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Effect of emulsifier additions in the course of the emulsion polymerization of styrene
EFFECT OF EMULSIFIER ADDITIONS IN THE COURSE OF THE EMULSION POLYMERIZATION OF STYRENE* G. D. BEREZHNOI, 1). M. KHOMIKOVSKII, S. S. MEDVEDEV and I. V. I)OLUYAN M. V. Lomonosov Moscow Institute of Fine Chemical Technology, L. Ya. Karpov PhysicoChemical Institute {Received 19 June 1963)
IN REFERENCES [1-3] it has been shown that emulsion (latex) polymerization of monomers not soluble in water is accomplished in the soap micelles and in a zone close to the surface of the polymer particles, the size of which remains approximately constant during the course of the polymerization. The rate of the process is determined b y the total concentration of emulsifier in the system (up to saturation of the particle surface b y emulsifier). All the data about the dependence of the rate of the process on emulsifier concentration were obtained in conditions when this change took place in the very start of the process. Ideas developed b y us lead to the conclusion that the rate of the process, being determined b y the total surface occupied b y emulsifier molecules, independent of whether the emulsifier is present in the form of micelles or in the form of absorbed layers on the polymer particles, will change with a change in the magnitude of this surface (that is, with emulsifier concentration). One m a y suppose that the addition of emulsifier in the course of polymerization will lead to an increase in the reaction rate corresponding to the increase in the surface of the polymer-monomer particles occupied b y adsorbed emulsifier molecules. The present work is devoted to a clarification of this question. Kolthoff et al. [4, 5] carried out work to clarify the effect of emulsifier additions made during polymerization on the rate of emulsion polymerization of styrene and the copolymerization of styrene with butadiene. The emulsifier was added in quantities which made possible the appearance of soap in the aqueous phase of the emulsion above the critical concentration for micelle-formation (CCM), and in smaller quantities. In the case of the butadiene-styrene latex, the emulsifier addition led to an increase in the rate of the process. An explanation of the accelerating action of the emulsifier was given only for the case when the quantity of additions ensured the appearance of soap in the aqueous phase of the emulsion in concentrations above the CCM. The addition of different amounts of emulsifier, * Vysokomol. soyed. 6: :No. 5, 891-895, 1964.
980
Emulsion polymerization of styrene
981
which could be adsorbed on the polymer particles, to the polystyrene latex did not lead to an increase in the rate of polymerization. This fact did not receive a n a d e q u a t e e x p l a n a t i o n i n [4].
EXPERIMENTAL Experiments were carried out in the apparatus shown in Fig. l. The ratio of the aqueous to the organic phase was equal to throe. Styrene was placed in the vessel 1; in vessel 2, a solution of the emulsifier M K (the sodium salt of sulphonic acids of an aliphatic series of average composition C~sH31SO~Na) at a low concentration; and in vessel 3, a solution of M K emulsifier at the m a x i m u m possible concentration (41°~).
B Vacuum
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982
G . D . BER~ZHNOI et al.
