Journal of Scientific & Industri al Research Vol. 58, May 1999, pp364-374

Kinetic Study of the ~vficroemulsion Polymerizations of Methyl Methacrylate and n-Butylacrylate

G.Y. Ramana Reddy Cent ra l Leather Research Institute, Adyar, Chennai 600020, India

and P. Manimalar, S . Prakash and R . Sriram

Coimbatore Institute of Tec hnology, Coimbatore 641 014, India

Received: 15 May 1998; accepted: 6 December 1998

Microemulsion po lymeri zati ons of methyl methacrylate (MMA ) and n-butyl acrylate (BA) have been studied , using sodium lauryl sulphate (SLS) as su rfac tant and potassiu m persulphate (KPS ) as initi ator. Cetyl alcohol (CA) and octyl alcohol (OA ) are used as cosurfac tants (CS) in the present study. The rates of mi croemulsion polymeri zati ons of M MA show 0.39 and 0.44 power dependencies on [KPS] with SLS-CA and SLS-OA as surfactant-cosurfactant pairs, respect ively. The rates of microemulsion polymeri zations of BA show 0.33 and 0.54 power dependencies on [KPS] with SLS-CA and SLS-OA as surfactant and cosurfactant pairs in the reaction medium, respectively. The latex solutions of MMA made by microemulsion polymeri zation methods using SLS-OA as surfac tants combination show that the particle sizes of latex solutions decre:1se with the increase of in iti ator concen­trati on in the reacti on medium. Whercas, the latex so lutions of BA do not show any regular trend in the variati on of part icle sizes with the initiator concentration. The rates of mic roeilluisi on pol ymerizations of MMA with SLS-CA and SLS-OA as surfactant­cosurfactant combinati ons in the reaction medium show 0.61 and 0.60 power dependenc ies on total surfactants concentrati on, {[SLS]+[CS]}, respectively. The rates of mi croemul sion pol ymerizations of I3 A show 0.5H and 0.60 power dependencies on {[SLS]+[CS]} with SLS-CA and SLS-OA as surfactants combinations, respecti vely. The effects of monomer concen trJti on Oil

microemulsion polymerizations of MMA and BA are also studi ed by keeping the oill~ constant , with the add ition of non ­reacti ve diluent, toluene, to the react ion medium. The overall acti va ti on energies, E""CI";oIl' for the mi croemul sion po lyme ri zJt ions of MMA and BA are evalu ated in the temperature region 60-75"C. The overall activation energies 5R.83 and 22.22 kl l mol arc obtained for the microemul sion polymerizations of MMA wi th CA and OA as cosurfactants in the react ion medium, respectively. The overall ac ti vat ion energies 29.12 and 47.87 kllmol arc obtained for the microclI1ul sion polymerizations of I3A with CA and OA as cosurfactant s in the reaction medium, respectively.

Introduction

E mul s ion polymerization has been an important and

widely used process for the production of a large number of polymers. The kine tics and mec hani sms of conven­tional emul sion polymerization have been studi ed ex­

tensively. The principal loc i for the initiation of latex

particles were proposed to be the monomer-swollen mi­celle 1,2 and aqueous phase3-6 .

A tik and Thomas 7 p o lym e ri ze d s tyren e o/w

microemulsion s using azobisisobutyronitrile (AIBN) or gamma-ray source for initiati on. Monodi sperse latexes

w ith average diameters of20 and 35 nm were produced. John son and Gulari 8 polymerized diluted styrene o/w

microemulsions using potass ium persulphate (KPS) and AIBN initiators and c ha racterize d the size of microemul sion droplets and latex particles by photon

corre lation spectroscopy. A bimodal latex partic le size

distribution was found and attributed to the presence of

two competing mechani sms for initiati on. Jayakri shnan and Shah9 studied the po lymerizati on of styrene and

methyl methacrylate (MMA) o/w mi croemul siom wi th AIBN and benzoy l peroxide initiators. A lower pc lym­

erization rate compared to conventi onal emulsion po .. Iymeri zation was found . The latexes obta ined were not

stable and could not be diluted. The photo ini tiated po­lymerization of styrene o/w microemulsions wa!', reported by Kuo et ai. 10 using dibenzyl ketone as ini tiaror and an ultraviolet light source . Rabag liati et a l. ll repolted the polyme rization of styrene in a three-ph ase

microemulsion- o il-water syste m u s in g pota ssi um

persulphate and AIBN. No autoaccelcrati on \-vas ob­served and the inc reas ing initiator concentrati on led to higher polyme rization rates and lowe r mo lec ular

