Polymer Association Structures - American Chemical Society

Chapter 11

Lyotropic Mesophases of Cellulose in the Ammonia—Ammonium Thiocyanate Solvent System

Downloaded by FUDAN UNIV on April 15, 2017 | http://pubs.acs.org Publication Date: December 30, 1989 | doi: 10.1021/bk-1989-0384.ch011

Effects of System Composition on Phase Types Kap-Seung Yang, Michael H. Theil, and John A. Cuculo Fiber and Polymer Science Program and Department of Textile Chemistry, North Carolina State University, Raleigh, NC 27695-8302 Cellulose can be dissolved i n an NH /NH SCN solvent over a solvent composition range 70-80% (w/w) NH SCN. Beyond certain minimum cellulose concentrations, depending on solvent composition, l i q u i d c r y s t a l l i n e phases w i l l form. Cellulose having a 210 degree of polymerization w i l l form a mesophase at a concentration of 3.5% (w/v) at 25°C i n a solvent containing 70% NH SCN. The minimum cellulose concentration f o r mesophase formation was highest when the solvent contained 75.5% NH SCN. Under most conditions, these mesophases were the twisted cholesterics that typically develop from c h i r a l mesogens. Their h e l i c o i d a l pitch was higher i n solvents richer in NH SCN and at lower cellulose concentrations. Nematic phases, which can be construed as untwisted c h o l e s t e r i c phases, were easily prepared from solutions i n which the cellulose concentration range was 816% and the solvent was 24.5% NH and 75.5% NH SCN (w/w). Evidence suggests that the nematic phase forms when interchain cellulose interactions are suppressed. Fibers extruded from nematic solutions were more highly oriented, more fibrillar in texture and appreciably s t i f f e r than those from cholesteric solutions. The former had moduli comparable to that of Fortisan, a strong regenerated cellulose fiber. 3

4

4

4

4

4

3

4

Mesophases c o n t a i n i n g high molecular weight components have been known f o r many years. The e a r l i e s t s t u d i e d i n v o l v e d p o l y e l e c t r o l y t e s such as tobacco mosaic v i r u s (1). Approximately 30 years ago mesophases were observed 0097-6156/89/0384-0156$08.00/0 * 1989 American Chemical Society

El-Nokaly; Polymer Association Structures ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


