Fractionation and Characterization of Commercial Cellulose Triacetate

24 Fractionation and Characterization of Commercial Cellulose Triacetate by Gel Permeation Chromatography 1

F. MAHMUD and E. CATTERALL Department of Applied Chemistry, Faculty of Applied Science, Coventry (Lanchester) Polytechnic, Coventry, England Downloaded by FUDAN UNIV on January 11, 2017 | http://pubs.acs.org Publication Date: March 30, 1984 | doi: 10.1021/bk-1984-0245.ch024

Commercial cellulose triacetate samples were frac tionated by both fractional precipitation and pre parative gel permeation chromatography (GPC). The triacetate fractions were characterized by visco metry, high speed membrane osmometry (HSMO) and GPC. A fair agreement has been found between the molecular weights of various triacetate fractions determined by the three procedures. All unfractionated cellulose triacetate samples and high molecular weight fractions showed a shoul der on the high molecular weight side of the GPC distribution. Material isolated from this region was found to be highly enriched in mannose and xylose, attributed to the presence of a hemicellu lose derivative. Cellulose triacetate from cotton linters did not show this behavior. The universal calibration approach ([η].Mvs elution volume) for polystyrene standards and narrow molecular triacetate fractions show slight deviation from linearity. This departure from linearity has been attributed to differences in both hydrodynamic behavior and the Mark-Houwink exponent 'a' for the two polymers in question. A l i t e r a t u r e survey (J_ - 11) on the f r a c t i o n a t i o n o f c e l l u l o s e t r i a c e t a t e by p r e c i p i t a t i o n i n d i c a t e s that i n most cases i t has been u n s u c c e s s f u l due t o the p o s s i b i l i t y o f hydrogen bonding b e t ween polymer and s o l v e n t i n s o l u t i o n s (10, 12). GPC has been a p p l i e d t o the f r a c t i o n a t i o n o f c e l l u l o s e d e r i v a t i v e s by many workers. Segal (13), Meyerhoff (14 - 16), M u l l e r and Alexander (17) have reported the f r a c t i o n a t i o n o f c e l l u l o s e n i t r a t e by GPC. M u l l e r and Alexander (17), Brewer, Tanghe, B a i l l y and Burr 1

Current address : Petroleum and Gas Technology Division, Research Institute, University of Petroleum and Minerals, Dhahran, Saudi Arabia 0097-6156/84/0245-0365506.00/0 © 1984 American Chemical Society

Provder; Size Exclusion Chromatography ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

366

SIZE E X C L U S I O N C H R O M A T O G R A P H Y

(18) have a l s o used GPC f o r the f r a c t i o n a t i o n of c e l l u l o s e acetate and c a r b a n i l a t e r e s p e c t i v e l y . Maley (19) and Cazes (20) reported some work on GPC f r a c t i o n a t i o n of c e l l u l o s e e s t e r s , but gave no data. I t i s worth mentioning here that the s u c c e s s f u l f r a c t i o n a t i o n of c e l l u l o s e t r i a c e t a t e has not been reported so f a r i n the literature. The prime object of the present study was t o determine the c o m p o s i t i o n a l p o l y d i s p e r s i t y of commercial c e l l u l o s e t r i a c e t a t e and t o examine the e f f e c t of molecular weight and molecular weight d i s t r i b u t i o n on the mechanical p r o p e r t i e s o f the f i b r e s .

Downloaded by FUDAN UNIV on January 11, 2017 | http://pubs.acs.org Publication Date: March 30, 1984 | doi: 10.1021/bk-1984-0245.ch024

Experimental M a t e r i a l s . C e l l u l o s e t r i a c e t a t e samples w i t h 61.7 - 62% a c e t y l v a l u e , were a l l commercial grade and were s u p p l i e d by C o u r t a u l d s L t d . , Coventry England. The chemicals and the solvents used i n t h i s work were a l l a n a l y t i c a l grade m a t e r i a l s . T

