Properties of polyelectrolyte complex films ofchitosan and gelatinYJ Yin,1 KD Yao,1* GX Cheng1 and JB Ma2

1Research Institute of Polymeric Materials, Tianjin University, Tianjin 300072, People’s Republic of China2The State Key Laboratory of Functional Polymer Materials for Adsorption and Separation, Nankai University, Tianjin 300072, People’sRepublic of China

Abstract: A series of chitosan±gelatin complexes was prepared by varying the ratio of constituents.

Differential scanning calorimetry was used to determine the amount of the different states of water.

The interaction between chitosan and gelatin was checked by IR and X-ray analysis and was related to

mechanical strength. The results indicate that the water take-up of a chitosan±gelatin complex is

depressed by strong interactions within networks. Chitosan can improve the tensile strength of

complex ®lms, and even with high water content these can keep appropriate tensile strength and higher

elongation.

# 1999 Society of Chemical Industry

Keywords: chitosan; gelatin; polyelectrolyte complex; water absorption; mechanical properties

INTRODUCTIONPolyelectrolyte complexes (PECs) are formed by the

reaction of a polyelectrolyte with an oppositely

charged polyelectrolyte in an aqueous solution. Poly-

saccharides, which have bulky pyranose rings and a

highly stereoregular con®guration in their linear back-

bone chains have frequently been studied. Chitosan, a

14-linked 2-amino-2-deoxy-b-D-glucan was prepared

by N-deacetylation of chitin. The cation charged

chitosan can interact with an anionic polyelectrolyte,

eg poly(acrylic acid),1 sodium alginate2 and pectin,3,4

etc, to form polyelectrolyte complexes (PECs). PECs

have numerous applications, such as membranes,

medical prosthetic materials, etc.

Considering that natural polymer±chitin exists in

the form of chitinproteinic complexes in crustacean

shells,5 we have developed a polyelectrolyte complex

of chitosan and gelatin via interaction between a rigid

poly(aminosaccharide) chitosan with ¯exible ampho-

lytic gelatin chains at pH higher than that of the

isoelectric point, pHiso=4.7.6 In the present work, we

focus our study on water absorption and mechanical

properties of chitosan±gelatin polyelectrolyte com-

plexes of various compositions.

EXPERIMENTALMaterialsChitosan was purchased from the Qingdao Institute of

Pharmaceuticals (China). The viscosity average mol-

ecular weight of the puri®ed chitosan was 7.2�105,

calculated by the Mark±Houwink equation:6 [�]=

KMa, where K =1.64�10ÿ30. DD14.0, a=ÿ1.02�10ÿ2 � DD�1.82 and the degree of N-deacetylation

(DD) was 84%. Gelatin (Mw=1.5�105, Mn=

1.0�105) was a biochemical reagent, and the other

reagents were of chemical grade.

Complex film preparationChitosan and gelatin were dissolved in 2wt% of

aqueous acetic acid solution. The mixture was poured

into a Te¯on frame model and maintained at 50°C for

®lm formation. The complex ®lm obtained was soaked

in a mixture of 10wt% of NaOH solution and

anhydrous ethanol (1:1 by volume) for 30min, washed

with deionized water and then dried at 50°C. The

mixing ratio (r) was de®ned as

r � WCS

WCS �WGel

where WCS and WGel are weights of chitosan and

gelatin, respectively.

