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Study of rheological properties of dilute and moderately concentrated solutions of chitosan
1546
A . M . S~,LYA~ e t a / .
18. H. R. ALLCOCK, R. W. ALLEN and I. I. MEIRTER, Conformational Analysis of Poly(dihalophosphazones), Macromolecules 9: 950, 1976 19. R. W. ALLEN and H. R. )ALI.COCK, Conformational Analysis of Poly(alkoxy- and Aryloxyphosphazones), Makromolecules 9: 950, 1976 20. C. C. PRICE and D. D. CARMELITE, J. Amer. Chem. Soc. 88: 4039, 1966 21. Metody issledovaniya polimerov (Methods of Investigating Polymers) ed. b y A. I. Pravednikova, Moscow, Izd. inostr, lit., 155, 1961 22. V. N. REIIKH and B. A. FAINBERG, Metody tekhnicheskogo kontrolya kachestva sinteticheskikh kauehukov i lateksov (Methods of Technical Control of the Quality of Synthetic Rubbers and Latexes). Moscow, Goskhimizdat, 61, 1951
PolymerScienceU.S.S.R. Vol.23, No. 6, pp. 1546-1554, 1981 Printed in Poland
0032-3950/81/061546-09507.50]0 © 1982PergamonPre~ Ltd.
STUDY OF RHEOLOGICAL P R O P E R T I E S OF D I L U T E A N D M O D E R A T E L Y CONCENTRATED SOLUTIONS OF CHITOSAN* A. M. SKLYAR, A. I. GAMZAZADE,L. Z. RooovI~A, L. V. TITKOVA,S.-S. A. PAVLOVA, S. V. ROOOZHI~"and G. L. SLOh~-VmK~ I n s t i t u t e of Hetero-organie Compounds, U.S.S.R. Academy of Sciences
(Received 24 April 1980) A study was made of the variation of rheological properties of chitosan solutions, according to the concentration of the polymer, the composition and type of solvent, the presence of various low-molecular weight additives (electrolyte, urea, etc.). I t was noted that some rheological properties of chitosan solutions in the concentration range studied depend on its polyelectrolyte nature. I t was shown that the viscosity variation of ehitosan solutions on keeping them over a period of time is not duo to the rupture of glycoside bonds of the polymer. I t was found that chitosan solutions have the highest stability over a period of time in salt media also containing a hydrogen bond accepter.
Cm'rosa_~ is a polyglucosamine of the following structure ["-
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Clq2OH
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which is an ionogenic polysaccharide with a complex of properties highly valuable * Vysokomol. s o y e d . . ~ 3 : No. 6, 1396-1403, 198I.
Dilute and moderately concentrated solutions of chitosan
1547
f r o m a practical p o i n t of view [1]. I n particular, c h i t o s a n offers considerable p o t e n t i a l in t h e a p p l i c a t i o n a n d p r o d u c t i o n of films, m e m b r a n e s a n d fibres w i t h ion-exchange properties, usually f o r m e d from p o l y m e r solutions. A s t u d y o f rheological properties of chitosan solutions in a p o ~ i b l y wider range o f concentrat i o n is therefore o f special urgency. A similar s t u d y is also of i m p o r t a n c e for t h e u n d e r s t a n d i n g of special features of v a r i o u s p o l y m e r - a n a l o g o u s conversions o f c h i t o s a n carried o u t in h o m o g e n e o u s m e d i u m . A n i n v e s t i g a t i o n of properties of chitosan solutions with high degrees of dilut i o n ( ) 0 . 1 ° / o ) shows t h a t this p o l y s a c c h a r i d c has v e r y high intrinsic viscosity (up to 24 dl/g) [2]. This p r o p e r t y is p r o b a b l y due to the v e r y high r i g i d i t y of t h e p o l y m e r chain a n d the high molecular w e i g h t of chitosan. This a n d t h e ionogenic n a t u r e of c h i t o s a n enable us to e x p e c t specific rheological properties of solutions, s t a r t i n g from low p o l y m e r c o n c e n t r a t i o n s ( ~ 1 ~/o)I n order to d e t e c t these features, we e x a m i n e d the dependence o f the v i s c o s i t y o f dilute acidic solutions o f c h i t o s a n on the rate of d i s p l a c e m e n t a n d the effect on this d e p e n d e n c e of the c o m p o s i t i o n and t y p e o f solvent, v a r i o u s low-molecular w e i g h t a d d i t i v e s (CHaCOO~Ta, urea, etc.). Two chitosan samples were examined: XT-1 and XT-2 with [t/l= 17 and 24 dl/g, r~spectively (intrinsic viscosity values were measured in 2% CI-IsCOOI-I using the method of "isoionic dilution"). Chitosan was obtained from chitin by treatment with a 48%o dilute solution of NaOH (in a 1 : 10 ratio) at 140° in argon for 1 hr [3]. The product obtained was washed with water (to neutral pH), thcn with methanol, ether and dried in air, then in vacuum at 60-70 °. The polymer was thcn reprecipitated from a hydrochloride solution (brought to a plC[ of 5.4 with ml alkaline solution) in 0-005 ~ dilute solution of NaOI-I. The finely dispersed residue was subjected to lyophilic drying, followed by drying in vacuum while heating. After reprecipitation the solubility of chitosan (amino-group content 78:[:3~o) increases, however, it was ~possible to obtain XT-2 solutions of definite homogeneity in 2 ~o CI:[:COOH: in a proportion over 7~b. This is why the chitosan concentration range selected was between 0.1 and 7~/o. Dilute solutions of hydrochloric, acetic and dichloroaeetic acids of varying concentration (ctfitosan does not dissolve in pure water) were used as solvents. The solution of ehitosazt samples studied was examined by two different methods to show that the general regularities obtained are independent of conditions of preparing the solutions. XT-1 solutions were prepared mainly from finely dispersed polymer powder by intensive agitation at 500 g for 30 rain, followed by centrifuging of tim solution obtained at 3200 g for 5 rain. Solutions of the X-2 sample were obtained mainly by spontazmons dissolving, keeping the polymer in the solvent for 16 hr. With this method of solution fairly homogeneous solutions could only be obtained in dilute solutions of acetic acid containing at least 2~o, and in the cas~, of hydrochloric acid, not more than 0.3 ~. Solutions of both ehitosan samples examined were therefore prepared i~l these limits of acid concentration. The viscosity of the solutions studied was measured in a polylTmr concentration range of 0"1-I'0%o using a precision rotary viscometer for the measurement of low viscosities (VMV-03) [4] and a "Rheotest-2" rheoviseometer (Gcrnmn Democrat ie Republic) for higlmr concentrations. M e a s u r e m e n t s carried o u t in a wide r a n g e of p o l y m e r c o n c e n t r a t i o n (Fig. l) i n d i c a t e t h a t c h i t o s a n solutions w i t h c o m p a r a t i v e l y low c o n c e n t r a t i o n s ( ~ 1~/o)
1548
A . M . Sg_LYA~ et al.
