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C o l l o i d P o l y m e r S c i e n c e olloid Polym Sci 270 :717~732 1992)

Lead ing ont r ibu t ionSusp ens ion em uls ion and d i spersion po lymeriza t ion: A m etho do lo g ica l surveyR . A r s h a d yD e p a r t m e n t o f C h e m i s t r y , I m p e r i a l C o l le g e o f S c ie n c e, T e c h n o l o g y a n d M e d i c i n e , L o n d o n , E n g l a n d

Abstract A m e t h o d o l o g i c a l d e s c r i p t i o n o f h e t e r o g e n e o u s p o l y m e r i z a t i o n p r o -cesses , inc luding suspens ion , emuls ion , d i spers ion , and prec ip i ta t ion polymer iza-t ion ~s presented . T he d iscu ss ion focuses on th e in i t i a l s ta te of the poly m er iza t io nm i x t u r e , m e c h a n i s m o f p a r t i c l e f o r m a t i o n , a n d t h e s h a p e a n d s i z e o f p o l y m e rpar tic les p rodu ced in d i f fe ren t he te rog eneou s poly me r iza t ion systems. The de-p e n d e n c e o f p a r ti c l e si z e a n d m o r p h o l o g y o n m a n u f a c t u r i n g p a r a m e t e r s s u c h a semulsif ier , stabil izer, reactor design, and st irr ing speed is discussed. Specialtop ics , in c luding emuls if ier - flee ( soapless ) emuls io n polym er iza t ion , seeded poly-me r iza t ion , an d the fo rm at ion o f core-she ll par tic les a re a l so br ie f ly covered .Key words H_ te rogene ous poly m er iza t ion ; _suspens ion polym er iza t ion ; em uls ionpolym er iza t ion ; d i sp ers ion p olym er iza t ion ; prec ip i ta t ion _polymer iza t ion ; _part ic le_[ormation; pa rt icle _size; core-shell part ic les

n t r o d u c t i o nD e t a i l s a n d p r o g r e s s o f s u s p e n s i o n , e m u l s i o n ,

a n d d i s p e r s i o n p o l y m e r i z a t i o n s a r e r e g u l a r l yr e v i e w e d i n s p e c i a l i s t p o l y m e r p u b l i c a t i o n s .H o w e v e r , a c o n c i s e t r e a t m e n t o f t h e s u b j e c t o fh e t e r o g e n e o u s p l y m e r i z a t i o n p r o c e s s e s c o v e r i n g a llo f t h e s e t e c h n i q u e s i n a c o m p a r a t i v e m a n n e r i ss u rp r i s i ng l y l ack i ng i n t he l i t e r a t u re . In add i t i on , o rp r o b a b l y f o r t h i s r e a s o n , t h e t e r m s s u s p e n s i o n ,e m u l s i o n , a n d d i s p e r s i o n a r e n o t a l w a y s u s e d c o n -s i s t en t l y i n t he l i t e r a t u re . S om e au t ho r s r e f e r t oe m u l s i o n p o l y m e r i z a t i o n a s d i s p e r si o n p o l y m e r i z a -t i o n o r v i c e v er s a , o r t o s u s p e n s i o n p o l y m e r i z a t i o na s e m u l s i o n p o l y m e r i z a ti o n . S o m e p o l y m e r i z a t i o nl i t e r a t u r e s h o w t h e t e r m s s u s p e n s i o n , e m u l s i o n ,a n d d i s p e r s i o n a s e q u i v a l e n t s o r s y n o n y m s . T h i ss i t u a t i o n i s p r a r t i c u l a r ly c o n f u s i n g f o r s t u d e n t s a n dn o n - p o l y m e r sc i e n ti s ts w h o s e , w o r k m a y i n v o l v et h e u s e o f p o l y m e r p r o d u c t s o b t a i n e d b y h e t e ro g e -n e o u s p o l y m e r i z a t i o n te c h n i q u e s . T h e p r e s e n t a r t i-c l e i s a i m e d t o p r o v i d e a c o n c i s e m e t h o d o l o g i c a ld e s c r i p t i o n o f a ll o f t h e m a j o r h e t e r o g e n e o u s p o l y -m e r i z a t i o n p r o c e s s e s , n a m e l y , s u s p e n s i o n , e m u l -s i o n , d i s p e r s i o n , a n d p r e c i p i t a t i o n p o l y m e r i z a t i o no f v in y l m o n o m e r s . A s i m i l a r c o v e r a g e o f s u s p e n -

s i o n , d i s p e r s i o n , a n d i n e r f a c i a l p o l y c o n d e n s a t i o np roces s es is t h e s u b j e c t o f a f o r t h c o m i n g p u b l i -cat ion [1] .

G e n e r a l f e a t u r e s o f h e t e r o g e n e o u sp o l y m e r i z a t i o n p r o c e s s e sH e t e r o g e n e o u s o r p a r t i c le f o r m i n g ) p o l y m e r i z a -t i o n p r o c e s s e s a r e u s u a l l y t w o - p h a s e s y s e t m s i nw h i c h t h e s ta r t i n g m o n o m e r s ) a n d / o r t h e r e s u l ti n gp o l y m e r a r e i n t h e f o r m o f a f in e d i s p e r s i o n in a ni m m i s c i b l e l i q u id . T h e p o l y m e r i z a t i o n i n i t i a to rm a y b e s o l u b le i n t h e m o n o m e r o r t h e l i q u id , a n d i tm a y o r m a y n o t b e p r e s e n t w i t h i n t h e p o l y m e r

p a r t ie l e s d u r i n g t h e i r f o r m a t i o n . I n a d d i t i o n t o t h em o n o m e r s ) , p o ly m e r i z a t io n m e d i u m a n d t h e i n it i-a t o r , o n e o r m o r e a d d i t i v e s i s a l s o a d d e d t o t h ep o l y m e r i z a t io n m i x t u r e t o e m u l s if y th e m o n o m e r ,a n d / o r t o s ta b i li z e t h e m o n o m e r d r o p l e t s a n d t h er e s u l t i n g p o l y m e r p a r t i c l e s . V a r i o u s c o m b i n a t i o n so f t h e a b o v e - n a m e d p o s s i b i l i t i e s a r e e m p l o y e du n d e r e m p i r i c a l l y a d j u s t e d c o n d i t i o n s t o p r o d u c es pher i ca l o r i r r egu l a r l y s haped ) po l y m e r pa r t i c l e sw i t h i n r e l a t i v e l y n a r r o w s i z e r a n g e s f r o m a b o u t5 0 n m t o 1 - 2 m m o r l a rg e r.

