Colloidal Dispersions and Micellar Behavior

11 Stability of Sterically Stabilized Dispersions at High Polymer Concentrations F. K. R. LI-IN-ON and B. VINCENT School of Chemistry, University of Bristol, Bristol BS8 ITS, England

Downloaded by PENNSYLVANIA STATE UNIV on May 13, 2012 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0009.ch011

F. A. WAITE I.C.I. (Paints Div.) Ltd., Slough SL2 5DS, England Introduction The use of polymer molecules, both natural and synthetic, to stabilise colloidal dispersions against coagulation has been widely practised i n industry, and is of fundamental importance in many natural processes, both biological and environmenta1(1). At present there is a need for a wide range of experimental work in this f i e l d , on well characterised systems, in order to test the several models and theories of steric s t a b i l i s a t i o n . One can distinguish at least two classes of experimental approach. F i r s t l y , one may compare the s t a b i l i t y of the dispersion in the presence and absence of polymer (at a given concentration). Stability is perhaps best defined in this context in terms of the ratio of the rate constants for aggregation in the two cases. Secondly, one may observe under what conditions a s t e r i c a l l y stabilised dispersion w i l l aggregate. There are perhaps four basic requirements for a dispersion to be effectively stabilised by adsorbed or anchoredt polymer chains : a) high surface coverage, i . e . no "bare" patches at the surface b) strong adsorption c) a "thick" adsorbed layer, to prevent the particles coming within a separation where the van der Waals interaction becomes effective. d) the stabilising chains should be i n "good" solvent environment. Such requirements may be met by using AB block and AB comb structures ( 1 - 3 ) · Here the (insoluble) A chains are the adsorbing t by "adsorbed" we refer to the case where the stabiliser is physically adsorbed to the particle surface; by "anchored", where the stabiliser is chemically bonded to the surface, or even (partially) incorporated into the bulk structure of the p a r t i c l e . n

165

In Colloidal Dispersions and Micellar Behavior; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.


166

C O L L O I D A L DISPERSIONS A N D M I C E L L A R

BEHAVIOR

(anchoring) component and serve to f u l f i l conditions (a) and (b) above, w h i l s t the (soluble) Β chains provide the a c t u a l s t e r i c b a r r i e r , f u l f i l l i n g conditions (c) and ( d ) . In p r i n c i p l e , a s t e r i c a l l y s t a b i l i s e d d i s p e r s i o n can be aggregated by a d j u s t i n g conditions such that one o r more of the p r e - r e q u i s i t e s (a) - (d) i s no longer met. Examples f o r each one are as f o l l o w s : a) at low surface coverages b r i d g i n g , f l o c c u l a t i o n may o c c u r . (In some cases polymer may be d i s p l a c e d l a t e r a l l y to create bare s p o t s ) . b) weakly adsorbed polymers, e . g . horaopolymers, may be d i s placed e n t i r e l y from the p a r t i c l e surface by s m a l l e r molecules, e . g . i o n i c s u r f a c t a n t s , which are themselves more s t r o n g l y adsorbed but which do not themselves provide adequate s t e r i c s t a b i l i s a t i o n . c) the thickness of the adsorbed l a y e r i s c r i t i c a l i n determining the depth of the p o t e n t i a l energy minimum i n the p a r t i c l e - p a r t i c l e i n t e r a c t i o n c u r v e . I t has been shown by Long, Osmond and Vincent (4) that a c r i t i c a l depth f o r t h i s p o t e n t i a l energy minimum e x i s t s , f o r a given p a r t i c l e number c o n c e n t r a t i o n , at and beyond which aggregation begins to o c c u r . Thus s u b t l e changes i n the thickness of the s t e r i c b a r r i e r , e . g . by changes i n molecular weight or solvency, can change a system from being s t a b l e to being (weakly) aggregated. d) i n a s e r i e s of papers (5-6) Napper has shown that by changing the solvency conditions f o r the s t a b i l i s i n g Β chains from a good solvent environment to a Θ - or near θ - s o l v e n t , l a t e x d i s p e r s i o n s s t a b i l i s e d by AB block copolymers may be aggregated. In a l l the systems s t u d i e d to date, as f a r as we are aware, the e f f e c t of the c o n c e n t r a t i o n of polymer i n the continuous phase on d i s p e r s i o n s t a b i l i t y has not been c o n s i d e r e d . Indeed i n most systems, the bulk polymer concentration i s n e g l i g i b l y s m a l l . However, t h i s i s a s i g n i f i c a n t point to consider i n many p r a c t i c a l d i s p e r s i o n s , e . g . i n drying l a t e x paint films where the concentration of polymer i n the continuous phase o b v i o u s l y must reach a high value as solvent evaporates. In t h i s work we set out to i n v e s t i g a t e whether high con c e n t r a t i o n s of homopolymer Β i n the continuous phase (approaching 100% polymer i n f a c t ) l e d to changes i n the s t a b i l i t y of d i s p e r s i o n s s t a b i l i s e d by AB comb polymers. The s p e c i f i c system i n v e s t i g a t e d was an aqueous polystyrene (PS) l a t e x , plus a comb s t a b i l i s e r with A • PS, Β » poly(ethylene oxide) (PEO), molecular weight 750, to which was added homopolymer PEO, of v a r i o u s , r e l a t i v e l y low molecular weights (200-4000), over a wide concentration range. Above a c e r t a i n c r i t i c a l PEO c o n c e n t r a t i o n , for a given molecular weight, i t was found that aggregation o c c u r r e d . We present a p r e l i m i n a r y report of the r e s u l t s obtained. I t i s intended that a more d e t a i l e d report and d i s c u s s i o n of other r e s u l t s w i l l be p u b l i s h e d s h o r t l y .


