Aqueous-Solution Behavior of Hydrophobically Modified Poly(acrylic

and the molar content of substitution ranged from 0 to. 10%. Above a ... 1 Current address : Beijing Institute of Chemical Technology,. Heping Street ...
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Chapter 14

Aqueous-Solution Behavior of Hydrophobically Modified Poly(acrylic Acid) T. Κ. Wang , I. Iliopoulos, and R. Audebert 1

Laboratoire de Physico-Chimie Macromoléculaire, Université Pierre et Marie Curie, URA Centre Nationale de Recherche Scientifique No. 278, ESPCI-10, rue Vauquelin, 75231 Paris Cedex 05, France Hydrophobically modified poly(acrylic acid) was obtained by reaction of a small amount of an alkylamine on the carboxyl groups of the polyacid. Alkylamines with 8, 14 or 18 carbon atoms were used and the molar content of substitution ranged from 0 to 10%. Above a c r i t i c a l polymer concentration the viscosity in pure water of the modified samples (in the sodium salt form) drastically increases and can be several orders of magnitude higher than the one of the corresponding precursor polymer. Further enhancement of viscosity is achieved by increasing the ionic strength of the solution or by addition of a surfactant. Solutions exibiting high viscosities also present a marked solubilization of pyrene. These results were interpreted in terms of conformational changes and interchain aggregation due to hydrophobic interactions. It is well known that thickening of solutions can be achieved by polymers of very high molecular weight. A more efficient viscosification or gelation can be obtained by the so-called "associating polymers". The most popular example of associating polymers in low-polarity solvents are the ionomers (1-4) : i . e . nonpolar macromolecules containing a low molar content of ionic groups (up to 5 %). In low-polarity solvents, these polymers aggregate because of the attraction between the ion pairs and at sufficiently high concentrations the intermolecular association leads to gel formation. Similarly, visco-s i f ication or gelation of aqueous solutions can be achieved by using amphiphilic polymers ( 5 13). That is water soluble polymers bearing a few of very hydrophobic groups, typically up to 5 mol%. In aqueous solution, aggregation of the hydrophobic parts may occur resulting in an increase in the apparent molecular weight. Such thickeners may 1

Current address : Beijing Institute of Chemical Technology, Heping Street, Beijing 100013, China 0097-6156/91/0467-0218$06.00/0 © 1991 American Chemical Society

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c o n t r o l v e r y e f f i c i e n t l y the f l o w p r o p e r t i e s o f a q u e o u s - b a s e d f l u i d s in many industrial applications/formulations, e.g. latex paints, drilling muds, hydraulic fracturing f l u i d s , f o o d s , c o s m e t i c s and drag r e d u c t i o n . The l a s t few y e a r s t h e r e has been s u b s t a n t i a l i n t e r e s t i n t h e s e amphiphilic associating polymers. Most of the s t u d i e s d e a l w i t h nonionic polymers obtained by m i c e l l a r copolymerization of a h y d r o p h i l i c and a h y d r o p h o b i c monomer (7.12). Because o f the l a r g e d i f f e r e n c e i n the water s o l u b i l i t y o f the two t y p e s o f monomer, b o t h molecular weight and l o c a l composition of the corresponding copolymers a r e d i f f i c u l t to c o n t r o l . In a recent work E z z e l l and McCormick (14) shown that copolymers obtained by micellar c o p o l y m e r i z a t i o n o r by s o l u t i o n c o p o l y m e r i z a t i o n e x i b i t differences in their solution properties. A m p h i p h i l i c polymers have a l s o been s y n t h e s i z e d by m o d i f i c a t i o n o f a p r e c u r s o r water s o l u b l e p o l y m e r . In t h i s way a r a t h e r random d i s t r i b u t i o n i s expected. S t i l l , most o f the s t u d i e d a s s o c i a t i n g polymers a r e n o n - i o n i c ( 5 . 9 ) . V e r y recently e f f o r t s were made t o d e s i g n a s s o c i a t i n g p o l y e l e c t r o l y t e s (15). In a p r e v i o u s p a p e r , we have r e p o r t e d a v e r y s i m p l e r e a c t i o n o f p o l y ( a c r y l i c a c i d ) (PAA) w i t h a l k y l a m i n e s ( 1 3 ) . By t h i s way a s e r i e s o f samples w i t h the same m o l e c u l a r weight but c o n t r o l l e d e x t e n t of m o d i f i c a t i o n were o b t a i n e d . In the p r e s e n t work we g i v e t y p i c a l v i s c o m e t r i c r e s u l t s r e l a t e d t o the aqueous s o l u t i o n behavior of these a m p h i p h i l i c poly(acrylic a c i d s ) . The i n f l u e n c e o f parameters such as e x t e n t o f modification, a l k y l c h a i n l e n g t h , i o n i c s t r e n g t h , pH, presence of a s u r f a c t a n t , i s discussed. In order to clearly point out the formation of hydrophobic microaggregates, the solubilization of a very hydrophobic probe, pyrene, i s a l s o s t u d i e d . Experimental Materials. Poly(acrylic acid) (PAA) in concentrated aqueous solution was purchased from Polysciences. The average m o l e c u l a r weight g i v e n by the s u p p l i e r was 150 000. S o l i d PAA i n the a c i d form was o b t a i n e d by u l t r a f i l t r a t i o n o f the c o m m e r c i a l s o l u t i o n a t first w i t h an aqueous HCI s o l u t i o n ( 0 . 0 1 M ) , then w i t h a l a r g e excess of pure water and finally by freeze-drying. Technical grade alkylamines, octyl-(Genamin 8R 100D) and o c t a d e c y l - ( G e n a m i n 18R 100D) were k i n d l y s u p p l i e d by Société Française Hoechst, and t e t r a d e c y l a m i n e was s u p p l i e d by A l d r i c h . A l l a l k y l a m i n e s were used without further purification. 99 % p u r i t y 1 - m e t h y l - 2 - p y r r o l i d o n e (MPD) and Ν , Ν ' - d i c y c l o h e x y l c a r b o d i i m i d e (CDI) from Janssen and dodecyltrimethylammonium bromide (DTAB) from A l d r i c h were u s e d . Pyrene ( r e f e n c e s t a n d a r d ) was p u r c h a s e d by P o l y s c i e n c e s . A l l other reagents were of analytical grade and water was p u r i f i e d by a M i l l i - Q system ( M i l l i p o r e ) . Modificiation of polv(acrvlic acid). The c l a s s i c a l r e a c t i o n of amines w i t h c a r b o x y l i c a c i d s i n an a p r o t i c s o l v e n t (MPD) and i n the presence of CDI was used for the modification of p o l y ( a c r y l i c acid) :