F r a c t i o n a t i o n Procedures. 1. F r a c t i o n a l p r e c i p i t a t i o n . A 10% (m/V) s o l u t i o n of a commercial grade t r i a c e t a t e sample was d i s solved i n 300 ml ϋ-methylpyrrolidone and 700 ml of acetone (30:70 V/V) and was thermostated f o r 2 hours a t 25°C p r i o r t o the a d d i t i o n of 460 ml of petroleum ether (60-80°) as p r e c i p i t a n t . The s o l u t i o n w i t h the p r e c i p i t a n t was gently warmed t o 45°C t o r e d i s s o l v e the p r e c i p i t a t e and g r a d u a l l y cooled i n the thermostat. Phase separation took p l a c e a f t e r a w h i l e , and the phases were i s o l a t e d from each other by f i l t r a t i o n . The g e l l i k e phase thus i s o l a t e d was the f i r s t primary f r a c t i o n . The subsequent f r a c t i o n s were i s o l a t e d i n the same way by the f u r t h e r s u c c e s s i v e a d d i t i o n s of p r e c i p i t a n t to the s o l u t i o n . The l a s t f r a c t i o n was i s o l a t e d by the a d d i t i o n of a l a r g e volume of the p r e c i p i t a n t and a l l o w i n g the s o l u t i o n t o stand f o r 72 hours before the phase separation i s a f f e c t e d by f i l t r a t i o n as s t a t e d above. Seven primary c e l l u l o s e t r i a c e t a t e f r a c t i o n s were i s o l a t e d by t h i s method. The f i r s t primary f r a c t i o n r i c h i n h e m i c e l l u l o s e was r e d i s s o l v e d and r e p r e c i p i t a t e d i n t o three s u b f r a c t i o n s i n the same way as described above. The r e f r a c t i o n a t i o n of the f i r s t f r a c t i o n was necessary t o i s o l a t e the h e m i c e l l u l o s e m a t e r i a l f o r subsequent a n a l y s i s and c h a r a c t e r i s a t i o n . 2. G e l Permeation Chromatography (GPC). Waters A s s o c i a t e Model 200 GPC was used w i t h 4 χ 3/8" s t y r a g e l * columns w i t h an i n t e r n a l diameter of 0.311" and r e f T a c t o m e t e r detector. The b a s i c c h a r a c t e r i s t i c s and o p e r a t i o n of the instrument have been pre v i o u s l y described i n d e t a i l (19-20). Some of the operating c o n d i t i o n s used i n t h i s study are o u t l i n e d below. f

Column e x c l u s i o n l i m i t s Mobile phase

1

5

6

: 7xl0 -5xl0 ,

6

5xl0 , 5xl0

3

3

& 2-5xl0 A

Dichloromethane


24.

M A H M U D A N D CATTERALL

Commercial Cellulose

Flow r a t e

: 1 ml/min.

Sample c o n c e n t r a t i o n

: 0.5% m/V

Triacetate

367

Sample s o l u t i o n p r e p a r a t i o n : Allowed to stand overnight and then f i l t e r e d through g l a s s s i n t e r No. I porosity. Operating temperature

: Ambient

I n j e c t i o n volume

: 2 ml


R e f r a c t i v e index a t t e n u a t o r : X 8 ( l / 1 6 " n u l l g l a s s ) Syphon s i z e

: 5 ml

Choice o f Solvent. ]£-Methylpyrrolidone (NMP) was i n i t i a l l y used as the mobile phase but proved t o be u n s a t i s f a c t o r y because of ( i ) h i g h s o l u t i o n v i s c o s i t i e s , ( i i ) exceedingly s m a l l d i f f e r e n c e s i n r e f r a c t i v e index between NMP and c e l l u l o s e t r i a c e t a t e s o l u t i o n s , ( i i i ) e r r a t i c base l i n e . In view of t h i s dichloromethane was employed. Some a d d i t i o n a l b e n e f i t s d e r i v e d from t h i s mobile phase a r e : ( i ) a decrease i n e l u t i o n volume due t o low s o l u t i o n v i s c o s i t i e s , ( i i ) f a s t s o l v e n t recovery due to low b o i l i n g p o i n t of dichloromethane and ( i i i ) ease of o b t a i n i n g p r e p a r a t i v e GPC cuts of c e l l u l o s e t r i a c e t a t e . P r e p a r a t i v e GPC of C e l l u l o s e T r i a c e t a t e Sample. A 1% (m/V) s o l u t i o n of c e l l u l o s e t r i a c e t a t e (medium) p r e f i l t e r e d through poro s i t y 3 g l a s s s i n t e r was f r a c t i o n a t e d by repeated i n j e c t i o n through the column s e t d e s c r i b e d above. Seven cuts c o v e r i n g t h e e n t i r e e l u t i o n curve were c o l l e c t e d . The flow r a t e , i n j e c t i o n time and the experimental c o n d i t i o n s were i d e n t i c a l to those s t a t e d above. A t o t a l of 50 i n j e c t i o n s were made. F r a c t i o n s were recovered by removing dichloromethane under vacuum a t low temperature. The cuts were c h a r a c t e r i z e d i n the same way as d e s c r i b e d p r e v i o u s l y for cellulose triacetate fractions. C a l i b r a t i o n of G e l Permeation Chromatograph. The chromatographic system was c a l i b r a t e d u s i n g : (1) P o l y s t y r e n e standars (2) Narrow molecular weight c e l l u l o s e t r i a c e t a t e f r a c t i o n s (3) A ' u n i v e r s a l ' c a l i b r a t i o n approach P o l y s t y r e n e standards. S o l u t i o n s o f t h e monodisperse p o l y s t y renes (Waters, Mass., USA) i n N-methylpyrrolidone (0.5$m/V) and dichloromethane {0.125% m/V) were_ used as c a l i b r a n t s . F i g u r e 1 shows a p l o t o f l o g (rj) v s . l o g Mn f o r c e l l u l o s e t r i a c e t a t e f r a c t i o n s i n dichloromethane a t 2 1 C.


