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Particle growth in the synthesis of acrylate latexes
PARTICLE GROWTH IN THE SYNTHESIS OF ACRYLATE LATEXES* V . I . YELISEYEVA, P . I . ZUBOV a n d V. F . I~ALOFEYEVSKAYA Institute of Physical Chemistry, U.S.R.R. Academy of sciences
(Received 5 September 1964) I N THE s t u d y of certain m e t h o d s of p r e p a r i n g a c r y l a t e latexes, which are o f i n t e r e s t as film formers, f e a t u r e s of this process s t a n d o u t in c o n t r a s t to t h e well k n o w n e m u l s i o n p o l y m e r i z a t i o n o f h y d r o p h o b i c m o n o m e r s . F o r instance, in t h e e m u l s i o n p o l y m e r i z a t i o n of a l k y l a c r y l a t e s , w h i c h are r e a d i l y soluble in w a t e r , t h e i r c o n c e n t r a t i o n in t h e r e a c t i o n s y s t e m influences t h e r a t e of t h e process, m o l e c u l a r w e i g h t of t h e p o l y m e r a n d s t a b i l i t y o f t h e s y s t e m [1]. T o e x p l a i n t h e s e features, we decided to s t u d y t h e g r o w t h kinetics o f p o l y m e r i c p a r t i c l e s in t h e s y n t h e s i s in this k i n d o f latex. EXPERIMENTAL
Particle growth was studied for different methods of emulsion polymerization of the following monomer mixes: methylacrylate (MA)-butylacrylate (BA)-acrylic acid (AA) ( 8 : 2 " 0 . 3 ) . The methods of polymerization were varied by adding the monomer in different ways to the reaction system. In one case it was added at the very beginning of the process and the polymerization proceeded in the presence of a monomer emulsion and the aqueous phase saturated with it. In the second case the monomer was added gradually and the reaction proceeded with the monomer concentration of the aqueous phase below saturation. Ammonium persulphate was used to initiate the polymerization, and as emulsifier in one case we used a mixture of the anionic emulsifier SPEK (sodium salt of sulphonaphthenic acids) and the nonionic wetter OP-10, and in the other, E-30 (sodium salt of the fatty sulpho acid Cls-- Cls). In comparable experiments the emulsifier and initiator concentrations, temperature and final phase ratios were the same. The size of the latex particles, and their size distribution at different stages of polymerization, were determined on an electron microscope~f. F i g u r e 1 shows electron p h o t o m i c r o g r a p h s of t h e l a t e x e s p r o d u c e d b y t h e m e t h o d s u n d e r c o m p a r i s o n . F i g u r e s 2 a n d 3 show t h e figures o b t a i n e d for t h e p a r t i c l e d i s t r i b u t i o n a t different stages of p o l y m e r i z a t i o n for t h e t w o m e t h o d s . I t c a n b e seen t h a t t h e r e is a difference in t h e particle g r o w t h l a w d e p e n d i n g o n t h e m e t h o d of a d d i n g t h e m o n o m e r s . W h e r e t h e m o n o m e r s are a d d e d all a t once, a t 50~/0 c o n v e r s i o n t h e r e is a v e r y definite m a x i m u m for m e a n size particles. On f u r t h e r c o n v e r s i o n t h e p o l y d i s p c r s i t y o f t h e l a t e x increases con* Vysokomol. soyed. 7: 1~o. 8, 1348-1353, 1965.
1"Electron microscope studies were performed in the electron microscopy department of V. M. Luk'yanovich. 1496
Synthesis of acrylate latexes
1497
siderably, due mainly to a growth in particles of mean or larger diameter. B u t at the final stages, the polydispersity falls again due to precipitation o f very large particles from the latex system, which is accompanied b y formation of a coagulume. For polymerization with gradual addition of the monomers, in the initial stage of the process there is a definite maximum for the smallest size particles. As the polymerization proceeds right up to the moment where the addition of the monomer ceases, the particle distribution curves shift to the right, indicating a uniform increase in particles of all sizes. In the latter stages, after the
FIG. 1. Electron photomicrograph for lstoxe~ a--prepared by a4idition of monomer all at once (0"58~/oof the dry material); 5--unde~ ~ e same conditions but 33.48~/o; c--with graclual addition of monomers (8.29%)| d--same conditions but 41%.