11. YANG ET AL.

Lyotropic Mesophases of Cellulose in NH —NHj5CN 3

i n v o l v i n g t h e n o n i o n i c h e l i c a l p o l y m e r ροΙν-γ-benzyl-Lg l u t a m a t e (PBLG) (2,3). S h o r t l y t h e r e a f t e r , F l o r y (4,5) developed a l a t t i c e theory that described mesophase f o r m a t i o n i n a system o f s t i f f chains and low m o l e c u l a r weight solvent. Stimulated by the formation of ultrahighstrength/high-modulus f i b e r s from nematic s o l u t i o n s o f r i g i d m a c r o m o l e c u l e s (6-8), a g r e a t d e a l o f a t t e n t i o n h a s been focused on t h e s t r u c t u r e and p r o p e r t i e s of macromolecular mesophases. More recently such a n i s o t r o p i c s o l u t i o n s have a l s o been o b t a i n e d from s o l u t i o n s o f s e m i - r i g i d p o l y m e r s s u c h a s c e l l u l o s e (9-11) and c e l l u l o s e d e r i v a t i v e s (12). C o n c u r r e n t l y t h e r e h a s b e e n much a c t i v i t y i n t h e a r e a o f new s o l v e n t s f o r c e l l u l o s e (10,13,14,15). Hudson e t a l . r e p o r t e d that ammonia (NH )/ammonium t h i o c y a n a t e (NH SCN) m i x t u r e s t o be an e x c e l l e n t s o l v e n t s f o r c e l l u l o s e . They d e m o n s t r a t e d t h a t t h i s f a m i l y o f s o l v e n t s has s e v e r a l s i g n i f i c a n t p r a c t i c a l advantages i n c l u d i n g conveniently low vapor p r e s s u r e s a t room t e m p e r a t u r e (16). Cellulose i n N H 3 / N H 4 S C N does n o t a p p e a r t o d e g r a d e , a n d e s s e n t i a l l y no o t h e r c h e m i c a l r e a c t i o n between c e l l u l o s e a n d t h e s o l v e n t o c c u r s (17). I s o t r o p i c c e l l u l o s e s o l u t i o n s have b e e n wet spun t o produce f i b e r s w i t h p r o p e r t i e s c l o s e t o t h o s e o f conventional r a y o n ( L i u , C. Κ.; C u c u l o , J . A. s u b m i t t e d t o J. Polym. Sci., Polym. Chem. Ed.). Recently a c h o l e s t e r i c mesophase i n c l u d i n g . c e l l u l o s e was f o r m e d i n a s o l v e n t c o n s i s t i n g o f 2 7 % NH and 73% o f NH SCN b y w e i g h t (18) . A cholesteric structure consists of a set of q u a s i - n e m a t i c l a y e r s (19) whose i n d i v i d u a l d i r e c t o r s a r e t u r n e d t h r o u g h a f i x e d a n g l e f r o m one l a y e r t o t h e n e x t . The l a y e r s w h i c h a r e t u r n e d t h r o u g h 2π a r e e q u i v a l e n t , and the distance between t h e s e p a r t i c u l a r l a y e r s i s defined a s t h e p i t c h o f a hélicoïdal c h o l e s t e r i c structure. The c h i r a l i t y of the constituent molecules of a t h e r m o t r o p i c mesophase d e t e r m i n e s i t s c h o l e s t e r i c s e n s e . When e q u a l m o l e s o f m i r r o r image i s o m e r s a r e m i x e d (a r a c e m i c m i x t u r e ) , t h e t w i s t i n g power f a l l s t o z e r o a n d the cholesteric structure changes t o a compensated nematic one (20) . More s u r p r i s i n g i s t h a t t h e c h o l e s t e r i c n a t u r e o f a l y o t r o p i c m e s o p h a s e o f PBLG d e p e n d s on t h e n a t u r e o f t h e s o l v e n t . F o r e x a m p l e , PBLG mesophases i n c h l o r o f o r m and i n d i o x a n e a r e r i g h t handed c h o l e s t e r i c s w h i l e i n methylene c h l o r i d e and i n e t h y l e n e c h l o r i d e , t h e y a r e l e f t handed. In a mixture o f dioxane and methylene chloride (20% dioxane, ν/ν), the c h o l e s t e r i c s t r u c t u r e changed t o a compensated nematic structure (21). S a m u l s k i a n d S a m u l s k i (22) h a v e d i s c u s s e d t h e mechanism o f f o r m a t i o n o f t h e compensated n e m a t i c phase by c o n s i d e r i n g t h e magnitude o f t h e d i e l e c t r i c c o n s t a n t s o f PBLG a n d s o l v e n t molecules. T o r i u m i e t al.(23,24) a l s o r e p o r t e d t h a t t h e c o n d i t i o n s 3

4

3

4


157

POLYMER ASSOCIATION STRUCTURES

158

for f o r m a t i o n o f t h e compensated nematic phase was markedly i n f l u e n c e d by t e m p e r a t u r e and by t h e hydrogen bonds formed between t h e c a r b o n y l e s t e r g r o u p i n PBLG a n d XH-cresol. Similarly, t h e c u r r e n t work describes t r a n s f o r m a t i o n s o f l y o t r o p i c mesophases a s e f f e c t e d b y varying the r a t i o of N H 3 / N H 4 S C N , cellulose concentration, d e g r e e o f p o l y m e r i z a t i o n (DP) o f c e l l u l o s e , s t o r a g e t i m e , and t e m p e r a t u r e .


