Emulsion Polymerization

13 Molecular Weight Development in Styrene and Methyl Methacrylate Emulsion Polymerization HOWARD L . JAMES, JR.

Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 25, 2018 | https://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0024.ch013

Standard Oil of Ohio, Cleveland, Ohio 44128 IRJA PIIRMA Institute of Polymer Science, The University of Akron, Akron, Ohio 44325

The "ideal" concept of emulsion polymerization was built on the assumption that the monomer was water insoluble and that in the absence of chain transfer, the number average degree of polymerization, x can be related to the rate processes of initiation and propagation by the steady-state relationship x = 2 R /R . Since R and R are both constant and termination is assumed to be instantaneous during the constant rate period described by Smith-Ewart kinetics, the above equation predicts the generation of constant molecular weight polymer. Data has been obtained which agrees with Smith-Ewart (1,2), but there is also a considerable amount of data which shows an increase in molecular weight with conversion during the constant rate period (3,4). Also, the molecular weight distribution has often been found to be very broad (3) contrary to the theoretically predicted most probable distribution. Molecular weight characterization of polymers requires various molecular weight averages and, preferably, a complete molecular weight distribution, such as that obtained by fractionation. Since these techniques are quite time consuming, molecular weight distributions were seldom obtained on a routine basis. Therefore dilute solution viscometry, coupled with MarkHouwink equations for the calculation of M , has been the main molecular weight identification method. The recent advent of gel permeation chromatography (GPC) has allowed a rapid, conve nient and reliable determination of molecular weight average and molecular weight distribution. Krackeler et al. (5), in com paring the results of GPC with those obtained using a fractional precipitation technique, found a good comparison between the two techniques. n

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Taken i n p a r t from the M.S. Thesis of H. L . James, granted at The U n i v e r s i t y of Akron, June 1974.

197

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


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Since both the v a r i o u s molecular weight averages and the molecular weight d i s t r i b u t i o n of a polymer are extremely s e n s i t i v e to any chain t r a n s f e r r e a c t i o n s , i n t h i s i n v e s t i g a t i o n an attempt was made to keep the polymerization i n g r e d i e n t s as pure as p o s s i b l e . The e m u l s i f i e r system, c o n s i s t i n g of a nonionic as w e l l as an i o n i c surface a c t i v e agent, had been tested f o r i t s chain t r a n s f e r a c t i v i t y and found " s a f e . " This surface a c t i v e agent combination a l s o has been found to y i e l d a l a t e x with q u i t e a narrow p a r t i c l e s i z e d i s t r i b u t i o n (2). The d e s i r a b i l i t y of u s i n g a monodisperse p a r t i c l e s i z e l a t e x i n k i n e t i c s t u d i e s has been discussed by s e v e r a l researchers (2t3t&» Experimental M a t e r i a l s and Polymerization. Styrene and methyl methacrylate were obtained from commercial sources and were d i s t i l l e d to remove i n h i b i t o r . A f t e r d i s t i l l a t i o n , the monomers were stored, under nitrogen, i n a r e f r i g e r a t o r . For the mixed e m u l s i f i e r system, Emulphogene BC840(GAF), t r i d e c y l o x y polye thylene-oxyethano1, was used as the nonionic c o n s t i t u e n t , and sodium l a u r y l s u l f a t e (K and Κ Labs) was used as the i o n i c constituent. The sodium l a u r y l s u l f a t e was at a concentration below i t s cms whereas the BD-840 was at a c o n c e n t r a t i o n above i t s cmc. This e m u l s i f i e r system has been shown to y i e l d mixed m i c e l l e s (T), having a low i o n i c change (2), which produce l a t i c e s with r a t h e r narrow p a r t i c l e s i z e d i s t r i b u t i o n s (2, Q). Potassium p e r s u l f a t e ( F i s h e r S c i e n t i f i c ) was used as the i n i t i a t o r and potassium hydroxide was added to insure e f f i c i e n t decom p o s i t i o n of the p e r s u l f a t e . The f o l l o w i n g r e c i p e was used f o r a l l polymerizations i n t h i s work: Material Monomer Water Emulphogene BC-840 Sodium L a u r y l S u l f a t e Potassium P e r s u l f a t e Potassium Hydroxide

Parts by Weight 100.0 180.0 5.0 0.05 0.30 0.075

The polymerizations were c a r r i e d out i n 4 oz. glass b o t t l e s with metal screw caps which were p e r f o r a t e d and l i n e d with s e l f s e a l i n g b u t y l gaskets. The t i g h t l y capped b o t t l e s , a f t e r being purged with nitrogen, were clamped i n t o a water bath thermos t a t t e d at 50°C and r o t a t e d end-over-end at 45 r.p.m. Percent conversion, at v a r i o u s r e a c t i o n times, were determined gravimetrically. C h a r a c t e r i z a t i o n . The polymer samples to be c h a r a c t e r i z e d were obtained by pouring v a r i o u s percent conversion samples i n t o