1498
V. I. YELISEYEYA e~ al.
monomer addition has ceased and polymerization entails its consumption from the reaction mix, there is preferred growth of small particles, due to which the maximum shifts towards the minimum sized particles.
x
26 0.I
0.,!.
0"3
d,/z
FIG. 2. Latex particle diameter distribution for addition of monombrs all at once. Concentration of dry material, %: 1--0.58, 2--21-6, 3--32.8, 4--38.40.
n,%
~0
o
ov
o..z
0.3 d,~
FIG. 3. Particle diameter distribution for latex prepared with gradual addition of monomers. Concentration of dry materials, %: 1--8-29; 2--15.9; 3--85.0, 4--41.0.
B y determining the mean diameter of the particles at each stage of polymer:ization, it was found to increase in proportion to time with gradual addition o f the monomer, b u t where this was added all at once the mean particle diameter passes through a maximum in the final stage of polymerization (Fig. 4). To find out the nature of the variation in the number of particles in the process of polymerization we calculated their mean amount per unit volume of l a t e x at different stages. The results showed that after reaching a cert~dn l a t e x concentration, the number of particles hardly alters at all in the process o f synthesis. Table 1 gives these figures for polymerization with gradual addition o f the monomers in experiments .with various emulsifiers.
Synthesis of acrylate latexes
1499
TABLE I . M E A N NUMBER OF PARTICLES PER 1 ML LATEX AT DIFFERENT STAGES OF POLYMERIZATION
WITH GRADUALADDITIONOF MONOMERS
Specimen No.
Polymerization time, min
Concentration of dry materialand latex, ~o
Mean particle Mean particle volume, diameter, cm X 1015 crux 104
Mean
iluin-
ber of particles, ems × I0 -z4
Mixture of emulsifiers S T E K + OP-10 80 140 175 270 360
1
2 3 4 5
8"29 15"90 22"10 35-00 41"00
0.094 0.110 0.143 0.156 0.170
0.43 0.7 1-53 2.0 2.6
1.9 2.3 1.7 2.3 2.28
0.124 0.40 0.701 1-82
3.0 2.52 3.09 2.62
Emulsifiers E-30 10 30 70 120•
1 2 3 4
3.71 9.58 21.65 34.0
0.062 0.092 0.119 0.151
I f this is true, t h e n t h e p o l y m e r i z a t i o n r a t e m u s t b e c h a r a c t e r i z e d b y a n increase in t h e m e a n w e i g h t or m e a n v o l u m e o f a single particle. U s i n g a n elect r o n microscope, we d e t e r m i n e d t h e v o l u m e o f p o l y m e r i c ( b u t n o t p o l y m e r i c m o n o m e r i c ) particles, a n d f o u n d t h e w e i g h t o f e a c h p a r t i c l e t o be p r o p o r t i o n a l to t h e e x p e r i m e n t a l l y f o u n d v o l u m e . F i g u r e 5 shows t h e p o l y m e r i z a t i o n r a t e curves as c a l c u l a t e d f r o m t h e increase in l a t e x c o n c e n t r a t i o n a n d in t h e m e a n v o l u m e o f t h e particle. T h e agreem e n t b e t w e e n t h e t w o curves d e r i v e d f r o m different e x p e r i m e n t a l r e s u l t s shows t h a t t h e a m o u n t of particles r e m a i n s c o n s t a n t in t h e process of p o l y m e r -
d,#
I
0.2. z
x
9.1
0
I00
ZOO
Pol#rneNz~[on t[mo, rn/n
300
FIG. 4. Time variation in particle diameter of the latex:/--addition of monomers all at o n c e (mixture of emulsifiers); £-- graclu~ addition of monomers (mixture of emulsif~rs); 3-- gradual addition of monomers (emulsifier E-30).
