Fluorescence Studies of Polymer Association in Water - American

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Downloaded by CORNELL UNIV on July 18, 2016 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/ba-1993-0236.ch018

Fluorescence Studies of Polymer Association in Water Mitchell A. Winnik and Françoise M. Winnik 1

2

Department of Chemistry and Erindale College, University of Toronto, Toronto, Ontario, Canada M5B 1A1 Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario, Canada L 5 K 2L1 1

2

Water-soluble polymers containing hydrophobic substituents form micellelike clusters in water. For rigid cellulosic polymers, the association is interpolymeric with little evidence for intramolecular association of the substituents. For more flexible linear polymers, the nature of the interaction can depend upon chain microstructure and the location of the hydrophobic substituents. Those at the chain ends, as in C -PEO-C where PEO is poly(ethylene oxide), undergo strong interpolymeric associations to form a network linking micellelike clusters. Polystyrene-poly(ethylene oxide) (PS-PEO) diblock and PEO- P S - P E Otriblock copolymers behave quite differently. They undergo a sharp association transition with increasing concentration to form spherical micelles with a very narrow size distribution. Their sizes and aggregation numbers seem to be well described by the star model of block copolymer micelles. n

n

W^ATER-SOLUBLE NONIONIC POLYMERS bearing h y d r o p h o b i c substituents undergo association i n aqueous solution ( J ) . T h i s association process has a p r o f o u n d effect o n the macroscopic properties o f the solutions, such as viscosity a n d c l o u d point. O n e o f the major technological applications o f these types o f materials is as viscosity modifiers f o r aqueous solutions i n areas as diverse as coatings a n d enhanced o i l recovery ( 2 ) . T h e nature o f these interactions depends sensitively o n the p o l y m e r microstructure as w e l l as o n the type a n d content o f h y d r o p h o b i c sub stituents. T o develop an understanding o f these s t r u c t u r e - p r o p e r t y relation0065-2393/93/0236-485$06.25/0 © 1993 American Chemical Society

Urban and Craver; Structure-Property Relations in Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

486

STRUCTURE-PROPERTY RELATIONS IN POLYMERS


ships, m u c h m o r e detailed i n f o r m a t i o n at the molecular level is needed. I n the w o r k described here, w e e m p l o y e d fluorescence spectroscopy i n conjunc t i o n w i t h other methods to obtain this k i n d o f i n f o r m a t i o n . T h e polymers hydroxypropylcellulose ( H P C ) a n d p o l y ( N - i s o p r o p y l acrylamide) ( P N I P A M ) share the p r o p e r t y that they precipitate f r o m their aqueous solutions u p o n heating. T h i s lower critical solution temperature ( L C S T ) is 42 °C f o r H P C a n d 32 °C f o r P N I P A M . B o t h polymers are relatively nonpolar b u t are s o l u b i l i z e d i n water b y hydrogen b o n d i n g . T h e s e H bonds are d i s r u p t e d b y heating, a n d this d i s r u p t i o n leads to phase separation. O n e o f the k e y differences between these polymers is that o f chain stiffness, H P C b e i n g substantially m o r e extended a n d less flexible than P N I P A M . T h e i n t r o d u c t i o n o f h y d r o p h o b i c substituents o n these polymers, either alkyl chains o r aromatic chromophores, perturbs the h y d r o p h o b i c h y d r o p h i l i c balance a n d has a n u m b e r o f interesting effects o n p o l y m e r behavior i n water. W e examined a n u m b e r o f these features, relying heavily o n fluorescence studies to reveal behavior at the molecular level. P o l y s t y r e n e - p o l y ( e t h y l e n e oxide) ( P S - P E O ) d i b l o c k a n d P E O - P S - P E O triblock copolymers have a very different microstructure; the h y d r o p h o b i c b l o c k o f the chain is at one e n d o r i n the m i d d l e . Samples containing m o r e than 50 w t % P E O dissolve i n water a n d associate to f o r m spherical micelles containing a dense P S core s u r r o u n d e d b y a corona o f solvent-swollen P E O chains ( 3 , 4). A n u m b e r o f features o f these micelles are o f interest, such as the aggregation n u m b e r , the effective size, a n d the concentration at w h i c h micelles first f o r m (the critical m i c e l l e concentration o r C M C ) . These same issues are also important for P E O containing t w o h y d r o p h o b i c e n d groups. H e r e this apparently small change i n microstructure leads to a complete change i n the nature o f the p o l y m e r association i n water. I n b o t h systems, fluorescence techniques provide important information, although i n the b l o c k c o p o l y m e r micelles, fight scattering plays a m u c h more important role i n d e t e r m i n i n g micelle size.

