Characterization of association phenomena in.pdf

Advances in Colloid and Interface Science .79 1999 81]103

Characterization of association phenomena inaqueous systems of chitosan of different

hydrophobicity q

Bo NystromU, Anna-Lena Kjniksen, Christian IversenDepartment of Chemistry, Uni ersity of Oslo, P.O. Box 1033, Blindern N-0315, Oslo, Norway

Abstract

Rheological, dynamical and structural properties of aqueous systems of chitosan UM-. .chitosan and hydrophobically-modified chitosans HM-chitosan are briefly reviewed. The

effects of pH, level of surfactant addition, polymer concentration, and temperature on therheological behavior, both in the linear and non-linear viscoelastic regime, have beenscrutinized. These variables have the strongest impact on the rheological properties of thehydrophobically-modified chitosans. We observe the formation of concentration-inducedgels for systems of UM-chitosan and HM-chitosans. Incipient gels are evolved at lowerconcentrations as the hydrophobicity of the polymer increases. Non-linear shear thinningbehavior is found in semidilute solutions of UM-chitosan and HM-chitosan at higher shearrates. The magnitude of this effect depends on factors, such as pH, amount of surfactant,polymer concentration, and hydrophobicity of the polymer. The dynamic light scatteringresults from semidilute solutions of UM-chitosan and HM-chitosan show that the inter-molecular association phenomena in the polymer solutions are promoted by decreasingtemperature and increasing polymer concentration and hydrophobicity. The intensity lightscattering measurements on semidilute solutions of UM-chitosan and HM-chitosans suggestthat the systems have a fractal structure and the fractal dimension is approx. 2. Q 1999Elsevier Science B.V. All rights reserved.

Keywords: Aqueous solutions; Hydrophobically associating; Chitosan; Rheology; Light scat-tering

q Part of this paper was presented at the conference on Associating Polymer, Fontevraud, France,November 1997.U Corresponding author. Tel.: q47 22 855522; fax: q47 22 855542; e-mail: [email protected]

0001-8686r99r$ - see front matter Q 1999 Elsever Science B.V. All rights reserved. .P I I: S 0 0 0 1 - 8 6 8 6 9 8 0 0 0 6 9 - 4

( )B. Nystrom et al. r Ad. Colloid Interface Sci. 79 1999 81]10382

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 822. Properties of chitosan and hydrophobically-modified chitosan systems . . . . . . . . . . . 833. Rheology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

3.1. Linear viscoelasticity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 843.1.1. Effects of pH and surfactant concentration . . . . . . . . . . . . . . . . . . . . 853.1.2. Concentration-induced gelation . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

3.2. Non-linear viscoelasticity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 914. Dynamic light scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 945. Static light scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 976. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

1. Introduction

Hydrophobically-modified water-soluble polymers or so-called associating po-lymers are important in a number of areas including enhanced oil recovery, drag

w xreduction, and for formulation in coatings and personal care items 1]5 . Thesenew materials are water-soluble polymers containing a small amount of highlyhydrophobic groups. Typically, less than 2% incorporation of the hydrophobicmonomer into the polymer backbone is necessary to yield associative behavior. Insemidilute aqueous solution the hydrophobic moieties associate, and this results innetwork formation and a rapid increase in solution viscosity. In this process,aggregates, presumably of micellar type, are formed and act as reversible cross-linksbetween the polymer chains. From these systems, very viscous solutions or gelsdisplaying a shear-thinning behavior can be obtained.

The regions of intermolecular hydrophobic association represent areas of highaffinity for surfactant interaction, and consequently, the addition of surfactant hasbeen observed to have a dramatic effect on the structural, dynamic, and rheologicalfeatures of solutions containing hydrophobically-associating polymers. Several pa-pers have been devoted to the study of physical properties of hydrophobically

w xmodified water-soluble polysaccharides 6]12 , especially the non-ionic cellulose .ether ethyl hydroxyethyl cellulose in the presence of an ionic surfactant has been

w x w xthoroughly investigated 13]23 , and a number of review articles 24,25 havealready appeared. Much less attention has been devoted to the static and dynami-cal behavior of solutions and gels of the polysaccharide chitosan, which is ahydrophilic cationic copolymer produced by deacetylation of chitin. This polymer

w xexhibits interesting association and gelling properties 26]32 in aqueous solutionunder various conditions.

