Viscosity Studies of Hydrophobically Modified (Hydroxyethyl)cellulose

6 Viscosity Studies of Hydrophobically Modified (Hydroxyethyl)cellulose Robert A . G e l m a n and H o w a r d G . Barth

Downloaded by UNIV OF AUCKLAND on May 3, 2015 | http://pubs.acs.org Publication Date: May 5, 1986 | doi: 10.1021/ba-1986-0213.ch006

Hercules, Inc., Research Center, W i l m i n g t o n , DE 19894

Complete dissolution of hydrophobically modified water-soluble polymers is sometimes difficult owing to intermolecular interactions mediated by the hydrophobic substituent groups. Studies on hydrophobically modified (hydroxyethyl)cellulose demonstrated that the hydrophobic interactions must be completely disrupted by the solvent to obtain meaningful intrinsic viscosity data. A number of solvent systems containing either methanol or surfactants were considered. The results indicate that several systems are appropriate for use with these macromolecules. Constants for the Mark-Houwink equation for hydrophobically modified (hydroxyethyl)cellulose in 0.1% sodium oleate are reported.

T H E

CONCEPT O F POLYMERIC SURFACTANTS

t h e e a r l y 1950s (1,2).

w a s first c o n s i d e r e d i n

S i n c e that t i m e , a s u b s t a n t i a l b o d y o f l i t e r a t u r e has

d e v e l o p e d that d e a l s w i t h s u r f a c e - a c t i v e o r a m p h i p h i l i c p o l y m e r s , p a r t i c u l a r l y as r e l a t e d t o t h e c o n c e p t o f s t e r i c s t a b i l i z a t i o n (3). W a t e r - i n s o l u b l e p o l y m e r s y s t e m s h a v e b e e n r e p o r t e d (3) a n d a r e o f interest i n the s t a b i l i z a t i o n o f p a r t i c l e s i n n o n a q u e o u s m e d i a . S t u d i e s (4, 5) o f w a t e r - s o l u b l e p o l y m e r i c surfactants w e r e m a d e ; these surfactants a r e u s u a l l y b a s e d u p o n polyelectrolytes. H o w e v e r , f e w reports of nonionic water-soluble polymers containing hydrophobic

groups

exist. L a n d o l l (6)

reported

w o r k o n t h e a d d i t i o n o f l o n g - c h a i n a l k y l e p o x i d e s t o c e l l u l o s e ethers. T h e a d d i t i o n o f l o w l e v e l s o f h y d r o p h o b i c g r o u p s t o p o l y m e r s , s u c h as ( h y d r o x y e t h y l ) c e l l u l o s e ( H E C ) , results i n a l t e r a t i o n s o f t h e m a t e r i a l s u c h that t h e p o l y m e r s n o w possess u n u s u a l r h e o l o g i c a l p r o p e r t i e s (6). S o l u tions 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 ( h y d r o x y e t h y l ) c e l l u l o s e

(HM-HEC)

e x h i b i t n o n - N e w t o n i a n b e h a v i o r at l o w shear rates, w h e r e a s s o l u t i o n s o f HEC

are N e w t o n i a n under similar conditions. T h e c o n c e p t of associa-

tive thickeners, m o l e c u l e s that associate i n solution a n d e x h i b i t a vastly d i f f e r e n t r h e o l o g y , l e d t o i n c r e a s e d i n d u s t r i a l interest i n this a r e a . T h e u n i q u e p r o p e r t y o f H M - H E C is t h e e n h a n c e d s o l u t i o n v i s c o s i t y , a result o f intermolecular h y d r o p h o b i c interactions a m o n g a l k y l groups. 0065-2393/86/0213-0101 $06.00/0 © 1986 American Chemical Society

In Water-Soluble Polymers; Glass, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1986.

102

WATER-SOLUBLE POLYMERS

F o r e x a m p l e , H M - H E C c o n t a i n i n g 1.2S5 ( b y w e i g h t ) c e t y l g r o u p s has a B r o o k f i e l d v i s c o s i t y (2^ a q u e o u s s o l u t i o n ) o f 3 X 1 0 c P s as c o m p a r e d to 3 X 10 cPs for a H E C sample of similar molecular weight a n d h y d r o x y ethyl m o l a r substitution. 4