weights. Guo el al .12 reported the kinetic s tudy on

REDDY el af.: KINETIC STUDY OF MICROEMULSION POLYMERIZATIONS 365

microemulsion polymerization of styrene olw system using KPS and 2,2'-azobis-(2-methyl butyronitrile) and the kinetic res ults were compared with conventional emulsion and miniemulsion polymerizati on systems. Loh et ol." studied the effect of surfactant concentra­tion on the microemulsion polymerization ofMMA and isobutyl methacrylate. Candau et al.'4 reported the in­verse acrylamide microemulsion polymerization initiated by KPS and AIBN. Small particle sizes and high mo­lecular weights were found . Some other publications have also appeared on wlo microemul s ion polymerizations during the past few years 15 - 1l).

This paper describes the kinetic inves ti gati on of olw microemulsion homopolymeri zati ons of methyl meth­acrylate (MMA) and n-butyl acrylate (BA) using potas­sium persul phate (KPS) as initiator, sodium lauryl sulphate(SLS) as surfactant, and cetyl alcoho l (CA) and octyl alcohol (OA) as cosurfactants. A comparative study of these microemulsion polymerizations with conven­tional emulsion and miniemulsion polymerizations is also reported.

Materials and Methods

Materials

The monomers, methyl methacrylate (M MA) (LR, Sisco-Chem Industries, Bombay, India) and n-butyl acr­ylate (BA) (LR, S.d. fine-chem Pvt Ltd , Boiscll-, India) were washed with 5% sodium hydroxide solulion to re­move inhibitor, followed by di stilled water. The washed monomers were dried over anhydrous sodium sulphate and the dried monomers were vacuum distilled under inert conditions.

Potassium persulphate (KPS), sod ium Iauryl sulphate (S LS ), cetyl alcoh ol (CA), hydroquinone (a ll LR and from S.d. fin e-chem, Pvt Ltd , Boisar, India ), octyl al sochol (OA) (2-ethyl hexanol), (LR, G.S . Chem Tes t­ing Lab & Allied Industries, Bombay, India) were used without furth er purification.

The solvents, methyl ethyl keton e (MEK) , (LR, Ranbaxy Laboratories Ltd , SAS Nagar, India), acetone (LR, Fischer Inorganics & Aromatics Ltd , Madras, In­dia) were di stilled before use . The commercial grade rectified spirit (E. I.D. Parry India Ltd, Madras, India) was distilled over calcium oxide before use.

Method

Sodium lauryl sulphate (SLS) in requi site quan tity (13.5 g) was dissolved in di stilled water (150 mL) to which the solution of a cosurfactant (0.75 g in the case of CA and 2.64 g in the case of OA) and monomer (8.55 g) was added and the mixture was stirred with magnetic stirrer bar for 40-50 min to ensure a uniform di spersion of monomer and cosurfactant solution. Thi s was used in microemulsion homopolymeri zati on experiments.

The microemulsion homopolymerizations of MMA and BA were carried out in 50 mL reaction tubes pro­vided with an inlet and outlet for the passage of nitro­gen. The requi site amount of microemulsion solution of monomer ( 15 mL) was taken in the reaction tube and deaerated by passing pure dry nitrogen through it . This deaerated microemulsion so lution was kept over a con­stant temperature water bath of 70uC and to thi s, after atta ining thermal equilibrium , the deaerated ini tiaror solution (0.2%) of KPS was added ( 1.5 mL). The reac­tion mi xture was shaken well. The homopolymerizat lon of the microemulsion so lution of the monomer was car­ried out at 70"C for the required time peri od and the pol yme rizati on reaction was stopp ed by addin g hydroquinone. The polymer from this microemulsion solution was precipitated with the addition of methanol and separated by fi ltration . The filtered pol ymer was washed with alcohol and water, and dried to a constant weight in a vacuum oven at 60°C. The conversion (%) of monomer into polymer was computed from the weight of th e polymer formed. The co nvers ions (7e ) of monomer for vari ous interval s of time were obtained and were plotted against tim e to y ie ld th e rat e of microemulsion homopolymerization (max imum rate), Rp'

from the steep portion of the graph (Figs I and 2).

Characteriwtion of Latexes

The latexes of polyMMA and polyBA, prepared by homopolymeri zing the microemulsion solutions of these monomers to lotal co nversions employing SLS-OA as surfactants cOl11bination and using various concentrat ions of initiator (KPS ) in the initial reac tion med ium , were analysed for their parti cle sizes usin g Joyce Loebl Disc Centrifuge (Tables land 2, Figs 3 and 4).

