining within the range of 0.2-0.8.
Thus, our studies have shown that the turbidity spectrum method can be applied not only to the final polymer dispersions resulting from the emulsion polymerization of acrylic monomers but also to the emulsion systems arising in the course of the reaction at various conversions. It is necessary to take into account multiple (secondary) light scattering, and the dependences of the scattering properties of particles on the polymerization depth, those of the refractive indices of the particle and medium on wavelength. Under these conditions, the turbidity spectrum method can be used in kinetic studies of this reaction.
Fig. 2 - Dependence of the fraction of multiple light scattering at 2 = 552 nm at the cell outlet with l = 0.1 cm on the dilution degree of the dispersion obtained by MA polymerization of 20% (1) and 10% (2, 3) with LS (1, 3) and Neonol (2) as the emulsifier. [APS] x 103 = 2 (2, 3) and 36 mol/l (1). 50oC
3 Results and Discussion
Our research showed that constant rate stage continues to the higher conversions q, depending on
monomer structure and reaction conditions. In some cases polymerization rate does not slow down at high conversions, but, on the contrary, increases. Such effects, occurring notwithstanding the fact that monomer concentration in PMP is decreased, indicate that average radicals' amount is growing up, which is possible when large enough particles are formed and when high viscosity takes place in PMP [11].
Simultaneous growth of several radicals in the particle leads to polymerization acceleration and initiation of the so-called "gel-effect". The probability of gel-effect, according to [12], can be judged by the value of parameter a, which is defined as: a = SV / k0N, where 0 - the total rate of radical
entrance to the particles, N and V - their number and average volume, k0 - rate constant of bimolecular
chain termination. Polymerization rate growth, caused by increasing of k0, starts to be noticeable when a >
10-4, at a > 10-2 gel effect is pronounced. If, as a first approximation, to assume that S is equal to the rate of radicals formation in water phase (initiation rate), and use the final dispersion data, than a parameter value can be estimated. The results are shown in the fig. 3. It indicates the existence of gel-effect, which is especially
strongly pronounced when Neonol AP9-12S and S-10 are used as emulsifiers. In the presence of these emulsifiers larger latex particles are formed, which promotes the origination of gel-effect. It weakens while emulsifier concentration is increased, but becomes stronger with the growth of initiation rate.
In the case of MA polymerization gel-effect starts already at initial reaction stages. MMA is characterized by lower propagation rate constant, and because of that conditions for gel-effect arise only at high enough conversion, when monomer concentration decreases and viscosity in PMP grows up. This leads to more pronounced influence on the shape of kinetic curves.
2
a 10
(1) (2, 3)
20
10
ШЕ], %
0
2
4
[APS] x 10 3 (mol/l)
Fig. 3 - Parameter a value against initiator concentration (1), emulsifier Neonol AP9-12S (2) and LS (3) concentrations, emulsion polymerization of MA. (Parameter a is the measure of gel-effect probability). [Neonol] = 1 %, 60oC (1); [APS] = 5 x 10-3 mole/l, 50oC (2, 3)
Thus, for the emulsion polymerization of studied monomers classical conceptions of instantaneous chain termination in PMP in the moment of second radical entrance are not valid.
The growth of emulsion polymerization kinetic order with respect to emulsifier (LS) with the increase of initiator concentration is reported in [13]. This is connected to the interaction of oligomeric radicals and chain termination in the water phase. In the present work, studying reaction rates, we also found similar relationships (fig. 4). ne
0,6
0,4
0,2
0
10
20 30
[APS] x 103 (mole/l) Fig. 4 - Kinetic order of MA emulsion polymerization with respect to emulsifier LS (1) and Neonol AP9-12S (2) versus initiator concentration. [MA] = 20 %; 50oC
Moreover, kinetic order with respect to
emulsifier ne is greater in the case of S-10 or Neonol,
than with LS. This is the result of partial solubility of oxyethylated alkylphenols in the monomer. We found [14] (using electronic spectroscopy) that with the growth of these emulsifiers' concentration the part of them staying in water also increases. This part is promoting PMP formation, which increases the influence of Neonol and S-10 concentration on polymerization rate comparing with LS.
Established dependences of kinetic order with respect to emulsifier on reaction conditions, emulsifier
properties indicate that ne, contrary to the prevailing
view, cannot characterize the emulsion polymerization of certain monomer.