Experimental Materials. O r i g i n and c h a r a c t e r i s t i c s o f t h e c e l l u l o s e samples used a r e l i s t e d i n T a b l e I . The l o w m o l e c u l a r w e i g h t c e l l u l o s e s a m p l e A, was p r e p a r e d f r o m American E n k a r a y o n b y h y d r o l y s i s i n 4 M H C 1 f o r 15 m i n u t e s a n d t h e n washed w i t h an e l u o t r o p i c series of solvents. S h e e t s o f s a m p l e s C a n d D were s h r e d d e d i n a W i l e y M i l l (40 mesh) b e f o r e u s e . Sample B, w i t h degree of p o l y m e r i z a t i o n (DP) o f 2 1 0 , was u s e d i n t h i s s t u d y e x c e p t where n o t e d . Ammonium t h i o c y a n a t e ( W i t c o C h e m i c a l ) was d i s s o l v e d i n c o n d e n s e d a n h y d r o u s ammonia ( A i r P r o d u c t s ) . U n l e s s o t h e r w i s e n o t e d , a l l c h e m i c a l s were ACS r e a g e n t grade. The s o l v e n t c o m p o s i t i o n s c h o s e n f o r s t u d y were i n t h e r a n g e o f 70 t o 80% (w/w) N H 4 S C N ( 0 . 3 3 to 0 . 4 7 mole f r a c t i o n ) r e p o r t e d b y C u c u l o a n d Hudson (15) t o be g o o d solvents f o r cellulose. P r i o r t o use, a l l c e l l u l o s e s a m p l e s a n d t h e N H 4 S C N were d r i e d u n d e r vacuum f o r 4 h o u r s a t 8 0 C e x c e p t where n o t e d . Solvents r e l a t i v e l y r i c h e r i n N H 4 S C N were p r e p a r e d b y a d d i n g N H 4 S C N t o a stock s o l u t i o n . e

Table I. C h a r a c t e r i s t i c s o f C e l l u l o s e Samples I n v e s t i g a t e d Sample S u p p l i e r A A m e r i c a n Enka (hydrolyzed)

DP 35

DP d e t e r m i n a t i o n method *Gel permeation chromatography

Β

Whatman C h e m i c a l 210 L t d . ( c e l l u l o s e powder, CC41, m i c r o g r a n u l a r grade)

*ASTM m e t h o d ( D e s i g n a t i o n D1795-62) (25)

C

Avtex

450

*TAPPI method 230 os-76

D

ITT R a y o n i e r ( C e l l u n i e r Q, s u l f i t e pulp)

765

*Sedimentation coefficient of a n i t r a t e d sample

Fiber

*Determined by the supplier


11. YANG ET A L

159 Lyotropic Mesophases of Cellulose in NH ~NHjSCN 3

Solution Preparation. A known amount o f c e l l u l o s e was combined with N H 3 / N H 4 S C N s o l v e n t i n a t i g h t l y capped c e n t r i f u g e tube and was evenly d i s p e r s e d by use o f a Vortex Genie a g i t a t o r . The tube was then placed i n d r y i c e f o r 24 hours and subsequently warmed i n hot water (ca. 50 C) t o b r i n g about flow. The s o l u t i o n was then inspected under a p o l a r i z i n g microscope. Upon complete d i s s o l u t i o n of the c e l l u l o s e , the sample was r o u t i n e l y frozen and brought t o and maintained at 25.0±0.1*C. This ensured a s i m i l a r thermal h i s t o r y f o r each sample and e s t a b l i s h e d a p r e c i s e reference time frame. A l l s o l u t i o n concentrations are reported as percents (g o f c e l l u l o s e per lOOmL of N H 3 / N H 4 S C N ) . Downloaded by FUDAN UNIV on April 15, 2017 | http://pubs.acs.org Publication Date: December 30, 1989 | doi: 10.1021/bk-1989-0384.ch011

