The effect of shear on the viscosity behaviour of high molecular weight amylose and amylose acetate

European Polymer Journal. 1970. Vol. 6, pp. 293-297. Pergamon Press. Printed in England.

THE EFFECT OF SHEAR ON THE VISCOSITY BEHAVIOUR OF HIGH MOLECULAR WEIGHT AMYLOSE AND AMYLOSE ACETATE* C. T. GREENWOOD, D. J. HOURSTON and A. R. PROCTER Department of Chemistry, The University of Edinburgh, Edinbttrgh, 9, Scotland

(Received 3 March 1969) Abstract--Viscosity measurements, at rates of shear between 0 and 1200 sec-t, have been carried out on samples of amylose with high apparent limiting viscosity numbers in 0" 15 M K O H (I-q] = 400 to 1000 ml g-t) in order to establish whether ['1] depends on the shear rate. Solutions of these amylose samples in 1.0 i K O H and 0"33 M KCI have also been examined, together with an investigation of the behaviour of high tool. wt. amylose acetate in nitromethane. It was found for all the systems studied that [,7] did not vary but that the Huggins' constant decreased as rate of shear increased.

INTRODUCTION IN TIrE c o u r s e o f s t u d i e s o f t h e h y d r o d y n a m i c b e h a v i o u r o f a m y l o s e a n d its a c e t y l derivative, we were concerned in establishing whether shear corrections to the limiting v i s c o s i t y n u m b e r , [,/], w e r e n e c e s s a r y , as e a r l y w o r k a p p e a r e d t o b e c o n t r a d i c t o r y 5 1 - s ~ This paper presents the results of these experiments.

EXPERIMENTAL

3¢aterials Starch was isolated from potatoes (var. Pentland Crown) as previously described. ~4) Fractionation of the starch granules into the two distinct components, amylose and amylopectin, was achieved by aqueous dispersion; the amylose was purified by recrystaUizing twice from hot, aqueous butan-l-ol, and finally dried in racuo at 50°. ~*) The dried amylose was dissolved in dimethylsulphoxide to yield a 0- 5 per cent solution. Subfractionation was then effected by the stepwise addition of ethanol at 25 ° as described by Everett and Foster. ~ Samples possessing an apparently high viscosity were selected for this investigation. The amylose acetate sample was prepared by a method already described ~ from a high tool. wt. amylose subfraction. Solutions Solid amylose was dissolved in 0" 15 i K O H at 0 ° by standing overnight at that temperature. The solution in 0.33 i KC1 was prepared by dissolving dried amylose in 0" 5 M K O H at 0 ° and allowing it to stand overnight at 0 ° prior to neutralizing to p H 7 with 1 M HC1 using a pH-meter. The amylose acetate dissolved directly in nitromethane on standing at room temperature. Solutions were carefully filtered through a sintered glass filter (Grade 4) before addition to the viscometer. Physical measurements (a) Capillary viscometry. The viscometry measurements at high rate of shear were carried out using a modified Ubbelohde suspended-level viscometer. C7) The kinetic energy correction for this instrument was shown to be less than 0-5 per cent and hence was ignored. All measurements were made at 25 ° + 0-01 °, and the effiux times were measured to the nearest 0 . I see. Concentrations in the range 0-4 mg rnl-* to ~ 1"0 mg ml-* were obtained by adding successive aliquots of the amylose solution to the solvent in the viscometer. * This is Part 49 in the series "Physicochemical Studies on Starches". 293 E.va. 6/2---u