11.

LI-IN-ON E T A L .

167

Sterically Stabilized Dispersions

Experimental


Materials. A l l solvents and monomers were r e d i s t i l l e d before use. The polyethylene oxides used were the polyethylene g l y c o l s e r i e s 200, 400, 600, 1000, 1500, 4000, ex B r i t i s h Drug House, plus polyethylene g l y c o l 800 ex S h e l l Chemicals L t d . Carbowax 750 was obtained from Union Carbide L t d . These m a t e r i a l s were used without f u r t h e r p u r i f i c a t i o n . Latex D i s p e r s i o n s . The polystyrene l a t e x used was prepared by a method which i s c u r r e n t l y the subject of a patent a p p l i c a t i o n by I . C . I . (Paints D i v i s i o n ) L t d . Nevertheless i t can be s t a t e d here that a free r a d i c a l p o l y m e r i s a t i o n route was used, and t h i s gave a polystyrene l a t e x d i s p e r s i o n , the p a r t i c l e s of which were s t a b i l i s e d by a comb s t a b i l i s e r , i n which the p o l y styrene backbone i s p h y s i c a l l y i n c o r p o r a t e d i n t o the s t r u c t u r e of the polystyrene p a r t i c l e s . The methoxy-terminated poly(ethylene oxide) (M.W. 750) chains are therefore t e r m i n a l l y anchored at the surface. C h a r a c t e r i s a t i o n of the L a t e x . The mean p a r t i c l e diameter of the l a t e x from e l e c t r o n microscopy was 140 nm. Although not as monodisperse as some of the c h a r g e - s t a b i l i s e d polystyrene l a t i c e s that have been prepared i n the absence of added s u r f a c t a n t s or polymers (7), we do not consider t h i s to be a s i g n i f i c a n t feature i n i n t e r p r e t i n g the r e s u l t s of the experiments we r e p o r t . M i c r o e l e c t r o p h o r e s i s and conductimetrie t i t r a t i o n i n d i c a t e d that the p a r t i c l e s c a r r i e d only a very small surface charge. The c a l c u l a t e d zeta p o t e n t i a l i n 10~ KC1, using the t a b l e s of O t t e w i l l and Shaw (8), was less than 15 mV, i . e . not s u f f i c i e n t to p l a y any s i g n i f i c a n t r o l e i n determining the s t a b i l i t y of the particles. 3