V. I . YELISEYEYA e$ al.
1500
ization. The additional number of particles at different stages of the process is determined by the continuous ultramicroscopy method*. The results once again confirmed the fact t h a t the number of particles remaind constant during the synthesis of latex.
uZ,lb% ~ o.zl. "~
.~ / - ~
o
I
/O-TIoo
zoo
300
T/me,m/n
FIG. 5. Polymerization rate: /--derived from concentration increase; 2--from increase in mean volume of particles.
20~I0"8
I
o
x
x'~---
2
x
P
0
100
!
,x x
200
I
300
PolymeNzat/ontime, rain FIG. 6. Polymerization rate v. particle surface: /--with addition of monomers all at once; 2--gra4ual increase of mo.nomers. Emulsifiers--mixture of STEK and OP-10. In view of the fact t h a t the polymerization process is continuous and t h a t an infinitesimal increase in the volume of each particle corresponds to an infinitesimal increase in time, the polymerization rate can be given in terms of the product of the particle volume and time dv/dt, where v is the mean volume of each particle. Since v=f(r), while r=~(~),
dv dv dr dr dv = dr dv =kr2 d-T From Figure 3 it follows t h a t where polymerization is performed with gradual addition of the monomer dr aTv --~const, * Determination of particles number with a continuous ultramicroscope was carried out by N. M. Kudryavtseva.
Synthesis of acryiate latexes
1501
since dv
dr Hence it follows that where the monomers are gradually added the polymerization rate should be proportional to the particle surface. Curves I and 2 of Fig. 6 show the ratio between the experimental mean polymerization rate and the mean particle surface for given periods of time with the two different methods of adding the monomers. The curves were plotted from the figures given in Table 2, which shows the pol.y.merization rates, particle sizes and molecular weight of the polymer at different stages of polymerization. The curves in Fig. 5 confirm the proportional relation between the free polymerization and the particle surface size for experiments with gradual monomer addition; this is not the case where the monomer is added all at once.
k
.~ 0.8} ~
Ix,'..
~ 04~I
| E
|
\\
",, \
\\
x..
\x
~',, 2
~"'- !
01
0"2 Papt[cle d/a.
~Xg. 7. Mean particle diszzu~r of ]~tex v. enx~]sJ~er eozxeen~ratJonof aqueous phase: ]--a~dJ-
tion of monomers all at once; 2--gradual addition of monomers. The molecular weight figures of Table 2 show that where the monomer is gradually added the polymer molecules pile up in particles right Ul~ until the completion of the process. Where the monomer is added all at once the accumulation of molecules in the particle passes through a maximum which is accompanied by precipitation of the largest particle from the latex system (Fig. 4). The number of molecules in the latex particles produced by gradual addition of the monomer is rather less than where this is done all at once. The difference in the particle size of the two latexes is due only to the smaller size of the molecules in the first case. As determined from measurements of the surface tension of the system, the emulsifier concentration showed that it was practically exhausted in the early stages of the process for both methods, after which the particles continued to increase in volume. This is illustrated by the curves for the mean particle diameter v. emulsifier concentration of the aqueous phase (Fig. 7). The results show that the emulsifier added to the system during polymerization is the only factor which dictates the colloidal stability of the latex.
V . I . Y~ISEY]~VA e$ al.