368

CHROMATOGRAPHY


SIZE E X C L U S I O N

Figure 1.

Log [η] versus l o g U r e l a t i o n s h i p f o r c e l l u l o s e triacetate fractions. n


24.

MAHMUD AND

CATTERALL


Triacetate

369

Narrow Molecular Weight T r i a c e t a t e F r a c t i o n s . Narrow molecular weight c e l l u l o s e t r i a c e t a t e f r a c t i o n s were obtained by both f r a c t i o n a l p r e c i p i t a t i o n and p r e p a r a t i v e GPC as described above. The number average molecular weight (MQ) of the v a r i o u s f r a c t i o n s and cuts was determined by h i g h speed membrane osmometry. A l i n e a r dependence of GPC e l u t i o n volume on l o g molecular weight f o r a l l c e l l u l o s e t r i a c e t a t e f r a c t i o n s was found i n both JSh-methylpyrrolidone and dichloromethane.


U n i v e r s a l C a l i b r a t i o n . A f u n c t i o n of the hydrodynamic volume [η]·Μ was p l o t t e d a g a i n s t the e l u t i o n volumes of c e l l u l o s e t r i a c e t a t e f r a c t i o n s and p o l y s t y r e n e standards run i n dichloromethane have a l l i n d i c a t e d s l i g h t d e v i a t i o n from l i n e a r i t y as shown i n Figure 2. Discussion F r a c t i o n a l P r e c i p i t a t i o n of C e l l u l o s e T r i a c e t a t e . The reported p a r t i a l or n o n - f r a c t i o n a t i o n of c e l l u l o s e t r i a c e t a t e from c h l o r i nated hydrocarbons or a c e t i c a c i d may be e x p l a i n e d i n terms of the polymer-solvent i n t e r a c t i o n parameter χ (1-11). The χ-values f o r cellulose triacetate-tetrachloroethane and c e l l u l o s e t r i a c e t a t e chloroform systems are reported (10,21) as 0.29 and 0.34 respec t i v e l y . The lower values of χ f o r such systems w i l l r e s u l t i n a s m a l l e r or negative heat of mixing (AHm) and t h e r e f o r e p a r t i a l or n o n - f r a c t i o n a t i o n of the polymer i n q u e s t i o n r e s u l t s . The poor f r a c t i o n a t i o n from a c e t i c a c i d has been a t t r i b u t e d to the i n t e r m o l e c u l a r hydrogen bonding between s o l v e n t molecules and thus a l e s s e r polymer-solvent i n t e r a c t i o n . This means the t o t a l heat evolved due to hydrogen bonding between polymer and s o l v e n t molecules w i l l be s m a l l e r than i n the case of chloroform and t e t r a chloroethane and hence AHm (22) w i l l be l a r g e r or more p o s i t i v e . The s t r u c t u r a l homogeneity of the v a r i o u s c e l l u l o s e t r i a c e t a t e f r a c t i o n s obtained by f r a c t i o n a l p r e c i p i t a t i o n was e s t a b l i s h e d by both i n f r a r e d and n u c l e a r magnetic resonance spectroscopy. C a l i b r a t i o n of Gel Permeation Chromâtograph P o l y s t y r e n e C a l i b r a t i o n . A p l o t of molecular s i z e i n (S) versus e l u t i o n volume f o r p o l y s t y rene standards i n dichloromethane showed d e v i a t i o n from l i n e a r i t y at about 2,200 Â which may be a t t r i b u t e d to imperfect column reso l u t i o n , peak broadening, a x i a l d i s p e r s i o n and skewing. The exten s i v e t a i l i n g of the chromatograms of h i g h molecular weight p o l y styrene standards observed i n dichloromethane has a l s o been r e ported i n the l i t e r a t u r e (23-26). Narrow Molecular Weight T r i a c e t a t e C a l i b r a t i o n . A l i n e a r r e l a t i o n s h i p was found when l o g R Q a g a i n s t the e l u t i o n volumes of v a r i o u s c e l l u l o s e t r i a c e t a t e f r a c t i o n s was p l o t t e d . For narrow molecular weight d i s t r i b u t i o n t r i a c e t a t e f r a c t i o n s , the GPC experimental average molecular weight, termed Μρ |ς can be expected to conform to the f o l l o w i n g equation β3