Hydroxypropylcellulose (HPC) Hydroxypropylcellulose ( F i g u r e 1) is soluble not only i n organic solvents such as tetrahydrofuran a n d methanol, b u t also i n c o l d water. A t t a c h i n g a small n u m b e r o f h y d r o p h o b i c substituents to this p o l y m e r has a significant effect o n its properties i n water. W h e n H P C carries a n average o f one o r fewer h y d r o p h o b i c groups p e r chain, its solubility i n water is hardly affected. H o w e v e r , a m u c h larger change i n the properties o f H P C occurs w h e n the level o f labeling is increased to a n average o f two o r m o r e groups p e r chain. T h e effect is very noticeable i n the pyrene-labeled p o l y m e r , H P C - P y / 5 6 (1 P y p e r 56 glucose units, F i g u r e 1). T h e solubility o f this p o l y m e r i n water is greatly r e d u c e d c o m p a r e d to that o f the u n l a b e l e d p o l y m e r . T h i s p o l y m e r


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Polymer Association in Water

487

(CH ) CH - N H - C O - C H 3

2

I

CH

2

!

R - NH - C O - C H I CH J ^ 2

PNIPAM-Py:

R = CH(CH3)2

R' = (CH )4-Py

PNIPAM-C :

R = CH(CH )

R' = C H

n

3

2

2

n

2 n +

1

PNIPAM-Py-N: R = CH(CH3)-N R' = (CH )4-Py 2

R' - NH - CO - CH I CH J ^


2

HPC-Py:

R = (CH ) Py

HPC-Flu:

R = CH -Flu

2

4

2

Flu Figure

1. Structures of the hydroxypropylcellulose (HPC) and isopropylacrylamide) {PNIPAM) derivatives.

poly(N-

undergoes extensive interchain aggregation i n water, a n d various aspects o f the heat-induced phase transition are p e r t u r b e d . T h r o u g h the use o f several fluorescence techniques w e demonstrated ( 5 - 7 ) that i n such hydrophobically m o d i f i e d H P C : (1) interchain aggregation b e l o w the L C S T occurs p r e d o m i nantly through association o f the substituents, (2) t h e polymers f o r m inter-


488


c h a i n aggregates even i n extremely dilute solutions, a n d (3) the association between h y d r o p h o b i c groups is destroyed w h e n the solutions are heated t h r o u g h their L C S T .

Aggregation of Labeled HPC-Py/56 in Cold Water. F l u o r e s cence spectra o f H P C - P y / 5 6 i n m e t h a n o l a n d i n water are presented i n F i g u r e 2. T h e spectrum shows two bands, one due to locally excited pyrene (intensity I , " m o n o m e r emission") w i t h the (0, 0) b a n d located at 376 n m a n d a b r o a d emission centered at 480 n m due to pyrene excimer emission (intensity I ) . F o r samples i n methanol, identical excitation spectra are obtained for emissions m o n i t o r e d at 396 a n d 480 n m , a n d the m a x i m a correspond to those i n the U V absorption spectrum. T w o differences are observed for aqueous solutions o f H P C - P y / 5 6 . T h e emission spectrum exhibits a m u c h stronger excimer b a n d . I n addition, the excitation spectra for the m o n o m e r a n d excimer are clearly different; the m o n o m e r s spectrum is shifted b y about 3 n m . T h e excimer excitation spectrum corresponds to the U V spectrum o f the sample. These features a n d other aspects o f the spectroscopy o f H P C - P y / 5 6 l e a d to the conclusion that the excimer emission originates f r o m aggregates o f pyrenes that exist before excitation. M


E

T h i s type o f p h e n o m e n o n is u n i q u e to aqueous solutions o f chrom o p h o r e - l a b e l e d polymers. A n important task is to distinguish w h e t h e r the pyrene association occurs intramolecularly or b e t w e e n chromophores o n different chains. T o address this question w e m o n i t o r e d the effect o f p o l y m e r concentration o n the ratio J / / . T h i s ratio decreases somewhat w i t h decreasing concentration, a feature i n d i c a t i n g interchain contributions. H o w ever, the results o f these experiments d i d not allow us to exclude the occurrence o f intramolecular p y r e n e - p y r e n e association, n o r to quantify the relative importance o f each contribution. A n o t h e r approach was devised to elucidate this point. E

