Determination of Latex Particle Size from Polymerization Rates

8 Determination of Latex Particle Size from Polymerization Rates

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 28, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0024.ch008

CARLTON G. FORCE Charleston Research Center, Westvaco Corp., North Charleston, S. C. 29406

Several techniques are a v a i l a b l e f o r measuring the p a r t i c l e s i z e of aqueous d i s p e r s i o n s of m a t e r i a l s having diameters cons i d e r a b l y smaller than the wavelength of v i s i b l e l i g h t . Of these, e l e c t r o n microscopy i s one of the more d e s i r a b l e because the p a r t i c l e images can a c t u a l l y be v i s u a l i z e d . However, the equipment r e q u i r e d i s expensive, measurement of the micrographs i s time consuming and tedious and sample p r e p a r a t i o n i s h i g h l y t e c h n i c a l and fraught with u n r e s o l v a b l e sources of e r r o r . The minimum d e v i a t i o n which can be expected i s 5% and accuracy i s u s u a l l y not n e a r l y t h i s good, p a r t i c u l a r l y with s o f t p a r t i c l e s l i k e polybutadiene or SBR. Such p a r t i c l e s must be brominated or otherwise t r e a t e d to harden them s u f f i c i e n t l y to maintain t h e i r s i z e and shape when d r i e d on the e l e c t r o n microscope g r i d . Bromination i n c r e a s e s p a r t i c l e s i z e an amount which v a r i e s with the type of polymer. A method that r e q u i r e s inexpensive equipment and produces a s a t i s f a c t o r y average p a r t i c l e s i z e f o r many a p p l i c a t i o n s i s based on the f a c t that l a t e x p a r t i c l e s w i l l adsorb soap molecules u n t i l each p a r t i c l e i s f u l l y coated with a monolayer before s u f f i c i e n t soap becomes f r e e i n the aqueous serum to produce soap m i c e l l e s . Determination of the onset of m i c e l l i z a t i o n provides a convenient end p o i n t to determine the q u a n t i t y of soap adsorbed. From knowledge of the area each soap molecule occupies on a l a t e x p a r t i c l e , a c a l c u l a t i o n of the volume to s u r f a c e area (V/S) average p a r t i c l e diameter of the l a t e x can be made (1). The g r e a t e s t problem i n t h i s technique i s accur a t e l y e s t a b l i s h i n g the area occupied by a soap molecule on a latex particle. T h i s has been accomplished by determination of soap s a t u r a t i o n requirements f o r a l a t e x whose average p a r t i c l e s i z e i s known from another technique. Because of the inherent i n a c c u r a c i e s i n l a t e x p a r t i c l e s i z e determination, d i f f e r e n c e s i n soap p u r i t y and probably unrecognized v a r i a b i l i t i e s i n l a t e x e s such as the number of charged groups generated i n the chains 122

Piirma and Gardon; Emulsion Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

8.

FORCE

Latex

Particle

Size

123

during p o l y m e r i z a t i o n , there a r e values i n the l i t e r a t u r e f o r ^ the area occupied by an o l e i c a c i d molecule ranging from 20.2 A to 47 (1,2,3). E l e c t r o n microscopy was the most popular method u t i l i z e d i n e s t a b l i s h i n g average l a t e x p a r t i c l e s i z e , but other techniques such as u l t r a c e n t r i f u g a t i o n were a l s o employed 04) f o r measuring p a r t i c l e s i z e f o r the c a l c u l a t i o n of these soap molecular areas. In t h i s paper, a means i s demonstrated f o r experimentally determining the percent of monomer conversion a t which soap m i c e l l e s disappear from the system during emulsion polymerization. By applying the mathematics and l a t e x p a r t i c l e s u r f a c e area which a soap molecule w i l l occupy, developed f o r the soap t i t r a t i o n p a r t i c l e s i z e measurement procedure, the average p a r t i c l e s i z e o f the f i n i s h e d l a t e x can be c a l c u l a t e d . In f a c t , under some c i r cumstances i t might even be f e a s i b l e to adjust monomer content, i f necessary, during the r e a c t i o n and short stop a t the s p e c i f i c average p a r t i c l e s i z e d e s i r e d f o r a given l a t e x .