e

e

M i c r o s c o p i c a l Examination. Solutions were aged at 25 C. A p o r t i o n of each c e l l u l o s e s o l u t i o n was c a r e f u l l y placed between a microscope s l i d e and a cover s l i p and then examined between the crossed p o l a r i z e r s o f an Olympus microscope, Model BHSP. C h o l e s t e r i c p i t c h s i z e s were measured from photomicrographs showing the c h a r a c t e r i s t i c f i n g e r p r i n t pattern (3). He-Ne Laser Beam D i f f r a c t i o n . Samples were p l a c e d i n a r e c t a n g u l a r quartz c e l l (path l e n g t h 2 mm) . The r i n g e d d i f f r a c t i o n p a t t e r n o f a t r a n s m i t t e d He-Ne l a s e r beam (Métrologie, λ=632.8 nm) recorded on P o l a r o i d f i l m a l s o served t o i d e n t i f y the c h o l e s t e r i c structures. C h o l e s t e r i c p i t c h s i z e s were c a l c u l a t e d by applying the sample t o f i l m distance and diameter of the d i f f r a c t i o n r i n g t o the Bragg equation. They were f a i r l y c l o s e t o those found from photomicrographs. O p t i c a l Rotatory Power Measurement. C e l l u l o s e s o l u t i o n s were placed between microscope cover s l i p s separated by a P a r a f i l m spacer (thickness 0.127 mm). Optical rotatory measurements of the s o l u t i o n s were made with a P e r k i n Elmer 241 polarimeter using a sodium l i g h t source (598 nm) and a mercury l i g h t source (356,436,546, and 578 nm). The s p e c i f i c r o t a t i o n at a given wavelength of l i g h t [ Θ ] was c a l c u l a t e d from the observed r o t a t i o n θ by

[Θ] = — where 1 = c e l l l e n g t h i n dm and C = c o n c e n t r a t i o n sample s o l u t i o n i n g/mL.

(1) of

Cent r i fugat i o n . The volume f r a c t i o n o f the a n i s o t r o p i c phase Φ was determined by c e n t r i f u g a t i o n at 15,000 rpm (RCF=26,890) and 25.0±0.5*C i n a du Pont Sorval Model RC5 centrifuge. Separation of phases was i n d i c a t e d by the



160

appearance o f a v i s i b l e phase boundary. The p e r i o d o f c e n t r i f u g a t i o n t o assure complete s e p a r a t i o n o f t h e phases was determined v i s u a l l y . Prior to centrifugation, the c e n t r i f u g e tube was marked t o i n d i c a t e the t o t a l v o l ume o f the c e l l u l o s e s o l u t i o n before c e n t r i f u g a t i o n V . A f t e r c e n t r i f u g a t i o n , the supernatant ( i s o t r o p i c phase) was decanted. The tube with the a n i s o t r o p i c phase s t i l l present was then q u i c k l y f i l l e d t o the mark with f r e s h solvent. The solvent was then removed and i t s volume which corresponded t o the volume o f the i s o t r o p i c phase ^iso measured. The a n i s o t r o p i c volume V ± , ^ i s o , and Φ are i n t e r r e l a t e d by t

w

a

s


an

so

V

^aniso " V ~ iso t

φ =

2

< *> (2b)

Minimum C e l l u l o s e C o n c e n t r a t i o n f o r Mesophase Formation. The approximate value o f t h e minimum cellulose concentration f o r mesophase formation (MCCM) was v i s u a l l y determined by p r e l i m i n a r y experiments. F o r the p r e c i s e d e t e r m i n a t i o n o f t h e MCCM, a s e r i e s o f c e l l u l o s e s o l u t i o n s was p r e p a r e d i n c o n c e n t r a t i o n increments 0.5g/100mL i n t h e neighborhood o f t h e i n c i p i e n c e o f mesophase f o r m a t i o n . The MCCM were determined by a v e r a g i n g two c e l l u l o s e c o n c e n t r a t i o n s , t h e h i g h e s t concentration showing the i s o t r o p i c phase and the lowest c o n c e n t r a t i o n showing t h e a n i s o t r o p i c phase. The presence of the mesophase was not always immediately d i s c e r n i b l e near the MCCM. However, i t could be i d e n t i f i e d upon h i g h speed c e n t r i f u g a t i o n . Biphasic systems, c o n s i s t i n g o f c o e x i s t i n g i s o t r o p i c and a n i s o t r o p i c phases, formed i n the s o l u t i o n s prepared from s o l v e n t containing 70.0-74.0% N H 4 S C N were s e p a r a b l e by centrifugation. However, mesophase s o l u t i o n s from solvent c o n t a i n i n g over 75.0% N H 4 S C N were not. In these cases, polarizing microscopical o b s e r v a t i o n s were substituted for centrifugation. The MCCM were determined by the averaging procedure just described. F i b e r Formation. S o l u t i o n s were extruded from a wet s p i n n i n g apparatus ( B r a d f o r d U n i v . Research L t d . , equipped with a s i x hole spinneret, hole diameter 0.23 mm) by wet o r a i r - g a p s p i n n i n g . Methanol was t h e coagulant. A l t e r n a t i v e l y , f i b e r s were extruded from a syringe i n t o methanol through a 2.5 cm air-gap. Mechanical P r o p e r t i e s o f F i b e r . T e n a c i t i e s and i n i t i a l moduli were obtained f o r f i b e r samples u s i n g an Instron t e n s i l e t e s t e r Model TT-B. The extension rate was 2.54