M

Aggregation of HPC-Py/438 and HPC-Flu/33 in Cold Wa ter. Energy-transfer experiments allow the detection o f association be tween fluorescent labels attached to different p o l y m e r i c chains. T o p r o c e e d , w e p r e p a r e d solutions containing a mixture o f two polymers, identical except for their fluorescent tags: O n e p o l y m e r carried a very small n u m b e r o f pyrene labels ( H P C - P y / 4 3 8 ) ; the second, fluorene labels ( H P C - F l u / 3 3 , F i g u r e 1). T h e two chromophores interact as d o n o r (fluorene) a n d acceptor (pyrene) b y nonradiactive energy transfer ( N R E T ) ( C M C ) these comprise the micelles a n d the clusters w

M ( a p p ) = W (micelles) + (1 - W w

M

M

) M

W

(clusters)

(4)

w h e r e W is the weight fraction o f p o l y m e r present as micelles. T o p r o c e e d w i t h the analysis, w e make three key assumptions: that the size distribution o f micelles is narrow (i.e., M « M f o r micelles), that the star m o d e l is valid, and that t h e density o f the m i c e l l e clusters is proportional to that o f t h e micelles themselves ( 1 9 ) . T h e first two assumptions are strongly supported b y the D L S experiment. T h e t h i r d assumption a n d t h e details o f t h e data analysis are discussed i n detail i n reference 19. T h e essence o f this analysis is M

w

N


500


CD

Figure 11. A plot of R /QNl N| vs. Q N | N | / for a series ofPS~PEO diblock and triblock copolymers. The lower data fit the star model and corre spond to the micelles. The upper data set is due to large secondary aggregates (clusters) of micelles. / 2 5

H


/ 2 5

/ 5

5

110

35 N

3 / 5

A

N

4 / 2 5

140

Q

Β

Table I. Characteristics of Block Copolymer Samples and Their Micelles in Water Sample Name

N (PS)

N (PEO)

M

110 36 36 17

400 236 450 155

28,700 14,100 23,600 8,500

290 120 120 64

23 14 19 9

12 5.6 6.0 3.5

100 200 X 180 X 164 X 240

13,100 20,000 20,000 18,000 26,000

130 67 130 120 150

12 9 13 13 18

5.8 3.6 6.0 5.5 6.8

B

Diblock DB23 DB40 DB41 jlm5 Triblock TB19 TB51 jlm4 jlm6 jlmll

40 18 41 35 47

A

2 2 2 2 2

X X

Ν

n

R

H

(nm)

R

c o r e

(nm)

that the proportionality assumption is not b a d , b u t the proportionality c o n stant for triblock copolymer micelles is different f r o m that for diblocks. A s a consequence, w e c a n calculate W , M (micelle), a n d the mean aggregation n u m b e r N for each micelle. Important characteristics o f the polymers a n d the micelles are collected i n T a b l e I. M

n

n

PEO with Hydrophobic End Groups P E O polymers containing h y d r o p h o b i c e n d groups are members o f a class o f polymers k n o w n as associative thickeners ( A T s ) . These substances are a d d e d at about 0 . 5 - 2 w t % to latex paint f o r m u l a t i o n to m o d i f y the rheology a n d to reduce splatter. A t higher concentrations the solutions w i l l gel, presumably because the e n d groups associate to f o r m micellelike aggregates, b r i d g e d b y the network o f P E O chains. A typical linear A T has the structure RO-(DI-0-PEO-0-) DI-OR n

1


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501

w h e r e D I is a diurethane, f o r m e d typically f r o m toluene diisocyanate or isophorone diisocyanate, a n d R is an alkyl or an alkylaryl (e.g., nonylphenyl) group. I n m a n y o f these materials, the P E O has M = 8000 w i t h a narrow molecular-weight distribution ( M W D ) . Average η values range f r o m 2 to 50. T h e final polymers, p r o d u c e d b y condensation o f the reactants, have a b r o a d M W D . R o h m and Haas markets an associative thickener ( R = C H ) u n d e r the trademark R M - 8 2 5 . n