-C00H

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H NR 2

60°C,

CDI

i n MPD

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- CONHR

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H,0 2

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A t y p i c a l example o f t h i s r e a c t i o n i s g i v e n i n r e f e r e n c e ( 1 3 ) . After p u r i f i c a t i o n the polymers were i s o l a t e d i n the sodium s a l t f o r m . The H NMR s p e c t r a o f the m o d i f i e d polymers i n d i c a t e t h a t the y i e l d of modification reaction r e a c h e s 100% ( 1 £ ) . Therefore the degree of modification (or hydrophobic group content) was a d j u s t e d by the molar ratio of alkylamine to a c r y l i c a c i d u n i t s . P r e c u r s o r and m o d i f i e d polymers a r e l i s t e d i n t a b l e 1. The sample designation is the f o l l o w i n g : e . g . PAA-150 i s the sodium s a l t o f the p r e c u r s o r polymer 150 000 m o l e c u l a r weight ; PAA-150-3-C18 i s d e r i v e d from the above polymer containing 3 mol% o f N - o c t a d e c v l a c r v l a m i d e g r o u p s . Since the modification r e a c t i o n was performed i n homogeneous s o l u t i o n , a random d i s t r i b u t i o n o f the a l k y l groups a l o n g the PAA chain c a n be expected. F u r t h e r m o r e we v e r i f i e d , by means of intrinsic viscosity measurements ( 1 6 ) , t h a t the m o d i f i e d polymers have the same p o l y m e r i z a t i o n degree as the o r i g i n a l PAA. Table 1

PAA-150

PAA-150-1-C18

PAA-150-3-C18

PAA-150-10-C18

PAA-150-3-C14 PAA-150-3-C8

Apparatus. U l t r a f i l t r a t i o n of PAA c o m m e r c i a l samples was carried out i n a P e l l i c o n C a s s e t t e system ( M i l l i p o r e ) u s i n g IRIS-3026 (Rhône Poulenc) u l t r a f i l t r a t i o n membranes of 10 000 nominal molecular weight c u t - o f f . Both a Contraves LS-30 Couette viscometer and a C a r r i - M e d c o n t r o l l e d s t r e s s rheometer w i t h a cone and p l a t e geometry were u s e d to o b t a i n viscosity values. Except f o r some s t u d i e s o f the s h e a r rate effect, a l l our v i s c o s i t y measurements were p e r f o r m e d a t low s h e a r r a t e s (1.28 s and 0.06 s " ) c o r r e s p o n d i n g t o the newtonian v i s c o s i t y o f the s y s t e m . F o r samples with v i s c o s i t i e s higher than about 1 000 cp t h e two t y p e s o f a p p a r a t u s g i v e v e r y s i m i l a r r e s u l t s . Measurements o f pH were performed w i t h a Tacussel T A T - 5 pH-meter using a glass-calomel unitubular electrode. UV-visible spectra for s o l u t i o n s c o n t a i n i n g p y r e n e , were r e c o r d e d between 200 and 450 nm on a U V - v i s s p e c t r o p h o t o m e t e r 552 ( P e r k i n E l m e r ) . The peak a t 334 nm were u s e d as a measurement o f the pyrene c o n c e n t r a t i o n . - 1

1

Conditions. Firstly, concentrated stock solutions were p r e p a r e d under m a g n e t i c stirring at least 24 hours before use. Then, s o l u t i o n s of desired concentration (by weight %) were o b t a i n e d by dilution of the appropriate stock solution w i t h water a n d , i f necessary, a d d i t i o n of s o l i d NaCI ( o r o f a concentrated surfactant s o l u t i o n ) . Solutions at v a r i o u s pH were o b t a i n e d by adjustement o f the s t o c k s o l u t i o n pH and t h e n by f u r t h e r d i l u t i o n w i t h w a t e r . For solubilization experiments the following p r o c e d u r e was adopted : in 3 ml of a solution with the desired polymer concentration, a very small amount of an e t h a n o l i c s o l u t i o n o f pyrene was injected i n order to obtain a well known final c o n c e n t r a t i o n i n p y r e n e . The f i n a l concentration of ethanol i n the