Many effects reported here are of a quite general nature, characteristic ofassociating polysaccharide systems, but emphasis in this brief review will be on

w xaqueous systems of chitosan, which we have recently studied 31,34,35 in somedetail. The effects of polymer concentration, surfactant addition, pH, temperature,and hydrophobicity on the linear and non-linear rheological properties for systems

( )B. Nystrom et al. r Ad. Colloid Interface Sci. 79 1999 81]103 83

.of unmodified chitosan UM-chitosan and hydrophobically-modified chitosans .HM-chitosan will be reviewed. Furthermore, some preliminary results fromintensity light scattering and dynamic light scattering on solutions of chitosan andhydrophobically-modified chitosans will be presented.

The paper is organized as follows. In Section 2, we will give some general aspectson the properties of the investigated chitosan systems. In Section 3, we will discussthe rheological features in the linear and non-linear viscoelastic regime forchitosan systems of different hydrophobicity at different conditions of pH, surfac-tant addition, polymer concentration, and temperature. Section 4 presents some

.recent results from dynamic light scattering DLS on solutions of UM-chitosanand HM-chitosan. The effects of polymer concentration, hydrophobicity, andtemperature on the dynamics will be scrutinized. The structural features, in the

.light of preliminary static light scattering SLS experiments, of UM-chitosan andHM-chitosan solutions are reviewed in Section 5. Finally, in Section 6, we summa-rize our main conclusions.

2. Properties of chitosan and hydrophobically-modified chitosan systems

w xChitosans are a family of linear, cationic polymers 36,37 obtained by deacylat-ing the biopolymer chitin, which is found in the skin or shell of anthropods. As the

.degree of deacetylation DD of chitinous material exceeds 50%, it becomessoluble in acidic aqueous solution and is called chitosan. It is a random copolymer,

.containing 1 4 linked 2-acetamide-2-deoxy-b-D-glucopyranose and 2-amino-2-deoxy-b-D-glucopyranose residues. Chitosan is water-soluble at an acidic pH andhas numerous applications in industry, pharmacy, and biotechnology.

This biopolymer is polycationic, i.e. positively charged at pH - 6. Due to thepresence of protonated amino groups the amino group in chitosan has a pK ofa

w x.approx. 6.5 38,39 , chitosan in dilute acid aqueous solution exhibits a polyelectro-lyte character at low pH, and its hydrodynamic and rheological behavior in solution

w xis intricate 40]42 . The physicochemical properties of solutions of chitosan areexpected to be governed by factors, such as temperature, pH, ionic strength,

w xsurfactant concentration, and DD. It is known 43,44 that the charge density alongthe chain increases with an increase in the DD, and that chain flexibility ofchitosan molecules can be manipulated by changing the DD. In addition, thehydrophobicity of chitosan should also play an important role for the dynamic andrheological behavior of this polymer. This biopolymer can be chemically modifiedw x45,46 on the free amino groups or the hydroxyl groups present along the mainchain to obtain a hydrophobically-modified polymer. In this review, chitosansamples with hydrophobic substitution degrees of 2.5, 5, and 10 mol% C12-aldehyde chains grafted to the polymer backbone will be discussed. An illustrationof the structure of chitosan is depicted in Fig. 1. The chitosan samples consideredin this review have a degree of N-deacetylation of 84%, and they have beendissolved in 1% acetic acid, yielding a pH of approx. 3. The weight-averagemolecular weight of the samples is approx. 4 = 105 and the polydispersity indexM rM s 2.7.w n


Fig. 1. A schematic illustration of the structure of UM-chitosan and the hydrophobically-modified .chitosan HM-chitosan .

3. Rheology

3.1. Linear iscoelasticity

In this section we will examine the effects of pH, temperature, polymer concen-tration, and addition of surfactant on the linear viscoelastic properties of systemsof chitosan of different degrees of hydrophobicity. The effect of hydrophobic

substitution is displayed in Fig. 2, where the viscosity at a low shear rate in the.zero-shear-rate Newtonian plateau is plotted as a function of polymer concentra-

tion for systems of UM-chitosan and HM-chitosans in 1% acetic acid. In all cases,the viscosity rises with increasing polymer concentration, but the viscosity enhance-ment is stronger as the hydrophobicity of the polymer increases. The hydrophobiceffect becomes more accentuated with increasing polymer concentration. The

rather restricted concentration range covered by the most hydrophobic sample 10

.Fig. 2. Concentraion dependence of the viscosity in the zero-shear-rate Newtonian plateau ofsemidilute acid aqueous solutions of UM-chitosan and HM-chitosans of different hydrophobicity at the

w xtemperature indicated. From 35 .