The effect of additions of emulsifier made during the process on the polymerization rate was studied with the usual different initiators--potassium persulphate, azoisobutyronitrile, and benzoyl peroxide. RESULTS AND DISCUSSION
The results of the investigations are set out in Figs. 2a, b and c, a n d in the Table. Emulsifier addition turned out to have an accelerating effect at all stages of the process up to a 50% degree of transformation, t h a t is, until drops of the monomer are contained in the latex (Fig. 2}. Emulsifier addition at a degree o f transformation greater than 50-60%, t h a t is, at the stage when all the monomer is to be found in the polymer-monomer particles, does not lead to an increase in the polymerization rate. As has been shown previously [6], with initiation of the polymerization by azoisobutyronitrile and benzoyl peroxide the rate of the process increases linearly with increase in initial emulsifier concentration up to 7%, and does not change with further increase in the emulsifier concentration. An increase in emulsifier concentration up to 7% during the process also leads to an increase in polymerization rate (see Table). In the case of emulsifier additions during the process at a degree of transformation of 10-20%, with an initial emulsifier concentration of 7~o, the polymerization rate is reduced (Fig. 2b). From Fig. 2c it can be seen that, on adding emulsifier up to 11%, a larger reduction in velocity takes place than on adding emulsifier up to 90/0. I t follows from the data presented in the Table t h a t at small emulsifier concentrations (initial concentration, 1 °/o; after the addition, 2.5-3%) the rate of polymerization after the addition of the soap was somewhat less t h a n with an initial emulsifier concentration equal to 2.5-3.0%. In the region of initial emulsifier concentrations equal to 2 . 5 - 3 ~ , and concentrations of 5-7% after additions, the increase in polymerization rate was proportional to the increase in emulsifier concentration (to the power one in the ease of initiation by azobisisobutyronitrile and benzoyl peroxide, or to the power two in the ease of potassium persulphate, in conformity with laws established previously [3]). Measurements of the surface of the polymer-monomer particles* on adding emulsifier from 1 to 3 % showed t h a t in this case the surface area of the particles was somewhat increased. This m a y be explained by the fact t h a t with an emulsifier concentration of 1% the adsorbed layers on the surface of the p o l y m e r monomer particles is strongly unsaturated even at the early stages of polymerization [6] and particle aggregates are contained in the system in considerable numbers. The addition of emulsifier leads to their peptization and an increase in the total surface. I t is also possible t h a t the formation of a small number of new particles takes place on addition of emulsifier. , * The surface determinations, as in the preceding work [6], were carried out by the method of titrating with soap.
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F i e . 2. Yield of polymer (q, grams of polymer per 100 ml of aqueous phase) with time. a - - E m u l s i f i e r a d d e d a t different stages of the process. Initial concentration of M K emulsifier, l~o; after the addition, 2%; Initiator, azoisobutyronitrile (1~o of the monomer), 50°C. b--Concentration of M K emulsifier increased during the course of the process from 7 to 9%, 1 - - B e n z o y l peroxide (1"5~o of monomer), 50°C; 2 - - p o t a s s i u m persulphato (0,4% of aqueous phase), 40°C; 3--azoisobutyronitrile (1.5% of monomer) 50°C. c - - D e pendence of yield on the amount of M K emulsifier added. / - - I n i t i a l latex with 7~o M K ; 2--emulsifier addition to bring concentration from 7 to 9%; 3 - - f r o m 7 to 11~o. Azoiso. butyronitrile (1"0~o of monomer), 50°C. Emulsifier added at 16-5~/o degree of polymeriza. tion.
Emulsion polymerization of styrene
985
Emulsifier addition from 1 up to 3% to the latex obtained in the presence of benzoyl peroxide and azobisisobutyronitrile leads to an increase in the total particle surface area from 2400 up to 3100 m~/100 ml of aqueous phase. The particle surface area with an initial emulsifier concentration of 2% is 3200 m2/100 ml of aqueous phase. The polymerization rate after the additional introduction of emulsifier also approximated to the reaction rate with an initial emulsifier concentration equal to 2%. I t was established t h a t in the case of emulsifier additions to latex obtained in the presence of 7 % emulsifier (in our experimental conditions [6], the adsorbed layers on the particle surface were completely saturated with emulsifier molecules at an emulsifier concentration of 6-7 %), the total surface of the polymer-monomer particles did not change. The reduction in polymerization rate thus observed m a y be connected with a change in diffusion conditions at the separation boundary in conditions of polymolecular adsorbed layers formed at emulsifier conceutrations higher t h a n 6-7 %. Thus, it does not matter whether polymolecular adsorbed layers are formed at the start of pol~nerization or in the course of the process, since with an increase in the initial emulsifier concentration above 7 % no change in the rate of polymerization occurs [3, 6]. In any case, the results obtained at large emulsifier concentrations indicate t h a t the formation of new particles does not take place on the addition of emulsifier (the total surface area of the particles does not change). The results obtained agree with the ideas expressed earlier about the progress of emulsion polymerization of monomers, not soluble in water, in soap micelles and in a certain zone close to the surface of the polymer particles, the size of which remains constant during the process and is determined by the emulsifier concentration. Increase in the surface of polymer-monomer particles in the course of polymerization by the method of adding emulsifier leads to an increase in the rate of polymerization, A reduction in the reaction rate on adding emulsifier to latex containing a large q u a n t i t y of emulsifier, in which the adsorbed layers on the particle surfaces are completely saturated, is connected with secondary effects. CONCLUSIONS (1) The effect of emulsifier additions, made during the course of the process, on the rate of emulsion polymerization of styrene, under the influence of the initiators benzoyl peroxide, azobisisobutyronitrile and potassium persulphate has been investigated. (2) I t has been shown that, for initial emulsifier concentrations for which the adsorbed layers on the polymer particle surface are unsaturated or near to saturation, the addition of emulsifier during the process leads to an increase in the polymerization rate. These results confirm ideas developed previously about the progress of emulsion polymerization close to the surface of polymer particles. Translated by G. MODLEN