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s o l u t i o n n e v e r exceeds 5 m l / 1 . We had v e r i f i e d t h a t the p r e s e n c e of such a low concentration of ethanol does n o t a l t e r the s o l u b i l i t y b e h a v i o r o f pyrene i n water. For s o l u t i o n s w i t h h i g h concentrations i n pyrene a s l i g h t d i f f u s i o n was observed. In t h i s case b a s e - l i n e c o r r e c t i o n was made t o o b t a i n c o n t r i b u t i o n due t o the pyrene peak o n l y (maximum a t 334 nm). A l l s o l u t i o n s were e q u i l i b r a t e d f o r about 24 h o u r s b e f o r e measurements have been made. V i s c o s i t y measurements were c o n d u c t e d a t 30°C ( ± 0 . 1 ° C ) , while solubilization experiments were performed at room temperature (- 2 0 ° C ) . A i r bubbles i n the h i g h l y v i s c o u s samples were r e a d i l y e l i m i n a t e d by b r i e f c e n t r i f u g a t i o n . Results Effects of Alkyl Chain Content and L e n g t h on Aqueous S o l u t i o n Properties. Typical viscosity results of m o d i f i e d and p r e c u r s o r p o l y ( a c r y l i c a c i d ) i n pure water a r e g i v e n i n f i g u r e 1 as a f u n c t i o n o f the polymer c o n c e n t r a t i o n . A semilogarithmic scale is suitable f o r adequate r e p r e s e n t a t i o n o f the o b s e r v e d v i s c o s i t y v a r i a t i o n s . A classical polyelectrolyte behavior is found for the precursor p o l y m e r , PAA-150 (figure 1). By i n c r e a s i n g polymer c o n c e n t r a t i o n , the viscosity firstly rises sharply ( C < 1%) due to the electrostatic repulsions between c h a r g e d groups a l o n g the polymer c h a i n s . F u r t h e r i n c r e a s e i n polymer c o n c e n t r a t i o n ( C > 1%) l e a d s to the smoothness of the slope of the v i s c o s i t y c u r v e due t o the progressive self-screening of the electrostatic interactions in semi-dilute solutions. Introduction of small amounts of a l k y l c h a i n s i n t o the PAA molecule completely changes its viscometric behavior. At c o n c e n t r a t i o n s lower than a c r i t i c a l value, C , depending on the degree o f m o d i f i c a t i o n and a l k y l c h a i n l e n g t h , the m o d i f i e d polymers behave s i m i l a r l y t o the p r e c u r s o r c h a i n . When polymer concentration exceeds C , the v i s c o s i t y o f the s o l u t i o n i n c r e a s e s s h a r p l y a n d , f o r some samples, geletion may o c c u r at sufficiently h i g h polymer c o n c e n t r a t i o n s . The c r i t i c a l polymer c o n c e n t r a t i o n and the s h a r p n e s s of the viscosity curve beyond C depend on the degree of m o d i f i c a t i o n and the a l k y l chain length ( f i g u r e 1 ) . The change o f the polymer m o l e c u l a r weight does n o t modify the g e n e r a l b e h a v i o r o f the s y s t e m s . However, f o r a g i v e n m o d i f i c a t i o n degree and alkyl c h a i n l e n g t h , the l a r g e r the polymer m o l e c u l a r w e i g h t , the stronger the v i s c o s i f i c a t i o n e f f i c i e n c y ( 1 6 . 1 7 ) . p

p

p

p

p

Effects of Ionic Strength and Shear Rate. Figure 2 displays typical viscometric behavior of o r i g i n a l and h y d r o p h o b i c a l l y m o d i f i e d polymers i n the p r e s e n c e o f NaCI and a t c o n s t a n t polymer c o n c e n t r a t i o n C = 2%. U n m o d i f i e d PAA as w e l l as P A A - 1 5 0 - 3 - C 8 show a continuous decrease in viscosity upon a d d i t i o n of NaCI. By i n c r e a s i n g a l k y l c h a i n l e n g t h o r a l k y l c h a i n c o n t e n t the enhancement o f v i s c o s i t y w i t h i o n i c s t r e n g t h becomes a l l the more pronounced and a maximum i n the c u r v e appears f o r the sample P A A - 1 5 0 - 3 - C 1 8 a t NaCI c o n c e n t r a t i o n o f 1%. At t h i s i o n i c s t r e n g t h , the v i s c o s i t y o f the m o d i f i e d polymer ( P A A - 1 5 0 - 3 - C 1 8 ) i s f o u r o r d e r s o f magnitude higher than the viscosity of the precursor polymer ( P A A - 1 5 0 ) . T h e v e r y hydrophobic PAA-150-10-C18, although it is a very efficient t h i c k e n e r i n pure water (completely g e l l e d a t C = 2%), loses i t s p

p

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222

3-C18

3-C14

ο­ υ

α

u

Figure 1 : V i s c o s i t y v e r s u s polymer concentration for PAA-150 s e r i e s i n p u r e w a t e r . Shear r a t e - 1.28 s " . (•) PAA-150, (•) P A A - 1 5 0 - 3 - C 8 , ( v ) PAA-150-1-C18 , ( • ) PAA-150-3-C14, ( Ο ) PAA150-3-C18, ( # ) PAA-150-10-C18. 1

