Structure-Property Relations in Polymers - American Chemical Society

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11 Film-Air and Film-Substrate Interfaces of Latex Films Monitored by Fourier Transform Infrared Spectroscopy Timothy A. Thorstenson, Kevin W. Evanson, and Marek W. Urban* Department of Polymers and Coatings, North Dakota State University, Fargo, N D 58105

The mobility of surfactants within latex films composed of ethyl acrylate and methacrylic acid is monitored via attenuated total reflectance-Fourier transform infrared spectroscopy. Ionic surfactants are found to exhibit a degree of incompatibility with the acrylic copolymer that results in their exudation to the film interfaces. Interfacial surface tension and elongation enhance exudation of surfactant to the interfaces. Neutralization of the copolymer acid functionality, however, inhibits the surfactant exudation. This inhibition is attributed to increased surfactant-copolymer compatibility that results from the formation of surfactant-copolymer complexes that become buried in the film during coalescene. In contrast to these observations, lattices prepared with a nonionic surfactant exhibit no exudation, which is attributed to a greater degree of surfactant-copolymer compatibility.

S U R F A C T A N T S PLAY A VITAL R O L E IN L A T E X T E C H N O L O G Y , b o t h i n the synthe

sis o f the latex a n d as postsynthesis additives. T h e postsynthesis additives have a major influence o n the stabilization o f the latex against coagulation, the modification o f rheological properties, a n d the dispersion a n d stabilization o f pigments. D e s p i t e their utility, these l o w m o l e c u l a r weight species c a n also * Corresponding author. 0065-2393/93/0236-0305$07.75/0 © 1993 American Chemical Society

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

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STRUCTURE-PROPERTY RELATIONS IN POLYMERS


give rise to a host o f undesirable properties. I f the surfactant is incompatible w i t h the p o l y m e r system, it can exude to the latex film interfaces, w h i c h results i n optical defects, premature degradation, or a loss o f adhesion. T h u s , s t r u c t u r e - p r o p e r t y relationships i n lattices are critical. A l t h o u g h the importance o f surfactants a n d the potential problems associated w i t h t h e i r use are w e l l established, relatively little w o r k has b e e n done w i t h regard to s u r f a c t a n t - p o l y m e r compatibility a n d surfactant d y n a m ics w i t h i n latex films. A series o f s t y r e n e - b u t a d i e n e ( S B ) lattices p r e p a r e d w i t h n o n y l p h e n o l ethylene oxide surfactants w e r e examined via electron microscopy ( I ) . T h e compatibility o f the surfactants w i t h the nonpolar S B lattices was f o u n d to decrease w i t h the increasing polarity o f surfactant. O n the other hand, i n the more polar v i n y l a c e t a t e - v i n y l acrylate lattices p r e p a r e d w i t h the same nonionic surfactants, surfactant adsorption increased w i t h the higher polarity o f the latex due to greater v i n y l acetate content ( 2 ) . Apparently, the hydrolysis o f v i n y l acetate groups to f o r m p o l y ( v i n y l alcohol) provides O H functionality w i t h w h i c h the polyether oxygens o f the surfactant interact. T h i s m e c h a n i s m is w h y the formation o f p o l y m e r - s u r f a c t a n t c o m plexes may affect c o p o l y m e r - s u r f a c t a n t compatibility a n d the classification o f various surfactants into penetrating a n d nonpenetrating types may be v a l i d (3). T h i s classification was p r o p o s e d (4) for a series o f v i n y l a c e t a t e - v i n y l acrylate lattices, w h i c h exhibit increasing c o p o l y m e r polarity that results i n i n d u c e d penetration o f anionic surfactants into the latex particles. These assessments were based o n viscosity studies, a n d the effect was attributed to the formation o f polyelectrolyte-type s o l u b i l i z e d p o l y m e r - s u r f a c t a n t c o m plexes. A p p a r e n t l y , the p o l y m e r chains u n c o i l , w h i c h results i n acetyl groups b e i n g p u s h e d into the aqueous phase, w h e r e surfactant molecules readily adsorb o n the p o l y m e r chains; this leads to the increased solubility o f these segments (3). A s expected, penetration o f the surfactant d e p e n d e d o n a critical size, the charge density at the p o l y m e r - w a t e r interface, and a shape conducive to penetration. I n the case o f n o n i o n i c surfactants, the surfactant concentration was f o u n d to decrease w i t h increasing polarity while, at the same t i m e , i n h i b i t i n g penetration b y the anionic surfactants. T h e presence o f interfacial surface tension at the p o l y m e r - w a t e r inter face may affect the degree o f surfactant adsorption o n various p o l y m e r surfaces (4, 5). T h e surface area p e r molecule o f s o d i u m dodecyl sulfate ( S D S ) surfactant o n a latex p o l y m e r particle increases w i t h the increasing polarity o f the p o l y m e r - w a t e r interface. T h e d r i v i n g force for the adsorption o f surfactant at various p o l y m e r - w a t e r interfaces was suggested to be related to the differences i n the interaction energy b e t w e e n the surfactant molecules a n d the surface i n question. O n a similar note, it is reasonable to expect that the surface free energy o f the substrate m a y influence the distribution o f surfactant w i t h i n the latex film. B r a d f o r d a n d V a n d e r h o f f ( 6 ) p r e p a r e d s t y r e n e - b u t a d i e n e c o p o l y m e r latex films o n a variety o f substrates i n c l u d i n g poly(tetrafluoroethylene)



11.