11. YANG ET AL.

Lyotropic Mesophases ofCellulose in NH ~NHJSCN 3

cm/min and the gauge length was 2.54 cm. Measurements were made under s t a n d a r d conditions/ 25 "C and 65% r e l a t i v e humidity. Scanning

Electron

Microscopy

(SEM).

An

ISI-40

SEM

was

used t o take photomicrographs of c e l l u l o s e fibers f r a c t u r e d i n l i q u i d n i t r o g e n and coated with a g o l d palladium a l l o y . Results

and

Discussion


Microscopical

Observation.

The

cellulose/NH3/NH4SCN

mesophase s o l u t i o n s d i s p l a y e d s e v e r a l types of p a t t e r n s . The p a t t e r n s are v a r i o u s l y i n d i c a t i v e of c h o l e s t e r i c / nematic, conjugated, and aggregated a n i s o t r o p i c phases. Due to the twisted arrangement of t h e i r molecular l a y e r s , c h o l e s t e r i c mesophases normally show s t r i a t e d p a t t e r n s under a p o l a r i z i n g microscope. In this study, c h a r a c t e r i s t i c c h o l e s t e r i c phase p a t t e r n s i n c l u d e d ones resembling f i n g e r p r i n t s (Figures l a and b) , and banded s p h e r u l i t e s (Figure 4) . The nematic phase showed high b i r e f r i n g e n c e o r i g i n a t i n g from i t s u n i a x i a l m o l e c u l a r orientation. I t may a l s o d i s p l a y S c h l i e r e n (Figure 2a), t h r e a d - l i k e (Figure 2b), and uniformly d i s p e r s e d h i g h l y biréfringent (Figure l c ) p a t t e r n s . The term, conjugated patterns (Figures 3a,b and c) , i s used t o represent combinations of d i f f e r e n t t y p e s of p a t t e r n s . The aggregated a n i s o t r o p i c phase p a t t e r n (Figure 5) was formed i n the s o l u t i o n prepared from hydrolyzed c e l l u l o s e of DP 35. The occurrence of s p e c i f i c p a t t e r n depends upon s o l v e n t composition, cellulose concentration, storage time, c e l l u l o s e DP, p r i o r chemical treatment such as h y d r o l y s i s , and storage temperature. General Behavior of S o l u t i o n s . Several i n t e r e s t i n g observations were made when v a r y i n g the ratio of N H 3 / N H 4 S C N i n the c e l l u l o s e / N H 3 / N H 4 S C N system at polymer concentrations of 12, 14, and 16g/100mL. With i n c r e a s i n g N H 4 S C N concentration i n the s o l v e n t , the following p r o g r e s s i o n of changes i n appearance were noted: the p i t c h s i z e , as i n d i c a t e d by f i n g e r p r i n t p a t t e r n spacings, increased (from Figure l a to l b ) , i t then transformed to a uniformly d i s p e r s e d h i g h l y biréfringent s o l u t i o n ( F i g u r e l c ) , which f i n a l l y y i e l d e d t o a c o n j u g a t e d combination of s p h e r u l i t i c p a t t e r n s and uniformly dispersed high b i r e f r i n g e n c e (Figure Id). A f i n g e r p r i n t p a t t e r n was observed i n each mesophasic s o l u t i o n i n which the N H 4 S C N content i n the s o l v e n t was l e s s than approximately 7 5 . 5 % (w/w). The r e q u i r e d time for formation of a f i n g e r p r i n t p a t t e r n depended on solvent composition, cellulose concentration, DP of c e l l u l o s e and storage temperature. I t decreased w i t h decreasing N H 4 S C N c o n c e n t r a t i o n and i n c r e a s i n g c e l l u l o s e