are non-l~lewtonian liquids, for which a reduction of viscosity is observed in the entire range of shear rates examined (from 1 × 10 -1 to 3 × l0 s see-l). For a number of samples viscosity reduction is higher at low rates of shear than at high rates of shear, when viscosity is almost independent of shear rate, while with fairly high rates of shear viscosity normally shows a marked reduction. This is ~ypical of the flow of pseudoplastic systems, such as solutions of long rigid macromolecules , particularly polyamino-acids and cellulose derivatives [5, 6]. In view of the absence from chitosan solutions of a clearly expressed range of m a x i m u m Newtonian viscosity, properties of these solutions were compared according to viscosity in the range of shear rates, where viscosity variation is comparatively slight, namely at 48.6 see- 1 (this is clearly seen from Fig. lb, where viscosity variation is shown in semi-logarithmic coordinates t/--log ~). Figure 2 shows the concentration dependence of relative viscosity (t/re1) at a given rate of shear for both chitosan samples in different solvents. It can be seen t h a t the dependences have a form, which is typical of most isotropic polymers examined [7, 8]. I t should be noted, however, t h a t rectification of curves 1--3 in coordinates log log t/tel-log c gives straight lines with about the same gradient (tan a=0.45q-0-03) both for various chitosan samples and when changing solvent composition, i.e. CH3COOH content in dilute medium. A study of the temperature dependence of the viscosity of a 2% solution o f chitosan in 2% CHtCOOH in the interval of 15-45 ° shows t h a t it conforms l~jq 8
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I~G. 1. Logarithmic dependence of viscosity (a) and the viscosity (b) of XT-2 chitosan solutions in 2% CH:sCOOH on the logarithmic rate of shear: a--concentration of XT-2, wt.%; 1--0.1; 2--0-3; 3--0.5; 4--0.7; 5--1.0; 6--2.0; 7--3.0; 8--4.0; 9--5.0; 10--7.0; b - - 3 % solution of XT-2; v/, poise; ~, soc-L
Dilute and moderately concentrated solutions of chitosan
1549
ts) the Arrhenius equation. This made it possible to determine thc apparent activation energy of viscous flow, which appeared to bc 33.5 kJ/mole. We note, for comparison, that in a viscose solution (SP~400-500), which is also a modified polysaccharide, similar activation energies are observed in the same temperature range with a considerably higher concentration (6-7°/o). It is kown that the viscosity of polymer solutions depends to a considerable extent on the type and hydrodynamic properties of the solvent [9]. I t was therefore interesting also to examine the effect on the viscosity of chitosan solutions on ~ l v e n t composition when changing acid content in the aqueous phase.
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FIe. 2. Concentration dependence of the relative viscosity of ohitamm solutions in 10% (1) and 2% CH=COOH (~-4); 1, 2, 4--XT-2", 3--XT-I; 4--with 0.2 CH,COONa; c, g/dl. Figure 3a indicates t h a t an increase in the concentration of acetic acid considerably increases solution viscosity. As shown b y Fig. 2 (curves 1 and 2), concentration dependenceg of the relative viscosity of chitosan solutions in acetic acid of varying concentration are similar. As a consequence of the low concentration of hydrogen ions in 2% CH3COOH ([H]----0.0025 g-ion/l.) aminogroups of chitosan in this solvent are protonated to a lesser extent than in 10~/o CH3COOH. With the same concentration of chitosan solutions an increase in the degree of protonation intensifies electrostatic repulsion of groups of the same charge, which as a result produces increased swelling of polymer macromolecules. Consequently, the probability of inter-chain contact and therefore the viscosity of the system increase.