Molecular Weight Determination

The polymer samples made usi ng various concentra­tions of monomers in the microemulsion solutions by keeping the oil phase constant with the addition of non­reactive diluent , toluene and employing SLS-CA as

366 J SCIIND RES VOL 58 MAY 1999

100 ~------'---------,

8 .0

;f! - 60

c o ... '" :> g 40 u

20

Tim.!:, min

Fig. I - Percent conversion vs time plots for the microemulsion polymerization of MMA at various KPS concentration

[MMA]=0.4583 M, [SLS]=0.2512M, [CA]=O.O 166M, T=700 C. Concentrations of KPS: (1)6.7263 X 1O'4M (2) 3.3610 X 1O'4M (3) 2.2197 X 1O'4M (4) 1.3453 X 1O'4M (5) 0,9417 X 1U,4M

100,---------------,

eo

~ 60 ~ o

= . :> ~ o U

10 o 10

Time, min

Fig. 2 - Percent conversion vs time plots for the microemulsion polymerization of I3A at various KPS concentrations

[BA]=0.3530 M, [SLS]=0.2477M, T = 70° C. Concentrations of KPS: (I) 15.1340X IO'W (2) 11.7710 X IO'W (3)9.0804 X 1O'4M (4) 6.7263 X lO'4M (5) 5.0447 X IO'W (6) 3.3631 X IO'W (7) 2.3545 X lOAM

surfactants combination were precipitated and purified by re-precipitation. The molecular weights were deter­mined by measuring the intrinsic viscosities in MEK at requisite temperatures and using the following Mark­Houwink relations :

,HfltI,"h"ii;,III., ... "11111'"'"1111111111111 :

.. "'.111/1,111111,,,",,,,,,,,:;;;:,, .url.,' ..... IIIIII.,I.,hll,II.,III,'::I,:11

";'I"lIlllIllIIWI'hlllllltllll"I,,'U:';"'IIoI ' ... '111, "11II~I.iiiihllll"'''"'b '111,1::11, '1IIII":i:,,, , ..

,,11111111 ~ 1!III'IIIlI"P.! gll""I'"'' II,;, ""I:'IIIIIo":!'IIII",I' 11,11, "'1 '" . . D.Min:O.l Scale: 1 0 Max:2.1

Fig. 3 - Evaluation of particle diameter-microemulsion polymer solution of MMA (sample 3)

• • dlll,IIIIIIIIIIII'" .1 1"1,.11.,1111111111111';1111'1,

,1 !llltllllllll l h l "IIIIIIIIIII".II::;ii ll·'II,,',.Io. I,I,IIIIIIUII,III'·""II,IIII.IIII1I1IIIIIIIIIIIIII"IIIIIiIIIIlII,'&', ..... . .

,11I111I11I1,11.llIhlllllllllll"II,'III'II'IIIIIIIII'III"III"III,1 11I1 111 1:,I"j./I . '11 '1IIIIIIIII'I'III)"I,IIIIIIIItIlIlIlIIlIIIlII,··III'II"'IIIIIIII'lo'"11'1110,1111 111111,

1IIII11III1I"I,I'111111"IIII"IIIIIIIIIII.'II'I,I"II'I'''''IIIIt'IIIII,hullIlllllIlllIII, 1I1I.11I1 ' 11..IIIIIIIIIIIIIII,tlftlilltlll,j,'t'1111hll ~111'1,11'lll1'JI""I'II,"III",,,,

o Min:O.5 Scale .1 o Max:l0.5

Fig. 4 - Evaluation of particle diameter-microemulsion polymer solution of BA (sample 2)

Results and Discussion

Features of Microemulsions and Resulting Latexes

In the present study, the microemulsion homopolymerizations of MMA were carried out by em­ploying SLS-CA and SLS-OA as surfactant-cosurfactant systems and KPS as water soluble initiator. The microemulsion solutions were translucent and after po­lymerization these solutions became white turbid. The latex solutions ofpolyMMA prepared by using SLS-CA combination were unstable while SLS-OA combinations were stable.

The microemulsions of BA turned into the white tur­bid solutions after polymerizations by using SLS-CA and SLS-OA surfactants combinations, respectively, in the microemulsion recipes. These latex solutions were sta­ble.

The change in colour of microemulsion solutions af­ter polymerizations might be due to the increase in par­ticle size of the latex particles during polymerizations.