986
V. YE. ESKIN et a~.
REFERENCES 1. A. P. SHEIKER and S. S. MEDVEDEV, Dokl. Akad. Nauk SSSR 97: 111, 1954; Zh. fiz. khimii 29: 250, 1955 2. Ye. V. ZABOLOTSKAYA, I. G. SOBOLEVA, N. V. M A K L E T S O V A and S. S. MEDVEDEV, Dokl. Akad. Iqauk SSSR 94: 81, 1954; Kolloidn. zh. 18: 420, 1956 3. G. D. BEREZHNOI, P. M. KHOMIKOVSKII and S. S. MEDVEDEV, Vysokomol. soyed. 2: 141, 1960 4. I. KOLTHOFF, E. MECHAN and C. CARR, J. Polymer Sci. 7: 577, 1951 5. F. BOVEY, I. KOLTHOFF, A. MEDALIA and E. MECI-IAN, Emulsion Polymerization, N.Y., London, 1955 6. G. D. BEREZHNOI, P. M. KHOMIKOVSKII and S. S. MEDVEDEV, Vysokomol. soyed. 3: 1839, 1961
THE COMPOSITIONAL HETOROGENEITY AND FRACTIONATION OF STYRENE-METHYL METHACRYLATE COPOLYMERS* V, Y E . E S K I N , I. A. ]~ARANOVSKAYA, A. D. LITMANOVICH a n d A. V. T O P C H I E V Institute for High Molecular Compounds, U.S.S.R. Academy of Sciences (Received 20 J u n e 1963)
IT IS well known that the combination of two monomers in one copolymer leads in a number of cases to new desirable technical properties being obtained, which are absent in each of the corresponding homopolymers separately. The optimum properties of the copolymer thus correspond to a fixed compositional make-up of its molecules. However, a different degree of heterogeneity in the eopolymer's composition (compositional dispersion) will be brought about by differing conditions of copolymerization. I n quantitative procedures, the necessity thus arises of determining the degree of compositional dispersion of copolymers. At the present time the study of light scattering by copolymer solutions is one such method. The basis of the method lies in the idea founded on experiment that the increment of refractive index of a copolymer solution in a given solvent v = d n / d c is additive (for sufficiently small concentrations) in the ratio of the increments vA and vB of the components (homopolymers) entering into the copolymer [1]: v = x v A + ( 1 - - x ) VB •
* Vysokomol. soyed. 6: No. 5, 896-900, 1964.