Flocculation. F l o c c u l a t i o n k i n e t i c s were followed using a conventional t u r b i d i m e t r i c technique (9-10), i n which the o p t i c a l density (OD) of the l a t e x was recorded continuously as a f u n c t i o n of time on the a d d i t i o n of a known amount of polymer s o l u t i o n . A Unicam SP 1800 spectrophotometer l i n k e d to a Unicam AR 25 pen recorder was used f o r t h i s purpose. A wavelength of 540 nm was used, and the c e l l - h o u s i n g of the spectrophotometer was thermos t a t t e d at 2 5 ° C . The i n i t i a l number concentration of the p a r t i c l e s Ν , was kept f i x e d at 3.5 χ Ι Ο p a r t i c l e s per ml (corresponding to a p a r t i c l e volume f r a c t i o n of 4 χ 10" *) f o r a l l the runs · Oster (11) has shown that f o r an aggregation d i s p e r s i o n i n which the p a r t i c l e s are Rayleigh s c a t t e r e r s , the t u r b i d i t y , τ , as a f u n c t i o n of time, t , i s given b y . 1 0

1


168


BEHAVIOR

2

τ - AN V ( l + 2N k t ) ο ο

(1)


where A i s an o p t i c a l constant r e l a t e d to the r e f r a c t i v e index o f the p a r t i c l e s and the medium, V i s the volume of a p a r t i c l e , and k the rate constant f o r the aggregation p r o c e s s . As i n d i c a t e d i n the I n t r o d u c t i o n , one may define the s t a b i l i t y , W, o f a d i s p e r s i o n as follows W - k /k ο

(2)

where k i s the rate constant under conditions o f r a p i d coagulation (maximum rate) i n the absence of added s t a b i l i s e r , and i s given by the Smoluchowski theory as f o l l o w s , (3) 1

where k and Τ are the Boltzmann constant and absolute and η i s the v i s c o s i t y o f the continuous phase. Combining Equations ( 1 ) , (2) and (3) leads to

temperature

8N k ' T τ = τ (1 • ο where τ or

—2— 3w

.t/η)

i s the i n i t i a l t u r b i d i t y (t

dOD

du7n)

(4) s

0) of the d i s p e r s i o n ,

8N k ' T =

( 5 )

~3w-

where ODr » 0D/0Dο = τ / τ ο . Thus the slope ν of a p ^l o t of OD versus t / η should d i r e c t l y r e f l e c t changes i n W, e f f e c t s due to changes i n A and η , on changing the bulk polymer c o n c e n t r a t i o n , having been e l i m i n a t e d . Since the conditions f o r Rayleigh s c a t t e r i n g are normally only f u l f i l l e d during the i n i t i a l stages of aggregation, and thus n o n - l i n e a r p l o t s are g e n e r a l l y obtained, i t i s normal p r a c t i c e to take the l i m i t i n g slope as t-*0, i . e . i n t h i s case r

dOD /d(t/n)

t-K)

V i s c o s i t y . The v i s c o s i t i e s o f the various PEO-water mixtures were determined at 25°C using a Cannon-Fenske c a p i l l a r y viscometer. The data obtained are shown i n F i g u r e 1.


11.

Li-iN-ON E T A L .


169

Results A t y p i c a l p l o t of 0 D versus t / η i s shown i n F i g u r e 2 f o r the l a t e x i n the presence of PEO 200. Below about 70% (by weight) PEO the system i s s t a b l e . Between 70 and 80% there i s a r a p i d increase i n the i n i t i a l aggregation r a t e . T h e r e a f t e r , the rate of aggregation decreases a g a i n . As discussed above these e f f e c t s cannot be a t t r i b u t e d to v i s c o s i t y or r e f r a c t i v e index changes i n the continuous phase, since these are e l i m i n a t e d i n the method of p l o t t i n g the d a t a . F i g u r e 3 shows the e f f e c t s described more c l e a r l y . Here (dOD / d C t / n ) . ) ^ i s p l o t t e d versus


