Influence of Soluble Polymer Residues in Crosslinked Carboxymethyl Starch on some Physical Properties of its Hydrogels

Christoph Heßa

Brigitte Hartmannb

M. Dieter Lechnerb

Wolfgang Nierlingb

Christian Seidelc

Werner-Michael Kulickec

a Biotec GmbH & Co. KG,Emmerich, Germany

b University of Osnabrück,Institute of Chemistry,Osnabrück, Germany

c University of Hamburg,Institute of Technical andMacromolecular Chemistry,Hamburg, Germany

Influence of Soluble Polymer Residues inCrosslinked Carboxymethyl Starch on somePhysical Properties of its Hydrogels

Crosslinked carboxymethyl starch (CMS) was synthesized from potato starch in a sin-gle-step procedure with mono- (MCA) and dichloroacetic acid (DCA), using the well-known Williamson reaction. The products varied in their degree of substitution DS(average number of carboxymethyl groups per monomer unit) and crosslinker ratio Fz

(number of crosslinker molecules offered per monomer unit). After neutralizing andremoval of the formed salt, one part of the synthesized CMS networks was pre-swollenin water in an additional purification step in order to wash out unlinked, soluble polymerchains. The rest of the product remained unwashed. Different swelling experimentswere performed with the two samples, before being dried and ground. Both, the FreeSwelling Capacity (FSC) and the Absorption Capacity Under Mechanical Load (AUL) ofthe hydrogels were strongly influenced by chemically unlinked CMS chains that wereonly physically entangled in the network structure. These mobile polymer segmentswere responsible for a significant weight loss of the swollen, unwashed hydrogels overthe course of time. Rheological oscillatory experiments showed that, in order toachieve comparable values for the storage and loss moduli (G0 and G0 0), the polymercontent of an unwashed hydrogel had to be more than twice as high as that of thecorresponding purified product. By using a special rheological test procedure with acyclic temperature program, the long-term stability of CMS gels could be measuredand verified.

Keywords: Hydrogel; Carboxymethyl starch; Free swelling capacity; Absorption underload

Starch/Stärke 59 (2007) 423–429 423

1 Introduction

Because of a wide range of possible applications, indus-trial interest increasingly focuses on three-dimensionallycrosslinked, starch-based polymer structures. Especiallyhydrophilic starch networks, often based on carboxy-methyl starch (CMS), show a promising behavior for dif-ferent applications. First of all, they can be used in theirdry state (xerogel), in which they are used as liquidabsorbers in food industry and medical care products(incontinence articles) [1-3]. On the other hand, there are arising number of established applications of the water-swollen derivatives (hydrogels), for example in medicalultrasonic diagnostics as sonogels where they bridge theair gap between skin and sonographic source [4, 5].

A successful development of new application areas ofstarch gels implies a fundamental knowledge of theirswelling behavior. Important parameters in this context

are the Free Swelling Capacity (FSC), the amount ofabsorbed liquid under load (Absorption Under Load, AUL)and rheological properties, such as the storage modulusG0 and the loss modulus G0 0.

Due to the required reaction environment, the carboxy-methylation and the crosslinking of starch are usually car-ried out under heterogeneous conditions in suspension [1,6-10]. The procedure then leads to an inhomogeneousproduct, containing areas with different crosslinking den-sities. Dependent on the amount of offered crosslinkermolecules, a certain number of the polymer chains are notcovalently linked to the network structure. Only physicallyentangled in the three-dimensional network, these solublechains contribute in a different way to the mechanical gelstability than the chemically fixed components do. Immo-bilized in the xerogel, the unlinked chains are easilywashed out in the swollen state by unraveling, in order tocompensate the gradient in polymer concentration be-tween hydrogel and surrounding solvent.

Evidently, the (time-dependent) loss of polymer influ-ences the hydrogel properties. This is a remarkableaspect, especially concerning applications, where the

Correspondence: M. Dieter Lechner, University of Osnabrück,Institute of Chemistry, Barbarastr. 7, 49069 Osnabrück, Ger-many. Phone: 149-(0)541-969-2819, Fax: 149-(0)541-969-3324, e-mail: [email protected].