REDDY et at.: KINETIC STUDY OF MICROEMULSION POLYMERIZATIONS 367

Table I - Study of the effect of initiator concentration on the microemulsion polymerization of MMA

[SLS]=0.2512M

[MMA]=0.4583M Reaction temperature=70"C

Cetyl alcohol as cosurfactant Octyl alcohol as cosurfactant [CA]=0.0166M [OA]=0.1079M

SI No. [KPS]xI04 Rpxl04 [KPS]x I Q4 Rpxl04 Dw Dn (mole/L) (mole/Us) (mole/L) (mole/Us) (Ilm) (Ilm)

I 6.7263 17.8170 15.1340 9.7022

2 3.3631 12.8324 11.7710 9.4892 1.2622 0.9993

3 2.2197 11.3047 9.0904 9.0958 1.3113 1.1080

4 1.3453 9.1660 6.7263 8.4896 1.3854 1.1896

5 0.9417 8.5549 5.0447 6.8220 13.3114 11.5622

6 3.3631 5.7607 16.8537 14.0671

7 2.3542 4.5479

Table 2 - Study of the effect of initiator concentration on the microemulsion polymerizatio of BA

[SLS]=0.25 12M

[BA]=0.3586M Reaction temperatur~=70"C

Cetyl alcohol as cosurfactant Octyl alcohol as cosurfactant [CA]=0.0l66M [OA]=0.1079M

SI No. [KPS]x I 04 Rpxl04 [KPS]x I 04 Rpxl04 Dw Dn (mole/L) (mole/Us) (mole/L)

6.7264 11.0570 15 .1340

2 3.3631 8.3673 11.7710

3 2.2197 7.1720 9.0804

4 1.3453 . 6.6939 6.7263

5 0.9417 5.9767 5.0447

6 3.3631

7 2.3542

Microemulsions, however, can be formed spontaneously and their stability is believed to be thermodynamic in origin. It has been proposed 12,20-22 that thermodynami­cally stable microemulsions can be formed when the negative free energy change, due to entropy of disper­sion and the negative change of chemical potential caused by the adsorption of surfactant and cosurfactant on the interface, overcomes the positive product of the low in­terfacial area and the large interfacial area generated by the dispersion of one phase into another.

(mole/Us) (Ilm) (Ilm)

7.5307

10.5900 5.0066 3.8769

8.9427 5.8967 4.5861

8.2507

6.5893 2.6807 2.1884

5.1773 1.8509 1.4966

4.0006 2.0998 1.6325

Effect of KPS Concentration

The effect of initiator (KPS) concentration on the microemulsion polymerization of MMA by using SLS­CA as surfactants combination was studied in the con­centration range of initiator as (O.94l7-6.7263)x lOAM.

The rates of microemulsion polymerizations, Rp, of MMA were evaluated for various initial initiator con­centrations in the reaction medium and were plotted against initiator concentrations on a logarithmic scale to evaluate the dependency of rate on [KPS] and was f~und

368 J SCI IND RES VOL 58 MAY 1999

c. Q:

1.3

1.1

2' 0 .9 + <t

0 .7

0 .5

1.0 1.2 1. 4 1.6 1.8 2.0 2.2

5 + lo g [K PS]

Fig. 5 - Plots of log HI' VS log IKPS] at 70°

( I) IMMA] = 0.4583 M, ISLS 1= 0.2512 M, rCA 1 = 0.0166 M (2) IMM A] = 0.4548M, ISLS I = 0.2493M, rOAI = 0. 1079M t3) [BA 1 = 0.3586M. rSLS] = 0.25 17M. [CAl = 0.0 166M (4) [BAI = o 3530M, [SLSj = 0.2477M, 10AI = 0. 1086M

80

-..e o ~ 60 c o

'" ... 41 :> g 40 o

O~~~~-L~--~--~--~--~---~

o 10

o 20 10

Time / min

30-1-3 20----.-4/5

Fi g. 6 - Percent conversion vs time plots for microemul sion polym­e ri z~ lli on of MMA at various SLS concentrations [MMAI = o 3788M, lOA] = 0.1090M, IK PSI = 6.7260 X IO-.IM, T = 70° C Conccntrati ons of SLS : ( I ) 0.2574M, (2) 0.2322M, (3) 0.2098M, (4) 0. 1 X75M, (5) 0. 1735M

to be 0.39 power on initi (l tor concentrati on (Table I, Figs I and 5) . This dependency of Rp on the initiator con­cent ration is in consistence with the prediction of initi a­tor exponent (0.40) in Smith-Ewart case2 hypothes is2

and thi s is probab ly du e to th e com parable size of

~ 0

c 0

'" ... Ql :> c: 0 0

80

60

40

20

15 25 o 10

Time, min

35-1-4 20-- 5

Fig. 7 - Percent convcrsion vs limc plol s fOI· the IllI CrOelll ubioll polymerization of BA at various SLS concentrati oll s