161



Figure 1. Phase transformation with v a r i a t i o n of solvent composition: 27.0/73.0 (a) , 26.0/74.0 (b) , 24.5/75.5 ( c ) , 23.0/77.0 (d); cellulose c o n c e n t r a t i o n , 12g/100mL; DP, 210; storage time at 25 °C, 10 days; c and d with I r e d p l a t e ; +ψ , d i r e c t i o n of the highest r e f r a c t i v e index o f the 1° red p l a t e . e


Lyotropic Mesophases of Cellulose in NH ~NHJSCN 3


YANG ET AU

F i g u r e 2. Nematic phase p a t t e r n s formed i n t h e cellulose solutions prepared from solvent c o m p o s i t i o n o f 24.5/75.5: c e l l u l o s e c o n c e n t r a t i o n , 14g/100mL; DP, 450 (a a n d b) ; i m m e d i a t e l y after dissolution.




Figure 3a. Conjugated pattern: solvent composition/ 27.0/73.0; c e l l u l o s e concentration, 12g/100mL; DP, 210; s t o r a g e t i m e a t 2 5 C , 14 d a y s . e

Figure 3b&c. Conjugated patterns formed at r e l a t i v e l y high cellulose concentration: solvent composition, 24.5/75.5; c e l l u l o s e concentration, 18g/100mL (b), 20g/100mL ( c ) ; DP, 210; s t o r a g e t i m e a t 2 5 C , 10 d a y s , w i t h I red p l a t e ; ^ /direction of the highest refractive index o f t h e 1° r e d plate. e

e



YANG ET AL.

Lyotropic Mesophases ofCellulose in NH ~NHjSCN 3

Figure 4. Spherulitic pattern: solvent composition, 24.0/76.0; c e l l u l o s e concentration, 14g/100mL; DP, 210; s t o r a g e t i m e a t 2 5 C , 20 d a y s . e

F i g u r e 5. Aggregated a n i s o t r o p i c phase p a t t e r n : s o l v e n t c o m p o s i t i o n , 23.6/75.0/1.4, NH /NH SCN/H 0; cellulose concentration, 20g/100mL, immediately after dissolution. 3