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.starch-journal.com

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DOI 10.1002/star.200400383

424 C. Heß et al. Starch/Stärke 59 (2007) 423–429

swollen gel should keep its stability and absorbance ca-pacity over a longer period of time (several hours, days orweeks). Washing out the soluble, unlinked polymer chainsafter crosslinking (for example by swelling in an excess ofsolvent), leads to a purified hydrogel which can be driedagain. In spite of the rising production costs, caused bythis additional washing and drying step, it may, in somecases, be useful to invest this effort. The goal of this paperis to show some significant differences in the physicalproperties of purified (washed) CMS hydrogels in com-parison with unwashed samples.

2 Experimental

2.1 Synthesis of carboxymethyl starch (CMS)gels

Based on potato starch (PS) (“Superior charge 149”, CHP,Carbohydrate Pirna GmbH & Co. KG, 01796 Pirna, Ger-many), CMS with different degrees of substitution DS andcrosslinker ratios Fz was synthesized.

First the potato starch was suspended in an alcohol/watermixture (15 %, w/w, total water content in the reactionbatch) and alkalized with NaOH (3 mol NaOH per molemonomer units) at 207C for 30 min. Both, carboxy-methylation and crosslinking were then carried out simul-taneously, using the Williamson etherification reaction[11]. This was done by adding monochloroacetic acid(MCA, for carboxymethylation [12]) and dichloroaceticacid (DCA, for crosslinking), and stirring the mixture at507C for 3 h. After neutralizing with aqueous HCl andwashing out the generated sodium chloride with an alco-hol/water mixture (80/20), the product was dried in anoven at 607C, ground (using a 500 mm mesh) and sepa-rated in two fractions. The synthesis parameters aresummarized in Tab. 1.

Tab. 1. Differences in the synthesis-parameters andresulting DS values for the examined samples.

Sample Fz nMCA/nAGU*

Solvent Starchconcentrationin the reactionmixture[%, w/w]

DS

260401 0.1 1 EtOH 15 0.45160501 0.1 1 EtOH 12.5 0.49131101 0.1 2.5 i-PrOH 10 1.20150402 0.06 2.5 i-PrOH 10 1.05

* Number of MCA molecules per monomer in the reactionmixture (monomer = anhydroglucose unit, AGU).

While one part of the synthesized samples was used di-rectly for the swelling experiments, the second fractionwas subjected to an additional washing step. To this end,it was suspended in an excess of water to ensure that theswelling capacity of the gel was low enough to excludemost of the liquid from the network structure, and thesystem so formed two phases. While the chemically fixedpolymer segments remained in the network, the unlinked,soluble CMS chains could now penetrate into the waterphase, in order to compensate the polymer concentrationdifference in- and outside the hydrogel. After decantingand adding fresh water several times (depending on thecontent of soluble CMS in the system and the amount ofadded water), most of the soluble, ionic polymer chainswere removed from the gel structure. This was ensured bymonitoring the conductivity of the surrounding solution.After drying and grinding the purified network was exam-ined in different swelling experiments in the same way asthe untreated product fraction.

2.2 Methods and equipment for thecharacterization of the hydrogels

2.2.1 Determination of the degree ofsubstitution DS

The amount of carboxymethyl groups, contained in theCMS network, was determined by titration with sodiumhydroxide according to [3, 4, 13].

2.2.2 Determination of the absorption underload (AUL)

The equipment used to determine the absorption ca-pacity of the xerogels under load, is shown in Fig. 1. ATeflon disc (1 = 30 mm), that fits exactly into a plasticsocket (one of the ends closed with metal gauze), wasloaded with a metal cylinder (total mass mt < 323 g).According to this, about 0.2 g of the xerogel, placed on afilter paper on the bottom of the socket, was put under apressure of about 4.5 kPa. Then the complete systemwas placed in a water bath. To ensure that the waterlevel just reached the bottom of the socket containingthe sample, the latter was placed on a base with a plas-tic filter and an additional filter paper. The gel now swellagainst the applied pressure (the water uptake occurredthrough the filter system and the metal gauze) and liftedthe Teflon disc with the metal cylinder up. In balance, theweight force of the load and the swelling force of thehydrogel compensated each other. After removing thesystem from the water bath (swelling time was 1 h or24 h, depending on the experiment), the AUL value wascalculated from [14]:


Starch/Stärke 59 (2007) 423–429 Influence of Soluble Polymer Residues on Carboxymethyl Starch Hydrogels 425

Fig. 1. Equipment for AUL determi-nation (see text).

qAUL ¼mt �msoc �mw

ms[1]

with mt: total mass of the socket with hydrogel, filterpaper and Teflon disc

msoc: mass of the empty, dry socket with filterpaper and Teflon disc

mw: mass of the water, adhered from the socketand the filter paper

ms: mass of the dry polymer sample

The values for msoc, mw and ms were determined beforestarting the swelling-procedure.

2.2.3 Determination of the free swellingcapacity (FSC)

About 0.2 – 0.4 g of the dry sample were exactly weighedin into a common teabag. The bag was locked with a clipand put into an excess of water at room temperature.After 1 h or 24 h (depending on the experiment) the bagwas taken out of the water and, hanging free by a thread,left for 5 min in order to release the surplus water. After-wards the total mass of the teabag plus hydrogel wasdetermined. The FSC now could be calculated with [15]:

qFSC ¼mt �mtb �mw

ms[2]

with mt: total mass of the gel containing teabag

mtb: mass of the empty, dry teabag

mw: mass of the water, absorbed by the empty,wet teabag

ms: mass of the dry polymer sample

The values for mtb, mw and ms were determined beforestarting the swelling-procedure.

2.2.4 Rheological measurements

The viscoelastic properties of the swollen gels wereexamined using rheological experiments [16, 17]. Themethod of choice for characterizing crosslinked struc-tures is a dynamic oscillatory measurement. Here thesubstance is subjected to a sinusoidal oscillation with adefined amplitude of deformation g0 and frequency o. Ifthe substance shows a viscoelastic behavior, a phaseshift d occurs. If the deformation is sufficiently small, anon-destructive characterization of the substance ispossible. The storage modulus (a measure for elasticity)and the loss modulus (correlating with viscosity) weredetermined from the oscillatory measurements. Thus theviscous and the elastic properties can be determinedseparately. From G0, G0 0 and o, the complex dynamic vis-cosity Z� was calculated with the following equation [16]:

Z� ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

G0� �2þ G00

� �2q

o[3]

The rheological measurements were carried out at T =257C with a strain-controlled Rheometer UDS 200 (PaarPhysica, Graz, Austria) and a cone-plate geometry(Ø 50 mm; cone angle 17). All measurements were madein the linear viscoelastic region (g = 0.09 % in the fre-quency range of o = 1 - 100 rad/s). Here the materialfunctions are independent of the applied deformation. Forthe rheological measurements samples of different con-centration were prepared from the xerogel. An idealoscillatory curve shows a rubber-elastic plateau in theintermediate frequency region. Here the larger moleculechains cannot slide by one another, because the relaxationability of the segments is smaller than the stress velocity.

According to a method described by Brummer et al. [18],the long-term stability of the hydrogels was tested with aspecial test procedure. While the samples passed a cyclictemperature program, storage and loss modulus were



measured at a constant oscillatory frequency (10 s–1). Thetime dependent course of the G’ and G’’ values allowed aprediction of the long-term stability of the gels.

2.2.5 Analysis of the washed out material

Approximately 5 g of the crosslinked unwashed starchderivative was added to 1 L distilled water and stirred atroom temperature during 1 h. After that the dispersionwas filtered with a cloth filter. The filtrate was con-centrated to 500 mL; 100 mL of the filtrate was used forthe conductometric determination of chloride. The con-ductometric measurement was done with silver nitrate asstandard solution (0.1028 mol/L) and the conductivityminimum designated the equivalence point of the titra-tion. The remaining filtrate was evaporated to drynessand weighed.