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Development

a large excess o f methanol which contained a s u f f i c i e n t q u a n t i t y of Pennstop RC 1866 to completely shortstop the l a t e x . The prec i p i t a t e d polymer was then washed s e v e r a l times with both methanol and water and d r i e d i n a vacuum oven. Tenth o f a percent polymer s o l u t i o n s were prepared f o r both viscometry and g e l permeation chromatography techniques. The s o l u t i o n process r e q u i r e d 2 to 4 days depending on the polymer molecular weight and the solvent. D i s s o l u t i o n was aided by gentle hand s w i r l i n g at i n t e r v a l s o f s e v e r a l hours. i. G e l Permeation Chromatography - The GPC data were obtained i n dime thy lformamide s o l u t i o n u s i n g a Waters A s s o c i a t e s Model 100 GPC equipped with a d i f f e r e n t i a l r e f r a c t i v e index d e t e c t o r and f i v e S t y r a g e l columns having Çhe f o l l o w i n g nominal e x c l u s i o n l i m i t d e s i g n a t i o n s : 1 0 Â, 1(P A, 1 0 A 3 χ 1 0 A, and 3 χ 1 0 A. Measurements were made a t 60°C u s i n g a flow r a t e of 1 ml/min. Sample c o n c e n t r a t i o n s were 0.1% (w/v) f o r both the polystyrene c a l i b r a t i o n standards and the polymers obtained i n t h i s work. P r i o r to i n j e c t i o n i n t o the GPC, the s o l u t i o n s were f i l t e r e d through a 5μ f i l t e r . This f i l t e r i n g process was c a r r i e d out very slowly so as not to shear degrade the polymer. The i n j e c t i o n time was 120 sec. i n a l l cases. The c a l i b r a t i o n curve used was constructed by u s i n g polystyrene standards from three d i f f e r e n t s u p p l i e r s ; Pressure Chemical Co., Waters Asso c i a t e s , and Duke Standards. The molecular weights o f these standards ranged from 51,000 to 7.1 m i l l i o n . F i g u r e 1 shows the GPC chromatogram f o r the standard with a molecular weight o f 7.1 million. Weight and number average molecular weights, uncorrected f o r instrument spreading, were c a l c u l a t e d u s i n g the f o l l o w i n g relationships: 7

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M

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- K H ^ ) / ^

M

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- Σί^/ΣίΗ./Μ.)

where i s the h e i g h t o f the chromatogram, measured from the b a s e l i n e , a t the i t h e l u t i o n count and i s the molecular weight a t t h i s count, determined by u s i n g the polystrene c a l i b r a t i o n curve. As i s common p r a c t i c e , i t was assumed that the peak maximum o f the c a l i b r a t i o n standards corresponded to the Mw o f the standard. The u n i v e r s a l c a l i b r a t i o n technique (9>) was not attempted i n the GPC study o f these h i g h molecular weight polymers due to the d i f f i c u l t i e s encountered by Slagowski e t a l . (10). They found that both types o f c a l i b r a t i o n curves, the conventional l o g M^ vs. e l u t i o n volume and the long [η] M v s . e l u t i o n volume, showed d i s t i n c t breaks i n l i n e a r i t y i n the 107 g/mole r e g i o n . ii. Viscometry - V i s c o s i t y measurements o f a l l the polymers were c a r r i e d out i n toluene a t 30°C i n a Cannon Ubbelohde d i l u t i o n viscometer. The c o r r e c t i o n term f o r k i n e t i c energy was l e s s than 0.2% and was t h e r e f o r e neglected i n