.mol% is due to the fact that this sample cannot be dissolved in this solvent above0.3 wt.%. We should note that even for the solution of UM-chitosan, a strongviscosification effect is registered at elevated concentration. At high concentra-tions, it is likely that the entanglement couplings will play an important role for the

w xviscosity. However, it has been argued 47,48 that as the polymer concentrationincreases, unmodified chitosan self-associates through intermolecular hydrophobicinteractions between the acetyl groups. Hence, the effect of hydrophobic associa-tions may also come into play for the unmodified polymer at higher concentrations.

3.1.1. Effects of pH and surfactant concentrationBefore we present and discuss the results, it may be instructive to give a brief

background on how the solution properties of chitosan are influenced by pH andsurfactant interaction. It is known that the association behavior of semidilutesystems of chitosan and HM-chitosan can be affected by pH and the level ofsurfactant addition. As a linear polyelectrolyte, chitosan has both reactive aminogroups and hydroxyl groups, and in addition HM-chitosan also contains hy-drophobic groups that can interact with the surrounding. Thus, we expect that thephysical and solution properties of UM-chitosan and HM-chitosan change with thesurrounding chemical environment. When the pH value is less than approx. 6.5,chitosan in solution carries a positive charge along its backbone. At pH valuesbelow 4, most of the amino groups of chitosan are supposed to be protonated, andsince this effect promotes electrostatic repelling between charged groups of the

w xsame sign, it leads to enhanced swelling 49,50 of the polymer network. AtpH-values greater than 6.5, which is approximately the pK of the amino group inachitosan, chitosan solutions exhibit phase separation.

The association behavior of chitosan systems can also be influenced byw xpolymer]surfactant interactions. In a recent viscosity study 51 on semidilute

solutions of unmodified chitosan, an interaction between chitosan and a non-ionicsurfactant was observed. In general, the polymer]surfactant interaction is strongerin the presence of an ionic surfactant, and this effect is usually further strength-ened in solutions of a hydrophobically-modified polymer. However, as was men-tioned above, signs of enhanced intermolecular hydrophobic interactions withincreasing concentration have also been reported for solutions of unmodified

w xchitosan. The general picture that emerges for this type of system 7,10,11,52 isthat the surfactant molecules interact with polymers at a critical aggregation

.concentration cac and the surfactant molecules will bind to the existing hy-drophobic substituents or microdomains. After saturation, the surfactant will startto associate to form mixed micelles involving substituents from one or morepolymer chains. In the semidilute polymer concentration regime, association ofsurfactant will tend to bring polymer chains together up to a maximum insurfactant concentration c , at which the optimum number of intermolecularsurf.,maxcross-links has been formed. This should correspond to a maximum in the viscosityof the system. At levels of surfactant addition above c , additional surfactantsurf.,maxaggregates are available for the solubilization of the polymer-bound hydrophobes.


As a result, the average number of bound hydrophobes per surfactant aggregatewill decrease, thereby reducing the number of effective micellar-like cross-links.This effect should result in a drop in the viscosity of the system.

w x . .Fig. 3 shows the effect of pH 31 on the storage G9 and loss G0 modulus andUw U 2 2 .1r2 .xthe complex viscosity h h s G9 q G0 r 2pv , where v is the frequency

. .of oscillation for 1 wt.% solutions of UM-chitosan and HM-chitosan 5 mol% inthe presence of 5 mmolal of the cationic surfactant cetyltrimethylammonium

.bromide CTAB . We should mention that similar trends in the rheologicalparameters were also observed at other moderate surfactant concentrations. The

U U w x w x .overlap concentration c , estimated from c s 1r h h is the intrinsic viscosity ,w xis approx. 0.05 wt.% 34 , i.e. these solutions are well located in the semidilute

concentration regime. For the solution of UM-chitosan, practically no pH depen-dence on the rheological quantities is observed. In the case of the HM-chitosan,the rheological parameters pass through minima, located at a pH around 4. Whena 1 wt.% solution of chitosan is prepared in 1% aqueous acetic acid, the solution

.assumes a pH value of 3]4. The pH value is raised by addition of base KOH tothe solution and neutralization of the excess acetic acid present in the solutiontakes place, and as a consequence the ionic strength of the chitosan solutionincreases. This effect may lead to screening of the charges on the HM-chitosanchains and poorer thermodynamic conditions, which will promote enhanced inter-molecular hydrophobic contacts and hence the complex viscosity and the dynamicmoduli are expected to rise as the value of pH increases. In the range 5.8 F pH -

w x 6.4, a gel zone evolves 31 at this stage the sample experiences no flow when a test.tube is turned upside-down for a longer time and at still higher values of pH an

incipient phase separation takes place. In an analogous way, the ionic strength willw .xalso increase when an acid HCl aq is added to the original chitosan solution,