1502
0
0 0
~.~
r~ 0
o
0
0 0
f~ 0
r~ 0
r~
0 0
~
~.~ ~°
f~
~.~
~
66~66
~6666
Synthesis of acrylate latexes
1503
DISCUSSION
As already established, the emulsion polymerization of low acrylates, which have considerable solubility in water, differs in dependence on the monomer concentra-' tion of the reaction system [1]. This affects the growth kinetics of the polymeric particles and their size distribution. If the aqueous phase is saturated with the monomer the rate of monomer diffusion during the reaction is higher at the reaction point, and this leads to the formation of large molecules and hence, large particles. If the monomer concentration of the aqueous phase is lower than saturation the reaction proceeds with a slow rate of monomer diffusion, and this produces a polymer with lower molecular weight, and consequently, small latex particles. In both cases, beginning at the early stages of polymerization, the number of particles formed in the latex remains practically unchanged, i.e., polymerization is due to the growth of each discrete particle. If the monomer is added gradually during the polymerization, the rate will be proportional to the particle surface, which is constantly increasing in size during the synthesis. This shows that the reaction takes place mainly on the surface of the polymeric particles[2, 3] in contrast to the case where the monomer is added all at once, in which bulk polymerization is very important [4-6]. The emulsifier added to the system is adsorbed f r o m the aqueous phase even in the early stages of polymerization, but this does not cause coalescence of the growing particles. This may happen if the polymeric particles are spontaneously stabilized by the polar groups of the polymer becoming oriented on the particle surfaces [7]. For low alkyl acrylates t h e polarity is due to the presence of carbonyl in the ester groups, and for a carboxyl-contajning polymer it is higher for this reason. Orientation of polar groups on the particle surface becomes more probable as this surface develops and the molecular weight of the polymer falls and, finally, as the molecule formation rate falls and the specific rate rises polymerization takes place on the surface. This makes plain the high colloidal stability of systems polymerized with gradual addition to the monomer. CONCLUSIONS
(1) The mechanism of particle growth in the process of the emulsion polymerization of the lower, highly water-soluble alkyl acrylates has been studied as a function of the monomer concentration of the reaction system. (2) Polymerization of the lower alkyl acrylates takes place mainly in discrete particles formed in the beginning of the process. After exhaustion of the emul, sifter, which occurs at the beginning of the polymerization, they are stabilized by the polar groups of the polymer macromolecules. (3) The topochemistry of the emulsion polymerization of these alkyl acrylates depends on the monomer concentration of the reaction system. If this
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R . S . MUROMOVAe$ al.
is less than the saturation concentration in the aqueous phase, polymerization occurs mainly on the particle surface, but if there is a monomer emulsion and its saturated solution in the aqueous phase, poljrmerization appears to occur inside them. Translated by V. ALFORD
REFERENCES 1. V. I. YELISEYEVA, N. G. ZHARKOVA, A. V. CHUBAROVA and P° I. ZUBOV, Vysokomol. soyed. 7: 156, 1965 2. S. S. MEDVEDEV, N. M. KHOMIKOVSKII, A. P. SHEINKER, Ye. V. ZABOLOTSKAYA and G. D. BEREZHNOI, Probl. fiz. khim. I : 5, 1958 3. J. G. BRODMYAN, J. A. CALA, T. KONEN and E. L. KELLEY, J. CoUoid. Sci. 18: 73, 1963 4. W. D. HARKINS, J. Amer. Chem. Soc. 69: 1428, 1947 5. W. D. HARKINS, J. Polymer Sci. 5: 217, 1950 6. B. M. E. van der HOFF, Advances in Chemistry, ser. 34, 6-31, 1962 7. V. I. YELISEYEVA, K. M. ZURABYAN and A. D. ZAIDES, Dokl. Akad. N a u k SSSR, 162: 1086, 1965
SYNTHESIS AND POLYCONDENSATION OF cis- AND tra~-ISOMERS OF ~,-(3-AMINOCYCLOHEXYL) BUTYRIC ACID*t R. S. MUROMOVA, I. D. P L E T N E V A , T. V. D E M I D O V A , I. V. SHKHIYANTS and G. A. TOKAREVA State Research a n d Projecting I n s t i t u t e of the Nitrogen I n d u s t r y and Organic Synthesis Products
(Received 5 October 1964)
OUR previous reports [1] dealt with the synthesis of cis- and trans-isomers of fl-(3-aminocyclohexyl)propionic acid (cis- and trans-3-ACP) and polyamides on their base. Comparison between the properties of polyamide prepared by the polycondensation of cis-3-ACP and that previously prepared based of cisfl-(4-aminocyclohexyl)propionic acid [2] (cis-4-ACP) showed that the first had considerably higher melting point and a higher state of erystallinity than the second. The literature contains no other information on the properties of polyamides based on cis-isomers of 3- and 4-~o-amino acids of the cyclohexane series, nor of any other polymers as the products of the polycondensation of cisisomers of bifunctional derivatives of cyclohexane containing functional groups * Vysokomol. soyed. 7: No. 8, 1354-1358, 1965. t I V t h report o f the series "Polyamides based on the amino acids of the cyclohexane series".