The residual material was analyzed by IR spectroscopy. Acomparison of this spectrum with those of the unwashedand washed products yields that the washed out residualproduct includes all characteristic absorption peaks ofthe unwashed and washed products (Fig. 2).

Fig. 2. IR spectroscopic analysis of the unwashed andwashed products together with the washed-out residual.

3 Results and Discussion

3.1 Determination of the AUL

In Fig. 3, the results of AUL measurements of washed(purified) and unwashed (containing unlinked polymerchains) hydrogels after a swelling time of 1 h and 24 h areshown. The samples differ in their DS and Fz values.

After 1 h of swelling, the examined samples alreadyreached about 25 % of their 24 h AUL value. As expected,

Fig. 3. Swelling behavior of washed and unwashed CMS-hydrogels under load at 1 and 24 h (absorption underload, AUL) according to Equation (1). a) DS = 0.49, Fz =0.1 ; b) DS = 1.20, Fz = 0.1 ; c) DS = 1.05, Fz = 0.06 (DS =degree of substitution, Fz = crosslinker ratio).



the absorption capacity of a gel grows with its hydrophiliccharacter (correlating with DS). Due to counter-ion con-densation [19-21], the effective DS values (charged frac-tion) for the samples with DS = 1.05 and 1.20 can beregarded as similar. Comparing the washed and un-washed derivatives makes evident that for both, shortand long swelling periods, significant differences occur inthe ability of the samples to absorb water under load.While a reduction of the crosslinker amount (from Fz = 0.1to 0.06) hardly reduces the 24 h-AUL value of the washedhydrogels by destabilizing the network, the unwashedderivative collapses under the mechanical load causedby the equipment. The pressure forces the unlinked poly-mer chains to unravel and to pass over into the surround-ing water (Figs. 3b and 3c). In case of the unwashed,slightly crosslinked sample (Fz = 0.06), nearly all of the gelhad disappeared after 24 h under load. The remainingamount was too small to determine the AUL value (Fig. 3c).

3.2 Determination of the FSC

The time-dependent weight loss for a swollen, purifiedCMS hydrogel, in comparison with its unwashed coun-terpart, is shown in Fig. 4. As a starting point (maximum),the 24-h FSC value was chosen. To accelerate the effects,this experiment was carried out in a water bath at 407C.

The two, nearly parallel curves in Fig. 4 show those overthe whole 50 day period the FSC values for the purifiedCMS hydrogel are significantly higher. The FSC of thewashed sample after 50 days is even higher than that ofthe unwashed sample at the beginning of the experiment.

Fig. 4. Free swelling capacity (FSC) of a washed andunwashed CMS-based hydrogel (DS = 0.45, Fz = 0.1) as afunction of time t according to Equation (2). The startingpoint of the FSC measurements was 24 h after swelling.Absolute FSC axis (DS = degree of substitution, Fz =crosslinker ratio).

Both samples exhibit a significant weight loss within thefirst ten days. The main reason for this is the fact that evenpurified CMS hydrogels consist of numerous small colloi-dal networks that are separated from each other (micro-gels). If these colloids are small enough, they are able topenetrate through the porous tissue of the teabag andpass over into the surrounding water bath. In this respect,the weight loss is systematically caused by the proce-dure of FSC determination. Of course, the relative FSCdecrease for the unwashed CMS hydrogel is muchhigher than that of the corresponding purified product.To show this, the ordinate of Fig. 4 was converted fromabsolute values to a percentual scale (Fig. 5). The start-ing points (maximum FSC) of both samples wereassigned to 100 %.

While the washed gel loses about 30 % of its initial FSC,the corresponding value for the unwashed derivative is40 %. The difference is caused by the unlinked polymerfraction of the latter.

3.3 Unwashed material

One washing process in distilled water yields approxi-mately 14 % sodium chloride and 11 % unlinked, washedout, water-soluble polymer.