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T h i s m e c h a n i s m of v i s c o s i t y increase m a y b e confirmed b y levelling t h e difference b e t w e e n solution viscosity values in solvents w i t h different concent r a t i o n s of C H a C O O H on a d d i n g to t h e m a l o w - m o l e c u l a r w e i g h t salt w h i c h screens t h e p o s i t i v e charges of p o l y m e r macromolecules. F i g u r e 3b shows, in f a c t , t h a t a n excess a m o u n t of sodium a c e t a t e (0.2 mole/l.) p r o d u c e s p r a c t i c a l l y t h e s a m e v i s c o s i t y of c h i t o s a n solutions in 2 and 10% C H s C O O H .
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FIo. 4. Dependence of the relative viscosity of a 2~o solution of XT-2 on the concentration of the sodium acetate added. FI(~. 5. Variation of the nominal viscosity ~/~H of solutions of XT-I (1, 3) and XT-2 (2, 4-6) samples, according to their retention time in different media (~--viscosity of freshly prepared solution). 1, 3, 5, 6--2~o, 2, d--0"5~o solution of chitosan in 20% (1), 10% (2, 5) and 2% (,wl-IaCOOI-I(3, 4, 6; 3--with 8 • urea).
Dilute and moderately concentrated solutions of chitosan
1551
degree of protonation of chitosan is, of course, achieved than in CH3COOH of the highest concentration. In this case, however, the effect of an excess number of chloride ions (counterions) predominates, which results in a compression of macromolecular spheres and a reduction in viscosity. Hence it follows that i~l the range of low chitosan concentrations rhcological properties of chitosan solutions are determined to a much greater extent by the polyelectrolytic nature of this polymer than by a conventional change in solvent properties. However, a change in thermodynamic properties of the solvent when adding a low molecular weight salt to chitosa~l solution evidently also affects rheological properties a~d this may be apparent in the range of higher chitosan concentrations. This may be confirmed, in particular, by the intersection of concentration curves showing the viscosity of XT-2 solutions with and without 0.2 M CH3COONa in 2% CH~COOH (Fig. 2, curves 2 and 4). From this and results in the literature [10] it may be assumed that the presence of salt causes a deterioration in thermodynamic properties of the solvent therefore, in the range of relatively dilute polymer solutions viscosity decreases on adding salt and above a certain critical concentration of polymer solutions--viscosity increases. Figure 4 indicates that an increase in salt content in this solution further increases the viscosity of the system. A study of viscosity variation over a period of time after applying stress to the system shows that no thixotropic properties are observed in chitomm solutions. Furthermore, solution viscosity decreases irreversibly on keeping the solutions without external shear field over a period of time. To explain whether this is the result of breakdown processes, or structural changes in the solution, we examined the dependence of the variation of the viscosity of chitosan solutions on the time of retention in various media. Figure 5 shows that the lower the concentration of acid in the solvent (curves 4-6) and the higher the concentration of the polymer in solution, the more significant the reduction of viscosity over a period of time. If in 20% CH.~COOH no viscosity reduction is observed for 2% polymer solutions, in 10~/o acetic acid with the same polymer concentration viscosity decreases although to a lower extent than in 2% acetic, acid. With lower polymer concentration (0.5%) solution viscosity is almost unchanged even in 10% CH3COOH (curve 2). l~gularitics observed in viscosity reduction over a period of time, apparently, are not the result of break(lown processes as it is difficult to imagine that in 2(~/o. CH3COOH the glycoside bond is ruptured or associates break down and thes(, processes are absent from media with a higher acetic acid content. It is more natural to relate changes in viscosity with structural regrouping. It may be assumed that in 2°/o CHaCOOH as a result of their considerable length chitosaa macromolecules are in a rather convoluted state consequently the formation of intramolecular hydrogen bonds is very likely. This results in a compressio~ of macromolecular spheres and therefore reduces viscosity. With art increase in the concentration of acetic acid swelling of macromolccules on the o t h e r
1552
A.M.
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hand impedes the formation of intra-chain hydrogen bonds while inter-chain contacts formed contribute to a structural stabilization of the solution. To explain the type of intra- and inter-molecular contact formed in the system studied, viscosity measurements of chitosan solutions were carried out in the presertce of various additives which break down hydrogen (urea, diehloroacetic acid) and ionic bonds (low-molecular weight salt). Thus, the addition to a ehitosan solution in 2% CH3COOtt of urea in the concentration range of 4 M to 8 M produces an almost linear increase in solution viscosity with the concentration of urea solution. Figure 6 shows t h a t for 8 ~I urea the viscosity of a 0'5~/o solution of XT-2 is more t h a n doubled. In the presence of u r e a a s a result of the rupture of hydrogen bonds chitosan macromolecules are straight~nod and this creates favourable'conditions for aggregation, probably as a result ~rel ~ I0 -z q'2- x \
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F I e . 6. Variation o f t h e r e l a t i v e viscosity" of ohitosan solutioxm according to t h e logarithmic r a t e of shear, ~ ( s e e - l ) : / - 3 - - 0 . 5 % solution of X T - 2 in 2 % C'~sCOOt~, 4 - 7 - - 2 % solution of X T - I in 2 % C ' ~ s C O O H (4, 7), in 0.1 ~¢ HC1 (5) and in 0.128 x •dichloroacetic acid (6). A d d i t i v e s : 1 - - 8 m u r e a a n d 0"2 s o d i u m acetate; 3, 7 - - 8 M urea.