(Received 5 September 1964) I N THE s t u d y of certain m e t h o d s of p r e p a r i n g a c r y l a t e latexes, which are o f i n t e r e s t as film formers, f e a t u r e s of this process s t a n d o u t in c o n t r a s t to t h e well k n o w n e m u l s i o n p o l y m e r i z a t i o n o f h y d r o p h o b i c m o n o m e r s . F o r instance, in t h e e m u l s i o n p o l y m e r i z a t i o n of a l k y l a c r y l a t e s , w h i c h are r e a d i l y soluble in w a t e r , t h e i r c o n c e n t r a t i o n in t h e r e a c t i o n s y s t e m influences t h e r a t e of t h e process, m o l e c u l a r w e i g h t of t h e p o l y m e r a n d s t a b i l i t y o f t h e s y s t e m [1]. T o e x p l a i n t h e s e features, we decided to s t u d y t h e g r o w t h kinetics o f p o l y m e r i c p a r t i c l e s in t h e s y n t h e s i s in this k i n d o f latex. EXPERIMENTAL
Particle growth was studied for different methods of emulsion polymerization of the following monomer mixes: methylacrylate (MA)-butylacrylate (BA)-acrylic acid (AA) ( 8 : 2 " 0 . 3 ) . The methods of polymerization were varied by adding the monomer in different ways to the reaction system. In one case it was added at the very beginning of the process and the polymerization proceeded in the presence of a monomer emulsion and the aqueous phase saturated with it. In the second case the monomer was added gradually and the reaction proceeded with the monomer concentration of the aqueous phase below saturation. Ammonium persulphate was used to initiate the polymerization, and as emulsifier in one case we used a mixture of the anionic emulsifier SPEK (sodium salt of sulphonaphthenic acids) and the nonionic wetter OP-10, and in the other, E-30 (sodium salt of the fatty sulpho acid Cls-- Cls). In comparable experiments the emulsifier and initiator concentrations, temperature and final phase ratios were the same. The size of the latex particles, and their size distribution at different stages of polymerization, were determined on an electron microscope~f. F i g u r e 1 shows electron p h o t o m i c r o g r a p h s of t h e l a t e x e s p r o d u c e d b y t h e m e t h o d s u n d e r c o m p a r i s o n . F i g u r e s 2 a n d 3 show t h e figures o b t a i n e d for t h e p a r t i c l e d i s t r i b u t i o n a t different stages of p o l y m e r i z a t i o n for t h e t w o m e t h o d s . I t c a n b e seen t h a t t h e r e is a difference in t h e particle g r o w t h l a w d e p e n d i n g o n t h e m e t h o d of a d d i n g t h e m o n o m e r s . W h e r e t h e m o n o m e r s are a d d e d all a t once, a t 50~/0 c o n v e r s i o n t h e r e is a v e r y definite m a x i m u m for m e a n size particles. On f u r t h e r c o n v e r s i o n t h e p o l y d i s p c r s i t y o f t h e l a t e x increases con* Vysokomol. soyed. 7: 1~o. 8, 1348-1353, 1965.
1"Electron microscope studies were performed in the electron microscopy department of V. M. Luk'yanovich. 1496
Synthesis of acrylate latexes
1497
siderably, due mainly to a growth in particles of mean or larger diameter. B u t at the final stages, the polydispersity falls again due to precipitation o f very large particles from the latex system, which is accompanied b y formation of a coagulume. For polymerization with gradual addition of the monomers, in the initial stage of the process there is a definite maximum for the smallest size particles. As the polymerization proceeds right up to the moment where the addition of the monomer ceases, the particle distribution curves shift to the right, indicating a uniform increase in particles of all sizes. In the latter stages, after the
FIG. 1. Electron photomicrograph for lstoxe~ a--prepared by a4idition of monomer all at once (0"58~/oof the dry material); 5--unde~ ~ e same conditions but 33.48~/o; c--with graclual addition of monomers (8.29%)| d--same conditions but 41%.