5 6

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718 Colloid and Polymer Science, Vol. 2 7 0 - N o . 8 1992)

T h e r e a r e b a s ic a l ly f o u r d i f f e r e n t t e c h n i q u e s e m -p l o y e d f o r t h e m a n u f a c t u r e o f p a r t ic u l a t e p o l y m e rp r o d u c t s b y a d d i t i o n p o l y m e r i z a t i o n o f v i n y lm o n o m e r s . T h e s e a r e c o n v e n t i o n a l ly t e r m e de m u l s i o n o r s u s p e n s i o n p r o c e ss e s o n t h e b a si so f t h e s i z e o f t h e p o l y m e r p a r t i c l e s p r o d u c e d . A na r b i t r a r y d i v i d i n g l i n e b e t w e e n e m u l s i o n a n d s u s -p e n s i o n is t h e d r o p l e t / p a r t i c l e s iz e o f 1 # m( 1 0 0 0 n m ) . F l u i d s c o n t a i n i n g d r o p l e t s / p a r t i c l e ss m a l le r th a n a b o u t 1 # m a r e k n o w n a s e m u l s i o n s( l a t ex o r co l l o i d s ) , and t hos e con t a i n i ng pa r t i c l e sl a rg e r t h a n a b o u t 1 # m a s s u sp e n s io n . T h e u p p e rl i m i t o f p a r t i c l e s i z e f o r s u s p e n s i o n s y s t e m s m a y ,f o r t h e p u r p o s e o f t h e p r e s e n t d i s c u s s i o n , b e r e -g a r d e d a s b e i n g a b o u t 1 - 2 m m . T h e t e r m d i s p e r-s i o n p o l y m e r i z a t i o n h a s b e en a d o p t e d b e c a u se t h et e c h n i q u e r e p l a c e d t h e p r o c e s s o f p o l y m e r d i s p e r -s i o n in t h e p a i n t i n d u s t r y . P o l y m e r i z a t i o n p r o c e s se sl e a d in g t o t h e f o r m a t i o n o f m a c r o s c o p ic a l ly a p p a r -e n t p o l y m e r p r e c i p i t a t e s a r e r e f e r r e d t o a s p r e -c i p it a ti o n p o l y m e r i z a t i o n . T h e w o r d d i s p e r s i o ni s a l s o u s ed t o des c r i be l i qu i d / s o l i d d i s pe r s ons i n agenera l s ens e .T h e t e r m i n o l o g y o f h e t e r o g e n e o u s p o l y m e r i z a -t i o n s y s t e m s a d o p t e d h e r e i s t h e o n e e m p l o y e d b ym o s t p o l y m e r c h e m i s ts , a n d t h e t e r m s s u s p e n -s i o n , e m u l s i o n , d i s p e r s o n , a n d p r e c i p i ta -t i o n a r e c le a r l y d i s t i n g u i s h e d o n t h e b a s is o f t h efo l l owi ng fou r c r i t e r i a .1) I n i ti a l s ta t e o f t h e p o l y m e r i z a t i o n m i x t u r e ;2 ) K i n e t ic s o f p o l y m e r i z a t i o n ;

3 ) M e c h a n i s m o f p a r ti c le f o r m a t i o n ; a n d4 ) S hape a nd s ize o f t he f i na l po l ym er pa r t ic l e s .T h e i n t e r d e p e n d e n c e o f t h e a b o v e f a c t o r s w i l lb e c o m e a b u n d a n t l y c l e a r th r o u g h o u t t h e p r e se n td i s c u s s io n , a n d t h e c l a r i fi c a ti o n is h o p e d t o e n c o u r -a g e a m o r e u n i v e r s a ll y c o n s i s t e n t us a g e o f t h e t e r m ss u s p e n s i o n , e m u l s i o n , a n d d i s p e r s i o n p o l y m e r i z a -t i o n . K i n e t ic f e a t u r e s a n d t h e s iz e r a n g e o f p o l y m e rp a r t ic l e s p r o d u c e d b y d i f f e r e n t h e t e r o g e n e o u s p o l y -m er i za t i on p roces s es a re p res en t ed i n F i g . 1 .I t i s he re neces s a ry t o ga t he r a b r i e f i dea o f t hen a t u r e a n d r o le o f v a r i o us c o m p o n e n t s o f h e te r o -g e n e o u s p o l y m e r i z a t i o n s y s t e m s in g e n e r a l , b e f o r ed i s c u s s in g t h e d e t a i l s o f i n d i v i d u a l p r o c e ss e s . T h u s ,t h e m o n o m e r is u s u a ll y r e f e rr e d t o a s t h e m o n o -

m e r p h a s e o r d i s p e r se d p h a s e . T h e l iq u i d p h a s ec o n t a i n i n g t h e d i s p e r s e d m o n o m e r i s d e f i n e d a s t h ep o l y m e r i za t io n m e d i u m , o r m e d i u m f o r t h es a k e o f b re v it y . T h e p o l y m e r i z a t io n m e d i u m m a ya l s o b e r e f e r r e d t o a s t h e c o n t i n u o u s p h a s e o r t h eo u t e r p h a s e . I n a d d i t i o n t o th e m o n o m e r ( s ) a n dt h e p o l y m e r i z a t i o n m e d i u m , a n o t h e r l i q u id ( w h i chm u s t b e m i s c i bl e w i t h t h e m o n o m e r a n d i m m i s c ib l ew i t h t h e m e d i u m ) m a y a l so b e a d d e d t o t h e m o n o -m e r . T h i s li q u id is k n o w n a s th e m o n o m e r d i-l u e n t , o r d i l u e n t f o r s h o r t . T h e u s e o f t h e t e r ms o l ven t i s de l i be ra t e l y avo i ded i n t he p res en t d i s -c u s s i o n , b e c a u s e t h e d i l u e n t m a y b e a g o o d , p o o r ,o r n o n - s o l v e n t , o r a p r e c i p i t a n t f o r t h e p o l y m e r .P o l y m e r i z a ti o n m i x t u r e s c o m p o s e d o f a n a q u e -o u s p h a s e a n d a n o n a q u e o u s p h a s e a r e g en e r a ll y