Differential scanning calorimetry (DSC)A Perkin Elmer DSC-2C (Norwork, USA) was used to

measure the phase transition of water sorbed on the

complex ®lms. The dried ®lms of known weight were

swollen in deionized water to equilibrium state. The

swollen specimens were wiped with a ®lter paper and

transferred into aluminium pans. The pans were

sealed to prevent water from evaporating and were

weighed on a microbalance to calculate the total water

content (W) of the ®lms. The weights of ®lm samples

were 2±10mg. Samples were cooled from room

Polymer International Polym Int 48:429±433 (1999)

* Correspondence to: KD Yao, Research Institute of Polymeric Materials, Tianjin University, Tianjin 300072, People’s Republic of ChinaContract/grant sponsor: National Natural Science Foundation of China; contract/grant numbers: 59633020 and 59583002(Received 2 November 1998; accepted 11 January 1999)

# 1999 Society of Chemical Industry. Polym Int 0959±8103/99/$17.50 429

temperature to 210K and then heated to 320K at a

heating rate of 10Kminÿ1. The fusion enthalpy of

water was evaluated from the DSC curve area using

pure water as a standard. Therefore, the weight of

freezing water in ®lm can be obtained. The weight of

non-freezing water (Wnf) was estimated by subtracting

the total fraction of freezing water (Wtf) from the total

water content (W) in the ®lm.7

Infrared (IR) spectraAn IR study was carried out on ®lms obtained by

evaporation of chitosan and gelatin solutions of 50%

formic acid using a Nicolet 5DX FTIR spectro-

photometer (Madison, USA).

X-ray diffraction observationFilm diffractograms for chitosan±gelatin complex

®lms were recorded using a Rigaku 2038 X diffract-

ometer (Tokyo, Japan), Cu-Ka radiation with a voltage

of 3.0kV and a current of 20mA. A typical 2y scan

ranged from 3° to 35°.

Mechanical propertiesStrip specimens (60�80�0.2mm3) of dried ®lms

were used to test their tensile strengths with a strain

rate of 10mmminÿ1 on a WD-1 electronic stress±

strain testing machine at 25°C. The swollen speci-

mens were taken out of deionized water and tested the

same way as quickly as possible, to minimize the loss of

water by evaporation.

RESULTS AND DISCUSSIONWater absorbing performanceComplex hydrogels are prone to absorb large amounts

of water and become swollen. The driving force is the

water chemical potential difference between the

polymer network and the aqueous phase. Several

theories address solute diffusion in hydrogels. Lee etal7 described three types of water in hydrogels, ie water

bound to polymer, intermediate water, and bulk water.

We reported that three states of water, ie non-freezing

water, freezing intermediate water and freezing bulk

water, exist in chitosan-based hydrogels Kim et al8

including chitosan hybrid polymer networks9 con-

cluded that diffusion of hydrophilic solutes through

hydrogel membranes depends on the molecular size of

the solute and the water content of the hydrogel. Here,

we focus on the change in the states of water.

The DSC curves of chitosan-gelatin complex ®lms

with a different relative chitosan content r, swollen in

deionized water are shown in Fig 1. The correspond-

ing water content (by weight) of total water (W),

freezing water (Wtf) and non-freezing water (Wnf) and

the fusion enthalpy of water (DH) are listed in Table 1.

It is noted that in the calculation of different water

contents, a small amount of mixing heat is omitted

because it is hard to resolve from the endothermic peak

of water. Data indicate that gelatin containing Ð

COOH and ÐNH2 groups absorbs more water than

does chitosan bearing ÐOH and ÐNH2. On adding

chitosan, the water take-up declines. This fact suggests

that there are strong interactions between chitosan and

gelatin which replace the macromolecular chain±water

interactions.

Figure 2 displays the same pattern of water state

Table 1. Contents (by weight) of totalwater (W), freezing water (Wtf), non-freezing water (Wnf), and the fusionenthalpy of water

r W (g/g)a Wtf (g/g)a Wnf (g/g)a DH (Jgÿ1)b

Gelatin 8.63 7.94 0.66 304.3

0.1 3.46 2.95 0.51 291.6

0.2 2.46 2.18 0.28 300.4

0.3 2.24 2.03 0.21 309.3

0.4 2.30 1.88 0.42 279.2

0.6 1.9 1.40 0.50 251.5

0.8 1.62 1.15 0.47 243.9

1.0 1.33 0.73 0.6 187.4

Chitosan 1.30 0.75 0.55 190.2

a g/g symbolizes gram of water per gram of dry polymer.b DH was estimated from the total endothermic areas of water divided by the total water content.