ο­ υ 3-C18

to ο υ

Figure 2 V i s c o s i t y versus NaCI c o n c e n t r a t i o n f o r PAA-150 s e r i e s . Polymer c o n c e n t r a t i o n = 2%. Shear r a t e = 0 . 0 6 s " . (•) PAA-150, ( O ) P A A - 1 5 0 - 3 - C 8 , ( A ) PAA-150 - 1-C18 , (Δ) PAA-150-3C14, ( · ) PAA-150-3-C18. 1

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v i s c o s i f y i n g p r o p e r t i e s as soon as t r a c e s of NaCI are added i n the solution. When the salt concentration exceeds 0.1%, the above polymer phase separates, giving a concentrated gel-like phase in e q u i l i b r i u m with a d i l u t e supernatant. Some examples o f the i n f l u e n c e o f shear r a t e on the viscosity o f m o d i f i e d polymers a r e g i v e n i n f i g u r e 3. A t y p i c a l pseudoplastic b e h a v i o r i s o b s e r v e d w i t h most o f t h e s e p o l y m e r s . Almost i n a l l the c a s e s a newtonian p l a t e a u i s found a t the low shear r a t e s , except f o r the v e r y m o d i f i e d P A A - 1 5 0 - 1 0 - C 1 8 . Influence of pH. Since polymer samples are obtained i n t h e i r sodium s a l t form, t h e i r aqueous s o l u t i o n s are b a s i c (pH > 9 ) . I n order to study the i n f l u e n c e o f pH on the viscometric behavior of these polymers the pH was adjasted by a d d i t i o n o f the r e q u i r e d amount o f a s t r o n g a c i d s o l u t i o n ( H C I ) . However, the a d d i t i o n o f HCI results in the f o r m a t i o n o f an e q u i v a l e n t amount o f NaCI i n the polymer s o l u t i o n and c o n s e q u e n t l y the v i s c o s i t y behavior w i l l be affected by b o t h decrease i n pH and i n c r e a s e i n i o n i c s t r e n g t h (NaCI). In order t o m i n i m i z e the importance o f the s a l t e f f e c t on the v i s c o s i t y , we have chosen a polymer sample ( P A A - 1 5 0 - 1 - C 1 8 ) w h i c h p r e s e n t p r a c t i c a l l y no v i s c o s i t y change w i t h a d d i t i o n o f N a C I . A t y p i c a l v a r i a t i o n of the viscosity versus pH, at f i x e d polymer c o n c e n t r a t i o n ( C = 2%), i s g i v e n i n f i g u r e 4. The viscosity remains p r a c t i c a l l y c o n s t a n t when the pH d e c r e a s e s from 9.7 t o 6.5 and t h e n i n c r e a s e s s h a r p l y and r e a c h e s a maximum a t about pH « 5. F u r t h e r d e c r e a s e i n pH r e s u l t s i n a dramatic decrease i n viscosity and f i n a l l y phase s e p a r a t i o n o c c u r s f o r pH v a l u e s lower t h a n 4. p

Effect of Surfactants. It has been reported that addition of surfactants to solutions of associative acrylamide copolymers (10.12) or to solutions of hydrophobically modified hydroxyethylcellulose (HMHEC) (9) results in a decrease in v i s c o s i t y . Other authors have c l a i m e d t h a t the v i s c o s i t y o f HMHEC (5) or hydrophobically modified ethoxylate u r e t h a n e s (8) can be enhanced i n the p r e s e n c e o f s u r f a c t a n t s . The v i s c o s i t y o f o u r polymers i n c r e a s e s by a d d i t i o n o f a n i o n i c , c a t i o n i c o r n o n i o n i c s u r f a c t a n t s . Even an a n i o n i c s u r f a c t a n t , which p r e s e n t s u n f a v o u r a b l e e l e c t r o s t a t i c r e p u l s i o n s w i t h the anionically c h a r g e d polymer c h a i n s , l e a d s t o a n o t i c e a b l e i n c r e a s e i n v i s c o s i t y . F o r i n s t a n c e , the v i s c o s i t y o f an aqueous s o l u t i o n o f PAA-150-1-C18 ( C = 3%) r i s e s from 23 cp i n pure water t o 300 cp i n the p r e s e n c e o f sodium d o d e c y l s u l f a t e (4 1 0 " mol.l" ). The most striking behavior is observed when a cationic s u r f a c t a n t i s u s e d , dodecyltrimethylammonium bromide (DTAB). T y p i c a l r e s u l t s are given i n f i g u r e 5 where the v i s c o s i t y o f the p r e c u r s o r (PAA-150) and o f a m o d i f i e d polymer ( P A A - 1 5 0 - 1 - C 1 8 ) i s p l o t t e d as a function of the polymer concentration i n pure water and in 4 10" mol.l" DTAB. A d d i t i o n o f DTAB t o the p r e c u r s o r polymer does n o t i m p l y any viscosification of the system, at least in the range of c o n c e n t r a t i o n s u s e d . F o r polymer c o n c e n t r a t i o n s h i g h e r t h a n 1% the viscosity is the same i n the absence o r i n the p r e s e n c e o f DTAB (lower c u r v e i n f i g u r e 5 ) . When the polymer c o n c e n t r a t i o n i s lower than 1%, corresponding to a ratio R = [acrylate residue] / [ s u r f a c t a n t ] < 26, a d d i t i o n o f DTAB r e s u l t s i n a p r e c i p i t a t i o n o f p