9 ~ . . . . . . . . . 111~ ...........i .Emulsion Polymerization

................. h.Emulsifier FreeEmulsion Polymerization

I P o l y m e rP a r t i c l e s

o

Precipitation Polymerization~u9 D i r e c t C o n v e r s i o n~ o f M o n o m e r D r o p l e t sm ........... ..... - ................................ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . suspension Polymerization

[ 1 [ i 1 f o 1 6 iP a r t i c l e S i z e Range ~m )

F i g . 1 . G e n e r a l k i n e t i c f e a t u r e s a n d p a r -t i c l e s i z e r a n g e s o f h e t e r o g e n e o u s ( p a r t i c lef o r m i n g ) p o l y m e r i z a t i o n p r o c e s s e s

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Arshady Suspension emu lsion and dispersion polymerization: A method ological survey 719c las s i f i ed in to o i l - in -w a te r (O /W) o r w a te r - in -o i l(W/O ) s ys tems . Th e us e o f the t e rms inve r s es u s p e n s io n a n d i n v e r s e e m u l s i o n s h o u l d b eavo ided , becaus e they imp ly a na r row de f in i tion o ft h e t e r m s s u s p e n s i o n a n d e m u l s i o n .

uspension polymerizationProcess descr ip t ion

In suspens ion polymer izat ion [2 , 3] ,* the in i t i -a to r i s s o lub le in the monomer , and thes e tw o a r ei n s o lu b l e i n t h e p o l y m e r i z a t io n m e d i u m . T h e v o l -u m e r a t i o o f t h e m o n o m e r p h a s e t o t h e po l y m e r i z a -t ion me d ium i s u s ua l ly kep t w i th in 0 .1 -0 .5 , bu t , i np r inc ip le , it can be a s h igh as un i ty o r even h ighe r .The monomer phas e i s , by means o f a s t i r r e r and asui table drople t s tabi l izer ( suspens ion agent) , sus -p e n d e d i n t h e m e d i u m i n t h e f o r m o f s m a l l d r o p l ts(mic rod rop le t s ) . The po lymer iza t ion i s then in -i t i a t ed a t the des i r ed t em pera tu re (20 -100 ~ andi s u s ua l ly a l low ed to p roceed to comple t ion( ~ 1 0 0 % ) . U n d e r t h e s e c o n d i t i o n s , t h e m o n o m e rmic rod r op le t s a r e conver t ed d ir ec t ly to the co r -r e s p o n d i n g p o l y m e r m i c r o b e a d s o f a p p r o x i m -ate ly the same s ize .Basic a s pec t s o f s u s pens ion po lym er iza t ion havebeen d i s cus s ed in ea r ly paper s by H ohens te in andM a r k [ 4 ] , T r o m m s d o r f f a n d c o w o r k e r s [ 5 ] , a n dla te r by H o pf f e t a l . [ 6 ] . O n the bas i s o f thes es tud ies, i t is gene ra l ly a s s umed tha t po lym er iza t ionk ine t i c s in s us pens ion po lym er iza t ion a r e s imi la r tothos e o f bu lk o r s o lu t ion po lymer iza t ion , depend-i n g o n t h e a b s e n c e o r p r e s e n c e o f a m o n o m e rd i luen t in the monomer d rop le t s . I n th i s s ens e ,s us pens ion po lymer iza t ion may be r ega rded asm i c r o b u l k o r m i c r o s o l u t i o n p o l y m e r i z a t i o n ,becaus e ind iv idua l monomer d rop le t s r ep res en tt iny bu lk o r s o lu t ion po lymer iza t ion r eac to r s . Thes u s p en s i on m e d i u m h o u s i n g t h e m i c r o r e a c t o r sacts as an ef f ic ient heat t rans fer agent . As a resul t ,h i g h r a t es o f p o l y m e r i z a t io n c a n b e m a i n t a i n e d t oach ieve comple te co nver s ion du r ing r e l a tive ly s ho r tpe r iods o f t ime .Examples o f indus t r i a l ly impor tan t po lymer sp roduced by o i l - in -w a te r (O /W) s us pens ion po ly -mer iza t ion inc lude po lys ty r ene , po ly (v iny l ch lo -r ide) (PVC), polyacryla ts and poly(vinyl aceta te) .