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and Film-Substrate

Interfaces

307

( P T F E ) , poly(ethylene terephthalate) ( M y l a r polyester), rubber, a n d m e r c u r y and examined the film-air a n d film-substrate interfaces v i a electron m i croscopy. A l t h o u g h differences w e r e observed for the film-substrate inter faces, they were assessed as b e i n g too small for further analysis a n d consider ation. It was c o n c l u d e d that the surfactant exudation a n d film formation behavior at the film-substrate interface was closely parallel to that at the film-air interface. H o w e v e r , as a recent study suggests, this is not the case ( 7 ) . Z h a o et al. (8, 9 ) e m p l o y e d attenuated total r e f l e c t a n c e - F o u r i e r trans f o r m i n f r a r e d spectroscopy, ( A T R - F T I R ) X - r a y photoelectron spectroscopy ( X P S ) , a n d secondary-ion mass spectrometry ( S I M S ) to examine the film-air and film-substrate interfaces o f b u t y l a c r y l a t e - m e t h l methacrylate lattices p r e p a r e d using the anionic surfactants s o d i u m d o d e c y l sulfate ( S D S ) a n d s o d i u m d o d e c y l d i p h e n y l disulfonate ( S D E D ) . I n this case, the latex films exhibited enrichment at b o t h the film-air a n d film-substrate interfaces. T h e extent o f e n r i c h m e n t was f o u n d to be dependent o n the nature o f the surfactant, the coalescence t i m e , the global concentration o f the surfactant, and the interface involved; the film-air interface showed a greater degree o f enrichment. S D S was f o u n d to f o r m a thick boundary layer at the filmsubstrate interface. T h i s " w e a k b o u n d a r y l a y e r " (9), w h i c h causes major adhesion problems, was attributed to a lesser degree o f compatibility w i t h the copolymer. Surfactant e n r i c h m e n t was attributed to three factors: (1) initial enrichment at b o t h interfaces to lower interfacial free energy, (2) e n r i c h m e n t at the film-air interface due to the transport o f nonadsorbed surfactant b y the water flux out o f the film, a n d (3) longer t e r m migration to b o t h interfaces due to surfactant incompatibility. T h e molecular level interactions governing surfactant behavior w e r e not addressed.

Surfactant-Copolymer Interactions A l t h o u g h the previous studies p r o v i d e d some insight into the factors govern ing surfactant compatibility a n d surfactant exudation behavior, they generally were confined to the study o f either a specific series o f lattices or a specific series o f surfactants and, as such, it is difficult to assess the ultimate c h e m i c a l factors that govern s u r f a c t a n t - p o l y m e r interactions. E v a n s o n a n d U r b a n ( 1 0 ) began a systematic examination o f s u r f a c t a n t - c o p o l y m e r interactions w i t h the study o f an ethyl acrylate-methacrylic a c i d ( E A - M A A ) latex p r e p a r e d w i t h sodium dioctyl sulfosuccinate ( S D O S S ) surfactant v i a attenuated total re flectance ( A T R ) a n d photoacoustic ( P A ) F o u r i e r transform i n f r a r e d ( F T I R ) spectroscopy. Surfactant behavior at the film-substrate interface was m o n i t o r e d d u r i n g coalescence v i a circle c e l l A T R - F T I R spectroscopy. F i g u r e 1 shows the 1 0 7 0 - 9 6 0 - c m ~ region o f the spectra r e c o r d e d d u r i n g coales cence. A l t h o u g h the spectrum a c q u i r e d 5 m i n after deposition o f the latex 1


308


1046

/

\


/y/\\ c^x

\ \

FILM-SUBSTRATE INTERFACE 1050

\ \ \

1020

990

960

Wavenumbers (cm"" ) 1

Figure 1. Circle ATR-FTIR spectra in the 1070-960-cm~ region collected as a function of time for the EA-MAA latex as it coalesces: A , 5 min; B, 30 min; and C, 4 h. (Reproduced with permission from reference 10. Copyright 1991 Wiley.) 1

film (trace A ) exhibits surfactant enrichment, as e v i d e n c e d b y the shoulder at 1046 c m

- 1

attributed to t h e symmetric S - O stretching m o d e o f S 0 , t h e 3

intensity o f this b a n d decreases w i t h time a n d is no longer present after 4 h o f coalescence. A s shown i n F i g u r e 2, t h e p r e c e d i n g observation parallels t h e intensity decrease o f the b r o a d b a n d at 3400 c m ~

1

I n contrast to these results, the spectrum o f the

due to water. film-air

interface o f the

fully coalesced latex film shown i n trace A o f F i g u r e 3 indicates surfactant e n r i c h m e n t to this interface, as demonstrated b y the presence o f the bands at 1046 a n d 1056 c m

- 1

, also attributed to t h e symmetric stretching m o d e o f

S 0 . T e m p o r a r i l y postponing the evaluation o f these bands u n t i l later, it was 3

f o u n d that t h e excess surfactant c o u l d b e r e m o v e d f r o m t h e surface o f the film b y washing w i t h dilute aqueous methanol. T h e A T R - F T I R spectrum o f a surface-washed sample is shown i n trace C o f F i g u r e 3, a n d the bands due to surfactant at 1046 a n d 1056 c m

- 1

are clearly absent. O n the basis o f these

results it was c o n c l u d e d that although there is an initial surfactant e n r i c h m e n t at t h e

film-substrate

interface, this excess o f water-soluble surfactant is

carried to the film-air interface b y the water flux that passes between the


11.

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and Film-Substrate

309

Interfaces


3400 I 3250

3800

3500

3200

2900

2600

Wavenumbers (cm~1) Figure 2. Circle ATR-FTIR spectra in the 3800-2600-cm ~ region collected as a function of time for the EA-MAA latex as it coalesces: A, 5 min; B, 30 min; and C, 4 h. (Reproduced with permission from reference 10. Copyright 1991 Wiley.) 1

latex particles d u r i n g coalescence, a n d the final coalesced latex film exhibits surfactant enrichment only at the film-air interface. It is apparent f r o m t h e p r e c e d i n g results that t h e spectra o f t h e film-substrate interface o f the latex film revealed a single b a n d d u e to t h e symmetric S - O stretch o f S 0 located at 1046 c m , w h i c h diminishes i n intensity as coalescence proceeds. T h e film-air interface, however, exhibited two bands located at 1046 a n d 1056 c m " . I n contrast to these results, the transmission F T I R spectrum o f neat S D O S S surfactant s h o w e d only a single b a n d centered at 1050 c m " . T h e s e observations indicate that t h e S - O stretching m o d e is sensitive to the local environment changes a r o u n d the sulfonate group that alter local symmetry i n the latex-water environment. - 1

3

1

1

I n a n effort t o determine the p r i m a r y causes o f these local symmetry changes, spectra o f solutions o f S D O S S i n b o t h H 0 a n d E t O H w e r e acquired, after solvent evaporation, via transmission F T I R spectroscopy. T h e 1 0 5 0 - c m " region o f the spectra (with solvent contributions subtracted out) are shown i n F i g u r e 4. I n t h e case o f the E t O H solution (trace B ) , S D O S S exhibits the anticipated b a n d due to the symmetric stretching m o d e o f S 0 at 2

1

3



310


1110

1080

1050

990

1020

Wave η umbers (cm

1

960

)