1498
V. I. YELISEYEYA e~ al.
monomer addition has ceased and polymerization entails its consumption from the reaction mix, there is preferred growth of small particles, due to which the maximum shifts towards the minimum sized particles.
x
26 0.I
0.,!.
0"3
d,/z
FIG. 2. Latex particle diameter distribution for addition of monombrs all at once. Concentration of dry material, %: 1--0.58, 2--21-6, 3--32.8, 4--38.40.
n,%
~0
o
ov
o..z
0.3 d,~
FIG. 3. Particle diameter distribution for latex prepared with gradual addition of monomers. Concentration of dry materials, %: 1--8-29; 2--15.9; 3--85.0, 4--41.0.
B y determining the mean diameter of the particles at each stage of polymer:ization, it was found to increase in proportion to time with gradual addition o f the monomer, b u t where this was added all at once the mean particle diameter passes through a maximum in the final stage of polymerization (Fig. 4). To find out the nature of the variation in the number of particles in the process of polymerization we calculated their mean amount per unit volume of l a t e x at different stages. The results showed that after reaching a cert~dn l a t e x concentration, the number of particles hardly alters at all in the process o f synthesis. Table 1 gives these figures for polymerization with gradual addition o f the monomers in experiments .with various emulsifiers.
Synthesis of acrylate latexes
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TABLE I . M E A N NUMBER OF PARTICLES PER 1 ML LATEX AT DIFFERENT STAGES OF POLYMERIZATION
WITH GRADUALADDITIONOF MONOMERS
Specimen No.
Polymerization time, min
Concentration of dry materialand latex, ~o
Mean particle Mean particle volume, diameter, cm X 1015 crux 104
Mean
iluin-
ber of particles, ems × I0 -z4
Mixture of emulsifiers S T E K + OP-10 80 140 175 270 360
1
2 3 4 5
8"29 15"90 22"10 35-00 41"00
0.094 0.110 0.143 0.156 0.170
0.43 0.7 1-53 2.0 2.6
1.9 2.3 1.7 2.3 2.28
0.124 0.40 0.701 1-82
3.0 2.52 3.09 2.62
Emulsifiers E-30 10 30 70 120•
1 2 3 4
3.71 9.58 21.65 34.0
0.062 0.092 0.119 0.151
I f this is true, t h e n t h e p o l y m e r i z a t i o n r a t e m u s t b e c h a r a c t e r i z e d b y a n increase in t h e m e a n w e i g h t or m e a n v o l u m e o f a single particle. U s i n g a n elect r o n microscope, we d e t e r m i n e d t h e v o l u m e o f p o l y m e r i c ( b u t n o t p o l y m e r i c m o n o m e r i c ) particles, a n d f o u n d t h e w e i g h t o f e a c h p a r t i c l e t o be p r o p o r t i o n a l to t h e e x p e r i m e n t a l l y f o u n d v o l u m e . F i g u r e 5 shows t h e p o l y m e r i z a t i o n r a t e curves as c a l c u l a t e d f r o m t h e increase in l a t e x c o n c e n t r a t i o n a n d in t h e m e a n v o l u m e o f t h e particle. T h e agreem e n t b e t w e e n t h e t w o curves d e r i v e d f r o m different e x p e r i m e n t a l r e s u l t s shows t h a t t h e a m o u n t of particles r e m a i n s c o n s t a n t in t h e process of p o l y m e r -
d,#
I
0.2. z
x
9.1
0
I00
ZOO
Pol#rneNz~[on t[mo, rn/n
300
FIG. 4. Time variation in particle diameter of the latex:/--addition of monomers all at o n c e (mixture of emulsifiers); £-- graclu~ addition of monomers (mixture of emulsif~rs); 3-- gradual addition of monomers (emulsifier E-30).