Figure 1. DSC curves of frozen water in chitosan–gelatin polyelectrolytecomplex films with different r values, swollen in deionized water, atequilibrium.

430 Polym Int 48:429±433 (1999)

KD Yao et al

change versus chitosan ratio r. The non-freezing water

content±r curve exhibits a minimum at r =0.3, where

the chitosan±gelatin polyelectrolyte complex reaches

the optimum interactive ratio of its components.

Moreover, the total water content attributed mainly

to freezing water declines with an increase in chitosan

content r in the complexes.

Study of interactions by IR spectrometryThe spectra of chitosan±gelatin polyelectrolyte com-

plex ®lms are given in Fig 3. The spectrum of chitosan

is characterized by its saccharide structure at 902cmÿ1

and 1155cmÿ1, an amino band at 1587cmÿ1 and an

amide I band of the acetyl group at 1651cmÿ1.10 The

spectrum of gelatin has peaks at 1537cmÿ1 (amino

Figure 2. Water states at equilibrium of water content (W), freezing watercontent (Wtf) and non-freezing water content (Wnf) of a chitosan–gelatinpolyelectrolyte complex film plotted against chitosan ratio r.

Figure 3. IR spectra of chitosan–gelatin complexes with different chitosanratios.

Figure 4. Effect of chitosan content r on X-ray diffraction patterns ofchitosan–gelatin polyelectrolyte complex films.

Figure 5. Tensile strength of chitosan–gelatin complex films againstchitosan content r, in the dry state (a) and the swollen state (b).

Polym Int 48:429±433 (1999) 431

Polyelectrolyte complex ®lms of chitosan and gelatin

group) and 1651cmÿ1 (carbonyl stretching).11 Incor-

poration of chitosan leads to small modi®cations in the

spectrum of gelatin, ie shifting of both carbonyl and

amino bands. This result implies that there are

interactions between gelatin and chitosan via a

polyelectrolyte complex.

X-ray diffraction patternFigure 4 shows X-ray diffraction pro®les of chitosan±

gelatin polyelectrolyte complexes with different chito-

san contents r. Chitosan (r =1.0) exhibits a single

crystal peak at 2y =20°,12 while a crystalline peak

starts to split with a 20% gelatin content. These peaks

become weaker with increasing gelatin content up to

60% (r =0.4). Moreover, two peaks appear centred

around 2y=11.7° and 8.6°, which may originate from

new crystals generated in the complex. The weakening

of the crystalline peaks induced by added gelatin

implies that the strong interactions between gelatin

and chitosan lead to their good compatibility.

Mechanical propertiesIntroducing rigid chitosan into ¯exible gelatin in-

creases their tensile strength, both in the dry and

swollen states as shown in Fig 5. The decrease of

tensile strength for the swollen specimens is attributed

to the plasticizing effect of the absorbed water.

It is worth noting that there is a maximum in the

curve of elongation versus chitosan content r (Fig 6).

The maximum occurs at about r =0.5, which corre-

sponds to optimum compatibility between chitosan

and gelatin, which has been veri®ed by the decrcase of

the crystalline peaks (cf. Fig 4).

CONCLUSIONSIR and X-ray results demonstrate that optimum

interactions between chitosan and gelatin exist over a

certain ratio range. The water absorption of gelatin±

chitosan can be depressed via polyelectrolyte complex

formation. Chitosan can improve the tensile strength

of the dry complex ®lm, and especially the mechanical

properties of swollen ®lms.

ACKNOWLEDGEMENTSThe authors thank the National Natural Science

Foundation of China for support of this research

through grants 59633020 and 59583002.

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Figure 6. Elongation of swollen chitosan–gelatin complex films as afunction of chitosan content r.

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