3.4 Rheological measurements

The frequency dependence of the storage modulus G’,the loss modulus G‚ and the complex dynamic viscosities)Z*) for two correlating (washed and unwashed) hydrogels

Fig. 5. FSC of a washed and unwashed CMS-basedhydrogel (DS = 0.45, Fz = 0.1) as a function of time taccording to Equation (2). The starting point of the FSCmeasurements was 24 h after swelling. Relative FSC-axis(DS = degree of substitution, Fz = crosslinker ratio).



(Fz = 0.1; DS = 0.45), each containing 3 % (w/w) of CMS,are shown in Fig. 6. As can be seen, removing the chainsof the soluble polymer from the CMS network influencesits rheological behavior. While the purified sample exhibitslinear curves, i.e. a gel-like rheological behavior, for G’, G’’and Z*, the unwashed hydrogel shows a non-linear be-havior (i.e. a non-gel-like rheological behavior) on a sig-nificantly lower level. Comparing the absolute values of G’and G’’, the differences between the two samples amountto up to five decades.

In Fig. 7 the results of a long-term stability test (a tem-perature cycle program) with two correlating (washedand unwashed) CMS hydrogels are shown. To achievecomparable storage and loss moduli, different polymerconcentrations had to be chosen (washed gel: 2 %,w/w; unwashed gel: 5 %, w/w). While the storage andloss moduli of the purified sample are not significantlyaffected by the temperature program (Fig. 6a), theunwashed hydrogel exhibits a maximum of G’ and G’’during the second period of heating, followed by a de-crease of these values down to lower levels than in thebeginning of the experiment (Fig. 7b). However, the dif-ferences (about half a decade for both, G’ and G’’) arenot so great as to markedly impair the usability of thisproduct as a stable gel, so that both samples exhibit acertain long-term stability. Nevertheless it is obviousthat the washed gels exhibit a greater stability than theunwashed ones.

4 Conclusions

Significant differences in their swelling behavior ofwashed and unwashed CMS based hydrogels werefound. Decomposition of physical entanglements inswollen, unwashed samples leads to a pronounced loss

Fig. 6. Rheological properties of a washed and unwa-shed CMS hydrogel (DS = 0.45, Fz = 0.1). CMS con-centration = 3 % (w/w) (DS = degree of substitution, Fz =crosslinker ratio).

Fig. 7. Rheological behavior of a washed (6a) and un-washed (6b) CMS hydrogel (DS = 0.49, Fz = 0.1) over thecourse of a cyclic temperature program (long stabilitytest). CMS concentration (washed gel) = 3 % (w/w); CMSconcentration (unwashed gel) = 5 % (w/w) (DS = degreeof substitution, Fz = crosslinker ratio).

of the liquid absorption capacity not only when loadingthe gels with external mechanical forces (where theeffects are particularly visible). A remarkable weight losscan also be noticed in free-swelling experiments, carriedout for example in teabags. The different rheological be-havior of washed and unwashed hydrogels is a strongindicator for the influence of chemically unlinked, solublepolymer chains on the sturdiness of the gels. To achievecomparable values for the storage and loss moduli, thepolymer concentration of an unwashed derivative has tobe more than twice as high as that of the corresponding,purified product. If this is considered, long-term stablehydrogels can be synthesized even with unwashed sam-ples. The user has to decide, whether, for his specialapplication, the effort of washing out the soluble polymerchains in an additional process step may be worthwhile.Undoubtedly, the purified CMS gels show a better be-



havior in every respect. Preferably, the purified CMS gelscould be used as sonogels in medical ultrasonic diag-nostics, as they are toxicologically unobjectionable forhuman beings. The unwashed products may be used forseveral other applications especially those where long-term stability is not required.

Acknowledgements

The authors would like to thank the Bundesministeriumfür Verbraucherschutz, Ernährung und Landwirtschaft(BMVEL), Berlin, Germany and the Fachagentur Nach-wachsende Rohstoffe (FNR), Gülzow, Germany for finan-cial support.

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(Received: November 25, 2004)(Revised: June 6, 2007)(Accepted: June 6, 2007)


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Influence of Soluble Polymer Residues in Crosslinked Carboxymethyl Starch on some Physical Properties of its Hydrogels