3

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1

1

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5

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1 10

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, 0

SHEAR

10

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r— 2

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Figure 3 : Dependence o f v i s c o s i t y on t h e s h e a r rate f o r three systems : 1 ( * ) P A A - 1 5 0 - 3 - C 1 8 , C = 2% i n pure w a t e r ; 2 ( • ) P A A - 1 5 0 - 1 0 - C 1 8 , C = 0.35% i n p u r e water ; 3 ( B a n d # ) P A A - 1 5 0 - 3 C18, C = 2% i n 0.6% NaCI. ψ, a , were obtained with a Contraves L S - 3 0 v i s c o m e t e r and · w i t h a C a r r i - M e d r h e o m e t e r . p

p

p

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F i g u r e 4 : V a r i a t i o n o f v i s c o s i t y v e r s u s pH f o r t h e PAA-150-1-C18 modified polymer. Polymer c o n c e n t r a t i o n = 2%. Shear rate = 1.28s" . 1

225

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Figure 5 : Effect of a cationic surfactant (DTAB) on the v i s c o s i t y o f the p r e c u r s o r and a m o d i f i e d p o l y m e r . Viscosity is p l o t t e d as a f u n c t i o n o f polymer c o n c e n t r a t i o n : c u r v e 1 ( O ) and 2 (#) f o r PAA-150 ; c u r v e s 3(D) and 4 ( • ) f o r PAA-150-1-C18. 1 ( O ) and 3 ( • ) were o b t a i n e d i n p u r e water w h i l e 2 (#) and 4 ( • ) i n an aqueous solution of DTAB (4 1 0 " mol.l" ). Shear r a t e = 1.28 s " . A l l measurements were p e r f o r m e d w i t h a C o n t r a v e s L S - 3 0 v i s c o m e t e r except f o r the two u p p e r p o i n t s o f the curve 4 obtained with a C a r r i - M e d rheometer. 3

1

1

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the complex DTAB/PAA-150. Such a b e h a v i o r i s w e l l known f o r m i x t u r e s o f a p o l y e l e c t r o l y t e and a o p p o s i t e l y charged s u r f a c t a n t (18-20). Undoubtedly, at a very low polymer concentrations, t h e system becomes homogeneous a g a i n . A l t h o u g h t h e l e s s m o d i f i e d PAA (PAA-1501-C18) does n o t e x h i b i t a very pronounced t h i c k e n i n g b e h a v i o r i n p u r e water as compared w i t h the p r e c u r s o r polymer, i n the presence o f DTAB (4 1 0 " m o l . l " ) i t shows a s u r p r i s i n g l y i m p o r t a n t increase i n v i s c o s i t y . F o r i n s t a n c e , when t h e polymer c o n c e n t r a t i o n i s 1.5%, the v i s c o s i t y i n t h e p r e s e n c e o f DTAB i s f o u r o r d e r o f magnitude h i g h e r t h a n i n p u r e w a t e r . F u r t h e r m o r e , p r e c i p i t a t i o n o f t h e complex DTAB/PAA-150-1-C18 occurs only a t polymer c o n c e n t r a t i o n s lower t h a n 0.3% (R = [ a c r y l a t e r e s i d u e ] / [ s u r f a c t a n t ] g 7 ) . 3

1

S o l u b i l i z a t i o n o f Hydrophobic A d d i t i v e s . I t i s w e l l known that s o l u b i l i t y o f a l i p h a t i c and a r o m a t i c h y d r o c a r b o n s i n water c a n be enhanced in the presence of surfactants or hydrophobically associating polymers (21.22). Pyrene is an a r o m a t i c h y d r o c a r b o n exhibiting a very low solubility in pure water ( [ P y ] water"* ) £ 2 3 . 2 4 ) and i t i s l a r g e l y u s e d as a probe f o r t h e ' s t u d y o f m i c e l l e s and o t h e r h y d r o p h o b i c a g g r e g a t e s i n w a t e r . We r e p o r t h e r e some r e s u l t s c o n c e r n i n g t h e s o l u b i l i z a t i o n o f pyrene by t h e h y d r o p h o b i c a l l y m o d i f i e d PAA. I n f i g u r e 6, t h e absorbance of pyrene a t t h e maximum of t h e peak (334 nm) i s p l o t t e d a g a i n s t pyrene c o n c e n t r a t i o n . Typical saturation curves are obtained : at f i r s t , the absorbance l i n e a r l y i n c r e a s e s w i t h pyrene c o n c e n t r a t i o n and f i n a l l y l e v e l s o f f . The s l o p e o f the l i n e a r p a r t o f the curves i s t h e same f o r a l l systems i n d i c a t i n g , t h a t the molar e x t i n c t i o n c o e f f i c i e n t does n o t change i n t h e p r e s e n c e o f p o l y m e r s . We w i l l c o n s i d e r t h a t the s a t u r a t i o n c o n c e n t r a t i o n of pyrene, [ P y ] , i s given by t h e upper l i m i t o f t h e l i n e a r p a r t o f t h e c u r v e . As a c o n s e q u e n c e , i n pure water ( f i g u r e 6a) the s a t u r a t i o n value found (~ 8 1 0 " M) i s c l o s e t o the l i t e r a t u r e data (~ 7 1 0 " M ) . Addition of unmodified PAA ( a t C = 0.5%) induces a very s l i g h t change i n t h e s a t u r a t i o n v a l u e : [ P y ] ^ 1.1 1 0 " M. On t h e o t h e r hand, i f 0.5% o f a modified polymer ( P A A - 1 5 0 - 3 - C 1 8 ) i s added, a noticeable increase i n [Py] i s found ( 1 . 9 1 0 " M ) . An i n c r e a s e i n m o d i f i e d polymer concentration results i n a higher s o l u b i l i t y of pyrene : [ P y ] = 2.8 1 0 " M f o r PAA-150-3-C18 a t C = 1% ( f i g u r e 6 a ) . 7

s

1

0

?