Figure 3. Circle ATR-FTIR spectra of EA-MAA latex in the 1130-950-cm~ region: A , Film-air interface; B, film-substrate interface; and C, film-air interface washed with MeOH-DDI H O solution. (Reproduced with permission from reference 10. Copyright 1991 Wiley.) 1

s

1050 c m . T h e aqueous solution, however, shows a b a n d at 1046 c m (trace A ) , w h i c h indicates that the b a n d at 1046 c m i n the latex spectrum is a result o f hydration o f the highly h y d r o p h i l i c sulfonate group. A close analysis o f transmission F T I R temperature study data ( F i g u r e 5) revealed that although there are no intensity changes o f the acid d i m e r b a n d at 1703 c m o n going f r o m 25 to 165 °C, the b a n d at 1765 c m , w h i c h is due to " f r e e " earboxylie acid, increases. T h i s observation indicates that only a fraction o f the h y d r o g e n - b o n d e d acid species is i n v o l v e d i n hydrogen b o n d i n g interactions w i t h other earboxylie a c i d species. T h i s observation was further substantiated b y the decrease o f the 1 7 3 5 - c m b a n d i n the same tempera ture range. T h e intensity decrease o f this b a n d i n attributed to the b r e a k i n g o f interactions, w h i c h results i n free carbonyl groups o f the acid functionality (11-13). I n essence, these results indicate the presence o f S O --ΗΟ interactions i n the latex system. T h i s assessment was further substantiated b y the results o f experiments i n w h i c h the latex a c i d functionality was e l i m i n a t e d either t h r o u g h synthesis or v i a the a d d i t i o n o f aqueous base. F i g u r e 6 shows the A T R - F T I R spectrum o f an " a c i d - f r e e " ethyl a c r y l a t e - m e t h y l methacry- 1

- 1

- 1

- 1

- 1

- 1

s



11.

THORSTENSON

E T AL.

311

Film—Air and Film—Substrate Interfaces

1100

950 Wavenumbers (cm""* ) 1

Figure 4. Transmission FTIR spectra of SDOSS after being dissolved in H 0 (A) and ethanol (B), followed by solvent evaporation. (Reproduced with permission from reference 10. Copyright 1991 Wiley.) 2

late ( E A - M M A ) latex along w i t h t h e spectrum o f t h e a c i d functional E A - M M A latex. A l t h o u g h the E A - M M A latex (trace B ) exhibits the familiar sphtting at 1046 a n d 1056 c m , t h e acid-free E A - M M A latex (trace A ) shows only a single b a n d centered at 1050 c m . T h u s , i t is the simultaneous presence o f b o t h t h e surfactant sulfonate group o f S D O S S a n d t h e a c i d functionality o f the E A - M M A copolymer that is responsible f o r t h e split t i n g o f the 1 0 5 0 - c m S - O stretching vibration to t w o bands at 1046 a n d 1056 c m . - 1

- 1

- 1

- 1

F u r t h e r insight into the interactions between t h e S O group o f S D O S S a n d the acid functionality o f the latex is gained b y considering the results o f an experiment i n w h i c h t h e acid functionality o f the latex suspension was neutralized v i a t h e addition o f aqueous base. T h i s neutralization process destroys the hydrogen b o n d - d o n a t i n g - O H groups present i n the latex, thus allowing the assessment o f the nature o f interactions between surfactant S 0 groups a n d C O O " functionality o f the neutralized copolymer. T o investigate the effect o f different cations o n the observed frequency shift, t w o different bases ( N a O H a n d N H O H ) were e m p l o y e d i n the neutralization procedure. F i g u r e 7a shows t h e 1 8 0 0 - 1 5 2 0 - c m region o f the spectra o f neutralized latex films. H e r e , neutralization is c o n f i r m e d b y a b r o a d b a n d d u e to t h e s

3

4

- 1



312

STRUCTURE-PROPERTY

1840

1780

1720

RELATIONS IN POLYMERS

1540

1660


Figure 5. Maximum-entropy restored spectra at various temperatures for the EA-MAA latex in the 1860-1530-cm' region: A, 25 °C; B, 105 °C; and D , 165 °C. (Reproduced with permission from reference 10. Copyright 1991 Wiley.) 1

C - O stretching m o d e o f the carboxylate salt group observed at 1590 c m for the latex neutralized w i t h N a O H (trace B ) a n d at 1570 c m for the latex neutralized w i t h N H O H (trace C ) . E x a m i n a t i o n o f the 1 0 5 0 - c m region ( F i g u r e 7b) shows that although t h e spectrum o f the nonneutralized film (trace A ) exhibits the familiar splitting at 1046 a n d 1056 c m , the neutral i z e d films b o t h exhibit only a single b a n d centered at 1046 c m . T h i s experiment confirms that the 1056 b a n d is due to S - O · · · H - O interactions between the copolymer acid functionality a n d the surfactant sulfonate groups. F u r t h e r m o r e , the b a n d at 1046 c m attributed to t h e S - O symmetric stretching m o d e o f hydrated S 0 3 ~ N a i o n pairs is insensitive to t h e ionic strength o f the neutralizing agent used. - 1

- 1

- 1

4

- 1

- 1

-

1

+

Effect of Surfactant Structure on Surfactant Mobility T h e effect o f surfactant structure was examined (14) v i a the preparation o f E A - M A A lattices using a variety o f surfactants i n c l u d i n g s o d i u m dioctyl sulfosuccinate ( S D O S S ) , s o d i u m dodecylbenzene sulfonate ( S D B S ) , s o d i u m dodecyl sulfate ( S D S ) , s o d i u m n o n y l p h e n o l ethylene oxide sulfonate (2 ethylene oxide units; S N P 2 S ) , a n d the nonionic surfactant n o n y l p h e n o l ethylene oxide (40 ethylene oxide units; N P 4 0 ) . T h e structures o f t h e



11.