IBAI = 0.2'+93M. ICAI = 0.0166M. IKPS I =3.363 1 X IO-IM,T =70" C Concentrations of SLS: ( I ) 0.2518M, (2) 0.2238M, (3) O. 195'JM, (4) 0.1679M. (5) 0.139'JM

microemulsion drop lets and micell es t 2 A similar va lue (0.40) for the miniemul sion polymeri zat ion ofMMA has been reported by Fontenot and Schork21 _

OA was used instead o f CA in th e above microemulsion polymerization experim ent s and th e initi a tor co nce ntr ati o n was va ri ed in th e ran ge (2 .354- 15.134) x 10--1 M. The dependency of the mi croemulsion polymerization or MMA on [KPS] was found to be 0.44 power on initi ator concen trati on (Table I, Fig.5). This hi gher va lue (0.44) for the power of ini­ti ator concen trati on compared to miniemul sion polym­eri za ti on of MMA2J (0.40) could be due to the higher free radicai absorption rate of smaller size microemuision droplets 12 .

For the microemulsion polymerization of BA, the in i­tiator (KPS ) co ncentration was vari ed in th e ra nge (0.94 17-6.7264) x I 0-4M and SLS-CA pair was employed as surfactants combination in microemulsion rec ipe. The rates of microemulsion pol ymeri zati ons for various ini­tial initi ator concentrations were plotted against the ini­tiator concentrati ons on a logarithmic scale and a 0.33

REDDY et (/1.: KINETIC STUDY OF MICROEMULSION POLYMERIZATIONS 369

0-0::

1.0

g' 0 .8

+ 'l"

0 .6

0 .2 0 .3 C.4 0 .5 · 0 .6 0 .7

1,3 : 1+I09{[SLS) + [CA]}; 2,4 : 1+109~SLS]+[OA]}

Fig. 8 - PIOIS of log Rp vs log ([SLS] + [CA)}; and log Rp vs log ([SLS) + lOA]) at 70° C

(1) [MMA) = 0.3291 M, rCA ) = ('.0 169M, [KPS] = 3.363 1 X ! 0·4M (2) [MMA) = 0. 3788M, [OA)= 0. I090M, [KPS) = 6.7260 X 10·4M (3) [BA) = 0.2493M, rCA) = 0.OI66M, [KPS] = 3.363 1 X 10·4M (4) [BA] = 0.2964M, lOA] = 0. 1097M, [KPS] = 6.7263 X 10·4M

'l-c 0

'on 0

~ > c 0

U

o 10 20

Time, min

Fig. 9 - Percent Wi". version vs time plots for the microemul sion pol ym<:l lZal lOn of MMA at various MMA concen trations

[SLS] = O.2493M, lOA] = 0.1079M, [KPS] = 6.7263 X 10.4 M, T = 70° C Concentrations of MMA: ( I ) 0.4550M, (2) O.3950M, (3) O.3400M, (4) O.2950M

power dependency o n [KPS] was obtained (Table 2,

Fig. 5). The initiator concentration was varied in the range

(2.354- l5.1 34)x I 0·4M for the microemul sion polymeri­

zation of BA and by using SLS-OA as surfactants com­

bination in the reaction med ium. The dependency of Rp on [KPS] was obtained from the log- log plot of Rp ver­sus [KPS] and was found to be 0 .54 power on [KPS]

(Table 2, Figs 2 and 5) . Hi ghe r va lue for the power of

[KPS] (0.74) has been reported for the mic roemul sion

polymerization of BA with anionic surfactants25 .

I

100

80

. . ;;;; - 60 c:

0 'w;

' 4; > c: 0 40 u

20

0 0 10 20 30

Time, min

Fig. 10 - Percent conversion vs time plots for the microemulsion polymeri zati on of BA at various BA concentrations [SLS] = O.2517M, [CAl = 0.01 66M, [KPS] = 3.363 1 X 10.4 M, T = 70° C Concentrations of BA: ( I ) 0.3586M, (2) 0.2680M, (3) 0.3133M, (4) 0.2341 M

The particle size variation with initiator concentra­tion was studied for the polymer latexes prepared by

microemulsion po lymerization ofMMA using SLS-OA

as surfactants combination, and allowing total conver­

sions of the mic roemulsion polymerizations (Tablel ).

The particle diameter, D w ' decreased with increase in the initiator concentration. It indicates that the number

of latex particles increases with the increase of initi ator concentration, which, in turn, causes the decrea se of

particle sizes of latex systems with increase of initiator

concentration.