S ty rene -bas ed re s in s u s ed fo r the p r epa ra t ion o f ionexchange r e s in s and po lymer s uppor t s [ 7 -9 ] a r ea l s o ob ta ined by O /W s us pens ion copo lymer iza -t ion o f s ty r ene and d iv iny lbenzene ( and a func t ion -a l monomer ) . F o r a l l o f thes e p r epa ra t ions thein i t i a to r i s u s ua l ly an azo com po und ( e.g. , azo -b is -2 -methy lp rop ion i t r i l e , A IBN ) , o r an o rgan ic pe rox -ide (e .g . , benzoyl peroxide) , and the poly-m e r i z a t i o n is p e r f o rm e d a t a t e m p e r a t u r e o f a b o u t50 -1 00 ~ A m ong typ ica l d rop le t s t ab il i zer s u s edfo r O /W s us pens ion po lymer iza t ion a r e po lyv iny l -py r ro l ido ne (P V P ) and po ly [ (v iny l a l coho l )- co -(v iny l ace ta t e ) ] . The l a t t e r po lymer i s ob ta ined bypar t i a l ( 8 5 -9 2% ) hydro lys i s o f po ly (v iny l ace ta t e ).A w ide r ange o f o the r w a te r s o lub le o r an ic po ly -mer s , i nc lud ing na tu ra l gums , ce l lu lo s e e the r s , ands yn the t i c po lymer s a r e a l s o u s ed . S ca rce ly so lub leinorganic sa l ts such as ta lc , phosphates , and su l-f a t e s may a l s o be employed , e i the r a lone o r incombinat ion with organic s tabi l izers [2 , 3] .M a j o r e x a m p l e s o f p o l y m e r s p r o d u c e d b y w a t e r -in -o il (W/O ) s us pens ion po lym er iza t ion inc ludepo lyac ry lam ide and w a te r s o lub le ac ry la t e s . A cry l -amide bas ed po lymer s uppor t s [ 10 -11 ] a r e a l s op r e p a r e d b y W / O s u s p en s io n c o p o l y m e r i z a t i o n o fac ry lam ide w i th b i s ac ry lam ide ( and a func t iona lm o n o m e r ) . H e r e , a n a q u e o u s s o l u t i o n c o n t a i n i n gthe monomer ( s ) and the in i t i a to r i s s u s pended inl iqu id pa ra f f in o r a ch lo roca rbon (po lymer iza t ionm e d i u m ) , f o l l o w e d b y p o l y m e r i z a t i o n a t a t e m p e r -a t u r e o f 2 0 - 5 0 ~ A w a t e r - s o lu b l e c a t a ly s t m a ya l s o be u s ed fo r thes e p r epa ra t ions . The mos ttyp ica l combina t ion o f in i t i a to r and ca ta ly s t f o rW/O s us pens ion po lymer iza t ion i s tha t o f po t -a s s i u m p e r o x y d i s u l f a t e a n d N , N , N ' , N ' - t e t r a -me thy le thy lened iamin e . S tab il i ze r s u s ed fo r W /Os us pens ion po lymer iza t ion inc lude e thy lce l lu lo s e ,ce l lu lo se ace ta t e bu ty ra te , an d va r ious am ph iph i l i co l igomer s s uch as S pan and Tw een [2 , 3 ] .F igu re 2 s how s a cy l ind r i ca l r eac to r ves se l w i ths y m m e t r i c a l l y m a t c h i n g s t i r r i n g a r r a n g e m e n tdes igned [3 , 12] for laboratory scale suspens ionpo lymer iza t ion . The s p i r a l s t i r r ing a r r angemen tp rov ides a r e l a tive ly tu rbu len t - f r ee and un i fo rmd i s t r i b u t i o n o f t h e m i x i n g f o r c e t h r o u g h o u t t h es us pens ion mix tu re . Th i s , i n tu rn , l eads to thef o r m a t i o n o f r e la t iv e l y u n i f o r m m o n o m e r d r o p l e tsa n d , h e n c e , p r o d u c t i o n o f p o l y m e r p a r t i c l e s w i t hco r r es pon d ing ly na r row s i ze d is t r ibu t ion . I n add i -

*) See also a recent review by Y uan HG , Kalfas G, Ray WH (1991) JMS-Rev Macrom ol Chem Phy s C31 (2-3):215

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6~ ] r

1 _ 1

E)~3mm

F i g. 2 . C y l i n d r i c a l g la s s r e a c t o r w i t h s y m m e t r i c a l l ym a t c h i n g s t i r r e r d e s i g n e d f o r l a b o r a t o r y s c a l e s u sp e n s i o n p o l y m e r i z a t io nt i o n , u n i f o r m d i s t r i b u t i o n o f t h e s t i rr i n g f o r c et h r o u g h o u t t h e m i x t u r e p r o d u c e s a r e la t iv e l y s ta b l es u s p e n s i o n s y s t e m , a n d m i n i m i z e s t h e c h a n c e o fi n a d v e r t e n t p a rt i c le c o a g u l a t i o n a n d e x p e r i m e n t a lf a i l u r e .

Q u a l i t y o f th e b e a d e d p o l y m e r p r o d u c t s o b -t a i n e d b y s u s p e n s i o n p o l y m e r i z a t i o n d e p e n d s , i na d d i t i o n t o r e a c t o r d e s i g n , o n o p e r a t i o n a l p a r a -m e t e r s g o v e r n i n g t h e o v e r a l l s t a b il i t y o f t h e s u s p e n -s i o n s y s t e m . I n p r a c t i c e , e f fi c ie n t m a n a g e m e n t o f as u s p e n s i o n p o l y m e r i z a t i o n p r o c e s s i s a s m u c h a na r t [ 3, 1 3] a s i t is b a s e d o n e x a c t s c i e n t i f i c p r i n c i -p l e s . F i g u r e 3 s h o w s e x a m p l e s o f p o l y m e r p a r t i c l e sp r o d u c e d b y s u s p e n s i o n p o l y m e r i z a t i o n , u s i n g t h er e a c t o r v e s s e l d e s c r i b e d a b o v e . I n m i c r o g r a p h A ,t h e p a r ti c l e s a r e i r r e g u l a r a g g l o m e r a t e s o f s m a l l e rm i c r o s p h e r e s . T h i s p r o d u c t w a s o b t a i n e d f r o m al o w v i s c o si t y m o n o m e r m i x t u r e a n d a p o o r l y s ta -b i l i z e d s u s p e n s i o n s y s t e m . M i c r o g r a p h B , o n t h eo t h e r h a n d , s h o w s r e l a t i v e l y u n i f o r m i n d i v i d u a lm i c r o s p h e r e s o b a t i n e d f r o m a n o r m a l p o l y m e r i z a -t i o n r u n u n d e r c a r e f u l l y c o n t r o l l e d c o n d i t i o n s [ 1 0 ,12] .D r o p l e t / p a r t i c l e s i z e c o n t r o l