(1)
IN REFERENCES [1-3] it has been shown that emulsion (latex) polymerization of monomers not soluble in water is accomplished in the soap micelles and in a zone close to the surface of the polymer particles, the size of which remains approximately constant during the course of the polymerization. The rate of the process is determined b y the total concentration of emulsifier in the system (up to saturation of the particle surface b y emulsifier). All the data about the dependence of the rate of the process on emulsifier concentration were obtained in conditions when this change took place in the very start of the process. Ideas developed b y us lead to the conclusion that the rate of the process, being determined b y the total surface occupied b y emulsifier molecules, independent of whether the emulsifier is present in the form of micelles or in the form of absorbed layers on the polymer particles, will change with a change in the magnitude of this surface (that is, with emulsifier concentration). One m a y suppose that the addition of emulsifier in the course of polymerization will lead to an increase in the reaction rate corresponding to the increase in the surface of the polymer-monomer particles occupied b y adsorbed emulsifier molecules. The present work is devoted to a clarification of this question. Kolthoff et al. [4, 5] carried out work to clarify the effect of emulsifier additions made during polymerization on the rate of emulsion polymerization of styrene and the copolymerization of styrene with butadiene. The emulsifier was added in quantities which made possible the appearance of soap in the aqueous phase of the emulsion above the critical concentration for micelle-formation (CCM), and in smaller quantities. In the case of the butadiene-styrene latex, the emulsifier addition led to an increase in the rate of the process. An explanation of the accelerating action of the emulsifier was given only for the case when the quantity of additions ensured the appearance of soap in the aqueous phase of the emulsion in concentrations above the CCM. The addition of different amounts of emulsifier, * Vysokomol. soyed. 6: :No. 5, 891-895, 1964.
980
Emulsion polymerization of styrene
981
which could be adsorbed on the polymer particles, to the polystyrene latex did not lead to an increase in the rate of polymerization. This fact did not receive a n a d e q u a t e e x p l a n a t i o n i n [4].
EXPERIMENTAL Experiments were carried out in the apparatus shown in Fig. l. The ratio of the aqueous to the organic phase was equal to throe. Styrene was placed in the vessel 1; in vessel 2, a solution of the emulsifier M K (the sodium salt of sulphonic acids of an aliphatic series of average composition C~sH31SO~Na) at a low concentration; and in vessel 3, a solution of M K emulsifier at the m a x i m u m possible concentration (41°~).
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U FIG. 1. D i a g r a m of the apparatus for filling the dilatometer. Explanation in text.. After the usual operations to free the styrene and emulsifier solutions from dissolved air, by freezing and melting in vacuum, with the tap 4 closed the styrene was poured in v a c u u m into the dilatometric vessel 7 from 1, and the low concentration emulsifier MK from 2. I n t o the right hand part of the system concentrated MK emulsifier solution was poured from the vessel 3; the tap 5 was closed and the graduated part of the burette filled with water. The system was filled with dry nitrogen at atmospheric pressure. The dilatometer was separated from the proportioning feeder (by removal along the line aal) and placed in a thermostat. The taps 4 and 5 and the cone 6 were greased with a paste of the MK emulsifier. Depending on the object of the experiment, the emulsifier addition was made at various degrees of polymerization "rod with a variable concentration of emulsifier in the initial latex. Tap 4 was first opened, the magnetic stirrer having been shut off as a preliminary, and then tap 5. A set amount of latex (usually not more than 1.1 ml) was squeezed into the cone a, by mercury pressure. Taps 4 and 5 were shut, the displaced latex was taken out through the top of the capillary, the magnetic stirrer was switched on and, by the m o v e m e n t of the level in the capillary dilatometer 8, the rate of polymerization was determined.
982
G . D . BER~ZHNOI et al.