There is no regular trend in variation of particl e di­

ameters with initiator concentration for the latexes made , by mic roemulsion polymeri zation of BA, using SLS-OA as surfact.ll1ts combination in the microemulsion rec ipe '\ . (\fabl e 2).

EJfect of'SlIrf'aclal/l COI/Cel/Imliofl

The surfactant (SLS) concentration was varied in the

range 0.1424-0 .2564M in the microemulsion recipe of

MMA containing CA as cosurfactant, maintaining all other parameters in the reac tion medium constant. The rates of microemul sion polymerizations of MMA with variou s SLS concentrations in the reaction medium were plotted against the total surfactants concentrati ons,

370 j SCI IND RES VOL 58 MAY 1999

Table 3 -- Study of the effect of monomer concentration on the microemulsion polymerization of MMA at 70"C

[KPS]=3.3631 x 10·4M [SLS]=0.2512M [KPS]=6.7263x IO·4M [CA]=0.0I66M [SLS]=0.2493M

[OA]=0.1079M

S. No. [MMA] Rpxl04 [Tl] M x10·5 w [MMA] Rpxl04

(mole/L) (mole/Us) (mUgm) (mole/L) (mole/Us)

0.4583 11.6100 307 27.4614 0.4550 8.4896 2 0.3980 9.2870 220 17.2873 0.3950 5.7933 3 0.3426 6.8520 208 15.9917 0.3400 4.0800 4 0.2973 5.9460 174 12.4807 0.2950 3.3433

Table 4 - Study of the effect of monomer concentration on the microemulsion polymerization of BA at 70"C

[SLS]=0.2477M [OA]=0.1073M [KPS]=6.7263x 10·4M

[SLS]=0.25 17M [CA]=0.0166M [KPS]=3.363I x 10·4M

S. No. [BA] Rpxl O4 [TlJ MxlO·4 w

[BA] RpxlO4

(mole/L) (mole/Us) (mUgm) (mole/L) (mole/Lis)

0.3586 8.1283 2 0.3133 6.7882 3 0.1680 5.5833 4 0.2341 4.994 1

{[SLS)+ (CA}}, in the reaction medium on logarithmic scale to evaluate the dependency of Rp on {[SLS]+ [CAl land was found to be 0.61 power on total surfactants concentration (Fig. 8). Similar studies were also carried out using OA as cosurfactant in the microemulsion recipe of MMA by varying the SLS coneentration in the range 0.1735-0.2574M. A 0.60 power depend,ency on tot~1 surfactants concentration, {[SLS]+[OA]}, for the microemulsion polymerization of MMA was 6btained (Figs 6 and 8). The probability of homogeneous nuclea­tion may become insignificant due to the large area pro­vided by the microemulsion droplets which will capture most of the radicals generated in the aqueous phase be­fore they reach the critical size for precipitation. In ad­dition, the concentratiol1l of MMA and cosurfactants in

212 159 130 85

14.5545 0.3530 8.2507 9.2850 0.3084 6.1680 6.7785 0.2638 5. 1001 3.4899 0.2304 5.0683

the aqueous phase may be less than expected from their solubilities, due to thermodynamic considerations 12. The thermodynamic stability of a microemulsion is a func­tion of the partitioning of the cosurfactant between the core and interphase of the microemulsion droplets for two reasons I2 ,22 ,24: (i) the dilution of oil phase by cosurfactant decreases the free energy and promotes additional solubilization of monomer; and (ii) the ad­sorption of cosurfactant at the interface decreases its chemical potential in the bulk phase and provides ex­cess surface area. Consequently, due to the partitioning of cosurfactant in the oil phase, the amounts of cosurfactant and monomer which exist in the aqueous phase can be less than their solubilities. In the present study, the dependency of Rp on total surfactants concen-

REDDY et at.: KINETIC ST UDY OF MICROEM ULSION POLYM ERIZATIONS 37 1

0.4

0 .2 L-._---1 __ --'-__ -.l.-__ -'-'

0.3 ' 0.4 0 .5 0 .6 0.7

1,2 : 1+log [MMA] i 3 ,4 : 1+109 [SA]

Fig. II - Plots of log Rp vs log ( [MMA] and log Rp vs log (BA] at 70° C

( I) [SLS] = 0.25 12M, rCA] = 0 .0 166M, [KPS ] = 3.363 1 X 10-4M (2) [SLS ] = 0.2493M, [OA]= 0.1079M, [KPS] = 6.7263 X 1O-4M (3) [SLS] = 0.2517M, [CAl = 0.0 166M, [KPS] = 3.363 1 X lOAM (4) [SLS] = 0.2477M, lOA] = 0. 1073M, [K PS] = 6.7263 X 1O-4M

trati on is quite consistent with the predicti on of 0 .6 fro m the Smith-Ewart case 2 hypothesis2. T his reduces the probabili ty fo r homogeneous nucleation in the present study. The latexes made by microemulsion polymeriza­tion ofMM A usi ng CA as cosurfac tant in the rec ipe were un stab le and thi s mi g ht be due to the reaso n that cosurfac tant in thi s case ac ted as destabili zer for the latex by desorbing the surfactant from the surface of the polymer partic les 9. 12.