S u s p e n s i o n p o l y m e r i z a t i o n c a n , i n p r i n c i p l e , b ee m p l o y e d t o p r o d u c e p o l y m e r p a rt i cl e s w i t h i n a n ys iz e r a n g e , f r o m a b o u t 1 0 0 n m u p t o a b o u t 1 - 2 m m

o r l a r g e r . F o r t h e r o u t i n e p r a c t i c e o f a d d i t i o np o l y m e r i z a t i o n o f v i ny l m o n o m e r s , h o w e v e r , s u s-p e n s i o n p o l y m e r i z a t i o n i s s u i t a b l e f o r p o l y m e rp a r t i c le s w i t h i n t h e s iz e r a n g e o f a b o u t 2 0 m -2 m m . P r e p a r a t i o n o f pa r t ic l e s s m a l l e r th a n a b o u t2 0 m b y s u s p e n s io n p o l y m e r i z a t i o n b e c o m e si n c r e a s i n g l y c o m p l i c a t e d a s a r e s u l t o f e m u l s i o np o l y m e r i z a t i o n [ 3 , 1 4 ] .T h e a v e r ag e s i ze o f t h e m o n o m e r d r o p l e t s ( a n d,h e n c e , t h a t o f t h e r e s u l t i n g p a rt i c le s ) c a n b e r e a d i l yc o n t r o l l e d b y v a r y i n g t h e s t i r r i n g s p e e d , v o l u m er a t io o f th e m o n o m e r t o s u s p e n s i o n m e d i u m , c o n-c e n t r a t i o n o f t h e s t a b il i z er , a n d t h e v i s c o s it i e s o fb o t h p h a s e s a c c o r d i n g t o E q . ( 1 ) . T h i s e q u a t i o nr e p r e s e n t s m o s t o f t h e e m p i r i c a l r e l a t i o n s h i p s r e -p o r t e d b y A r s h a d y a n d L e d w i t h [ 3 ] , H o p f f a n dc o w o r k e r s [6 ], K a v a r o v a n d B a b a n o v [ 15 ],M e r s m a n n a n d G r o s s m a n [ 1 6 ] , a n d S c u l l e s [ 1 7 ] :

d = k D v R v a ~ (1)Ds N- V m 9 C s '

w h e r e d = a v e r a g e p a r t i c l e s iz e ; k = p a r a m e t e r ss u c h a s a p p a r a t u s d e s i g n , t y p e o f s t i r r e r , s e lf s t a b i l -i z a t i o n , e t c . ; D v = d i a m e t e r o f v e s s e l; D s = d i a -m e t e r o f st i rr e r ; R = v o l u m e r a t i o o f th e d r o p l e tp h a s e t o s u s p e n s i o n m e d i u m ; N = s t i r r i n g s p e e d( o r p o w e r o f m i x i n g ) ; v a = v i s c o s it y o f t h e d r o p l e tp h a s e ; v m = v i s c o s it y o f t h e s u s p e n s i o n m e d i u m ;

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Arshady Suspension emulsion and dispersion polymerization: A methodological survey 721

500

400

3 0 0

200

IO0' | l Ii00 200 300 400 500

Stirring Speed rpm)Fig. 4. Effect of stirrer speed on particle size in suspensionpolymerization of styrene in the presence of 0.2 (m), 0.3(Q), or 0.4 (A) of a hydrophobical ly modified polyoxy-ethylene stabilizer (adapted fr om [18])

Am ong the param eters ind icated in Eq. 1), stir-rer speed is by far the most convenient means ofcontrolling particle size. A typical example of thedependence of particle size on stirrer speed isprovid ed by the g raphs in Fig. 4 for suspensio~polymerization of s tyrene [18]. This pattern ofparticle size control by stirrer speed is generallyobserved for well-stabilized suspension systems;although the slope of the curve may vary, depend-ing on the magnitude of other parameters ofEq. 1).

Fig. 3. Scanning electron micrographs of typical polymer sam-ples produced by suspension polymerization. A) broad sizedistribution and aggregation resulting from a poorly stabilizeddroplet suspension: B) individually spherical particles withnarrow particle size distribution resulting from a well-stabil-ized droplet suspension

= interfacial tension between the two immisciblephases; and Cs = stabilizer concentration.Quantitative expressions reported by differentauthor s [3, 6, 15-17] for various paramet ers in Eq.1) differ from each other to some extent, depend-ing on the details of the suspen sion system studied.How eve r, Eq. 1) provides a useful guide for plan-ning a new suspension polymerization process, aswell as a semiquantitative basis for understandingpart ic le s ize data of the product thus produced.

article morphologyAn important aspect of polymer part ic les ob-tained by suspension polymerization is the surfaceand bulk morpho logy of the individual beads. Thismorphology is basically related to the degree bywhich the polymer dissolves, swells or precipitates

in the monomer phase. When the polymer is sol-uble or swellable) in its mo nom er mixture, theresulting polymer particles have a smooth surfaceand a relatively homog eneous nonporous) texture.On the other hand, w hen the polymer is not solubleor swellable) in its monomer mixture, the finalparticles have a rough surface and a porousmorphology. For example, polystyrene andpoly meth yl methacry late) dissolve swell) in theirown monomers and produce smooth and t rans-lucent microbeads. On the other hand, poly vinyl

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722 Col lo id and P olym er Sci ence , Vol . 270 9 No. 8 (1992)

chloride) and polyacry lonitrile are insoluble i.e.,precipita te) in their own monomers, and producecorr ugat ed or porous) particles.The degree of resin porosity and the detai ls ofpore structure is of particular interest in the pro-duction of crossl inked ion exchange resins andpolymer supports. For these materials, particlemorph ology can be strongly influenced by the useof a suitable mon ome r diluent not to be confusedwith wate r used as polymerization medium). Thus,for the preparation of polymer supports and ionexchange resins, the mo nom er is di luted by an inertl iquid which may be a good or poor solvent, or aprecipitant for the result ing polymer part ic les. Inthis way, polymer part ic les with a wide range ofporosit ies can be produced, depending on the

nature and the percentage of the monomer diluentand the extent of polym er crosslinking [19, 20].Figure 5 shows scanning electron micrographsSEM) of cross-sections of nonporo us and p orousbeads produced by suspension copolymerization ofstyrene with 2,4,5-trichlorophenyl acrylate and di-vinylbenzene. The polymers were obtained in thepresence of chlorobenzene A, a good solvent) , orchlorobenzene-octane B, a poor solvent) [9].