CHROMATOGRAPHY


370

F i g u r e 2.

U n i v e r s a l c a l i b r a t i o n (Μ·[η] i n dichloromethane at 21 C.

vs e l u t i o n volume)


MAHMUD A N D

24.

CATTERALL

M


. - M - M peak w ν

M

371

Triacetate

η


However, f o r u n f r a c t i o n a t e d t r i a c e t a t e samples and f o r f r a c t i o n s o f broader molecular weight_ d i s t r i b u t i o n , t h i s equation " w i l l not h o l d and t h e r e f o r e t a k i n g M o r M as M . w i l l lead to serious η ν ne ale e r r o r s . This f a c t i s evident from the r e s u l t s shown i n Table I . The apparent d i f f e r e n c e between v i s c o s i t y average (M ) and number average molecular weight (M ) f o r u n f r a c t i o n a t e d t r i a c e t a t e samples and h i g h molecular weight f r a c t i o n s may be a t t r i b u t e d t o the presence of h e m i c e l l u l o s e and p o l y d i s p e r s i t y e f f e c t i n these m a t e r i a l s as shown i n the t a b l e i n question and i n F i g u r e 3 . U n i v e r s a l C a l i b r a t i o n . P l o t s of [η]·Μ a g a i n s t e l u t i o n volumes i n d i c a t e that p o l y s t y r e n e and c e l l u l o s e t r i a c e t a t e f o l l o w d i f f e r ent c a l i b r a t i o n s as shown i n Figure 2. This d e v i a t i o n from l i n e a r i t y may be due to the f o l l o w i n g reasons. 1. L i n e a r polymers, p o l y s t y r e n e and c e l l u l o s e t r i a c e t a t e e x h i b i t d i f f e r e n c e s i n hydrodynamic behavior i n s o l u t i o n . Cellu l o s e and i t s d e r i v a t i v e s are known to have h i g h l y extended and s t i f f chain molecules below a Dp of about 300, but as the Dp i n c r e a s e s above 300 the chain tends to assume the character of a random c o i l (27,28). The assumption that hydrodynamic volume c o n t r o l f r a c t i o n a t i o n i n GPC may not be t r u e f o r p o l y s t y r e n e and c e l l u l o s e t r i a c e t a t e , though i t has been found s a t i s f a c t o r y f o r non-polar polymers i n good s o l v e n t s (29). 2. The Mark-Houwink exponent 'a' f o r c e l l u l o s e t r i a c e t a t e i n dichloromethane was found 1.10-1.14 compared to p o l y s t y r e n e w i t h a = 0.71. These values were obtained e x p e r i m e n t a l l y i n the present work. P a r i k h (12) has found higher values f o r the exponent a using the f o l l o w i n g polymer-solvent systems: ( i ) C e l l u l o s e t r i a c e t a t e - c h l o r o f o r m at 25°C : 'a = 1.33 ( i i ) C e l l u l o s e t r i a c e t a t e - t e t r a c h l o r o e t h a n e at 25°C : a = 1.24 ( i i i ) C e l l u l o s e t r i a c e t a t e - A c e t i c a c i d at 25°C: a = 1.18. Though the a values f o r c e l l u l o s e t r i a c e t a t e - d i c h l o r o m e t h a n e system appear h i g h i n the present study, i t i s s t i l l not s u r p r i s ing when compared to the above s t a t e d a values reported by P a r i k h (12). The d i f f e r e n c e i n the values of 'a f o r p o l y s t y r e n e and c e l l u l o s e t r i a c e t a t e may account p a r t l y f o r the d e v i a t i o n i n slopes as shown i n Figure 2 . E t h y l c e l l u l o s e and c e l l u l o s e t r i a c e t a t e have been shown to form hydrogen bonded a s s o c i a t e s w i t h dichloromethane (10,30). I f t h i s i s so, then c e l l u l o s e t r i a c e t a t e - d i c h l o r o m e t h a n e i n t e r a c t i o n w i l l be favored over p o l y s t y r e n e - c e l l u l o s e t r i a c e t a t e i n t e r a c t i o n and thus no a d s o r p t i o n should be expected. The p a r t i a l b l o c k i n g of the GPC column w i t h 5 x 10 A e x c l u s i o n l i m i t i n both dichloromethane and ϋ-methylpyrrolidone may be a t t r i b u t e d to the presence of h e m i c e l l u l o s e i n both u n f r a c t i o n a t e d t r i a c e t a t e samples and high molecular weight f r a c t i o n s used i n t h i s work. The b l o c k i n g of the column i n question was i n d i c a t e d 1