U . . . .Fig. 3. Effect of pH on the complex viscosity h , storage G9 and loss G0 moduli v s 1.5 Hz of 1 .wt.% solutions of UM-chitosan and HM-chitosan 5 mol% in the presence of 5 mmolal CTAB at 258C.

w xFrom 31 .


and this favor enhanced hydrophobic association of the HM-chitosan at low valuesof pH. In this context it should be mentioned that at pH f 0.7, a transparent gel is

w xformed 31 , while at pH values - 0.5 a phase separation occurs. These findingssuggest that the effect of pH on the rheological parameters is related to pH-in-duced hydrophobic associations.

The evolution of the viscoelasticity during the pH-induced gelation process for 1 .wt.% HM-chitosan 5 mol% in 5 mmolal CTAB is depicted in Fig. 4. The curves

are shifted horizontally by a factor 10B of the value listed in the insert to avoidoverlap. The general trend is that at low frequencies, a viscous behavior withG0 ) G9 is observed, while at higher frequencies, depending on the value of pH,G9 increases to cross G0, and above this frequency, G9 exceeds G0, which suggeststhat the elastic response dominates. These results indicate that the HM-chitosansolutions become more elastic at higher frequencies, which is typical for networks

.containing entangled or crosslinked polymer chains. At pH s 6.1 in the gel zoneG9 ) G0 over the considered frequency domain.

U .The frequency of intersection v G9 s G0 , may be determined from G9 andG0 data. In the inset plot of Fig. 4, the equivalent quantity, the time of intersection

U U U . Ut t s 1rv is plotted as a function of pH. The value of t is at its minimumat pH f 4 and increases toward lower and higher pH values. The quantity t U canbe considered as a relaxation time for chain disengagement of the network. Theincrease of t U at low and high values of pH is probably another manifestation ofenhanced hydrophobic associations at these values of pH.

Fig. 4. Frequency dependences of the storage modulus G9 and the loss modulus G0 at different pH .values of 1 wt.% solutions of UM-chitosan and HM-chitosan 5 mol% in the presence of 5 mmolal

CTAB at 258C. The curves have been shifted horizontally by a factor 10B of the value listed in the inset.U . w xThe inset plot shows the pH dependence of the time of intersection t see text . From 31 .


In Fig. 5, the effect of addition of CTAB on hU is illustrated for 1 wt.% solutionsof UM-chitosan and HM-chitosan at different values of pH. The complex viscosityis virtually independent of the level of surfactant addition for the UM-chitosan,while a significant polymer]surfactant interaction is observed for the HM-chito-san-CTAB system, at all the pH values, with at first an increase and then adecrease in the value of hU. The maximum of hU is always located at a surfactantconcentration of approx. 1 mmolal, close to the critical micelle concentration for

CTAB in water. This surfactant concentration, probably corresponds to c cf.surf.,max.the discussion above , where the optimum number of intermolecular cross-links has

been formed. The decrease in hU at higher surfactant concentrations can beattributed to a disruption of the polymer network. Similar trends in the viscosityhave recently been reported for other types of hydrophobically-modified polymers.

3.1.2. Concentration-induced gelationHydrogels of chitosan and the rheological behavior of chitosan systems under

various gelling conditions have attracted a great deal of interest in recent years. Inmost studies, the chitosan gels were chemically cross-linked by utilizing different

w xcross-linking techniques 26,28]30,32,33,49,53]58 . The effects of factors, such asw xthe type and concentration of cross-linking agent 26,29,30,56 , cosolvent, concen-

w xtration and molecular weight of chitosan 26,27,30,56,57 , degree of deacetylationw x w x w x26,29,30,57 , pH 30,31,49,55 , and temperature 26,32,55,57,58 on the gelationprocess have been reported.

w xIt has recently been observed 27,35 that physical gels of chitosan can be formed

U .Fig. 5. Effect of surfactant addition and pH on the complex viscosity h v s 1.5 Hz of 1 wt.% . w xsolutions of UM-chitosan and HM-chitosan 5 mol% at 258C. From 31 .


by increasing the polymer concentration. The general feature observed for thegelling UM-chitosan and HM-chitosan systems in Fig. 6 is that the loss tangent, tand s G0rG 9, decreases during the gel formation, indicating that the solutionsbecome more and more elastic. This type of behavior is frequently observed forgelling systems of both physical and chemical natures. The concentration-induced

w xgel point is determined by observation of a frequency independent value 59,60 oftan d obtained from a multifrequency plot of tan d vs. polymer concentration. It isevident from Fig. 6 that the value of the gel concentration decreases with