Dilute and moderately concentrated solutions of chitosan
1553
of ionic bonds. It may be assumed therefore that the system studied contains complex macro-ions formed as a result of the addition of ionized macromolecules to non-dissociated macromolecules (with .combined counterions) of chitosan. Ionic aggregation is supported by the fact that further addition of a low molecular weight salt to the system examined reduces viscosity to the level of chitosarl solution without urea or salt. A reduction in viscosity with addition of salt is not evidently the result of convolution of chitosan m a c r o molecules in this case by the action of ionic strength since on comparing viscosities in hydrochloric and diehloroacetic acids of the same ionic strength, viscosity values in dichloroacetic acid appear to be higher than in hydrochloric acid (Fig. 6, curves 5 and 6). I t is interesting that on keeping acetic acid solutions of chitosan with 8 M urea over a period of time the viscosity of the system decreases at a lower rate than in solutions free from urea (Fig. 5, curves 3, 6). In the cases examined the mechanism of solution visoosity reduction apparently varies. I n the presence of 8 M urea the breakdown of associates results probably in a reduction of viscosity. The addition of a low-molecular weight salt impedes the formation of these associates; in the presence of 8 M urea and salt in the solution viscosity therefore shows little variation over a period of time. Dichloroacetic acid has a similar effect since it explains a certain ionic strength of the medium and is an effective accepter of the hydrogen bond. Measurement of viscosity in dilute solutions of dichloroacetic acid with a concentration of 0-1 to 0.3 M shows that the viscosity of chitosan solutions is practically unchanged for a prolonged period of time (up to 7 days); viscosity shows a slight reduction after 15 days which is probably due to the breakdown of polymers. Therefore, a viscosity variation of chitosan solutions on retention for a period of time is mainly related to variations in the form and dimensions o f macromolecules and to the formation and breakdown of their assooiates. No breakdown processes are observed in practice under the conditions investigated. This is in agreement with results in the literature [II] confirming increased stability to acid hydrolysis of the glycoside bond containing an amino-group a t the s e c o n d carbon atoms. I t should be noted finally that the regularities established in this study, which are evidently general for solutions of ehitosan polymer-homologues may, however, vary within certain limits, according to the molecular weight of chitosan, polydispersion and the homogeneity of distribution of amino-groups in its macromoleeules. T r a ~ / a ~ by E. S z ~ REFERENCES
1. R. MUZZARF.LLI, Chitin, London, Pergamon Press, 309, 1977 2. A. I. GAMZAZADE, A. M. SKLYAR, S.-S. A. PAVLOVA and S. V. ROGOZHIN, Vysokernel, soyed. A23: 594, 1981 (Translated in Polymer Sci. U.S.S.R. 28: 3, 665, 1981)
G. M. B A l O N e Y and Yu. A. SrmcmrnvA
1554
3. L. A. NUD'GA, Ye. A. PLISKO and S. N. DANILOV, Zh. obshch, khimii 41: 2555~ 1971 4. L . V . TITKOVA, A. B. BYSTROV, (book) Tezisy dokladov sominara " I n s t r u m e n t a l ' n y y o m e t o d y reologii" (Proceedings of Papers of the Seminar " I n s t r u m e n t a l Methods o f Rhoology"). Moscow, Izd. VDNKh, 16, 1977 5. Tsollyuloza i eye proizvodnyye (Cellulose and its Derivatives). Edited b y N. Baikaz and L. Me Segala, Mir, vol. 1, p. 440, 1974 6. G. KISS and R. PORTER, J. Polymer Sei. C: 65, 193, 1.(}78 7. S. P. PAPKOV, Fiziko-khimieheskiye osnovy pererabotki rastvorov polimerov, Moscow, Khimiya, 239, 1971 8. V. G. KULICHIKHIN, A. Ya. MALKIN', Ye. G. KOGAN and A. V. VOLOKHINA, Khimieh. volokna, No. 6, 26, 1978 9. A. A. TAGER, Fizikokhimiya polimerov, Moscow, Khimiya, 392, 1978 10. A. A. TAGF~, V. Ye. PREVAL', G. D. BOTVINNIK, S. B. KENINA, V. I. NOVITSK~YA, L. K. SIDOROVA and T. A. USOL'TSEVA, Vysokomol. soyed. A I 4 : 1381, 1972 11. A . FOSTER, D. HORTON a n d M. STACEY, J. Chem. Soc., 81, 1957
Polymer Science U.S.S.R. Vol. 23, No. 6, pp. 1554-1561. 1981
Prha~l in Poland
0032-3950/81/061554-08507.50/{~ © 1982 Pergamon Pre~ Ltd.
RELAXATION NATURE AND REGULARITIES OF BREAKDOWN OF CROSSLINKED AND NON-CROSSLINKED POLYMERS IN THE, HIGH-ELASTIC STATE* G. M. BA-RTE.'~EVand Yu. A. S~'ICHKI~A I n s t i t u t e of Physical CheraistlT, U.S.S.R. Academy of Sciences Scientific Research Tnstitute of the R u b b e r Industry"
(Received 20 April 1980) A s t u d y was made of the durability" and strength of crosslinked and non-crosslinked elmstomers with varying degrees of crosslinkmg. A power law is observed for d u r a b i l i t y and strength with exponents showing a slight dependence on the number of chemical crosslinks in 1 em: polymer N. The actiw~tion energy of d u r a b i l i t y is independent of _N and coincides with the activation energy of relaxation 2-trausitions, but strength and relaxation characteristics show a m a r k e d dependence on the degree of crosslinking passing through a maximum, which is explained b y the effect of chemical network on non-temperature factors of d u r a b i l i t y and relaxation times and therefore on dimensions of micro-razlges of supermolecular microstructure. Results confirm a unique relaxation t y p e process of visco-elasticity and fracture. RESULTS o f i n v e s t i g a t i n g t e m p e r a t u r e - t i m e depende~ccs of the complex of m e e h a I l i c a l p r o p e r t i e s o f e l a s t o m e r s ( s t r e s s r e l a x a t i o n , v i s c o u s flow, p r o c e s s e s * Vysokomol. soyed. A23: ~ o . 6, 1404-1410, 1981.