V. I . YELISEYEYA e$ al.
1500
ization. The additional number of particles at different stages of the process is determined by the continuous ultramicroscopy method*. The results once again confirmed the fact t h a t the number of particles remaind constant during the synthesis of latex.
uZ,lb% ~ o.zl. "~
.~ / - ~
o
I
/O-TIoo
zoo
300
T/me,m/n
FIG. 5. Polymerization rate: /--derived from concentration increase; 2--from increase in mean volume of particles.
20~I0"8
I
o
x
x'~---
2
x
P
0
100
!
,x x
200
I
300
PolymeNzat/ontime, rain FIG. 6. Polymerization rate v. particle surface: /--with addition of monomers all at once; 2--gra4ual increase of mo.nomers. Emulsifiers--mixture of STEK and OP-10. In view of the fact t h a t the polymerization process is continuous and t h a t an infinitesimal increase in the volume of each particle corresponds to an infinitesimal increase in time, the polymerization rate can be given in terms of the product of the particle volume and time dv/dt, where v is the mean volume of each particle. Since v=f(r), while r=~(~),
dv dv dr dr dv = dr dv =kr2 d-T From Figure 3 it follows t h a t where polymerization is performed with gradual addition of the monomer dr aTv --~const, * Determination of particles number with a continuous ultramicroscope was carried out by N. M. Kudryavtseva.
Synthesis of acryiate latexes
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since dv
dr Hence it follows that where the monomers are gradually added the polymerization rate should be proportional to the particle surface. Curves I and 2 of Fig. 6 show the ratio between the experimental mean polymerization rate and the mean particle surface for given periods of time with the two different methods of adding the monomers. The curves were plotted from the figures given in Table 2, which shows the pol.y.merization rates, particle sizes and molecular weight of the polymer at different stages of polymerization. The curves in Fig. 5 confirm the proportional relation between the free polymerization and the particle surface size for experiments with gradual monomer addition; this is not the case where the monomer is added all at once.
k
.~ 0.8} ~
Ix,'..
~ 04~I
| E
|
\\
",, \
\\
x..
\x
~',, 2
~"'- !
01
0"2 Papt[cle d/a.
~Xg. 7. Mean particle diszzu~r of ]~tex v. enx~]sJ~er eozxeen~ratJonof aqueous phase: ]--a~dJ-
tion of monomers all at once; 2--gradual addition of monomers. The molecular weight figures of Table 2 show that where the monomer is gradually added the polymer molecules pile up in particles right Ul~ until the completion of the process. Where the monomer is added all at once the accumulation of molecules in the particle passes through a maximum which is accompanied by precipitation of the largest particle from the latex system (Fig. 4). The number of molecules in the latex particles produced by gradual addition of the monomer is rather less than where this is done all at once. The difference in the particle size of the two latexes is due only to the smaller size of the molecules in the first case. As determined from measurements of the surface tension of the system, the emulsifier concentration showed that it was practically exhausted in the early stages of the process for both methods, after which the particles continued to increase in volume. This is illustrated by the curves for the mean particle diameter v. emulsifier concentration of the aqueous phase (Fig. 7). The results show that the emulsifier added to the system during polymerization is the only factor which dictates the colloidal stability of the latex.
V . I . Y~ISEY]~VA e$ al.