The classic emulsion polymerization theory quantitatively describes the influence of emulsifier and initiator on the number of formed latex particles. Including the reaction rate into the same description pattern presumes that it is proportional to the number of particles. However, the reaction rate is determined not only by number of particles, but also by the rate of monomer conversion inside them, which can change due to the conditions and depth of polymerization (for example, due to different degree of gel-effect).
Our studies of emulsifier concentration effect on latex particles number in the final dispersion N100 (q =100 %) showed, that kinetic order with respect to
emulsifier, obtained using this data, is much larger than calculated using rate values (especially in the case of Neonol and S-10).
It's important to keep in mind that emulsion polymerization rate and N100 value characterize the process and forming dispersion at different conversions and comparison between them is justified only when one of the basic postulate of classical theory is working - number of PMP during the reaction after completion of its first stage is constant. However, our research showed that number of particles in the emulsion depends on polymerization depth. The most common effect - decreasing of particles number during the third stage of the reaction (fig. 5, 6).
W x 10 mole/(l x s) N x 10 -13 (sm) J
20 -,
[M]part (moWF
w x 10 (sm/s
10
80 100 q(%)
Fig. 5 - Polymerization rate (1), number of particles in emulsion (3), monomer concentration and rate in the particle (5, 2) against monomer-polymer conversion depth, emulsion polymerization of MA. 4 - Specific rate in the particle at n = 0,5. [APS] = 0,25 x 10-3 mole/l; [LS] = 1 %; 60oC
This indicates the changing of interphase surface condition and decreasing of its stabilization degree with the conversion growth. Such effects can be caused by various reasons, for example by changing the ratio of monomer/polymer concentrations in the particle. In addition, all the time during polymerization new and new charged oligomer radicals are entering PMP from water phase and sometimes start the reaction in the particle, sometimes terminate it. Such radicals are increasing the charge on the particles' surface and strengthening the electrostatic repulsion of surface active substances molecules (SAS), that hinders their absorption and can cause even desorption of emulsifier. As a result, flocullation of particles occurs, not only at the initial conversions, as assumed in [1], but also at more advanced stages of the reaction.
When partially soluble in monomer S-10 and Neonol AP9-i2S are used as stabilizers, the conversion growth is followed at first by increase of PMP number, and only after that by its decrease (fig. 6). In this case during polymerization the emulsifier, dissolved in monomer, is released. This emulsifier is stabilizing new particles. At the same time the emulsifier replenishment promotes flocculation due to the growth of interphase surface, and also because new small particles are less stable [15].
N x1013(sm-3 )
15
10
0
20 40
60
80 100 q
Fig. 6 - Number of particles in the emulsion versus monomer-polymer conversion depth, polymerization of MA.[APS] x 103 = 0,1 (1), 0,25 (2), 1 (3), 4 (4), 8 mole/l (5); [Neonol] = 1 %; 60oC
Flocculation is the cause of abnormal high kinetic orders with respect to emulsifier (> 0,6), found from particles number data in final dispersion, because at high emulsifier concentrations number of particles decreases due to the flocculation at third reaction stage not so heavily as when emulsifier concentration is low.
We determined specific reaction rate in the particles, comparing reaction rate and number of particles at the same conversion level. Specific reaction rate was found to change with the conversion. This is related to a considerable degree to the decreasing of monomer concentration in the particle [M]part . At the
initial polymerization stages, when monomer droplets are still present in the reaction system, monomer concentration in the particle is determined by equilibrium degree of dissolving in polymer (equilibrium swelling degree) and remains constant. At
5
5
2
1
higher conversions it decreases. If we consider all monomer and polymer to sit in PMP, the dependence of [^W (mole/l) on polymerization depth at third
reaction stage will be given by:
= lom^ _-
m0 1 -q ■ 1 - PMjPp ) (P M, P P - densities of monomer and polymer, mo -
monomer molecular mass).
Specific rate in the particle w = W/ (N [M]part) grows with monomer-polymer conversion degree (fig. 5, 7). This means that radicals' number in PMP is increasing. Moreover, the extrapolation of specific rate to the initial conversion gives the value corresponding to the specific rate at n = 0,5 (dotted line 4 in fig. 5 is calculated assuming kp for MA is equal to 1190 l/mole
x sec [16]). Reaction rate in the particles grows with the conversion depth depending on initiator concentration -higher concentrations lead to higher growth (fig. 7), indicating the increase of gel-effect. Consequently, our data confirm the previous conclusion that in the case of MA emulsion polymerization gel-effect emerges already at low conversions and than grows gradually as particle size in increasing at second polymerization stage, and as viscosity is growing at third stage due to increasing of polymer concentration. When MMA is polymerized, gel-effect starts to be noticeable only at high conversions and is much more manifested than with MA (fig.7).