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and Film-Substrate

313

Interfaces

Wavenumbers (cm" ) 1

Figure 6. ATR spectra in the 1110-950-cm~~ region: A, EA-MAA latex; B, EA-MMA latex; and C, SDOSS surfactant. (Reproduced with permission from reference 10. Copyright 1991 Wiley.) 1

surfactants are shown i n C h a r t I. T o identify potential spectral features that may be used to identify the presence o f surfactant at the latex film interfaces, it is first necessary to identify the relevant spectral features o f the c o p o l y m e r a n d surfactants. F i g u r e 8 shows the 1 8 0 0 - 5 0 0 - e m "

region to the spectra o f

1

the c o p o l y m e r a n d surfactants, whereas T a b l e I fists the tentative b a n d assignments f o r these species. E x a m i n a t i o n o f F i g u r e 8 shows that, i n t h e case o f the anionic surfactants, all spectra exhibit characteristic absorbance bands i n the regions o f 1 2 5 0 - 1 1 5 0 ( S - O asymmetric stretching), 1 0 6 0 - 1 0 4 5 (S-O

symmetric stretching), a n d 7 5 0 - 5 5 0 c m

-

1

( S - O bending). A s is

demonstrated i n F i g u r e 8, the latter region w i l l b e particularly useful because it is essentially free o f interfering absorbance bands due to the copolymer. I n the case o f t h e n o n i o n i c N P 4 0 surfactant, identification c a n b e readily facilitated b y m o n i t o r i n g the b a n d at 947 c m " " d u e to the C H ~ 0 stretching 1

2

m o d e o f the polyether units o f this surfactant. I n a n effort to investigate t h e effect o f surfactant structure o n sur factant exudation behavior, films o f the lattices synthesized w i t h the various surfactants were p r e p a r e d o n a poly (tetrafluoroethylene)

substrate a n d

examined via rectangular A T R - F T I R spectroscopy. F i g u r e 9 a shows t h e


314


1 1 3 5 - 5 0 0 - c m region o f the spectra o f the film-air interfaces o f these latex films. T h e S D O S S latex (trace A ) exhibits slight enrichment, as demonstrated b y the presence o f bands at 1046 a n d 1056 (symmetric S - O stretch o f S O ) , 652 ( S - O b e n d i n g m o d e o f S 0 ) , a n d 581 c m " ( S O scissors.) Similarly, the S D B S latex (trace B ) exhibits weak bands at 614 a n d 5 8 3 c m " due to t h e S - O b e n d i n g a n d S 0 scissor vibrational modes o f this surfactant, respec tively. T h e S D S latex (trace D ) also shows slight surfactant e n r i c h m e n t at the film-air interface as shown b y the weak bands at 631 a n d 5 8 5 c m " ( S - O b e n d i n g modes o f S 0 ) . I n contrast, t h e S N P 2 S latex (trace C ) has none o f the characteristic surfactant bands, w h i c h indicates that there is no surfactant enrichment at t h e film-air interface. T h e latex p r e p a r e d w i t h the n o n i o n i c surfactant N P 4 0 also shows a trace o f surfactant at t h e film-air interface (trace Ε o f F i g u r e 9a), w h i c h is e v i d e n c e d b y the presence o f a weak b a n d at 947 c m " that has b e e n assigned to t h e C H - 0 (ether) stretch o f this surfactant. T h e spectra o f the film-substrate interfaces o f the same latex films are presented i n F i g u r e 9b. T h e spectrum o f the S D O S S latex (trace A ) exhibits - 1

a

1

3

a

1

2


1

4

1

2

(a)

l/Au ^ III \ i \ II 11 \ /I/ 11 \ \\

e

c

* °^ Neutralization:

* ^- ^ 'y* B. Latex w/NaOH. C. Latex w/NH 0H

A

a

ex

on

4

\\ \

tseo 1 5 7 0

1

\\

1^— C.

^

1820

1760

1700

^ "

1640

!

'

1580

ι

ι

ι

1520

Wave η umbers (cm " ) 1

Figure 7a. ATR-FTIR spectra in the 1800-1520-cm~ region of A, EA-MAA latex; B, EA-MAA latex neutralized with NaOH; and C, EA-MAA latex neutralized with NH OH. (Reproduced with permission from reference 10. Copynght 1991 Wiley.) 1

4


11.

THORSTENSON ET AL.

Film-Air

and Film-Substrate

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315

(b)

1046 \

/

\

i\

1056/

EA/MM Latex Neutralized with:


Λ \\ ι \ \

\

A. Latex Only

\

B. NaOH

NH 0H

\ C .

1100

'

4

'

950


Figure 7b. ATR-FTIR spectra in the 1100-950-cm~ region of A, EA-MAA latex; B, EA-MAA latex neutralized with NaOH; and C, EA-MAA latex neutralized with NH OH. (Reproduced with permission from reference 10. Copyright 1991 Wiley.) 1

4

the same characteristic absorbance bands due to surfactant that w e r e o b served i n the spectrum o f the film-air interface. F u r t h e r m o r e , t h e intensity o f these bands is similar at b o t h interfaces, w h i c h indicates that a similar concentration o f S D O S S is present at b o t h interfaces. A greater intensity o f the bands at 616 a n d 583 c m i n the spectrum o f the S D B S latex (trace B ) indicates a somewhat higher surfactant concentration at t h e film-substrate interface. I n addition, the b a n d at 1046 c m due to t h e symmetric S - O stretch o f S 0 becomes m o r e p r o n o u n c e d at this interface. T h e S N P 2 S latex (trace C ) , w h i c h exhibited no surfactant e n r i c h m e n t to the film-air interface, shows considerable exudation to the film-substrate interface as illustrated by the intense bands at 1056 a n d 614 c m (symmetric S - O stretching a n d S - O b e n d i n g modes o f S 0 , respectively). T h e S D S latex (trace D ) also exhibits significant surfactant exudation t o the film-substrate interface, again demonstrated b y the intense bands due to the S - O b e n d i n g m o d e o f S 0 at 631 a n d 5 8 5 c m . Similarly to t h e spectrum at t h e film-air interface, t h e N P 4 0 latex (trace E ) shows a trace o f surfactant at t h e film-substrate interface as e v i d e n c e d b y t h e weak b a n d at 9 4 7 c m . I n this case i t was - 1

- 1

3

- 1

3

4

- 1

- 1


316


Sodlui Oioctyl Sulfoeuccinate 0 C H -0-i—CH 8

1 7

2

O^H_SO -N. 3

C H 8

1 7

(SDOSS)

+

_JB-i/


Sodlui Dodecyl Benzene Sulfonate

(SOBS) Sodlui Nonylphenol Ethylene Oxide

(SNP2S)

r—\ Sulfonate . , CgH —/Q\—0CH CH 0CH CH -S0 Na t

19

2

2

2

2

^

3

Sodlui Dodecyl Sulfate

(SOS)