0

Experimental The f o l l o w i n g r e c i p e was used i n polymerizing the p o l y butadiene l a t e x e s f o r t h i s study. Butadiene Water O l e i c type potassium soap Potassium c h l o r i d e Tamol Ν Ferrous hepto s u l f a t e EDTA t e t r a sodium s a l t Sodium s u l f o x y l a t e p-menthane hydroperoxide t-dodecyl mercaptan

100 200 4.35 0.4 0.15 0.012 0.0326 0.0335 0.065 0.25

The polymerizations were c a r r i e d out at 5°C i n 28 f l u i d ounce s o f t d r i n k b o t t l e s r o t a t e d end over end i n a constant temperature water bath. With t h i s r e c i p e , the i n i t i a l r e a c t i o n r a t e i s d i f f e r e n t than the r a t e a f t e r p o l y m e r i z a t i o n has proceeded f o r a p e r i o d of time. In order to study t h i s phenomenon, i t was necessary to f r e q u e n t l y determine percent conversion during the coarse o f the r e a c t i o n . Samples f o r the t o t a l s o l i d s r e q u i r e d were obtained every 10 minutes f o r the f i r s t 80 minutes o f the p o l y m e r i z a t i o n . The sampling p e r i o d was then extended to 20 minutes f o r the next 40 minutes and 30 minutes t h e r e a f t e r up to a t o t a l p o l y m e r i z a t i o n time o f 300 minutes. Hourly samples were taken beyond 300 minutes. C o n d u c t i v i t y measurements were a l s o made during some of the p o l y m e r i z a t i o n s . For t h i s work, c i r c u l a r e l e c t r o d e s were cut



124

EMULSION

POLYMERIZATION

from platinum f o i l . Each of these was s i l v e r soldered to the end of a platinum wire. The platinum wire was i n s e r t e d i n a g l a s s tube shorter than the wire and p r e v i o u s l y bent i n a hot flame to conform r e l a t i v e l y c l o s e l y to the shape of the 28 oz. s o f t d r i n k b o t t l e used as the polymerization v e s s e l . The platinum wire extending beyond the g l a s s tube was then i n s e r t e d through the s e l f s e a l i n g rubber gasket and a hole punched i n the metal snap-on s o f t d r i n k pressure cap used to s e a l the b o t t l e . Two e l e c t r o d e s were thus placed i n each b o t t l e i n p o s i t i o n s such that the g l a s s tubing would maintain them a maximum d i s t a n c e apart near opposite s i d e s of the b o t t l e . T h i s d i s t a n c e was necessary f o r s u i t a b l e c o n d u c t i v i t y readings. In making these measurements, lead wires from a General Radio Co. Impedance Bridge type 1650A were attached to the platinum wires p r o t r u d i n g from the b o t t l e with a l i g a t o r c l i p s . A f t e r q u i c k l y measuring the c o n d u c t i v i t y of the s o l u t i o n with the bridge, a sample was removed f o r s o l i d s determination and the b o t t l e was replaced i n the polymerization bath f o r c o n t i n u a t i o n of the r e a c t i o n . Results P l o t t i n g the percent conversion against time, as shown i n F i g u r e 1, gave s t r a i g h t l i n e s c h a r a c t e r i z i n g each of the two rates. In Figure 2, c o n d u c t i v i t y i s p l o t t e d against percent conv e r s i o n . C o n d u c t i v i t y i s high during the e a r l y stages of p o l y m e r i z a t i o n because soap i s present i n m i c e l l a r form producing b e t t e r e l e c t r i c a l transport than a f t e r i t has become p r i m a r i l y attached to more bulky l a t e x p a r t i c l e s . Beyond the CMC, c o n d u c t i v i t y decreases over a few percent conversion to a c o n s i d e r a b l y smaller v a l u e . C o n d u c t i v i t y remains r e l a t i v e l y high i n these systems because of the s a l t s other than soap which are present. However, the soap i s by f a r the highest concent r a t i o n of i o n i z a b l e s a l t present and i t s bonding i n a l e s s mobile form to the growing rubber p a r t i c l e s causes a c o n s i d e r able decrease i n c o n d u c t i v i t y . The i n t e r s e c t i o n of the r a t e curves f o r the polymerization on which the data f o r F i g u r e 2 was obtained was at 8.0% conv e r s i o n . T h i s compares c l o s e l y with 8.3% conversion at the c r i t i c a l m i c e l l e c o n c e n t r a t i o n (CMC) as determined by conduct i v i t y i n F i g u r e 2. Comparative data f o r other systems are shown i n Table I. These r e s u l t s s t r o n g l y suggest that the i n t e r s e c t i o n of the p o l y m e r i z a t i o n r a t e curves occurs at the CMC of the o l e a t e soap e m u l s i f i e r . The most widely accepted t h e o r i e s of emulsion polymerization Ob 6) point out that new l a t e x p a r t i c l e s are generated only as long as soap i n excess of the p a r t i c l e surface adsorption requirements i s present i n the system. A f t e r s u f f i c i e n t p a r t i c l e surface area has developed to adsorb the soap i n the aqueous