T he microemul sion polymeri zat ions of BA were car­ried out by empl oy in g [S LS] in the ranges, 0 . 1399-0.25 18M and 0. 1745-0 .2589M, usi ng CA and OA as cosurfac tants, respecti ve ly, in the reac ti on media. T he de pe nde ncies of Rp fo r th e mi c roe mul s io n poly meri zations of BA were fo und to be 0.58 and 0.60 powers on [total surfac tants] when CA and OA were used as cosurfac tants, respecti ve ly (Figs 7 and 8) . T hese values were consistent w ith the surfac tant exponent (0.6) in Smith-Ewart case 2 hypothes is2. In thi s case, the latexes are stab le and the probabili ty of homogeneous nucleation is reduced l2 .

EffeCT of Monomer Concentration

To study the effect o f MMA concentration on the k inetics of microemul s ion polymerization, toluene was

7.0 b

II c> 6 .0 ~

5.0 1.4 1.5 1.6 1.7 1.8

0 : 2+log [MMA] ; b : 2 + log [BA]

Fig. 12 - Plots of log Mw vs log [MMA] and log Mw vs log [B A] at

70° C

(a) [SLS] = 0.2512M, rCA] = 0.OI66M, IKPS] = 3.3631 X I O-~M

(b) [SLS] = 0.25 17M, [CAl = 0.0 166M, [KPS] = 3.363 1 X IO -4 M

used as a non- reacti ve d iluent to ma inta in the amount of o il ph ase constan t in the rec ipe . It is expected that microemuls ion with si milar size droplets will resu lt by keeping o il content constant , which can lead to sim ilar competition for radicals l 2.

A hi gher monomer concentration leads to a hi gher polymeri zati on rate, and the dependencies of 1.5 8 and 2.56 powers were obta in ed from log Rp versus log [MM A] plots with CA and OA as cosurfac tants, respec­tive ly, in the reaction media (Figs 9 and II ).

T he poly merization rate decreases more than can be accounted fo r by dilut ion alone l2 . T he effect of non­reacti ve additi ves on the kinetics of emuls ion polymeri­zati on was studied by Azad and Fitch26 . It was con­cluded26 that the depress ion in rates of po lymeri zation were not onl y due to dilution of monomer, but also to chain transfer with the diluent as it accumul ates in the partic les. C hain transfer leads to format ion of small radi­cals which may di ffuse out of the partic le, lowering the va lue of II , the rad icalllumber. T hus, the lower the po­lymeri zati on rate w ith increas ing amoun t of toluene can be al so accoun ted fo r the d ilut ion effec t and dec rease of I/. by c hain transfer reacti on and desorpt ion.

T he molecular we ights of the po lymers, poly MMA, obta ined with d ifferent initial monomer concentrat ions in the microemul sions usi ng CA as cosurfacta nt were de te rmined from the ir intr in sic v iscos ities measure­ments27 in M EK at 25°C. T he higher MMA concentra­tion led to higher molecular weights, as expected (Table 3) and the dependency was found to be the 1.85

372 J SCIIND RES VOL 58 MAY 1999

l00~--------·----~

80

~ . c~ 60 0 .. .... .. > c 0

U

Time, min

Fig. 13 - Percent conversion vs time plots for the microemulsion polymerization of MMA at various temperatures

MMA = 0.4583M, [SLS] = 0.2512M, rCA] = 0.01 69M, [KPS] = 3.3631 X 10-4 M, T(OC): ( I )75, (2) 70, (3) 65, (4) 60

power of monomer concentration (Fig.12).

Similarly, the effects of monomer concentrations for the microemulsion polymerizations of BA were studied using toluene as diluent to keep the oil phase constant and the dependencies were 1.26 and 1.53 powers on monomer concentration, respectively, with CA and OA as cosurfactants in the reaction media (Figs . 10 and II ).