W/W
10 -
5 -
20
40
60
80 100
q
Fig. 7 - Behavior of specific reaction rate in the particle at the third stage of MA (1-3) and MMA (4) emulsion polymerization. [APS] x 103 = 1 (1), 1,5 (4), 4 (2), 8 mole/l (3); [Neonol] = 1 (1-3) h 2 % (4); 60oC
Initiation rate also influences emulsion polymerization of (meth)acrylates in an unusual way. The parameters of this process and polymer dispersion differently depend on initiator concentration. The maximal rate value W , as a rule, increased with the
max ' '
growth of APS, but differently in the various ranges of PSA concentration change. As a result, the graph of initiation rate versus initiator concentration in logarithmic scale (used to calculate the order with respect to initiator ni ) has the form of broken line with
the point of inflexion (fig. 8). Value of nt at the
relatively low initiator concentrations turned out to be
higher than expected theoretical value of 0,4 (nj = 0,6 -
0,7), but in the relatively high concentration region -
lower (ni = 0,25 - 0,35). Thus, kinetic order of the
emulsion polymerization with respect to initiator depends not only on monomer properties, nature and concentration of emulsifier, but also on the region of initiator concentrations where it is measured.
1
4+lgW
2,0
1,5
1,0-
0,5-
1 2 4+lg[APS]
Fig. 8 - Emulsion polymerization of MA: rate against initiator concentration.Emulsifier - LS (0,4 %) (1), Neonol (1 %) (2), S-10 (1 %) (3). 60oC
Number of particles in the polymer dispersion N100 is growing initially, but later starts to decrease with the growth of PSA concentration in the presence of Neonol or S-10. The stationary rate curve also passes maximum. The observed extremal curves are related to the flocculation processes, taking place in the varius polymerization stages. For instance, dependence of N100 on intiator concentration is determined by flocculation at all stages of the process, while stationary rate is influenced by flocculation only at initial stages. Behavior of maximum rate versus [APS] is influenced both by flocculation growth and by initiator concentration increase, which promotes acceleration of the reaction in the particles. Therefore, at relatively low initiator concentrations, when flocculation is still weak, the gel-effect intensification causes more drastic dependence of reaction rate on initiator concentration than is expected based on classic concepts (ni > 0,4). At
the high initiator content, depending on flocculation -gel-effect relative efficiency correlation, reaction rate continue to grow (though with low ni value), or even
starts to decrease.
Number of particles in emulsion at the conversion corresponding to maximum polymerization rate also depends extremally on initiator concentration. This fact, together with the stationary rate passing maximum, indicates that flocculation takes place at the initial reaction stages. At high [APS] flocculation completely compensates the growth of particles' number due to the initiation rate increase; at low [APS] this is not the case. Thus, flocculation at initial polymerization stages increases with the initiation rate growth and reduces the ni value. Flocculation at the
third stage, as a rule, almost does not influence Wmax
c 7 7 mar
0
and, consequently, nt, because the rate reaches
maximum at lower conversions. However, it reduces the number of particles in final dispersion. The reducing effect is more striking with emulsifiers S-10 or Neonol, than with LS, because the first two emulsifiers cause stronger flocculation at the third reaction stage, due to emulsifier replenishment from monomer droplets or PMP. In the case of LS N100 value is increasing in all studied initiator concentration range and does not pass maximum.
It is shown that kinetics behaviour of the reaction is determined by: different nucleation mechanisms, initiation of gel-effect, bimolecular chain termination in water phase, flocullation of polymer-monomer particles during all polymerization stages, partial emulsifier solubility in the monomer. These effects lead to the dependance of particles number and reaction rate in particles on conversion and to the influence of polymerization conditions on kinetics orders with respect to initiatior and emulsifier concentrations [17].
4 Conclusions
The authors' experimental study was conducted by dilatometry and the turbidity spectrum method. The possibility and conditions of applying the turbidity spectrum method in kinetic studies of emulsion polymerization are discussed. It is necessary to take into account the spectral dependence of the refractive index of the polymer-monomer particles and dispersion medium, secondary light scattering, as well as the use of approximating analytic expressions for the characteristic functions of light scattering, since the refractive index of the scattering centers depends on the depth of polymerization.