C H -0-S0 'Na • 1 2

2 5

3

Nonylphenol Ethylene Oxide (40 units) C

9 i9 H

— stretch ( S 0 ) asym. C - O - C stretch sym. C - O - C stretch sym. S - O stretch ( S 0 ) sym. S - O stretch ( S 0 ) C - O - C (ester) = C - H in-plane deformation C H - 0 (ether) = C - H out-of-plane S - O - C stretch ester skeletal vibration -(CH ) -(n>3) C H out-of-plane S - O bending ( S 0 ) S - O bending ( S 0 ) S 0 scissor alkyl chain skeletal vibrations 3

4

3

2

2

n

3

4

2

Reproduced with permission from reference 14. Copyright 1991 Wiley.

the previous spectra at 947 c m . Because this b a n d is almost the same intensity as that observed for the film-air interface o f the nonneutrafized latex film ( F i g u r e 9a, trace E ) , it appears that, i n the case o f the n o n i o n i c surfactant, the surfactant concentration at the film-air interface is not af fected b y neutralization. - 1

T h e spectra o f the film-substrate interfaces o f the n e u t r a l i z e d latex films are presented i n F i g u r e 10b. A t this interface, none o f the lattices p r e p a r e d w i t h anionic surfactants exhibits characteristic surfactant bands. Apparently,


11.

THORSTENSON E T AL.

Film-Air

and Film-Substrate

Interfaces

319

surfactant exudation to this interface is also i n h i b i t e d . A s was observed i n the previous spectra, the N P 4 0 latex again shows the presence o f a surfactant b a n d at 947 c m ~ w i t h a similar intensity to that seen previously. It appears that the neutralization process again does not exert any significant effect o n the distribution o f this n o n i o n i c surfactant. 1

A l t h o u g h t h e spectra o f the ionic surfactants i n F i g u r e 10b d o n o t exhibit characteristic surfactant bands, a l l spectra o f the film-substrate interfaces show that bands at 635 a n d 521 c m , w h i c h are attributed to the wagging vibrations o f the l o n g chain α-methyl carboxylate salts f o r m e d u p o n neutral ization o f the c o p o l y m e r a c i d functionality. T h e spectra presented i n F i g u r e 10a a n d b also indicate that these bands are detected o n l y i n the film-sub strate interface spectra. T h i s observation is likely attributed to the fact that the salt groups are quite h y d r o p h i l i c a n d this provides a driving force f o r migration a n d orientation t o w a r d the film-substrate interface w h e r e , d u r i n g coalescence, the aqueous phase remains f o r the longest p e r i o d o f t i m e .


- 1

B a s e d o n these results i t is apparent that, i n the case o f the i o n i c surfactants, neutralization results i n the i n h i b i t i o n o f surfactant exudation. I n

520 Wavenumber (cm—1) (a) Figure 9. ATR-FTIR spectra in the 1135-500-cm~ region recorded at the film-air interface (a) and at the film-substrate interface (b) of latex films prepared on PTFE: A, SDOSS latex; B, SDBS latex; C , SNP2S latex; D , SDS; and Ε, NP latex. (Reproduced with permission from reference 14. Copyright 1991 Wiley.) Continued on next page. 1


320

STRUCTURE-PROPERTY



1046

ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι 1 120 1020 920 820 720 620 520

Wavenumber (cm—1) (b) Figure 9. Continued. (Reproduced with permission from reference 14. Copyright 1991 Wiley.)

the nonneutralized lattices, the ionic surfactants are incompatible w i t h t h e c o p o l y m e r environment, a n d exudation to the interfaces o f the latex films occurs. H e r e , the c o p o l y m e r environment is quite nonpolar o w i n g to the fact that it is c o m p o s e d p r i m a r i l y o f ethyl acrylate. I n comparison, the ionic surfactants, o w i n g to their negatively charged sulfate o r sulfonate heads, are highly polar a n d h y d r o p h i l i c . T h i s m a y lead to a certain inherent degree o f incompatibility, so is is possible that d u r i n g coalescence these surfactants assemble i n the interstices o f an inhomogeneous p o l y m e r matrix. M o r e o v e r , the ionic surfactants have relatively l o w molecular weight and, i n comparison w i t h the nonionic N P 4 0 , they are quite water soluble. T h e c o m b i n a t i o n o f these factors makes their diffusion to the film interfaces easy i f there exists a d r i v i n g force f o r t h e m to d o so. W h e n the c o p o l y m e r a c i d functionality is neutralized, however, the c o p o l y m e r becomes m u c h m o r e polar d u e to the formation o f carboxylate salt groups. T h i s increased hydrophificity causes the latex particles to swell: their hydrodynamic v o l u m e increases a n d c h a i n extension occurs. A s this process continues, the anionic surfactants m a y b e c o m e displaced f r o m the surface o f the latex particles a n d reside i n the aqueous phase. H o w e v e r , as chain extension proceeds further, h y d r o p h o b i c ethyl groups pendant o n the copolymer chains are f o r c e d out into the


11.

Film-Air

THORSTENSON E T AL.

and Film-Substrate

Interfaces

321


aqueous phase a n d water diffuses into the latex particles. T h e water-soluble anionic surfactants may then penetrate into the latex particles and associate w i t h t h e m through h y d r o p h o b i c interactions, w h i c h leads to the formation o f solubilized p o l y m e r - s u r f a c t a n t complexes that then b e c o m e b u r i e d w i t h i n the latex film as coalescence proceeds. Additionally, ionic interactions b e tween the carboxlate salt groups and the surfactant sulfate o f sulfonate groups may further enhance compatibility.

Effect of Substrate Surface Tension on Surfactant Mobility A s was m e n t i o n e d earlier, another factor that m a y influence surfactant m o b i l i t y is the surface tension o f the substrate. T o establish i f i n h i b i t i o n o f the exudation o f t h e ionic surfactants d u e to neutralization was substrate related, films o f the neutralized lattices were p r e p a r e d o n glass a n d m e r c u r y substrates. F i g u r e 11 shows the 1 1 3 5 - 4 8 0 - c m region o f the spectra ac q u i r e d at the film-substrate interfaces o f the films allowed to coalesce o n glass, P T F E , a n d m e r c u r y substrates. I n this case, the S D O S S surfactant was _ 1

A. B. C. D. Ε.

rA

/

/A

/ U^^JEy

U

^

^

J

Y

Λ I

\

\

\

1080

631

585

Λ

947

ι

ι ι ι ι ι I I I l I

SDOSS Latex SDBS Latex SNP2S Latex SDS Latex NP-40 Latex

/

I

\

I I I I I I I I I I I I I I I l I I I l I I

980 880 780 680 Wavenumber (cm—1)