1 2

2 5

A n o t h e r class o f associative thickeners is based u p o n cellulose derivatives. F o r example, A q u a l o n produces a hydrophobically m o d i f i e d hydroxyethylcellulose ( H M H E C ) containing a small amount ( < w t % ) o f alkyl chains ( C to ^ 2 4 > * ë * ' Natrasol 250 G R ) . Fluorescent p r o b e experiments w e r e e m p l o y e d (23) to study the networks f o r m e d b y these polymers i n water. T h e fluores cence data show that the h y d r o p h o b i c groups o f H M H E C associate to f o r m clusters above a critical p o l y m e r concentration o f 500 p p m . 1 2


e

O u r experiments (24) e m p l o y e d a series o f m o d e l materials ( R = C H ) p r e p a r e d b y the Bassett group at U n i o n C a r b i d e . W e a d d e d these polymers to aqueous solutions containing a fixed concentration o f pyrene (6 X 10 ~ M ) a n d examined the changes i n the p r o b e fluorescence as a function o f c for two samples o f 1 w i t h M = 40,000 a n d 47,000 ( M / M = 1.3 a n d 1.5, respectively). T o estimate the onset o f association, w e took advantage of the shift i n the excitation spectrum o f P y akin to that shown i n F i g u r e 2: B y choosing X == 338 n m P y is selectively excited i n a h y d r o p h o b i c environment. A plot o f fluorescence intensity I versus l o g c is sigmoidal ( F i g u r e 12). T h e rising p o r t i o n o f the curve indicates the onset o f end-group association c o u p l e d w i t h P y partitioning into the m i c e l l e l i k e clusters ( M L C ' s ) that f o r m . 1 6

3 3

7

w

w

n

e x

A m o r e elegant, b u t less general, approach to this p r o b l e m was reported recently b y R i c h e y et al. (25), w h o p r e p a r e d a derivative o f 1 w i t h 4 - ( l pyrenebutyl) e n d groups ( P y A T ) . Viscosity studies suggest that P y A T a n d 1 with R = C H have e n d groups o f comparable hydrophobicity. W i t h P y groups serving as the h y d r o p h o b i c substituents, fluorescence experiments report directly u p o n association w i t h o u t complications due to p r o b e partition i n g between phases. I n very dilute solution, P y A T forms a small amount o f excimer (concentration-independent) because o f self-cyclization. A t higher concentrations I / i increases w i t h c due to intermolecular end-group association. T h e crossover for P y A T ( M = 60,000; M / M = 2) occurs at c = 5 p p m . T h i s value is i n the same range as that i n f e r r e d for o u r samples shown i n F i g u r e 12. T h e most important i n f o r m a t i o n about these polymers is their aggrega tion n u m b e r , the n u m b e r o f c h a i n ends p e r M L C . W e have as yet n o information about w h e t h e r a closed-association m o d e l , w i t h a narrow distri b u t i o n o f ends groups p e r M L C , or an open-association m o d e l , involving aggregates whose size depends u p o n end-group concentration, provides a better description o f association i n this system. 1

2

2

5

E

M

w

w

n


502

STRUCTURE-PROPERTY RELATIONS IN POLYMERS 350

300

·= 1 0

250 200

h 150

c

01 Downloaded by CORNELL UNIV on July 18, 2016 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/ba-1993-0236.ch018

A

h 100

-1.5 log C

-0.5 ( g/L,

polymer)

Figure 12. Plot of the fluorescence intensity (lower two curves) and mean fluorescence decay times (upper two curves) as a function of polymer concentrations for 1 (R = C H ). Key: open symbols, M = 40,000; and closed symbols, M = 47,000. 16

33

w

w

T o begin, m e a n aggregation numbers, N must b e d e t e r m i n e d . R i c h e y et al. ( 2 5 ) approached this p r o b l e m b y a d d i n g i n c r e m e n t a l amounts o f P y A T to solutions o f R M - 8 2 5 , k e e p i n g the total c fixed at 0.02 w t % . T h i s approach enhances the probability o f finding two o r m o r e P y p e r micelle, w h i c h s h o u l d lead to excimer emission, as opposed to m o n o m e r emission f r o m M L C containing o n l y one P y . I n this way the authors ( 2 5 ) calculated N = 6 e n d groups p e r m i c e l l e f r o m analysis o f their I /I data. T h i s n u m b e r depends u p o n t h e unfortunate assumption that m o n o m e r a n d excimer have identical q u a n t u m yields a n d that an excimer w i l l b e f o r m e d whenever t w o pyrenes occupy o n e M L C . W e n o w k n o w the second assumption is n o t correct (26). n