.increasing hydrophobicity of the polymer see also Table 1 . We should mentionthat the gel concentrations obtained by this procedure are consistent with those

w xobserved with the test tube tilting method 61,62 , where the gelation concentra-tion was determined by tilting the test tube containing the solution. In thisapproach the concentration at which the solution no longer flows is taken as theconcentration of gelation. These results suggest that the hydrophobicity of thepolymer plays an important role in the gel-forming process. Our conjecture is thatthe gel network is controlled by an intricate interplay between entanglementcouplings and crosslinks formed by hydrophobic associations.

At the gel point, G9 and G0 curves are expected to become parallel to each n.other, and a power law behavior G9 ; G0 ; v in frequency should be observed

.see Fig. 7 . The value of the viscoelastic exponent n is virtually the same for the

Fig. 6. Viscoelastic loss tangent as a function of polymer concentration for the systems and frequenciesw xindicated. From 35 .


Table 1Characteristic data for incipient chitosan and HM-chitosan gels

nU w x .Hydrophobicity c s lr h Gel conc. n S Pas df . . . mol% wt.% wt.% calculated from calculated from

.. ..Eq. 2 Eq. 1

0 0.044 3.9 0.36 232 2.22.5 0.024 3.0 0.46 83 2.05.0 0.027 1.75 0.46 24 2.0

.HM-chitosans, but somewhat smaller for the UM-chitosan system see Table 1 .Since the gel concentration is higher for UM-chitosan than for the HM-chitosans,it is possible that this system possesses stronger entanglement couplings than the

w xhydrophobically-modified samples. It has been argued 63 that an increasingentanglement density may give rise to lower values of n.

The values of n reported for these chitosan systems are considerably smaller . w xthan that 0.7 predicted for percolating networks 64,65 . Recently, Muthukumar

w x66 developed a theoretical model to rationalize values of n deviating from thepercolation value. In this approach it is assumed that the variations in the strandlength between cross-linking points of the incipient gel network give rise tochanges in the excluded volume interactions and the value of the power lawexponent. The hypothesis is that increasing strand length will enhance the excludedvolume effect. If the excluded volume interaction is fully screened, the relaxation

w xexponent for a polydisperse system 66 can be written as

. . .n s d d q 2 y 2 d r2 d q 2 y d 1f f

Fig. 7. Plot of G9 and G0 vs. frequency for the indicated systems at the gel point, showing the powerw xlaw behavior. From 35 .


.where d d s 3 is the spatial dimension and d is the fractal dimension thatfrelates the mass of a molecular cluster to its radius of gyration by Rd f ; M. In the

.framework of Eq. 1 , all the values of the exponent 0 - n - 1 are possible for afractal in the physically realizable domain 1 F d F 3. The value of the fractalf

.dimension, calculated with the aid of Eq. 1 , for the UM-chitosan is higher than .those of the hydrophobically-modified chitosans see Table 1 . These findings

w xindicate 66,67 that the incipient network of the UM-chitosan is more dense, witha higher value of d , than those of the HM-chitosans. In this context we shouldf

w xnote that recent intrinsic viscosity results 34 on dilute solutions of the samepolymer samples, suggest that the molecules of UM-chitosan are more compactthan those of HM-chitosans. It is possible that the introduction of hydrophobicgroups produces a gel structure with some branches, leading to a more opennetwork structure.

The linear viscoelastic properties of incipient gels can be characterized by the gelw xstrength parameter S 59,60 , that depends on the cross-linking density and the

molecular chain flexibility. This material parameter can be defined by means of thefollowing relationship

n . .G9 s Sv G 1 y n cosd 2

.where G 1 y n is the gamma function. An inspection of Table 1, reveals that Sincreases significantly with decreasing degree of hydrophobic substitution. Thismay be an indication that the entanglement couplings have a stronger impact onthe formation of the gel network than the specific interactions in form of hy-drophobic associations.

Temperature is a variable that in many cases affects the viscoelasticity ofassociating polymer systems. The effect of temperature on the dynamic viscosity h9for solutions of chitosan of different hydrophobicity is shown in Fig. 8. A compar-ison of the temperature dependence of h9 for solutions of a fixed concentration of

.1.5 wt.%, and for concentrations corresponding to the same value 18.5 of theratio crcU is displayed. In both cases, the value of h9 drops with increasingtemperature, and this effect is practically the same independent of hydrophobicsubstitution. These results suggest weakening of the networks at elevated tempera-

w xtures. In this context we may note a recent NMR study 57 on gelling chitosansystems, where an increase in temperature was found to favor the molecularmobility of the chains.