Emulsion polymerizationrocess descriptionIn classical) emu lsion polym erizati on [21-23],the mon om er is insoluble or scarcely soluble) in

A)

B

Fig. 5. C ross-sections of nonpo rous A) andporous B) particles produ ced by suspensionpolymerization of styrene with 2,4,5-tri-chlorophenyl acrylate and divinylbenzene inthe presence of a monomer diluent: chloro-benzene A), or chlorobenzen e-n-octane B)

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Arshady Suspension emulsion and dispersion polymerization: A methodological survey 7 2 3

t h e p o l y m e r i z a t i o n m e d i u m , b u t i t is e m u l s i fi e d i ni t by t he a i d o f a s u r fac t an t ( em u l s i f i e r o r s oap ) .T h e i n i t ia t o r i s , u n l i k e i n s u s p e n s i o n p o l y m e r -i z a t i o n , s o l u b l e i n t h e m e d i u m , a n d n o t i n t h em o n o m e r . U n d e r t h e s e c o n d i t i o n s , t h e m o n o m e ri s p r e s e n t i n t h e m i x t u r e p a r t l y i n t h e f o r m o fd r o p l e t s ( a b o u t 1 - 1 0 ~ m o r l a r g e r ) , a n d p a r t l y o i nt h e f o r m o f s o a p - c o a t e d m i c e ll e s (c a. 5 0 - 1 0 0 A ) ,d e p e n d i n g o n t h e n a t u r e a n d c o n c e n t r a t i o n o f t h ee m u l s i f i e r . A s m a l l p e r c e n t a g e o f t h e m o n o m e r i sa l so m o l e c u l a r l y d i s so l v e d in t h e m e d i u m . F o r e x -am p l e , so l ub i l i t y o f s ty rene in wa t e r a t 70 ~ i sab ou t 4 g /1 .T h e v o l u m e r a t io o f t h e m o n o m e r p h a s e t o t h em e d i u m i n e m u l s i o n p o l y m e r i z a t i o n i s u s u a l l yw i t h i n a b o u t 0 . 1 - 0 . 5 , a n d t h e p o l y m e r i z a t i o n isc a r r ie d o u t a t 4 0 - 8 0 ~ T h e s t a te o f t h e p o l y m e r -i z a t i o n m i x t u r e i n t h e e a r l y s t a g e s o f e m u l s i o npo l y m e r i za t i o n is s chem at i ca l l y p res e n t ed i n F i g. 6.S i n c e t h e i n i t i a t o r i s p r e s e n t o n l y i n t h e m e d i u m ,t h e i n i ti a l lo c u s o f p o l y m e r i z a t i o n is i n t h e m e d i u m( i. e., o u t s i de t he d rop l e t s and m i ce l l e s ). The o l i go -r a d ic a ls f o r m e d i n t h e p o l y m e r i z a t i o n m e d i u m a r ee i t h e r s u r r o u n d e d b y t h e d i s s o l v e d m o n o m e r a n de m u l s i f i e r m o l e c u l e s , o r t h e y a r e a b s o r b e d b y t h ea l r e a d y p r e s e n t s o a p - c o a t e d m o n o m e r m i c e l l e s .In e i t he r cas e , t he i n i t i a l l y fo rm ed o l i go rad i ca l sp roduce s t ab i l i zed nuc l e i (p r i m ary pa r t i c l e s ) [24 ,25 ]. S ub s eque n t l y , em u l s i f i e r - s t ab i li zed po l y m e rn u c l e i b e c o m e t h e m a i n l o c i o f p o l y m e r i z a t i o n b ya b s o r b in g f u r t h e r o l i g o ra d i ca l s a n d m o n o m e r m o l -e c u l e s f r o m t h e m e d i u m , o r i n e f f e c t f r o m t h em o n o m e r d r o p l e t s . I n th i s w a y , t h e n u c l e i / p a r ti c l e sg r o w g r a d u a l ly u n t i l t h e m o n o m e r is c o m p l e t e lyc o n s u m e d . T h e s i z e o f t h e l a t e x p a r t i c l e s t h u sp r o d u c e d is u s u a ll y i n th e r a n g e o f 5 0 - 3 0 0 n m . h isn o t e w o r t h y , h o w e v e r , t h a t e x a c t d e t a i ls o f p a r ti c l ef o r m a t i o n a n d g r o w t h a r e s t ro n g l y d e p e n d e n t o nt h e n a t u r e o f t h e p o l y m e r i z a t i o n m i x t u r e , a s h a sb e e n o u t l i n e d r e c e n t l y b y L a g a l y a n d c o l l e a g u e s[22].F o r O / W e m u l s i o n p o l y m e r i z a t i o n (e .g ., t h o s e o fs t y r e n e o r m e t h y l m e t h a c r y l a t e i n w a t e r ) , p o t -a s s iu m p e r o x y d i s u l fa t e a n d s o d i u m d o d e c y l su l f o n -a t e a r e c o m m o n l y u s e d a s i n i t i a t o r a n d e m u l s if i e r,r e s p e c t i v e l y . C o m b i n a t i o n s o f i o n i c a n d n o n i o n i cem ul s i f i e r s m ay a l s o be u s ed . An i n t e res t i ng ex -a m p l e i s t h e u s e o f s o d i u m d o d e c y l s u l fo n a t e a n dT r i t o n X - 1 0 0 , a s r e p o r t e d b y W o o d s e t a l . f o r t h ep r e p a r a t i o n o f m o n o d i s p e r s e p o l y s t y r e n e p a r t i c l e s(ca. 250 nm ) [26].