580

480

(a) Figure 10. ATR-FTIR spectra in the 1135~480-cm~ region recorded at the film-air interface (a) and at the film-substrate interface (b) of neutralized latex films prepared on PTFE: A, SDOSS latex; B, SDBS latex; C, SNP2S latex; D, SDS latex; and Ε, NP latex. (Reproduced with permission from reference 14. Copyright 1991 Wiley.) Continued on next page. 1



322

S T R U C T U R E - P R O P E R T Y RELATIONS IN

POLYMERS

used. A l l three spectra exhibited similar features a n d s h o w e d no signs o f absorbance bands characteristic o f the S D O S S surfactant. Similar behavior was observed for the other lattices. These data indicate that the i n h i b i t i o n o f surfactant exudation to the film-substrate interface observed u p o n neutraliza tion is not substrate related. I n an effort to determine i f the observed i n h i b i t i o n of exudation following neutralization is i n f l u e n c e d b y mechanical strain, films o f the neutralized lattices were subjected to elongations o f u p to 5 0 % a n d then examined using rectangular A T R - F T I R spectroscopy. F i g u r e 12 shows the 1 1 3 5 - 4 8 0 - c m region o f spectra a c q u i r e d f r o m the films o f the neutralized S D B S latex that w e r e subjected to mechanical elongation o f 0, 30, a n d 5 0 % (traces A , B , a n d C , respectively). A s can be seen, no significant spectral changes are observed as the degree o f elongation is increased. Similar results w e r e obtained w i t h the other lattices. A s w i l l be shown later o n , elongation o f the nonneutrafized latex films can result i n significant surfactant exudation, so it appears that neutralization o f the copolymer acid functionality results i n enhanced compatibility between the surfactant and the copolymer. - 1

T h e effects o f substrate surface tension a n d elongational strain are, however, m u c h m o r e p r o n o u n c e d for nonneutrafized lattices (15). L e t us



11.

Film-Air

THORSTENSON ET AL.

11 / /

Interfaces

323

621

\\

λ

\\

^·

Λ Λ—ν 635 7 Α \ / \ \j

IV

Β.

7α\

c.

/ \y

V 1100

and Film-Substrate

1000

900

/ " Λ

800

\

700

600

500

Woven umbers (cm"'' ) Figure 11. ATR-FTIR spectra in the 1135-480-cm region recorded at the film-substrate interface of neutralized SDOSS latex films prepared on different substrates: A, glass; B, PTFE; and C, mercury (Eg). (Reproduced with permission from reference 14. Copyright 1991 Wiley.) 1

again consider the results obtained for the film-substrate interfaces o f latex films p r e p a r e d o n a P T F E substrate. T h e spectra are shown i n F i g u r e 9 a a n d b . H e r e , all o f the anionic surfactants except S D O S S exhibit a greater degree o f exudation to t h e film-substrate interface. T h i s behavior was very p r o n o u n c e d i n t h e cases o f S D S a n d S N P 2 S , whereas t h e S D B S latex showed only slight preferential enrichment to this interface. A l t h o u g h the migration o f surfactant to this interface may b e accounted f o r i n terms o f the water solubility o f the surfactants, w h i c h provides a d r i v i n g force for migration to the film-substrate interface where the aqueous phase is present f o r t h e longest p e r i o d o f t i m e , it is necessary to also consider the effects o f substrate surface tension. Because P T F E has a l o w surface tension (15 m N / m ) , deposition o f a film o f latex o n the P T F E substrate creates a h i g h degree o f interfacial tension at the l a t e x - P T F E interface, a n d this may provide a d r i v i n g force for the migration o f surfactant molecules to this interface so as to reduce the interfacial tension. T o establish i f the interfacial surface tension does i n d e e d provide a d r i v i n g force f o r surfactant exudation to the film-substrate interface, latex films were p r e p a r e d directly o n t h e thallous b r o m i d e iodine ( K R S - 5 ) A T R element. K R S - 5 has a surface tension similar to that o f glass (approximately



324

STRUCTURE-PROPERTY

1100

1000

900

800

700

R E L A T I O N S IN

600

POLYMERS

500

Wave η umbers ( c m " 1 ) Figure 12. ATR-FTIR spectra in the 1135-480-cm~ film-substrate interface of the neutralized SDBS latex film elongation: A, 0% elongation; B, 30% elongation; and C, (Reproduced with permission from reference 14. Copyright 1

region for the as a function of 50% elongation. 1991 Wiley.)

70 m N / m ) , a n d this higher substrate surface tension w i l l result i n a lower degree o f interfacial tension between the latex a n d the substrate, thus r e d u c i n g the driving force for the migration o f surfactant to this interface. F i g u r e 13 shows A T R - F T I R spectra acquired at the film-substrate interfaces o f these films. A s can be seen, the S D O S S a n d S N P 2 S lattices (traces A a n d C ) exhibit no previously detected characteristic surfactant bands. T h e S D B S a n d S D S films, however, do show some enrichment to this interface, b u t a comparison w i t h the results observed for the P T F E substrate ( F i g u r e 9b) indicates that the degree o f e n r i c h m e n t is significantly smaller than that for the K R S - 5 substrate. I n contrast to these spectral changes, the latex p r e p a r e d w i t h the n o n i o n i c N P 4 0 surfactant shows a familiar b a n d at 947 c m ( C H - 0 stretching m o d e o f the surfactant ether units) that is about the same intensity as that observed for the latex deposited o n P T F E substrate. - 1

2

T o further investigate the observed effects of substrate surface tension, latex films were p r e p a r e d o n a l i q u i d m e r c u r y substrate. A T R - F T I R spectra a c q u i r e d at the film-substrate interfaces of these films are shown i n F i g u r e 14. A s shown b y the e n h a n c e d intensity o f the bands at 1046 a n d 1056 c m (symmetric S - O stretching m o d e o f S O ) , the S D O S S latex (trace A ) reveals - 1

a



11.

THORSTENSON E T AL.

ι

ι

1100

r

Film-Air

t

i

and Film-Substrate

i

1

1

I

.

325

Interfaces

.

.

(

950 800 650 Wavenumbera (cm-1)

500

Figure 13. ATR-FTIR spectra in the region from 1150 to 500 cm recorded at the film-substrate interface of latex films prepared on glass: A, SDOSS; B, SDBS; C, SNP2S; D, SDS, and Ε, NP. (Reproduced with permission from reference 15. Copyright 1991 Wiley.) 1

considerable

exudation to t h e

film-substrate

interface.