r

PEO c o n c e n t r a t i o n f o r various molecular weights. F o r each M.W. there i s a c r i t i c a l minimum PEO c o n c e n t r a t i o n and only above t h i s i s aggregation observed. This c r i t i c a l c o n c e n t r a t i o n i s p l o t t e d as a f u n c t i o n of PEO molecular weight i n F i g u r e 4. From F i g u r e 3 i t can a l s o be seen that the f a l l o f f i n aggregation rate at higher polymer concentrations occurs f o r a l l the molecular weight samples s t u d i e d . P r e l i m i n a r y r e s u l t s have been obtained with a polystyrene l a t e x dispersed i n n-hexane, f o r which the s t a b i l i s e r was a p o l y s t y rene-polyisoprene (PIP) block copolymer. The PIP chains i n the block had a molecular weight of the order of 1000, and homo polymer PIP of the same molecular weight was added to the continuous phase at i n c r e a s i n g c o n c e n t r a t i o n s . Again i t was observed that above a c r i t i c a l minimum PIP c o n c e n t r a t i o n , aggregation o c c u r s . Discussion Of the four mechanisms, o u t l i n e d i n the I n t r o d u c t i o n , concerning the various ways i n which a s t e r i c a l l y s t a b i l i s e d d i s p e r s i o n may be d e - s t a b i l i s e d , one can probably only r u l e out with any confidence at t h i s stage mechanism ( b ) , i . e . desorption of the s t a b i l i s e r on p a r t i c l e c o n t a c t . I t would seem (3) that the s t a b i l i s e r i s w e l l anchored at the s u r f a c e . Any of the other three mechanisms, e i t h e r s i n g l y or i n combination, or indeed some other mechanism not so f a r considered, could be responsible f o r the behaviour observed. However at t h i s stage much more work needs to be done on the e f f e c t s of e . g . molecular weight of the s t a b i l i s i n g group, number concentration of the p a r t i c l e s i n d i s p e r s i o n , p a r t i c l e s i z e and temperature, before any firm conclusions can be reached. I t would a l s o seem e s s e n t i a l that other systems be s t u d i e d . Work i n a l l these d i r e c t i o n s i s c u r r e n t l y underway, and i t i s hoped to report these r e s u l t s i n the l i t e r a t u r e i n the near f u t u r e . Acknow1edgement s The authors would l i k e to acknowledge the many u s e f u l and



170


BEHAVIOR

Figure 1. Viscosity of aqueous poly(ethylene oxide) solutions

Figure 2. Corrected OD v s . time plots for the latex in the presence of various concen trations of PEO 200



11.

LI-IN-ON ET

AL.


25

50

75

%PEO

171

100

Figure 3. Stability curves for the latex as a function of poly(ethylene oxide) concentration

Figure 4. Critical poly(ethylene oxide) concen trations for flocculation as a function of molec ular weight


172

C O L L O I D A L DISPERSIONS A N D M I C E L L A R B E H A V I O R

s t i m u l a t i n g d i s c u s s i o n s w i t h M r . D . W . J . Osmond ( I . C . I . , P a i n t s D i v i s i o n ) concerning t h i s work. One of us ( F . K . R . L . ) would l i k e to thank the U n i v e r s i t y of E r i s t o l f o r the award of a U n i v e r s i t y S c h o l a r s h i p to help finance t h i s work.


Literature 1. 2. 3. 4. 5.

6. 7. 8. 9. 10. 11.

Cited

V i n c e n t , Β., A d v . C o l l o i d I n t e r f a c e Sci., (1974) 4 , 193 Waite, F . A . , J.Oil Colour Chem.Ass., (1971) 54, 342. B a r r e t t , K . E . J . , Br.Polymer J., (1973) 5, 259. Long, J.A., Osmond, D.W.J. and V i n c e n t , B . , J.Colloid Inter face Sci. (1973) 42, 545. Napper, D.H. and Hunter, R.J. i n " M . T . P . I n t e r n a t i o n a l Review of S c i e n c e , P h y s i c a l Chemistry, Surface Chemistry and Colloids, S e r . 1 , Vol.7",M. K e r k e r , Ed., p. 289, B u t t e r w o r t h s , London, 1972. Napper, D . H . , Ind.Eng.Chem.Prod.Res.Dev., (1970) 9 , 467. Goodwin, J.W., Hearn, J., Ho, C . C . and Ottewill, R.H., Br.Polymer J (1973) 5 , 347. O t t e w i l l , R . H . and Shaw, J.N., J.Electroanal.Chem. Interfacial Electrochem. (1972) 37, 133. R e e r i n k , H. and Overbeek, J.Th.G., D i s c . F a r a d a y S o c . (1954) 18, 74. O t t e w i l l , R . H . and Shaw, J.N. D i s c . F a r a d a y S o c . , (1966) 42, 154. Oster, G . , J.Colloid Sci., (1960) 15, 512.


Colloidal Dispersions and Micellar Behavior

Recommend Documents