The kinetics and mechanism of the emulsion polymerization of (meth) acrylates in the presence of various emulsifiers (sodium lauryl sulfate, sulfated oxyethylated alkylphenols) were studied. It has been shown that the kinetics and mechanism of emulsion polymerization contradict to the classical concepts of this reaction due to a variety of nucleation mechanisms, the presence of several growing radicals in the polymer-monomer particles, the manifestation of gel effect, flocculation of the particles at different stages of polymerization, the partial solubility of some of the tested emulsifiers in the monomer, interactions between radicals in the aqueous phase, resulting in the formation
of surfactant oligomers that act as a "self' emulsifier as well as chain termination. These effects lead to the number of particles and the reaction rate therein depending on the conversion degree, the influence of polymerization conditions on the kinetic orders by the emulsifier and initiator concentrations.
References
1. Eliseeva, V.I. Polymeric Dispersions (Moscow, 1980). 296 pp. (in Russian).
2. Eliseeva, V.I., Ivanchev, S.S., Kuchanov S.I. and Lebedev, A.V. Emulsion Polymerization and its Application in Industry (Moscow, 1976). 240 pp. (in Russian).
3. Gladyshev, G.P. and Popov, V.A. Radical Polymerization at High Degrees of Conversion (Moscow, 1974), 244 (in Russian).
4. Smith, W.V. and Ewart, R.H. J. Chem. Phys., 16 (6), 592599 (1948).
5. Klenin, V.I., Shchegolev, S.Yu. and Lavrushin, V.I. Characteristic functions of light scattering from disperse systems. Saratov: Saratov Univ. Press, 1977. 177 p.
6. Bohren, C.F. and Huffman, D.R. Absorption and Scattering of Light by Small Particles (Wiley-Interscience, 1983). 11. Ivanchev, S.S. Radical Polymerization (Leningrad, 1985). 280 pp. (in Russian).
7. Ramazanov, K.R., Khlebtsov, N.G., Shchegolev, S.Yu. and Klenin, V.I. Colloid Journal, 45 (3), 473-479 (1983).
8. Khlebtsov, N.G., Melnikov, A.G. and Shchegolev, S.Yu. Colloid Journal, 53 (5), 928-933 (1991).
9. Klenin, V.I., Shchegolev, S.Yu. and Lebedeva, L.G/ Optics and Spectroscopy, 35 (6), 1161-1166 (1973).
10. Joffe, B.V. Refractometric methods in chemistry. Leningrad: Khimiya. 1983. 352 p.
11. Ivanchev, S.S. Radical Polymerization (Leningrad, 1985). 280 pp. (in Russian).
12. Friis, N. and Hamielec, A.E.Amer. Chem. Soc. Polymer Preprints, 16 (1), 192 (1975).
13. Kozhevnikov, N.V., Goldfein, M.D., Zyubin, B.A. and Trubnikov, A.V. Vysokomolek. Soed. A., 33 (6), 12721280 (1991).
14. Kozhevnikov, N.V., Terekhina, N.V. and Goldfein, M.D. Proceedings of Russian Higher Educational Establishments. Chemistry and chemical technology, 41 (4), 83-87 (1998).
15. Dunn, A.S. and Chong, L.C-H. Brit. Polym. Journal., 2 (12), 49-59 (1970).
16. Bagdasariyan, H.S. Theory of Radical Polymerization (Moscow, 1966). 300.
17. Kozhevnikov, N.V., Kozhevnikova, N.I. and Goldfein, M.D., Proceedings of Russian Higher Educational Establishments. Chemistry and chemical technology, 53 (2), 6468 (2010).
© M. D. Goldfein - Doctor of Chemistry, Full Professor, Saratov State University, Saratov, Russia, N. V. Kozhevnikov - Doctor of Chemistry, Full Professor, Saratov State University, Saratov, Russia, N. I. Kozhevnikova - Ph.D., Associate Professor, Saratov State University, Saratov, Russia, G. E. Zaikov - Doctor of Chemistry, Full Professor, Plastics Technology Department, Kazan National Research Technological University, Kazan, Russia, [email protected].
© М. Д. Гольдфейн - доктор химических наук, профессор, Саратовский государственный университет, Саратов, Россия, Н. В. Кожевников - доктор химических наук, профессор, Саратовский государственный университет, Саратов, Россия, Н. И. Кожевникова - кандидат химических наук, доцент, Саратовский государственный университет, Саратов, Россия, Г. Е. Заиков - доктор химических наук, профессор, кафедра Технологии пластических масс, Казанский национальный исследовательский технологический университет, Казань, Россия. [email protected].