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c a l c u l a t i o n s of i n t r i n s i c v i s c o s i t y . Although viscometry can be a q u i t e simple and r a p i d method f o r o b t a i n i n g i n f o r m a t i o n on polymers, polymers having molecular weights greater than about 5 χ 10^ g/mole are known to shear degrade (11, 12). This non-Newtonian behavior of polymer s o l u t i o n s has been discussed i n the l i t e r a t u r e f o r both high molecular weight polystyrene (12, 13, 14) and poly (methyl methacrylate) (15, 16). The i n t r i n s i c v i s c o s i t i e s obtained i n the Cannon Ubbelohde viscometer were c o r r e c t e d f o r shear e f f e c t s by u s i n g the curve shown i n Figure 2. This curve was obtained by u s i n g a Zimm-type low shear viscometer (17) to determine the i n t r i n s i c v i s c o s i t i e s o f four emulsion polymerized polystyrene samples 02). The curve was l i n e a r over the range of i n t r i n s i c v i s c o s i t y r e s u l t s f o r polystyrene samples encountered i n t h i s study. Results and D i s c u s s i o n The emulsion p o l y m e r i z a t i o n at 50°C o f styrene and methyl methacrylate u s i n g concentrations of i n g r e d i e n t s as l i s t e d i n experimental s e c t i o n gave p o l y m e r i z a t i o n r a t e s of 11.5%/hr and 33.6%/hr r e s p e c t i v e l y . Figures 3 and 4 show smooth, continuous r a t e curves i n d i c a t i n g the presence o f three d i s t i n c t stages i n these p o l y m e r i z a t i o n r a t e s . The three stages were designated i n t e r v a l I, I I , and I I I s i m i l a r to Gardon (18). I n t e r v a l I represents the p a r t i c l e - f o r m i n g period, i n t e r v a l I I the polymer i z a t i o n period, and i n t e r v a l I I the p e r i o d f o l l o w i n g the constant r a t e . Although our i n v e s t i g a t i o n i s p r i m a r i l y concerned with the molecular weight development during i n t e r v a l I I , we have shown data covering a l l i n t e r v a l s . From Figures 3 and 4 i t can be seen that a constant r a t e p e r i o d extends from about 25 to 60 percent conversion i n the case of styrene and from 13 to 35 percent conversion f o r methyl methacrylate^ The data f o r the molecular weight averages, and obtained by g e l permeation chromatography f o r the samples o f polystyrene and poly(methyl methacrylate) are shown i n Table I and I I , r e s p e c t i v e l y . Although the values are not absolute, they provide the a b i l i t y to study the molecular weight develop ment i n the emulsion p o l y m e r i z a t i o n o f these monomers i n a r e l a t i v e sense. Figures 5 and 6 provide g r a p h i c a l represent a t i o n f o r the data.


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E l u t i o n Volume (counts)

Figure 1. GPC chromatogram of a polystyrene standard

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6 Γ η]

8 Ubbelohde

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Viscometer

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Figure 2. Calibration curve for shear-rate effect (polystyrene in toluene at30°C)


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Samples f o r Conversion Only

Time

(Min.)

Figure 3. Polymerization rate of styrene at50°C

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Samples f o r Conversion Only

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Samples f o r M o l e c u l a r Weight A n a l y s i s

Time

Figure 4.

(Min.)

Polymerization rate of methyl methacrylate at 50°C


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Figure 6. Molecular weight vs. conversion for poly(methyl methacrylate)


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Table I Molecular Weights f o r Polystyrene Determined by GPC


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%

M^xlO" (g/mole)

M xl0" (g/mole)

3.05 7.28 16.44 31.71 43.18 53.63 68.19 85.06

2.31 3.52 4.55 4.64 4.61, 4.64 4.74, 4.62 4.98, 4.72 4.87, 4.88

0.98 1.44 2.31 2.56 2.29, 2.41 2.51, 2.48 2.61, 2.75 2.25, 2.43

Conversion

6

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Table I I Molecular Weights f o r Poly(Methyl Determined by GPC

Methacrylate)

%

M xl0-6 (g/mole)

MnXlO" (g/mole)

2.42 4.71 8.73 12.10 13.93 22.45 28.24 33.84 43.92 87.86

3.24 4.18, 4.30 5.46 5.97 6.50 6.90 6.83 6.93 7.66, 7.60 8.31

1.52 2.08, 2.09 2.94 3.14 4.09 4.40 4.10 4.14 4.96, 4.94 5.15

Conversion

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6

The change i n i n t r i n s i c v i s c o s i t y with conversion i s shown i n F i g u r e s 7 and 8. Although the data i n Figure 8 appears to be q u i t e s c a t t e r e d , a l l values o f i n t r i n s i c v i s c o s i t y between and i n c l u d i n g 13.9 and 33.8% conversion agree to ± 3.1% o f the average value o f 5.21 d l / g , w e l l w i t h i n the experimental e r r o r of the method. In c a r r y i n g out molecular weight development s t u d i e s i n emulsion polymerization, care must be taken that the e m u l s i f i e r s do not a c t as chain t r a n s f e r agents and thus lower the molecular weight averages o f the samples. Kamath (2) and Wang (7) have shown that Emulphogene BC-840 does not a c t as a c h a i n t r a n s f e r agent. The high values f o r i n t r i n s i c v i s c o s i t i e s obtained i n t h i s study a l s o i n d i c a t e that BC-840 i s " s a f e . "


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Ubbelohde Viscometer Low Shear Viscometer

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70

I 80

1

Emulsion Polymerization

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