n

E

M

W e also used P y excimer emission to determine m e a n aggregation n u m b e r s . I n the course o f o u r experiments w i t h pyrene as a probe (24), w e n o t i c e d that over a l i m i t e d range o f p o l y m e r concentration, P y excimer emission c o u l d b e observed. T h i s observation is i n many ways remarkable. W i t h the b u l k P y concentration h e l d at 6 X 1 0 ~ M , a strong partitioning o f P y into a relatively small n u m b e r o f micellelike aggregates w o u l d b e neces sary to achieve local concentrations h i g h e n o u g h f o r excimers to b e f o r m e d . A plot o f I /I versus l o g c is shown i n F i g u r e 13. T h e m a x i m u m i n this plot can easily b e understood: at l o w c most o f the P y is i n the aqueous phase. M L C ' s c o m p o s e d o f e n d groups f o r m as c increases, a n d P y partitions b e t w e e n the micellar a n d aqueous phases. A t h i g h c values most o f the P y is located i n the M C L ' s , b u t because the n u m b e r o f M C L ' s vastly exceeds the n u m b e r o f P y molecules, the probability o f finding t w o P y i n t h e same micelle is negligible, a n d I drops to zero. 7

E

M

E


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503



0.12

0.00 H -3.5

•

1 -2.5

»

1 -1.5

log

—

c

1 -0.5

·

ι 0.5

i

1 1.5

( g/L, polymer)

Figure 13. The intensity ratio l%/l as a function of polymer concentration for the two samples of 1 described in Figure 12. M

T h e m a x i m u m i n the ί / ί versus c plot s h o u l d occur at the point at w h i c h t h e n u m b e r o f M L C ' s is h a l f the n u m b e r o f P y molecules. F r o m this assumption a n d knowledge o f the molecular weight o f the polymers, m e a n aggregation numbers can b e d e t e r m i n e d . These refer to the average n u m b e r o f c h a i n ends p e r M L C . W e find N = 15 f o r the sample w i t h the lower molecular weight a n d N = 2 5 f o r the sample w i t h the higher molecular weight. These numbers are small c o m p a r e d to those f o r n o r m a l nonionic surfactants, a n d somewhat larger than that calculated b y R i c h e y et a l . f o r their p o l y m e r at m u c h higher p o l y m e r concentration. Ε

Μ

n

n

T h e s e types o f experiments n o w n e e d to b e extended to a broader range o f p o l y m e r concentrations, e n d groups, a n d c h a i n lengths. I n addition, m o r e reliable M values are n e e d e d f o r the A T s to calculate the aggregation numbers. Values d e t e r m i n e d b y size exclusion chromatography alone d e p e n d u p o n c o l u m n calibration a n d other factors that make the three aggregation numbers reported here only estimates o f their true values. In summary, the essential features o f associative thickener behavior i n water are first, that the e n d groups associate to f o r m micellelike aggregates; second, the onset o f association occurs i n the p a r t s - p e r - m i l l i o n concentration range; a n d , finally, the m e a n aggregation numbers are m u c h smaller than those o f corresponding alkyl P E O n o n i o n i c surfactants. n


504


Summary Water-soluble p o l y m e r containing h y d r o p h o b i c substituents f o r m micellelike clusters i n water. F o r rigid cellulosic polymers, the association is interpolym e r i c w i t h little evidence for intramolecular association o f the substituents. F o r m o r e flexible linear polymers, the nature o f the interaction can d e p e n d u p o n chain microstructure a n d the location o f the h y d r o p h o b i c substituents. Those at the chain ends, as i n C - P E O ~ C , undergo strong interpolymeric associations to f o r m micellelike clusters c o m p o s e d of a small n u m b e r o f e n d groups (6 to 25). O n l y at p a r t s - p e r - m i l l i o n concentrations is there a d o m i nance o f intramolecular end-group interactions. n

n

I n the P N I P A M containing a r a n d o m distribution o f C substituents, h y d r o p h o b i c association seems to be intramolecular i n origin. N o detectable association is seen w h e n the chains are short (e.g., C ) , but p r o n o u n c e d association is seen for longer chains (e.g., C a n d C ) . These micellelike aggregates are m o r e rigid, a n d presumably smaller i n size, than traditional surfactant micelles. U p o n heating solutions o f hydrophobically m o d i f i e d P N I P A M a n d H P C above their respective L C S T s , the micelles are disrupted. P E O - P S a n d P E O - P S - P E O b l o c k copolymers behave quite differently. I f these polymers have a n a r r o w M W D , they undergo a sharp association transition (a true C M C ) w i t h increasing concentration. T h e y f o r m spherical micelles w i t h a very n a r r o w distribution o f sizes. T h e i r sizes a n d N values seem to be w e l l described b y the star m o d e l o f b l o c k c o p o l y m e r micelles.


n

1 0

1 4

1 8

n

Acknowledgments W e thank the N a t i o n a l Science a n d E n g i n e e r i n g Research C o u n c i l o f C a n a d a and the Province o f O n t a r i o for their financial contributions i n support o f this research. W e acknowledge the contributions o f J. V e n z m e r a n d H . R i n g s d o r f to part o f this w o r k .