1502
0
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~6666
Synthesis of acrylate latexes
1503
DISCUSSION
As already established, the emulsion polymerization of low acrylates, which have considerable solubility in water, differs in dependence on the monomer concentra-' tion of the reaction system [1]. This affects the growth kinetics of the polymeric particles and their size distribution. If the aqueous phase is saturated with the monomer the rate of monomer diffusion during the reaction is higher at the reaction point, and this leads to the formation of large molecules and hence, large particles. If the monomer concentration of the aqueous phase is lower than saturation the reaction proceeds with a slow rate of monomer diffusion, and this produces a polymer with lower molecular weight, and consequently, small latex particles. In both cases, beginning at the early stages of polymerization, the number of particles formed in the latex remains practically unchanged, i.e., polymerization is due to the growth of each discrete particle. If the monomer is added gradually during the polymerization, the rate will be proportional to the particle surface, which is constantly increasing in size during the synthesis. This shows that the reaction takes place mainly on the surface of the polymeric particles[2, 3] in contrast to the case where the monomer is added all at once, in which bulk polymerization is very important [4-6]. The emulsifier added to the system is adsorbed f r o m the aqueous phase even in the early stages of polymerization, but this does not cause coalescence of the growing particles. This may happen if the polymeric particles are spontaneously stabilized by the polar groups of the polymer becoming oriented on the particle surfaces [7]. For low alkyl acrylates t h e polarity is due to the presence of carbonyl in the ester groups, and for a carboxyl-contajning polymer it is higher for this reason. Orientation of polar groups on the particle surface becomes more probable as this surface develops and the molecular weight of the polymer falls and, finally, as the molecule formation rate falls and the specific rate rises polymerization takes place on the surface. This makes plain the high colloidal stability of systems polymerized with gradual addition to the monomer. CONCLUSIONS
(1) The mechanism of particle growth in the process of the emulsion polymerization of the lower, highly water-soluble alkyl acrylates has been studied as a function of the monomer concentration of the reaction system. (2) Polymerization of the lower alkyl acrylates takes place mainly in discrete particles formed in the beginning of the process. After exhaustion of the emul, sifter, which occurs at the beginning of the polymerization, they are stabilized by the polar groups of the polymer macromolecules. (3) The topochemistry of the emulsion polymerization of these alkyl acrylates depends on the monomer concentration of the reaction system. If this
1504
R . S . MUROMOVAe$ al.
is less than the saturation concentration in the aqueous phase, polymerization occurs mainly on the particle surface, but if there is a monomer emulsion and its saturated solution in the aqueous phase, poljrmerization appears to occur inside them. Translated by V. ALFORD
REFERENCES 1. V. I. YELISEYEVA, N. G. ZHARKOVA, A. V. CHUBAROVA and P° I. ZUBOV, Vysokomol. soyed. 7: 156, 1965 2. S. S. MEDVEDEV, N. M. KHOMIKOVSKII, A. P. SHEINKER, Ye. V. ZABOLOTSKAYA and G. D. BEREZHNOI, Probl. fiz. khim. I : 5, 1958 3. J. G. BRODMYAN, J. A. CALA, T. KONEN and E. L. KELLEY, J. CoUoid. Sci. 18: 73, 1963 4. W. D. HARKINS, J. Amer. Chem. Soc. 69: 1428, 1947 5. W. D. HARKINS, J. Polymer Sci. 5: 217, 1950 6. B. M. E. van der HOFF, Advances in Chemistry, ser. 34, 6-31, 1962 7. V. I. YELISEYEVA, K. M. ZURABYAN and A. D. ZAIDES, Dokl. Akad. N a u k SSSR, 162: 1086, 1965
SYNTHESIS AND POLYCONDENSATION OF cis- AND tra~-ISOMERS OF ~,-(3-AMINOCYCLOHEXYL) BUTYRIC ACID*t R. S. MUROMOVA, I. D. P L E T N E V A , T. V. D E M I D O V A , I. V. SHKHIYANTS and G. A. TOKAREVA State Research a n d Projecting I n s t i t u t e of the Nitrogen I n d u s t r y and Organic Synthesis Products
(Received 5 October 1964)
OUR previous reports [1] dealt with the synthesis of cis- and trans-isomers of fl-(3-aminocyclohexyl)propionic acid (cis- and trans-3-ACP) and polyamides on their base. Comparison between the properties of polyamide prepared by the polycondensation of cis-3-ACP and that previously prepared based of cisfl-(4-aminocyclohexyl)propionic acid [2] (cis-4-ACP) showed that the first had considerably higher melting point and a higher state of erystallinity than the second. The literature contains no other information on the properties of polyamides based on cis-isomers of 3- and 4-~o-amino acids of the cyclohexane series, nor of any other polymers as the products of the polycondensation of cisisomers of bifunctional derivatives of cyclohexane containing functional groups * Vysokomol. soyed. 7: No. 8, 1354-1358, 1965. t I V t h report o f the series "Polyamides based on the amino acids of the cyclohexane series".