~- : - ~_

Mo no me r ~ ~..'.~: , : . ~D r o p l e t

L

~ - ~ ~ . ~ r ~ 5 ~ x , ~

.C'~~ S 9

-~7~ .. ~ / . i~x" . " . ~ , : . . . . ~&,~ 9~to~ o ~.. .::... ..:.

9 9

9 o ~ s ~~ ~ N

9 M

~...'..... :.... :. ,~, u - - 0 . 0 5 ~ n - - - q ~ ' : - " " . . . . . : ' -~ ~ - . ~ i ~ . ~~ : ' . ' . . . . : : : ' v : : d ,

9 = M o n o m e r m o l e c u l e , M = M o n o m e r c o n t a i n i n g m i c el l eS = S o a p m i c e l l e t ,9-0 = G r o w i n g m a c r o r a d i c a l sN = P o l y m e r n u c l e u s p L = G r o w i n g l a t e x p a r t i c l e

F i g . 6 . S c h e m a t i c p r e s e n t a t i o n o f e a r ly s t a g e s o f e m u l s i o np o l y m e r i z a t i o n ( a d a p te d f r o m B . V o l l m e r t , P o l y m e r C h e m i s -t r y , S p r i n g e r V e r l a g , N e w Y o r k , 1 9 73 )

F o r w a t e r s o lu b l e m o n o m e r s , a n a q u e o u s s o l u -t i o n o f t h e m o n o m e r i s e m u l s i f i e d i n a w a t e ri m m i s c i b l e l i qu i d ( an o i l ) , i n t he p res ence o f awa t e r - i n -o i l (W / O) em u l s i f i e r , and an o i l s o l ub l ei n it ia t o r. E x a m p l e s o f W / O e m u l s i o n p o l y m e r iz a -t i o n [ 2 7 - 2 9 ] a r e th o s e o f a c r y l a m i d e a n d s o d i u m 4 -v i n y l b e n z e n e s u t f o n a t e i n t o l u e n e , i n t h e p r e s e n c e o fbenzo y l pe ro x i de i n i t i a t o r . F a t t y e s t e r s o f po l y -h y d r o x y c o m p o u n d s ( e . g . , s o r b i t a n m o n o o l e a t e )a re o f t en u s ed as W / O em ul s i f i e r s . A recen t r ec i ped e s c r i b e d b y P i c h o t e t a l . [ 2 7 ] u s e s a n a q u e o u ss o l u t i o n o f a c r y l a m i d e a n d m e t h a c r y l i c a c i d i nt o l u e n e , i n t h e p r e s e n c e o f s o r b i t a n e s t e r s a n dA I B N . T h i s p o l y m e r i z a t i o n is c a r r ie d o u t a t 4 0 ~f o r 1 - 4 h t o o b t a i n a h i g h y ie l d o f t h e la t e x c o p o l y -m e r c o n t a i n in g 3 0 - 7 0 m e t h a c r y li c a c id .

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724 Colloid and Polymer Science, Vol. 270 - No. 8 1992)

lkyl cyanoacrylate nanospheresAn in te r est ing example o f O/W emuls ion po ly -mer izat ion is that of a lkyl cyanoacrylates by an-

ionic in i t ia t ion i .e ., HO - ions presen t in water ) . Avery low conce n t r a t ion o f H O - is requ i r ed fo r thepo lymer iza t ion to p roceed in a con t ro l l ed manner .In pract ice , pH is adjusted to 1-3 to achieve aH O - c o n c e n t ra t i o n o f a b o u t 1 0 - 1 3 _ 1 0 - t 1 m o le /1 .Poly alkyl cyanoacrylate) la tex nanosphe res,nanocapsules) are biodegradable , and have at t rac-t ed cons iderab le in t er es t f o r po ten t i a l pharm aceu t -ical use [30, 31].Douglas et a l . have studied the detai ls of emul-sion polymer izat ion of butyl cyanoacrylate in thepresence o f po lyoxye thy lene-b -po lyoxypropy lenecopo lymers o r low molecu la r we igh t po lysacchar -ides as em ulsif ier [32]. Since these em ulsif iers carryf ree hyd rox y groups, i t is reasoned [33] that theemulsi f ier may act a lso as in i t ia tor and, hence,covalent a t tac hm ent to the la tex par t ic les Fig . 7) .Particle siz e control

The size of la tex par t ic les in emulsion polymer-izat ion has no di rect re lat ionship wi th the size ofthe in i t ia l ly fo rme d m ono me r d rop le t s o r micel les .These do no t con ta in any in i t i a to r and , hence , a r eno t d i r ec t ly conver t ed to the co r r espond ing po ly -mer par t ic l es. I ns t ead , t he f r ac t ion o f the m ono me rmolecular ly dissolved in the polymer izat ionmed ium p lays a key ro le in de te rmin ing the s i ze o fthe f inal polymer par t ic les . The size of the la texpar t ic les in emulsion polymer izat ion is a lso inf lu-enced by a number o f o ther f ac to r s , i nc lud ingemuls i f i e r concen t r a t ion and po lymer iza t ion t em-pera tu re . F igure 8 shows e l ec t ron microgaphs o fpo lyv iny l to luene par ti c les p r epared by em uls ion

polymer izat ion at d i f ferent temperatures, underotherwise simi lar cond i t ions [34].The micrographs in Fig . 8 indicate that the sizeof the par t ic les decreases as the temperature ofpolymer izat ion increases. This observat ion is inaccordance wi th the nuc lea t ion mechan i sm ou t -

l ined above [24, 25] , and reflects the depen denc e ofpar t ic le s ize on the rate of nuclea t ion _~ rate ofpo lymer iza t ion) . Other k ine t i c pa ramete r s whichcontro l the rate of polym er izat ion react ion i .e .,concentrat ions of in i t ia tor , emulsi f ier , and sal t ) ,also influence the size of the latex particles [22, 24,25, 34, 35] . Salt con centra t ion con trols the viscosi tyand ion ic s t r eng th o f the med ium, bo th o f whichinf luence the course of the nucleat ion process.Soapless emulsion po lymerization