A . M . S~,LYA~ e t a / .
18. H. R. ALLCOCK, R. W. ALLEN and I. I. MEIRTER, Conformational Analysis of Poly(dihalophosphazones), Macromolecules 9: 950, 1976 19. R. W. ALLEN and H. R. )ALI.COCK, Conformational Analysis of Poly(alkoxy- and Aryloxyphosphazones), Makromolecules 9: 950, 1976 20. C. C. PRICE and D. D. CARMELITE, J. Amer. Chem. Soc. 88: 4039, 1966 21. Metody issledovaniya polimerov (Methods of Investigating Polymers) ed. b y A. I. Pravednikova, Moscow, Izd. inostr, lit., 155, 1961 22. V. N. REIIKH and B. A. FAINBERG, Metody tekhnicheskogo kontrolya kachestva sinteticheskikh kauehukov i lateksov (Methods of Technical Control of the Quality of Synthetic Rubbers and Latexes). Moscow, Goskhimizdat, 61, 1951
PolymerScienceU.S.S.R. Vol.23, No. 6, pp. 1546-1554, 1981 Printed in Poland
0032-3950/81/061546-09507.50]0 © 1982PergamonPre~ Ltd.
STUDY OF RHEOLOGICAL P R O P E R T I E S OF D I L U T E A N D M O D E R A T E L Y CONCENTRATED SOLUTIONS OF CHITOSAN* A. M. SKLYAR, A. I. GAMZAZADE,L. Z. RooovI~A, L. V. TITKOVA,S.-S. A. PAVLOVA, S. V. ROOOZHI~"and G. L. SLOh~-VmK~ I n s t i t u t e of Hetero-organie Compounds, U.S.S.R. Academy of Sciences
(Received 24 April 1980) A study was made of the variation of rheological properties of chitosan solutions, according to the concentration of the polymer, the composition and type of solvent, the presence of various low-molecular weight additives (electrolyte, urea, etc.). I t was noted that some rheological properties of chitosan solutions in the concentration range studied depend on its polyelectrolyte nature. I t was shown that the viscosity variation of ehitosan solutions on keeping them over a period of time is not duo to the rupture of glycoside bonds of the polymer. I t was found that chitosan solutions have the highest stability over a period of time in salt media also containing a hydrogen bond accepter.
Cm'rosa_~ is a polyglucosamine of the following structure ["-
I,
Clq2OH
o-I
"7
i
which is an ionogenic polysaccharide with a complex of properties highly valuable * Vysokomol. s o y e d . . ~ 3 : No. 6, 1396-1403, 198I.
Dilute and moderately concentrated solutions of chitosan
1547
f r o m a practical p o i n t of view [1]. I n particular, c h i t o s a n offers considerable p o t e n t i a l in t h e a p p l i c a t i o n a n d p r o d u c t i o n of films, m e m b r a n e s a n d fibres w i t h ion-exchange properties, usually f o r m e d from p o l y m e r solutions. A s t u d y o f rheological properties of chitosan solutions in a p o ~ i b l y wider range o f concentrat i o n is therefore o f special urgency. A similar s t u d y is also of i m p o r t a n c e for t h e u n d e r s t a n d i n g of special features of v a r i o u s p o l y m e r - a n a l o g o u s conversions o f c h i t o s a n carried o u t in h o m o g e n e o u s m e d i u m . A n i n v e s t i g a t i o n of properties of chitosan solutions with high degrees of dilut i o n ( ) 0 . 1 ° / o ) shows t h a t this p o l y s a c c h a r i d c has v e r y high intrinsic viscosity (up to 24 dl/g) [2]. This p r o p e r t y is p r o b a b l y due to the v e r y high r i g i d i t y of t h e p o l y m e r chain a n d the high molecular w e i g h t of chitosan. This a n d t h e ionogenic n a t u r e of c h i t o s a n enable us to e x p e c t specific rheological properties of solutions, s t a r t i n g from low p o l y m e r c o n c e n t r a t i o n s ( ~ 1 ~/o)I n order to d e t e c t these features, we e x a m i n e d the dependence o f the v i s c o s i t y o f dilute acidic solutions o f c h i t o s a n on the rate of d i s p l a c e m e n t a n d the effect on this d e p e n d e n c e of the c o m p o s i t i o n and t y p e o f solvent, v a r i o u s low-molecular w e i g h t a d d i t i v e s (CHaCOO~Ta, urea, etc.). Two chitosan samples were examined: XT-1 and XT-2 with [t/l= 17 and 24 dl/g, r~spectively (intrinsic viscosity values were measured in 2% CI-IsCOOI-I using the method of "isoionic dilution"). Chitosan was obtained from chitin by treatment with a 48%o dilute solution of NaOH (in a 1 : 10 ratio) at 140° in argon for 1 hr [3]. The product obtained was washed with water (to neutral pH), thcn with methanol, ether and dried in air, then in vacuum at 60-70 °. The polymer was thcn reprecipitated from a hydrochloride solution (brought to a plC[ of 5.4 with ml alkaline solution) in 0-005 ~ dilute solution of NaOI-I. The finely dispersed residue was subjected to lyophilic drying, followed by drying in vacuum while heating. After reprecipitation the solubility of chitosan (amino-group content 78:[:3~o) increases, however, it was ~possible to obtain XT-2 solutions of definite homogeneity in 2 ~o CI:[:COOH: in a proportion over 7~b. This is why the chitosan concentration range selected was between 0.1 and 7~/o. Dilute solutions of hydrochloric, acetic and dichloroaeetic acids of varying concentration (ctfitosan does not dissolve in pure water) were used as solvents. The solution of ehitosazt samples studied was examined by two different methods to show that the general regularities obtained are independent of conditions of preparing the solutions. XT-1 solutions were prepared mainly from finely dispersed polymer powder by intensive agitation at 500 g for 30 rain, followed by centrifuging of tim solution obtained at 3200 g for 5 rain. Solutions of the X-2 sample were obtained mainly by spontazmons dissolving, keeping the polymer in the solvent for 16 hr. With this method of solution fairly homogeneous solutions could only be obtained in dilute solutions of acetic acid containing at least 2~o, and in the cas~, of hydrochloric acid, not more than 0.3 ~. Solutions of both ehitosan samples examined were therefore prepared i~l these limits of acid concentration. The viscosity of the solutions studied was measured in a polylTmr concentration range of 0"1-I'0%o using a precision rotary viscometer for the measurement of low viscosities (VMV-03) [4] and a "Rheotest-2" rheoviseometer (Gcrnmn Democrat ie Republic) for higlmr concentrations. M e a s u r e m e n t s carried o u t in a wide r a n g e of p o l y m e r c o n c e n t r a t i o n (Fig. l) i n d i c a t e t h a t c h i t o s a n solutions w i t h c o m p a r a t i v e l y low c o n c e n t r a t i o n s ( ~ 1~/o)
1548
A . M . Sg_LYA~ et al.
are non-l~lewtonian liquids, for which a reduction of viscosity is observed in the entire range of shear rates examined (from 1 × 10 -1 to 3 × l0 s see-l). For a number of samples viscosity reduction is higher at low rates of shear than at high rates of shear, when viscosity is almost independent of shear rate, while with fairly high rates of shear viscosity normally shows a marked reduction. This is ~ypical of the flow of pseudoplastic systems, such as solutions of long rigid macromolecules , particularly polyamino-acids and cellulose derivatives [5, 6]. In view of the absence from chitosan solutions of a clearly expressed range of m a x i m u m Newtonian viscosity, properties of these solutions were compared according to viscosity in the range of shear rates, where viscosity variation is comparatively slight, namely at 48.6 see- 1 (this is clearly seen from Fig. lb, where viscosity variation is shown in semi-logarithmic coordinates t/--log ~). Figure 2 shows the concentration dependence of relative viscosity (t/re1) at a given rate of shear for both chitosan samples in different solvents. It can be seen t h a t the dependences have a form, which is typical of most isotropic polymers examined [7, 8]. I t should be noted, however, t h a t rectification of curves 1--3 in coordinates log log t/tel-log c gives straight lines with about the same gradient (tan a=0.45q-0-03) both for various chitosan samples and when changing solvent composition, i.e. CH3COOH content in dilute medium. A study of the temperature dependence of the viscosity of a 2% solution o f chitosan in 2% CHtCOOH in the interval of 15-45 ° shows t h a t it conforms l~jq 8
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Dilute and moderately concentrated solutions of chitosan
1549
ts) the Arrhenius equation. This made it possible to determine thc apparent activation energy of viscous flow, which appeared to bc 33.5 kJ/mole. We note, for comparison, that in a viscose solution (SP~400-500), which is also a modified polysaccharide, similar activation energies are observed in the same temperature range with a considerably higher concentration (6-7°/o). It is kown that the viscosity of polymer solutions depends to a considerable extent on the type and hydrodynamic properties of the solvent [9]. I t was therefore interesting also to examine the effect on the viscosity of chitosan solutions on ~ l v e n t composition when changing acid content in the aqueous phase.
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FIe. 2. Concentration dependence of the relative viscosity of ohitamm solutions in 10% (1) and 2% CH=COOH (~-4); 1, 2, 4--XT-2", 3--XT-I; 4--with 0.2 CH,COONa; c, g/dl. Figure 3a indicates t h a t an increase in the concentration of acetic acid considerably increases solution viscosity. As shown b y Fig. 2 (curves 1 and 2), concentration dependenceg of the relative viscosity of chitosan solutions in acetic acid of varying concentration are similar. As a consequence of the low concentration of hydrogen ions in 2% CH3COOH ([H]----0.0025 g-ion/l.) aminogroups of chitosan in this solvent are protonated to a lesser extent than in 10~/o CH3COOH. With the same concentration of chitosan solutions an increase in the degree of protonation intensifies electrostatic repulsion of groups of the same charge, which as a result produces increased swelling of polymer macromolecules. Consequently, the probability of inter-chain contact and therefore the viscosity of the system increase.