8.

FORCE

Latex Particle

125

Size

..4-

I

ι

t

0

2

4 6 8 10 7» Conversion

Figure 2.

»

i

t

l 20

i 30

t

»

40

Conductivity vs. % conversion for latex polym erized with oleate soap


126

EMULSION

TABLE I .

COMPARISON URGENT CONVERSION AT CMC BY CONDUCTIVITY I*IJLASuREMENTS AND POLYMERIZATION RATES % Conversion a t


POLYMERIZATION

CMC

Conductivity

Polymerization Rate

12.1 12.2 7.5 11.1 9.5 8.2 12.5 9.2 13.4

12.0 12.2 7.4 11.5 9.3 7.6 12.6 9.3 12.7

phase to below i t s c r i t i c a l p o i n t f o r s t a b i l i z a t i o n of new p a r t i c l e s , e s s e n t i a l l y a l l of the p o l y m e r i z a t i o n continues i n the l a t e x p a r t i c l e s already formed i n the system. For soaps having low CMCs, such as the f a t t y a c i d soaps used i n t h i s study, i t i s g e n e r a l l y agreed t h i s c r i t i c a l p o i n t c l o s e l y c o i n c i d e s with the CMC. From the f o l l o w i n g c a l c u l a t i o n s , i t can be seen that the s u r f a c e area occupied by each soap molecule and the percent conversion at the CMC allows determination of the number of p a r t i c l e s present i n the l a t e x . From t h i s number, the necessary amount of monomer can be c a l c u l a t e d to produce any d e s i r e d f i n a l average p a r t i c l e s i z e . Symbols A

« p a r t i c l e surface area occupied by each soap molecule. A = 47 Â /molecule f o r potassium o l e a t e (2) C = soap c o n c e n t r a t i o n adsorbed to polymer p a r t i c l e s P • percent conversion a t CMC. P = 15.7% f o r F i g u r e 1 Pf - percent conversion a t end of p o l y m e r i z a t i o n . Pf 92.2% f o r Figure 1 latex S - weight of soap present i n p o l y m e r i z a t i o n b o t t l e . S 5.575g for Figure 1 latex Q » a c i d number of f a t t y a c i d . Q = 191.0 f o r Westvaco 1480 CJJJ » soap c o n c e n t r a t i o n f r e e i n l a t e x serum a t CMC. = 1.6 χ 10"^ moles f o r Westvaco 1480 Avogadros number - 6.02 χ 10^3 molecules/mole W = weight of polymer present at CMC Wf weight of polymer present at end of p o l y m e r i z a t i o n Τ « s u r f a c e area a t CMC i n square angstrums per gm of polymer D « p a r t i c l e diameter at CMC Df p a r t i c l e diameter a t end of p o l y m e r i z a t i o n Ν « number of polymer p a r t i c l e s per gm of polymer 2

a

c

c

β

s

c

β

c

s


8.

Latex Particle

FORCE

Size

127

« polymer d e n s i t y . G * 0.92 gm/cm χ 1 0 - 2 4 3 / Â 3 = 0.92 χ 10"24gm/A3 f polybutadiene Weight o f monomer i n b o t t l e • 148 gms f o r Figure 1 l a t e x 3

G

cm

o

r

Calculations


C

a -

Determination of Latex Particle Size from Polymerization Rates

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