1

f

f

f

1

f

f

f

f

f

f

1

o


1

SIZE EXCLUSION CHROMATOGRAPHY

372


— CTA/T6 3323/2/F2/FB/FA . — - CTA/S/ONLY (FREE OF SHOULDER MATERIAL) CTA/COTTON LINTERS CTA/M/I/F2 — CTA/M/UNF

ELUTION VOLUMES (5ml. counts) F i g u r e 3.

G e l permeation chromatograms of v a r i o u s t a t e samples and f r a c t i o n s .

triace


24.

MAHMUD AND CATTERALL

Table 1.

373

A n a l y t i c a l Data of Various C e l l u l o s e T r i a c e t a t e Samples and F r a c t i o n s 21°C [nl


Commercial Cellulose Triacetate

M

N

M V

E.V. D (5ml) Ρ (DCM)

(GPC) (DCM)

M* (GPC) (DCM)

M /M w η (DCM)

Sample

CH.2 C I 2

CTA/M/1/UNF

2.04

72000 105680 18.60 250

46000 121334

2.63

CTA/M/1/F1

2.88

114200 115880 18.22 396

97420 165639

1.70

CTA/M/1/F1/FA 3.20

228375 151360 17.80 793 1110474 217469

1.96

CTA/M/1/F1/FB 2.72

188720 137090 18.10 655

108525 186029

1.71

83946 18.64 285

91907 109982

1.19

111090 113760 18.22 385

91000 124957

1.37

CTA/M/1/F1/FC 1.68

82210

CTA/M/1/F2

1.95

CTA/M/1/F3

1.58

93916

85310 18.64 326

70362

99050

1.40

CTA/M/1/F4

1.38

87000

77804 19.00 302

60000

88982

1.48

CTA/M/1/F5

1.18

66300

67143 19.64 230

46620

55926

1.19

CTA/M/1/F6

0.60

35658

36658 20.60 123

19800

36158

1.80

CTA/M/1/F7

0.236

14170

16136 23.00

18600

19196

1.03

49

by a r a p i d r i s e i n the system pressure which n e c e s s i t a t e d the removal of t h i s column i n order t o overcome the problem s t a t e d above. Meyerhoff (14-16) has a l s o observed s i m i l a r b l o c k i n g of the g e l column using c e l l u l o s e t r i n i t r a t e f r a c t i o n s w i t h molecular weight above 1.4 * 10^, w h i l e f r a c t i o n s w i t h molecular weight 4.2 x 10" could not be separated. I t i s obvious from these r e s u l t s that he d i d n o t , however, r e a l i z e the presence of the h e m i c e l l u l o s e d e r i v a t i v e s i n the wood-pulp based c e l l u l o s e n i t r a t e and i t s r o l e i n b l o c k i n g of the high p o r o s i t y column as shown i n t h i s study. Acknowledgments Many people and o r g a n i s a t i o n have c o n t r i b u t e d t o t h i s work, n o t a b l y C o u r t a u l d s L t d . , Coventry, England. 1