M

g

7

7

p

6

g

s

6

6

p

The most interesting result i s shown i n t h e f i g u r e 6b. The lower c u r v e i s a l r e a d y g i v e n i n t h e f i g u r e 6a and c o r r e s p o n d s t o t h e system PAA-150-3-C18, C = 0.5%, i n water. The u p p e r c u r v e was o b t a i n e d w i t h t h e same polymer and a t t h e same c o n c e n t r a t i o n b u t 1% NaCI solution i s used as solvent. It is c l e a r that i n the p r e s e n c e o f N a C I , t h e s o l u b i l i t y o f pyrene i s v e r y i n c r e a s e d . I t must be n o t e d t h a t a d d i t i o n o f NaCI b r i n g s about an enhancement o f pyrene s o l u b i l i t y o n l y i f a m o d i f i e d PAA i s p r e s e n t i n t h e s o l u t i o n . p

Discussion The d r a s t i c v i s c o s i t y i n c r e a s e i n t h e aqueous s o l u t i o n s o f modified p o l y ( a c r y l i c a c i d ) beyond a c r i t i c a l polymer c o n c e n t r a t i o n , C , c a n be ascribed t o the i n t e r c h a i n a s s o c i a t i o n through formation of h y d r o p h o b i c m i c r o d o m a i n s . I n f a c t , i t has been s u g g e s t e d t h a t water soluble polymers with a low c o n t e n t o f v e r y h y d r o p h o b i c groups a g g r e g a t e i n a s i m i l a r way as s u r f a c t a n t m o l e c u l e s m i c e l l i z e ( 9 . 1 1 ) . p

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CPy] * 1 0 / m o l . 6

Γ

227

1

Figure 6 Solubility curves for pyrene expressed as the absorbance at 334 nm v e r s u s pyrene concentration. 1 i n pure water ; 2 i n aqueous s o l u t i o n o f PAA-150, C = 0.5% ; 3 i n aqueous s o l u t i o n of PAA-150-3-C18, C = 0.5% ; 4 i n aqueous s o l u t i o n of PAA-150-3-C18, C = 1% ; 5 i n 1% NaCl s o l u t i o n o f PAA-150-3-C18, C = 0.5%. p

p

p

?

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WATER-SOLUBLE P O L Y M E R S

Our v i s c o m e t r i c results plotted in figure 1, c a n be compared qualitatively t o the m i c e l l i z a t i o n o f ionic surfactants. The v i s c o s i f i c a t i o n o f o u r samples i n pure water and c o n s e q u e n t l y the formation of hydrophobic aggregates, i s a l l t h e more pronounced as the a l k y l c h a i n length or the a l k y l chain content i n c r e a s e s . In a s i m i l a r way, t h e m i c e l l i z a t i o n o f i o n i c s u r f a c t a n t s becomes more e f f e c t i v e by i n c r e a s i n g t h e s u r f a c t a n t c o n c e n t r a t i o n o r the a l k y l chain length (22). On t h e o t h e r hand, it i s very i n t e r e s t i n g t o compare t h e behavior of o u r polymers with that of hydrophobically modified n o n i o n i c polymers (HMNIP) ( 9 . 1 0 . 1 2 ) . Despite the d i f f e r e n c e s i n the molecular weight, ionic and n o n i n o n i c h y d r o p h o b i c a l l y modified p o l y m e r s show s i m i l a r t r e n d s in their b e h a v i o r i n water s o l u t i o n (effect of alkyl chain content and l e n g t h ) (9.12). However, v i s c o s i f i c a t i o n o f a HMNIP s o l u t i o n o c c u r s a t polymer c o n c e n t r a t i o n s and a l k y l chain contents o f one o r two o r d e r s o f magnitude lower than in hydrophobically modified PAA (HMPAA) (10.12). These d i f f e r e n c e s i n t h e b e h a v i o r o f HMNIP and HMPAA c a n be compared t o the differences i n the c . m . c . value of nonionic and i o n i c s u r f a c t a n t s . In g e n e r a l , the c . m . c . i s lower f o r n o n i o n i c t h a n f o r i o n i c s u r f a c t a n t s (22 . 2 5 ) . The t h i c k e n i n g o f aqueous s o l u t i o n s o f h y d r o p h o b i c a l l y m o d i f i e d water s o l u b l e polymers c a n be a t t r i b u t e d t o t h e f o r m a t i o n o f highly branched m u l t i c h a i n aggregates o r o f a p h y s i c a l g e l i n which crosslinking is assured by h y d r o p h o b i c i n t e r c h a i n a g g r e g a t i o n . As a consequence, v i s c o s i f i c a t i o n occurs o n l y a t polymer concentrations above t h e c r i t i c a l o v e r l a p concentration, C . In pure water, the h i g h e r t h e number o f i n t e r c h a i n hydrophobic aggregates, the h i g h e r the v i s c o s i t y o f t h e s y s t e m . A d d i t i o n o f a s a l t t o an aqueous s o l u t i o n o f HMPAA r e s u l t s i n the s c r e e n i n g o f t h e e l e c t r o s t a t i c r e p u l s i o n s between c h a r g e s along the polymer c h a i n and b r i n g s a b o u t , a t t h e same t i m e , a retraction o f t h e p o l y e l e c t r o l y t e c h a i n and a more e f f e c t i v e a g g r e g a t i o n o f t h e alkyl chains. As a c o n s e q u e n c e , a d d i t i o n o f NaCI t o a s s o c i a t i n g polyelectrolytes solutions leads either t o a more pronounced decrease in viscosity or t o a very e f f e c t i v e t h i c k e n i n g o f the s o l u t i o n . F o r r e l a t i v e l y low polymer c o n c e n t r a t i o n s ( C < 0.5%), the enhancement o f h y d r o p h o b i c a g g r e g a t i o n upon a d d i t i o n o f NaCI r e s u l t s r a t h e r i n an i n t r a c h a i n a s s o c i a t i o n w h i c h causes a more pronounced viscosity shift ( n o t shown i n figure 2). When t h e polymer c o n c e n t r a t i o n i s h i g h enough, i n g e n e r a l h i g h e r t h a n C , a d d i t i o n o f NaCI b r i n g s about i n c r e a s i n g i n t e r c h a i n h y d r o p h o b i c a g g r e g a t i o n and f i n a l l y l e a d s t o a s u b s t a n t i a l enhancement i n t h e v i s c o s i t y (curves 3-C14 and 3-C18 i n figure 2). However, chain r e t r a c t i o n i s i n c o m p e t i t i o n w i t h i n t e r c h a i n a g g r e g a t i o n r e g a r d i n g t h e e f f e c t on t h e v i s c o s i t y o f t h e s y s t e m . Above a g i v e n l e v e l o f NaCI concentration, depending on t h e h y d r o p h o b i c a l l y m o d i f i e d PAA u s e d , the c h a i n retraction effect prevails over the a g g r e g a t i o n e f f e c t and t h e viscosity of t h e system starts t o d e c r e a s e (see c u r v e 3-C18 i n f i g u r e 2 ) . At higher s a l t concentrations ( > 3%) phase separation may o c c u r . A s i m i l a r maximum i n v i s c o s i t y upon a d d i t i o n o f NaCI was reported recently for ionic terpolymers of acrylamide, N-ndecylacrylamide and sodium a c r y l a t e o r sodium 3-acrylamido-3methylbutanoate (3J5). The HMNIP e x h i b i t only small viscosity enhancement w i t h a d d i t i o n o f s a l t s ( 1 2 . 2 6 ) . On t h e o t h e r h a n d , the p