Additionally, t h e

bands at 652 ( S - O b e n d i n g mode) a n d 5 8 1 c m " ( S - O scissors) are 1

observed. T h e S D B S latex (trace B ) also shows significant surfactant e n r i c h ment to the m e r c u r y interface, as demonstrated b y the b a n d at 616 c m " d u e 1

to the S - O b e n d i n g m o d e o f S O . Similar results are seen f o r the S D S latex a

(trace D ) , as shown b y the intense bands at 631 a n d 585 c m "

1

( S - O bending

mode of S 0 ) . 4

I n contrast to the ionic surfactants just m e n t i o n e d , t h e S N P 2 S latex spectrum (trace C ) exhibits a degree o f enrichment, as seen b y the intensity o f the S - O b e n d i n g m o d e at 614 c m

- 1

, that is less than that observed i n the

case o f the P T F E substrate. I n this case, however, a t h i n , c l o u d y film was observed o n the surface o f the m e r c u r y after removal o f the coalesced latex film. Analysis o f this material using transmission F T I R spectroscopy revealed it to b e c o m p o s e d almost exclusively o f S N P 2 S surfactant. T h u s , the degree o f enrichment to the

film-substrate

interface is greater than was indicated b y

the A T R - F T I R spectrum.



326

STRUCTURE-PROPERTY

»

ι

1050

1

1

1

900

1

1

1

750

ι

1

ι

600

RELATIONS IN

r

r

POLYMERS

Γ

450

Wavenumberg ( c m * " ) 1

Figure 14. ATR-FTIR spectra in the region from 1150 to 500 cm' recorded at the film-substrate interface of the latex films prepared on mercury: A, SDOSS; B, SDBS; C, SNP2S; D, SDS; and Ε, NP. (Reproduced with permission from reference 15. Copynght 1991 Wiley.) 1

A g a i n i n contrast w i t h the p r e c e d i n g results, the nonionic N P 4 0 exhibits no surfactant enrichment to the m e r c u r y interface ( F i g u r e 14, trace E ) . T h i s spectrum does, however, exhibit a b r o a d b a n d at 938 c m attributed to the O H ··· Ο out-of-plane deformation vibrations o f the earboxylie acid groups. F u r t h e r m o r e , the bands at 664, 600, and 575 c m assigned to the O - C O in-plane vibration o f α-branched aliphatic earboxylie acids are observed. These features are also observed for the S D O S S latex (trace A ) . E x a m i n a t i o n of the carbonyl region o f this spectrum (not shown) reveals that a large portion o f the acid groups present at the film-substrate interface are involved i n hydrogen b o n d i n g interactions w i t h the surfactant a n d other acid groups. - 1

- 1

I n the case o f the incompatible anionic surfactants, the substrate e m p l o y e d i n film preparation exerts a significant influence o n the degree o f surfactant enrichment that w i l l be observed at the film-substrate interface o f the latex films. T h i s p h e n o m e n o n can be understood i n terms of the surface tension differences between the c o p o l y m e r a n d various substrates. I n the case o f P T F E , w h i c h has a very l o w surface free energy o f 18.5 m N / m , a considerable driving force exists f o r t h e migration o f surfactant t o t h e film-substrate interface to lower the h i g h interfacial surface tension present


11.

THORSTENSON E T AL.

Film-Air

and Film-Substrate

327

Interfaces

there. I n the case o f the glass substrate, w i t h a surface free energy approxi mately 70 m N / m , the p o l y m e r (surface free energy approximately 30 m N / m ) may readily wet t h e glass substrate a n d thus, the d r i v i n g force f o r surfactant migration to this interface is significantly r e d u c e d . T h e very h i g h surface tension (416 m N / m ) o f the m e r c u r y substrate allows the p o l y m e r to initially wet the surface, b u t once coalescence

begins, a solid latex

film-liquid

m e r c u r y interface exists, a n d this leads to a h i g h interfacial tension that gives


rise to a higher d r i v i n g force f o r surfactant enrichment. T h i s hypothesis c a n also explain the presence o f acid d i m e r species that are observed i n the case o f the N P 4 0 latex film. A s the aforementioned results show, the degree o f compatibility b e t w e e n the N P 4 0 surfactant a n d the E A - M A A c o p o l y m e r is m u c h greater than that observed f o r t h e anionic surfactants. D u e to this factor, it is possible that the driving force for exudation p r o d u c e d b y the surface free energy o f m e r c u r y substrate is insufficient to i n d u c e surfactant migration to the

film-substrate

interface.

Because the interfacial tension is not r e d u c e d b y the assembly o f surfactant at this interface, the c o p o l y m e r may respond b y orientation o f its acid function ality toward the mercury. T h e acid groups, w h i c h have significantly greater polarity than the other species, may increase the surface free energy o f the copolymer

film-substrate

interface, a n d thus reduce the interfacial excess o f

energy present at the solid latex

film-liquid

m e r c u r y interface.

Similar

conclusions c a n b e tentatively d r a w n f o r the S D O S S latex. D u e to the presence o f two h y d r o p h o b i c tails, it is possible that this surfactant is unable to p r o p e r l y align itself at the c o p o l y m e r - m e r c u r y interface so as to effectively reduce the interfacial surface tension. A g a i n , the orientation o f acid-func tional species t o w a r d this interface appears to be a p r i m a r y consideration.

Effect of Elongation on Surfactant Mobility A s was i n d i c a t e d i n the previous sections, the m o b i l i t y o f surfactants i n neutralized latex films is insensitive to mechanical strain. I n contrast, i t was f o u n d ( 1 5 ) that considerable surfactant exudation m a y result u p o n the elongation o f nonneutrafized latex films. I n a n effort to examine surfactant mobility u p o n elongation, latex films w e r e p r e p a r e d o n a P T F E substrate. A f t e r coalescence, the films were r e m o v e d a n d w a s h e d w i t h dilute aqueous m e t h a n o l to remove any surfactant that may have assembled at the interfaces d u r i n g coalescence. A f t e r washing, the films were elongated to the stated degree, h e l d at this elongation f o r 5 m i n , a n d t h e n allowed to relax. T h e spectral results for the S D O S S latex are shown i n F i g u r e 15. A t 1 0 % elongation (trace A ) , the appearance o f a weak b a n d at 529 c m