References 1. Polymers in Aqueous Media: Performance through Association; Glass, J. E . , E d . ; Advances in Chemistry 223; American Chemical Society: Washington, D C , 1989. 2. (a) Emmons, W . D . ; Stevens, T. E . ; "Polyurethane Thickeners in Latex Composi tions", U.S. Patent N o . 4, 079, 028, 1978. (b) Hoy, K . L . ; Hoy, R. C . "Polymers with Hydrophobe Bunches", U.S. Patent N o . 4, 426, 485, 1984. 3. (a) Tuzar, Z.; Kratochvil, P. Adv. Colloid Interface Sci. 1976, 6, 201. (b) Tuzar, Z.; Kratochvil, P. Colloids Surf., in press. 4. Riess, G . ; Hurtrez, G.; Bahadur, P. Encyclopedia of Polymer Science and Engineering; 2nd ed.; Wiley: New York, 1985; Vol. 2, pp 324-434. 5. Winnik, F . M. Macromolecules 1987, 20, 2745. 6. Winnik, F . M.; Winnik, Μ. Α.; Tazuke, S.; Ober, C . K. Macromolecules 1987, 20, 38. ;



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7. Winnik, F . M. Macromolecules 1989, 22, 734; Winnik, F . M.; Tamai, N.; Yonezawa, J.; Nishimura, Y.; Yamazaki, J. J. Phys. Chem. 1992, 96, 1967. 8. Berlman, I. B. Energy Transfer Parameters of Aromatic Molecules; Academic Press: Orlando, FL, 1981. 9. Winnik, F. M. Macromolecules 1990, 23, 233. 10. Schild, H. G . ; Tirrell, D . A. Polym. Prepr. Am. Chem. Soc. Div. Polym. Chem. 1989, 30(2), 350. 11. Fujishige, S. Polym. J. 1987, 19, 297. 12. Winnik, F. M. Polymer, 1990, 31, 2125. 13. Ringsdorf, H.; Venzmer, J.; Winnik, F. M. Macromolecules 1991, 24, 1678. 14. Winnik, F. M.; Winnik, Μ. Α.; Ringsdorf, H.; Venzmer, J. J. Phys. Chem. 1991, 95, 2583. 15. Kalyanasundaram, K.; Thomas, J. K . J. Am. Chem. Soc. 1977, 99, 2039. 16. Dong, D . C.; Winnik, M. A. Can. J. Chem. 1985, 62, 2560. 17. Georgescauld, D.; Desmasez, J. P.; Lapouyade, R.; Babeau, Α.; Richard, H.; Winnik, M. A. Photochem. Photobiol. 1980, 31, 539. 18. Zachariasse, Κ. Α.; Vaz, W. L. C.; Sotomayor, C.; Kühnle, W . Biochem. Biophys. Acta 1982, 688, 323. 19. Xu, R.; Winnik, Μ. Α.; Riess, G.; Croucher, M. D.; C h u , Β. Macromolecules 1992, 25, 644. 20. Zhao, C.. L.; Winnik, Μ. Α.; Riess, G.; Croucher, M. D. Langmuir 1990, 6, 514. 21. Wilhelm, M.; Zhao, C. L.; Wang, Y.; Xu, H.; Winnik, Μ. Α.; Riess, G.; Mura, L.; Croucher, M. D . Macromolecules 1991, 24, 1033. 22. Xu, R. L.; Winnik, Μ. Α.; Hallet, F . R.; Riess, G.; Croucher, M. D . Macro molecules 1991, 24, 87. 23. Sivadasan, K.; Somasundaran, P. Colloid Surf. 1990, 49, 229. 24. Wang, Y.; Winnik, M. A. Langmuir 1990, 6, 1437. 25. Riehey, B.; Kirk, A. B.; Eisenhart, E. K.; Fitzwater, S.; Hook, J. W . J. Coatings Technol. 1991, 63(798), 31. RECEIVED 1992.

f o r review M a y 22, 1991.

ACCEPTED

revised manuscript June 1,


Fluorescence Studies of Polymer Association in Water - American

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