p

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HMPAA sample, w h i c h does n o t e x h i b i t associating behavior i n pure w a t e r , i n the p r e s e n c e o f NaCI behaves l i k e the p r e c u r s o r polymer (see c u r v e s 3-C8 and PRECURSOR i n f i g u r e 2 ) . The i n f l u e n c e o f pH on the v i s c o s i t y o f HMPAA ( f i g u r e 4) c a n be e x p l a i n e d i n the same terms as the i n f l u e n c e o f s a l t s . By d e c r e a s i n g pH the a c t u a l charge density of the PAA c h a i n d e c r e a s e s b r i n g i n g about a r e t r a c t i o n o f the c h a i n and a more e f f e c t i v e h y d r o p h o b i c a g g r e g a t i o n . Even the l e s s m o d i f i e d polymer (PAA-150-1-C18) which does n o t e x h i b i t any n o t i c e a b l e t h i c k e n i i g b e h a v i o r upon a d d i t i o n o f N a C I , p r e s e n t s an increase i n v i s c o s i t y o f one o r d e r o f magnitude when the pH s h i f t s from 9.7 t o - 5. P r e s u m a b l y , a d d i t i o n o f NaCI or decrease i n pH do n o t have e x a c t l y the same i n f l u e n c e on the association of the hydrophobic groups and on the polymer c h a i n r e t r a c t i o n . The d e t a i l s o f t h i s phenomenon a r e t o be e l u c i d a t e d . Whatever the hydrophobically modified PAA, the most s t r i k i n g t h i c k e n i n g b e h a v i o r was o b s e r v e d i n t h e p r e s e n c e o f s u r f a c t a n t s . It i s w e l l e s t a b l i s h e d t h a t s u r f a c t a n t s i n t e r a c t w i t h n o n i o n i c polymers mainly through hydrophobic interactions and w i t h i o n i c polymers through both hydrophobic and e l e c t r o s t a t i c i n t e r a c t i o n s (27) · I n general, the more hydrophobic the polymer i s , the s t r o n g e r the interaction with surfactants. In a sense, our p o l y m e r s (HMPAA) behave as polymeric surfactants and may form a k i n d o f mixed a g g r e g a t e s w i t h low m o l e c u l a r weight s u r f a c t a n t s . T h i s seems t o be a c t u a l l y the c a s e as emerged from the v i s c o s i t y b e h a v i o r o f m o d i f i e d and p r e c u r s o r polymer i n the p r e s e n c e o f DTAB shown i n f i g u r e 5. In p u r e w a t e r , because o f the s t r o n g e l e c t r o s t a t i c r e p u l s i o n s between the c h a r g e d groups o f the c h a i n s , o n l y a s m a l l f r a c t i o n o f polymera l k y l groups i s involved i n hydrophobic aggregates. A d d i t i o n of a s u r f a c t a n t l e a d s t o the f o r m a t i o n o f mixed a g g r e g a t e s and thus t o a more effective c r o s s - l i n k i n g and t o the v i s c o s i f i c a t i o n o f the system : mixed aggregates containing a t l e a s t two p o l y m e r - a l k y l groups c a r r i e d on d i s t i n c t polymer c h a i n s a c t as e f f e c t i v e crosslinks. If a l a r g e excess o f s u r f a c t a n t i s added, mixed a g g r e g a t e s containing only one polymer-alkyl group can be formed and c o n s e q u e n t l y the e f f e c t i v e c r o s s - l i n k i n g and the v i s c o s i t y o f the system d e c r e a s e . Such a b e h a v i o r f o r a s s o c i a t i n g polymers i n the p r e s e n c e o f s u r f a c t a n t s was r e p o r t e d by L u n d b e r g , G l a s s and E l e y (8) and by Sau (5) and o b s e r v e d r e c e n t l y i n our l a b o r a t o r y ( T . K . WANG, unpublished r e s u l t s ) . The i d e a o f mixed a g g r e g a t e s f o r m a t i o n i s a l s o s u p p o r t e d by the differences in the value of the ratio R = [acrylate residue]/ [ s u r f a c t a n t ] a t w h i c h phase s e p a r a t i o n o c c u r s . This r a t i o is much lower f o r the m o d i f i e d PAA-150-3-C18 (- 7) t h a n f o r the precursor PAA (- 2 6 ) , i n d i c a t i n g that the nature of a s s o c i a t i o n between anionic polymers and c a t i o n i c surfactants is changed when h y d r o p h o b i c groups a r e a t t a c h e d on the polymer c h a i n . On the other hand, the hydrophobically modified nonionic polymers a g g r e g a t e v e r y e f f i c i e n t l y i n pure water ( p r o b a b l y most of the a l k y l groups a r e i n v o l v e d i n a g g r e g a t e s ) and thus a d d i t i o n of surfactants and f o r m a t i o n of mixed aggregates lead to the d e s t r u c t i o n o f some c r o s s - l i n k s and, consequently to a decrease of the v i s c o s i t y . The p s e u d o p l a s t i c behavior of our polymers (figure 3) i s s i m i l a r t o t h a t o b s e r v e d w i t h o t h e r a s s o c i a t i n g systems ( 1 1 . 1 2 ) and confirm the picture of a physical reversible highly branched