- 1

(alkyl c h a i n

vibration) is detected. A s the degree o f elongation increases, however, additional characteristic bands, i n c l u d i n g 1046 a n d 1056 (symmetric S - O


328

STRUCTURE-PROPERTY

stretch), 653 ( S - O b e n d i n g mode), and 583 c m

R E L A T I O N S IN

(S0

- 1

scissors),

2

POLYMERS

become

evident a n d increase considerably f r o m 30 to 5 0 % elongation (traces Β a n d C , respectively). S i m i l a r behavior is observed for the S D B S latex ( F i g u r e 16) a n d the S N P 2 S latex ( F i g u r e 17). I n the case o f the S D S latex ( F i g u r e 18), characteristic surfactant bands at 631 a n d 585 c m

- 1

( S - O bending mode of S 0 ) 4

increase w i t h elongation. H o w e v e r , a b a n d at 921 c m

are also observed a n d - 1

is observed at 5 0 %


elongation (trace C ) . T h i s b a n d is assigned to the bisulfate i o n , a n d its presence is not surprising because S D S is k n o w n to hydrolyze d u r i n g the e m u l s i o n polymerization process to f o r m s o d i u m bisulfate a n d dodecanol (16). T h e exudation o f u n h y d r o l y z e d surfactant is c o n f i r m e d b y examination o f the C - H stretching region o f these spectra ( F i g u r e 19). T h e intensity o f the surfactant bands at 2956, 2919, a n d 2852 c m

increases w i t h the degree

- 1

o f elongation. T h i s observation indicates the exudation o f u n h y d r o l y z e d surfactant that results f r o m elongation. T h e exudation o f anionic surfactants that accompanies

film

elongation

may also be addressed i n terms o f surface tension. W h e n the latex film is elongated, the total surface area o f the film is increased a n d the concentration

ι—η

1100

1

ι

ι

950

1

1

ι

800

1

1

1 — τ 1

650

ι

500

Figure 15. ATR-FTIR spectra in the region from 1135 to 500 cm' of the SDOSS latex films recorded as a function of percent elongation: A, 10%; R, 30%; and C, 50%. (Reproduced with permission from reference 15. Copyright 1991 Wiley.) 1


THORSTENSON E T AL.

Film-Air

and Film-Substrate

329

Interfaces


11.

1100

950

800

650

500

Figure 17. ATR-FTIR spectra in the region from 1135 to 500 cm' of the SNP2S latex films recorded as a function of percent elongation: A , 10%; B, 30%; and C, 50%. (Reproduced with permission from reference 15. Copynght 1991 Wiley.) 1



330

STRUCTURE-PROPERTY

r

1

ι

1

1100

r—ι

ι

950

1

1

1

800

1

1


r

1

650

500

Figure 18. ATR-FTIR spectra in the region from 1135 to 500 cm" of the SDS latex films recorded as a function of percent elongation: A, 10%; B, 30%; and C, 50%. (Reproduced with permission from reference 15. Copyright 1991 Wiley.) 1

I

3150

1

I

!

1

3000

I I

1

2850

""I

1

2700

ι

ι

«

2550

Wavenumbera (cm-1)

Figure 19. ATR-FTIR spectra in the region from 3150 to 2550 cm' of the SDS latex films recorded as a function of percent elongation: A , 10%; C, 30%; C, 50%; and D, SDS only. (Reproduced with permission from reference 15. Copyright 1991 Wiley.) 1


11.

THORSTENSON

E T AL.

Film-Air

and Film-Substrate

Interfaces

331

of surfactant at the interfaces o f the film is therefore decreased. T h i s results i n a h i g h e r surface tension at the latex film interfaces, a n d thus provides the necessary d r i v i n g force for the exudation o f surfactant; the c o m b i n a t i o n o f c o p o l y m e r - s u r f a c t a n t i n c o m p a t i b i l i t y a n d l o w m o l e c u l a r weight makes f o r ready diffusion to the interfaces o f the elongated film. A similar p h e n o m e n o n is observed for plasticizers i n thermoplastic polymers ( 1 7 ) . I n contrast to the anionic surfactants, films o f the N P 4 0 latex show no exudation u p o n elonga


t i o n (not shown). H e r e , the greater compatibility a n d the h i g h e r m o l e c u l a r weight o f this surfactant again o v e r w h e l m the d r i v i n g force for surfactant exudation.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Vanderhoff, J. W. Br. Polym. J. 1970, 2, 161-173. Vijayendran, B. R.; Bone, T. L.; Gajria, C. J. Appl. Polym. Sci. 1981, 26, 1351. Arai, H.; Horin, S. J. Colloid Interface Sci. 1969, 30, 372. Vijayendran, B. R.; Bone, T.; Gajria, C. J. Appl. Polym. Sci. 1981, 26, 1351-1359. Vijayendran, B. R. J. Appl. Polym. Sci. 1979, 23, 733-742. Bradford, E. B.; Vanderhoff, J. W. J. Macromol. Sci. Phys. 1972, B6(4), 671-694. Urban, M . W.; Evanson, K. W. Polym. Commun. 1990, 31, 279. Zhao, C. L.; Holl, Y.; Pith, T.; Lambla, M . Colloid Polym. Sci. 1987, 265, 823-829. Zhao, C. L.; Holl, Y.; Pith, T.; Lambla, M . Br. Polym. J. 1989, 21, 155-160. Evanson, K. W.; Urban, M . W. J. Appl. Polym. Sci. 1991, 42, 2287-2296. Lee, Y. J.; Painter, P. C.; Coleman, M . M . Macromolecules 1988, 21, 346-354. Lee, Y. J.; Painter, P. C.; Coleman, M . M . Macromolecules 1988, 21, 954-960. Lichkus, A. M . ; Painter, P. C.; Coleman, M . M . Macromolecules 1988, 21, 2636-2641. Evanson, K. W.; Thorstenson, Τ. Α.; Urban, M . W. J. Appl. Polym. Sci. 1991, 42, 2297-2307. Evanson, K. W.; Urban, M . W. J. Appl. Polym. Sci. 1991, 42, 2309-2320. Nakagaki, M.; Yokoyama, S. Bull. Chem. Soc. Jpn. 1986, 59, 935-936. Ludwig, B.; Urban, M . W. Polymer 1992, 33(16), 3343.

RECEIVED for review July 15, 1991. ACCEPTED revised manuscript September 9, 1992.


Structure-Property Relations in Polymers - American Chemical Society

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