The effect of additions of emulsifier made during the process on the polymerization rate was studied with the usual different initiators--potassium persulphate, azoisobutyronitrile, and benzoyl peroxide. RESULTS AND DISCUSSION
The results of the investigations are set out in Figs. 2a, b and c, a n d in the Table. Emulsifier addition turned out to have an accelerating effect at all stages of the process up to a 50% degree of transformation, t h a t is, until drops of the monomer are contained in the latex (Fig. 2}. Emulsifier addition at a degree o f transformation greater than 50-60%, t h a t is, at the stage when all the monomer is to be found in the polymer-monomer particles, does not lead to an increase in the polymerization rate. As has been shown previously [6], with initiation of the polymerization by azoisobutyronitrile and benzoyl peroxide the rate of the process increases linearly with increase in initial emulsifier concentration up to 7%, and does not change with further increase in the emulsifier concentration. An increase in emulsifier concentration up to 7% during the process also leads to an increase in polymerization rate (see Table). In the case of emulsifier additions during the process at a degree of transformation of 10-20%, with an initial emulsifier concentration of 7~o, the polymerization rate is reduced (Fig. 2b). From Fig. 2c it can be seen that, on adding emulsifier up to 11%, a larger reduction in velocity takes place than on adding emulsifier up to 90/0. I t follows from the data presented in the Table t h a t at small emulsifier concentrations (initial concentration, 1 °/o; after the addition, 2.5-3%) the rate of polymerization after the addition of the soap was somewhat less t h a n with an initial emulsifier concentration equal to 2.5-3.0%. In the region of initial emulsifier concentrations equal to 2 . 5 - 3 ~ , and concentrations of 5-7% after additions, the increase in polymerization rate was proportional to the increase in emulsifier concentration (to the power one in the ease of initiation by azobisisobutyronitrile and benzoyl peroxide, or to the power two in the ease of potassium persulphate, in conformity with laws established previously [3]). Measurements of the surface of the polymer-monomer particles* on adding emulsifier from 1 to 3 % showed t h a t in this case the surface area of the particles was somewhat increased. This m a y be explained by the fact t h a t with an emulsifier concentration of 1% the adsorbed layers on the surface of the p o l y m e r monomer particles is strongly unsaturated even at the early stages of polymerization [6] and particle aggregates are contained in the system in considerable numbers. The addition of emulsifier leads to their peptization and an increase in the total surface. I t is also possible t h a t the formation of a small number of new particles takes place on addition of emulsifier. , * The surface determinations, as in the preceding work [6], were carried out by the method of titrating with soap.
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F i e . 2. Yield of polymer (q, grams of polymer per 100 ml of aqueous phase) with time. a - - E m u l s i f i e r a d d e d a t different stages of the process. Initial concentration of M K emulsifier, l~o; after the addition, 2%; Initiator, azoisobutyronitrile (1~o of the monomer), 50°C. b--Concentration of M K emulsifier increased during the course of the process from 7 to 9%, 1 - - B e n z o y l peroxide (1"5~o of monomer), 50°C; 2 - - p o t a s s i u m persulphato (0,4% of aqueous phase), 40°C; 3--azoisobutyronitrile (1.5% of monomer) 50°C. c - - D e pendence of yield on the amount of M K emulsifier added. / - - I n i t i a l latex with 7~o M K ; 2--emulsifier addition to bring concentration from 7 to 9%; 3 - - f r o m 7 to 11~o. Azoiso. butyronitrile (1"0~o of monomer), 50°C. Emulsifier added at 16-5~/o degree of polymeriza. tion.