3.2. Non-linear iscoelasticity

A typical feature of many associating polymer systems is the strong shear ratedependence of the viscosity. In many cases, the shear thinning phenomenon isobserved at higher shear rates. Shear thinning begins at the shear rates at whichshear forces disrupt the network by reducing the number of interchain bonds,lowering the resistance to the flow as well as the viscosity. The effect of shear rateon the measured viscosity is depicted in Fig. 9 for 1 wt.% solutions of UM-chitosan


.Fig. 8. Temperature dependence of the dynamic viscosity for the systems indicated see text .

and HM-chitosan at different conditions of pH and CTAB concentration. Apractically Newtonian behavior is observed for the UM-chitosan at the consideredconditions in the shear rate range examined. For solutions of HM-chitosan, on theother hand, a non-Newtonian shear thinning effect is found at higher shear rates.This effect is most pronounced at low surfactant concentrations in solutions of pHvalues of 1.2 and 5.0, while shear thinning is less dominant at pH s 4.0 and in

.solutions of high surfactant concentrations disruption of the network .Fig. 10 shows the effect of shear rate on the measured viscosity for solutions of

UM-chitosan and HM-chitosans in 1% acetic acid at different polymer concentra-tions. At low concentrations, all the systems exhibit Newtonian behavior, but as theconcentration raises a feature of non-Newtonian shear thinning is observed athigher shear rates, and this effect is first visible for the most hydrophobically-mod-ified polymer. These results reveal an augmentation of the shear thinning effectwith increasing polymer concentration and hydrophobicity. The observation thatshear thinning becomes stronger as the concentration increases has previously

w xbeen reported 27,68,69 for different chitosan systems.The relative degree of shear thinning for the UM-chitosan and HM-chitosan

systems can be characterized in the framework of a simple power law relationship,which can be expressed as

ay1. .h ; g 3


Fig. 9. Shear rate dependence of the viscosity for 1 wt.% solutions of UM-chitosan and HM-chitosan 5. w xmol% at 258C and at the conditions indicated. From 31 .

where g is the shear rate and a is the power law index. The values of a ,determined from the shear thinning parts of the curves, for the solutions ofUM-chitosan and HM-chitosans under different conditions have been collected inTable 2. For the systems discussed here, no disturbing hysterises effects wereobserved when the shear-rate was first increased and then decreased back to itsinitial value. The results in Table 2 reveal that at pH values favoring intermolecu-

w xFig. 10. Shear rate dependence of the viscosity for the concentrations and systems indicated. From 35 .


Table 2 .Values of the characteristic exponent a describing the non-Newtonian behavior of chitosan and

HM-chitosans

CTAB concentration s 0 mmolal, pH ( 3.5 Polymer concentration a . ..Hydrophobicity wt.% see Eq. 3

.mol%

0 3.00 0.473.75 0.20

2.5 1.50 0.562.25 0.503.25 0.25

5.0 1.50 0.391.75 0.312.25 0.11

Polymer concentration s 1.00 wt.%Hydrophobicity s 5.0 mol%pH CTAB concentration a

. ..mmolal see Eq. 31.2 5.0 0.284.0 5.0 0.585.0 5.0 0.34

1.2 30.0 0.464.0 30.0 0.565.0 30.0 0.46

lar association, low CTAB concentrations, increasing polymer concentration andhydrophobicity all promote the shear thinning effect of these systems at elevatedshear rate. The picture that emerges is that the decrease in viscosity with increas-ing shear rate can be attributed to the disruption of the network junctions, that is,the rate of junction disruption exceeds the rate at which hydrophobic associationsor entanglement couplings can be re-formed.