In emulsif ier-free or soapless) em ulsi on poly-mer izat ion [36-38] , the polymer izat ion is car r iedout in the same way as in c lassical emulsion poly-mer izat ion, except that no emulsi f ier i s used. Ac-cordingly, nucleat ion takes place by precipi ta t ionof macrorad ica l s and macromolecu les) , a s com-pared wi th mice l l e fo rmat ion in normal emuls ion

Fig. 7. Schematic presentation of stabilizer-grafted poly alkylcyanoacrylate) particles produced by emulsion polymerizationin the presence of dextran used as emulsifier Fig, 8. Polyvinyltoluene particles prod uced b y emulsion poly-merization at 23 ~ A), 50 ~ B), 70 ~ D), and 80 ~ E) [84]

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Arshady Suspension emulsion and dispersion polymerization: A methodological survey 7 2 5po lym er iza t ion . S ince the re i s no emuls i fi e r p r es en ti n t h e m e d i u m , t h e n u c l e i t h u s f o r m e d a r e n o ts tabi l ized by any emuls i f ier or s tabi l izer . As a res -u l t , t he in i t i a lly fo rmed p o lym er nuc le i co l lide andfo rm la rge r and l a rge r pa r t i c l e s a s the po lymer iz -a t ion p roceeds .H o w ev er , l a t ex pa r t i c l e s p rod uced in the abs enceo f emuls i f ie r a r e , to s om e ex ten t , s t ab i li zed by theo r i e n t a t i o n o f t h e i r o w n p o l y m e r c h a i n s , n o t a b l ythe cha in ends o r ig ina t ing f rom in i t i a to r mo lecu les .F o r example , in the cas e o f po tas s ium pe roxyd i -s u l f a te in i t i a to r , the cha in end g roups a r e-~ fV x- -O S O ~ K + . A s the pa r t i c l e s g row , the i r s u r-face charge increases , and a t a cer ta in s ize (usual ly> 100 nm) , the pa r t i c le s becom e s tab i l ized by the i row n e lec t ro s ta t i c cha rge . La tex s t ab i l i za t ion by theus e o f amid in ium in i t i a to rs (Fig. 9) p roduc ing pos -i tive ly cha rged l a t ex pa r t i c l e s has a l s o been s tud iedby G oo dw in e t a l. [ 36b ] . M ono m er to w a te r r a t io inemuls i f i e r - f r ee emuls ion po lymer iza t ion i s u s ua l lym u c h s m a l l e r t h a n t h a t i n n o r m a l e m u l s i o n p o l y -m e r i z a t io n . I n m o s t c a se s , m o n o m e r c o n c e n t r a t i o nis less than 5 , and th e s ize of the resul t ingpar t i c l e s i s in the r eg ion o f abou t 100 -1000 nm.

D is pe r s ion polymeriz tionrocess descr ip t ion

In d i s pe r s ion po lymer iza t ion [39 -41 ] the mono-mer and the in i t i a to r a r e bo th s o lub le in the po ly -m e r i z a t i o n m e d i u m , b u t t h e m e d i u m i s a p o o rH 2 N H 2H 2 N y ~ H 22 , 2 ~ - A z o - b i s - i s o b u t y r a m i d i n ed i c h l o r i d e

, IH 2 C - H ~ ' ~ q ~ IH - C H 2R = C ( C H 3 ) 22 ,2 ' - A z o - b i s - N , N - d i m e t h y l -e n e i s o b u t y r a m i d i n e d i c h l o r i d eF i g . 9 . A m i d i n i u m - a z o i n i t i a to r s u s e d i n s o a p l e ss e m u l s i o np o l y m e r i z a t i o n t o p r o d u c e p o s i t i v e ly c h a r g e d l a t e x p a r t ic l e s[36b]

s o lven t fo r the r e s u l t ing po lym er . A cco rd ing ly , ther e a c t io n m i x t u r e i s h o m o g e n e o u s a t t h e o n s e t, a n dt h e p o l y m e r i z a t i o n i s i n i t i a t e d i n h o m o g e n e o u ss o l u ti o n . D e p e n d i n g o n t h e s o lv e n c y o f t h e m e d i u mf o r t h e r e s u l t i n g m a c r o r a d i c a l s a n d m a c r o m o l -cu les , phas e s epa ra t ion occu r s a t an ea r ly s t age .Th i s l eads to nuc lea t ion and the fo rmat ion o fp r imary pa r t i c l e s , a s i l lu s t r a ted in F ig . 10 . H ow -ever , p r imary pa r t i c l e s thus fo rmed in d i s pe r s ionp o l y m e r i z a t i o n a r e s w o l l e n b y t h e p o l y m e r i z a t i o nm e d i u m a n d / o r t h e m o n o m e r . A s r e s u l t , p o l y -mer iza t ion p roceeds l a rge ly w i th in the ind iv idua lpa r t i c l e s , l ead ing to the fo rmat ion o f s pher ica lp a r ti c le s i n th e r e g i o n o f a b o u t 0 . 1 - 1 0 / l m .Typ ica l examples o f d i s pe r s ion po lymer iza t iona re thos e o f s ty rene and me thy l me thac ry la te inhydroca rbons o r in C1-C5 a lcoho l s . V ar ious a s -pec t s o f d i s pe r s ion po lymer iza t ion in pe t ro leumh y d r o c a r b o n s w e r e d i s c u s s e d i n a m o n o g r a p hedi ted by Barre t [39] . Alm og et al . [40] , and Ob er e ta l . [ 41] have r epo r ted on d i s pe r s ion po lym er iza t iono f s t y r e n e a n d m e t h y l m e t h a c r y l a t e i n a l c o h o ls a n dvar ious a lcoho l - e the r o r a l coho l -w a te r mix tu res .M ore r ecen t ly , Ts eng e t a l . have s tud ied thee f f ec t s o f med ium s o lvency , and concen t r a t ions o f

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