294

C. T. GREENWOOD, D. J. HOURSTON and A. R. PROCTER

The Huggins' interaction constant (k) was calculated from the graph ofr/,p/C vs. C using the relation '7,,,tC = Jr/] + k ['7]zC. In capillary viscometry, the shear stress, r,, expressed in units of dynes cm -z, is defined as ~-, = (0ghR)/21, where 0 is the liquid density, g is the gravitational constant, R is the radius of the viscometer capillary and l is its length, while h is the mean hydrostatic head which is calculated ~s~ from h = (lt--lz)/ln(l t/lz), where It and 12 are the distances between the top and the bottom markings of the viscometer bulb and the end of the capillary. The maximum shear rate, G, in sec-z, is given by G = (h0gR)/2l r/o'7r~t = K/r/rel, where r/r,t is the relative viscosity andr/o is the viscosity of the solvent. K is a constant for a given viscometer and a given solvent. It follows therefore that ,, = '7 G, where '7 is the solution viscosity. In this work, the applied stress is given in terms of the average rate of shear (3, i.e. ~ G, and, for the viscometer used, was 1200 sec-t. (b) Rotational viscometry. The experiments at low rates of shear were carried out using a Couettetype viscometer, as described by Ogston-Stanier.¢9~ This viscometer was capable of measuring shear rates as low as ~ 0" 5 sec-t, and of course had the usual advantages of rotational-viscometry in that corrections for kinetic energy and surface tension were not rtecessary. Much greater sensitivity was achieved when the Eureka wire of the original design was replaced ct°~ by a copper-beryllium alloy wire (Johnson Matthey, beryllium 2.0-2-5 per cent, nickel 0" 25-0.50 per cent, iron 0" I per cent and the balance copper; S.W.G. 43). This wire showed no permanent distortion even after severe twisting. The constancy of the drive speed was improved by use of an electronically-controlled constant-speed motor (England Hanovia Lamps, Ltd.). To avoid the influence of slightly-magnetized steel bearings around the cup, a gold-plated brass bob was used. Much more reproducible results were achieved when a Perspex draught-excluder was fitted around the cup and the torsion wire. The temperature was controlled at 25 ° +_ 0.02 °. The working volume of solution was pipetted directly into the clean, dried viscometer. Dilutions were made externally, and the diluted solution added to the instrument. For a Couette viscometer with a bob of radius Rb in a rotating cup of radius Re, the couple acting on the bob is given by the Margules equation¢~z~ C.O = 4~rLr/o~/[(l/R~)---(l/R~)], where C is the couple per unit angular displacement, O the angular displacement of the bob in radians, L the length of the torsion wire, ,7 the absolute viscosity of the fluid in the cup, and co is the rotational speed of the cup in rad/sec. This equation neglects the end-effect, which in this instance is minimized by the concave lower end of the bob trapping an air bubble when the bob is lowered into the cup. The above equation reduces to 0 = ~'7os,where a is the combined instrument constant. This can be readily obtained by calibration using Newtonian liquids (i.e. deionized- and degassed-water and redistilled butan1-oi) with accurately-known absolute viscosities,cz2~ The mean value of the rate of shear, Ca, in this instrument is given by (3 = 2o~ReRb/(Re_Rb).2 " The cup and bob were constructed so that the speed of the cup in rev/min equalled twice the average rate of shear. Deflections, 0, were measured on a circular scale of 100 cm-radius by means of an image of a hair-line projected onto a mirror fixed to the torsion wire and reflected back onto the scale; the scale was read to the nearest 1 ram.

RESULTS

AND

DISCUSSION

T h e results i n T a b l e I s h o w t h a t o v e r the 0 - 1 2 0 0 sec -z rate o f s h e a r r a n g e there is n o c h a n g e i n the l i m i t i n g viscosity n u m b e r s - - w i t h i n e x p e r i m e n t a l e r r o r - - f o r all the a m y l o s e f r a c t i o n s i n a q u e o u s s o l u t i o n . I f t h e s o l u t i o n s were s h o w i n g n o n - N e w t o n / a n b e h a v i o u r in the c o n c e n t r a t i o n a n d m o l e c u l a r size r a n g e s u n d e r e x a m i n a t i o n t h e n a significant c h a n g e i n [~7] w o u l d b e expected o v e r this fairly wide s h e a r rate r a n g e , b u t e v e n for the s a m p l e s o f largest m o l e c u l a r size (P1 a n d A1) this was n o t e v i d e n t . T h e v a l u e o f ['7] for the a m y l o s e acetate s a m p l e was also f o u n d to be i n d e p e n d e n t o f shear r a t e ; l-q] in n i t r o m e t h a n e was 405 m l g-~ at G = 1200 see -z a n d 408 m l g-Z at zero rate o f shear. F i g u r e 1 shows t y p i c a l g r a p h s o f viscosity n u m b e r vs. c o n c e n t r a t i o n w h i c h i n d i c a t e s t h a t , a l t h o u g h Jr/] d i d n o t c h a n g e , the H u g g i n s ' c o n s t a n t , k, was d e p e n d e n t o n the r a t e o f shear. T y p i c a l v a l u e s o f the H u g g i n s ' c o n s t a n t for the a q u e o u s systems a r e s h o w n i n T a b l e 2; i n the case o f the a m y l o s e acetate, k was 0 . 3 9 at zero shear a n d 0 . 2 7 at G ----- 1200 see -z.