2

A l t h o u g h the h y d r o p h o b i c i n t e r a c t i o n s g i v e H M - H E C its u n i q u e


s o l u t i o n c h a r a c t e r i s t i c s , these f o r c e s m a k e i t r a t h e r d i f f i c u l t to d e t e r m i n e accurate intrinsic viscosities a n d m o l e c u l a r w e i g h t distributions. F u r t h e r m o r e , the s o l u b i l i t y o f H M - H E C is h i g h l y d e p e n d e n t o n b o t h the a m o u n t a n d c h a i n l e n g t h o f the a l k y l g r o u p s . S m a l l increases i n e i t h e r a m o u n t o r c h a i n l e n g t h c a n l e a d to p a r t i a l l y i n s o l u b l e p r o d u c t s . B r o o k f i e l d viscosity measurements are extremely valuable for det e r m i n i n g t h e s o l u t i o n p r o p e r t i e s o f H M - H E C f o r end-use e v a l u a t i o n . T h e s e m e a s u r e m e n t s r e f l e c t the extent o f h y d r o p h o b i c i n t e r m o l e c u l a r i n t e r a c t i o n s a n d , to a s l i g h t d e g r e e , m o l e c u l a r w e i g h t . H o w e v e r , w h e n the s o l u t i o n p r o p e r t i e s o f H M - H E C are c h a r a c t e r i z e d , i n f o r m a t i o n r e g a r d i n g the m o l e c u l a r s i z e o f c h a i n s is i m p o r t a n t . T h e s e d a t a c a n b e obtained f r o m intrinsic viscosity determinations. Intramolecular interact i o n s m a y o c c u r at l o w c o n c e n t r a t i o n , w h i c h w o u l d result i n a n u n d e r e s timation of m o l e c u l a r size. F u r t h e r m o r e , p o o r solubility of H M - H E C w i l l l e a d to e r r o n e o u s r e s u l t s . T h u s , the o b j e c t i v e o f o u r studies w a s to determine a suitable solvent system for measuring reliable intrinsic viscosities o f H M - H E C s a m p l e s .

Experimental Section The H M - H E C samples were prepared as previously reported (6). The chain length and level of the linear alkyl hydrophobic group are listed in Tables I-V. Viscosity data were determined with Brookfield and Ubbelohde viscometers at 25 °C. Intrinsic viscosity was measured by a five-point dilution procedure; no shear-rate corrections were made for these data. Solutions were made by first dissolving the polymer in water, usually overnight, and then adding the appropriate solvent to obtain the desired final solvent and polymer concentrations.

Results and Discussion Viscosity Studies in Aqueous Solutions. T h e s t a n d a r d m e t h o d o f d e t e r m i n i n g i n t r i n s i c v i s c o s i t i e s o f H E C is t o use w a t e r (7) o r d i l u t e salt s o l u t i o n (8) as the s o l v e n t . T a b l e I lists t h e r e s u l t o f i n t r i n s i c v i s c o s i t y d e t e r m i n a t i o n s m a d e b y u s i n g a series o f H M - H E C s a m p l e s i n d i l u t e salt s o l u t i o n ; these m a t e r i a l s w e r e p r e p a r e d s u c h that the m o l e c u l a r w e i g h t o f these s a m p l e s w o u l d b e s i m i l a r . T h e d a t a i n d i c a t e , h o w e v e r , that a p r o g r e s s i v e d e c r e a s e i n the i n t r i n s i c v i s c o s i t y o c c u r s w i t h b o t h the a m o u n t a n d c h a i n l e n g t h o f the substituent h y d r o p h o b e . I n t r a m o l e c u l a r a s s o c i a t i o n o f h y d r o p h o b i c g r o u p s , w h i c h t e n d s to f o r c e the p o l y m e r i n t o a m o r e c o m p a c t c o i l , c o n t r i b u t e s to the l o w e r v i s c o s i t y . I n a d d i t i o n , the s o l u t i o n c l a r i t y o f these s a m p l e s i n w a t e r v a r i e s f r o m c l e a r to h a z y . T h e


6.

G E L M A N A N D BARTH

Viscosities of Hydrophobically Modified HEC

103

Table I. Intrinsic Viscosity of H M - H E C Samples Using 0.02 M K H P 0 as the Solvent 2

4

Hydrophobe Sample Cl A B

Type

c

Ci6 Ci6 Ci6 Ci6

1.0 2.5 3.4 0.5 0.6 0.9 1.1


D E F G

c c c

8 8 8

M

0

c

c

C

Amount

h

9.28 8.75 8.28 4.12 7.55 6.24 3.78 3.31

?The amount is in units of weight percent of alkyl present. b [rj] is the intrinsic viscosity in 0.02 M K H P 0 , p H 5.5, at 25 °C. T h e control was unmodified H E C of the same furnish and preparation conditions as the H M - H E C samples. 2