4. Dynamic light scattering

.A dynamic light scattering DLS experiment probes the relaxation times ofprocesses which relax on some length scale qy1, where q is the wave vector defined

. .as q s 4p nrl sin ur2 , here l is the wavelength of the incident light in avacuum, u is the scattering angle, and n is the refractive index of the medium.When the scattering light obeys Gaussian statistics, the measured homodyne

2 .intensity autocorrelation function g q,t is related to the theoretically amenable1 .first-order electric field correlation function g q,t through the Siegert relation

2 . < 1 . < 2 .g q ,t s 1 q B g q ,t 4


.where B F 1 is the coherence factor that depends on the experimental geometry.This DLS study on semidilute solutions of UM-chitosan and HM-chitosan

revealed the existence of two relaxation modes. Initially, the decay of the correla-tion function is described by a single exponential, followed at longer times by astretched exponential

b1 . . . .g t s A exp y1rt q A exp y trt 5f f s se

with A q A s 1. The parameters A and A are the amplitudes for the fast andf s f sthe slow relaxation mode, respectively. Analyses of the time correlation functionsof the concentration fluctuations at long wavelengths in the semidilute concentra-

.tion regime have shown that the first term short-time behavior on the right-hand . y1 2 .side of Eq. 5 is related to a collective diffusion coefficient D t s D q , whichc f c

reflects a concerted motion of the polymer chains relative to the solvent. The .second term long-time behavior is expected to be associated with disengagement

w x w xrelaxation of individual chains 70,71 or cluster relaxation 72 . The variable t isse .some effective relaxation time, and b 0 - b F 1 is a measure of the width of the

distribution of relaxation times. The distribution of relaxation times for the presentsystems is rather narrow with b s 0.8]0.9. The mean relaxation time is given by

`b . . . .t ' exp y trt d t s t rb G 1rb 6Hs se se0

y1 . y1where G b is the gamma function of b . .Fig. 11 shows time correlation function data at a scattering angle of 608 at

various polymer concentrations, together with the corresponding curves fitted with . .the aid of Eq. 5 , for solutions of UM-chitosan and HM-chitosan 2.5 mol% . The

time correlation function is always bimodal and can for all chitosan systems, .initially be described by a single exponential fast mode followed by a stretched

.exponential slow mode at longer times. We note that there is a progressiveslowing down of the long-time relaxation process as the polymer concentrationincreases. At a given concentration, the slowing down feature is more pronouncedfor the hydrophobically-modified polymer.

The effect of hydrophobicity is depicted in Fig. 12 for two different concentra-tions. The shift of the relaxation process toward longer times is strong when thedegree of hydrophobic substitution is raised from 0 to 2.5 mol%, and this effect isenhanced with increasing polymer concentration. However, when the hydrophobicsubstitution is increased from 2.5 to 5 mol%, there is only a small additional effect.

The temperature dependence of the reduced collective diffusion coefficient .D h rT h is the solvent viscosity and T is the absolute temperature for 1 wt.%c 0 0

solutions of UM-chitosan and HM-chitosans is displayed in Fig. 13a. The reduceddiffusion coefficient has been used to eliminate trivial temperature and solventviscosity effects. The reduced diffusion coefficient falls off with increasing tempera-ture for all the polymer samples, and the values of D seem to be higher for thechydrophobically modified polymers. The same type of behavior is also observed forthe other polymer concentrations.


.Fig. 11. First-order electric field correlation function vs. time every second point is shown for thedisplayed systems at the scattering angle, temperature, and concentrations indicted. The solid curves are

.fitted with the aid of Eq. 5 .

The concentration dependence of D for solutions of UM-chitosan and HM-c .chitosans at 358C the same trend is also found for the other temperatures is

depicted in Fig. 13b. D is virtually independent of concentration in the consideredcrange.

The concentration and temperature dependences of the slow relaxation time tsfor solutions of UM-chitosan and HM-chitosans are shown in Fig. 14. An increaseof t with concentration is observed for all the systems, but this effect is morespronounced for the solutions of HM-chitosans. These findings suggest that in-creased polymer concentration and hydrophobicity promote interpolymer associa-tion, and thereby hamper the process of chain disengagement. The reduced slowrelaxation time decreases with increasing temperature for both UM-chitosan andHM-chitosans, and this tendency is consistent with the behavior of the dynamicviscosity in Fig. 8. A temperature rise seems to promote the disengagement of thepolymer chains.

Fig. 15 shows the q dependences, in a reduced form, of the correlation functionsfor 0.5 wt.% solutions of UM-chitosan and HM-chitosans at 258C. The separationof the curves at longer times suggests that the slow mode exhibits a stronger q

3 .dependence approximately a q dependence than that of a diffusive process. This


Fig. 12. Effect of hydrophobicity on the correlation functions at the scattering angle, temperature, andconcentrations indicated.

type of behavior of the slow relaxation mode is often observed for associatingsystems. A more detailed analysis of the angular dependence of the slow relaxation

w xtime will be presented in a forthcoming paper 73 .