The Effect of Shear on the Viscosity Behaviour of High Molecular Weight Amylose

295

TABLE 1. T~m EFFECT OF SI-IEARON THE LIMITING VISCOSITY ,NUMBERS (irt m i g - t ) OF AMYLOSE IN VARIOUS SOLVENTS

Solvent

0-15 M K O H

PI P2 P3 P4 P5 A1 A2 A3

0"15 M KCl

1"0 M K O H

['7]*0

[~]'1=0o

['7]0

['Th:oo

['7]0

['7]t:oo

1040 855 760 n.d.t 510 1060 626 430

1010 855 770 600 510 1090 626 430

810 690 520 5 I0 450 n.d. n.d. n.d.

810 695 550 510 450 n,d. n,d. n,d.

360 310 250 220 190 275 n.d. n.d.

350 300 255 220 190 277 n.d. n.d.

* ['7]0 -- ['7]-value at zero rate of shear; t n.d. = not determined.

['Th~oo

= [~7]-value at G = 1200 sec-L

TABLE 2. VALUES OF HUGGINS' CONSTAbVI', k, FOR AMY-LOSE FRACTION P1 IN VARIOUS SOLVEN"IS

0.15 M K O H

1.0 M K O H

0"15 ~ KCI

G SeC- t

[~]* 0 1200

1040 1010

k

[,fl*

k

C~]*

k

0.62 0.42

810 810

0.61 0-40

360 350

1.00 0.76

* Units of ['7] are ml g-t.

Figure 2 shows the results for the sample of largest molecular size (A1) in 0.15 r4 KOH graphed according to the method of Schurz "3) which emphasizes that the viscosity number was independent of shear rate over the range studied in the Couette viscometer (0-50 see-t). I

i

I

I

I

I

500

2000 ~500

400

IO00

0 .~~~~__~e~1200

60(

sec -I sec-t

~

-2

sec-I

. - - - - - 0 s e c -I 1200 sec -I

300

200 I 0.5

I I0 Concentration

0 (q ml-lx

I 0.5

t I0

I I'$

ZOO

I0 3)

FIG. 1. The effect of shear rate on the limiting viscosity number for (a) amylose samples A1, A2 and A3 in 0-15 M KOH, and Co) the amylose acetate fractionin nitromethane (Q), and amylose A1 in 0.33 M KC1 (©).

296

C. T. GREENWOOD, D. J. HOURSTON and A. R. PROCTER I

25OO

I

I

2000 !

1500

I000

- -

... 0

i

l

I

2

4

6

ConcentrotJon (g rnl-lxl03)+IO-{ G (see)

Fro. 2. Schurz(*s) plot of shear data for amylose A1 in 0.15 MK O H in the Couette viscometer. It is of interest that the amylose-subfractions reported here show no shear dependence of [~], although they are much larger in molecular size than those used in our hydrodynamic measurements, ca' 14. ~5) There are several examples in the literature where, for polysaccharide derivatives, the shear dependence of [~] for a shear rate of up to a few thousand reciprocal seconds has been shown to be negligible. Lohmander and Svensson "s) found for cellulose nitrate in ethyl acetate ([~] = 6380 ml/g) that in the rate of shear range 0-200 sec-~, [~7] changed by only 2 to 3 per cent, but the Huggins' constant changed from 0-84 to 0"56, whilst Goldberg and Fuoss c17) reported for nitrocellulose in butyl acetate that for shear rates up to ,,, 7000 sec-~ there is no shear dependence of [~7],but k is markedly shear dependent. Peterlin and (~opi~" 8) have presented a review of the experimental factors governing the gradient dependence of [~], and have argued that for an entirely flexible coil a gradient dependence would not be expected because, as the molecule is distorted, its statistical restoring force in the flow direction is enhanced whilst the extension in the direction perpendicular to the direction of flow is decreased. This hypothesis is in agreement with the results presented here for our hydrodynamic mea~-urements have shown that amylose behaves as a random coil in all the solvents used in these experiments.C6. ,4, 15~ Despite the fact that for the amylose-solvent systems under investigation there is no dependence of [~?]on the rate of shear, there is an appreciable shear dependence of the Huggins' constant, k. As yet there is no complete theoretical understanding of what