Nomenclature of C e l l u l o s e T r i a c e t a t e Samples and F r a c t i o n s Samples: In CTA/S/UNF, CTA/M/UNF, CTA/3060/UNF and CTA/TG 3323/UNF CTA stands f o r c e l l u l o s e t r i a c e t a t e . The d e s i g n a t i o n of S,M,3060 and TG 3323 are batch numbers given t o these samples by Courtauld s L t d . UNF i s the symbol f o r u n f r a c t i o n a t e d sample. 1


374


CHROMATOGRAPHY

Fractions: Each t r i a c e t a t e f r a c t i o n , l i k e the r e s p e c t i v e sample, s t a r t s w i t h the symbol CTA ( f i r s t column from l e f t t o r i g h t ) and i s then followed by the batch number (2nd column), f r a c t i o n a t i o n number ( 3 r d column), f r a c t i o n number (4th column) and s u b - f r a c t i o n number (5th column r e s p e c t i v e l y ) .


Literature Cited 1. Elod, E. and Schmidt-Bielenberg, J. Physik. Chem. B25, 38, 1934. 2. Lachs, H.J., Kolloid, J . , 79, 91, 1937. 3. Levi, G. R., Gazz. Chim. Ital, 68, 589, 1938. 4. Bezzi, S., Atti 1st. Veneto Sci., 99, 905, 1939-40; Chemical Abstracts, 38, 1356, 1944. 5. Munster, Α., J. Polym. Sci., 5, 58, 1950. 6. Cumberbirch, R. J. Ε., Shirley Institute Memoirs, 31, 1958. 7. Thinius, Κ., Plaste Kautschuk, 6, 547, 1959. 8. Okunev, P.P. and Tarakanov, O. G., Vysokomol, Soed., 4, 5, 688, 1962. 9. Dymarchuk, N.P., Zhurnal Prikladnoi Khimi, 37, No. 10, pp 2263-2268, English Translation, October 1964. 10. Howard, P., and Parikh, R. S., J. Polym. Sci., A-1, 4, 407-418, 1966. 11. Geller, Β. Ε., Khimicheski Volokna, 11, No. 5, pp 1-6, English Translation, 1969. 12. Parikh, R.S., Ph.D. Thesis, University of Surrey, 1965. 13. Segal, L., J. Polym. Sci., B4, 1011, 1966. 14. Meyerhoff, G., Makromolek. Chem., 89, 282, 1965. 15. Meyerhoff, G. and Jovanovic, S., J. Polym. Sci., B5, 495, 1967. 16. Meyerhoff, G., Makromolek. Chem., 134, 129, 1970. 17. Muller, Τ. Ε., and Alexnader W. J., in "Analytical Gel Permea tion Chromatography"(J. Polym. Sci. C, 21), Johnson, J. F. and Porter, R.S., Eds., Interscience, New York, p. 283, 1968. 18. Brewer, R. J . , Tanghe, L. J . , Bailey, S. and Burr, J. T., J. Polym. Sci., A-1, 6, 1697, 1968. 19. Maley, L. Ε., Analysis and Fractionation of Polymers, J. Polym. Sci., C-8, 253-268, 1965. 20. Gazes, J . , J. Chem. Educ., 43, A567, 1966. 21. Hager, O. and Vander Wyk, A. J. Α., Helv. Chim. Acta, 1940 23, 484. 22. Hildebrand, J. and Scott, R., "The Solubility of Non-Electro lytes," 3rd Ed., Reinhold, 1949. 23. Harmon, D. J . , in "Analysis and Fractionation of Polymers," J. Polym. Sci., C-8, Mitchell, J . , Jr. and Billmeyer, F.W., Jr., Eds., Interscience, New York, p. 243, 1965. 24. Tung, L. J., J. Polym. Sci. 10, 375, 1261, 1274, 1966. 25. Hess, M. and Kratz, R. F., J. Polym. Sci. A-2, 4, 73, 1966. 26. Smith, W. N., J. Appl. Polym. Sci., 11, 639, 1967.


24. MAHMUD AND CATFERALL

375 Commercial Cellulose Triacetate

27. Flory, P. J., "Principles of Polymer Chemistry," Cornell Univ. Press, Ithaca, New York, Chap. 13, 1953. 28. Stamm, A. J., "Wood and Cellulose Science," Ronald, New York, pp 96, 108, 1966. 29. Dawkins, J. V., Br. Polym. J . , 4, 87-101, 1972. 30. Brookshaw, A. P., Br. Polym. J . , 5, 229-239, 1973. December

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Fractionation and Characterization of Commercial Cellulose Triacetate

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