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T h i s m e c h a n i s m of v i s c o s i t y increase m a y b e confirmed b y levelling t h e difference b e t w e e n solution viscosity values in solvents w i t h different concent r a t i o n s of C H a C O O H on a d d i n g to t h e m a l o w - m o l e c u l a r w e i g h t salt w h i c h screens t h e p o s i t i v e charges of p o l y m e r macromolecules. F i g u r e 3b shows, in f a c t , t h a t a n excess a m o u n t of sodium a c e t a t e (0.2 mole/l.) p r o d u c e s p r a c t i c a l l y t h e s a m e v i s c o s i t y of c h i t o s a n solutions in 2 and 10% C H s C O O H .
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Dilute and moderately concentrated solutions of chitosan
1551
degree of protonation of chitosan is, of course, achieved than in CH3COOH of the highest concentration. In this case, however, the effect of an excess number of chloride ions (counterions) predominates, which results in a compression of macromolecular spheres and a reduction in viscosity. Hence it follows that i~l the range of low chitosan concentrations rhcological properties of chitosan solutions are determined to a much greater extent by the polyelectrolytic nature of this polymer than by a conventional change in solvent properties. However, a change in thermodynamic properties of the solvent when adding a low molecular weight salt to chitosa~l solution evidently also affects rheological properties a~d this may be apparent in the range of higher chitosan concentrations. This may be confirmed, in particular, by the intersection of concentration curves showing the viscosity of XT-2 solutions with and without 0.2 M CH3COONa in 2% CH~COOH (Fig. 2, curves 2 and 4). From this and results in the literature [10] it may be assumed that the presence of salt causes a deterioration in thermodynamic properties of the solvent therefore, in the range of relatively dilute polymer solutions viscosity decreases on adding salt and above a certain critical concentration of polymer solutions--viscosity increases. Figure 4 indicates that an increase in salt content in this solution further increases the viscosity of the system. A study of viscosity variation over a period of time after applying stress to the system shows that no thixotropic properties are observed in chitomm solutions. Furthermore, solution viscosity decreases irreversibly on keeping the solutions without external shear field over a period of time. To explain whether this is the result of breakdown processes, or structural changes in the solution, we examined the dependence of the variation of the viscosity of chitosan solutions on the time of retention in various media. Figure 5 shows that the lower the concentration of acid in the solvent (curves 4-6) and the higher the concentration of the polymer in solution, the more significant the reduction of viscosity over a period of time. If in 20% CH.~COOH no viscosity reduction is observed for 2% polymer solutions, in 10~/o acetic acid with the same polymer concentration viscosity decreases although to a lower extent than in 2% acetic, acid. With lower polymer concentration (0.5%) solution viscosity is almost unchanged even in 10% CH3COOH (curve 2). l~gularitics observed in viscosity reduction over a period of time, apparently, are not the result of break(lown processes as it is difficult to imagine that in 2(~/o. CH3COOH the glycoside bond is ruptured or associates break down and thes(, processes are absent from media with a higher acetic acid content. It is more natural to relate changes in viscosity with structural regrouping. It may be assumed that in 2°/o CHaCOOH as a result of their considerable length chitosaa macromolecules are in a rather convoluted state consequently the formation of intramolecular hydrogen bonds is very likely. This results in a compressio~ of macromolecular spheres and therefore reduces viscosity. With art increase in the concentration of acetic acid swelling of macromolccules on the o t h e r
1552
A.M.
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hand impedes the formation of intra-chain hydrogen bonds while inter-chain contacts formed contribute to a structural stabilization of the solution. To explain the type of intra- and inter-molecular contact formed in the system studied, viscosity measurements of chitosan solutions were carried out in the presertce of various additives which break down hydrogen (urea, diehloroacetic acid) and ionic bonds (low-molecular weight salt). Thus, the addition to a ehitosan solution in 2% CH3COOtt of urea in the concentration range of 4 M to 8 M produces an almost linear increase in solution viscosity with the concentration of urea solution. Figure 6 shows t h a t for 8 ~I urea the viscosity of a 0'5~/o solution of XT-2 is more t h a n doubled. In the presence of u r e a a s a result of the rupture of hydrogen bonds chitosan macromolecules are straight~nod and this creates favourable'conditions for aggregation, probably as a result ~rel ~ I0 -z q'2- x \
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F I e . 6. Variation o f t h e r e l a t i v e viscosity" of ohitosan solutioxm according to t h e logarithmic r a t e of shear, ~ ( s e e - l ) : / - 3 - - 0 . 5 % solution of X T - 2 in 2 % C'~sCOOt~, 4 - 7 - - 2 % solution of X T - I in 2 % C ' ~ s C O O H (4, 7), in 0.1 ~¢ HC1 (5) and in 0.128 x •dichloroacetic acid (6). A d d i t i v e s : 1 - - 8 m u r e a a n d 0"2 s o d i u m acetate; 3, 7 - - 8 M urea.