The Effect of Shear on the Viscosity Behaviour of High Molecular Weight Amylose

297

parameters quantitatively a n d qualitatively determine the m a g n i t u d e o f k, b u t G o l d berg a n d Fuoss (17~ attribute the n o n - N e w t o n i a n b e h a v i o u r of p o l y m e r solutions as arising from one, or both, of two sources: a shear dependence o f the interaction between the solute molecules, and a shear dependence of [,/] due to molecular deform a t i o n . G r a p h i n g ~ j C vs. c o n c e n t r a t i o n at various fixed shear rates distinguishes between these, because if the intercepts o n the ,Tsp/C-axis are different then the latter cause holds, a n d vice versa. I f this theory is correct, the constancy o f [7] at G equal to 0 sec -1 a n d at G with a n average value o f ca. 1200 see-* indicates that the shear dependence is due to the shear dependence o n the solute interactions. Acknowledgements--The Science Research Council and the Potato Marketing Board are thanked for maintenance grants (to D.H. and A.R.P.). REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (t3) (14) (15) (16) (17) (18)

J. M. G. Cowie, Makromolek. Chem. 42, 230 (1960). J. M. G. Cowie, J. Polym. Sci. 49, 455 (1961). A. R. Procter, Ph.D. Thesis, University of Edinburgh (1965). W. Banks, C. T. Greenwood and J. Thomson, Makromolek, Chem. 31, 197 (1959). W. W. Everett and J. F. Foster, J. Am. chem. Soc. 81, 3459, 3464 (1959). W. Banks, C. T. Greenwood and D. J. Hourston, Trans. Faraday Soc. 66, 363 (1968). W. E. Davis and J. H. Elliott, J. Coll. Sci. 4, 313 (1949). J. Schurz and E. H. Immergut, J. Polym. Sci. 9, 279 (1952). A. G. Ogston and J. E. Stanier, Biochem. J. 53, 4 (1953). A. G. Ogston, personal communication. J. R. Van Wazer, J. W. Lyons, K. Y. Kim and R. E. Colwell, Viscosity and Flow Measurement, Interscience, New York (1963). International Critical Tables, McGraw-Hill, New York (1926). J. Schurz. RheoL Acta 1, 43 (1963). W. Banks and C. T. Greenwood, Carbohydrate Res. 7, 349 (1968). W. Banks and C. T. Greenwood, Carbohydrate Res. 7, 414 (1968). I. Lohmander and A. Svensson, Makromolek. Chem. 65, 202 (1963). P. Goldberg and R. M. Fuoss, J. phys. Chem. 58, 648 (1954). A. Peterlin and M. (~opi~, J. appL Phys. 27, 434 (1956).

R&um6---On a effectu~, dans la potasse 0,15 M, des mesures de viscosit~ sur des ~hantillons d'amylose ~ indices de viscosit~ lirnite apparents ~lev6s ([,i] = 400 ~ 1000 cms g-t) avee des gradients de vitesse compris entre 0 et 1200 see-~ dans le but d'&ablir de quelle maniCre [,/] d~pend du gradient de vitesse. Des solutions de ces ¢,chantillons d'amylose darts la potasse 1,0 Met dans du KCI 0,33 M ont ¢galement ~t~ examines, paraU61ement/t tree &ude du comportement d'un ac&ate d'amylose de forte masse mol&:ulaire darts le n/trom&hane. On a trouv~ pour tousles syst&nes &udi~ que [,7] ne varie pas mais que la constante d'Huggins d~croit lorsque le gradient de vitesse augmente. Sommario---Misure della viscositA a fattori di taglio tra 0 e 120 sec-t sono state effettuate per carnpioni di amilosa con alti humeri apparenti di viscositA limitanti, in 0,15 t,l KOH (,/ = 400 a I000 ml g-t), per stabilire s e n dipende dal fattore di taglio. Soluzioni di questi campioni di amilosa in 1,0 M KOH e 0,33 M KCI sono state anche esaminate, insieme ad una investigazione del comportamento di acetato di amilosa di alto peso molecolare in nitrometano. I~.stato trovato nel caso di tutti i sistemi studiati chen non sie variata ma che il costante Huggins diminuisce con l'aumento del fattore di taglio. Zusammenfassung--Viskosit-~itsmessungen,bei Schergeschwindigkeiten zwischen 0 und 1200 sec-1, wurden an Amyloseproben mit hohen [scheinbaren Grenzviskosit~tszahlen in 0,15 M KOH (['q] ---400 bis 100 ml g-'*) durchgef/.ihrt, urn festzustellen, ob [~] von der Schergrschwindig,keit abhgmgt. LSsungen dieser Amylosen in 1,0 M KOH und 0,33 MKCI wurden ebenfalls untersueht, zusammen mit der Untersuchung des L6sungsverhaltens yon hochmolekularem Amyloseacetat in Nitromethan. F/ir alle untersuchten Systeme wurde festgestellt, dab [,7]sich nicht ver~nderte, dab abet die Hugging Konstante mit steigender Schergeschwindigkeit abnahm.