4


2

165


166


concentration. Supporting evidence that t h e a n i s o t r o p i c solutions with fingerprint patterns incorporated cholesteric mesophases included t h e i r high negative s p e c i f i c r o t a t i o n s when compared w i t h t h e l o w s p e c i f i c r o t a t i o n o f +35* f o r c e l l o b i o s e i n t h e 30.0/70.0 s o l v e n t . F u r t h e r , a l i n e a r r e l a t i o n s h i p was f o u n d between s p e c i f i c r o t a t i o n a n d t h e r e c i p r o c a l o f t h e s q u a r e o f t h e wave length of the incident l i g h t . Such a r e l a t i o n s h i p i s i n t e r p r é t a b l e b y de V r i e s ' t h e o r y (26) t o show t h a t t h e s e mesophases were cholesteric. Finally, the ring d i f f r a c t i o n p a t t e r n g e n e r a t e d b y t h e i n c i d e n t He-Ne l a s e r beam a l s o p r o v i d e d e v i d e n c e o f a c h o l e s t e r i c phase (3,27,28) . N e m a t i c p h a s e b i r e f r i n g e n c e , w h i c h was o b s e r v e d i n t h e c e l l u l o s e s o l u t i o n s made f r o m t h e 24.5/75.5 s o l v e n t , appeared immediately f o l l o w i n g d i s s o l u t i o n and p e r s i s t e d t h r o u g h o u t a two week p e r i o d . The u n i f o r m l y dispersed b i r é f r i n g e n t p a t t e r n s were t h e most p r e v a l e n t ones i n t h e n e m a t i c s o l u t i o n s a t a l l DPs s t u d i e d , b u t S c h l i e r e n a n d thread-like patterns i n d i c a t i n g nematic phases were r e a d i l y o b s e r v e d i n s o l u t i o n s o f DP 450 c e l l u l o s e . C o n j u g a t e d p a t t e r n s formed i n s o l u t i o n s o f moderate c e l l u l o s e concentration, 12g/100mL-16g/100mL, i n s o l v e n t s containing l e s s t h a n 75.5% N H 4 S C N ( F i g u r e 3a), ori n fairly concentrated cellulose solutions, 18 a n d 20g/100mL, prepared from solvents containing a p p r o x i m a t e l y 75.5% N H 4 S C N ( F i g u r e s 3b a n d c ) . The a g g r e g a t e d p h a s e p a t t e r n was e v i d e n t w i t h i n a few minutes a f t e r d i s s o l u t i o n f o r s o l u t i o n s p r e p a r e d from hydrolyzed cellulose of DP 35 i n 23.6/75/1.4 N H 3 / N H 4 S C N / H 2 O (see F i g u r e 5 ) . S p h e r u l i t i c phase p a t t e r n s (Figure 4) a p p e a r e d i n s o l u t i o n s p r e p a r e d from s o l v e n t s c o n t a i n i n g g r e a t e r than a p p r o x i m a t e l y 75.5% N H 4 S C N o r i n s o l u t i o n s p r e p a r e d f r o m solvent containing a p p r o x i m a t e l y 75.5% N H 4 S C N i f t h e y were s t o r e d o v e r two months. The d e n s i t y o f s p h e r u l i t e content increased with increasing N H 4 S C N concentration and cellulose concentration. Anisotropic cellulose s o l u t i o n s c o n t a i n i n g t h e s p h e r u l i t e s showed a h i g h n e g ative specific rotation. On t h a t b a s i s , t h e c e l l u l o s e s o l u t i o n s w i t h t h e s p h e r u l i t i c p a t t e r n s were e s t a b l i s h e d as c h o l e s t e r i c . V a r y i n g t h e s o l v e n t c o m p o s i t i o n a l s o a f f e c t s t h e MCCM and t h e a n i s o t r o p i c p h a s e volume f r a c t i o n . T h e MCCM showed a maximum a t 75.5% N H 4 S C N o v e r t h e r a n g e o f 0.70 t o 0.79 w e i g h t f r a c t i o n N H 4 S C N i n N H 3 / N H 4 S C N ( F i g u r e 6 ) . The t u r b i d i t y of the cellulose solutions increased with i n c r e a s i n g d e v i a t i o n i n e i t h e r d i r e c t i o n from t h e 24.5/75.5 s o l v e n t c o m p o s i t i o n ( F i g u r e 7) a n d c o r r e l a t e d d i r e c t l y w i t h t h e o b s e r v e d magnitude o f [ Θ ] . Increasing storage time increased the t u r b i d i t y of the solutions p r e p a r e d f r o m s o l v e n t c o n t a i n i n g e i t h e r more o r l e s s t h a n


YANG ET AL.

Lyotropic Mesophases of Cellulose in NHf-NHjSCN

9.0 r


δδ

j_

0.70

0.71

0.72

0.73

0.74

0.75

0.76

δ

δ

_l_

J

0.77

0.78

0.79

Weight Fraction NH SCN in NH /NH SCN 4

3

4

F i g u r e 6. Minimum c e l l u l o s e c o n c e n t r a t i o n f o r mesophase formation as d e t e r m i n e d by s o l v e n t c o m p o s i t i o n : DP, 210; s t o r a g e t i m e a t 2 5 C , 30 days. e

Figure 7. Turbidity dependence o f cellulose solution on solvent c o m p o s i t i o n : 30/70 ( a ) , 28/72 (b), 26/74

Polymer Association Structures - American Chemical Society

Recommend Documents