Dilute and moderately concentrated solutions of chitosan
1553
of ionic bonds. It may be assumed therefore that the system studied contains complex macro-ions formed as a result of the addition of ionized macromolecules to non-dissociated macromolecules (with .combined counterions) of chitosan. Ionic aggregation is supported by the fact that further addition of a low molecular weight salt to the system examined reduces viscosity to the level of chitosarl solution without urea or salt. A reduction in viscosity with addition of salt is not evidently the result of convolution of chitosan m a c r o molecules in this case by the action of ionic strength since on comparing viscosities in hydrochloric and diehloroacetic acids of the same ionic strength, viscosity values in dichloroacetic acid appear to be higher than in hydrochloric acid (Fig. 6, curves 5 and 6). I t is interesting that on keeping acetic acid solutions of chitosan with 8 M urea over a period of time the viscosity of the system decreases at a lower rate than in solutions free from urea (Fig. 5, curves 3, 6). In the cases examined the mechanism of solution visoosity reduction apparently varies. I n the presence of 8 M urea the breakdown of associates results probably in a reduction of viscosity. The addition of a low-molecular weight salt impedes the formation of these associates; in the presence of 8 M urea and salt in the solution viscosity therefore shows little variation over a period of time. Dichloroacetic acid has a similar effect since it explains a certain ionic strength of the medium and is an effective accepter of the hydrogen bond. Measurement of viscosity in dilute solutions of dichloroacetic acid with a concentration of 0-1 to 0.3 M shows that the viscosity of chitosan solutions is practically unchanged for a prolonged period of time (up to 7 days); viscosity shows a slight reduction after 15 days which is probably due to the breakdown of polymers. Therefore, a viscosity variation of chitosan solutions on retention for a period of time is mainly related to variations in the form and dimensions o f macromolecules and to the formation and breakdown of their assooiates. No breakdown processes are observed in practice under the conditions investigated. This is in agreement with results in the literature [II] confirming increased stability to acid hydrolysis of the glycoside bond containing an amino-group a t the s e c o n d carbon atoms. I t should be noted finally that the regularities established in this study, which are evidently general for solutions of ehitosan polymer-homologues may, however, vary within certain limits, according to the molecular weight of chitosan, polydispersion and the homogeneity of distribution of amino-groups in its macromoleeules. T r a ~ / a ~ by E. S z ~ REFERENCES
1. R. MUZZARF.LLI, Chitin, London, Pergamon Press, 309, 1977 2. A. I. GAMZAZADE, A. M. SKLYAR, S.-S. A. PAVLOVA and S. V. ROGOZHIN, Vysokernel, soyed. A23: 594, 1981 (Translated in Polymer Sci. U.S.S.R. 28: 3, 665, 1981)
G. M. B A l O N e Y and Yu. A. SrmcmrnvA
1554
3. L. A. NUD'GA, Ye. A. PLISKO and S. N. DANILOV, Zh. obshch, khimii 41: 2555~ 1971 4. L . V . TITKOVA, A. B. BYSTROV, (book) Tezisy dokladov sominara " I n s t r u m e n t a l ' n y y o m e t o d y reologii" (Proceedings of Papers of the Seminar " I n s t r u m e n t a l Methods o f Rhoology"). Moscow, Izd. VDNKh, 16, 1977 5. Tsollyuloza i eye proizvodnyye (Cellulose and its Derivatives). Edited b y N. Baikaz and L. Me Segala, Mir, vol. 1, p. 440, 1974 6. G. KISS and R. PORTER, J. Polymer Sei. C: 65, 193, 1.(}78 7. S. P. PAPKOV, Fiziko-khimieheskiye osnovy pererabotki rastvorov polimerov, Moscow, Khimiya, 239, 1971 8. V. G. KULICHIKHIN, A. Ya. MALKIN', Ye. G. KOGAN and A. V. VOLOKHINA, Khimieh. volokna, No. 6, 26, 1978 9. A. A. TAGER, Fizikokhimiya polimerov, Moscow, Khimiya, 392, 1978 10. A. A. TAGF~, V. Ye. PREVAL', G. D. BOTVINNIK, S. B. KENINA, V. I. NOVITSK~YA, L. K. SIDOROVA and T. A. USOL'TSEVA, Vysokomol. soyed. A I 4 : 1381, 1972 11. A . FOSTER, D. HORTON a n d M. STACEY, J. Chem. Soc., 81, 1957
Polymer Science U.S.S.R. Vol. 23, No. 6, pp. 1554-1561. 1981
Prha~l in Poland
0032-3950/81/061554-08507.50/{~ © 1982 Pergamon Pre~ Ltd.
RELAXATION NATURE AND REGULARITIES OF BREAKDOWN OF CROSSLINKED AND NON-CROSSLINKED POLYMERS IN THE, HIGH-ELASTIC STATE* G. M. BA-RTE.'~EVand Yu. A. S~'ICHKI~A I n s t i t u t e of Physical CheraistlT, U.S.S.R. Academy of Sciences Scientific Research Tnstitute of the R u b b e r Industry"
(Received 20 April 1980) A s t u d y was made of the durability" and strength of crosslinked and non-crosslinked elmstomers with varying degrees of crosslinkmg. A power law is observed for d u r a b i l i t y and strength with exponents showing a slight dependence on the number of chemical crosslinks in 1 em: polymer N. The actiw~tion energy of d u r a b i l i t y is independent of _N and coincides with the activation energy of relaxation 2-trausitions, but strength and relaxation characteristics show a m a r k e d dependence on the degree of crosslinking passing through a maximum, which is explained b y the effect of chemical network on non-temperature factors of d u r a b i l i t y and relaxation times and therefore on dimensions of micro-razlges of supermolecular microstructure. Results confirm a unique relaxation t y p e process of visco-elasticity and fracture. RESULTS o f i n v e s t i g a t i n g t e m p e r a t u r e - t i m e depende~ccs of the complex of m e e h a I l i c a l p r o p e r t i e s o f e l a s t o m e r s ( s t r e s s r e l a x a t i o n , v i s c o u s flow, p r o c e s s e s * Vysokomol. soyed. A23: ~ o . 6, 1404-1410, 1981.