5. Static light scattering

Semidilute solutions of associating systems may be viewed as transient networksformed by more or less interpenetrating clusters. In the regime qj ) 1, the length

y1 scale q is associated with more local properties of the system one sees inside of.the clusters and the scattering intensity depends strongly on the length scale. In a

.static light scattering SLS experiment one probes density correlations on a lengthscale qy1, and j is a length scale characteristic of the range of density fluctuationsoccurring in the solution. In the regime qj ) 1, the angular dependence of the

.scattered intensity or the static structure factor S q can give us a direct access tothe fractal dimension d of the clusters. The fractal structure of heterogeneousf

.systems is homologous self-similar in a shape or a distribution of mass over arelatively wide range of space size. From an analogy with a fractal self-avoidingwalk of randomly branched macromolecules in semidilute solutions, a percolation


Fig. 13. Effect of temperature and concentration on the reduced collective diffusion coefficients for thesystems and conditions indicated.

Fig. 14. Effect of temperature and concentration on the reduced slow relaxation time for the systemsand conditions indicated.


2 Fig. 15. Plot of the first-order electric field correlation function as a function of q t every third data.point is shown for the displayed systems at the scattering angles, temperature, and concentration

indicated.

w x .model has been developed 74 which predicts a power law behavior of S q

. ym .S q ; q 7

w xwhere m ' d s 2 75 for a monodisperse solution of clusters with excludedf .volume interactions. In the case of a polydisperse solution m ' d 3 y t s 1.6,f

where t s 2.2 is the exponent that characterizes the distribution of cluster sizes inw xthe percolation model 76 .

.The diffusion-limited cluster]cluster aggregation DLCA and reaction-limited .cluster]cluster aggregation RLCA models have been developed to describe

association in complex systems. The cluster]cluster aggregates of the DLCA modelw xhave very open structures with d s 1.75]1.80 77,78 . If the sticking probability isf

very small, the clusters will need to collide many times before they stick, and thisw xtype of aggregation is called RLCA and yields denser aggregates with d s 2.1 79 .f

.In Fig. 16 the intensity profiles in the form of log]log plots of S q vs. q aredisplayed for solutions of UM-chitosan and HM-chitosans of different concentra-tions in the semidilute regime. The solid lines represent the best fit of the

.experimental data with Eq. 7 . The values of m are around 2, virtually independentof polymer concentration and hydrophobicity, and fairly close to the values de-termined from the oscillatory shear-results for the incipient gels of UM-chitosan

.and HM-chitosans see Table 1 . However, in contrast to the values of the fractal


Fig. 16. A log]log plot of the q dependence of the scattered intensity function for the displayed systemsat 258C and for the concentrations indicated. The solid lines represent the best fits of the experimental

..data see Eq. 7 .

dimension obtained from the rheological experiments, the values of d determinedffrom SLS do not allow us to draw a distinction between UM-chitosan andHM-chitosan systems. The reason for these may be that the concentrations utilizedin SLS are far below the concentrations for the formation of incipient gels. In thiscontext we should note that these values of d are significantly higher than thatf . w x1.3 reported 80 from a small-angle X-ray scattering study on solutions of

.chitosan with a degree of deacetylation of 79% in 2 wt.% acetic acid aqueoussolution. Our samples have a deacetylation degree of 84%. The cause for thisdifference in fractal dimension is not clear, but it was found that the heteroge-neous structure was very sensitive to the degree of deacetylation of the chitosansample.

6. Conclusions

This review has shown that association phenomena in systems of chitosan . .UM-chitosan and hydrophobically-modified chitosan HM-chitosan can be wellcharacterized by rheology and light scattering. The linear and non-linear viscoelas-


ticity are affected by factors, such as pH, level of surfactant addition, polymerconcentration, and temperature. These effects have a stronger influence on sys-tems of HM-chitosan. The concentration-induced incipient gels are characterizedby the following power-law G9 ; G0 ; v n, where n is equal to 0.36 and 0.46 forUM-chitosan and HM-chitosans, respectively. Shear thinning behavior was observedat higher shear rates for systems of both UM-chitosan and HM-chitosan. Themagnitude of this effect is governed by pH, surfactant and polymer concentration,and hydrophobicity of the polymer.

The dynamic light scattering results from semidilute solutions revealed that theassociation effects are promoted by decreasing temperature and increasing po-lymer concentration and hydrophobicity. Intensity light scattering results fromsemidilute solutions of UM-chitosan and HM-chitosan indicate a fractal dimensionof approx. 2.

Acknowledgements

This work was supported by Norsk Hydro and the Norwegian Research Councilthrough the program PROSMAT.

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