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structure or gel formed by interchain hydrophobic aggregation. Practically, the same plots of viscosity were obtained by increasing or decreasing the shear rate. Finally, enhanced solubilization of pyrene in aqueous solutions of HMPAA, clearly bear out the statement of hydrophobic aggregation. By increasing polymer concentration as well as by addition of NaCI enhanced solubility of pyrene was observed (figure 6). This is in agreement with the viscosity results of figures 1 and 2. In pure water, near to the viscosification threshold (C = 0.5%) a relatively small increase in pyrene solubility is observed (figure 6a) indicating some extent of hydrophobic aggregation. Addition of NaCI (1%) promotes hydrophobic aggregation, mainly intrachain in the range of concentration studied, and consequently i t leads to lower viscosity and increased pyrene solubility (figure 6b). p

Conclusions The classical viscosity reduction of polyelectrolyte solutions upon addition of salts can be prevented i f a small amount of highly hydrophobic groups are covalently bounded on the polyelectrolyte chain. Under suitable conditions the hydrophobic moieties aggregate and the solution viscosity is found increased. Addition of a salt or a surfactant or decrease in pH promotes hydrophobic groups aggregation and may result in a very important enhancement of viscosity and in an increased solubilization power towards aromatic hydrocarbons. Acknowledements This study was supported by the "Société Française HOECHST". We wish to acknowledge Miss M.N. CHAUSSET for performance of solubilization experiments. Literature Cited 1.

Lundberg, R.D. ; Phillips, R.R. J . Polym. Sci. Polym. Phys. 1982, 20, 1143. 2. Peiffer, D.G. ; Kim, M.W. ; Schulz, D.N. J . Polym. Sci. Polym. Phys. 1987, 25, 1615. 3. Hara, M. ; Wu, J . L . ; Lee, A.H. Macromolecules 1988, 21, 2214 4. Granville, M. ; Jerome, R.J. ; Teyssie, P. ; De Schryver, F.C. Macromolecules 1988, 21, 2894. 5. Sau, A.C. Proceedings ACS Div. Polym. Mater. Sci. Engin. 1987, 57, 497. 6. King, M.T. ; Constien V.G. Proceedings ACS Div. Polym. Mater. Sci. Engin. 1986, 55, 869. 7. Bock, J . ; Siano, D.B. ; Schulz, D.N. ; Turner, S.R. ; Valint, P.L. ; Pace, S . J . Proceedings ACS Div. Polym. Mater. Sci. Engin. 1986, 55, 355. 8. Lundberg, D.J. ; Glass, J . E . ; Eley, R.R. Proceedings ACS Div. Polym. Mater. Sci. Engin. 1989, 61, 533. 9. Landoll, L.M. J . Polym. Sci. Polym. Chem. 1982, 20, 443. 10. Schulz, D.N. ; Kaladas, J.J. ; Maurer, J.J. ; Bock, J. ; Pace, S.J. ; Schulz, W.W. Polymer 1987, 28, 2110. 11. Valint, P.L. ; Bock, J . Macromolecules 1988, 21, 175.

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