The molecular weights of polymers, poly BA, ob­tained with different initial monomer concentrations in the microemulsions usi ng CA as cosurfactant were de­termined from their instrinsic viscosities measurements28

in MEK at 35°C. The higher BA concentration led to higher molecul ar weights (Table 4) and the dependency was 3.43 power of monomer concentration (Fig. 12) .

Eff ect of Temperature

The effec t of polymerizat ion temperature was stud­ied on the microel11ulsion polymerizations of MMA us­ing CA and OA as cosurfactants, respectively, at vari-

l00~--------------------~

80

~ • c 60 0

OIl ~ .. > c

40 0 u

o 10 20 30·

Time 1 min

Fig. 14 - Percent conversion vs time plots for the microemulsion polymerization of BA at various temperatures

[BA] = 0.3530M, [SLS] = 0.2477M, lOA] = 0.1 073M. [KPS] = 6.7263 X 10.4 M T(OC): (I )75, (2) 70, (3) 65, (4) 60

ous temperatures (60, 65, 70 and 75°C) . The rates of microemulsion polymerizations ofMMA at various tem­peratures are evaluated from the percent conversion ver­sus time plots. The overall activation energies for the microemulsion polymerizations ofMMA were obtained from the Arrhenius plots, log Rp vs liT, the s lopes of which gave (-E 11/2.303 R), where E II is the over-overa overa all activation energy and R is the gas constant (R=8.3 14 Joules K"1 .mol ·J). The overall activation energies ob­tained for the microemulsion polymerizations of MMA, using CA and OA as cosurfactants in the microemulsion recipes , were 58.83 and 22.22 kllmol, respectively (Figs 13 and IS ).

Similarly, the effect of tempe rature o n th e microemulsion polymeri zations of B A using CA and OA as cosllrfactants, respec ti ve ly, in the microemul sion recipes was studied at various temperatures (60, 65, 70 and 75°C) . The overall activation energ ies obtained for the microemulsion polymeri zat ions of BA were 29.12

REDDY et af.: KINETIC STUDY OF MICROEMULSION POLYMERIZATIONS 373

0. cr

1.1

O. 5·'--z....l.e-..I..--3.L-O --1.-3-'-=. 2---'-_..I..--'-:1-:!,2

2 .8 3'.0-3,4

Fig. 15 - Plots of log Rp vs (117) for the determination of overall

activation energy

( I) [MMA] = 0.4583M, [SLS] = 0.2512M,

[CA]=0.0166M,[KPS]=3 .3631 X 10-4M

(2) [MMA] = 0.4548M, [SLS] = 0.2493M, [OA]= 0.1 079M, [KPS] = 6.7263 X 10-4M

(3) [BA] = 0.3586M, [SLS] = 0.2517M, [CAl = 0.01 66M, [KPS] = 3.3631 X 10-4M

(4) [BA] = 0.3530M, [SLS] = 0.2477M, [~Al = 0.1073M, [KPS] = 6.7263 X 10-4M

are 0.31 and 0.54 powers on [KPS] using CA and OA as cosurfactants in the recipes, respectively. These values of initiator exponent in the present study deviate from the initiator exponent (0.4) in Smith-Ewart theory. The dependencies of Rp on emulsifier concentration for the microemulsion polymerizations of MMA are 0.61 and 0.60 powers on total surfactants concentration using CA and OA as cosurfactants in the recipes, respectively. The exponents of total surfactants concentration for the microemulsion polymerizations ofBA are 0 .58 and 0.60, us ing CA and OA as cosurfactants in the recipes, re­spectively. These are in consistence with the surfactant exponent (0.6) in Smith-Ewart case 2 hypothesi s. The effect of monomer concentration on the microemulsion polymerization of MMA shows 1.58 and 2.56 power dependencies on [MMA] when CA and OA are employed as cosurfactants, respectively, in the recipes. The effect of monomer concentration on the microemulsion polym­erization of BA shows 1.26 and 1.53 power dependen­cies on [BA] when CA and OA are e mployed as

cosurfactants, respectively, in the recipes. The molecu­lar weights of polymers obtained by employing various monomer concentrations at the same oil-water phase ra­tio and with CA as cosurfactant in the recipes show 1.85 and 3.43 power dependencies on [MMA] and [BA], respectively.Arrhenius plots for temperature variation yield the overall activation energies, 58.83 and 22.22 kllmol for the microemulsion polymerizations ofMMA, using CA and OA as cosurfactants in the recipes, re­spectively. The overall activation energies obtained for microemulsion polymerizations of BA are 29.12 and 47.87 kllmol when CA and OA are used as cosurfactants in the recipes, respectively.

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