Emulsion polymerization of styrene
985
Emulsifier addition from 1 up to 3% to the latex obtained in the presence of benzoyl peroxide and azobisisobutyronitrile leads to an increase in the total particle surface area from 2400 up to 3100 m~/100 ml of aqueous phase. The particle surface area with an initial emulsifier concentration of 2% is 3200 m2/100 ml of aqueous phase. The polymerization rate after the additional introduction of emulsifier also approximated to the reaction rate with an initial emulsifier concentration equal to 2%. I t was established t h a t in the case of emulsifier additions to latex obtained in the presence of 7 % emulsifier (in our experimental conditions [6], the adsorbed layers on the particle surface were completely saturated with emulsifier molecules at an emulsifier concentration of 6-7 %), the total surface of the polymer-monomer particles did not change. The reduction in polymerization rate thus observed m a y be connected with a change in diffusion conditions at the separation boundary in conditions of polymolecular adsorbed layers formed at emulsifier conceutrations higher t h a n 6-7 %. Thus, it does not matter whether polymolecular adsorbed layers are formed at the start of pol~nerization or in the course of the process, since with an increase in the initial emulsifier concentration above 7 % no change in the rate of polymerization occurs [3, 6]. In any case, the results obtained at large emulsifier concentrations indicate t h a t the formation of new particles does not take place on the addition of emulsifier (the total surface area of the particles does not change). The results obtained agree with the ideas expressed earlier about the progress of emulsion polymerization of monomers, not soluble in water, in soap micelles and in a certain zone close to the surface of the polymer particles, the size of which remains constant during the process and is determined by the emulsifier concentration. Increase in the surface of polymer-monomer particles in the course of polymerization by the method of adding emulsifier leads to an increase in the rate of polymerization, A reduction in the reaction rate on adding emulsifier to latex containing a large q u a n t i t y of emulsifier, in which the adsorbed layers on the particle surfaces are completely saturated, is connected with secondary effects. CONCLUSIONS (1) The effect of emulsifier additions, made during the course of the process, on the rate of emulsion polymerization of styrene, under the influence of the initiators benzoyl peroxide, azobisisobutyronitrile and potassium persulphate has been investigated. (2) I t has been shown that, for initial emulsifier concentrations for which the adsorbed layers on the polymer particle surface are unsaturated or near to saturation, the addition of emulsifier during the process leads to an increase in the polymerization rate. These results confirm ideas developed previously about the progress of emulsion polymerization close to the surface of polymer particles. Translated by G. MODLEN
986
V. YE. ESKIN et a~.
REFERENCES 1. A. P. SHEIKER and S. S. MEDVEDEV, Dokl. Akad. Nauk SSSR 97: 111, 1954; Zh. fiz. khimii 29: 250, 1955 2. Ye. V. ZABOLOTSKAYA, I. G. SOBOLEVA, N. V. M A K L E T S O V A and S. S. MEDVEDEV, Dokl. Akad. Iqauk SSSR 94: 81, 1954; Kolloidn. zh. 18: 420, 1956 3. G. D. BEREZHNOI, P. M. KHOMIKOVSKII and S. S. MEDVEDEV, Vysokomol. soyed. 2: 141, 1960 4. I. KOLTHOFF, E. MECHAN and C. CARR, J. Polymer Sci. 7: 577, 1951 5. F. BOVEY, I. KOLTHOFF, A. MEDALIA and E. MECI-IAN, Emulsion Polymerization, N.Y., London, 1955 6. G. D. BEREZHNOI, P. M. KHOMIKOVSKII and S. S. MEDVEDEV, Vysokomol. soyed. 3: 1839, 1961
THE COMPOSITIONAL HETOROGENEITY AND FRACTIONATION OF STYRENE-METHYL METHACRYLATE COPOLYMERS* V, Y E . E S K I N , I. A. ]~ARANOVSKAYA, A. D. LITMANOVICH a n d A. V. T O P C H I E V Institute for High Molecular Compounds, U.S.S.R. Academy of Sciences (Received 20 J u n e 1963)
IT IS well known that the combination of two monomers in one copolymer leads in a number of cases to new desirable technical properties being obtained, which are absent in each of the corresponding homopolymers separately. The optimum properties of the copolymer thus correspond to a fixed compositional make-up of its molecules. However, a different degree of heterogeneity in the eopolymer's composition (compositional dispersion) will be brought about by differing conditions of copolymerization. I n quantitative procedures, the necessity thus arises of determining the degree of compositional dispersion of copolymers. At the present time the study of light scattering by copolymer solutions is one such method. The basis of the method lies in the idea founded on experiment that the increment of refractive index of a copolymer solution in a given solvent v = d n / d c is additive (for sufficiently small concentrations) in the ratio of the increments vA and vB of the components (homopolymers) entering into the copolymer [1]: v = x v A + ( 1 - - x ) VB •
* Vysokomol. soyed. 6: No. 5, 896-900, 1964.
(1)