4

c

s o l u b i l i t y c h a r a c t e r i s t i c s o f these s a m p l e s , as e x p e c t e d , a p p e a r to h a v e a p r o n o u n c e d e f f e c t o n i n t r i n s i c v i s c o s i t y results. I n r e l a t e d w o r k , w e f o u n d that t h e a d d i t i o n o f e i t h e r a l c o h o l o r s u r f a c t a n t t o H M - H E C solutions d i s r u p t s a s s o c i a t i o n b e t w e e n h y d r o p h o b i c groups. These components, particularly alcohols, are generally u s e d i n l i q u i d c h r o m a t o g r a p h y to m o d e r a t e h y d r o p h o b i c i n t e r a c t i o n s b e t w e e n solute a n d t h e s t a t i o n a r y p h a s e . A t h i g h c o n c e n t r a t i o n , a l c o h o l o r s u r f a c t a n t m o l e c u l e s c o m p e t e e f f e c t i v e l y f o r h y d r o p h o b i c sites o n the p o l y m e r ; this a c t i o n p r e v e n t s i n t r a - o r i n t e r m o l e c u l a r a s s o c i a t i o n . V i s c o s i t y Studies U s i n g M e t h a n o l . B R O O K F I E L D VISCOSITIES. T h e B r o o k f i e l d v i s c o s i t y o f H M - H E C i n a q u e o u s solutions is p r e d o m i n a n t l y a m e a s u r e o f i n t e r m o l e c u l a r i n t e r a c t i o n s . B e c a u s e o f the m o d e r a t e shear rates i n v o l v e d i n these m e a s u r e m e n t s , t h e r h e o l o g i c a l p r o p e r t i e s o f H M - H E C s o l u t i o n s also p l a y a r o l e . I n t e r m o l e c u l a r i n t e r a c t i o n s i n c r e a s e w i t h p o l y m e r c o n c e n t r a t i o n s , that i s , t h e h i g h e r t h e c o n c e n t r a t i o n o f h y d r o p h o b e s o r H M - H E C i n s o l u t i o n , t h e greater t h e p r o b a b i l i t y o f h y d r o p h o b e - h y d r o p h o b e contacts. T o test this h y p o t h e s i s t h e B r o o k f i e l d v i s c o s i t y o f H E C ( c o n t r o l ) , 0.60!? ( b y w e i g h t ) C w H M - H E C , a n d 0.75% C H M - H E C , w h e r e C is t h e l e n g t h o f t h e h y d r o p h o b i c s i d e c h a i n , w a s m e a s u r e d as a f u n c t i o n of p o l y m e r concentration. T h e t w o H M - H E C samples were prepared f r o m the c o n t r o l ; a l l three samples, therefore, h a d the same c h a i n l e n g t h a n d h y d r o x y e t h y l s u b s t i t u t i o n l e v e l . T h e results a r e s h o w n i n T a b l e I I . B r o o k f i e l d v i s c o s i t i e s o f b o t h H M - H E C s a m p l e s , as e x p e c t e d , r a p i d l y a p p r o a c h e d the v i s c o s i t y o f the c o n t r o l as the c o n c e n t r a t i o n w a s d e c r e a s e d . F o r these s a m p l e s , p o l y m e r c o n c e n t r a t i o n s o f 1

8


M

104

WATER-SOLUBLE POLYMERS

Table II. Effect of H M - H E C Concentration on Brookfield Viscosity Using Water as the Solvent Concn (%) HEC (Control) 0.6!I Cm HM-HEC 2.0 1.0 0.50 0.25 0.125

9.3 3.4 1.9 1.4 1.2

0.75% Cm HM-HEC 246.0 7.1 2.2 1.4 1.1

110.0 7.0 2.1 1.4 1.1


N O T E : The solvent viscosity was 0.9 cP.

w e r e s u f f i c i e n t l y l o w t o e l i m i n a t e p o l y m e r a s s o c i a t i o n . H o w e v e r , t h e B r o o k f i e l d v i s c o s i t i e s at these c o n c e n t r a t i o n s () a r e c l o s e to the s o l v e n t v i s c o s i t y . T h u s , m e a s u r e m e n t s a r e n o t v e r y a c c u r a t e at these p o l y m e r c o n c e n t r a t i o n l e v e l s , a n d the d a t a m u s t b e i n t e r p r e t e d w i t h caution. T h e B r o o k f i e l d v i s c o s i t i e s o f t h e c o n t r o l a n d t h e 0.602 C i H M - H E C s a m p l e w e r e d e t e r m i n e d i n 502 m e t h a n o l - w a t e r , a n d the results are listed i n T a b l e III. A s i n d i c a t e d , b o t h the c o n t r o l a n d H M - H E C h a d s i m i l a r v i s c o s i t i e s o v e r this c o n c e n t r a t i o n r a n g e . T h e h i g h e r viscosities e x h i b i t e d b y the c o n t r o l a n d b y the H M - H E C s a m p l e at

Viscosity Studies of Hydrophobically Modified (Hydroxyethyl)cellulose

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